Cavitation and immobilised photo-catalysis for effluent treatment: A comparative study of individual and combined operations

Background Unsustainable strain on freshwater sources globally could be reduced if industries reuse their wastewater after treatment. Wastewater treatment (WWT) plants, which remove pollutants from wastewater, commonly use conventional physico-chemical processes to treat wastewater. These conventional processes are energy intensive, expensive or use environmentally unfavourable chemicals for treatment and hence an alternative has to be sought ^1,2. Advanced oxidation processes (AOPs) are a form of advanced treatment which are gaining attention as a potential cost-effective method of treating wastewater. AOPs are characterised by their in situ generation of hydroxyl radicals to mineralise organic pollutants. Various AOPs include Fenton, photo-Fenton, photocatalysis, cavitation, ozonation and electrochemical treatment. In Fenton and ozonation based processes, chemicals are added to induce hydroxyl radical production, whilst electrochemical utilises electricity to treat wastewater by inducing oxidation processes with electrodes. Cavitation and photocatalysis are both processes which have been extensively studied for water treatment, and recent studies show great potential with large scale operation ^1,3,4. Cavitation is the formation, growth and subsequent collapse of microbubbles in a liquid, and is of interest for WWT due to its ability to completely mineralise organic pollutants without the need of complex catalysts or extreme operating conditions. The two commonly studied types of cavitation are acoustic cavitation (AC) and hydrodynamic cavitation (HC). HC relies on cavity generation through induced pressure variations in a flowing liquid using a cavitating device whereas AC relies on the propagation of ultrasound in a liquid medium for cavity formation. HC can be easily scaled up for large scale operation due to its continuous nature and low energy requirement when compared to AC. Photocatalysis is another emerging AOP which is a light driven chemical reaction capable of producing reactive radical species through irradiation of a semi-conductor. Key characteristics of photocatalysis are its ambient operating conditions, low operating costs, and complete mineralisation of pollutants without the formation of undesired intermediates, as such, this process has feasible applications in wastewater treatment ^4-7. Commonly, photocatalysts are suspended in solution to obtain an improved irradiated area to volume ratio to perform photocatalysis. However, the need to separate these nanoparticles is a major limiting step for scale-up. Conversely, immobilised photocatalyst (ImPC) systems that do not require a separation process could be used. Poor mass transfer in ImPC and poor diffusivity of OH radicals in bulk⁸ are however issues faced by ImPC. These limitations can be partially overcome by operating it in combination with other AOPs, or as a continuous or recirculating batch systems to enhance mass transfer. The extent of degradation of pollutants is dependent on the quantity of hydroxyl radicals produced, which is increased significantly when combinations of AOPs are combined ⁹. HC is an effective AOP which is operated in continuous flow, this characteristic is ideal for coupling with ImPC process. By combining HC with ImPC, mass transfer limitations can be negated through the continuous flow, whilst the degradation efficiency of both processes is increased through increased production of hydroxyl radicals. Fig. 1 depicts such a coupled process. Figure 1 - schematic diagram of cavitation coupled with an immobilised photocatalyst system Scope of present work Both HC and AC produces in situ OH radicals as a result of cavity collapse. Although methods of cavity generation are different, the chemistry of the reactions upon bubble collapse is equivalent in both cases. Hence, for the ease of experiments, this paper will focus on AC coupled with ImPC (sonophotocatalysis) which will then be used for translating it to a HC-ImPC system. Sonophotocatalysis has been extensively studied in the past for degrading a range of pollutants and proves to be an effective hybrid technique. However, the only combinations reported have used suspended photocatalysts with AC ¹⁰. Due to the issues surrounding the scale-up of a suspended photocatalyst system, ImPC coupled with AC/HC is suggested in this work. By effectively degrading a model pollutant through ImPC and cavitation, an effective hybrid system can be employed which has neither mass transfer limitations nor a necessary separation process. The results will provide a sound basis for design of such hybrid systems for effluent treatment. Experimental methods Acetone was used as the model pollutant for degradation experiments. The photocatalyst used was TiO₂ P25. The ImPC/cavitation system is described Fig. 1., the pollutant will be initially passed through a cavitating device (Sonics VCX 500 Ultrasonic processor) before being treated with photocatalysis. Given the nature of this system, treatment will consist of multiple passes occurring within a certain time. Individual processes (AC and ImPC) and their optimal operating parameters will be determined (amplitude, catalyst coating) through initial experiments. Sonophotocatalysis experiments will then be carried out and optimised. By comparing degradation data for individual and hybrid processes, the extent of synergy of the two processes can be quantitively compared and assessed against other hybrid systems in literature. Results and discussion Processes were compared on a basis of acetone degradation. Acetone was selected as a model pollutant as it is a common industrial solvent, it also possesses a simple organic structure and has a well-documented reaction scheme with hydroxyl radicals, reducing the likelihood of unknown intermediates forming. Furthermore, sonophotocatalysis has been established as a process which can degrade a range of pollutants from a variety of industries, which does not need to be further established by investigating complex pollutants. Initial experiments with AC have been carried out using 20kHz Sonics VCX 500 Ultrasonic processor, where the amplitude has been optimised (90%) for acetone solution (1000ppm) during 15 mins of sonication. Analysis was performed using a Cary 300 Scan, UV-vis Spectrophotometer at 263 nm, with a scan rate of 400 nm/min. Detailed results including optimised conditions for individual ImPC catalyst coating, and hybrid process will be presented at AIChE2018. Summary and outlook This work focuses on comparing the performance of a batch recirculating system hosting an ultrasound/hydrodynamic cavitation reactor and an immobilised photoreactor in series. The performance of this sonophotocatalytic system is compared to the individual processes based on the degradation of a model pollutant - acetone. Additionally, the effect of operating parameters of cavitation reactors and weight of photocatalyst coating on the degradation performance will be tested and optimised. This comparison will allow the extent of synergy between the systems to be determined and would form the basis for developing a mathematical model to understand sonophotocatalytic degradation performance for a broader range of effluents. This approach and work presented is useful for future development of immobilised photocatalytic - hydrodynamic cavitation hybrid systems.