Dissolved iodide in marine waters determined with Diffusive Gradients in Thin-films technique

Abstract

For the first time, Diffusive Gradient in Thin-films (DGT) focuses on the inorganic iodine species iodate (IO₃^-) and iodide (I^-). A silver-doped Cl resin (AgdCl), which is known to selectively accumulate I^-, was used to make a binding gel. Laboratory investigations were designed to verify the suitability of the AgdCl-DGT method to measure the total I^- concentration in environmental waters. Total recovery of I^- was obtained using an elution solution containing 100 mmol L^-1 KCN. DGT validation experiments in 10 mmol L^-1 NaCl showed linear accumulation of I^- over time, contrary to IO₃^-, thus confirming the selectivity of AgdCl-binding gel. The AgdCl-DGT measurement of total I^- concentration was independent of pH (4.5-8.8) and was not impacted by the presence of bicarbonate (1-5 mmol L^-1). Finally, the performance of AgdCl-DGT samplers were tested in two continental waters and a synthetic seawater. The AgdCl-DGT samplers measured 27-33% of the total I^- concentration in the two continental waters up to 24 h of deployment time, whereas the AgdCl-DGT response retrieved the total I^- concentration in seawater up to 72 h (106 ± 7%). The difference in DGT response was attributed to the low ionic strength of the two continental waters, limiting the application of AgdCl-DGT method to media with higher ionic strength.

Keywords: Diffusive gradients in thin-films; Iodide; Iodine; Laboratory validation; Marine waters; Redox speciation; Time-series experiment.