Rates of Hydrous Ferric Oxide Crystallization and the Influence on Coprecipitated Arsenate

Arsenate coprecipitated with hydrous ferric oxide (HFO) was stabilized against dissolution during transformation of HFO to more crystalline iron (hydr)oxides. The rate of arsenate stabilization approximately coincided with the rate of HFO transformation at pH 6 and 40 °C. Comparison of extraction data and X-ray diffraction results confirmed that hematite and goethite were the primary transformation products. HFO transformation was significantly retarded at or above an arsenate solid loading of 29 455 mg As/kg HFO. However, HFO transformation proceeded at a significant rate for arsenate solid loadings of 4208 and 8416 mg As/kg HFO. At a solid loading of 8416 mg As/kg HFO, XRD results suggested arsenate primarily partitioned to hematite. Comparison of HFO transformation rates observed in this research to rates obtained from the literature at pH 6 and temperatures ranging from 24 to 70 °C suggests that arsenate stabilization could be realized in oxic environments with a significant fraction of iron (hydr)oxides. While this process has not been documented in natural systems, the predicted half-life for transformation of an arsenic-bearing HFO is approximately 300 days at 25 °C at solid loading < 8415 mg As/kg HFO. The projected time frame for arsenate stabilization indicates this process should be considered during development of conceptual and analytical models describing arsenic fate and transport in oxic systems containing reactive iron (hydr)oxides. The likelihood of this process would depend on the chemical dynamics of the soil or sediment system relative to iron (hydr)oxide precipitation-dissolution reactions and the potential retarding/competing influence of ions such as silicate and organic matter

Use of Hydrochloric Acid for Determining Solid-Phase Arsenic Partitioning in Sulfidic Sediments

We examined the use of room-temperature hydrochloric acid (1−6 M) and salt solutions of magnesium chloride, sodium carbonate, and sodium sulfide for the removal of arsenic from synthetic iron monosulfides and contaminated sediments containing acid-volatile sulfides (AVS). Results indicate that acid-soluble arsenic reacts with H2S released from AVS phases and precipitates at low pH as disordered orpiment or alacranite. Arsenic sulfide precipitation is consistent with geochemical modeling in that conditions during acid extraction are predicted to be oversaturated with respect to orpiment, realgar, or both. Binding of arsenic with sulfide at low pH is sufficiently strong that 6 M HCl will not keep spiked arsenic in the dissolved fraction. Over a wide range of AVS concentrations and molar [As]/[AVS] ratios, acid extraction of arsenic from sulfide-bearing sediments will give biased results that overestimate the stability or underestimate the bioavailability of sediment-bound arsenic. Alkaline solutions of sodium sulfide and sodium carbonate are efficient in removing arsenic from arsenic sulfides and mixed iron−arsenic sulfides because of the high solubility of arsenic at alkaline pH, the formation of stable arsenic complexes with sulfide or carbonate, or both

The Nature of Zn Precipitates Formed in the Presence of Pyrophyllite

The partitioning of Zn to the pyrophyllite surface was studied as a function of surface loading for periods up to 4 months. Examination of the reaction products using X-ray absorption fine structure spectroscopy (XAFS) indicated the formation of a Zn precipitate at each surface loading. Comparison of the local structure of the surface precipitates to the structure of various hydroxide- and carbonate-bearing phases indicates the formation of a Zn−Al layered double hydroxide (LDH). The solubility of Zn following aging in pyrophyllite systems indicated that the initial Zn−Al LDH precipitates transformed to a more stable form. Increased Zn stability in these experimental systems may be attributed to an increase in LDH crystallinity (Ostwald ripening) or incorporation of Si within the LDH interlayer leading to transformation to a phyllosilicate-like phase. Our results support formation of an LDH precipitate as a precursor to Zn fixation in soils abundant in aluminosilicate minerals. These results augment recent findings that transition metals may form layered hydroxide and phyllosilicate-like precipitates during sorption to clay minerals. Acknowledgment of this process as a potential metal sequestration mechanism in certain soil types is important to assessment of contaminant attenuation. Development of a more comprehensive database of solubilities for these surface precipitates will facilitate more reliable estimates

Examination of Arsenic Speciation in Sulfidic Solutions Using X-ray Absorption Spectroscopy

Both thioarsenites and thioarsenates have been demonstrated to exist in sulfidic waters, yet there is uncertainty regarding the geochemical conditions that govern the formation of these arsenic species. The purpose of this research was to use advanced spectroscopy techniques, speciation modeling, and chromatography to elucidate the chemical speciation of arsenic in sulfidic solutions initially containing arsenite and sulfide. Results of X-ray absorption spectroscopy (XAS) show that experimental solutions contained mixtures of arsenite and thioarsenites with increasing substitution of sulfur for oxygen on arsenic as the sulfide concentration increased. Experimental samples showed no evidence of polymeric arsenic species, or transformation of thioarsenites to thioarsenates. The arsenic speciation measured using XAS was similar to predictions obtained from a thermodynamic model for arsenic speciation, excluding thioarsenate species in sulfidic systems. Our data cast some doubt on the application of chromatographic methods for determining thioarsenates and thioarsenites (or mixtures) in natural waters in cases where the arsenic oxidation state cannot be independently verified. The same chromatographic peak positions proposed for thioarsenates can be explained by thioarsenite species. Furthermore, sample dilution was shown to change the species distribution and care should be taken to avoid sample dilution prior to chromatographic analysis

Chromium-Removal Processes during Groundwater Remediation by a Zerovalent Iron Permeable Reactive Barrier

Solid-phase associations of chromium were examined in core materials collected from a full-scale, zerovalent iron permeable reactive barrier (PRB) at the U.S. Coast Guard Support Center located near Elizabeth City, NC. The PRB was installed in 1996 to treat groundwater contaminated with hexavalent chromium. After eight years of operation, the PRB remains effective at reducing concentrations of Cr from average values >1500 μg L-1 in groundwater hydraulically upgradient of the PRB to values -1 in groundwater within and hydraulically downgradient of the PRB. Chromium removal from groundwater occurs at the leading edge of the PRB and also within the aquifer immediately upgradient of the PRB. These regions also witness the greatest amount of secondary mineral formation due to steep geochemical gradients that result from the corrosion of zerovalent iron. X-ray absorption near-edge structure (XANES) spectroscopy indicated that chromium is predominantly in the trivalent oxidation state, confirming that reductive processes are responsible for Cr sequestration. XANES spectra and microscopy results suggest that Cr is, in part, associated with iron sulfide grains formed as a consequence of microbially mediated sulfate reduction in and around the PRB. Results of this study provide evidence that secondary iron-bearing mineral products may enhance the capacity of zerovalent iron systems to remediate Cr in groundwater, either through redox reactions at the mineral−water interface or by the release of Fe(II) to solution via mineral dissolution and/or metal corrosion

Nonbiological Removal of <i>cis</i>-Dichloroethylene and 1,1-Dichloroethylene in Aquifer Sediment Containing Magnetite

The U.S. EPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater emphasizes biological reductive dechlorination as the primary mechanism for destruction of chlorinated solvents. However, biological reductive dechlorination could not explain the removal of cis-dichloroethylene (cis-DCE) and 1,1-DCE from a plume of contaminated groundwater in Minnesota. Several recent laboratory studies have demonstrated that common iron minerals such as magnetite can also transform chlorinated alkenes. Laboratory microcosms were constructed with sediment from three depth intervals in the aquifer near the source of the plume. The microcosms were autoclaved to prevent biological transformations. In these autoclaved sediments, the rates of removal of cis-DCE in samples from the shallow, intermediate, and deeper depth intervals in the aquifer were 0.58 ± 0.09, 2.29 ± 0.26, and 0.31 ± 0.08 per year at 95% confidence. The rate of removal of 1,1-DCE in sediment from the shallow interval was 1.37 ± 0.50 per year. The rates of removal in the microcosms are similar to the rates of attenuation observed in the field. Magnetite was identified in the sediment by X-ray diffraction and optical microscopy. Published rates of transformation of cis-DCE by magnetite are consistent with the rates of removal in the microcosm study