Cadmium ions removal from acid aqueous water by chicken eggshells

Chicken eggshells (ES) were used to remove cadmium ions from acid aqueous water. The presence of (Ca, Cd)CO3 crystals has been corroborated by scanning electron microscope with element mapping. Combining with the decreasing pH values as Cd2+ concentration increases, the removal process was dominated by precipitation and cation exchange. The cation exchange occurred at low concentrations, and precipitation dominated the removal mechanism at high concentrations. In binary metals competitive experiment of Cd2+-Cu2+ and Cd2+-Pb2+, at low concentrations of Cu2+ and Pb2+, the removal capacity of Cd2+ did not decrease obviously; while at high concentrations of Cu2+ and Pb2+, the Cd2+ ions were at a competitive disadvantage. The Langmuir model fitted equilibrium data better than Freundlich model. The certified endothermic and favorable sorption had maximum uptake of 2.40 mg g(-1) for Cd2+ on ES at 298 K. The adsorption process better followed pseudo-second-order kinetics, and the activation energy was 23.67 kJ mol(-1); both showed the chemical sorption being the rate-controlling step. The results indicated that the removal efficiency of cadmium ions had reached above 90% on ES of diameter 0.45-1.0 mm in solution of pH 4.5 and 7.6 mg L-1 Cd2+.