Insight into mechanisms of heavy metal-induced natural clay aggregation

Transfer risk of heavy metal in soil–water–organism systems strongly depend on natural clay aggregation/dispersion. An important characteristic of natural clay minerals is their strong electric field of 10 ⁸–10 ⁹ V m ⁻¹ in water or 10 ¹⁰–10 ¹¹ V m ⁻¹ in vacuum at their surfaces. The outer-shell orbitals of surface O atoms of the natural clay mineral are significantly modified in such strong electric fields, the energies of the lone-pair electrons therefore enhanced, which results in the polarization-induced covalent bonding between surface O atoms and heavy-metal cations. This study revealed that polarization-induced covalent bonding force, electrostatic force, and cation size jointly determine the adsorption energies of heavy-metal cations on the outer surface of layered natural clay particles, and thus determine natural clay aggregation. The observed CCC and activation energies of aggregation decreased in the order of Pb ²⁺ < Cu ²⁺ < Zn ²⁺ < Cd ²⁺ , while the reverse sequence was exhibited with respect to aggregation rates. Moreover, the polarization-induced covalent bonding adsorption of heavy-metal cations plays the most important role in clay aggregation. This study revealed that polarization-induced covalent bonding force, electrostatic force, and cation size jointly determined the adsorption energies of heavy-metal cations on the outer surface of layered natural clay particles, and thus determine natural clay aggregation. However, only polarization-induced covalent bonding adsorption of heavy-metal cations plays a critical role in the natural clay aggregation. Important insight at a sub-atomic scale into natural clay aggregation induced by heavy-metal adsorption is provided. The polarization-induced covalent bonding can be influenced by regulating the electric field strength by changing parameters such as temperature, electrolyte type, electrolyte concentration, and pH. The findings of this study would therefore be helpful in future studies on evaluating and controlling transfer risks of heavy-metal cations in soil–water–organism systems.