Release dynamics of As, Co, and Mo in a biochar treated soil under pre-definite redox conditions

This study assessed the impact of pre-definite redox potential (E_H) on the release dynamics and distribution of As, Co, and Mo between the dissolved and colloidal phases as well as their potential mobility and phytoavailability in the sediment phase of a mining soil treated with rice hull biochar (BC). The experiment was conducted from controlled moderately-reducing to oxidizing conditions using an automated biogeochemical microcosm system. Arsenic and Mo were more abundant in the dissolved phase due to their predominant in potential mobile fractions, while Co was more abundant in the colloidal phase due to its association with Fe-(hydr)oxides. Biochar increased the dissolved and colloidal concentrations of As, the dissolved concentration of Co, and the colloidal concentration of Mo under oxidizing condition. On the other hand, the application of BC decreased the dissolved concentration of Mo and the colloidal concentration of Co in the first redox cycle under reducing-acidic condition, due to lower pH values, and chemistry of sulfide-sulfate and Fe/Mn oxides. The phytoavailability of As and Co were higher than their potential mobility in the sediment phase, while the same trend was not discerned for Mo. The potential mobility and phytoavailability of As and Co were high under oxic-acidic conditions. The potential mobility and phytoavailability of Mo might be increased under oxic condition due to the dissolution of Fe and Mn oxides under lower pH conditions, especially in the BC treated soil. Application of such rice hull BC to soil might stimulate the release of As, Co, and Mo under flooding conditions, which might increase the environmental and health risks in such wetland ecosystems.