Dual nonradical pathways in enhanced PDS activation by Fe@BC-S: Fe7S8 mediated the relationship between 1O2 and electron transfer

Abstract

Biochar (BC)-based catalysts have been widely utilized in the activation of peroxydisulfate (PDS), representing an effective water treatment technique for decontamination purposes. However, the mechanisms of PDS activation by S and Fe co-doped biochar hybrid catalysts were still ambiguous. Therefore, Fe-S biochar by exogenous doping (Fe@BC-S) was prepared to activate PDS for sulfamethazine (SMT) degradation, and Fe₇S₈ sites were formed in Fe@BC-S acting as the main active center. The SMT degradation efficiency within 50 min in Fe@BC-S/PDS system (95.3 %) was much higher than that in Fe@BC/PDS system (62.5 %). It was found that the dual nonradical pathways by ¹O₂ and electron transfer dominated in the more efficient PDS activation and SMT degradation. The detailed mechanisms in the activation process could be determined to: (i) Successive ¹O₂ generation mediated via Fe(II)/Fe(III) redox cycle with a steady-state concentration of 5.66 × 10^-13 M. And the ¹O₂ was derived from not only reaction process of PDS → O₂^·-→ ¹O₂, but also the oxidation of the oxygen molecules in the sub-stable complexes; (ii) The doping of S promoted the electrons transfer capability, enabling the formation of sub-stable complexes between Fe@BC-S and PDS through outer-sphere interactions, with Fe₇S₈ acting as an electron intermediate mediating the electron transfer process from the pollutants to PDS. Further, the dual nonradical pathways by ¹O₂ and electron transfer selectively attacked the electron-rich sites of SMT. In addition, Fe@BC-S exhibited high environmental adaptability and stability. This work suggested a new strategy for biochar modification and provided new insights into the enhanced water decontamination by nonradical pathways.