Efficiency And Mechanism Of Removal Of Cu(?)-organic Complexes By UV/Chlorine Advances Oxidation Process

With the rapid development of industry,heavy metal pollution has become a hot and difficult issue in environmental abatement.Heavy metal pollution resulting from many industries such as electroplating,printed circuit boards,chemicals and printing and dyeing causes serious negative impacts on human health and ecological environment due to the effects of carcinogenesis,teratogenesis and mutagensis.Heavy metals such as copper,nickel,and chromium in these industrial wastewaters usually presents in the form of stable complexes with organics because of the massive use of organic complexing agents such as ethylenediaminetetraacetic acid?EDTA?,ammonia triacetic acid?NTA?,citric acid,and tartaric acid.Those metal complexes are difficult to be effectively removed by conventional methods such as precipitation,adsorption,ion exchange,so new strategies are in urgent need for efficient removal of heavy metal complexes from industrial effluents.In recent years,the UV/chlorine advance oxidation process?AOP?has received increasing interest,because the simultaneous production of diverse reactive species,including hydroxyl radicals?HO·?and active chlorines(e.g.,Cl·,Cl₂^·-and ClO·)exerts complementary effects in degrading persistent organic pollutants including pharmaceuticals,pesticides,and tatse and odor compounds,and it is expected to achieve the decomplexation of heavy metal complexes.In this study,copper complexes?such as Cu-EDTA,Cu-NTA,Cu-citric acid and Cu-tartaric acid?were chosed as target compounds,and the performance and mechanism of UV/chlorine AOP in removing metal complexes were systematically studied.The results showed that UV/chlorine can achieve complete removal of Cu-EDTA within 15 min,and Cu-EDTA degradation exhibits atwo-stage kinetics;The remaining Cu is less than 1mg/L within 60 min,which are much higher than single UV irradiation or a single chlorination.The increase of chlorine dosage promoted the degradation of Cu-EDTA and the removal of Cu.The decrease of initial pH of solution accelerated the degradation of Cu-EDTA,but it was not conducive to the removal of Cu.Coexisting sulfate,nitrate and NOM showed some inhibition on Cu removal.In addition,UV/chlorine was capable of destructing other Cu complexes such as Cu-NTA,Cu-citric acid and Cu-tartaric acid and copper complexes of actual electroplating wastewater and the remaining Cu is less than 1 mg/L at the condition of the molar ratio of chlorine dosage to initial Cu located in 60¹50.With the rapid degradation of Cu-EDTA,the rate of chlorine decomposition is also accelerated,indicating that the degradation products of Cu-EDTA promote chlorine decomposition to produce active oxide species.During the degradation of CuEDTA,Cu?I?was detected from several?M to several tens of?M,and its formation accelerated with the increase of pH,and both exhibited a good linear relationship.Those indicated that Cu?I?plays a key role in the degradation of Cu-EDTA and the generation of free radicals,and is the rate-limiting step of Cu-EDTA.The degradation of Cu-EDTA is little affected by the increase of Cl^-,indicating the negligible contribution of Cl₂^·-;100 mM HCO₃^-and tert-butanol significantly inhibited the degradation of Cu-EDTA,indicating that the role of ClO·can be ruled out,and HO·and Cl·are the main reactive species in the Cu-EDTA degradation by UV/chlorine.The competitive kinetics of Cu-EDTA,NB and BA showed that Cl·is the main reactive speices and it fully accounted for Cu-EDTA degradation.The reaction rate of Cu-EDTAwith Cl·was estimated to be the diffusioncontrolled rate(¹0¹00 M^-1s^-1).Electron paramagnetic resonance?EPR?spectra further confirm that Cl·was the dominated speices in Cu?I?/UV/chlorine and Cu?II?/UV/chlorine.11 key products such as Cl·-attacked Cu-complexes products,Cu?I?-ClO complexes and decarboxylated products were detected by high resolution mass spectrometry during Cu-EDTA degradation,indicating that Cu-EDTA degradation is a successive decarboxylation process caused by Cu?II?/Cu?I?redox cycle.Moreover,the Cu?II?/Cu?I?redox cycle involving LMCT and MLCT enhances the production of Cl·,leading to self-enhaced degradation of Cu-EDTA.