Desorption of metals from suspended material in the rhine estuary

Abstract

Aliquots of suspended matter collected from Rhine river water were resuspended in distilled water, in diluted artificial sea water (1 : 1), in artificial sea water, and in ‘nitrate sea water’, respectively at pH's 7.5 and 8.0. The desorption of heavy metals into these solutions was calculated in relation to the concentrations as determined by extraction in 4 M hydrochloric acid. The heavy metal concentrations after resuspension and the relative desorption percentages were compared with calculated inorganic metal ion speciation. These comparisons, though quantitatively not quite satisfactory, demonstrated that complex formation is important in desorption processes during estuarine mixing. The order of decreasing desorption of metals into 1 : 1 diluted sea water and sea water in the experiments is : Cd > Zn > Mn > Ni > Co > Cu > Cr; fo for Fe and Pb no desorption was found. Analyses of samples collected at a tidal station at Hoek van Holland show that both suspended matter and filtered water are depleted in most heavy metals. Certainly, in this tidal area suspended matter and adsorbed and dissolved heavy metals do not show a conservative mixing behaviour. In order to correlate experimental results and field obsevartions, large scale flocculation and sedimentation of suspended matter must be accepted at an early stage of estuarine mixing. It is not certain whether this can be proved; future research should include more chemical indicators for testing these estuarine processes.