Bromide Adsorp on by Reference Minerals and Soils

2010, Vol. 9 Bromide Adsorp on by Reference Minerals and Soils Bromide, Br−, adsorp on behavior was inves gated on amorphous Al and Fe oxide, montmorillonite, kaolinite, and temperate and tropical soils. Bromide adsorp on decreased with increasing solu on pH with minimal adsorp on occurring above pH 7. Bromide adsorp on was higher for amorphous oxides than for clay minerals. Shi s in point of zero charge (PZC) were observed on amorphous Al and Fe oxide following Br− adsorp on, sugges ng an inner-sphere adsorp on mechanism for Br− on these surfaces. Ionic strength eff ects indicated an inner-sphere adsorp on mechanism for Br− on kaolinite and an outersphere adsorp on mechanism on amorphous Fe oxide. Two chemical surface complexa on models, the constant capacitance model and the triple layer model, were able to describe Br− adsorp on as a func on of solu on pH on all materials. For the oxides and clay minerals and most of the soils the fi t of the constant capacitance model, containing an inner-sphere adsorp on mechanism, was improved over that of the triple layer model, containing an outer-sphere adsorp on mechanism, as measured by the overall variance, VY. Bromide adsorp on on amorphous Fe oxide as a func on of solu on pH and solu on ionic strength was well described using the triple layer model. Our results indicate that Br− would most likely not act as a conserva ve tracer at soil solu on pH values below 7. Therefore, we suggest that researchers carefully evaluate the pH regime and mineralogy of their study site before assuming that Br− can be applied as a conserva ve tracer for transport experiments.