KINETICS OF TRACE-ELEMENT COMPLEXATION WITH SUSPENDED MATTER AND WITHFILTERABLE LIGANDS IN FRESH-WATER

Using tagged river suspended matter with trace elements ((Mn, Fe, Co, Zn, Ag, Cd, and Cs), the kinetic binding of these elements onto partic les as well as the kinetics of the speciation within the filterable ph ase (<0.45 mu m) were investigated. Quasi in-situ experiments have bee n performed over a period of 120 h on a relatively rich organic river water collected in winter and during a bloom event in summer. The part itioning of trace elements between the dissolved and the particulate p hase was determined by the usual distribution coefficient (K-d), and t he speciation in the filterable phase has been studied from the select ive extraction of the dissolved complexing species using macroporous r esins (Sep-Pak: C18, NH2, and QMA). A seasonal variation of the distri bution coefficient vs time K-d(t) was observed, as shown by the signif icantly higher K-d(t) values in summer for Mn, Co, and Fe than in wint er and conversely for Cd, Zn, Cs, and Ag. The removal of Mn, Co, and F e appeared controlled by the binding (and subsequent oxidation) with p articulate organic matter or biogenic particles. The Mn-Co K-d(t) valu es covariation and their linearization using an overall first-order re action model indicated that a single and common kinetic process is con trolling the removal of Mn and Co in winter while (at least) two kinet ic processes are involved in summer. Binding was also found to be cont rolled by several kinetic processes for Fe, Cd, and Cs for both season s. Using the above resins, some features of the speciation of trace el ements in the filterable phase have been pointed out. Except for Cs, e lements were chiefly and rapidly complexed by filterable organic ligan ds. According to the element, these ligands belong to distinct types o f organic matter sometimes corresponding to a small percentage of the total filterable organic species. A gradual and total change of the sp eciation in time was observed for Mn and Co in winter (and to a lesser extent for Mn in summer). This supports the hypothesis of a transfer of Mn and Co from fast but weak ligands to slower but stronger ligands in the filterable phase.