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Role of solution chemistry in the retention and release of graphene oxide nanomaterials in uncoated and iron oxide-coated sand
Citation:
Wang, D., C. Shen, Y. Jin, C. Su, L. Chu, AND D. Zhou. Role of solution chemistry in the retention and release of graphene oxide nanomaterials in uncoated and iron oxide-coated sand. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 579:776-785, (2017).
Impact/Purpose:
Graphene oxide nanoparticles (GONPs) are an important 2-D nanomaterials used for many applications. Fat and transport of GONPS are dependent on solution chemistry as well as intrinsic properties of GONPs. Deposition and remobilization of GONPs were thoroughly studied in this paper. Interested people include program and regional partners, general public, and academia.
Description:
Upon increasing production and use of graphene oxide nanoparticles (GONPs), concerns agitate over their potential impacts and risks to the environment, ecosystem, and human health. An improved understanding of the fate and transport including remobilization of GONPs in the subsurface environment enables us to better expedite the benign use of GONPs in a sustainable fashion but also evaluate their environmental impacts and health risks. In this study, the deposition and release of GONPs were systematically examined in water-saturated columns packed with either uncoated sand or iron oxide-coated sand (U-S and Fe-S, respectively) at environmentally relevant solution chemistry conditions (1–100 mM KCl and 0.1–10 mM CaCl2 at pH 7.0). Our results indicate that, in line with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, increasing influent ionic strength (IS) resulted in the reduced mobility of GONPs; and the impact of monovalent K+ was less than divalent Ca2+ in weakening the mobility of GONPs in both U-S and Fe-S. The positively charged iron oxide coating on the sand surface strongly immobilized the negatively charged GONPs at pH 7.0, producing the hyperexponential retention profiles for GONPs particularly in the presence of CaCl2 due primarily to the synergistic effects between iron oxide coating and Ca2+ (e.g., aggravate physical straining). A stepwise decreasing in pore-water transient IS initiated detachment of GONPs that were previously retained in the column when the initial influent ISs were high. The overall released mass of GONPs during each detachment step correlated positively with the difference (ΔΦmin2) of secondary minimum (Φmin2) under different ISs, suggesting that the secondary minimum mediated the release of GONPs. Most of the retained GONPs were reentrained upon lowering pore-water IS in U-S; but decreasing transient IS released only a small portion of the retained GONPs in Fe-S due to the reservoir role of primary minimum in capturing GONPs. Nevertheless, introducing 1 mM NaOH (pH 11) released most of the retained GONPs in Fe-S due largely to the enhanced electrostatic repulsion interactions between GONPs and between GONPs and sand grains under highly unfavorable conditions. The hydrodynamic diameters of detached GONPs upon injecting 1 mM NaOH were significantly smaller than those of GONPs in the influents and retentates, indicating that NaOH induced the disaggregation of retained GONPs. The threshold value of diameter ratio between colloid and sand grain is determined to be 0.0013 for sheet-shaped GONPs, above which the hyperexponential retention profile occurs. The findings from this study advance our current knowledge of the processes and mechanisms controlling NPs deposition and remobilization in porous media. This knowledge will extend our capability to better forecast NPs release under highly variable solution chemistry conditions such as during rainfall events and in the mixing zones between sea water and fresh water.
URLs/Downloads:
http://dx.doi.org/10.1016/j.scitotenv.2016.11.029
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