Environmental Pollution xxx (2018) xxx-xxx
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Environmental Pollution
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journal homepage: www.elsevier.com
Uranium (VI) transport in saturated heterogeneous media: Influence of kaolinite and humic acid☆
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Chong Chen1, a, Kang Zhao1, a, Jianying Shanga, ∗, Chongxuan Liub, Jin Wangc, Zhifeng Yand, Kesi Liue, Wenliang Wuf a
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Key Laboratory of Plant-Soil Interactions, The Ministry of Education, Key Laboratory of Arable Land Conservation in North China, The Ministry of Agriculture, College of Resources and Environmental Sciences, China Agricultural University, Beijing, 100193, China b School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China c School of Environmental Science and Engineering, Guangzhou University, Guangzhou, 510006, China d Institute of Surface-Earth System Science, Tianjin University, Tianjin, 300072, China e Department of Grassland Science, China Agricultural University, Beijing, 100193, China f Beijing Key Laboratory of Farmland Soil Pollution Prevention and Remediation, College of Resources and Environmental Sciences, China Agricultural University, Beijing, 100193, China
ARTICLE INFO
ABSTRACT
Article history: Received 23 January 2018 Received in revised form 31 March 2018 Accepted 22 April 2018 Available online xxx
Natural aquifers typically exhibit a variety of structural heterogeneities. However, the effect of mineral colloids and natural organic matter on the transport behavior of uranium (U) in saturated heterogeneous media are not totally understood. In this study, heterogeneous column experiments were conducted, and the constructed columns contained a fast-flow domain (FFD) and a slow-flow domain (SFD). The effect of kaolinite, humic acid (HA), and kaolinite/HA mixture on U(VI) retention and release in saturated heterogeneous media was examined. Media heterogeneity significantly influenced U fate and transport behavior in saturated subsurface environment. The presence of kaolinite, HA, and kaolinite/HA enhanced the mobility of U in heterogeneous media, and the mobility of U was the highest in the presence of kaolinite/HA and the lowest in the presence of kaolinite. In the presence of kaolinite, there was no difference in the amount of U released from the FFD and SFD. However, in the presence of HA and kaolinite/HA, a higher amount of U was released from the FFD. The findings in this study showed that medium structure and mineral colloids, as well as natural organic matter in the aqueous phase had significant effects on U transport and fate in subsurface environment.
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Keywords: Uranium (VI) Transport Heterogeneous medium Kaolinite Humic acid
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1. Introduction
Uranium (U) is a long-lived radionuclide that occurs naturally at very low concentrations in soil, rock, and water. However, the concentration of U in environment tends to become higher owing to human activities such as uranium milling and mining, production and disposal of radioactive materials, and application of phosphate fertilizers (Zielinski et al., 2006; Bachmaf and Merkel., 2011). Excess levels of U can contaminate soil and groundwater, and pose a great threat to human health. The permissible level of U in drinking water, according to the guideline value set by the World Health Organization, is very low (i.e., 2–30 μg L−1), and exposure to U above these levels can cause nephrotoxic effects and damage to the proximal tubulus (Gilman et al., 1998; Kurttio et al., 2002; Vicente-Vicente et al., 2010). Thus, uranium pollution is an ongoing concern in environmental research.
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This paper has been recommended for acceptance by.Sarah Harmon.
∗
Corresponding author. 1These authors contributed equally to this work.
Email address:
[email protected] (J. Shang)
https://doi.org/10.1016/j.envpol.2018.04.095 0269-7491/ © 2018.
© 2018.
Heterogeneity is a common phenomenon that occurs in natural aquifers in the presence of earthworm burrows, cracks in soil, and layered soils, which causes contrasting hydraulic conductivities in some regions. Medium heterogeneity can considerably influence the reaction, transport, and fate of U in the environment. Mikake et al. (2000) showed that the distribution of uranium in a subsurface environment was likely controlled by the hydraulic conductivity field. Medium heterogeneity is a key factor for determining the rates of U bioreduction (Li et al., 2011). Liu et al. (2014) found that the rate of U desorption in heterogeneous medium was significantly lower than that in relatively homogeneous medium. These results are mainly due to the presence of preferential flow and low reaction rates in heterogeneous medium (Allaire et al., 2009; Liu et al., 2013, 2014). Furthermore, the above findings highlight the importance of understanding the transport behaviors of U in heterogeneous medium for accurately assessing the risk posed by U in subsurface environment. Mineral colloids and humic acid (HA, organic macromolecules) are ubiquitous in the environment. U can be immobilized on a mineral surface by either surface complexation or precipitation (Duff and Amrheim, 1996; Barnett et al., 2000; Moyes et al., 2000; Bachmaf and Merkel., 2011; Shang et al., 2011). HA is soluble in natural waters and can form soluble HA–U complexes with U through the pres
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sonic dispersion for 10 min, and then filtered with a 0.45 μm nylon membrane filter (Jinlong, Tianjin, China) before use. Four types of U-spiked influent solutions or suspensions (i.e., U only, U+kaolinite, U+HA, and U+HA+kaolinite) were prepared in 1 mM NaCl solution. The concentrations of the components in U-spiked influents were U = 1 mg L−1, HA = 20 mg L−1 (the HA-containing influents), and kaolinite = 100 mg L−1 (the kaolinite-containing influents), and the pH of the influents was adjusted to 6 using 0.1 M HCl and 0.1 M NaOH.
2. Materials and methods
Column experiments were conducted in a transparent acrylic column (length 12.4 cm, inner diameter 2.2 cm). The base and top of the column were equipped with two layers of nylon screen (pore size 50 μm) to seal the sands and distribute the flow. The preferential flow system was set up following the procedure in the literature (Lv et al., 2016). The sand specimens were packed into two regions under saturated condition with vibration to minimize layering and air entrapment (Fig. S1). During the packing process, a plastic sheet (
