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

## TL;DR

A new 3D electrochemical reactor efficiently removes selenium from coal plant wastewater, outperforming traditional methods.

## Contribution

A three-dimensional electrochemical reactor design with carbon-based electrodes for selective selenite reduction is developed and optimized.

## Key findings

- The 3DER achieved 68.1% selenium removal in 12 hours from synthetic wastewater.
- Real FGD wastewater saw 51.7% average hourly removal without regeneration.
- Peak removal of 74.4% was achieved under high selenium loadings.

## 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, 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.