Electrochemical chromium removal using nanodiamond enhanced flow-through electrosorption

Effective removal of hexavalent chromium (Cr(VI)) from water is challenging due to the need for a highly selective process. Efficient chromium removal may potentially be achieved through the use of redox-assisted flow-through electrosorption. In this study, graphitized nanodiamonds (NDs) were annealed under various conditions and the ND with the lowest internal resistance was applied to an activated carbon cloth electrode. The ND-modified electrode was then used as the cathode in a flow-through electrode cell with pristine carbon cloth as the anode. Effective chromium removal was found through a dual pathway mechanism, whereby Cr(VI) is directly electrosorbed at the anode while Cr(VI) is reduced to Cr(III) at the cathode, and subsequently precipitated as Cr(OH)₃ under the locally high cathodic pH conditions. The effects of flow rate and charging/discharging voltage on Cr(VI) removal were further investigated, revealing that sufficiently low flow rates (i.e., ≤1 mL min⁻¹) are required for effective Cr(OH)₃ precipitation and sufficiently high (i.e., >1.4 V) reverse voltages are needed during the discharging period to enable full desorption. Overall, the system displayed high Cr removal capacity (i.e., up to 39.51 µmol g⁻¹) and favorable redox of Cr(VI) to trivalent chromium Cr(III) and Cr(III) to Cr(VI). These results suggest that the ND-modified electrode is a promising tool for the removal of toxic metal ions from wastewater.