EFFECTS OF PH, IONIC-STRENGTH AND A FULVIC-ACID ON SIZE DISTRIBUTION AND SURFACE-CHARGE OF COLLOIDAL QUARTZ AND HEMATITE

Size distribution of colloidal quartz and hematite (initial diameter 140-190 nm and 70-360 nm, respectively) were determined by Photon Correlation Spectroscopy (PCS) in well-defined laboratory systems. Effects on the mean diameter and the size distribution were measured at various values of pH (2-12) and ionic strength (0.001-0.7 mol/1) in an inert medium (NaClO4). The effect of an aquatic fulvic acid on the stability of the colloids was included in the study. In the absence of fulvic acid the colloids flocculate at a pH close to pH(zpc) of the respective mineral. This increase in mean diameter is due to electrostatic interactions. Hematite colloids are also destabilised at a pH much below pH(zpc). The quartz colloids are stabilised in the presence of fulvic acid. The hematite/fulvic acid systems are stable at high pH, but flocculate at low pH. Increasing ionic strength induces agglomeration of the quartz colloids at 0.1 mol/l, independently of the presence of fulvic acid (2 mg/1). Hematite is destabilised at 0.02 mg/l in the absence of fulvic acid, while in the presence of fulvic acid the colloids aggregate at ionic strength >0.07 mol/l.