Synthesis Of Bacillus Subtilis-based Compound Photocatalytic Materials By Using Bacillus Subtilis As Biological Carriers And Their Application In The Degradation Of Dyeing Wastewater

Dyeing wastewater was one of the industrial effluents which are difficult to decompose and gradually pollute the water system. The utilization of photocatalytic materials through catalytic oxidation methods is a promising technology to degrade the dyeing wastewater. Due to its low cost, excellent biodegradability, environmental friendly and high efficiency, photocatalytic materials give a huge advantage over other common approaches. In the present study, Bacillus subtilis, as a novel bacteria template, were studied to understand the dye adsorption mechanism for methylene blue. A series of TiO₂@Bacillus subtilis and ZnS@Bacillus subtilis compound photocatalytic materials were prepared by using Bacillus subtilis as biological carriers. Their structure and surface chemical-physical properties were characterized by employing a variety of characterization methods(i.e., Fourier transform infrared spectroscopy（FTIR）, scanning electron microscope（SEM）, X-ray diffraction（XRD） and energy dispersive spectrometer（EDS） to research into their synthesis mechanisms. Moreover, batch-mode adsorption experiments were conducted by using rhodamine B as a target dye to investigate the degradability of the Bacillus subtilis-based compound photocatalytic materials. The observed results are summarized as follows:（1）For the methylene blue adsorption onto Bacillus subtilis, the adsorption capacity was sharply increased with the increase of initial dye concentration at first 30 minutes, and then equilibrium adsorption was reached as time prolonged. It could be observed that the pH values of dye solutions played an important role in dye adsorption of Bacillus subtilis for methylene blue, viz., the adsorption capacity increased with the increase of pH values of dye solutions from 2 to 12. These results confirmed a better application of the pseudo-second-order kinetic model than the pseudo-first-order kinetic model, and the adsorption rate was directly proportional to the square of solute concentration. The negative value of Gibbs free energy change（ΔG） and enthalpy change（ΔH） indicated that the adsorption process was spontaneous and endothermic, respectively. The activation energy was equal to 11.30 KJ·mol-1（between 5 KJ·mol-1 and 50 KJ·mol-1）, manifesting that the dye adsorption was mainly in the form of physical absorption. Owing to the abundant hydrophilic groups including hydroxyl（–OH）, amino（–NH）, carboxyl（–COO-） and carbonyl（C=O） groups on the cell wall, Bacillus subtilis had a good adsorption capacity for methylene blue. The results also informed us that the optimal adsorption condition was at 308 K with an initial concentration of 100 mg/L where the adsorption capacity was up to 39 mg/g. Therefore, the Bacillus subtilis are a novel and potential adsorbent for dyestuffs, duo to their low cost, high adsorption capacity and excellent suspension and dispersion.（2）TiO₂@Bacillus subtilis compound photocatalytic materials were successfully prepared via a single-step strategy based on electrostatic interaction driving self-assembling heterocoagulation. Such materials had a rod-shaped microstructure with uniform size（1.4 ± 0.1 μm in length; 450 ± 50 nm in width）. The SEM images also showed that TiO₂ nanoparticles were attached on the cell wall of Bacillus subtilis in monolayer, but not covered all of the cell wall. The crystalline of TiO₂ nanoparticles remained unchanged before and after the preparation process, as illustrated by XRD spectra. The FTIR spectra suggested that –OH, –CONH2, –COO-, and –OPO32- groups were the major contributors for the anchoring of the TiO₂ nanoparticles. To further examine the application of TiO₂@Bacillus subtilis composite particles in the removal of water contaminants, the aqueous solution of rhodamine B（RB）, was taken as an example. The removal ratio of RB by TiO₂@Bacillus subtilis compound photocatalytic materials was approximately 89 %. All the findings indicated that TiO₂@Bacillus subtilis compound photocatalytic materials integrated the propertied of acillus subtilis and TiO₂ nanoparticles, and equipped with well suspension property and dispersibility, large contact area, favourable bio-sorption capacity and photocatalytic activity.（3）ZnS@Bacillus subtilis compound photocatalytic materials were fabricated through biochemical adsorption and aging process. As can be seen, the composite materials were 1.4 ± 0.1 μm in length and 500 ± 50 nm in width. The SEM images reflected that the winkles on the surface of cell wall were reduced obviously after reaction. Zn and S elements were found by EDS, and the diffraction peak of sphalerite structure of ZnS was also characterized by XRD, which testified a successful load of ZnS semiconductor photocatalyst on the Bacillus subtilis. The FTIR spectra indicated that–CONH2 groups played a dominate role in the preparation process. Moreover, ZnS@Bacillus subtilis compound photocatalytic materials could efficiently degrade rhodamine B（RB） under visible light irradiation.In our paper, the results of experiments showed that Bacillus subtilis had a good adsorption capability for cationic dye. Owing to the excellent physical and chemical properties of Bacillus subtilis, a series of Bacillus subtilis-based photocatalytic compound materials were prepared by simple synthetic methods. Such-prepared compound materials integrated the biosorption capacity of Bacillus subtilis and catalytic property of semiconductor materials, and were successfully applied in the degradation of dyestuffs. By now, there is little literature about Bacillus subtilis-based photocatalytic compound materials. Hence, our studies firstly report the fabrication of micro/nano photocatalytic composite materials by using Bacillus subtilis as the biological carriers, which are valuable for the development of a wide range of photocatalysis and micro/nano innovative materials.