Agricultural Byproducts Used as Low-Cost Adsorbents for Removal of Potentially Toxic Elements from Wastewater: A Comprehensive Review

Potentially toxic elements (PTEs) are ubiquitous chemical compounds in the environment due to contamination of air, water, or soil. They are primarily sourced from fossil fuel combustion, mining and smelting, electroplating, dyes and pigments, agricultural treatments, and plastic and metallic industries. These chemical contaminants can produce various adverse effects when they enter the human body and can also affect crops and aquatic ecosystems. To address these issues, researchers are developing various techniques, including ion exchange, membrane filtration, photocatalysis, electrochemical methods, bioadsorption, and combinations of these processes, to reduce the levels of these contaminants, especially from wastewater. Among these methods, bioadsorption has gained much attention due to its high efficiency, low cost, and abundance of adsorbent materials. Agricultural byproducts used as biosorbents include rice husk and bran, citrus peel, banana peel, coconut husk, sugarcane bagasse, soybean hulls, walnut and almond shells, coconut fiber, barley straws, and many others. Biosorption capacity can be described using adsorption kinetic models such as Elovich, Ritchie’s, and pseudo-second-order models, as well as different adsorption isotherm models such as Freundlich, Langmuir, Temkin isotherm, and BET models. Both conventional processes and adsorption models are influenced by parameters such as pH, agitation speed, contact time, particle size, concentration of the adsorbent material, initial concentration of the contaminant, and the type of modifying agent used. This review paper aims to examine the low-cost adsorbents and their removal efficiency and bioadsorption capacity for different PTEs present in wastewater, and their potential as decontamination methods.