Photocatalytic Detoxification of Antibiotic- and Bacteria-Contaminated Water using Cobalt-Doped Ni–Zn Ferrites Magnetic Separable Nanopowders


 Here, metronidazole (MZ) antibiotic degradation and bactericidal efficacy of Co–Ni0.5Zn0.5Fe2O4 (Co–NZF) with and without photoactivation by UV light is reported as a viable cost-competitive water disinfection solution. Co–NZF has a total pore volume of 0.298 cm3 g− 1, a specific surface area of 70.2 m2 g− 1 and sufficiently high magnetic properties (80.35 emu g− 1). After 360 min of UV–assisted irradiation at pH 3, 10 mg Co–NZF, and 4 mM H2O2, the maximum MZ degradation was reached (92.8%). The adsorption result of 10 mg Co–NZF in the dark for 12 h resulted in 70.2% MZ removal, whereas MZ self-degradation was significantly minimal in a blank trial. In the presence of interfering anions and very high molecular weight tylosin antibiotic, Co–NZF maintained 51.7–75.4% degradation efficiency. The effect of the Co–NZF dosage on the viability of Staphylococcus aureus and Escherichia coli strains showed that 15 mg of the catalyst was sufficient to cause bactericidal activity after 180 min in the presence of UV light, while 25 mg is needed under dark conditions. In addition, when compared to Escherichia coli strains, Co–NZF showed higher inhibition against Staphylococcus aureus in time-kill experiments under dose variation.


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Pharmaceutical antibiotics are becoming increasingly important globally due to their effectiveness 38 in preventing or treating infections in humans and promoting animal growth (Serna-Galvis et al.   To start, the co-precipitation method was used to synthesize Ni0.5Zn0.5Fe2O4 (NZF) nanoparticles 130 as shown in Scheme 1 (Mustafa and Oladipo, 2021). In 120 mL distilled water, 5.5 g ZnSO4.7H2O, 131 6 g NiSO4.6H2O, and 5.7 g FeCl3.6H2O were dissolved. The solution was stirred at 250 rpm 132 continuously until it became homogeneous, then 20 mL of 0.3 M acetic acid was added to prevent 133 particle aggregation. The reaction flask was then transferred to an oil bath and vigorously stirred 134 at 800 rpm for 180 min at 80°C. The solution was cooled and dried overnight at 80°C before being 135 calcined in a muffle furnace for 120 min at 800°C. Ni0 Following that, Co-Ni0.5Zn0.5Fe2O4 was prepared by mixing 5 g of NZF with 50 mL of 0.1 143 M Co(NO3)2.6H2O; then stirred the solution at 250 rpm at room temperature for 45 min. The 144 reaction temperature was then raised to 70°C, and 30 mL of 75 mM NaOH was added dropwise to 145 the mixture and stirred for another 60 min. The precipitate was filtered, washed with water-50% 146 ethanol mixture several times then dried at 100°C for 24 h. The final product was ground and 147 sieved to a uniform size then labelled (Co-NZF). The Co-NZF was characterized by UV-vis diffuse reflectance spectroscopy (DRS) using a UV-153 2450 spectrometer (Shimadzu, Japan) from 200 nm to 800 nm. The SEM-EDX was acquired by 154 JSM-6390 scanning electron microscope-coupled EDX (JEOL, Japan). The X-ray powder 155 diffraction (XRD) patterns were recorded using X-ray diffractometer Bruker D8 (Bruker-AXS, 156 Ettlingen, Germany) with a Cu Kα (λ = 1.54187 Å) monochromatic radiation at 40 kV and 30 mA.

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The measurement was recorded at 2θ = 5−70° at a rate of 2° min −1 and the crystalline phases were 158 clarified using standard JCPDS files. The electrochemical impedance spectroscopy (EIS) was 159 investigated using the Palmsens Sensit smart potentiostat (PalmSens BV, Netherlands) in the 160 frequency range of 50 Hz-200 kHz at room temperature. Fourier Transform Infrared (FTIR) spectra were recorded in the range 4000-400 cm −1 using an FTIR-8700 spectrophotometer 162 (Perkin-Elmer, Japan). Photoluminescence spectrum (PL) was collected by fluorescence 163 spectrophotometer F-7000 (Hitachi High-Tech, Japan) with optical radiation at λ= 340 nm. At 164 room temperature, a MicroSense Vibrating Sample Magnetometer (Model 10, MicroSense USA) 165 was used to investigate the magnetic properties of samples ranging within ±10,000 Oe.    Each experiment was replicated twice, with an average outcome of ±1.5% error. The photocatalytic 216 degradation efficiency, η (%), degradation kinetics and the electrical energy per order, EEO (kWh 217 m −3 order −1 ) required to decrease the concentration of MZ were calculated following Eqs. 1-3: where Ct and Co (mg/L) denote the MZ concentrations at time t (min) and before the reaction, 224 respectively; while k (min −1 ) is the apparent rate constant, V (L) denotes the reaction volume, P is 225 the electrical power consumed during the process (kW) and t represents reaction time (min).

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Results and discussion 229 The NZF and Co−NZF FTIR spectra are shown in Fig. 1a. We used the abbreviations ν and δ to 230 denote stretching and bending vibrations, respectively, when interpreting infrared (IR) spectra.

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Main assignments of IR wave-numbers are listed accordingly for NZF and Co−NZF;   shown in Fig.1c. When the irradiation light was turned on for both samples, the photocurrent density increased rapidly but decreased to a constant value when the light was turned off. Co−NZF, 278 in particular, showed a photocurrent response of 1.73 A cm -2 , which is 1.4 times higher than NZF.

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The photoluminescence (PL) technique was used to investigate the transfer and   aqueous solutions due to its high magnetic properties.

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The electrochemical properties of the samples were investigated using impedance Also, note that the Co−NZF exhibited higher crystallinity, slightly lower X-ray density, 347 decreased dislocation density and increased average lattice constant (Table 1)   maximum degradation was observed at pH 3 which decreased to 32.6% at pH 9 (Fig.4b). Except 387 in the presence of K2S2O8, the pH of the final solutions increased before remaining marginally 388 stable at pH 6.87.

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The following is an interpretation of the removal behaviour as a function of pH variation:  Fig.4c   The 10 mg Co−NZF/H2O2/UV and 10 mg Co−NZF/K2S2O8/UV systems reached 92.8% 465 and 50.8% degradation, respectively. However, according to the kinetic studies shown in Fig.5c, 466 the Co−NZF/H2O2/UV system had a faster degradation rate and reached the maximum before 6 h.

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When compared to photolysis (UV alone), H2O2/UV and K2S2O8/UV systems, the 468 photodegradation activity of Co−NZF/H2O2/UV increased remarkably by 15.8, 2.74 and 1.74   469 times, respectively. The increased performance is attributed to Co−NZF's activation of H2O2 that 470 generated • OH radicals to enhance the degradation of MZ as represented in Eqs. 8−10.
In comparison to using Co−NZF alone, the Co−NZF/K2S2O8 system showed less 475 degradation. K2S2O8 is available in the form of S2O8 2− in a wide pH range between 2 and 14 due 476 to its low pKa = −1.30 and may firmly attach to the positively charged catalyst's surface (pHpzc 477 =6.69) at pH 3, decreasing MZ adsorption for photocatalytic degradation. Based on these findings, 478 we can deduce that while photolysis and oxidants play minor roles in MZ degradation individually, 479 the synergistic effects of photocatalysis (Co−NZF/H2O2/UV) are critical.    Considering the radical scavenging results and UV-vis DRS data, the likely mechanism of MZ 523 photodegradation by Co−NZF is postulated. Eqs. 11 and 12 were used to calculate the conduction 524 band (Cb) and valence band (Vb) potentials of Co−NZF, which were found to be -0.53 eV and 2.31 525 eV, respectively; and χ represents the calculated absolute electronegativity of Co−NZF.
In the initial dark adsorption step, the reactive sites of Co−NZF absorbed both MZ species 529 and oxygen. Electrons are promoted from the Vb to the Cb when the Co−NZF was irradiated with 530 photon energy greater than that of its bandgap. As indicated in Eq.8, this mechanism produces 531 electron (e−)−holes (h + ) in the exterior shell of Co−NZF, which generates extremely reactive 532 radicals by oxidizing O2 and H2O molecules as shown in Eqs. 13−14.

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Under optimal conditions, different salts (Na2SO4, KNO3 and NaCl) and antibiotics (tetracycline 561 and tylosin) were added into the system as interference. In the presence of ionic salts and 562 antibiotics, the Co−NZF degradation efficiency decreased to 53.3-75.4% (SO4 2− > NO3 − > Cl − ) 563 and 51.7-67.3%, respectively, after 360 min of interaction (Fig.6b). The size of the interfering was subsequently reduced as a result of this. The Co−NZF was effectively recycled for five 571 photocatalytic reuse cycles and retained more than 50% after the third cycle but the performance 572 decreased remarkably beyond this stage (Fig. 6c). This decrease in degradation efficiency could 573 be related to the diminishing reactive sites and Co−NZF concentration in the bulk solution as a 574 result of sequential separation, filtration, and washing operations.  Co−NZF/UV system, respectively. But when the time was extended to 120 min; both 15 and 25 586 mg mL −1 of Co−NZF inactivated nearly 98% of the E. coli colonies (Fig.7a). After 120 min 587 (Fig.7b), 5 mg of Co−NZF had a bactericidal effect against S. aureus, which was most likely owing 588 to the interaction of the positively charged Co−NZF + with the bacteria proteins or/and the induction 589 of ROS, which resulted in bacterial death.