Study On The Degradation Of Dimethyl Phthalate And3,3’-dichlorobenzidine In Aqueous Solution By Ozonation

Phthalic acid esters （PAEs） are widely used organic substances, which have been mainlyused as plasticizers, and also in pesticide, paint, dyeing, cosmetic and spices industries. Dueto the characteristics of teratogenicity, mutagenicity, carcinogenicity and reproductive toxicity,PAEs have aroused the wide attention of many countries. The solubility in water of dimethylphthalate （DMP） is the best during the five most commonly used PAEs, and the quantityneeded of DMP is large. DMP has been included on the list of priority pollutants of the USEnviromental Proteetion Agency and China.3,3′-dichlorobenzidine （DCB） is an importantintermediate in synthesis process of azo dyes and organic pigments, and DCB-based organicpigments account for about27%of the total. But DCB has been included on the list of prioritypollutants of the US Enviromental Proteetion Agency due to its highly toxic and carcinogenicpotential.A large amount of production wastewater will be produced during DMP and DCBproduction process. It is a serious threat to human health and the environment if thewastewater is not properly treated and discharged into the environment. Ozone is a kind ofclean and strong oxidizing agent. And under certain conditions, ozone can decomposeinto·OH that is a stronger oxidizing agent. In this paper, we had studied the degradationeffects, degradation efficiency and degradation kinetics of DMP and DCB in aqueous solutionby ozonation, which could be divided into six aspects as follow:（1） Degradation of DMP in aqueous solution by ozonation was investigated. The effectsof pH, DMP initial concentration, ozone dosage, reaction temperature, cations and anions onthe degradation of DMP were discussed. Experimental results showed that the degradationefficiency of DMP firstly increased then decreased with the increasing of pH from2to12,and the degradation efficiency of DMP was best at pH9. The degradation efficiencydecreased with the increasing DMP initial concentration, but the degradation quantityincreased gradually. The degradation efficiency increased with increasing ozone dosage, butthe ozone utilization efficiency decreased. Rising temperature increased the degradation efficiency at low temperature, but the reaction temperature had no remarkable effect on thedegradation efficiency over24oC. Low concentration cations and anions had no effects on thedegradation efficiency of DMP. But high concentration Cl-and HCO3-inhibited thedegradation and the inhibitory action of HCO3-was stronger than Cl-.（2） Degradation efficiency and degradation kinetics of DMP in aqueous solution byozonation were investigated. DMP in aqueous was removed efficiently by ozonation. Thedegradation efficiency could reach up to85%under the experimental conditions of pH510,DMP concentration1466mg/L and ozone dosage1424mg/min at2030oC after40min.The degradation of DMP by ozonation followed pseudo-first-order kinetics under the aboveexperimental conditions. A kinetics model for degradation of DMP by ozonation was derivedby power index equation:c=c0exp(-2328.5exp（-30418.4/RT）QO31.989c00.751cII-0.0190t)The rate constants for the reaction of DMP with ozone and hydroxyl radical （·OH） weredetermined by competition kinetics method, in which nitrobenzene （NB） was selected as thereference organic compound. The rate constants of DMP with ozone and·OH were（0.064±0.014） L/（mol·s） and3.59×109L/（mol·s） respectively. There was no positivecorrelation between kobsand pH in the range of pH212because of the different reactionmechanism of ozonation. DMP was oxidized by ozone in the range of pH25, the kobswassmaller, but the kobsincreased quickly. Then kobsincreased slightly in pH59, DMP wasoxidized by·OH. The decrease of kobswas observed over pH9, the biggest kobswas0.0929min-1at pH9.（3） The degradation mechanism of DMP was investigated. Because the reaction wasinhibited by tert-butyl alcohol （TBA）, the degradation of DMP was·OH indirect oxidationmechanism. A large number of intermediates resulted in the removal rate of COD laggingbehind the degradation efficiency of DMP, and parts of the intermediates were difficult to bemineralized.（4） Degradation of DCB in aqueous solution by ozonation was investigated. The effectsof pH, DCB initial concentration, ozone dosage and reaction temperature on the degradationof DCB were discussed. Experimental results showed that the degradation efficiency of DCBfirstly increased then decreased with the increasing of pH from1to9, and the degradationefficiency of DCB was best at pH4. The degradation efficiency decreased with the increasing DCB initial concentration, but the degradation quantity increased gradually. The degradationefficiency increased with increasing ozone dosage. Rising temperature increased thedegradation efficiency at low temperature, but the reaction temperature had no remarkableeffect on the degradation efficiency over26oC.（5） Degradation efficiency and degradation kinetics of DCB in aqueous solution byozonation were investigated. The DCB in aqueous was removed efficiently by ozonation. Thedegradation efficiency could reach up to85%under the experimental conditions of pH4,DCB concentration1.26.5mg/L and ozone dosage3.75.1mg/min at1630oC after50min.The degradation of DCB by ozonation followed pseudo-first-order kinetics under the aboveexperimental conditions. A kinetics model for degradation of DCB by ozonation was derivedby power index equation:c=c0exp(-190.3exp（-30460.0/RT）QO33.408c0-0.794t)The kobsfirstly increased then decreased with the increasing of pH from1to9because ofthe different reaction mechanism of ozonation and dissociation of DCB. The biggest kobswas0.235min-1at pH4, it was6.9times and35.8times higher than pH1and7, respectively. Thereaction order between ozone and different forms of DCB was:(C12H10Cl2N2)2>(C12H10Cl2N2)2·HCl>(C12H10Cl2N2)2·2HCl.（6） The degradation mechanism of DCB was investigated. Because the reaction was notinhibited by TBA and Cl-, the degradation of DCB was ozone direct oxidation mechanism atpH4. The removal rate of UV285lagged behind the degradation efficiency of DCB. Thedetection results of UV and HPLC showed that benzene ring of DCB could not beendestroyed in the initial stage of the degradation of DCB. While the removal rate of UV285reached95.8%after50min, the aromatic organic compounds were almost completelydegraded. But the removal rate of TOC was13.6%when degradation efficiency of DCB was99%. The small molecular substances produced by the degradation of DCB could not bemineralized by Ozone direct oxidation.