Chemical Properties of Ozone

1. Ozone is chemically extremely unstable, slowly decomposing into oxygen in both air and water. The reaction is:

2O3 → 3O2 + 285kJ (1-2)

Because decomposition releases a large amount of heat, it is prone to explosion when its concentration exceeds 25%. However, the ozone concentration in ozonated air is generally unlikely to exceed 10%, and there has never been a case of oxygen explosion in the long history of ozone's use in drinking water treatment.

Ozone concentrations below 1% have a decomposition half-life of approximately 16 hours in air at normal temperature and pressure. The decomposition rate accelerates with increasing temperature; above 100℃, decomposition is very rapid, and at 270℃, it can immediately convert into oxygen. Ozone decomposes much faster in water than in air. In aqueous solutions containing impurities, ozone rapidly reverts to the oxygen it forms. When the ozone concentration in water is 6.25 × 10⁻⁵ mol/L (3 mg/L), its half-life is 5–30 min. However, the decomposition rate is slower in pure water; for example, the half-life in distilled or tap water is approximately 20 min (20℃). In double-distilled water, only 10% of the ozone decomposes after 85 min. Ozone becomes more stable when the water temperature is close to 0℃. The decomposition rate of ozone in water increases with water temperature and pH. Figure 1-3 shows the relationship between water temperature and decomposition rate at pH=7. To improve ozone utilization, slower ozone decomposition is required during water treatment, while faster decomposition of ozone in the treated exhaust gas is required to reduce ozone pollution.

2. Oxidizing Power of Ozone

Ozone has extremely strong oxidizing power; its redox potential is second only to F₂. This characteristic is primarily used in its applications. Ozone's standard electrode potential is higher than that of oxygen, chlorine, chlorine dioxide, and potassium permanganate, except for being lower than that of fluorine. This indicates that ozone is the most powerful oxidizing agent among commonly used oxidants. Furthermore, the product of ozone reactions is oxygen, making ozone a highly efficient oxidant without secondary pollution.

3. Oxidation Reactions of Ozone

a. Oxidation Reactions with Inorganic Compounds

⑴ Reaction of ozone with ferrous iron

⑵ Reaction of ozone with Mn²⁺

⑶ Reaction of ozone with sulfides

⑷ Reaction of ozone with thiocyanate

⑸ Reaction of ozone with cyanide

Overall reaction:

⑹ Reaction of ozone with chlorine

b. Reaction of Ozone with Organic Compounds

The reactions of ozone with organic compounds in aqueous solution are extremely complex. The following are just a few generally accepted reaction equations for reference. (1) Reaction of Ozone with Alkenes: Ozone readily reacts with double-chain alkenes. The reaction mechanism is described below: where G represents OH, OCH3, OCCH3, etc. The final product may be a mixture of monomeric, polymeric, or interleaved ozonides. Ozone decomposes into aldehydes and acids.

(2) Reaction of Ozone with Aromatic Compounds: The reaction between ozone and aromatic compounds is relatively slow. The rate constant increases gradually in the order benzene < naphthalene < phenanthrene < dianaphthalene < anthracene. (3) Reactions to nucleoproteins (amino acids)

(4) Oxidation of organic amines

The oxidation order of ozone in the following mixtures is: alkenes > amines > phenols > polycyclic aromatic hydrocarbons > alcohols > aldehydes > alkanes

c. Toxicity and Corrosivity of Ozone

Ozone is a harmful gas. At a concentration of 6.25 × 10⁻⁶ mol/L (0.3 mg/m³), it causes irritation to the eyes, nose, and throat; at concentrations of (6.25-62.5) × 10⁻⁵ mol/L (3-30 mg/m³), it causes headaches and local paralysis of the respiratory organs; at ozone concentrations of 3.125 × 10⁻⁴ to 1.25 × 10⁻³ mol/L (15-60 mg/m³), it is harmful to the human body. Its toxicity is also related to the duration of exposure. For example, long-term exposure to ozone concentrations below 1.748 × 10⁻⁷ mol/L (4 ppm) can cause permanent heart damage, but exposure to ozone concentrations below 20 ppm for no more than 2 hours poses no permanent harm to humans. Therefore, the permissible ozone concentration is set at 4.46 × 10⁻⁹ mol/L (0.1 ppm) for 8 hours. Because ozone has a very strong odor, people can detect it at a concentration of 4.46 × 10⁻⁹ mol/L (0.1 ppm). Therefore, ozone has been used worldwide for over a hundred years, and to date, there have been no reported cases of death due to ozone poisoning.

Ozone has strong oxidizing properties; except for gold and platinum, ozonated air corrodes almost all metals. Aluminum, zinc, and lead are strongly oxidized upon contact with ozone, but chromium-containing ferroalloys are largely unaffected by ozone corrosion. For this reason, ferrochrome alloys (stainless steel) containing 25% Cr are commonly used in production to manufacture components in ozone generators and dispensing equipment that come into direct contact with ozone. Ozone also has a strong corrosive effect on non-metallic materials; even materials like PVC plastic filter plates, which are quite stable elsewhere, show signs of loosening, cracking, and perforation after a short period of use in ozone dispensing equipment. Ordinary rubber cannot be used as a sealing material in ozone generators and metering equipment; highly corrosion-resistant silicone rubber or acid-resistant rubber must be used.