Thermodynamics of bromide exchange on ferrihydrite: Implications for bromide transport

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.