| Nowadays,an increasing discharge of acidic wastewaters from various industries,e.g.,electroplating,metallurgy,printing and dyeing,chemical industry,pesticide and pharmacy,which contain a variety of refractory organics including metal-organic complexes,pesticides,pharmaceuticals,dyes,PCBs,PAHs,nitro-aromatics,and chloro-aromatics,have shown potential risks to the ecosystem and human health.Therefore,the development of a high efficient and economic treatment technology for acidic wastewater has received great interests in the field of water pollution control.Advanced oxidation processes(AOPs)such as ozonation are widely used in advanced water treatment for organics due to its safety,cleanliness,high oxidizing reactivity,fast reactions and the ability to increase biodegradability.Ozonation is usually applied under neutral and alkaline conditions,but is greatly limited in acidic circumstance because of the low efficiency of hydroxyl radicals(·OH)production.To overcome the inefficient degradation of organics from acidic wastewater by ozonation,this study proposed a new strategy to utilize electron-deficient aromatics,and the coexisting complexing agent(EDTA)along with its metal complexes to enhance the ozonation process.The behaviors of ozone decomposition and the degradation of aromatics substituted with electron-withdrawing group(Ph-E W G)were systematically investigated in this dissertation.Moreover,the typical influencing factors,reaction kinetics and the mechanisms during ozone activation by Ph-EWG,complexing agent(EDTA)and its metal complexes were also studied.In addition,acidic electroplating wastewater was employed to evaluate the practical feasibility to remove metal-EDTA complexes by ozonation.The results showed that one or more EWG such as-Cl,-COOH or-NO2 on aromatics significantly enhanced both ozone decomposition and the degradation of Ph-EWG.Both ozone decomposition and the degradation of Ph-EWG increased firstly and then slowed down with the increase of the number of-Cl on aromatics.The kinetics of ozone decomposition and the degradation of Ph-EWG were both enhanced by increasing concentrations of aromatics.The degradation of Ph-EWG was promoted by increasing ozone concentration.The increase of solution pH from 2.3 to 7.0 weakened the enhancement of ozone decomposition by the intermediates.With the increase of the concentration of Ph-EWG,ozone exposure gradually decreased,while·OH exposure exhibited the trend of increase first followed by a decline.A linear relationship between ·OH exposure,the polarizability and the most negative charge of Ph-EWG was observed.Essentially,the degradation of Ph-EWG during ozonation is a HO·-mediated reaction.Excluding the possible contributions of H2O2 and phenol-like intermediates to-OH production,the reaction mechanism involved the formation of ozone ion(O3·-),an effective precursor of-OH,was thus proposed.The hydroxycyclohexadienyl-type radicals generated during the attack of Ph-EWG by HO· may lead to the formation of O3·-.Meanwhile,O3·-could also be possibly formed from the reaction between ozone and organic(e.g.,ROO·)or inorganic peroxyl radicals(e.g.,HO2·).In addition,the hydroxylated products like phenol-like intermediates also played a positive role in·OH production.A marked enhancement of ·OH production from ozone at acidic pHs by EDTA was observed,and the enhanced ·OH production in the presence of EDTA was further confirmed by EPR.EDTA significantly accelerated the degradation of benzoic acid(BA),a ·OH probe,by ozonation in the pH range of 2.3-5.5,and the efficiency was higher than O3/H2O2.The efficiency was less dependent upon temperature than that of O3 and O3/H2O2,and the activation energy was 45.3 kJ mol-1.The production of ·OH was promoted by increasing ozone dosage,whereas was inhibited by the presence of Cu2+,Cr3+ and NOM.In contrast,the influence induced by Ni2+ and Cr(VI)was marginal.At an O3 dosage of 1.0 mg/L,Rct was significantly increased by a factor of 8-50 by increasing EDTA concentration from 1.0 μM to 40 μM at pH 2.3.Continuous batch runs were carried out with only one-time addition of EDTA,and similar enhancement of BA degradation by ozone was observed,suggesting that the intermediates of EDTA enhanced ozone decomposition to produce ·OH.With excess ozone dosage relative to the stoichiometric amount for complete transformation from EDTA into NO2-,-OH production increased linearly with increasing ozone concentration,indicating that generated nitrogen-and/or carbon-centered radicals played an important role in ozone decomposition and ·OH production.Based on these results,a plausible mechanism taking into account the organonitrogen intermediates and nitrogen-or carbon-centered radicals was proposed to interpret the HO·generation in the system.O3·-or O2·-was derived from the interaction between O3 and organonitrogen intermediates of EDTA.Meanwhile,O2·-could be possibly generated from the reactions between nitrogen-and/or carbon-centered radicals and O2 and/or O3.Ozonation could result in efficiently degradation of Cu-EDTA and simultaneous removal of Cu(II)not only from acidic solution(pH 3.1-6.3)but also from an electroplating effluent(pH 3.6).A complete Cu-EDTA removal could be achieved from a synthetic solution in 10 min at the initial Cu(II)of 64 mg/L and ozone dosage of 30 mg min-1 L"1,with 75-80%of TOC removal in 40 min.During the ozonation process,solution pH was on the increase from 3.1-6.3 to 6-7,resulting in 90-97%of Cu(II)removal through precipitation finally.HO--mediated oxidation was predominantly responsible for the degradation of Cu-EDTA,as proved by the radical scavenging experiments.Cu-EDTA and its intermediates promoted ozone decomposition to produce-OH demonstrated by HO· probe and EPR.Based on the evolution of HPLC spectra and HPLC-MS analysis,a stepwise decarboxylation was found to be the main pathway for the ozonation of Cu-EDTA.Moreover,efficient removal of Cu-EDTA,Cu(Ⅱ)and TOC from a practical electroplating effluent was achieved by ozonation,with the removal of 100%,85%and 96%,respectively. |
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