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

Chlorophenols (CPs) have strong toxicity because of the presence of chlorine atom. Although the dechlorination can eliminate their toxicity, by-product organics maybe bring 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 solution was almost completely removed and the mineralization rate reached to 91.5% under the optimal conditions of unadjusted initial pH (5.4), S-nZVI dosage 2.5 g·L-1, and PDS concentration 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 the more predominate 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 electrons transformation and oxidation degradation involving single electron transfer, radical adduct formation, and hydrogen atom abstraction. This study demonstrated that the noval S-nZVI pre-reduction and sequential S-nZVI/PDS process is a very promising and efficient approach for complete removal of CPs in water.