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Kinetics and mechanism of reductive dehalogenation of carbon tetrachloride using zero-valence metals
Abstract
Elemental iron and zinc reduced part-per-thousand levels of aqueous-phase carbon tetrachloride to chloroform in a few hours. Free metal ions, chloride ion and hydrogen gas were produced in the reaction; protons were consumed. Process kinetics were dependent on solution pH, surface area of the elemental metal, carbon tetrachloride concentration, buffer selection and solvent composition (volume fraction 2-propanol). Reaction rate was first-order with respect to carbon tetrachloride at concentrations less than 7.5 mM. This class of reactions offers promise as a means for initiating the destruction of heavily halogenated organic compounds.
... In the early 1990s, metallic iron or elemental iron (Fe 0 ), then termed as zero-valent iron (ZVI), was presented as a new material for water treatment including environmental remediation (Reynolds et al. 1990, O'Hannesin 1993, Lipczynska-Kochany et al. 1994, Gillham and O'Hannesin 1994, Matheson and Tratnyek 1994, Schreier and Reinhard 1994, Cantrell et al. 1995, Powells et al. 1995, Warren et al. 1995. Moreover, Fe 0 was presented as an environmental reducing agent (Matheson and Tratnyek 1994, Roberts et al. 1996, Weber 1996. ...
... However, the key issue of the operating mode of Fe 0 -based systems was not (and is still not) resolved (Lipczynska-Kochany et al. 1994, Warren et al. 1995, Qiu et al. 2000, Lavine et al. 2001, Furukawa et al, 2002, Ghauch et al. 2010, Ghauch et al. 2011, Gheju and Balcu 2011, Noubactep 2013, Ghauch 2015, Ebelle et al. 2018). ...
... The discussion on the mechanism of Fe 0 in removing contaminants was initiated by Matheson and Tratnyek (1994) and was actively conducted for some four years (Lipczynska-Kochany et al. 1994, Cantrell et al. 1995, Warren et al. 1995, Eitzer 1996, Roberts et al. 1996, Weber 1996, Gu et al. 1998, O'Hannesin and Gillham 1998. Although a 'broad concensus' was made on 'reducing Fe 0 ' around 1998 (O'Hannesin and Gillham 1998) the discussion has continued until the present day (Noubactep 2007, Noubactep 2008, Gheju and Balcu 2001, Giles et al. 2011, Ghauch 2015, Noubactep 2015. ...
Metallic iron (Fe 0) has been suggested as an affordable, applicable and efficient material for environmental remediation. Mixed to soil or filled in reactive walls, Fe 0 is a feasible pathway to control contamination in seepage waters. Available information in the literature however presents discrepant evidence on the efficiency of this (still innovative) technology. On basis of a profound literarture study over the past 160 years, it is outlined that these discrepancies are explained by the aqueous chemistry of iron (corrosion). Neglected aspects contributing to the apparent complexity of the Fe 0 /H2O system are outlined. It appears that designing an efficient and sustainable Fe 0 remediation system is a pure site-specific issue and that available data are not (really) comparable. In particular, it was made clear that Fe 0 barriers that have been successfully working for more than one decade were not designed according to any scientific basis. While the success or failure of implemented reactive walls can be rationalized, more systematic research is needed for a science-based design of efficient and sustainable Fe 0-based systems for environmental remediation.
Cited as:
Noubactep C. (2019): Metallic Iron for Environmental Remediation: Prospects and Limitations. Chap. 36, A Handbook of Environmental Toxicology: Human Disorders and Ecotoxicology. J.P.F. D’Mello (ed), CAB International, 531–544.
... Further results questioning the validity of the reductive transformation concept were presented by several authors including Cantrell et al. (1995), Warren et al. (1995), Fiedor et al. (1998), Qiu et al. (2000), Farrell et al. (2001), Lavine et al (2001), and Jiao et al. (2009). ...
... V), and Zn 0 (E 0 = -0.76 V) are more powerful reducing agents than Fe 0 (Cutler 1987, Shreier and Reinhardt 1994, Warren et al. 1995. ...
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).
... The question then arises, whether progress in knowledge can be achieved without a common starting point? Unfortunately, all warnings from investigators suggesting that Fe 0 might not be an own reducing under environmental conditions [28][29][30][31][32][33][34] were overlooked by the majority of active researchers. In particular, Lavine et al. [31] used differential pulse polarography to simultaneously monitor the disappearance of nitrobenzene and the appearance of Fe 2+ in the Fe 0 /H 2 O system. ...
... V) are by far stronger reducing agents than Fe 0 (E 0 = -0.440 V) and have been reported to be initially more reactive for contaminant reductive transformations [8,28]. However, as corrosion proceeds, Al 0 and Zn 0 are progressively covered by impervious layers of hydroxide (Al(OH) 3 ) or oxide (ZnO), respectively. ...
Scientific collaboration among various geographically scattered research groups on the broad topic of "metallic iron (Fe0) for water remediation" has evolved greatly over the past three decades. This collaboration has involved different kinds of research partners including researchers from the same organization, domestic researchers also from non-academic organizations as well as international partners. The present analysis of recent publications of some few leading scientists shows that after a decade of frank collaboration in search of ways to improve the efficiency of Fe0/H2O systems, the research community has divided itself into two schools of thought since about 2007. Since then, progress in knowledge has stagnated. The first school maintains that Fe0 is a reducing agent for some relevant contaminants. The second school argues that Fe0 in-situ generates flocculants (iron hydroxides) for contaminant scavenging, and reducing species (e.g. FeII , H2 , Fe3O4) but reductive transformation is not a relevant contaminant removal mechanism. The problem encountered in assessing the validity of the views of both schools arises from the quantitative dominance of the supporters of the first school who mostly ignore the second school in their presentations. The net result is that the various derivations of the original Fe0 remediation technology may be collectively flawed by the same thinking mistake. While recognizing that the whole research community strives for the success of a very promising, but yet to established technology, annual review articles are suggested as an ingredient for successful collaboration.
... The reductive transformation concept has never been univocally accepted (Warren et al. 1995, Lavine et al. 2001. For example, Warren et al. (1995) wrote, "a convincing mechanism for the reductive dehalogenation of haloorganics by zero-valence metals has not yet been proposed. ...
... The reductive transformation concept has never been univocally accepted (Warren et al. 1995, Lavine et al. 2001. For example, Warren et al. (1995) wrote, "a convincing mechanism for the reductive dehalogenation of haloorganics by zero-valence metals has not yet been proposed. Matheson & Tratnyek (1994) maintained that dehalogenation was not mediated by H 2 (g) or Fe(II) in the bulk aqueous-phase solution, suggesting that observed reactions take place at the metal surface." ...
This thesis deals with the use of metallic iron (Fe0) for water treatment in general and the use of Fe0 for safe drinking water production in particular. The provision with safe drinking water is a real problem for 800 millions of people all over the world.Chapter 1 presents the concept of water treatment with Fe0 in a broader scientific context and
reveals research needs. Chapter 2 presents the 21 peer-reviewed journal articles on which the thesis is based in relation to their contribution to solve the problems from Chapter 1. Chapter 3 presents the same articles in the perspective of using Fe0 for safe drinking water production.Chapter 4 summarizes the major findings or the present work. An outlook is given in form of specific recommendations for future works. Chapter 5 gives an epilogue which is a sort of responses to the comments made by the referees on the submitted thesis. Chapter 6 lists cited references. The 21 papers on which this thesis is formulated are not appended to this version.
The experimental research was carried out at the Department of Applied Geology of the
University of Göttingen (Prof. Martin Sauter) between July 2005 and March 2009 and partly was financed by the German Research Foundation (DFG) under the Grant number DFG NO 626/2-1 and DFG NO 626/2-2.
I would like to thank Angelika Schöner, Paul Waofo and Sabine Caré for the scientific
collaboration during the study.
My acknowledgements also go to my colleagues of the Department of Applied Geology at the University of Göttingen, to my friends and collaborators for religious, cultural and sportive issues in Göttingen (and Krebeck), in Freiberg (Sachsen) and elsewhere. They provided the excellent atmosphere for this work.
Special thanks to: (i) my family for his endless support and (ii) Léonard Kwuida, Sabine Caré, and Ewa Lipczynska-Kochany for reading and re-reading the draft of this thesis.
... Since 1994 when the first peer-reviewed mechanistic discussion by Matheson and Tratnyek (1994) was published, there has never been any convincing report on the contaminant reducing nature of Fe 0 (Lipczynska-Kochany et al., 1994;Gillham and O'Hannesin, 1994;Warren et al., ...
... However, since the 1990s, Fe 0 has been widely considered as a reducing agent for the treatment of groundwaters polluted with chlorinated organic compounds (RCl) (Guan et al., 2015;Cao et al., 2020;Cao et al., 2021b;Hu et al., 2021a;Noubactep, 2022). Other metals, such as Al 0 , Mg 0 and Zn 0 have also been reported to mediate the reductive dehalogenation of RCl (Schreier and 1994; Warren et al., 1995;Sarathy et al., 2012;Mogharbel and Yestrebsky, 2019;Aghaei et al., 2021). ...
An innovative approach to characterize the reactivity of metallic iron (Fe0) for aqueous contaminant removal has been in use for a decade: The methylene blue method (MB method). The approach considers the differential adsorptive affinity of methylene blue (MB) for sand and iron oxides. The MB method characterizes MB discoloration by sand as it is progressively coated by in-situ generated iron corrosion products (FeCPs) to deduce the extent of iron corrosion. The MB method is a semi-quantitative tool that has successfully clarified some contradicting reports on the Fe 0 /H2O system. Moreover, it has the potential to serve as a powerful tool for routine tests in the Fe 0 remediation industry, including quality assurance and quality control (QA/QC). However, MB is widely used as a 'molecular probe' to characterize the Fe 0 /H2O system, for instance for wastewater treatment. Thus, there is scope to avoid confusion created by the multiple uses of MB in Fe 0 /H2O systems. The present communication aims at filling this gap by presenting the science of the MB method, and its application and limitations. It is concluded that the MB method is very suitable for Fe 0 material screening and optimization of operational designs. However, the MB method only provides semi-quantitative information, but gives no data on the solid-phase characterization of solid Fe 0 and its reaction products. In other words, further comprehensive investigations with microscopic and spectroscopic surface and solid-state analyses are needed to complement results from the MB method.
... Khudenko demonstrated the feasibility of using Cu 2+ cementation as a novel tool to generate Fe 2+ and H + for the reductive transformation of organics (Khudenko 1991). Khudenko (1991) was not considered while discussing the mechanism of RCl reduction from the middle of the 1990s onwards (Matheson and Tratnyek 1994, Warren et al. 1995, Weber 1996, O'Hannesin and Gillham 1998). ...
... In these efforts the intrinsic reactivity of both materials are not considered. Earlier warnings by experienced chemists have been ignored (Lipczynska-Kochany et al., 1994;Warren et al., 1995;Farrell et al., 2001;Lavine et al., 2001). In particular, the excellent studies by Lavine et al. (2001) using the polarographic method to investigate the remediation of nitroaromatics has not been properly considered. ...
The global effort to mitigate the impact of environmental pollution has led to the use of various types of metallic iron (Fe(0)) in the remediation of soil and groundwater as well as in the treatment of industrial and municipal effluents. During the past three decades, hundreds of scientific publications have controversially discussed the mechanism of contaminant removal in Fe(0)/H2O systems, with the large majority considering Fe(0) to be oxidized by contaminants of concern. This view assumes that contaminant reduction is the cathodic reaction occurring simultaneously with Fe 0 oxidative dissolution (anodic reaction). This view contradicts the century old theory of the electrochemical nature of aqueous iron corrosion and hinders progress in designing efficient and sustainable remediation Fe(0)/H2O systems. The aim of the present communication is to demonstrate the fallacy of the current prevailing view based on articles published before 1910. It is shown that properly reviewing the literature would have avoided the mistake. Going back to the roots is recommended as the way forward and should be considered first while designing laboratory experiments.
... The idea that Fe 0 reduces selected contaminants under operational conditions was introduced in the peer-reviewed literature in 1994 [5] and was controversially discussed for the following five (5) years [6][7][8][9][10][11]. For example, Warren et al. [7] used several elemental metals (Al 0 , Fe 0 , Sn 0 and Zn 0 ) to treat carbon tetrachloride (CT) and reported on "unresolved mechanistic complexities". ...
... The idea that Fe 0 reduces selected contaminants under operational conditions was introduced in the peer-reviewed literature in 1994 [5] and was controversially discussed for the following five (5) years [6][7][8][9][10][11]. For example, Warren et al. [7] used several elemental metals (Al 0 , Fe 0 , Sn 0 and Zn 0 ) to treat carbon tetrachloride (CT) and reported on "unresolved mechanistic complexities". CT was one of the organic 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 compounds used in the 1994 seminal work [5]. ...
A survey of the literature on using metallic iron (Fe 0) for environmental remediation suggests that the time is ripe to center research on the basic relationship between iron corrosion and contaminant removal. This communication points the main problem which is based on the consideration that contaminant reductive transformation is the cathodic reaction of iron oxidative dissolution. It is recalled that properly considering the inherent complexities of the Fe 0 /H 2 O system will favor an appropriate research design that will enable more efficient and sustainable remediation systems. Successful applications of Fe 0 /H 2 O systems require the collective consideration of progress achieved in system' understanding. More efforts should be made to decipher the long-term kinetics of iron corrosion, so as to provide better approaches to accurately predict the performance of the next generation Fe 0-based water treatment systems.
... In retrospect, Fe 0 remediation research was born with a mistake as the reductive transformation paradigm has never been convincingly established [13][14][15][16], but the flaw has been 'difficult' to recognize in the midst of success stories [17][18][19][20][21][22][23][24]. During the past two decades, Fe 0 remediation researchers have (re) demonstrated the potential of Fe 0 and Fe 0 -based filtration to remediate many cases of pollution including wastewater [10][11][12] and safe drinking water [25][26][27][28][29][30]. ...
... The idea that electrons from Fe 0 mediate contaminant reductive transformations was adopted from a seminal work by Matheson and Tratnyek [4]. Although controversial views were published during 1994 and 1995 [13,71,72] Weber [6] validated and generalized the results of Matheson and Tratnyek [4]. Despite this pseudo-scientific demonstration supported by further reports [5,73,74], O'Hannessin and Gillham [75] acknowledged that the reductive transformation concept was a 'broad consensus'. ...
The well-accepted assumption of metallic iron (Fe0) acting as electron donor for environmental remediation has created an unstable domain of knowledge for the past 23 years. This assumption is discouraging some outstanding and prospective scientists from correctly interpreting their experimental results. Such a situation is a recipe for long-term decline. The critical situation cannot be solved with simplistic approaches. It is now imperative to develop an understanding to defend the difficulties of this assumption and re-orient Fe0 mediated remediation research as a whole.
... Two other daughter products (CH 3 Cl, CH 4 ) were also detected; however, their relative moles (C/C 0 ) are not shown in Fig. 5 because their contribution is negligible compared to CT, CF, and DCM. The dechlorination (reduction) process consists of at least three elements: electron donor (NZVI), electron carriers, and electron acceptor (CT) [55,56] can be an electron carrier [55]. However, it should be noted that the adsorption of CT on NZVI could also facilitate electron transfer [57]. ...
... Two other daughter products (CH 3 Cl, CH 4 ) were also detected; however, their relative moles (C/C 0 ) are not shown in Fig. 5 because their contribution is negligible compared to CT, CF, and DCM. The dechlorination (reduction) process consists of at least three elements: electron donor (NZVI), electron carriers, and electron acceptor (CT) [55,56] can be an electron carrier [55]. However, it should be noted that the adsorption of CT on NZVI could also facilitate electron transfer [57]. ...
Zero-valent iron nanoparticles (NZVI) were synthesized and dispersed in solutions of sodium oleate (SO), sodium laurate (SL), sodium dodecyl phosphonate (SDP), and sodium dodecyl sulfate (SDS). The reactivity of these dispersions was evaluated to assess the impact of surfactants on the reduction rate of hydrophilic reactive black 5 (RB5) and hydrophobic carbon tetrachloride (CT) model contaminants. SO and SL, used at their critical micelle concentration (CMC), lowered the reduction rate of RB5 by two and three orders of magnitude, respectively. SO and SL also decreased the reduction rate of CT by up to one order of magnitude. SDS and SDP, at their CMC, decreased the reduction rate of RB5 by approximately 50-fold, but increased the reduction rate of CT. The decrease in RB5 reduction rate might be explained by the formation of adsorbed surfactant species on the surface of NZVI that could hinder the transport of RB5 and other hydrophilic species. For SO and SL, the inhibition of RB5 and CT reduction might also be explained by the binding of carboxylates to NZVI. The increase in CT reduction rate with SDS and SDP suggests that providing a non-binding lipophilic environment on the surface of NZVI would improve the reduction rate and selectivity towards the reduction of hydrophobic contaminants.
... Matheson and Tratnyek (1994) presented the first mechanistic investigations. Despite some parallel sceptical views (Lipczynska-Kochany et al. 1994, Warren et al. 1995, Lavine et al. 2001) reductive transformation was consensually adopted (O'Hannesin and Gillham 1998, Kang and Choi 2009, Chen et al. 2013, since then, the suitability of Fe 0 for water treatment has been tested on a case-by-case basis and results are tabulated in numerous overview and review articles (Scherer et al. 2000, Richardson and Nicklow 2002, Henderson and Demond 2007, Chen et al. 2013). However, this pragmatic research approach was performed to understand why contaminant reductive transformation occurs despite the presence of the oxide scale (Scherer et al. 2000 and ref. cited therein). ...
... Colombo et al. (2015) stated explicitly that the review covered the literature from "from 1994 up to now, is not exhaustive, but allows the non-specialist to get into this specific field and to understand its great potentiality". Any critical evaluation is missing as the review started by accepting the reductive transformation concept introduced in 1994 without any allusion on the concerns which have been raised over the years (Warren et al. 1995, Lavine et al. 2001, Jiao et al. 2009). In particular, the efficiency of multi-metallic particles for decontamination has been presented as a proof, that the reductive transformation does not result from electrons from Fe 0 (Noubactep 2009h). ...
This article critically evaluates recent review articles on using metallic iron (Fe(0)) for environmental remediation in order to provide insight for more efficient Fe(0)-based systems. The presentation is limited to peer-reviewed articles published during 2014 and 2015, excluding own contributions, dealing mostly with granular Fe(0). A literature search was conducted up to June 15th 2015 using Science Direct, SCOPUS, Springer and Web of Science databases. The search yielded eight articles that met the final inclusion criteria. The evaluation clearly shows that seven articles provide a narrative description of processes occurring in the Fe(0)/H20 system according to the concept that Fe(0) is a reducing agent. Only one article clearly follows a different path, presenting Fe(0) as a generator of adsorbing (hydroxides, oxides) and reducing (Fe(II), H/H2) agents. The apparent discrepancies between the two schools are identified and extensively discussed based on the chemistry of the Fe(0)/H20 system. The results of this evaluation indicate clearly that research on 'Fe(0) for environmental remediation' is in its infancy. Despite the current paucity of reliable data for the design of efficient Fe(0)-based systems, this review demonstrates that sensible progress could be achieved within a short period of time, specific recommendations to help guide future research are suggested.
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... This generally assumed trend is not univocally accepted. As an example, Warren et al.[62] worked with Fe 0 and Zn 0 and came to the conclusion that the overall rate of reaction may have been mass-transfer limited in the experiments involving Fe 0 , and reaction-limited in the Zn 0 experiments. Concordantly to the results of Warren et al.[62] (34) and evidences from the open corrosion literature ...
... As an example, Warren et al.[62] worked with Fe 0 and Zn 0 and came to the conclusion that the overall rate of reaction may have been mass-transfer limited in the experiments involving Fe 0 , and reaction-limited in the Zn 0 experiments. Concordantly to the results of Warren et al.[62] (34) and evidences from the open corrosion literature ...
Despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe0/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe0-materials (material A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe0/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min-1 and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (kEDTA). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on kEDTA from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe0 surface.
... It is important to notice that the reductive transformation concept has never been univocally accepted [17,18]. For example, Warren et al. [18] wrote that "a convincing mechanism for the reductive dehalogenation of haloorganics by zero-valence metals has not yet been proposed. ...
... Accordingly, there should have been no need to discuss the active form of rate control in the process of contaminant removal in the presence of metallic iron under subsurface conditions. Clearly, attempts to determine whether the process of contaminant removal in the presence of Fe 0 in a field reactive wall is mass transfer or reaction-limited [9,17,35] was not necessary as this was well-documented before the event of the iron remediation technology [25]. ...
The further development of Fe0-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe0/H2O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both process is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe0 as remediation agent compared to thermodynamically more favourable Al0 and Zn0. The validity of the adsorption/co-precipitation concept is corroborated.
... In both cases, the efficiency of Fe 0 /H2O systems relies on the electrochemical oxidative dissolution of Fe 0 in aqueous solutions which is necessarily coupled with precipitation of iron hydroxides/oxides (at pH > 4.5). Noubactep (2015) clearly showed that the reductive transformation concept has never been universally accepted (Lipczynska-Kochany et al. 1994, Warren et al. 1995, Farrell et al. 2001, Lavine et al. 2001, Mantha et al. 2001, Furukawa et al. 2002, and that its general acceptance was based on a 'broad consensus' (O'Hannessin and Gillham 1998). ...
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.
... However, these applications are mainly perceived to be derived from the Fe 0 PRB technology for organic pollutants (Obiri-Nyarko et al. 2014, Naseri et al. 2017. The concept that Fe 0 is an electron donor under environmental conditions has never been experimentally established (Warren et al. 1995, Farrell et al. 2001, Lavine et al. 2001, Jiao et al. 2009, Naseri et al. 2017). For example, while investigating the reductive dechlorination of carbon tetrachloride (CT) in inherent relationship between the dechlorination of CT and the corrosion of iron is attributed to the fact that the adsorbed hydrogen atoms produced during the iron corrosion process are necessary for the dechlorination process of CT". ...
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 addition to ZVI, zero-valent zinc (ZVZ) has also been studied for the degradation of COCs in the 1990s. Reduction of chlorinated methanes has been observed by Warren et al. (1995), as well as reduction of chlorinated ethanes (Fennelly and Roberts 1998;Arnold et al. 1999), ethylenes (Arnold and Roberts 1998) and propanes Cushman 2014). Zinc is a good alternative to iron for in situ application (Cheng and Wu 2000) as its storage is easier and its tolerable concentration in drinking water is high (for example, the maximum admissible value in drinking water for iron is 200 µg L -1 according to the Council Directive 98/83/EC of 3 November 1998 and 3 mg L -1 for zinc according to the Guidelines for Drinking-Water Quality). ...
... In addition to ZVI, zero-valent zinc (ZVZ) has also been studied for the degradation of COCs in the 1990s. Reduction of chlorinated methanes has been observed by Warren et al. (1995), as well as reduction of chlorinated ethanes (Fennelly and Roberts 1998;Arnold et al. 1999), ethylenes (Arnold and Roberts 1998), and propanes Cushman 2014). Zinc is a good alternative to iron for in situ application (Cheng and Wu 2000) as its storage is easier and its tolerable concentration in drinking water is high (for example, the maximum admissible value in drinking water for iron is 200 μg L À1 according to the Council Directive 98/83/EC of 3 November 1998 and 3 mg L À1 for zinc according to the Guidelines for Drinking-Water Quality). ...
Chlorinated organic compounds (COCs) are common anthropogenic
contaminants encountered in soil and groundwater. COCs were
industrially produced for different applications, such as dry cleaning,
degreasing, or as pesticides. The presence of COCs in the environment is a major concern because of their toxicity and persistence. The most widely used method for their removal is the conventional pump-and-treat approach. However, this technology can hardly achieve a complete remediation because of geological characteristics and the presence of pore space pollution/adsorbed pollution, leading to a residual saturation.
Hence, in addition to the improvement of pump-and-treat systems, In situ chemical processes have been largely developed. These chemical processes involve the injection of chemical reagents for the removal of residual pollution source and/or the treatment of contamination plume. Chemical degradation of COCs can be achieved by oxidative or reductive processes. If chemical oxidation has been first developed for in situ application, chemical reduction is one of the most important emerging remediation techniques for COCs treatment. Due to the electronegative character of chlorine substituents, COCs can effectively be transformed via reductive pathways. Moreover, reductive dechlorination has shown higher efficiency on highly chlorinated compounds.
This chapter focuses on the presentation of the chemical reduction of the most common COCs pollutants, followed by kinetic and mechanistic approaches related to the use of iron-based particles. Developments of in situ chemical reduction technologies aiming to enhance remediation rates are also exposed. Influence of environmental conditions for in situ applications is then developed. Finally, a case study is presented.
... Furthermore, the experimental conditions used in the pioneering studies were not appropriate for traceable conclusions [39][40][41]66]. Despite some serious warnings [28,[67][68][69][70][71][72][73], the inherent error of the pioneers has been perpetuated and propagated through the scientific literature [1][2][3][4][5][6][7][8][9][10]14,29,23,26,27,74,75]. The aim of the present contribution is to demonstrate from a historical perspective that indirect reduction should have merited more attention in the Fe 0 remediation research than other mechanisms. ...
Elemental iron (Fe0) has been widely used in groundwater/soil remediation, safe drinking water provision and wastewater treatment. It is still mostly reported that a surface-mediated reductive transformation (direct reduction) is a relevant decontamination mechanism. Thus, the expressions "contaminant removal" and "contaminant reduction" are interchangeably used in the literature for reducible species (contaminants). This contribution reviews the scientific literature leading to the advent of the Fe0 technology and shows clearly that reductive transformations in Fe0/H2O systems are mostly driven by secondary (FeII, H/H2) and tertiary/quaternary (e.g. Fe3O4, green rust) reducing agents. The incidence of this original mistake on the Fe0 technology and some consequences for its further development are discussed. It is shown in particular that characterizing the intrinsic reactivity of Fe0 materials should be the main focus of future research.
... CHCl3, CH2Cl2 or CH3Cl are observed [20]. On the 115 other hand, it was reported that the reduction of CHCl3 on an iron surface electrode occurs at a 116 slower rate compared to CCl4 [73,74]. The latter observation is in accord with the report that the 117 de-chlorination of CCl4 with iron powder does not produce CH4 [67]. ...
The de-halogenation processes of CH2BrCHBrCOO− and of CH2ClCH(OH)COO− on Zero Valent Iron (ZVI) powders and porous iron electrodes were studied. The results suggest that by applying a negative voltage bias on the electrode, the composition of the products obtained is dramatically changed. Furthermore, the applied potential inhibits the passivation of the ZVI. Thus, it is recommended that the application of a negative potential to porous ZVI is desirable in batch treatment of halo-organic pollutants.
... Research on water treatment and environmental remediation has been wrongly directed from the beginning on. Warnings from chemists [116][117][118][119] were simply ignored. Whenever a mistake is identified, starting on a new basis is always a good decision. ...
Since its introduction about 25 years ago, metallic iron (Fe0) has shown its potential as the key component of reactive filtration systems for contaminant removal in polluted waters. Technical applications of such systems can be enhanced by numerical simulation of a filter design to improve, e.g. the service time or the minimum permeability of a prospected system to warrant the required output water quality. This communication discusses the relevant input quantities into such a simulation model, illustrates the possible simplifications and identifies lack of relevant thermodynamic and kinetic data. As a result, necessary steps are outlined that may improve the numerical simulation and, consequently, the technical design of Fe0 filters. Following a general overview on the key reactions in a Fe0 system, the importance of iron corrosion kinetics is illustrated. Iron corrosion kinetics, expressed as a rate constant kiron, determines both the removal rate of contaminants and the average permeability loss of the filter system. While the relevance of a reasonable estimate of kiron is thus obvious, information is scarce. As a conclusion, systematic experiments for the determination of kiron values are suggested to improve the data base of this key input parameter to Fe0 filters.
... The barrier filling material must have specific physical and chemical properties. In particular, the design of PRBs relies on three main technical aspects: (a) the reactive media has to be matched to the target pollutant, (b) the PRB must be wide enough to ensure a sufficient residence time of the pollutant plume [14,15], and (c) the barrier must be large and deep enough and the reactive material must have a sufficient hydraulic conductivity so as to allow for the interception of the whole plume [6,16]. Moreover, clogging phenomena due to mineral precipitates in the barrier, if zero-valent iron is used as reactive media, have to be avoided [17]. ...
This work presents an innovative configuration of a permeable adsorptive barrier (PAB) for the in situ remediation of benzene-contaminated groundwater in the area of Naples (Italy). A PAB is a type of permeable reactive barrier (PRB) made with adsorbing materials (e.g. activated carbon). This particular PAB is a discontinuous permeable adsorptive barrier (PAB-D), consisting in an array of deep passive adsorptive wells whose hydraulic conductivity is higher than the surrounding soil. The design was based on COMSOL Multi-physics® simulations, which allow for the description of pollutant transport in groundwater and adsorption onto the barrier by means of a 2D model solved using a finite element approach. Based on a hydrological and geotechnical characterization of the entire polluted aquifer, the design and optimization of PAB-D parameters (location, orientation, number of wells and dimensions) were defined. The influence of hydraulic conductivity and dispersivity on the total number of wells for a complete aquifer remediation was investigated. Finally, a comparison with a continuous barrier (PAB-C), i.e. a wall of adsorptive material, in terms of total adsorbing material needed, is presented.
... Zero-valence state metals, such as Fe 0 , Zn 0 , Sn 0 and Al 0 , are effective for the remediation of contaminants in contaminated groundwater. 59,71 Bimetallic combination (Fe 0 /Ni 0 : the ratio of Fe to Ni being 3 : 1) has provided better degradation rates. 72 Bimetallic particles are composed of two types of zero-valent iron (ZVI). ...
... The reductive transformation model for remediation Fe 0 has never been univocally established [51][52][53][54] and was severely questioned/refuted in the peer-reviewed literature in 2007 [29,30]. [49,58,59] in the remediation Fe 0 community to help resolving these questions and contribute to progress in knowledge. ...
The premise of this research note is that current research on metallic iron (Fe0) for environmental remediation and water treatment has started on a biased basis. Before expecting experienced researchers to correct flawed approaches compromising the future of the technology, the attention of new researchers should be drawn on the prevailing flawed conceptual models. There are guides on how to select good research topics, to perform good literature review, to select good mentors, and to write good scientific papers. But critically reviewing the published material is part of the competence of any new researcher in a given field of research. This research note summarizes the most critical issues of research on Fe0 for water treatment as asks some key questions which would help research beginners to find their way.
... As mentioned in chapter 1, contamination of groundwater with toxic chlorinated compounds may be eliminated via reductive dechlorination reactions that transform chlorinated solvents into their nonchlorinated analogs and chloride ions ( Cl − ) (Vogel et al., 1987). Reductive dechlorination may occur via solvent reaction with Fe(II)-bearing minerals in aquifer sediments (Butler and Hayes, 1998;Sivavec and Horney, 1997;Sivavec, 1995), or it may be promoted in engineered remedial systems using corroding iron Gillham and Ohannesin, 1994;Matheson and Tratnyek, 1994;Warren et al., 1995) or metal cathodes Li and Farrell, 2001;Liu et al., 2000) as electron donors. Due to its ubiquity in contaminated groundwater, much research has been done to investigate dechlorination reactions of carbon tetrachloride ( CCl 4 ). ...
... On the contrary, only few practical tools are available in the literature for the design of funnel-and-gate PRBs and they are particularly focused on the hydraulic behaviour of PRBs [6][7][8]. However, the design of such PRBs relies on three technical aspects: (a) the reactive media must be appropriate to the pollutants, (b) the filters' size must be large enough to ensure a sufficient residence time [9][10][11], and (c) the reactive material must have a sufficient hydraulic conductivity to prevent any bypass of the system. The first aspect is a key issue in the design of PRBs and a particular attention has to be dedicated to the selection of the reactive or sorbent material when installing any PRB system [12]. ...
Permeable Reactive Barriers represent an innovative remediation technique of contaminated aquifers. Three geometric configurations are encountered in the literature: a continuous wall, a funnel-and-gate system, and a caisson configuration. The present paper is focused on the design of the second and third geometric configurations and presents an analytical solution of the flow in a Permeable Reactive Barrier based on the Schwarz-Christoffel transformation. This analytical solution is coupled to residence time calculations to define a methodology of design taking into account the most important parameters on the design of a PRB: cutoff width, slenderness of the reactive cell, and hydraulic conductivity. Finally, the study provides a guidance diagram for the design of funnel-and-gate or caisson configurations, as well as a case study.
... and less than 1% of secondary metal (catalyst) plated on the surface Zero valent zinc may be more effective for PCP dechlorination than iron because of its higher reduction potential. In comparing with iron higher reactivities of zinc towards highly chlorinated compounds such as tetrachloroethene (PCE) and carbon tetrachloride were reported [8][9][10][11][12][13]. Zinc also dechlorinated octachlorodibenzo-p-dioxin (OCDD) to hexa-and penta-CDD under basic and neutral environments [14]. ...
Reductive dechlorination of chlorinated organic compounds, trichloroethene (TCE), tetrachloroethene (PCE), Pentachlorophenol (PCP), three tetrachlorophenols (TeCPs), six trichlorophenols (TCPs)) with zero valent metals (ZVMs) was examined through batch experiments. Bimetals showed enhanced reactivates for TCE and non-toxic compounds were detected as products. Zinc showed much higher reactivity towards PCP than iron and amended iron indicating that zero valent zinc can be a good candidate for reductive dechlorination of chlorinated phenols. Chlorophenols were sequentially dechlorinated and less chlorinated phenols were identified as reduction products. ZVMs could be employed as permeable reactive barrier materials.
... Zero valent state metals (such as Fe 0 , Zn 0 , Sn 0 and Al 0 ) are surprisingly effective agents for the remediation of contaminated ground water [124,125]. In particular, it has been the subject of numerous studies over the last 10 years and this is an interestingly popular choice for treatment of hazardous and toxic wastes, and for remediation of contaminated sites. ...
... Except economic considerations, the design of such PRBs relies on three technical aspects: (1) the reactive media must be appropriate to the pollutants and its selection must essentially be based on factors such as site-specific geochemical, biological, and hydrogeological conditions, (2) the filters' size must be large enough to ensure a residence sufficient time for rate-limited reactions to occur (Shoemaker et al., 1995, O'Hannesin and Gillham, 1998, Warren et al., 1995, (3) the reactive material must have a hydraulic conductivity greater than the surrounding aquifer to prevent any bypass of the system (Starr and Cherry, 1994). ...
This last equation is similar to the Darcy's law expressed in a filter with an axial flow and an apparent hydraulic conductivity k' for the reactive media. The flow rate Q is thus expressed as is equal to the multiplication of the apparent hydraulic conductivity k', the cross section p(Rext²-Rint²) and the gradient (Hext-Hint)/L. In a classical filter, the expression of the flow rate would be the similar but k' would be replaced by k. As the Fig. 6 illustrating the Eq.13 shows, the apparent hydraulic conductivity k' can be several times higher than the hydraulic conductivity (up to 80 when considering a radius of 25 cm and a length of 2 m). As a consequence, the radial filter appears to be "more permeable" than a classical filter at constant volume. This last result is particularly interesting in case of clogging, as it can be induced by colloids or precipitates in the reactive media. Indeed, several simulations performed on PRBs in a funnel-and-gate configuration show that the flow rate in a filtering gate can be considered constant while the hydraulic conductivity of the filter remains above a critical value (Courcelles, 2007). As a consequence of its hydraulic performances, the lifetime of a radial filter could be longer than a classical filter's.
... The use of zero-valent metals to degrade contaminants represents an active research area (28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38). In large part, this was sparked by the suggestion of Gillham and coworkers that Fe(0) could be utilized as an immobilized reagent in a passive approach to groundwater remediation. ...
To date it does not appear to have been demonstrated in the literature that halogenated ethylenes can undergo reductive {beta}-elimination to alkynes under environmental conditions. The purpose of this paper is to provide experimental evidence that such pathways may be involved in the reaction of chloroethylenes with zero-valent metals as well as to speculate on the significance of the products that may result. Calculations indicate that reductive {beta}-elimination reactions of chloroethylenes are in fact comparable energetically to hydrogenolysis at neutral pH. Experiments were therefore initiated to assess whether {beta}-elimination reactions of chlorinated ethylenes could occur in the presence of two zero-valent metals, Fe and Zn. 76 refs., 3 figs., 1 tab.
... Presently, there is considerable interest in reductive dehalogenation reactions because of their potential applicability in the treatment of PHA wastes (11,12), as well as in remediation approaches to removing PHAs from contaminated soils and aquifers (13,14). In this context, the reductive transformation of selected PHAs has been investigated by several groups using a variety of model reductants including reduced organic and inorganic sulfur species (15)(16)(17)(18)(19), biologically relevant transition-metal complexes (predominantly iron (20)(21)(22)(23)(24) and cobalt species (25)(26)(27)(28)(29)(30)(31)), and zero-valent metals, in particular, iron metal (12,(32)(33)(34)(35). Generally, these studies focused on the evaluation of the kinetics and/or product identification of the reduction of a few compounds. ...
Methyl tert-butyl ether (MTBE) is a gasoline oxygenate additive used to enhance gasoline combustion by lowering carbon monoxide and hydrocarbon emissions, thus reducing air pollution. However, it has been identified as the second most common volatile organic contaminant of urban aquifers in the United States. Methyl tertbutyl ether has also been blended into two types of gasoline sold in Mexico by the state oil company (Petróleos Mexicanos) but is currently not monitored in groundwater. Early research on MTBE determined that it is unable to sorb to soils and sediments. The objective of this study is to determine if fine-grained materials with high organic matter (0.25-15.3%) have the potential for sorption of MTBE. The experiment consisted of sorption isotherms of loess from DeKalb, Illinois, and lacustrine sediments from Chalco, Mexico. Experiments were performed with various concentrations of MTBE and benzene (10, 50, 100, 500, and 1000 μg/L) at 25°C and 10°C. Methyl tert-butyl ether showed a retardation factor (R) as high as 1.856 ± 0.0130 for lacustrine sediments and 1.095 ± 0.0010 for loess at 25°C. Benzene showed retardation factors as high as 1.996 ± 0.0150 in lacustrine sediments and 1.775 ± 0.0050 in loess at 25°C. These results showed that sorption, and therefore, the retardation of MTBE in groundwater, is possible in fine-grained materials especially with high organic matter. This research increases the understanding of the fate and transport of MTBE and improves the knowledge to implement the optimal remediation method for sites contaminated by MTBE. Copyright ©2006. The American Association of Petroleum Geologists/Division of Environmental Geosciences. All rights reserved.
... This does not preclude the utility of the encapsulated buffer because there may be applications where changes in pH may result in less than optimum conditions and may negatively impact the overall goals of the process. For example, researchers have investigated the use of zero-valent metals to induce the in situ remediation of halogenated organic compounds ͑e.g., Matheson and Tratnyek 1994;Warren et al. 1995;Schlimm and Heitz 1996;Siantar et al. 1996͒. The corrosion of the metal releases electrons that induce the reduction of the halogenated compound, and the pH rises as the reaction proceeds ͑Matheson and Tratnyek 1994͒. ...
A one-dimensional mathematical model was developed to simulate pH control using an encapsulated phosphate buffer during denitrification in a sand column. The parameters required for the model obtained from direct physical measurement, from a tracer study to characterize the dispersion coefficient in the column, and from batch experiments designed to obtain an empirical expression describing the variation of the first-order rate constant for the encapsulated buffer core release with pH. First-order kinetic constants describing the rates of denitrification and ethanol biodegradation were obtained by fitting the model to column runs without the encapsulated buffer. With these parameters, the model was subsequently used to predict the performance of column runs containing the encapsulate buffer. Since denitrification was essentially complete in the sand columns, an increase in the effluent pH was observed. This pH increase was counteracted by the controlled release of the acidic core of the encapsulated buffers added in the columns. The model reasonably predicted the release of the encapsulated buffer core and the performance of the encapsulated buffer for controlling pH in the column.
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.
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.
Chemical oxidation and advanced oxidation technologies are important processes for the abatement of refractory and/or toxic pollutants in wastewaters. The species participating in advanced oxidation technologies are powerful radicals, such as the hydroxyl radical, HO•, which can attack virtually all organic compounds at very high reaction rates. In this chapter, oxidative technologies, such as permanganate, ozonation, Fenton and related processes, electrochemical oxidation, γ-radiolysis and electron-beam treatment, nonthermal plasma, persulfate, wet air oxidation, supercritical water oxidation, ultrasound, zero-valent iron, ferrate, water photolysis under vacuum ultraviolet, periodate, chlorine and heterogeneous photocatalysis, some of them combined with UV light, are briefly described, together with examples of their applications. The most important and recent papers are cited together with review papers. These oxidative technologies can process wastewaters resistant to conventional treatments and are complementary to them.
Precipitation and dissolution are two main chemical reactions determining the fate of chemicals or contaminants in natural waters and in many water and wastewater treatment processes. Mineral precipitation and dissolution are the prime factors altering the chemical composition of natural water (Stumm and Morgan, 1996). Treatment processes, such as lime-soda softening, iron removal, coagulation with hydrolyzed metal salts and phosphate precipitation, are all based on the precipitation (Snoeyink and Jenkins, 1980).Likewise, the behavior of compounds containing carbon,nitrogen,sulfur,iron and manganese in the treatment processes and natural water is largely govermed by redox conditions in polluted areas is often a prerequisite in predicting the fate of pollutantss and for selecting suitable treatment or mitigation approaches. For instance, the redox condition information of a contaminant plume in subsurface allows evaluation of further plume development and potential risks to downgradient groundwater resources, and assessment of the appropriateness of natural attenuation as a remediation option.
This chapter presents three different approaches to responsible nanotechnology. These approaches involve nanomaterials (NMs) in manufacturing, NMs for environmental remediation, and a sociotechnical integration of creativity and responsibility in the use of NMs. The chapter presents some examples of recent research and developments, including inorganic materials, organic materials, and composite materials. The chapter pays special attention to NMs produced by thermal plasma technology based on the author’s experience. It then describes recent information about self-replicating NMs. Some of the most common NMs used for the environmental detection of contaminants are described and classified in four categories: carbon-based materials, metallic nanoparticles (NPs), silicon-based NPs, and semiconductor NPs. Nanotechnology-based products have evolved quickly and their use in composite materials, solar cells, energy storage units, fuel cells, clothes, environmental remediation, etc., is imminent. © 2015 by the American Society of Civil Engineers. All rights reserved.
Catalysts (homogeneous or heterogeneous) can be utilized to improve the performance of conventional advanced oxidation processes (AOPs). In general, catalyst activity, selectivity, stability, simplicity of preparation, preparation time, cost, nontoxicity, availability, recycling capability, environmental suitability, etc. can be the important parameters in the catalyst selection. High costs, cumbersome preparations, and environmental unsuitability can usually hinder the industrial applicability of a catalyst. In this chapter, catalytic AOPs (Fenton-based processes, catalytic ozonation, heterogeneous photocatalysis, catalytic wet air oxidation, and catalytic supercritical water oxidation), related catalytic materials, and cost-effective catalytic materials used in these processes are discussed.
The haloacetamides (HAcAms), an emerging class of nitrogen-containing disinfection byproducts (N-DBPs), are highly cytotoxic and genotoxic, and typically occur in treated drinking waters at low μg/L concentrations. Since many drinking distribution and storage systems contain unlined cast iron and copper pipes, reactions of HAcAms with zero-valent iron (ZVI) and metallic copper (Cu) may play a role in determining their fate. Moreover, ZVI and/or Cu are potentially effective HAcAm treatment technologies in drinking water supply and storage systems. This study reports that ZVI alone reduces trichloroacetamide (TCAcAm) to sequentially form dichloroacetamide (DCAcAm) and then monochloroacetamide (MCAcAm), whereas Cu alone does not impact HAcAm concentrations. The addition of Cu to ZVI significantly improved the removal of HAcAms, relative to ZVI alone. TCAcAm and their reduction products (DCAcAm and MCAcAm) were all decreased to below detection limits at a molar ratio of ZVI/Cu of 1:1 after 24 h reaction (ZVI/TCAcAm = 0.18 M/5.30 μM). TCAcAm reduction increased with the decreasing pH from 8.0 to 5.0, but values from an integrated toxic risk assessment were minimised at pH 7.0, due to limited removal MCAcAm under weak acid conditions (pH = 5.0 and 6.0). Higher temperatures (40 °C) promoted the reductive dehalogenation of HAcAms. Bromine was preferentially removed over chlorine, thus brominated HAcAms were more easily reduced than chlorinated HAcAms by ZVI/Cu. Although tribromoacetamide was more easily reduced than TCAcAm during ZVI/Cu reduction, treatment of tribromoacetamide resulted in a higher integrated toxicity risk than TCAcAm, due to the formation of monobromoacetamide (MBAcAm).
Dithionite can be used to reduce Fe(II) and produce nanoscale zero-valent iron (nZVI) under conditions of high pH and in the absence of oxygen. The nZVI is coprecipitated with a sulfite hydrate in a thin platelet. The nanoparticles formed are not pure iron but this feature does not appear to affect their degradation performance under air or N(2) gas conditions. The efficiency of trichloroethylene (TCE) degradation, when one is employing nanoparticles manufactured using dithionite (nZVI(S2)O(4)), is similar to if not slightly better than that of the more conventional borohydride procedure (nZVI(BH4)). The other advantages of the dithionite method are that (i) it uses a less expensive and widely available reducing agent, and (ii) there is no production of potentially explosive hydrogen gas. Oxidation of benzoic acid using the nZVIS2O4 particles results in different byproducts than those produced when nZVI(BH4) particles are used. The low oxidant yield based on hydroxybenzoic acid generation is offset by the production of higher concentrations of phenol. The high concentration of phenol compared to hydroxybenzoic acids suggests that OH(center dot) addition is not the primary oxidation pathway when one is using the nZVI(S2O4) particles. It is proposed that sulfate radicals (SO(4)(center dot-)) are produced as a result of hydroxyl radical attack on the sulfate matrix surrounding the nZVI(S2O4) particles, with these radicals oxidizing benzoic acid via electron transfer reactions rather than addition reactions.
Dithionite can be used to reduce Fe(II) and produce nanoscale zero-valent iron (nZVI) under conditions of high pH and in the absence of oxygen. The nZVI is coprecipitated with a sulfite hydrate in a thin platelet. The nanoparticles formed are not pure iron but this feature does not appear to affect their degradation performance under air or N2 gas conditions. The efficiency of trichloroethylene (TCE) degradation, when one is employing nanoparticles manufactured using dithionite (nZVIS2O4), is similar to if not slightly better than that of the more conventional borohydride procedure (nZVIBH4). The other advantages of the dithionite method are that (i) it uses a less expensive and widely available reducing agent, and (ii) there is no production of potentially explosive hydrogen gas. Oxidation of benzoic acid using the nZVIS2O4 particles results in different byproducts than those produced when nZVIBH4 particles are used. The low oxidant yield based on hydroxybenzoic acid generation is offset by the production of higher concentrations of phenol. The high concentration of phenol compared to hydroxybenzoic acids suggests that OH• addition is not the primary oxidation pathway when one is using the nZVIS2O4 particles. It is proposed that sulfate radicals () are produced as a result of hydroxyl radical attack on the sulfite matrix surrounding the nZVIS2O4 particles, with these radicals oxidizing benzoic acid via electron transfer reactions rather than addition reactions.
This article gives an overview of the application of nanomaterials in environmental remediation. In the
area of environmental remediation, nanomaterials offer the potential for the efficient removal of
pollutants and biological contaminants. Nanomaterials in various shapes/morphologies, such as
nanoparticles, tubes, wires, fibres etc., function as adsorbents and catalysts and their composites with
polymers are used for the detection and removal of gases (SO2, CO, NOx, etc.), contaminated chemicals
(arsenic, iron, manganese, nitrate, heavy metals, etc.), organic pollutants (aliphatic and aromatic
hydrocarbons) and biological substances, such as viruses, bacteria, parasites and antibiotics.
Nanomaterials show a better performance in environmental remediation than other conventional
techniques because of their high surface area (surface-to-volume ratio) and their associated high
reactivity. Recent advances in the fabrication of novel nanoscale materials and processes for the
treatment of drinking water and industrial waste water contaminated by toxic metal ions,
radionuclides, organic and inorganic solutes, bacteria and viruses and the treatment of air are
highlighted. In addition, recent advances in the application of polymer nanocomposite materials for the
treatment of contaminants and the monitoring of pollutants are also discussed. Furthermore, the
research trends and future prospects are briefly discussed.
Reduction rate constants of seventeen polyhalogenated methanes and ethanes were measured in aqueous solutions containing bulk reductants and the electron-transfer mediators iron porphyrin or juglone (5-hydroxy-1,4-naphthoquinone). Rate constants varied 4 orders of magnitude in the case of reaction with iron porphyrin and 7 orders of magnitude in the reaction with mercaptojuglone, an addition product of juglone and hydrogen sulfide. For the iron porphyrin system the results support a reaction mechanism in which one electron is transferred in an outer-sphere process to the polyhalogenated alkane with bond breakage in the transition state. In the juglone system two competing reaction mechanisms are proposed: an outer-sphere electron transfer to the polyhalogenated alkane and an SN2 reaction at the halogen. The kinetic results are directly applicable to environmental systems containing common reductants such as natural organic matter and reduced iron species, providing a “fingerprint” of reactivity for examination of reactive species and rate-limiting steps in those systems.
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.
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The use of zero-valent iron in the treatment of hazardous water contaminants is reviewed. The review concentrates on experimental procedures employed in the investigation of a range of applications for zero -valent iron and the efficacy of the processes. Investigations reviewed include those performed with halogenated aliphatic compounds, halogenated aromatic compounds, nitro-aromatic organic compounds, and high-valency toxic metals. Experiments employing bimetallic and cementation reductants as well as zero-valent iron alone are covered. The need for zero-valent iron technology is discussed, as is the need for more rigorous investigation of operational parameters. Despite a paucity of information from the literature it has been shown that efficacy is greatest for well-mixed batch or continuous column systems employing neutral to acidic pH and a high specific surface area of iron. The literature also reveals a need to focus future work on enhancing the chemical reaction taking place on the iron surface in order to enhance the rate of reductive remediation.
Addition of sodium dithionite (Na2S2O4) to aqueous carbon tetrachloride solutions (similar to 650 mg/L) resulted in CCl4 concentrations lower than 2.5 mg/L after 2.5 h and in undetectable concentrations after 4-5 h of reaction. The main product of the reductive dehalogenation of CCl4 by Na2S2O4 was found to be the trichloromethanesulfinate (CCl3SO2-) anion.
This article summarizes and systematizes the current understanding of abiotic and biotic chemistry of halogenated aliphatic compounds. Knowledge of abiotic transformations can provide a conceptual framework for understanding biologically mediated transformations. Most abiotic transformations are slow, but they can still be significant within the time scales commonly associated with ground water movement. In contrast, biotic transformations typically proceed much faster, provided that there are sufficient substrate and nutrients and a microbial population that can mediate such transformation. Recent studies, which describe transformations of halogenated aliphatic compounds in microbial and mammalian systems, are also discussed. These studies reveal broad patterns of transformation in biological systems in general. 114 references, 8 figures, 12 tables.
The formation of radicals from carbon tetrachloride (CT) is often invoked to explain the product distribution resulting from its transformation. Radicals formed by reduction of CT presumably react with constituents of the surrounding milieu to give the observed product distribution. The patterns of transformation observed in this work were consistent with such a hypothesis. In cultures of Escherichia coli K-12, the pathways and rates of CT transformation were dependent on the electron acceptor condition of the media. Use of oxygen and nitrate as electron acceptors generally prevented CT metabolism. At low oxygen levels (approximately 1%), however, transformation of [14C]CT to 14CO2 and attachment to cell material did occur, in accord with reports of CT fate in mammalian cell cultures. Under fumarate-respiring conditions, [14C]CT was recovered as 14CO2, chloroform, and a nonvolatile fraction. In contrast, fermenting conditions resulted in more chloroform, more cell-bound 14C, and almost no 14CO2. Rates of transformation of CT were faster under fermenting conditions than under fumarate-respiring conditions. Transformation rates also decreased over time, suggesting the gradual exhaustion of transformation activity. This loss was modeled with a simple exponential decay term.
The purpose of this investigation was to develop a new colorimetric method for zinc and copper utilizing the compound 2-carboxy-2′-hydroxy-5′-suIfoformazylbenzene (Zincon). Both elements form a blue complex with this reagent. The zinc complex is stable over the pH range 8.5 to 9.5 while the copper complex is stable in the pH range 5.0 to 9.5. This difference in effect of pH permits the determination of zinc and copper in the presence of each other. Both complexes follow Beer's law over the concentration range 0.1 to 2.4 p.p.m. of the element. The sensitivity is 0.003 γper sq. cm. for both zinc and copper. An ion exchange procedure is described for the separation of zinc from interfering ions.
Nine bacteria were tested for the ability to dehalogenate tetrachloromethane (CT), tetrachloroethene (PCE), and 1, 1, 1-trichloroethane (TCA) under anaerobic conditions. Three bacteria were able to reductively dehalogenate CT. Dehalogenation ability was not readily linked to a common metabolism or changes in culture redox potential. None of the bacteria tested were able to dehalogenate PCE or TCA. One of the bacteria capable of dehalogenating CT, Shewanella putrefaciens, was chosen as a model organism to study mechanisms of bacterially catalyzed reductive dehalogenation. The effect of a variety of alternate electron acceptors on CT dehalogenation ability by S. putrefaciens was determined. oxygen and nitrogen oxides were inhibitory but Fe (III), trimethylamine oxide, and fumarate were not. A model of the electron transport chain of S. putrefaciens was developed to explain inhibition patterns. A period of microaerobic growth prior to CT exposure increased the ability of S. putrefaciens to dehalogenate CT. A microaerobic growth period also increased cytochrome concentrations. A relationship between cytochrome content and dehalogenation ability was developed from studies in which cytochrome concentrations in S. putrefaciens were manipulated by changing growth conditions. Stoichiometry studies using {sup 14}C-CT suggested that CT was first reduced to form a trichloromethyl radical. Reduction of the radical to produce chloroform and reaction of the radical with cellular biochemicals explained observed product distributions. Carbon dioxide or other fully dehalogenated products were not found.
The kinetics of reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols have been investigated in aqueous solution containing reduced sulfur species and small concentrations of either a naphthoquinone or an iron porphyrin. Boith the two naphthoquinones and the iron porphyrin used in this study mediated the reduction of the nitro group. In all three cases, the rate of reduction of the nitrobenzenes and of the nitrophenols was strongly pH dependent. Dissociated nitrophenols were reduced ~ 3-4 times more slowly as compared to the nondissociated species. For the substituted nitrobenzenes, the effect of substitution on the reaction rate could be described by a linear free energy relationship (LFER) of the general form log k = aE(h)(1') + b, where k is the second-order rate constant for reaction with the hydroquinone monophenolate or the iron porphyrin, respectively, and E(h)(1') is the one-electron reduction potential of the nitroaromatic compound. Competition between different nitrobenzenes was observed in the case of the iron porphyrin, while no effects were found for the reaction with the hydroquinones. The results of this study form an important base for the evaluation and interpretation of reductive transformation processes of nitroaromatic compounds in the environment.
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.
The present study was designedto further elucidate the potential of transition-metal coenzymes in biodehalogenation by systematically investigating the dechlorination of polychlorinated ethanes by vitamin B 12 and hematin
Tetrachloromethane (CT) is transformed in biological systems via two major pathways: hydrogenolysis to trichloromethane (CF) and hydrolysis to carbon dioxide. The mechanism of hydrolysis is poorly understood. One possibility is a hydrolytic reduction of CT to formate or carbon monoxide followed by oxidation to carbon dioxide. In biological systems, formate and carbon monoxide are readily oxidized to carbon dioxide, making their identification difficult, but in an electrolysis cell, reduction is physically separated from oxidation, and the production of formate and carbon monoxide at a cathode can be observed. Measurements of current, chloride, CF, dichloromethane, formate, and CO upon electrolysis of CT in aqueous solution supported the conclusion that hydrogenolysis to CF and hydrolytic reduction to CO and formate are significant and competitive reductive processes. Similar findings were obtained for the reduction of 1,1,1-trichloroethane via hydrogenolysis to 1,1-dichloroethane and to an unidentified dechlorinated product via a two-electron parallel pathway. It is suggested that electrolysis in aqueous environments may have value as a useful tool for the understanding and control of reductive dehalogenation and as a novel treatment technology.
The bacterial transition-metal coenzymes vitamin Bââ (Co), coenzyme Fâââ (ni), and hematin (Fe) catalyzed the reductive dechlorination of polychlorinated ethylenes and benzenes, whereas the electron-transfer proteins four-iron ferredoxin, two-iron ferredoxin, and azurin (Cu) did not. For vitamin Bââ and coenzyme Fâââ, reductive dechlorination rates for different classes of perchlorinated compounds had the following order: carbon tetrachloride > tetrachloroethylene > hexachlorobenzene. For hematin, the order of reductive dechlorination rates was carbon tetrachloride > hexachlorobenzene > tetrachloroethylene. Within each class of compounds, rates of dechlorination decreased with decreasing chlorine content. Regio- and stereospecificity were observed in these reactions. In the reductive dechlorination of trichloroethylene, cis-1,2-dichloroethylene was the predominant product formed with vitamin Bââ, coenzyme Fâââ, and hematin. Pentachlorobenzene and pentachlorophenol were each dechlorinated by vitamin Bââ to yield two out of three possible isomeric tetrachlorobenzenes. Similar relative kinetics and dechlorination products have been observed in anaerobic cultures, suggesting a possible role of transition-metal coenzymes in the reductive dechlorination of poly-chlorinated compounds in natural and engineered environments.
The transformability of trihalomethanes, carbon tetrachloride, 1,1,1-trichloroethane, 1,2-dibromoethane, tetrachloroethylene, hexachloroethane, and dibromochloropropane was studied under conditions of denitrification, sulfate respiration, and methanogenesis. These compounds at concentrations commonly found in groundwater were continuously administered to anoxic biofilm columns that resembled groundwater environments. Acetate was the primary substrate to support microbial growth. All of the compounds studied were transformed under methanogenesis. Bromoform, bromodichloromethane, carbon tetrachloride, and hexachloroethane were transformed even under the less reducing conditions of denitrification. Some of the compounds were partially mineralized to CO2. However, reductive dehalogenation appeared to be the predominant mechanism for removal. Characterization of the available electron acceptors in the subsurface is important for assessing organic micropollutant biotransformation. Reaction rates observed in the laboratory biofilms indicate that biotransformation could be responsible for significant removals of these halogenated compounds in the subsurface.
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.
In an earlier publication, we reported that corrinoids catalyze the sequential reduction of CCl4 to CHCl3, CH2Cl2, CH3Cl, and CH4 with titanium(III) citrate as electron donor [Krone, U. E., Thauer, R. K., & Hogenkamp, H. P. C. (1989) Biochemistry 28, 4908-4914]. However, the recovery of these products was less than 50%, indicating that other products were formed. We now report that, under the same experimental conditions, CCl4 is also converted to carbon monoxide. These studies were extended to include FREONs 11, 12, 13, and 14. Corrinoids were found to catalyze the reduction of CFCl3, CF2Cl2, and CF3Cl to CO and, in the case of CFCl3, to a lesser extent, to formate. CF4 was not reduced. The rate of CO and formate formation paralleled that of fluoride release. Both rates decreased in the series CFCl3, CF2Cl2, CCl4, and CF3Cl. The reduction of CFCl3 gave, in addition to CO and formate, CHFCl2, CH2FCl, CH3F, C2F2Cl2, and C2F2Cl4. The product pattern indicates that the corrinoid-mediated reduction of halogenated C1-hydrocarbons involves the intermediacy of dihalocarbenes, which may be a reason why these compounds are highly toxic for anaerobic bacteria.
Dehalogenation of chlori-nated organics by zero-valence metals
- R G Arnold
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- R J Kuhler
- E A Betterton
- K D Warren
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Reductive dehalogenation using a chemically modified electrode
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G.A. Santo, Reductive dehalogenation using a chemically modified electrode, Masters Thesis, Civil Engineering Department, University of Arizona, 1993.
Analytical Applications of 1,10-Phenanthroline and Related Compounds
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Reductive degradation of halogenated pesticides
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Dehalogenation of chlorinated organics by zero-valence metals
- Arnold