Electric fields within clay materials: How to affect the adsorption of metal ions.

Electric fields exist ubiquitously in chemical and biological systems while how to affect the interfacial adsorption processes remain elusive. Here, molecular dynamics simulations are used to understand at a molecular level the adsorption of metal ions at the interface of aqueous solutions and clay materials that are generally endowed with strong electric fields. In absence of electric fields, even Cs+, one of the strongest adsorbed metal ions, is facile to detach from solvated clay surfaces. Electric fields are critical to construct stable inner-sphere complexes and enhance pronouncedly the adsorption strengths of both inner- and outer-sphere complexes. Heavy metal ions such as Pb2+ that are exclusively outer-sphere adsorbed are driven inner-sphere by electric fields, causing the adsorption strengths to surpass Cs+ and explaining partially the serious pollution to clay systems. Adsorption quantities of inner-sphere alkali ions increase with electric fields and in the case of Pb2+, are closely correlated with the intensities of electric fields. Previous models fail to account for Hofmeister effects occurring at solvated clay surfaces, and forceful supports are given to polarization effects that correctly interpret Hofmeister series (Cs+>Na+) and respond towards the change of electric fields.