Titanium Dioxide-Based DGT Technique for In Situ Measurement of Dissolved Reactive Phosphorus in Fresh and Marine Waters

Abstract

A new diffusive gradients in a thin film (DGT) technique for measuring dissolved reactive phosphorus (DRP) in fresh and marine waters is reported. The new method, which uses a commercially available titanium dioxide based adsorbent (Metsorb), was evaluated and compared to the well-established ferrihydrite-DGT method (ferrihydrite cast within the polyacrylamide gel). DGT time-series experiments showed that the mass of DRP accumulated by Metsorb and ferrihydrite was linear with time when deployed in simple solutions. Both DGT methods showed predictable uptake across the pH (4.0-8.3) and ionic strength (0.0001-1 mol L(-1) NaNO(3)) ranges investigated, and the total capacity of the Metsorb binding phase (∼40,000 ng P) was 2.5-5 times higher than the reported total capacity of the ferrihydrite binding phase. The measurement of DRP in synthetic freshwater and synthetic seawater by Metsorb-DGT over a 4 day deployment period showed excellent agreement with the concentration of DRP measured directly in solution, whereas the ferrihydrite-DGT method significantly underestimated (23-30%) the DRP concentration in synthetic seawater for deployment times of two days or more. Field deployments of Metsorb-DGT samplers with various diffusive layer thicknesses allowed accurate measurement of both the diffusive boundary layer thickness and DRP concentration in situ. The Metsorb-DGT method performs similarly to ferrihydrite-DGT for freshwater measurements but is shown to be more accurate than the ferrihydrite method for determining DRP in seawater.

Progress in understanding the use of diffusive gradients in thin films (DGT) – back to basics

“…Different slopes of mass v. time have been observed when using DGT devices with different binding agents, as for measurements of Hg in simple solutions using Chelex and Spheron-thiol [54] and, for times exceeding 24 h, when measuring phosphate in seawater using ferrihydrite and titanium oxide. [72] If capacity was approached in the latter case, competitive binding by the anions in seawater may be responsible, as suggested. However, the former case of different slopes throughout the time regime is more puzzling.…”

Sampling of ammonium ion in water samples by using the diffusive-gradients-in-thin-films technique (DGT) and a zeolite based binding phase

“…The combined thickness of nylon membrane and diffusive gel was calculated in DGT equation. Before usage, the membrane was pretreated as mentioned previously [7]. Detailed information for preparation of diffusive gel and binding gel is available in ESM.…”

“…(2)). The calculated concentration is an average concentration over the deployment period which can better represent the original level than instantaneous concentration, especially for concentrations with wide fluctuation [7,8]. Binding layer is the most important part of DGT technique.…”

Evaluation and application of Diffusive Gradients in Thin Films (DGT) technique using Chelex®-100, Metsorb™ and Diphonix® binding phases in uranium mining environments

“…This U uptake diminution in the presence of a greater PO 4 3À concentration is presumably a consequence of the direct competition for the binding sites between uranium phosphate complexes and dihydrogen phosphate ion[19,58]. The greater impact of increasing PO 4 3À concentration on the performance of Metsorb™-DGT is most likely due to the fact that Metsorb™ adsorbent was successfully used beforehand for the measurement of the dissolved reactive phosphorus in an aqueous solution[14,60]. The U uptake on the Diphonix ® -DGT was quantitative over experimental PO 4 3À concentration range.It is also noteworthy that small, but significant decrease in U concentration at high PO4 3À concentrations has been observed, which is most likely attributed to the precipitation of U. Uranyl phosphate complexes are extremely insoluble under circumneutral pH conditions (e.g.…”