Cr(iii)-bearing schwertmannite transformation by Fe(ii)-oxalic acid catalysis: complexation of Fe(iii)/oxalate and nanoscale redistribution of Cr/C

It has been widely reported that the Fe(ii)-catalyzed heavy metals bearing Fe minerals affect the redistribution of heavy metals and the phase transition of minerals in the acid mine drainage (AMD) environment. However, the effects and mechanisms of interactions among organic acids, dissolved Fe(ii), and Fe oxide minerals on the heavy metal fate under different pH conditions are still unclear. In this study, using oxalic acid (OA) as a surrogate, batch in situ attenuated total reflectance-Fourier transform infrared spectrometry experiments and spherical aberration corrected scanning transmission electron microscopy were employed to investigate the influence of OA on Fe(ii)-induced Cr(iii)-bearing schwertmannite (Cr-Sch) transformation and Cr/C redistribution at pH 3.5-7.0. The overall relative intensity of OA adsorption on the mineral surface decreased in the presence of Fe(ii), and the proportion of the bidentate binuclear complex (BB) increased. The binding mode of BB and the bidentate mononuclear complex diminished with increasing pH. However, the outer-sphere complex (OS) and aqueous oxalate (Ox) ions gradually became dominant, regardless of whether or not Fe(ii) was present. OA was adsorbed on the goethite surface and Cr was mostly aqueous and extractable state in the presence of Ox. The synergism of OA and Fe(ii) accelerated mineral dissolution but impeded Fe(iii) recrystallization at pH > 5.5 and directly transformed to goethite from Sch. This study provides new insights into the transformation of Cr-Sch and the fate of Cr(iii) under the coexistence of Fe(ii) and OA in an AMD environment, particularly in a watershed obviously changed by pH.

Automated summary

It has been reported that Fe(ii)-catalyzed Fe-bearing minerals play a role in the redistribution of heavy metals and phase transition of minerals in acid mine drainage (AMD) environments. However, the effects and mechanisms of interactions among organic acids, Fe(ii), and Fe oxide minerals on heavy metal fate under different pH conditions are still not well understood. In this study, oxalic acid (OA) was used as a surrogate to investigate the influence of OA on the transformation of Fe(ii)-induced Cr(iii)-bearing schwertmannite (Cr-Sch) and Cr/C redistribution at pH 3.5-7.0. The results showed that the presence of Fe(ii) decreased the overall relative intensity of OA adsorption on the mineral surface and increased the proportion of a bidentate binuclear complex (BB). The binding mode of the BB and a bidentate mononuclear complex decreased with increasing pH. However, regardless of the presence of Fe(ii), the dominance gradually shifted to an outer-sphere complex (OS) and aqueous oxalate (Ox) ions. The study also found that OA was adsorbed on the goethite surface and Cr was mostly in an aqueous and extractable state in the presence of Ox. The synergy of OA and Fe(ii) accelerated mineral dissolution but impeded Fe(iii) recrystallization at pH > 5.5, leading to a direct transformation from schwertmannite to goethite. This study provides new insights into the transformation of Cr-Sch and the fate of Cr(iii) under the coexistence of Fe(ii) and OA in an AMD environment, particularly in a watershed that has experienced significant pH changes.