Addition of polysaccharides to soils or their formation in situ through microbial activity increases the stability of soil aggregates. Like other components of soil organic matter, polysaccharides are expected to affect strongly the flocculation and dispersion behavior of clays, which are the most important soil component. The influence of polysaccharide concentration, molecule characteristics, and pH, on flocculation and rheological properties of Na-montmorillonite were studied. Flocculation values (FV) increased with increasing concentrations of the anionic polysaccharide polygalacturonic acid (PGA). The pattern was similar at pH 4 and 8: a relatively sharp increase in FV with small additions of PGA up to a concentration of 10 mg L−1 (≈136.0 and 160 mmol L−1) and a slight to moderate increase thereafter. At pH 6 a different pattern was observed. The FV changes at the lower PGA concentrations (up to 10 mg L−1) were similar to those observed for pH 4. At higher concentrations, a further sharp increase in the FV was found. Addition of 1 mg L−1 PGA at pH 10 resulted in a FV increase to 37.0 mmol L−1. For higher PGA concentrations, the FV remained basically unchanged.
Results obtained by measurements of the rheological properties of suspensions to which PGA was added provided additional proof for the clay-PGA interaction and followed similar trends. The Na-montmorillonite suspension exhibited non-Newtonian flow properties. When PGA was added to the Na-montmorillonite suspension, the flow behavior gradually changed from non-Newtonian to Newtonian as the PGA concentration increased. The mechanisms proposed to explain the interactions between Na-montmorillonite and the anionic polysaccharide (PGA) are (i) edge charge reversal and (ii) mutual flocculation (heteroflocculation). These mechanisms are similar to those proposed previously for suspensions of Na-montmorillonite to which humic substances (HS) were added. The dissimilarities are, apparently, a result of the differences between structure and characteristics of the molecules of these organic materials.
Additions of noncharged polysaccharides (dextrans) resulted in completely different responses. Additions of the low molecular weight dextran (T-40, MW 40000) did not result in significant changes in the FVs of Na-montmorillonite. In contrast, two neutral polysaccharides tested (T-500, MW 500,000 and T-2000, 2,000,000) enhanced clay flocculation and consequently caused a sharp decrease in the FV.