An evaluation of ferrihydrite- and Metsorb™-DGT techniques for measuring oxyanion species (As, Se, V, P): effective capacity, competition and diffusion coefficients

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.