Advances in photocatalytic reduction of hexavalent chromium: From fundamental concepts to materials design and technology challenges

Toxic Cr(VI) polluted wastewater is worldwide recognized because of the wide application of chromium substances in various industrial sectors. Sustainable water pollution approaches are on demand now more than ever. In terms of Cr(VI), many homogenous and heterogenous techniques are applying to intensify its removal form water. Photocatalytic reduction of Cr(VI) has been suggested widely by the scientific community over the last two decades. This report addresses the current research state of the photocatalytic Cr(VI) reduction by addressing the most advances in terms of materials design and mechanistic pathways depending on the working conditions. The photocatalytic activity of different classes of materials such as single or heterojunction semiconductors, hybridization of photocatalyst nanoparticles (NPs) with sorbing materials, i.e., carbonaceous materials, metalorganic frameworks (MOFs), and conjugated polymers, is discussed in depth. The photodeposition of photoproduced Cr(III) on the surface of photocatalysts, and approaches to boost its desorption was addressed as well. The review discusses also the simultaneous photocatalytic reduction of Cr(VI) and oxidation of organic pollutants. A critical analysis of the current state and how to transfer substantial fundamental research to present world application was given by addressing the pros and cons of photocatalytic technology for Cr(VI) reduction compared to existing technologies. Positive research suggestions were provided to enhance the ability of photocatalytic technology for possible use, even for the purification of small volumes of industrial wastewater.