Novel insights into the dehalogenation mechanism and ecotoxicity assessment of 6:2 Cl-PFESA from density functional theory calculations

The electroplating industry is the main source of 6:2 chlorinated polyfluorinated ether sulfonate (6:2 Cl-PFESA) pollution, which presents risks to human health and the environment. It is therefore crucial to develop effective 6:2 Cl-PFESA degradation techniques. Persulfate oxidation is a potential treatment method for 6:2 Cl-PFESA due to its outstanding oxidative degradability following the generation of the sulfate radical (SO4•−) and hydroxyl radical (•OH). It has proven difficult to acquire a full understanding of the reaction mechanism and formation of intermediate (IM) products through conventional experimental studies because they are costly and time-consuming. Therefore, a theoretical analysis method based on density functional theory (DFT) calculations was applied. The DFT results showed that electron transfer for the degradation of 6:2 Cl-PFESA could be initiated by the protonated sulfate radical (HSO4•, ΔG≠SET = 9.16 kcal/mol), rather than SO4•− (ΔG≠SET = 41.60 kcal/mol). After desulfonation, the reaction underwent stepwise decarboxylation cycles under the action of •OH, leading to the elimination of the CF2 units until there was complete mineralization into HCl, HF, and CO2. Furthermore, the IMs and the end products of 6:2 Cl-PFESA were evaluated using ECOSAR and TEST software. The low bioaccumulation of the short-chain IMs meant that they could be considered safe in terms of ecotoxicity and health effects. This research determined the theoretical and mechanistic basis of the effects of persulfate in the treatment of water containing 6:2 Cl-PFESA, and its structural analogues.