Carbon nanotubes and graphene-based materials for adsorptive removal of metal ions – A review on surface functionalization and related adsorption mechanism

Enhanced use of heavy metals for commercial and non-commercial applications has resulted in industrial and domestic discharge loaded with toxic metal ions creating increased environmental concern. Owing to the wide array of heavy metals that can be effectively removed, relatively low cost and economical process conditions, removal of heavy metals by adsorption has attracted the scientific community over years. Search for novel adsorbents which are cost-effective with higher adsorption affinity and reusability has been an area of active research. Carbon based adsorbents, which offer enhanced surface area, higher surface to volume ratio and ease of surface functionalization, have proven to be more effective than conventional adsorbents for removal of both inorganic and organic contaminants. Among the various carbon based adsorbents, carbon nanotubes, graphene and modified graphene oxide composites are gaining increasing importance as effective adsorbents for heavy metal removal from waste waters. This paper presents a comprehensive review on use of these adsorbents for adsorptive removal of various heavy metals. Optimal conditions for maximum removal of various metal ions are highlighted. Case studies on approach to surface functionalization of adsorbents and the mechanism of metal ion removal by the functionalized surface are discussed for practical application.