Recent advances in adsorptive removal and catalytic reduction of hexavalent chromium by metal–organic frameworks composites

Metal-organic frameworks (MOFs) as porous or spongy materials have recently attracted research interests and industrial applications in different disciplines owing to the well-known characteristics as high surface area, good stability, low density-formed skeleton, spongy like structures and tunable micro-porosity. The reactivity of MOFs derivatives and composites are directly referred to their structural composition and combination of the major two constituents, namely transition metal ions and ligands. Therefore, MOFs are regarded as perfect functional porous materials for applications in removal of metal ions, catalytic reduction of toxic pollutants, sensing and detection of a number of analytes. Toxic heavy metals (Cr, As, Cd, Hg and Pb) and their species in water represent a prevalent environmental problem and their complete removal from water by conventional treatment methods is sometimes very expensive and tedious. Therefore, it is necessary to find out and design more efficient treatment techniques for removal of toxic heavy metals. Hexavalent chromium is a highly toxic and carcinogenic species which recently attracted an increasing number of focused publications on removal process by adsorption and catalytic reductive techniques. For this reason, the present review is devoted to cover the state-of-the-art for application of modified and functionalized MOFs composites for hexavalent chromium removal in order to provide the necessary basis for future in-depth understanding of the recent advances in this field. The additional focus of this review is also extended to canvass a variety of modified MOFs composites in the photocatalytic reduction and conversion of hexavalent chromium ions (highly toxic) into trivalent chromium species (less toxic).