Understanding the Relationship between Organic Structure and Mineralization Rate of TiO2-mediated Photocatalysis

Global crisis of clean water supply has attracted great attention to reuse and recycle wastewater effluent. However, the effort has been limited by the presence of toxic organics in the effluent which in turn inhibits the public acceptance of recycling water. The existence of those organic pollutants, which cannot be eliminated by conventional primary and secondary treatment processes, can be problematic. Photocatalysis offers a promising process by breaking down low-level organic pollutants in water into nontoxic compounds such as CO2, H2O and mineral acids. The process involves excitation of a photocatalyst with near UV (300–400 nm) light to promote the formation of highly reactive chemical species such as holes, hydroxyl radicals, superoxide radical anions that are capable to mineralize the organic pollutants. An extensive variety of compounds can be satisfactorily degraded such as carboxylic acids, alcohols, aromatics, dyes, natural organic matters, etc.The study shows the promising application of photocatalysis to treat water and wastewater with a series of organic compounds of carboxylic acids and alcohols was selected as the substrate model. The products resulted from the mineralization of the substrates have been identified and the effect of the formation of those intermediates to the overall mineralization rate has been discussed. The results showed the mineralization rates were greatly influenced by molecule structure of organics being degraded. For dicarboxylic acid group, the presence of hydroxyl functional groups increases the residence time of a carboxylate anion adjacent to the hydroxyl group and thus, rapid mineralization rate was observed. For alcohols, only one hydroxyl group attached to the surface and hence, the relative slow mineralization was recorded in the system. It can be concluded that the surface approach of the organic molecule determines the type of intermediates formed and thus, its overall mineralization rate.