Advanced oxidation processes via catalysis

Abstract

Advanced oxidation processes (AOPs) are defined as "near ambient temperature and pressure water treatment processes which involve the generation of hydroxyl radicals in sufficient quantity to effective water purification". Hydroxyl radical is a powerful, non selective oxidant, which reacts rapidly with most organic compounds as halogenated hydrocarbons, aromatic compounds, phenol-compounds, detergents, pesticides, etc... The most important AOPs via catalysis are: i) Ozonation coupled with catalysts - O3/H2O2/UV (Λ < 310 nm) utilizing the oxidizing power of ozone by addition of H2O2. The former acts as homogeneous catalyst causing rapid decomposition of ozone with high output of OH. radicals. In addition ferrous/ferric ions might be added to the system. ii) Hydrogen peroxide based catalytic processes - Fenton's reagent (H2O2/Fe2+) and Fenton's like reagents. Fenton reaction is based on an electron transfer between H2O2 and a metal (iron, copper, etc.) acting as a homogeneous catalyst. Fixation of metal ions on various supports is one of the current topics in the field. iii) Photo-Fenton's reagent (H2O2/Fe2+/UV or Vis, Λ < 580 nm) - improves the efficiency of dark Fenton process by means of interaction with radiation in UV or visible range. iv) Photocatalysis - applies inexpensive semiconductors (TiO2, ZnO, etc.) to mineralize even very stable organic compounds. Photocatalytic reactions occur when charges separation is induced in a large band gap semiconductor (TiO2, hν ≥ Eg = 3.2 eV). The absorption of light by the photocatalyst greater than its band gap energy (for TiO2 Λ < 390 nm) excites an electron from the valence band of the irradiated particle to its conduction band, producing a positively charged hole, h+ in the valence band and an electron, e- in the conduction band. The hole, h+ in the valence band may react with water absorbed on the surface to form hydroxyl radicals, and on the other hand, the conduction band electron, e-, can reduce absorbed oxygen to form peroxide radical anions, that can further disproportionate to form OH. radical through various pathways. The OH• radicals react with contaminants via oxidative processes, and the band electron may also react directly with the contaminants via reductive processes. There are several technologies based on AOPs for the near ambient degradation of soluble organic compounds from waste waters and contaminated soils. These technologies are at different level of development and commercialization. © 2011 by Nova Science Publishers, Inc. All rights reserved.