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

Abstract

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.

Metal speciation from stream to open ocean: modelling v. measurement

“…Agreements for Co II are especially poor, perhaps because much of the metal was present as Co III in marine samples. [16] The variable n/[M] is poorly predicted for Cd in the open ocean.…”

“…The data set is summarised in Tables 1 and S1, with further details in references. [14][15][16] Chemical speciation modelling We used WHAM [10] incorporating Humic Ion-Binding Model VII [18] to perform the speciation calculations; previously, the open-ocean calculations [16] were done using Model VI. [17] Models VI and VII use structured formulations of discrete, chemically plausible binding sites for protons in humic and fulvic acids (HA, FA), in order to allow the creation of regular arrays of bidentate and tridentate binding sites for metals.…”

“…For estuarine and coastal waters, some missing DOC concentrations had to be estimated from other studies carried out at the same location, [15] and for open ocean samples, [DOC] was taken to be the value at the nearest location for which data were available. [16] The binding activity of DOM was estimated by assuming DOM to be 50 % carbon, and that 65 % of the DOM behaves like isolated FA whereas the rest is inert. [36,37] For example, a DOC concentration of 5 mg L À1 corresponds to [DOM] of 10 mg L À1 , and so the concentration of FA for modelling is 6.5 mg L À1 .…”

“…A less direct but more widely applicable approach is the comparison of analytical results with outputs from the same model, and this also provides a test of the model. We have previously performed three such studies focussing on data from fresh waters, [14] estuarine and coastal waters [15] and the open ocean, [16] and comparing them with predictions obtained using WHAM6 [17] or WHAM7. [18] We found that measurements and model predictions based only on metal binding by humic-type ligands (i.e.…”

Lead solubility in seawater: an experimental study

“…More complex activity corrections based on Pitzer's equations [14] are appropriate but are not available as part of geochemical speciation codes such as Visual Minteq. Stockdale et al [14] compared the extended Debye-Hückel and Pitzerbased activity corrections for the calculation of inorganic lead speciation in seawater and found less than 16 % difference between the calculated free ion activities using the two approaches. This level of uncertainty was regarded as being acceptable for chemical modelling as applied in the present study.…”