SORPTION OF CHROMATE IONS DIFFUSING THROUGH BARITE-HYDROGEL COMPOSITES: IMPLICATIONS FOR THE FATE AND TRANSPORT OF CHROMIUM IN THE ENVIRONMENT

Abstract

The interaction of Cr(VI) with barite is studied by quantifying the effect of this mineral on the net flux of chromate ions diffusing through an artificial porous medium consisting of barite grains embedded in a matrix of silica hydrogel. The gel suppresses convection and advection, only allowing diffusion of the aqueous ions, which eventually can be sorbed on the surface of the embedded grains. We find that long-term Cr(VI) uptake by barite occurs by epitaxial overgrowth of a Ba(CrO4,SO4) solid solution with the barite structure. In these particular experiments, the epitaxial crystallites have compositions around BaCr0.89S0.11O4. Sorption on barite reduces the net flux of chromate ions in relation to the flow through an equivalent (with the same porosity and tortuosity) but unreactive quartz-gel composite. A linear sorption model with a factor Kd = 0.291 was used to account for the experimental results. This factor is a complex measure that depends on the bulk medium characteristics and on the tendency of CrO42- to partition into barite under the precipitation conditions. Here, we assess the operating precipitation conditions in terms of possible limiting scenarios of supersaturation and discuss their influence on the partitioning of CrO42- ions into barite. The results demonstrate that precipitation of Ba(SO4,CrO4) solid solutions may be an option to control the concentration of Cr(VI) in natural waters. Neglecting to consider such solid solution formation will lead to overestimates of the availability and mobility of Cr(VI) in the environment.