Suspension of Multi-Walled Carbon Nanotubes (CNTs) in Freshwaters: Examining the Effect of CNT Size

Abstract

Global demand for carbon nanotubes (CNTs) is increasing dramatically. As CNTs become commonplace, the range of uses is expected to expand as will the potential for release into ecosystems. Recent research suggests that CNTs display increased suspension in water in the presence of natural organic matter (NOM), thus increasing their ability for transport and dispersion. However, it remains unclear how CNT size will affect the suspension of CNTs in natural waters. Here we examine the effect of CNT diameter (10–500 nm) and CNT length (1–40 μm) in the presence of 1% sodium docecyl sulfate (SDS), and two different freshwater NOM extracts on suspension of CNTs in water. Absorbance spectrometry (UV-VIS) was used to determine CNT concentration in solutions over a 68 h period in the dark. Seventy to ninety-five percent of the CNTs settled out of the 1% SDS solution as compared to 23–54% in each of the NOM solutions. The half-life of suspension in solution increased with decreasing CNT diameter (from 13.9 to 138.8 h−1 for solutions containing NOM). These results demonstrate that settling rates are strongly determined by NOM presence in solution as well as CNT size.