Insight into Hofmeister effects on aggregation of 2:1 and 1:1 type clay minerals

摘要

Abstract 2:1 and 1:1 types of clay minerals are ubiquitous in soil, and their different surface properties result in distinct environmental behaviours. In this study, dynamic light scattering (DLS) technology was adopted to determine the aggregation kinetics of 2:1 type (Montmorillonite Mt) and 1:1 type (Kaolinite Ka) clay minerals in different electrolytes. Hofmeister effects on their aggregation characteristics and mechanisms were explored. The results showed that the activation energy and critical coagulation concentration (CCC) for Mt and Ka aggregation followed Hofmeister sequences of Na+ > K+ > Cs+, Mg2+ > Ca2+ > Zn2+ ≈ Cd2+ > Cu2+> Pb2+. The analyses of physical insights into those differences revealed that the observed Hofmeister sequence was produced from the different non‐classical polarisation of cations and surface O atoms in the strong electric field of clay, which led to non‐electrostatic force adsorption of cations at the surface (or in the Stern layer). In this study, the effective charge coefficient γ was employed to characterise the intensity of the non‐electrostatic adsorption forces of cations arising from the non‐classical polarisation, and the γ values followed Na+ < K+ < Cs+, Mg2+ < Ca2+ < Zn2+ = Cd2+ < Cu2+ < Pb2+. For alkali and alkali earth metal cations with same valence, the more the electron layers, the ‘softer’ the electron cloud, the stronger the non‐classical polarisation effects in the electric field and the stronger ability to shield the electric field around the particle, which finally lead to the larger γ and lower CCC for clay minerals aggregation. For heavy metal cations, different s/p/d empty orbitals provided the basis for the difference in polarisation‐induced coordination between heavy metal cations with surface O atoms on clay. The results also showed that, the Hofmeister effects on Mt/Ka aggregation were different, it was from the differences in Hamaker constant and surface charge density for the two clay minerals. Highlights Mt and Ka aggregations exhibited different Hofmeister effects. Repulsive force among Mt particles was much larger than Ka. Cationic polarisation played critical role in particle interactions of Mt. Polarisation‐induced covalent adsorption of cations was observed at Mt surface.