Complete degradation of 2,4-dichlorophenol in a sequential sulfidated nanoscale zero-valent iron/peroxydisulfate system: dechlorination, mineralization and mechanism†

Chlorophenols (CPs) have strong toxicity because of the presence of chlorine atoms. Although dechlorination can eliminate their toxicity, their by-products may cause secondary pollution. In this study, a two-step process of pre-reduction dechlorination and oxidation, reductive dechlorination by sulfidated nanoscale zero-valent iron (S-nZVI) and advanced oxidation by S-nZVI-activated peroxydisulfate (PDS), was innovatively adopted to achieve efficient and complete mineralization of 2,4-dichlorophenol (2,4-DCP). The pre-reduction of S-nZVI achieved 80% dechlorination of 2,4-DCP. With the subsequent addition of PDS, 2,4-DCP and its dechlorination by-products in the solution were almost completely removed, and the mineralization rate reached 91.5% under the optimal conditions of unadjusted initial pH (5.4), S-nZVI dosage 2.5 g L⁻¹, and PDS concentration of 1.8 mM. The electron spin resonance (ESR) and radical quenching experiments demonstrated that both ·OH and SO₄⁻ were involved in the degradation of 2,4-DCP, while SO₄⁻ played a more predominant role. Based on the transformation products of 2,4-DCP identified by GC-MS, the degradation mechanism of 2,4-DCP in this system included two steps, namely, reductive dechlorination induced by electron transformation and oxidation degradation involving single electron transfer, radical adduct formation, and hydrogen atom abstraction. This study demonstrated that the novel S-nZVI pre-reduction and sequential S-nZVI/PDS process is a very promising and efficient approach for the complete removal of CPs in water.