Factories across the world are dumping thousands of tonnes of untreated dyes into rivers and waterways every year. The majority of these dyes are toxic to the environment and may lead to mutations and cancers in animals. Particularly in textile industries where considerable amounts of water and chemicals are used during the dyeing process the wastewater contains about 20% of dye as well as organic matter, salts and other substances. Also since synthetic dyes are used to resist bleaching by UV-light and chemicals to improve the quality of the textiles, they are also persistent in the environment and some dyes can be biologically modified into carcinogenic compounds. For example azo dyes, a commonly used dye to color fabrics can cause cancer if released into the environment with wastewater.

The release of untreated wastewater has high color, high chemical oxygen demand, low biodegradabilityand high variability, it poses a threat to the animal and human health, environment and the most serious problems are ground water and surface water pollution. Further, the discharge of colored effluents into water bodies affects the sunlight penetration which in turn decreases both the photosynthetic activity and dissolved oxygen levels. The removal of dyes from wastewater is one of the major environmental challenges.

Wastewater containing dye is conventionally filtered using activated carbon. However, the carbon can only be used once and is then commonly disposed of in landfill sites. Biotechnological treatment methods called dye remediation can be used for the treatment of dyes using biological and physico-chemical techniques. Different techniques are adopted to treat dye wastewater including adsorption, catalytic oxidation, chemical oxidation, photocatalysis, electrochemical process, biodegradation and catalytic wet oxidation by adding catalysts and oxidants to improve the oxidation rate.
Catalytic wet oxidation

Catalytic wet oxidation process is usually carried out at high temperature and pressure, which restrict its wide application. More and more efforts have been focused on developing new processes to improve the efficiency of CWO, such as the preparation of new type heterogeneous catalysts with high catalytic activity. CeO2 or CeO2-based oxides materials by virtue of their large surface area exhibit greater catalytic activity in CWO.

It is very hard to recover pure CeO2 or CeO2-based oxides powders from water when they are used in aqueous systems. Coating the particles onto other materials is the promising method to resolve this problem. Supports of silica and γ-Al2O3 have been used to prepare the CeO2-based catalysts, but, the supports, synthesized by chemical reactions have inherent defects such high cost, time consuming reaction and low surface area.

Attapulgite (ATP) is a crystalline hydrated magnesium aluminum silicate with reactive –OH groups on its surface with a structure of zeolite-like channels. Due to its regular structure and large specific surface area, ATP has been used as absorbent, catalyst and catalyst support. Zhao et al. prepared copper modified palygorskite/TiO2 photocatalyst by hydrolysis method, which exhibited much higher activity than that of the pure titanium dioxides in the degradation of methylene blue. In addition, it was reported that the redox couple (Ce3+/Ce4+) in contact with metal particles promoted the catalytic activity in ceria-based materials. It is effective way to enhance the catalytic reaction rate that modified the palygorskite clay with copper ions since addition of rare metal ions to CeO2-based catalytic systems.