With the development of society, industrial wastewater, agricultural pesticides, and improperly treated household waste have caused serious pollution of water bodies, while traditional pollution treatment methods such as carbon adsorption and separation are only a transfer and dilution of pollutants, and do not fundamentally remove pollutants. Photocatalysis is an ecofriendly technique that emerged as a promising alternative for the degradation of many organic pollutants. This technology can be applied to low biodegradability, high complexity, and high concentration of pollutants in the wastewater. In the field of photocatalyst research, coordination complexes have received a lot of attention because of their outstanding catalytic effects. Structurally diverse coordination complexes can act as catalysts for photodegradation of organic pollutants, which brings new approaches to solve environmental pollution problems.
Among all the organic pollutant degradation methods, photocatalytic methods have received a lot of attention. This is mainly because photocatalytic degradation has the following characteristics:
- Wide range of application. The ·OH produced in the photocatalytic process is the main active oxide species, with strong oxidation capacity, which can effectively oxidize difficult to degrade organic pollutants with complex molecular structure.
- Low cost and mild reaction conditions. Photocatalytic reaction can use sunlight or ultraviolet light as light source, which is an efficient and energy-saving organic pollutant treatment technology.
- Easy to control, no secondary pollution. No other chemicals were added to the photocatalytic degradation of organic pollutants, so no secondary pollution was generated. In addition, during the reaction process, the organic matter is completely degraded to CO2 and H2O, and there is no need to consider the disposal and collection of reaction products.
- Fast reaction rate. Under the action of high-performance photocatalyst, degradation of organic pollutants usually takes only a few minutes to several hours, far less than the time required for other traditional methods. This greatly improves the value of practical application.
Coordination complexes have fascinating topologies, strong thermal/chemical stability, complex and diverse steric structures, and metal central ions with variable valence electrons and empty d orbitals. Therefore, these coordination complexes have great advantages as photocatalysts. In recent years, there have been numerous studies on coordination complexes as photocatalysts to degrade organic pollutants. For example, ruthenium, iron, and osmium complexes are not only able to catalyze the degradation of hazardous substances such as formaldehyde and toluene into CO2 and H2O, thus purifying the air. These complexes also have a good degradation effect on water pollutants, which helps in the treatment of water pollution. Many research teams also began to synthesize new complex catalysts. For example, a new copper (II) pyridyl complex (CP1) has been synthesized by Harshita Jain's group  for adsorption as well as photocatalytic degradation of Methylene Blue (MB) and Rhodamine B (RhB) dyes. The photocatalytic degradation was performed under visible light irradiation which reveals that 95.8% of MB (16 mg/L) and 93.7% of RhB (23.95 mg/L) can be decolorized in 35 minutes using 0.3 g/L of catalyst. The adsorption and photocatalytic degradation processes of the two dyes are shown in figure1. The results show that compared with other materials, copper complexes as photocatalysts have the advantages of easy synthesis, high efficiency, and fast removal time.
Figure 1. Adsorption and photocatalytic degradation of MB and RhB dyes
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- Harshita J.; et al. Synthesis of a highly efficient multifunctional copper (II)-pyridyl complex for adsorption and photocatalytic degradation of organic dyes. Chemistry Select. 2019, 4: 4952-4961.