In situ sampling and speciation method for measuring dissolved phosphite at ultratrace concentrations in the natural environment

Abstract

Phosphite (P) is of emerging chemical interest due to its importance within the global phosphorus cycle. Yet, to date, precise/accurate measurements of P are still lacking due to the inherent analytical challenges linked to its instability/ease of oxidation and ultra-trace concentration. Here, we present the first in-situ sampling and speciation analysis method, for dissolved P, using the diffusive-gradients-in-thin-films (DGT) technique, combined with capillary-column-configured-dual-ion-chromatography (CC-DIC). Method optimization of the DGT elution regime, to simultaneously maximize desorption efficiency and CC-DIC sensitivity, along with the characterization of diffusion coefficients for P, were undertaken before full method validation. Laboratory-performance testing confirmed DGT-P acquisition to be independent of pH (3.0-10.0) and ionic strength (0-500 mM). The capacity for P was 45.8 μg cm, while neither P (up to 10 mg L) nor As (up to 1 mg L) impacted the DGT-P measurement. This novel method's functionality stems from the herein confirmed speciation preservation and double pre-concentration of P, resulting in quantification limits as low as 7.44 ng L for a 3-day deployment. Applications of this method in various terrestrial/aquatic environments were demonstrated and simultaneous profiles of P and P across a sediment-water interface were captured at mm resolution in two contrasting redox-mesocosm systems.

Simultaneous measurement of aqueous redox-sensitive elements and their species across the soil-water interface

“…Arsenite, P(V), and S(-II) (> 70%) represent the dominant As, P and S in soil porewater, respectively. These results agree well with previous reports from multiple soils when the HPLC-ICP-MS, IC or colorimetric methods were used (Chen et al, 2019;Han et al, 2018;Xu et al, 2017). Besides, a susceptible P peak was detected in both solution and porewater with a retention time of 200 s (Fig.…”

“…4). The As, P and S trends agree with their fates in flooded soils, and the results revealed by other methods (Arsic et al, 2018;Han et al, 2018;Mcadams et al, 2016;Robertson et al, 2008).…”

“…Although the redox-sensitive elements speciation has received much attention, most analytical methods for those species were developed for single element only Han et al, 2018;Jackson and Bertsch, 2001). In comparison with those methods, the optimized method in the current study has higher measurement throughput, which can simultaneously measure As, P and S species within 10 min in a single run.…”

Developments in the diffusive gradients in thin-films technique for the speciation of oxyanions and platinum group elements in aquatic systems

“…Also, the separation of P(III) and P(V) in J o u r n a l P r e -p r o o f 9 environmental samples is of emerging interest due to the often-overlooked importance of P(III) within the global P cycle. Han and co-workers [56] found that, by coupling Zr-DGT with capillary-columnconfigured-dual-ion-chromatography (CC-DIC), P(III) could be selectively measured even at ultratrace levels in natural samples. The elution of P(III) from zirconia binding gel was optimized by using 2 mL of 50 mmol L -1 NaOH to increase the sensitivity of the CC-DIC measurement and to maximize the elution efficiency at the same time.…”

“…The elution of P(III) from zirconia binding gel was optimized by using 2 mL of 50 mmol L -1 NaOH to increase the sensitivity of the CC-DIC measurement and to maximize the elution efficiency at the same time. However, P(III) only stays stable in the binding gel or eluent for about three days [56].…”