Compatible real-time rates of mineral dissolution by Atomic Force Microscopy (AFM)

Abstract

The Fluid Cell attachment to the Atomic Force Microscope can be used to emulate batch and flow-through reactors to observe the progression of mineral-water interaction processes on individual surfaces. Recent applications of this method include the in-situ measurement of mineral dissolution or growth rates by comparing time-sequenced images. Because AFM images are collected as lines of information by the physical rastering of a lever over a mineral surface, there are inherent limitations to the range of reaction rates compatible with in-situ AFM methods. This investigation examines the AFM-compatible range of monolayer formation or removal rates on individual mineral surfaces. We estimate that AFM can be used to observe dissolution or growth processes occurring at rates in the range of 10 −10 − 10 −6 mol m −2 s −1. This calculated estimate of the reaction rate range was compared with estimates of dissolution rates from time-sequenced in-situ observations of dissolution on specific cleavage faces of calcite, barite and celestite. Our AFM observations show a dissolution rate trend which follow the order calcite (at near-zero P CO 2 ) > celestite > barite in deionized water at 30°C. These observations of relative differences in dissolution rates are consistent with studies of bulk dissolution rates using geochemical reactors. Yet, the absolute rates estimated from time-sequenced images are considerably slower than bulk reaction rates in all cases. This is probably because our experiments: (1) use relatively smooth areas which are less reactive than roughened areas with large step, defect or pit densities; and (2) cannot simultaneously observe the reactivity of other, possibly more reactive, mineral surfaces. Our findings demonstrate the sensitivity of Fluid Cell AFM as a probe of small differences in the reactivity of individual steps and surfaces. They also suggest that AFM cannot be used to observe reactions on individual surfaces which have dissolution rates outside of the estimated compatible rate boundaries of 10 −10 to 10 −6 mol m −2 s −1. Mineral surfaces with reactivities outside of this span of rates may be adjusted into the AFM-compatible range by controlling chemical affinity through saturation state or solution pH. These observations offer guidelines for designing new in-situ AFM studies of mineral-water interactions.