Trace metal speciation predictions in natural aquatic systems: incorporation of dissolved organic matter (DOM) spectroscopic quality
Kristin K. Mueller A , Stephen Lofts B , Claude Fortin A and Peter G. C. Campbell A C
+ Author Affiliations
- Author Affiliations
A INRS Eau Terre et Environnement, Université du Québec, 490 de la Couronne, Québec QC, G1K 9A9, Canada.
B Centre for Ecology and Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, LA1 4AP, United Kingdom.
C Corresponding author. Email: peter.campbell@ete.inrs.ca
Environmental Chemistry 9(4) 356-368 https://doi.org/10.1071/EN11156
Submitted: 13 December 2011 Accepted: 5 June 2012 Published: 20 August 2012
Journal Compilation © CSIRO Publishing 2012 Open Access CC BY-NC-ND
Environmental context. To assess the risk posed by environmental contaminants such as metals, one needs to be able to identify the key chemical species that prevail in natural waters. One of the recognised stumbling blocks is the need to quantify the influence of heterogeneous dissolved organic matter (DOM). Here we explore the possibility of using the optical signature of DOM to determine its quality, to alleviate the need to make assumptions about its metal-binding properties and to improve the prediction of trace metal species distributions in natural waters.
Abstract. To calculate metal speciation in natural waters, modellers must choose the proportion of dissolved organic matter (DOM) that is actively involved in metal complexation, defined here as the percentage of active fulvic acid (FA); to be able to estimate this proportion spectroscopically would be very useful. In the present study, we determine the free Cd2+, Cu2+, Ni2+ and Zn2+ concentrations in eight Canadian Shield lakes and compare these measured concentrations to those predicted by the Windermere Humic Aqueous Model (WHAM VI). For seven of the eight lakes, the measured proportions of Cd2+ and Zn2+ fall within the range of values predicted by WHAM; the measured proportion of Cu2+ falls within this range for only half of the lakes sampled, whereas for Ni, WHAM systematically overestimated the proportion of Ni2+. With the aim of ascribing the differences between measured and modelled metal speciation to variations in DOM quality, the percentage of active FA needed to fit modelled and measured free metal concentrations was compared with the lake-to-lake variation in the spectroscopic quality of the DOM, as determined by absorbance and fluorescence measurements. Relationships between the percentage of active FA and DOM quality were apparent for Cd, Cu, Ni and Zn, suggesting the possibility of estimating the percentage of active FA spectroscopically and then using this information to refine model predictions. The relationships for Ni differed markedly from those observed for the other metals, suggesting that the DOM binding sites active in Cd, Cu and Zn complexation are different from those involved in Ni complexation. To our knowledge, this is the first time that such a distinction has been resolved in natural water samples.
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