Photocatalytic Degradation of Phenol Using a Nanocatalyst: The Mechanism and Kinetics

The study of photocatalytic degradation of phenol was exploited with nano-ZnO as immobilized photocatalysts in a laboratory scale photocatalytic reactor. The photocatalytic degradation mechanism and kinetics of phenol in water were studied using the solid-phase microextraction (SPME) technique. Based on optimized headspace SPME conditions, phenol in water was first extracted by the fibre, which was subsequently inserted into an aqueous system with immobilized photocatalysts (nano-ZnO) exposed to an irradiation source (i.e., ultraviolet A (UVA) lamps). After different irradiation times (5–80？min), four main intermediates of photocatalytic degradation generated on the fibre were determined by GC-MS. 1. Introduction Phenolic compounds were a constant concern from the environmental point of view with regard to their toxicity, bioaccumulation, and persistency in the environment. Among this class of compounds, the representative was phenol which was widely used as a raw material in the petrochemical industries and oil refineries. Phenol and its degradation products were major aquatic pollutants in the environment. Polychlorinated phenol could occur as a result of the chlorination of phenol. Therefore, the increasing presence of phenol represented significant hazardous environmental toxicity. The World Health Organization (WHO) has limited phenol concentration in drinking water to 1？μg/L [1]. However, traditional water treatment techniques including active carbon adsorption, chemical oxidation, and biological digestion had difficulty in the removal of phenol to the safe levels. Over the last two decades, application of advanced oxidation process (AOP) to degrade organic pollutants in wastewater is a relatively new process. Among AOP, heterogeneous photocatalysis using various semiconductor nanoparticles has been shown to be potentially advantageous and useful for the degradation of wastewater pollutants. Several advantages of this process are: (1) complete mineralization of organic pollutants to CO2, water, and mineral acids, (2) no waste-solid disposal problem, and (3) only mild temperature and pressure conditions are necessary. In photocatalysis, water and hydroxide ions react with the electron holes to form hydroxyl radicals, proven to be the primary oxidant in the photocatalytic oxidation of organics [2]. These hydroxyl radicals have an oxidation potential higher than that of ozone or hydrogen peroxide, second only to fluorine [3]. Repeated hydroxyl radical attacks can eventually lead to complete oxidation of the organic contaminants. Even if complete