Department of Civil and Environmental Engineering, Washington State University , Pullman, Washington 99164, United States.
Departments of Materials Science and Engineering, Chemistry, and Medicine, Northwestern University , Evanston, Illinois 60208, United States.
Environ Sci Technol. 2015 Sep 15;49(18):10886-93. doi: 10.1021/acs.est.5b01866. Epub 2015 Sep 1.
The aggregation and stability of graphene oxide (GO) and three successively reduced GO (rGO) nanomaterials were investigated. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, influence of pH, ionic strength, ion valence, and presence of natural organic matter (NOM) were studied. Results show that stability of GO in water decreases with successive reduction of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concentration of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10 mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca(2+) ions on the surface functional groups of GO which reduces the surface charge of GO. As the concentration of rGO functional groups decreased, so did the influence of Ca(2+) ions on rGO stability. Critical coagulation concentrations (CCC) of GO, rGO-1h, and rGO-2h were determined to be ∼ 200 mM, 35 mM, and 30 mM NaCl, respectively. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amount of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca(2+), Mg(2+)), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concentration and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.
研究了氧化石墨烯（GO）和三种连续还原的 GO（rGO）纳米材料的聚集和稳定性。还原的 GO 物种为部分还原的 GO（rGO-1h）、中间还原的 GO（rGO-2h）和完全还原的 GO（rGO-5h）。具体而言，研究了 pH 值、离子强度、离子价态以及天然有机物（NOM）的存在对 GO 稳定性的影响。结果表明，GO 在水中的稳定性随着官能团的连续还原而降低，其中 pH 值对 rGO 稳定性的影响最大。稳定性还取决于离子价态和表面官能团的浓度。虽然 pH 值在添加 10mM NaCl 时对 GO 稳定性没有明显影响，但添加 0.1mM CaCl2 会降低 GO 在增加 pH 值时的稳定性。这是由于 Ca(2+)离子吸附在 GO 的表面官能团上，降低了 GO 的表面电荷。随着 rGO 官能团浓度的降低，Ca(2+)离子对 rGO 稳定性的影响也随之降低。GO、rGO-1h 和 rGO-2h 的临界聚集浓度（CCC）分别为约 200mM、35mM 和 30mM NaCl。然而，在 CaCl2 存在下，GO 和 rGO 的 CCC 值非常相似。长期研究表明，大量 rGO-1h 和 rGO-2h 在 Call's Creek 地表水中保持稳定，而废水则容易使 rGO 不稳定。在 NOM 和二价阳离子（Ca(2+)、Mg(2+)）存在下，GO 团聚物由于 GO 官能团与 NOM 和二价离子桥接而从悬浮液中沉降。然而，由于 rGO-1h 和 rGO-2h 的官能团浓度较低，因此 NOM-二价离子桥接减少，rGO-1h 和 rGO-2h 仍保持悬浮状态。总的来说，pH 值、二价阳离子和 NOM 可以在 rGO 和 GO 的命运中发挥复杂的作用。
