R. Kretzschmar et al., FLOCCULATION OF KAOLINITIC SOIL CLAYS - EFFECTS OF HUMIC SUBSTANCES AND IRON-OXIDES, Soil Science Society of America journal, 57(5), 1993, pp. 1277-1283
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.