The effect of ionic strength and pH on the stability of tannic acid-facilitated carbon nanotube suspensions

Carbon nanotubes (CNTs) are prone to aggregation and precipitation in water due to their high hydrophobicity and aspect ratio. However, the addition of dissolved organic matter (DOM) has been reported to disperse and stabilize certain CNTs, suggesting the potential transport and bioavailability of CNTs in natural aqueous environments. For a better understanding of the CNT-DOM interaction, five multiwalled CNTs with outer diameters of 10 (MWCNT10), 10-20 (MWCNT20), 20-40 (MWCNT40), 40-60 (MWCNT60), and 60-100 nm (MWCNT100) were used to investigate their sorptive and suspension behavior in tannic acid (TA, as a DOM surrogate) solution; the effects of ionic strength and pH on the TA-CNT interaction were examined. Suspension of MWCNTs sharply improved with increasing TA concentration and leveled off at an initial TA concentration ca. 20 mg/L. Suspension of MWCNTs was in the order of MWCNT40 > MWCNT60 > MWCNT20 > MWCNT100 > MWCNT10. The TA-stabilized CNTs were stable within pH 5-11, while they quickly precipitated at pH < 5. Different ions (Na+, Mg2+, Ca2+, or La3+) aggregated the stabilized CNTs, with a critical coagulation concentration exponentially correlated to ionic valence. Changes of steric repulsion and electrostatic interaction with the added TA could account for the variation of CNT stability. (C) 2009 Elsevier Ltd. All rights reserved.