Efficient activation of persulfate by N, S, Fe co-doped rice husk biochar for degradation of sulfamethazine: Synergistic effect and degradation mechanism

Nanoscale zerovalent iron (nZVI) has attracted extensive attention in the oxidative decomposition of organic matters as the persulfate (PS) activator due to its tunable physical/chemical properties and unique electronic structures. However, the trend of agglomeration and ease of oxidation for nZVI seriously hinder its further application. Herein, a composite catalyst composed of polyethyleneimine modified rice husk biochar (RHB-PEI) loaded with sulfide modified nZVI (S-nZVI) was constructed and its micro-structure was characterized by STEM, SEM, XRD, FT-IR and XPS. Results show that a layer of FeS shell with a thickness of ∼ 3 nm was formed around nZVI, and the whole core-shell structure was uniformly loaded on the N-doped RHB. The as-synthesized catalyst was used as a PS activator to remove sulfamethazine (SMT), a typical sulfonamide antibiotic. Compared with nZVI, the removal rate ( k obs ) of S-nZVI@RHB-PEI increased from 0.091 min⁻ 1 to 0.287 min⁻ 1 within 60 min, and the removal efficiency enhanced by 10.45 times in the fifth cycle experiment. The improvement of degradation performance in S-nZVI@RHB-PEI/PS system is not a simple accumulation caused by different heteroatom doping, but the synergistic effect from different heteroatom doping. The activation of PS by S-nZVI@RHB-PEI can effectively generate reactive oxygen species (ROS), including 1 O 2 , O 2 • ⁻, SO 4 • ⁻ and •OH, in which 1 O 2 played a major role. To be specific, under the attack of ROS, SMT was oxidized through a series of ring opening, SO 2 extrusion, Smiles rearrangement, and bond cleavage. This study verified the synergistic effect of N, S and Fe co-doped RHB on SMT degradation, the resultant catalyst demonstrated a higher electron transfer efficiency and thus exhibited excellent degradation efficiency and sustainable utilization over a wider pH range, which will advance the rational design of biochar supported nZVI catalysts and offer insights into the PS activation mechanism for the efficient degradation of SMT.