Graphene-based (nano)catalysts for the reduction of Cr(VI): A review

Abstract Hexavalent chromium (Cr(VI)) contamination in water resources is among the most serious environmental problems due to its the great carcinogenicity of chromium to humans and aquatic systems. Since Cr(VI) is released into water from several industrial sources such as alloy production, mining, etc., the removal of this metal from water based solutions is a universal priority. Reduction of Cr(VI) has shown excellent effectiveness in its elimination from aqueous systems and has been greatly focused on during the recent years, as the resultant product is Cr(III), which is a non-toxic compound and a nutrient. Novel catalysts based on nanostructures of carbonaceous compounds, particularly graphene and its derivatives, have attracted significant attention in heavy metal reduction and water purification given their large surface area and appropriate conductivity, which gives rise to their high catalytic efficiency and durability. Therefore, a comprehensive review of various graphene-based catalysts for hexavalent chromium reduction in water is presented in this study. A classification of reduction methods for the examination of graphene catalysts is demonstrated including electroreduction, photoreduction, adsorption-reduction, chemical reduction (formic acid mediated reactions) and bio-reduction. Graphene-based catalysts have been found to show enhanced photocatalytic activities and long term stabilities in comparison with plenty of other conventional and benchmark catalysts commercially being employed.