Abstract
ABSTRACT Hexavalent chromium (Cr(VI)) ion is extremely toxic to man and the environment. Its hazardous nature has attracted intensive attention cumulating to the design of different techniques for its removal, which include adsorption on different natural and man-made absorbents. This review is focused on the contributions of metal-organic frameworks (MOFs) and multi-walled carbon nanotubes (MWCNTs) as efficient adsorbents in the uptake of Cr(VI) from water bodies. A comparison of the available resources revealed that the application of MOFs in water treatment via the adsorptive approach is relatively new. The uptake of Cr(VI) onto MOFs and MWCNTs is strongly dependent on parameters that influence adsorption. The efficiency of adsorption of MOFs and MWCNTs increased with an increase in contact time, adsorbent dosage and sorbate concentration. Studies revealed that both MOFs and MWCNTs are more efficient in an acidic environment with optimum pH of <2.5 and <4.0 respectively. Generally, the kinetics and isotherm data that best described both adsorbents are pseudo-second-order and Langmuir models. The uptake of Cr(VI) onto MOFs or MWCNTs is endothermic and highly disordered. In the adsorptive removal of Cr(VI), MOFs show a superior capacity to MWCNTs. The adsorptive mechanism of either MOFs or MWCNTs to trap Cr(VI) is dependent on the quality of surface modification. The adsorption capacity can be enhanced by the adoption of a green and effective modification. The application of these adsorbents in column adsorption containing the analyte of varied chemistry should attract new research efforts. This review emphasises the cardinal parameter of adsorption and the adsorptive mechanism responsible for the uptake of Cr(VI) onto MOFs and MWCNTs.
Published Version
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