|Paikaray, S; Schröder, C; Peiffer, S (2017): Schwertmannite stability in anoxic Fe(II)-rich aqueous solution, Geochim. Cosmochim. Acta, 217, 292-305, DOI: https://doi.org/10.1016/j.gca.2017.08.026 [Link]|
Schwertmannite (SHM) is a powerful scavenger for As(III) leading to As(III)-enriched precipitates around acid mine drainage environments that may become exposed to aqueous Fe(II). In this study we have investigated the stability of pure SHM and SHM containing 0.92 wt% As(III) under Fe(II)aq-rich (0.4-1.0 mM) anoxic conditions using XRD, SEM, Mössbauer and FTIR spectroscopic techniques. Schwertmannite transformation proceeded through an alkalinity-driven pathway releasing sulfate and a Fe(II)-catalyzed pathway that generated lepidocrocite and goethite at pH 6 and 6.9 in the presence of 1 mM Fe(II)aq. Lepidocrocite was found to be needle shaped if the SHM contained As(III) and platy for pure SHM. Goethite had a poor degree of crystallinity in As(III) containing SHM. Pre-adsorption of As(III) inhibited the extent of SHM transformation. Fe(II) sorption onto SHM was pH dependent and reflected a sorption edge with complete consumption at pH 6.9, while only ∼20% were adsorbed at pH 5. Surface coverage with Fe(II) appears to be the key parameter controlling extent and products of the transformation process. As(III) concentrations in solution are controlled by two mechanisms: 1) exchange of As(III) for sulfate upon alkalinity-driven transformation of schwertmannite and 2) re-adsorption to new phases formed upon Fe(II)-catalyzed transformation. The adsorbed As(III) has inhibited the extent of transformation and was partly released with the maximum release at pH 5 (0.5%) in the absence of Fe(II)aq.