The relationship between humic acid (HA) adsorption on and stabilizing multiwalled carbon nanotubes (MWNTs) in water: effects of HA, MWNT and solution properties

Abstract

This study was aimed to explore the relationship between humic acid (HA, as a model NOM) adsorption on and stabilizing multiwalled carbon nanotubes (MWNTs) in water with a focus on the effects of HA, MWNT and solution properties. It was found that MWNT-surface area-normalized adsorption of HAs (Q(SA)) increased with increasing outer-diameter of the MWNTs and decreasing polarity of the HAs. However, at low pH values (ca. <4) or high ionic strengths (ca. >1mmolL(-1) Ca(2+)), the HA adsorption decreased with decreasing polarity of the HAs. The MWNT stabilization increased with increasing Q(SA), but the increase leveled off when Q(SA) exceeded a threshold value markedly lower than the maximum Q(SA), especially for the MWNTs with relative large outer-diameters. On the whole, the Q(SA)-normalized MWNT stabilization, presenting the capability of the MWNT-adsorbed HAs for the MWNT stabilization, increased with increasing HA polarity and solution pH, but with decreasing Ca(2+) concentration. However, the stabilized MWNTs by the HAs with greater polarity could be more subject to destabilization by Ca(2+). The results of this study are believed to shed light on predictive understanding the interaction between MWNTs and NOM and the environmental behavior of MWNTs.