Non-conventional adsorbents for the removal of metal compounds from wastewaters

Heavy metals in liquid effluents are dangerous for both the human health and the environment, and their removal from wastewaters is an issue of great environmental relevance. To this end, adsorption is a consolidated technique since it is economical, effective, simple and versatile. To reduce the costs associated with the use of activated carbon as adsorbent, the employment of cheaper non-conventional adsorbents (like industrial residues, natural materials, agricultural by-products and other biomasses) has been proposed. This can also take advantage from proper beneficiation techniques (such as chemical modification, demineralization, gasification, mechanical sieving, pyrolysis) aimed at improving the physical, chemical and morphological characteristics of the material. The aim of this review is to critically report a selection of results, starting from the adsorbent properties and their (if any) beneficiation treatments, and then considering as relevant parameters the adsorption capacities, the thermodynamic and kinetic data. Finally, a discussion concerning the controlling mechanisms is dealt with.