Effect of solution composition, flow and deployment time on the measurement of trace metals by the diffusive gradient in thin films technique

Abstract

Diffusive gradients in thin films (DGT) is a recently developed technique capable of in situ measurement of labile metal species in water. Tests of the performance of DGT were extended in this work to cover a wide range of environmentally relevant conditions. DGT accurately measured Cd concentrations in 0.01 M NaNO 3 solutions in the pH range 5–10 and Co, Mn and Zn concentrations, between pH 3.5 and 10. Cu was measured accurately between pH 2 and 10. Above pH 11 prolonged deployments were impractical due to swelling of the diffusive gel, but even at pH 12.9 significant fractions of the total Cd and Cu concentrations were measured by DGT units deployed for a day or less. Measurements of trace metals by DGT were unaffected by the presence of Ca at concentrations found in hard water. Errors due to changes in flow velocity were less than 10% above a flow of 0.02 m s −1, where the formation of a diffusive boundary layer becomes negligible for diffusive gels of ≥0.7 mm thickness. Therefore, DGT devices prepared with this diffusive gel thickness can confidently be used in most streams and rivers. However, in quiescent solutions the DGT measurement may only be 50% of the true concentration, confirming the need in quiet waters for parallel deployments of devices with different diffusive layer thickness. Quantitative DGT measurements of Cd over a 1 month period showed that neither the diffusive nor resin gel deteriorated with time. Collectively these results show that DGT behaves predictably over a wide range of solution conditions. For Cu it can be used in very acidic solutions. The upper pH limit for DGT measurement is governed by the stability of the gel. For this gel composition the realistic limit is pH 11.