Synthesis of solid-supported silver nanocomposites for antimicrobial and dye adsorption applications

Abstract

Dyes and pathogenic bacteria from industrial effluents and pharmaceutical waste are a major concern when they are not efficiently removed by conventional methods. This is because many of these compounds can adversely affect human health. Nanotechnology-driven materials are being explored as efficient adsorbers and antimicrobial agents for removal of dye and microorganisms in water bodies. Silver nanoparticles (AgNPs) are known for their broad spectrum of antimicrobial and adsorption properties. However, AgNPs tend to agglomerate in aqueous media, thus reducing their stability and eventually deteriorate their capability to be used as adsorbent and antibacterial agents. In this study, to overcome aggregration problem, silver nanoparticles were loaded on kaolinite (Kln) and synthesized using chemical and physical methods producing silver kaolinite naocomposites (Ag/Kln NCs). To further enhance stability, biopolymer Chitosan (Cts) was added to the Ag/Kln NCs resulting in silver kaolinite bionanocomposites (Ag/Kln BNCs). Ag/Kln NCs and BNCs were successfully developed using two synthesis methods which using sodium borohydride and gamma-irradiation in an aqueous system at room temperature and under ambient pressure. The effect of different concentration of silver nitrate solution, irradiation dose and addition of biopolymer Cts was also investigated. The synthesized Ag/Kln NCs were characterized by UV-Vis spectrophotometer, X-Ray Diffraction, Transmission Electron Microscopy, Field Emmission Scanning Electron Microscopy and Fourier Transform Infra-Red Spectroscopy analysis. The successful formation of Ag/Kln NCs was confirmed by the colour change due to reduction and band appearance at the range of 386-425 nm due to surface plasmon resonance. The intensity of the surface plasmon resonance peak increased with the increase of either irradiation dose or AgNO3 molar concentration. The XRD pattern indicated both Ag/Kln NCs and BNCs had a face-centred cubic crystalline structure. Shapes of AgNCs and BNCs produced were approximately spherical with the average diameter range of 2.92 – 5.81 nm depending on the irradiation dose and concentration. The characterization of the samples revealed the successful loading of Ag on the surface of the Kln instead of being intercalated between the layers of the Kln framework. The addition of Cts reduced the minimum size range of both chemical and gamma synthesized Ag BNCs and extended their storage stability. Generally, the structure of Ag/Kln NCs and BNCs remained same as the raw Kln in both synthesis method. The resultant Ag/Kln NCs and BNCs displayed excellent adsorption capacity evaluated through elimination of Methylene Blue dyes from the liquid phase. This observation was confirmed by a decrease in absorbance maximum values. The rate constant for reduction reaction for both dyes using the synthesized NCs and BNCs was found to increase with the increase of Ag molar concentration and irradiation dose. The synthesized NCs and BNCs showed efficient antimicrobial activity against Gram- negative and Gram-positive bacteria represented by Escherichia coli, Enterococcus faecalis, Proteus vulgaris, Methicilin resistant Staphylococcus aureus and Candida albicans. Overall, stable Ag/Kln NCs and BNCs with adsorbent and antibacterial agent were synthesized using a clean and safe process and material. This indicates that these NCs have the potential of removing dyes and pathogenic microorganisms in water.