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During X-ray absorption spectroscopy (XAS) measurements of arsenic (As), beam-induced redox transformation is often observed. In this study, the As species immobilized by poorly crystallized mackinawite (FeS) was assessed for the susceptibility to beam-induced redox reactions as a function of sample properties including the redox state of FeS and the solid-phase As speciation. The beam-induced oxidation of reduced As species was found to be mediated by the atmospheric O2and the oxidation products of FeS [e.g.Fe(III) (oxyhydr)oxides and intermediate sulfurs]. Regardless of the redox state of FeS, both arsenic sulfide and surface-complexed As(III) readily underwent the photo-oxidation upon exposure to the atmospheric O2during XAS measurements. With strict O2exclusion, however, both As(0) and arsenic sulfide were less prone to the photo-oxidation by Fe(III) (oxyhydr)oxides than NaAsO2and/or surface-complexed As(III). In case of unaerated As(V)-reacted FeS samples, surface-complexed As(V) was photocatalytically reduced during XAS measurements, but arsenic sulfide did not undergo the photo-reduction.

Cis-1,2,-dichloroethylene (cis-DCE) is a toxic, persistent contaminant occurring mainly as a daughter product of incomplete degradation of perchloroethylene (PCE) and trichloroethylene (TCE). This paper reports on abiotic reductive dechlorination of cis-DCE by mackinawite (FeS1−x ), a ferrous monosulfide, under variable geochemical conditions. To assess in situ abiotic cis-DCE dechlorination by mackinawite in the field, mackinawite suspensions prepared in a field groundwater sample collected from a cis-DCE contaminated field site were used for dechlorination experiments. The effects of geochemical variables on the dechlorination rates were monitored. A set of dechlorination experiments were also carried out in the presence of aquifer sediment from the site over a range of pH conditions to better simulate the actual field situations. The results showed that the suspensions of freshly prepared mackinawite reductively transformed cis-DCE to acetylene, whereas the conventionally prepared powder form of mackinawite had practically no reactivity with cis-DCE under the same experimental conditions. Significant cis-DCE degradation by mackinawite has not been reported prior to this study, although mackinawite has been shown to reductively transform PCE and TCE. This study suggests feasibility of using mackinawite for in situ remediation of cis-DCE-contaminated sites with high S levels such as estuaries under naturally achieved or stimulated sulfate-reducing conditions.

The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) in the presence of natural oxidants, such as oxygen, Fe(III) hydroxides, and nitrite. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(IV) solid phases. Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO2) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO2 in the presence and absence of synthetic mackinawite (FeS) under simulated groundwater conditions of pH = 7, P O 2 = 0.02 atm, and P CO 2 = 0.05 atm. The kinetic profiles of dissolved uranium indicate that FeS inhibited UO2 dissolution for about 51 h by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mossbauer, and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO2 oxidative dissolution occurred at an initial rate of rm = 1.2 ± 0.4 × 10−8 mol g−1 s−1, higher than rm = 5.4 ± 0.3 × 10−9 mol g−1 s−1 in the control experiment where FeS was absent. XAS analysis confirmed that soluble U(VI)-carbonato complexes were adsorbed by iron oxyhydroxides (i.e., nanogoethite and lepidocrocite) formed from FeS oxidation, which provided a sink for U(VI) retention. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events.

A novel bioreactor system, consisting of two biologically active carbon (BAC) reactors in series, was developed for the simultaneous removal of nitrate and arsenic from a synthetic groundwater supplemented with acetic acid. A mixed biofilm microbial community that developed on the BAC was capable of utilizing dissolved oxygen, nitrate, arsenate, and sulfate as the electron acceptors. Nitrate was removed from a concentration of approximately 50 mg/L in the influent to below the detection limit of 0.2 mg/L. Biologically generated sulfides resulted in the precipitation of the iron sulfides mackinawite and greigite, which concomitantly removed arsenic from an influent concentration of approximately 200 ug/L to below 20 ug/L through arsenic sulfide precipitation and surface precipitation on iron sulfides. This study showed for the first time that arsenic and nitrate can be simultaneously removed from drinking water sources utilizing a bioreactor system.

Iron sulfide minerals commonly found in natural anoxic Fe-S systems have been shown to reductively transform chlorinated hydrocarbons including trichloroethylene (TCE). In the present study, we tested the feasibility of applying an Fe(II) solution to a TCE-contaminated aquifer groundwater under simulated sulfide reducing conditions to enhance reductive transformation of TCE to nontoxic compounds. To achieve this goal, iron sulfide particles were precipitated under a range of pH and Fe:S molar ratios in aquifer groundwater samples from the Dugway Proving Grounds, Utah. Batch tests for abiotic reductive dechlorination of TCE were performed using the precipitates to establish the conditions for most favorable solids for dechlorination. Under all experimental conditions, the solids formed consisted mainly of mackinawite, a tetragonal reduced iron monosulfide FeS1-x . However, the precipitation conditions strongly affected the reactivity of the mackinawite particles formed. The results indicated that addition ...

Reaction of aqueous uranium(VI) with iron(II) monosulfide mackinawite in an O(2) and CO(2) free model system was studied by batch uptake measurements, equilibrium modeling, and L(III) edge U X-ray absorption spectroscopy (XAS). Batch uptake measurements showed that U(VI) removal was almost complete over the wide pH range between 5 and 11 at the initial U(VI) concentration of 5 × 10(-5) M. Extraction by a carbonate/bicarbonate solution indicated that most of the U(VI) removed from solution was reduced to nonextractable U(IV). Equilibrium modeling using Visual MINTEQ suggested that U was in equilibrium with uraninite under the experimental conditions. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the U(IV) phase associated with mackinawite was uraninite. Oxidation experiments with dissolved O(2) were performed by injecting air into the sealed reaction bottles containing mackinawite samples reacted with U(VI). Dissolved U measurement and XAS confirmed that the uraninite formed from the U(VI) reduction by mackinawite did not oxidize or dissolve under the experimental conditions. This study shows that redox reactions between U(VI) and mackinawite may occur to a significant extent, implying an important role of the ferrous sulfide mineral in the redox cycling of U under sulfate reducing conditions. This study also shows that the presence of mackinawite protects uraninite from oxidation by dissolved O(2). The findings of this study suggest that uraninite formation by abiotic reduction by the iron sulfide mineral under low temperature conditions is an important process in the redistribution and sequestration of U in the subsurface environments at U contaminated sites.

As(III) uptake from solution by synthetic mackinawite is examined as a function of pH and initial As(III) concentration using X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). XAS data indicate that when mackinawite is reacted at pH 5, 7, and 9 with 5 x 10(-4) M As(III), arsenic is reduced from its original +3 valence state and is primarily coordinated as As-S (approximately 2.26 angstroms) and As-As (approximately 2.54 angstroms), which is consistent with the formation of a realgar-like phase in agreement with XRD data. At 5 x 10(-5) M As(III), samples are markedly different from those collected at an order of magnitude higher concentration and differ at each pH value. The XAS analysis of mackinawite samples reacted with 5 x 10(-5) M As(III) shows a transition from As-O coordination to As-S coordination as pH decreases, with the sample reacted at pH 5 resembling realgar. Under alkaline conditions, arsenic retains its original valence state of +3 and is primarily coordinated to oxygen at a distance of 1.75 angstroms. This may be attributed to uptake by adsorption as an As(III) oxyanion. These results provide the basis for selecting the reactions needed for modeling and are beneficial in understanding the mechanisms of arsenite uptake by mackinawite under anoxic sulfidic conditions.