(117ap) Analyzing the Rationale and Assembly for Combined Oxidation Processes: Photocatalytic and Peroxide Treatment Coupled with Cavitation for Effluent Treatment

The production of wastewater is inevitable as a result of industrial and agricultural activities. The introduction of organic molecules in the form of solvents, pharmaceuticals, pesticides lead to generation of wastewaters necessitating its treatment. The treatment of these wastewaters requires multiple processing steps. Conventional treatment methods are not effective in treating wastewaters containing recalcitrant pollutants [1]. Advanced oxidation processes (AOPs) are processes which can generate reactive oxygen species, which can efficiently breakdown organic pollutants in wastewater. In recent years, there has been an avid interest in the use of semiconductor photocatalysis and hydrodynamic cavitation based techniques for effluent treatment [2].

Photocatalysis is a technique of generating reactive oxidant species without the catalyst being consumed. Titania and zinc oxide are common photocatalysts employed for semiconductor photocatalysis. UV light or sunlight can be used to illuminate photocatalysts and drive effluent breakdown. Research of coupling photocatalysis with hydrodynamic cavitation has been attempted in fewer than 10 studies in the recent years, with the objective of achieving required scalability with cavitation reactors and (chemical) oxidant effects with photocatalysis. This coupling is promising and has shown synergy and is effective in breakdown of phenolics [3]. Studies which investigate the breakdown of organic pollutants in these coupled studies use concentrations typically less than 100 ppm. However, in order to effectively treat effluents and be relevant to industrial processing, higher initial concentrations need to be considered and additional oxidants such as hydrogen peroxide or ozonation are required. The use of hydrogen peroxide in conjunction with photocatalysis does not lead to proper utilization of oxidant species, as hydrogen peroxide gets consumed in scavenger type of reactions. Thus, it is important to demarcate the phase at which these (hydrogen peroxide and photocatalysis) are present in the reaction mixture, in order to efficiently utilize these oxidants.

A hydrodynamic cavitation loop was developed to investigate the coupling of cavitation with hydrogen peroxide and photocatalysis. A pool of organic pollutants comprising – chlorocresol, dichloroaniline and paracetamol was prepared to simulate wastewater. The characterization of individual processes of i) cavitation and peroxide, and ii) cavitation and photocatalysis was first carried out. After determining the optimal parameters of the individual processes, a matrix of tests where photocatalysis was introduced at different stages of the reaction was performed. Similarly, peroxide was introduced at different stages of the reaction. The concentration of water matrix was quantified using TOC based techniques. An initial result of the coupling of peroxide and photocatalysis (shown below) does show a monotonic decrease with increase in peroxide concentration, which was on expected lines for the system. A complete characterization of the system with the test matrices is underway and will provide useful information for the strategy whilst coupling this powerful strategy of AOPs to treat effluents.

References

[1] V.P. Sarvothaman, Hydrodynamic Cavitation for Effluent Treatment: Using Vortex-based Cavitation Devices, PhD Thesis, Queen's University Belfast, Queen's University Belfast, 2020.

[2] V.V. Ranade, Bhandari, V.M., Nagarajan, S., Sarvothaman, V.P., and Simpson, A.T., Hydrodynamic Cavitation: Devices, Design and Applications, Wiley-VCH2022.

[3] M.V. Bagal, P.R. Gogate, Degradation of diclofenac sodium using combined processes based on hydrodynamic cavitation and heterogeneous photocatalysis, Ultrasonics sonochemistry 21(3) (2014) 1035-1043.