Exploring the limits of DGT-speciation concept for arsenic – application in the Scheldt estuary

Two approaches are generally applied to in-situ speciation by the DGT technique: (i) utilization of sorbents selective for individual species or (ii) utilization of unselective sorbents and separation of species after their elution by analytical methods (HPLC) [1]. The first approach is less likely to be affected by speciation changes during sample processing and is generally preferred. Although finding sorbents with specific affinity for each redox species may be challenging, this DGT-speciation concept has been mastered for certain oxyanions – or hasn’t it? For instance, the speciation of As by the DGT technique is generally recommended by a combination of sorbents for total As (e.g., iron oxide-based sorbents or ZrO2), AsIII (3-mercaptopropyl functionalized silica (3-MFS)), and AsV (Amberlite IRA 910 (dimethyl ethanol ammonium chloride)). Even though the utilization of the latter sorbent is regularly suggested in the literature, there is no reference to its actual application in the natural environment besides the original work of Rolisola et al. [2]. Nevertheless, our preliminary results (unpublished) evaluating the performance of the above-mentioned sorbents simultaneously deployed in natural waters with elevated As concentrations show poor repeatability of Amberlite-DGTs and thus raise doubts about the applicability of the sorbent in the natural environment. Although we believe this is caused by improper elution protocol suggested for this resin which can be easily optimized, the current state-of-the-art in As speciation by the DGT technique only allows the application of sorbents for total As and for AsIII, and subsequent calculation of AsV fraction by their difference. However, the assumption that the remaining fraction is entirely represented by AsV may be short-sighted as the presence of methylated species is not considered. Moreover, by having a selective sorbent for AsV, we will be able to gain better insight into which species are actually determined by the DGT technique.
This work, therefore, brings optimization of the elution protocol for Amberlite IRA 910 sorbent and evaluation of its applicability for selective sorption of AsV. To evaluate the speciation of As in the natural waters, the DGTs with various sorbents for total As (ZrO2, Metsorb, Lewatit FO 36, Bayoxide E33) as well as for selective accumulation of AsIII (3-MFS) and AsV (Amberlite IRA 910) were deployed in the Scheldt estuary along the salinity gradient and in the coastal zone (Zeebrugge harbour). Water samples were taken at the sampling stations and were processed by filtration (on 0.45 μm) in order to describe As distribution between particulate, dissolved as well as labile (DGT-determined) fractions in the water column. The results will be presented at the conference.

[1] Gao, Y., et al., Trends in Analytical Chemistry, 2021, p 116513.
[2] Rolisola, A. M., et al., Analyst, 2014, 139 (17), p. 4373-4380.