Measurement of electrokinetic and size characteristics of estuarine colloids by dynamic light scattering spectroscopy

The combined use of flow-through (0.4 μm) and cross-flow (~1 nm) filtration techniques allowed the transfer of estuarine colloids into electrolyte solutions suitable for their characterization by dynamic light scattering (DLS) measurement techniques. Acid-base titrations monitored by laser doppler electrophoresis revealed the existence of two classes of acid groups having pKa values of 2.6–2.7 and 8.8–9.1 respectively. Similar centres of pKa distributions as well as similar dependencies of pKa on ionic strength were observed on humic acids isolated from the same estuary. This suggests that the (de)protonation behaviour of the natural colloidal material from the Beaulieu estuary is determined by that of humic substances. Measurements of their effective mean size by photon correlation spectroscopy revealed that individual humic molecules were held together in units of 2–300 nm in size, the precise value increasing with time and decreasing with increasing pH or ionic strength. On the other hand, the same measurements on the raw colloidal material gave a completely different picture: only at very low ionic strength (I = 0.006) was there an unambiguous dependence of size on pH; at higher ionic strength (I = 0.06) the combined effect of pH and time was complex. These results suggest that the humic molecules which so profoundly influence the surface properties of Beaulieu estuary colloids are either strongly bound to or irreversibly incorporated into their colloidal carrier.