Study On Refractory Wastewater Degradation By Discharge Plasma Coupling Electrolysis

The environmental problems have garnered increasingly attention. At present,refractory organic wastewater decomposition has became one of the focuses of environmental engineering field. Chlorophenols are widely used as chemical raw materials with the characteristics of stable chemical properties, molecular structures and strong biological toxicities. They are preferentially monitored as refractory organic pollutants. Therefore, carrying out the treatments of chlorophenols wastewater have a crucial theory and practical significance.In this paper, atmospheric pressure discharge plasma coupling electrolysis was proposed as a new type coupling technology which was used to oxidize the chlorophenol and actual refractory wastewater effectively. Moreover, the major factors and effect of the factors were studied by the determination of degradation effect, bubble image acquisition, Danckwerts-plot technique and the analysis of active species, etc. In addition,the corresponding mechanisms were also studied. The major research conclusions were made as following:(1) By optimizing the power supply voltages in coupling system, the optimal voltages of 4-chlorophenol(4-CP) degradation were obtained: 8.0 V of electrolytic voltage and 14.2 kV of plasma peak voltage. By comparison, 4-CP removal was much higher than that in single electrolysis system and single plasma discharge system.(2) By studying the relationship between the salt content in electrolytic cell and changes of 4-CP degradation, the optimal salt content in coupling system was 80 mg/L Na Cl.(3) The influence rules of initial pH on 4-CP degradation rate, COD and TOC removal of the solution were studied. The 4-CP removal reached 88.28% at pH 5.90 in the same treatment time(The average energy efficiency of the 4-CP degradation was 0.72g/k Wh), which was much higher than 81.32% at pH 6.50 and 79.99% at pH 3.65. The COD removal reached 61.10% in alkaline condition(The average energy efficiency of COD removal was 1.35 g/kWh), which was the highest. The removal of COD were59.44% and 49.29% in neutral and acidic conditions respectively. The TOC removal was the highest in alkaline condition, the second in acidic condition, and the worst in neutral condition.(4) The active species of O3, H2O2 and ?OH generated in the discharge processes of coupling system were determined by potassium indigo trisulfonate spectrophotometry,titanium potassium oxalate colorimetry and p-hydroxybenzoic acid capture method,respectively. The generation rate of O3 was faster at pH 3.65 and 9.23, and it was faster in alkaline solution than that in acid solution. In acidic condition, the yield of H2O2 was the largest in 90 min, which was 0.0507 mmol/L, and it was the second in neutral condition, and the least in alkaline condition. The ?OH generation rate reached 2.67×10-5mmol/(L s) in alkaline condition, and the energy yield was as high as 1.16×10-7mol/J.Therefore, the major active species to promote the degradation of 4-CP were O3 and ?OH in alkaline condition, and it was ?OH and H2O2 in acidic and neutral conditions.(5) Bubble image acquisition technique was carried out to obtain the instantaneous bubble images in plasma discharge system. The bubble size distributions and characteristic parameters at different experimental conditions were counted and calculated. The gas-liquid mass transfer parameters were determined by Danckwerts-plot,implying that the theoretical calculations were consistent with the statistical results of bubble images. Therefore, it is more reasonable to use the two results to interpret mass transfer and reaction processes in the discharge plasma reactor.(6) The degradation processes of 4-CP was inferred according to the aromatic intermediates and organic acids of 4-CP decomposition identified by HPLC and IC. In acidic and neutral solutions, ortho or para electrophilic substitution reactions were induced mainly by ?OH and H2O2, which led to the production of 4-chlorocatechol and hydroquinone respectively. In alkaline solution, electrophilic substitutions are initiated by O3 and triggered minorly by ?OH. The ortho or para of oxhydryl could be attacked by O3 easily, causing the dechlorination and production of hydroquinone and phenol.4-chlorocatechol was produced by the substitution of the ortho hydrogen atom.(7) The application research of actual refractory wastewater degradation was studiedin the coupling system. It can be seen that the degradation effect of wastewater was better without the catalyst. The COD removal was higher, but it was decreased with increasing the peak voltage. At 14.2 kV, COD removal was up to 67.68%, and the energy consumption was the least. Meanwhile, by optimizing the power supply voltages, the optimal coupling voltages were obtained: 5.0 V of electrolytic voltage and 16.6 kV of plasma peak voltage. Under these conditions, the COD removal of wastewater was up to a maximum of 43.33%, energy efficiency was 0.48 g/k Wh.