FLOCCULATION OF KAOLINITIC SOIL CLAYS - EFFECTS OF HUMIC SUBSTANCES AND IRON-OXIDES

Dispersion of clay affects soil physical properties and can result in clay transport with surface runoff or with percolating water moving th rough soil macropores. The water-dispersible fine clay (<0.2 mu m) is probably the most mobile fraction with percolating water. This study w as conducted to characterize the mineralogy and flocculation behavior of water-dispersible fine clays isolated from surface horizons (Ap) of three Ultisols in North Carolina, and to study the effects of treatin g the clays for removal of organic substances and Fe oxides. Critical coagulation concentrations (CCC) of dilute suspensions (0.5 g clay L(- 1)) in CaCl2 and KCl solutions were determined for: (i) untreated clay s; (ii) clays treated with NaOCl for removal of organic. substances; a nd (iii) clays treated with NaOCl and dithionite-citrate-bicarbonate f or removal of organic substances and Fe oxides. Suspension pH was vari ed between 4.5 and 9.0. Analysis by x-ray diffraction (XRD), thermogra vimetry, and Mossbauer spectroscopy showed that the mineralogy of the clays was dominated by kaolinite and hydroxy-Al-interlayered 2:1 clay minerals (HIM), with smaller amounts of gibbsite and poorly crystallin e, Al-substituted Fe oxides (hematite and goethite). The untreated cla ys contained between 17 and 32 g total C kg(-1), and difference infrar ed spectra between the untreated and NaOCl-treated clays were typical for humic substances. Treat. ment of the clays for removal of organic substances markedly decreased the CCC values in both CaCl2 and KCl sol utions in the entire pH range studied. Treatment for removal of Fe oxi des had little effect on CCC values. All fine clays exhibited high ele ctrophoretic mobilities (u) at pH 6 (+/- 0.6) in 0.001 M CaCl2, with u ranging between -2.95 x 10(-8) and -5.11 x 10(-8) m(2) s(-1) V-1. For two of the soil clays, u was not clearly affected by either of the cl ay treatments, indicating that there was little change in surface char ge density. The results of this study strongly support the hypothesis that naturally occurring humic substances increase the colloidal stabi lity of kaolinitic soil fine clays in aqueous suspensions. The data su ggest that a combination of electrostatic stabilization and steric sta bilization is responsible for this effect, with steric stabilization p ossibly being the more important mechanism.