Fate of Cadmium and Lead Nitrates in Water

Cadmium, lead, and the nitrate (NO₃^-) ions rank among the most hazardous water pollutants. Natural sources of these species include cadmium and lead nitrates, which are highly soluble in water. Experimental data on the corresponding aqueous solutions are scarce, and previous computational studies have only addressed the hydration of either the metal or the NO₃^- ions. Here, we employ molecular modeling and ab initio molecular dynamics simulations with umbrella sampling to investigate how, and how readily, the component ions are released from the metal nitrates in aqueous solutions. Our model corresponds to a solute concentration of ∼0.4 M at 350 K. Our results show that the dissociation pathway is characterized by several subspecies, all clearly identifiable as contact-ion pairs and nearly degenerate within the accuracy of our computational scheme. We estimate free-energy barriers of no more than ∼4 kcal/mol for the various steps of the decomplexation reaction. Our findings, while providing additional valuable information, support suggestions, derived from experimental observations, regarding the presence of monodentate nitrates in the case of cadmium and also the nature of the ion-pairing in both systems.