Interactions of biofilms with dissolved and suspended aquatic matter play an important role in many water work technologies. In order to get a better insight into the driving forces for these interactions, the adsorption of polysaccharides to kaolin clay was studied. The linear adsorption coefficients, obtained at various pH levels, were all substantially lower than those obtained in other studies for humic materials(HA) at pH 6 k=60ml/g for polysaccharide, but for HA k=700ml/g. This is explained in light of the higher hydrophobicity of humic materials. The adsorption coefficient for polysaccharides increases with ionic strength, with the addition of calcium ion, and when the pH is reduced. All these phenomena are explained by conformational changes, of the polysaccharide macromolecules, which result in respective changes in its electric charge. These phenomena are also in accord with the results obtained by determination of the micro-electrophoretic mobility of the clay and clay-polysaccharide particles. For instance, at reduced pH levels the enhanced adsorption was accompanied by decreasing of the negative electrophoretic mobility. Both of these results point to a reduced electric density of polysaccharide and clay-polysaccharide particles when the pH is reduced. All these results may also explain the effect of polysaccharides on the clay coagulation-flocculation by alum.

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