Investigation of ionic activity behavior in aqueous solutions containing heavy metal salts at 298.15 K

Modeling ionic activity is essential for understanding ion-specific effects in environmental and industrial systems. The microscopic interactions of heavy metal salts in aqueous solutions become increasingly complex with rising salt molality, posing significant challenges for accurately modeling ionic activity. This study applies, for the first time, an electrolyte version of the cubic-plus-association equation of state to calculate the mean ionic activity coefficients of several heavy metal salts in water at 298.15 K. Experimental data for four types of salts-nickel, cadmium, cobalt, and zinc salts-were compiled, and the ion-water binary interaction parameters are regressed from these measurements. The model successfully predicts water activity, osmotic coefficients, and mean ionic activity coefficients across most systems. Without explicit incorporation of ionic association, the average calculation relative average deviation of the mean ionic activity coefficient for nickel and cobalt salts is 5.9% and 6.3%, respectively, extending the applicable molality range up to 5.0 mol/kg water and 5.5 mol/kg water. For solutions of cadmium salts and zinc salts that have been confirmed to contain ion pairs, ion association was introduced into the model for calculation. The calculation results indicate that the consideration of ion association has significantly reduced the calculation deviations. Within the calculated salt molality ranges, the average relative deviation of the mean ionic activity coefficient for cadmium salts is all below 2.0%, while that for zinc salts is all less than 5.5%, respectively. This study provides valuable insights into the micro-level factors influencing activity coefficients and offers recommendations for improving model accuracy under complex conditions.