Carbon fiber-based flow-through electrode system (FES) for Cr(VI) removal at near neutral pHs via Cr(VI) reduction and in-situ adsorption of Cr(III) precipitates

Square-Wave Alternating Voltage for Enhancing Cr(VI) Reduction in a Carbon Fiber-Based Flow-Through Electrode System

“…, where E 0 is the standard electrode potential for HCrO 4 − reduction. 9,34 Obviously, the E absolute values decreased with decreasing solution pH, meaning that HCrO 4 − reduction can be enhanced at higher H + ion concentrations under smaller absolute values of cathode potential (Figure 3A).…”

“…According to the currents in Figure S3B in the SI, the solution pH values of the anode-treated influent were further calculated and are shown in Figure S7 in the SI from 140 to 180 min (after the adsorption saturation of Cr species), and around 0.32 mM H + ions were generated and accumulated in the anode-treated influent, which was coincident with the observation from pH measurement for the anode-treated influent. Meanwhile, the generated H + ion concentration was around 10% compared to that in the influent (3.16 mM and a pH of 2.50), and the effect of the H + concentration on the equilibrium potential ( E ) of HCrO 4 – reduction was further evaluated by the Nernst equation ( E = E 0 + 0.0197 log­([HCrO 4 – ]­[H + ] 7 /[Cr 3+ ])), where E 0 is the standard electrode potential for HCrO 4 – reduction. , Obviously, the E absolute values decreased with decreasing solution pH, meaning that HCrO 4 – reduction can be enhanced at higher H + ion concentrations under smaller absolute values of cathode potential (Figure A).…”

“…Also, under anoxic conditions, still ∼10% energy efficiency was attributed to the side reactions. This phenomenon suggested that the O 2 molecule formation via H 2 O oxidation also contributed to the energy consumption during the subsequent cathodic reactions. , Hence, with the periodic polarity reversal of oxidation–reduction reactions on the electrodes, the O 2 generation via anodic oxidation and O 2 in situ consumption via cathodic reduction avoided effectively the gas bubble formation in the electrodes, resulting in a shorter time (60 min) to reach the stable levels for HCrO 4 – reduction and a higher surface area for electroreactions than the Red-Ox and Ox-Red systems (150 min) under the DV power supply (Figure A,B).…”

“…Among various Cr­(VI) reduction technologies, electrochemical reduction has been acknowledged as an environment-friendly method for Cr­(VI) detoxification since electrons were employed to regulate the redox reactions on the electrodes with nonsignificant disturbance to the environment. , A flow-through electrode system (FES) has been well developed with a graphene sponge, carbon nanotube membrane, or carbon felt/cloth , as cathodes to promote the Cr­(VI) reduction performance via enhancing the convective mass transfer of Cr­(VI) anions in the porous cathodes. However, the Cr­(VI) anion (HCrO 4 – or CrO 4 2– ) reduction efficiency was strongly suppressed by poor accessibility to the cathode due to electrostatic repulsion. , …”

“…However, the Cr(VI) anion (HCrO 4 − or CrO 4 2− ) reduction efficiency was strongly suppressed by poor accessibility to the cathode due to electrostatic repulsion. 9,14 As an influent solution was pumped continuously through the FES under a direct voltage power supply, the electrochemical removal of amphoteric ions was significantly influenced by the order of the anode and cathode, 15 which were involved with the sequential reduction−oxidation (Red-Ox) or oxidation−reduction (Ox-Red) systems. Additionally, it was well known that the anodic reaction was related to the adsorption of HCrO 4 − (the major Cr(VI) species at pH values between 2.0 and 4.0), 16 which was also accompanied by generating H + and O 2 species via H 2 O oxidation.…”