Civil and Environmental Engineering, Massachusetts Institute of Technology, 15 Vassar St, Cambridge, MA 02139, USA.
Environ Sci Process Impacts. 2019 Oct 16;21(10):1764-1776. doi: 10.1039/c9em00244h.
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.
在非常规石油和天然气开采过程中产生的产出水中，含有复杂的天然和人为潜在有毒化学成分，包括砷（As）、铬（Cr）和镉（Cd）、天然放射性物质（NORMs），包括 U 和 Ra，以及无数有机化合物。通过了解对潜在危险天然页岩成分溶解到产出水中的过程的地球化学控制，可以减轻与产出水处置相关的人类-生态健康风险和挑战。在这里，我们通过一系列批处理实验研究了 As、Se、Cu、Fe、Ba、Cr、Cd 和放射性核素 U、Ra 从页岩到产出水的浸出行为。具体来说，研究了四种主要控制因素对元素迁移性的影响：（1）溶液 pH 值，（2）溶液的离子强度，（3）好氧-缺氧条件，和（4）压裂液中使用的添加剂。在含有过硫酸钠的处理中，金属和类金属从页岩中的迁移最大，过硫酸钠是一种氧化剂，也是压裂液中常用的添加剂。在高离子强度处理中，溶解的 Ba 浓度比低离子强度处理增加了 5 倍。总的来说，缺氧条件与低 pH 值叠加导致溶解的金属和放射性核素（如 Ra）的增加最大。总的来说，我们的结果表明，（1）通过注入碱性流体来限制碳酸盐含量低的页岩中孔隙水的酸化，以及（2）在页岩地层中最小化强氧化条件，可能会导致产出水污染物的原位有效保留，从而降低成本。
