Pristine silver nanoparticles (AgNPs) are not chemically stable in the environment and react strongly with inorganic ligands such as sulfide and chloride once the silver is oxidized. Understanding the environmental transformations of AgNPs in the presence of specific inorganic ligands is crucial to determining their fate and toxicity in the environment. Chloride (Cl(-)) is a ubiquitous ligand with a strong affinity for oxidized silver and is often present in natural waters and in bacterial growth media. Though chloride can strongly affect toxicity results for AgNPs, their interaction is rarely considered and is challenging to study because of the numerous soluble and solid Ag-Cl species that can form depending on the Cl/Ag ratio. Consequently, little is known about the stability and dissolution kinetics of AgNPs in the presence of chloride ions. Our study focuses on the dissolution behavior of AgNPs in chloride-containing systems and also investigates the effect of chloride on the growth inhibition of E.coli (ATCC strain 33876) caused by Ag toxicity. Our results suggest that the kinetics of dissolution are strongly dependent on the Cl/Ag ratio and can be interpreted using the thermodynamically expected speciation of Ag in the presence of chloride. We also show that the toxicity of AgNPs to E.coli at various Cl(-) concentrations is governed by the amount of dissolved AgCl(x)((x-1)-) species suggesting an ion effect rather than a nanoparticle effect.

Hydraulic fracturing of unconventional hydrocarbon reservoirs is critical to the United States energy portfolio; however, hydrocarbon production from newly fractured wells generally declines rapidly over the initial months of production. One possible reason for this decrease, especially over time scales of several months, is the mineralization and clogging of microfracture networks and pores proximal to propped fractures. One important but relatively unexplored class of reactions that could contribute to these problems is oxidation of Fe(II) derived from Fe(II)-bearing phases (primarily pyrite, siderite, and Fe(II) bound directly to organic matter) by the oxic fracture fluid and subsequent precipitation of Fe(III)-(oxy)hydroxides. The extent to which such reactions occur and their rates, mineral products, and physical locations within shale pore spaces are unknown. To develop a foundational understanding of potential impacts of shale iron chemistry on hydraulic stimulation, we reacted sand-sized (150-250 μm) and whole rock chips (cm-scale) of shales from four different formations

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Hydraulic fracturing of unconventional shale reservoirs increases the fracture network surface area to access hydrocarbons from the low permeability rock matrix. Porosity and permeability of the matrix, through which hydrocarbons migrate to fractures, are important for determining production efficiency and can be altered by chemical interactions between shale and hydraulic fracturing fluids (HFFs). Here, we present results from an experimental study that characterizes the thickness of the alteration zone in the shale matrix after shale−HFF interactions. Experiments were conducted with whole cores submerged in HFF both with and without added barium and sulfate to promote barite scale formation. After 3 weeks of reaction at 77 bar and 80 °C, the cores were characterized using X-ray microtomography, synchrotron X-ray fluorescence microprobe imaging, and synchrotron X-ray absorption spectroscopy. Our results show that the thickness of the altered zone depends on shale mineralogical composition and varies for different chemical reactions. For reactions between the low-carbonate Marcellus shale and HFF, pyrite (FeS 2 ) oxidation manifests as both a thick zone of sulfur oxidation (>0.5 cm) and a thinner zone of iron oxidation (100−150 μm). Carbonate dissolution extended 100−200 μm into the matrix, with the resulting observable secondary porosity localized at the shale−fluid interface where mineral grains were removed by either dissolution or mechanical erosion. In solutions oversaturated with respect to barite, barite precipitates were observed in the reaction fluid and at the shal− HFF interface. In contrast, the carbonate dissolution zone in the high-carbonate Eagle Ford was only 30−40 μm thick, within which a uniform texture of increased porosity was observed. Pyrite oxidation in the Eagle Ford was evident from an iron oxidation zone (150−200 μm thick), while sulfur oxidation was minor and hard to observe. Barite precipitation extended 1−2 mm into the matrix when the initial HFF was oversaturated with respect to barite, filling shale microcracks down to the submicrometer length scale. Our findings provide a scientific basis to predict the extent of chemical alteration in shale reservoirs during hydraulic fracturing and its impacts on hydrocarbon production.

Mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes provides a new tool for tracing Hg in contaminated environments such as mining sites, which represent major point sources of Hg pollution into surrounding ecosystems. Here, we present Hg isotope ratios of unroasted ore waste, calcine (roasted ore), and poplar leaves collected at a closed Hg mine (New Idria, CA, U.S.A.). Unroasted ore waste was isotopically uniform with δ(202)Hg values from -0.09 to 0.16‰ (± 0.10‰, 2 SD), close to the estimated initial composition of the HgS ore (-0.26‰). In contrast, calcine samples exhibited variable δ(202)Hg values ranging from -1.91‰ to +2.10‰. Small MIF signatures in the calcine were consistent with nuclear volume fractionation of Hg isotopes during or after the roasting process. The poplar leaves exhibited negative MDF (-3.18 to -1.22‰) and small positive MIF values (Δ(199)Hg of 0.02 to 0.21‰). Sequential extractions combined with Hg isotope analysis revealed higher δ(202)Hg values for the more soluble Hg pools in calcines compared with residual HgS phases. Our data provide novel insights into possible in situ transformations of Hg phases and suggest that isotopically heavy secondary Hg phases were formed in the calcine, which will influence the isotope composition of Hg leached from the site.

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