Environmental Chemistry Environmental Chemistry Society
Environmental problems - Chemical approaches
RESEARCH ARTICLE

Trace metals in the open oceans: speciation modelling based on humic-type ligands

Anthony Stockdale A C D , Edward Tipping A , John Hamilton-Taylor B and Stephen Lofts A

A Centre for Ecology and Hydrology, Lancaster Environment Centre, Bailrigg, Lancaster LA1 4AP, UK.

B Lancaster Environment Centre (LEC), Lancaster University, Bailrigg, Lancaster LA1 4YQ, UK.

C Present address: School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK.

D Corresponding author. Email: tony@biogeochemistry.org.uk

Environmental Chemistry 8(3) 304-319 http://dx.doi.org/10.1071/EN11004
Submitted: 8 January 2011  Accepted: 23 May 2011   Published: 22 June 2011

Environmental context. Speciation of trace metals in the oceans is typically explained by invoking the concept of metal binding to specific organic ligands. Here, using a speciation model widely used for freshwaters, we assess the extent to which non-specific humic-type ligands found in the ocean may explain chemical speciation of cationic metals. We found that the model can give good fits in some cases, and that experimental results do not give consistent variation from the model. This has implications for the way that the availability of trace elements is considered in ocean environments.

Abstract. The speciation of trace metals in the oceans is typically explained by invoking the concept of metal binding to specific organic ligands, but a lack of detailed knowledge about the ligands has impeded the formulation of comprehensive models to predict speciation chemistry. The aim of our study was to shed further light on the possible role of humic-type ligands in trace metal complexation in the oceans by comparing published seawater (open ocean) speciation measurements with predictions obtained using a speciation model typically used for freshwater and soil systems (Windermere Humic Aqueous Model; WHAM). We show that in some cases, speciation of trace metals in seawater environments may be reasonably predicted using this model with its default parameter set, without any model fitting. The results support the idea that humic-type ligands may account for much of the observed organic binding at least in the cases of Fe, Cu and Pb. Although the model does not consistently provide agreement with the measured values, it provides a useful benchmark to compare different datasets and to examine variation in speciation as a result of varying levels of competing metal ion concentration and fulvic acid activity.


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