Thermodynamics of Bromide Exchange on Ferrihydrite: Implications for Bromide Transport
- S. C. Brooks ,
- P. M. Jardine and
- D. L. Taylor
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
Copyright © .