Modeling of heavy metals removal from aqueous solution using activated carbon produced from cotton stalk

Activated carbon produced from cotton stalks was examined for the removal of heavy metal contaminants. Adsorption studies in completely mixed batch reactors were used to generate equilibrium pH adsorption edges. Continuous flow experiments using the activated carbon in fixed beds were conducted to determine heavy metal breakthrough versus bed volumes treated. At given pH value in the range 5–7, the adsorption capacity was similar for copper and lead and clearly greater than for cadmium. A surface titration experiment indicated negative surface charge of the activated carbon at pH > 6, meaning that electrostatic attraction of the divalent heavy metals can occur below the pH required for precipitation. Substantive metal removal below the pH of zero charge might be due to surface complexation. Accordingly, a surface complexation model approach that utilizes an electrostatic term in the double-layer description was used to estimate equilibrium constants for the protolysis interactions of the activated carbon surface as well as equilibria between background ions used to establish ionic strength and the sorbent surface. Pb(II) adsorption edges were best modeled using inner-layer surface complexation of Pb²⁺, while Cd(II) and Cu(II) data were best fit by outer-layer complexes with Me²⁺. The full set of equilibrium constants were used as input in a dual-rate dynamic model to simulate the breakthrough curves of the target metals (Pb, Cu and Cd) from fixed bed experiments and to estimate external (or film) diffusion and internal (surface) diffusion coefficients.