Waste calcite sludge as an adsorbent for the removal of cadmium, copper, lead, and zinc from aqueous solutions

Sludge residues, an industrial waste material for the removal of cadmium (Cd²⁺), copper (Cu²⁺), lead (Pb²⁺), and zinc (Zn²⁺) from aqueous solutions were investigated using batch method. Batch mode experiments were carried out as a function of solution pH, adsorbent dosage, initial concentration, and contact time. The results indicated that the adsorbent showed good sorption potential and maximum metal removal was observed at pH ≥ 3. Within 120 min of operation, about 63.7, 95.2, 99.9, and 88.2% of Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions were removed from the solutions, respectively. Sorption curves were well fitted to the Langmuir and Freundlich models. The adsorption capacities for Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions at optimum conditions were 121.2, 1067.8, 566.4, and 534.2 mg g⁻¹, respectively. The kinetics of Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ adsorption from aqueous solutions was analyzed by fitting the experimental data to pseudo-first- and pseudo-second-order kinetic models. However, the pseudo-first-order kinetics model provided much better R ² values and the rate constant was found to be 0.001 min⁻¹ for Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions. The results revealed that sludge residues can adsorb considerable amount of Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions and it could be an economical method for the removal of these ions from aqueous systems.