Iron/manganese binary metal oxide-biochar nano-composites with high adsorption capacities of Cd2+: Preparation and adsorption mechanisms

Biochar has been widely used as an adsorbent to remove cadmium (Cd) from polluted waters because of its high specific surface area and rich functional groups. Efforts have been made to modify biochar to further improve its metal adsorption capacity. In this work, biochars prepared using cotton straw, corn straw, and rice husk, were first modified with ball-milling and iron/manganese (Fe/Mn) oxides to prepare Fe/Mn binary metal oxide-ball milled biochar (Fe/Mn-BMBCs). Compared with pristine biochars, Fe/Mn-BMBCs had rough surface with deposits of Fe/Mn oxide nanoparticles, larger specific surface areas (226.50–331.50 m2 g−1), and more oxygen-containing functional groups. The adsorption kinetics and isotherm of Cd2+ on both the pristine biochars and Fe/Mn-BMBCs were best described by the pseudo-second-order model and the Langmuir model, respectively, indicating that the adsorption of Cd2+ was primarily monolayer chemisorption. The adsorption capacities of Fe/Mn-BMBCs were 4.8–6.1 times higher than those of pristine biochars, respectively. When the solution ionic strength and valence of coexisting cation increased, the adsorption capacities of Fe/Mn-BMBCs decreased because of cation competition. In contrast, the adsorption capacities of Fe/Mn-BMBCs increased with the increasing solution pH value. Complexation with oxygen-bearing functional groups, ion exchange, Cd2+-π interaction and chemical precipitation were the main mechanisms of Cd2+ adsorption on Fe/Mn-BMBCs. Therefore, Fe/Mn-BMBCs prepared with different agricultural wastes have different Cd2+ adsorption capacities and can be used as environmentally friendly and effective adsorbents for Cd2+ removal from aqueous media.