Synergistic antibacterial activity of a combination of silver and copper nanoparticle impregnated activated carbon for water disinfection

Abstract

Silver and copper nanoparticles impregnated separately on plasma-treated activated carbon (AC) (named Ag-AC and Cu-AC, respectively), when mixed in an optimized 50 : 50 weight ratio, show synergistic water disinfection compared to only Ag-AC or only Cu-AC. Time-dependent cryo-SEM imaging and reactive oxygen species (ROS) measurement of E. coli prove that (in both batch mode and continuous flow column) Cu-AC forms pits faster on the cell membrane compared to Ag-AC and together with the latter facilitates faster cell death. The composite of Ag-AC and Cu-AC in batch mode showed a 4 log reduction of live E. coli cells in the treated water in 8 minutes, as confirmed by both plate count and fluorescence spectroscopy. Scaling up this result in a continuous flow column (of 8 cm diameter and 25 cm height, packed with the composite), one could achieve zero live cell concentration in only 7 minutes of contact time, translating into a very high flow rate of 5.32 L h⁻¹ of treated water. Therefore, plate count, fluorescence spectroscopy, time-dependent ROS measurement and cryo-SEM imaging establish the mechanism of synergy and contact time required for cell death. The disinfection performance to zero live E. coli cells could be maintained with an even higher inlet E. coli concentration of 10⁶ CFU ml⁻¹ (albeit at a lower throughput from the column, as expected) and also non-stop for at least 5 days, demonstrating a potential long-term performance, with more than adequate Ag and Cu left in the column. Finally, the steady state concentrations of Ag and Cu in the treated water were 23 and 56.4 μg L⁻¹, respectively, which are well within the permissible limits of 100 and 1000 μg L⁻¹ for Ag and Cu, respectively. Therefore, the synergistic antibacterial activity of the composite resulted in production of decontaminated water at the highest throughput over a long time period.