Oxidation Mechanism Of Carbon Nanotube/Peroxydisulfate System And Control Of Halogenated Byproducts Formation

Peroxodisulfate?PDS?as one common oxidant has gained increasing attention in the field of water treatment in recent years,because it is environmentally friendly,low cost,and easy for storage and transportation.Usually,the application of PDS requires certain external conditions to stimulate its decomposition to produce sulfate radical(SO₄^·-)with a strong oxidation ability,which is responsible for the efficient degradation of organic micropollutants in water.A few studies have reported that some carbon materials can enhance the oxidation ability of PDS through a nonradical pathway,but the underlying mechanism is not clear.In this study,commeicial carbon nanotubes?CNT?were used as the activitor of PDS.The nonradical mechanism of CNT activiting PDS was deeply explored and the generated reactive oxidative species were identified.Oxidation efficiencies and products of various organic micropollutants by the PDS/CNT system were investigated.The transformation of inorganic halogen ions and typical halogenated organics in the PDS/CNT system was emphatically studied,and accordingly the formation potential of halogenated byproducts was evaluated.The PDS/CNT system shows a significant selective reactivity toward organic compounds oxidation.It has a high reactivity toward electron-rich phenolic contaminants and sulfonamide antibacterials,while recalcitrant aromatic compounds such as benzoic acid are hardly oxidized.Meanwhile,similar substrate-selective reactivities are observed in cases of PDS activation by different types of CNTs.By means of radical and singlet oxygen scavenging experiments,electron paramagnetic resonance spectroscopy,Fourier transform infrared spectroscopy as well as monitoring PDS decomposition,it is demonstrated that the nonradical mechanism of CNT activating PDS is mainly via the interaction of PDS with certain CNT surface active sites?e.g.,the adjacent domains of nitrogen atoms and defective sites?to form reactive complexes.The relatively mild oxidation capacity of these complexes is responsible for their strong selectivity toward organic compounds oxidation.Acetaminophen?ATP?was selected as the representative of phenolic contaminants,and its oxidation kinetics and products by the PDS/CNT system were systematically investigated.Results suggest that the PDS/CNT system has an appreciable reactivity toward ATP.Under the condition investigated,the maximum oxidation rate of ATP is observed at neutral p H condition.The observed pseudo-first-order rate constant?kobs?is 0.6682 min^-1.Both the increase and decrease of p H result in reduction of ATP oxidation rate.The kobs values at p H 5 and 9 are 0.3045 and 0.4567 min^-1,respectively.With the increase of PDS dosage,the oxidation rate of ATP in PDS/CNT system first increases and then remains constant.In other words,the PDS/CNT system exhibits the saturation kinetics with respect to PDS dosage in organics oxidation.ATP oxidation rate increases linearly with the increase of CNT dosage.The presence of chloride ion and carbonate/bicarbonate hardly impacts the degradation of ATP,while humic acid shows a certain degree of inhibition on ATP oxidation.The oxidation of ATP by the PDS/CNT system leads to the formation of various intermediate products.The reaction pathways mainly involve hydroxylation,coupling of radicals,intramolecular dehydration,as well as transformation of acetylamino to nitroso groups.The PDS/CNT system has an extremely low reactivity toward bromide ion?Br^-?,and their reaction does not lead to the formation of hypobromous acid?HOBr?or bromate?BrO₃^-?.Brominated organics such as 2-bromophenol?2-Br P?,3-bromophenol?3-Br P?,4-bromophenol?4-Br P?can be effectively degraded by the PDS/CNT system.Under the condition investigated,their kobs values at neutral p H are 0.1870,0.0951,0.1223 min^-1,respectively.Multiple polymeric products,maily involving hydroxylated polybrominated diphenyls as well as hydroxylated polybrominated biphenyl ethers,are produced during reactions of Br Ps with the PDS/CNT system.Their formation pathways maily involve the generation of bromophenoxyl radicals through one-electron oxidation of Br Ps by the PDS/CNT system,and their subsequent coupling reactions.Despite the enhanced toxicological effects of these brominated polymeric products compared to the precursor compounds,they can be nearly completely removed by CNT adsorption owing to the good adsorption capability of CNT.So,these products are not detected in treated water effluents.In addition,the oxidation products of bromophenols by SO₄^·-are quite different from those formed in the PDS/CNT system,revealing the essential difference between radical vs nonradical oxidation processes.In contrast to Br^-,iodide ion?I-?fast reacts with the PDS/CNT system to generate hypoiodous acid?HOI?at an equal stoichiometry.But the further transformation of HOI to iodate?IO₃^-?is negligible.The presence of I-accelerates the oxidation of phenolic compounds in the PDS/CNT system.For instance,under the condition investigated,the oxidation rate of p-hydroxybenzoic acid?p-HBA?is increased from 0.0058 to 0.0122 min^-1 by the presence of 10 ?M I-;that is,kobs value is increased by about two times.This is primarily attributed to the reason that HOI derived from I-is considerably reactive toward phenolic compounds.p-Hydroxybenzoic acid?p-HBA?was selected as the representative to explore the formation of iodinated oxidation byproducts during organics oxidation by the PDS/CNT system in the presence of I-.Results suggest that the transformation of p-HBA by the PDS/CNT/I-system results in formation of multiple iodinated aromatic products.Their formation pathways mainly include the substitution of HOI on aromatic ring of p-HBA,decarboxylation,one-electron transfer as well as coupling of radicals.Similar iodinated aromatic products are also detected in iodide-spiked real water samples.Based on semi quantitative analysis of these products,it is found that their levels first increase as the reaction proceeds,and reaches the maxima and then gradually declines to zero.This indicates that the PDS/CNT system is able to further degrade the formed iodinated aromatic byproducts.Moreover,the PDS/CNT system can hardly oxidize iopamidol?IPM?that is selected as a representative of iodinated organics.This can be attributed to the low reactivity of nonradical reactive species toward macromolecular organic compounds with complex structures.Therefore,the PDS/CNT oxidation technology has a relatively low risk of generating brominated or iodinated oxidation byproducts when used in treatment of organic micropollutants in water.