Insights into the contrasting effects of sulfidation on dechlorination of chlorinated aliphatic hydrocarbons by zero-valent iron.

Contrasting effects of sulfidation on contaminants reduction by zero-valent iron (ZVI) has been reported in literature but the underlying mechanisms remain unclear. Here, under well-controlled conditions, we compared the performance of ZVI and sulfidated ZVI (S-ZVI) toward a series of chlorinated compounds. Results revealed that, although S-ZVI was more reactive than ZVI toward hexachloroethane, pentachloroethane, tetrachloroethylene, and trichloroethene, sulfidation hindered the dechlorination of the other ten tested chlorinated aliphatics by a factor of 1.5-125. Moreover, S-ZVI may lead to an accumulation of toxic partially-dechlorinated products. Analogous to its effects on ZVI reactivity, sulfidation also exerted positive, negligible, or negative effects on the electron efficiency of ZVI. Solvent kinetic isotope effect analysis suggested that direct electron transfer rather than reaction with atomic hydrogen was the dominant reduction mechanism in S-ZVI system. Hence, the sulfidation enhancing effects could be expected only when direct electron transfer is the preferred reduction route for target contaminants. Furthermore, linear free energy relationships analysis indicated one-electron reduction potential could be used to predict the transformation of chlorinated ethanes by S-ZVI, whereas for chlorinated ethenes, their adsorption properties on S-ZVI determined the dechlorination process. All these findings may offer guidance for the decision-making regarding the application of S-ZVI.

Abiotic reductive removal of organic contaminants catalyzed by carbon materials: A short review.

Since the observation that carbon materials can facilitate electron transfer between reactants, there is growing literature on the abiotic reductive removal of organic contaminants catalyzed by them. Most of the interest in these processes arises from the participation of carbon materials in the natural transformation of contaminants and the possibility of developing new strategies for environmental treatment and remediation. The combinations of various carbon materials and reductants have been investigated for the reduction of nitro-organic compounds, halogenated organics, and azo dyes. The reduction rates of a certain compound in carbon-reductant systems vary with the surface properties of carbon materials, although there are controversial conclusions on the properties governing the catalytic performance. This review scrutinizes the contributions of quinone moieties, electron conductivity, and other carbon properties to the activity of carbon materials. It also discusses the contaminant-dependent reduction pathways, that is, electron transfer through conductive carbon and intermediates formed during the reaction, along with possibly additional activation of contaminant molecules by carbon. Moreover, modification strategies to improve the catalytic activity for reduction are summarized. Future research needs are proposed to advance the understanding of reaction mechanisms and improve the practical utility of carbon material for water treatment. PRACTITIONER POINTS: Reduction rates of contaminants in carbon-reductant systems and modification strategies for carbon materials are summarized. Mechanisms for the catalytic activity of carbon materials are discussed. Research needs for new insights into carbon-catalyzed reduction are proposed.

Enhanced trichloroethylene dechlorination by carbon-modified zero-valent iron: Revisiting the role of carbon additives.

Given that there are still some debates on the influence of carbon modification on zerovalent iron (ZVI) decontamination process, the roles of carbon on trichloroethylene (TCE) reduction by ZVI were re-investigated in this work. Compared to activated carbons (AC) with high adsorption ability, carbon fibers (CF) with good electronic conductivity performed much better in enhancing ZVI performance in terms of both reactivity and selectivity. Moreover, it was interesting to observe that a low carbon loading is sufficient to effectively improve TCE reduction and this promoting effect would decline with further increasing the carbon amounts from 1.0 wt.% to 50 wt.%. Regarding to the ZVI selectivity, a relatively high carbon loading (especially for CF, it may be as high as 50 wt.%) was needed to protect ZVI from non-productive reactions with H2O/H+ effectively. However, a mixture of 10 wt.% AC and 1.0 wt.% CF could combine their respective merits of inhibiting side reactions and enhancing TCE reduction, and thus simultaneously enhanced the reactivity and selectivity of ZVI. Mechanistic investigations revealed that carbon modification could enhance the ZVI performance through improving TCE adsorption and/or accelerating electron transfer, while the latter one may play a more important role especially at high carbon loadings.

Ferrous ion mitigates the negative effects of humic acid on removal of 4-nitrophenol by zerovalent iron.

In this study, Fe2+ addition was employed to overcome the negative effects of humic acid (HA) on contaminant removal by zerovalent iron (ZVI), and its feasibility to improve electron efficiency of ZVI was also tested. HA at high concentrations suppressed the removal of 4-nitrophenol (4-NP) by ZVI, while the addition of 0.25-1.0 mM Fe2+ could greatly mitigate this inhibitory effect and enhance 4-NP reduction. Specifically, with a mixed-order model, global fitting results showed that the addition of Fe2+ increased the rate constant from 0.124 × 10-2-0.219 × 10-2 mM/min to 0.227 × 10-2-0.417 × 10-2 mM/min and shortened lag period from 19.7-47.9 min to 8.0-15.2 min for 4-NP removal. The mechanistic investigation revealed this trend could be explained by the following aspects: i) Fe2+ can facilitate the generation of Fe(II)-containing oxides, which can act as an electron mediator or direct electron donor for 4-NP reduction; ii) the presence of Fe2+ could lead to aggregation of HA particles and accordingly reduced its coverage on ZVI surface. But the results of respike experiments indicate that Fe2+ addition did not show remarkable effect on the electron efficiency of 4-NP by ZVI, which should be associated with that Fe2+ was not able to favor the enrichment of 4-NP on ZVI surface.

Effects of Sulfidation and Nitrate on the Reduction of N-Nitrosodimethylamine by Zerovalent Iron.

Competition among oxidizing species in groundwater and wastewater for the reductive capacity of zerovalent iron (ZVI) makes the selectivity of ZVI for target contaminant degradation over other reduction pathways a major determinant of the feasibility of ZVI-based water treatment processes. The selectivity for reduction of contaminants over water is improved by sulfidation, but the effect of sulfidation on other competing reactions is not known. The interaction between these competing reactions was investigated using N-nitrosodimethylamine (NDMA) as the target contaminant, nitrate as a co-contaminant, and micrometer-sized ZVI with and without sulfidation. Unsulfidated ZVI reduced NDMA to dimethylamine via N,N-dimethylhydrazine, but the addition of nitrate decreased the rate of NDMA reduction and increased the quantity of intermediate observed. With sulfidated ZVI, the kinetics and products of NDMA reduction were similar to those with unsulfidated ZVI, but no inhibitory effect of nitrate was observed. Conversely, the reduction of nitrate-which dominated NDMA reduction in unsulfidated ZVI systems-was strongly inhibited by sulfidation. H2 and Fe2+ generation by sulfidated ZVI was almost independent of nitrate concentration. Therefore, sulfidation improved the efficiency of NDMA reduction by ZVI in the presence of nitrate mainly by inhibiting nitrate reduction. The shift in selectivity of ZVI for NDMA over nitrate upon sulfidation was due to replacement of Fe0/FexOy surface sites with FeS.

Characterization methods of zerovalent iron for water treatment and remediation.

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.

Modeling the Kinetics of Hydrogen Formation by Zerovalent Iron: Effects of Sulfidation on Micro- and Nano-Scale Particles.

The hydrogen evolution reaction (HER) that generates H2 from the reduction of H2O by Fe0 is among the most fundamental of the processes that control reactivity in environmental systems containing zerovalent iron (ZVI). To develop a comprehensive kinetic model for this process, a large and high-resolution data set for HER was measured using five types of ZVI pretreated by acid-washing and/or sulfidation (in pH 7 HEPES buffer). The data were fit to four alternative kinetic models using nonlinear regression analysis applied to the whole data set simultaneously, which allowed some model parameters to be treated globally across multiple experiments. The preferred model uses two independent reactive phases to match the two-stage character of most HER data, with rate constants ( k's) for each phase fitted globally by iron type and phase quantities ( S's) fitted as fully local (independent) parameters. The first, faster stage was attributed to a reactive mineral intermediate (RMI) phase like Fe(OH)2, which may form in all experiments during preequilibration, but is rapidly consumed, leaving the second, slower stage of HER, which is due to reaction of Fe0. In addition to providing a deterministic model to explain the kinetics of HER by ZVI over a wide range of conditions, the results provide an improved quantitative basis for comparing the effects of sulfidation on ZVI.

Coupled Effect of Ferrous Ion and Oxygen on the Electron Selectivity of Zerovalent Iron for Selenate Sequestration.

Although the electron selectivity (ES) of zerovalent iron (ZVI) for target contaminant and its utilization ratio (UR) decide the removal capacity of ZVI, little effort has been made to improve them. Taking selenate [Se(VI)] as a target contaminant, this study investigated the coupled influence of aeration gas and Fe(II) on the ES and UR of ZVI. Oxygen was necessary for effective removal of Se(VI) by ZVI without Fe(II) addition. Due to the application of 1.0 mM Fe(II), the ES of ZVI was increased from 3.2-3.6% to 6.2-6.8% and the UR of ZVI was improved by 5.0-19.4% under aerobic conditions, which resulted in a 100-180% increase in the Se(VI) removal capacity by ZVI. Se(VI) reduction by Fe0 was a heterogeneous redox reaction, and the enrichment of Se(VI) on ZVI surface was the first step of electron transfer from Fe0 core to Se(VI). Oxygen promoted the generation of iron (hydr)oxides, which facilitated the enrichment of Se(VI) on the ZVI particle surface. Therefore, the high oxygen fraction (25-50%) in the purging gas resulted in only a slight decrease in the ES of ZVI. Fe(II) addition resulted in a pH drop and promoted the generation of lepidocrocite and magnetite, which benefited Se(VI) adsorption and the following electron transfer from underlying Fe0 to surface-located Se(VI).

Combined Effect of Weak Magnetic Fields and Anions on Arsenite Sequestration by Zerovalent Iron: Kinetics and Mechanisms.

In this study, the effects of major anions (e.g., ClO4-, NO3-, Cl-, and SO42-) in water on the reactivity of zerovalent iron (ZVI) toward As(III) sequestration were evaluated with and without a weak magnetic field (WMF). Without WMF, ClO4- and NO3- had negligible influence on As(III) removal by ZVI, but Cl- and SO42- could improve As(III) sequestration by ZVI. Moreover, the WMF-enhancing effect on As(III) removal by ZVI was minor in ultrapure water. A synergetic effect of WMF and individual anion on improving As(III) removal by ZVI was observed for each of the investigated anion, which became more pronounced as the concentration of anion increased. Based on the extent of enhancing effects, these anions were ranked in the order of SO42- > Cl- > NO3- ≈ ClO4- (from most- to least-enhanced). Furthermore, the inhibitory effect of HSiO3-, HCO3-, and H2PO4- on ZVI corrosion could be alleviated taking advantage of the combined effect of WMF and SO42-. The coupled influence of anions and WMF was associated with the simultaneous movement of anions with paramagnetic Fe2+ to keep local electroneutrality in solution. Our findings suggest that the presence of anions is quite essential to maintaining or stimulating the WMF effect.

Premagnetization for Enhancing the Reactivity of Multiple Zerovalent Iron Samples toward Various Contaminants.

Premagnetization was applied to enhance the removal of various oxidative contaminants (including amaranth (AR27), lead ion (Pb(2+)), cupric ion (Cu(2+)), selenite (Se(4+)), silver ion (Ag(+)), and chromate (Cr(6+))) by zerovalent iron (ZVI) from different origins under well-controlled experimental conditions. The rate constants of contaminants by premagnetized ZVI (Mag-ZVI) samples were 1.2-12.2-fold greater than those by pristine ZVI (Pri-ZVI) samples. Generally, there was a linear correlation between the specific reaction rate constants (kSA) of one particular contaminant removal by various Pri-ZVI or Mag-ZVI samples and those of the other contaminant, which could be successfully employed to predict the kSA of one contaminant by one ZVI sample if kSA of the other contaminant by this ZVI sample was available. The specific rate constant of Fe(II) release at pH 4.0 was proposed in this study to stand for the intrinsic reactivity of a ZVI sample. All Mag-ZVI samples had higher intrinsic reactivity than their counterparts without premagnetization. There were strong correlations between the intrinsic reactivity of various Pri-ZVI/Mag-ZVI samples and the removal rate constants of a specific contaminant by these ZVI samples not only at pH 4.0 when the intrinsic reactivity was determined but also at other pH levels. This correlation could be employed to predict the removal rate constant of this contaminant by a ZVI sample that was not included in the original data set once the intrinsic reactivity of the ZVI sample was known.

Weak magnetic field accelerates chromate removal by zero-valent iron.

Weak magnetic field (WMF) was employed to improve the removal of Cr(VI) by zero-valent iron (ZVI) for the first time. The removal rate of Cr(VI) was elevated by a factor of 1.12-5.89 due to the application of a WMF, and the WMF-induced improvement was more remarkable at higher Cr(VI) concentration and higher pH. Fe2+ was not detected until Cr(VI) was exhausted, and there was a positive correlation between the WMF-induced promotion factor of Cr(VI) removal rate and that of Fe2+ release rate in the absence of Cr(VI) at pH4.0-5.5. These phenomena imply that ZVI corrosion with Fe2+ release was the limiting step in the process of Cr(VI) removal. The superimposed WMF had negligible influence on the apparent activation energy of Cr(VI) removal by ZVI, indicating that WMF accelerated Cr(VI) removal by ZVI but did not change the mechanism. The passive layer formed with WMF was much more porous than without WMF, thereby facilitating mass transport. Therefore, WMF could accelerate ZVI corrosion and alleviate the detrimental effects of the passive layer, resulting in more rapid removal of Cr(VI) by ZVI. Exploiting the magnetic memory of ZVI, a two-stage process consisting of a small reactor with WMF for ZVI magnetization and a large reactor for removing contaminants by magnetized ZVI can be employed as a new method of ZVI-mediated remediation.

The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zero-valent iron technology in the last two decades (1994-2014).

Over the past 20 years, zero-valent iron (ZVI) has been extensively applied for the remediation/treatment of groundwater and wastewater contaminated with various organic and inorganic pollutants. Based on the intrinsic properties of ZVI and the reactions that occur in the process of contaminants sequestration by ZVI, this review summarizes the limitations of ZVI technology and the countermeasures developed in the past two decades (1994-2014). The major limitations of ZVI include low reactivity due to its intrinsic passive layer, narrow working pH, reactivity loss with time due to the precipitation of metal hydroxides and metal carbonates, low selectivity for the target contaminant especially under oxic conditions, limited efficacy for treatment of some refractory contaminants and passivity of ZVI arising from certain contaminants. The countermeasures can be divided into seven categories: pretreatment of pristine ZVI to remove passive layer, fabrication of nano-sized ZVI to increase the surface area, synthesis of ZVI-based bimetals taking advantage of the catalytic ability of the noble metal, employing physical methods to enhance the performance of ZVI, coupling ZVI with other adsorptive materials and chemically enhanced ZVI technology, as well as methods to recover the reactivity of aged ZVI. The key to improving the rate of contaminants removal by ZVI and broadening the applicable pH range is to enhance ZVI corrosion and to enhance the mass transfer of the reactants including oxygen and H(+) to the ZVI surface. The characteristics of the ideal technology are proposed and the future research needs for ZVI technology are suggested accordingly.