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Chemical Oxidation

Chemical oxidation and Advanced Oxidation Processes(AOPs)

Chemical oxidation includes the technologies based on:

• ozonation (O₃)
• UV light with oxidants (e.g. hydrogen peroxide)
• Fenton mechanisms
• on the different combinations of these treatment technologies
• catalytical oxidation

The efficiency of the technology and the consumption of oxidants are influenced by  various factors, including the properties and concentrations of the compounds of interest, as well as the general characteristics of the matrix treated. The matrix also impacts on the type and distribution of intermediates and by-products. In particular pH, turbidity, alkalinity, temperature and the nature and amount of the organic and inorganic matter are important properties.

Chemical oxidation technologies have numerous advantages, e.g.

• disinfection (bacterias, molds, viruses, biofilm)
• enhancement of processes
• oxidation of toxic and refractory compounds
• minimizing the sludge production
• the improvement of biological processes
• sum effects (e.g. colour, odour, microbes, toxicity, organics)

Examples of case studies done in the laboratory:

Integrated technologies for the treatment of contaminated soils (PAHs, CPs, Syanides), ground waters and industrial landfill leachates Ozone treatment of circulation waters and effluents in the pulp and paper industry – removal of resin acids, EDTA and microorganisms Impact of ozonation on the colour and COD of pulp and paper mill waters Inactivation of microbes and fungus in a Finnish fish farm (O3, UV ja H2O2) The oxidation of malodorous odours from compost (prestudy) The oxidation of quicksilver wastes (prestudy)

Ozonation

Ozone (O₃) has been used as a chemical reagent, an industrial chemical and as an oxidant for water treat­ment over a century.  Ozone is a powerful oxidant and disinfectant, with the highest thermodynamic oxidation potential of the common oxidants. In principle, ozone should be able to oxidize some of the organic substances to their highest stable oxidation states and organic com­pounds to carbon dioxide and water. Hence, ozonation rarely results the mineralization to CO₂, salts and water under the conditions typically present in practical processes.  Recently the treatment of industrial effluents and landfill leachates including refractory and ”hard” COD compounds is expected to be one of the most promising use of ozone.

Ozone reacts both directly by the ozone molecule with the specific compounds present in water or indirectly primarily through hydroxyl radicals generated by ozone decomposition. The reaction mechanism depends on the pH of the solution and on the level of active chain initiators, such as hydrogen peroxide and ultraviolet radiation. Even at near neutral pH, the reactions may involve radical character because peroxide is often a by-product of the ozonolysis process. Very short-lived, extremely high oxidizing power hydroxyl radicals formed during ozonation, will react unselectively with almost all substances.

Advanced Oxidation processes

The processes that are based on the utilization of secondary oxidants, such as hydroxyl radicals are called advanced oxidation processes (AOPs). AOPs have been suggested as an alternative, particularly for the treatment of landfill leachates and biorefractory organic pollutants, such as aromatics.  By AOPs it is possible to oxidize a larger spectrum of compounds, by the highly reactive and unselective radical pathway than by direct ozonation. The generation hydroxyl radicals (^.OH) can be considerably intensified via various combination of oxidants, radiation and catalyst. Especially in water remediation, a number of OH-radical generating systems are currently in use, or under study eg. O₃/H₂O₂, UV/H₂O₂, Fe²⁺/ H₂O₂, Fe²⁺/H₂O₂ + hv, UV/O₃, UV/TiO₂ and ionizing radiation. The efficiency by which hydroxyl radicals are formed during reactions between ozone and H₂O₂ depend on the pH and on the amount of scavengers.

The reaction rate of a compound in hydroxyl radical mediated oxidation is usually several orders of magnitude higher than the reaction rate with molecular ozone under same conditions. The reaction rate constants between hydroxyl radicals and organic species are in the range of 10⁸ – 10¹⁰ M^-1s^-1 M. Reaction with alkenes and aromatics is even faster, in the range of 10⁹ – 10¹⁰ M^-1s^-1

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