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

Paikaray, Susanta, Peiffer, Stefan
Applied geochemistry 2012 v.27 no.3 pp. 590-597
X-ray diffraction, acid mine drainage, arsenic, crystal structure, ferrihydrite, geochemistry, goethite, infrared spectroscopy, pH, phase transition, ripening, scanning electron microscopy, temperature
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 (0.92wt% As(III)) during transformation, compared with SHM devoid of As(III). Ageing experiments were conducted at room temperature in controlled reactors for 120days at pH5, 6, 7 and 8. Transformation was indicated by a steady SO₄ ²⁻ release of⩾90% of solid phase SO₄ ²⁻ at pH8 after 75days from both specimens. The SO₄ ²⁻ release correlated with OH⁻ consumption, indicating OH⁻–SO₄ ²⁻ exchange and was facilitated as pH increased from 5 to 8. No significant structural change could be indicated after 75days 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 120days with traces of ferrihydrite and goethite in As(III)-free specimens, which was more prominent at higher pH. Significant release of sorbed As (1.9wt% at pH5) 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 120days at lower pH values in the case of As(III)-sorbed specimens indicating an inhibitory effect of As on the nucleation of new phases.