Is Bromide a Conservative Tracer in Soils?.

Bromide, Br^-, adsorption behavior was investigated on amorphous Al and Fe oxide, montmorillonite, kaolinite, and temperate and tropical soils.  Bromide adsorption decreased with increasing solution pH with minimal adsorption occurring above pH 7.  Bromide adsorption was higher for amorphous oxides than for clay minerals.  Shifts in point of zero charge were observed on amorphous Al and Fe oxide following Br^- adsorption suggesting an inner-sphere adsorption mechanism for Br^- on these surfaces.  Two chemical surface complexation models, the constant capacitance model and the triple layer model, were able to describe Br^- adsorption as a function of solution pH on all materials.  For the oxides and clay minerals and most of the soils the fit of the constant capacitance model, containing an inner-sphere adsorption mechanism was improved over that of the triple layer model, containing an outer-sphere adsorption mechanism, as measured by the overall variance, V_Y.  Our results indicate that Br^- would, most likely, not act as a conservative tracer at soil solution pH values below 7.  Therefore, we suggest that researchers carefully evaluate the pH regime of their study site before assuming that Br^- can be applied as a conservative tracer for transport experiments.