CHARACTERIZATION OF THE SUBMICROMETER PHASE IN SURFACE WATERS - A REVIEW

The need for improved knowledge about the composition and behaviour of solid matter in the colloidal size range (1 nm to 1 mu m) in natural waters has been identified by several authors in the literature. The increased interest in these colloids is due to their large impact on the transport, redistribution and bioavailability of numerous chemical compounds (such as trace metals, nutrients and organic pollutants). This paper presents a review of different analytical techniques available for determination of some of the properties of interest (e.g., colloid concentration, size distribution, mineral composition and surface properties). It is obvious that most methods suffer from limitations and that the only feasible way to overcome these problems is by using a combination of different techniques. Sampling in a Swedish lake, as an example, illustrates one promising combination including light scattering and gravimetric analysis for determination of colloid concentration, photon correlation spectroscopy and scanning electron microscopy for size distribution measurements and energy dispersive microscopy; powder X-ray diffraction and infrared spectroscopy in order to obtain information about mineral constituents and general surface properties. Size fractionations were performed by filtration and centrifugation to gain information regarding the distribution of metals and organic matter between dissolved and solid phases. The study indicated, for instance, a seasonality of the size distribution of the colloidal matter, with smallest colloids (range 120-340 nm) found in spring, while the colloid samples during summer and autumn have a size distribution in the range 280-700 nm. This result also illustrates the limitations when using filters with, e.g., a pore size of 0.45 mu m for the distinction between particulate and dissolved substances.