Development Of A New Diffusive Gradients In Thin-films （DGT） Technique For Selectively Measuring Cr^Ⅵ In Waters And Soils

In soil and aquatic environments, Cr mainly occurs in two oxidation states as hexavalent chromium (CrⅥ) and trivalent species(CrⅢ), which are drastically different in chemical properties. CrvⅥ is much more toxic and mobile than the CrⅢ, they could transform to each other and easily influenced by pH, redox potential, photochemical reactions and so on. Consequently, in situ monitoring of CrⅥ can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a kind of in-situ and non-destrutive tool for determining element. Compared with commonly used active sampling methods, the DGT technique has specific advantages, including its convenient deployment for obtaining time-integrated concentrations of analytes and its low effective detection limits for trace species. DGT is also powerful for determining metal(loid) speciation, but a binding phase that absorbs only one specific species of is needed.In this study, synthesized N-Methyl-D-glucamine(NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of CrⅥ.28.5% acrylamide (w/v) and 1.5% N,N-methylene bis(acrylamide) (w/v) were used as gel solution to prepare the binding gel.The diffusion coefficient of CrⅥ in the polyacrylamide diffusive gel was firstly reported here, which is 8.18 ×10-6 cm2 s-1 at 25℃. This NMDG-DGT sampler exhibited atheoretically linear accumulation of CrⅥ, with negligible accumulation (< 5%) of CrⅢ, even after 72 h deployment. Under the situation of pH 3-10 and ionic strength 0-50 mmol L-1NO3-, the developed DGT technique could also successfully measure CrⅥ. The good prediction of CrⅥ concentration in synthetic freshwater with NMDG-DGT, even in the presence of 10-time more CrⅢ, further indicated the sampler’s reliability in selective detection of CrⅥ. Moreover, its high capacity for CrⅥ, which exceeded 230 μg cm-2, facilitates measurement of CrⅥ in both uncontaminated natural waters and in slightly and heavily contaminated (mg L-1 level) waters. Field deployment of the NMDG-DGT sampler in such waters allowed accurate measurement of time-averaged CrⅥ concentration. The method detection limit for NMDG-DGT, was 0.02 μg L-1 for a 72 h deployment, much lower than the usually used DPC method (4 μg L-1).NMDG-DGT could also be used to measuring the labile CrⅥ in soils. In this research, we found the DGT measured CrⅥ has good liner relationship with the water-soulable and exchangeable CrⅥ(R2 > 0.7), which were both bioavailable. This indicates its further application in the risk assessment of Cr.This development of a DGT method for CrⅥ is complementary to the existing method for CrⅢ using Chelex-100. The pair of them could be used for investigating Cr biogeochemistry in various media, including groundwater, soils and sediments.