Ozonated Water: Chemistry, Stability, and Health Effects

Ozonated water is gaining attention for its applications in disinfection, food safety, and medical treatments. It is created by dissolving ozone gas (O₃) into water, producing a powerful oxidizing solution that breaks down contaminants and pathogens. Its effectiveness depends on concentration, exposure time, and the presence of organic or inorganic compounds.

Understanding ozonated water requires examining its chemical formation, stability, and interactions with other substances. Measuring its concentration accurately is crucial for safe and effective use.

Chemistry Of Ozone Formation In Water

Ozone (O₃) is introduced into water through ozone dissolution, where gaseous ozone is generated and transferred into the liquid phase. The most common production method, corona discharge, uses high-voltage electricity to split oxygen molecules (O₂) into individual atoms, which then recombine to form ozone. Once generated, ozone is injected into water using diffusers, venturi injectors, or contact chambers to maximize dissolution and oxidative potential.

Upon entering water, ozone undergoes reactions that determine its solubility and reactivity. The Henry’s Law constant for ozone at 20°C is approximately 0.021 mol/(L·atm), indicating moderate solubility. Temperature, pressure, and water composition influence how much ozone remains dissolved. Higher temperatures reduce solubility, while increased pressure enhances retention. Organic matter, dissolved ions, and pH variations further affect ozone’s persistence before decomposing into oxygen.

Dissolved ozone acts as a strong oxidant, reacting with water to form hydroxyl radicals (•OH) under specific conditions. This occurs primarily in alkaline environments, where ozone undergoes self-decomposition via a chain reaction involving hydroxide ions (OH⁻). The reaction initiates with ozone interacting with OH⁻ to form the superoxide anion (O₂⁻), leading to hydrogen peroxide (H₂O₂) and hydroxyl radicals. These radicals exhibit even greater oxidative strength than ozone, breaking down organic contaminants, microbial cell walls, and inorganic pollutants.

Chemical Stability Factors

Ozone persistence in water is influenced by variables that govern its degradation rate and oxidative capacity. Temperature plays a key role, as higher temperatures accelerate decomposition by increasing molecular motion, breaking ozone down into oxygen. At 20°C, the half-life of dissolved ozone is about 20 minutes, decreasing to 15 minutes at 30°C. This instability limits storage, necessitating immediate use in many applications. Lower temperatures enhance stability, with near-freezing conditions extending ozone’s half-life, making chilled ozonated water preferable for prolonged antimicrobial effects.

pH also dictates ozone breakdown kinetics. In acidic environments (pH < 4), ozone remains relatively stable due to minimal hydroxide ion (OH⁻) availability, slowing radical-driven degradation. At neutral pH, decomposition occurs at a moderate rate, while in alkaline conditions (pH > 8), ozone rapidly converts into hydroxyl radicals (•OH), which are highly reactive but short-lived. This transformation is relevant in water treatment, where enhanced oxidation is desired, though it reduces molecular ozone presence. Regulatory guidelines, such as those from the U.S. Environmental Protection Agency (EPA), emphasize pH control to optimize disinfection while minimizing unwanted decomposition.

Dissolved organic and inorganic substances further influence ozone stability. Natural organic matter (NOM), such as humic and fulvic acids, react with ozone, leading to partial oxidation and smaller, more biodegradable molecules. Inorganic species like iron (Fe²⁺), manganese (Mn²⁺), and bromide (Br⁻) catalyze ozone decay through redox reactions, often producing secondary oxidants like hypobromous acid (HOBr). This is particularly relevant in drinking water treatment, where controlling byproduct formation is necessary to meet safety standards set by organizations like the World Health Organization (WHO).

Hydrodynamic conditions also impact ozone retention. High turbulence, such as in aerated water systems or flowing pipelines, increases surface area exposure, leading to rapid volatilization. Enclosed systems with minimal agitation retain ozone longer, making them suitable for sustained disinfection. This principle is applied in medical and dental settings, where ozonated water is stored in sealed reservoirs to maintain potency. Additionally, higher pressures favor ozone retention, a factor leveraged in industrial-scale ozonation processes to maximize efficiency.

Typical Byproducts From Oxidation

When ozone reacts with organic and inorganic substances in water, it generates oxidation byproducts that affect water quality and safety. One major byproduct is aldehydes, such as formaldehyde and acetaldehyde, which form when ozone partially oxidizes organic matter. These compounds are of interest in drinking water treatment because they can promote microbial regrowth in distribution systems. Ozonated water containing aldehydes may require additional filtration, such as granular activated carbon (GAC), to prevent bacterial proliferation.

Bromate (BrO₃⁻) is another significant byproduct, especially in water sources containing bromide (Br⁻). Ozone oxidizes bromide through a multi-step reaction producing hypobromous acid (HOBr) and, under certain conditions, bromate. The U.S. Environmental Protection Agency (EPA) has set a maximum contaminant level (MCL) of 10 µg/L for bromate in drinking water due to its classification as a potential carcinogen. Treatment facilities mitigate bromate formation by adjusting pH, limiting ozone dosage, or incorporating ammonia to promote the formation of less harmful brominated species.

Carboxylic acids, including oxalic and formic acids, emerge as secondary oxidation products when ozone breaks down natural organic matter (NOM). Unlike aldehydes, these acids are more stable and can alter water acidity. Their presence is relevant in industrial applications where ozonated water is used for cleaning or food processing, as residual acidity may require neutralization. Studies on ozonated beverages indicate that elevated carboxylic acid concentrations can alter taste profiles, necessitating careful control over ozonation parameters.

Analytical Methods For Concentration Measurement

Accurately determining ozone concentration in water is essential for ensuring efficacy and safety. Since ozone decomposes quickly, measurement techniques must be rapid and precise. Spectrophotometry is widely used, relying on ozone’s strong absorption of ultraviolet (UV) light at 254 nm. This technique measures the reduction in UV transmission as ozone interacts with the light source, providing a direct assessment of concentration. Instruments like the Hach DR6000 UV-Vis spectrophotometer are commonly used in laboratories and water treatment facilities for real-time measurements.

Electrochemical sensors offer another reliable approach, particularly for continuous monitoring in industrial and medical settings. These sensors detect the oxidation potential of ozone as it interacts with an electrode surface, producing an electrical signal proportional to concentration. Amperometric and potentiometric sensors are frequently used in automated systems where real-time adjustments to ozone dosing are necessary. Companies like Rosemount and Yokogawa manufacture advanced ozone sensors that integrate into water treatment systems, ensuring precise control over disinfection processes.