The Effect of Aeration on Mn(II) Sorbed to Clay Minerals and Its Impact on Cd Retention

Manganese is a redox-sensitive element in soils and sediments that plays an important role in the retention of trace elements. Under anoxic conditions, clay minerals were shown to increase Cd retention by favoring the precipitation of Mn(II) phases. In this study, we investigated the influence of aeration on anoxically formed Mn solid phases and its impact on Cd retention in the presence of two clay minerals with low Fe contents, a natural kaolinite (KGa-1b) and a synthetic montmorillonite (Syn-1). Ca-saturated KGa-1b and Syn-1 were pre-equilibrated with Mn2+ and Cd2+ under anoxic conditions for 1 or 30 days and subsequently exposed to air for 1 or 30 days. The analysis with synchrotron X-ray absorption spectroscopy (XAS) revealed that extended anoxic pre-equilibration (30 days) partially prevented the oxidation of sorbed Mn(II) (MnSiO3 and Mn(II)Al-LDH). Extended exposure to ambient air and short anoxic pre-equilibration favored the formation of feitknechtite (β-MnOOH) and birnessite (δ-MnO2). Aeration resulted in a decrease of pH and a net release of Cd2+ into the solution, indicating that Cd re-sorption by Mn(III/IV)-phases was insufficient to compensate for the release of Cd2+ due to dissolution of Mn(II)-containing phases and the decrease in pH. Our results demonstrate the significance of clay minerals in the (trans)formation of Mn-containing phases and their impact on trace metal retention in environments undergoing fluctuating redox conditions.