Synthesis Of MWNTs-TiO₂ Nanocomposites And Photocatalytic Degradation Of PPCPs

Pharmaceuticals and Personal care products (PPCPs) are a new kind of organic pollutants, enter into the environment with different pathway through human excretion, abandoned by production and medical treatment, even using the illegal and outdated of pharmaceuticals. The characteristic of ingredient complexity, high toxicity, difficult biochemistry degradation, therefore, PPCPs will easily gather in the environment and have chronic toxicity to human by food chain. Recently, due to TiO2 has the advantage of non-secondary pollution, efficient, and so on, it has raisd more concern in depollution of environment application. However, TiO2 can only excited by UV light because of its wide energy gap (3.2 eV), which limit its application under visble light. Therefore, how to increase the sunlight use ratio, enlarge the visble light response of catalysts will be the key point of developing a new photocatalysts. In this work, different diameters and lengths of multi-walled carbon nanotubes (MWNTs) modified TiO2 nanocomposites (MWNTs-TiO2) were prepared by hydrothermal method and their photocatalytic activities under the UV light, the visble light and the sunlight, respectively. Furthermore, photocatalytic degradation activities and mechanisms of propranolol, tetracycline and sulfonamides with MWNTs-TiO2 nanocomposites were investigated. The detail researchs and conclusions are as follows:1. MWNTs-TiO2 nanocomposites with different diameters and lengths of MWNTs were prepared by hydrothermal method and characterized by XRD, SEM, Raman, FTIR, BET, their photocatalytic activities were evaluated by degradation of RhB under different light source and their degradation mechanism. It found that TiO2 has well distributed particle size, which has intimate contact with MWNTs. Through the degradation experiments, the shorter of MWNTs have the better photocatalytic activities, because their have the better electrical conductivity lead to decrease the recombination of electron-hole pairs. When under the UV light irradiation, the small diameter of MWNTs (SM1) has the better activities. However, the large diameter of MWNTs has the better activities under the visble light irradiation. When under the sunlight irradiation, the photoctatlytic activities is similar with the result under the visble light.2. Photocatalytic degradation of propranolol in aqueous MWNTs-TiO2 suspension (MWNTs:diameter<8 nm, length 30μm) using UV light irradiation, was investigated to determine the optimal conditions with the initial concentration of propranolol, pH, and catalyst loading, even preliminary conjectured the probable degradation progress. MWNTs can enhance TiO2 photocatalytic activities under the UV light, and the optimal conditions was 10 wt% MWNTs-TiO2 loading concentration of 2.0 g/L and pH 11, approximately 95% of 20 mg/L propranolol was degraded with UV light irradiation 180 min. Completely removed with 300 min, while nearly 35% of propranolol was mineralized. Moreover, propranolol was oxidated by hydroxyl radical was the major reaction in photocatalytic degradation of propranolol.3. Simulation visble light photocatalytic degradation of tetracycline with MWNTs-TiO2 nanocomposites, was investigated to determine the optimal conditions with the initial concentration of tetracycline, pH, and catalyst loading. Radical scavengers even have been used to assess information about the participation of different radicals. It found that MWNTs can enhance TiO2 photocatalytic activities under simulation visble light irradiation, and the optimal conditions was MWNTs-TiO2 loading concentration of 2.0 g/L and pH 9, approximately 95% of 20 mg/L tetracycline was degraded with 30 min. Completely degraded with 90 min, while nearly 23% of propranolol was mineralized. The hydroxyl radical and holes play a significant role in the photocatalytic degradation of tetracycline, while holes were the leading role in the direct oxidation of tetracycline. The mechanism of photocatalytic degradation was also discussed. There found seven types of photocatalytic degradation intermediates of tetracycline with 10 min irradiation were detected by HPLC-MS, also presumed the probable pathways of degradation tetracycline with MWNTs-TiO2 nanocomposites.4. Optimize the mass ratio and calcining heat of the best photocatalytic activities of MWNTs-TiO2 nanocomposites (MWNTs:diameter>50 nm, length 0.5-2μm), used them in photocatalytic degradation of sulfonamides, and investigated the effect of differents structures on degradation rates under differents light source with quantum chemical theory. The results indicated that sulfonamides can be degraded in the suspension containing MWNTs-TiO2 under the visble light irradiation, while has no effect by P25 TiO2 (Degussa, Germany). Through the results combined with quantum chemical theory, it discovered that the degradation rates were depended on the structures of sulfonamides, for example n-octanol/water partition coefficient, moment of dipole, and orbital energy.Furthermore, it's hopeful to remove PPCPs with MWNTs-TiO2 nanocomposites under sunlight irradiation due to its preferable reuse and visble-light photocatalytic activites, provides a new approach to remove organic pollutants.