Construction of bubbles enrichment sites and gas-diffusion-microchannel to assist Cr(VI) electrochemical reduction in a flow-through electrochemical system

Abstract

Due to characteristics of the forced convection and enhanced mass transfer, graphite felt cathode based flow-through electrochemical system presented high electroreduction efficiency for aqueous Cr(VI). However, disadvantage is that bubbles generated by water electrolysis cannot pass through the liquid filled microchannel in time, which is easy to cause gas accumulation and an unstable electrochemical operation. Herein, we demonstrated graphite felt flow-through cathode designed with hydrophobic polytetrafluoroethylene (PTFE) as bubbles enrichment sites and construction of gas-diffusion-microchannel to assist in Cr(VI) electrochemical reduction. Cr(VI) reduction kinetics constant by graphite felt flow-through cathode increased 4.6 times after loading PTFE and was 2.6 ∼ 5.7 times higher than parallel-plate electrodes system. Typical electrochemical analysis suggested that PTFE can enrich and accelerate the dissolved oxygen mass transfer rate at PTFE-electrode-solution micro-interface, realizing oxygen digestion via two-electron reduction reaction. The unconsumed bubbles were transported out of electrochemical cell along gas-diffusion-microchannel to avoid gas accumulation and maintained a stable operation. Batch experiments indicated the ∼ 99.9% Cr(VI) reduction efficiency when the initial pH 2 and 100 mg/L Cr(VI) feed was treated under a single-pass pattern with flow rate of 1.5 mL/min and current density of 12.74 mA/cm2. Mechanism study revealed that faraday reaction was the main driving force for Cr(VI) reduction, while Cr(VI)-induced activation of in-situ electro-generated H2O2 contributed to formations of •OH and intermediate chromium species. This work also shows the feasibility of remediation for Cr(VI)-organic coexistence environment by a catalyst-free flow-through electrochemical system.