Preparation Of Crosslinked Starch/TiO₂ Composite And Synergic Effect On Dye Wastewater Treatment

Titanium dioxide (TiO2) is an effective photocatalyst which has great potential in dye waste water treatment, and many organics can be converted to CO2 and H2O by TiO2 under the UV light, Unfortunately, the number of its adsorption active sites is not sufficiently high, resulting in longer photocatalysis time that limits its application. In order to improve it’s photocatalysis, two kinds of modified starch/TiO2 composites have been proposed. The photocatalysis efficiency of TiO2 would be effectively improved by making full advantage of the adsorption capacity of modified starch.Cosslinked starch microspheres (CSM)/TiO2 composite and freeze-drying crosslinked porous starch (FDPS)/TiO2 composite were prepared via in-situ synthesis in sol-gel system. We investigated the effect of different amounts of modified starch on the composites’ morphology, crystal form and infrared characteristics. The results suggested the success of covalent cross-linking between TiO2 and modified starch. Futhermore, most of TiO2 was located on the surface of modified starch, which is favourable to photocatalysis.The effects of different modified starch loadings on the specific surface area, adsorption capacity, and photocatalysis performance of composites were investigated, A hypothesis that the composites show the monolayer adsorption of MB has been proposed, and the optimum modified starch content was determined. As the CSM loading amount was 25.3%, the CSM/TiO2 composite exhibited the best performances, i.e., a specific area of 255.1 m2/g, a maximum adsorption capacity of 147.68 mg/g, and a MB removal ratio of 98.86%. Additionally, the FDPS/TiO2 composite displayed the best performances with the FDPS loading of 10wt%, i.e., a specific surface area of 176.9 m2/g, a maximum adsorption capacity of 80.55 mg/g, and a MB removal ratio of 99.92%.The comparison between the adsorption capacities and the MB removal efficiencies of composites and individual components indicated two kinds of composites exhibited much shorter photocatalysis reaction time and enhanced photocatalysis ratio of MB. Both composites can show MB removal ratio of 99% with the photocatalysis reaction times of CSM/TiO2 and FDPS/TiO2 be 12.5% and 12% of those of TiO2, respectively. Concurrently, CSM/TiO2 and FDPS/TiO2 showed the removal efficiencies contributed by adsorption of 92.41% and 99.7, respectively.To further investigate the removal mechanism of the composites on MB, the thermodymanics and dynamics models for adsorption of the composites on MB were fitted, indicating the adsorption of the both composites on MB conform to the Langmuir adsorption model. Moreover, the adsorption-photocatalysis synergy dynamics of both composites on MB conform to pseudo-second order model and Weber-Morrris internal diffusion model. Molecule internal diffusion is the rate-limiting step in the reaction process. Based on the above description, the adsorption-photocatalysis synergy mechanism was proposed, in which the diffusion of the dyes adsorpted on the surface of adsorbents to active sites of TiO2 promoted the photocatalysis efficiency, and controlling step of the adsorption-photocatalysis synergy mechanism.