Effect of low‐concentration rhamnolipid biosurfactant on Pseudomonas aeruginosa transport in natural porous media

Abstract

Enhanced transport of microbes in subsurface is a focus in bioaugmentation applications for remediation of groundwater. In this study, the effect of low‐concentration monorhamnolipid biosurfactant on transport of Pseudomonas aeruginosa ATCC 9027 in natural porous media (silica sand and a sandy soil) with or without hexadecane as the nonaqueous phase liquids (NAPLs) was studied with miscible‐displacement experiments using artificial groundwater as the background solution. Transport of two types of cells was investigated, glucose‐grown and hexadecane‐grown cells with lower and higher cell surface hydrophobicity (CSH), respectively. A clean‐bed colloid deposition model was used to calculate deposition rate coefficients (k) for quantitative assessment on the effect of the rhamnolipid on the transport. In the absence of NAPLs, significant cell retention was observed in the sand (81% and 82% for glucose‐grown and hexadecane‐grown cells, respectively). Addition of low‐concentration rhamnolipid enhanced cell transport, with 40 mg/L of rhamnolipid reducing retention to 50% and 60% for glucose‐grown and hexadecane‐grown cells, respectively. The k values for both glucose‐grown and hexadecane‐grown cells correlated linearly with rhamnolipid‐dependent CSH quantitatively measured using a bacterial‐adhesion‐to‐hydrocarbon method. Retention of cells by the soil was nearly complete (>99%). Forty milligrams per liter of rhamnolipid reduced the retention to 95%. The presence of NAPLs in the sand enhanced the retention of hexadecane‐grown cells with higher CSH. Transport of cells in the presence of NAPLs was enhanced by rhamnolipid at all concentrations tested, and the relative enhancement was greater than in the absence of NAPLs. This study shows the importance of hydrophobic interaction on bacterial transport in natural porous media and the potential of using low‐concentration rhamnolipid for facilitating cell transport in subsurface for bioaugmentation efforts.

Transport of Pseudomonas aeruginosa in Porous Media Mediated by Low‐Concentration Surfactants: The Critical Role of Surfactant to Change Cell Surface Hydrophobicity

“…The background solutions used in the all experiments were artificial ground water (AGW). It consists of (per liter): 0.006 g NaCl, 0.012 g CaSO 4 , 0.012 g NaHCO 3 , 0.002 g KNO 3 , and 0.035 g MgSO 4 ·7H 2 O (pH 6.8) (Liu, Zhong, et al, ). The CMCs of the SDBS and TX‐100 in AGW are 204 and 195 mg/L, respectively (Zhong, Zhang, et al, ).…”

“…The procedures for the cell transport experiments were similar to that described by Liu, Zhong, et al (). In brief, the columns were preequilibrated with at least 30 pore volumes (PVs) of bacteria‐free AGW solutions before each experiment.…”

Advances in applications of rhamnolipids biosurfactant in environmental remediation: A review

“…The successful bioremediation (bioaugmentation in particular) depends on effective transport of injected bacteria over considerable distances and adequate distribution of the injected bacteria throughout the contaminated area (Liu, Zhong, et al, ). However, the soil has a great adsorption capacity and straining potential for bacterial cells, which may significantly reduce the concentration of suspended bacterial cells as they transport (Zhong, Liu, et al, ).…”

“…For example, it was reported that low concentration of rhamnolipid significantly attenuated hydrophobic interactions between P. aeruginosa cells and glass beads, and hence reduced cell adsorption and enhanced cell transport (Zhong, Jiang, et al, ; Zhong, Liu, et al, ). The effect of low‐concentration mono‐rhamnolipid on transport of P. aeruginosa cells in natural porous media (silica sand and a sandy soil) with or without hexadecane as the nonaqueous phase liquid (NAPL) was further investigated in the study of Liu, Zhong, et al (), wherein it was observed that transport of cells in natural porous media was enhanced by rhamnolipid through decreasing bacterial CSH, and the relative enhancement in the presence of NAPL was greater than that in the absence of NAPL.…”

“…As discussed in section 2.2 that the morphology of rhamnolipid aggregates is closely related to rhamnolipids concentration, pH, ionic strength and DOM, and the bioavailability of hydrocarbons may be different under different aqueous phase conditions. Moreover, the concentration of rhamnolipid, DOM, temperature, pH and ionic strength of the aqueous phase have a significant influence on degradation activity and transport of bacteria in soils (Liu, Zhong, et al, ; Singh et al, ; Zhong et al, ), which are of importance for successful bioremediation.…”