Arsenic sorption to nanoparticulate mackinawite (FeS): An examination of phosphate competition

Nanoparticulate mackinawite (FeS) can be an important host-phase for arsenic (As) in sulfidic, subsurface environments. Although not previously investigated, phosphate (PO₄^3-) may compete with As for available sorption sites on FeS, thereby enhancing As mobility in FeS-bearing soils, sediments and groundwater systems. In this study, we examine the effect of PO₄^3- on sorption of arsenate (As(V)) and arsenite (As(III)) to nanoparticulate FeS at pH 6, 7 and 9. Results show that PO₄^3- (at 0.01-1.0 mM P) did not significantly affect sorption of either As(V) or As(III) to nanoparticulate FeS at initial aqueous As concentrations ranging from 0.01 to 1.0 mM. At pH 9 and 7, sorption of both As(III) and As(V) to nanoparticulate FeS was similar, with distribution coefficient (K_d) values spanning 0.76-15 L g^-1 (which corresponds to removal of 87-98% of initial aqueous As(III) and As(V) concentrations). Conversely, at pH 6, the sorption of As(III) was characterized by substantially higher K_d values (6.3-93.4 L g^-1) than those for As(V) (K_d = 0.21-0.96 L g^-1). Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that up to 52% of the added As(V) was reduced to As(III) in As(V) sorption experiments, as well as the formation of minor amounts of an As₂S₃-like species. In As(III) sorption experiments, XANES spectroscopy also demonstrated the formation of an As₂S₃-like species and the partial oxidation of As(III) to As(V) (despite the strictly O₂-free experimental conditions). Overall, the XANES data indicate that As sorption to nanoparticulate FeS involves several redox transformations and various sorbed species, which display a complex dependency on pH and As loading but that are not influenced by the co-occurrence of PO₄^3-. This study shows that nanoparticulate FeS can help to immobilize As(III) and As(V) in sulfidic subsurface environments where As co-exists with PO₄^3-.