Review of Advanced Oxidation Process

Advance oxidation process has come out as the most effective treatment technology for complete removal of organic contaminants from waste water. It is a broad-spectrum technique, since it is non-selective towards organic compounds. In this process hydroxyl radical having very high oxidation potential of 2.7eV are produced in situ. It offers a powerful water treatment solution for the reduction and/or removal of residual organic compounds as measured by COD, BOD or TOC. Generally, treatment efficiency depends upon type of effluent, its physical and chemical properties, on the basis of which type of AOP system to be used is decided. As Fenton process is pH sensitive, it provides a very limited range of working about 2-4, to overcome this draw back some complexes of Fe with the ligands like EDTA, EDDS, oxalates, tartrates are reported which gives a wide range of pH. Nano particle technology is another means for the quick degradation of refractory organic wastes in much lesser time. This paper is a review of all such technologies used in past, their drawbacks and new techniques overtaking them.

Ever growing population has cost ever growing industrialization which causes water pollution, resulting in undesirable impact on our environment. To meet the water requirement the concept of reduce, reuse, and recycle has taken firm roots in the concept of water treatment and management. Over a decade social organization, NGOs, government bodies, have laid various do’s and don’ts to protect environment and sustainable development plays a key role. The fulfillment of severe quality standards is claimed for toxic chemicals which effect biosphere adversely and causes landfills. It has been generally observed that pollutants not agreeable to biological treatments may be characterized by high chemical stability and/or by strong difficulty to be completely mineralized. Various separation techniques consist of conventional phase separation techniques (adsorption processes, stripping techniques) and methods which destroy the contaminants (chemical oxidation/ reduction). Adsorption processes cause more of landfills, whereas chemical oxidation /reduction aims at the mineralization of the contaminants to carbon dioxide, water and inorganics or, at least, at their transformation into harmless products. Hence a technique which produces complete degradation of non-biodegradable waste without any phase change is a need of hour.

In 1987, Glaze et al. defined AOPs as “near ambient temperature and pressure water treatment processes which involve the generation of hydroxyl radicals in sufficient quantity to effect water purification”. Previously it was used for portable water treatment only, later its application in industrial water waste was studied and it proved to be an excellent technique for the degradation of non-biodegradable organic compounds. In AOP treatment, hydroxyl radicals (OH•) or sulfate radicals (SO42-) are generated in sufficient quantity to oxidize refractory organic matters into carbon dioxide, water and biodegradable inorganic wastes. Hydroxyl radical differs from common oxidants such as chlorine and ozone that have a dual role of decontamination and disinfection, AOPs are applied primarily for destruction of organic or inorganic contaminants in water and wastewater. AOPs are not used as disinfectant as the hydroxy radicals are very short lived, but when subjected to industrial waste water they act as strong oxidizing agent and are expected to destruct the pollute to less toxic compound. The hydroxyl radical (.OH) is a powerful, non-selective chemical oxidant (Table 1), which acts very rapidly with most organic compounds. Once generated, the hydroxyl radicals attack virtually all organic compounds unselectively. Depending upon the nature of the organic species, two types of initial attack are possible: the hydroxyl radical can abstract a hydrogen atom from water, as with alkanes or alcohols, or it can add itself to the contaminant, as in the case of olefins or aromatic compounds. The reaction rate constants of molecular ozone with different organic compounds are also given in Table 2. These reaction rate constants vary in quite a wide range from 0.01 to 104 M–1 s–1. Non-photochemical methods involve generation of hydroxy radicals without using light energy. Out of four methods generally used for water treatment, two of these involve the reaction of ozone while one uses Fe2+ /Fe3+ions as the catalyst. These methods are ozonation, carried out at elevated values of pH (>8.5), combining ozone with hydrogen peroxide, ozone + catalyst, and the Fenton system.

Ozone (O3) itself is a strong oxidant with an oxidation potential of 2.07 V vs. SCE. Ozone can oxidize compound by either or both modes in aqueous solutions: direct oxidation by molecular ozone (O3), or indirect oxidation by hydroxyl free radicals produced during the decomposition of ozone. However, oxidation with molecular ozone is reaction specific. It preferentially reacts with the neutral form of organic compounds, rather than ionized and dissociated forms. Ozone shows high decomposition rate in water, it’s half-life can be less than 1 min at the raised pH 10. The rate of attack by .OH radical is 106 to 109 times faster than molecular ozone. Ozone and hydroxide ions reacts to form super-oxide anion radical O2.-and hydroperoxyl radical HO2.

Another ozone molecule reacts with super-oxide anion radical to form ozonide anion radical O3, which decomposes immediately giving .OH radical, which causes the oxidation of organic species present in waste water. Summarizing, three ozone molecules produce two .OH radicals:3O3 + OH– + H+®2.OH + 4O2 (7). Certain species like Bicarbonate and carbonate play as scavengers of .OH radicals in natural systems. Tert-Butyl alcohol also suppresses the chain reaction, if present. The major operating cost for the ozone oxidation process is the cost of electricity for ozone generation. The energy requirement for ozone synthesis using air as a feed gas ranges from 22 to 33 kWh/kg O3, including air handling and ozone contacting with water. The energy requirement for ozone production from pure oxygen is in the range from 12 to 18 kWh/kg O3, to which the cost of oxygen should be added. Fishy smell during the production of ozone is unbearable.