Ozone Reactions

Ozone is a strong oxidant that can react directly as molecular ozone in a slow and selective fashion or via activation to a more active form known as the hydroxyl radical that reacts faster and less selectively. The formation of the hydroxyl radical is important in the decomposition of ozone in aqueous media. These reactions are also important in advanced oxidation processes.

Molecular ozone will act as a dipole, as an electrophilic agent, and as a nucleophilic agent.

As a result of its dipolar structure, the ozone molecule may lead to 1-3 dipolar cyclo addition on unsaturated bonds, with the formation of primary ozonide. In a protonic solvent such as water, this primary ozonide decomposes into a carbonyl compound (aldehyde or ketone) and to a hydroxy-hydroperoxide stage that, in turn, decomposes into a carbonyl compound and hydrogen peroxide.

Electrophilic reaction. The electrophilic reaction is restricted to molecular sites with a strong electronic density and, in particular, certain aromatic compounds. Aromatics substituted with electron donor groups (OH, NH2, and similar compounds) show high electronic densities on carbons located in the ortho and para positions, and so are highly reactive with ozone at these positions. On the contrary, the aromatics substituted with electron-withdrawing groups (- COOH, -NO2) are weakly ozone reactive. In this case, the initial attack of the ozone molecule takes place mainly on the least deactivated meta position. The result of this reactivity is that the aromatic compounds bearing the electron donor groups (for example, phenol and aniline) react quickly with the ozone. This initial attack of the ozone molecule leads first to the formation of ortho- and para-dydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions.

Nucleophilic reaction. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron withdrawing groups.

In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups.

Example groups of molecules that ozone is reactive with are:

C=C, C≡C, SH-, S2-, NH2-, NH-2, OH- (phenolic), CHO- Fe+2, Mn+2, Cr+3, SO3-2, NO2-, I- (ozone analytical).

Some of the principal reactions of ozone are:

Fe+2 + O3 + H2O = Fe+3 + O2 + 2OH-
Fe+3 + 3H2O = Fe(OH)3 + 3H+
Mn+2 + O3 + H2O = Mn+4 + OH- + O2
Mn+4 + 4OH- = Mn(OH)4 = MnO2 + 2H2O
CN- + O3 = CNO- + O2
2(CNO-) + 2H2O = 2CO2 + N2 + 4H+
3S-2 + 4O3 = 3 SO4-2
NO-2 + O2 = NO3- + O2
Ozone is a strong oxidizing agent as shown in the table below:

Reaction Potential (V)
O3 + 2H+ + 2e- = O2 + H2O 2.07
H2O2 + 2H+ + 2e- = 2H2O 1.77
MnO4- + 4H+ + 3e- = MnO2 + 2H2O 1.69
O2 + 4H+ + 4e- = 2H2O 1.23
HOCl + H+ + e- = 1/2Cl2 + H2O 1.63
HClO2 + 2H+ + 2e- = HClO + H2O 1.64
Cr2O7 + 14 H+ +6e- = 2Cr3+ + 7H2O 1.33

Oxidation of organic materials by ozone is selective and incomplete at the concentrations and pH values of aqueous ozonation. Unsaturated and aromatic compounds are oxidized and split at the double bonds, producing carboxylic acids and ketones as products. Because of the high reactivity of ozone, oxidation of organic matter in the aqueous environment, whether it be potable water or wastewater, will consume ozone in varying amounts. Therefore, one of the most significant parameters for evaluating ozone is the determination of the immediate ozone demand. Oxidation of the (organic) material is usually incomplete. It is estimated that the reduction in TOC may be only 10-20 percent although decreases in COD and BOD are generally greater, ranging up to 50 percent COD reduction.

There are instances where BOD has appeared to increase, resulting from conversion to more readily oxidized compounds. Ozone causes the formation of assimilable organic carbon (AOC). AOC are compounds that are more readily digested by bacteria.

Ozone is an effective bleaching agent against organic compounds that contribute to color in wastewater and potable water. The ability to attack these compounds, including humates and fulvates, makes ozone a good wastewater polishing agent. The ability of ozone to destroy taste forming phenolic compounds is an important contribution to the field of potable water treatment. Ozone is capable of destroying other taste forming such as Geosmin and MIB which are formed by algae.