Influence of experimental conditions to obtain silver-modified zeolite-rich tuffs on the antimicrobial activity for Escherichia coli suspended in aqueous media

Abstract

The conditioning parameters used to obtain silver-modified clinoptilolite-rich tuffs and the characterization as well as the bactericidal activity behavior of these zeolitic materials were investigated, choosing Escherichia coli (ATCC 8739) as an indicator of fecal contamination of water. Natural zeolites (clinoptilolite type) with a grain size between 1.68–1.41 and 0.71–0.60 mm were conditioned with 0.01, 0.1 and 0.3 M silver nitrate (AgNO3) solutions at room temperature in static contact or under reflux, manipulated in darkness or visible light and reduced by exposure to γ-irradiation or contact with sodium borohydride (NaBH4). The characterization was performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and ultraviolet–visible (UV–Vis) spectroscopy. Neutron activation analysis (NAA) was also considered. The experimental data were fitted to Chick’s model to estimate the disinfection kinetics and to Higuchi’s model to evaluate the release of silver (Ag) from the modified zeolitic materials. The results showed the presence of Ag in different chemical species, Ag＋ and nanoparticles of silver (NpsAg) in the zeolite crystalline network, and their concentrations were variable. It was found that the Escherichia coli inhibition rate depends on Ag release from the silver-modified zeolite-rich tuffs, which is related to the sites that are occupied by the metallic ion in the zeolite network. In general, the silver-modified zeolite-rich tuffs conditioned at room temperature in static contact presented the highest bacterial inhibition rate. Under these experimental conditions the total mortality of Escherichia coli was observed in a shorter time compared with that of the other silver-modified zeolitic materials.