Environmental Fate and Exposure Assessment for Arsenic in Groundwater (Addendum)

The original document contains color images. Prepared in cooperation with Integrated Science & Technology and University of California, Merced, CA.

This one-year project extension was aimed at identifying and characterizing end-member As- and Fe- sulfide precipitates and co-precipitated As+Fe sulfide phases as potential sinks for As in sulfate-reduced environments. Precipitation experiments used starting solutions with As3+ (as arsenite, AsO3 3-), sulfide (S2--), and either Fe2+ or Fe3+, and As-only or Fe-only end-members at pH 3 or 4, 6, and 8 and aged for up to 210 d. Products were characterized by synchrotron X-ray absorption spectroscopy (XAS) and by synchrotron and laboratory XRD. Results from precipitation experiments under strict anoxic conditions showed that the primary solid products at low pH (3, 4) were mixtures of amorphous-to-crystalline Fe and As sulfides, with no evidence for extensive solid solution. With starting solutions containing As3+ + Fe2+ + S2--, mixtures of Fe or As sulfides, a green rust-type phase (Fe2+, Fe3+-hydroxide), and a fraction of As3+ bonded to oxygen formed at higher pH (6, 8). With starting solutions containing As3+ + Fe3++ S2--, no As sulfide was precipitated and all As was identified as the arsenite species, probably present mostly as a sorbed complex. Iron precipitated as a mixture of Fe sulfide and an Fe(III)-oxide phase that with aging formed poorly crystalline hematite. Precipitates were initially amorphous to X-ray diffraction and became more crystalline with aging for end members, but the rate of transformation of Fe sulfides and oxides from amorphous to crystalline was generally much slower (weeks to months) when As was present in solution. Characterization of precipitation products by XAS at aging times up to one month indicated that the local structure around Fe or As was indicative of the final solid products after 210 d of aging. These data suggest that small (perhaps nano-to-micro-meter sized) particles are nucleating on short time scales consistent with overall