Abstract
Schwertmannite is an Fe(III)-oxyhydroxysulfate which is common in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. Natural schwertmannite is often enriched in Cr(III), yet the effects of Cr(III) substitution on schwertmannite transformation to more stable Fe(III) minerals has not been addressed. Here we examine, for the first time, the effects of Cr(III) substitution on the Fe(II)-accelerated transformation of schwertmannite. X-ray diffraction (XRD) and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy shows that Cr(III) substitution inhibits schwertmannite transformation. Substitution at a Cr(III):Fe(III) ratio of 0.025 decreased schwertmannite transformation (at pH 6.5) by 18–49% (depending on Fe(II) concentrations) relative to that of Cr(III)-free schwertmannite. Formation of crystalline secondary phases (predominantly goethite) caused associated decreases in solid-phase Fe and Cr extractability by 1 M HCl. The extractability of Cr was consistently greater than that of Fe, suggesting some accumulation of Cr(III) at the residual schwertmannite surface–a phenomenon which passivates the surface against Fe(II)/Fe(III) electron transfer and atom exchange required for the Fe(II)-accelerated transformation process. The finding that Cr(III)-substitution inhibits schwertmannite transformation implies that it may also significantly impact associated Fe, S and trace metal(loid) behaviour.
Highlights
Schwertmannite is a poorly-ordered Fe(III)-oxyhydroxysulfate which precipitates from acidic, Fe- and SO42—rich waters [1,2,3]
The total Fe and S content of the initial schwertmannites ranged from 7.9–8.2 mmol g-1 and 1.63–1.82 mmol g-1, respectively (S1 Table), which is consistent with the compositional variability of natural schwertmannite [1, 4, 40, 45]
The total Cr content in the low, medium- and high-Cr(III) schwertmannite was 0.02, 0.05 and 0.21 mmol g-1, respectively (S1 Table). These total Cr concentrations bracket those reported in previous studies on natural schwertmannite formed in acid-mine drainage (AMD) systems [27,28,29,30]
Summary
Schwertmannite is a poorly-ordered Fe(III)-oxyhydroxysulfate which precipitates from acidic, Fe- and SO42—rich waters [1,2,3]. We examine the effects of Cr(III) substitution on the Fe(II)-accelerated transformation of schwertmannite. The first derivative of the XANES spectra revealed the presence of additional edge features for the Cr(III)-substituted schwertmannite (Fig 4B).
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