Cadmium(II) adsorption on esterified spent grain: Equilibrium modeling and possible mechanisms

A novel adsorbent—esterified spent grain (ESG) was used to remove Cadmium(II) from aqueous solution. Cd(II) adsorption behavior and mechanisms were investigated in this paper. The sorption process was very fast and the equilibrium was established in 15 min. The maximum Cd(II) adsorption capacity of ESG calculated from Langmuir isotherm was 473.93 mg g−1 that was higher than other adsorbents reported. The sorption kinetics was well described by the pseudo-second order kinetic model. The calculated activation energy (Ea) implied that adsorption of Cd(II) on ESG was a chemical adsorption. The thermodynamic parameters of sorption systems indicated a spontaneous and endothermic process. More importantly, Cd(II) binding mechanisms on ESG were discussed by FT-IR and XPS studies. The results showed that the carbon–oxygen (CO) in carboxyl group of ESG directly attached to the cadmium ion that led to most of the adsorption.

► Esterified spent grain (ESG) is efficient for Cd(II) adsorption.
► The maximum Cd(II) adsorption capacity is 473.93 mg g−1.
► The adsorption equilibrium can be established in 15 min.
► ESG shows good reusability during nine cycles of sorption–desorption.
► Most of Cd(II) is directly attached to the oxygen atom of CO in carboxyl group.