Chemical composition-dependent removal of cationic surfactants by carbon nanotubes

Qian Gao; Fan Wu; Jing Hu; Weixiao Chen; Xinyu Zhang; Xiaoying Guo; Bin Wang; Xilong Wang

doi:10.1016/j.scitotenv.2020.137017

Chemical composition-dependent removal of cationic surfactants by carbon nanotubes

Sci Total Environ. 2020 May 10:716:137017. doi: 10.1016/j.scitotenv.2020.137017. Epub 2020 Jan 30.

Authors

Qian Gao¹, Fan Wu¹, Jing Hu¹, Weixiao Chen¹, Xinyu Zhang¹, Xiaoying Guo¹, Bin Wang², Xilong Wang³

Affiliations

¹ Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.
² School of Public Health, Peking University, Beijing 100191, China.
³ Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China. Electronic address: xilong@pku.edu.cn.

PMID: 32036136
DOI: 10.1016/j.scitotenv.2020.137017

Abstract

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_{efficiency-max}) 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_{efficiency-max} 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_{efficiency-max} of cationic surfactants. The MWCNTs with a smaller OD have a higher R_{efficiency-max} 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.

Keywords: Aggregation; Application; Carbon nanotubes; Cationic surfactant; Removal.