Enhanced degradation of sulfamethoxazole by activation of peroxodisulfate with red mud modified biochar: Synergistic effect between adsorption and nonradical activation

Abstract

Since the discovery of the potential environmental risk associated with large quantities of red mud, there has been an urgent need to find a rational disposal technology for this waste. In this study, low-cost iron-based biochar catalysts (RM@BC-x) were synthesized by co-pyrolysis of waste red mud and spent coffee grounds after ball milling. RM@BC-800 exhibited remarkable adsorption capacity, reaching 4.70 mg/g in 120 min. The adsorbed SMX was degraded simultaneously after adding peroxodisulfate (PDS), and sulfamethoxazole (SMX) was completely removed from the solution in only 30 min. Correlation analysis and pre-sorption experiment proved that the adsorption of SMX significantly enhanced the oxidation effect. Moreover, the Fe0, defects and CO groups of RM@BC-800 were the main active sites. Singlet oxygen (1O2) played a dominant role in the degradation of SMX rather than the traditional radicals (•OH or SO4•−). In particular, O2•− was shown to be used mainly as a precursor for the production of 1O2 without directly oxidizing SMX. The steady state concentration of 1O2 has been calculated to be hundreds or thousands of times higher than that of other radicals. Five degradation pathways of SMX were proposed (including NS bond cleavage, hydroxylation, and –NH2 oxidation). RM@BC-800 has the advantages of strong anti-interference ability, low iron leaching based on energy calculations and environmental impact assessments. This work provides a new option for the resource utilization of red mud and designs a green material with fast adsorption and efficient catalysis for the practical treatment of antibiotic wastewater.