Acid Erosion of Carbonate Fractures and Accessibility of Arsenic-Bearing Minerals: In Operando Synchrotron-Based Microfluidic Experiment

Deng, Hang; Fitts, Jeffrey P.; Tappero, Ryan V.; Kim, Julie J.; Peters, Catherine A.

doi:10.1021/acs.est.0c03736

Title: Acid Erosion of Carbonate Fractures and Accessibility of Arsenic-Bearing Minerals: In Operando Synchrotron-Based Microfluidic Experiment

Journal Article · 26 August 2020 · Environmental Science and Technology

DOI: https://doi.org/10.1021/acs.est.0c03736 · OSTI ID:1658975

^[1]; Fitts, Jeffrey P. ^[2]; Tappero, Ryan V. ^[3]; Kim, Julie J. ^[4];

^[4]

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Columbia University, New York, NY (United States)
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Princeton University, NJ (United States)

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 an acidic fluid over a surface of mineralogically heterogeneous rock from the Eagle Ford shale. Over 48 h, calcite was preferentially dissolved, forming an altered layer 200–500 μm thick with a porosity of 63–68% and surface area >10× higher than that in the unreacted shale as shown by xCT analyses. Calcite dissolution rate quantified from the attenuation data was 3 × 10^–4 mol/m²s and decreased to 3 × 10^–5 mol/m²s after 24 h because of 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.

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Research Organization:: Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source and National Synchrotron Light Source II (NSLS-II); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)

Sponsoring Organization:: National Science Foundation (NSF); USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division