Models, kinetics, and thermodynamics for the adsorption of Pb²⁺ and Cd²⁺ metal ions by solid tofu waste immobilized on silica's surface

This research aimed to study the models, kinetics, and thermodynamics for the adsorption of Pb²⁺ and Cd²⁺ metal ions by solid tofu waste immobilized on silica surface. The isothermal adsorption model developed in this study revealed that a temperature of 27°C was required for the adsorption of Pb²⁺ and Cd²⁺ metal ions in a batch system with pH = 5.3 and pH = 6 respectively. The initial concentration of the metal ions were 5, 10, 20, 30, 40, and 50 ppm respectively in a volume of 50 mL containing 0.5g of adsorbent mass. The optimum contact time for adsorption was 120 minutes. Absorption model of both ions followed BiLangmuir Model. The real adsorption capacity was between 0.337 mg/L to 3.474 mg/L for Pb²⁺ ions and 0.437 mg/L to 3.687 mg/L for Cd²⁺ ions. The Gibbs free energy for Pb²⁺ ions adsorption was 3015.45 J/mol within the initial concentration range of 5, 10 and 20 ppm and 3995.62 J/mol in the initial concentration range of 30,40, and 50 ppm, while for Cd²⁺ ion adsorption was 578.17 J/mol I the initial concentration range of 5 and 10 ppm and 2376.86 J/mol within the initial concentration range of 20, 30, 40, and 50 ppm. The results of the adsorption kinetics study showed that the average rate of initial adsorption of Cd²⁺ ions was greater than that of Pb²⁺ ions. The order of the adsorption kinetics for Pb²⁺ ions varied between: pseudo first order and pseudo second order. While the adsorption kinetics for Cd²⁺ ions were second order for a concentration range of 5-50 ppm while the thermodynamic study of adsorption carried out at temperature variations (from 30 to 60°C) was used to determine the average enthalpy adsorption values for Pb²⁺ ions (95.553 kJ/mol) and Cd²⁺ ions (122.249 kJ/mol). While the pH of zero charge potential (pH_PCZ of adsorbent was 8.40) and the optimum pH for the adsorption of Pb²⁺ ion and Cd²⁺ ion is 6.