Flocculation of Kaolinitic Soil Clays: Effects of Humic Substances and Iron Oxides

Abstract

AbstractDispersion of clay affects soil physical properties and can result in clay transport with surface runoff or with percolating water moving through soil macropores. The water‐dispersihle fine clay (<0.2 µm) is probably the most mobile fraction with percolating water. This study was 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 treating 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 clays; (ii) clays treated with NaOCl for removal of organic substances; and (iii) clays treated with NaOCl and dithionite‐citrate‐bicarbonate for removal of organic substances and Fe oxides. Suspension pH was varied between 4.5 and 9.0. Analysis by x‐ray diffraction (XRD), thermogravimetry, and Mössbauer 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 crystalline, Al‐substituted Fe oxides (hematite and goethite). The untreated clays contained between 17 and 32 g total C kg−1, and difference infrared spectra between the untreated and NaOCl‐treated clays were typical for humic substances. Treatment of the clays for removal of organic substances markedly decreased the CCC values in both CaCl2 and KCl solutions in the entire pH range studied. Treatment for removal of Fe oxides had little effect on CCC values. All fine clays exhibited high electrophoretic mobilities (u) at pH 6 (±0.6) in 0.001 M CaCl2, with u ranging between ‐2.95 × 10−8 and −5.11 × 10−8 m2 s−1 V−1. For two of the soil clays, u was not clearly affected by either of the clay treatments, indicating that there was little change in surface charge density. The results of this study strongly support the hypothesis that naturally occurring humic substances increase the colloidal stability of kaolinitic soil fine clays in aqueous suspensions. The data suggest that a combination of electrostatic stabilization and steric stabilization is responsible for this effect, with steric stabilization possibly being the more important mechanism.