The surface charge of suspended particles in estuarine and coastal waters

Abstract

The colloidal stability of particles transported in suspension by rivers is controlled by surface electrical properties¹ which are conventionally regarded as undergoing rapid change during estuarine mixing^1–3 by interaction with polyvalent ions, especially Ca²⁺ and Mg²⁺. Although wide differences in rates of coagulation of clay minerals as a function of salinity have been demonstrated in the laboratory^4,5, Gibbs⁶ and others^7,8 have concluded that field studies provide no real evidence for the importance of such differential flocculation. Instead, changes in clay mineral distribution with distance from the source river can be accounted for by differences in the particle size spectra and the resulting sedimentation velocities. The effects of differing electrical and surface properties on coagulation rates are apparently suppressed by the formation of uniform films of metal oxides and/or organic matter around the particles⁶. We present here direct measurements of the electrokinetic charge on river and estuarine particles which confirm this hypothesis.