Effect of inorganic electrolytes on the stability of allophane in aqueous solutions of anionic xanthan polysaccharide

The polymeric (a) weak-stabilization (PWST), (b) floccelation (PFL), and (c) stabilization (PST) of ailophane with the anionic xanthan (GX) polysaccharide (He and Horikawa 1996a, b: Soil Sci. Plant Nutr., 42, 603-612, 637-644) were investigated further with respect to the effect of 1-1, 2-1, and 1-2 electrolytes (MA). At pH 4.5, the PST was (i) high in 0.1 mM NaCl, MgCl₂, ZnCl₂, and PbCl₂ solutions, whereas (ii) considerably low in 100 mM NaCl, 0.1 mM Na₂SO₄ and NaH₂PO₄ solutions, respectively. At pH 6.5 (i.e.p.), the flocculated ailophane showed (i) a very low PWST in 0.01 mM NaCl, MgCl₂, and Na₂SO₄ solutions, and also (ii) a low PST in 0.01 mM MA except for NaH₂PO₄. In 0.01 mM PbCl₂ and NaH₂PO₄ solutions, ailophane was well dispersed, respectively, and marked PFL and PST were observed with further addition of xanthan. The changes and decrease of PWST and PST were discussed in relation to the iocculation of ailophane due to the (a) effect of MA on the diffuse double layer of the particles, (b) specific adsorption of phosphate and lead ions on the surface, and (c) weakening of the repulsive force between anionic GX chains by MA.