(56f) Green Oxidation Processes for Industrial Wastewater Treatments

In industrial wastewater treatment processes, Fenton’s reaction is an effective method for the treatment of dye wastewater, phenols and other recalcitrant pollutants which cannot be removed by using biological processes. In the reaction, H₂O₂ is injected into the wastewater in the presence of ferrous ions to produce powerful hydroxyl free radicals (Ë‘OH). However, Fenton’s reaction only happens in a very narrow pH range (2.8-3.2). At neutral pH, ferrous ions precipitate and become sludge. Alternatively, H₂O₂ can be combined with UV and ozone to produce hydroxyl free radicals, but the treatment cost is very high.

To address the issues in Fenton’s reaction, we developed new metal-based catalysts which can be used in industrial wastewater treatment by using H₂O₂ as an oxidant. The catalysts are able to complete a redox cycle in the presence of H₂O₂ and generate both free radicals and high-valence metal ions. These highly reactive species are able to convert hydrocarbons, especially unsaturated organic molecules, into organic acids and carbon dioxide with high molecular efficiency. These metal-based catalysts work over a wide range of pH (3-11) and do not produce any sludge. They are soluble and can be mixed with wastewater homogenously at parts-per-million (ppm) level at neutral pH to degrade the pollutants rapidly. Because of the fast reaction rate, the hydraulic residence time of the process in a wastewater treatment plant is usually less than one hour. They can also be immobilized on solid carriers to treat the wastewater continuously in a fluidized bed. Currently, the new process has been tested in a full-scale plant in Singapore and that leads to better treatment efficiency and 50% cost-saving compared to current treatment processes.

In addition to industrial wastewater treatment, the catalysts can also be utilized in manufacturing processes such as decolorization of vegetable oils and bleaching fabrics. They are also potentially useful as chemical disinfectant for the inhibition of algae growth and deactivation of bacterial endospores. There are many opportunities for the green catalysts to improve our lives in the future.