Impact of Synthesis Methods on the Transport of Single Walled Carbon Nanotubes in the Aquatic Environment

In this study, a systematic approach has been followed to investigate the fate and transport of single walled carbon nanotubes (SWCNTs) from synthesis to environmentally relevant conditions. Three widely used SWCNT synthesis methods have been investigated in this study including high pressure carbon monoxide (HiPco), SWeNT CoMoCat, and electric arc discharge technique (EA). This study relates the transport of three SWCNTs (HiPco-D, SG65-D, and P2-D) with different synthesis methods and residual catalyst content revealing their influence on the subsequent fate of the nanotubes. To minimize nanotube bundling and aggregation, the SWCNTs were dispersed using the biocompatible triblock copolymer Pluronic, which allowed the comparison in the transport trends among these SWCNTs. After purification, the residual metal catalyst between the SWCNTs follows the trend: HiPco-D > SG65-D > P2-D. The electrophoretic mobility (EPM) and hydrodynamic diameter of SWCNTs remained insensitive to SWCNT type, pH, and presence of natural organic matter (NOM); but were affected by ionic strength (IS) and ion valence (K+, Ca2+). In monovalent ions, the hydrodynamic diameter of SWCNTs was not influenced by IS, whereas larger aggregation was observed for HiPco-D with IS than P2-D and SG65-D in the presence of Ca2+. Transport of HiPco-D in the porous media was significantly higher than SG65-D followed by P2-D. Release of HiPco-D from porous media was higher than SG65-D followed by P2-D, though negligible amount of all types of SWCNTs (<5%) was released. Both transport and release patterns follow a similar trend to what was observed for residual metal catalysts in SWCNTs. Addition of NOM increased the transport of all SWCNTs primarily due to electrosteric repulsion. HiPco-D was notably more acidic than SG6S-D followed by P2-D, which is similar to the transport trend. Overall, it was observed that the synthesis methods resulted in distinctive breakthrough trends, which were correlated to metal content. These findings will facilitate the safe design of environmental friendly SWCNTs by minimizing mobility in aquatic environments.