Study On Degradation Of High Concentration Nitrobenzenes' Wastewater With Catalytic Ozonation

Nitrobenzene compounds are environmental pollutants of very high concern in the world due to their carcinogenicity,mutagenicity,and chemical stability,and it is well known that the compounds are unbiodegradable.Unfortunately,there is no commercial technique to treat wastewater with high concentration of nitrobenzenes' in industrial scale efficiently and economically although a lot of techniques have been studied for degradation of wastewater.Therefore,catalytic ozonation was put forward to degrade nitrobenzenes' wastewater with calcium oxide as catalytic in this thesis.To study the feasibility of degradation high concentration of nitrobenzenes' wastewater efficiently with heterogeneous catalytic ozonation,the effect of experimental conditions on degradation efficiency was studied firstly,then the optimum degradation conditions were obtained from orthogonal experiments,and finally,the degradation mechanism,degradation kinetics and degradation pathways of nitrotoluene were discussed.There are main conclusions below:?1?The degradation data of nitrobenzenes' wastewater(COD_cr value is 2914 mg/L,the major pollutants are nitrotoluene and dinitrotoluene,and the concentration measured in the form of nitrobenzene is 225 mg/L)showed that the removal ratio of COD_cr was 96.38% and chroma removal ratio up to 100% under the conditions of pH=12.0,calcium oxide 2g,ozone dosage 200 mg/min,and degradation 60 min,which suggested that the degradation of high concentration of nitrobenzenes' wastewater is feasible through catalytic ozonation with calcium oxide as catalyst.?2?The optimum conditions from orthogonal experiments were as follows: the initial pH=12.5,the dosage of ozone 180 mg/ min,the mass of calcium oxide 3 g,and the treatment time was 60 min.Under this optimized conditions,the COD_cr and nitrobenzene removal ratio of the wastewater were 97.98% and 99.96%,respectively.In addition,the color of treated wastewater was disappeared.?3?The degradation data of p-nitrotoluene?PNT?revealed that the degradation of PNT was in accordance with the quasi-first-order kinetic model if ozone was kept constantly,and the catalyst?calcium oxide?promoted the reaction rate constant by nearly three times.?4?Hydroxyl radical scavenging experiments suggested that the degradation of PNT followed the ·OH mechanism in ozonation,and calcium oxide promoted ozone to produce hydroxyl radical?·OH?.?5?The intermediate products showed that PNT was first converted to p-nitrobenzaldehyde,which was then decomposed to acetic acid and finally mineralized to carbon dioxide and water.?6?The catalytic activity of calcium oxide decreased with catalytic cycle increased due to party of catalyst converted to calcium hydroxide.It is speculated that the catalyst is calcium oxide rather than calcium hydroxide in the process of catalytic ozonation.