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

Abstract

AbstractSorption 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 pKa 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 pKa of the isomer and the pH of the solution. A technique for determining the intrinsic partitioning coefficient (Kd) of an non‐ionized species was derived to predict the equilibrium sorption of a family of compounds over a range of pH and pKa. For the diflourobenzoic acids used in this study, Kd 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 pKa. 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 pKa (i.e., >99% ionized).