Colloidal carrier substances : Properties and impact on trace metal distribution in natural waters

Sammanfattning: Colloidal (<0,45 J.Im) and suspen~ed matter (>0,45 J.Im) are important constituents in the aquatic environment, due to their impact on the mobility and bioavailability of associated components (e.g. metals). The aim of the present study was to improve the understanding of processes that control the stability of colloidal particulate matter as well as of themechanisms that govern the distribution of elements (e.g. heavy metals) between suspended solids and water in the aquatic environment.Two different methods for isolation and characterization of the colloidal phase were improved and applied in surface and groundwaters as well as in laboratory systems. Fractionation of the colloidal phase with screen filters in the range 5-0.015 J.Im required a careful control of the conditions, in terms of maximum loading capacity, pH, gas pressure, redox state etc. for each pore size in order to maintain the original sizedistribution and composition of the colloidal phase. Technique for measurements of size distribution and concentration of the solid fraction by photon correlation spectroscopy was improved, and it was demonstrated that this technique can be used for quantitative characterisation (size distribution and concentration) of colloidal matter in natural aquatic systems even at low concentrations ( < I mg/1).The results from well-defined laboratory experiments demonstrated that the stability (size distribution) of the colloidal phase and its affinity for dissolved trace elements were effected by the geologic composition of the colloids, hydrochemical parameters like pH, total salinity and the presence of organic matter (fulvic acid). The adsorption of trace metals was less influenced by the stability of the colloids but dependent on the formation of surface coatings and the chemical distribution of dissolved species.Studies of colloidal suspended matter in a surface water (Lake Risten, Bersbo, Sweden) illustrated the impact of water discharge on both concentration and size distribution of the colloidal phase. Decreasing water velocity in the inlet to the lake resulted in a reduction in colloid concentration (from above 10 mg/1 to below 1 mg/1) as well as in an increase in average diameters (from 230 nm to 950 nm). The latter was attributed to precipitation of AI, Fe and Mn hydrous oxides in combination with adsorption/coprecipitation of organic matter on a clay matrix. A seasonality in particle diameters as well as in the distribution of associated trace metals was observed. The average diameters of the colloids varied within the range 200-1160 nm, with largest particles in the summer. The fraction of cadmium associated with the colloidal phase in the lake varied within the range 11-76% with an average of 26%. Corresponding values for zinc and copper were 3-30% and 0-54% with average values of 14% and 20%, respectively.