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Acid erosion of carbonate fractures and accessibility of arsenic-bearing minerals: In Operando synchrotron-based microfluidics experiment.

Underground flows of acidic fluids through fractured rock can create new porosity and increase accessibility to hazardous trace elements such as arsenic. In this study, we developed a custom microfluidic cell for an in operando synchrotron experiment using x-ray attenuation. The experiment mimics reactive fracture flow by passing acidic fluid over a surface of mineralogically heterogeneous rock from the Eagle Ford shale. Over 48 hours, calcite was preferentially dissolved, forming an altered layer 200 to 500 μm thick with a porosity of 63-68% and surface area >10x higher than in the unreacted shale as shown by xCT analyses. Calcite dissolution rate quantified from the attenuation data was 3x10-4 mol/m2s, and decreased to 3x10-5 mol/m2s after 24 hours due to increasing diffusion limitations. Erosion of the fracture surface increased access to iron-rich minerals, thereby increasing access to toxic metals such as arsenic. Quantification using XRF and XANES microspectroscopy indicated up to 0.5 wt% of As(-I) in arsenopyrite and 1.2 wt% of As(V) associated with ferrihydrite. This study provides valuable contributions for understanding and predicting fracture alteration and changes to the mobilization potential of hazardous metals and metalloids.