Degradation of Disperse Dyestuff Wastewater by Homogeneous Catalytic Ozonation
|Keywords||disperse dyes wastewater ozonation homogeneous catalytic ozonation catalyst bubbles|
Disperse dyes are widely used commercial dyestuffs. In recent years, with the increased demand for disperse dyes, the textile and dyeing industry and its wastewaters have been increasing proportionally. Since disperse dyestuffs are intentionally designed to resistant to photodegradation, oxidation and biodegradation, a marked resistance of these dyes to decolorization and biological degradation is not unusual. Wastewater containing these dyes cannot be treated by conventional processes (biological, chemical, or physical) effectively.In this paper, ozonation alone and homogeneous catalytic ozonation were employed for treatment of industrial disperse dyestuff wastewater effluents of physicochemical and sedimentation tank, using gas washing bottle and a self-made porous sintered glass sparger column as ozonation reactor. Chemical oxygen demand (COD) removal, total organic carbon (TOC) removal, color removal, UV254 removal, and system pH value of the effluent was employed. Influences on decolorization and mineralization such as initial pH value, ozone dosage, reaction time, the type and concentration of catalysts, ozone bubble size and distribution were investigated. The reaction mechanisms were discussed. Results indicated that the decolorization and mineralization of dyestuffs wastewater and the reaction rate increased as the system pH value and ozone dosage increased. The pH of the solution decreased and the COD removal、TOC removal、UV254 removal increased gradually during the ozonation process, but after a certain time the increasing trend decelerated and the removal rate increased no longer, which could be regarded as oxidation has reached the end. The ozonation of wastewater was correctly modelled by a pseudo second-order model kinetic model.The dynamic high-speed motion analyzer microscope was applied to characterize ozone bubbles both in gas washing bottle and sintered glass sparger column. A fine porosity, sintered glass plate diffuser in the bubble column was good for the dispersion of ozone bubbles. Compared with gas washing bottle, the amount of bubbles with diameter of submicron was huge, and bubbles was distributed evenly in space and time. The number of bubbles in gas washing bottle was small and the bubble diameter was in millimeters. The color removal and the degradation of organic contaminants in the bubble column were much higher than in the gas washing bottles. It required 3 min ozonation for bubble column system and 17 min for gas washing bottle to reach 90% color removal. The UV254 removal was 64.27% after 3 min degradation in the bubble column, and 64.49% after 30 min reaction in gas washing bottle. The COD removal and TOC removal of the effluent in gas washing bottle were 27.51% and 6.49% after 30 min, while in the bubble column were 42.35% and 23.61%. It indicated that gas-liquid contact is much better in the bubble column than in the gas washing bottle, and the bubble column system increased zone mass transfer rate and improved the oxidation of dye molecules. Results showed that color removal reached 99.37% within 7 min, UV254 removal was 69.87% and the system pH changed to 4.20 after 10 min ozonation when initial pH was 9.15, and ozone dosage was 5.66 mg/min, gas flow rate 1.5 L/min using ozonation alone. In addition, within 10 min COD removal and TOC removal were 41.00% and 30.41%, respectively.The experimental results of homogeneous catalytic ozonation showed that Mn(Ⅱ)、Fe(Ⅱ)、Co(Ⅱ) and Ni(Ⅱ) has catalytic effect when optimum amount of catalyst was added. Co(Ⅱ) and Fe(Ⅱ) have much stronger catalytic oxidation under weak alkali condition, and of which the reaction mechanism was consistent with the hydroxyl radical hypothesis. In O3/Mn(Ⅱ) system, Mn(Ⅱ) can react with ozone to form hydrous manganese oxide and high-valent manganese ion Mn7+ and Mn3+. The degradation depends on the synergetic oxidation of hydroxyl radical mechanism and high-valent manganese oxidation.Using ozonation for decolorizing wastewater has the following advantages:(1) It possesses strong oxidative properties; (2) It can decolorize water-soluble synthetic dyes effectively; (3) It removes color and reduces the organic matter in one-step; (4) It does not increase the volume of wastewater and sludge. Compare to ozonation alone, metal-catalyzed homogeneous ozonation is capable of achieving much better results for removal many organic pollutants from treated water. It can increase ozone utilization, allowing a quicker removal of color and contaminants, improving the oxidizing power of ozone, reducing the economic cost and increasing in biodegradable organic carbon ahead of biological stages. Therefore, metal-catalyzed homogeneous ozonation a promising approach to dyestuff wastewater treatment, either as a pre-treatment or a final polishing step before discharge.