Elution and disinfection of silver and zinc nanoparticles in co-fired ceramic water filters

Ceramic water filters (CWFs) are decentralized water treatment technologies commonly used in resource-restricted geographies. Inclusion of silver nanoparticles (AgNP) assists with disinfection but can substantially increase costs. This research investigates AgNP supplementation with zinc oxide (ZnO) as a low-cost bactericide alternative. CWF disks were impregnated with varying AgNP and/or ZnO concentrations and challenged against Escherichia coli. Effluent bacteria were enumerated and monitored over 72 h while eluted metal concentrations were measured and scaled according to surface area to establish 'pot-equivalent' estimates (0-50 ppb Ag and 0-1200 ppb Zn). Ag addition correlated to subsequent measured release values, though Zn impregnation did not. Background Zn was thus evidently present. Meanwhile, the eluted metal concentration related to disinfection: a CWF with a pot-equivalent elution estimate of 2 ppb Ag and 156 ppb Zn achieved a Log Removal Value (LRV) of 2.0 after 60 min of filtration and 1.9 after 24 h of storage while a CWF with a pot-equivalent elution estimate of 20 ppb Ag and 376 ppb Zn achieved LRVs of 3.1 and 4.5 after the same filtration and storage times, respectively. Clay elemental composition may therefore impact filter performance more than previously considered This trend was further confirmed by batch experiments with Ag and Zn in concentrations of 0-20 ppb Ag and 0-800 ppb Zn, respectively: bacterial regrowth was only observed when Ag and Zn were each below 5 ppb and 160 ppb while 1 ppb Ag and 800 ppb Zn maintained complete disinfection for 72 h. Increased Zn concentrations thus reduced Ag required to maintain disinfection over time. Overall, it is recommended to include Zn with Ag for CWF to improve short-term and long-term disinfection efficacy and associated water safety.