Suspending Multi-Walled\nCarbon Nanotubes by Humic\nAcids from a Peat Soil

Abstract

Suspension of the pristine and COOH-substituted multi-walled\ncarbon\nnanotubes (P- and C-MWCNTs) with different outer diameters (ODs) by\nhumic acids (HAs) from a peat soil was examined. Under shaking condition,\nMWCNTs were not suspended within 5 d. Without HAs, C-MWCNTs were slightly\nsuspended by sonication within 16 h, but no suspension was observed\nfor the pristine ones (P-MWCNTs). HAs greatly enhanced suspension\nof both P- and C-MWCNTs. The suspension enhancement was attributed\nto HA sorption, which increased electrostatic repulsion and steric\nhindrance between individual MWCNTs. Introduction of O-containing\nhydrophilic moieties to MWCNTs via HA sorption enhanced the interactions\nof their surfaces with water through H-bonding. Suspending capability\nof various MWCNTs on suspended mass concentration basis by four HAs\nshowed inconsistent orders with the increasing or decreasing trend\nof their ODs. However, the suspended surface area concentrations of\nboth P- and C-MWCNTs by individual HAs consistently followed an order\nof P8 > P30 > P50, and C8 > C30 > C50 (P and C, respectively,\nrefer\nto P- and C-MWCNTs, and the numbers represent their ODs). These data\nimplied that MWCNTs with smaller OD could be more strongly suspended\nby a given HA relative to those with larger OD under sonication condition.