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

Abstract

Cr(VI) reduction to Cr(III) and subsequent Cr(III) precipitation is an extensively employed strategy to treat Cr(VI)-contaminated wastewater, which generally generates large volumes of sludge that need expensive disposal, and requires high chemical cost for pH adjustment. Herein, flow-through electrode system (FES) with a sequential oxidation-reduction process was employed to simultaneously reduce Cr(VI) and in-situ precipitate Cr(III), thereby simplifying the treatment processes and providing potentials to recollect Cr resource. The influent (10 mg/L Cr(VI)) with near neutral pHs (6–8) was first oxidized by the anode with H+ ions generation via H2O oxidation, leading to the anode-treated effluent turned into strong acidic with pH below 3.5. Subsequently, under acidic condition, the Cr(VI) species were prone to reduce to Cr(III) on the cathode. The multi-cathode FESs (1A + nC) equipped with one counter anode (1A) and multiple cathodes (nC) were constructed to enhance the Cr(VI) reduction by providing more cathode reactive sites and in-situ Cr(III) precipitation by maintaining near neutral effluent pHs. The flow-through flux and applied voltage were optimized to be 500 L/h/m2 and 2.6 V, under which Cr species were undetectable in effluent of 1A + 4C FES within continuous operation time of 30 min. Regenerating the Cr(OH)3-passivated CFF electrodes and recollecting the Cr waste resource were easily accomplished by using a small volume of acidic solution with reversed applied voltage. This study suggested that carbon fiber-based FES with a sequential oxidation-reduction process can be a feasible approach for remediation and recovery of Cr(VI) contamination.