key: cord-0979553-y7vfoqny
authors: Lu, Siyi; Meng, Ge; Wang, Can; Chen, Hong
title: Photocatalytic inactivation of airborne bacteria in a polyurethane foam reactor loaded with a hybrid of MXene and anatase TiO(2) exposing {001} facets
date: 2020-08-05
journal: Chem Eng J
DOI: 10.1016/j.cej.2020.126526
sha: da3309908bd9ef813d4cda345a04b9fc7d34ea45
doc_id: 979553
cord_uid: y7vfoqny

A hybrid of TiO(2) exposing {001} facets and monolayer Ti(3)C(2)T(x) nanosheet (MXene) was synthesized, characterized and used as a photocatalyst in this study. The introduction of MXene (3.4 wt.%) helped to reduce the recombination of photo-induced electrons and holes, and thus enhanced the photocatalytic activity by 30%. A continuous flow-through reactor loaded with the as-prepared photocatalyst coated onto polyurethane foam was developed to inactivate airborne bacteria. The photocatalytic inactivation efficiency of airborne Escherichia coli (E. coli) achieved 3.4 lg order under ultraviolet (UV) irradiation at 254 (UV254), which was superior to that using UV254-only treatment with 2.5 lg order under the same operating condition (95% relative humidity and retention time of 4.27 s). The effect of humidity and bacteria species on inactivation performance was also investigated. The thick cell membrane could protect bacteria from photocatalytic oxidation while high humidity increased the photocatalytic inactivation efficiency by generating more reactive oxygen species. The phenomena of photo reactivation and dark repair of airborne E. coli using UV254-only treatment was observed. However, no reactivation occurred after UV photocatalytic inactivation, and even a continuous decline under visible light. These results suggested a different inactivation mechanism between UV irradiation and UV photocatalysis that the former inactivated bacteria by damaging their DNA, whereas photocatalysis physically damaged their cell structure.

Bioaerosols are airborne microbial cells with fragments and particulate matter of biological origin, such as virus, bacteria, and fungal spores [1, 2] . These small particles cause infectious diseases, acute toxic reactions, and allergies and thus affect human health [3] . Outbreaks of severe acute respiratory syndrome (SARS) and influenza H1N1 viral infections across the globe have attracted worldwide attention for airborne microbial prevention and control measures [4] .

Ultraviolet (UV) irradiation, especially UVC, has been recommended to inactivate various infectious organisms due to its ability to inactivate airborne organism by effectively damaging their DNA [5] [6] [7] . However, other studies have reported that the "dead" state of bacteria caused by exposure to UVC irradiation may not be permanent. Inactivated bacteria may be reactivated after a period of time; thus, the survival of bacteria and the risk must be re-assessed [8, 9] . Photocatalysis has been applied in previous studies and turns out to be a promising purification technique for water disinfection [10] , statistic surface sterilization [11, 12] , and antimicrobial materials development [13] . However, the research in the field of dynamic bioaerosols disinfection within a flow-through device is very limited [13, 14] . Therefore, this study aimed to establish a continuous flow-through reactor to inactivate airborne bacteria dynamically under UV photocatalysis.

Catalyst is an important part of the photocatalytic reactor [15] [16] [17] . Titanium dioxide (TiO 2 ), especially commercial Degussa P25, has been widely used as a photocatalyst since Matsunaga [18] and his coworkers first applied photocatalysis to inactivate microorganism. Reports [19] have shown that the photocatalytic reactivity of TiO 2 can be further improved by metal/nonmetal doping and crystal facet regulation. Both theoretical and experimental evidence show that {001} facets are much more reactive than the thermodynamically stable {101} facets due to their higher surface energy.

Anatase TiO 2 nanosheets with reactive {001} facets exhibited a superior 4 photocatalytic activity to that of P25 [20] . Furthermore, Sun et al. [21] discovered that introducing a two-dimensional material could enhance the electron transfer yield and thus improving the photocatalytic performance of TiO 2 .

A new family of two-dimensional early transition metal carbides and/or nitrides (MXenes) has attracted intensive interest since they were synthesized [22] . Most studied nanosheet among MXenes is Ti 3 C 2 T x (T=OH, F or O) [23] . Density functional theory (DFT) calculations predict that Ti 3 C 2 T x exhibits metallic conductivity, which is favorable for electron-hole separation and charge transfer [24, 25] . With the special structure of few layered or monolayer nanosheets, MXene may well improve the photocatalytic performance [26, 27] . Mashtalir et al. [28] first reported the adsorption and photocatalytic degradation of dyes by multilayered MXene. Cai et al. [29] fabricated Ag 3 PO 4 /Ti 3 C 2 interface materials as a Schottky catalyst with enhanced activity and stability because the Schottky junction can transfer electrons to the Ti 3 C 2 surface in a timely manner through the built-in electric field, inhibiting the photocorrosion of Ag 3 PO 4 caused by photogeneration electrons. A hybrid of multilayer Ti 3 C 2 T x and TiO 2 has been synthesized and reported to have high photocatalyst activity [30, 31] .

Rasool et al. presented antimicrobial activities of Ti 3 C 2 T x MXene against Escherichia coli(E. coli) and Bacillus subtilis(B. subtilis) in water [32, 33] . However, the effect of monolayer Ti 3 C 2 T x MXene on photocatalysts and its application performance in airborne bacteria inactivation have not been reported.

In this study, a hybrid of monolayer Ti 3 C 2 T x and TiO 2 exposing {001} facets was prepared as a photocatalyst. Its structure and property were characterized, and the function of monolayer Ti 3 C 2 T x was analyzed. A continuous flow-through UV/photocatalysis reactor, with polyurethane (PU) foam as support to load as-prepared photocatalysts, was established. The photocatalytic inactivation performance on different bacteria and spore under different UV wavelengths was investigated. The photo reactivation and dark repair of after-inactivated microbe were 5 also explored to reveal the difference in inactivation mechanism between UV irradiation and photocatalysis.

2.1 Material preparation and characterization 2.1.1 Preparation of MXene Ti 3 AlC 2 (purchased from 11 technology Co., Ltd, Jilin, China) was used as the raw material to prepare multilayered Ti 3 C 2 T x . Ti 3 C 2 T x was produced by immersing 2 g of Ti 3 AlC 2 in 40 mL of hydrochloric acid solution containing 2 g of LiF and 9 M of HCl for 24 h at 35°C [34] . During this process, Al species in Ti 3 AlC 2 were selectively etched by HF. The obtained multilayered Ti 3 C 2 T x suspension was centrifuged at 3500 rpm for 10 min and washed with deionized water until the pH of the solution was higher than 6. Then the precipitate was dispersed in dimethyl sulfoxide (DMSO) for intercalation. Next, the precipitate was dispersed successively in ethanol and deionized water by ultrasonication (500 W) for 1 h with ice bath. The suspension was then centrifuged at 9000 rpm for 20 min, and a dark colloidal Ti 3 C 2 T x nanosheets (MXene) solution was obtained [35] .

Anatase TiO 2 nanoparticles with exposed {001} facets were incorporated with MXene through the one-step hydrothermal method [20] . A certain amount (0.9 wt.%, BaSO 4 was used as reflectance reference for the UV-Vis spectrophotometer. The band gap energy of the sample was determined by plotting [F(R ∞ )hv] 2 against the photon energy. X-ray photoelectron spectroscopy (XPS) was analyzed by a Thermo ESCALAB 250xi spectrometer at a spot size of 500 μm, using an Al Kα source with a resolution of 0.05 eV. Photoluminescence (PL) spectra were obtained using a fluorescence spectrometer (F-4600, Hitachi, Japan). The excitation wavelength was set at 254 nm.

Photocatalytic reactivity was evaluated through the degradation of methylene blue (MB) under UV radiation. Detailed information can be found in supplementary material (Text S1 and Fig. S1 , SI). For the analysis of different oxidation pathways in the photocatalytic process, trapping tests for radicals and holes were carried out, in which 0.1 M of tert-butanol (t-BuOH) and 0.1 M of EDTA-2Na were used as scavengers of hydroxyl radical (HO·) and holes (h + ), respectively. 7 The as-prepared {001}TiO 2 /MXene catalyst was loaded onto the PU foam by dipping in a 1% ethanol base catalyst solution for 10 min, dried in vacuum at 55 ℃ for 8 h, and fabricated to develop a quartz reactor for bacteria inactivation in bioaerosols. The preparation and coating process mentioned above is presented in Fig.   1 . impinger (Qingdao Junray Intelligent Instrument Co., Ltd., China) at a flow rate of 12.5 L/min for 10 min. The nutrient agar plates were then incubated for 24 h at 37 ℃. 8 The number of colony-forming units (CFUs) on each nutrient agar plate was manually counted. Each experiment was conducted in duplicate.

A schematic representation of the experimental setup for inactivating airborne bacteria is shown in Fig. S2 in SI. The photoreactor is a cylinder-shaped double-layer quartz casing with dimensions of 7 cm diameter × 35 cm length. PU foam was used as a catalyst carrier to support the as-prepared photocatalyst to increase the contact area with which the reactor was packed up. To test the filtration of PU foam and the inactivation of UV irradiation, experiments were also conducted under PU foam-only and UV-only conditions. The bioaerosols containing bacteria passed through the photoreactor from bottom to top, and were subsequently collected and measured.

Other parameters are listed in Table S1 in SI.

The inactivation performance was measured by log inactivation efficiency (E), which is defined as the logarithmic order of bacteria concentration decrease after inactivation [36] :

where N 0 and N t are the specific bacteria concentration (CFU/m 3 ) in bioaerosols before and after inactivation treatment, respectively.

The electrical efficiency per log order (EE/O) in the process of UV/photocatalytic inactivation was applied to evaluate the energy consumption. EE/O is defined as the electrical energy (kW·h) required to reduce the concentration of microbes by one order of magnitude in 1 m 3 air using the following equation [37] . In this study, only the energy consumed by UV lamps was considered and the other consumption was not included. 9 where P is the lamp power output. Only the lamp power was considered, and the UV lamp with a wavelength of 365 nm (UV365) is 8 W, while the UV lamp with wavelength of 254 nm (UV254) is 15 W. V is the reactor volume. t is the retention time.

Reports indicate that the bacteria after UV treatment might not be really "dead" and can be reactivated after a period time of repair under certain circumstances [38, 39] .

The method of repair and cultivation was applied to determine the resuscitation potential of the E. coli inactivated by UV irradiation and the photocatalytic treatment 

3.1 Structure and properties of Ti 3 C 2 T x and {001}TiO 2 Fig. 2(a) shows the morphology of multilayered Ti 3 C 2 T x , indicating that the Al element in Ti 3 AlC 2 was removed after LiF/HCl etching and therefore exhibited an accordion-like shape [ Fig. 2(b) ]. The removal of Al was also verified by EDS [ Fig.   S4 ], indicating that the content decreased to 3%. The Ti 3 C 2 T x nanosheet (MXene) was prepared after the DMSO intercalation and ultrasound delamination of the multilayered Ti 3 C 2 T x , as shown in Fig. 2(c) . The TEM images clearly demonstrated the morphology and size of MXene. The low contrast indicates few atomic layers and uniform lamellae, with a nanosheet size of approximately 100 nm. The MXene film was prepared by the self-assembly Ti 3 C 2 T x nanosheets under vacuum filtration ( Fig. S5) , after which they were separated from the colloidal solution. In this study, the monolayer Ti 3 C 2 nanosheets were verified by the HRTEM images (Fig. S6) . The flexible MXene film has a smooth surface. Fig. 2(d) shows the XRD pattern of 10 Ti 3 AlC 2 , multilayer Ti 3 C 2 T x , and MXene. Meanwhile, the production of MXene is verified by (002) peak at around 10° (2θ), where the peak shifted to a lower angle after Ti 3 C 2 T x delamination [32, 40, 41] . of Ti 3 C 2 T x at 7.5° can be observed, suggesting the existence of MXene, although its intensity is weak due to the low contents.

As shown in Fig. 4(b) , the optical absorption properties of the photocatalysts with different MXene contents were investigated using a UV-Vis diffuse reflectance spectrometer. Intense ultraviolet characteristic absorptions were observed from 200-400 nm and are consistent with the range of the anatase TiO 2 absorptions [42] . 12 Clearly, the absorption intensity was significantly strengthened by the addition of 3.4% and 7.2% MXene, suggesting the enhancement of the anatase TiO 2 catalytic ability by introducing MXene. The bandgap energy of the photocatalysts was determined by plotting [F(R ∞ )hv] 2 against the photon energy in Fig. 4(c) . The bandgap of TiO 2 obtained by the transverse intercept of the extension line of the straight part is 3.5 eV, which is higher than the theoretical value ( [42, 43, 44] . 13 The photocatalytic activity of the catalysts with different MXene contents in anatase TiO 2 is shown in Fig. 4(d) . A rather low activity was found in the absence of UV irradiation, indicating that the effect of adsorption by the catalyst can be neglected.

Approximately 85% of the MB was photodegraded by the {001}TiO 2 catalyst after 180 min reaction. Photocatalytic activity was improved by adding MXene, and the highest degradation efficiency was observed when the {001}TiO 2 /3.4% MXene catalyst was used, which was 30% higher than that using the {001}TiO 2 catalyst. Su et al. [44] also observed much better property of monolayer Ti 3 C 2 T x than multilayer hard to be observed in C 1s spectra because of the high content of TiO 2 [47, 48] . To further study the effects of MXene on photocatalytic activity, PL spectra of the 14 TiO 2 /MXene and pure TiO 2 are tested and shown in Fig. S7 . The recombination of photo-induced electrons and holes lead to the release of photons and heat, resulting in PL [49] . The PL intensity decreases after loading with MXene, compared with that of the pure {001}TiO 2 . These results suggested that the monolayer MXene in {001}TiO 2 improved electrons transport between the contacting surfaces, and thus inhibiting the recombination of photo-induced electrons and holes [20] . [31, 49, 50] , with high electrical conductivity and two-dimensional structure, at the interface. Therefore eand h + were 15 more likely to react to produce OH·, or other free radicals, that can be utilized for degradation of organics, thus leading to the enhancement of photocatalytic activity [44, 51] . Notably, the photocatalyst-coated PU foam had a high inactivation efficiency of 3.4 and 2.5 lg order under UV254 and UV365, respectively.

Rasool and Yury [32] reported the antimicrobial activity of Ti 3 C 2 MXene to waterborne bacteria. However, no obvious antimicrobial activity to airborne bacteria was observed in our previous study. In this study, {001}TiO 2 /MXene(3.4%) showed good photocatalytic activity in inactivating bacteria under both UV365 and UV254.

The enhancement of inactivation efficiency from 3.0 lg to 3.4 lg order under UV254, or from 0.6 to 2.5 under UV365 (Fig. 7) was attributed to the addition of the {001}TiO 2 /MXene catalyst. In other words, the presence of photocatalysis significantly improved the inactivation performance, especially for UV365. Therefore, the inactivation performance obtained under UV365/photocatalysis can be considered a photocatalysis-dominated damage, whereas that obtained under UV254/photocatalysis was the sum of photocatalytic oxidation and UV photolysis.

As for EE/O under UV irradiation and photocatalysis, Fig. 7(b) shows that the addition of a photocatalyst remarkably reduced the energy consumption for inactivating E. coli compared with the condition of UV365-only inactivation. The EE/O using UV365 fell by half when the PU foam was added, because the inactivation efficiency (UV365/foam) increased by double due to PU filtration effects, while the energy consumption stayed unchanged. Interestingly, the electrical efficiencies of UV/photocatalysis under UV254 and UV365 were nearly the same, 17 indicating that photocatalytic inactivation under UV365 is also an economical and efficiently method.

To investigate the influence of bacteria species, inactivation experiments for E.coli and B. subtilis and its spore were conducted (Fig. 8(a) ), which were representative of gram-negative bacteria, gram-positive bacteria, and bacteria in a dormant state, respectively. Results show that the degree of difficulty of photocatalytic inactivation followed the order E.coli < B. subtilis < spore of B. subtilis regardless of the UV source adopted. A widely recognized theory is that photocatalysis inactivates microorganisms by reactive oxygen species (ROS), which first compromises the cell membrane or cell wall, and then penetrates to reach the internal cellular component and eventually causes cell death [52] . subtilis and its spore than that of E. coli. Moreover, the effect of relative humidity was also studied [ Fig. 8(b) ]. The lg inactivation efficiency improved with the increasing relative humidity from 30% to 95%. Most of the other studies showed that ROS was responsible for the photocatalytic inactivation of microbes and hydroxyl radical (·OH) is dominant [52, 53] . [54] revealed that the increase in atmospheric humidity progressively increased the photodegradation rates of VOCs given that water vapors 

Species 19 can be adsorbed on the TiO 2 active sites as resources for ·OH, and therefore the enhancement of inactivation performance may result from the promoted generation of ·OH under high humidity [55] . 

The performance of photo reactivation and dark repair after UV254, UV365, UV254/photocatalysis and UV254/photocatalysis treatment, respectively, are shown in Fig. 10 . Detailed pictures of culture plates are shown in Fig. S8. Figs. 10 Zhang et al. [63] reported that many pathogens can enter the viable-but-nonculturable (VBNC) state in response to environmental stresses, such as UV irradiation, chloramine, and free radical. The resuscitation of VBNC cells induced by chloramine was not observed, but resuscitation was observed when UV254 was used in their study. A similar phenomenon was also found in our study, and Fig. 11 presents a diagram of the inactivation mechanism of UV irradiation and UV photocatalysis. UV irradiation inactivates bacteria by damaging their DNA, especially the pyrimidine dimer in it, which can be repaired under certain circumstances, such as under visible light. However, UV photocatalysis disinfects microorganisms by ROS oxidation and causes physical damage to their outer membrane [64] , and the membrane cannot be rebuilt without nutrition. Therefore, UV photocatalytic inactivation can be considered a more thorough disinfection approach than UV irradiation. The photo reactivation and dark repair performance of the after-inactivated microbe were also investigated in this study. The bacteria after photocatalytic oxidation were difficult to reactivate, whereas those after UV-only treatment can be repaired and become culturable again. These results suggested a different inactivation mechanism between UV irradiation and UV photocatalysis that the former inactivated bacteria by damaging their DNA, whereas photocatalysis physically damaged their cell structure. Therefore, photocatalytic inactivation is considered as a more thorough disinfection technology compared with single UV inactivation.

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This study was supported by the National Natural Science Foundation of China (51678402) and the key technologies R & D program of Tianjin (20ZXGBSY00100). 23