Chromium removal at neutral pHs via electrochemical Cr(VI) reduction and subsequent Cr(III) adsorption with MoS2 nanoflowers-modified graphite felt

Abstract

The operation performance and stability of electrochemical Cr(VI) reduction are strongly restricted at neutral pHs (e.g., drinking water and groundwater) by the high Cr(VI) oxidation potentials and cathode passivation of Cr(OH)3 precipitates. Herein, we fabricated MoS2 nanoflowers-modified graphite felt (GF-MoS2) to construct the electrochemical apparatus (EA) and adsorption column (AC), attempting to stable and effective Cr(VI) removal at neutral pHs via electrochemical Cr(VI) reduction and subsequent Cr(III) adsorption. In EA with a sequential oxidation-reduction process, Cr(VI)-contaminated influent (5 mg/L) at neutral pHs (6.0–8.0) was oxidized first by anode to generate large amounts of H+ ions via H2O oxidation, decreasing the pH of anode-oxidized influent to ∼2.5 at 2.6 V and 1000 L/m2/h. Subsequently, the acidic anode-oxidized influent was further reduced by GF-MoS2 cathode, promoting significantly Cr(VI) reduction via decreasing Cr(VI) oxidation potentials and alleviating Cr(III) precipitation on cathode. These results enabled the stable and effective operation of GF-MoS2-based EA with almost Cr(VI) reduction to Cr(III). With further assembling GF-MoS2-based AC, Cr(III) species in EA effluent were easily adsorbed or intercepted by GF-MoS2, achieving undetectable Cr species in AC effluent. Combination techniques of GF-MoS2-based electrochemical reduction and adsorption can be an effective approach for remediating Cr(VI)-contaminated water at neutral pHs.