The Growth of Gypsum in the Presence of Hexavalent Chromium: A Multiscale Study

The sorption of dissolved inorganic pollutants into the structure of minerals is an important process that controls the mobility and fate of these pollutants in the Earth’s crust. It also modifies the surface structure and composition of the host mineral, affecting its crystallization kinetics. Here, we investigate the effect of hexavalent chromium, Cr(VI), on the nucleation and growth of gypsum by conducting two types of experiments: (i) in situ atomic force microscopy (AFM) observations of the growth of gypsum {010} surfaces in the presence of Cr(VI) and (ii) gypsum precipitation experiments by mixing aqueous solutions containing variable amounts of Cr(VI). Gypsum precipitation is progressively delayed when occurring from solutions bearing increasing Cr(VI) concentrations. Chemical analyses of gypsum precipitates show that gypsum incorporates small Cr(VI) amounts that correlate with the content of this ion in the aqueous solution. Gypsum cell parameters variation reflects this incorporation. At the molecular scale, Cr(VI) induces a slowdown of step advance rates on gypsum {010} surfaces accompanied by the roughening of nanostep edges and the so-called “template effect”. This effect involves the reproduction of the original nanotopography after the completion of individual advancing monolayers and appears as a general nanoscale phenomenon occurring during growth of solid solutions from aqueous solutions even in the case of compositionally-restricted solid solutions.