Explore the unique chemistry of ozone (O3) and its role in water treatment. Unlike other chemicals, ozone is oxygen-based and leaves no lasting residues, making it highly effective yet environmentally friendly. Its reactive yet short-lived nature ensures effectiveness without downstream concerns.
Ozone, or triatomic oxygen (O3), is a colorless or pale blue gas and is a powerful oxidant (E° = 2.07 V). Ozone is an unstable gas that takes anywhere from seconds to minutes to decompose in water, depending on water quality. Ozone is safe for the environment and your crops/livestock, because the end product of its breakdown is oxygen (O2) which is both safe and beneficial. To better understand ozone, let’s start with it’s precursor – oxygen.
Oxygen is a familiar element that has been known for hundreds of years, and is essential to life on earth as we know it. In the atmosphere, the oxygen that we breathe exists in a di-atomic form (O2), whereas the elemental form (O) is much more reactive and scarce in nature. Oxygen is the element with the second-strongest electronegativity (other than fluorine), meaning that electrons are strongly attracted to it. When oxygen interacts with other elements, electrons are pulled or shifted away from other atoms towards the oxygen atom. This property makes oxygen a reactive molecule that combines readily with many of the other elements in nature to form what are called ‘oxides’.
In some of the examples below, addition of oxygen to molecules is defined as oxidation. Over time, the definition of oxidation was broadened to include any reaction where a chemical loses an electron in a reaction (whether or not oxygen is involved). Every atom has a nucleus, and electrons that circle around it in ‘orbitals’, similar to how the planets orbit around the sun. Electrons in the outermost shell are referred to as ‘valence’ electrons. The number of valence electrons held by an atom affects how it interacts with other atoms. In turn, this affects how molecules form and the chemical reactions that occur between them. Oxidation occurs when a valence electron is removed. The removal of electrons through oxidation breaks bonds between atoms and re-arranges molecules in a destructive manner. These oxidative reactions make ozone useful for disinfection or purification of water, since it breaks down pathogens or pollutants.
• Iron that is exposed to O2 and H2O in the air will slowly rust, forming iron oxide.
4 Fe + 3 O2 + 6H2O → 4Fe(OH)3
• Carbon dioxide (CO2) is an oxide of carbon (C). It can be produced many ways, such as cellular respiration – the air you exhale when you breathe.
C6H12O6 + 6O2 → 6CO2 + 6H2O
• Water (H2O) is an oxide of hydrogen (H+). It can be produced in many ways, for example through combination of hydrogen and oxygen gasses
2H2 + O2 → 2H2O
When ozone accepts an electron (e–), oxidizing a target, it produces oxygen and hydroxyls. These by-products do not accumulate in solution and are not harmful to plants or livestock.
O3 + H2O + 2e– → O2 + 2 OH–
As shown below, when chlorine and chlorine dioxide are used for water treatment, they leave chloride residues behind in water. Chlorides are toxic to plants or livestock above certain concentrations and they accumulate in solution.
• When chlorine accepts an electron (e–), oxidizing a target, it produces chlorides.
Cl2 + 2e– → 2Cl–
• When chlorine dioxide accepts an electron (e–), oxidizing a target, it produces chlorites. Chlorites then undergo reactions that produce chlorides.
ClO2 + e– → ClO2–
ClO2– + 2H2O + 4e– → Cl– + 4OH–
Oxidation Potential is measured in volts (V) and measures the tendency for a chemical species to gain electrons. As shown below, ozone is a more powerful oxidant than other disinfectants like peroxide, chlorine or chlorine dioxide.
Substance | Oxidation Potential (V) |
Hydroxyradical (OH) | 2.86 |
Oxygen atom (O) | 2.42 |
Ozone molecule (O3) | 2.07 |
Hydrogen peroxide (H2O2) | 1.78 |
Chlorine (Cl) | 1.36 |
Oxygen molecule (O2) | 1.23 |
Chlorine dioxide (ClO2) | 0.95 |
Ozone is produced by splitting the two atoms in atmospheric oxygen (O2) apart to form singlet oxygen (O). The singlet oxygen then reacts with remaining molecular oxygen (O2) to form ozone (O3). This process requires either UV Light (UV bulbs, sunlight in nature) or corona discharge (high voltages, thunderstorms in nature). Corona discharge units are much superior, providing higher ozone purity and output as well as being more cost-effective and durable. UV generators are more useful for light-duty applications whereas corona discharge generators are the preferred choice for moderate-to heavy-duty applications.
The overall chemical reaction that occurs from adding ozone to water is O3 ↔ O + O2, however numerous intermediary reactions also occur. By using ozone, other short-lived oxygen species such as hydroxyls (OH–) form naturally as chemical intermediates. These intermediates are themselves oxidants that contribute to water purification. Addition of ozone treats water directly, but also indirectly through its degradation products.
Ozone oxidizes organic (carbon-based) contaminants selectively – mainly with double bonds, activated aromatic systems and non-protonated amines. In contrast, hydroxyls are unselective and react in a nearly diffusion-controlled manner. Both ozone and hydroxyls contribute significantly to water treatment, the ozone by disinfecting and the hydroxyls by purifying.
