Intensification of Wastewater Treatment using Sono-hybrid Processes: An Overview of Mechanistic Synergism

Sono-hybrid advanced oxidation processes (AOPs) have recently attracted significant attention of researchers for degradation of bio-recalcitrant pollutants. Most widely studied sono-hybrid processes are: sonocatalysis, sono-Fenton and sono-photocatalysis. However, exact synergy between these AOPs and the underlying physical mechanism remains largely unexplored. This paper has addressed this fundamental issue. With approach of coupling experimental results to simulation of cavitation bubble dynamics, an attempt is made to identify synergistic links between individual mechanisms of AOPs. It is revealed that in all sono-hybrid AOPs, physical effect of ultrasound and cavitation (generation of micro-turbulence) contributes more than chemical effect (radical production). In sonocatalysis, the catalyst merely acts as adsorbent, while sono-photocatalysis shock waves produced by transient cavitation bubble have an adverse effect of desorption of pollutant molecules from catalyst surface. The phenomena of radical conservation and scavenging of radicals also play an important role. Externally added H2O2 (as Fenton reagent) scavenges oxidising radicals produced by cavitation, while dissolved oxygen as well as externally added Fe2+ (as Fenton reagent) help either generation of additional oxidising radicals (such as O-center dot and HO2 center dot) or regeneration of (OH)-O-center dot radicals through Fenton reaction induced by in-situ produced H2O2 through combination of (OH)-O-center dot radicals.