Effect Of Reverse Irradiation With UV Light On The Photocatalytic Oxidation System Of Supported TiO₂ And The Efficacy In The Degradation Of Organic Pollutants

In recent years,research into the degradation of organic pollutants in water based on the photocatalytic effect generated by semiconductor materials has received widespread attention.Compared to traditional water treatment technologies such as chemical and biodegradation methods,photocatalytic oxidation has the advantages of clean energy,stable reaction process and complete degradation.However,there are still technical bottlenecks in conventional photocatalytic oxidation,the application scope of which is greatly restricted by the light permeability of water bodies and the severe attenuation of light energy in water will lead to the reduction of effective energy utilization.To solve these problems,this study changed the irradiation method of the light source.In a thin-film loaded photocatalytic system,the original light propagation path,which requires light to penetrate through the water to reach the surface of the catalytic film,is changed to one that directly penetrates the catalytic film and the photocatalytic reaction occurs at the film-water interface,thus providing a new way of thinking to break through the limitations of traditional loaded photocatalytic technology.Since the path of light irradiation in this mode is exactly opposite to that of conventional photocatalytic systems,it is referred to as reverse irradiation.In this study,TiO₂ was selected to prepare photocatalytic films,and the optimal film preparation parameters,main contributors to degradation and film recycling performance in the reverse irradiation mode were investigated;the energy efficiency of photocatalytic oxidation for the treatment of printing and dyeing wastewater and water with high algal density was investigated.The photocatalytic degradation efficiency of the conventional forward irradiation and reverse irradiation modes were also compared.The following research results were obtained:（1）By characterizing the crystalline structure,elemental composition and other properties of TiO₂ films prepared under different parameter conditions,the preparation parameters of the liquid phase deposition method should be controlled within 1.5h of deposition time and the heat treatment temperature should not exceed 800℃in order to obtain films with good optical properties.The wavelength preference of UV light source shows that 365nm is suitable for the photocatalytic oxidation system in reverse irradiation mode due to its better penetration ability than 254nm UV light.The degradation rate of rhodamine B was used to reflect the photocatalytic performance,and the degradation process was in accordance with the quasi-level kinetic model.Calculation of the kinetic constants led to a controlled temperature of 60°C deposition for 1.5h and a heat treatment at 700°C for 1h,which was able to obtain the film with the best catalytic effect.Under the reverse irradiation condition,the photocatalytic oxidation system produced a large amount of·OH and its contribution to the degradation of pollutants was more than 80%;the cyclic experiments showed that the TiO₂ films with the best catalytic performance prepared under the reverse irradiation mode had good stability and the performance only decreased by 7.5%after five times of reuse.（2）The photocatalytic system with reverse irradiation has good degradation efficiency for printing and dyeing wastewater under different pollutant concentrations,different initial p H and different chemical conditions of co-existing anions.At low concentrations,the degradation rate is faster.The best degradation was achieved under slightly alkaline conditions（p H≈8）.For co-existing anions,NO₃^2-has little effect,while Cl^-and SO₄^2-slightly hinder degradation due to the consumption of·OH,and CO₃^2-makes the solution alkaline and promotes degradation.A comparison of the UV spectra of the UV photolysis alone and the photocatalytic degradation of dyeing wastewater by forward and reverse irradiation showed that the characteristic peaks of the functional groups representing each dye molecule in the aqueous humour were somewhat weakened.（3）The photocatalytic system using reverse irradiation mode was able to show relatively good results for algal waters,with chlorophyll a removal rates exceeding 28%and overall cell inactivation rates reaching 19.79%for high algal densities within 12h.Through the detection of intracellular protein content,superoxide dismutase activity and extracellular organic matter,it was found that the mechanism of inactivation of algae by the photocatalytic system in reverse irradiation mode was to destroy the antioxidant enzyme system of algae and cause oxidative damage to intracellular proteins and other substances.The system not only breaks down some of the algal cells,but also removes all kinds of algal organic matter released into the water column,but it lacks a sustained inhibitory effect on algal growth.（4）Comparison of the treatment efficiency of the photocatalytic system in forward and reverse irradiation modes shows that for printing and dyeing wastewater,the degradation efficiency of forward and reverse irradiation is similar when the thickness of the liquid layer is reduced to 1.2cm or the concentration of each dye is reduced to 3mg/L.When the thickness of the liquid layer is reduced to 1.2cm or the concentration of each dye is reduced to 3mg/L,the photocatalytic degradation efficiency of reverse irradiation is not as good as that of forward irradiation.However,when the thickness of the liquid layer was increased to 2.2 cm or the concentration of each dye was increased to 10 mg/L,the degradation efficiency of the reverse irradiation was significantly higher than that of the forward irradiation.Similar results were obtained in the same type of experiments in algal waters,with the only difference being that the conditions that brought them close to each other were a controlled algal density of 2.8×10⁷cells·L^-1 and a light transmission range of 1.75cm.The proposed photocatalytic oxidation system in reverse irradiation mode is therefore more advantageous in the case of longer light transmission range or higher pollutant concentration in water.