Organic compounds oxidation may be effected by means of Fenton reagent assisted, or not assisted, by photolysis [57,60-62]. The Fenton reaction consists of a reaction of hydrogen peroxide with ferrous sulfate at a low pH [57,61]. As a result, free hydroxyl radicals, characterized by high reactivity, are produced according to the following reaction :
Then, in chain reactions (4) and (5), organic compounds, marked as HRH, are oxidized, resulting in organic radicals RH production. Subsequently, oxygen is required. Unless the recombination
of organic radicals takes place [reaction (6)], the decomposition of organic compounds cannot proceed:
With the Fenton reagent, very different organic compounds present in industrial wastewater [63,64], rain water , as well as in landfill leachate are oxidized [20,60,66]. Therefore, the reaction is completed at different doses of ferrous sulfate and hydrogen peroxide. The reciprocal relation between ferrous sulfate and hydrogen peroxide is established experimentally. Although the molar ratio [H2O2]: [Fe2+] in Fenton reagent is applied usually within range from 5:1 to 10:1 , it is often accepted even at the level 1:1. However, a dose of hydrogen peroxide is varied with oxygen demand for oxidation of particular amounts of organic compounds, that is, with particular values of COD . Normally, a low pH of 3-3.5 is recommended because a higher pH would foster coagulation domination of the oxidation process [20,63].
When landfill leachates are treated with a Fenton reagent, 60-80% of COD removal can be achieved [20,63]. The process efficiency depends, among other facts, on molecular weight of organic compounds subjected to oxidation. The higher the molecular weight reached by means of the Fenton reaction, the better it would be. Yoon and colleagues  showed that using a Fenton reagent removes 72-89% of organic compounds having molecular weight over 500 and no more than 43% of organic compounds characterized by molecular weight below 500.
The combination of a typical Fenton reaction with UV irradiation is also called the PhotoFenton reaction. Additional UV irradiation at different wavelengths increases the organic compounds' oxidation rate. UV light of wavelengths lower than 300 nm generates more free hydroxyl radicals because of extra photolysis of hydrogen peroxide; it is also responsible for direct photolysis of organic compounds. However, UV light of wavelengths above 300 nm allows ferrous ions to regenerate [57,61]. The mechanism of the Photo-Fenton reaction is illustrated in Fig. 12.
With the Photo-Fenton reagent, it is possible to increase the rate of organic compound oxidation tenfold in comparison with the traditional Fenton reagent. However, favorable results can be achieved if the content of carbonates in treated leachate is lowered. This is necessary since carbonates react with hydroxyl radicals making them inactive, according to reactions (7) and (8):
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