Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes

Hexavalent chromium is a contaminant of concern in water. Electrochemical methods are being developed to reduce toxic Cr(vi) to benign Cr(iii) at the point of generation or point of use. The effectiveness of glassy carbon electrodes to detect and reduce Cr(vi) in cyclic voltammetry was recently demonstrated. Herein, we report that the nature of the buffer system used, at a fixed pH, has unexpected impacts on the electrochemical reduction of Cr(vi) in water. At low concentrations of Cr(vi), the buffer influences the PCET step gating Cr(vi) reduction on the timescale of cyclic voltammetry experiments. At higher concentrations of Cr(vi), the effect is more complex. Data suggests impacts on both the chemical steps of Cr(vi) reduction and the nature of the products formed, hypothesized to be due to chelation effects. In particular, evidence of adsorption on the electrode surface is seen through cyclic voltammetry studies in certain buffers. Chronoamperometry studies confirm the adsorption of chromium containing species on the electrode surface during Cr(vi) electroreduction. XPS confirms Cr(iii) as the product. The activity of the electrode is regained after an acid wash step, without the need for re-polishing. This work provides a framework to understand the impact of the presence of small organic acids on Cr(vi) reduction for water purification.