HUMIC SUBSTANCES AND PH EFFECTS ON SODIUM-MONTMORILLONITE AND CALCIUM-MONTMORILLONITE FLOCCULATION AND DISPERSION

Humic substances (HS) are important factors in soil particle associati ons. Little is known, however, about the mechanism of clay-HS interact ion and their effects on aggregation at various pH levels have not bee n adequately studied to date. In this study, flocculation and dispersi on characteristics of homoionic montmorillonite were studied as a func tion of the exchangeable cation (Na+ and Ca2+), humic acid (HA), and f ulvic acid (FA) concentration (0-40 mg L-1), and pH (4, 6, 8, and 10). Flocculation values (FV) of Na-montmorillonite increased with increas ing concentrations of HS at all pH levels. For pH values of 4, 6, and 8, a sharp increase in FV was observed with increasing HS concentratio n up to 10 mg L-1 (37.5 g kg-1 clay). Only slight increases in FV were observed for HS concentration >10 mg L-1 at these pH values. In contr ast, for suspensions at pH 10, HS concentrations up to 10 mg L-1 had l ittle effect on the FV, but further addition of HS caused an increase in FV up to 141 mmol L-1. In contrast to Na-montmorillonite suspension s, addition of Ca-humate or Ca-fulvate to Ca-montmorillonite resulted in no effect on the FV. The effect of HS on the stability of Na-montmo rillonite suspensions is explained by interactions between negatively charged HS molecules and the edge charge of the clay. These interactio ns are affected by pH and by the charge and configuration of the HS ma cromolecule. In pure clay systems at pH values below the point of zero charge (PZC) of edge sites, edge-edge (E-E) and edge-face (E-F) inter actions among clay platelets are the dominant flocculation mechanisms. At pH values above the PZC of the edge sites, only F-F associations o ccur. In the presence of HS at pH values below the PZC, negative HS mo lecules are adsorbed to positive edge sites causing edge-charge revers al (from positive to negative). Consequently, increased electrolyte co ncentrations are required for flocculation relative to Na-montmorillon ite suspensions without HS. The second flocculation mechanism we propo se is based on a random distribution of the HS macromolecules among th e montmorillonite tactoids. The FV of such a mixture is higher than th e FV of montmorillonite suspensions. The term mutual flocculation (het eroflocculation) has been suggested to describe this mechanism.