Ozone (O3) is recognized as a powerful oxidant and an excellent sanitizer. Depending on reaction conditions, ozone delivers oxidizing power through the generation of hydroxyl free radicals in the decomposition of ozone.
Ozone is more stable in acidic conditions, at pH levels of less than six. As the pH increases above seven, ozone becomes less stable and generates hydroxyl free radicals. The oxidation-reduction potential of the hydroxyl free radical (2.76 volts) is 33% stronger than the decomposition of ozone in acidic conditions (2.076 V).
As the applications for ozone have advanced, so have the challenges. Over the years, ozone has been applied to increasingly difficult tasks. Ozone is becoming an important player in the growing field of advanced oxidation technology (AOT), also called advanced oxidation processes.
What is AOT?
Simply put, AOT is the application of ozone and other chemicals or processes in combination to yield higher concentrations of powerful hydroxyl ions through catalyzed, coupled reactions. Some organic compounds that are resistant to oxidation are readily oxidized by hydroxyl free radicals, including halogenated and volatile organic compounds such as benzene and tetrachloroehtylene. Catalyzed AOT reactions tend to increase versatility because they produce hydroxyl-free radicals rapidly over a wide pH range, providing greater oxidative punch under a broad range of conditions.
A variety of means can be used to catalyze the generation of hydroxyl-free radicals in AOT. Some of the more common methods include the following:
- Ozone + ultraviolet (UV) light;
- Peroxone: Ozone + hydrogen peroxide (H2O2);
- H2O2 + UV;
- Ozone + H2O2 + UV; and
- Catalysis with titanium dioxide (TiO2).
These and other methods are reported on in the Journal of Advanced Oxidation Technologies. The role of ozone in AOT is well-covered in the official journal of the International Ozone Association (IOA), Ozone Science and Engineering. In addition, a wide variety of publications are available from the IOA, including the proceedings of all major world and regional congresses, as well as specific conferences on AOT.
A Myriad of Applications
Once one starts looking for AOT, it seems to be everywhere, addressing an extensive array of challenging applications. While AOT could be applied to virtually any oxidation application, it is best reserved for the most challenging applications because of the additional cost and com-plexity of implementation and operation.
IOA-IUVA World Congress
As stated in the post-congress report in the October issue of Water Quality Products, an interesting intersection occurred between the topics of advanced oxidation technologies and emerging contaminants at the World Congress on Ozone and Ultraviolet Technologies held this past August in Los Angeles.
Prescription medicines, over-the-counter (OTC) drugs, personal care products, fire retardants and other sources contribute a growing body of complex compounds to wastewater. These chemicals ultimately return to the raw water supply and can cause unintended side effects in humans and wildlife, such as disruption of endocrine function.
Many emerging contaminants are not easily removed or destroyed by conventional treatment regimens. Several papers presented in the emerging contaminates sessions at the congress showed recent efforts to denature these compounds with ozone, UV and AOT processes. Many of the results in the presentations showed promise. Some presenters concluded that more work is needed in this area of research to better understand the kinetics and byproducts of these reactions.
Several sessions were devoted to advances in AOT processes and applications. Papers presented in these sessions included endocrine disputing compounds; drinking water treatment; taste and odor control in drinking water; organophosphate plasticizers and flame retardants; pesticides; and pharmaceuticals and OTC medicines. Advanced oxidation technologies were also a hot topic in the session on groundwater and soil remediation.
Pros and Cons of AOT
The benefits and challenges of AOT are expressed in a paper presented at the World Congress on Ozone and Ultraviolet Technologies by Professor James Malley Jr.
The pros of AOT include:
- Very rapid processes requiring relatively little time and small footprints to treat high flow rates;
- Potential for complete detoxification of a wide range of emerging organic pollutants and pathogens (disinfection);
- No generation of solid residuals; and
- The relatively nonselective oxidative nature of AOT makes it an attractive tool for simultaneously addressing multiple treatment objectives, such as control of tastes and odors, disinfection and oxida- tion of emerging organic compounds.
The cons of AOT include:
- Limited track record in water reuse and drinking water applications;
- Significant capital and operation and maintenance costs (particularly power);
- Radical chemical reactions can be difficult to predict due to site-specific water quality conditions that can result in free radical initiators or scavengers;
- Site-specific questions about chemical byproducts and biologically stable water after the AOT; and
- Removal of hydrogen peroxide residuals after the AOT needs to be addressed in the integrated design of the treatment facilities.
The Bottom Line
The total costs of AOT processes can be high, and additional research and development is needed to optimize the kinetics and efficiencies of these technologies. Even so, we undoubtedly will be hearing more about advanced oxidation technologies and processes in the future. Given increased abilities to detect environmental contaminants and the global struggle to expand and improve worldwide supplies of fresh water, the accelerated sanitizing power of AOT will be sorely needed.