Transport of Bacteria in Column for in situ Bioremediation Application: Effect of dissolved surfactant and microbubble on bacterial mobility in sand

Abstract

The absence of bacteria capable of degrading pollutants and the shortage of oxygen in contaminated soil have been identified as key factors that can limit the efficiency of in situ bioremediation. Inoculating contaminated soils with exogenous bacteria and oxygen purging are common approaches for stimulating in situ bioremediation. The use of surfactant and microbubble offers a way of addressing these limitations by promoting bacteria transport while at the same time providing oxygen. This study investigates the effect of surfactant solution and surfactant microbubble on bacteria transport in sand columns with the aim of understanding the factors and mechanisms that affect bacteria transport in soil. This study also introduces an innovative measurement probe, the optrode, for in situ monitoring of bacteria distribution. Rhamnolipid and tergitol were used as surfactant in this study, and Pseudomonas putida (a hydrophilic bacterium) and Rhodococcus erythropolis (a hydrophobic bacterium) were the chosen bacteria. The different types of surfactant altered bacterial hydrophobicity to differing extents but had an equivalent effect on the hydrophobicity of sand particles. In the presence of surfactant solution, a higher proportion of hydrophilic P. putida (76.7%) was eluted out of the column compared to the hydrophobic R. erythropolis (60.3%). Both rhamnolipid and tergitol enhanced the transport of both bacterial strains. However, the larger enhancement in bacteria transport observed for rhamnolipid was attributed to increased Lifshitz-van der Waals and acid-base (LW-AB) interaction energies between bacteria and sand particles. A continuous supply of microbubbles was found able to transport bacteria into the soil using a smaller amount of surfactant. In comparison to a propeller mixer, microbubbles generated by a flat spinning disc mixer were more stable and could elute a greater proportion of bacteria. Microbubbles of rhamnolipid eluted a greater percentage of the injected bacteria compared to tergitol, because rhamnolipid microbubbles showed more stability and higher affinity for bacteria. Hydrophilic P. putida was more likely to be transported with microbubble than hydrophobic R. erythropolis due to the favourable adhesion of hydrophobic R. erythropolis to sand particles. Bacterial transport increased considerably in porous media with higher porosity. Bacteria transport in columns was monitored using both ex situ measurement with a microplate photometer and in situ measurement with the optrode. The optrode measurement demonstrated that sample collection, which is necessary for measurement with conventional ex situ approaches, could affect the local bacterial transport in sand columns. In columns with ex situ sampling, optrode measurements differed from the ex situ concentrations, with the breakthrough curves for optrode showing smaller bacterial concentrations and a larger lag than those obtained via ex situ measurements. However, when the optrode and ex situ measurements were conducted in independent columns, their concentrations and breakthrough curves were almost identical. The transport parameters obtained by fitting the breakthrough curves showed that estimates from the optrode data were more consistent with less error. This study provides very useful information on bacteria transport with surfactant solutions and microbubbles in sand columns simulating soil environment. It has demonstrated that optimizing parameters including surfactant type and form, and bacteria strain can contribute to enhanced bacteria transport when surfactant solution and microbubble are used as the bacteria carrier during soil bioremediation. This study also validates the operation of an innovative technique, the optrode, for real time monitoring of bacteria transport without the need to take bacterial samples.