Fractionation and composition of colloidal and suspended particulate materials in rivers

The association of pollutants (nutrients, heavy metals and organic compound s) with colloidal and suspended particle matter (SPM) plays a dominant role in determining their transport, fate, biogeochemistry, bioavailability and toxicity in natural waters. A scheme for the fractionation and composition of colloidal and SPM from river waters has been tested. All four separatio n methods, i.e. sieving, continuous flow centrifugation, tangential flow fi ltration, sedimentation field-flow fractionation, were for the first time u sed to separate five size particulate fractions from river. Significant (gr am) amounts of colloidal material (<1 mu m) in three size ranges, nominally 1-0.2, 0.2-0.006 and 0.006-0.003 mu m were obtained. The separation scheme was able to process large samples (100 l), within reasonable times (1 day) and the apparatus was portable. The aquatic colloid size was also characte rized with high resolution by using sedimentation field-flow fractionation technique. The mass-based particle size distribution for the water sample s howed a broad size distribution between 0.05 and 0.4 mu m with the maximum around 0.14 mu m. There was a systematic increase in the content of organic carbon (estimated by loss on ignition), Mg, Ca, Na, Cu and Zn with decreas ing particle size, highlighting the importance of the colloidal (<1 mu m) f raction. It was concluded that the colloidal Cu and Zn concentrations in ri vers might be much higher than those reported before. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.