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

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 (AS(Ex)). 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 approximate to 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 >= 100 mu M 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 As-Ex, attenuation of As-(aq) and TEM-EDX spectra indicate concurrent formation of nano-scale zones variably enriched in As (similar to 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 (<1 h) 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 in-direct 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. (C) 2012 Elsevier B.V. All rights reserved.