L-Cysteine boosts Fe2+/Fe3+ cycling in Fe-doped cellulose-derived hydrothermal carbonation carbon for enhanced peroxydisulfate activation and diclofenac degradation.

L-cysteine (L-Cys) is an ideal cocatalyst that enhances the redox cycle of transition metals and accelerates chemical processes, widely used for promoting pollutant degradation in homogeneous systems. However, its role in heterogeneous systems, mechanisms and pathways, remains unclear. In this study, iron-based hydrothermal carbonation carbon (HTCC-Fe) was prepared by reconstituting reed cellulose through hydrothermal carbonization, acid etching, and Fe doping methods. L-Cys promoted the conversion of Fe3+ to Fe2+, which in turn strengthened peroxydisulfate (PDS) activation and increased reactive species formation in the HTCC-Fe/PDS system, with •OH serving as the primary contributor to diclofenac (DCF) degradation, followed by 1O2 and O2•-. This degradation occurred through pathways including hydroxylation, dehydration, dechlorination, and C-N bond cleavage. As a result, the HTCC-Fe/PDS/L-Cys system achieved a 41.5% higher DCF degradation efficiency than the HTCC/PDS system, coupled with broad pH applicability, strong resistance to anions interference, and effective DCF removal (>70%) in real water matrices. By elucidating the previously unclear role of L-Cys in heterogeneous systems, this work revealed its co-catalytic mechanism in promoting electron transfer and regulating radical/non-radical pathways for enhanced pollutant degradation.