Abiotic schwertmannite transformation kinetics and the role of sorbed As(III)

Schwertmannite (SHM) precipitated from acid mine drainage often contains high levels of As(III). The present study is aimed at understanding the fate of As(III) preadsorbed to bio-synthesized schwertmannite (0.92 wt% As(III)) during transformation, compared with SHM devoid of As(III). Ageing experiments were conducted at room temperature in controlled reactors for 120 days at pH 5, 6, 7 and 8. Transformation was indicated by a steady SO42- release of ⩾90% of solid phase SO42- at pH 8 after 75 days from both specimens. The SO42- release correlated with OH− consumption, indicating OH--SO42- exchange and was facilitated as pH increased from 5 to 8. No significant structural change could be indicated after 75 days using XRD, IR spectroscopy and scanning electron microscopy, suggesting the formation of an initial transformation product of low crystallinity. Incomplete ripening of the transformation product was determined after 120 days with traces of ferrihydrite and goethite in As(III)-free specimens, which was more prominent at higher pH. Significant release of sorbed As (1.9 wt% at pH 5) could be detected during the initial phase of the transformation of As-containing SHM which was rapidly readsorbed probably to the new phase. No detectable ripening occurred after 120 days at lower pH values in the case of As(III)-sorbed specimens indicating an inhibitory effect of As on the nucleation of new phases.

► Schwertmannite transformation is strongly triggered by enhancing aqueous pH. ► Schwertmannite transformed incompletely to goethite even after 120 days at pH 8. ► Only <2% of sorbed As(III) get released and inhibits schwertmannite transformation. ► Aqueous pH inversely affect As(III) release which re-adsorbed within 40 days ageing. ► Prolonged schwertmannite exposure in alkaline media brings high sulfate loading.