Application of Photocatalytic Degradation of Nano-TiO2 in Treatment of Wastewater

It is generally believed that the mechanism of nano-TiO ₂ photocatalytic degradation follows: anatase TiO ₂ nanoparticles having a higher photocatalytic oxidation. Band gap of Eg = 3.2eV, energy corresponding to the wavelength of 387nm light, which is just in the ultraviolet region, therefore, as TiO ₂ with photocatalytic oxidation reactions for an ultraviolet light source, e.g., sunlight, halogen, mercury lamp, etc. . Under the action of ultraviolet light, electrons in the valence band are excited to the conduction band, and holes are generated in the valence band to form a free electron-hole pair. That is, under the irradiation of ultraviolet rays, the freely moving negatively charged electron (e ^- ) and the positively charged hole (h+) can be decomposed by themselves, and a series of chemical reactions occur to form an electron-hole pair, and the adsorption is dissolved in TiO. ₂ Oxygen trapping electrons on the surface forms O ₂ ^- , and holes oxidize OH ^- and H ₂ O absorbed on the surface of TiO ₂ to HO·, and the generated atomic oxygen and hydroxyl radicals have strong chemical activity, especially Atomic oxygen reacts with most organic substances, causing organic matter to be oxidatively decomposed and eventually decomposed into CO ₂ , H ₂ O and/or inorganic substances.

According to statistics, it has been found that more than 3,000 kinds of refractory organic substances can be rapidly degraded by UV TiO ₂ under ultraviolet irradiation, especially when the concentration of organic pollutants in water is high or it is difficult to be degraded by other methods. More obvious advantages.

Nano TiO ₂ has photocatalytic degradation properties, which can not only treat organic pollutants in wastewater, but also treat pollutants in the air. Such as nitrogen oxides (NO _x ), oil, ethanol, methanol, formaldehyde, acetone, mercaptans, etc., wherein the degradation rate of nitrogen oxides reaches 80%. By treating these contaminants, they can be directly converted into non-toxic, harmless CO ₂ , H ₂ O and/or inorganic substances without separating the contaminants from the mainstream liquid (gas). Therefore, it is possible to avoid secondary pollution that may occur due to purification methods such as absorption and adsorption. At the same time, the operation process can be simplified. Therefore, the use of nano-TiO ₂ photocatalytic decontamination has become one of the most promising methods for controlling the environment. The advantages of nano-TiO ₂ photocatalytic degradation of organic wastewater treatment technology are as follows: 1 has a large specific surface area, and thus has more sufficient contact with organic pollutants in the wastewater, and can adsorb organic matter on its particle surface to a great extent; It has stronger UV absorption capacity and thus has stronger photocatalytic degradation ability, which can quickly decompose organic matter adsorbed on its surface.

1. Treatment of dye wastewater

The dye wastewater contains carcinogens such as benzene rings, amino groups, and azo groups. The degradation efficiency of conventional methods for treating water-soluble dyes is usually low. It has been found that the R-6G dye can be degraded very rapidly with the TiO ₂ /SiO ₂ system. Photocatalytic degradation of dyes by TiO ₂ /SiO ₂ can not only effectively destroy the chromophore groups in the dye, but also destroy the aromatic groups in the dye molecules to achieve complete degradation.

2. Treatment of pesticide wastewater

When nano-TiO ₂ photocatalytic degradation of organophosphorus pesticides, only a small amount of Fe ³⁺ is added to the reaction solution, which can greatly improve the removal rate of COD and the recovery rate of inorganic phosphorus.

3. Treatment of wastewater containing surfactant

Surfactant-containing wastewater is not only prone to odor and foam, but also affects the biodegradability of wastewater. Nonionic and cationic surfactants are not only difficult to biodegrade, but sometimes produce toxic or insoluble intermediates. The photocatalytic decomposition of surfactants by nano-TiO ₂ has achieved good results. It is found that the benzene ring is more susceptible to chain-breaking degradation than alkyl or alkoxy groups to achieve inorganicization, and the degradation rate of the linear part is extremely slow. This is because benzene The Ï€ electrons in the ring may be moved by holes to the surface of TiO _{2 to} form cationic radicals. Although the paraffin fraction in the surfactant is more difficult to completely oxidize to CO ₂ by photocatalytic degradation reaction, the surface activity and toxicity are greatly reduced with the destruction of the benzene ring portion of the surfactant, and the long-chain alkane by-product formed. The harm to the environment has been significantly reduced. At present, it is recognized at home and abroad that this method is very attractive for the treatment of surfactants in wastewater.

4. Treatment of oily wastewater

The continuous development of the petroleum industry has promoted the economic development of various countries. At the same time, it has also produced very serious environmental pollution problems. Oily wastewater is one of the more serious pollutions. Dealing with such oil-insoluble and less dense oil than water has always been a concern. The treatment of oily wastewater with nano-TiO ₂ powder can achieve a degradation rate of 94.74%. If it can be supported on a carrier and floated on the water surface, the photocatalytic activity of TiO ₂ can be effectively improved. The TiO ₂ film is supported on the surface of the hollow glass sphere by dip coating or heat treatment, or the nano TiO _{2 is} firmly adhered to the surface of the carrier hollow ceramic microsphere by silicon coupling, and the photocatalytic degradation of octane can achieve satisfactory results.

5. Handling papermaking wastewater

Papermaking industry wastewater discharge is large, difficult to handle, and serious pollution, and it is one of the major industrial pollution sources in China. The papermaking wastewater was treated by nano-TiO ₂ photocatalytic oxidation method. The results showed that the COD concentration of papermaking wastewater after photocatalytic deep treatment decreased from 20.0 mg/L to 81.0 mg/L, and the color was 86.0. % fell to 50.0%, fully meeting national wastewater discharge standards.

6. The nano-TiO ₂ surface of the high activity of the electron e ^- has a strong reducing ability, can be reduced and removed some of the heavy metals from wastewater

7. Sewage from the photographic industry is also toxic, harmful and difficult to degrade. Photocatalytic degradation of nano-TiO ₂ has been available in recent years. In addition, hydrocarbons, halogenated hydrocarbons, organic acids, nitrogen-containing organic compounds, phenols, wood preservatives, etc. can also be photocatalyzed by nano-TiO ₂ .

Nano-TiO ₂ photocatalytic oxidation treatment of wastewater has a wide application prospect. Domestic research on the use of nano-TiO ₂ photocatalytic technology for wastewater treatment has been more than a decade old, but most of them have remained in the laboratory initial stage, and there is no practical application report. The reason is that industrial application is restricted: 1 quantum yield is low, about 4%, the highest is less than 10%, it is difficult to handle industrial waste gas and wastewater with large amount and high concentration; 2 low solar energy utilization rate, TiO ₂ -based light The catalyst can only absorb the ultraviolet portion of the sunlight, and cannot fully utilize the visible portion. How to improve quantum yield, improve solar energy utilization rate, improve the photocatalytic efficiency of nano-TiO ₂ and reduce environmental treatment costs are still very important issues.