Microbial mortality behavior promoted by silver (Ag+/Ago)-modified zeolite-rich tuffs for water disinfection

In developing countries, death due to diseases caused by fecal-oral ingestion can be avoided by taking action on drinking water issues. Adequate access to water treatment systems to reduce infections is a critical cause. Silver has been used as an antibacterial product, including biomedical applications. Therefore, in this paper, the effect of the chemical speciation of silver from silver-modified zeolite-rich tuffs on the mortality of Escherichia coli (E. coli), Streptococcus faecalis (S. faecalis) and Candida albicans (C. albicans) suspended in aqueous solution was investigated for disinfection purposes. The following aspects were considered to develop the investigation: a) the technique to prepare the modified zeolitic materials, either with ionic silver or silver nanoparticles, which were obtained in two ways: one, with grapefruit extract and the second, by using non-thermal plasma generated in a dielectric barrier discharge reactor of parallel plates; b) the response of the prokaryotes (bacteria) and eukaryote (yeast) microorganisms to disinfectant agents in batch systems; c) the disinfection processes as a function of time to obtain kinetics parameters; and d) the kinetics of the silver release from the silver-modified zeolite-rich tuffs, considering the models of Higuchi and Korsmeyer. The zeolitic materials were characterized by low-vacuum scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The non-thermal plasma reduced ionic silver is more efficient at generating silver compounds with several oxidation states, which are essential during the microbial inhibition process. For the bacterial (E. coli and S. faecalis), the materials with nanoparticles were efficient to inactivate them. However, the yeast (C. albicans) reaches the total inactivation when the zeolitic material contains ionic silver in the crystalline network. The E. coli, S. faecalis and C. albicans survival behavior suspended in aqueous solutions after contact with Ag-modified natural zeolites depends on the chemical speciation of the silver present in these materials, Ag+1 in the case of OAgiZ or nanoparticles of Ago promoted by the grapefruit extract (OAgnpTZ), as well as by non-thermal plasma generated in a dielectric barrier discharge reactor of parallel plates (OAgnpPZ). In general, the concentration of silver in the aqueous solution after the disinfection process cannot exceed the recommended levels established for international organizations. The OAgnpPZ is a potential microbicide agent against E. coli and C. albicans, and the OAgnpTZ for F. faecalis.