Enhanced Adsorption of Anionic Polymer on Montmorillonite by Divalent Cations and the Effect of Salinity

Adsorption of polymers from an aqueous solution onto clay minerals is of great interest to many applications such as water purification and soil conditioning. Molecular dynamics simulations were performed to study the adsorption of anionic polyacrylamide (APAM) on anionic montmorillonite, in an aqueous solution containing monovalent or divalent salts. Compared with monovalent salts (NaCl), the enhancement of APAM adsorption brought by divalent salts (CaCl₂) was significant, which could not be explained by the Poisson-Boltzmann theory alone. Each solvated Ca²⁺ was coordinated by 4-6 water oxygens in its first coordination shell. One to two of these water molecules were displaced when APAM formed a complex with Ca²⁺. Ca²⁺ ions in the adsorbed Ca²⁺-APAM complexes did not serve as bridges sandwiched between APAM and Mt; instead, the complexes carried a residual positive charge and were subsequently attracted to montmorillonite. The number of adsorbed Ca²⁺-APAM complexes changed with salinity in a nonmonotonic manner, due to the modulation of apparent charges of montmorillonite and APAM by Ca²⁺. Increasing adsorption of Ca²⁺-APAM complexes also promoted APAM adsorption through direct hydrogen bonding with montmorillonite. The findings provided new molecular insights into the long-standing debates on the role of divalent ions in promoting polymer adsorption on like-charged solid surfaces.