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Diversity of contaminant reduction reactions by zerovalent iron: role of the reductate
Abstract
The reactions of eight model contaminants with nine types of granular Fe(0) were studied in batch experiments using consistent experimental conditions. The model contaminants (herein referred to as "reductates" because they were reduced by the iron metal) included cations (Cu2+), anions (CrO4(2-), NO3(-), and 5,5',7,7'-indigotetrasulfonate), and neutral species (2-chloroacetophenone, 2,4,6-trinitrotoluene, carbon tetrachloride, and trichloroethene). The diversity of this range of reductates offers a uniquely broad perspective on the reactivity of Fe(0). Rate constants for disappearance of the reductates vary over as much as four orders of magnitude for particular reductates (due to differences in the nine types of iron) but differences among the reductates were even larger, ranging over almost seven orders of magnitude. Various ways of summarizing the data all suggest that relative reactivities with Fe(0) vary in the order Cu2+, 5,5',7,7'-indigotetrasulfonate > 2-chloroacetophenone, 2,4,6-trinitrotoluene > carbon tetrachloride, CrO4(2-) > trichloroethene > NO3(-). Although the reductate has the largest effect on disappearance kinetics, more subtle differences in reactivity due to the type of Fe(0) suggests that removal of CrO2(2-) and NO3(-) (the inorganic anions) involves adsorption to oxides on the Fe(0), whereas the disappearance kinetics of all other types of reductants is favored by reduction on comparatively oxide-free metal. Correlation analysis of the disappearance rate constants using descriptors of the reductates calculated by molecular modeling (energies of the lowest unoccupied molecular orbitals, LUMO, highest occupied molecular orbitals, HOMO, and HOMO-LUMO gaps) showed that reactivities generally decrease with increasing E(LUMO) and increasing E(GAP) (and, therefore, increasing chemical hardness eta).
... The Fe 0 content of particles was determined by measuring the quantity of H 2 evolved upon particle digestion in concentrated HCl. 32 Ions such as SO 4 2− , SO 3 2− , and S 2 O 3 2− in the solution were analyzed using a Dionex ion chromatograph (IC) (Thermo, ICS-1100) equipped with a Dionex IonPac AS11-HC and a potassium hydroxide ion eluent generator. S 2− in the solution was determined by the methylene blue spectrophotometric method and analyzed using a PERSEE spectrophotometer (TU-1810) at 665 nm. ...
... Furthermore, in the presence of Fe 0 , S 2 O 3 2− is expected to be reduced to S 2− and SO 3 2− . 17,19 Thus, the overall result of S 0 disproportionation in an aqueous suspension containing ZVI could be to produce stoichiometric quantities of SO 4 2− and/or SO 3 2− . ...
... Furthermore, in the presence of Fe 0 , S 2 O 3 2− is expected to be reduced to S 2− and SO 3 2− . 17,19 Thus, the overall result of S 0 disproportionation in an aqueous suspension containing ZVI could be to produce stoichiometric quantities of SO 4 2− and/or SO 3 2− . However, after 24 h reaction of S 0 and mZVI, the sulfur content in the aqueous phase was negligible (<0.1% of total sulfur). ...
Sulfidation can enhance both the reactivity and selectivity (i.e., electron efficiency, εe) of zero-valent iron (ZVI) in contaminant removal, which may make this technology cost-effective for a wider range of water treatment applications. However, current sulfidation methods involve either hazardous or unstable sulfidation agents (e.g., Na2S, Na2S2O3, and Na2S2O4) or energy-intensive preparations (e.g., mechanochemical sulfidation with elemental sulfur). In this study, we demonstrate that very efficient sulfidation of microscale ZVI (mZVI) can be achieved at all S/Fe molar ratios (∼100% sulfidation efficiency, εs) simply by direct reaction between elemental sulfur (S0) and ZVI in an aqueous suspension at ambient temperature. In comparison, the εs values obtained using Na2S, Na2S2O3, or Na2S2O4 as the sulfidation agents were only ∼23, ∼75, and ∼38%, respectively. The sulfidated mZVI produced using the new method reacts with trichloroethylene (TCE) with very high rates and electron efficiencies: rate constants and electron efficiencies were 800- and 79-fold higher than those of the unsulfidated mZVI. The enhanced performance of this material, together with the operational advantages of S0 for sulfidation (including safety, stability, and cost), may make it a desirable product for full-scale engineering applications.
... Ideally, such innovative technologies should be simple to operate and maintain, be able to function without electricity and be based on local resources and skills [2,4,5]. Filtration on metallic iron (Fe 0 ) 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 2 ...
... Reliable information about the intrinsic reactivity of Fe 0 materials is crucial in designing efficient and sustainable Fe 0 -based filters [9,18,20,22,[42][43][44][45][46][47][48][49]. Li et al. [22] recently reviewed information about appropriately selecting Fe 0 materials for water treatment and environmental remediation. ...
... However, this effort was not really continued and the Fe 0 reactivity was mainly characterized using selected Fe 0 specimens and various kinds of pollutants (e.g. Cr VI , methylene blue, nitrate, trichloroethylene) [44,47]. The reactivity of the materials was then reflected by the rate constants of their degradation. ...
Metallic iron (Fe 0) materials have been industrially used for water treatment since the 1850s. There are still many fundamental challenges in affordably and reliably characterizing the Fe 0 intrinsic reactivity. From the available methods, the one using Fe 0 dissolution in ethylenediaminetetraacetic acid (EDTA-2 mM) was demonstrated the most applicable as it uses only four affordable chemicals: ascorbic acid, an ascorbate salt, EDTA and 1,10-Phenanthroline (Phen). A careful look at these chemicals reveals that EDTA and Phen are complexing agents for dissolved iron species. Fe III-EDTA is very stable and difficult to destabilize; ascorbic acid is one of the few appropriate reducing agents, therefore. On the other hand, The Fe II-Phen complex is so stable that oxidation by dissolved O 2 is not possible, This article positively tests Fe 0 (0.1 g) dissolution in 2 mM Phen (50 mL) as a characterization tool for the intrinsic reactivity, using 9 commercial steel wool (Fe 0 SW) specimens as probe materials. The results are compared with those obtained by the EDTA method The apparent iron dissolution rate in EDTA (k EDTA) and in Phen (k Phen) were such that 0.53 k EDTA (g h-1) 4.81 and 0.07 k Phen (g h-1) 1.30. Higher k EDTA values, relative to k Phen , are a reflection of disturbing Fe III species originating from Fe II oxidation by dissolved O 2 and dissolution of iron corrosion products. It appears that the Phen method considers only the forward dissolution of Fe 0. The Phen method is reliable and represents the most affordable approach for characterizing the suitability of Fe 0 for water treatment.
... Moreover, ZVI can also produce H 2 O 2 and trigger the Fenton reaction starting from dissolved O 2 (reactions (1-4)), even in the absence of H 2 O 2 addition. The use of ZVI in the sequestration process for contaminants removal from water and wastewater has been studied widely, exploiting adsorption, reduction and co-precipitation phenomena (e.g., As(V) [42], Cr(VI) and Sb(III) [43], Cd(II) [49], Cu 2+ , CrO 4 2− , 2-chloroacetophenone, 2,4,6-trinitrotoluene, carbon tetrachloride, trichloroethene [50], NO 3 − and NO 2 − [23,50]). An electron energy loss spectroscopy (EELS) analysis of ZVI has shown that surface-bonded OH groups (Fe−OH) are generated on the ZVI surface in the presence of water, and that they provide effective exchange sites for pollutants in the sequestration process [42]. ...
... Moreover, ZVI can also produce H 2 O 2 and trigger the Fenton reaction starting from dissolved O 2 (reactions (1-4)), even in the absence of H 2 O 2 addition. The use of ZVI in the sequestration process for contaminants removal from water and wastewater has been studied widely, exploiting adsorption, reduction and co-precipitation phenomena (e.g., As(V) [42], Cr(VI) and Sb(III) [43], Cd(II) [49], Cu 2+ , CrO 4 2− , 2-chloroacetophenone, 2,4,6-trinitrotoluene, carbon tetrachloride, trichloroethene [50], NO 3 − and NO 2 − [23,50]). An electron energy loss spectroscopy (EELS) analysis of ZVI has shown that surface-bonded OH groups (Fe−OH) are generated on the ZVI surface in the presence of water, and that they provide effective exchange sites for pollutants in the sequestration process [42]. ...
... At pH values lower than the pH PZC , the ZVI surface is positively charged and it can adsorb anions simply via electrostatic attraction. In contrast, at pH > pH PZC the negatively charged surface of ZVI attracts and adsorbs positively charged species, while at the same time one has electrostatic repulsion between anions and the ZVI surface [42,50]. 4 and FeO(OH) species on the ZVI surface at pH < 5, together with a strong electrostatic attraction between positively charged iron oxides and negatively charged tetracycline molecules [52]. ...
Heterogeneous Fenton processes with solid catalysts have gained much attention for water and wastewater treatment in recent years. In the field of solid catalysts, zero valent iron (ZVI) is among the most applicable due to its stability, activity, pollutant degradation properties and environmental friendliness. The main limitation in the use of ZVI in heterogeneous Fenton systems is due to its deactivation in neutral and alkaline conditions, and Fenton-like processes have been developed to overcome this difficulty. In this review, the effect of solution pH on the ZVI-Fenton performance is discussed. In addition, the pH trend of ZVI efficiency towards contaminants removal is also considered in oxic solutions (i.e., in the presence of dissolved O2 but without H2O2), as well as in magnetic-field assisted Fenton, sono-Fenton, photo-Fenton and microwave-Fenton processes at different pH values. The comparison of the effect of pH on ZVI performance, taking into account both heterogeneous Fenton and different Fenton-like processes, can guide future studies for developing ZVI applications in water and wastewater treatment.
... The reactivity of nZVI particles was evaluated in anaerobic batch experiments using Cu(II) and Cr(VI), which were selected, in part, be- cause their suitability as model contaminants for ZVI testing has been systematically investigated in previous work [50]. The stock solutions of Cu(II) and Cr(VI) were prepared from copper sulfate pentahydrate (CuSO 4 ·5H 2 O, p.a., Sigma-Aldrich) and potassium chromate (K 2 CrO 4 , p.a., Sigma-Aldrich) using deoxygenized DI water. ...
... Two such probe compounds are Cu(II) and Cr(VI), both of which are readily reduced and sequestered by ZVI, resulting in the disappearance of the parent compounds from solution that is easily and unambiguously monitored. The mechanisms, kinetics, and practical utility of these re- actions have been studied extensively in previous works (e.g., [65]), and their representativeness when used as reactivity assays has been systematically evaluated [50]. In the latter study, it was shown that Cu (II) and Cr(VI) were removed very fast and moderately fast, respectively (compared with a wide range of contaminants); the rates of Cu(II) se- questration correlated surprisingly well with the rates of TCE reduction (considering that the two reactions are fundamentally different in many ways); and the rates of Cr(VI) sequestration were greatest with impure ZVI that was coated with significant iron oxide (in contrast to non- oxyanion contaminants, which are generally are more rapidly reduced by ZVI that is more pure Fe(0)). ...
... The capacity experiments (Fig. 7b and Fig. S6b) involved measuring contaminant concentration vs. dose of nZVI, all with 24 h of contact time, which was chosen mainly as a typical and convenient exposure time for laboratory-based reactivity assays. The time series data in the kinetics experiments and the dose series data in the capacity experiments exhibit a variety of complications that make them un- suitable for fitting to a comprehensive model, and even the method of fitting initial rates, which is often used for these reactions [50,65], could not be applied reliably to these data. However, there are quali- tative trends in the data that are clear and consistent and with sig- nificant implications for the design of nZVI with optimal properties for water treatment. ...
The optimization of nanoscale zero-valent iron (nZVI) for groundwater remediation applications requires consideration of properties that influence its longevity and transport in porous media and reactivity with contaminants. Here, we report on the stabilization of nZVI by controlled growth of oxide shells of varying thickness and characterization of the resulting materials’ structure and reactivity. Using a thermal oxidation method, nZVI was prepared with shell thickness varying between 4 and 10 nm. These nZVI materials, together with pyrophoric nZVI (without a passivating oxide coating) and two commercial nZVI materials (NANOFER STAR and NANOFER 25), were characterized in detail with respect to morphology, shell thickness, structure, magnetism, stability, and reactivity. The results show that increasing oxidation temperature results in thicker oxide coatings on the particles, but these coatings also have more fractures and other defects. The reactivity of these particles, demonstrated on Cr(VI) and Cu(II) removal, increases with increasing shell thickness, probably as a result of higher extent of defects in thicker shell. Therefore the ability to control thickness and character of the shell leads to possibility to controlling reactivity while keeping comparable content of Fe(0) in the material. These nZVI materials with 7 and 10 nm oxide shell prepared via simple solid-gas synthesis can be used as a suitable alternative to common air-stable nZVI without additional activation steps.
... by ZVI through oxidation, reduction, adsorption, and/or co-precipitation (Miehr et al., 2004;Noubactep, 2008;Yoon et al., 2011;Fu et al., 2014;Liang et al., 2014b;Guan et al., 2015;Li et al., 2015;. In these processes of contaminants removal, ZVI corrodes and then is converted to a variety of iron (hydr)oxides (Liang et al., 2014b;Liu et al., 2022). ...
... According to Fig. 2(c), it was found that there was a positive correlation between the lgi corr and the surface normalized rate constants (lgk SA , calculated using Eq. (1)) of the ASZVI samples (Miehr et al., 2004;Li et al., 2015;. This relationship implies that the ASZVI sample is highly dependent on its electron transfer to Cr(VI), as evidenced by the high percentage (over 72.3%) of Cr(III) measured in the Cr-reacted ASZVI samples ( Fig. 2(b)). ...
Sulfated zero-valent iron (SZVI) has shown promising applications in wastewater treatment. However, the rapid decline in the reactivity of SZVI with time limits its real practice. To mediate this problem, partial aging was proposed to improve the reactive durability of SZVI. Taking Cr(VI) as the target contaminant, we found that the aged ZVI (AZVI) gradually lost reactivity as aging time increased from 0.5 to 2 d. Counter-intuitively, the partially aged SZVI (ASZVI) showed greater reactivity than SZVI when exposed to oxygenated water for a period ranging from 0.5 to 14 d. In addition, the ASZVI with 0.5 d of aging time (ASZVI-0.5) not only maintained reactivity in successive runs but also increased the Cr(VI) removal capacity from 9.1 mg/g by SZVI to 19.1 mg/g by ASZVI-0.5. Correlation analysis further revealed that the electron transfer from the Fe ⁰ core to the shell was mediated by the conductive FeS and FeS 2 in the subshell of ASZVI. Meanwhile, the lepidocrocite and magnetite on the surface of ASZVI facilitated Cr(VI) adsorption and subsequent electron transfer for Cr(VI) reduction. Moreover, the iron (hydr)oxide shell could retain the conductive FeS and FeS 2 in the subshell, allowing ASZVI to reduce Cr(VI) efficiently and sustainably. In general, partial aging can enhance the reactive durability of ZVI when coupled with sulfidation and this synergistic effect will be beneficial to the application of SZVI-based technology for wastewater treatment.
... It is hoped that in establishing that contaminant removal in Fe 0 /H 2 O systems is due to flocculation (adsorption and co-precipitation) ( Figure 6) [89], the results presented herein will redirect research for the design of next generation of Fe 0based remediation systems accounting for the ion-selective nature of the systems [14,84,[89][90][91][92]. The view that Fe 0 is a reducing agent [15,93,94] should be immediately abandoned [85]. This view has mediated or supported wrong wordings like zero-valent iron for metallic [14] or reductates for species reducible by Fe 0 according the relative electrode potentials [94]. ...
... The view that Fe 0 is a reducing agent [15,93,94] should be immediately abandoned [85]. This view has mediated or supported wrong wordings like zero-valent iron for metallic [14] or reductates for species reducible by Fe 0 according the relative electrode potentials [94]. In 2013, the notion of electron efficiency was introduced [86,95,96] and is progressively used by several research groups [86]. ...
There is growing interest in using pyrite minerals (FeS 2) to enhance the efficiency of metallic iron (Fe 0) for water treatment (Fe 0 /H 2 O systems). This approach contradicts the thermodynamic predicting suppression of FeS 2 oxidation by Fe 0 addition. Available results are rooted on time series correlation between aqueous and solid phases based on data collected under various operational conditions. Herein, the methylene blue method (MB method) is used to clarify the controversy. The MB method exploits the differential adsorptive affinity of MB onto sand and sand coated with iron corrosion products to assess the extent of Fe 0 corrosion in Fe 0 /H 2 O systems. The effects of the addition of various amounts of FeS 2 to a Fe 0 /sand mixture (FeS 2 method) on MB discoloration were characterized in parallel quiescent batch experiments for up to 71 d (pH 0 = 6.8). A pristine and an aged FeS 2 specimens were used. Parallel experiments with methyl orange (MO) and reactive red 120 (RR120) enabled a better discussion of achieved results. Results clearly showed that FeS 2 induces a pH shift, and delays Fe precipitation and sand coating. Pristine FeS 2 induced a pH shift to values lower than 4.5, but no quantitative MB discoloration occurred after 45 d. Aged FeS 2 , could not significantly shift the pH value (final pH 6.4), but improved MB discoloration. The used systematic sequence of experiments demonstrated that adsorption and co-precipitation are the fundamental mechanisms of contaminant removal in Fe 0 /H 2 O systems. This research has clarified the reason why FeS 2 addition enhances the efficiency of Fe 0 environmental remediation.
... However, shortterm laboratory experiments are always a simplification and this should be borne in mind when interpreting achieved results [76,129,131,132]. Given the diversity of operational parameters that have been proven to influence iron corrosion from individual studies, one can be overwhelmed by their number and the fact that each material is unique in its corrosion behaviour [11,124,125,133]. Therefore, a first attempt toward a systematic investigation of relevant influencing factors goes through the consideration of the electrochemical nature of aqueous iron corrosion. ...
... These considerations clearly show that the H 2 evolution method [163,164] is an approximation, while the EDTA method [170] is disturbed by dissolved O 2 . All other methods are contaminant-specific, and thus of low value [133,160,161]. Lufingo et al. [11] then proposed iron dissolution in a dilute (2 mM) 1,10 Phenanthroline solution (Phen test) as a facile method free from the inherent shortcomings of all available methods. ...
A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe 0) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe 0 specimens for water treatment are typically small in size. Before the advent of this technology and it application for environmental remediation, such small Fe 0 particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe 0 was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe 0 research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe 0 research back to the highway of mainstream corrosion science, where the fundamentals of Fe 0 technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe 0 technology are demonstrated. The misuse of the terms "reactivity", and "efficiency", and adsorption kinetics and isotherm models for Fe 0 systems is also elucidated. The immense importance of Fe 0 /H 2 O systems in
... In both cases, the tests are validated by comparing the results to those of the removal of various kinds of pollutants used as probing agents. Currently, using relevant contaminants as probing agents to assess the reactivity of Fe 0 materials is the most common approach [6,31,32]. However, rationally selecting a probing agent is controversial due to the following reasons: (i) hundreds of compounds are relevant contaminants [29], (ii) metabolites may form in-situ [13] and (iii) at many sites, water is polluted by several species [33]. ...
... Figure 5 further shows that for the same material k EDTA > k Phen (section 1). Figure 5a shows no monotonous decrease of a values with increasing particle size as theoretically expected [50]. This corresponds to the observations of the past 30 years that the particle size alone is not enough to assess the reactivity of a Fe 0 specimen [6,17,31,51]. The results in Fig. 5b conveys the same message as the particle size is directly related to the specific surface area [50]. ...
There is a burgeoning interest in reliably characterizing the intrinsic reactivity of metallic iron materials (Fe 0) used in the water treatment industry. The present work is a contribution to a science-based selection of Fe 0 for water treatment. A total of eight (8) granular Fe 0 materials (ZVI1 to ZVI8) were tested. Fe 0 dissolution in ethylenediaminetetraacetic acid (EDTA test) and 1,10-Phenanthroline (Phen test) is characterized in parallel experiments for up to 250 hours (10 days). 50 mL of each solution and 0.1 g of each Fe 0 material are equilibrated in quiescent batch experiments using 2 mM EDTA or Phen. Results indicated a far higher extent of iron dissolution in EDTA than in Phen under the experimental conditions. The tested materials could be grouped into three reactivity classes: (i) low (ZVI4, ZVI6, ZVI7 and ZVI8), (ii) moderate (ZVI1 and ZVI5) and (iii) high (ZVI2 and ZVI3). The order of reactivity was the same for both tests: ZVI2 ZVI3 > ZVI1 ZVI5 > ZVI4 ZVI6 ZVI7 ZVI8. Phen results revealed for the first time the time-dependent variation of the kinetics of iron corrosion (corrosion rate) in short-term batch experiments. Overall, the results demonstrated the superiority of the Phen test for evaluating the initial stage of Fe 0 dissolution. Long-term term column experiments are recommended to deepen the acquired knowledge.
... A better understanding of the long-term corrosion process could hold clues for engineering improved Fe 0 -based remediation systems. A variety of Fe 0 specimens have been tested and used for environmental remediation and water treatment [89][90][91][92][93]. However, these studies failed to pay particular attention to the iron corrosion rate [94,95]. ...
... While the details of manufacturing conditions are typically not accessible to the researcher, all other parameters can be analytically determined [93,118,119]. It is essential to recall that all relevant parameters are interdependent and none of them could be proven superior in determining the reactivity of Fe 0 materials [89,92,[120][121][122] The evidence that E 0 = −0.44 V is the driving force for all Fe 0 materials implies that the nature of Fe 0 is a stand-alone variable in investigating the efficiency of Fe 0 for environmental remediation. ...
Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) summarize the state-of-the-art knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0.
... The raw water (B) is The large majority of investigations pertaining to use of Fe 0 for water treatment are conducted in the batch mode. In such studies, various types of Fe 0 materials were used, including; iron foam, iron nails, scrap iron, steel wool, and various composites [86][87][88][89][90][91]. Although batch laboratory studies provide useful information in treatability studies for column experiments, continuous column studies provide more practical information for the design of Fe 0 filters. ...
... This transient pseudo-equilibrium is related to the apparent corrosion kinetics (intrinsic reactivity) of the filter material (e.g., Fe 0 ). Unlike for pure adsorbents [95], in Fe 0 /H 2 O systems, the established equilibrium is not static in nature, as it changes further with time ('rust never rests') [95,86]. Contrary to the transient pseudoequilibrium in batch systems, column experiments are dynamic, and solution continuously enters and leaves the column. ...
Inadequate access to safe drinking water is one of the most pervasive problems currently afflicting the developing world. Scientists and engineers are called to present affordable but efficient solutions, particularly applicable to small communities. Filtration systems based on metallic iron (Fe0) are discussed in the literature as one such viable solution, whether as stand-alone system or as a complement to slow sand filters (SSFs). Fe0 filters can also be improved by incorporating biochar to form Fe 0-biochar filtration systems with potentially higher contaminant removal efficiencies than those based on Fe0 or biochar alone. These three low-cost and chemical-free systems (Fe0 , biochar, SSFs) have the potential to provide universal access to safe drinking water. However, a well-structured systematic research is needed to design robust and efficient water treatment systems based on these affordable filter materials. This communication highlights the technology being developed to use Fe0-based systems for decentralized safe drinking water provision. Future research directions for the design of the next generation Fe0-based systems are highlighted. It is shown that Fe0 enhances the efficiency of SSFs, while biochar has the potential to alleviate the, loss of porosity and uncertainties arising from the non-linear kinetics of iron corrosion. Fe0-based systems are an affordable and applicable technology for small communities in low-income countries, which could contribute towards attaining self-reliance in clean water supply and universal public health.
... The obtained k' values correspond to k SA values of 2.8 ± 0.3 × 10 −3 L m −2 d −1 for the PRB experiment and 1.6 ± 0.1 × 10 −3 L m −2 d −1 for both the MQ and pH4 experiments. For the latter experiments, the obtained k SA values are within the range reported for chemical nitrate reduction by ZVI (1.1 × 10 −4 to 7.2 L m −2 d −1 for pH between 7 and 9.5, and 2.4 × 10 −3 to 12.3 L m −2 d −1 for pH values of 3-6.5, Alowitz and Scherer, 2002;Su and Puls, 2004;Choe et al., 2004;Ginner et al., 2004;Miehr et al., 2004). ...
... Both abiotic batch experiments (pH4 and MQ) showed the same reaction rate, suggesting that at the tested conditions (pH = 4 for the pH4 experiment, pH = 5.5 for the MQ experiment, respectively), ZVIdriven nitrate reduction was independent of the pH. Nevertheless, at pH values relevant to ZVI-PRBs (pH from 6.5 to 9), slower rates have generally been observed at higher pH values (Hu et al., 2001;Alowitz and Scherer, 2002;Miehr et al., 2004;Westerhoff and James, 2003;Ginner et al., 2004). ...
Permeable reactive barriers (PRBs) filled with zero-valent iron (ZVI) are a well-known remediation approach to treat groundwater plumes of chlorinated volatile organic compounds as well as other contaminants. In field implementations of ZVI-PRBs designed to treat these contaminants, nitrate consumption has been reported and has been attributed to direct abiotic nitrate reduction by ZVI or to denitrification by autochthonous microorganisms using the dissolved hydrogen produced from ZVI corrosion. Isotope tools have proven to be useful for monitoring the performance of nitrate remediation actions. In this study, we evaluate the use of isotope tools to assess the effect of ZVI-PRBs on the nitrate fate for the further optimization of full-scale applications. Laboratory batch experiments were performed using granular cast ZVI and synthetic nitrate solutions at pH 4–5.5 or nitrate-containing groundwater (pH = 7.0) from a field site where a ZVI-PRB was installed. The experimental results revealed nitrate attenuation and ammonium production for both types of experiments. In the field site, the chemical and isotopic data demonstrated the occurrence of ZVI-induced abiotic nitrate reduction and denitrification in wells located close to the ZVI-PRB. The isotopic characterization of the laboratory experiments allowed us to monitor the efficiency of the ZVI-PRB at removing nitrate. The results show the limited effect of the barrier (nitrate reduction of less than 15–20%), probably related to its non-optimal design. Isotope tools were therefore proven to be useful tools for determining the efficacy of nitrate removal by ZVI-PRBs at the field scale.
... (5)), which appeared to offer the most practical and general descriptor of contaminant degradation kinetics by ZVI. Since then, k SA has been considered as the primary measure to describe and generalize the contaminants removal kinetics in Fe 0 -H 2 O systems (Li et al., 2015b;Miehr et al., 2004). ...
... The authors showed that the efficiency of ZVI materials, determined by the EDTA test, was comparable to that of U(VI) removal by these ZVI materials . Based on the abundant kinetic data that are now available for sequestering various compounds by different ZVI materials, it is concluded that there is a wide range of reactivity for Fe 0 (Johnson et al., 1996;Li et al., 2015b;Miehr et al., 2004). Nevertheless, very few attempts have been made to explore the relationship between the disappearance rates of the contaminant by ZVI and the properties of the ZVI materials via a simple descriptor variable. ...
Appropriately selecting methods for characterizing the reaction system of zerovalent iron (ZVI) favors its application for water treatment and remediation. Hence, a survey of the available ZVI characterization techniques used in laboratory and field studies are presented in this review for clarifying the characteristic properties, (in-situ) corrosion processes, and corrosion products of ZVI system. The methods are generally classified into four broad categories: morphology characterization techniques, (sub-)surface and bulk analysis mainly via the spectral protocols, along with the (physio)electrochemical alternatives. Moreover, this paper provides a critical review on the scopes and applications of ZVI characterization methodologies from several perspectives including their suitable occasions, availability, (semi-)quantitative/qualitative evaluations, in/ex-situ reaction information, advantages, limitations and challenges, as well as economic and technical remarks. In particular, the characteristic spectroscopic peak locations of typical iron (oxyhydr)oxides are also systematically summarized. In view of the complexity and variety of ZVI system, this review further addresses that different characterization methods should be employed together for better assessing the performance and mechanisms of ZVI-involved systems and thereby facilitating the deployment of ZVI-based installations in real practice.
... Most of research on metal sequestration by ZVI has used pseudofirst-order kinetics to describe the kinetics [65,66], which was, therefore, also chosen here for the removal of Sb. in Fig. 3(A) shows the kinetics of Sb removal by S-ZVI that was "aged" for different times in contact with the medium. Synchronous measurements of the four most important "geochemical" properties of the system (DO, Fe 2+ , Eh, and pH) were also measured for each treatment, shown in Fig. 3(B)-(E). ...
... Initially, immersion of the S-ZVI initiates the dissolution processes of the oxide layer and leads to the increase of the corrosion rate, which in turn increases the rates of Sb(III) sequestration. Analogous effects have been founded in previous studies including a variety of ZVIs and contaminants [66]. After depassivation of the ZVI, the oxide film appears to have entered into a relatively stable and metastable state. ...
Zerovalent iron (ZVI) is commonly used for water treatment under aerobic conditions such as sequestration of metals. Sulfide-modified ZVI (S-ZVI) is attracting increasing attention for its easy preparation and high reactivity with environmental pollutants. The processes responsible for contaminant removal can be a complex mixture of redox, sorption, and coprecipitation processes. In this paper, ZVI and S-ZVI were used to sequester antimonite (Sb(III)). The rates of Sb(III) sequestration were determined in open, well-mixed, batch reactors. The effects of various experimental variables were investigated, including pH, iron dose, initial concentrations of Sb(III), aging time of the ZVI and S-ZVI, addition of Fe²⁺, mixing rate, etc. The results showed that S-ZVI can significantly enhance the Sb(III) sequestration, and under basic conditions in this study, the kobs (0.018 min⁻¹) obtained in the S-ZVI system was approximately 15 times higher than the 0.0012 min⁻¹ obtained in the ZVI system. Solid phase characterizations were conducted to assess the influence of sulfidation on the morphology and surface geochemistry of ZVI. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of sulfur. X-ray photoelectron spectroscopy (XPS) indicated the oxidation of Sb(III) to Sb(V) and adsorption and coprecipitation onto the iron oxides is the mainly sequestration process. The FeS layer on ZVI is more conductive than oxides and therefore accelerates electron transfer. In addition sulfidation promotes the corrosion of iron and the formation of ferrous iron, which further enhances the ferric iron oxide formation, thereby favoring adsorption and oxidation of Sb(III).
... cited therein). It has been demonstrated that none of the chemical or physical characteristics alone can enable the assessment of the intrinsic reactivity (Reardon 1995, Miehr et al. 2004, Rahman et al. 2013, Kim et al. 2014, Birke et al. 2015, Li et al. 2016, Moraci et al. 2016. Moreover, experiments for characterizing the Fe 0 intrinsic reactivity should last for several weeks (Noubactep 2010c). ...
... A central question that should be scientifically answered before engineers can accurately select appropriate Fe 0 materials for a specific application is the characterization of the intrinsic reactivity and its time-dependent changes. Again, information on elemental composition, particle morphology and size, and surface area are not really helpful in characterizing the reactivity (Miehr et al. 2004, Kim et al. 2014, Velimirovic et al. 2014, Birke et al. 2015, Li et al. 2016. In essence, the intrinsic reactivity is a characteristic of each material and cannot be measured, but just assessed by appropriate procedures. ...
The use of metallic iron (Fe0) for environmental remediation is applied industrially with a great degree of empiricism. Rules of thumb seem to be the only guide followed by the greater part of the Fe0 remediation community. The present communication demonstrates the validity of such a harsh statement and hopes that the research community will now follow the 10-year-old path concealing Fe0 remediation and mainstream science. A promising application is safe drinking water provision on a decentralized manner.
... Note however, that the strong linear correlation between CHC reduction potential (i.e., LUMO values) and the pseudo-first order rate constants obtained for nZVI in this study is likely specific to the nZVI material tested here. CHC degradation trends with nZVI can vary greatly as a result of synthesis procedure (Velimirovic et al., 2013;Islam et al., 2020) and we refer the readers to earlier studies that extensively studied nZVI QSAR with CHC Song and Carraway (2008); Scherer et al. (1998); Arnold and Roberts (1998);Chaplin et al. (2012); Miehr et al. (2004). ...
Sulfidated nanoscale zerovalent iron (S-nZVI) exhibits low anoxic oxidation and high reactivity towards many chlorinated hydrocarbons (CHCs). However, nothing is known about S-nZVI reactivity once exposed to complex CHC mixtures, a common feature of CHC plumes in the environment. Here, three S-nZVI materials with varying iron sulfide (mackinawite, FeSm) shell thickness and crystallinity were exposed to groundwater containing a complex mixture of chlorinated ethenes, ethanes, and methanes. CHC removal trends yielded pseudo-first order rate constants (kobs) that decreased in the order: trichloroethene > trans-dicloroethene > 1,1-dichlorethene > trichloromethane > tetrachloroethene > cis-dichloroethene > 1,1,2-trichloroethane, for all S-nZVI materials. These kobs trends showed no correlation with CHCs reduction potential based on their lowest unoccupied molecular orbital energies (ELUMO) but absolute values were affected by the FeSm shell thickness and crystallinity. In comparison, nZVI reacted with the same CHCs groundwater, yielded kobs that linearly correlated with CHCs ELUMO (R² = 0.94) and that were lower than S-nZVI kobs. The CHC selectivity induced by sulfidation treatment is explained by FeSm surface sites having specific binding affinities towards some CHCs, while others require access to the metallic iron core. These new insights help advance S-nZVI synthesis strategies to fit specific CHC treatment scenarios.
... The reliance on physical (e.g. particle size, specific surface area, and surface state) and chemical parameters (e.g. chemical composition) alone to assess the intrinsic reactivity of Fe 0 specimen has been categorically demonstrated to be unsuitable (Richardson and Nicklow, 2002;Miehr et al., 2004;Btatkeu-K et al., 2013;Li et al., 2016;. This is because physico-chemical parameters fail to properly characterize the reactivity of Fe 0 materials. ...
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.
... A variety of hazardous oxyanions including chromate, pertechnetate, arsenate, and selenate can be removed from aqueous solutions with Fe 0 by reduction, adsorption, coprecipitation, and precipitation processes (Blowes et al., 2000;Miehr et al., 2004;O'Carroll et al., 2013). In particular, the kinetics of Fe 0 transformations in the presence of pertechnetate (TcO 4 -) at variable pH conditions is a focus of this study due to the need for treatment options in complex, radioactive waste streams. ...
Elemental iron Fe⁰ is a promising reductant for removal of Tc from complex aqueous waste streams that contain sulfate, halides, and other inorganic anions generated during processing of legacy radioactive waste. The impact of sulfate on the kinetics of oxidation and reduction capacity of Fe⁰ in the presence of Tc has not been examined. We investigated the oxidative transformation of Fe⁰ and reductive removal of TcO4⁻ in 0.1 M Na2SO4 as a function of initial pH (i.e., pHi 4, 7, and 10) under aerobic conditions up to 30 days. Tc reduction was the fastest at pHi 7 and slowest at pHi 10 (Tc reduction rate pHi 7 > 4 > 10). Aqueous fraction of Tc was measured at 0.4% at pHi 7 within 6 hours, whereas ≥ 97% of Tc was removed from solutions at pHi of 4 and 10 within 24 hours. Solid phase characterization showed that magnetite was the only oxidized crystalline phase for the first 6 hours regardless of initial pH. Lepidocrocite was the most abundant oxidized product for pHi 10 after 5 days, but was not observed at pH of 4 or 7.
... This formulation and terminology have been adopted to describe the kinetics of the contaminant reaction with a wide range of heterogeneous reductants, including iron oxides. 194,361,425,426,526 One way that this formulation has been extended is by plotting log k SA vs log k M , which has proven useful for comparing reactivity among contaminants, among reductants, and across a variety of other operational variables. 508,527−529 Note that the reactivity of iron modified forms or their composite minerals is beyond the scope of this review, but readers are referred to several reviews on this topic. ...
Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.
... (V H2 ) 24 is the H 2 volume after 24 hours, v max is the maximum H 2 production rate at time t vmax ; [Fe] is the aqueous iron concentration at the end of the H 2 production experiment (28 hours). k EDTA is the corrosion rate in a 2 mM EDTA solution[42]. ...
Metallic iron (Fe0) has been demonstrated as an excellent material for decentralized safe drinking water provision, wastewater treatment and environmental remediation. An open issue for all these applications is the rational material selection or quality assurance. Several methods for assessing Fe0 quality have been presented, but all of them are limited to characterizing its initial reactivity. The present study investigates H2 evolution in an acidic solution (pH 2.0) as an alternative method, while comparing achieved results to that of uranium removal in quiescent batch experiments at neutral pH values. The unique feature of the H2 evolution experiment is that, quantitative H2 production ceases when the pH reached a value of 3.1. A total of twelve Fe 0 specimens was tested. The volume of molecular H2 produced by 2.0 g of each Fe0 specimen in 560 mL H2SO4 (0.01 M) was monitored for 24 hours. Additionally, the extent of U(VI) (0.084 mM) removal from an aqueous solution (20.0 mL) by 0.1 g of Fe0 was characterized. All U removal experiments were performed at room temperature (22 ± 2°C) for 14 days. Results demonstrated the difficulty of comparing Fe0 specimens from different sources, and confirmed that the elemental composition of Fe0 is not a stand-alone determining factor for reactivity. The time-dependent changes of H2 evolution in H2SO4 confirmed that, tests in the neutral pH range just address the initial reactivity of Fe0 materials. In particular, materials initially reacting very fast would experience a decrease in reactivity in the long-term, and this aspect must be incorporated in designing novel materials and sustainable remediation systems. An idea is proposed which could enable the manufacturing of intrinsically long-term efficient Fe0 materials for targeted operations as a function of the geochemistry.
... Coated sand depicts a lower adsorptive affinity for MB than clean sand [46,48]. The merit of the three named tests (EDTA, MB and Phen) is that, like H 2 evolution [50,51], they do characterize an intrinsic property of each Fe 0 (intrinsic reactivity) and results are transferable to all systems, unlike characterization tools based on the Fe 0 removal efficiency for individual contaminants [52][53][54][55]. ...
Studies were undertaken to characterize the intrinsic reactivity of Fe 0-bearing steel wool (Fe 0 SW) materials using the ethylenediaminetetraacetate method (EDTA test). A 2 mM Na 2-EDTA solution was used in batch and column leaching experiments. A total of 15 Fe 0 SW specimens and one granular iron (GI) were tested in batch experiments. Column experiments were performed with four Fe 0 SW of the same grade but from various suppliers and the GI. The conventional EDTA test (0.100 g Fe 0 , 50 mL EDTA, 96 hours) protocol was modified in two manners: (i) decreasing the experimental duration (down to 24 h) and (ii) decreasing the Fe 0 mass (down to 0.01 g). Column leaching studies involved glass columns filled to 1/4 with sand, on top of which 0.50 g of Fe 0 was placed. Columns were daily gravity fed with EDTA and effluent analyzed for Fe concentration. Selected reactive Fe 0 SW specimens were additionally investigated for discoloration efficiency of methylene blue (MB) in shaken batch experiments (75 rpm) for 2 and 8 weeks. The last series of experiments tested six selected Fe 0 SW for water defluoridation in Fe 0 /sand columns. Results showed that (i) the modifications of the conventional EDTA test enabled a better characterization of Fe 0 SW; (ii) after 53 leaching events the Fe 0 SW showing the best k EDTA value released the lowest amount of iron; (iii) all Fe 0 specimens were efficient at discoloring cationic MB after 8 weeks; (iv) limited water defluoridation by all six Fe 0 SW was documented. Fluoride removal in column systems appears to be a viable tool to characterize the Fe 0 long-term corrosion kinetics. Further research should include correlation of the intrinsic reactivity of SW specimens with their efficiency at removing different contaminants in water.
... It is difficult for a critical reader to identify signs of common efforts, even from some critical review articles. As an example, while the intrinsic reactivity of Fe 0 materials has been recognized as a crucial design parameter in the early phase of technology development [55,56] only some few contaminant independent tools have been presented and none of them is really adopted [57][58][59][60]. Meaning that researchers have even not tried to characterize granular materials (mm in size) before manufacturing granular bimetallics and nano-scale materials [30,50]. ...
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.
... Over the last few years, zero-valent iron (ZVI) has attracted substantial attention for its ability to remove organic or metal/metalloid pollutants from water [9][10][11][12][13][14][15]. In regard to the different removal mechanisms of various contaminants, ZVI has been widely reported as a reagent for treating various environmental pollutants under both aerobic and anaerobic conditions [16,17]. ...
... Storage of NZVI in suspension requires at least anaerobic conditions but is always associated with iron corrosion. For converting suspended NZVI into a dry product, flash drying (i.e., rinsing with organic solvents during suction filtration with drying under inert gas) or freeze-drying under vacuum is applied (e.g., Miehr et al. 2004;Liu and Lowry 2006;Wang et al. 2010). ...
This chapter provides an overview of NZVI types used to date for environmental restoration. The particle types are introduced systematically from bare NZVI to the manifold modifications leading to NZVI-containing composites or emulsions. Properties of these NZVI types which are important for the intended use as water treatment reagent and methods for their characterization are compiled. For each of the main NZVI groups – bare and bimetallic NZVI, polymer-modified NZVI, supported NZVI and emulsified NZVI, approved synthesis strategies and resulting NZVI properties are described.
... Available laboratory data are mostly irrelevant for field situations, mainly because of the lack of any reference Fe 0 material and the fact that experiments were performed under very different conditions, very far to those occurring under field conditions [31,46,71,139,142]. Additionally, it has been acknowledged that Fe 0 materials currently used for water treatment and environmental remediation (e.g., Gotthart-Maier GmbH or iPutech -Rheinfelden/Germany, Connelly-GPM Inc. -Chicago/USA, Peerless Metals Powder Inc. -Detroit/USA) have not been specially produced for these applications [10,21,145,146]. For example, Gotthart-Maier GmbH and later iPutech widely used at many test sites in Germany are a mixture of scrap materials. ...
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.
... Or regarding NZVIs, which are originally produced for application in the electronic or even food indus try, therefore are very often associated with different kinds and amounts of trace elements. Thus, their reactivity regarding cVOC dehalogenation in groundwater can vary significantly, which has already been addressed and documented extensively (Johnson et al., 1996;Miehr et al., 2004;Ebert et al., 2006). ...
Remediation of groundwater is complex and often challenging. But the cost of pump and treat technology, coupled with the dismal results achieved, has paved the way for newer, better technologies to be developed. Among these techniques is permeable reactive barrier (PRB) technology, which allows groundwater to pass through a buried porous barrier that either captures the contaminants or breaks them down. And although this approach is gaining popularity, there are few references available on the subject. Until now. Permeable Reactive Barrier: Sustainable Groundwater Remediation brings together the information required to plan, design/model, and apply a successful, cost-effective, and sustainable PRB technology. With contributions from pioneers in this area, the book covers state-of-the-art information on PRB technology. It details design criteria, predictive modeling, and application to contaminants beyond petroleum hydrocarbons, including inorganics and radionuclides. The text also examines implementation stages such as the initial feasibility assessment, laboratory treatability studies (including column studies), estimation of PRB design parameters, and development of a long-term monitoring network for the performance evaluation of the barrier. It also outlines the predictive tools required for life cycle analysis and cost/performance assessment. A review of current PRB technology and its applications, this book includes case studies that exemplify the concepts discussed. It helps you determine when to recommend PRB, what information is needed from the site investigation to design it, and what regulatory validation is required.
... Or regarding NZVIs, which are originally produced for application in the electronic or even food indus try, therefore are very often associated with different kinds and amounts of trace elements. Thus, their reactivity regarding cVOC dehalogenation in groundwater can vary significantly, which has already been addressed and documented extensively (Johnson et al., 1996;Miehr et al., 2004;Ebert et al., 2006). ...
Zero-valent iron (ZVI) applied in engineered permeable reactive barriers (PRBs) as well as application of nano and/or micro scale ZVI emulsions (NZVIs) by in situ injection are effective in situ technologies for remediating groundwater plumes or source zones, respectively, which are contaminated by chlorinated volatile organic compounds (cVOCs) or certain heavy metals.
... The core is responsible for the reduction of the pollutants as its oxidation reaction in an aqueous environment generates appropriate reactants such as Fe(II), H/H 2 , and iron hydroxides and oxides [63]. The determined pollutant elimination pathways of nZVI consist of adsorption, complexation, coprecipitation, and surface-mediated chemical reduction [65]. One of the crucial factors controlling the complex interfacial reactions of degradation rate is pH [66À70]. ...
... Micro-and nano-scale zero-valent iron can also reduce nitrate (Young et al., 1964;Huang et al., 1998;Till et al., 1998;Alowitz and Scherer, 2002;Westerhoff, 2003) through the following equation: 4Fe 0 + NO 3 À + 7H 2 O $ 4Fe 2+ + NH 4 + + 10OH À (11) Alkalinity is produced in this process and, therefore, pH is an important parameter which can influence the reaction kinetics (Huang et al., 1998;Huang and Zhang;Choe et al., 2004;Miehr et al., 2004). Choe et al. (2004) investigated nitrate reduction by ZVI under anaerobic and various pH conditions. ...
... Tables 1 and 2 clearly relate the diversity among Fe 0 material tested or used for environmental remediation and water treatment. Although Fe 0 has been used in some 200 PBRs, little progress has been made toward characterizing the variability in reactivity among Fe 0 samples from different sources [132][133][134]142,143]. ...
There are many factors to consider for the design of appropriate water treatment systems including: cost, the concentration and type of biological and/or chemical contamination, concentration limits at which contaminant(s) are required to be removed, required flow rate, level of local expertise for on-going maintenance, and social acceptance. An ideal technology should be effective at producing clean, potable water; however it must also be low-cost, low-energy (ideally energy-free) and require low-maintenance. The use of packed beds containing metallic iron (Fe0 filters) has the potential to become a cheap widespread technology for both safe drinking water provision and wastewater treatment. Fe0 filters have been intensively investigated over the past two decades, however, sound design criteria are still lacking. This article presents an overview of the design of Fe0 filters for decentralized water treatment particularly in the developing world. A design for safe drinking water to a community of 100 people is also discussed as starting module. It is suggested that Fe0 filters have the potential for significant worldwide applicability, but particularly in the developing world. The appropriate design of Fe0 filters, however, is site-specific and dependent upon the availability of local expertise/materials.
... Reactive Fe 0 materials used for water filters are characterized mostly by their size/thickness of less that 5.0 mm [46][47][48][49]. A widely used Fe 0 sample from iPuTech (Rheinfelde, Germany) has a grain size varying from 0.3 to 2.0 mm [16,47]. ...
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.
... In the past decades, zero valent iron (ZVI) technologies have been proved as effective approaches to remove various aqueous organic/inorganic contaminants 1 . As a common and cost-effective transition metal, ZVI can act as not only reducing agent directly but also electron-donor in attending oxidative reactions [2][3][4][5][6][7][8] . It is well established that ZVI can be applied in ground water remediation for direct reduction of heavy metal ions e.g. ...
This study demonstrated the synergistic degradation of 4-chlorophenol (4-CP) achieved in a magnetic field (MF) enhanced zero-valent iron (ZVI)/H2O2 Fenton-like (FL) system and revealed an interesting correlative dependence relationship between MF and the pristine iron oxides layer (FexOy) on ZVI particles. First, a comparative investigation between the FL and MF-FL systems was conducted under different experimental conditions. The MF-FL system could suppress the duration of initial lag degradation phase one order of magnitude in addition of the significant enhancement in overall 4-CP degradation. Monitoring of intermediates/products indicated that MF would just accelerate the Fenton reactions to produce hydroxyl radical more rapidly. Evolutions of simultaneously released dissolved iron species suggested that MF would not only improve mass-transfer of the initial heterogeneous reactions, but also modify the pristine ZVI surface. Characterizations of the specific prepared ZVI samples evidenced that MF would induce a special evolution mechanism of the ZVI particles surface depending on the existence of FexOy layer. It comprised of an initial rapid point dissolution of FexOy and a following pitting corrosion of the exposed Fe0 reactive sites, finally leading to appearance of a particular rugged surface topography with numerous adjacent Fe0 pits and FexOy tubercles.
... The first records of the use of ZVI as a reactive material in PRBs were stated in the 1990s. Initially, the success of ZVI as a reactive media in PRB systems was typically related to the remediation of chlorinated organic contaminants, metals and radionuclides (Cundy et al, 2008) but rapidly was extend to denitrification, being, as well, a full-studied process since decades (Miehr et al, 2004;Puls, 2006;Henderson and Demond, 2007;Cundy, 2008). ...
It is unquestionable that an effective decision concerning the usage of a certain environmental clean-up technology should be conveniently supported. Significant amount of scientific work focussing on the reduction of nitrate concentration in drinking water by both metallic iron and nanomaterials and their usage in permeable reactive barriers has been worldwide published over the last two decades.This work aims to present in a systematic review of the most relevant research done on the removal of nitrate from groundwater using nanosized iron based permeable reactive barriers.The research was based on scientific papers published between 2004 and June 2014. It was performed using 16 combinations of keywords in 34 databases, according to PRISMA statement guidelines. Independent reviewers validated the selection criteria.From the 4161 records filtered, 45 met the selection criteria and were selected to be included in this review.This study's outcomes show that the permeable reactive barriers are, indeed, a suitable technology for denitrification and with good performance record but the long-term impact of the use of nanosized zero valent iron in this remediation process, in both on the environment and on the human health, is far to be conveniently known. As a consequence, further work is required on this matter, so that nanosized iron based permeable reactive barriers for the removal of nitrate from drinking water can be genuinely considered an eco-efficient technology.
... Correlations were generally best when compound subsets were considered separately (e.g., chlorinated alkanes and ethenes, chloro-and bromoalkanes) (17,28,35). This agrees with the finding that overarching relationships for different compound classes are intrinsically difficult to establish (41,42). ...
Degradation of chlorinated organic contaminants by natural and engineered reductive dechlorination reactions can occur via numerous biotic and abiotic transformation pathways giving rise to either benign or more toxic products. To assess whether dechlorination processes may lead to significant detoxification (a) the thermodynamic feasibility of a reaction, (b) rates of transformation, and (c) product formation routes need to be understood. To this end, fundamental knowledge of chlorohydrocarbon (CHC) reaction mechanisms is essential. We review insight from reaction thermodynamics, structure-reactivity relationships, and applications of radical and carbene traps, as well as of synthetic probe molecules. We summarize the state-of-knowledge about intermediates and reductive dechlorination pathways of vicinal and geminal haloalkanes, as well as of chlorinated ethenes. Transformation conditions are identified under which problematic products may be avoided. In an outlook, we discuss the potential of stable carbon and chlorine isotope fractionation to identify initial transformation mechanisms, competing transformation pathways, and common branching points.
... However, properties can be different even between manufacturing batches of the same nZVI type, and NP storage and handling can also affect particle reactivity. The reactivity realised in the subsurface is also a function of subsurface conditions, in particular of key water chemistry factors like solution pH, oxidationreduction potential and the presence of other reductates or anions such as carbonate and sulphate , Miehr 2004, US EPA 2008. ...
This report seeks to develop an understanding of the “value proposition” for iron nanoparticles/ nanoscale zero valent iron (nZVI) in remediation in terms of a risk benefit appraisal of its use given the current state of knowledge. Scenario analysis is used to explore likely market potential and the factors affecting this over the short, medium and longer term. An analysis of strengths, weaknesses, opportunities and threats (SWOT) and how these might change over time has been used to draw some conclusions about the broad actions that might support better exploitation of nanoremediation.
This report focusses on nZVI, as the most frequently used nanoparticle type. Future NanoRem work will involve undertaking similar analyses for other nanoparticles being developed by NanoRem. This report provides a brief overview and status quo of nZVI and an ¬outline risk benefit appraisal for the technology. It describes a scenario approach to understanding possible nanoremediation market trends. The process and findings of an expert consultation workshop conducted by NanoRem are discussed. A SWOT analysis is provided utilising collected information to establish the strengths, weaknesses, opportunities and threats of/to the nanoremediation market. Next steps are suggested.
Metallic iron (Fe0) is a reactive material for treating polluted water. The effect of water salinity on the efficiency of Fe0-based remediation systems is not yet established. This work aims to clar-ify the reasons why Cl– ions are often reported to improve the efficiency of Fe0/H2O remediation systems. Quiescent batch experiments were carried out to characterize the effect of chloride (Cl–) ions on the efficiency of methylene blue (MB) discoloration in the presence of Fe0. Cl– was used in the form of NaCl at concentrations ranging from 0 to 40 g L–1. The MB concentration was 10 mg L–1, the Fe0 loading was 5 g L–1, and the duration of the experiment varied from 2 to 46 days. Four different Fe0 materials were tested in parallel experiments. Tests with different NaCl levels were performed in parallel with three other organic dyes: Methyl orange (MO), orange II (OII), and reactive red 120 (RR 120). The results clearly show that the presence of Cl– reduces the extent of dye discoloration in all systems investigated. The efficiency of the dyes increased in the order MB < MO < RR 120 < OII. In systems with varying NaCl concentrations, dye discoloration ini-tially decreases with increasing NaCl and slightly increases for [NaCl] > 30 g L–1. However, the extent of dye discoloration for [NaCl] = 40 g L–1 remains much lower than for the system with [NaCl] = 0 g L–1. The results clearly demonstrate that the presence of Cl– fundamentally delays the process of contaminant removal in Fe0/H2O systems, thus improving the understanding of the contaminant interactions in Fe0-based remediation systems. These results also suggest that the effects of other inorganic anions on the efficiency of Fe0/H2O systems should be revisited for the design of field applications.
Granular metallic iron (gFe0) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe0 is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe0 based on the extent of iron dissolution in an ascorbic acid (AA-0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for FeII and a reducing agent for Fe III , AA induces the oxidative dissolution of Fe0 and the reductive dissolution of FeIII oxides from gFe0 specimens. In other words, Fe0 dissolution to FeII ions is promoted while the further oxidation to FeIII ions is blocked. Thus, unlike the EDTA method that promotes Fe0 oxidation to FeIII ions, the AA method promotes only the formation of FeII species, despite the presence of dissolved O2. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe0 specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g. accuracy in assessing Fe0 dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe0 in individual studies.
Zero-valent iron (ZVI) is one of the most widely used engineered materials for the remediation of chlorinated ethenes in the subsurface environment. The material has been widely used in various in situ remediation technologies including Fe-permeable reactive barriers (Fe-PRB) and subsurface injection of nanoscale zero-valent iron (nZVI) for contaminant plume attenuation and source zone remediation. However, there are two serious drawbacks when ZVI is used for the remediation of chlorinated ethenes: (i) ZVI tends to undergo rapid passivation which undermines its longevity for remediation applications, and (ii) ZVI has low dechlorination reactivity in the absence of catalyst additives. Bimetallic nanoparticles (BNPs), prepared by doping a small amount of catalytic metals (e.g., Pd or Ni), can significantly enhance the particles’ dechlorination reactivity. However, BNPs suffer rapid deactivation when exposed to groundwater media, and the BNPs deactivation mechanisms are still poorly understood.
The first part of this dissertation aims to investigate Ni-Fe BNPs and Pd-Fe BNPs deactivation mechanisms when exposing them to common groundwater solutes. Aging experiments were conducted by pre-immersing fresh prepared BNPs in solutions containing different groundwater solutes for 24 h prior to reacting the particles with trichloroethene (TCE) to assess their dechlorination reactivity. Analyses of reaction kinetics and product distribution and stable carbon isotope fractionation measurements suggest that Pd-Fe BNPs were sensitive to solute-induced deactivation, particularly in solutions containing chloride, bicarbonate, nitrate or sulfite ions. Although Ni-Fe BNPs possess higher electrochemical stability than Pd-Fe BNPs in the aqueous media, strong deactivation was observed in sulfate, nitrate, and phosphate solutions. Multiple modes of BNP deactivation were proposed for the two types of BNPs.
To overcome the intrinsic limitations of conventional and bimetallic ZVI materials, the second part of this dissertation aims to develop a new form of ZVI with higher dechlorination reactivity without the use of catalyst additives and a greater resistance to environmental passivation. A surface sulfidation treatment was designed and optimized for laboratory-made nZVI and commercial ZVI. The sulfided nZVI demonstrates remarkable improvements in dechlorination rates for chlorinated ethenes. Aging experiments indicated that sulfided nZVI possesses greater stability and maintains its dechlorination reactivity over long-term aging processes. Applying sulfidation treatment to commercial iron results in more efficient tetrachloroethene (PCE) and TCE degradation. Sulfidation treatment therefore represents a simple yet promising approach to increase the reactivity of ZVI using earth-abundant reagents in place of precious catalyst metals.
The U.S. Army is phasing out legacy munitions compounds that are prone to accidental detonation and replacing them with insensitive munitions compounds (IMCs). The major IMCs, namely 3‐nitro‐1,2,4‐triazol‐5‐one (NTO), 2,4‐dinitroanisole (DNAN), and nitroguanidine (NQ), are not compatible with existing munitions wastewater treatment technologies such as granular activated carbon due to their high water solubilities. In this study, a two‐stage process employing nanoscale zero‐valent iron (nZVI) and hydrogen peroxide (H2O2) was evaluated as a potential technology for the destructive treatment of IMC wastewater. In the first stage, nZVI rapidly and completely degraded all three IMCs and generated dissolved Fe(II). NTO and DNAN were degraded via nitro reduction to 3‐amino‐1,2,4‐triazol‐5‐one and 2,4‐diaminoanisole, respectively. In the second stage, H2O2 was added to oxidize the IMC reduction products through Fenton reaction utilizing the dissolved Fe(II) from the first stage. nZVI‐treated NTO and DNAN samples showed 66 % and 63 % TOC removal after oxidation, respectively. In contrast, NQ reduction products exhibited negligible mineralization. The results with individual IMCs were confirmed by an experiment using synthetic wastewater containing all three IMCs. This study illustrates the potential feasibility of a synergistic and destructive nZVI−H2O2 technology for treating IMC‐laden wastewaters at military facilities.
Environmental contamination is one of the important issues that the world is facing today and it is widening with each passing year and leading to deadly and harmful effect to the earth. Water is one of the world’s most abundant resources, but less than 1% of the global supply of water is available and adequate for human use. Owing to the rapid growth of urbanization, industrialization and modern agricultural practices, contamination of stream water or groundwater is on the rise. An increasing number of drinking water sources are showing evidence of pollution with organic and inorganic compounds. These environmental contaminants have a harmful effect on the human health; therefore, technologies for the removal of hazardous pollutants from waters are highly desired.
The Great East Japan Earthquake of magnitude 9 and resulting tsunami on March 11, 2011, triggered the Fukushima Daiichi Nuclear Power Plant disaster. A very large amount of radioactive materials was vented into the environment and subsequently caused serious radioactive contamination of the surrounding land, seawater and groundwater. In addition, with the rapidly increasing dependence on nuclear energy and the application of radionuclides in medicine, industry, agriculture and researches, treatment and disposal of radioactive wastewater has become one of the most significant challenges in nuclear waste management and nuclear industry to almost all countries. Some of the important radioactive isotopes in the liquid wastes are cesium (134Cs and 137Cs) and strontium (90Sr). Cesium and strontium are considered as the most dangerous radionuclides to human health due to their long half-lives, high solubility, high transferability, high concentrations in the wastewaters and easy assimilation in living organisms. The reported materials for removal of radioisotopes, such as zeolites, activated carbon, kaolinite and biomaterials have limited application since separation the suspended fine solids of these adsorbents from the medium after use is very difficult, which may increase the risk of secondary environmental pollution in practice. Accordingly, there is an urgent need to develop new technologies that more efficient and economic for the treatment of toxic environmental pollutants. The application of nanotechnology in environmental fields gives a new solution for cleaning pollution and improving the performance of traditional water and wastewater treatment techniques.
Nanoscale zero valent iron (nZVI) has been shown to be effective for environmental remediation of a wide variety of contaminants present in water resources including nitroaromatic compounds, perchlorate, azo dyes, halogenated hydrocarbons, nitrate, phosphate, arsenic and heavy metal ions. Due to its large surface area and high number of active sites, nZVI enhances the removal efficiencies of those contaminants significantly via diverse mechanisms. Moreover, recent investigations demonstrated nZVI can be utilized as a promising technology in the clean-up of radionuclides contaminated waters. Nevertheless, nZVI exhibits strong tendency to agglomerate to micron size particles because of the high surface energy and intrinsic magnetic interaction, which decrease efficiency by reducing surface area for reaction and limit their field scale application. Modifying the surface of nZVI and supporting nZVI particles on a carrier have been proposed to improve the reactivity of these nanoparticles and resolve the aforementioned issues.
Therefore, in this study, nZVI-based materials were applied to examine their effectiveness for removal of radioactive isotopes such as cesium and strontium from contaminated waters. In addition, the performance of nZVI-based materials for removal of nitrate in porous media using an upflow packed sand column was tested. The nZVI-based materials were synthesized and their physicochemical characteristics were analyzed by transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning electron microscope-elemental analysis (SEM-EDS). The effect of various operating variables such as initial metal ion concentration, contact time, initial solution pH, temperature, influence of competing ions and dosage of nZVI-based materials was investigated. The kinetic, thermodynamic and isotherm parameters of the removal process were also evaluated. Furthermore, the removal capacity was compared between synthesized nanomaterials and other reported adsorbents. Finally, nZVI-based materials were applied in simulated seawater or groundwater to demonstrate the reliability of the materials. The obtained results indicated that nZVI-based materials could be employed as promising methods for the removal of cesium and strontium from radioactive wastewaters. In addition, the results showed that nitrate removal in porous media could be enhanced effectively by using nZVI on the full length of porous media or using nano-Fe/Cu in multilayer porous media.
Due to the increasing diversity of organic contaminants discharged into anoxic water environments, reactivity prediction is necessary for chemical persistence evaluation for water treatment and risk assessment purposes. Almost all quantitative structure activity relationships (QSARs) that describe rates of contaminant transformation apply only to narrowly-defined, relatively homogenous families of reactants (e.g., dechlorination of alkyl halides). In this work, we develop predictive models for abiotic reduction of 60 organic compounds with diverse reducible functional groups, including nitroaromatic compounds (NACs), aliphatic nitro-compounds (ANCs), aromatic N-oxides (ANOs), isoxazoles (ISXs), polyhalogenated alkanes (PHAs), sulfoxides and sulfones (SOs), and others. Rate constants for their reduction were measured using a model reductant system, Fe(II)-tiron. Qualitatively, the rates followed the order NACs > ANOs ≈ ISXs ≈ PHAs > ANCs > SOs. To develop QSARs, both conventional chemical descriptor-based and machine learning (ML)-based approaches were investigated. Conventional univariate QSARs based on a molecular descriptor ELUMO (energy of the lowest-unoccupied molecular orbital) gave good correlations within classes. Multivariate QSARs combining ELUMO with Abraham descriptors for physico-chemical properties gave slightly improved correlations within classes for NCs and NACs, but little improvement in correlation within other classes or among classes. The ML model obtained covers reduction rates for all classes of compounds and all of the conditions studied with the prediction accuracy similar to those of the conventional QSARs for individual classes (r² = 0.41-0.98 for univariate QSARs, 0.71-0.94 for multivariate QSARs, and 0.83 for the ML model). Both approaches required a scheme for a priori classification of the compounds for model training. This work offers two alternative modelling approaches to comprehensive abiotic reactivity prediction for persistence evaluation of organic compounds in anoxic water environments.
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.
This study reports the equilibrium, long-term performance and mechanisms in removing Pb(II) ions by metallic iron/carbon (Fe⁰/C) ceramsites (FCC). The Pb(II) removal equilibrium data was analyzed using the Langmuir, Freundlich and Dubinin–Radushkevich isotherms. At the FCC dosage of 1.14 g L⁻¹, 95.97% of Pb(II) ions were removed from 50 mg L⁻¹ Pb(II) solution at initial pH 6.0. The Langmuir isotherm could fit well with the data at initial pH 3.0 with a maximum monolayer adsorption capacity of 112.36 mg g⁻¹ at 25 °C, while the data obtained at initial pH 6.0 could be described by the Freundlich model, indicating multilayer adsorption of Pb species on the FCC. Column tests demonstrated that FCC achieved the highest Pb(II) removal of 65.86% after 12 days' run compared to 32.35% for Fe⁰/activated carbon couples and only 1.24% for activated carbon. The X-ray diffraction and X-ray photoelectron spectroscopy analysis revealed that the PbO (dominant Pb species), Pb⁰, asisite and plumbojarosite appeared after Pb(II) removal. Scanning electron microscopy with energy dispersive X-ray spectroscopy showed that PbO particles with numerous structures were deposited on the FCC surface in a high amount. The decrease of the Fe/C mass ratio from 7.5 : 1 to 0.298 : 1 revealed that microscale Fe⁰ could been readily corroded by forming galvanic couples between Fe⁰ and carbon. The mechanisms of Pb(II) removal by the FCC were proposed.
Researchers on metallic iron (Fe0) for environmental remediation and water treatment are walking in a valley of confusion for 25 years. This valley is characterized by the propagation of different beliefs that have resulted from a partial analysis of the Fe0/H2O system as (i) a reductive chemical reaction was considered an electrochemical one and (ii) the mass balance of iron has not been really addressed. The partial analysis in turn has been undermining the scientific method while discouraging any real critical argumentation. This communication re-establishes the complex nature of the Fe0/H2O system while recalling that, finally, proper system analysis and chemical thermodynamics are the most confident ways to solve any conflicting situation in Fe0 environmental remediation.
Magnetic nanomaterials are making significant impact on improving the quality of human health that is tangible from a wide range of applications in various fields of medicine and biology. In recent years, nanoparticles successfully demonstrated outstanding applications due to having excellent magnetic properties of the iron oxide nanoparticles-based counterparts. Zero-valent iron nanoparticles toxicity is less than the toxicity of other nanoparticles. These nanoparticles are considerably potential for functionalization due to their highly reactive surfaces and this feature can facilitate targeted functionalization. This is very promising for various applications in different fields. Use of polymers as a protective agent is increasing sharply. Polymer magnetic field of zero-valent metal nanoparticles is still in its early stages. In this study, the syntheses of zero-valent iron nanoparticles are presented in different ways. Nanoparticles can be identified and evaluated through their size, shape, morphology, syntax, and various laboratory methods. In this context, magnetic nanoparticles can be potentially used in hyperthermia, magnetic resonance imaging (MRI), diagnosis and treatment of tumor diseases or cancer, labeling, biological separation, biotechnology, and eliminating major organic, inorganic and radioactive pollutants because of their high biocompatibility. With respect to the importance and the need to draw the attention of researchers to zero-valent iron magnetic nanoparticles, a brief description of various methods of synthesis, characterization, and their application in medicine and biology are reviewed. © 2016, Mazandaran University of Medical Sciences. All rights reserved.
The high reactivity of nano zerovalent iron (nZVI) leads to inefficient treatment due to competition with various natural reductant demand (NRD) processes, especially the reduction of water to hydrogen. Here we show that this limitation can be alleviated by sulfidation (i.e., modification by reducing sulfur compounds). nZVI synthesized on carboxylmethylcelluose (CMC-nZVI) was sulfidated with either sulfide or dithionite. The reactivity of the resulting materials was examined with three complementary assays: (i) direct measurement of hydrogen production, (ii) reduction of a colorimetric redox probe (indigo disulfonate, I2S), and (iii) dechlorination of trichloroethylene (TCE). The results indicate that sulfidation at S/Fe molar ratios of ≥0.3, effectively eliminates reaction with water, but retains significant reactivity with TCE. However, sulfidation with sulfide leaves most of the nZVI as Fe(0), whereas dithionite converts majority of the nZVI to FeS (thus consuming much of the reducing capacity originally provided by the Fe(0)). Simplified numerical models show that the reduction kinetics of I2S and TCE are mainly dependent on the initial reducing equivalents and that the TCE reduction rate is affected by the aging of FeS. Overall, the results suggest that pretreatment of nZVI with reducing sulfur compounds could result in substantial improvement in nZVI selectivity.
Dehalogenation is one of the highly important degradation reactions for halogenated organic compounds (HOCs) in the environment, which is also being developed as a potential type of the remediation technologies. In combination with the experimental results, intensive efforts have recently been devoted to the development of efficient theoretical methodologies (e.g. multi-scale simulation) to investigate the mechanisms for dehalogenation of HOCs. This review summarizes the structural characteristics of neutral molecules, anionic species and excited states of HOCs as well as their adsorption behavior on the surface of graphene and the Fe cluster. It discusses the key physiochemical properties (e.g. frontier orbital energies and thermodynamic properties) calculated at various levels of theory (e.g. semiempirical, ab initio, density functional theory (DFT) and the periodic DFT) as well as their connections to the reactivity and reaction pathway for the dehalogenation. This paper also reviews the advances in the linear and nonlinear quantitative structure-property relationship models for the dehalogenation kinetics of HOCs and in the mathematical modeling of the dehalogenation processes. Furthermore, prospects of further expansion and exploration of the current research fields are described in this article.
Published by Elsevier Ltd.
One of the important questions in the chemistry of pollutant degradation is the identity and distribution of chemical agents that are responsible for reduction reactions in the environment. Reduction occurs primarily in water-saturated environments, such as sediments, soils, and sludges. Redox indicators can be used (i) as chemical probes to obtain fundamental insights into biogeochemical processes and (ii) as the basis for demonstrations suitable for teaching aspects of environmental chemistry. This paper explores the latter with examples that involve a variety of indicators (indigo sulfonates, resazurin, etc.), environmental media (anaerobic sediments and granular iron metal), and physico-chemical processes (oxidation-reduction, adsorption, and diffusion). The results show that reduction by either media (sediments or iron metal) is primarily, although not always entirely, a surface reaction. This situation results in indicator behavior that is interesting and challenging for students with a wide range of backgrounds.
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 effect of adsorption to elemental iron on the reductive transformation of 2,4,6-trinitrotoluene (TNT) and hexahydro-1.3.5-trinitro-1.3,5-triazine (royal demolition explosive [RDX]) in aqueous solution Was Studied with scrap iron and high-purity iron. In batch experiments with the same total iron surface area and a mixing rate of 100 rpm. TNT and RDX were removed from the solution within 30 min, With high-purity iron, adsorbed TNT was reduced to 2,4.6-triaminotoluene (TAT) rapidly, with little accumulation of intermediates at the Surface. With scrap iron. the extent of adsorption of TNT and its daughter products was more significant and reduction of these adsorbed molecules to TAT was slower. Distribution of the intermediates indicated that the reaction with scrap iron Occurred primarily through reduction of the ortho nitro group. Kinetic analysis suggests that mass transfer or adsorption of TNT controlled the overall rate of TNT reduction to TAT with pure iron, whereas with scrap iron. the rate of TAT formation was probably limited by other processes. Compared to TNT. transformation of adsorbed RDX was more rapid and less affected by iron type. The RDX was reduced to an unidentified. water-soluble intermediate and NH(4)(+) which accounted for approximately 50%, of the RDX nitrogen. No total organic carbon reduction was observed before and after RDX transformation with scrap iron.
Specific features of cementation processes observed in numerous experimental studies are explained on the basis of a recently developed theory. This theory considers the kinetics of the oxidation reduction reactions and the possibility of either migrational or diffusional transport of the noble species. In particular, synergistic and antagonistic effects in simultaneous reductions of noble species, effects associated with anionic promoters, and effects of atmosphere are considered in the present paper.
Pathways and kinetics through which chlorinated ethylenes and their daughter products react with Fe(O) particles were investigated through batch experiments. Substantial intra- and interspecies inhibitory effects were observed, requiring the use of a modified Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model in which species compete for a limited number of reactive sites at the particle-water interface. Results indicate that reductive β-elimination accounts for 87% of tetrachloroethylene (PCE), 97% of trichloroethylene (TCE), 94% of cis-dichloroethylene (cis-DCE), and 99% of trans-dichloroethylene (Trans-DCE) reaction. Reaction of 1,1-DCE gives rise to ethylene, consistent with a reductive α-elimination pathway. For the highly reactive chloro- and dichloro-acetylene intermediates produced from the reductive elimination of TCE and PCE, 100% and 76% of the reaction, respectively, occur via hydrogenolysis to lessen chlorinated acetylenes. The branching ratios for reactions of PCE or TCE (and their daughter products) with iron particles are therefore such that production of vinyl chloride is largely circumvented. Reactivity of the chlorinated ethylenes decreases markedly with increasing halogenation, counter to the trend that might be anticipated if the rate-limiting step were to involve dissociative electron transfer. The authors propose that the reaction of vinyl halides proceeds via a di-Ï-bonded surface-bound intermediate. The reactivity trends and pathways observed in this work explain why lesser-chlorinated ethylenes have only been reported as minor products in prior laboratory and field studies of PCE and TCE reaction with Fe(O).
Nitrate transformation in artificial saturated soils made from sand and/or clay mixed with nitrate-contaminated water was investigated using (1) zero-valent iron treatment, (2) electrokinetics treatment alone, and (3) electrokinetics coupled with an iron treatment wall placed near the cathode or anode. Approximately 75-98% of the nitrates were transformed using zero- valent iron to treat the artificial soils. The major products in the zero- valent-assisted transformation were nitrogen gases (75-81%) and ammonia- nitrogen (2-18%). Nitrite-nitrogen was less than 1% in all these experiments. With electrokinetics alone, some nitrates were transformed (about 30%). Nitrate transformation was about 60% with the iron wall placed near the cathode, and was about 95% with the iron wall placed near the anode. The major products were nitrogen gases and ammonia. Nitrite-nitrogen was less than 1% in these runs. Electroosmotic permeability (Ke) variation appeared to be dependent on the voltage in the reactor. The pH variations over time in electrokinetics (control and iron wall near cathode) indicated the movement of a small acid front. However, the presence of an iron wall near the anode tended to increase the pH slightly. The pH variations over time in the electrokinetics coupled with an iron wall depend on the location of the iron wall. This study demonstrates that electrokinetics coupled with an iron wall near the anode is capable of remediating nitrate contamination in low permeability soils.
The recent successful adaptation of mainline computational chemistry codes to parallel computers introduces a new era of cost-effective, computer-intensive chemistry applications and paves the way for future applications on massively parallel centralized computers being developed under the High Performance Computer and Communications Initiative. Parallel computer architecture offers the promise of inexpensive supercomputing for the price of effort in algorithm adaptations to parallelism. In Chemical Sciences-supported work at Argonne, beginning efforts at algorithm changes in computational chemistry codes has resulted in program performances on the Group`s 12-processor Alliant computer superior to that on one-processor Cray X-MP or Y-MP computers. The effort so far has focused on sophisticated and highly accurate electronic structure production codes for determining the forces between atoms and molecules responsible for chemical structure, spectra, and reactivity. Some effort has also been invested in trajectory simulations of molecular dynamics. The American-made Alliant computer (model FX/2812) is one of the latest generation of shared-memory group- or division-size computers that generally cost about an order of magnitude less than the laboratory- or university-size computers such as Crays.
Medium basis sets based upon contractions of Gaussian primitives are developed for the third-row elements K through Zn. The basis functions generalize the 6-31G and 6-31G∗ sets commonly used for atoms up to Ar. They use six primitive Gaussians for 1s, 2s, 2p, 3s, and 3p orbitals, and a split-valence pair of three and one primitives for valence orbitals, which are 4s and 5p for atoms K and Ca, and 4s, 4p, and 3d for atoms Sc through Zn. A 6-31G∗ set is formed by adding a single set of Gaussian polarization functions to the 6-31G set. They are Cartesian d-functions for atoms K and Ca, and Cartesian f-functions for atoms Sc through Zn. Comparison with experimental data shows relatively good agreement with bond lengths and angles for representative vapor-phase metal complexes. © 1998 American Institute of Physics.
A set of rules for determining the atomic radii of spheres used to build the molecular cavities in continuum solvation models are presented. The procedure is applied to compute the hydration free energy for molecules containing H, C, N, O, F, P, S, Cl, Br, and I at a computational level (Hartree–Fock with a medium size basis set) allowing the study of relatively large systems. The optimized radii reduce the mean error with respect to the experimental solvation energies below 0.20 kcal/mol for a set of 43 neutral solutes and around 1 kcal/mol for 27 ions. Moreover the correct trends are observed for the solvation energies of homolog series, like the series ammonia–trimethylamine, that are not correctly reproduced by usual solvation models. © 1997 American Institute of Physics.
Irrigation drainage and industrial wastewaters often contain elevated levels of toxic oxyanions and oxycations such as selenate, chromate, and uranyl. A potential remediation method is to react contaminated water with zero-valent iron, which transforms the mobile contaminants into immobile forms. In this work, iron foil was exposed to aqueous solutions containing the relevant ions, and the reacted surfaces were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM images collected in situ show that the protrusions on the foil surface associated with iron oxides are smoothed out by the reaction. XPS indicates that partially reduced Se(IV) and Cr(III) are adsorbed on the surface, while uranium is deposited as U(VI), i.e., without reduction. More Se and Cr are deposited when the atmospheric gases are removed from solution because of the elimination of a competing process in which dissolved O2 increases the thickness of the iron oxide overlayer to the point where the reduction reaction is quenched. The amount of U deposited is greatly increased when the atmospheric gases are removed because of the elimination of dissolved CO2, which can form carbonate complexes with uranium.
This study evaluates the potential of using granular iron metal for the abiotic removal of the organic ground water pollutant trichloroethene (TCE) in the presence of the common inorganic co-contaminants chromate and nitrate, respectively. Our long-term column experiments indicate a competitive process between TCE dechlorination and reductive transformation of chromate and nitrate, which is reflected in a significantly delayed onset of TCE dechlorination. Delay times and therefore the ranges of the nonreactive flowpaths increased with increasing experimental duration, resulting in a migration of the contaminants through the iron metal treatment zone. The present investigation also indicates that the calculated migration rates of TCE and the added cocontaminants chromate and nitrate are linearly related to the initial content of the cocontaminants. With an average pore water velocity of 0.6 m/d and a surface area concentration of 0.55 m2/mL in the column, the calculated migration rates varled between 0.10 cm/d and 5.86 cm/d. The particular similarity between the values of TCE migration and the migration of the strong oxidants chromate and nitrate and the long-term steady state of the TCE dechlorination in the absence of the chromate and nitrate indicates that these competitive transformations are the driving force for the gradual passivation of the granular iron due to the buildup of an electrically insulating Fe(III)-oxyhydroxide. Based on these passivation processes, general formulae were developed that allow a simplified approximation of breakthrough times for the contaminants TCE, chromate, and nitrate.
The rate reduction of hexavalent chromium (Cr(VI)) by metallic iron under a range of conditions was studied in batch systems. The chemical variables studied were the Cr(VI) concentration, hydrogen ion concentration and surface area of iron. The influence of ionic strength and mixing rate was also examined. The reaction kinetics were found to be dependent on hydrogen ion concentration, hexavalent chromium concentration and iron surface area and to adhere to the following kinetic expression..The rate constant was evaluated and found to have a value of 5.45 × 10−5 1 cm−2 min−1 over a wide range of conditions.The rate constant was found to increase as mixing rate increased up to a maximum value beyond which the rate was essentially independent of mixing. Increases in ionic strength were found to result in a rapid decrease in the rate constant at ionic strengths below 0.1 M. Further increases in ionic strength had no detectable impact on the rate constant. All rate determination studies were run in the mixing and ionic strength independent regions of these systems.Reaction stoichiometry was found to be, with one exception, independent of environmental conditions. In general, 1.33 mol of iron dissolved for each mol of Cr(VI) reduced. This highly efficient utilization of iron in the reduction suggests that hydrogen generated during iron dissolution may be acting as a reductant for the Cr(VI). The single parameter which influenced the reaction stoichiometry was the initial Cr(VI) concentration. The ratio of Cr(VI) reduced to iron dissolved increased rapidly as the Cr(VI) concentration increased. This observation was taken as being consistant with a surface interaction between the hexavalent chromium and some metastable hydrogen species at the iron surface.
This review is predicated upon the need for a detailed process-level understanding of factors influencing the reduction of anthropogenic organic chemicals in natural aquatic systems. In particular, abiotic reductions of anthropogenic organic chemicals are reviewed. The most important reductive reaction is alkyl dehalogenation (replacement of chloride with hydrogen) which occurs in organisms, sediments, sewage sludge, and reduced iron porphyrin model systems. An abiotic mechanism involving a free radical intermediate has been proposed. The abstraction of vicinal dihalides (also termed dehalogenation) is another reduction that may have an abiotic component in natural systems. Reductive dehalogenation of aryl halides has recently been reported and further study of this reaction is needed. Several other degradation reactions of organohalides that occur in anaerobic environments are mentioned, the most important of which is dehydrohalogenation. The reduction of nitro groups to amines has also been thoroughly studied. The reactions can occur abiotically, and are affected by the redox conditions of the experimental system. However, a relationship between nitro-reduction rate and measured redox potential has not been clearly established. Reductive dealkylation of the N- and O-heteroatom of hydrocarbon pollutants has been observed but not investigated in detail. Azo compounds can be reduced to their hydrazo derivatives and a thorough study of this reaction indicates that it can be caused by extracellular electron transfer agents. Quinone-hydroquinone couples are important reactive groups in humic materials and similar structures in resazurin and indigo carmine make them useful as models for environmental redox conditions. The interconversion of sulfones, sulfoxides, and sulfides is a redox process and is implicated in the degradation of several pesticides though the reactions need more study. Two reductive heterocyclic cleavage reactions are also mentioned. Finally, several difficulties (both semantic and experimental) that recur in the studies reviewed are discussed. The subtle effects of various sterilization techniques on extracellular biochemicals and complex chemical reducing agents in sediment have stifled attempts to separate abiotic from biological degradation reactions. The characterization of redox conditions in a natural system is still problematic since measured redox potential is not adequate. Suggestions for future research toward a process-level understanding of abiotic chemical reductions are made.
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.
For neutral and charged species, atomic and molecular, a property called absolute hardness η is defined. Let E(N) be a ground-state electronic energy as a function of the number of electrons N. As is well-known, the derivative of E(N) with respect to N, keeping nuclear charges Z fixed, is the chemical potential μ or the negative of the absolute electronegativity χ: μ = (∂E/∂N)Z = -χ. The corresponding second derivative is hardness: 2η = (∂μ/∂N)Z = -(∂χ/∂N)Z = (∂2E/∂N2)Z. Operational definitions of χ and η are provided by the finite difference formulas (the first due to Mulliken) χ = 1/2(I + A), η = 1/2(I - A), where I and A are the ionization potential and electron affinity of the species in question. Softness is the opposite of hardness: a low value of η means high softness. The principle of hard and soft acids and bases is derived theoretically by making use of the hypothesis that extra stability attends bonding of A to B when the ionization potentials of A and B in the molecule (after charge transfer) are the same. For bases B, hardness is identified as the hardness of the species B+. Tables of absolute hardness are given for a number of free atoms, Lewis acids, and Lewis bases, and the values are found to agree well with chemical facts.
Sorption and reduction kinetics of trichloroethylene (ICE) and tetrachloroethylene (PCE) with metallic (zero-valent) iron were determined in a closed, well-mixed, anaerobic batch system by measuring aqueous and total system concentrations of the respective chlorinated solvent as a function of time. The reaction orders with respect to TCE and PCE total system concentrations were 2.7 and 1.3, respectively, indicating that the reaction mechanisms are complex. Both compounds exhibited nonlinear sorption behavior and could be fitted by the generalized Langmuir isotherm expression. After accounting for the mass sorbed to the iron, the reduction rates of PCE and TCE are first-order. This indicates that the bulk of sorption is to nonreactive sites. Competitive sorption was observed when both PCE and TCE were present; however, no competition for reaction was detected. The design and study of treatment systems for chlorinated solvents using metallic iron requires consideration of sorption processes.
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.
Chemical reduction is an alternative technique to remove nitrogen oxides from contaminated groundwater and closed-surface water body. Metallic iron was employed as a reductant for the reduction of nitrite in water in this study. The effect of pH on the rate and products of nitrite reduction was investigated with a fixed dosage of metallic iron powder (12 mol-Fe/mol-N, size of the powder: 80 mesh). The reduction of nitrite by metallic iron was a pseudo-zero-order reaction under the experimental conditions. The reduction rate of nitrite was increased with decreasing the pH of reaction solution, and the pseudo-zero-order reaction rate constants were 180, 130, 60, 15, 10, and 1 mM/h at pH = 2, 3, 4, 5, 6, and 7, respectively. The reduction products of nitrite were nitrogen gas and ammonium. The yields of nitrogen gas from nitrite reduction were 0.63, 0.74, 0.81, 0.87, 0.92, and 0.98 as molar ratios of nitrogen atom at pH =2, 3, 4, 5, 6, and 7, respectively. Neutral condition enhanced the formation of nitrogen gas from nitrite reduction.
The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.
Permeable walls of granular iron are a new technology developed for the treatment of groundwater contaminated with dissolved chlorinated solvents. Degradation ofthe chlorinated solvents involves a charge transfer process in which they are reductively dechlorinated, and the iron is oxidized. The iron used in the walls is an impure commercial material that is covered with a passive layer of Fe2O3, formed as a result of a high-temperature oxidation process used in the production of iron. Understanding the behaviour of this layer upon contact with solution is important, because Fe2O3 inhibits mechanisms involved in contaminant reduction, including electron transfer and catalytic hydrogenation. Using a glass column specially designed to allow for in situ Raman spectroscopic and open circuit potential measurements, the passive layer of Fe2O3 was observed to be largely removed from the commercial product, Connelly iron, upon contact with Millipore water and with a solution of Millipore water containing 1.5 mg/l trichloroethylene (TCE). It has been previously shown that Fe2O3 is removed from iron surfaces upon contact with solution by an autoreduction reaction; however, prior to this work, the reaction has not been shown to occur on the impure commercial iron products used in permeable granular iron walls. The rate of removal was sufficiently rapid such that the initial presence of Fe2O3 at the iron surface would have no consequence with respect to the performance of an in situ wall. Subsequent to the removal of Fe2O3 layer, magnetite and green rust formed at the iron surface as a result of corrosion in both the Millipore water and the solution containing TCE. The formation of these two species, rather than higher valency iron oxides and oxyhydroxides, is significant for the technology. The former can interfere with contaminant degradation because they inhibit electron transfer and catalytic hydrogenation. Magnetite and green rust, in contrast, will not inhibit the mechanisms involved in contaminant reduction, and hence their formation is beneficial to the long-term performance of the iron material.
The effect of precipitates on the reactivity of iron metal (Fe0) with 1,1,1-trichloroethane (TCA) was studied in batch systems designed to model groundwaters that contain dissolved carbonate species (i.e., C(IV)). At representative concentrations for high-C(IV) groundwaters (approximately 10(-2) M), the pH in batch reactors containing Fe0 was effectively buffered until most of the aqueous C(IV) precipitated. The precipitate was mainly FeCO3 (siderite) but may also have included some carbonate green rust. Exposure of the Fe0 to dissolved C(IV) accelerated reduction of TCA, and the products formed under these conditions consisted mainly of ethane and ethene, with minor amounts of several butenes. The kinetics of TCA reduction were first-order when C(IV)-enhanced corrosion predominated but showed mixed-order kinetics (zero- and first-order) in experiments performed with passivated Fe0 (i.e., before the onset of pitting corrosion and after repassivation by precipitation of FeCO3). All these data were described by fitting a Michaelis-Menten-type kinetic model and approximating the first-order rate constant as the ratio of the maximum reaction rate (Vm) and the concentration of TCA at half of the maximum rate (K(1/2)). The decrease in Vm/K(1/2) with increasing C(IV) exposure time was fit to a heuristic model assuming proportionality between changes in TCA reduction rate and changes in surface coverage with FeCO3.
Sufficient kinetic data on abiotic reduction reactions involving organic contaminants are now available that quantitative structure-activity relationships (QSARs) for these reactions can be developed. Over 50 QSARs have been reported, most in just the past few years, and they are summarized as a group here. The majority of these QSARs concern dechlorination reactions, and most of the rest concern nitro reduction reactions. Most QSARs for reduction reactions have been developed mainly as diagnostic tools for determining reduction mechanisms and pathways. So far, only a few of these QSARs are sufficiently precise in formulation, yet general in scope, that they might be useful for predicting contaminant fate. Achieving the goal of developing predictive models for the kinetics of contaminant reduction in the environment will require a delicate balance between process-level rigor and practical levels of approximation.
- R M Baxter
Baxter, R. M. Water Poll. Res. J. Canada 1989, 24, 299-322.
- J G Hering
- S Kreamer
- D L Macalady
- Ed Oxford
Hering, J. G.; Kreamer, S. In Perspectives in Environmental
Chemistry; Macalady, D. L., Ed.; Oxford: New York, 1998; pp
57-74.
In Handbook of Property Estimation Methods for Chemicals: Environmental and Health Sciences
- P G Tratnyek
- D L Macalady
Tratnyek, P. G.; Macalady, D. L. In Handbook of Property
Estimation Methods for Chemicals: Environmental and Health
Sciences;
- M M Scherer
- K Johnson
- J C Westall
- P G Tratnyek
Scherer, M. M.; Johnson, K.; Westall, J. C.; Tratnyek, P. G. Environ.
Sci. Technol. 2001, 35, 2804-2811.
- W J Hehre
- R Ditchfield
- J A Pople
Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257-2261.
- J D Dill
- J A Pople
Dill, J. D.; Pople, J. A. J. Chem. Phys. 1975, 62, 2921-2923.
- M M Francl
- W J Pietro
- W J Hehre
- J S Binkley
- M S Gordon
- D J Defrees
- J A Pople
Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654-3665.
- C Lee
- W Yang
- R G Parr
Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785-789.
- A D Becke
Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
- N Godbout
- D R Salahub
- J Andelm
- E Wimmer
Godbout, N.; Salahub, D. R.; Andelm, J.; Wimmer, E. Can. J.
Chem. 1992, 70, 560-571.
- A Klamt
- G J Schüü Rmann
Klamt, A.; Schüü rmann, G. J. Chem. Soc., Perkin Trans. 2 1993,
799-803.
- W F Wü St
- R Köber
- O Schlicker
- A Dahmke
Wü st, W. F.; Köber, R.; Schlicker, O.; Dahmke, A. Environ. Sci.
Technol. 1999, 33, 4304-4309.
- R M Allen-King
- R M Halket
- D R Burris
Allen-King, R. M.; Halket, R. M.; Burris, D. R. Environ. Toxicol.
Chem. 1997, 16, 424-429.
- D R Burris
- R M Allen-King
- V S Manoranjan
- T J Campbell
- G A Loraine
- B Deng
Burris, D. R.; Allen-King, R. M.; Manoranjan, V. S.; Campbell,
T. J.; Loraine, G. A.; Deng, B. J. Environ. Eng. 1998, 124, 1012-1019.
- T L Johnson
- M M Scherer
- P G Tratnyek
Johnson, T. L.; Scherer, M. M.; Tratnyek, P. G. Environ. Sci.
Technol. 1996, 30, 2634-2640.
- R W Gillham
- S F O'hannesin
Gillham, R. W.; O'Hannesin, S. F. Ground Water 1994, 32, 958-967.
- T J Campbell
- D R Burris
- A L Roberts
- Wells
Campbell, T. J.; Burris, D. R.; Roberts, A. L.; Wells, J. R. Environ.
Toxicol. Chem. 1997, 16, 625-630.
- B M Khudenko
Khudenko, B. M. J. Environ. Eng. 1987, 113, 681-702.
- Y Ku
- C.-H Chen
Ku, Y.; Chen, C.-H. Separ. Sci. Technol. 1992, 27, 1259-1275.
- A Kuhn
Kuhn, A. Met. Finishing 1993, 91, 25-31.
- D W Blowes
- C J Ptacek
- S G Benner
- C W T Mcrae
- T A Bennett
- R W Puls
Blowes, D. W.; Ptacek, C. J.; Benner, S. G.; McRae, C. W. T.;
Bennett, T. A.; Puls, R. W. J. Contam. Hydrol. 2000, 45, 123-137.
- A R Pratt
- D W Blowes
- C Ptacek
Pratt, A. R.; Blowes, D. W.; Ptacek, C. J. Environ. Sci. Technol.
1997, 31, 2492-2498.
- T Astrup
- S L S Stipp
- T H Christensen
Astrup, T.; Stipp, S. L. S.; Christensen, T. H. Environ. Sci. Technol.
2000, 34, 4163-4168.
- C.-P Huang
- H.-W Wang
- P.-C Chiu
Huang, C.-P.; Wang, H.-W.; Chiu, P.-C. Water Res. 1998, 32,
2257-2264.
- D P Siantar
- C G Schreier
- C.-S Chou
- M Reinhard
Siantar, D. P.; Schreier, C. G.; Chou, C.-S.; Reinhard, M. Water
Res. 1996, 30, 2315-2322.
- L I Zawaideh
- T C Zhang
Zawaideh, L. I.; Zhang, T. C. Water Sci. Technol. 1998, 38, 107-115.
- P G Tratnyek
- N L Wolfe
Tratnyek, P. G.; Wolfe, N. L. Environ. Toxicol. Chem. 1990, 9,
289-295.
- P G Tratnyek
- D L Macalady
Tratnyek, P. G.; Macalady, D. L. J. Agric. Food Chem. 1989, 37,
248-254.