In Situ Measurements of Metal Complex Exchange Kinetics in Freshwater

Abstract

Trace metals were measured in situ in a freshwater river draining a peat catchment (DOC = 15 mg L(-1)) using diffusive gradients in thin-films (DGT) devices with a range of gel layer thicknesses (0.16-2.0 mm). The reciprocal of the accumulated mass of each metal varied linearly with the thickness of the diffusive layer. These plots allowed calculation of the thickness of an apparent diffusive boundary layer (ADBL). A constant value was obtained from the plots of Cd, Pb, and Zn. The observed increase in the ADBL for the other metals (Mn<Co<Ni<Cu<AI<Fe) was consistent with increasing kinetic limitation of metal supplied from complexes with humic substances. The results could be presented as a kinetic signature, which reflects the nature of the binding ligands available in the water. A theoretical basis for deriving kinetic constants from the DGT measurements was developed. The derived association rate constants for Ni, Mn, and Cu were consistent with values calculated by assuming that the Eigen mechanism holds and that the mean stability constants of the complexes are related to the binding constants of each metal with fulvic acid. These first in situ measurements of dissociation rate constants provided larger values than those obtained by different techniques in the laboratory, suggesting that trace metal complexes in natural waters may be more labile than previously thought.