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

Abstract

The nature of the buffer influences the PCET step gating Cr(vi) reduction in water at pH 4.75, as well as the extent of deposition on carbon electrodes. Electrode activity is recovered without polishing, through a simple acid wash step.

Advancing Cr(VI) Electroreduction: A Redox Mediator to Catalyze the Electrochemical Reduction of Cr(VI) in Water While Preventing Fouling of Carbon Electrodes

“…An estimate of the rate constant can be obtained from the slope, which here yields k = 4.9 × 10 2 M –1 s –1 (Figure S8). We have previously shown that Cr(VI) reduction in similar conditions is initiated by the transfer of 1e – /1H + . , The electron here is supplied by the catalyst, and the H + is expected to be supplied by the buffer (pH 4 citrate). A solution chemistry report investigating the use of Cr(VI) as a chemical oxidant for Fe(II) at 303 K proposed a rate law which would translate to a rate constant of 8.5 × 10 2 M –1 s –1 in our conditions (see calculation in the SI page SI-8) .…”

“…In prior work, we have observed rapid fouling of carbon electrodes in bulk electrolysis conditions, with the presence of Cr(III) confirmed by X-ray photoelectron spectroscopy. 15 Here we report the impact of using a redox mediator as a molecular electrocatalyst S2). An isosbestic point is observed at 466 nm, which further supports the clean reduction of Cr(VI) by [Fe(CN) 6 ] 4− and the generation of [Fe(CN) 6 ] 3− .…”

“…Following previously reported procedures, , all electrochemical experiments were performed using a SP-300 Biologic potentiostat. Working electrodes used were 3 mm diameter glassy carbon disks (CH Instruments) or 100 PPI reticulated vitreous carbon (RVC, Duocel).…”

“…16−20 In particular, some buffers were shown to disfavor adsorption of Crcontaining species on the electrode surface on the time scale of cyclic voltammetry experiments. 15 However, in bulk electrolysis conditions relevant to water purification, deposition is observed, which fouls the carbon electrodes and shuts down the electrochemical Cr(VI) reduction activity. treat solutions contaminated with Cr(VI) and as a method to decontaminate large bodies of water.…”

“…The reduction is initiated by a proton-coupled electron transfer (PCET) process in a pH range from 3 to 6, in agreement with data reported on Au electrodes in acidic media . The acid/base couple used to create the buffer in these experiments has an impact on the observed chemistry, even at a fixed pH, highlighting the noninnocence of buffers and electrolytes which continues to garner more attention in molecular electrocatalytic water purification and energy storage reactions. − In particular, some buffers were shown to disfavor adsorption of Cr-containing species on the electrode surface on the time scale of cyclic voltammetry experiments . However, in bulk electrolysis conditions relevant to water purification, deposition is observed, which fouls the carbon electrodes and shuts down the electrochemical Cr­(VI) reduction activity.…”