Abstract:

Objective Highly toxic and refractory trace antibiotics in agricultural and medical wastewater seriously affect the ecological environment, which makes it necessary to develop efficient antibiotic removal methods. Photocatalytic technology can use solar energy to degrade pollutants in water. However, in most studies, photocatalysts are usually used in powder form for photocatalytic degradation of pollutants, which is difficult to remove from water. This research sets out to investigate the natural nanofiber network of bacterial cellulose (BC) as a support skeleton for supporting nano-catalytic materials, with potential applications in water purification.
Method Biocultured bacterial cellulose nanofiber membranes were made into nanofiber dispersion, and Au nanoparticles were deposited on the dispersed nanofibers by chemical reduction to obtain flexible BC/Au conductive nanofiber materials. After that, the tungsten trioxide (WO₃) photocatalyst was fixed by adsorption method, and the BC/Au-WO₃ nanofiber membrane was formed by vacuum-assisted filtration.
Results A large number of adherent WO₃ nanoparticles were found on BC nanofibers, as a good flexible support role for photocatalytic materials (Fig. 2). BC showed standard cellulose crystals and the composite nanofibers show obvious diffraction peaks of Au and WO₃. No other impurities were found in the sample by XRD (Fig. 3). The electrochemical impedance and mechanical test indicated that the addition of Au could improve the electron transfer resistance and mechanical properties of nanofiber membrane and the interface charge transfer impedance of BC/Au nanofiber membrane was 32.353 Ω (Fig. 4). The light absorption capacity of BC/Au-WO₃ nanofiber membrane for visible light was improved with the addition of Au, and there was a distinct characteristic absorption peak at 550 nm, consistent with the local surface plasmon resonance effect of Au nanoparticles in the visible light (Fig. 5). In this work, tetracycline hydrochloride was used as the target substrate to study the catalytic degradation of nanofiber membrane. The results showed that the addition of Au was beneficial to improve the photocatalytic degradation rate of the material (Fig. 6(a)). The photocatalytic and photoelectrocatalytic degradation rates of tetracycline hydrochloride by nanofiber membrane in 3 h were 60.2% and 78.4%, respectively (Fig. 6(b)). Compared with photocatalytic degradation alone, the catalytic degradation efficiency was greatly improved after adding additional electric fields, and the photoelectrocatalytic efficiency was greater than the sum of the efficiency of photocatalysis and electrocatalysis (Fig. 6(c)). Coupling was shown between photocatalysis and electrocatalysis. With the addition of additional electric field, the photogenerated electrons generated by the photocatalytic reaction were controlled by the voltage and transferred from the fiber membrane side to the platinum electrode side through the external circuit, thus playing the role of separating the photogenerated electrons and hole pairs. In addition, BC/Au-WO₃ nanofiber membrane also showed the good reusability, and the results showed that its catalytic degradation performance remained unchanged after 6 times of reuse (Fig. 6(d)).
Conclusion BC can play a good support role for WO₃ as a three-dimensional biomass flexible skeleton. Au nanoparticles modification can effectively improve the photocatalytic efficiency of BC/Au-WO₃ composite nanofiber membrane. After light radiation, the Au nanoparticles and the WO₃ Photocatalyst on the nanofiber network can form Mott-Schottky barriers to trap photogenerated electrons, thus reducing the recombination of photcogenerated electron-hole pairs. Moreover, Au nanoparticles can generate local plasma resonance effects in visible light and inject thermal electrons into WO₃ to improve photocatalytic efficiency. Under light excitation and additional electric field, the catalytic degradation rate of tetracycline hydrochloride by composite nanofiber membranes was greatly improved. This is because the directional movement of photogenerated electrons under an additional electric field can force photogenerated electron hole pairs to efficient separate. When the additional voltage is 2.0 V, the photocatalytic degradation rate of tetracycline hydrochloride can reach 78.4% within 3 h. Compared with the single photocatalytic reaction, photoelectric catalytic reaction can accelerate the reaction and improve the degradation rate, which has potential application value in the water treatment of antibiotic.

Key words: tungsten trioxide, bacterial cellulose, photoelectric catalysis, nanofiber film, degradation of antibiotic wastewater, Au nanoparticle