Three-Dimensional Packed-Bed Electrochemical Reactor Design for Selective Selenite Reduction in Water

Abstract

Selenium (Se) contamination in flue-gas desulfurization (FGD) wastewater from coal-fired power plants poses significant environmental and regulatory challenges. Here, we developed and optimized a three-dimensional electrochemical reactor (3DER) with carbon-based particle electrodes (PEs) to remove Se(IV). Compared with conventional two-dimensional systems, the 3DER provides an enlarged electrode surface area, enabling faster removal kinetics and higher resilience without regeneration. Reactor performance was systematically evaluated as a function of PE geometry, recirculation rate, cell potential, and anode-to-cathode (A:C) chamber ratio. The optimized configuration (A:C = 1:2, Ecell = –2.1 V, recirculation rate 3.3 mL min–1) balanced cathodic efficiency while minimizing anodic parasitic reactions. In synthetic wastewater containing 0.1 mM Se(IV), the single-pass 3DER achieved steadily increasing performance, with hourly removal improving from 61.3% in the first hour to 68.1% by the 12th hour. Applied to real FGD wastewater, the system maintained an average hourly removal of 51.7% (4.2 mg of Se L–1 h–1) without regeneration and reached a specific energy consumption as low as 0.03 kWh g–1 Se despite high chloride levels. Competing ions, including Mn and Si, further enhanced the Se reduction by forming oxide layers and rejecting Cl– from the electrode surface. Enhanced kinetics under elevated Se(IV) loadings yielded a peak removal of 74.4% (17.5 mg of Se L–1 h–1). These results demonstrate robust and efficient removal performance of the 3DER, supporting its promise for selenium-rich wastewater treatment and future scale-up.