The relevance of ligand exchange kinetics in the measurement of iron speciation by CLE-AdCSV in seawater

CLE-AdCSV (competitive ligand exchange-adsorptive cathodic stripping voltammetry) has proved to be a decisive technique in recognising the key role that organic matter plays in modulating the biogeochemical cycle of iron in aquatic ecosystems. As the method requires equilibrium among all the constituents of the solution before measurements are taken, accurate knowledge of the kinetics of the undergoing ligand exchange is essential. The kinetics of the process has been most frequently described in terms of a dissociative (or Eigen-Wilkins) mechanism, with some authors even claiming that, under such approach, iron complexes would dissociate during CLE so slowly that the method would be invalidated. The complexity of the processes largely exceeding the simple mechanistic description of the Eigen-Wilkins mechanism is discussed on the basis of a broad overview of possible reaction routes. Evidence of the role of matrix components of natural waters is presented and the impossibility of exporting kinetic constants to systems characterised by different molecular mechanics, mostly ignored in previous studies, is highlighted. Experimental evidence obtained from various methodological approaches allows inferring the range of CLE equilibration times to be applied in CLE-AdCSV measurements and further proves that the Eigen-Wilkins mechanism is just one of the many reaction routes applicable in natural waters. Studying iron CLE mechanisms for proposed competing ligands before designing any experiment using CLE-AdCSV is recommended. (C) 2014 Elsevier B.V. All rights reserved.