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Long-Term Performance of Zero-Valent Iron Permeable Reactive Barriers: A Critical Review
Abstract
Permeable reactive barriers (PRBs) have shown great promise as an alternative to pump and treat for the remediation of groundwater containing a wide array of contaminants including organics, metals, and radionuclides. Analyses to date have focused on individual case studies, rather than considering broad performance issues. In response to this need, this study analyzed data from field installations of in situ zero-valent iron (ZVI) PRBs to determine what parameters contribute to PRB failure. Although emphasis has been placed on losses of reactivity and permeability, imperfect hydraulic characterization was the most common cause of the few PRB failures reported in the literature. Graphical and statistical analyses suggested that internal EH, influent pH, and influent concentrations of alkalinity, NO3 − and Cl− are likely to be the strongest predictors of PRBs that could be at risk for diminished performance. Parameters often cited in the literature such as saturation indices, dissolved oxygen, and total dissolved solids did not seem to have much predictive capability. Because of the relationship between the predictive parameters and corrosion inhibition, it appears that reactivity of the ZVI, rather than the reduction in permeability, is more likely the factor that limits PRB longevity in the field. Due to the sparseness of field monitoring of parameters such as EH, the data available for these analyses were limited. Consequently, these results need to be corroborated as additional measurements become available. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/63236/1/ees.2006.0071.pdf
... Dissolved species and pathogens are mostly quantitatively removed from aqueous solutions in the presence of metallic iron (Fe 0 ) (Devonshire 1890, Baker 1934. This century-old knowledge has been intensively applied during the past three decades in solving the challenges for a clean environment and safe drinking water (Wakatsuki et al. 1993, Henderson and Demond 2007, Chen et al. 2014, Guan et al. 2017, Singh et al. 2022, Lawrinenko et al. 2023a). Despite such a broad application, the science of the Fe 0 remediation technology has not yet been achieved (Cao et al. 2022, Lawrinenko et al. 2023b. ...
... several other authors made similar conclusion which finally led to the "broad concensus" (O'Hannesin and Gillham 1998) that Fe 0 oxidative dissolution is the anodic reaction occurring simultaneously to the cathodic reduction of the contaminant of interest (e.g., an electrochemical reaction). It should be pointed out that this reasoning is limited to reducible contaminants, while several other non-reducible species, including pathogens, have been quantitatively eliminated from the aqueous phase in the presence of Fe 0 (You et al. 2005, Henderson and Demond 2007, Noubactep 2007. Thus, the removal of non-reducible contaminants and microbes cannot be explained by the concept of reductive transformation or degradation. ...
... This is when the time to material exhaustion (t∞), is lower than the time to clogging (tclog). A Fe 0 volumetric ratio of 25 % in a Fe 0 /sand system roughly corresponds to (i) 50 % Fe 0 (w/w) which has been used at several field installations (Henderson andDemond 2007, Guan et al. 2015) and (ii) the solution to the special case (t∞ = tclog, where clogging occurs promptly at Fe 0 complete exhaustion). The temporal issue, that is the extent of porosity loss at any time (or time to clogging) is difficult or even impossible to address because the corrosion kinetics is not known and has even not been really investigated in the Fe 0 remediation context (Moraci et al. 2016, Noubactep 2016, Yang et al. 2022. ...
Permeable reactive barriers (PRBs) containing metallic iron (Fe0) as reactive materials are currently considered as an established technology for groundwater remediation. Fe0 PRBs have been introduced by a field demonstration based on the fortuitous observation that aqueous trichloroethylenes are eliminated in Fe0-based sampling vessels. Since then, Fe0 has been tested and used for treating various biological (e.g., bacteria, viruses) and chemical (organic and inorganic) contaminants from polluted waters. There is a broad consensus on the view that "reactivity loss" and "permeability loss" are the two main problems hampering the design of sustainable systems. However, the view that Fe0 is a reducing agent (electron donor) under environmental conditions should be regarded as a distortion of Corrosion Science. This is because it has been long established that aqueous iron corrosion is a spontaneous process and results in the Fe0 surface being shielded by an oxide scale. The multi-layered oxide scale acts as a conduction barrier for electrons from Fe0. Accordingly, "reactivity loss", defined as reduced electron transfer to contaminants must be revisited. On the other hand, because "stoichiometric" ratios were considered while designing the first generation of Fe0 PRBs (Fe0 as reductant), "permeability loss" should also be revisited. The aim of this communication is to clarify this issue and reconcile a proven efficient technology with its scientific roots (i.e., corrosion science).
... Therefore, following these steps is essential; nonetheless, it must accurately represent the field condition, resulting in unexpected physical performances/failures. Despite the fact that many researchers are studying the long-term performances of PRB using modeling tools, there are still numerous gaps between predicted data and field observations (Henderson & Demond, 2007;Madzin et al., 2016). Short-term column studies and geochemical modeling cannot accurately represent the aging of PRB materials and the processes involved in a real-world scenario that affect the performance of a PRB system (Farrell et al., 2000;Henderson & Demond, 2007). ...
... Despite the fact that many researchers are studying the long-term performances of PRB using modeling tools, there are still numerous gaps between predicted data and field observations (Henderson & Demond, 2007;Madzin et al., 2016). Short-term column studies and geochemical modeling cannot accurately represent the aging of PRB materials and the processes involved in a real-world scenario that affect the performance of a PRB system (Farrell et al., 2000;Henderson & Demond, 2007). The reactive materials in the system are theoretically sufficient to remove pollutants for a long time; however, the situation may differ in practice. ...
... They are evaluated by investigating the system's geochemical conditions, monitoring hydraulic field measurements at the existing field, and results obtained from the laboratory column experiments. As a result, monitoring the condition and long-term performance of a PRB system is essential to achieve the desired goals of a PRB by assessing the temporal variation of the system's permeability, porosity, and reactivity (Henderson & Demond, 2007;Regmi et al., 2009). However, the long-term monitoring of permeable barriers is rarely done due to the lack of financial and technical aid. ...
Waste generation is increasing rapidly worldwide; compared to 2018, there will be a 19% upsurge in developed countries and a 40% increase in developing countries by 2050. Currently, 33% of municipal solid waste (MSW) generated globally is dumped on open sites; in contrast, nearly all the waste generated in developing countries is openly dumped, where no pollution prevention mechanisms are available. The leachate may contain various unspecified contaminants, including pharmaceuticals, heavy metals, volatile organic compounds, xenobiotic organic compounds, and other organic and inorganic compounds; thus, the pH and ionic strength of leachate may vary spatially and temporally. According to the literature, the review has identified the natural attenuation of heavy metals such as Fe2+, Cu2+, and Cr3+ and inorganic ions (NO3−, Cl−) in and around dumpsites were significant and discussed the consequences caused by pH and ionic concentration changes of leachate on contaminant transport processes. However, several cases noted groundwater pollution at open dumpsites due to the deep percolation of leachate. Thus, the significance of having an effective, perceptible groundwater remediation treatment method and the limitations of currently available leachate treatment technologies were discussed. This review highlights the permeable reactive barriers (PRB) as a promising technology in the remediation of already contaminated groundwater at open dumpsites and various possible PRB materials for leachate treatment that can withstand a wide range of pH and ionic strength. The significant attributes of continuous monitoring of PRB at open dumpsites were recognized while discussing the impact of damage caused by the leachate. Novel technologies and limitations in PRB applications have been highlighted, suggesting adaptability to different environmental conditions. The review provides up-to-date information on groundwater remediation at open dumpsites using PRBs. A shortfall of data on lab, pilot scale experiments, and field monitoring data on PRB studies was noticed during this review study, recommending fulfilling these research gaps.
... Permeable reactive barriers are expensive in its purchase, too, but after installement in the aquifer they work for long periods of time. The barriers are inserted downgradient of the contaminant source and work in-situ, decontaminating the groundwater by physical, chemical or biological processes (Jambor et al., 2005;Henderson et al., 2007). There are the so called funnel-and-gate systems as well as reactive curtains. ...
... Elemental iron (Fe 0 ) is the most widely used material for contaminant removal in permeable reactive barriers. Efficient decontamination by Fe 0 has been reported for organic and inorganic contaminants, including heavy metals and radionuclides (Jambor et al., 2005;Henderson et al., 2007). The successful removal of viruses has also been reported (You et al. 2005). ...
... The use of elemental iron (Fe 0 ) for the remediation of contaminated groundwater has been an area of intensive research during the past fifteen years (Gillham et al., 1994;Matheson et al., 1994;Jambor et al. 2005, Henderson et al., 2007. Reductive degradation/precipitation has been considered to be the main removal process in dealing with reducible contaminants. ...
The reactivity of elemental iron, Fe0 is widely investigated for use in permeable
reactive barriers. Fe0 is known for its efficient removal of a wide range of contaminants, like organic and inorganic substances, heavy metals and viruses.
Beside its efficacy, Fe0 is unexpensive and available in quantities huge enough for
the use in permeable reactive barriers. Aqueous decontamination by Fe0 (e.g., in Fe0-H2O systems) may proceed by (i) contaminant adsorption onto Fe0, aged and nascentFe0 corrosion products, (ii) contaminant co-precipitation with nascent Fe0
corrosion products, (iii) contaminant reduction by Fe0 (direct reduction), and (iv) contaminant reduction by FeII and atomic or molecular hydrogen (indirect reduction). Investigations for the efficacy of Fe0 are abundantly performed under shaken conditions.
In this work the influence of shaking intensities on the decontamination in “Fe0-H2O” systems was investigated. Methylene blue (MB) was used as a model contaminant for the characterization of the removal efficiency of Fe0. MB is both
easy in handling and inexpensive and its adsorption behavior is comparable with
several organic contaminants. Besides, it is a contaminant of the textile industry
itself. MnO2 and GAC were added as comparable systems to the “Fe0-H2O” system
for a better understanding of proceeding processes in this system. Investigations were performed under non-shaken, mild and violent shaken conditions. Shaking duration was either one day, three or five days.
It could be shown, that shaking intensity as well as shaking duration has a significant influence on the discoloration of MB and thus decontamination. The generation of corrosion products was accelerated and the contaminant removal, which is mainly due to adsorption and co-precipitation on in-situ generated corrosion products, increased significantly. Furthermore, the pattern of discoloration, i.e. decontamination, under shaken conditions differed strongly from the pattern seen in the undisturbed experiments.
Data obtained by this work indicate, that experiments performed under shaken conditions might be difficult in transfer to reality and when compared with each other. Therefore, investigation of decontamination efficiency or removal mechanisms should be performed under undisturbed (non-shaken) conditions
... In the last thirty years, substantial research has been done on novel and sustainable in-situ passive water treatment approaches, such as Permeable reactive barriers (PRBs) and bio-thermal treatments. (Henderson and Demond, 2007). ...
... There have since been several published papers and reviews on PRBs. Most focus on removing a specific contaminant species with a suitable reactive barrier and the contaminants removal mechanism (Henderson and Demond, 2007;Noubactep and Caré, 2010). However, at this time, there seems to be a lack of a study that thoroughly discusses all critical aspects of PRB design and functioning. ...
... Due to the effectiveness of this approach on these contaminants, it was applied to other groundwater pollutants (Obiri-Nyarko et al., 2014). In the underlying research period, ZVI was the most used reactive material, as it could remediate various contaminants (Henderson and Demond, 2007). ...
Groundwater quality is deteriorating due to contamination from both natural and anthropogenic sources. Traditional "Pump and Treat" techniques of treating the groundwater suffer from the disadvantages of a small-scale and energy-intensive approach. Permeable reactive barriers (PRBs), owing to their passive operation, offer a more sustainable strategy for remediation. This promising technique focuses on eliminating heavy metal pollutants and hazardous aromatic compounds by physisorption, chemisorption, precipitation, denitrification, and/or biodegradation. Researchers have utilized ZVI, activated carbon, natural and manufactured zeolites, and other by-products as reactive media barriers. Environmental parameters, i.e., pH, initial pollutant concentration, organic substance, dissolved oxygen, and reactive media by-products, all influence a PRB's performance. Although their long-term impact and performance are uncertain, PRBs are still evolving as viable alternatives to pump-and-treat techniques. The use of PRBs to remove anionic contaminants (e.g., Fluoride, Nitrate, etc.) has received less attention since precipitates can clog the reactive barrier and hinder groundwater flow. In this paper, we present an insight into this approach and the tremendous implications for future scientific study that integrates this strategy using sustainability and explores the viability of PRBs for anionic pollutants.
... Over the last three decades, permeable reactive barriers (PRBs) containing zero valent iron (Fe 0 , ZVI) [1][2][3][4][5][6][7] and injected n-Fe 0 [8][9][10] have been used to decontaminate (confined and unconfined) aquifers and soils. This technology is addressed by >1000 patents and patent applications, together with >10,000 academic publications (e.g., references [1][2][3][4][5][6][7][8][9][10]). ...
... Over the last three decades, permeable reactive barriers (PRBs) containing zero valent iron (Fe 0 , ZVI) [1][2][3][4][5][6][7] and injected n-Fe 0 [8][9][10] have been used to decontaminate (confined and unconfined) aquifers and soils. This technology is addressed by >1000 patents and patent applications, together with >10,000 academic publications (e.g., references [1][2][3][4][5][6][7][8][9][10]). A PRB places a reactive permeable wall or barrier containing Fe 0 into an aquifer, in order to intercept the flowing, contaminated water plume. ...
... The PRB remediation approach permanently reduces the aquifer porosity and permeability [4,5,[15][16][17]. This reduction occurs within the treatment areas (PRB, injected zone and the adjacent sediments). ...
Polluted aquifers can be decontaminated using either ZVI (zero valent iron) permeable reactive barriers (PRB) or injected ZVI. The placement of ZVI within the aquifer may take several decades to remediate the contaminant plume. Remediation is further complicated by ZVI acting as an adsorbent to remove some pollutants, while for other pollutants, it acts as a remediation catalyst. This study investigates an alternative aquifer decontamination approach to PRB construction or n-Fe 0 injection. The alternative approach reconstructs the potentiometric surface of the aquifer containing the contaminant. This reconstruction confines the contaminant plume to a stationary, doughnut shaped hydrodynamic mound. Contaminated water from the mound is abstracted, decontaminated, and then reinjected, until all the water confined within the mound is decontaminated. At this point, the decontaminated mound is allowed to dissipate into the surrounding aquifer. This approach is evaluated for potential use in treating the following: (i) immiscible liquid plumes; (ii) miscible contaminant and ionic solute plumes; (iii) naturally contaminated aquifers and soils; and (iv) contaminated or salinized soils. The results indicate that this approach, when compared with the PRB or injection approach, may accelerate the decontamination, while reducing the overall amount of ZVI required.
... Based on site conditions, such as groundwater chemistry and hydrological characteristics, the expected performance of PRBs can be predicted. Site-specific data can be used to conduct life-cycle analysis of PRBs (Henderson and Demond, 2007). Under ideal conditions such as pH and redox conditions within the PRB, sufficient residence time coupled with low total dissolved solids and negligible oxygen in the influent groundwater can lead to sustained PRB performance that surpasses the lifetime of contaminant plumes (Wilkin et al., 2014). ...
... In 2002, the United States Environmental Protection Agency (EPA) adopted PRBs as a standard remediation technology for the long-term treatment of polluted groundwater. Since then, PRBs have been used at over 90 sites in the United States and over 200 sites worldwide, of which 120 PRBs are iron-based (ITRC, 2005;Henderson and Demond, 2007;Chen et al., 2011;Fu et al., 2014). Most of these PRBs were installed for groundwater remediation of chlorinated hydrocarbons (Weber et al., 2013). ...
... Most of these PRBs were installed for groundwater remediation of chlorinated hydrocarbons (Weber et al., 2013). The PRBs are generally effective; however, challenging groundwater conditions and resulting in-situ changes have led to their varied performance and shorter than expected service life in some cases (Henderson and Demond, 2007). ...
Permeable reactive barriers (PRBs) are used for groundwater remediation at contaminated sites worldwide. This technology has been efficient at appropriate sites for treating organic and inorganic contaminants using zero-valent iron (ZVI) as a reductant and as a reactive material. Continued development of the technology over the years suggests that a robust understanding of PRB performance and the mechanisms involved is still lacking. Conflicting information in the scientific literature downplays the critical role of ZVI corrosion in the remediation of various organic and inorganic pollutants. Additionally, there is a lack of information on how different mechanisms act in tandem to affect ZVI-groundwater systems through time. In this review paper, we describe the underlying mechanisms of PRB performance and remove isolated misconceptions. We discuss the primary mechanisms of ZVI transformation and aging in PRBs and the role of iron corrosion products. We review numerous sites to reinforce our understanding of the interactions between groundwater contaminants and ZVI and the authigenic minerals that form within PRBs. Our findings show that ZVI corrosion products and mineral precipitates play critical roles in the long-term performance of PRBs by influencing the reactivity of ZVI. Pore occlusion by mineral precipitates occurs at the influent side of PRBs and is enhanced by dissolved oxygen and groundwater rich in dissolved solids and high alkalinity, which negatively impacts hydraulic conductivity, allowing contaminants to potentially bypass the treatment zone. Further development of site characterization tools and models is needed to support effective PRB designs for groundwater remediation.
... Permeable Reactive Barriers (PRBs) are among the most efficient and promising technologies for in-situ remediation of contaminated aquifers and groundwater worldwide . Since the first pilotscale PRB application in Borden, Ontario, Canada, in 1991 and the first commercial full-scale PRB emplacement in Sunnyvale, California, in 1994California, in -1995 PRBs have been installed worldwide, especially in North America and Europe (Birke et al., 2003;Birke and Parbs, 2006;Gavaskar et al., 2000;Gillham and O'Hannesin, 1994;Henderson and Demond, 2007;ITRC, 2005;Ludwig et al., 2009;Phillips et al., 2010;Sivavec et al., 2003;USEPA, 1998;Wang et al., 2016;Wilkin et al., 2014). In the last three decades, PRBs have evolved from "innovative" remedial measures for contaminant management to a "developed" remediation technology that has already been erected at various contaminated sites around the globe (Warner, 2014). ...
... The long-term performance of a PRB system depends a lot on the selection of suitable reactive materials for the target contaminants (Henderson and Demond, 2007). Several reactive materials have been used for field applications of PRBs, treating a variety of pollutants around the world, such as zero-valent iron (ZVI), granular activated carbon (GAC), limestone, zeolites, and various organic materials (Birke et al., 2007(Birke et al., , 2003; Faisal et al., 2018;RUBIN, 2020;Thiruvenkatachari et al., 2008;USEPA, 1998;Wang et al., 2016). ...
... Several reviews on PRBs have been published in the last few years, however, most of these papers focus on specific issues of ZVI without providing extensive field performance information on various sites around the world (Henderson and Demond, 2007;Korte, 2001;Scherer et al., 2000). However, Birke et al. (2003) provided comprehensive insights into European field-scale PRB applications and their performances after certain years of operations. ...
Permeable reactive barriers (PRBs) are significant among all the promising remediation technologies for treating contaminated aquifers and groundwater. Since the first commercial full field-scale PRB emplacement in Sunnyvale, California, in 1994–1995, >200 PRB systems have been installed worldwide. The main working principle of PRB is to treat a variety of contaminants downstream from the contaminated source zone (“hot spot”). However, to accurately assess the longevity of PRB, it is essential to know the total contaminant mass in the source area and its approximate geometry. PRBs are regarded as both a safeguarding technique and an advanced decontamination technique, depending on the contamination scenario and its outcome during the operational life of the barrier. In the last three decades, many PRBs were performed very well and provided a likely designated performance for the contaminated sites. However, there is still the necessity of its potential implications for different PRBs worldwide. Therefore, this study presents a comprehensive overview of field-scale PRBs applications and their long-term performance after on-site emplacements. This paper provides in-depth insight into PRBs as a potential passive remedial measure, covering all significant dimensions, for eliminating the contaminated plume over a long time in the subsurface. The overview will help all the stakeholders worldwide to understand the implications of PRB's field-scale application and help them take all the required measures before its on-site application to avoid any potential failure.
... As a result of volumetric expansion, Fe°-filters are subject to loss of porosity and, therefore, loss of permeability [35,[47][48]. The solid CPs are iron hydroxides and iron oxides such as Fe 3 O 4 , Fe 2 O 3 , FeOOH, Fe(OH) 2 , and Fe(OH) 3 [26][27][48][49][50]. These minerals have little or no positive permanent surface charge generated by the adsorption of protons on the surface of the corroded material [51][52]. ...
... (Si-O-Si) are hydrophobic groups with low reactivity and therefore are hardly involved in surface chemistry in aqueous solutions [71]. Silica is very often added to Fe° to remedy the problem of chemical compaction of the iron bed which causes the clogging of the pores [26,[72][73][74], generally caused by the CPs of iron which are responsible for decontamination, among other things. ...
... Furthermore, 25% S also seems to be sufficient within a ternary device with 25%/25%/50% Fe°/S/Pz. Some studies were found that the presence of different functional groups in the dyes such as OM, can be an important factor for selective interaction with iron oxide nanoparticles [2,[26][27]94]. The properties to serve the system as receptacles for Fe°CPs, makes Fe°/S/Pz stable device, which allow to reduce the proportion of Fe° in the RZ, Figure 5. However, silica is very often added to Fe° to remedy the problem of chemical compaction of the iron bed that causes clogging of the pores [26,[72][73][74], which is usually caused by iron CPs that are responsible for decontamination, among other things; therefore, it is important to retain a sufficient proportion of sand to ensure this does not occur early. ...
... The PRB technology generally involves the emplacement of a barrier filled with a reactive material(s) across the flow trajectory of the contaminant plume. As the contaminated groundwater flows passively through the barrier under the influence of the natural hydraulic gradient, the contaminants are removed (via mechanisms such as sorption, biodegradation, ion exchange, precipitation, etc.), allowing treated groundwater to emerge downstream of the barrier (Henderson and Demond, 2007;Obiri-Nyarko et al., 2014). Several forms/designs of the PRB technology have emerged since its inception, and these are based on factors such as the employed removal processes and the nature of the contamination. ...
... Many studies have shown that the operational life of the PRB can be truncated, owing to early exhaustion of the capacity or passivationinduced loss of the reactive medium (Kamolpornwijit et al., 2003;Obiri-Nyarko et al., 2014;Yang et al., 2021). Some researchers have also reported changes in the pore geometry (i.e. increase or reduction of porosity) due to reorganization of grains by the infiltrating solution, dissolution/degradation of media, biofouling or production of secondary products during PRB operation (e.g., Eykholt et al., 1999;Kamolpornwijit et al., 2003;Henderson and Demond, 2007;Vignola et al., 2011). Obiri-Nyarko et al. (2014) reviewed reactive materials used in PRBs and noted that when mixtures of materials are used, they sometimes tend to show antagonistic (inhibitory) instead of synergistic (stimulatory) effects. ...
... Evaluating the hydraulic performance of potential PRB materials for in situ groundwater remediation is crucial because many of the reported PRBs failures were caused by changes (increase/decrease) in porosity and hydraulic conductivity of the medium (e.g., Henderson and Demond, 2007). Table 9 displays the hydraulic conductivity values measured before and after solute injection into the columns. ...
Permeable adsorptive barriers (PABs) consisting of individual (compost, zeolite, and brown coal) and composite (brown coal-compost and zeolite-compost) adsorbents were evaluated for their hydraulic performance and effectiveness in removing aqueous benzene using batch and column experiments. Different adsorption isotherms and kinetic models and different formulations of the equilibrium advection-dispersion equation (ADE) were evaluated for their capabilities to describe the benzene sorption in the media. The batch experiments showed that the adsorption of benzene by the adsorbents was favourable and could be adequately described by the Freundlich and Langmuir isotherms and the pseudo-second-order kinetic model. Particle attrition and structural reorganization occurred in the columns, possibly introducing preferential flow paths and resulting in slight changes in the final hydraulic conductivity values (4.3 × 10⁻⁵ cm s⁻¹–1.7 × 10⁻³ cm s⁻¹) relative to the initial values (4.2 × 10⁻⁵ cm s⁻¹–2.14 × 10⁻³ cm s⁻¹). Despite the fact that preferential flow appeared to have an impact on the performance of the investigated adsorbents, the brown coal-compost mixture proved to be the most effective adsorbent. It significantly delayed benzene breakthrough (R = 29), indicating that it can be applied as a low-cost effective adsorbent in PABs for sustainable remediation of benzene-contaminated groundwater. The formulated transport models could fairly describe the behaviour of benzene in the investigated media under dynamic flow conditions; however, model refinement and additional experimental studies are needed before pilot/full-scale applications to improve the fits and verify the benzene removal processes. Our results generally demonstrate how such studies can be useful in evaluating potential reactive barrier materials.
... Previous studies have proven that ZVI is a strong reducing material for organic contaminants such as trichloroethylene (TCE) and nitrobenzene (Xin et al. 2018;Yin et al. 2016). The proven effectiveness of ZVI led to its practical application as an effective and inexpensive medium for in situ treatment technologies for the remediation of hazardous contaminants, such as permeable reactive barriers (PRB) (Henderson and Demond 2007;Phillips et al. 2010). Our research group recently optimized the reduction of NTO using acid-pretreated ZVI in batch assays, providing complete conversion of NTO to ATO within 10 min at pH 3. ...
... influent after extended operation (29% loss after 10,900 PVs or 180 days) ( Table 2). Previous studies have reported that iron corrosion products caused a decrease of the hydraulic conductivity of permeable reactive barriers (PRB) packed with ZVI (Henderson and Demond 2007;Phillips et al. 2010;Santisukkasaem and Das 2019;Xin et al. 2018;Yang et al. 2021). Corrosion of ZVI to maghemite and magnetite led to a 50% decrease in hydraulic conductivity of a ZVI column fed with tap water for 1 month (Santisukkasaem and Das 2019). ...
The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the US Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for 6 months (ca. 11,000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1 M HCl, fully removing NTO. After the experiment, solid-phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.
Graphical abstract
... In this regard, the use of metallic iron (Fe 0 ) and hexavalent iron (Fe(VI)), in particular, has attracted interest from researchers involved in the fields of environmental remediation and water/wastewater treatment. Iron species can act as potent reductants, oxidants, adsorption materials and coagulants exhibiting high efficiency in the removal of various water contaminants including arsenic, bromate, chromate, halogenated organics, mercury, nitrate, nitroaromatics, pesticides, phenolic compounds, phosphates, selenium, uranium and zinc [1][2][3][4][5]. ...
... Wire diameter (mm) = 0.06 Specific weight (g/m) = 0.0076 Specific surface area (m 2 The effect of iron surface passivation was assessed in terms of Cr(VI) reduction efficiency in relation to treated water volume, expressed in terms of bed volume (BV = inflow water volume/reactive material volume). The techniques that were evaluated to counter iron material passivation were (i) chemical cleaning using HCl 36%, (ii) mechanical cleaning using a wire brush and (iii) cleaning via application of ultrasounds using a 200 W Hielscher (Teltow, Germany) ultrasonic processor, model UP200S. ...
Iron species can act as electron donors, electron acceptors or serve as a sorbent to co-precipitate contaminants. These properties, along with its relatively low cost as a material, make iron an ideal compound for environmental applications in the removal of pollutants from water and wastewater. This study assesses the use of metallic iron as a reductant for the removal of toxic Cr(VI) from aqueous solutions, as well as the use of hexavalent iron (ferrates) for the removal of organic compounds, turbidity and biological contaminants from water and wastewater. Laboratory-scale experiments show that the Cr(VI) removal efficiency of metallic iron filling materials, such as scrap iron fillings, via reduction to Cr(III) and the subsequent precipitation/filtration of aggregates can reach values over 99.0%. Moreover, the efficiency of ferrates, in situ synthesized via a low-cost Fe0/Fe0 electrochemical cell, in the removal of organic compounds, turbidity and biological contaminants from high-strength industrial wastewater, biologically treated wastewater and natural water can also reach values over 99.0%. The results showed that iron species can be applied in low-cost and environmentally friendly technologies for natural water remediation and wastewater treatment. Furthermore, the study showed that the challenge of an iron material’s surface passivation, as well as of ferrates’ procurement cost and stability, can be resolved via the application of ultrasounds and via in situ ferrate electrosynthesis.
... • the life span of ZVI [2,4,[16][17][18][19][20][21][22]; • the necessity for and relevance of an in-depth knowledge of hydrogeology and the nature of contaminated plumes [13,16,17]; • the necessity for detailed and comprehensive field monitoring to evaluate the effectiveness of the PRBs [2,23]; • the necessity for a better understanding of the removal mechanisms activated by ZVI-based aqueous systems [22,[24][25][26][27]. ...
... These issues have arisen from the awareness that although PRBs generally perform well after over a decade of operation [28,29], their long-term performance is still not well understood, especially in terms of hydraulic conductivity, (this statement has remained unchanged from 2007 until now [23]). For this reason, this review aimed to investigate the hydraulic behavior of PRBs composed of ZVI in light of the most recent findings. ...
Permeable reactive barriers (PRBs) based on the use of zero valent iron (ZVI) represent an efficient technology for the remediation of contaminated groundwater, but the literature evidences “failures”, often linked to the difficulty of fully understanding the long-term performance of ZVI-based PRBs in terms of their hydraulic behavior. The aim of this paper is to provide an overview of the long-term hydraulic behavior of PRBs composed of ZVI mixed with other reactive or inert materials. The literature on the hydraulic performance of ZVI-based PRBs in full-scale applications, on long-term laboratory testing and on related mathematical modeling was thoroughly analyzed. The outcomes of this review include an in-depth analysis of factors influencing the long-term behavior of ZVI-based PRBs (i.e., reactive medium, contamination and the geotechnical, geochemical and hydrogeological characteristics of the aquifer) and a critical revision of the laboratory procedures aimed at investigating their hydraulic performance. The analysis clearly shows that admixing ZVI with nonexpansive granular materials is the most suitable choice for obtaining a long-term hydraulically efficient PRB. Finally, the paper summarizes a procedure for the correct hydraulic design of ZVI-based PRBs and outlines that research should aim at developing numerical models able to couple PRBs’ hydraulic and reactive behaviors.
... By adding ISCR reagents to the subsurface environment, a sequence of different processes can create very strong (e.g., Eh < −550 mV) reducing conditions that stimulate the reduction of contaminant concentrations of interest to a desired level [318]. In general, the process is equivalent to In Situ Chemical Oxidation (ISCO), although this method has only been applied to the treatment of contaminant plumes, particularly through the use of permeable reactive barriers [319][320][321]. Injections upstream of the contaminant source have also been developed in the last decade [322]. ...
The pollution of groundwater and soil by hydrocarbons is a significant and growing global problem. Efforts to mitigate and minimise pollution risks are often based on modelling. Modelling-based solutions for prediction and control play a critical role in preserving dwindling water resources and facilitating remediation. The objectives of this article are to: (i) to provide a concise overview of the mechanisms that influence the migration of hydrocarbons in groundwater and to improve the understanding of the processes that affect contamination levels, (ii) to compile the most commonly used models to simulate the migration and fate of hydrocarbons in the subsurface; and (iii) to evaluate these solutions in terms of their functionality, limitations, and requirements. The aim of this article is to enable potential users to make an informed decision regarding the modelling approaches (deterministic, stochastic, and hybrid) and to match their expectations with the characteristics of the models. The review of 11 1D screening models, 18 deterministic models, 7 stochastic tools, and machine learning experiments aimed at modelling hydrocarbon migration in the subsurface should provide a solid basis for understanding the capabilities of each method and their potential applications.
... The reactive media is usually made from a hydraulically permeable material or barrier which will react with the contaminated groundwater. As the contaminated groundwater passes through the reactive zone of the PRB under natural hydraulic gradient, the contaminants in the water are either retained or absorbed in the barrier or degraded and released downstream through the barrier in a less toxic level (Henderson and Demond, 2007). ...
... Most in-situ PRB projects register service life from a few years to over 100 years (Henderson & Demond, 2007;Obiri-Nyarko et al., 2014;Zhang et al., 2022), a time span which is not readily duplicated in a laboratory experiment. However, the feasibility of SIP technique on the long-term performance monitoring is crucial for engineering applications, and was addressed as follows. ...
The municipal solid waste (MSW) landfill in Hangzhou, China utilized zeolite and activated carbon (AC) as permeable reactive barrier (PRB) fill materials to remediate groundwater contaminated with MSW leachates containing ammonium, chemical oxygen demand (COD), and heavy metals. The spectral induced polarization (SIP) technique was chosen for monitoring the PRB because of its sensitivity to pore fluid chemistry and mineral-fluid interface composition. During the experiment, authentic groundwater collected from the landfill site was used to permeate two columns filled with zeolite and AC, and the SIP responses were measured at the inlet and outlet over a frequency range of 0.01 to 1000Hz. The results showed that zeolite had a higher adsorption capacity for COD (7.08mg/g) and ammonium (9.15mg/g) compared to AC (COD: 2.75mg/g, ammonium: 1.68mg/g). Cation exchange was found to be the mechanism of ammonium adsorption for both zeolite and AC, while FTIR results indicated that π-complexation, π-π interaction, and electrostatic attraction were the main mechanisms of COD adsorption. The Cole-Cole model was used to fit the SIP responses and determine the relaxation time (τ) and normalized chargeability (mn). The calculated characteristic diameters of zeolite and AC based on the Schwarz equation and relaxation time (τ) matched the pore sizes observed from SEM and MIP, providing valuable information on contaminant distribution. The mn of zeolite was positively linear with adsorbed ammonium (R2=0.9074) and COD (R2=0.8877), while the mn of AC was negatively linear with adsorbed ammonium (R2=0.8192) and COD (R2=0.7916), suggesting that mn could serve as a surrogate for contaminant saturation. The laboratory-based real-time non-invasive SIP results showed good performance in monitoring saturation and provide a strong foundation for future field PRB monitoring.
... In this context, zero valent iron (ZVI), a versatile reactive medium supplied by different manufacturers, known and used worldwide in laboratory or in full scale PRB meets all the requirements aforementioned (Fu et al., 2014;Makota et al., 2017;Ullah et al., 2020a;Zhu et al., 2022). Although there are examples of a good longevity of this reactive medium in full scale (Wilkin et al., 2014), there are numerous cases when a significant reduction of the barrier permeability occurred (Henderson and Demond, 2007). This phenomenon is mainly caused by the formation of iron oxides and hydroxides which, due to their expansive nature, reduce the porosity and permeability of the barrier (Cao et al., 2021;Hu et al., 2018). ...
... On the contrary, sand can be added to reduce the cost or increase the permeability of the barrier [11,12] in case the permeable reactive barrier (PRB) methodology requires a more permeable material used in the barrier than the aquifer soil [8,[13][14][15][16][17][18][19] to control and treat the contaminated groundwater while passing through, based on its reactivity against the heavy metals [20,21], chlorinated organics [22], radionuclides [23] etc in the soil. This alternative passive technology may involve one or more reactive materials in a barrier such as zero-valent iron (ZVI) [24][25][26][27], hydroxyapatite [28], and activated carbon [29] or natural rocks; limestone [30], attapulgite [31], sepiolite [32][33][34], zeolite [12,[33][34][35][36] which have been mechanically brought to a certain grain size or processed minerals such as organoclay [37][38] and organozeolite [38]. ...
Areas vulnerable to catastrophic disasters such as hurricane, landslide and earthquake require ready and sustainable solutions for the post-pollution scenarios. Clinoptilolite type zeolite resources of Türkiye can serve economical and sustainable solutions as a quick response. While the studies on application of zeolite-bentonite mixtures in the landfill liners or zeolite with sand in permeable reactive barrier (PRB)s are common, the slurry form of fine-grained zeolite in subsurface reactive barriers is not received an attention by the researchers. In this context, this laboratorial study presents the preliminary findings on one-dimensional consolidation and hydraulic conductivity tests on crushed zeolite samples S1 and S2 having 33 and 84% fine particle size fraction, respectively. S2 shows the higher compression index than S1, without a significant change in swelling index attributed to less than 4% clay contents. A self-designed rigid wall type permeameter was used to study on reconstituted slurry like materials under the benefit of back pressure saturation without the consolidation during testing that encountered in flexible wall permeameter. Falling head – rising tail water procedure was adopted under the back pressure in between 200 and 700 kPa. S2 samples reconstituted under 25, 50, 100 and 200 kPa show a gradual decrease in kv from 3×10-8 to 2×10-9 m/s. Previous observations on the sample of S1 revealed 8 times higher kv values under the same sv'. Since the fine content of zeolite limits kv, the proposed permeameter will be beneficial to determine the proper grain size distribution of fill materials considering the barrier height and in-situ stress conditions before the environmental studies with leachate.
... Desorption of the Cd under high acidic condition. Cd-loaded biomaterials are lastly removed and landfilled, increasing the risk of hazard compounds in the environments, depending on their hazard assessment (27) Conclusion: Cadmium treatment is needed for water and more effective methods. Water treatment through physical, chemical, and biological techniques is an important aspect of water cleansing, and many studies have been undertaken to identify and develop treatment technologies. ...
... Introduction 42 Metallic iron (Fe 0 ) is an effective reactive medium for the remediation of aqueous 43 systems (e.g. groundwater, rainwater, stormwater, wastewater) polluted with numerous 44 species, including chlorinated solvents, trace metals, nutrients and pathogens [1][2][3][4][5][6][7][8][9][10] . One problem has been that the terms 66 "efficiency" and "reactivity" have been often randomly interchanged [18, [23][24][25]. ...
Granular metallic iron (gFe0) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe0 is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe0 based on the extent of iron dissolution in an ascorbic acid (AA-0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for FeII and a reducing agent for Fe III , AA induces the oxidative dissolution of Fe0 and the reductive dissolution of FeIII oxides from gFe0 specimens. In other words, Fe0 dissolution to FeII ions is promoted while the further oxidation to FeIII ions is blocked. Thus, unlike the EDTA method that promotes Fe0 oxidation to FeIII ions, the AA method promotes only the formation of FeII species, despite the presence of dissolved O2. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe0 specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g. accuracy in assessing Fe0 dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe0 in individual studies.
... GRAPHICAL ABSTRACT INTRODUCTION and renewable energy concept. Permeable reactive barrier (PRB) technology could be an alternative to traditional pumping and treatment systems to treat contaminated groundwater (Henderson & Demond 2007). Since 1990, many PRBs have been designed and implemented in different forms, including funnel and gat (Thiruvenkatachari et al. 2008). ...
The quality of groundwater resources is at catastrophic risk. The proper performance of iron nanoparticles has made a permeable reactive barrier (PRB) an alternative to conventional filtration methods. The performance of nanozerovalent iron (nZVI) PRBs is limited by particle aggregation, instability, and phase separation, even at low iron concentrations. Therefore, the precipitation of reactive materials and a decrease in the longevity of PRB are fundamental challenges. A laboratory setup is used to compare the performance of bare nZVI and xanthan gum (XG)-nZVI + Mulch PRB to simultaneously remove nitrate, sulfide, and arsenic in groundwater. nZVI (average diameter of 35–55 nm) particles are used as reactive media. The objectives are (1) to develop a method for treating nitrate, sulfide, and arsenic simultaneously in groundwater using organic mulch and XG-nZVI; and (2) to evaluate the longevity performance of the XG-nZVI + Mulch and bare nanoparticles treatment system over 10 days. The results showed that the XG-nZVI + Mulch barrier's performance for eliminating NO3-, As, and S2− was generally improved compared to the bare nZVI barriers by 5.7, 19.2, and 10.9%, respectively. Finally, despite the need for long-term sustainability assessment, XG-nZVI PRB performance is impressive, and this stability promises to improve the longevity of nanoparticles while used in PRBs.
HIGHLIGHTS
Permeable reactive barriers (PRBs) based on XG-nZVI + Mulch can lead to adequate remediation of NO3, As(V), S2− compared to the bare nZVI barriers by 5.7, 19.2, and 10.9%, respectively.;
The stability and longevity of the XG-nZVI + Mulch barrier are outstandingly better than the bare nanoparticles barrier.;
XG-nZVI + Mulch PRB's footprint is green and sustainable because of using recycled materials.;
... Previous studies have proven that ZVI is a strong reducing material for organic contaminants such as trichloroethylene (TCE) and nitrobenzene (Xin et al. 2018;Yin et al. 2016). The proven effectiveness of ZVI led to its practical application as an effective and inexpensive medium for in-situ treatment technologies for the remediation of hazardous contaminants, such as permeable reactive barriers (PRB) (Henderson and Demond 2007;Phillips et al. 2010). Our research group recently optimized the reduction of NTO using acid-pretreated ZVI in batch assays, providing complete conversion of NTO to ATO within 10 min at pH 3. ...
The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the U.S. Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for six months (ca. 11000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when only 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1 M HCl, fully removing NTO. After the experiment, solid phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.
... The in-situ remediation technology for MSW landfill leachate contaminated soil and groundwater demand removal of multi-component contaminants, which is tackled preferably by permeable reactive barriers (PRBs), especially on hydrogeological conditions of abundant groundwater flow. This is challenging as traditional permeable reactive barrier usually removes single component contaminants, such as chlorinated solvents by zero-valence iron (Shen and Wilson, 2007;Henderson and Demond, 2007) or activated carbon (Erto et al., 2009;Bortone et al., 2013Bortone et al., , 2014, ammonium by zeolite (Chen et al., 2022;Zhang et al., 2022), and iron by calcite , while multi-layer PRBs is desired. Preliminary research shows that organic matters, often characterized by chemical oxygen demand (COD), are often abundant in fresh leachates, which can be oxidized and adsorbed (Ye et al., 2019;Renou et al., 2008;Kjeldsen et al., 2002). ...
Albeit the widely-used zeolite permeable reactive barriers (PRBs) in remediating ammonium in groundwater from mining industry and municipal solid waste landfills, the engineering design is primarily based on the traditional maximum adsorption capacity method and the residence time method. Both methods could predict neither the NH4+ saturation versus time evolution, nor the breakthrough behavior of a zeolite PRB. This adds uncertainty to the PRB performance, on top of the conventional clogging and preferential flow problems. In this study, a kinetic-based method was proposed to tackle above challenges. An adsorption kinetic model was obtained based on two-variables batch test results, whereas the rate constant k was 0.1728 L/(min ⋅ mol), and the adsorption exponents with respect to both NH4+ concentration and the zeolite adsorption site molarity were unity. Effective diffusion coefficient D∗ (1 × 10⁻⁹ m²/s) and mechanical dispersion (αL=8×10⁻³ m) were calibrated by Cl− tracer tests. Three column tests with inlet NH4+ concentrations of 200, 1000 and 2000 mg/L were performed to obtain the breakthrough curves, which agreed well (R2 > 0.93) with those simulated by the proposed method. Indeed, breakthrough curves considering kinetics were also more precise than those with instantaneous adsorption assumption (R2=0.712-0.863). The proposed method was used for calculating the required thickness of a PRB for a municipal solid waste landfill, which was more conservative than those calculated by traditional methods.
... Based on the extent of the iron corrosion process, porosity reduction could lead to the clogging of the reactive medium and, as a consequence, barrier hydraulic conductivity decreases, reaching values not compatible with the PRB working. In some PRBs, hydraulic conductivity reduction occurred after a few years after installation [24,25], whereas in other cases, the PRB still effectively fulfils its function, as described by [26] after 21 years of operation for the treatment of chlorinated compounds. ...
Zero valent iron (ZVI) is widely used in permeable reactive barriers (PRBs) for the remediation of contaminated groundwater. The hydraulic conductivity of ZVI can be reduced due to iron corrosion processes activated by water and its constituents including pollutants. To overcome this issue, ZVI particles can be mixed with granular materials that avoid a drastic reduction in the hydraulic conductivity over time. In light of the most recent studies concerning iron corrosion processes and recalling the basic principles of century-old chemistry of iron corrosion, we have revised the results of 24 long-term column tests investigating the hydraulic and reactive behavior of granular mixtures composed of ZVI and pumice or lapillus. From this analysis, we found a clear correlation between the reactive behavior, described by the retardation factor (i.e., the ratio between flow velocity and propagation velocity of the contamination front), and the hydraulic behavior, described by means of the permeability ratio of the reactive medium (i.e., the ratio between the final and initial value of hydraulic conductivity). In particular, the permeability ratio decreased with the increase in the retardation factor. Moreover, it was found that the retardation factor is a useful parameter to evaluate the influence of flow rate, contaminant concentration, and ZVI content on the reactive behavior of the granular medium.
... nitrate or oxygen) not targeted for remediation at contaminated sites, which might be another potential problem (Henderson and Demond 2013). On the contrary, Henderson and Demond (2007) stated that imperfect hydraulic characterization is the main cause of failure. Therefore, the key aspect for the design of PRBs is adequate site characterization (Obiri-Nyarko et al. 2014). ...
Zero-valent iron has been used for more than 130 years for water treatment. It is based on redox reactions as well as on sorption to the corrosion products of iron. It is successfully applied for the removal of metals and organic pollutants from groundwater and wastewater. There are different variations how zero-valent iron can be used, especially (i) permeable reactive barriers, (ii) fluidized bed reactors and (iii) nanoscale zero-valent iron. Permeable reactive barriers are used for in situ treatment of groundwater in trench-like constructions or in a funnel and gate system. Their advantages are low maintenance cost, inexpensive construction and prevention of excavation wastes, and their disadvantages are surface passivation and clogging of pores by corrosion products. Zero-valent iron nanoparticles are injected directly in contaminated soil or groundwater. Their advantages are a higher reactivity than coarse-grained zero-valent iron and their mobility in the subsurface to reach the contaminated areas. However, they also have some major disadvantages like fast ageing in the system, phytotoxicity, agglomeration during migration and high costs. The latest development is a fluidized bed process (“ferrodecont process”) which avoids the passivation and clogging observed in permeable reactive barriers as well as the high costs and toxicity issues of nanoscale zero-valent iron. First results of this technology for Cr(VI) and organically contaminated groundwaters and metal removal from industrial wastewaters are highly promising.
... Early studies showed that zero-valent iron can effectively reduce nitrate [10][11][12] because it is a good reductant under anoxic conditions. As a result, it has been used in permeable reactive barriers for the passive reduction of nitrate-contaminated groundwater [9,13]. More recently, it has been added to filter media to remove pathogens from drinking water and can remove several co-occurring chemical pollutants (along with nitrate) from domestic sewage [14][15][16]. ...
Elevated nitrate concentrations in groundwater and surface water supplies can negatively impact the quality of the environment and human health. Recent studies have examined the use of zero-valent iron technology to treat nitrate-contaminated groundwater. Mechanistic aspects of nitrate reduction by zero-valent iron are unresolved. This project investigated the kinetics and mechanism of nitrate reduction by zero-valent iron under anoxic conditions and under oxic conditions. Stirred-batch reactions were studied over environmentally relevant ranges of reactant concentration, pH, and temperature. A complex rate expression was derived with a 1.8 order dependence on nitrate, a 1.4 order dependence on zero-valent iron, and a fractional order (0.8) dependence on proton concentrations under anoxic conditions. An apparent activation energy of 35 kJ mol−1 was observed indicating that nitrate reduction was diffusion controlled under our conditions. Furthermore, the calculated entropy of activation value of −162 J mol−1K−1 indicates that this reaction occurred by an associative mechanism. Under oxic conditions, there was a lag period in nitrate reduction where oxygen was preferentially utilized, leading to a slower rate of nitrate reduction when compared with anoxic conditions. These rate data can be used in predicting nitrate disappearance in nitrate-contaminated groundwater and wastewater treated with zero-valent iron.
... In recent years, scholars have performed considerable research on PRB fillers, among which zero-valent iron (ZVI) is commonly used as a reaction medium (Wilkin et al. 2010;Sun et al. 2019); it was also reported that ZVI can be applied in practical engineering applications (Henderson and Demond 2007). ZVI has the advantages of high reaction rates, a large specific surface area, and ample reducing capability. ...
In this research, a new composite adsorbent (SC@ZVI@CS-AA) was designed and synthesized, and its application for the removal of Cr(VI) in groundwater was investigated. The interaction between SC@ZVI@CS-AA and Cr(VI) conformed to a pseudo-second-order model, and the adsorption process was dominated by chemisorption. The effects of material ratios, pH, temperature, SC@ZVI@CS-AA dosage, and coexisting ions on the removal of Cr(VI) were investigated. The removal efficiency of Cr(VI) by SC@ZVI@CS-AA reached 95%, and the reaction was significantly inhibited when SO4²⁻ was present. Thermodynamically, the adsorption of Cr(VI) proceeded spontaneously above 35 °C (ΔGθ < 0). According to scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometry, and synchronous thermal analysis, the removal mechanism of Cr(VI) by SC@ZVI@CS-AA was attributed to electrostatic attraction and reduction. In addition, SC@ZVI@CS-AA had good cyclic adsorption performance. Overall, the SC@ZVI@CS-AA composite showed great potential in the remediation of Cr(VI)-contaminated groundwater.
The term "waste management" refers to any trash that isn't in a gaseous or liquid state, although it also includes container - based gaseous and gaseous waste. Solid waste generated trash, agricultural residues, industrial garbage, ashes from thermal plants, and toxic materials are the principal categories of solid waste. Biological treatment is well-defined as the process of biologically degrading organic wastes in controlled circumstances to a benign state or to concentrations lower then regulatory concentration limits. Because biological treatment is only effective when conditions are favorable for microbial growth and activity. There are basically two types of remediation in situ and ex situ remediation. In situ remediation have landfill, aerobic composting, anaerobic digestion. Ex-situ remediation has biopile and bioreactors. But there are the limitations for the bioremediation. For example, bioremediation is only possible with biodegradable chemicals.
Water pollution is calling for a sustainable remediation method such as the use of metallic iron (Fe ⁰ ) to reduce and filter some pollutants, yet the reactivity and hydraulic conductivity of iron filters decline over time under field conditions. Here we review iron filters with focus on metallic corrosion in porous media, flaws in designing iron filters, next-generation filters and perspectives such as safe drinking water supply, iron for anaemia control and coping with a reactive material. We argue that assumptions sustaining the design of current Fe ⁰ filters are not valid because proposed solutions address the issues of declining iron reactivity and hydraulic conductivity separately. Alternatively, a recent approach suggest that each individual Fe ⁰ atom corroding within a filter contributes to both reactivity and permeability loss. This approach applies well to alternative iron materials such as bimetallics, composites, hybrid aggregates, e.g. Fe ⁰ /sand, and nano-Fe ⁰ . Characterizing the intrinsic reactivity of individual Fe ⁰ materials is a prerequisite to designing sustainable filters. Indeed, Fe ⁰ ratio, Fe ⁰ type, Fe ⁰ shape, initial porosity, e.g. pore size and pore size distribution, and nature and size of admixing aggregates, e.g. pumice, pyrite and sand, are interrelated parameters which all influence the generation and accumulation of iron corrosion products. Fe ⁰ should be characterized in long-term experiments, e.g. 12 months or longer, for Fe dissolution, H 2 generation and removal of contaminants in three media, i.e., tap water, spring water and saline water, to allow reactivity comparison and designing field-scale filters.
Quiescent batch experiments were conducted to evaluate the influences of Cl–, F–, HCO3–, HPO42–, and SO42– on the reactivity of metallic iron (Fe0) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe0 and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0 to 45 d), (ii) 4 different types of Fe0, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO3– enhanced the extent of MB discoloration, while Cl–, F–, HPO42–, and SO42– inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO3– inhibited O-II discoloration. The increased MB discoloration in the HCO3– system was justified by considering the availability of FeCPs as contaminant scavengers, pH-increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe0–based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.
Sulfidated nano- and microscale zero-valent iron (S-(n)ZVI) has shown enhanced selectivity and reactive lifetime in the degradation of chlorinated ethenes (CEs) compared to pristine (n)ZVI. However, varying effects of sulfidation on the dechlorination rates of structurally similar CEs have been reported, with the underlying mechanisms remaining poorly understood. In this study, we investigated the β-dichloroelimination reactions of tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and trans-1,2-dichloroethene (trans-DCE) at the S and Fe sites of several S-(n)ZVI surface models by using density functional theory. Dechlorination reactions were both kinetically and thermodynamically more favorable at Fe sites compared to S sites, indicating that maintaining the accessibility of reactive Fe sites is crucial for achieving high S-(n)ZVI reactivity with contaminants. At Fe sites adjacent to S atoms, the reactivity for CE dechlorination followed the order trans-DCE ≈ TCE > cis-DCE > PCE. PCE degradation was hindered at these sites due to the steric effects of S atoms. At the S sites, the energy barriers correlated with the CEs’ energy of the lowest unoccupied molecular orbital in the order PCE < TCE < DCE isomers. Our findings reveal that the experimentally observed selectivity of S-(n)ZVI materials for individual CEs can be explained by an interplay of the varying reactivities of Fe and S sites in CE dechlorination reactions.
Metallic iron (Fe0) is readily available worldwide and it has shown promise for water treatment in filtration systems. Fe0 filters remove physical contamination (e.g. colloids, suspended particles), pathogens (e.g. bacteria, viruses), and micro-pollutants (e.g. arsenic, nitrate, pesticides, pharmaceuticals) from polluted waters. Accordingly, Fe0 filters can be used for water treatment applications where other materials (e.g. activated carbon, biochar, bone char) are economically or logistically infeasible. Therefore, Fe0 filters are a good candidate to help low-income communities in their efforts to achieve universal access to safe drinking water by 2030. The objective of this chapter is to summarize available knowledge on the design of Fe0 filters in order to booster their large scale application at household and small community levels. Optimal conditions for Fe0 filters include the rational choice of the used materials building the reactive zone (Fe0 and other aggregates), the Fe0 ratio in the reactive zone, the Fe0 mass (e.g. size of the filter or number of filters in series), and the contact time (flow velocity). The proper combination of these design parameters is discussed. Results show that: (i) all reactive Fe0 can be used for efficient water filters, (ii) only porous Fe0 materials are suitable for sustainable water filters, (iii) well-designed hybrid Fe0/aggregate systems are also sustainable, (iv) the major limitation of Fe0 filters is the lack of knowledge on the long-term corrosion rate. Future research efforts should last for months or years.
Advances in Drinking Water Purification
Small Systems and Emerging Issues
1st Edition - January 17, 2024
Editor: Sibdas Bandyopadhyay
Paperback ISBN: 9780323917339
9 7 8 - 0 - 3 2 3 - 9 1 7 3 3 - 9
eBook ISBN: 9780323972024
Description
Advances in Drinking Water Purification: Small Systems and Emerging Issues captures the knowledge and impact on the performance of various types of water purification technologies and identities the need for further development with a view to carry forward the SDG global targets of achieving safe and affordable drinking water. The book bridges the knowledge gap between various types of treatability options which is essential for selection of suitable treatment systems and augmentation in the desirable levels of specific contaminants. It focuses on providing the scope of selecting location specific technology options by presenting multiple approaches for treatment of most crucial toxic contaminants/pathogens.
In addition, it provides insights into the effect of nature of impurities and selection of treatment options on the global quality of drinking water, comprising its possible impacts on the efficiency of the techniques used and thus on the safety of drinking water. This information is indispensable in identifying the appropriate technology depending on the socioeconomic conditions to address the problem of decontamination in drinking water.
Sustainability has become the conscientious and future-oriented principle of modern resource management and environmental protection because caring for the future is tantamount to providing manageable and healthy surroundings for ourselves. For the foreseeable future, geotechnical and environmental engineers must therefore be concerned with ensuring a healthy balance between extraction, processing, manufacturing, utilization, recycling, and disposal of materials and products.
The removal mechanisms of contaminants in Fe0/H2O are still poorly understood, and characterized by contradictory findings. Therefore, the present study aims to improve the understanding of the processes involved in phosphate removal in Fe0/H2O system. Herein, the methylene blue method (MB method) is used to trace the dynamics within the investigated systems. The MB method utilizes the differential adsorptive affinity of MB onto sand and sand coated with iron corrosion products to evaluate the degree of Fe0 corrosion in Fe0/H2O systems. The extent of MB discoloration and phosphate removal in various Fe0-based systems was characterized in parallel quiescent batch experiments for two weeks and two months. Parallel experiments with Orange II (O-II) as a model contaminant allowed an improved discussion of the results. The investigated systems were: (i) Fe0 alone, (ii) MnO2 alone, (iii) sand alone, (iv) Fe0 + sand, (v) Fe0 + MnO2, and (vi) Fe0 + sand + MnO2. Additional experiments were conducted to test the influence Fe0 type, Fe0 and sand mass loadings on dye discoloration and phosphate removal in Fe0/sand system. Each system was characterized by: (i) pH value, (ii) Fe concentration, (iii) dye discoloration (MB, O-II), and (iv) phosphate concentration. Results showed that the MB method was capable of tracing the extent of iron corrosion in the various investigated systems and clarified the role of in-situ generated iron corrosion products (FeCPs) in the removal of phosphate. The suitability of mixing sand aggregates with Fe0 for efficient phosphate removal is demonstrated through the MB method. Overall, the appropriateness of the MB method to characterize the dynamics of Fe0/H2O systems is validated.
Nanoscale zero-valent iron (nZVI)-based advanced oxidation processes (AOPs) are limited by the rapidly formed surface layer of iron (oxyhydr) oxides. This restriction can be broken by the simultaneous activation of H2O2 and peroxydisulfate (PDS, S2O8²⁻) over sulfidated nanoscale ZVI (S-nZVI), which displayed a synergistic effect to alleviate the drawbacks of the oxidants used alone. In this work, a biochar-supported S-nZVI (noted as [email protected]) was employed to simultaneously activate PDS and H2O2 for methyl tert-butyl ether (MTBE) degradation, and the rate constant for [email protected]/Bi-ox (Bi-ox, bi-oxidant at 1:1 molar ratio of PDS and H2O2) was 3.7-, 4.5-, and 12.8-fold higher than that of [email protected]/Bi-ox, [email protected]/PDS, and [email protected]BC/H2O2. According to electron paramagnetic resonance (EPR), X-ray photoelectric spectroscopy (XPS), and in-situ oxygen detection analyses, oxygen vacancies were generated over the shell of [email protected] during PDS activation, and the oxygen vacancy-contained surface layers promoted H2O2 adsorption and dissociation to produce surface-bound ·OH (·OHads), thus significantly improving H2O2 utilization efficiency and accelerating MTBE degradation. These findings provide promising S-nZVI-based AOPs by combining H2O2 and peroxydisulfate activation for environmental remediation and bring insights for the creation of oxygen vacancy-containing materials for peroxide activation.
The concentrations of U in natural waters are usually low, being typically less than 4 μg/L in river water, around 3.3 μg/L in open seawater, and usually less than 5 μg/L in groundwater. Higher concentrations can occur in both surface water and groundwater and the range spans some six orders of magnitude, with extremes in the mg/L range. However, such extremes in surface water are rare and linked to localized mineralization or evaporation in alkaline lakes. High concentrations in groundwater, substantially above the WHO provisional guideline value for U in drinking water of 30 μg/L, are associated most strongly with (i) granitic and felsic volcanic aquifers, (ii) continental sandstone aquifers especially in alluvial plains and (iii) areas of U mineralization. High-U groundwater provinces are more common in arid and semi-arid terrains where evaporation is an additional factor involved in concentrating U and other solutes. Examples of granitic and felsic volcanic terrains with documented high U concentrations include several parts of peninsular India, eastern USA, Canada, South Korea, southern Finland, Norway, Switzerland and Burundi. Examples of continental sandstone aquifers include the alluvial plains of the Indo-Gangetic Basin of India and Pakistan, the Central Valley, High Plains, Carson Desert, Española Basin and Edwards-Trinity aquifers of the USA, Datong Basin, China, parts of Iraq and the loess of the Chaco-Pampean Plain, Argentina. Many of these plains host eroded deposits of granitic and felsic volcanic precursors which likely act as primary sources of U. Numerous examples exist of groundwater impacted by U mineralization, often accompanied by mining, including locations in USA, Australia, Brazil, Canada, Portugal, China, Egypt and Germany. These may host high to extreme concentrations of U but are typically of localized extent.
The overarching mechanisms of U mobilization in water are now well-established and depend broadly on redox conditions, pH and solute chemistry, which are shaped by the geological conditions outlined above. Uranium is recognized to be mobile in its oxic, U(VI) state, at neutral to alkaline pH (7–9) and is aided by the formation of stable U–CO3(±Ca, Mg) complexes. In such oxic and alkaline conditions, U commonly covaries with other similarly controlled anions and oxyanions such as F, As, V and Mo. Uranium is also mobile at acidic pH (2–4), principally as the uranyl cation UO2²⁺. Mobility in U mineralized areas may therefore occur in neutral to alkaline conditions or in conditions with acid drainage, depending on the local occurrence and capacity for pH buffering by carbonate minerals. In groundwater, mobilization has also been observed in mildly (Mn-) reducing conditions. Uranium is immobile in more strongly (Fe-, SO4-) reducing conditions as it is reduced to U(IV) and is either precipitated as a crystalline or ‘non-crystalline’ form of UO2 or is sorbed to mineral surfaces. A more detailed understanding of U chemistry in the natural environment is challenging because of the large number of complexes formed, the strong binding to oxides and humic substances and their interactions, including ternary oxide-humic-U interactions. Improved quantification of these interactions will require updating of the commonly-used speciation software and databases to include the most recent developments in surface complexation models. Also, given their important role in maintaining low U concentrations in many natural waters, the nature and solubility of the amorphous or non-crystalline forms of UO2 that result from microbial reduction of U(VI) need improved quantification.
Even where high-U groundwater exists, percentage exceedances of the WHO guideline value are variable and often small. More rigorous testing programmes to establish useable sources are therefore warranted in such vulnerable aquifers. As drinking-water regulation for U is a relatively recent introduction in many countries (e.g. the European Union), testing is not yet routine or established and data are still relatively limited. Acquisition of more data will establish whether analogous aquifers elsewhere in the world have similar patterns of aqueous U distribution. In the high-U groundwater regions that have been recognized so far, the general absence of evidence for clinical health symptoms is a positive finding and tempers the scale of public health concern, though it also highlights a need for continued investigation.
Nanoscale particles of zero-valent iron were used to form a permeable reactive barrier whose performance in dechlorinating a solution of trichloroethylene was compared with that of a barrier formed from limestone. The iron was combined with kaolin by calcination. The test liquid contained sewage sludge, and also added NH4Cl and KH2PO4. The average removal rates of trichloroethylene and phosphorus over 365 days both exceeded 94%. Chemical oxygen demand was reduced by 92% and ammonium nitrogen by 43.6%. All were significantly greater than the removals with the limestone barrier. The ceramsite barrier retained 85% of its effectiveness even after 365 days of use. Dechloromonas sp. was the main dechlorinating bacterium, but its removal ability is limited. The removal of trichloroethylene in such a barrier mainly depends on reduction by the zero-valent iron and biodegradation. The results show that the prepared ceramsite is stable and effective in removing trichloroethylene from water. It is a promising in-situ remediation material for groundwater.
Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change, and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), the remediation Fe0/H2O system, a consensus on electrochemical contaminant reduction was established during the 1990s and is still prevailing. Arguments against the reductive transformation concept has been regarded for more than a decade as 'science denial'. However, is it the prevailing concept that had denied the science of aqueous iron corrosion? This communication retraces the path used by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are interrelated as the Fe0 surface is permanently shielded by an oxide scale acting as diffusion barrier to dissolved species and conductive barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and call for alternatives.
Permeable reactive barrier (PRB) is one of the most promising in-situ groundwater remediation technologies due to its low costs and wide immobilization suitability for multiple contaminants. Reactive medium is a key component of PRBs and their selection needs to consider removal effectiveness as well as permeability. Zeolites have been extensively reported as reactive media owing to their high adsorption capacity, diverse pore structure and high stability. Moreover, the application of zeolites can reduce the PRBs fouling and clogging compared to reductants like zero-valence iron (ZVI) due to no formation of secondary precipitates, such as iron monosulfide, in spite of their reactivity to remove organics. This study gives a detailed review of lab-scale applications of zeolites in PRBs in terms of sorption characteristics, mechanisms, column performance and desorption features, as well as their field-scale applications to point out their application tendency in PRBs for contaminated groundwater remediation. On this basis, future prospects and suggestions for using zeolites in PRBs for groundwater remediation were put forward. This study provides a comprehensive and critical review of the lab-scale and field-scale applications of zeolites in PRBs and is expected to guide the future design and applications of adsorbents-based PRBs for groundwater remediation.
Acid drainage is one of the most dangerous and costly environmental problems that reduce soil and water quality. Acid drainage begins when pyrite and other sulfides are exposed and, in the presence of oxygen and water, undergo oxidation to form hydrated sulfates. This can occur in tailing piles, sterile or other materials moved by mining activities, cuts of pyrite materials on roads, tunnels, etc., where the oxidized compounds appear as white and yellowish crusts on the exposed surface of rocks and weathered sediment. Sulfide oxidation products, besides being highly soluble, have a strongly acidic reaction, so that they are easily dissolved in the liquid phase, acidifying the water bodies into which they discharge. Thus, the waters, when dissolving the salts produced by oxidation, become acidic and with high concentrations of sulfate and iron. Due to the low pH values (which may drop below 2.0), other elements such as aluminum, manganese, copper, zinc, lead, mercury, cadmium, etc., if present in the medium, are solubilized and mobilized in the drainage waters and may become toxic. By reaching lakes and rivers, acid drainage can compromise the ecological balance of water resources, with the potential to incorporate heavy metals in the food chain. Remediation of acid drainage involves (i) admixture of sufficient alkaline material to neutralize acidity and (ii) reducing sulfide contact with the atmosphere through a water layer or chemical or biological precipitation of sulfates and oxides. Adoption of preventive and corrective strategies should be based on balance between acidity and alkalinity generation, as well as the reaction kinetics.
This paper describes reactive transport simulations conducted to assess the impact of mineral fouling on the long-term performance of permeable reactive barriers employing granular zero valent iron (ZVI). Three minerals were assumed to form in the pore space (CaCO3, FeCO3, and Fe(OH) 2) and the inflowing groundwater was assumed to have the following composition: DO = 10-8 M, Fe2+ = 10-10 M, Ca2+ = 10-3 M, OH- = 10-7M, HCO 3- = 10-3 M, and CO32- = 10-7 M. Results of the simulations show that the porosity and hydraulic conductivity of the ZVI decrease over time and that flows are redistributed throughout the PRB in response to fouling of the pore space. Seepage velocities in the PRB increase, and residence times decrease, due to porosity reductions caused by accumulation of minerals in the pore space. Under the assumed conditions, only subtle changes occur within the first 10 years (i.e. the duration of the current field experience record with PRBs) and the most significant changes do not occur until the PRB has operated for at least 30 years. However, after 30-50 years, reductions in residence time of the order of 50% occurred. More rapid and extensive changes are likely to occur for conditions that result in greater precipitation rates (e.g. groundwater with higher ionic strength, higher velocity).
Concretions are common in some of the modern intertidal sediments on the Lincolnshire coast of the Wash. The mineralogy and geochemistry of numerous examples of these concretions have been studied in detail. The majority have metallic nuclei and those that do not, exhibit textures which suggest that they originally did so. Petrographic observations indicate that the cements within these concretions precipitated in a distinct sequence that is spatially developed around the metallic nucleus and that are arranged from core to periphery: (a) a ferrous hydroxy chloride mineral (similar in bulk composition to akaganeite) together with iron monosulphide (amorphous FeS and mackinawite), pyrite and possibly elemental sulphur; (b) ferroan carbonate cements (including siderite, ankerite and calcite); (c) mixed ferrous and ferric minerals (“green rust” together with magnetite and possibly greigite); (d) fully oxidized minerals (including akaganeite, goethite, hematite, gypsum and a complex basic sulphate of Fe, Ca, Mg, Si and Al). This latter material has not been possible to characterize fully but is probably an amorphous mixture.
The initiating reaction for the precipitation of these cements is anaerobic corrosion of iron at zero valence state with sulphate as an oxidant. Metal, hydroxide and monosulphide are found in close spatial association, indicating the reaction: 4 Fe 0 + S O 4 2 − + 4 H 2 O → 3 Fe ( OH ) 2 + 2 OH − + FeS
Further away from the metallic nucleus, carbonates cement the host clastic sediment. These are the products of reaction between the first-formed hydroxides and pore water solutes (principally HCO ⁻³ , Mg ²⁺ and Ca ²⁺ ). When oxygen gains access to the growing concretion, Fe(II) minerals are replaced very rapidly by akaganeite and either goethite or hematite. Gypsum and other sulphate minerals also precipitate as new cements around the periphery of the concretions.
The cementation process in these concretions is driven by the extreme instability of metallic iron (here, relic military armaments and shrapnel fragments) in contact with saline, anaerobic water and is indicative more of cathodic corrosion than the growth of ancient carbonate–sulphide concretions.
The working lifetime of permeable reactive barriers (PRBs) using Fe-0 as the reactive media is limited by precipitation of secondary minerals, due to reaction of groundwater with Fe-0. Since PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of their formation, can vary widely. Using plausible phases obtained from field PRBs, the study shows that chemical equilibrium modeling can correctly predict the amounts of precipitates formed, based on the thermodynamic properties of Fe-0 and groundwater constituents. These predictions were compared to the results from the solid phase analysis from a field column experiment and from a field-installed PRB at Y-12 Plant, Oak Ridge, TN. Using the column chemical data molar distributions of the precipitates along the flow path were modeled. The maximum precipitation at the Fe-0-sand interface at the influent end was predicted, where pore water showed high saturation index (SI) with respect to calcite and iron (oxyhydr)oxide. In the absence of flow information, the field sampling data were used to construct an SI-pH diagram, from which the extent of reaction with Fe-0, the potential for precipitate buildup, and relative residence time for the pore water were identified. Kinetic and heterogeneous flow effects were also discussed. To illustrate the application of chemical equilibrium modeling to the design and planning phase of PRBs, groundwater data from four PRB sites were analyzed. The analysis shows that up to 0.63 cm(3)/L solid could form in pore water using an average Fe-0 dissolution rate, leading to severe clogging of Fe-0 medium over a 10-yr period of operation.
The purpose of this project is to conduct collaborative research to evaluate and maximize the effectiveness of permeable reactive barriers (PRBs) with a broad-based working group including representatives from the U.S. Department of Energy (DOE), U.S. Department of Defense (DoD), and the U.S. Environmental Protection Agency (EPA). The Naval Facilities Engineering Service Center (NFESC) and its project partner, Battelle, are leading the DoD effort with funding from DoD's Environmental Security Technology Certification Program (ESTCP) and Strategic Environmental Research and Development Program (SERDP). Oak Ridge National Laboratory (ORNL) is coordinating the DOE effort with support from Subsurface Contaminant Focus Area (SCFA), a research program under DOEs Office of Science and Technology. The National Risk Management Research Laboratory's Subsurface Protection and Remediation Division is leading EPA's effort. The combined effort of these three agencies allows the evaluation of a large number of sites. Documents generated by this joint project will be reviewed by the participating agencies' principal investigators, the Permeable Barriers Group of the Remediation Technologies Development Forum (RTDF), and the Interstate Technology and Regulatory Cooperation (ITRC). The technical objectives of this project are to collect and review existing field data at selected PRB sites, identify data gaps, conduct additional measurements, and provide recommendations to DOE users on suitable long-term monitoring strategies. The specific objectives are to (1) evaluate geochemical and hydraulic performance of PRBs, (2) develop guidelines for hydraulic and geochemical characterization/monitoring, and (3) devise and implement long-term monitoring strategies through the use of hydrological and geochemical models. Accomplishing these objectives will provide valuable information regarding the optimum configuration and lifetime of barriers at specific sites. It will also permit development of site-specific monitoring and performance plans, thus optimizing operation and maintenance (O&M) costs while increasing confidence of both regulators and end users in applying the barrier technology.
A review with 32 refs. concerning remediation pollution groundwater with 0-valent metals (corroded iron) is given. Topics discussed include: rediscovering corrosion, the biogeochem. context, sizing an iron wall, variations new and old, and beyond chlorinated solvents. [on SciFinder(R)]
Battelle, under contract to the Air Force, has prepared a design guidance document for the proposed use of site managers, contractors, and regulators. The design guidance covers permeable barrier application to sites contaminated with dissolved chlorinated solvents in groundwater. The document does not purport to replace the scientific judgment of the on-site engineer or hydrogeologist, but provides guidance on the issues and options that need to be considered based on the current understanding of the technology. The guidance addresses design, emplacement, and monitoring of permeable barriers in various hydrogeologic settings. The Remediation Technologies Development Forum's Permeable Barriers Work Group and the Interstate Technology and Regulatory Cooperation Group provided advisory support during the preparation of this document.
The primary objective of this report is to describe the results of the last round of monitoring conducted in July 2004, their relationship to the results of pervious rounds, and their implications for the longevity and hydraulic performance of the permeable reactive barrier (PRB).
This report discusses soil and ground-water sampling methods and procedures used to evaluate the long-term performance of permeable reactive barriers (PRBs) at two sites, Elizabeth City, NC, and the Denver Federal Center near Lakewood, CO. Both PRBs were installed in 1996 and have been monitored and studied since installation to determine their continued effectiveness for removing contaminants from ground water. An effective monitoring program requires appropriate soil and ground-water sampling techniques. For ground-water sampling, water quality indicator parameters must be monitored to determine when formation water has been accessed. Geochemical parameters include oxidation-reduction potential (ORP), pH, specific conductance, dissolved oxygen (DO), and turbidity. Field analytical methods are discussed along with interferences and issues which may arise when using certain electrodes or instruments in the field. Detailed field analytical procedures for hexavalent chromium, ferrous iron, alkalinity, hydrogen sulfide, and dissolved oxygen are described. Also included are laboratory methods for sample analyses for organics, cations, anions, and carbon. Sample collection methods, sample containers, preservation methods, and sample storage techniques are also discussed.
The purpose of this research was to address the anoxic oxidation of metallic iron and stainless steel powder by nitrate, nitrite, and anaerobic mixed cultures. In sterile batch reactors, both nitrate and nitrite (10 mg N/L) could chemically oxidize metallic iron, with a concomitant reduction to ammonium. Nitrate or nitrite reduction coupled to metal corrosion was not observed in the case of stainless steel powder. Combination of an anaerobic mixed culture and metallic iron led to (cathodically produced) H2 consumption and a complete nitrate or nitrite reduction (mainly to NH4+). This caused a slightly enhanced metal oxidation. In the case of stainless steel, corrosion caused by denitrifying microorganisms was evidenced by data on nitrate/nitrite ions and solubilized iron. Experiments with increasing nitrite concentrations indicated that nitrite in the range of 50 mg of NO2- -N/L inhibited the corrosion processes. Moreover, at concentrations above 140 mg NO2- -N/L, a significant production of nitric oxide (NO) was detected. Differences between iron and stainless steel powder at low concentrations of nitrate or nitrite are most probably due to differences in kinetics: metallic iron exhibited faster chemical than biological reactions as opposed to stainless steel. It is postulated that the inhibitory effect of higher nitrite concentrations could partly be due to the chemical formation of NO and its toxic effect on the microorganisms acting at the steel surface.
This chapter presents a study in which a permeable iron reactive barrier was installed in late November 1997 at the U.S. Department of Energy's Y-12 National Security Complex in Oak Ridge, Tennessee. The overall goal of this study was to determine the effectiveness of the use of zero valent iron (Fe0) to retain or remove uranium and other contaminants such as technetium and nitrate in groundwater. The long-term performance issues were investigated by studying the biogeochemical interactions between Fe0 and groundwater constituents and the mineralogical and biological characteristics over an extended field operation. Results from nearly 3 years of monitoring indicated that the Fe0 barrier was performing effectively in removing contaminant radionuclides such as uranium and technetium. In addition, a number of groundwater constituents such as bicarbonates, nitrate, and sulfate were found to react with the Fe0. Both nitrate and sulfate were reduced within or in the influence zone of the Fe0 with a low redox potential. An increased anaerobic microbial population was also observed within and in the vicinity of the Fe0 barrier, and these microorganisms were at least partially responsible for the reduction of nitrate and sulfate in groundwater. Decreased concentrations of Ca2+ and bicarbonate in groundwater occurred as a result of the formation of minerals such as aragonite (CaCO3) and siderite (FeCO3), which coincided with the Fe0 corrosion and an increased groundwater pH. A suite of mineral precipitates was identified in the Fe0 barrier system, including amorphous iron oxyhydroxides, goethite, ferrous carbonates and sulfides, aragonite, and green rusts, which were found to be responsible for the cementation and possibly clogging of Fe0 filings observed in a number of core samples from the barrier.
Investigation of the hydraulic performance of a funnel-and-gate permeable reactive barrier (PRB), packed with zero-valent iron (Fe0) and installed at the Vapokon site, Denmark, has been conducted with a natural gradient tracer study. The purposes of this study were to evaluate the flow pattern and determine the seepage velocity (vx) of groundwater through the PRB. After collecting and analysing about 13 000 groundwater samples over a period of 10 months, the moving path of the tracer (i.e. lithium, Li+) was identified in which the Li+ plume was observed passing through the reactive barrier. However, probably owing to clogging caused by mineral precipitates, there was a preferential path inside the Fe 0 PRB. Comparison of the water table contour in the Vapokon site obtained in March 2000 and January 2003 showed formation of a low permeability zone within the reactive barrier, thereby further verifying clogging caused by mineral precipitates. Spatial moments analysis of the Li+ distribution illustrates the similarity of the maximum Li+ mass passing through the upgradient and downgradient locations of the Fe0 PRB and thereby indicated the conservative of the Li+ across the reactive barrier as well as the absence of bypassing flow. Based upon the results of first moment analysis, a vx of 99.5 m year-1 within the PRB was calculated. The vx just upgradient of the reactive barrier, however, was only about 6.86 m year-1, most likely owing to the effect of disturbance exerted by the clogged upgradient interface of the upper part of the Fe0 PRB.
Chemical reduction, using a catalyzed metal powder as the reductant, has been found to degrade many toxicants found in chemical plant waste streams to non-toxic forms. The process is particularly useful for the toxicants dissolved in water ( mu g/l to mg/l levels) which cannot be removed by physical means. Degradation to 1 mu g/l and less is commonly achieved. This allows reuse or safe discharge of the waters. The process appears to be simple, effective and economical. Reductants such as catalyzed iron, zinc or aluminum powders have been used in effecting treatment by such mechanisms as hydrogenolysis, hydroxylation, saturation of aromatic structure, condensation, ring opening and rearrangements to change the toxicants to an innocuous form. The toxicants are chemically changed, rather than remaining in a concentrated solution, transferred to a solvent phase, or transmitted to the air.
The first commercial full-scale permeable zero-valent iron reactive barrier (PRB) was installed in 1994 in Sunnyvale, California to treat dissolved chlorinated hydrocarbons present in groundwater. Performance monitoring results, including recently measured dissolved hydrogen concentrations are presented. Research conducted by others indicates that hydrogen evolution may be an indicator of the performance of the dehalogenation reaction. Chlorinated hydrocarbons concentrations are consistently below groundwater cleanup standards within the treatment zone. Treatment appears not to decrease from precipitation of inorganic species in the upgradient gravel zone. The high levels of dissolved hydrogen within the PRB indicate that the corrosion process remains strong more than six years after installation.
The hydraulic and chemical performance at 10 years of operation for the permeable reactive barrier (PRB) composed of granular iron metal was described. Performance assessment techniques, including the use of tracer dilution testing, analysis of groundwater samples for dissolved gases and inorganic constituents showed that the system continued to function successfully after nearly 10 years since its installation. The evaluation of the analytical results indicated that at 10 years, groundwater within the iron core of the PRB was limited in biological activity, strongly reducing, high in pH and low in calcium, and magnesium compared to ambient groundwater outside the PRB.
Chemical reduction, using a catalytically-activated metal powder as the reductant, has been found to degrade many toxicants found in chemical plant waste streams to non-toxic forms. The toxicants are chemically changed to a form innocuous to the environment, rather than remaining in a concentrated solution or transmitted to the air. The chemistry of the process and the means for carrying out the reaction are described. Several applications of the process are described. The allegedly carcinogenic trihalomethanes (THM's) (chloroform, bromoform) and intermediates were degraded in a six-month pilot test from an average 242 mu g/l to well below the EPA control level, with 75% of the samples analyzing less than 5 mu g/l THM's. Reduction of trichloroethylene, tetrachloroethylene and trichloroethane from about 250 mu g/l to less than 5 mu g/l is also described. Chlorobenzene was shown to be effectively degraded, with identification of the principal products as the vastly less toxic cyclohexanol. Successful degradation of a PCB waste to detection limits was shown, as well as the pesticide chlordane.
Granular zero-valent iron was used for the treatment of groundwater pollution caused by chlorinated ethylenes, mainly TCE, cis-DCE and VC at an industrial site. The rapidly decreasing rates of de-chlorination in the initial phase were attributed to the precipitation of carbonates and the development of hydrogen by anaerobic corrosion. After 70 pore volumes, sulphate was reduced by bacteria. From this point in time, the degradation of TCE was slightly accelerated whereas the de-chlorination rates of the other chlorinated ethylenes decreased only slowly. This relative improvement was assumed to be caused by the uptake of electron-transfer-blocking hydrogen by bacteria. Because the overall trend of the degradation rates is negative we conclude that the inhibitive effect of carbonate precipitation and hydrogen formation cannot be compensated for by the positive influence of the activity of sulphate-reducing bacteria.
Since 1998, the Department of Energy's (DOE) Office of Environmental Management has funded the Accelerated Site Technology Deployment (ASTD) Program to expedite deployment of alternative technologies that can save time and money for the environmental cleanup at DOE sites across the nation. The ASTD program has accelerated more than one hundred deployments of new technologies under 76 projects that focus on a broad spectrum of EM problems. More than 25 environmental restoration projects have been initiated to solve the following types of problems: characterization of the subsurface using chemical, radiological, geophysical, and statistical methods; treatment of groundwater contaminated with DNAPLs, metals, or radionuclides; and other projects such as landfill covers, purge water management systems, and treatment of explosives-contaminated soils. One of the major goals of the ASTD Program is to deploy a new technology or process at multiple DOE sites. ASTD projects are encouraged to identify subsequent deployments at other sites. Some of the projects that have successfully deployed technologies at multiple sites focusing on cleanup of contaminated groundwater include: Permeable Reactive Barriers (Monticello, Rocky Flats, and Kansas City), treating uranium and organics in groundwater; and Dynamic Underground Stripping (Portsmouth, and Savannah River), thermally treating DNAPL source zones. Each year more and more new technologies and approaches are being used at DOE sites due to the ASTD program. DOE sites are sharing their successes and communicating lessons learned so that the new technologies can replace the baseline or standard approaches at DOE sites, thus expediting cleanup and saving money.
Groundwater remediation using permeable reactive barriers (PRB) can provide a cost- and energy-effective and environmentally friendly technology. The targeted pollutants to be removed from the groundwater by the reac- tive materials in the permeable barrier are either decomposed to less dangerous compounds or efficiently attached to the reactive material. The most thoroughly studied decontamination processes are chemical reduction, oxidation, precipitation, sorption and biodegradation, while halogenated hydrocarbons, (nitro)aromatic compounds, heavy metals, nitrates, arsenates and phosphates have so far been treated successfully. PRBs are capable of treating groundwater and leachates polluted with uranium compounds or other radionuclides, so the technology is applica- ble to the remediation of former uranium mining sites. At present it is little known about the long-term behaviour of such systems: changes in porosity and reactivity of the reactive media may impair their long-term functioning. The PEREBAR research project has been initiated within the 5th Framework Programme of EU to look into these issues by developing accelerated testing methods, increasing efficacy by introducing chemical ligands into the reactive material and combining the reactive barrier principle with electrokinetic effects. Bench- and floor-scale testing as well as on-site investigations on a former uranium mining site in southern Hungary will help move the results towards practical application.
Growth of a biofilm in a porous medium reduces the total volume and the
average size of the pores. The change in the pore size distributions is
easily quantified when certain geometric assumptions are made. Existing
models of permeability or of relative permeability can be manipulated to
yield estimates of the resulting reduction in permeability as a function
of biofilm thickness. The associated reductions in porosity and specific
surface can be estimated as well. Based on a sphere model of the medium,
the Kozeny-Carman permeability model predicts physically realistic
results for this problem. Using a cut-and-random-rejoin-type model of
the medium, the permeability model of Childs and Collis-George yields
qualitatively reasonable results for this problem, as does a
generalization of the relative permeability model of Mualem.
Permeability models of Kozeny-Carman and of Millington and Quirk lead to
unrealistic results for a cut-and-random-rejoin-type medium. The Childs
and Collis-George and the Mualem models predict that the permeability
reduction for a given volume of biomass is greatest when the porous
medium has uniform pore sizes.
There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe° reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe° reactive barrier and (2) to examine the cementation and corrosion of Fe° filings in order to estimate the lifespan of this barrier. This field scale barrier (225-ft long x 2-ft wide x 31-ft deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe° portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe° section of the barrier. After 15 months in the barrier, most of the Fe° filings in the core samples were loose, and a little corrosion of Fe° filings was observed in most of the barrier. However, larger amounts of corrosion (10-150 m thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2-50 m thick) were also found on the uncemented individual Fe° filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe° filings in the more corroded sections is 5 to 10 years, while the Fe° filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe° filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe° section of the barrier.
Zero-valent iron can be used to remove inorganic arsenic from aqueous solutions. The mechanism for arsenic removal appears to be a surface precipitation or adsorption of arsenic with iron. This mechanism differs significantly from the removal mechanisms of chlorinated hydrocarbons (reductive dechlorination) and chromium(VI) reduction accompanied by hydroxide precipitation. Arsenic removal efficiencies of greater than 95% are observed in laboratory and field-column studies. Arsenate is removed more effectively than arsenite; however, effective arsenite removal occurs even under anoxic conditions. The removal efficiency is related to the surface area or the type of iron used and improved over time, possibly due to pitting of the iron surface and increased surface area for sorption due to iron corrosion and ferrous iron adsorption/precipitation. Preliminary results indicate that arsenic is tightly bound to the iron filings, and only a small percentage is readily leached. This finding coupled with the results of field experiments and spectroscopic analysis (SEM/EDX and XPS) provide evidence that surface precipitation is the predominant removal mechanism.
dOver a six-month period, the performance of a pilot-scale, zero-valent iron permeable reactive barrier (PRB) at the Somersworth, NH Sanitary Landfill Superfund Site was evaluated. The 21-ft long x 34-ft deep x 28-inch wide PRB was installed in November 1999 to test a construction technique that used a biodegradable polymer slurry to support an open excavation while a granular iron/sand mixture was placed into the subsurface. Criteria used to assess the PRB's performance included (1) hydraulic testing to evaluate potential fluid viscosity effects related to the use of the biopolymer, (2) monitoring. of VOCs, groundwater parameters (TOC, TDS, pH, DO, ORP, specific conductance, viscosity) and inorganic parameters (including metals, major ions and nutrients), (3) microbial characterization of groundwater, (4) reactivity testing of cored iron material, and (5) advanced surface analysis of cored iron material.
The radiotracer method described previously was applied for investigations of adsorption processes occuring in the system: iron electrodeposited electrode and CO2-saturated (14C-labelled) neutral electrolyte. In addition to adsorption of 14C-containing species in the electrodeposited film, both reversible and irreversible adsorption of those species was identified. The irreversible adsorption was interpreted to result from incorporation of 14C-species (most probably HCO−3 ions) to the passive layer formed on the iron electrode. The reversible adsorption was concluded to occur due to the weak interactions of carbonic acid with the oxidized iron surface. Lewis acid and base concept of adsorption was used to account for the results. The role of the reversible adsorption process in the accelerated corrosion of steel due to the presence of carbon dioxide in aqueous solutions is discussed.
This document addresses the factors that have been found to be relevant for successfully implementing PRBs for contaminant remediation. Additionally, it provides sufficient background in the science of PRB technology to allow a basic understanding of the chemical reactions proposed for the contaminant transformations that have been witnessed both in the laboratory and in field settings. It contains sections on PRB-treatable contaminants and the treatment reaction mechanisms, feasibility studies for PRB implementation, site characterization for PRBs, PRB design, PRB emplacement, monitoring for both compliance and performance, and summaries of several field installations. The appendices supplement this information with a detailed table of information available in the literature through 1997, summarizing the significant findings of PRB research and field studies (Appendix A), a further examination of the physical and chemical processes important to PRBs, such as corrosion, adsorption, and precipitation (Appendix B), and a set of scoping calculations that can be used to estimate the amount of reactive media required and facilitate choosing among te possible means of emplacing the required amount of media (Appendix C). Appendix D provides a list of acronyms and Appendix E a glossary of terms that are used within this document.
The dehalogenation of chlorinated solvents by zero-valence iron has recently become the subject of intensive research and development as a potentially cost-effective, passive treatment for contaminated groundwater through reactive barriers. Because of its successful application in the laboratory and other field sites, the X-625 Groundwater Treatment Facility (GTF) was constructed to evaluate reactive barrier technology for remediating trichloroethylene (TCE)-contaminated groundwater at the Portsmouth Gaseous Diffusion Plant (PORTS). The X-625 GTF was built to fulfill the following technical objectives: (1) to test reactive barrier materials (e.g., iron filings) under realistic groundwater conditions for long term applications, (2) to obtain rates at which TCE degrades and to determine by-products for the reactive barrier materials tested, and (3) to clean up the TCE-contaminated water in the X-120 plume. The X-625 is providing important field-scale and long-term for the evaluation and design of reactive barriers at PORTS. The X-625 GTS is a unique facility not only because it is where site remediation is being performed, but it is also where research scientists and process engineers can test other promising reactive barrier materials. In addition, the data collected from X-625 GTF can be used to evaluate the technical and economic feasibility of replacing the activated carbon units in the pump-and-treat facilities at PORTS.
The overall goal of this portion of the project was to package one or more unit processes, as modular components in vertical and/or horizontal recirculation wells, for treatment of volatile organic compounds (VOCs) [e.g., trichloroethene (TCE)] and radionuclides [e.g., technetium (Tc){sup 99}] in groundwater. The project was conceived, in part, because the coexistence of chlorinated hydrocarbons and radionuclides has been identified as the predominant combination of groundwater contamination in the US Department of Energy (DOE) complex. Thus, a major component of the project was the development of modules that provide simultaneous treatment of hydrocarbons and radionuclides. The project objectives included: (1) evaluation of horizontal wells for inducing groundwater recirculation, (2) development of below-ground treatment modules for simultaneous removal of VOCs and radionuclides, and (3) demonstration of a coupled system (treatment module with recirculation well) at a DOE field site where both VOCs and radionuclides are present in the groundwater. This report is limited to the innovative treatment aspects of the program. A report on pilot testing of the horizontal recirculation system was the first report of the series (Muck et al. 1996). A comprehensive report that focuses on the engineering, cost and hydrodynamic aspects of the project has also been prepared (Korte et al. 1997a).
The IronRefs database covers published research on remediation applications of 0-valent metals and is accessible at http://cgr.ese.ogi.edu/ironrefs/. The rapid growth of groundwater remediation applications of 0-valent metals is reflected in the rapid growth of this database, which exceeded 500 records in Dec. 2001. This is much more complete coverage of this topic than can be obtained from any of the major com. databases of the scientific literature. Specialized databases such as IronRefs, prepd. by experts in a particular field, are becoming increasingly common on the Web and offer distinctive benefits over traditional options for keeping up with the literature in rapidly growing fields of science and engineering. [on SciFinder(R)]
The operating life of an Fe(0)-based permeable reactive barrier (PRB) is limited due to chemical reactions of Fe(0) in groundwater. The relative contributions from mineral precipitation, gas production, and microbial activity to the degradation of PRB performance have been uncertain. In this controlled field study, nitrate-rich, site groundwater was treated by Fe(0) in large-volume, flow-through columns to monitor the changes in chemical and hydraulic parameters over time. Tracer tests showed a close relationship between hydraulic residence time and pH measurements. The ionic profiles suggest that mineral precipitation and accumulation is the primary mechanism for pore clogging around the inlet of the column. Accumulated N(2) gas generated by biotic processes also affected the hydraulics although the effects were secondary to that of mineral precipitation. Quantitative estimates indicate a porosity reduction of up to 45.3% near the column inlet over 72 days of operation under accelerated flow conditions. According to this study, preferential flow through a PRB at a site with similar groundwater chemistry should be detected over approximately 1 year of operation. During the early operation of a PRB, pH is a key indicator for monitoring the change in hydraulic residence time resulting from heterogeneity development. If the surrounding native material is more conductive than the clogged Fe-media, groundwater bypass may render the PRB ineffective for treating contaminated groundwater.
This research investigated the long-term performance of zero-valent iron for mediating the reductive dechlorination of trichloroethylene (TCE). Over a 2-year period, rates of TCE dechlorination in columns packed with iron filings were measured in simulated groundwaters containing either 3 mM CaSO4, 5 mM CaCl2, or 5 mM Ca(NO3)2. At early elapsed times, TCE reaction rates were pseudo-first-order in TCE concentration and were independent of the solution pH. With increasing elapsed time, reaction rates deviated from pseudo-first-order behavior due to reactive site saturation and increased iron surface passivation toward the influent end of each column. The extent of passivation was dependent on both the TCE concentration and the background electrolyte solution. For most of the investigation, TCE reaction rates in 3 mM CaSO4 and 5 mM CaCl2 solutions were statistically identical at the 0.05 confidence level. However, TCE reaction rates in 5 mM Ca(NO3)2 were slower. In columns operated using chloride- and sulfate-containing waters, the effective half-life for TCE dechlorination increased from approximately 400 min after 10 days elapsed to approximately 2500 min after 667 days. The effective TCE half-life in the nitrate-containing water increased from approximately 1500 min after 10 days to approximately 3500 min after 667 days. Measurements of iron corrosion rates in nitrate and chloride solutions showed that nitrate contributed to increased iron surface passivation and decreased rates of iron corrosion. Corrosion current measurements indicated that halocarbon reduction on fresh iron surfaces was cathodically controlled, whereas on aged iron surfaces, iron corrosion was anodically controlled. Anodic control of iron corrosion contributed to the development of reactive site saturation with time and to similar reaction rates for TCE and perchloroethylene. Passivation of the iron surfaces was found to be dependent on the adhering tendency of the corrosion products and not on the overall mass of corrosion products in the columns. The decrease in TCE reaction rates over time can be attributed to anodic control of iron corrosion and not to increasing reactant mass transfer limitations associated with diffusion through porous corrosion products.
The impact of microbiological and geochemical processes has been a major concern for the long-term performance of permeable reactive barriers containing zero-valent iron (Fe0). To evaluate potential biogeochemical impacts, laboratory studies were performed over a 5-month period using columns containing a diverse microbial community. The conditions chosen for these experiments were designed to simulate high concentrations of bicarbonate (17−33 mM HCO3-) and sulfate (7−20 mM SO42-) containing groundwater regimes. Groundwater chemistry was found to significantly affect corrosion rates of Fe0 filings and resulted in the formation of a suite of mineral precipitates. HCO3- ions in SO42--containing water were particularly corrosive to Fe0, resulting in the formation of ferrous carbonate and enhanced H2 gas generation that stimulated the growth of microbial populations and increased SO42- reduction. Major mineral precipitates identified included lepidocrocite, akaganeite, mackinawite, magnetite/maghemite, goethite, siderite, and amorphous ferrous sulfide. Sulfide was formed as a result of microbial reduction of SO42- that became significant after about 2 months of column operations. This study demonstrates that biogeochemical influences on the performance and reaction of Fe0 may be minimal in the short term (e.g., a few weeks or months), necessitating longer-term operations to observe the effects of biogeochemical reactions on the performance of Fe0 barriers. Although major failures of in-ground treatment barriers have not been problematic to date, the accumulation of iron oxyhydroxides, carbonates, and sulfides from biogeochemical processes could reduce the reactivity and permeability of Fe0 beds, thereby decreasing treatment efficiency.
Arsenite [As(III)] and arsenate [As(V)] are highly toxic inorganic arsenic species that represent a potential threat to the environment and human health. Iron oxides including poorly crystalline oxides, e.g., ferrihydrite, play a significant role in controlling dissolved As concentration and limit the mobility and bioavailability of As(III) and As(V). Adsorption occurs by ligand exchange of the As species for OH2 and OH- in the coordination spheres of surface structural Fe atoms. The objective of this study was to evaluate H+/OH- release stoichiometry and changes in surface charge properties of the adsorbent during the adsorption of arsenite and arsenate on ferrihydrite in the pH range of 4−10. This information, which is not directly accessible through spectroscopic studies, provides important clues to bonding mechanism. While arsenate adsorption resulted in the net release of OH- at pH 4.6 and 9.2, arsenite adsorption resulted in net OH- release at pH 9.2 and net H+ release at pH 4.6. The amount of H+ or OH- release per mole of adsorbed As varied with the As surface coverage, indicating that different mechanisms of arsenic adsorption predominate at low versus high coverage. The experimentally observed surface charge reduction and net OH- release stoichiometry were compared with the theoretical stoichiometry of the surface adsorption reactions that might occur. The results provide evidence that during arsenite adsorption at low pH, i.e., pH 4.6, the oxygen of the Fe−O−As bond remained partially protonated as Fe−O(H)−As. There is evidence that the monodentate bonding mechanism might play an increasing role during arsenate adsorption on ferrihydrite with increasing pH (at pH > 8). The results of this study have provided ancillary evidence to support the experimentally observed reduced adsorption of arsenite at low pH and of arsenate at high pH.
Granular iron has been determined to be a potentially useful reductant for the removal of common organic contaminants from groundwater. This research is aimed at improving our understanding of the processes that control the reactivity and longevity of the iron particles when they are used for groundwater treatment. A suite of nitroaromatic compounds (NACs) including 4-chloronitrobenzene (4ClNB), 4-acetylnitrobenzene (4AcNB), nitrobenzene, 2-methylnitrobenzene (2MeNB), and 2,4,6-trinitrotoluene (TNT) was used to investigate granular iron reactivity in anoxic pH 10, 0.008 M KNO3 solution. Master Builder's brand of granular iron with a surface area of about 1 m2/g was used in all experiments. The NACs were reduced rapidly to anilines that were found to sorb reasonably strongly to the solid particles and to interfere with the reduction of NACs. The granular iron was found to lose reactivity quite rapidly over the first few days of exposure and then more slowly over the next several months. Reactivity loss due to reversibly sorbed products was minimized by flushing the system with background electrolyte between experiments. Competition experiments with binary mixtures of 4ClNB and each one of the other NACs were performed to investigate relative affinities of these compounds for the solid surface. Despite the overall loss in reactivity observed for the granular iron, the relative rate constants in the competition experiments appeared to remain constant in time.
The properties of iron metal that make it useful in remediation of chlorinated solvents may also lead to reduction of other groundwater contaminants such as nitro aromatic compounds (NACs). Nitrobenzene is reduced by iron under anaerobic conditions to aniline with nitrosobenzene as an intermediate product. Coupling products such as azobenzene and azoxybenzene were not detected. First-order reduction rates are similar for nitrobenzene and nitrosobenzene, but aniline appearance occurs more slowly (typical pseudo-first-order rate constants 3.5 × 10-2, 3.4 × 10-2, and 8.8 × 10-3 min-1, respectively, in the presence of 33 g/L acid-washed, 18−20 mesh Fluka iron turnings). The nitro reduction rate increased linearly with concentration of iron surface area, giving a specific reaction rate constant (3.9 ± 0.2 × 10-2 min-1 m-2 L). The minimal effects of solution pH or ring substitution on nitro reduction rates, and the linear correlation between nitrobenzene reduction rate constants and the square-root of mixing rate (rpm), suggest that the observed reaction rates were controlled by mass transfer of the NAC to the metal surface. The decrease in reduction rate for nitrobenzene with increased concentration of dissolved carbonate and with extended exposure of the metal to a particular carbonate buffer indicate that the precipitation of siderite on the metal inhibits nitro reduction.
Laboratory experiments were conducted to evaluate materials used in the construction of groundwater monitors for their potential to cause sampling bias. Ten materials were exposed to low concentrations of five halogenated hydrocarbons in water for periods up to 5 weeks. Borosilicate glass was the only material that did not diminish the halocarbon concentrations. Three metals, including stainless steel, apparently transformed the compounds. Six synthetic polymers, including poly(tetrafluoroethylene) and rigid poly(vinyl chloride), absorbed the compounds. The sorption rates were dependent on flexibility of the polymer, water solubility of the compound, solution volume to polymer surface area ratio, and temperature. A diffusion model explained the concentration histories of solutions exposed to polymers, and the diffusion mechanism was confirmed by direct measurement of halocarbon distributions in several of the polymers. The experimentally determined diffusivities and polymer-water partition coefficients for polyethylene were consistent with literature data.
Permeable reactive barriers (PRBs) are a relatively recent development of a passive system to remediate subsurface waters containing organic or inorganic contaminants. Groundwater fl ow under a natural gradient passes through a permeable curtain of treatment medium that either precipitates the contaminants as relatively insoluble compounds or transforms the contaminants into environmentally acceptable or benign species. The most widely adopted treatment medium is submillimetric zero-valent iron, a substance that is highly reactive, environmentally acceptable, and is readily available as a manufactured product derived from the recycling of scrap iron and steel. Organic compost wastes have also been used to ameliorate inorganic contaminants, and two case studies of the utilization of composts to reduce sulfate and precipitate metals are presented, primarily from a mineralogical perspective. In cores of the reacted treatment media, the most abundant secondary product formed in situ is Fe oxyhydroxide, but a variety of precipitates has been identifi ed. For example, secondary pyrite, greigite, and native nickel are present at a site at which replacement of organic material by sulfi des is common. At an industrial site, secondary pyrite, covellite, chalcopyrite, and bornite have formed in the treatment medium, and whereas replacement of organic material by Fe oxyhydroxides is widespread, replacement by sulfi des is rare. The secondary sulfi des and metals are volumetrically small and are unlikely to impede the perme-ability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of zero-valent iron requires further monitoring to determine whether the secondary precipitates and the consumption of Fe 0 will appreciably lessen the effectiveness of such PRBs over the long term. Current indications are that PRBs are both an environmentally effective and a cost-effective technique of remediation.
The degradation of 2,4,6-trinitrotoluene (TNT) and other explosives by zero-valent iron (ZVI) is rapid and, as a result, potentially useful for both ex situ ground water “pump-and-treat” systems and in situ permeable reactive barriers. However, the usefulness of ZVI in either configuration may be limited by reaction-induced reduction in both hydraulic conductivity (K) and reactivity (as represented by the surface area–normalized rate constant, ksa). The impacts of dissolved oxygen and TNT on K and ksa are examined here using field and laboratory columns. The data suggest that K reduction in ZVI columns can be significant when dissolved oxygen is present. However, when TNT is present at approximately the same concentration (10 mg/L), it does not cause significant reduction in K. In contrast, TNT causes a significant reduction in ksa, while dissolved oxygen appears to have relatively little impact on the reactivity of the columns toward TNT.