Dual activation of persulfate by ZnFe₂O₄ and sulfur reductants for pesticide-contaminated groundwater

Persulfate (PS)-based treatment of pesticide-contaminated groundwater is often limited by short-lived radical generation and poor performance under environmentally relevant conditions. This study develops a dual activation system by integrating ZnFe₂O₄ with sulfur-containing reductants (SCRs), specifically bisulfite (BS) and dithionite (DTN), to enhance PS activation for the degradation of co-contaminated atrazine (ATZ), alachlor (ALA), and malathion (MLT). The system operates through complementary pathways, where SCRs promote rapid initial radical formation while ZnFe₂O₄ sustains continuous activation via Fe³⁺/Fe²⁺ redox cycling. Distinct differences were observed between the reductants. BS-enabled activation exhibited more stable and controlled degradation behavior, maintaining high efficiency across a wide pH range (3–11), under oxygen-limited conditions, and in natural groundwater matrices. In contrast, DTN induced rapid initial oxidation but led to less stable performance due to stronger reductive conditions and transient redox imbalance. Mechanistic analysis revealed that oxygen availability governs reactive species distribution, with SO₄^•⁻ dominating under anaerobic conditions and additional contributions from ^•OH and oxygen-derived species under aerobic environments. Transformation pathway analysis indicated that MLT was converted into less toxic intermediates, which was further supported by improved cell viability in REM134 and MDCK assays. Although initial Zn release was observed, rapid stabilization in subsequent cycles indicates surface equilibration rather than structural degradation. Overall, the ZnFe₂O₄/PS/BS system demonstrates superior stability and adaptability, providing a robust strategy for pesticide remediation in complex groundwater environments.