Molecular Simulations of Understanding Separation of Cadmium and Lead Ions from Aqueous Waste Water Using Directional Solvent Extraction

In this paper, we present an all-atomistic moleculardynamics simulationstudy to understand the separation of cadmium (Cd2+) andlead (Pb2+) from the aqueous waste using directional solventextraction (DSE) using decanoic acid (DEC) as an extractant. Differentstructural, dynamic, and thermodynamic properties were evaluated suchas mass density profiles, radial distribution functions, self-diffusion,solvation free energy (& UDelta;G (Solv)),and transfer free energy (& UDelta;G (Transfer)). The peak intensity of mass density distribution of Cd(NO3)(2) and Pb(NO3)(2) shows an increasewith an increase in temperature (T). The radial distributionfunction between the Cd2+ and NO3 (-) and Pb2+ and NO3 (-) showsa significantly higher interaction with the maximum interaction betweenCd(2+) and NO3 (-). Compared tothe intermolecular structure between bulk water and solvent mixedsystems, we observe that metal ion water interactions are higher inbulk water, which start to decrease in the presence of a solvent.We further investigate thermodynamic analysis using free energy simulations.The & UDelta;G (Solv) of Cd2+ andPb(2+) becomes favorable with an increase in T in DEC, while it becomes unfavorable in H2O. The & UDelta;G (Transfer) of Cd2+ and Pb2+ from H2O to DEC becomes favorable with an increase in T. The partition coefficient (log P) valuesshow positive values for Cd2+ and Pb2+. Theself-diffusion (D) of metal ions shows a 60% decreasefor Cd2+ and a 75% decrease for Pb2+ comparedto their D values in bulk water. The D values of H2O increase with T. Overall,we observe that the H2O, solvent, and metal ions are sensitiveto the temperature. These molecule-level results help in guiding thesolvent design for separating heavy metal ions using DSE.