Trace metal (Cd, Cu, Ni and Pb) partitioning, affinities and removal in the Danshuei River estuary, a macro-tidal, temporally anoxic estuary in Taiwan

Using physical (0.1 and 0.45 μm cut-off filters) and ion exchange (Chelex-100 and AG MP1 resins) separation techniques, detailed distribution patterns and chemical affinities of selected trace metals (Cd, Cu, Ni, Pb) were investigated under different redox conditions in the Danshuei River estuary, a macro-tidal, temporally anoxic estuary, passing though the metropolitan area of Taipei in Northern Taiwan. A distinct partitioning and removal behavior of the different trace metals was revealed, in that under high river flow and oxic conditions, a large fraction of these trace metals that were passing through a < 0.1 μm pore size filter was found to be Chelex-labile, while during the dry season, low river flow and anoxic conditions, elevated fractions of trace metals, especially Cd and Cu, were found to be strongly complexed to inert ligands such as sulfides, with higher percentages present also in the colloidal (0.1–0.45 μm), and particulate (> 0.4 μm) size fractions. Large fractions of both particulate and dissolved metals were removed within the estuary, especially during anoxic conditions in the dry season. Anoxic conditions in the upper Danshuei estuary resulted in very high sulfide concentrations that likely were responsible for the high removal of trace metals as metal-sulfides in the water column during that time. The variations in distribution and fractionation patterns of the different metals were reflected by both their affinities to ion exchange resins and their size distributions.