Enhanced oxidative capacity of red mud-lignin composite/peroxydisulfate to degrade sulfamethazine through electron transfer mechanism.

• The catalyst activated PDS efficiently with high stability and generalizability. • The complexation of AL with RM favored the exposure of Fe active sites and adsorption of PDS. • The electron transfer pathway was the main pathway for PDS activation and SMZ degradation. • DFT demonstrated that Fe-active species are the primary site where electron transfer occurs. • T.E.S.T and biological experiments predicted lower toxicity of degradation intermediates. Advanced oxidation processes based on peroxydisulfate (PDS-AOPs) have been used for the remediation of organic wastewater. However, the mechanism for the generation of electron transfer sub-stable complexes in the metal–carbon/PDS system has not been fully elucidated. Herein, two environmental solid wastes were selected to synthesize a novel Fe-carbon catalyst (RM@KC 1:2 –800) using alkali lignin as the carrier and red mud (RM) as the iron source, and applied for peroxydisulfate (PDS) activation in sulfamethazine (SMZ) removal. The complexation of alkali lignin with RM favored the exposure of Fe active sites and adsorption of PDS. 97% of SMZ could be removed efficiently in the RM@KC 1:2 –800/PDS system, and the degradation efficiency remained at 80.81% after 5 cycles without metal leaching, which was suitable for a variety of natural water bodies. Combined with density functional theory (DFT) and other analyses confirmed that the electron transfer pathway is the main pathway for PDS activation and SMZ degradation. And the reactive Fe species were the active sites that contributed to the adsorption of PDS on the RM@KC 1:2 –800 surface to form sub-stable complexes (RM@KC-PDS*), whereby the electron transfer occurred. Surprisingly, the prediction of environmental toxicity of the degradation intermediates based on T.E.S.T software and biological experiments proved their environmental friendliness. Overall, this study follows the concept of "waste-to-waste" and proposes a strategy for a suitable and cost-effective Fe-carbon catalyst for large-scale production, while demonstrating its potential application in the treatment of organic wastewater. [ABSTRACT FROM AUTHOR]