During oxidation of organic compounds by means of ozone at high pH, some chain reactions occur that result in hydroxyl radicals, among others. Free radicals are responsible for oxidation of organic substances according to reactions (4) and (5), but the decomposition of organic matter can also be caused by ozone alone . Ozone oxidation requires a high pH; carbonate ions will therefore disturb the process since they are scavangers of free radicals. This was described in the discussion of Fenton reagent. For landfill leachate, characterized by COD of 900-2100 mg/L, ozone doses are included in the range 1.8-3.5 g O3/g COD. They allow the removal of 25-90% of COD and AOX .
Ozone oxidation enhanced by photolysis is also a very efficient method that produces favorable results. Chemical oxygen demand and AOX reduction may be achieved at the level of 80% with ozone dose of 1.6 g O3/g COD. This method offers the technical possibility for COD values in biologically treated leachate to decrease to lower than 60 mg/L and for AOX to be less than 70 pg/L . It is also possible to remove 100% of phenols, 23-96% of hydrocarbons and biphenyl, as well as 74% of dioxins and furans .
Combination of ozone and hydrogen peroxide also results in free hydroxyl radicals according to reactions (9) to (12) : H2O2^HO2-+H+
Summarily, it can be written as: H2O2+2O3^2OH+3O2
Free radicals can also be produced by means of hydrogen peroxide UV irradiation alone, as described in reaction (14):
as well as by ozone irradiation in a water environment according to reaction (15) : Oj + H2Q % Oi + HiOi 20ET" , s
Ozone irradiated with UV light decomposes to oxygen and hydrogen peroxide, which further decomposes to free radicals. Moreover, hydrogen peroxide reacts with ozone, according to summary reaction (13), producing another extra free radical.
Application of hydrogen peroxide oxidation with photolysis for biologically pretreated leachate allows removal of 60-95% of COD as well as 85-90% of AOX. Because oxidation results from free radical activity, an acidic pH of 2-4 is required. The decrease of pH results in low concentrations of carbonate ions HCO3- and CO32-, which are scavengers of free radicals.
The efficiency of hydrogen peroxide oxidation enhanced by photolysis depends on the energy of emitted radiation. However, energy consumption depends on the kind of the lamps applied. With low-pressure lamps, energy consumption varies from 100 to 200 kWh/kg COD, and for high-pressure lamps, the amounts range from 400 to 700 kWh/kg CODremoved. Because only high-pressure lamps can be used on a technical scale for landfill leachate treatment, the high consumption of energy is a drawback of this application method [67,72].
Chemical methods are applied for landfill leachate treatment after biological pretreatment, which allows for removal of nitrogen and biodegradable organic compounds. Thus, the chemical treatment methods are focused on removal of refractory organic compounds as a main goal . However, the advanced oxidation processes also have disadvantages, which are presented in Table 6. Exploitation difficulties can be classified as follows:
• The formation of scale on UV lamps as a result of precipitation. This weakens UV light penetration into the place of reaction.
• Limitation of the reaction rate because of free hydroxyl radicals inactivation as a result of reaction with carbonate ions.
These drawbacks are eliminated by upgrading the reactors, that is, separating the source of UV light from the reaction environment  or applying extra beds made of catalyst, accelerating mineralization of adsorbed organic compounds by radicals produced on the catalyst surface .
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