InterPore2025

Microbial transport toward residual chemical contamination trapped in low permeable regions of a microfluidic device with layered structural heterogeneity is optimal at moderate Peclet numbers

22 May 2025, 09:35

15m

Oral Presentation (MS11) Microfluidics and nanofluidics in porous systems MS11

Speaker

Roseanne Ford

Description

Groundwater pollutants that become trapped in low-permeable clay lenses are difficult to remediate using conventional pump-and-treat methods. Water flows preferentially through more permeable sandy layers leaving behind residual contamination in clay lenses that then slowly leaches out over time creating a long-term contamination problem. An alternative approach to pumping contaminants above ground for treatment is to deliver microorganisms that biodegrade the chemical pollutants in place. Chemotactic bacteria can swim from the highly permeable layers to access residual contamination that is trapped at the boundaries of low-permeable layers. These bacteria detect the presence of chemical pollutants and migrate toward increasing concentrations to accumulate at the source.
To analyze the complex interactions between fluid flow, bacterial transport, and contaminant transport in porous media for this scenario, we used a microfluidic device designed with a highly permeable region sandwiched between two low-permeable layers representing clay lenses within a sandy aquifer. A chemical pollutant (naphthalene) dissolved in a non-aqueous phase liquid (NAPL) was flushed from the device leaving behind residual droplets, especially at the boundaries between the high- and low-permeable layers. A suspension of bacteria chemotactic toward naphthalene (Pseudomonas putida G7) was then flowed through the device. Pore-scale imaging was used to quantify accumulation of chemotactic bacteria near the residual contamination over time at several positions along the length of the microfluidic device. Experimental observations over a range of fluid velocities and pore configurations were compared to computer simulations of Darcy-scale transport equations solved for the microfluidic device geometry.
Our analysis revealed an optimal fluid flow velocity for bacterial accumulation that balanced fluid penetration into dead-end pores blocked by residual NAPL and bacterial transport by chemotaxis within the quiescent fluid of those dead-end pores. We initially expected as the fluid velocity increased bacterial accumulation would decrease monotonically because the independent swimming motion of chemotaxis was overcome by convective flow. However, what we found was a more complex role of fluid velocity that increased accumulation and complemented chemotaxis up to a maximum value before decreasing as originally anticipated. We found that bacterial accumulation in the vicinity of residual naphthalene-containing NAPLs was greatest for values of a chemotactic Peclet number around 10.
To relate our laboratory-scale observations to larger-scale field studies we used dimensional analysis. Simulated outcomes from the microfluidic device over a range of velocities, observation times, and positions along the flow path were fit to a logistic equation using dimensionless parameters. Bacterial transport dynamics were effectively described by two timescales, one associated with fluid flow and one with chemotaxis, highlighting that directed migration in porous media can be determined by two key processes: convection that carries bacteria to the vicinity of contaminant sources and chemotaxis that is driven by pore-scale chemical gradients.
Outcomes from our study suggest that flow rates for pump-and-treat scenarios may be adjusted to maximize bioavailability of residual contaminants for chemotactic bacteria.