RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The sorption of Cu²⁺, Pb²⁺, Ni²⁺, and Cd²⁺ ions on the aluminum(III) phosphate was observed to increase with increases in the concentration, temperature, and pH of the system. The apparent dissociation (pK_a), binding (pK_b) and exchange (pK_ex) constants of aluminum(III) phosphate were evaluated and found to be dependent upon the temperature and nature of the metal cations. The values of the dissociation constants (pK_a) followed the order Pb²⁺<Cu²⁺<Cd²⁺<Ni²⁺<K⁺, opposite to the values of the binding constants and selectivity shown by the solid. Ion exchange between protons from the surface and metal cations from aqueous solution was found to be responsible for the metal ions sorption by AlPO₄. The values of the dissociation (pK_a) and exchange (pK_ex) constants were used to estimate the corresponding apparent thermodynamic parameters (ΔH°_(diss), ΔS°_(diss) and ΔH°_(exch), ΔS°_(exch)). The values of ΔH°_(diss) and ΔH°_(exch) were found to be positive, which showed that both the dissociation and exchange processes were endothermic in nature. The ΔS°_(diss) and ΔS°_(exch) were also found to be positive, which showed that both the deprotonation and exchange processes on the AlPO₄ were entropy driven and spontaneous.