Novel insights into ferrate (VI) activation by Mn-modified sludge biochar for sulfamethoxazole degradation: Dominance of hydroxyl group and Mn-O bond in the non-radical pathway

Ferrate (Fe (VI)), as a versatile oxidizer, has been widely employed for water treatment. However, the rapid self-decomposition of Fe (VI) diminishes its practical application efficiency. In this study, a novel Mn-modified sludge biochar (MSBC) was synthesized for the first time to activate Fe (VI) and generate highly reactive Fe (IV)/Fe (V) for the rapid removal of sulfamethoxazole (SMX). The results showed that MSBC (0.1 g/L) effectively activated Fe (VI) (100 μM), and 87.39 % of SMX (20 μM) and 40.26 % of total organic carbon (TOC) were removed within 10 min. Notably, raising the solution pH (e.g., from 6.0 to 11.0) would result in decreasing the reactivity of Fe (VI) and a lower removal efficiency of SMX. The quenching, electron paramagnetic resonance and probe experiments suggested that •O2– and high-valent iron species (Fe (V)/Fe (IV)) were identified as the major contributions to the removal of SMX. The detailed activation sites of MSBC were –OH and Mn-O, as corroborated by density functional theory (DFT) calculation and characterization. These activation sites facilitated activation of Fe (VI) through efficient electron transfer. The Fukui index indicated that the N, S, and O atom of SMX were the primary attacked sites. Subsequently, five potential degradation pathways were proposed, with the cleavage of the S-N bond being the predominant one. The toxicity of these products was examined using ECOSAR program, revealing that main products showed low toxicity or non-toxicity. The Cl-, SO42-, and NO3– had negligible effect on SMX degradation, although excessive concentrations of HCO3– and humic acid (HA) showed slightly inhibition. Additionally, the Fe (VI)/MSBC system also effectively removed 87.34 % of sulfadiazine (SDZ) and 93.91 % of sulfamethoxypyridazine (SMP). Overall, this study offered a practical and cost-effective approach for the activation of Fe (VI) and provided new insights to the degradation mechanism.