Colloidal particles play an important role in the distribution of pollutants in natural aquatic systems because they may sorb significant quantities of both inorganic and organic substances due to their large surface areas relative to their masses. Currently, only a limited amount of data is available on particle number and size distributions of natural submicron particles, owing to the many problems involved in their experimental determination. In the present paper, experimental results on particle size distribution and characterization of submicron colloidal particles from two different natural water systems will be discussed and compared with theoretical predictions based on a classical coagulation/sedimentation model. This theoretical model is well suited to describe the behaviour of particles greater than 100 nm but fails to predict the existence of colloids smaller than this. The existence of such small colloids have been observed by high resolution TEM. They seem to be embedded in large organic matrices, and probably for this reason they have neither been detected by other experimental techniques nor predicted by classical coagulation models. Improvement of the measurement techniques currently available as well as development of the theory necessary to describe this type of colloid-organic matrix associations are therefore definitely needed.

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