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The coupling of hydrodynamic cavitation (HC) and photocatalysis (PC) predominantly exhibits a complementary nature, which is highlighted through a synergistic index. However, this calculation, based on pseudo-first order kinetics, fails to accurately represent the concentration data, often skewed by the formation of reaction intermediates. To address this, an ‘extended’ kinetic approach previously developed with two parameters, which accounts for the formation of intermediates, was adopted. The PC process was optimized for a simulated wastewater containing phenol of concentration (C0) of 100 ppm by applying a UV-A light source of intensity 175 ± 8 W/cm2 on an operating volume (VL) of 200 mL. Catalyst loading, solution pH and initial concentration (C0) were optimized. These optimal parameters were used to operate HC-PC (VL = 3500 mL) at a comparable illumination intensity across the 2 techniques’ reactors. It was observed that phenol conversion was observable only with halved C0 (= 50 ppm) for the HC-PC system, increasing catalyst loading from 0.5 to 1.0 g/L exhibited no increase in phenol conversion. The obtained results from experiments were interpreted with the help of the two-parameter model. For the PC system, the initial rate constant (k0) exhibits a similar trend to the final oxidation extent, however, it did not compare quantitatively. The second parameter – ‘y’ showed a high finite value, denoting the need for dosing external oxidants. The trends of the two-parameters with respect to different relevant parameters presented will help leverage this kinetic approach for AOPs, while optimizing and translating processes to larger scales of operation.

Phenols are recalcitrant compounds that constitute the majority of organic contaminants in industrial wastewaters. Their removal at large scales require a combination of various processes to reach the desired discharge quality. An extensive body of work has already been published in the area of phenol removal from wastewater, however none of them have focussed on a truly ‘sensible’ approach for coupling advanced oxidation processes (AOPs). Rather, a higher removal efficiency was targeted by unduly complicating the process by combining multiple AOPs. The most influential AOP as the primary method typically driven by the nature of the pollutant should form the basis for a hybrid AOP followed by a complementary AOP to intensify the oxidation process. This strategy is lacking in current literature. We address this knowledge gap directly by systematically identifying the best hybrid process for ZnO mediated photocatalysis of phenol. Either a cavitation mediated pre-treatment of ZnO or cavitation-photocatalysis-peroxide based hybrid AOP was investigated. While the pre-treatment approach led to >25% increase in phenol oxidation compared to bare ZnO photocatalysis, the hydrodynamic cavitation-photocatalysis-peroxide based system was found to have a cavitational yield 5 times higher than its acoustic cavitation counterpart. A new phenomenon known as the ‘pseudo staggered effect’ was also observed and established in hydrodynamic cavitation mediated photocatalysis-peroxide hybrid process for the first time. While we demonstrated that cavitation is a truly ‘sensible’ choice to enhance photocatalysis, the nature of the pollutant under investigation must always be the key driver when designing such hybrid AOPs.

Stimulated by the capacity of Zn to improve the adoption of CO2 and CH4, we doped a Ni-supported ZrO2 catalyst with Zn to enhance its performance and stability in the dry reforming of methane. We prepared a set of catalysts with different Ni:Zn:Zr proportions and conducted extensive ex situ and in situ characterizations to prove that a Ni–Zn alloy was formed at 750 °C under reductive conditions. Combining a tailored morphology of the alloy nanoparticles, strong metal–support (ZnO–ZrO2) interactions, and additional oxygen vacancies created by Zn inclusion resulted in an enhanced catalyst with 15% higher initial activity and higher stability for over 100 h on stream than Zn-free catalyst. Our experimental and modeling results demonstrated that the catalyst with adjusted Ni:Zn:Zr proportion improves the adsorption and reaction rates of CH4 and CO2 while extending its lifetime through enhanced coke precursor gasification compared to its Zn-free counterpart.