Application of Microfluidics for the Study of Bacterial Chemotaxis to NAPL in Porous Media

Nonaqueous phase liquid (NAPLs) contaminants are difficult to eliminate from the groundwater system due to their low solubility, low intrinsic reactivity and low release rates from soil or sediments. In situ bioremediation is an economical method to deal with NAPL pollution, but its efficiency is hindered due to the heterogeneous structure in soil. Chemotaxis, the microbial property by which microorganisms sense the concentration gradient of chemicals and migrate towards the preferential regions for their survival and growth, may be a key factor to achieve more efficient bioremediation. A heterogeneous microfluidic device (H-µChip) was designed to mimic features of the natural contaminated groundwater system, with NAPL contaminant trapped in designated locations within the low permeable region. Chemotaxis facilitated the bacteria (P. putida F1) to accumulate adjacent to the contaminated low permeable region as well as in vicinity of the NAPL contaminant sources. Chemotactic bacteria were also more difficult to be washed out from the contaminated region due to their interaction with attractant gradient. Bacteria in the device were also subjected to different flow rates within typical range of groundwater flow rates. A higher flow rate reduced the apparent effect of chemotaxis.

Another microfluidic device was fabricated that created a convection free channel (CF-µChip), in order to measure the two essential chemotaxis parameters: chemotactic sensitivity coefficient χ0 and chemotactic receptor constant Kc. These two parameters are the key to quantifying the impact of chemotaxis; however, their values for P. putida F1 and toluene were not previously measured. This device provided a way to enhance the accuracy of the measurement to a great extent compared to conventional methods. By performing the experiment at two different values of the attractant concentration, the two chemotaxis parameters could be determined independently. Chemotactic bacteria exhibited a larger accumulation when the attractant concentration was closer to the correct value of Kc; therefore, Kc could be determined first, and then χ0 could be easily derived after fitting the chemotactic patterns with the correct Kc in the mathematical models. Both Pseudomonas putida and Escherichia coli strains were used, and the predicted chemotaxis parameters were consistent with the published results and the values derived from related work.

The numerical solution of mathematical models using COMSOL algorithms yielded outcomes that were consistent with the experimental results, and statistical analysis also supported the experimental comparisons. There is no direct observation on biodegradation, but because toluene is degradable by P. putida F1, the experimental observations of biased accumulation of chemotactic bacteria around the NAPL sources within the low permeable region is expected to lead to an increase of contaminant consumption, which can improve the efficiency of bioremediation.