Comparing dissolved reactive phosphorus measured by DGT with ferrihydrite and titanium dioxide adsorbents: Anionic interferences, adsorbent capacity and deployment time

Two adsorbents (Metsorb and ferrihydrite) used in binding layers with the diffusive gradients in a thin film technique were evaluated for the measurement of dissolved reactive phosphorous (DRP) in synthetic and natural waters. Possible interferences were investigated with Cl− (up to 1.35 mol L−1) and SO42− (up to 0.056 mol L−1) having no affect on either DGT binding layer, and HCO3− (up to 5.7 mmol L−1) having no effect on Metsorb-DGT, over 4 days. However, HCO3− interfered with the ferrihydrite-DGT measurement at concentrations typical of many natural waters (≥0.7 mmol L−1) after a deployment period of 1–2 days. The capacity of the Metsorb binding phase for DGT response was ∼37,000 ng P, whereas the capacities of a low-mass (17.8 mg of adsorbent per DGT sampler) and high-mass (29.2 mg of adsorbent per DGT sampler) ferrihydrite binding phase were substantially lower (∼15,000 ng P and ∼25,000 ng P, low-mass and high-mass, respectively). Increasing the capacity of the ferrihydrite adsorbent allowed the ferrihydrite-DGT to be utilized for up to 3 days before interference by HCO3− was observed. Seawater deployments demonstrated that even high-capacity ferrihydrite-DGT devices underestimated the DRP concentration by 37%, whereas Metsorb-DGT measurements were accurate. The Metsorb-DGT is superior to the ferrihydrite-DGT for determining DRP over deployment times greater than 1 day and in waters with ≥0.7 mmol L−1 HCO3−. Based on the experience obtained from this detailed validation process, the authors propose a number of key requirements that need to be considered when developing new DGT binding layers, with testing the performance over longer deployment times being critical.

The accuracy of the DGT technique for the measurement of DRP in natural waters is dependent on the adsorbent, adsorbent capacity and deployment time.Figure optionsDownload as PowerPoint slideHighlights
► Titanium dioxide based adsorbent (Metsorb) is superior to ferrihydrite adsorbent.
► Bicarbonate interferes with measurement of DRP by ferrihydrite-DGT but not Metsorb-DGT.
► Metsorb has a higher capacity and is more accurate than ferrihydrite.
► Accuracy of DGT measurements is dependent on adsorbent capacity and deployment time.
► Testing the performance of DGT over longer time periods is critical.