Kinetics of Trace Element Complexation with Suspended Matter and with Filterable Ligands in Freshwater

Using tagged river suspended matter with trace elements (Mn, Fe, Co, Zn, Ag, Cd, and Cs), the kinetic binding of these elements onto particles as well as the kinetics of the speciation within the filterable phase (<0.45 μm) were investigated. Quasi in-situ experiments have been 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 partitioning of trace elements between the dissolved and the particulate phase was determined by the usual distribution coefficient ( K d ), and the speciation in the filterable phase has been studied from the selective extraction of the dissolved complexing species using macroporous resins (Sep-Pak: C18, NH2, and QMA). A seasonal variation of the distribution coefficient vs time K d ( t ) was observed, as shown by the significantly higher K d ( t ) values in summer for Mn, Co, and Fe than in winter and conversely for Cd, Zn, Cs, and Ag. The removal of Mn, Co, and Fe appeared controlled by the binding (and subsequent oxidation) with particulate organic matter or biogenic particles. The Mn−Co K d ( t ) values covariation and their linearization using an overall first-order reaction model indicated that a single and common kinetic process is controlling the removal of Mn and Co in winter while (at least) two kinetic processes are involved in summer. Binding was also found to be controlled by several kinetic processes for Fe, Cd, and Cs for both seasons. Using the above resins, some features of the speciation of trace elements in the filterable phase have been pointed out. Except for Cs, elements were chiefly and rapidly complexed by filterable organic ligands. According to the element, these ligands belong to distinct types of organic matter sometimes corresponding to a small percentage of the total filterable organic species. A gradual and total change of the speciation 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.