Redox transformation of soil minerals and arsenic in arsenic-contaminated soil under cycling redox conditions.

Changes in the saturation degree of aquifers control the geochemical reactions of redox-sensitive elements such as iron (Fe), sulfur (S), and arsenic (As). In this study, the effects of redox conditions and the presence of Fe and S on the behavior of As in a soil environment were investigated by observation in a batch experimental system. Arsenic was stable on Fe(III) solid surface in an oxidizing environment but was easily released into the aqueous phase following the reductive dissolution of Fe during an anoxic period. The alternating redox cycles led to a change in the concentrations of Fe, S, and As in both the aqueous and solid phases. The composition of Fe minerals changed to a less crystalline phase while that of solid phase As changed to a more reduced phase in both the As-contaminated natural soil and FeS-amended soil batch systems. This tendency was more prominent in the batch containing higher amounts of total Fe and S. These results show that a redox cycle can increase the possibility of As contamination of groundwater during dissolution and reprecipitation of Fe minerals and simultaneous microbial reduction of S and/or As species.

pH impact on reductive dechlorination of cis-dichloroethylene by Fe precipitates: an X-ray absorption spectroscopy study.

The pH impact on reductive dechlorination of cis-dichloroethylene (cis-DCE) was investigated using in situ Fe precipitates formed under iron-rich sulfate-reducing conditions. The dechlorination rate of cis-DCE increased with pH, which was attributed to changes in the solid-phase Fe concentration, the composition of Fe minerals, and the surface speciation of Fe minerals. With increasing pH, larger quantities of Fe minerals, having much greater reactivity than dissolved Fe(II), were produced. Fe-K edge X-ray absorption spectroscopy (XAS) analysis of Fe precipitates revealed the presence of multiple Fe phases with their composition varying with pH. Correlation analyses were performed to examine how the solid-phase Fe concentration, the composition of Fe minerals, and their surface speciation were linked with the cis-DCE dechlorination rate. Such analyses revealed that neither mackinawite (FeS) nor magnetite (Fe3O4) was reactive with cis-DCE dechlorination, but that Fe (oxyhydr)oxides including green rusts and Fe(OH)2 were reactive. Based on a proposed model of the surface acidity of Fe minerals, the increasing deprotonated surface Fe(II) groups with pH correlated well with the enhanced cis-DCE dechlorination.

Kinetic study of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) dechlorination using green rusts formed under varying conditions.

Abiotic degradation of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) was investigated using Fe hydroxides obtained by hydrolyzing Fe(II) salts over a pH range of 7.7-8.0. Within this narrow pH range, a green rust (GR) precipitated. The dechlorination reactivity of the resulting GR precipitates increased with the dissolved Fe(II) concentration remaining in solution after precipitation. Controls run using only the dissolved Fe(II) supernatant were not reactive, suggesting the relative amount of Fe(II) on the surface of precipitated GRs was the causative agent in the relative reactivity. To test this, a series of GR batches with varying dissolved Fe(II) concentrations were prepared by acid-base titration and examined for cis-DCE and VC dechlorination kinetics under reducing conditions. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses of these batches were performed to characterize the bulk mineralogy and the excess surface Fe(II), respectively. Cis-DCE and VC dechlorination results along with solid phase characterization show that different surface Fe(II)/Fe(III) compositions are responsible for the different reactivity of GRs formed within the GR precipitation zone.

X-ray absorption and photoelectron spectroscopic study of the association of As(III) with nanoparticulate FeS and FeS-coated sand.

Iron sulfide (FeS) has been demonstrated to have a high removal capacity for arsenic (As) in reducing environments. However, FeS may be present as a coating, rather than in nanoparticulate form, in both natural and engineered systems. Frequently, the removal capacity of coatings may be different than that of nanoparticulates in batch systems. To assess the differences in removal mechanisms between nanoparticulate FeS and FeS present as a coating, the solid phase products from the reaction of As(III) with FeS-coated sand and with suspensions of nanoparticulate (NP) FeS were determined using x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. In reaction with NP FeS at pH 5, As(III) was reduced to As(II) to form realgar (AsS), while at pH 9, As(III) adsorbed as an As(III) thioarsenite species. In contrast, in the FeS-coated sand system, As(III) formed the solid phase orpiment (As(2)S(3)) at pH 5, but adsorbed as an As(III) arsenite species at pH 9. These different solid reaction products are attributed to differences in FeS concentration and the resultant redox (pe) differences in the FeS-coated sand system versus suspensions of NP FeS. These results point to the importance of accounting for differences in concentration and redox when making inferences for coatings based on batch suspension studies.

Abiotic reductive dechlorination of cis-dichloroethylene by Fe species formed during iron- or sulfate-reduction.

This study investigated reductive dechlorination of cis-dichloroethylene (cis-DCE) by the reduced Fe phases obtained from in situ precipitation, which involved mixing of Fe(II), Fe(III), and S(-II) solutions. A range of redox conditions were simulated by varying the ratio of initial Fe(II) concentration ([Fe(II)](o)) to initial Fe(III) concentration ([Fe(III)](o)) for iron-reducing conditions (IRC) and the ratio of [Fe(II)](o) to initial sulfide concentration ([S(-II)](o)) for sulfate-reducing conditions (SRC). Significant dechlorination of cis-DCE occurred under highly reducing IRC and iron-rich SRC, suggesting that Fe (oxyhydr)oxides including green rusts are highly reactive with cis-DCE but that Fe sulfide as mackinawite (FeS) is nonreactive. Relative concentrations of sulfate to chloride were also varied to examine the anion impact on cis-DCE dechlorination. Generally, slower dechlorination occurred in the batches with higher sulfate concentrations. As indicated by higher dissolved Fe concentration, the slower dechlorination in the presence of sulfate was probably due to the decreased surface-complexed Fe(II). This study demonstrates that the chemical form of reduced Fe(II) is critical in determining the fate of cis-DCE under anoxic conditions.

Microscopic and spectroscopic characterization of Hg(II) immobilization by mackinawite (FeS).

This study investigated the solid-phase Hg formed by reacting 0.005 or 0.01 M Hg(II) with 10 g/L mackinawite (FeS) as a function of pH in 0.2 M chloride solutions using X-ray diffraction (XRD), transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) analyses. Under all experimental conditions, XRD analysis showed formation of metacinnabar (ß-HgS) as a bulk-phase sorption product, in agreement with the results from high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and selected area electron diffraction (SAED) in TEM analysis. HAADF-STEM and energy dispersive X-ray (EDX) analyses also suggested formation of Hg(II) surface precipitates. EXAFS analysis indicated that metacinnabar was the dominant product under most conditions, with Hg(II) chlorosulfide-like surface precipitates having increased contribution at lower Hg(II) concentration and higher pH. This finding is consistent with the results of desorption experiments using Hg(II)-complexing ligands. Considering the low solubility and high stability of metacinnabar, our results support the potential application of mackinawite for sequestering Hg(II) in anoxic environments.

X-ray absorption and X-ray photoelectron spectroscopic study of arsenic mobilization during mackinawite (FeS) oxidation.

In this study we investigated the speciation of the solid-phase As formed by reacting 2 x 10(-4) M As(III) with 1.0 g/L mackinawite and the potential for these sorbed species to be mobilized (released into the aqueous phase) upon exposure to atmospheric oxygen at pH 4.9, 7.1, and 9.1. Before oxygen exposure, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) analyses indicated that As(III) was removed from the aqueous phase by forming As(0), AsS, and surface precipitates as thioarsenites at pH 4.9 and As(0) and thioarsenite surface precipitates at pH 7.1 and 9.1. When oxygen was introduced, XAS analysis indicated that As(0) and the surface precipitates were quickly transformed, whereas AsS was persistent. During intermediate oxygen exposure times, dissolved As increased at pH 4.9 and 7.1 due to the rapid oxidation of As(0) and the slow precipitation of iron (oxyhydr)oxides, the oxidation products of mackinawite. This indicates that oxidative mobilization is a potential pathway for arsenic contamination of water at acidic to neutral pH. The mobilized As was eventually resorbed by forming edge-sharing and double-corner-sharing surface complexes with iron (oxyhydr)oxides.

Characterization of synthetic nanocrystalline mackinawite: crystal structure, particle size, and specific surface area.

Iron sulfide was synthesized by reacting aqueous solutions of sodium sulfide and ferrous chloride for 3 days. By X-ray powder diffraction (XRPD), the resultant phase was determined to be primarily nanocrystalline mackinawite (space group: P4/nmm) with unit cell parameters a = b = 3.67 Å and c = 5.20 Å. Iron K-edge XAS analysis also indicated the dominance of mackinawite. Lattice expansion of synthetic mackinawite was observed along the c-axis relative to well-crystalline mackinawite. Compared with relatively short-aged phase, the mackinawite prepared here was composed of larger crystallites with less elongated lattice spacings. The direct observation of lattice fringes by HR-TEM verified the applicability of Bragg diffraction in determining the lattice parameters of nanocrystalline mackinawite from XRPD patterns. Estimated particle size and external specific surface area (SSA(ext)) of nanocrystalline mackinawite varied significantly with the methods used. The use of Scherrer equation for measuring crystallite size based on XRPD patterns is limited by uncertainty of the Scherrer constant (K) due to the presence of polydisperse particles. The presence of polycrystalline particles may also lead to inaccurate particle size estimation by Scherrer equation, given that crystallite and particle sizes are not equivalent. The TEM observation yielded the smallest SSA(ext) of 103 m(2)/g. This measurement was not representative of dispersed particles due to particle aggregation from drying during sample preparation. In contrast, EGME method and PCS measurement yielded higher SSA(ext) (276-345 m(2)/g by EGME and 424 ± 130 m(2)/g by PCS). These were in reasonable agreement with those previously measured by the methods insensitive to particle aggregation.

Sorption of mercuric ion by synthetic nanocrystalline mackinawite (FeS).

Iron sulfides are known to be efficient scavengers of heavy metals. In this study, Hg(II) sorption was investigated using synthetic nanocrystalline mackinawite (a disordered phase) as a function of initial Hg(II) concentration [Hg(II)]0, initial FeS concentration [FeS]0, total chloride concentration CIT, and pH. Hg(II) sorption mechanisms are dependent on relative concentrations of [Hg(II)]0 and [FeS]0 (the molar ratio of [Hg(II)0/[FeS]0). When the molar ratio of [Hg(II)]0/[FeS]o is as low as 0.05, adsorption is mainly responsible for Hg(II) removal, with its contribution to the overall sorption increasing at lower Cl(T). As the molar ratio increases, the adsorption capacity becomes saturated, resulting in precipitation of a sparingly soluble HgS(s). XRD analysis indicates formation of metacinnabar (beta-HgS). Concurrently with HgS(s) precipitation, the released Fe(II) from FeS(s) is resorbed by adsorption at acidic pH and either adsorption or precipitation as Fe (hydr)-oxides at neutral to basic pH. Subsequently, the Fe precipitate formed at neutral to basic pH serves as an adsorbent for Hg(II). Under the conditions where either adsorption or HgS(s) precipitation is dominant, more than 99% of [Hg(II)]0 is immobilized. When the molar ratio of [Hg(II)]0/[FeS]0 exceeds 1, the sulfide concentration is no longer sufficient for HgS(s) precipitation, and formation of chloride salts (Hg2Cl2 at acidic pH and HgCl2 x 3HgO at basic pH) occurs.

Reductive dechlorination pathways of tetrachloroethylene and trichloroethylene and subsequent transformation of their dechlorination products by mackinawite (FeS) in the presence of metals.

Because of frequent co-occurrence of metals with chlorinated organic pollutants, Fe(II), Co(II), Ni(II), and Hg(II) were evaluated for their impact on the dechlorination pathways of PCE and TCE and the subsequent transformation of the initial dechlorination products by FeS. PCE transforms to acetylene via beta-elimination, TCE via hydrogenolysis, and 1,1-DCE via alpha-elimination, while TCE transforms to acetylene via beta-elimination and cis-DCE and 1,1-DCE via hydrogenolysis. Acetylene subsequently transforms in FeS batches, but little transformation of cis-DCE and 1,1-DCE was observed. Branching ratio calculations indicate that the added metals decrease the reductive transformation of PCE and TCE via beta-elimination relative to hydrogenolysis, resulting in a higher production of the toxic DCE byproducts. Nonetheless, acetylene is generally the dominant product. Production of highly water-soluble compound(s) is suspected as a significant source for incomplete mass recoveries. In the transformation of PCE and TCE, the formation of unidentified product(s) is most significant in Co(II)-added FeS batches. Although nearly complete mass recoveries were observed in the other FeS batches, the subsequent transformation of acetylene would lead to the formation of unidentified product(s) over long time periods.

Reductive dechlorination of tetrachloroethylene and trichloroethylene by mackinawite (FeS) in the presence of metals: reaction rates.

Reductive dechlorination by mackinawite (FeS) is an important transformation pathway for chloroethylenes in anoxic environments. Yet, the impact of metals on reductive dechlorination is not well understood, despite their frequent cooccurrence with chloroethylenes. Fe(II), Co(II), Ni(II), and Hg(II) were evaluated for their impact on the dechlorination rates of PCE and TCE by FeS. Compared with unamended FeS batches, the dechlorination rates of both chloroethylenes decreased by addition of 0.01 M Fe(II). Relative to 0.01 M Fe(II)-added FeS batches, the dechlorination rates increased in FeS batches amended with 0.01 M of Co(II) and Hg(II), whereas the rates decreased in 0.01 M Ni(II)-added batches. While significantly impacting the dechlorination rates, the amended metals were quantitatively sequestered by FeS mainly because of formation of metal sulfides. Comparison of the dechlorination rates between metal-added FeS batches and metal sulfide batches suggests that discrete metal sulfides do not form in metal-added FeS batches. The observed exceptionally high reactivity of CoS suggests that it may be useful in reactive permeable barrier applications because of its stability in anoxic waters. The dechlorination rates of PCE and TCE significantly varied with Fe(ll) amendment concentrations (Fe(II)0), indicating the presence of different types of solid-bound Fe phases with Fe(II)o.

Impact of transition metals on reductive dechlorination rate of hexachloroethane by mackinawite.

Mackinawite, an iron monosulfide, has been shown to be a potential reductant for chlorinated organic compounds under anaerobic conditions. Chlorinated organic compounds are often found with inorganic contaminants. This study investigates the impact of various transition metals on the reductive dechlorination by mackinawite using a readily degradable chlorinated organic compound, hexachloroethane (HCA). Different classes of transition metals show distinct patterns in their impact on the HCA dechlorination: 10(-3) M Cr(III) and Mn(II) (hard metals) decreased the dechlorination rates, while 10(-4), 10(-3), and 10(-2) M Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) (intermediate/soft metals) increased the rates. The tested hard metals, due to their weak affinity for sulfides, are thought to form surface precipitates of hydroxides around FeS under the experimental conditions with these hydroxides hindering the electron transfer between FeS and HCA. Due to their high affinity for sulfides, however, the tested intermediate/soft metals can react with FeS in various ways: precipitation of pure metal sulfides (MS), formation of metal-substituted FeS by lattice exchange, and coprecipitation of the mixed sulfides in a Fe-M-S system. Fe(II), released as a result of the interaction of FeS with intermediate/soft metals, enhances the HCA dechlorination at the doses of 10(-4) and 10(-3) M through sorbed or dissolved Fe(II) species, while Fe(OH)2(s) formed at the higher dose of 10(-2) M also enhances the reductive dechlorination. Rate increases observed in Co(II)-, Ni(II)-, and Hg(II)-amended systems are not simply explained by the formation of pure MS; instead, metal-substituted FeS or coprecipitated sulfides are thought to be responsible for the significantly increased rates observed in these systems.