Ultrasonic Inactivation of <i>Escherichia coli</i> with Multi-Walled Carbon Nanotubes: The Effect of Solution Chemistry

This study explored inactivation of Escherichia coli by 5 mg/L pristine short or long multi-walled CNTs (MWCNTs) or 20 kHz ultrasound individually or through a combination in DI water, Suwannee River natural organic matter (SRNOM), or sodium dodecyl sulphate (SDS) solutions. The results indicated that dispersity of MWCNTs was the single most important factor determining the inactivation rate of E. coli. The dispersity of short MWCNTs in solutions increased in the order of DI < 10 mgC/L SRNOM < 2 mM SDS. Correspondingly, the greatest log inactivation of E. coli was achieved in SDS when short MWCNTs were used alone (0.67 ± 0.12) and combined with ultrasound (1.80 ± 0.02) in 10 min. Short MWCNTs alone had a slightly greater inactivation (0.29 ± 0.07) in SRNOM solution than in DI water (0.18 ± 0.05). However, long MWCNTs had a little higher inactivation in DI (0.24 ± 0.03) than short ones, because of better dispersity in DI. The observed synergistic inactivation when ultrasound and short MWCNTs were used together in 2 mM SDS, although SDS and MWCNTs can occupy the reaction sites at the cavitational bubble–water interfacial regions and scavenge •OH radicals. The results suggest that sonophysical effects are more important to inactivate E. coli than sonochemical effects. Ultrasound inactivates E. coli and/or energizes MWCNTs through the mechanisms of acoustic streaming, microstreaming, microstreamers, and transient cavitation collapse generated shock waves and micro jets and localized hot temperature. The results of study suggest the cytotoxicity of CNTs is impinging bacterial cells.