The behavior of clays in soils, groundwater aquifers, and aquatic environments is controlled to a large extent by flocculation and disper- sion phenomena. Dispersed clay particles can become mobile and facilitate the translocation of strongly sorbed pollutants in the environ- ment. In this study, the adsorption of a soil humic acid to Na-kaolinite and the resulting effects on surface charge and colloidal stability of kaolinite were investigated in dilute aqueous suspensions. The pH dependence (pH 3-11) of humic acid adsorption to kaolinite in a NaClO4 background electrolyte was studied with batch experiments at three different ionic strengths (0.001, 0.01, and 0.1 M). Sorption of humic acid increased with decreasing pH or increasing ionic strength, a behavior that is typical for anionic polyelectrolytes with carboxylic functional groups. Electrophoretic mobility measurements showed that pure kaolinite had positive net total particle surface charge at low pH and negative surface charge at high pH, with an isoelectric point at pH 4.8 (in 0.01 M NaCIO4). With increasing amounts of humic acid sorbed to the kaolinite surface, the electrophoretic mobility was contin- uously shifted to more negative values. At low pH, this resulted in charge reversal from positive to negative net total particle surface charge. Additions of small amounts of humic acid to the kaolinite suspensions resulted in large increases in colloidal stability. At low pH, this was probably due to reversal of edge charge from positive' to negative preventing edge-to-face flocculation. In addition, a general increase in negative surface charge density and steric stabilization effects may contribute to the influence of adsorbed humic acid on the colloidal stability of kaolinite.
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