Andrea L. Foster

Quantitative arsenic speciation in mine tailings using X-ray absorption spectroscopy

Abstract Polarized infrared absorption spectra of thin, oriented single-crystal slabs of pectolite and serandite were recorded between 4000 and 350 cm-1 at 298 and 83 K. The spectra of both minerals show a broad absorption region parallel to the silicate chains (b direction) that is centered around 1000 cm-1 which is interrupted by a transmission window, and which is superimposed by sharp silicate, lattice, and overtone modes. This band is assigned to the OH stretching mode consistent with the alignment of the O-H O hydrogen bond parallel to b and the short O···O distance of 2.45-2.48 Å that was found in previous X- ray structure refinements. At 1396 cm-1 (pectolite) and 1386 cm-1 (serandite) an OH bending mode is observed in the IR spectra parallel to c. At low temperatures, this mode shifts up to higher frequencies (1403 cm-1 at 83 K in pectolite). whereas the down-shift of the OH stretching mode cannot be observed due to the extremely broad band width. The slightly higher energy of the bending mode in pectolite indicates a slightly stronger hydrogen bond with respect to serandite. However, the bond length in serandite is slightly shorter than that in pectolite. An asymmetric O-H ··· O bond is confirmed in pectolite and serandite through comparison with different materials with similar, very strong hydrogen bonds and low-energy OH stretching modes.

X-ray absorption fine structure study of As(V) and Se(IV) sorption complexes on hydrous Mn oxides

X-ray absorption fine structure (XAFS) spectroscopic analysis at the As, Se, and Mn K-edges was used to study arsenate [As(V)O43−] and selenite [Se(IV)O32−] sorption complexes on the synthetic hydrous manganese oxides (HMOs) vernadite (δ-MnO2) and K-birnessite (nominal composition: K4Mn14O27 · 9H2O). No significant changes were observed in sorption complex structure as a function of sorbent, pH (5 to 8), surface coverage (0.04 to 0.73 μmol/m2), or reaction time (5 to 22 h) in the arsenate or selenite systems. In the arsenate/HMO system, extended XAFS parameters indicate an average second-neighbor As(V) coordination of 2.0 ± 0.4 Mn at an average distance of 3.16 ± 0.01 A, which is consistent with formation of As(V)O4 sorption complexes sharing corners with two adjacent Mn(IV)O6 surface species (i.e., bidentate, binuclear). In the selenite/HMO system, selenite surface complexes are surrounded by two shells of Mn atoms, which could represent two different adsorption complexes or a precipitate. The first shell consists of 1.6 ± 0.4 Mn at 3.07 ± 0.01 A, which is consistent with the selenite anion forming bidentate (mononuclear) edge-sharing complexes with Mn(II)O6 or Mn(III)O6 octahedra. The second shell consists of 1.4 ± 0.4 Mn at 3.49 ± 0.03 A, consistent with selenite forming monodentate, corner-sharing complexes with Mn(II)O6 or Mn(III)O6 octahedra. Pauling bond valence analysis that uses the extended XAFS-derived bond lengths for As(V)-O, Se(IV)-O, and Mn-O bonds indicates that the proposed surface complexes of selenite and arsenate on HMOs should be stable. Although a nearly identical Se(IV) coordination environment is found in a crystalline Mn(II)-Se(IV) precipitate (which has a structure similar to that of MnSeO3 · H2O), there are significant differences in the X-ray absorption near-edge structure and extended XAFS spectra of this precipitate and the selenite/HMO sorption samples. These differences coupled with transmission electron microscopy results suggest that if a precipitate is present it lacks long-range order characteristic of crystalline MnSeO3 · H2O.

XANES Evidence for Rapid Arsenic(III) Oxidation at Magnetite and Ferrihydrite Surfaces by Dissolved O2 via Fe2+-Mediated Reactions

To reduce the adverse effects of arsenic on humans, various technologies are used to remove arsenic from groundwater, most relying on As adsorption on Fe-(oxyhydr)oxides and concomitant oxidation of As(III) by dissolved O(2). This reaction can be catalyzed by microbial activity or by strongly oxidizing radical species known to form upon oxidation of Fe(II) by dissolved O(2). Such catalyzed oxidation reactions have been invoked to explain the enhanced kinetics of As(III) oxidation in aerated water, in the presence of zerovalent iron or dissolved Fe(II). In the present study, we used arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy to investigate the role of Fe(II) in the oxidation of As(III) at the surface of magnetite and ferrihydrite under oxygenated conditions. Our results show rapid oxidation of As(III) to As(V) upon sorption onto magnetite under oxic conditions at neutral pH. Moreover, under similar oxic conditions, As(III) oxidized upon sorption onto ferrihydrite only after addition of Fe(II)(aq) within the investigated time frame of 24 h. These results confirm that Fe(II) is able to catalyze As(III) oxidation in the presence of dissolved O(2) and suggest that oxidation of As(III) upon sorption on magnetite under oxic conditions can be explained by an Fe(2+)-mediated Fenton-like reactions. Thus, the present study shows that magnetite might be an efficient alternative to the current use of oxidants and Fe(II) to remove As from aerated water. In addition, this study emphasizes that special care is needed to preserve arsenic oxidation state during laboratory sorption experiments as well as in collecting As-bearing samples from natural environments.

Arsenic Speciation in Bituminous Coal Fly Ash and Transformations in Response to Redox Conditions

The risk of the mobilization of coal ash into the environment has highlighted the need for the assessment of the environmental behavior of coal ash, particularly with respect to toxic trace elements such as arsenic (As). Here, we examined As speciation in coal fly ash samples and transformations in response to aquatic redox conditions. X-ray absorption spectroscopy indicated that 92-97% of total As occurred as As(V), with the remainder present as As(III). Major As-bearing hosts in unamended ashes were glass, iron (oxyhydr)oxides, and calcium arsenate. Oxic leaching resulted in immediate As mobilization to the aqueous phase, reprecipitation of As-iron ferrihydrite, and As adsorption to mineral surfaces. Under anoxic conditions, the (reductive) dissolution of As-bearing phases such as iron ferrihydrite resulted in increased dissolved As compared to oxic conditions and reprecipitation of iron arsenate. Overall, As in coal ash is not environmentally stable and can participate in local biogeochemical cycles.

Copper Speciation in Variably Toxic Sediments at the Ely Copper Mine, Vermont, United States

At the Ely Copper Mine Superfund site, Cu concentrations exceed background values in both streamwater (160-1200 times) and sediments (15-79 times). Previously, these sediment samples were incubated with laboratory test organisms, and they exhibited variable toxicity for different stream sites. In this study we combined bulk- and microscale techniques to determine Cu speciation and distribution in these contaminated sediments on the basis of evidence from previous work that Cu was the most important stressor in this environment and that variable observed toxicity could have resulted from differences in Cu speciation. Copper speciation results were similar at microscopic and bulk scales. The major Cu species in the more toxic samples were sorbed or coprecipitated with secondary Mn (birnessite) and Fe minerals (jarosite and goethite), which together accounted for nearly 80% of the total Cu. The major Cu species in the less toxic samples were Cu sulfides (chalcopyrite and a covellite-like phase), making up about 80-95% of the total Cu, with minor amounts of Cu associated with jarosite or goethite. These Cu speciation results are consistent with the toxicity results, considering that Cu sorbed or coprecipitated with secondary phases at near-neutral pH is relatively less stable than Cu bound to sulfide at lower pH. The more toxic stream sediment sites were those that contained fewer detrital sulfides and were upstream of the major mine waste pile, suggesting that removal and consolidation of sulfide-bearing waste piles on site may not eliminate all sources of bioaccessible Cu.

Laboratory data from testing parameters of EPA Method 3060A on Soils Contaminated with Chromium or Processing Residue

It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from solids containing chromium ore processing residue (COPR). We systematically tested modifications to prescribed parameters of EPA 3060A towards improving extraction efficiency of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil from New Jersey (NJ). The alkaline extraction fluid leached Al, Si, and B from the prescribed borosilicate glass vessels which interfered with Cr(VI) extraction from COPR. The use of polytetrafluoroethylene (PTFE) vessels increased the extraction efficiency. Intensive grinding of NIST 2701 resulted in the extraction of 730±30 mg kg-1 Cr(VI), which is substantially greater than the certified Cr(VI) value of 551±35 mg kg-1 but still less than the Cr(VI) value of ~3000 mg kg-1 previously determined by X-ray absorption near edge structure (XANES) spectroscopy. Increasing the extraction fluid to sample ratio also increased the efficiency of Cr(VI) extraction. Ratios similar to the 20 mL g-1 prescribed by 3060A resulted in low and highly variable extraction efficiencies. Ratios of 900 mL g-1 or greater resulted in relatively consistent extraction, yielding as much as ~950 mg kg-1 Cr(VI) from intensively ground NIST 2701 after 2.25 hours of extraction. Increasing the extraction time to 48 hours resulted in up to 1274 mg kg-1.

Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-Contaminated Soils

It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from chromium ore processing residue (COPR). We modified various parameters of EPA 3060A toward understanding the transformation of COPR minerals in the alkaline extraction and improving extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si were the major elements dissolved from NIST 2701, and their concentrations in solution were correlated with Cr(VI). The extraction fluid leached additional Al and Si from the method-prescribed borosilicate glass vessels which appeared to suppress the release of Cr(VI). Use of polytetrafluoroethylene vessels and intensive grinding of NIST 2701 increased the amount of Cr(VI) extracted. These modifications, combined with an increased extraction fluid to sample ratio of ≥900 mL g-1 and 48-h extraction time resulted in a maximum release of 1274 ± 7 mg kg-1 Cr(VI). This is greater than the NIST 2701 certified value of 551 ± 35 mg kg-1 but less than 3050 mg kg-1 Cr(VI) previously estimated by X-ray absorption near edge structure spectroscopy. Some of the increased Cr(VI) may have resulted from oxidation of Cr(III) released from brownmillerite which rapidly transformed during the extractions. Layered-double hydroxides remained stable during extractions and represent a potential residence for unextracted Cr(VI).