PNNL

Abstract

An electrodynamics-based model was formulated for simulation of ion diffusion in microbial polysaccharides with fixed charges and elecrostatic double layers. The model extends a common multicomponent ion diffusion model that is based on irreversible thermodynamics under a zero ionic charge flux condition, which is only applicable to the regions without fixed charges and electrostatic double layers. An efficient numberical procedure was presented to solve the differential equations in the model. The model well described key features of experimental observations of ion diffusion in negatively charged microbial polysaccharides including accelerated diffusive transport of cations, exclusion of anions, and increased rate of cation transport with increasing negative charge density. The simulated diffusive fluxes of cations and anions were consistent with a classic exchange diffusion concept in negatively charged polysaccharides at the interface of plant roots and soils; and the developed model allows to mathematically study such diffusion phenonmena. Numerical simulations also showed that ion diffusive transport within a becterial cell wall polysaccharide may induce an ionic current that compresses or expands the bacterial electrostatic double layer at the interface of the cell wall and bulk solution.