Arsenic mobilization and iron transformations during sulfidization of As(V)-bearing jarosite

Abstract

Jarosite (KFe3(SO4)2(OH)6) is an important host-phase for As in acid mine drainage (AMD) environments and coastal acid sulfate soils (CASS). In AMD and CASS wetlands, jarosite may encounter S(−II) produced by sulfate reducing bacteria. Here, we examine abiotic sulfidization of As(V)-bearing K-jarosite at pH 4.0, 5.0, 6.5 and 8.0. We quantify the mobilization and speciation of As and identify corresponding Fe mineral transformations. Sulfide-promoted dissolution of jarosite caused release of co-precipitated As and the majority of mobilized As was re-partitioned to a readily exchangeable surface complex (AsEx). In general, maximum As mobilization occurred in the highly sulfidized end-members of all treatments and was greatest at low pH, following the order pH 5.0≈4.0>8.5>6.5. X-ray absorption spectroscopy revealed that most solid-phase As remained as oxygen-coordinated As(V) when pH values were >5.0 — even during latter stages of sulfidization and the presence of ≥100μM dissolved S(−II). In contrast at pH 4.0, As transitioned from oxygen-coordinated As(V) to a sulfur-coordinated orpiment-like phase. This transition coincided with a marked decrease in AsEx, attenuation of As(aq) and TEM-EDX spectra indicate concurrent formation of nano-scale zones variably enriched in As (~1–15%). Although discordant with geochemical modeling, the formation of an orpiment-like precipitate appears to be a primary control on As mobility during the late stages of complete jarosite sulfidization under acidic conditions (pH 4.0).Mackinawite was the main Fe-mineral end product in all pH treatments. However, at pH 8.0, jarosite rapidly (<1h) transformed to a lepidocrocite intermediary. Although lepidocrocite efficiently adsorbed As(aq), the transformation process itself was incongruent with electron transfer to Fe(III). Further investigation is required to determine whether the electron donor triggering this transformation was direct via S(−II), or indirect via surface complexed Fe(II) and hence akin to the widely-known Fe(II)-catalyzed transformation of Fe(III) minerals. The results demonstrate that abiotic sulfidization of As(V)-co-precipitated jarosite can mobilize substantial As and that pH exerts a major control on the subsequent As solid-phase speciation, electron transfer kinetics and Fe mineralization pathways and products. The findings are particularly relevant to heterogeneous sediments in which As-bearing jarosite encounters dissolved sulfide under a range of pH conditions.