Sulfate and hydroxyl radicals-initiated degradation reaction on phenolic contaminants in the aqueous phase: Mechanisms, kinetics and toxicity assessment

Abstract

Phenolic organic contaminants (POCs) in water environment have been usually degraded by advanced oxidation processes (AOPs) based on sulfate radicals (SO4·-) and hydroxyl radicals (OH). In this paper, the first step of SO4·-/·OH-initiated oxidation reactions toward 19 POCs were investigated using density functional theory (DFT) in order to explore and compare the reactivity of POCs with two free radicals in the aqueous phase. The oxidation reactions initiated by SO4·-/·OH were confirmed that POCs can follow three reaction mechanisms: radical adduct formation (RAF), hydrogen atom abstraction (HAA), and single electron transfer (SET). The rate constants of all primary oxidation reactions were calculated using transition state theory (TST). The results turn out that the stronger the electron donating effects of the substituent on POCs, the better the reactivity of POCs with two free radicals. SET mechanisms are main reaction pathways for SO4·--initiated oxidation reactions. Furthermore, the ecotoxicity assessment shows that most OH adducts have higher toxic on aquatic organisms than corresponding reactants. For all the POCs covering in this work, the order of acute toxicity is p-DP > o-AP > m-AP (p-AP) > MP > CP > NP > m-DP (o-DP) > phenol > YP, while the chronic toxicity is in the order p-DP > o-AP > m-AP (p-AP) > MP > CP > NP > YP > phenol > m-DP (o-DP). Thus, the application of AOPs for the removal of POCs should be taken seriously.