An atomic-scale understanding of the initial stage of nucleation of heavy metal cations on clay edges

For the purpose of revealing the microscopic mechanism of the early stage in the nucleation of heavy metals on clay edges, we performed first principles molecular dynamics (FPMD) simulations systematically to characterize the chemistry of the incipient clustering processes by taking Pb²⁺ and Ni²⁺ as the model cations. We found that Pb²⁺ and Ni²⁺ have different complexation mechanisms: Pb²⁺ cations are adsorbed onto different sites with the similar probability whereas Ni²⁺ cations prefer octahedral vacancy to the other sites. The reason is that Pb²⁺ does not enter the vacancy due to the large ionic size. pKa calculations indicate that Ni²⁺ adions can provide available complexing sites (i.e., OH− groups) for the subsequent cations through hydrolysis and free energy calculations show that the complexation of subsequent cations is thermodynamically favored, which can thus result in nucleation and precipitation eventually. In contrast, the first adsorbed Pb²⁺ cannot provide available complexing sites due to the extremely high pKa, thus making nucleation impossible. Our findings, consistent with the experiment, reveal the role of the surface in heterogeneous nucleation, which should also apply to other similar systems. The results derived in this study are directly useful for understanding the mobility and fate of heavy metals in surficial environments.