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Thermodynamics of Bromide Exchange on Ferrihydrite: Implications for Bromide Transport

Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, MS 6038, Oak Ridge, TN 37831-6038

*Corresponding author (3sb{at}ornl.gov)

ABSTRACT

Because Br^- is often assumed to be nonreactive with mineral surfaces, it is frequently employed as a tracer in transport experiments. We investigated the thermodynamics of Br^--Cl^- exchange on the synthetic ferric oxide ferrihydrite [Fe₅O₇(OH)·4H₂O]. Even with 475 times more Cl^- than Br^- in solution, sites on the ferrihydrite surface selectively adsorbed Br^-. The implications of these results for Br^- transport were examined in a series of column flow experiments. During flow through columns packed with ferrihydrite-coated silica, Br^- retardation increased from 0.912 to 2.42 as the pH of column experiments decreased from 7.8 to 5. This behavior is consistent with the variable-charge nature of ferrihydrite, which exhibits increasing positive surface charge below the pH of its zero point of charge (pH_zpc 7.5). These results show that Br^- can behave as a reactive tracer under certain circumstances, thus leading to erroneous estimates of transport parameters that rely solely on the use of Br^- as a nonreactive solute.

This research was sponsored by the Subsurface Science Program, Co-Contaminant Chemistry Subprogram of the Environmental Science Division, Office of Biological and Environmental Research, U.S. Dep. of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp. Further support was provided by an appointment to the Oak Ridge National Lab. Postdoctoral Research Associates Program administered jointly by the Oak Ridge National Lab. and the Oak Ridge Inst. for Science and Education. Publication no. 4760, Environmental Sciences Division, Oak Ridge National Lab.

Received for publication September 22, 1997.

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