Trends in the mitigation of heavy metal ions from aqueous solutions using unmodified and chemically-modified agricultural waste adsorbents

Abstract

One of the many methods that have been utilized in the remediation of heavy metals is adsorption. This paper focuses specifically on the chemically-modified means of adsorption using several agricultural wastes as the adsorbents for the uptake of various heavy metal ions. This review focuses on the effect of chemical modification on several agricultural wastes adsorbents for the uptake of various heavy metal ions, it examines the progress made over the years in the adsorption of heavy metal ions using unmodified and chemically-modified agricultural wastes. Herein, we described the range of technologies for heavy metal remediation, the effect of chemical modification of agricultural wastes, kinetic and isotherm studies, and the associated environmental issues. Our findings revealed that the isotherm that best describes the adsorption process is the Langmuir isotherm followed by the Freundlich isotherm and the kinetics was better described by the pseudo-second order model. Also, with respect to the adsorption energy requirement, agricultural wastes such as eucalyptus wood/H2O2, Lansium domesticum peel, and sugarcane bagasse showed some remarkable promises especially for industrial scale-up. As regards the toxicity of the modifying agent, this review suggests that potassium carbonate should be assisted with surface-improving additives as a replacement for potassium hydroxide. In addition, other chemicals such as phosphoric acid and citric acid seem promising for use as modifiers for agricultural waste as they are less toxic. Moreover, it was observed that the adsorption capacity of most of the chemically-modified agricultural wastes was higher than that of the unmodified biomass owing to the increase in biomass active sites that chemical modification provides. This present study provides perspectives on the understanding of the effects of chemical modification of biomass, identification of knowledge gaps and directions for future research.