Dual role of pyrogenic carbon in mediating electron transfer from clay minerals to chromium in aqueous and solid media

Abstract

Clay minerals (CMs) and pyrogenic carbons (PCs) often co-exist in the environment and participate in the redox cycling of pollutants. This study unveiled the dual role of PCs in CM-dominated chromium transformation in both aqueous and agar solidification media. The findings showed that CMs and PCs adsorbed minimal Cr(VI), while reduced CMs and PCs displayed a substantial difference by directly reducing Cr(VI) to solid/dissolved Cr(III) through reactive structural Fe(II) and functional groups, respectively. Moreover, dissolved PCs were found to mediate electron transfer from reduced CMs to Cr(VI) in aqueous and solid media. Interestingly, the effect of solid PCs on Cr(VI) reduction by reduced CMs was concentration-dependent. At lower concentrations, solid PCs dispersed reduced CMs, acting as electron mediators and facilitating both direct and indirect Cr(VI) reduction, resulting in solid Cr(III) rather than dissolved Cr(III). Conversely, at higher concentrations, solid PCs served as redox buffers, storing electrons transferred from reduced CMs to Cr(VI). In either case, the transformed chromium was primarily immobilized on the surface of CMs rather than PCs. These findings offer valuable insights into pollutant transformations associated with CMs and PCs, deepening our understanding of their geochemical processes.