DC Electric Fields Promote Biodegradation of Waterborne Naphthalene in Biofilter Systems

Abstract

Biofiltration is a simple and low-cost method for the cleanup of contaminated water. However, the reduced availability of dissolved chemicals to surface-attached degrader bacteria may limit its efficient use at certain hydraulic loadings. When a direct current (DC) electric field is applied to an immersed packed bed, it invokes electrokinetic processes, such as electroosmotic water flow (EOF). EOF is a surface-charge-induced plug-flow-shaped movement of pore fluids. It acts at a nanometer distance above surfaces and allows the change of microscale pressure-driven flow profiles and, hence, the availability of dissolved contaminants to microbial degraders. In laboratory percolation columns, we assessed the effects of a weak DC electric field (E = 0.5 V·cm–1) on the biodegradation of waterborne naphthalene (NAH) by surface-attached Pseudomonas fluorescens LP6a. To vary NAH bioavailability, we used different NAH concentrations (C 0 = 2.7, 5.1, or 7.8 × 10–5 mol·L–1) and Darcy velocities typical for biofiltration ( U̅ = 0.2–1.2 × 10–4 m·s–1). In DC-free controls, we observed higher specific degradation rates (q c) at higher NAH concentrations. The q c depended on U̅ , suggesting bioavailability restrictions depending on the hydraulic residence times. DC fields consistently increased q c and resulted in linearly increasing benefits up to 55% with rising hydraulic loadings relative to controls. We explain these biodegradation benefits by EOF-altered microscale flow profiles allowing for better NAH provision to bacteria attached to the collectors even though the EOF was calculated to be 100–800 times smaller than bulk water flow. Our data suggest that electrokinetic approaches may give rise to future technical applications that allow regulating biodegradation, for example, in response to fluctuating hydraulic loadings.