Interactions Between DOM And MWCNTs And Mechanisms Of Its Impact On MWCNTs Suspension

Sorption of sequentially extracted humic acids (HAs) from a peat soil by multi-walled carbonnanotubes (MWCNTs) and the suspension of pristine and COOH-substituted MWCNTs by theHAs under sonication conditions were examined in this work. Sorption rate of HAs to MWCNTswas dominantly controlled by their diffusion from liquid-MWCNT boundary to MWCNT surfaces.Size exclusion chromatography analysis did not detect preferential sorption of HA fractions toMWCNTs at equilibrium, whereas the components with lower molecular weight in some HAfractions (e.g., HA1) would more preferentially be sorbed to MWCNTs at the initial sorption stage.Equilibrium sorption intensity of HAs by MWCNTs was dependent on their surface area and asum of meso-and macropore volume. The surface area and sum of meso-andmacroporosity-normalized sorption coefficient (Kd) values of a given HA by MWCNTs increasedwith increasing outer diameter of MWCNTs, because MWCNTs with larger outer diameter weremore strongly dispersed by HAs thereby making more sorption sites exposed for HA sorption. Vander Waals interaction between the alkyl components rather than the aromatic ones of HAs withMWCNTs was likely the key driving force for their sorption.Aside from the pristine MWCNTs and HAs used in the equilibrium sorption experiments, thecarboxylic group-substituted MWCNTs with outer diameters (OD) ranging from <8nm,20-30nm, and>50nm were also used to probe the underlying mechanisms controlling suspension ofMWCNTs by dissolved organic matter (e.g., HAs). Simple mixture of HAs with MWCNTsfollowed by5d shaking resulted in no detectable suspension. In the absence of HAs, nosuspension of the pristine MWCNTs was observed with sonication within16h, butCOOH-substituted MWCNTs (F-MWCNTs) were suspended under the same condition. It wasobserved that HAs greatly enhanced suspension of both pristine and F-MWCNTs. The suspensionenhancement was attributed to HA sorption, which increased the electrostatic repulsion and sterichindrance between individual MWCNTs. Introduction of O-containing hydrophilic moieties toMWCNTs via HA sorption enhanced interactions of their surfaces with water molecules throughH-bonding. Suspending capability of various pristine and F-MWCNTs on suspended massconcentration basis by four HAs showed inconsistent orders with the increasing or decreasingtrend of their ODs. However, the suspended surface area concentrations of both pristine andF-MWCNTs by individual HAs consistently followed an order of MWCNT8> MWCNT30> MWCNT50, and F-MWCNT8> F-MWCNT30> F-MWCNT50(the numbers after each MWCNTrepresent their ODs). These data implied that MWCNTs with smaller OD could be more stronglysuspended by a given HA relative to those with larger OD under sonication condition. Results ofthis study highlights the sorption rate-controlling step of HAs from a same source to MWCNTsand the major factors affecting their sorption intensity at equilibrium, and the roles of sonicationand ODs of MWCNTs in their suspension by HAs.