Preparation Of Nano-zinc Oxide And Photocatalytic Degradation Of Dyeing Wastewater

Due to the rapid development of dyestuff and dyeing industry in China,the economic and efficient treatment of dyeing wastewater has become one of the urgent problems to be solved.Because of its small size,strong catalytic activity,high repeating utilization factor and environmental protection,nano-ZnO has a broad application prospects in the field of water treatment.In this paper,methylene blue(MB),a representative dye,was selected as the target pollutant,and nano-zinc oxide(ZnO)was used as catalyst to carry out a series of photocatalytic studies.The research contents and results are as follows:(1)Spherical nano-ZnO powders were synthesized by the method of direct precipitation with zinc chloride and ammonia water.The nano-ZnO powders were characterized by X-ray diffraction(XRD)and scanning electron microscopy(SEM).Under ultraviolet light,the effects of calcination temperature,mechanical stirring speed,ammonia adding speed,nano-ZnO dosage,photocatalytic reaction time,initial p H and initial concentration of solution on MB removal rate were studied.The results showed that the catalytic activity of zinc oxide was the strongest when the calcination temperature was400?(now the morphology of ZnO is spherical and flaky mixing)and the mechanical stirring speed was 100r·min^-1and the speed of ammonia water was 13s.When the dosage of ZnO was 1 g·L^-1and the initial concentration of solution was 5 mg·L^-1and the p H was 9,the maximum removal rate of MB by nano-ZnO was 93.11%under UV irradiation for 180minutes.Moreover,the removal rate of MB by nano-ZnO was still 88%when it was recycled for 4 times.Free radical quenching experiments showed that the hole was the main factor for MB degradation.(2)Flower-like nano-ZnO powders were synthesized by the reaction of zinc chloride and sodium hydroxide in low temperature water bath.The nano-ZnO powders were characterized by XRD and SEM.The photocatalytic degradation of MB solution was studied under the same experimental conditions.In addition,the effects of the addition of hydrogen peroxide,Na₂S₂O₈,HCO₃^-and CO₃^2-on the degradation of MB were also studied.The results showed that the catalytic activity of zinc oxide was the strongest when the water bath two hours and the water bath temperature was 45?and the ratio of n(OH^-)to n(Zn²⁺)was 8.When the initial concentration of MB solution was 5 mg·L^-1,the p H was 9and catalyst dosage was 0.6 g·L^-1,the maximum removal rate of MB was 97.46%under UV irradiation for 150 minutes.And the removal rate of MB by nano-ZnO still reach 89%after four times of recycling.The addition of H₂O₂and Na₂S₂O₈promoted the degradation of MB,while the addition of HCO₃^-and CO₃^2-inhibited the degradation of MB.(3)The kinetics of MB degradation was studied by using the flower-like nano-ZnO as catalyst.The results showed that the concentration of MB was linear with the reaction rate,which follow a pseudo-first-order kinetic reaction.The influence of the dosage of catalyst,the concentration of MB solution and the initial p H of MB solution on the photocatalytic rate were investigated.The catalyst surface reaction rate constant K₀is 0.45 min^-1,the zinc oxide adsorption equilibrium constant K_ZnOis 0.06 mg^-1·L,the MB adsorption equilibrium constant K_MBis 73.787 mg^-1·L,and the photocatalytic reaction rate constant K_Cis 0.034mg/L^-1min^-1.Compared with the spherical nano-ZnO,the f lower-like nano-ZnO has stronger catalytic activity,and the low temperature water bath method is simpler and more economical than direct precipitation method.It has a certain reference value for industrial production and actual water treatment.