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

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, E cell = -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.