APPLICATIONS OF NANOPARTICLES IN WATER TREATMENT: REACTION KINETICS AND PATHWAYS

Abstract

A wide variety of organic contaminants have been detected in the aquatic environment, which are discharged through municipal waste-water, industrial effluents, commercial operations, runoff from agricultural lands, chemical spills etc. The conventional water treatment methods, such as, coagulation, flocculation, adsorption biological treatment techniques and chlorination etc. have not been able to get rid of these persistent organic contaminants from the environment. However, these techniques may produce potentially harmful by-products during the water treatment process. Hence, advanced oxidation process is considered to be best method for treatment of recalcitrant organic contaminants present in waste-water, industrial effluent etc. using semiconductor as photocatalyst to completely degrade toxic organic contaminant to CO2 and H2O. Kinetic investigations related to water treatment are drawing much attention in the recent years, as they provide insight into the detailed reaction mechanisms. The transformation of organic contaminants will not take place if the reaction is slow with the oxidants used in the process, on the other hand, if reaction is fast then the transformation or degradation of the organic contaminant takes place. Therefore, it becomes necessary for researchers to understand the factors controlling the rates. In the proposed research, we intend to study degradation kinetics and mechanism of some of the reactions involving organic contaminants using advanced oxidation processes. Advanced oxidation processes (AOPs) have been successfully employed for the degradation of many hazardous organic contaminants in aqueous environment with some acceptable levels, without generating harmful by-products. AOPs depend on in-situ generation of reactive hydroxyl radicals (OH·) using light or chemical energy. The so formed OH· radicals are used to decompose indiscriminately a variety of organic contaminants present in water and waste waters. The AOP process involves a semiconductor stimulated by UV or visible light to destruction of environmental contaminants is called photo catalysis which results in complete or partial demineralization of the organic molecules. Titanium dioxide (TiO2) is an excellent metal oxide semiconductor photocatalyst, due to its chemical and thermal stability and it is relatively inexpensive, biocompatible and non toxic. It is characterized by a band gap ranging from 3 to 3.2 eV, which allows UV light to pump the electrons from the valence bond to conduction band leading to the formation of electron hole pair. Thus generated electron-hole pair facilitates the formation vi of OH· radicals leading to the degradation of organic contaminants. The high rate of electron-hole recombination in TiO2, however, limits the efficiency of the photo catalyst, which can be prevented, to some extent, by doping with noble metals such as silver, ruthenium, copper, gold, platinum etc. In order to improve the efficacy of photocatalyst and inhibit the electron-hole recombination metal oxide semiconductors are doped with rare earth metals. Metal oxides and metal doped metal oxides have been used for the degradation of many organic contaminants. In the present thesis we report synthesis and characterization of doped and undoped metal oxide semiconductors and their application in the photocatalytic degradation of some organic contaminants.