Sc. Brooks et al., THERMODYNAMICS OF BROMIDE EXCHANGE ON FERRIHYDRITE - IMPLICATIONS FORBROMIDE TRANSPORT, Soil Science Society of America journal, 62(5), 1998, pp. 1275-1279
Because Br- is often assumed to be nonreactive with mineral surfaces,
it is frequently employed as a tracer in transport experiments. We inv
estigated the thermodynamics of Br--Cl- exchange on the synthetic ferr
ic 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 co
lumns packed with ferrihydrite-coated silica, Br- retardation increase
d 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). Thes
e results show that Br- can behave as a reactive tracer under certain
circumstances, thus lending to erroneous estimates of transport parame
ters that rely solely on the use of Br- as a nonreactive solute.