NMR relaxometry to monitor chromium (VI) reduction by hydrogen peroxide, ascorbic acid and aluminum powder

Chromium is released in the effluents of several industries. One strategy to decrease the toxicity of chromium in water is the reduction of Cr(VI) to Cr(III). The monitoring of the reduction is often carried out by measuring the Cr(VI) concentration using the carbazide test. This technique is sensitive but destructive for the sample and can only be used for clear solutions. Here we propose another method for the Cr(VI) reduction monitoring, using the magnetic properties of Cr(VI) and Cr(III): Cr(VI) is diamagnetic while Cr(III) is paramagnetic. From the proton NMR relaxation point of view, a solution of Cr(VI) ions is similar to pure water with long relaxation times (⁓2s), while a solution of Cr(III) ions presents faster relaxation, because of the coupling of proton spin with the paramagnetic ion. This allows the use of NMR relaxometry for the follow-up of chromium reduction using only a small volume of sample, and even for turbid solutions. Low-resolution NMR devices were used for the measurement of T₁ and T₂ at 20 MHz and 29 MHz. UV-Vis spectroscopy was used to measure the remaining Cr(VI) concentration. The reduction of K₂Cr₂O₇ solutions by H₂O₂ was studied at different pHs. 1/T₁ and 1/T₂ increased during the reduction of Cr(VI) into paramagnetic Cr(III). The correlation between the relaxation rates and the remaining Cr(VI) concentration in the sample was excellent. Moreover, it was possible to study the reduction kinetics. The reduction of Cr(VI) by ascorbic acid was also successfully monitored by NMR relaxometry. The presence of complexing molecules in the reaction mixture drastically influenced the relaxation induced by Cr(III). Therefore, the comparison of results obtained in different reaction mixtures must be done carefully. Finally, as a proof of concept for a turbid solution, the kinetics of the reduction of a K₂Cr₂O₇ solution by aluminum powder was successfully monitored.