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). 4H(2)O], Even with 475 times mure 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 now 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) approximate to 7.5). These results show that Br- can behave as a reactive tracer under certain circumstances, thus lending to erroneous estimates of transport parameters that rely solely on the use of Br- as a nonreactive solute.