Sorption and Competition of Aromatic Compounds and Humic Acid on Multiwalled Carbon Nanotubes

Abstract

Sorption of humic acid (HA) and aromatic compounds by multiwalled carbon nanotubes (MWCNTs), and their competition on MWCNTs were examined. HA sorption by MWCNTs was regulated by their surface area (SA). Hydrophobic and pi-pi attractions of HA with MWCNTs were main driving forces for their interactions. K(d)/K(ow) values of phenanthrene (Phen), naphthalene (Naph), and 1-naphthol(1-Naph) by individual MWCNTs were positively correlated with their molecular size, suggesting that micropore-filling could not be a predominant mechanism. HA had the lowest competition with Phen and 1-Naph on MWCNT20, due to its greatest abundance of sorption sites. Competition between HA and 1-Naph followed an order MWCNT40 < MWCNT60 < MWCNT100, due to their reduction in SA and porosity. Micropore blockage and direct competition by HA increased with deceasing SA and porosity of MWCNTs. MWCNT20 had much more sorption sites than other MWCNTs, leading to insignificant difference in competition between 1-Naph and Phen with HA. Also, HA had higher competition with Phen on MWCNT40, MWCNT60, and MWCNT100 than 1-Naph. Our results highlight the significance of MWCNT SA for HA sorption and the associated influence on sorption of aromatic compounds. Further, molecular size and hydrophobicity of aromatic compounds strongly affected their competition with HA on MWCNTs.