Suspending Multi-Walled
Carbon Nanotubes by Humic
Acids from a Peat Soil
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Abstract
Suspension of the pristine and COOH-substituted multi-walled
carbon
nanotubes (P- and C-MWCNTs) with different outer diameters (ODs) by
humic acids (HAs) from a peat soil was examined. Under shaking condition,
MWCNTs were not suspended within 5 d. Without HAs, C-MWCNTs were slightly
suspended by sonication within 16 h, but no suspension was observed
for the pristine ones (P-MWCNTs). HAs greatly enhanced suspension
of both P- and C-MWCNTs. The suspension enhancement was attributed
to HA sorption, which increased electrostatic repulsion and steric
hindrance between individual MWCNTs. Introduction of O-containing
hydrophilic moieties to MWCNTs via HA sorption enhanced the interactions
of their surfaces with water through H-bonding. Suspending capability
of various MWCNTs on suspended mass concentration basis by four HAs
showed inconsistent orders with the increasing or decreasing trend
of their ODs. However, the suspended surface area concentrations of
both P- and C-MWCNTs by individual HAs consistently followed an order
of P8 > P30 > P50, and C8 > C30 > C50 (P and C, respectively,
refer
to P- and C-MWCNTs, and the numbers represent their ODs). These data
implied that MWCNTs with smaller OD could be more strongly suspended
by a given HA relative to those with larger OD under sonication condition