ORGANIC-MATTER ROLE IN ILLITIC SOIL COLLOIDS FLOCCULATION .2. SURFACE-CHARGE

The ability to predict the effect of various factors on soil flocculation depends on a knowledge of the mechanisms by which those factors operate. The purpose of this study was to investigate the mechanisms that control the effects of organic matter content, pH, and bivalent cation type on soil flocculation. The electrophoretic mobility of an illitic soil before and after treatment with H2O2 was measured at a soluble Ca or Mg charge fraction of 0.0 to 0.9 at pH 6, and at pH 4 to 9 and a soluble Ca charge fraction of 0.0 or 0.5 The change in the net particle charge as pH was increased from 6 to 8 at a soluble Ca concentration of 0.65 and 2.26 mol m(-3) was determined from proton and Ca ion titrations. Electrophoretic mobility was independent of both Ca vs. Mg and organic matter content, which suggests that the influence of these factors on flocculation was caused by steric effects. The effect of pH on soil flocculation and electrophoretic mobility in the presence or absence of organic matter, on the other hand, was consistent with an electrostatic mechanism. The change in net particle charge from pH 6 to 8 became more positive as the soluble Ca concentration was increased, and this result was modeled in terms of competitive binding of H+ and Ca2+ by soil functional groups.