Humic Substances and pH Effects on Sodium- and Calcium-Montmorillonite Flocculation and Dispersion

Abstract

Humic substances (HS) are important factors in soil particle associations. Little is known, however, about the mechanism of clay-HS interaction and their effects on aggregation at various pH levels have not been adequately studied to date. In this study, flocculation and dispersion characteristics of homoionic montmorillonite were studied as a function of the exchangeable cation (Na+ and Ca2+), humic acid (HA), and fulvic acid (FA) concentration (0–40 mg L-1), and pH (4, 6, 8, and 10). Flocculation values (FV) of Na-montmorillonite increased with increasing 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 concentration 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 contrast, for suspensions at pH 10, HS concentrations up to 10 mg L-1 had little 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 suspensions, 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-montmorillonite suspensions is explained by interactions between negatively charged HS molecules and the edge charge of the clay. These interactions are affected by pH and by the charge and configuration of the HS macromolecule. 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) interactions among clay platelets are the dominant flocculation mechanisms. At pH values above the PZC of the edge sites, only F-F associations occur. In the presence of HS at pH values below the PZC, negative HS molecules are adsorbed to positive edge sites causing edge-charge reversal (from positive to negative). Consequently, increased electrolyte concentrations are required for flocculation relative to Na-montmorillonite suspensions without HS. The second flocculation mechanism we propose is based on a random distribution of the HS macromolecules among the montmorillonite tactoids. The FV of such a mixture is higher than the FV of montmorillonite suspensions. The term mutual flocculation (heteroflocculation) has been suggested to describe this mechanism.