Distribution of trace elements associated with dissolved compounds (<0.45 μm-1 nm) in freshwater using coupled (frontal cascade) ultrafiltration and chromatographic separations

After tagging with Mn, Co, Fe, Zn, Ag, Cd, and Cs colloids of the dissolved phase (<0.45 mu m) of three samples of a river water were investigated by way of the amount of trace elements associated with them. Using coupled frontal cascade ultrafiltration and chromatographic separations, the size distribution of colloids and the affinity of the colloidal organic matter for the trace elements were investigated. The size distribution was determined using membranes with a nominal molecular cutoff of 100, 10, and 1 kDa (1 kDa = 1000 daltons). For each fraction, the trace elements associated with organic complexes were studied by the selective isolation of complexing species using a macroporous resin (Acell plus QMA, Sep-Pak, Waters). The mass of colloids in the >10-kDa fractions of one sample was measured from the concentrate of 200 L. This allowed the determination of a distribution coefficient (Kc) between colloids and the "true" dissolved phase (10-kDa filtrate). Depending on the element, the percentage of colloids in each size class indicated that Cs was mainly present 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 distributed amongst all the colloidal fractions, including the 1-kDa filtrate. In comparison to the distribution of the trace elements in the different inorganic colloid size classes (using a UV-irradiated natural sample), the distribution of trace elements in the >1-KD fractions of natural samples appeared to be controlled by organic matter. This was particularly evident for Co and Cd and, to a lesser extent, for Mn. Using the QMA resin, it appeared that the variable distribution of trace elements in the different colloid size classes is related to their affinity with the dissolved organic matter. The affinity depends on the natura and/or on the distribution of functional groups in the natural organic matter isolated (about 85% of the total <0.45 mu m) or not isolated by this resin. The reactivity of the colloidal phase has been determined using Kc for three different colloid concentrations of the same sample. Kc decreased as the concentration increased, indicating an apparent so called "particle concentration effect".