Strategy and mechanisms of sulfamethoxazole removal from aqueous systems by single and combined Shewanella oneidensis MR-1 and nanoscale zero-valent iron-enriched biochar

Sulfamethoxazole (SMX, a sulfonamide antibiotic) is ubiquitously present in various aqueous systems, which can accelerate the spread of antibiotic resistance genes, induce genetic mutations, and even disrupt the ecological equilibrium. Considering the potential eco-environmental risk of SMX, this study explored an effective technology using Shewanella oneidensis MR-1 (MR-1) and nanoscale zero-valent iron-enriched biochar (nZVI-HBC) to remove SMX from aqueous systems with different pollution levels (1-30 mg·L-1). SMX removal by nZVI-HBC and nZVI-HBC + MR-1 (55-100 %) under optimal conditions (iron/HBC ratio of 1:5, 4 g·L-1 nZVI-HBC, and 10 % v/v MR-1) was more effective than its removal by MR-1 and biochar (HBC) (8-35 %). This was due to the catalytic degradation of SMX in the nZVI-HBC and nZVI-HBC + MR-1 reaction systems because of accelerated electron transfer during oxidation of nZVI and reduction of Fe(III) to Fe(II). When SMX concentration was lower than 10 mg·L-1, nZVI-HBC + MR-1 effectively removed SMX (removal rate of approximately 100 %) when compared to nZVI-HBC (removal rate of 56-79 %). In addition to oxidation degradation of SMX by nZVI in the nZVI-HBC + MR-1 reaction system, MR-1-driven dissimilatory iron reduction accelerated electron transfer to SMX, thereby enhancing reductive degradation of SMX. However, a considerable decline in SMX removal from the nZVI-HBC + MR-1 system (42 %) was observed when SMX concentrations ranged 15-30 mg·L-1, which was due to the toxicity of accumulated degradation products of SMX. A high interaction probability between SMX and nZVI-HBC promoted the catalytic degradation of SMX in the nZVI-HBC reaction system. The results of this study provide promising strategies and insights for enhancing antibiotic removal from aqueous systems with different pollution levels.