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This article in SSSAJ

  1. Vol. 62 No. 5, p. 1275-1279
     
    Received: Sept 22, 1997


    * Corresponding author(s): 3sb@ornl.gov
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doi:10.2136/sssaj1998.03615995006200050018x

Thermodynamics of Bromide Exchange on Ferrihydrite: Implications for Bromide Transport

  1. S. C. Brooks ,
  2. P. M. Jardine and
  3. D. L. Taylor
  1. Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, MS 6038, Oak Ridge, TN 37831-6038

Abstract

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 (pHzpc ≈ 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.

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