Catalytic Air Oxidation of Refractory Organics in Wastewater

Abstract

Catalytic air oxidation (CAO) is an economical, environmentally friendly, and efficient technology used to treat wastewater that contains refractory organics. This review analyzes recent studies regarding five common types of CAO that use external energy sources (heat, light radiation, microwave, and electricity) or non-oxidizing chemical promoters (nitrites and sulfites). Methods include hydrothermal, electro-assisted, photocatalytic, microwave-assisted, and non-oxidizing chemical-assisted CAO. The associated catalytic mechanisms are discussed in detail in order to explain the connections between CAO catalytic pathways. Mechanisms include O2 activation via excitation, free-radical autocatalytic reactions, and coordination catalysis. Classical kinetic mechanisms, including Mars-van Krevelen and Langmuir-Hinshelwood, are also proposed to reveal overall CAO dynamic processes. The catalysts used in each CAO technology are summarized, with a focus on their catalytic pathways and the methods by which they might be improved. Finally, important challenges and research directions are proposed. The proposals focus on further research regarding catalyst mechanisms, mechanism-guided catalyst design, and process improvement.