SIZE-DISTRIBUTION ANALYSIS OF SUBMICRON COLLOIDAL PARTICLES IN RIVER WATER

The particle size spectra of sub-micron colloidal particles collected from a river water, the Water of Leith in Dunedin, New Zealand, have b een determined by transmission electron microscopy (TEM). Particles we re isolated onto hydrophilic carbon films by ultracentrifugation at 30 0,000 g for 1 hr. Size spectra were found to conform to a power-law si ze distribution dn/dr = A r-(B), behaviour also found for much larger- sized particles in natural waters and marine colloids. Values of the e xponent B for unfiltered samples were in the range 2.3 - 2.9, consiste nt with Brownian aggregation as the principal mechanism maintaining th e size spectrum. Comparison with spectra obtained from sampled prefilt ered through Nuclepore 0.4 mu m filters showed clear evidence of parti cle removal down to 100 - 120 nm radius by filtration. Particles exhib ited a range of electron densities under TEM, indicating a mixture of inorganic minerals and organic matter in the colloidal size range. Exa mples of fractal aggregates were also found.