tThis study investigated several knowledge gaps with respect to the diffusive gradients in thin films(DGT) technique for measurement of oxyanions (As(III), As(V), Se(IV), Se(VI), PO43−, and V(V)) usingthe ferrihydrite and MetsorbTMbinding layers. Elution efficiencies for each binding layer were higherwith 1:20 dilutions, as analytical interferences for ICP-MS were minimised. Diffusion coefficients mea-sured by diffusion cell and by DGT time-series experiments were found to agree well and generallyagreed with previously reported values, although a range of diffusion coefficients have been reportedfor inorganic As and Se species. The relative binding affinity for both ferrihydrite and MetsorbTMwasPO43−≈ As(V) > V(V) ≈ As(III) > Se(IV) ≫ Se(VI) and effective binding capacities were measured in singleion solutions, and spiked synthetic freshwater and seawater, advising practical decisions about DGT mon-itoring. Under the conditions tested the performance of both ferrihydrite and MetsorbTMbinding layerswas directly comparable for As(V), As(III) Se(IV), V(V) and PO43−over a deployment spanning ≤2 daysfor both freshwater and seawater. In order to return quantitative data for several analytes we recom-mend that the DGT method using either ferrihydrite or MetsorbTMbe deployed for a maximum of 2 daysin marine waters likely to contain high levels of the most strongly adsorbing oxyanions contaminants.The high pH, the competitive ions present in seawater and the identity of co-adsorbing ions affect thecapacity of each binding layer for the analytes of interest. In freshwaters, longer deployment times can beconsidered but the concentration and identity of co-adsorbing ions may impact on quantitative uptake ofSe(IV). This study found ferrihydrite-DGT outperformed Metsorb-DGT while previous studies have foundthe opposite, with variation in binding materials masses used being a likely reason. Clearly, preparationof both binding layers should always be optimised to produce the highest capacity possible, especiallyfor seawater deployments.



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