19    SOLUBILIZATION AND BIODEGRADATION OF
OCTADECANE IN THE PRESENCE OF TWO
COMMERCIAL SURFACTANTS
Le T. Thai, Research Assistant
Walter J. Maier, Professor
Department of Civil and Mineral Engineering
University of Minnesota
Minneapolis, Minnesota 55455
INTRODUCTION
Compounds are usually only biodegraded when they are dissolved in an aqueous solution or at least
in direct contact with water.1"6 Accessibility is a particular concern with hydrocarbons that have low
aqueous solubility because in a soil environment, they will tend to sorb or partition to the soil matrix,
or form separate organic phases in the pore spaces. In order to enhance biodegradation of these
compounds, it is necessary to accelerate their desorption and/or solubilization rate. Desorption and/
or solubilization can be accelerated with the addition of surfactants, which have been shown to be
effective in enhancing desorption and solubilization of hydrophobic compounds.7,8
Surfactants, or surface active agents, are amphipathic molecules consisting of a polar/hydrophilic
head and a nonpolar/hydrophobic tail. When added to water, a surfactant molecule may dissolve as a
monomer, adsorb at an interface with its hydrophobic end pointing away from the water, or aggregate
with other surfactant molecules into clusters called micelles.910 The first two forms dominate below
the critical micelle concentration, and the third above. The critical micelle concentration (CMC)
corresponds to the surfactant concentration at which micelles are formed. Micelles are organized
aggregates of surfactant molecules with the molecules' hydrophobic groups directed toward the
interior of the aggregates and their hydrophilic groups directed toward the water.10 The CMC value is
unique for each surfactant and temperature. Furthermore, at the CMC, there are sharp breaks in
many physical properties of the surfactant solution such as its electrical conductivity and surface
tension.10
Surfactants can enhance the apparent solubility of a hydrophobic compound through at least two
mechanisms. The first mechanism is dominant below the critical micelle concentration. It involves the
"association" of the hydrophobic compound with the hydrophobic groups of the surfactant molecules. The second mechanism is important above the CMC and involves the partitioning of the
hydrophobic compound into the hydrophobic/nonpolar centers of the surfactant micelles."
While surfactants' ability to enhance desorption and solubilization of hydrophobic compounds has
been studied by many researchers,912 the effect of surfactants on biodegradability has been less
thoroughly studied. The purpose of this research was to characterize both the potential for enhancement of solubilization of a sparingly soluble compound through surfactant addition, and the potential
for enhancing rates of biodegradation.
MATERIALS AND METHODS
Solubility Enhancement Experiments
Solubilities of octadecane were measured in a series of solutions containing from 5 to 300 mg/L of
surfactants. Octadecane (purity = 99%) was obtained from Aldrich Chemical Company, Milwaukee,
WI. Carbon-14 octadecane, labelled in theC-1 position (specific activity: 3.6 mCi per mmol; purity >
98%) was obtained from Sigma Chemical Company, St. Louis, MO. Triton X-l 14 (manufactured by
Rohm and Haas) was also obtained from Sigma Chemicals, St. Louis, MO; Corexit 0600 was
obtained from Exxon Chemical Company, Houston, TX. Surfactants were used as received without
further purification. Hexane was obtained from Fisher Scientific Co., Pittsburgh, PA. Acrodisc HT
Tuffryn 0.2 /tm polysulfone membrane filters (manufactured by Gelman) were obtained from Fisher
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