Anionic polysaccharide sorption by clay minerals

An influence of clay mineral composition on C turnover in surface soils is widely assumed but poorly documented. The objective of this study was to evaluate the effect of various clay minerals on polysaccharide sorption under different environmental conditions, including pH, ionic strength, and cation type. Xanthan, an anionic polysaccharide produced by Xanthomonas campestris, was used to represent soil microbial exopolysaccharides. Highly significant effects (P > F < 0.0001) were observed for type of day mineral, pH, xanthan concentration, and electrolyte concentration. Sorption decreased with increase in pH from 3 to 8, consistent with an increase in the negative charge of both the clay surface and xanthan molecules. The presence of 10 mmol L ^-1 Ca(NO ₃) ₂ made sorption possible at pH values above the pKa of xanthan. Divalent cations (Sr ²⁺, Ca ²⁺, and Mg ²⁺) enhanced sorption to a greater degree than monovalent cations (K ⁺, Na ⁺, and Li ⁺) at the same ionic strength, indicating that cations participated in the binding of xanthan to clay surfaces. Generally, sorption was smallest with kaolinite and greatest with a low-charge (0.62 e layer charge per unit cell) smectite where layer charge originated mostly in the tetrahedral positions. Average sorption was two times greater for smectite than for kaolinite, indicating that clay mineral composition influenced polysaccharide sorption; however, contributions may not be significant on a field scale.