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Effect of pH on Sorption and Transport of Fluorobenzoic Acid Ground Water Tracers

Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Bethel Valley Rd., Oak Ridge, TN 37831-6036.
Dep. of Geological Sciences, Univ. of Tennessee, 1412 Circle Dr., Knoxville, TN 37996-1410.
Theta Technologies, Inc., 101 E. Tennessee Ave., Oak Ridge, TN 37830.

^* Corresponding author (mccarthyjf{at}ornl.gov).

ABSTRACT

Sorption and transport behavior of fluorobenzoic acid (FBA) tracers were evaluated as ground water tracers in a highly weathered and fractured clay-rich saprolite under both acidic and neutral conditions in laboratory batch tests and column transport studies. The FBAs are a group of 14 organic acids with pK_a from 2.71 to 3.83, and may thus be potentially retarded in weathered, acidic soils. The degree of sorption was directly related to the pK_a of the isomer and the pH of the solution. A technique for determining the intrinsic partitioning coefficient (K_d) of an non-ionized species was derived to predict the equilibrium sorption of a family of compounds over a range of pH and pK_a. For the diflourobenzoic acids used in this study, K_d was 2.50 L/kg. The intact column transport experiments confirmed that at neutral pH, none of the FBA isomers were retarded relative to a nonreactive tracer (Br^–). Under acidic conditions (pH = 5.2), significant retardation was observed for isomers with higher pK_a. Because the use of FBA tracers could be limited by regulatory concerns about potential toxic effects in surface water receiving discharge from ground water containing FBA, aquatic toxicity was assessed using a Ceriodaphnia 96-h acute toxicity test. The LD50 (the concentration resulting in mortality of 50% of the test organisms) for the four FBA isomers tested was above 100 mg/L. The results of these equilibrium isotherm and column transport experiments demonstrate that FBAs can be very useful as nonreactive ground water tracers in these materials as long as the pH is approximately 2 pH units above the pK_a (i.e., >99% ionized).

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