EFFECTS OF ADSORBED HUMIC-ACID ON SURFACE-CHARGE AND FLOCCULATION OF KAOLINITE

The behavior of clays in soils, groundwater aquifers, and aquatic envi ronments is controlled to a large extent by flocculation and dispersio n phenomena. Dispersed clay particles can become mobile and facilitate the translocation of strongly sorbed pollutants in the environment. I n this study, the adsorption of a soil,humic acid to Na-kaolinite and the resulting effects on surface charge and colloidal stability of kao linite were investigated in dilute aqueous suspensions. The pH depende nce (pH 3-11) of humic acid adsorption to kaolinite in a NaClO4 backgr ound electrolyte was studied with batch experiments at three different ionic strengths (0.001, 0.01, and 0.1 M). Sorption of humic acid incr eased with decreasing pH or increasing ionic strength, a behavior that is typical for anionic polyelectrolytes with carboxylic functional gr oups. 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 NaClO4). With increasing amounts of humic acid sorbed to the kaol inite surface, the electrophoretic mobility was continuously shifted t o more negative values. At low pH, this resulted in charge reversal fr om positive to negative net total particle surface charge. Additions o f 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 ed ge-to-face flocculation. In addition, a general increase in negative s urface charge density and steric stabilization effects may contribute to the influence of adsorbed humic acid on the colloidal stability of kaolinite.