A critical review in the features and application of photocatalysts in wastewater treatment

Rapidly escalating environmental contamination is considered as the most urgent challenge over the world which between them, water contamination is the top priority. As a result, the return of investment on developing novel methods for the effective removal of various kinds of unfavorable contaminants from wastewater would be enormous. In view of the current scenario, among different branches of AOPs for wastewater treatment, the photocatalytic process has emerged as an effective and highly promising technology. The ability to remove pollutants from various environments, the formation of nontoxic products, demand minimum chemicals at a reasonable time, effective degradation, generation of the minimum by-products as pollutants, and energy-efficient operation are some of the major advantages of the photocatalytic process. However, because of several serious challenges concerned with the semiconductor metal oxides, the performance of the solar photocatalyst is still inadequate. As a result, the photocatalyst method is facing some serious challenges, including material instability, ineffective charge transformation and separation, and a discrepancy between solar spectrum and semiconductors' bandgap, which needs to be further explored. In this review paper, photocatalyst doping, coupling, doping with nonmetal, transition metals, noble metals, and co-doped are investigated as methods for enhancing process efficiency. Moreover, developing some immobilized nanocatalyst systems for fixing semiconductors are briefly discussed. Integrated photocatalyst absorbents as an effective method for overcoming some significant shortcomings of individual photocatalyst besides standards for selection of major adsorbents like graphene, clays, zeolite, and activated carbon are investigated. In addition, several crucial parameters that have demonstrated substantial influence on governing the photocatalytic mechanism are discussed. The main parameters for evaluating process effectiveness are summarized. Finally, different serious challenges associated with the photocatalyst method, some strategies for improving photocatalytic activity besides proposing future research direction, and critical recommendations for addressing those challenges are discussed to further broaden the horizon of such a promising method.