Monitoring Cr(VI) photoreduction at different depths by operando low-field NMR relaxometry

Chromium (VI) (Cr(VI)), a toxic metal ion, is widely present in industrial wastewater. To reduce the contamination of Cr(VI), many technologies for the photocatalytic reduction of Cr(VI) to Cr(III) have been developed in the past decades. However, the practical application of photocatalysts for the reduction of Cr(VI) in wastewater treatment is often hindered by the complicated photoreduction processes due to the sedimentation and stratification of catalyst particles that present during the treatment of the wastewater. Probing and understanding the influences of the sedimentation and stratification of the catalyst particles on the photoreduction processes are long-term challenges in the field. Herein, we demonstrate that this issue can be solved by using layer location integrated low-field time-domain nuclear magnetic resonance (LF-NMR) relaxometry. With paramagnetic Cr (III) cation as the molecular probe, we successfully monitored the Cr(VI) photoreduction processes by operando probing the ¹H T₂ relaxation time of the photoreduction systems. The influences of catalyst sedimentation and the light wavelength on photocatalysis were studied and discussed. The results showed the great potential of LF-NMR relaxometry in the study of Cr (VI) photoreduction processes during industrial wastewater treatments.