Study On Preparation Of Co-doped TiO₂ Supported By Chitosan And Its Application In The Treatment Of Papermaking Wastewater

TiO2 has widely been used in various fields of wastewater treatment, air purification, disinfection and clean energy production because of its non-toxicity, chemical stability, high efficiency, easy synthesis and low cost. However, the two drawbacks of wide-band gap TiO2 (Eg= 3.0-3.2 eV) which severely blocking up its practical application. One is that TiO2 is confined to absorb the ultraviolet (UV) light of overall solar spectrum (only 3-5% in total solar energy); the other is that the photo-generated electrons and holes are prone to recombination and thus resulting in low utilization of light quantum. In addition, the problems of easy agglomeration, difficult separation and recovery also exist in the nano-TiO2 based materials. The papermaking wastewater is an important pollution source in our country, which is complex components, difficult to degrade organic matter, high COD concentration, high toxicity, low biodegradability and difficult to deal with. It is of great practical significance and has wide application values to study on the application of TiO2 photocatalytic technology in the treatment of papermaking wastewater.In this paper, the easily separated and recycled S-La-TiO2-chitosan composite photocatalysts were prepared by coupling the co-doped TiO2 as photocatalytic material with the biomass chitosan as support. Firstly, the S or La single-doped (S/La-TiO2) and the S, La co-doped TiO2 (S-La-TiO2) were prepared by using the sol-gel method (Sol-Gel) with the reaction precursors of tetrabutyl titanate, lanthanum nitrate and thiourea. Then investigating the effects of doping amount of S and La and annealing temperature on the photocatalytic activity of the doped TiO2 by employing the methyl orange solution (50 mg/L) as the degradation target and the 400 W metal halide lamp as the simulated visible light source. The result shows that the photocatalytic performance of co-doped TiO2 was better than that of single-doped one under visible light. Next, the S-La-TiO2-chitosan (S-La-TiO2/CS) composite photocatalysts were prepared by loading the S-La-TiO2 on the chitosan microspheres support, and the visible-light photocatalytic activity of the composites were optimized by tuning the loading amount. Finally, the photocatalytic degradation of the S-La-TiO2/CS composites on papermaking waste was studied under the illumination of 500 W xenon lamp (simulated solar light source). Meanwhile, the prepared single-doped TiO2, co-doped TiO2, and S-La-TiO2/CS were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and UV-Vis diffuse reflectance spectra (UV-Vis DRS), and so on.The main research contents and conclusions of this paper are listed as follows:(1) The S/La single-doped, La and S co-doped TiO2 photocatalyst were prepared by a sol-gel method with the alcohol solution of tetrabutyl titanate precursor mixed with the S source (CN2H4S) or La source (La(NO3)3) or the S and La sources together. The technological conditions were optimized through the single factor experiments based on the parameters of doping amount and calcination temperature. The results showed that the best doping mass ratio of thiourea and tetrabutyl titanate, and lanthanum nitrate and tetrabutyl titanate was 5% and 3% for the thiourea-based and lanthanum nitrate-based single-doped materials annealed at 500 ℃ for 1 h, respectively. When thiourea, lanthanum nitrate, tetrabutyl titanate mass ratio was 0.03:0.03:1, the TiO2 co-doped with lanthanum and sulfur obtained at 500 ℃ for 1 h shown the highest visible-light photocatalytic activity compared (MB degradation rate is 93.22%) with pure TiO2, S-TiO2 and La-TiO2.(2) The TiO2 nanoparticles modified by thiourea and lanthanum nitrate, were immobilized on chitosan (CS) using cross-linking processing method to form S, La co-doped TiO2 chitosan ball (S-La-TiO2/CS), and the optimal loading amount ratio of co-doped photocatalyst and chitosan was determined by the photocatalytic degradation decolorization rate of methyl orange solution. The prepared photocatalysts were characterized by XRD, UV-Vis DRS, SEM and FT-IR. The results showed that the decolorization rate can reach 83.16% for the photocatalytic degradation of 50 mg/L methyl orange solution on the S-La-TiO2/CS (the ratio of co-doped catalyst and chitosan was 1:3). The anatase crystal morphology of nano-TiO2 was found to change after co-doping with thiourea and lanthanum nitrate from the XRD characterization results, which significantly improve the photocatalytic activity of co-doped TiO2. This is because that the co-doping of S and La in doped photocatalysts lead into the synergetic effect and thus obviously shifting the absorption edge to longer wavelength. The results show that the degradation of methyl orange solution conforms to the first-order reaction kinetics for different concentrations using S-La-TiO2/CS as photocatalyst.(3) The papermaking wastewater was treated using the best S-La-TiO2/CS composite photocatalysts under simulated sunlight (500 W high-pressure xenon lamp). The effects of H2O2 dosage, pH of the solution, illumination time and amount of catalyst on CODcr removal rate were investigated through the single factor and orthogonal experiments. The results showed that these factors had significant effects on the CODcr removal rate based on S-La-TiO2/CS, and the optimal technological conditions obtained by orthogonal tests were listed as follows:pH of the papermaking wastewater was 6, the amount of catalyst was 7 g/L, dosage of H2O2 was 8 mL/L and the illumination time was 6 h. The CODcr removal rate reached 65.54% using 500 W xenon lamp under the above condition, and the effect of various factors on photocatalytic performance were following this turn:pH value, illumination time, H2O2 amount and catalyst amount. The experimental results showed that the S-La-TiO2/CS composite photocatalysts have good activity for the degradation of papermaking wastewater and can be used repeatedly after separation and recovery in our system. Therefore, it is feasible to treat the actual wastewater using the prepared S-La-TiO2/CS photocatalysts.