Comprehensive analysis of photocatalytic and photoreactor challenges in photocatalytic wastewater treatment: A case study with ZnO photocatalyst

Photocatalytic wastewater treatment is an affordable, low-cost, and environmentally friendly process that does not generate secondary waste. This review article categorizes challenges in photocatalytic wastewater treatment into two main areas: photoreactor design and photocatalyst synthesis. It specifically focuses on the proposed solutions by researchers, using ZnO photocatalyst as a case study. For this purpose, a comprehensive literature review was conducted to analyse the significant and impactful challenges in reactor design, photocatalytic reactions over photocatalysts, and scaling up the process. The obstacles existing in photoreactor design include light distribution, adverse effects of temperature increase, mass transfer limitations, photocatalyst separation from treated effluent, and recovery of photocatalyst. Strategies to overcome the challenges in photoreactor design include implementing new configurations of photoreactors, utilizing various substrate geometries, employing different light sources, etc. From a catalysis perspective, challenges include the wide bandgap of photocatalysts, electron-hole recombination, and mass transfer limitations within the photocatalyst cavities, insufficient adsorption of contaminants, the instability of certain photocatalysts under operating conditions, and minimal adsorption of photons by photocatalysts. This review article aims to contribute to enhancing the photocatalytic wastewater treatment efficiency in photodegradation of pollutants and scaling up its application as a primary method for wastewater treatment. The review indicates the influential role of photoreactor configuration on light distribution, the delivery of photons to the photocatalyst, and the generation of active species, thereby impacting process efficiency. Additionally, it reveals the impact of the physicochemical properties of the photocatalyst on the kinetics and mass transfer of the photocatalytic reaction.