Merits of photocatalytic and antimicrobial applications of gamma-irradiated Co x Ni1- x Fe2O4/SiO2/TiO2; x = 0.9 nanocomposite for pyridine removal and pathogenic bacteria/fungi disinfection: implication for wastewater treatment

Abstract

In this paper, we report a layer-by-layer approach for the preparation of a concentric recyclable composite (Co _x Ni_1-x Fe₂O₄/SiO₂/TiO₂; x = 0.9) designed for wastewater treatment. The prepared composite was investigated by X-ray diffraction spectroscopy, high-resolution transmission electron microscopy and scanning electron microscopy (SEM) supported with energy dispersive X-ray (EDX) spectroscopy to analyze crystallinity, average particle size, morphology and elemental composition, respectively. The antimicrobial activities of the prepared composite have been investigated against multi-drug-resistant bacteria and pathogenic fungi using a variety of experiments, such as zone of inhibition, minimum inhibitory concentration, biofilm formation and SEM with EDX analysis of the treated bacterial cells. In addition, the effects of gamma irradiation (with different doses) and UV irradiation on the antibacterial abilities of the prepared composite have been evaluated. Moreover, the effect of gamma irradiation on the crystallite size of the prepared composite has been studied under varying doses of radiation (25 kGy, 50 kGy and 100 kGy). Finally, the photocatalytic efficiency of the prepared composite was tested for halogen-lamp-assisted removal of pyridine (artificial wastewater). Various parameters affecting the efficiency of the photocatalytic degradation, such as photocatalyst dose, pyridine concentration, pH, point of zero charge and the presence of hydrogen peroxide, have been studied. Our results show that the synthesized composite has a well-crystallized semi-spherical morphology with an average particle size of 125.84 nm. In addition, it possesses a high degree of purity, as revealed by EDX elemental analysis. Interestingly, the prepared composite showed promising antibacterial abilities against almost all the tested pathogenic bacteria and unicellular fungi, and this was further improved after gamma and UV irradiation. Finally, the prepared composite was very efficient in the light-assisted degradation of pyridine and its degradation efficiency can be tuned based on various experimental parameters. This work provides a revolutionary nanomaterial-based solution for the global water shortage and water contamination by offering a new wastewater treatment technique that is recyclable, cost effective and has an acceptable time and quality of water.

Fig. 1. HRTEM analysis of the prepared Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 hybrid nanocomposite concentric structure, where yellow and black arrows display the two shell layers and the white circles represent the core Co_xNi_1−xFe₂O₄; x = 0.9 NPs.

Fig. 2. SEM and corresponding EDX elemental analysis of the synthesized Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite. [A–C] Different magnifications of the core–multi-shell structure. [D] The corresponding EDX elemental analysis of the synthesized nanocomposite.

Fig. 3. SEM-EDX elemental mapping of the synthesized Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite.

Fig. 4. Antibacterial and antifungal activities of gamma-irradiated Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite against pathogenic microbes for [A] E. coli, [B] Staphylococcus aureus and [C] C. albicans, measured as ZOI (mm).

Fig. 5. XRD pattern showing the effect of gamma irradiation, with doses of 25, 50 and 100 kGy, on the crystal size of the prepared Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite, compared with the non-irradiated composite.

Fig. 6. Antimicrobial abilities of UV-irradiated Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite against different pathogens: [A] E. coli, [B] S. aureus and [C] C. albicans.

Fig. 7. Antibiofilm activity of 100 kGy gamma-irradiated Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite using the test tube method against [A] E. coli and [B] C. albicans. The steps were reported as follows. (a) Growth of the bacterial and yeast cells and biofilm formation (rings) without treatment with the synthesized Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite and the inhibition of bacterial and yeast growth after treatment with Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite. (b) Staining of the adherent bacterial and yeast cells with crystal violet. (c) Removing and dissolving the adherent bacterial and yeast cells by ethanol for determination of semi-quantitative biofilm inhibition (%) (as shown in Table 3).

Fig. 8. SEM and corresponding EDX elemental analysis of E. coli. [A] Normal bacterial cell without treatment with 100 kGy gamma-irradiated Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite. [B] The depressed and deformed bacterial cell after treatment with 100 kGy gamma-irradiated Co_xNi_1−xFe₂O₄; x = 0.9/SiO₂/TiO₂ nanocomposite (yellow square represents complete lysis of E. coli cells). [C] The corresponding EDX elemental analysis of the treated E. coli cell, confirming the cellular internalization of the 100 kGy gamma-irradiated Co_xNi_1−xFe₂O₄; x = 0.9/SiO₂/TiO₂ nanocomposite.

Fig. 9. UV-visible spectra of pyridine showing its degradation with time (10 mg of nanocomposite, 50 ml Py solution (100 ppm), 25 °C and pH 7).

Fig. 10. Effect of initial concentration of pyridine solution on the degradation efficiency (10 mg of composite, 50 ml Py solution, 25 °C and pH 7).

Fig. 11. The first-order kinetics of pyridine degradation (10 mg of composite, 50 ml Py solution, 25 °C and pH 7).

Fig. 12. Apparent first-order rate constants vs. initial concentration of pyridine.

Fig. 13. Effect of photocatalyst dose on pyridine degradation efficiency (50 ml Py solution (100 mg l⁻¹), 25 °C and pH 7).

Fig. 14. [A] Influence of initial pH on the removal of pyridine (50 ml pyridine (100 mg l⁻¹), 10 mg nanocomposite and 100 min irradiation time) and [B] the point of zero charge (PZC) of Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 at different pH values.

Fig. 15. Effect of H₂O₂ on pyridine degradation (initial concentration of pyridine C₀ = 100 ppm, 50 ml, 10 mg of nanocomposite and pH 9).

Fig. 16. Photocatalyst mechanisms of pyridine using Co_xNi_1−xFe₂O₄/SiO₂/TiO₂; x = 0.9 nanocomposite and a possible degraded product.