A comprehensive review on sequestration of aqueous Cr (VI) over graphene based adsorbents

Abstract

Cr (VI) causes several chronic diseases like skin, lung, bladder, and kidney cancer as it is extremely carcinogenic. The effluents containing Cr (VI) from various industries like leather tanneries, electroplating, stainless steel etc. contaminate the nearby natural water ecosystems for which the aquatic lives and downstream users suffer from plethora of diseases. Among the existing technologies such as ion exchange, solvent extraction, adsorption, chemical precipitation and electrochemical reduction, adsorption is found the most promising. Graphene based adsorbents (GBAs) are suitable for capturing Cr (VI) due to their excellent chemical stability, large surface area and innumerable surface active sites. Additionally, the OH, NH₂ and SH groups present on GBA surfaces sequester effectively the oxyanionic Cr (VI) species through ionic interaction, H bonding and ion exchange. Moreover, the presence of π electrons of carbocyclic rings and electron donating groups such as OH, COOH, OCH₃ etc. reduce these species to less toxic Cr (III). The present review consists of eight parts. The introduction section briefly depicts the sources of Cr (VI) contamination, its toxic effects, strengths and weaknesses of graphene. In the second part, the advantageous features of GBAs are highlighted. Third part comprises of the development of GBAs through functionalization and composite formation. Their adsorption performances are then systematically presented in terms of sorption parameters. Fifth part of the review emphasizes on the adsorption kinetics and isotherms models suitable for Cr (VI) capture. The plausible mechanism for efficient Cr (VI) uptake and its simultaneous detoxification is focused in the sixth part. The toxicity of GBAs on organisms was briefly out lined in the next part. Various challenges faced by these potential adsorbents and future research directions were addressed along with conclusive summary in the eighth part.