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Simultaneous removal of nitrate and heavy metals by iron metal
Abstract
Great attention should be paid now to simultaneously removing common pollutants, especially inorganic pollutants such as nitrate and heavy metals, as individual removal has been investigated extensively. Removing common pollutants simultaneously by iron metal is a very effective alternative method. Near neutral pH, heavy metals, such as copper and nickel, can be removed rapidly by iron metal, while nitrate removal very much slower than that of copper and nickel, and copper can accelerate nitrate removal when both are removed simultaneously. Even a little amount of copper can enhance nitrate removal efficiently. Different mechanisms of these contaminants removal by iron metal were also discussed.
... Nitrates jeopardize all water reservoirs. Recently, contamination of surface and groundwater with nitrates has emerged as an insistent problem on a global scale [1][2][3][4][5][6], due to its toxicity and various pernicious health hazards on both human and animal beings causing in some cases fatal results [7][8][9][10]. Furthermore, the existence of elevated levels of nitrates in water has been globally linked to numerous environmental risks such as eutrophication [11]. ...
... Based on methodologies described by [2,[51][52][53][54] with some modification, five bottles, each one contains 50 ml of an aqueous solution of NaCl (0.01 M), and fixed sorbent loading (2.0 g L −1 in case of AlTFC and AlBWS and 3.0 g L −1 in case of ACPP and ACSB), were prepared then the starting pH of the solution (pH i ) in each bottle was adjusted by using NaOH (0.5 M) or HCl (0.5 M) solutions at pH range (2)(3)(4)(5)(6)(7)(8)(9)(10) in case of AlTFC and AlBWS and (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) in case of ACPP and ACSB. The final pH of the solutions (pH f ) was measured after 48 h of agitation with the sorbent (long enough time is allowed for diffusive mixing). ...
... Based on methodologies described by [2,[51][52][53][54] with some modification, five bottles, each one contains 50 ml of an aqueous solution of NaCl (0.01 M), and fixed sorbent loading (2.0 g L −1 in case of AlTFC and AlBWS and 3.0 g L −1 in case of ACPP and ACSB), were prepared then the starting pH of the solution (pH i ) in each bottle was adjusted by using NaOH (0.5 M) or HCl (0.5 M) solutions at pH range (2)(3)(4)(5)(6)(7)(8)(9)(10) in case of AlTFC and AlBWS and (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) in case of ACPP and ACSB. The final pH of the solutions (pH f ) was measured after 48 h of agitation with the sorbent (long enough time is allowed for diffusive mixing). ...
Purpose
Two types of active acidic alumina derived from aluminum building wire scraps (AlBWS) and aluminum take-out food container waste (AlTFC), as well, two effective types of activated carbon prepared from potato peels (ACPP) and sugarcane bagasse (ACSB) for removal of nitrate ions were utilized in this study.
Methods
Adsorbent samples were prepared with highly porous structure and large specific surface area above 200 m² g⁻¹. Also, low calcinations temperatures and low-priced chemicals were used.
Results
The results showed that the adsorption of nitrates were rapid. The qmax were found to be 12.4, 8.8, 7.1 and 3.9 (mg g⁻¹) for AlTFC, AlBWS, ACPP and ACSB samples, respectively. The thermodynamic parameters showed the endothermic nature and adsorption of nitrate ions and are considered chemisorption. The total cost of the samples production were 13 USD kg⁻¹ for acidic alumina and 6 USD kg⁻¹ for active carbon.
Conclusion
The adsorption of nitrates were utilized in real and synthetic samples which give an excellent removing capability for nitrate pollution from groundwater samples.
... Zero-valent metal (Fe 0 , Al 0 ) will remove [5,38,40] nitrates [41][42][43][44][45][46][47][48][49][50][51][52][53][54], nitrites [55,56], bromates [54], chlorates [54], organic compounds (including organo-halides [4][5][6][7]16,[57][58][59][60][61][62][63]) and metals (Ag [64], Al [5], As [40,65,66], Ba [5], Cd [12,64], Co [40], Cr [5,63], Cu [5,42], Fe [5], Hg [64], Mn [5], Ni [42], Pb [5], Se [67], U [68], Zn [69]), phosphates [40], sulphates [5] from groundwater, by changing the redox environment (Eh, pH). Adding Al salts to Fe 0 increases the rate of organic contaminant removal [38,70,71]. ...
... Zero-valent metal (Fe 0 , Al 0 ) will remove [5,38,40] nitrates [41][42][43][44][45][46][47][48][49][50][51][52][53][54], nitrites [55,56], bromates [54], chlorates [54], organic compounds (including organo-halides [4][5][6][7]16,[57][58][59][60][61][62][63]) and metals (Ag [64], Al [5], As [40,65,66], Ba [5], Cd [12,64], Co [40], Cr [5,63], Cu [5,42], Fe [5], Hg [64], Mn [5], Ni [42], Pb [5], Se [67], U [68], Zn [69]), phosphates [40], sulphates [5] from groundwater, by changing the redox environment (Eh, pH). Adding Al salts to Fe 0 increases the rate of organic contaminant removal [38,70,71]. ...
... Zero-valent metal (Fe 0 , Al 0 ) will remove [5,38,40] nitrates [41][42][43][44][45][46][47][48][49][50][51][52][53][54], nitrites [55,56], bromates [54], chlorates [54], organic compounds (including organo-halides [4][5][6][7]16,[57][58][59][60][61][62][63]) and metals (Ag [64], Al [5], As [40,65,66], Ba [5], Cd [12,64], Co [40], Cr [5,63], Cu [5,42], Fe [5], Hg [64], Mn [5], Ni [42], Pb [5], Se [67], U [68], Zn [69]), phosphates [40], sulphates [5] from groundwater, by changing the redox environment (Eh, pH). Adding Al salts to Fe 0 increases the rate of organic contaminant removal [38,70,71]. ...
Socio-economic, climate and agricultural stress on water resources have resulted in increased global demand for water while at the same time the proportion of potential water resources which are adversely affected by sodification/salinisation, metals, nitrates, and organic chemicals has increased. Nano-zero-valent metal (n-ZVM) injection or placement in aquifers offers a potential partial solution. However, n-ZVM application results in a substantial reduction in aquifer permeability, which in turn can reduce the amount of water that can be abstracted from the aquifer. This study using static diffusion and continuous flow reactors containing n-ZVM and m-ZVM (ZVM filaments, filings and punchings) has established that the use of m-ZVM does not result in a reduction in aquifer permeability. The experimental results are used to design and model m-ZVM treatment programs for an aquifer (using recirculation or static diffusion). They also provide a predictive model for water quality associated with specific abstraction rates and infiltration/injection into an aquifer. The study demonstrates that m-ZVM treatment requires 1% of the weight required for n-ZVM treatment for a specific flow rate. It is observed that 1 t Fe0 will process 23,500 m3 of abstracted or infiltrating water. m-ZVM is able to remove >80% of nitrates from flowing water and adjust the water composition (by reduction) in an aquifer to optimize removal of nitrates, metals and organic compounds. The experiments demonstrate that ZVM treatment of an aquifer can be used to reduce groundwater salinity by 20 –> 45% and that an aquifer remediation program can be designed to desalinate an aquifer. Modeling indicates that widespread application of m-ZVM water treatment may reduce global socio-economic, climate and agricultural stress on water resources. The rate of oxygen formation during water reduction [by ZVM (Fe0, Al0 and Cu0)] controls aquifer permeability, the associated aquifer pH, aquifer Eh and the degree of water treatment that occurs.
... Such lag periods were repeatedly reported in the literature over the years (e.g. Huang et al. 1998, Noubactep et al. 2003, Hao et al. 2005. In Fe 0 /H2O systems, the lag time is indicative of the time required for the generation of FeCPs following Fe 0 immersion into a polluted water. ...
... The presentation until now has demonstrated that no single electron from Fe 0 can be transferred to COCs because of the presence of the universal oxide scale which is never electronically conductive (Table 4). If electrons from Fe 0 were transferred to any COC, there would have not been a lag time between the start of the experiment and the start of reductive transformation of COCs (Schreier and Reinhard 1994, Huang et al. 1998, Hao et al. 2005. Consequently, COCs, O2 and co-contaminants are reduced by Fe II , H2, Fe3O4, green rust, and other reducing species generated in the Fe 2+ can donate one electron and H2 two electrons. ...
The suitability of remediation systems using metallic iron (Fe0) has been extensively discussed during the past 3 decades. It has been established that aqueous Fe0 oxidative dissolution is not caused by the presence of any contaminant. Instead, the reductive transformation of contaminants is a consequence of Fe0 oxidation. Yet researchers are still maintaining that electrons from the metal body are involved in the process of contaminant reduction. According to the electron efficiency concept, electrons from Fe0 should be redistributed to: i) contaminants of concern (COCs), ii) natural reducing agents (e.g., H2O, O2), and/or iii) reducible co-contaminants (e.g. NO3-). The electron efficiency is defined as the fraction of electrons from Fe0 oxidation which is utilized for the reductive transformations of COCs. This concept is in frontal contradiction with the view that Fe0 is not directly involved in the process of contaminant reduction. This communication recalls the universality of the concept that reductive processes observed in remediation Fe0/H2O systems are mediated by primary (e.g., FeII, H/H2) and secondary (e.g., Fe3O4, green rusts) products of aqueous iron corrosion. The critical evaluation of the electron efficiency concept suggests that it should be abandoned. Instead, research efforts should be directed towards tackling the real challenges for the design of sustainable Fe0-based water treatment systems based on fundamental mechanisms of iron corrosion.
... In particular, at pH values that are relevant to natural systems, iron readily precipitates and the adsorptive affinity of the oxide-film for cations is increased (pH > pH pzc ). Therefore, it is surprising that adsorption and co-precipitation has received less attention in the discussion results of cation removal by Fe 0 /H 2 O systems [43,44]. Hao et al. [44] reported that Cu 2+ enhances nitrate reduction by Fe 0 . ...
... Therefore, it is surprising that adsorption and co-precipitation has received less attention in the discussion results of cation removal by Fe 0 /H 2 O systems [43,44]. Hao et al. [44] reported that Cu 2+ enhances nitrate reduction by Fe 0 . Even though this is possible through an indirect reaction, "NO 3 removal" and "NO 3 reduction" are not randomly interchangeable without precise investigations. ...
A review of the approach used by environmental remediation researchers to evaluate the reactivity of Fe0-based alloys reveals the lack of consideration of the results available from other branches of science. This paper discusses the limitations of the current approach. The discussion provided here suggests that the current assumption that redox-sensitive species serve as corrosive agents for Fe0 maybe incorrect because water as the solvent is also corrosive. A new approach is proposed in which water is considered as the primary Fe0 oxidizing agent and the impact of individual relevant solutes (including contaminants) should be assessed in long-term laboratory experiments. It is expected that the application of the proposed approach will help to reliably characterize the reactivity of Fe0 materials within a few years.
... In particular, at pH values that are relevant to natural systems, iron readily precipitates and the adsorptive affinity of the oxide-film for cations is increased (pH > pH pzc ). Therefore, it is surprising that adsorption and co-precipitation has received less attention in the discussion results of cation removal by Fe 0 /H 2 O systems [43,44]. Hao et al. [44] reported that Cu 2+ enhances nitrate reduction by Fe 0 . ...
... Therefore, it is surprising that adsorption and co-precipitation has received less attention in the discussion results of cation removal by Fe 0 /H 2 O systems [43,44]. Hao et al. [44] reported that Cu 2+ enhances nitrate reduction by Fe 0 . Even though this is possible through an indirect reaction, " NO 3 − removal " and " NO 3 − reduction " are not randomly interchangeable without precise investigations. ...
A review of the approach used by environmental remediation researchers to evaluate the reactivity of Fe0-based alloys reveals the lack of consideration of the results available from other branches of science. This paper discusses the limitations of the current approach. The discussion provided here suggests that the current assumption that redox-sensitive species serve as corrosive agents for Fe0 maybe incorrect because water as the solvent is also corrosive. A new approach is proposed in which water is considered as the primary Fe0 oxidizing agent and the impact of individual relevant solutes (including contaminants) should be assessed in long-term laboratory experiments. It is expected that the application of the proposed approach will help to reliably characterize the reactivity of Fe0 materials within a few years.
... The experimental duration has an efficient role, when the contact time was few days, that mean there was no available enough quantity of the removal agents, which are the corrosion products (Hao et al. 2005, Ghauch 2015, Konadu-Amoah et al. 2022). ...
The concept that metallic iron (Fe0) is a reducing agent under environmental conditions has urged the large-scale application of Fe0 filters for environmental remediation and water treatment. During the past two decades, some 3,000 scientific articles have widely discussed the importance of processes yielding water treatment in Fe0/H2O systems. The current state-of-the-art knowledge is that Fe0 is the generator of (i) contaminant scavengers (iron hydroxides/oxides), and (ii) reducing agents (e.g. H/H2, FeII, green rusts, Fe3O4). In other words, contaminant reductive transformation in the presence of Fe0 is not mediated by electrons from the metal body (direct reduction).
The realization that Fe0 is not the reducing agent in Fe0/H2O systems has redirected fundamental researches on the operating mode of Fe0 filters. In this effort, a cationic azo dye (methylene blue, MB) has been presented as reactivity indicator to characterize changes in Fe0/H2O systems. The present study investigates the impact of contact time on the efficiency of Fe0/H2O systems. The research questions are "is there any direct relationship between experimental duration and system's efficiency?" If yes, how is the efficiency modified in the presence of natural additives such as manganese oxides (MnO2) and sand? Both research questions are justified by the evidence that the Fe0 surface is constantly shielded by an oxide scale which has been reported to mediate a 'reactivity loss' of Fe0 materials.
The methodology consists of (i) varying the experimental duration, and (ii) modifying the Fe0/H2O system by amending it with various amounts of MnO2 and sand. The efficiency of Fe0/sand/MnO2 systems for water treatment is characterized using methylene blue (MB) as reactivity indicator. Batch experiments using various weight ratios of Fe0 and the two additives were performed for up to six weeks (47 days). The impact of the intrinsic reactivity of MnO2 was characterized by using different types of MnO2. The MB discoloration process was investigated both under shaking and non-disturbed conditions.
The results clearly demonstrate the impact of increased contact time on the extent of MB discoloration in all tested systems (Fe0, Fe0/sand, Fe0/MnO2 and Fe0/sand/MnO2). As a rule, MB discoloration was improved by increased experimental duration. It was noted that the extent of MB discoloration is influenced by the diffusive (or advective) transport of MB from the solution to the reactive materials at the bottom of the test tubes. Without shaking, more time is needed for the transport of MB to the particles of tested materials. For experiments lasting for longer times, sand addition prevented Fe0 particles from compaction (cementation) at the bottom of the test-tubes. This enabled the long-term generation of iron hydroxides (new iron corrosion products) for the discoloration of MB by adsorption and co-precipitation. The same observation was made for Fe0/MnO2 systems. In other words, the addition of non-expansive materials (e.g. MnO2, sand) is necessary to sustain the efficiency of Fe0 filters. Shaking the test tubes increased the extent of MB discoloration by two different mechanisms: (i) speeding up the mass transport of MB solution towards the adsorptive materials, and (ii) speeding up the kinetics of Fe0 corrosion, creating new corrosion products. Discoloration processes occur due to MB diffusion and advection which are accelerated during the shaking operation. The results clearly delineate the complexity of the Fe0/MnO2/sand system and suggest that varying the experimental conditions will give more opportunities to discuss the efficiency of Fe0/H2O systems.
... This makes iron filings a potential reducing agent for many labile substances. In the case of nitrate, the interaction of iron filings with the nitrate in the stormwater may result in the reduction of nitrate to ammonia, nitrite, or nitrogen gas as described in Eq. (5) (Hao et al. 2005;Xu et al. 2017). Kassaee et al. (2011) found the reduction of nitrate to nitrogen gas in the presence of zero valent iron in acid pH. ...
During storm events, contaminants and sediments from roadways, curbside, parking lots, and lawns in urban environments are mobilized and transported by the stormwater runoff. These contaminants are finally discharged in waterbodies, such as rivers and lakes, with adverse effects on public health and ecosystems. Several studies have reported high levels of heavy metals and nutrients in urban stormwater runoff. Best management practices such as sedimentation and bioretention are not practical in urban environments due to the lack of adequate space; however, filtration systems, such as an in-ground permeable filter system, are being developed because they are practical and feasible. Four different filter materials (calcite, zeolite, sand, and iron filings) were effective in removing individual contaminants (Cd, Cu, Ni, Cr, Zn, nitrate, and phosphate) in tests at 24 h. The present investigation assessed the removal kinetics of contaminants from a simulated stormwater consisting of multiple heavy metals and nutrients by the four filter materials. Batch experiments were conducted to evaluate the removal kinetics of co-existing heavy metals and nutrients from the simulated stormwater. The rate of contaminant removal and overall removal efficiency was found to be dependent on the filter material and contaminant nature, as well as the induced pH changes caused by the filter media. The zero-order kinetic model best described the removal rate of Cu and Ni by sand. The first-order kinetic model was only applicable for nitrate removal by iron filings, and the second-order kinetic model described the removal rates of other contaminants and filter media combinations.
... In that period Fe 2+ is not released and its concentration in solution is zero. Analogous kind of "induction period" was reported in previous study (Lavine et al. 2001, Noubactep et al. 2003, Hao et al. 2005, Touomo-Wouafo et al., 2018. In that study, only 0.1 g of Fe 0 was used, the "induction period" lasts the first 1 -2 h, the pH increased up to 6.0 and then returned back to 5.5. ...
Metallic iron (Fe 0) is a reactive material that is widely used for industrial water treatment. The course of the metal ion removal process using Fe 0 (iron powder) was monitored electrochemically (differential pulse polarography). As probe species, Zn 2+ , Pb 2+ and Cd 2+ were selected for their different (i) adsorptive affinity to iron corrosion products (FeCPs), (ii) redox properties, (iii) precipitation ability at various pH. Batch experiments were carried out with binary (Zn 2+ / Pb 2+ and Zn 2+ / Cd 2+) and ternary (Zn 2+ /Cd 2+ /Pb 2+) systems in order to reveal the mutual interference of these cations. Detailed time monitoring of iron ageing for up to 14 d as well as concentration decay of individual removed cations represent important data for mechanistic discussions. The aqueous concentration of Fe 2+ was also monitored. FeCPs were characterized using X-ray Photoelectron Spectroscopy (XPS) and Scanning electron microscopy (SEM). Results showed that the presence of Pb 2+ delays the Zn 2+ removal whereas the presence of Cd 2+ in solution accelerates its removal. The removal of Pb 2+ by FeCPs was not affected by the presence of Zn 2+ and Cd 2+ , moreover, the Pb 2+ inhibited the effect of Cd 2+ on the removal of Zn 2+. XPS has proven existence of Fe2O3 and hydrated Fe oxidic phase, whilst the SEM showed that the original Fe grains were partly dissolved into buffered ambient under formation of fine particles of FeCPs. Results confirm that reductive transformation of any contaminant in a Fe 0 /H2O system is the consequence and not the cause of iron corrosion.
... Nitrates menace all water resources. Contamination of surface and groundwater with nitrates has emerged as an insistent problem on a global scale over the past three decades [5][6][7][8]. Elevated nitrate concentrations in potable water supplies can lead to various pernicious health hazards on both human and animal beings causing in some cases fatal results [4,[9][10]. Moreover, the existence of high levels of nitrates in water stimulates numerous environmental risks such as eutrophication [11]. ...
Abstract
Purpose Two types of active acidic alumina derived from aluminum building wire scraps (AlBWS) and aluminum takeout
food container waste (AlTFC), as well, two effective types of activated carbon prepared from potato peels (ACPP) and
sugarcane bagasse (ACSB) for removal of nitrate ions were utilized in this study.
Methods Adsorbent samples were prepared with highly porous structure and large specific surface area above 200 m2 g−1.
Also, low calcinations temperatures and low-priced chemicals were used.
Results The results showed that the adsorption of nitrates were rapid. The qmax
were found to be 12.4, 8.8, 7.1 and 3.9
(mg g−1) for AlTFC, AlBWS, ACPP and ACSB samples, respectively. The thermodynamic parameters showed the endothermic
nature and adsorption of nitrate ions and are considered chemisorption. The total cost of the samples production
were 13 USD kg−1 for acidic alumina and 6 USD kg−1 for active carbon.
Conclusion The adsorption of nitrates were utilized in real and synthetic samples which give an excellent removing capability
for nitrate pollution from groundwater samples.
Keywords Nitrate removal · Alumina · Activated carbon · Building scraps wire · Aluminum take-out food container ·
Potato peels
... In the first few 20 days there was no availibilty of enough iron oxides for the dye removal. The initial quantitative breakthrough corresponds to the time lag between the commencement of the experiment and of quantitative generation of removing agents (iron oxides) (Schreier and Reinhard 1994, Hao et al. 2005, Ghauch et al. 2010, Ghauch et al. 2011, Ghauch 2013). RR 120 dyes shows the similar trend as Orange II. ...
Elemental iron (Fe0) has been successfully tested and used for water treatment for decades due to its worldwide availability and inexpensiveness. Fe0-based filtration technology has been used for (i) environmental remediation (e.g. subsurface permeable reactive barriers) (ii) wastewater treatment and (iii) safe drinking provision. The evidence that Fe0 oxidative dissolution and subsequent precipitation at pH > 4.0 is a volumetric expansive process (Voxide > Viron) implies that Fe0 should be amended with non-expansive aggregates such as activated carbon, manganese oxides, pumice or sand. Only such hybrid systems are likely to be sustainable.
The present work focuses on the characterization of the ion selective nature of Fe0-based filters using three azo dyes: methylene blue (cationic), Orange II and Reactive Red 120 (anionic). The dyes are used as indicators for the reactivity of Fe0/H2O system in both batch and column experiments. The idea is to demonstrate that downwards from a Fe0 / sand system; available aggregates are in-situ coated with iron oxide, such that in the medium to long term, the whole system is ion-selective. The selectivity being fixed by positively charged iron oxides.
The characterization of the Fe0/H2O system is realized herein by amending (i) Fe0 with sand and MnO2 in batch experiments and (ii) Fe0 with sand in column experiments. Sand is a pure adsorbent with a negatively charged surface while MnO2 is reactive in nature. MnO2 addition enables the control of the availability of in-situ generated iron corrosion products and thus the role of corrosion product in the process of contaminant removal. The investigated systems in batch mode are (i) pure sand, (ii) pure MnO2, (iii) pure Fe0, (iv) Fe0/sand mixture, (v) Fe0/MnO2 mixt ure and (vi) Fe0/sand/MnO2 mixtures with various amounts of sand and MnO2 loadings. Column experiments were performed with the following systems: (i) pure sand (0 % Fe0), (ii) pure Fe0 (100 % Fe0), and (iii) Fe0/sand (50 % Fe0- vol/vol).
Results of batch experiment showed that sand is a good adsorbent for MB and has negligible effect on anionic dyes. MnO2 also favors MB discoloration. Pure Fe0 favors discoloration of both cationic and anionic dyes but shows best discoloration efficiency for Orange II. Among the Fe0 amended systems, the Fe0/sand system is most efficient for dye discoloration. The discoloration efficiency in Fe0-based systems is 75 % for MB and > 95 % for Orange II and RR120. Results confirmed quantitative adsorptive MB discoloration and negligible adsorption of anionic dyes on negatively charged sand. Quantitative discoloration of the anionic dyes (Orange II and RR120) in Fe0-based systems was attributed to high affinities of both species to positively charged iron corrosion products. The ion selective nature of the Fe0/H2O system is elegantly demonstrated.
... Generally, the mixing intensity is considered of utmost importance when it leads to attrition of the reactive media [25]. However, even this aspect is not usually taken into account in most studies, and mixing intensities higher than 300 -1 min have been used in characterising redox processes in Fe 0 /H 2 O systems [43,44]. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe0). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min-1 was directly compared. Investigated materials were scrap iron (Fe0), granular activated carbon (GAC), and deep sea manganese nodules (MnO2). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0 to 9.0 g/L of each reactive material (systems I, II and III), or (iii) 5 g/L Fe0 and 0 to 9.0 g/L GAC or MnO2 (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8 to 5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min-1, a plateau between 50 and 150 min-1, and a sharp increase of MB discoloration at shaking intensities ≥ 200 min-1. At 300 min-1, increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe0/MnO2 and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe0 surface should not exceed 50 min-1.
... From the perspective of rendering Fe 0 accessible by adding EDTA, the effect of Cu II addition as discussed by Gyliene et al. [1] can be improved. In fact, Cu II removal by Fe 0 is a very welldocumented metallurgical process [7], that has also been reported in the context of remediation by Fe 0 [8]. Accordingly, EDTA keeps Fe 0 surface free for Cu II reduction (cementation). ...
This letter presents an improved discussion of the data provided in a recent article on EDTA removal from aqueous solutions using elemental iron (Fe0) by O. Gyliene and his co-workers. It is shown that the authors have furnished a brilliant validation of the concept that dissolved contaminants are primary removed in Fe0/H2O systems by adsorption onto iron corrosion products and co-precipitation with iron corrosion products. It is reiterated that “contaminant removal” and “contaminant reduction” should not be interchanged randomly.
... Contrary to the simulated case of this work (5 g L -1 Fe 0 ), studies using less than 20 g L -1 Fe 0 are scarce. Furthermore, experiments are sometimes performed under high mixing conditions (up to 500 min -1 )[28,29], possibly yielding a larger extent of Fe 0 consumption (> 10 %) for tested experimental durations and thus data that are more difficult to interpret as shown in the next section. ...
Aqueous contaminant removal in the presence of metallic iron (e.g. in Fe0/H2O systems) is characterized by the large diversity of removing agents. This paper analyses the synergistic effect of adsorption, co-precipitation and reduction on the process contaminant removal in Fe0/H2O systems on the basis of simple theoretical calculations. The system evolution is characterized by the percent Fe0 consumption. The results showed that contaminant reduction by Fe0 is likely to significantly contribute to the removal process only in the earliest stage of Fe0 immersion. With increasing reaction time, contaminant removal is a complex interplay of adsorption onto iron corrosion products, co-precipitation or sequestration in the matrix of iron corrosion products and reduction by Fe0, FeII or H2/H. The results also suggested that in real world Fe0/H2O systems, any inflowing contaminant can be regarded as foreign species in a domain of precipitating iron hydroxides. Therefore, current experimental protocols with high contaminant to Fe0 ratios should be revisited. Possible optimising of experimental conditions is suggested.
... 37 From the perspective of rendering Fe 0 accessible by adding EDTA, the effect of Cu II addition 38 as discussed by Gyliene et al. [1] can be improved. In fact, Cu II removal by Fe 0 is a very well39 documented metallurgical process [7], that has also been reported in the context of 40 remediation by Fe 0 [8]. Accordingly, EDTA keeps Fe 0 surface free for Cu II reduction 41 (cementation). ...
... Less than 2.0 mg N L −1 nitrite accumulated in the beginning and subsequently decreased rapidly in the presence of Fe 2+ or Fe 3+ . It might be because Cu 2+ or Cu 0 could catalyze nitrate reduction to nitrite but it was weak or not in catalyzing nitrite to ammonia [30], which led to the accumulation of nitrite at the beginning of the reaction. Subsequently, Cu 2+ or Cu 0 lost its catalytic activity due to precipitation and passivation by nonconducting substances. ...
... This hypothesis is supported by previous studies, which showed that Ni was mainly scavenged by Fe oxyhydroxides (e.g. Xu et al., 2007; Gunsinger et al., 2006; Sterckeman et al., 2006; Dries et al., 2005; Hao et al., 2005; Stipp et al., 2002; Li et al., 2000). Nachtegaal and Sparks (2002) noted that this phenomenon was enhanced by the presence of humic acids. ...
This paper presents geochemical profiles of a tephra-bearing minerotrophic peat column from NE-Iceland obtained using various elemental analyses of the solid phase and the pore water. The influence of tephra grain size, thickness and composition of each tephra on the peat geochemistry was investigated. Interpretations are supported by a statistical approach, in particular by autocorrelation, and by microscopy observations. Minerotrophic peat geochemistry may be strongly dependent upon post-depositional mobilization and possible leaching of elements as demonstrated by Fe and trace metal concentration profiles. Chemical elements, and more specifically potentially harmful metals, can be slowly leached out of volcanic falls during their weathering and re-accumulate downwards. It is emphasised that a tephra deposit can act as an active geochemical barrier, blocking downward elemental movements and leading to the formation of enriched layers. In this study, the formation of poorly amorphous Fe phases above the Hekla 3 tephra is shown. These poorly crystalline Fe phases scavenged Ni.
... • Chromium (Li & Zhang, 2000b;Niu, Liu, Xu, & Lou, 2005) and nickel (Li & Zhang, 2000b) from spent electroplating solutions by magnetically stabilized beds of iron-zinc mixtures. • Nitrates deposition on iron particles Hao, Xu, Jin, et al., 2005). • Arsenic removal by iron coated sands and iron-oxide loaded polymers (Zouboulis & Katsoyiannis, 2002) and magnetite (Ohe, Tagai, Nakamura, Ohshima, & Baba, 2005). ...
This article deals with problems relevant to implementation of magnetically stabilized beds (MSB) as separation devices. The main issues discussed are: bed mechanics, bed structure, possibilities to create controllable filter media, etc. As examples several separation techniques are discussed: dust filtration—magnetic and non-magnetic, ion-exchange, copper cementation, yeast filtration from biological liquids, particle separation by density and magnetic properties, dangerous wastes removal. Only key publications will be quoted that provide a basis for further reading and study and relevant information.
... ZVM research has focused on determining n-ZVM kinetic reaction rates [28] over a period of minutes or hours [29,30] or months [28]. Reaction kinetics are important in a simple closed system reactor design, but are largely irrelevant in the design of IZVM (injected ZVM program) or a ZVM PRB in a more complex open or semi-open aquifer buffer environment. ...
Sixteen static diffusion reactors containing n-ZVM (Fe0, Cu0, Al0) establish a common equilibrium redox (Eh-pH) trajectory which is directly linked to the aquifer pore volume, volume of injected n-ZVM, throughflow rate within the aquifer and time. The effect of NaCl and Ca-montmorillonite on the trajectory is considered. The trajectory can be directly linked to TDS (EC) and to the equilibrium removal of contaminants. In each example, the progressive oscillation between reduction and oxidation reactions (including Fenton reactions) creates the catalytic nuclei (and redox environment) required for the decomposition of organic pollutants and their reconstruction as simple alkanes and oxygenates.
... Generally, the mixing intensity is considered of utmost importance when it leads to attrition of the reactive media [25]. However, even this aspect is not usually taken into account in most studies, and mixing intensities higher than 300 −1 min have been used in characterising redox processes in Fe 0 /H 2 O systems [43,44]. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe(0)). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min(-1) was directly compared. Investigated materials were scrap iron (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min(-1), a plateau between 50 and 150 min(-1), and a sharp increase of MB discoloration at shaking intensities >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1).
... Contrary to the simulated case of this work (5 g L −1 Fe 0 ), studies using less than 20 g L −1 Fe 0 are scarce. Furthermore, experiments are sometimes performed under high mixing conditions (up to 500 min −1 ) [28,29], possibly yielding a larger extent of Fe 0 consumption (>10%) for tested experimental durations and thus data that are more difficult to interpret as shown in the next section. ...
Aqueous contaminant removal in the presence of metallic iron (e.g. in Fe(0)/H(2)O systems) is characterized by the large diversity of removing agents. This paper analyses the synergistic effect of adsorption, co-precipitation and reduction on the process contaminant removal in Fe(0)/H(2)O systems on the basis of simple theoretical calculations. The system evolution is characterized by the percent Fe(0) consumption. The results showed that contaminant reduction by Fe(0) is likely to significantly contribute to the removal process only in the earliest stage of Fe(0) immersion. With increasing reaction time, contaminant removal is a complex interplay of adsorption onto iron corrosion products, co-precipitation or sequestration in the matrix of iron corrosion products and reduction by Fe(0), Fe(II) or H(2)/H. The results also suggested that in real world Fe(0)/H(2)O systems, any inflowing contaminant can be regarded as foreign species in a domain of precipitating iron hydroxides. Therefore, current experimental protocols with high contaminant to Fe(0) ratios should be revisited. Possible optimising of experimental conditions is suggested.
... From the perspective of rendering Fe 0 accessible by adding EDTA, the effect of Cu II addition as discussed by Gyliene et al. [1] can be improved. In fact, Cu II removal by Fe 0 is a very well-documented metallurgical process [7], that has also been reported in the context of remediation by Fe 0 [8]. Accordingly, EDTA keeps Fe 0 surface free for Cu II reduction (cementation). ...
This letter presents an improved discussion of the data provided in a recent article on EDTA removal from aqueous solutions using elemental iron (Fe(0)) by O. Gyliene and his co-workers. It is shown that the authors have furnished a brilliant validation of the concept that dissolved contaminants are primary removed in Fe(0)/H(2)O systems by adsorption onto iron corrosion products and co-precipitation with iron corrosion products. It is reiterated that "contaminant removal" and "contaminant reduction" should not be interchanged randomly.
Pomegranate peel (PP), a by-product of agro-food consumption, has a low adsorption capacity for nitrate and phosphate ions in aqueous media, but its surface is very rich in alcohol functional groups. In this work, the surface of pomegranate peels was functionalized by chemo-grafting 3-(2-Aminoethylamino) propyl] trimethoxy silane (AEAPTES) using the availability of alcohol groups to increase the adsorption capacity of the resulting adsorbent (PP/AEAPTES) towards nitrate and phosphate ions. The prepared PP/AEAPTES adsorbent was analyzed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Zeta potential, and X-ray photoelectron spectrometry (XPS). Under experimental conditions, the adsorption capacity of PP/AEAPTES has been found to be 124.57 mg/g and 94.65 mg/g for NO3− and PO43−, respectively, at pH 6 over a wide temperature range, and adsorption is exothermic for NO3− and endothermic for PO43−, as well as spontaneous and physical in nature. The adsorptions of NO3− and PO43− were also correctly described by the Langmuir isotherm and followed the pseudo-second-order model. The ability of PP/AEAPTES to adsorb NO3− and PO43− ions under real conditions was evaluated, and efficient regeneration and repetitive use of PP/AEAPTES was successfully achieved up to 5 cycles.
Groundwater contamination is a worldwide concern and the development of new materials for groundwater remediation has been of great interest. This study investigated removal kinetics and mechanisms of nitrate, copper ion and hexavalent chromium (20-50 mg L-1) by particles of Al-Fe alloy consisting of 20% Fe in batch reactors from a single KNO3, CuSO4, Cu(NO3)2, K2Cr2O7 and their mixed solutions. The effects of contaminant interactions and initial pH of the solution were examined and the alloy particles before and after reaction were characterized by X-ray diffraction spectrometer, scanning electron microscopy and X-ray photoelectron spectroscopy. The removal mechanisms were attributed to chemical reduction [Cu(II) to Cu, NO3- to NH3 and Cr(VI) to Cr(III)] and co-precipitation of Cr(III)-Al(III)-Fe(III) hydroxides/oxyhydroxides. Cu(II) enhanced the rates of NO3- and Cr(VI) reduction and Cr(VI) was an inhibitor for Cu(II) and NO3- reduction. This study demonstrates that Al-Fe alloy is of potential for groundwater remediation.
Zero valent iron (ZVI) and ZVI-based materials have been widely used for the reduction of nitrate, a major contaminant commonly detected in groundwater and surface water. The reduction of nitrate by ZVI is influenced by various factors, such as the physical and chemical characteristics of ZVI and the operational parameters. There are some problems for the nitrate reduction by ZVI alone, for example, the formation of iron oxides on the surface of ZVI at high pH condition, which will inhibit the further reduction of nitrate; in addition, the end reduction product is mainly ammonium, which itself needs to be concerned. Several strategies, such as the optimization of the structure of ZVI composites and the addition of reducing assistants, have been proposed to increase the reduction efficiency and the selectivity of end product of nitrate reduction in a wide range of pH, especially under neutral pH condition. This review will mainly focus on the high efficient reduction of nitrate by ZVI-based materials. Firstly, the reduction of nitrate by ZVI alone was briefly introduced and discussed, including the influence of physical and chemical characteristics of ZVI and some operational parameters on the reduction efficiency of nitrate. Then, the strategies for enhancing the reduction efficiency and the N 2 selectivity of the reductive products of nitrate were systematically analyzed and evaluated, especially the optimization of the structure of ZVI composites (e.g., doped ZVI composite, supported ZVI composite and premagnetized ZVI), and the addition of reducing assistants (e.g., metal cations, ligand, hydrogen gas and light) were highlighted. Thirdly, the mechanisms and pathways of nitrate reduction were discussed. Finally, concluding remarks and some suggestions for the future research were proposed.
Polymeric ion exchanger has been proven to be an excellent carrier for metal nanoparticles due to chemical stability and robust mechanical strength. However, polymeric ion exchanger containing non-diffusible negatively or positively charged groups can only permit cation or anion contaminants to permeate into the polymer phase due to Donnan exclusion effect. In this study, zero-valent iron nanoparticles (NZVI) were immobilized within a chelating resin DOW 3N with pyridine functional groups through NaBH4 reduction. TEM indicated that Fe-0 particles were clearly discrete and well dispersed on the surface of DOW 3N with grain size ranging from 10 to 30 nm. The reduction ability of NZVI-DOW 3N for Pb2+, NO3- and their mixture were evaluated, respectively. The results showed that NO3- and Pb2+ can be reduced efficiently. In a binary solution, the removal efficiencies of NO3- and Pb2+ can reach 87% and 97%, respectively.
Research and applications of magnetically stabilized and fluidized beds begun several
decades ago, and have maintained their rate of interest during all these years. During last years their
applications was diversified, and the application of these beds in the processing of biological
materials and in environmental issues has nowadays focused the main attention within this area. In
this work we present a brief review of magnetically fluidized and stabilized beds, mainly
concerning their theoretical background, heat and mass transfer topics, some industrial and
laboratorial setups and applications.
The effect of the ionic charge on the efficiency of Fe0/sand systems for dye discoloration was investigated in column studies. Tested systems for each dye were: (i) pure sand (0 % Fe0), (ii) pure Fe0 (100 % Fe0), and (iii) Fe0/sand (50 % Fe0 - vol/vol). Tested dyes were methylene blue (MB - cationic), orange II (anionic) and reactive red 120 (RR 120 - anionic). Used dye concentration was 31 mM and used Fe0 mass was 100 g. Each system was characterized by the time-dependent changes of the pH value, the iron breakthrough, the dye breakthrough, and the hydraulic conductivity (permeability). The experiments lasted for 93 days during which a total of 26.12 L of the dye solution flowed through each column (809.7 mM dye in total). No significance changes in pH value, Fe breakthrough and permeability could be documented. In pure sand systems (0 % Fe0) 15, 21 and 140 mM of RR 120, Orange II and MB were discolored respectively. The discoloration efficiency in Fe0-based systems was 75 % for MB and more than 95 % for RR120 and Orange II. Results confirmed quantitative adsorptive MB discoloration and negligible adsorption of anionic dyes onto negatively charged sand. Quantitative discoloration of anionic dyes (Orange II, RR 120) in Fe0/sand systems was attributed to high affinities of both species to positively iron corrosion products. UV-vis spectra of effluent solutions revealed a quantitative chemical reaction of RR 120 in the Fe0/H2O system. The yellow-colored reaction products were not active in the range 800 - 200 nm and their breakthroughs were quantitative over the whole experimental duration. Results confirmed the ion-selective nature of the Fe0/H2O system and are regarded as a cornerstone for the design of next generation Fe0-based filtration systems.
Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl(-), SO(4)(2-), and ClO(4)(-) suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO(3)(-) and HCO(3)(-) concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO(3)(-) suspensions. Markedly different trends in Cr(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr(VI) removal capacities and rates of Fe(II) production measured in the absence of Cr(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe(II) species are primarily responsible for Cr(VI) reduction.
Zero-valent iron nanoparticles (INP) were investigated as a remediation strategy for a uranium-contaminated waste effluent from AWE, Aldermaston. Nanoparticles were introduced to the effluent, under both oxic and anoxic conditions, and allowed to react for a 28-d period during which the liquid and nanoparticle solids were periodically sampled. Analysis of the solution indicated that under both conditions U was removed to <1.5% of its initial concentration within 1h of introduction and remained at similar concentrations until approximately 48 h. A rapid release of Fe into solution was also recorded during this initial period; attributed to the limited partial dissolution of the INP. XPS analyses of the reacted nanoparticulate solids between 1 and 48 h showed an increased Fe(III):Fe(II) ratio, consistent with the detection of iron oxidation products (akaganeite and magnetite) by XRD and FIB. XPS analysis also recorded uranium on the recovered particulates indicating the chemical reduction of U(VI) to U(IV) within 1h. Following the initial retention period U-dissolution of U was recorded from 48 h, and attributed to reoxidation. The efficient uptake and retention of U on the INP for periods up to 48 h provide proof that INP may be effectively used for the remediation of complex U-contaminated effluents.
Chemical treatment of para-nitrochlorobenzene (p-NCB) by palladium/iron (Pd/Fe) bimetallic particles represents one of the latest innovative technologies for the remediation of contaminated soil and groundwater. The amination and dechlorination reaction is believed to take place predominantly on the surface site of the Pd/Fe catalysts. The p-NCB was first transformed to p-chloroaniline (p-CAN) then quickly reduced to aniline. 100% of p-NCB was removed in 30 min when bimetallic Pd/Fe particles with 0.03% Pd at the Pd/Fe mass concentration of 3g 75 ml(-1) were used. The p-NCB removal efficiency and the subsequent dechlorination rate increased with the increase of bulk loading of palladium and Pd/Fe. As expected, p-NCB removal efficiency increased with temperature as well. In particular, the removal efficiency of p-NCB was measured to be 67%, 79%, 80%, 90% and 100% for reaction temperature 20, 25, 30, 35 and 40 degrees C, respectively. Our results show that no other intermediates were generated besides Cl(-), p-CAN and aniline during the catalytic amination and dechlorination of p-NCB.
Steel manufacturing byproducts and commercial iron powders were tested in the treatment of Ni(2+)-contaminated water. Ni2+ is a priority pollutant of some soils and groundwater. The use of zero-valent iron, which can reduce Ni2+ to its neural form appears to be an alternative approach for the remediation of Ni(2+)-contaminated sites. Our experimental data show that the removal efficiencies of Ni2+ were 95.15% and 94.68% at a metal to solution ratio of 20 g/L for commercial iron powders and the steel manufacturing byproducts in 60 min at room temperature, respectively. The removal efficiency reached 98.20% when the metal to solution ratio was 40 g/L for commercial iron powders. Furthermore, we found that the removal efficiency was also largely affected by other factors such as the pHs of the treated water, the length of time for the metal to be in contact with the Ni(2+)-contaminated water, initial concentrations of metal solutions, particle sizes and the amount of iron powders. Surprisingly, the reaction temperature appeared to have little effect on the removal efficiency. Our study opens the way to further optimize the reaction conditions of in situ remediation of Ni2+ or other heavy metals on contaminated sites.
A novel in situ membrane technology was developed to remove nitrate (NO3-) from groundwater. Membrane-fed hydrogen gas (H2) was used as an electron donor to stimulate denitrification. A flow-through reactor fit with six hollow-fiber membranes (surface area = 93 cm2) was designed to simulate groundwater flowing through an aquifer with a velocity of 0.3 m/day. This membrane technology supported excellent NO3- and nitrite (NO2-) removal once H2 and carbon limitations were corrected. The membrane module achieved a maximum H2 flux of 1.79 x 10(-2) mg H2/m2 s, which was sufficient to completely remove 16.4 mg/L NO3(-)-N from a synthetic groundwater with no NO2- accumulation. In addition, this model in situ treatment process produced a high quality water containing <0.5 mg/L total organic carbon.
The chemical and microbial activity of corroding iron metal is examined in the acid rock drainage (ARD) resulting from pyrite oxidation to determine the effectiveness in neutralizing the ARD and reducing the load of dissolved heavy metals. ARD from Berkeley Pit, MT, is treated with iron in batch reactors and columns containing iron granules. Iron, in acidic solution, hydrolyzes water producing hydride and hydroxide ion resulting in a concomitant increase in pH and decrease in redox potential. The dissolved metals in ARD are removed by several mechanisms. Copper and cadmium cement onto the surface of the iron as zerovalent metals. Hydroxide forming metals such as aluminum, zinc, and nickel form complexes with iron and other metals precipitating from solution as the pH rises. Metalloids such as arsenic and antimony coprecipitate with iron. As metals precipitate from solution, various other mechanisms including coprecipitation, sorption, and ion exchange also enhance removal of metals from solution. Corroding iron also creates a reducing environment supportive for sulfate reducing bacteria (SRB) growth. Increases in SRB populations of 5,000-fold are observed in iron metal treated ARD solutions. Although the biological process is slow, sulfidogenesis is an additional pathway to further stabilize heavy metal precipitates.
Cementation of copper on iron powder was shown to be a feasible process to achieve a high degree of copper removal over a broad operational range. First-order kinetics were followed for both the copper concentration and the surface area of iron. To minimize the effect of copper-hydroxyl formation and excess iron consumption, the cementation process was found to be more practical in weak acidic conditions.
Experiments have been conducted to investigate the chemical reduction of nitrate under conditions relevant to the often low organic carbon environment of groundwaters. At pH 8 and 20 ± 2°C, in the presence of Cu(II), NO3− was chemically reduced by Fe(II) to NH4+ with an average stoichiometric liberation of 8 protons. The rate of the reaction systematically increased with pH in the range pH 7–8.5. The half-life for nitrate reduction, , was inversely related to the total molar copper concentration, [CuT], by the equation log log [CuT] −2.616, for all measured values of from 23 min to 15 days. At the Cu(II) concentrations used of 7 × 10−6 −10−3 M, Cu was present mainly as a solid phase, either adsorbed to the surfaces of precipitated iron oxides or as a saturated solid. It is this solid phase copper rather than CU2+ in solution which is catalytically active. Neither magnetite, which was formed as a product of the reaction, nor freshly prepared lepidocrocite catalysed the reaction, but goethite did. Although traces of oxygen accelerated the reaction, at higher partial pressures (>0.01 atm) the reduction of nitrate was inhibited, probably due to competition between NO3− and O2 for Fe(II). Appreciable catalytic effects were also observed for solid phase forms of Ag(I), Cd(H), Ni(H), Hg(II), and Pb(II). Mn(II) enhanced the rate slightly, and there was evidence for slow abiotic reduction in the absence of any added metal catalysts. These results suggest that the chemical reduction of nitrate at catalytic concentrations and temperatures appropriate to groundwater conditions is feasible on a timescale of months to years.
The reduction of nitrate to ammonia occurs with nearly complete conversion at room temperature and pressure under aerobic conditions in the presence of iron and either HCl or a pH buffer. A 50.0 mL solution of 12.5 millimolar nitrate is rapidly reduced to ammonia when exposed to 4.00 g of 325 mesh iron at pH 5.0, 0.05 M sodium acetate/acetic acid. The pseudo-first order rate constant was 0.053 min−1, Under conditions of pH 6.0 buffer, (i.e. 0.1 M 4-morpholineethanesulfonic acid adjusted to pH 6.0) and pH 7.0 buffer (0.1 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid adjusted to pH 7.0), the rate constants were 0.0408 min−1 and 0.0143 mint, respectively. In unbuffered solutions there was no loss in nitrate and no production of ammonia. A more concentrated nitrate solution (100 mL of 1.0 M sodium nitrate) was also reduced to ammonia in the presence of 2.5 M HCl with the slow addition of 50.0 g of 325 mesh iron.
Chemical reduction of nitrate by metallic iron (Fe0) was studied as a potential technology to remove nitrate from water. The effects of pH and the iron-to-nitrate ratio on both nitrate reduction rate and percent removal were investigated. Rate constants and the apparent reaction order with respect to nitrate were determined and a mass balance was obtained. Rapid nitrate reduction by iron powder was observed only at pH≤4. pH control with sulfuric acid significantly prolonged nitrate reduction and increased the percent removal. At high nitrate loadings, both the rate and the percent removal increased with decreasing pH. An iron-to-nitrate ratio of 120 m2 Fe0/mol NO3 or higher was required to completely remove nitrate within an hour. An apparent reaction order of 1.7 with respect to nitrate was observed, which may be partly due to the inhibitory effect of sulfate. Ammonia was the end product of nitrate reduction and accounted for all nitrate transformed under our experimental conditions. Acidity is the principal factor which controls the rate and the extent of nitrate removal by Fe0. The rapid reduction of nitrate at low pH was most likely due to either direct reduction by Fe0 or indirect reduction by surface hydrogen derived from proton. Ferrous species, Fe2+ and Fe(OH)2, were probably not involved in this reaction.
Zero-valent iron (Fe0), metallic iron, is being evaluated as a permeable reactive barrier material to mitigate the transport of a wide array of highly mobile contaminants in groundwater. Zero-valent iron has previously been shown to destroy effectively numerous chlorinated hydrocarbon compounds via reductive dehalogenation. No references could be found regarding the ability of zero-valent iron to reduce UO2+2, MoO2−4, or TcO−4.A series of kinetic-batch studies was conducted to determine the capability of particulate Fe0 to remove UO2+2, MoO2−4, TcO−4, and CrO2−4 from groundwater. Particulate Fe0 effectively removed each of these contaminants from solution; removal rates decreased as follows: CrO2−4 > TcO−4 > UO2+2 ⪢ MoO2−2. The removal mechanism appears to be reductive precipitation. Thermodynamic equilibrium calculations indicated that the rate of removal of the metals from solution increased as the difference in pe (Δpe) increased between the redox half reaction for the redox couple of interest and the couple. Furthermore, the pe value for a redox couple provided a qualitative indication of the reduction rate by Fe0. These results indicate that the rate of removal of CrO2−4, TcO−4, and UO2+2 from groundwater is rapid, permitting an inexpensive barrier of practical dimensions to be used for in situ remediation purposes.
Reduction of chlorinated solvents by fine-grained iron metal was studied in well-mixed anaerobic batch systems in order to help assess the utility of this reaction in remediation of contaminated groundwater. Iron sequentially dehalogenates carbon tetrachloride via chloroform to methylene chloride. The initial rate of each reaction step was pseudo-first-order in substrate and became substantially slower with each dehalogenation step. Thus, carbon tetrachloride degradation typically occurred in several hours, but no significant reduction of methylene chloride was observed over 1 month. Trichloroethene (TCE) was also dechlorinated by iron, although more slowly than carbon tetrachloride. Increasing the clean surface area of iron greatly increased the rate of carbon tetrachloride dehalogenation, whereas increasing pH decreased the reduction rate slightly. The reduction of chlorinated methanes in batch model systems appears to be coupled with oxidative dissolution (corrosion) of the iron through a largely diffusion-limited surface reaction.