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Issue 10, 2019
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Geochemical conditions conducive for retention of trace elements and radionuclides during shale–fluid interactions

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Abstract

Produced water generated during unconventional oil and gas extractions contains a complex milieu of natural and anthropogenic potentially toxic chemical constituents including arsenic (As), chromium (Cr), and cadmium (Cd), naturally occurring radioactive materials (NORMs) including U and Ra, and a myriad of organic compounds. The human-ecological health risks and challenges associated with the disposal of produced water may be alleviated by understanding geochemical controls on processes responsible for the solubilization of potentially hazardous natural shale constituents to produced water. Here, we investigated, through a series of batch treatments, the leaching behavior of As, Se, Cu, Fe, Ba, Cr, Cd, and radioactive nuclides U, Ra from shale to produced water. Specifically, the effect of four major controls on element mobility was studied: (1) solution pH, (2) ionic strength of the solution, (3) oxic–anoxic conditions, and (4) an additive used in fracking fluid. The mobilization of metals and metalloids from shale was greatest in treatments containing sodium persulfate, an oxidant and a commonly used additive in fracture fluid. In the high ionic strength treatments, dissolved Ba concentrations increased 5-fold compared to low ionic strength treatments. Overall, anoxic conditions superimposed with low pH resulted in the largest increase of dissolved metals and radionuclides such as Ra. Overall, our results suggest that (1) limiting pore water acidification by injection of alkaline fluid in carbonate-low shale and (2) minimizing strong oxidizing conditions in shale formations may result in cost-effective in situ retention of produced water contaminants.

Graphical abstract: Geochemical conditions conducive for retention of trace elements and radionuclides during shale–fluid interactions

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Publication details

The article was received on 25 May 2019, accepted on 24 Aug 2019 and first published on 03 Sep 2019


Article type: Paper
DOI: 10.1039/C9EM00244H
Environ. Sci.: Processes Impacts, 2019,21, 1764-1776
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    Geochemical conditions conducive for retention of trace elements and radionuclides during shale–fluid interactions

    N. Mehta and B. D. Kocar, Environ. Sci.: Processes Impacts, 2019, 21, 1764
    DOI: 10.1039/C9EM00244H

    This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Material from this article can be used in other publications provided that the correct acknowledgement is given with the reproduced material and it is not used for commercial purposes.

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