| Abstract
| - After tagging with Mn, Co, Fe, Zn, Ag, Cd, and Cs,colloidsof the dissolved phase (<0.45 μm) of three samples of ariver water were investigated by way of the amount oftrace elements associated with them. Using coupledfrontalcascade ultrafiltration and chromatographic separations,the size distribution of colloids and the affinity of thecolloidal organic matter for the trace elements wereinvestigated. The size distribution was determinedusingmembranes with a nominal molecular cutoff of 100, 10,and 1 kDa (1 kDa = 1000 daltons). For each fraction,thetrace elements associated with organic complexes werestudied by the selective isolation of complexing speciesusinga macroporous resin (Acell plus QMA, Sep-Pak, Waters).The mass of colloids in the >10-kDa fractions of onesample was measured from the concentrate of 200 L.Thisallowed the determination of a distribution coefficient(Kc) between colloids and the “true” dissolved phase(10-kDa filtrate). Depending on the element, thepercentageof colloids in each size class indicated that Cs wasmainlypresent in the 1-kDa filtrate, in contrast to Fe and Ag,which were mainly associated with colloids of large size(>100 kDa). Cd, Zn, Co, and Mn were distributedamongstall the colloidal fractions, including the 1-kDa filtrate.Incomparison to the distribution of the trace elements inthe different inorganic colloid size classes (using a UV-irradiated natural sample), the distribution of traceelementsin the >1-KD fractions of natural samples appeared tobe controlled by organic matter. This was particularlyevidentfor Co and Cd and, to a lesser extent, for Mn. UsingtheQMA resin, it appeared that the variable distribution oftrace elements in the different colloid size classes isrelatedto their affinity with the dissolved organic matter.Theaffinity depends on the nature and/or on the distributionoffunctional groups in the natural organic matter isolated(about 85% of the total <0.45 μm) or not isolated by thisresin.The reactivity of the colloidal phase has beendeterminedusing Kc for three different colloid concentrations of thesame sample. Kc decreased as the concentrationincreased, indicating an apparent so called “particleconcentration effect”.
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