Ozonation at Elevated pH
As the pH rises, the decomposition rate of ozone in water increases. For example, at pH 10, the half-life of ozone in water can be less than 1 min. Oxidation of organic species may occur due to a combination of reactions with molecular ozone and reactions with OH radicals.
The reaction between hydroxide ions and ozone leads to the formation of super-oxide anion radical and hydroperoxyl radical. By the reaction between ozone and the super-oxide anion radical the ozonide anion radical is formed, which decomposes immediately giving .OH radical. Summarizing, three ozone molecules produce two OH radicals:
3 O3 + OH- + H+ –> 2 .OH + 4O2
Bicarbonate and carbonate play an important role as scavengers of OH radicals in natural systems. The products of a reaction between OH radical and carbonate or bicarbonate ions are passive carbonate or bicarbonate radicals, which do not interact further with ozone or organic compounds. tert-Butyl alcohol also suppresses the chain reaction, if present.
The rate of the attack by OH radicals is typically 106 to 109 times faster than the corresponding reaction rate for molecular ozone. The major operating cost for the ozone oxidation process is the cost of electricity for ozone generation.
The energy requirement for the synthesis of ozone using air as a feed gas, ranges from 22 to33 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 to18 kWh/kg O3, to which the cost of oxygen should be added.