THE ROLE OF ORGANIC-MATTER AND IONIC COMPOSITION IN DETERMINING THE SURFACE-CHARGE OF SUSPENDED PARTICLES IN NATURAL-WATERS

The surface charge behaviour of riverine and estuarine particles from the Yarra River system (Australia) was monitored using particle microelectrophoresis. The surface charge was invariably negative and decreased in magnitude as the ionic strength of the water increased. Laboratory mixing experiments involving river water and salt solutions revealed that the minor cations in seawater (Ca2+ and Mg2+) were much more effective than the major cation (Na+) in reducing the magnitude of the surface charge of suspended particles during estuarine mixing. The charge on the particles appears to be controlled by an adsorbed layer of natural organic matter and the concentration of divalent cations in the water. Evidence for this was found by the fact that the charge behaviour of both freshwater and estuarine particles can be modelled closely by a goethite colloid coated with an adsorbed layer of humic substances. © 1990.