Biosurfactant- and Cosolvent-Enhanced Remediation of

Chapter 7

Biosurfactant- and Cosolvent-Enhanced Remediation of Contaminated Media

Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0594.ch007

Mark L. Brusseau, Raina M . Miller, Yimin Zhang, Xiaojiang Wang, and Gui-Yun Bai Soil and Water Science Department, University of Arizona, Tucson, AZ 85721

The use of water flushing (pump and treat, in situ soil washing) is one of the predominant methods currently in use for remediation of contaminated subsurface environments. While this method has been successful in some cases, its effectiveness is often constrained by one or more factors related to contaminant transport and fate. Recent research has focused on chemical additives that might be useful for enhancing contaminant removal during flushing. Examples include the addition of surfactants, cosolvents, and complexing agents. We are involved in the study of cosolvents and microbially produced surfactants (biosurfactants) and their effects on solubilization, biodegradation, and removal of residual phases from the subsurface. In this paper, we summarize our recent results and provide a comparison of the advantages and disadvantages in the use of biosurfactants and cosolvents in remediation of contaminated subsurface environments.

The use of water flushing (pump and treat, in situ soil washing) is one of the predominant methods currently in use for remediation of contaminated subsurface environments. While this method has been successful in some cases, its effectiveness is often constrained by one or more factors related to contaminant transport and fate. These factors include porous-media heterogeneity, dissolution of residual inuniscible liquid, and rate-limited desorption (cf., 1-3). Recent research has focused on chemical additives that might be useful for enhancing contaminant removal during flushing. Examples include the addition of surfactants, cosolvents, and complexing agents. The basis for using chemical additives to enhance recovery of organic compounds

0097-6156/95/0594-0082$12.00/0 © 1995 American Chemical Society

In Surfactant-Enhanced Subsurface Remediation; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Remediation of Contaminated Media

83

from porous media was established in petroleum science and engineering with the development of enhanced oil recovery (EOR) techniques. While the basic concepts developed for EOR are useful, it is not necessarily possible to apply them directly to environmental systems. Discussions of chemical enhancement techniques for environmental applications have been presented by several authors (3-7). Surfactants are currently the focus of the research effort on chemical enhancements and, based on preliminary laboratory data, appear to have promise for enhancing pump-and-treat remediation in some situations. The use of dissolved organic matter (DOM) and of solvents is also being investigated, albeit at a smaller scale. Miscible solvents, such as ethanol, reduce the net polarity of the mixed solvent when added to water and thereby increase the quantity of a nonionic organic compound that can dissolve in the mixed solvent. This increase, in turn, results in a smaller equilibrium sorption constant and less attendant retardation. Thus, the addition of a cosolvent can reduce the volume of water required to flush a contaminant from porous media by altering the equilibrium phase distribution. A similar result is obtained with surfactants and DOM, although by different mechanisms. Hence, surfactants, D O M , and cosolvents act to increase the aqueous-phase concentration of organic compounds, the so-called "solubilization" effect This effect is of special interest for the removal of residual phases of immiscible liquids and of highly sorbed solutes. The other major method of removing trapped residual phases is mobilization. We are involved in the study of cosolvents and microbially produced surfactants (biosurfactants) and their effects on solubilization, biodégradation, and removal of residual phases from the subsurface. In this paper, we summarize our recent results and provide a comparison of the advantages and disadvantages in the use of biosurfactants and cosolvents in remediation of contaminated subsurface environments. Biosurfactants: Rhamnolipids Biosurfactants are a class of surfactants that are produced by microorganisms, plants, and animals (8). Of particular interest in remediation are bacterial biosurfactants, a structurally diverse class of anionic or nonionic compounds rangingfrom500 to 1500 MW (7). Studies in our laboratory have shown that at least one type of biosurfactant, rhamnolipids, increases the apparent aqueous solubility of a variety of organic compounds (9-11). Rhamnolipids are produced by Pseudomonas aeruginosa strains, often in mixtures of several rhamnolipid types. Structure-function studies of rhamnolipids suggest that structural changes (Figure 1) caused differences in the mode of action and impact of the rhamnolipid on the apparent solubility of hexadecane (Figure 2). For example, the dirhamnolipid methyl ester acts primarily by emulsification, which explains its much greater impact on hexadecane concentration. The monorhamnolipid acid has been tested for effectiveness of removal of residual non-aqueous phase liquids (NAPL). As shown in Figure 3 (from


84

SURFACTANT-ENHANCED SUBSURFACE REMEDIATION

HO Δ

Ο OCH - CH COO - CH - CHgCOOR 2

OH

OH OH

(ÇH ) 2


C H

HO

Λ

ι

3

CH.

Ο

>-

Β.

OCH - CH COO - ÇH - CH C00R

OH ϋ

/ — 4Ο

OH Figure 1.

M

2

H

m

ι

CH.

Ο

o

H

Ç 2>n

CH,

OH

The basic rhamnolipid structures A) monorhamnolipid where R = Η or C H , and B) dirhamnolipid where R = Η or C H . 3

3

0.5

"35

u

ft

ce

I Rhamnolipid concentration (mM) Figure 2.

Solubilization (dispersion) of hexadecane by monorhamnolipid-acid, dirhamnolipid acid, and dirhamnolipid-methyl ester.


7. BRUSSEAU ET AL.

Remediation of Contaminated Media

85


1

12), monorhamnolipid add (1 mM, 500 mg Γ )) removed approximately 22% of residual hexadecane from sand columns in 120 pore volumes. A comparison of monorhamnolipid with two synthetic surfactants reveals that sodium dodecyl sulfate was ineffective in removing residual, and that polyoxyethylene(20) sorbitan monooleate was 4-fold less effective than the monorhamnolipid (see Figure 3). Removal of the hexadecane residual was primarily by mobilization. Although biosurfactants are effective solubilization and emulsification agents, the more intriguing aspect of biosurfactants is the potential for enhanced biodégradation of organic compounds during remediation. We have shown that rhamnolipids are effective in increasing biodégradation rates of organic compounds in batch solution culture (9,10,13). There are two important factors in determining enhancement of biodégradation; the effect of the rhamnolipid on solubilization/emulsification (or bioavailability) of substrate, and the effect of the rhamnolipid on cell surfaces and the uptake of solubilized substrate. Enhancement of biodégradation, like enhancement of solubilization/emulsification, is dependent on the structure of the rhamnolipid. We have observed increases in hexadecane biodégradation of up to 30-fold by the dirhamnolipid methyl ester (0.05 mM), and 22-fold by the dirhamnolipid acid (0.05 mM). For all organisms and organic compounds tested thus far, the dirhamnolipid methyl ester has been most effective in solubilization/emulsification and enhancement of biodégradation. One problem in working with the dirhamnolipid methyl ester is its low water solubility (

Biosurfactant- and Cosolvent-Enhanced Remediation of

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