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

ABSTRACT

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.

HIGHLIGHTS

• Valorization of coffee wastes for applications in water treatment is reviewed

• Carboxyl and hydroxyl surface functional groups improve metal adsorption

• Adsorption capacity as high as 302 mg/g for Cr(VI) has been reported

• Gaps and recommendations are provided for future research

GRAPHICAL ABSTRACT

KEYWORDS:

• Coffee residue
• charred and uncharred adsorbents
• chemical modification
• remediation
• heavy metals

Acknowledgements

The support from the Vaal University of Technology research directorate is gratefully appreciated.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Authors contributions

Vusumzi Emmanuel Pakade: Conceptualization, Data acquisition, Funding acquisition, Literature search, Writing – original draft, review & editing. Somandla Ncube: Conceptualization, data analysis, review & editing. Michael John Klink: Conceptualization, data analysis, review & editing. Lawrence Mzukisi Madikizela: Conceptualization, data analysis, review & editing.

Consent for publication

The authors declare that this work has not been submitted to any other journal for publication and that all authors consent to the publication of the manuscript.

Additional information

Funding

The South African National Research Foundation (NRF) through [grant number 120425] is gratefully appreciated for the financial support.

Notes on contributors

Vusumzi Emmanuel Pakade

Vusumzi Emmanuel Pakade is an associate professor with over ten years of experience working in the department of Biotechnology and Chemistry at the Vaal University of Technology, South Africa. Among other things, his research work focuses on the fabrication of materials for environmental remediation. He is currently working on the synthesis of organic-inorganic hybrids, smart polymers, and the valorization of agricultural waste products to address social and technical challenges using the principle of adsorption. He has hosted a few postdoctoral fellows and supervised several doctoral and master's students. Besides publishing over 60 peer-reviewed scientific journal articles and five book chapters, Prof. Pakade contributes to scholarship in many roles, including editorial roles in specific journals. When not working, Prof. Pakade enjoys swinging a club.

Lawrence Mzukisi Madikizela

Prof Lawrence Mzukisi Madikizela is an Associate Professor of Analytical and Environmental Research at the University of South Africa and holds a PhD in Chemistry from the University of Witwatersrand. His research focus is largely on the development of sample preparation methods and the analysis of organic pollutants and metals in aquatic environments. In recent years, he has developed an interest in the development, characterization, and application of various adsorbents for the removal of emerging contaminants in water. Some of his research includes ecological risk assessment based on the levels of emerging contaminants found in water samples. In addition, Prof. Madikizela investigates the fate of organic contaminants such as pharmaceuticals in the environment through the monitoring of their occurrence in sediments, aquatic plants, and vegetables grown in contaminated regions. He further performs research that addresses issues of food quality and safety. The scope of this research focuses on monitoring the uptake of heavy metals and pharmaceuticals by vegetables and the occurrence of toxic organics in food products. To date, he has published 59 research articles, 4 book chapters, and graduated several postgraduate students. His contributions in the research world have seen him scoop several research awards and grants while attaining a Google Scholar H-index of 26.

Michael John Klink

Prof. Michael John Klink obtained his doctoral degree in physical chemistry from the University of the Western Cape (South Africa) in 2007, where he demonstrated exceptional research skills and a deep understanding of his area of expertise. As an Associate Professor, Prof. Klink has continued to make significant contributions to his field of research. His research interests encompass adsorption, sensors, and biosensors, with a particular emphasis on environmental studies. Prof. Klink has an extensive publication record, with numerous articles, book chapters, and conference proceedings to his name. His research has been widely cited and has had a profound impact on the scientific community. He is frequently invited to present his work at international conferences and collaborate with renowned researchers and institutions worldwide. In addition to his research achievements, Prof. Klink is dedicated to mentoring and guiding the next generation of researchers. He has successfully supervised and mentored numerous graduate students and postdoctoral fellows, fostering a collaborative and stimulating research environment. His mentorship has nurtured the development of young scientists and contributed to their success in academia and industry.

Somandla Ncube

Dr. Somandla Ncube is a research lecturer at the Durban University of Technology, Durban, South Africa. He graduated with a PhD in 2018 from the University of the Witwatersrand, Johannesburg, South Africa. Somandla does research in Environmental Analytical Chemistry with interests in sample preparation and determination of both organic compounds and metals in environmental, plant and food samples. Current focus is on monitoring, health risk assessment and remediation of pollutants of emerging concern in environmental samples and food sources. His research outputs are available on the following links: https://www.scopus.com/authid/detail.uri?authorId=57191162956; https://scholar.google.co.za/scholar?hl=en&as_sdt=0%2C5&q=Somandla+ncube.