Experimental and molecular dynamics study on anion diffusion in organically modified bentonite

Abstract

The work presents experimental and molecular modeling study of the anion diffusion with nitrate as a model probe in the organically modified bentonite. Organoclay samples with different loadings of the two cations Hexadecyltrimethylammonium (HDTMA+) and Hexadecylpyridinium (HDPy+) were prepared. The diffusivity of NO3− in the interfacial area of the organo-bentonite is strongly affected by the packing density of the organic coating. Through-diffusion experiments verified that the retardation capacity of the organoclays increases with the increasing amount of both organic cations on the clay surface. Observed NO3− diffusion in HDTMA+- and HDPy+-bentonite is in a similar range. Further, classical molecular dynamics studies, performed on sets of HDTMA+- and HDPy+-montmorillonite models, explained a molecular mechanism of the diffusion process of the nitrate anion at the organoclay–water interface. The molecular simulations showed comparable trends of the dependence of the NO3− diffusion coefficient on the content of organic cations on the surface as observed in experiment. Calculations explained how the organic cations are arranged on the surface and revealed that the positively charged head groups of the organic cations play a key role in the decreasing of the diffusivity of nitrate ions.