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Thermodynamics of Bromide Exchange on Ferrihydrite: Implications for Bromide Transport
Author(s) -
Brooks S. C.,
Jardine P. M.,
Taylor D. L.
Publication year - 1998
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1998.03615995006200050018x
Subject(s) - ferrihydrite , bromide , chemistry , thermodynamics , inorganic chemistry , environmental chemistry , physics , adsorption
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 [Fe 5 O 7 (OH)·4H 2 O]. 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.