Effects Of Persulfate Oxidation On Halogenation Of Natural Organic Matter And Control Of Halogenated By-products

Activated persulfate（PS）oxidation is a promising in situ remediation technology for groundwater and soils.Sulfate radical(SO₄^·-)has a high redox potential（E⁰=2.5～3.1V,vs.NHE）which react with organic compounds by mechanisms of Habstraction,electron transfer,and addition.Application of this technology to contaminated zones may result in a large quantity of PS residue in subsurface environment due to inefficient activation and repeated injection during remediation.Bicarbonate is one of the anions ubiquitously present in groundwater environment.In this study,we demonstrated that natural organic matter（NOM）molecules could be reconfigured due to exposure to unactivated PS and bicarbonate,resulting in reduced disinfection by-products（DBPs）formation potential in post chlorination process.Fourier transformed inferred spectrometry（FTIR）,size exclusive chromotraghy（SEC）,and mass spectrometry（MS）analysis revealed that hydroxylation and carboxylation of NOM occurred,followed by inter-molecular coupling via ether bonds.The change of both the reactivity toward free chlorine and molecular structure of NOM during PS/bicarbonate treatment was well mimicked by 3,5-dihydroxylbenzoic acid（DHBA）,suggesting that phenolic moieties in NOM molecules were the main sites underwent transformation in the PS/bicarbonate system.It is proposed that peroxymonocarbonate（HCO₄^-）formed upon the reaction between PS and bicarbonate was the main reactive species responsible for the reconfiguration of NOM.It selectively attacked the phenolic moieties via single-electron abstracting mechanism,leading to phenoxy radical intermediates which couple to each other via C-O-C bonds.Halides are also ubiquitous presented in natural waters.In activated PS oxidation processes,SO₄^·-can oxidize halides to reactive halogen species（RHS）,including halogen radicals(X·,X₂^·-)and free halogens（X₂,HXO）.RHS immediately react with NOM by substitution or addition,resulting in the formation of toxic halogenated DBPs.In this study,it was revealed that the formation of brominated DBPs（Br^-DBPs）in heatactivated PS process decreased with the addition of hydrogen peroxide（H₂O₂）,and the yields of Br^-DBPs were negatively related with the dosage of H₂O₂.Kinetic modeling indicated that the Br·and free bromine can be rapidly reduced back to Br^-by H₂O₂.However,MS analysis found that addition of H₂O₂ could not completely inhibit the formation of brominated intermediates.According to model simulation,it is likely that a fraction Br·reacted with Br^-generating Br₂^-· and BrOH^·-.These bromine radicals react with organics to form brominated intermediates.The findings of this study shed light on the environmental behaviors and impacts of PS in groundwater environment.