Size Fractionation (Dissolved, Colloidal and Particulate) of Trace Metals in the Thur River, Switzerland

Abstract

The distribution of some trace metals (Cu, Zn, Ni, Co,Fe, Mn) and of DOC over a particulate (> 1 μm),a colloidal (size < 0.45 μm and molecular weight > 10 kD) and an ultrafiltered fraction (< 10 kD)was determined at several sites on the Thur River,Switzerland, at various times of the year. Thecomplexation of Cu by strong ligands in theultrafiltrate and in the conventional filtrate (<0.45 μm) was compared using a ligand-exchange/CSV method.

The <0.45 μm concentrations of Cu (from anaverage of 7 nM to 24 nM), Zn (<5–23 nM), Ni (5–13 nM),Co (1.5–3 nM) and Mn (7–92 nM)increased downstream. The major part of Cu, Zn, Niand Co usually occurred in the ultrafiltratefraction at all sites, whereas Fe and Mn were mostlyin the particulate fraction, under conditions of lowsuspended matter content (< 10 mg L^-1) in theriver. The percentage of metal in the colloidalfraction, with respect to the 0.45-μm filtrate,decreased in the order: Cu (median 11%) > Zn ≈Ni(median 5–6%) > Mn ≈ Co (median < 5%). DOCalso consisted mostly of molecules in the < 10 kDrange.

Cu was strongly complexed by natural organic ligandsin all filtrate and ultrafiltrate samples. A largepart of the strong Cu binding ligands consisted ofcompounds in the < 10 kD range, but colloidalligands with similar properties also occurred. Cu wasdistributed among the dissolved and the colloidalligands, roughly in proportion to organic carbon.The colloidal fraction (as defined here) did notincrease in its proportional amount downstream and wasonly of limited significance in transporting traceelements in the Thur River under low discharge conditions.