Study On Photocatalytic Treatment Of Heavy Metal Ions In Industrial Wastewater

Water pollution caused by industrial wastewater discharge has been widespread concerned by the whole society.Photocatalytic technology has opened up a new approach to the treatment of poisonous heavy metal ions in water.Ti O₂ is the most used photocatalyst due to its stable nature,non-toxicity,harmlessness,and low price.However,because of the wide bandgap,Ti O₂ is that it only absorbs a small portion of solar spectrum in the ultraviolet（UV）region,which limits its effective applications.In this work,the photocatalytic degradation ability of nano titanium dioxide（P25）as a photocatalyst to heavy metals industrially produced was studied.A series of characterizations were used to analyse its morphology,crystal form and optical properties.The mechanism of photocatalytic reduction of heavy metal ions was explored through photocatalytic performance and free radical capture experiments.The specific research content are as follows:1.As a photocatalyst,P25 that surface in situ modified with sodium formate was used to degrade free Cd（II）under ultraviolet light,and characterized by XRD,TEM,XPS,UV-vis,FT-IR.The experimental results show that the pure P25 cannot photocatalytically degrade Cd（II）in 60 minutes under UV light.While modified with sodium formate,P25 had a strong absorption at wavelength of 400-600 nm,and showed the high catalytic activity.When the optimal concentration of sodium formate was 1.0 g/L,Cd（II）was completely degraded in 30 minutes.Theoretical analysis showed that Cd（II）was adsorbed on the catalyst surface by the bidentate chelate structure formed between P25 and sodium formate.The strong reducing substance that formed in the reaction of carboxyl groups of sodium formate with photogenerated holes,and photogenerated electrons transfered to heavy metal ions,achieving reduction of heavy metal ions.2.Fixed irradiation of UV light for 30 min,the effect of P25 degradation of Cr（VI）under UV light was investigated by changing the single factor method.The results showed that the optimal conditions of every factors were p H value 4.0,the concentration of Cr（VI）0.2 mg/L,and the amount of P25 1.5 g/L,respectively.The supreme degradation rate for Cr（VI）reached 91.20%under the optimal conditions.3.The Ag-P25 composite photocatalyst was prepared by vacuum activated photodeposition method and ethanol reduction method,respectively.The as-prepard samples were tested for photocatalytic performance.Under the optimal conditions,the samples prepared by the ethanol reduction method could completely degrade Cr（VI）after 90 min under visible light irradiation.However,for the sample prepared by the photodeposition method,it was difficult to control the formation of Ag,which leads to the agglomeration of the Ag particles,resulting in a decrease in photocatalytic performance.4.The Ag-P25 prepared by the ethanol reduction method were characterized through a variety of properties such as the structure,morphology,binding energy,and absorbance.The results showed that Ag nanoparticles were successfully loaded on the catalyst surface,and the absorption of catalyst was broadened to visible light region.The possible mechanism involved that the Ag nanoparticles accelerated the efficiency of electron and hole separation,and citric acid reacted with holes to produce a strong reducing free radical CO₂^·-,which degraded Cr（VI）.5.The photoelectrocatalysis experiments approveded that electrocatalysis and photocatalysis had synergistic effects in reducing heavy metal ions.For 3.0-Ag-P25prepared by the ethanol reduction method,the p H 3.0 and the current intensity 0.01 A were the optimum values.Under these optimal conditions,the degradation rate of Cr（VI）was 88.57%within 20 minutes.After 30 minutes,Cr（VI）were completely degraded under any current conditions.The results above mentioned showed that the photoelectric synergy improved the reaction rate of the photocatalyst,greatly shortened the degradation time of heavy metal ions.This provided a feasible method for the degradation of heavy metal ions.