Understanding the Chemical Reactions of Ozone

With water being a crucial resource for our health and environment, water treatment using ozone systems offers a well-documented and proven way of solving the increasing need for water purification.

Oxidation reactions initiated by ozone in water are generally rather complex. In the following, this complexity will be elaborated upon, to delve into the functions of ozone as a promising water treatment method.

The principles of ozone reactions in water and with solutes

In water, only part of the ozone reacts directly as molecular ozone with dissolved solutes. Another part decomposes before solute reaction, catalyzed by hydroxide ions (OH^-) or by superoxide anion radicals O_2^{\,\cdot-} liberated from reactions of molecular ozone with certain organic and inorganic compounds. This decomposition of molecular ozone results in the formation of the highly reactive secondary oxidant – the OH-radical. Hence, any degradation of a solute during ozonation will be a result of the solutes reaction with both molecular ozone and OH-radical.

The reactions of molecular ozone and OH-radicals with solutes follow second-order reaction kinetics, corresponding to the following equations:

S + O_3 \rightarrow S_{ox} \quad (1)

-\frac{dS}{dt} = k_{s,o_3} \times [S] \times [O_3] \quad (2)

S + HO^\cdot \rightarrow S_{ox} \quad (3)

-\frac{dS}{dt} = k_{s,HO^\cdot} \times [S] \times [HO^\cdot] \quad (4)

Where, k_{s,o_3} and k_{s,HO^\cdot} are the elementary second-order rate constants for the reaction of a solute with molecular ozone and the OH-radical respectively. The overall elimination of the solute during ozonation can therefore be described by the following equation:

-\frac{dS}{dt} = k_{s,o_3} \times [S] \times [O_3] + k_{s,HO^\cdot} \times [S] \times [HO^\cdot] \quad (5)

The degree of solute oxidation by either of the two oxidant species will depend on the water matrix composition.

The temperature and pH conditions as well as the concentration of background dissolved organic matter and inorganic ions will influence the exposure of a solute to molecular ozone and OH-radicals through competing reactions. The degree of competition from the water matrix depends on the numerical values of the elementary second-order rate constant for the reaction of background dissolved organic matter and inorganic ions with molecular ozone and the OH-radical as well as on their concentration.

Furthermore, for dissociating solutes, the numerical values of the elementary second-order rate constant will vary with pH as the non-protonated solutes (S-) often react many orders of magnitude faster than their corresponding protonated solute SH. Consequently, the acid dissociation constant (pKa) of dissociating solutes among target solute compounds is also an important parameter when ozonation processes are to be assessed.

Ozone research in the R&D department

The R&D department of ULTRAAQUA has worked extensively to understand the complexity of ozone water treatment for numerous years. Today, ozone technology is often included in new projects dealing with water treatment, both on a research basis and for full-scale solutions. If properly dealt with, ozone in water treatment poses many advantages, which the R&D department has first-hand insights and data on.

If you are interested in hearing more about how we can help with ozone water treatment, you are always welcome to reach out to our sales engineers at sales@ultraaqua.com – or use the contact formula below.

An overview of our ozone product line can be found by clicking here.