Adsorption of Several Metal Ions onto a Low-Cost Biosorbent:  Kinetic and Equilibrium Studies

Sugar beet pulp generated by sugar-refining factories has been shown to be an effective adsorbent for the removal of heavy metals from aqueous solutions. The structural components related to the metallic adsorption being determined, batch adsorption studies were performed for several metal ions, namely, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Ni²⁺ cations. Two simple kinetic models, that is, pseudo-first- and pseudo-second-order, were tested to investigate the adsorption mechanisms. The kinetic parameters of the models were calculated and discussed. For an 8 × 10^-4 M initial metal concentration, the initial sorption rates (v₀) ranged from 0.063 mmol·g^-1·min^-1 for Pb²⁺ to 0.275 mmol·g^-1·min^-1 for Ni²⁺ ions, in the order Ni²⁺ > Cd²⁺ > Zn²⁺ > Cu²⁺ > Pb²⁺. The equilibrium data fitted well with the Langmuir and Freundlich models and showed the following affinity order of the material:  Pb²⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺ > Ni²⁺. The metal removal was strongly dependent on pH and, to a lesser extent, ionic strength. Ion exchange with Ca²⁺ ions neutralizing the carboxyl groups of the polysaccharide was found to be the predominant mechanism, added with complexation for Pb²⁺, Cu²⁺, and Zn²⁺ metals.