Kinetics of Divalent Metals (Cd2+, Cu2+, Pb2+, Zn2+) Adsorption onto a Modified Brick

Abstract

Brick which originated from Bangui Region (in Central African Republic), was pre-activated with HCl at 90°C and coated with ferrihydrite. This material was used in the present work as an adsorbent mainly for the removal of metallic cations (Me2+ = Cd2+; Cu2+; Pb2+; Zn2+) from aqueous solutions due to their bad effect on the environment. Experimental results were compared to those obtained for other divalent cations (Ca2+, Fe2+, Mg2+, Mn2+, Sr2+). Freundlich and Langmuir models were applied to describe adsorptions isotherms, and Langmuir model was found to provide the best fitting. Langmuir adsorption capacities (Qmax) of these ions onto ferrihydrite-coated brick increased in the following order: Cu > Zn >Pb>Ca> Cd >Mg. Kinetic studies revealed that experimental data were better fitted by the pseudo second-order rate equation than the pseudo first-order rate one. Thermodynamic analyses indicated that the adsorption kinetics process was endothermic. Because of acid/hydrolysis characteristics of metallic cations, the kinetic constant, k2, was found to be affected by H+ and OH- (i.e., solution pH) as potential-determining ions by regulating proportions of negative and positive charges at the brick surface. The adsorption capacity (Qe) was determined from kinetic data and the adsorption affinity of the studied eight elements onto ≡S-OH sites in modified brick had the following order: Pb> Cu > Cd ≈ Mn ≈ Zn > Mg >Ca> Sr. These Qevalues were found to be instead related to the surface binding constant, K(≡SOMe), relative to the equilibrium: S-OH) + Me2+ ↔ ( S-O)Me+ + H+), suggesting the implication of hydroxyl groups from brick metakaolinite in addition to –OH and/or –ONa functions mainly present in ferrihydrite deposits.