Unraveling the cation adsorption of geopolymer binder: A molecular dynamics study

Geopolymers play a significant role in remediation of heavy metal contamination and are attracting increasing interests. Sodium aluminosilicate hydrate (NASH) is the prime hydration substance of geopolymers which exhibits excellent adsorption capacity, however, the mechanism of metal cation adsorption at the NASH interface remains unclear. In this study, the adsorption behavior of cations at the NASH interface was investigated in depth, and the effects of Si/Al ratios, ion concentration and ion type on adsorption behavior were also analyzed. Furthermore, three Si/Al ratio models of NASH gel were modified and developed by molecular dynamics simulation, and validated by experiments. The result showed that electrostatic attraction and ion exchange played the major role in adsorbing three cations on the surface of NASH gel. For cations with the same charge number, ionic radius was inversely proportional to the cation exchange and adsorption capacity. Cations with lower ionic potential, among those with different charge numbers, were easier to be adsorbed onto the NASH surface. Therefore, the adsorption capacity of NASH for the three adsorbents was in the order of Na+ > Cs+ > Pb2+. The adsorption capacity of NASH gel for cations increased with the increasing of Al/Si and decreased with the increasing of cation concentration, which was attributed to the increased electrostatic attraction on the NASH surface and the limited number of adsorption sites. The derived microstructure and dynamics information are beneficial for profoundly understanding the adsorption mechanisms of geopolymers on cations.