Nanoscale mechanism of ions immobilized by the geopolymer: A molecular dynamics study

Geopolymer with sodium aluminosilicate hydrate (NASH) gel as the main hydration product exhibits good waster immobilizing ability. To provide a fundamental understanding of the immobilization mechanism of geopolymer materials, molecular dynamics approach is used to study the density distribution, dynamics properties and local structures of Na+, K+ and Cs+ ions in the vicinity of the NASH gel interface. Plenty of non-bridging oxygen sites made by the cleaved silicate and aluminate branch structures are exposed forming Hydrogen bonds with the water molecules nearby. The water molecules strongly confined in the silicate-aluminate channel of hydrophilic NASH surface exhibit enlargement of dipolar moment because of the strong H-bond restriction. Furthermore, the O atoms in silicate-aluminate network can restrict the cations by a strong adsorption. On the other hand, Cl- ions exhibited a weak adsorption, repulsed by the negatively charged silicate surface, which is primarily due to the formation of the cation-anion ionic pair near the interface. The calculation results of density profile and resident-time shows that the adsorption capacities of cations on the NASH surface and the ionic radius are negative related, with the sequence of Na+> K+> Cs+. The Na+ and K+ ion with smaller hydration shell can adsorbed at the cavity and have oxygen atoms in silicate skeleton as their nearest neighbors, contributing to inner sphere adsorption with long resident time on the surface. On the contrary, the Cs+ ions can only be immobilized by a weak Cs-O bond in the outer sphere of the NASH surface, which results in frequently dissociation from the NASH surface and high diffusivity of ions. This study provides a fundamental understanding of immobilization mechanism of geopolymer material at the molecular level. (C) 2019 Elsevier B.V. All rights reserved.