Photodegradation

Photodegradationconcerns the breaking of a molecule to smaller molecules (degradation and/or mineralization) by means of the absorption of photons of suitable wavelength such as that found in the infrared radiation, visible light and ultraviolet light of the solar spectrum. Photodegradation also includes the change of the shape of a molecule such as protein denaturation and the addition of other atoms or molecules. One type of photodegradation reaction is oxidation which is used to purify some drinking water and wastewater to destroy pollutants. Phototodegradation can be photochemical and photocatalytical: in the first case only light is used while in the second case a combination of light and a semiconductor photocatalyst is used. In the photocatalytic oxidation process, organic pollutants are destroyed in the presence of semiconductor photocatalysts (e.g., TiO2 and ZnO), an energetic light source, and an oxidising agent such as oxygen or air. Photocatalytic degradation in aqueous medium is mainly dependent on the solution pH, catalysts and their composition, organic substrate type (usually pollutants) and concentration, light intensity, catalyst loading, ionic composition of waste water, types of solvent, oxidant concentration, and calcination temperatures of the photocatalyst (Ahmed et al 2010). In the degradation of organic pollutants various compounds such as dyes (Molinari et al 2003), drugs (Molinari et al 2006), pesticide and herbicide derivatives dominant in storm water and wastewater effluent have been treated. In the photocatalytic process the hydroxyl radical, which is generated from the oxidation of adsorbed water as OH−, is the primary oxidant species while the presence of oxygen prevents an electron–hole pair recombination. In the photocatalytic degradation of pollutants, when the reduction process of oxygen and the oxidation of pollutants do not proceed simultaneously, there is an electron accumulation in the CB, thereby causing an increase in the rate of recombination of e−and h+. Thus it is of paramount importance to prevent electron accumulation in efficient photocatalytic oxidation. In photocatalysis, TiO2 has been studied extensively because of its high activity, desirable physical and chemical properties, low cost, and availability. Among the TiO2 crystalline forms, anatase and rutile forms have been investigated extensively as photocatalysts. Anatase has been reported to be more active as a photocatalyst than rutile. Heterogeneous photocatalysis involving zinc oxide (ZnO) emerges as a promising new route for water purification process in terms of the degradation and mineralization of dyes discharged into wastewater from textile and other industrial processes (Lam et al. 2012), although ZnO can give rise to anodic photocorrosion in aqueous systems, depending on the experimental conditions chosen. Removal of nitrogen-containing organic compounds in wastewater by TiO2 photocatalytic degradation is the result of a strong interaction among pollutant structures, TiO2 properties and photocatalytic reaction conditions. It is also clear that for each organic pollutant, a unique set of conditions may be needed for the optimal performance (Jing et al. 2011). Solid-phase photocatalytic degradation for eco-friendly disposal of polymer such as waste plastics has been also studied. Composite plastics embedding photocatalysts into polymer matrix have excellent photocatalytic degradation activity. They could be degraded effectively in ambient air under sunlight exposure (Yang et al. 2011). Photoelectrochemical cell and the factors that affect efficient production of electricity and hydrogen by photocatalytic degradation of (principally) organic and (secondarily) inorganic waste materials is reported by Lianos. Materials used for making a photoanode and the rest of the components of the cell as well as the models of cell efficiency and photodegradation procedures during cell operation have been also described (Lianos, 2011). Efficient photochemical systems for environmental remediation can be constituted by a semiconductor and a surface-adsorbed antenna molecule (dyes or other color species). The major advantage of these systems is that they are able to achieve the degradation of organic pollutants by using visible light from the sun as energy and O2 in the air as the oxidant under ambient conditions (Chen et al. 2010). References Ahmed S, Rasul MG, Martens WN, Brown R, Hashib MA(2010) Heterogeneous photocatalytic degradation of phenols in wastewater: A review on current status and developments. DESALINATION 261: 3-18 Chen CC, Ma WH, Zhao JC(2010) Semiconductor-mediated photodegradation of pollutants under visible-light irradiation. CHEMICAL SOCIETY REVIEWS 39: 4206-4219 Jing JY, Liu MH, Colvin VL, Li WY, Yu WW(2011) Photocatalytic degradation of nitrogen-containing organic compounds over TiO2. JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL 351: 17-28 Lam SM, Sin JC, Abdullah AZ, Mohamed AR (2012) Degradation of wastewaters containing organic dyes photocatalysed by zinc oxide: a review. DESALINATION AND WATER TREATMENT 41: 131-169 Lianos P(2011) Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a photoelectrochemical cell. The concept of the Photofuelcell: A review of a re-emerging research field. JOURNAL OF HAZARDOUS MATERIALS 185: 575-590 Molinari R , Pirillo F , Falco M , Loddo V , Palmisano L(2003) Photocatalytic oxidation of dyes in a membrane reactor. J. Catal. Mat. Env. 2:77-80 Molinari R , Pirillo F , Loddo V , Palmisano L(2006) Heterogeneous photocatalytic degradation of pharmaceuticals in water by using polycrystalline TiO2 and a nanofiltration membrane reactor. Catalysis Today 118:205-213 Yang CJ, Peng TY, Deng KJ, Zan L (2011) Solid-Phase Photocatalytic Degradation of Waste Plastics. PROGRESS IN CHEMISTRY 23: 874-879