Oxalate-Induced Photoreduction Dissolution and Transformation of Schwertmannite: Change of Mineral Phase and Elemental Fate

Abstract

Schwertmannite (Sch), a common metastable ferric sulfate secondary mineral in acidic mine drainage (AMD) matrix, can be transformed under solar irradiation, thus playing a significant role in elemental redistribution. However, the effects of oxalic acid on the converted mineral products and the fate of S and C during the dynamic process of Sch photoreduction transformation remain unclear. In this study, we investigated the transformation process and the elemental fate of Sch under the synergistic effect of oxalate and light radiation. Characterizations with X-ray diffraction (XRD), scanning transmission electron microscopy (SEM), Fourier transform infrared (FTIR), and Raman spectra at different concentrations of oxalate indicated that solar irradiation and oxalate resulted in the formation of goethite and/or humboldtine. Upon treatment with lower oxalate concentrations of 0.5 and 1.0 mM, Sch could be dissolved and recrystallized to goethite by Fe(II) generated from iron-oxalate complex under ultraviolet radiation (365 nm) at pH 3.0. However, on increasing the oxalate concentration to 5 and 10 mM, goethite proportion decreased and new mineral humboldtine appeared. Therefore, the change of oxalate dosage altered the oxalate-induced Sch dissolution and photoreduction transformation pathway and end product. To sum up, these results provide critical information for understanding the environmental behavior of S and C during the conversion process of secondary minerals under complex natural influencing factors in AMD environments.