| As one of major sources of pollution in industrial waste water, heavy metal pollution has the features of long contamination time and difficult to repair. It has brought a lot of environmental problems which trouble people's daily life and production. Therefore, it is many researchers' goal to find an effective way for purifying the water contaminated by heavy metal. Compared with traditional processes, photocatalytic technology comes into researchers' vision for its high efficient, environmental friendly and feasible advantages. Titanium dioxide(TiO2) is considered as a new type semiconductor photocatalyst. And it usually was prepared into nanoscale particles to get higher photocatalytic reduction efficiency. While TiO2 still has flaws which are difficulty in recycling nano-scale particles and low utilization efficiency of visible light in practical application. Effective ways that have already been proven to improve this technology include that use porous mineral to load the nano-TiO2 and modify the surface properties of TiO2 by doping. However, current research of nano-TiO2 loaded by porous minerals mainly focuses on degradation of dye, phenol, toluene and other organic compounds. Using photocatalytic reducibility of TiO2 to remove heavy metal ions in the water has rarely reported. Furthermore, study is not quite focus on the interaction mechanism between adsorption capacity of porous minerals and photocataltic ability of TiO2. Therefore, the research, carried out to study the effects of different mineral carriers on photocatalytic activity of TiO2 and clarify the relationship between adsorption capacity and photocatalytic activity, is of an important significance for the selection of non-metallic mineral carriers. Considering energy crisis and environmental problems in the world today, it is important to study TiO2/porous mineral photocatalysts which have response to visible light in aims of developing and promoting application in the daily life.Based on the above actual needs and current research status, nano-TiO2/porous mineral photocatalysts were prepared by modified TiCl4 hydrolysis precipitation method at low temperature using the porous mineral such as diatomite, zeolite and silica residue of demagging boron mud as the research object in this dissertation. And Cr(?) and Cu2+ were manually added to the solution as the simulated pollutants. XRD, SEM-EDS, low-temperature nitrogen adsorption, TG-DSC, TEM, UV-Vis, XPS and other characteristic approaches were applied for the outline research described in the following parts:1. Effects of TiO2 loading amount, calcination temperature and calcination time on the photocatalytic ability of nano-TiO2/diatomite photocatalyst were studied by doing orthogonal experiments. The results showe that TiO2 loading amount and calcination temperature are significant factors affecting the photocatalytic properties of the samples. And high calcination temperature would easier cause the increasing grain size of TiO2 and agglomeration of particles, which hence affect the photocatalytic properties of the sample. The optimum condition for the preparation of nano-TiO2/diatomite photocatalyst was finally determined by single-factor experiments: 20% TiO2 loading amount, calcination temperature at 700oC and calcination time of 2h. The sample obtained under this condition is anatase which has a good dispersibility. The grain size of TiO2 in nano-TiO2/diatomite and pure TiO2 which calcined at 700oC are 13.1nm and 37.4nm, respectively. The changes of heat, weight and phase transitions during calcination process in TiO2 and nano-TiO2/diatomite photocatalyst were studied through TG-DSC and XRD analysis. The results show that phase structure of diatomite calcined at 300~900oC has no change. While the diatomite could slow the process of TiO2 phase transition, increase the phase transition temperature and decrease the TiO2 grain size.2. Effects of pH on Zeta-potential on the sample surface and status of Cr(?) and Cu2+ in the solution were studied by Zeta potential analysis and Visual MINTEQ 3.1 simulation software. And the influence of pH value on the adsorption rate and mechanism was analysed. The results indicate that the catalyst is more beneficial to adsorb ion via electrostatic attraction when the Zeta potential on surface of nano-TiO2/diatomite is dissimilarity with Cr(VI) and Cu2+. The mechanism is that H+ or OH- can react with the Si-OH along with the pH value lowering or raising. Then results show that Zeta potential of nano-TiO2/diatomite photocatalytic displays positive charge or negative charge. Due to chromium anions mostly present in forms of anion(HCrO4-, Cr2O72-, CrO42-) in the solution, chromium ions are favor to be adsorbed in the acidic conditions. Copper ions mostly present in forms of Cu2+and CuOH+ in the acidic solution and Cu(OH)2(aq) in neutral or alkaline solution. So, copper ions are favor to be adsorbed in higer pH values conditions. Adsorption removal rate of 10mg/L Cr(?) is 16.9% at pH of 2 and the adsorption removal rate of 10mg/L Cu2+ is 26.4% at pH of 5 by 2g/L nano-TiO2/diatomite photocatalyst.3. According to the Nernst equation, thermodynamic calculations were conducted for the photocatalytic process of Cr(?) and Cu2+ at different pH values. The results suggest that the reduction potential of Cr(VI) is closer to the potential of TiO2 conduction band with the increasing of pH which is not conducive to the photocatalytic reduction reaction. The reduction potential of Cu2+ does not vary with the change of pH. So the photocatalytic reduction of Cu2+ is not affected by pH. By contrast of the adsorption and photocatalytic removal of by nano-TiO2/diatomite under different p H values, it is concluded that the adsorption process has synergistic effect on the photocatalytic reaction. The optimization pH values of photocatalytic reduction of Cr(?) and Cu2+ are 2 and 5, respectively. It can find that the degradation process fits Langmuir-Hinshelwood(L-H) kinetic model very well through analysing the kinetic of nano-TiO2/diatomite photocatalyst for Cr( ?) degradation. And the calculated adsorption rate constant K=0.3479 was larger than that of photocatalytic reduction rate constant k=0.3244. That means the photocatalytic reduction reaction is the controlling step. After the photocatalytic reaction, 55.6% of Cr(?) has been reduced to Cr(?). Finally, reusing test of the catalyst indicates that nano-TiO2/diatomite photocatalyst can be reused four times.4. Using zeolite as carrier, the preparation process for nano-TiO2/zeolite photocatalyst was investigated. And the optimal loading amount of TiO2 is 30% and the optimal calcination temperature is 500°C. The nano-TiO2 particles with the crystal of anatase phase are well loaded on zeolite and the average grain size of TiO2 is 7.6nm. The photocatalytic removal efficiency of Cr(VI) is 98.25% within 3h under the illumination intensity of 300 W. The structural properties of sample under different preparation conditions were characterized by XRD, SEM, FT-IR and low-temperature nitrogen adsorption methods. The results show that the grain size and agglomeration of TiO2 increases with the increasing of calcination temperature. And calcination has activation effect on carrier of zeolite. When the calcination temperature is lower than 500 °C, the adsorbed water and impurities are removed from zeolite and the specific surface area of the sample is significantly improved. At the same time TiO2 has relative low crystallinity. However, when the calcination temperature is greater than 600 °C, the aluminosilicate framework structure of the zeolite will be destroyed and Si(Al)-O changes to Si-O-Si and the pore structure will collapse. These will result in the decrease of surface area.5. Studies of nano-TiO2/zeolite photocatalyst adsorption kinetics and thermodynamics for Cr( ?) show that the dynamic model is in line with the pseudo-second-order kinetic model and the thermodynamic model conforms to the Langmuir isotherm model. The equilibrium adsorption capacity qe and the maximum adsorption capacity Qm of samples for Cr(VI) decrease with increasing calcination temperatures. The maximum of qe(2.73mg/g) and Qm(2.78mg/g) are obtained at 300 °C. Nano-TiO2/zeolite photocatalysts prepared by different calcination temperatures were studied for the photocatalytic Cr(?). And the kinetic follows the simplified L-H kinetic model(apparent first-order kinetic model). The maximum of apparent reaction rate is obtained at 500 °C(kapp=0.0096min-1). The relationship among kapp of nano-TiO2/zeolite photocatalyst, kin of intrinsic TiO2 photocatalytic reduction rate and Qm of the maximum adsorption was calculated by formula of in app m Lk ?k/(Q k). The result shows that the higher the calcination temperature(within the range 300-700 °C), the greater TiO2 crystallinity and intrinsic reduction rate there are. But the adsorption capacity decreases with the increasing of calcination temperatures. The apparent catalytic performance of TiO2/zeolite photocatalyst is influenced by the synergistic effect of the adsorption properties of zeolite and TiO2 intrinsic reaction rate.6. The preparation of nano-TiO2/silica residue of demagging boron mud was studied. The optimal preparation conditions of nano-TiO2/silica residue of demagging boron mud are 40% TiO2 loading amount, calcination temperature of 700 °C and calcination time of 2h. TiO2 particles have an average grain size of 11.4nm and anatase phase in samples prepared under this condition. The structures of obtained samples were characterized using XRD, SEM-EDS, low-temperature nitrogen adsorption and TEM. The results show that the silica residue of demagging boron mud is fine particle with agglomeration structure and it has a larger surface area because of the accumulation hole. Because part of TiO2 piled into the hole, it makes the largest optimal loading amount of TiO2 in the three porous mineral composite.7. Compared with the photocatalytic properties of composite prepared by the three porous mineral under optimal preparation condition, TZE-30-500 gives the highest photocatalytic activity. UV-vis, specific surface area and pore size, Zeta potential, adsorption properties of TDIA-20-700, TZE-30-500 and TPSI-40-700 samples were studied. The results show that UV-visible light absorption properties of the composite have no direct relationship with photocatalytic activity but effect by TiO2 loading. The photocatalytic performance of the samples affected by their adsorption performance is significant. And adsorption properties of the catalysts are affected by the combined influence of the specific surface area, pore structure and surface potential. TDIA-30-700, TZE-30-700 and TPSI-30-700 which were prepared under the same conditions were taken out for photocatalytic experiments. The results show that, excluding the impact of the adsorption properties, TZE-30-700 with zeolite as supporter still performs a relatively superior photocatalytic properties. The hydroxyl groups density test show that hydroxyl groups density on diatomite, zeolite and silica residue of demagging boron mud are 2.45nm-2?15.17 nm-2 and 1.44 nm-2, respectively. It has a positive correlation between the surface hydroxyl groups density and photocatalytic property. The surface hydroxyl groups could combine with photo-generated holes and inhibit recombination of electrons and holes. Thereby it can promote the photocatalytic activity. Adding formic acid as the extra hole shows that it significantly promotes the photocatalytic removal efficiency of Cu2+. And removal efficiency of Cu2+ by TDIA-20-700 can reach 100% within 90 min. It indicates that the separation of holes in favour of improving the photocatalytic performance significantly. And it has verified the function of porous surface hydroxyl for promoting photocatalytic performance.8. Effect of calcination temperature on the preparation of g-C3N4 by urea decomposition was studied and samples were characterized by XRD, low-temperature nitrogen adsorption, UV-vis, TEM. The results show that g-C3N4 phase generates at the calcination temperature of 500 °C and 550 °C. With increasing the calcination temperature, impurity phase will appear. The g-C3N4 obtained at 500 °C has the best visible absorption and the forbidden gap of 2.64 eV. Under TEM, g-C3N4 shows graphite like layer, relatively fluffy structure, nanoscale thickness and width and nearly 20 nm mesoporous structure. The g-C3N4 specific surface area is 44.46m2/g and the average pore size is 7.71 nm.9. Effects of the amount of urea and calcination temperature on the visible photocatalytic properties of g-C3N4/TiO2/diatomite were studied. The 50% dosage of urea(TiO2/diatomite: urea =2:1) and the 500 °C calcination temperature were determined as optimal process. The band gap of the sample obtained under this condition is 2.96 eV. Compared to the P25 and undoped sample, g-NTDIA-1-500 exhibites good visible catalytic performance and removal efficiency of Cr(?) is 51.04% after visible irradiation for 5h. Finally, XPS and HRTEM show that the part of N element exists on surface of g-C3N4/TiO2/diatomite in g-C3N4 form having a thickness of 0.92 nm, so that g-C3N4 and TiO2 form a semiconductor heterostructure. The heterojunction can rapidly transfer electrons from g-C3N4 to the surface of TiO2, which inhibits the direct combination of holes and electrons on the surface or in the inside of g-C3N4. The other part of N element which replaces of O enters into the TiO2 lattice and it resultis in forming an N2 p isolated band and reducing the band gap of TiO2. Thus the response range of TiO2 diffuses into the visible light range and it works together with g-C3N4 to improve the visible-light activity of TiO2/diatomite. |
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