Countering major challenges confronting photocatalytic technology for the remediation of treated palm oil mill effluent: A review

Abstract Recently, research interest has shifted to the development of effective remediation technologies for treated palm oil mill effluent (TPOME) due to the failure of the existing treatment methods to comply with the standard discharge limits. Photocatalytic technology has been recognized as an extremely attractive and sustainable technology for the remediation of TPOME. However, inefficient harvesting of visible or solar light, very low remediation efficiency under visible light, incomplete recovery of the photocatalyst after the treatment, lack of property-performance relationship, and lack of process understanding as well as parametric optimization are the major challenges which hinder the practical application of photocatalytic technology for the remediation of TPOME. Many investigations have been carried out to overcome the existing challenges and enhance the overall remediation efficiency of TPOME. This review provides an in-depth and critical analysis of the recently developed strategies to countermeasure the limitations and improve the overall performance of photocatalytic technology for the remediation of TPOME with respect to the preparation of visible light active photocatalyst, strategies to eliminate the need for the recovery of photocatalyst, and parametric optimization of the photocatalytic process. Various properties of the photocatalysts were reviewed and their correlation with the photocatalytic performance for the remediation of TPOME was evaluated. Also, the specific removal rate (mg of COD removed/g catalyst h) was calculated to provide an inclusive and quantitative comparison of individual studies using various photocatalysts, which are barely deliberated in the published literatures. Prospective research needs for resolving the major challenges and improving the competency of photocatalytic technology for practical application in remediation of TPOME are suggested accordingly. This review will enhance our understanding of the photocatalytic technology for the remediation of TPOME and extend the boundary towards the practical application of this technology on an industrial scale.