Application of TiO2 photocatalytic oxidation in wastewater treatment

As a non-biological technology, titanium dioxide photocatalytic oxidation method can destroy and mineralize a variety of organic pollutants.
This new pollution control technology has the following advantages:

①The reaction conditions are mild and can occur under ultraviolet light or sunlight;

②The reaction rate is fast, generally only a few minutes to a few hours;

③There is no selectivity in degradation, almost any organic matter can be degraded;

④Simple operation can reduce secondary pollution
.

The titanium dioxide photocatalytic oxidation method has opened up a new research direction for the treatment of complex polymers and toxic and harmful pollutants
.

4.
3.
1 Photocatalytic degradation of organic compounds in water

(1) Dye wastewater.
Many dyes are often left in wastewater discharged from factories that produce and apply dyes.
For example, dyes containing benzene rings, amino groups, and azo groups are carcinogens and cause serious environmental pollution.
Biochemical treatment of water soluble The degradation efficiency of sex dyes is generally very low
.

At present, there have been many reports on the use of semiconductor photocatalytic degradation of dyes

The large amount of water demand and wastewater discharge has always been a major problem that plagued the printing and dyeing industry
.

The Environmental Science Institute of Wuhan University of Science and Technology has developed microwave electrodeless ultraviolet photocatalytic oxidation technology to solve the problem of recycling high-temperature colored printing and dyeing water

Malato et al.
studied the effect of TiO ₂ /Fenton system combined with biological treatment to treat industrial wastewater as a basis for designing industrial water plants
.

The CPC reactor has an area of ​​100m2 and a processing capacity of 250L/h

(2) Pesticide wastewater Pesticides are generally divided into herbicides and insecticides.
They have a wide range of harm and stay in the atmosphere, soil and water for a long time, so their decomposition and removal have attracted much attention
.

Although the use of photocatalysis cannot completely mineralize all pollutants, at least it will not produce more toxic intermediate products

(3) Chloride Organic chloride is the most important type of pollutant in water, with high toxicity and wide distribution.
Its treatment is an important subject of water pollution treatment
.

The photocatalytic process has shown a good application prospect in the treatment of organic chlorides.

(4) Surfactant Wastewater containing surfactants is not only easy to produce peculiar smell and foam, but also affects the biochemistry of the waste water.
Non-ionic and cationic surfactants are not only difficult to biodegrade, but sometimes also produce toxic or insoluble Intermediate
.

The currently widely used synthetic surfactants usually include different carbon chain structures, and the photocatalytic degradation performance often varies greatly depending on the structure

Although it is difficult to completely oxidize the paraffin part of the surfactant to CO ₂ by photocatalytic degradation reaction , with the destruction of the benzene ring part of the surfactant, the surface activity and toxicity are greatly reduced, and the long-chain alkane is produced as a by-product.
The harm to the environment is significantly reduced.
It is recognized at home and abroad that this method is very attractive for the treatment of surface active agents in wastewater
.

(5) The existence of Freon Freon (CFCs) will destroy the ozone layer, create a hole in the ozone layer, lead to a series of environmental problems such as global warming, and seriously disturb the global ecological balance
.

Therefore, the research on photodegradation of freon is of great significance and has become a relatively active field in recent years

(6) Oily wastewater With the development of the petroleum industry, a large amount of oil flows into the ocean every year, causing serious pollution to water bodies and the coastal environment
.

The treatment of such oils and organic pollutants that are insoluble in water and floating on the water surface is also a topic of great concern in recent years

Table 4-1 Photodegradation of different types of organic compounds on semiconductor catalysts

4.
3.
2 Photocatalytic redox of inorganic pollutants in water

In addition to organic substances, many inorganic substances also have photochemical activity on the surface of TiO _2.
For example, the treatment of Cr ₂ O ₇ ^2- ion aqueous solution was reported as early as 1977
.
Miyaka et suspensions were Ti02 ₂ powder, by the light of Cr ₂ O _.
7 ^2- restore of Cr ³⁺ work
.
Yoneyama et al.
used a variety of photocatalysts to study the Cr ₂ O ₇ ^2- photocatalytic reduction reaction
.
Dai Xiaming et al.
studied the effect of ZnO/TiO ₂ ultrafine powder on the reduction of hexavalent chromium in aqueous solution under different reaction conditions , and discussed the feasibility of this method in the process
.
The treatment of cyanide-containing wastewater is also a content that has been studied more
.
Frank et al studied using Ti02 ₂ as the photocatalyst to the CN ^- oxidation of OCN ^- , then further reacted to CO.
'S ₂ , N ₂ and NO _.
3 ^- process
.
Serpone et al.
reported the use of TiO ₂Photocatalytic from Au (the CN) _.
4 ^-
.
Restore Au, while the oxidation of the CN ^- is NH2 _.
3 and CO.
'S ₂ process, noting the electroplating method for processing industrial wastewater, not only capable of reducing a noble metal plating solution, but also eliminate cyanide bath environment The pollution is a practical method of treatment
.
Hidaka et al.
studied the photocatalytic oxidation process of cyanide and cyanide-containing industrial wastewater through the intermediate product OCN ^-to produce CO ₂ and N ₂ , and discussed the possibility of photocatalytic oxidation to treat large-scale cyanide-containing wastewater
.
The reaction formula of photocatalytic oxidation applied to inorganic pollutant wastewater is as follows:

Of Cr ₂ O _.
7 ^2- + 16H ⁺ + 12e ^- = cr ₂ O _.
3 + 8H ₂ O (4-16)

Hg ²⁺ + 2E ^- = Hg (4-17)

Pb ²⁺ +2H ₂ O+2h ⁺ =PbO ₂ +4H ⁺                  (4-18)

Mn ²⁺ +2H ₂ O+2h ⁺ =MnO ₂ +4H ⁺                   (4-19)

2CO ²⁺ +3H ₂ O+2h ⁺ =Co ₂ O ₃ +6H ⁺                  (4-20)

A large number of experimental results have proved that the TiO ₂ photocatalytic reaction has a strong ability to treat industrial wastewater
.
However, it is worth mentioning that since the photocatalytic reaction is based on the absorption of light energy by the system, the treated system is required to have good light transmittance, etc.
If there are many impurities, high turbidity, and poor light transmittance, the reaction will be difficult to proceed
.
Therefore, this method is suitable for advanced treatment in the actual wastewater treatment
.
For example, Spain first used biological methods for pre-treatment of wastewater discharged from factories, and then used photocatalytic methods for degradation, and obtained very satisfactory results
.