Novel Speciation Method Based on Diffusive Gradients in Thin-Films for in Situ Measurement of Cr^VI in Aquatic Systems

Hexavalent chromium (Cr^VI) is much more toxic and mobile than the trivalent species (Cr^III) and consequently, in situ monitoring of Cr^VI can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a powerful tool for determining metal­(loid) speciation, but a binding phase that absorbs only one specific species of Cr is needed. N-Methyl-d-glucamine (NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of Cr^VI. This NMDG-DGT sampler exhibited a theoretically linear accumulation of Cr^VI, with negligible accumulation (<5%) of Cr^III, even after 72 h deployment. The good prediction of Cr^VI concentration in synthetic freshwater with NMDG-DGT, even in the presence of 10-time more Cr^III, further indicated the sampler’s reliability in selective detection of Cr^VI. Moreover, its high capacity for Cr^VI, which exceeded 230 μg cm^–2, facilitates measurement of Cr^VI in both uncontaminated natural waters and in slightly and heavily contaminated (ppm level) waters. Field deployment of the NMDG-DGT sampler in such waters allowed accurate measurement of time-averaged Cr^VI concentration, indicating its robustness for in situ measurements of Cr speciation and its potential for further application in the risk assessment of Cr.