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.
Mining wastes or
combustion ash are materials of high carbon sequestration
potential but are also known for their toxicity in terms of heavy
metal content. To utilize such waste materials for engineered carbon
mineralization purposes, there is a need to investigate the fate and
mobility of toxic metals. This is a study of the coprecipitation of
metals with calcium carbonate for environmental heavy metal mitigation.
The study also examines the stability of precipitated phases under
environmentally relevant acid conditions. For a wide range of cadmium
(Cd) and zinc (Zn) concentrations (10 to 5000 mg/L), induced coprecipitation
led to greater than 99% uptake from water. The calcium carbonate phases
were found to contain amounts as high as 9.9 wt % (Cd) and 17 wt %
(Zn), as determined by novel synchrotron techniques, including X-ray
fluorescence element mapping and three-dimensional (3D) nanotransmission
X-ray microscopy (TXM). TXM imaging revealed first-of-a-kind observations
of chemical gradients and internal nanoporosity within particles.
These observations provided new insights into the mechanisms leading
to the retention of coprecipitated heavy metals during the dissolution
of calcite in acidic (pH 4) solutions. These observations highlight
the feasibility of utilizing carbonate coprecipitation as an engineered
approach to the durable sequestration of toxic metals.
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