Particles of clay minerals were studied due to their importance carrying capacity of contaminants (4). In nature they always in geochemical processes in natural waters, such as adsorption exist in mixtures of clays, quartz, carbonates, and metal and transfer of ionic contaminants, stabilization by organics, and oxides, particularly aluminum, iron and manganese. This flocculation and sedimentation phenomena. Information on the paper is an investigation of two typical, pure, single clay behavior of clays was sought by experiments with model systems. minerals: two models that contribute to the understanding Measurements of electrophoretic mobilities in relation to pH, at of behavior of clays in the environment. varying concentrations of well-characterized fulvic acid (FA), As a physical model clay minerals attract attention due to were performed on two structurally well defined, representative their heterogeneity of surfaces. The basic model of clay clay minerals prepared with clean surfaces: ripidolite (a wellknown trioctahedral nonswelling chlorite) and beidellite (a typical surfaces was originally presented by van Olphen (5) for dioctahedral smectite). Natural ripidolite and beidellite show high kaolinite and was supported and elaborated on in many subnegative electrokinetic potentials in the range pH 2 (010 and 020 sequent and recent investigations (2, 6-8). The model has mV, respectively) to pH 10 (060 and 050 mV, respectively). come to be known as the Constant Basal Surface Charge Experiments utilizing mechanical particle disintegration (dry mill-(CBSC) (7) model. The basal surface (siloxane surface) of ing), mimicking natural wear and physical weathering, resulted a clay mineral carries a constant negative charge which is in increases of specific surface area (12.3 and 1.5 times, respecattributed to the isomorphous substitution of Si 4/ by Al 3/ tively) and of cation exchange capacity (3.2 and 1.2 times, respecand which is pH independent. The charge of broken bonds tively). Such small-sized particles, shown by SEM figures, retain (edges) depends on pH and shows an amphoteric character. their crystal structure (X ray) and the nature of their structural It arises from protolytic equilibrium, adsorption and desorpbonds (FTIR), exhibiting an IEP (at pH 6.0 and 3.0, respectively).
tion. The hydroxyl groups at the edges are considered to be This was interpreted to be the creation of positively charged edge surfaces. Exposed to fulvic acid in solutions of 10 03 NaCl at pH the major reactive sites (9) in clay surfaces. The fraction of Å 6.5, these new surfaces showed an increase in negative z-potenthe edge surfaces in the total surface area varies depending tial for ripidolite, and, to a smaller extent, for beidellite. In the on the type of the clay mineral. For kaolinite this fraction interaction of clay mineral particles with aqueous medium, it is is 12% (10) to 14% (11), while for smectites it is no more concluded that the degree of mechanical wear is more decisive than 1% (12). For chlorite minerals there are no data in the than the type of the mineral.