Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.
School of Public Health, Peking University, Beijing 100191, China.
Sci Total Environ. 2020 May 10;716:137017. doi: 10.1016/j.scitotenv.2020.137017. Epub 2020 Jan 30.
How to attenuate water surfactant pollution using carbon nanomaterials (CNMs) has been gaining increasing research attention in recent years. However, how the composition of cationic surfactants and physicochemical properties of CNMs may affect cationic surfactant maximum removal efficiency (R) with minimal cost from the aqueous phase and the associated mechanisms remain largely unclear. To address this knowledge gap, we compared removal efficiency of three cationic surfactants including dodecyl dimethyl benzyl ammonium chloride (DDBAC), tetradecyl dimethyl benzyl ammonium chloride (TDBAC) and hexadecyltrimethylammonium bromide (CTAB) by various carbon nanotubes (CNTs), including pristine and OH- or COOH-functionalized multiwalled- (MWCNTs) and single-walled (SWCNTs) CNTs. The results showed that R of CTAB by pristine MWCNTs with an outer diameter OD < 8 nm is 50.36 ± 0.56%, while that by OH-MWCNTs with OD < 8 nm is merely 22.72 ± 0.21%. Surface area and porosity of CNTs strongly affect R of cationic surfactants. The MWCNTs with a smaller OD have a higher R than that with a larger one especially for CTAB, due to their larger surface area and porosity. Among various CNTs, SWCNTs is an ideal choice for removing cationic surfactants, especially for non-aromatic CTAB. Interestingly, for most cases, cationic surfactant removal by CNTs decreased when the amount of CNTs added exceeded a certain level, attributable to their aggregation. This implies that it is impossible to completely remove some cationic surfactants even when excess CNTs were added. The π-π bonding dominates over hydrophobic interaction in regulating cationic surfactant removal especially for those with aromatic structure. Aromatic cationic surfactants such as DDBAC and TDBAC can be removed more readily by CNTs than those without a benzene ring due to their strong π-π interactions. TDBAC has a longer hydrophobic chain relative to DDBAC, leading to a better removal efficiency by CNTs, due to stronger hydrophobic interaction.
近年来，如何利用碳纳米材料（CNMs）来减轻水表面活性剂污染受到了越来越多的关注。然而，阳离子表面活性剂的组成和 CNMs 的物理化学性质如何影响阳离子表面活性剂从水相中以最小成本达到最大去除效率（R），以及相关的机制在很大程度上仍不清楚。为了解决这一知识空白，我们比较了三种阳离子表面活性剂（包括十二烷基二甲基苄基氯化铵（DDBAC）、十四烷基二甲基苄基氯化铵（TDBAC）和十六烷基三甲基溴化铵（CTAB））在不同碳纳米管（CNTs）上的去除效率，包括原始的和 OH-或 COOH 功能化的多壁（MWCNTs）和单壁（SWCNTs）CNTs。结果表明，原始 MWCNTs 对 CTAB 的 R 为 50.36±0.56%，而 OD<8nm 的 OH-MWCNTs 对 CTAB 的 R 仅为 22.72±0.21%。CNTs 的表面积和孔隙率强烈影响阳离子表面活性剂的 R。OD 较小的 MWCNTs 的 R 比 OD 较大的 MWCNTs 的 R 高，特别是对于 CTAB，这是由于它们具有更大的表面积和孔隙率。在各种 CNTs 中，SWCNTs 是去除阳离子表面活性剂的理想选择，特别是对于非芳香族 CTAB。有趣的是，对于大多数情况，当添加的 CNTs 量超过一定水平时，CNTs 对阳离子表面活性剂的去除效果会降低，这归因于它们的聚集。这意味着，即使添加过量的 CNTs，也不可能完全去除某些阳离子表面活性剂。π-π 键合在调节阳离子表面活性剂去除方面比疏水相互作用更为重要，特别是对于具有芳香结构的阳离子表面活性剂。由于它们之间的强 π-π 相互作用，具有芳香族结构的 DDBAC 和 TDBAC 等阳离子表面活性剂比没有苯环的阳离子表面活性剂更容易被 CNTs 去除。与 DDBAC 相比，TDBAC 具有更长的疏水链，因此与 CNTs 之间的疏水相互作用更强，去除效率更高。
