Nano TiO2 Photocatalytic Oxidation Technology

This paper introduces the principles of nanotechnology, particularly nano-TiO2 photocatalytic oxidation technology and nanofiltration membrane technology, as well as their roles and applications in water treatment, suggesting that the application of novel nanotechnology for water treatment is imminent.

Nanotechnology studies the unique properties of materials in the 0.1–100 nm scale range and how to utilize these properties. Broadly speaking, nanomaterials refer to materials in three-dimensional space where at least one dimension reaches the nanoscale range or are composed of materials with nanoscale as their basic units. Nanomaterials differ significantly from ordinary materials in mechanical, magnetic, optical, electrical, and thermal properties, exhibiting novel characteristics such as radiation, absorption, catalysis, and adsorption. Many scientists have studied these properties of nanomaterials and their effects on certain pollutants in water, indicating that nanotechnology may bring about a breakthrough in water treatment technology.

Photocatalytic oxidation technology of nano-TiO2

1.1 Principle and characteristics

Since J.H. Cary et al. [2] reported in 1976 that nano-TiO2 could dechlorinate recalcitrant polychlorinated biphenyls under ultraviolet light irradiation, hundreds of organic pollutants have been found to be treatable by photocatalysis. The working principle [3] is that under ultraviolet light irradiation, the surface of nano-TiO2 generates highly oxidizing hydroxyl radicals (·OH), which oxidize and degrade organic pollutants in water into harmless CO2 and water. The advantages of nano-TiO2 photocatalytic oxidation technology are: ① Fast degradation speed, generally only tens of minutes to a few hours to achieve good wastewater treatment effect; ② Non-selective degradation, it can degrade almost any organic matter, especially suitable for chlorinated organic matter, polycyclic aromatic hydrocarbons, etc.; ③ Mild oxidation reaction conditions, low investment, low energy consumption, photocatalytic oxidation reaction can be carried out by ultraviolet light irradiation or exposure to sunlight; ④ No secondary pollution, organic matter is completely oxidized and degraded into CO2 and H2O; ⑤ Wide range of applications, almost all sewage can be treated.

1.2 Experimental Research Status

(1) Treatment of organophosphorus pesticide wastewater. Organophosphorus pesticides, which were developed in the 1970s, account for more than 80% of my country's pesticide production. A large amount of toxic wastewater is generated in the production process. At present, the treatment of organophosphorus pesticide wastewater mostly adopts biochemical methods. After treatment, the content of organophosphorus in the wastewater is still as high as 30 mg/L. So far, there is no ideal solution. According to reports [4], using nano-TiO2*.SiO2 supported composite photocatalysts, the photocatalytic activity and high adsorption efficiency of the catalysts can rapidly enrich organophosphorus pesticides on their surface. With the extension of light exposure time, the photolysis rate of organophosphorus pesticides gradually increases. After 80 min of light exposure, the tested trichlorfon was completely degraded.

(2) Treatment of wool textile dyeing and finishing wastewater. Glass packing material coated with nano-TiO2 film is filled into a glass reactor, and the wastewater is circulated in the reactor by a submersible pump for photocatalytic oxidation treatment [5]. Due to the huge specific surface area of nano-TiO2, it has more sufficient contact with organic matter in wastewater, and can adsorb them to the maximum extent on its surface, and rapidly decompose organic matter into CO2 and H2O. The treatment effect is better than biological treatment and suspended photocatalytic oxidation treatment, and the COD removal rate and decolorization rate are both high. The catalyst can be used continuously without separation and recovery, which is convenient for industrial application.

(3) Treatment of chlorinated organic wastewater. Makoto Noguchi of the University of Tokyo used a combination of nano-TiO2 photocatalyst and ozone for water purification [6]. In a simulated wastewater treatment experiment, a 16 mg/L aqueous solution of 3-chlorophenol was used as the simulated wastewater. Three methods were used to treat it: a combination of nano-TiO2 photocatalyst and ozone, nano-TiO2 photocatalyst alone, and O3 alone. After 2 h of treatment with nano-TiO2 photocatalyst and ozone, the residual concentration of 3-chlorophenol was 0, which was significantly better than the other two methods. An experiment using a ceramic tube with nano-TiO2 photocatalyst coated on the inner surface to treat a 5.5 mg/L aqueous solution of phenol and trichloroethylene showed that phenol was completely decomposed after 1.5 h, and trichloroethylene was also completely decomposed within 2 h.

(4) Treatment of oily wastewater. Oily wastewater contains aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, organic acids, phenols, and other organic compounds that are difficult to degrade. Using nano-TiO2, its photocatalytic degradation function can rapidly degrade these organic compounds [7].

1.3 Application Prospects

Nano-TiO2 photocatalytic oxidation technology has outstanding advantages in the complete degradation of organic pollutants in water and the utilization of solar energy. Especially when the concentration of organic pollutants in water is high or difficult to treat by other methods, it has even more obvious advantages, which are unmatched by other traditional methods. In particular, the research and development of high-efficiency photocatalysts, nanoparticle loading and metal doping, photoelectric combined catalytic methods, and solar energy technology in recent years have made the application of nano-TiO2 photocatalytic oxidation in the field of water treatment promising. Currently, Japan, the United States, Canada, and other countries have attempted to use nano-TiO2 photocatalytic oxidation technology for water treatment, but most are still in the laboratory research stage, and there are few reports on industrial-scale application development. How to quickly realize its engineering application requires further efforts from researchers in related fields.