Pb(II) and Cd(II) Removal from Aqueous Solutions Using Activated Carbon Developed from Coffee Residue Activated with Phosphoric Acid and Zinc Chloride

Coffee residue, a low-cost agricultural byproduct, was tested as a precursor for the production of porous carbons in a chemical scheme using phosphoric acid and zinc chloride. The raw material was impregnated with increasing impregnation ratio (mass of ZnCl2 or H3PO4/mass of coffee residue) from (0 to 100) % followed by pyrolysis at 600 °C for 1 h. The products were characterized by adsorption of N2 at 77 K and proved to be highly microporous with high surface area. The impregnation ratio had a strong influence on the pore structure of these activated carbons, which could be easily controlled by simply varying the proportion of activating agents used in the activation. Thus, a low impregnation ratio led to essentially microporous activated carbons. At intermediate and high impregnation ratios, activated carbons with a wider pore size distribution (from micropores to mesopores) were obtained with high surface area. These low-cost adsorbents developed with ZnCl2 and H3PO4 were used for the removal of lead(II) and cadmium(II), and they showed a substantial capability to adsorb lead(II) and cadmium(II) ions from aqueous solution. The kinetics of adsorption and extent of adsorption at equilibrium were dependent on the physical and chemical characteristics of the adsorbent, adsorbate, and experimental parameters. The effect of contact time and initial concentrations of adsorbate on the uptake of lead and cadmium was studied in batch experiments. The kinetic data were fitted to pseudofirst-order and pseudosecond-order models and follow closely the pseudosecond-order model. Equilibrium adsorption isotherms of Pb(II) and Cd(II) were analyzed by the Langmuir and Freundlich isotherm models. The Langmuir model gives a better fit than the Freundlich model.