Effect of Exopolymers on the Liquid Limit of Clays and Its Engineering Implications

Abstract

An experimental study aimed at understanding the interactions between exopolymers exuded by microorganisms and clays, particularly kaolinite, is described. Two biopolymers–-xanthan gum, an anionic bacterial extracellular polysaccharide, and guar gum, a neutrally charged plant polysaccharide–-were used as exopolymer analogs. Liquid limits of a kaolinite clay were measured, with varied biopolymer concentrations and background cations (Ca2+, Na+, or K+) in the pore fluid, to study the influence of exopolymers on clay behavior. Results indicate that the liquid limit of kaolinite generally increases with the biopolymer concentration of the pore fluid as a result of increased viscosity, and the background cation present in the pore fluid alters the liquid limit. Five types of active nanoscale interactions between clay particles, cations, and biopolymers were adopted to interpret the results: biopolymer-induced aggregation of clay particles tends to decrease the liquid limit; polymer cross-linking caused by divalent cations significantly increases the biopolymer solution viscosity and hence the liquid limit; the formation of a clay–polymer interconnected network via cation bridging and hydrogen bonds increases the liquid limit; a reduction in the thickness of the electric double layer on the clay surface decreases the liquid limit; and preferred adsorption of monovalent cations over biopolymer molecules decreases the liquid limit. The variations in the liquid limit reflect the macroscopic response of these interactions competing with each other at the nanoscale. Engineering implications also are discussed on the basis of the experimental results and the clay–biopolymer interactions.