Adsorption of toxic heavy metals using charred and uncharred coffee waste adsorbents: a review

The presence of toxic heavy metals in water bodies is a subject of international concern due to the deleterious effects caused by these contaminants on both humans and the ecosystem. Adsorption is among the most successfully used methods for the abatement of heavy metals from water and low-cost adsorbents are sought after. Coffee waste is generated in ton volumes and its inherent chemical structural groups have elicited interest from researchers to investigate it for developing low-cost adsorbents for heavy metal removal. Functional groups of interest are the dominant carboxyl and hydroxyl groups which have been explored for metal ion binding. In this review, we attempt to correlate the treatment methods (physical (pyrolysis) and chemical) of charred and uncharred adsorbents with adsorption efficiency. The adsorption efficiencies of the different classes of modifications are not clearly distinguishable, which then dwarfs the purpose of surface modification. There have been some studies where excellent adsorption performances were reported, with adsorption capacity reaching 302 mg/g for chromium six but these are very few. The comprehensive comparison data provided in this review shows that more work still needs to be done in extending the adsorption to other metal ions and improving the surface modification reactions to include grafting, crosslinking, and the fabrication of composite materials incorporating nanoparticles other than magnetite to improve the adsorption efficiency as well as analyzing the cost-effectiveness of coffee waste adsorbents.HIGHLIGHTSValorization of coffee wastes for applications in water treatment is reviewedCarboxyl and hydroxyl surface functional groups improve metal adsorptionAdsorption capacity as high as 302 mg/g for Cr(VI) has been reportedGaps and recommendations are provided for future research