Chemotactic Effect on Bacterial Migration to a Low Permeable Contaminated Zone in a Heterogeneous Porous Microfluidic Device

Bacterial Chemotaxis and Biodegradation to Residual Toluene in a Porous Micromodel

Abstract

Organic solvents such as toluene are the most widely distributed pollutants in groundwater. Subsurface bioremediation is often limited by the poor contact between injected microorganisms and residual pollutants, which may be dispersed as pore-size organic-phase droplets within the saturated soil matrix. Chemotaxis toward chemical pollutants provides a mechanism for bacteria to migrate to locations of high contamination, which may not normally be accessible to bacteria carried along by groundwater flow, and thus it may enhance the overall effectiveness of bioremediation. A porous microfluidic device (µ-chip) was used to mimic the dissolution of an organic-phase contaminant from a pore network into a larger macropore representing a preferred pathway for microorganisms that are carried along by groundwater flow. The µ-chip successfully trapped the organic pollutant in a regular and reproducible pattern within the porous network, and its glass windows allowed direct image analysis of bacterial distributions within the vicinity of the organic contaminant at different points along the flow pathway. Enhanced bacterial migration of P. putida F1 near the organic pollutant network was observed relative to that of the control experiments. Velocities in the macropore pathway were varied over a typical range of groundwater flow rates, which are from 0.5 to 10 m/d.  Bacterial chemotaxis was observed at lower flow rates comparable to natural groundwater flow rates. At the higher velocities, accumulation of chemotactic bacteria was similar to the control experiments. Computer-based simulation using finite element analysis software  (COMSOL) was also performed to understand the effects of various model parameters on bacterial chemotaxis to NAPL. There was good agreement between the simulations (generated using reasonable values of the model parameters) and the experimental data for the bacterial strains.

Figure 1. Residual toluene trapped within the microfluidic device. The toluene droplets have been dyed red and are trapped within the circular pore bodies of the micromodel network.