Clay minerals (CMs) and pyrogenic carbons (PCs), common in various environmental settings, play significant roles in the redox cycling of pollutants like chromium (Cr). This study explores the complex interactions between CMs and PCs, particularly focusing on their capacity to transform toxic hexavalent chromium (Cr(VI)) into the less harmful trivalent form (Cr(III)).

The research demonstrates that both CMs and PCs adsorb minimal amounts of Cr(VI). However, their reduced forms are pivotal in directly reducing Cr(VI) to Cr(III), primarily through the reactive structural iron (Fe(II)) and functional groups present within them. Notably, dissolved PCs facilitate the electron transfer from reduced CMs to Cr(VI) in both liquid and solid agar media. This mediation is crucial for the transformation process where the formed Cr(III) is largely immobilized on the surface of CMs rather than PCs, highlighting the selective nature of chromium immobilization.

Further findings reveal that the impact of solid PCs on Cr(VI) reduction is concentration-dependent. At lower concentrations, PCs disperse reduced CMs and act as electron mediators, enhancing both direct and mediated Cr(VI) reduction and favoring the formation of solid Cr(III). Conversely, at higher concentrations, PCs function as redox buffers, storing electrons and diminishing Cr(VI) transformation.

These insights are instrumental for environmental science, offering a deeper understanding of the geochemical processes involving CMs and PCs in pollutant transformations. The study’s methodologies, which include a combination of wet chemistry techniques and spectroscopy, provide a robust framework for future research on the interactions between these minerals and their roles in environmental remediation.

Overall, the dual role of PCs, whether as electron mediators or redox buffers, underscores their complex behavior in environmental processes and their potential utility in the biogeochemical cycling of contaminants.