Speciation Analysis of Cr(VI) and Cr(III) in Water with Surface-Enhanced Raman Spectroscopy

Identifying and quantifying chromium in water are important for the protection of precious water resources from chromium pollution. Standard methods however are unable to easily distinguish toxic hexavalent chromium, Cr(VI), from innocuous trivalent chromium, Cr(III), are time-consuming, or require large sample quantity. We show in this report that Cr(VI) and Cr(III) in water can be differentiated based on their distinct spectral features of surface-enhanced Raman scattering (SERS). Their SERS signals exhibit different pH dependences: the SERS features of Cr(VI) and Cr(III) are most prominent at pH values of 10 and 5.5, respectively. The obtained limit of detection of Cr(VI) in water is below 0.1 mg/L. Both concentration curves of their SERS signals show Langmuir sorption isotherm behavior. A procedure was developed to quantify Cr(VI) concentration based on the direct retrieval or addition method with an error of 10%. Finally, the SERS detection of Cr(VI) is shown to be insensitive to co-present Cr(III). The developed SERS procedure offers potential to monitor toxic chromium in fields.

Automated summary

Identification and quantification of chromium in water, especially the differentiation between toxic Cr(VI) and harmless Cr(III), can be achieved through surface-enhanced Raman scattering (SERS) based on distinct spectral features. The method shows high sensitivity and selectivity at pH 10 and 5.5 for Cr(VI) and Cr(III) detection, with a limit of detection below 0.1 mg/L and an error of 10% in quantification. This SERS technique is also insensitive to the presence of co-existing Cr(III) and holds potential for monitoring toxic chromium in various fields.