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An efficient photocatalyst coating strategy for intimately coupled photocatalysis and biodegradation (ICPB): Powder spraying method
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An efficient photocatalyst coating strategy for intimately coupled photocatalysis and biodegradation (ICPB): Powder spraying method Fuqiang Lia, Xuefang Lana, Lili Wangb, Xiaoying Konga, Ping Xua, Yun Taia, Guoqing Liua, ⁎ Jinsheng Shia, a b
College of Chemistry and Pharmaceutical Science, Qingdao Agricultural University, PR China Science and Information College, Qingdao Agricultural University, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Catalyst was firmly adhered to the • surface of carrier by powder spraying method.
internal biofilms avoid suffering • The from ROS attack caused by photocatalysis.
photocatalysts remained firmly on • The the carrier even after six cycles. removal rate was maintained at • TCH 94.3% after six cycles.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Biodegradation Powder spraying Tetracycline hydrochloride
Intimately coupled photocatalysis and biodegradation (ICPB) is an attractive wastewater treatment technology, and the key to a successful ICPB is a carrier capable of accumulating biofilm inside and adhering photocatalyst firmly on its exterior. However, the photocatalysts coated on the carrier by the conventional method not only occupy the internal pores which should be left for microorganism loading, but also possess an insufficient adhesion to the carrier. To solve this problem, a powder spraying method has been proposed for the first time, which allows the photocatalyst to adhere more firmly on the exterior surface of the carrier, and to reserve the inside pore structures for biofilm accumulation. In the end, when using visible-light-induced ICPB (called VPCB), the degradation rate of tetracycline hydrochloride achieved 97.2%, and the stability of recycled carrier was still excellent even after 6 cycles of operation. This work provides a novel avenue for the design of a highly stable ICPB system with low-cost and high degradation efficiency.
1. Introduction Antibiotics have been widely used to treat human and animal infections in the past few decades [1]. Among the antibiotics, tetracycline hydrochloride (TCH) is a well-known broad-spectrum antibacterial
⁎
agent, which is inexpensive and effective [2,3]. TCH molecules are extensively accumulated in the aquatic environment after abuse, causing serious damage to human and animal health by triggering bacterial resistance [4,5]. However, due to the stable chemical structure and the high refractory to biodegradation, TCH cannot be
Corresponding author at: Qingdao Agricultural University, College of Chemistry and Pharmaceutical Science, Chengyang District, Qingdao, PR China. E-mail address: [email protected] (J. Shi).
https://doi.org/10.1016/j.cej.2019.123092 Received 25 June 2019; Received in revised form 28 September 2019; Accepted 6 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Fuqiang Li, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123092
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Nomenclature ICPB VPCB TCH AOPs HPLC PCBBR P
AD VPC TOC DOC CLSM PBS SEM EPSs m/z
intimately coupled photocatalysis and biodegradation visible-light-induced ICPB tetracycline hydrochloride advanced oxidation processes high-performance liquid chromatography photocatalytic circulating-bed biofilm reactor photolysis
adsorption visible-light-induced photocatalysis total organic carbon dissolved organic carbon confocal laser scanning microscopy phosphate buffer saline scanning electron microscopy extracellular polymer substances mass charge ratio
bonded to polyurethane sponges due to similar compatible principle [23]. Therefore, the combination of polyurethane waterproof coating and polyurethane sponge can be selected as an alternative to the traditional photocatalyst coating procedure. In terms of the photocatalysts, an environment-friendly, low-cost and high visible-light-induced catalyst was selected for ICPB. According to our previous work, BiOCl/Bi2WO6/Bi is an excellent visible-light-driven and easy-prepared photocatalyst for pollutant degradation: the BiOCl/Bi2WO6 heterojunction and Bi formation accelerate the efficiency of carriers separation and transfer; the presence of Bi also broaden the light absorption scope from UV to Vis [24]. Besides, the catalyst can be obtained by onestep solvothermal synthesis and is a non-noble metal catalyst [25]. Therefore, BiOCl/Bi2WO6/Bi was selected as the photocatalyst in our ICPB investigation. In this study, we provided a simple photocatalyst coating strategy that endows strong adhesion between the photocatalyst and the carrier. As shown in Scheme 1, a powder spraying method was first developed as a photocatalyst coating procedure, which allows the attachment of photocatalysts only to the exterior surface of the carrier and leaves the inside pores for bacteria loading and accumulating. This method enhances the survive opportunity for the internal bacteria and reduces the consumption of photocatalysts. To demonstrate the feasibility of this strategy, the adhesion strength and distribution situation of loaded catalysts, the biomass accumulation situation, the VPCB’s efficiency, and the stability of prepared system were evaluated in detail. Meanwhile, the degradation pathway of VPCB was investigated in depth. The results confirm that the new photocatalyst coating methodology significantly enhanced the TCH degradation efficiency by ICPB system and provided a solid platform for the technology development of ICPB.
effectively removed by conventional wastewater treatment [6,7]. Therefore, effective techniques are required to treat TCH in wastewater. Advanced oxidation processes (AOPs) are effective methods for degrading TCH, but it is difficult to achieve complete mineralization [8]. Combining AOP with biodegradation has been proposed as a more promising approach for mineralizing TCH [9], in which the biodegradation is usually a necessary downstream process after AOP degrades TCH [6]. However, the AOP is indiscriminate and fast acting, which makes it impossible to obtain the desired biodegradable intermediates for subsequent biological treatment [10]. Intimately coupled photocatalysis and biodegradation (ICPB) developed in recent years has overcome the challenge of AOP, i.e. the indiscriminate reactivity [11,12], which allows the AOP and the biodegradation occur simultaneously in the same reactor [13]. In this process, the photocatalyst and the microorganism are coated on the same carrier, where the photocatalytic reaction occurs on the exterior surface and the biodegradation occurs inside [11]. The key of ICPB is to protect microorganisms from ultraviolet radiation, free radical attack and original pollutants (such as TCH). This requires the microorganisms to be protected as a biofilm in the core of macroporous carrier, and the photocatalyst to be adhered to the exterior surface of carrier with high strength [14,15]. So far, many photocatalyst coating procedures have been reported, such as low-temperature sintering and sol-gel methods. Nevertheless, in these preparation methods, the internal carrier was also loaded by catalysts, which contributes to an adverse effect on the attachment and accumulation of microorganisms since it allows the microorganism destruction incurred by the free radicals generated from photocatalyst [15,16]. Besides, when these preparation methods were used, a poor adhesion strength between the photocatalyst and the carrier was obtained, indicating a low stability of ICPB system. Therefore, it is imperative to develop an efficient photocatalyst coating strategy for ICPB. Previously, all studies in ICPB used ultraviolet light as an excitation source for photocatalysis [10,13,17]. In 2015, ICPB driven by visible light was first developed with the Er3+:YAlO3/TiO2 as photocatalyst [8], which is called visible-light-induced photocatalysis and biodegradation (VPCB). This new VPCB approach has demonstrated excellent biofilm protection and energy savings merits [8,18], and has been extensively investigated until now [19]. However, using the rare earth upconversion material as a light converter is costly and the TiO2 cannot fully utilize the upconversion emissions, especially those in visible light range [20]. As to noble metal doped photocatalysts, the problems like high cost, bactericidal effect, complex operation and so on, are also troublesome [21]. Therefore, visible-light-induced photocatalyst with low-cost and environmentally friendly properties should be developed to improve ICPB performance [22]. ICPB is an attractive technology for wastewater treatment, nevertheless many challenges are still need to be solved. Since Li et al. first employed commercial polyurethane sponge as carriers for ICPB, economically applicable polyurethane sponge has been extensively used as carriers [10]. In addition, commercial single component polyurethane waterproof coating is an excellent polyurethane adhesive with many advantages such as solvent-free volatilization, non-toxicity, odorlessness, and rapid curing. Polyurethane waterproof coating can be easily
2. Material and methods 2.1. Chemicals and reagents The tetracycline hydrochloride (C22H24N2O8•HCl, purity of 98%) was obtained from Sigma-Aldrich Co. LLC. (USA). The methanol, acetonitrile and glacial acetic acid used were of high-performance liquid chromatography (HPLC) grade and were purchased from
Scheme 1. Schematic representation of photocatalyst coating procedure by powder spraying method. 2
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Sinopharm Group Co., Ltd. (Shanghai, China). The other chemicals were of analytical grade without further purification. Ultrapure water was used throughout the whole experimental process.
2.2. Carrier and BiOCl/Bi2WO6/Bi coating procedure The carrier used in this study was a commercially available polyurethane sponge cube (HSYUAN, Jiangxi, China) with an average length of 10.0 mm (Fig. S1). The sponge had a specific surface area of 2.3 m2/g, a large pore size of 1–2 mm and a porosity of 95%. The BiOCl/Bi2WO6/Bi ternary complex photocatalyst was prepared based on our previous work [25]. The detailed preparation method (Section S1 and S2) and characterization of BiOCl/Bi2WO6/Bi (Fig. S2–S5) were illustrated in the Supplementary information. The BiOCl/Bi2WO6/Bi was coated onto the sponge carrier according to powder spraying method. 1.0 g of blank sponge cube was completely immersed in a polyurethane waterproof coating (Jingfu Co., Ltd., Shanghai, China) for five seconds, then quickly taken out and shaken off to prevent the cross-linking of the sponge skeleton. After that, the sponge was loaded by 1.0 g of BiOCl/Bi2WO6/Bi via powder spraying method, in which process the BiOCl/Bi2WO6/Bi powder was sealed in a tiny spray bottle and evenly sprayed on the exterior surface of the sponge. Then the coated sponge was placed at room temperature for half an hour to allow the polyurethane waterproof coating solid. Finally, the coated carriers were rinsed with ultrapure water to remove any poorly fixed photocatalyst. Sponge carriers coated by sol-gel method were also prepared as a control experiment (details in Section S3).
Fig. 1. Sponge carrier before (a) and after (b) BiOCl/Bi2WO6/Bi coating by the powder spraying method.
2.6. Analytical methods Analytical methods for TCH (analyzed by HPLC) and TCH-degradation intermediates (analyzed by UPLC-MS) were given in Section S5. Pretreatment procedures for carrier microstructure observation by scanning electron microscopy (SEM) and biofilm staining procedure for confocal laser scanning microscopy (CLSM) imaging were presented in Section S6 and Section S7, respectively. The total organic carbon (TOC) was measured using a TOC analyzer (Shimadzu, Japan). Prior to analysis, each sample was filtered through a 0.22 μm membrane filter, so the measured TOC was assumed to be dissolved organic carbon (DOC). 3. Results and discussion
2.3. Inoculum and biofilm cultivation
3.1. Photocatalyst distribution and biomass accumulation
The activated sludge was obtained from the secondary sedimentation tank of the Chengyang Wastewater Treatment Plant, Qingdao, China. The BiOCl/Bi2WO6/Bi coated carriers were immersed in the activated sludge for 24 h to adsorb the microorganisms, and then the biofilm was cultured in an internal loop airlift-driven fluidized bed reactor with aeration [26]. Biofilms were cultured using synthetic nutrients containing 330 mg/L sodium acetate, 29 mg/L NH4Cl, 8 mg/L Na2HPO4·2H2O and 4 mg/L NaH2PO4. The biofilm culture lasted for one week and the nutrient solution was refreshed every two days.
The Fig. 1a shows an uncoated sponge carrier; it had a black color, a macroporous structure, and a wet density of 1.01–1.02 g/cm3, which allowed a good circulation in the PCBBR. In comparison to the blank sponge, the exterior surface of the carrier became gray and rough after the attachment of BiOCl/Bi2WO6/Bi nanoparticles (Fig. 1b). According to SEM images, the exterior surface of the sponge cube was covered by photocatalyst particles while the interior surface was kept clean after the photocatalyst loading (Fig. 2). This confirming the viability of powder spraying method for photocatalyst coating, which on one hand, reduces the amount of catalysts consumed, and on the other hand, provides a non-toxic environment for biofilm growth. Fig. S7a and b exhibit the SEM images of catalyst-coated carrier after biofilm cultivation, which suggests that the microorganisms were successfully accumulated inside the carrier without significant loss of BiOCl/Bi2WO6/Bi on the exterior surface. Fig. S7c and d reveal that carrier overcame the previously reported obstacle of VPCB, i.e. the insufficient adherence of the photocatalyst [13]. Therefore, the sponge carrier coated with BiOCl/Bi2WO6/Bi by the powder spraying method could meet basic requirements for ICPB.
2.4. Photocatalytic circulating-bed biofilm reactor (PCBBR) setup The PCBBR was an internal loop airlift-driven fluidized bed reactor with a working volume of 540 mL, which was operated at room temperature. Its configuration and size details are shown in Fig. S6 and Section S4.
2.5. Experimental protocols The initial concentration of TCH for the degradation test was 30.0 mg/L. The TCH removal by various phenomena in the PCBBR were evaluated using the following protocols: (1) photolysis (P), which was conducted under visible light without carriers; (2) adsorption (AD), which used catalyst-coated carriers without biofilm and light; (3) biodegradation (B) was evaluated using catalyst-coated carriers after biofilm cultivation but in the dark; (4) Visible-light-induced photocatalysis (VPC) was evaluated using catalyst-coated carriers that have not been subjected to biofilm cultivation; (5) the coupled effect (VPCB) was evaluated using catalyst-coated biofilm carriers illuminated with visible light [27]. The time of TCH-degradation experiments with all the protocols was 10 h. All of above protocols were conducted in the separate reactor with the same configuration and operated at room temperature.
3.2. Fabrication of VPCB for TCH degradation Fig. 3 shows the distribution of BiOCl/Bi2WO6/Bi and biofilm on the carrier during the six operating cycles of VPCB. Prior to the VPCB operation, abundant biofilm matrix was found on the interior surface of the carrier (Fig. 3a) due to the bridging effect of the large amount of extracellular polymer substances (EPSs) generated by microbial [27], and abundant BiOCl/Bi2WO6/Bi particles were adhered to the exterior surface of the sponge skeleton due to the adhesive effect of polyurethane waterproof coating (Fig. 3b). During VPCB operation, the loading amount of photocatalysts on the surface was almost stable (Fig. 3d and f), while the amount of biofilm loaded in the core was decreased to some extent (Fig. 3c and e). This suggested that the 3
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Fig. 2. SEM images of middle section of BiOCl/Bi2WO6/Bi-coated carrier. BiOCl/Bi2WO6/Bi located in the core (a) and on the surface (b) of the carrier; the insert images are magnified view of red squares. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Enhanced adherence strength of photocatalyst
powder spraying method imparted excellent adhesion between catalysts and carrier and provide high abrasion resistance to the composite of catalyst-coated-carrier. Thus, the powder spraying method led to an ideal case for the VPCB degradation of TCH: BiOCl/Bi2WO6/Bi photocatalysts were coated on the exterior surface of sponge carrier, and the active microorganisms were cultivated inside the carrier. In this case, the biodegradable intermediates produced by photocatalysis could be immediately biodegrade by the internal microorganisms.
To evaluate the adhesion strength of the powder spraying method, the weight of VPC carriers prepared via the powder spraying method and sol-gel method was tested and compared. As shown in Fig. 4, the weight of carriers prepared by the powder spraying method barely reduced throughout the experiment. In contrast, the weight of carrier prepared by the sol-gel method decreased to 73% of initial weight
Fig. 3. SEM images of the BiOCl/Bi2WO6/Bi-coated biofilm carriers of the VPCB during six operating cycles. Samples were collected at the end of the noted cycle. Rad highlights areas of biofilms at different locations. 4
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protocol B was due to adsorption rather than biodegradation. Obviously, TCH was difficult to degrade in the natural environment. In contrast, TCH removal rate of the protocol VPC reached ~70% within 10 h, indicating the high-efficiency photocatalytic degradation of TCH by BiOCl/Bi2WO6/Bi. When protocol VPCB was used, 95.2% of the TCH was rapidly removed within the first 2 h. TCH removal rate reached 97.2% finally within 10 h. The TCH removal of the protocol VPCB was superior to the protocol VPC. This trend supported that the biodegradation of the photocatalytic produced intermediates reduces the competition for photocatalytically generated free radicals, which makes these free radicals more available for TCH degradation [27]. The stability of VPCB with carriers prepared by our strategy and conventional sol-gel method were tested by cycle experiments. The degradation efficiency declined as the recycle time increases likely due to the detachment of biofilms or the decrease in photocatalytic activity. After six cycles experiments, the removal rate of TCH in VPCB prepared our strategy was stabilized at 94.3% (Fig. 5b). The high efficiency and stability of the VPCB confirms that the adhesion between catalysts and carriers introduced by polyurethane waterproof coating is strong enough to acclimate to the moving aquatic environment [11]. On the contrary, the removal rate of TCH in VPCB by sol-gel method decreased more significant with 81.5% removal rate achieved after six cycles (Fig. 5c). This suggests that the carrier prepared by sol-gel method has a worse stability. Apparently, the powder spraying method was superior to the sol-gel method in practical applications. Dissolved organic carbon (DOC) was used to represent the concentration of residual organic in the treated liquid. The decrease in DOC concentration followed a similar trend under the VPC and VPCB protocols (Fig. 6). The DOC removal rate of the VPC was 53.4% after 10 h, while the DOC removal rate of the VPCB eventually reached 70%. The higher DOC removal rate of VPCB indicates that the microorganisms play a significant role in enhancing TCH mineralization, as they can degrade the photocatalytic intermediates and increase TCH degradation and mineralization by lowering competition for free radicals [28]. Although biodegradation could enhance the mineralization of TCH, the complete removal of DOC is still hard to achieve likely because some
Fig. 4. The weight change of carrier by the different coating methods in the protocol VPC over 6 operating cycles (60 h).
(Table S1), indicating a significant catalyst detachment during 60 h VPC experiments. In case of sol-gel prepared carriers, catalysts were loaded on the carrier via weak force of electrostatic adsorption [27], which could lead to the pealing and cracking of catalysts during long-term operation under strong water disturbance. However, the powder spraying coating strategy significantly enhanced the adhesion strength between catalysts and carriers via glue binder, leading to high stability of catalysts during VPCB operation. 3.4. Enhancing TCH and DOC removal using VPCB For the degradation test, the concentration changes for different protocols during 10 h are shown in Fig. 5a. Visible light photolysis (P) did not degrade TCH. For the absorption (AD), the final removal rate of the carriers was 25.9% within 10 h. The concentration reduction trend in protocol B was similar to AD, which means that the TCH loss in
Fig. 5. (a) TCH degradation (points) and pseudo-first-order kinetic simulation (solid lines) during operation. Cycling experiment of TCH degradation for VPCB’ carrier coated catalysts by (b) powder spraying method and (c) sol-gel method. The initial TCH concentration for each experiment was 30.0 mg/L. 5
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TCH-transformed intermediates were non-biodegradable.
3.5. Microbial survival during VPCB operation After the degradation test, the exterior surface of VPCB carrier was still uniformly coated by BiOCl/Bi2WO6/Bi nanoparticles (Fig. 3f). At the same time, the core of carrier was still densely covered by microbes (Fig. 3e). The microbial survival on the catalyst-coated biofilm carrier was visualized by CLSM (Fig. S8). Prior to operating the VPCB, a large amount of active bacteria was present in the catalyst-coated carrier after biofilm cultivation (Fig. S8a and b). After the illumination of VPCB, the death of microbes occurred on the exterior surface of the carrier due to photocatalytic sterilization (Fig. S8d) [29–31]. In contrast, the biofilm inside the carrier was well preserved after irradiation (Fig. S8c). The high survival rate of microbes confirms that the powder spraying method provides a friendly environment for the cultivation and subsistence of microbes. What’s more, the microbes inside carrier showed a gradient from death to live, indicating the self-sacrificing protection mechanism of microbes when photocatalytic products invade the carrier [10]. These trends were typical of ICPB implementation that the biofilm inside the carrier was still biodegradable despite
Fig. 6. The DOC concentrations versus time plots under VPC and VPCB protocols.
Fig. 7. The proposed transformation pathways of TCH degradation. 6
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the death of the biofilm on the exterior surface during the operation [10,13].
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3.6. TCH degradation pathways Fig. 7 presents a potential TCH degradation pathway proposed on the base of photocatalytic products identified by UPLC-MS (Fig. S8). Briefly, the m/z values of TCH photocatalytic initial products were 461.15, 417.13 and 426.14, formed by hydroxylation, dealkylation and dehydration of TCH, respectively [32–34]. The molecule with m/ z = 461.15 was further attacked by the •OH to form hydroxylated product with m/z = 477.15. Afterwards the 477.15 fragment was then transformed to the product of m/z = 449.12 by dealkylation. Subsequent products with m/z values of 401.11 and 383.14 were produced via deamination and N-C bond cleavage, respectively [27]. Ringopening products with m/z values of 363.13, 375.10, 279.12 and 359.10 were formed through ·OH attack, the further oxidation and dehydration. The TCH of the 4-ring molecular structure was very stable and resistant to biodegradation (Fig. 5a). However, the 3-ring products obtained by VPC degradation were capable of being degraded by microorganisms. Small molecule intermediates with m/z values of 168.12, 120.01, 102.07, and 59.05 were generated, mainly due to biodegradation. Some of these small molecule products were finally degraded by microorganisms into CO2 and H2O [35]. Some maybe non-biodegradable contributing to the residue DOC in the final products. Since the microorganisms in the activated sludge are less tolerant to TCH (EC50 is 2.2 mg/L) [36], photocatalysis occurred on the exterior surface of the carrier effectively protects the internal biofilm. Moreover, the efficient degradation of the TCH by BiOCl/Bi2WO6/Bi allowed a continuous stream of biodegradable intermediates to be obtained for subsequent biodegradation. The intimately coupled of the BiOCl/ Bi2WO6/Bi and biofilm supported the long-term benefits of VPCB in the treatment of TCH degradation and mineralization. 4. Conclusions In summary, we have proposed a new strategy for adhering catalysts to the carriers of the VPCB system: powder spraying method. In this strategy, the BiOCl/Bi2WO6/Bi photocatalyst was firmly coated on the exterior surface of carrier, while the biofilm inside the carrier was excellently protected. The intimately coupled photocatalysis and biodegradation contributed to a high degradation efficiency of TCH, reaching 97.2% throughout the experiment. Even after six cycles, the photocatalyst on the exterior surface and the interior biofilm maintained good stability during experiment. Therefore, our work demonstrated a practical catalysts coating method for ICPB technology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements This work was supported by Doctoral Foundation of Natural Science Foundation of Shandong Province (No. ZR2017BEM013) and National Natural Science Foundation of China (No. 11804180). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123092. 7
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
An efficient photocatalyst coating strategy for intimately coupled photocatalysis and biodegradation (ICPB): Powder spraying method Fuqiang Lia, Xuefang Lana, Lili Wangb, Xiaoying Konga, Ping Xua, Yun Taia, Guoqing Liua, ⁎ Jinsheng Shia, a b
College of Chemistry and Pharmaceutical Science, Qingdao Agricultural University, PR China Science and Information College, Qingdao Agricultural University, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Catalyst was firmly adhered to the • surface of carrier by powder spraying method.
internal biofilms avoid suffering • The from ROS attack caused by photocatalysis.
photocatalysts remained firmly on • The the carrier even after six cycles. removal rate was maintained at • TCH 94.3% after six cycles.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Biodegradation Powder spraying Tetracycline hydrochloride
Intimately coupled photocatalysis and biodegradation (ICPB) is an attractive wastewater treatment technology, and the key to a successful ICPB is a carrier capable of accumulating biofilm inside and adhering photocatalyst firmly on its exterior. However, the photocatalysts coated on the carrier by the conventional method not only occupy the internal pores which should be left for microorganism loading, but also possess an insufficient adhesion to the carrier. To solve this problem, a powder spraying method has been proposed for the first time, which allows the photocatalyst to adhere more firmly on the exterior surface of the carrier, and to reserve the inside pore structures for biofilm accumulation. In the end, when using visible-light-induced ICPB (called VPCB), the degradation rate of tetracycline hydrochloride achieved 97.2%, and the stability of recycled carrier was still excellent even after 6 cycles of operation. This work provides a novel avenue for the design of a highly stable ICPB system with low-cost and high degradation efficiency.
1. Introduction Antibiotics have been widely used to treat human and animal infections in the past few decades [1]. Among the antibiotics, tetracycline hydrochloride (TCH) is a well-known broad-spectrum antibacterial
⁎
agent, which is inexpensive and effective [2,3]. TCH molecules are extensively accumulated in the aquatic environment after abuse, causing serious damage to human and animal health by triggering bacterial resistance [4,5]. However, due to the stable chemical structure and the high refractory to biodegradation, TCH cannot be
Corresponding author at: Qingdao Agricultural University, College of Chemistry and Pharmaceutical Science, Chengyang District, Qingdao, PR China. E-mail address: [email protected] (J. Shi).
https://doi.org/10.1016/j.cej.2019.123092 Received 25 June 2019; Received in revised form 28 September 2019; Accepted 6 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Fuqiang Li, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123092
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Nomenclature ICPB VPCB TCH AOPs HPLC PCBBR P
AD VPC TOC DOC CLSM PBS SEM EPSs m/z
intimately coupled photocatalysis and biodegradation visible-light-induced ICPB tetracycline hydrochloride advanced oxidation processes high-performance liquid chromatography photocatalytic circulating-bed biofilm reactor photolysis
adsorption visible-light-induced photocatalysis total organic carbon dissolved organic carbon confocal laser scanning microscopy phosphate buffer saline scanning electron microscopy extracellular polymer substances mass charge ratio
bonded to polyurethane sponges due to similar compatible principle [23]. Therefore, the combination of polyurethane waterproof coating and polyurethane sponge can be selected as an alternative to the traditional photocatalyst coating procedure. In terms of the photocatalysts, an environment-friendly, low-cost and high visible-light-induced catalyst was selected for ICPB. According to our previous work, BiOCl/Bi2WO6/Bi is an excellent visible-light-driven and easy-prepared photocatalyst for pollutant degradation: the BiOCl/Bi2WO6 heterojunction and Bi formation accelerate the efficiency of carriers separation and transfer; the presence of Bi also broaden the light absorption scope from UV to Vis [24]. Besides, the catalyst can be obtained by onestep solvothermal synthesis and is a non-noble metal catalyst [25]. Therefore, BiOCl/Bi2WO6/Bi was selected as the photocatalyst in our ICPB investigation. In this study, we provided a simple photocatalyst coating strategy that endows strong adhesion between the photocatalyst and the carrier. As shown in Scheme 1, a powder spraying method was first developed as a photocatalyst coating procedure, which allows the attachment of photocatalysts only to the exterior surface of the carrier and leaves the inside pores for bacteria loading and accumulating. This method enhances the survive opportunity for the internal bacteria and reduces the consumption of photocatalysts. To demonstrate the feasibility of this strategy, the adhesion strength and distribution situation of loaded catalysts, the biomass accumulation situation, the VPCB’s efficiency, and the stability of prepared system were evaluated in detail. Meanwhile, the degradation pathway of VPCB was investigated in depth. The results confirm that the new photocatalyst coating methodology significantly enhanced the TCH degradation efficiency by ICPB system and provided a solid platform for the technology development of ICPB.
effectively removed by conventional wastewater treatment [6,7]. Therefore, effective techniques are required to treat TCH in wastewater. Advanced oxidation processes (AOPs) are effective methods for degrading TCH, but it is difficult to achieve complete mineralization [8]. Combining AOP with biodegradation has been proposed as a more promising approach for mineralizing TCH [9], in which the biodegradation is usually a necessary downstream process after AOP degrades TCH [6]. However, the AOP is indiscriminate and fast acting, which makes it impossible to obtain the desired biodegradable intermediates for subsequent biological treatment [10]. Intimately coupled photocatalysis and biodegradation (ICPB) developed in recent years has overcome the challenge of AOP, i.e. the indiscriminate reactivity [11,12], which allows the AOP and the biodegradation occur simultaneously in the same reactor [13]. In this process, the photocatalyst and the microorganism are coated on the same carrier, where the photocatalytic reaction occurs on the exterior surface and the biodegradation occurs inside [11]. The key of ICPB is to protect microorganisms from ultraviolet radiation, free radical attack and original pollutants (such as TCH). This requires the microorganisms to be protected as a biofilm in the core of macroporous carrier, and the photocatalyst to be adhered to the exterior surface of carrier with high strength [14,15]. So far, many photocatalyst coating procedures have been reported, such as low-temperature sintering and sol-gel methods. Nevertheless, in these preparation methods, the internal carrier was also loaded by catalysts, which contributes to an adverse effect on the attachment and accumulation of microorganisms since it allows the microorganism destruction incurred by the free radicals generated from photocatalyst [15,16]. Besides, when these preparation methods were used, a poor adhesion strength between the photocatalyst and the carrier was obtained, indicating a low stability of ICPB system. Therefore, it is imperative to develop an efficient photocatalyst coating strategy for ICPB. Previously, all studies in ICPB used ultraviolet light as an excitation source for photocatalysis [10,13,17]. In 2015, ICPB driven by visible light was first developed with the Er3+:YAlO3/TiO2 as photocatalyst [8], which is called visible-light-induced photocatalysis and biodegradation (VPCB). This new VPCB approach has demonstrated excellent biofilm protection and energy savings merits [8,18], and has been extensively investigated until now [19]. However, using the rare earth upconversion material as a light converter is costly and the TiO2 cannot fully utilize the upconversion emissions, especially those in visible light range [20]. As to noble metal doped photocatalysts, the problems like high cost, bactericidal effect, complex operation and so on, are also troublesome [21]. Therefore, visible-light-induced photocatalyst with low-cost and environmentally friendly properties should be developed to improve ICPB performance [22]. ICPB is an attractive technology for wastewater treatment, nevertheless many challenges are still need to be solved. Since Li et al. first employed commercial polyurethane sponge as carriers for ICPB, economically applicable polyurethane sponge has been extensively used as carriers [10]. In addition, commercial single component polyurethane waterproof coating is an excellent polyurethane adhesive with many advantages such as solvent-free volatilization, non-toxicity, odorlessness, and rapid curing. Polyurethane waterproof coating can be easily
2. Material and methods 2.1. Chemicals and reagents The tetracycline hydrochloride (C22H24N2O8•HCl, purity of 98%) was obtained from Sigma-Aldrich Co. LLC. (USA). The methanol, acetonitrile and glacial acetic acid used were of high-performance liquid chromatography (HPLC) grade and were purchased from
Scheme 1. Schematic representation of photocatalyst coating procedure by powder spraying method. 2
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Sinopharm Group Co., Ltd. (Shanghai, China). The other chemicals were of analytical grade without further purification. Ultrapure water was used throughout the whole experimental process.
2.2. Carrier and BiOCl/Bi2WO6/Bi coating procedure The carrier used in this study was a commercially available polyurethane sponge cube (HSYUAN, Jiangxi, China) with an average length of 10.0 mm (Fig. S1). The sponge had a specific surface area of 2.3 m2/g, a large pore size of 1–2 mm and a porosity of 95%. The BiOCl/Bi2WO6/Bi ternary complex photocatalyst was prepared based on our previous work [25]. The detailed preparation method (Section S1 and S2) and characterization of BiOCl/Bi2WO6/Bi (Fig. S2–S5) were illustrated in the Supplementary information. The BiOCl/Bi2WO6/Bi was coated onto the sponge carrier according to powder spraying method. 1.0 g of blank sponge cube was completely immersed in a polyurethane waterproof coating (Jingfu Co., Ltd., Shanghai, China) for five seconds, then quickly taken out and shaken off to prevent the cross-linking of the sponge skeleton. After that, the sponge was loaded by 1.0 g of BiOCl/Bi2WO6/Bi via powder spraying method, in which process the BiOCl/Bi2WO6/Bi powder was sealed in a tiny spray bottle and evenly sprayed on the exterior surface of the sponge. Then the coated sponge was placed at room temperature for half an hour to allow the polyurethane waterproof coating solid. Finally, the coated carriers were rinsed with ultrapure water to remove any poorly fixed photocatalyst. Sponge carriers coated by sol-gel method were also prepared as a control experiment (details in Section S3).
Fig. 1. Sponge carrier before (a) and after (b) BiOCl/Bi2WO6/Bi coating by the powder spraying method.
2.6. Analytical methods Analytical methods for TCH (analyzed by HPLC) and TCH-degradation intermediates (analyzed by UPLC-MS) were given in Section S5. Pretreatment procedures for carrier microstructure observation by scanning electron microscopy (SEM) and biofilm staining procedure for confocal laser scanning microscopy (CLSM) imaging were presented in Section S6 and Section S7, respectively. The total organic carbon (TOC) was measured using a TOC analyzer (Shimadzu, Japan). Prior to analysis, each sample was filtered through a 0.22 μm membrane filter, so the measured TOC was assumed to be dissolved organic carbon (DOC). 3. Results and discussion
2.3. Inoculum and biofilm cultivation
3.1. Photocatalyst distribution and biomass accumulation
The activated sludge was obtained from the secondary sedimentation tank of the Chengyang Wastewater Treatment Plant, Qingdao, China. The BiOCl/Bi2WO6/Bi coated carriers were immersed in the activated sludge for 24 h to adsorb the microorganisms, and then the biofilm was cultured in an internal loop airlift-driven fluidized bed reactor with aeration [26]. Biofilms were cultured using synthetic nutrients containing 330 mg/L sodium acetate, 29 mg/L NH4Cl, 8 mg/L Na2HPO4·2H2O and 4 mg/L NaH2PO4. The biofilm culture lasted for one week and the nutrient solution was refreshed every two days.
The Fig. 1a shows an uncoated sponge carrier; it had a black color, a macroporous structure, and a wet density of 1.01–1.02 g/cm3, which allowed a good circulation in the PCBBR. In comparison to the blank sponge, the exterior surface of the carrier became gray and rough after the attachment of BiOCl/Bi2WO6/Bi nanoparticles (Fig. 1b). According to SEM images, the exterior surface of the sponge cube was covered by photocatalyst particles while the interior surface was kept clean after the photocatalyst loading (Fig. 2). This confirming the viability of powder spraying method for photocatalyst coating, which on one hand, reduces the amount of catalysts consumed, and on the other hand, provides a non-toxic environment for biofilm growth. Fig. S7a and b exhibit the SEM images of catalyst-coated carrier after biofilm cultivation, which suggests that the microorganisms were successfully accumulated inside the carrier without significant loss of BiOCl/Bi2WO6/Bi on the exterior surface. Fig. S7c and d reveal that carrier overcame the previously reported obstacle of VPCB, i.e. the insufficient adherence of the photocatalyst [13]. Therefore, the sponge carrier coated with BiOCl/Bi2WO6/Bi by the powder spraying method could meet basic requirements for ICPB.
2.4. Photocatalytic circulating-bed biofilm reactor (PCBBR) setup The PCBBR was an internal loop airlift-driven fluidized bed reactor with a working volume of 540 mL, which was operated at room temperature. Its configuration and size details are shown in Fig. S6 and Section S4.
2.5. Experimental protocols The initial concentration of TCH for the degradation test was 30.0 mg/L. The TCH removal by various phenomena in the PCBBR were evaluated using the following protocols: (1) photolysis (P), which was conducted under visible light without carriers; (2) adsorption (AD), which used catalyst-coated carriers without biofilm and light; (3) biodegradation (B) was evaluated using catalyst-coated carriers after biofilm cultivation but in the dark; (4) Visible-light-induced photocatalysis (VPC) was evaluated using catalyst-coated carriers that have not been subjected to biofilm cultivation; (5) the coupled effect (VPCB) was evaluated using catalyst-coated biofilm carriers illuminated with visible light [27]. The time of TCH-degradation experiments with all the protocols was 10 h. All of above protocols were conducted in the separate reactor with the same configuration and operated at room temperature.
3.2. Fabrication of VPCB for TCH degradation Fig. 3 shows the distribution of BiOCl/Bi2WO6/Bi and biofilm on the carrier during the six operating cycles of VPCB. Prior to the VPCB operation, abundant biofilm matrix was found on the interior surface of the carrier (Fig. 3a) due to the bridging effect of the large amount of extracellular polymer substances (EPSs) generated by microbial [27], and abundant BiOCl/Bi2WO6/Bi particles were adhered to the exterior surface of the sponge skeleton due to the adhesive effect of polyurethane waterproof coating (Fig. 3b). During VPCB operation, the loading amount of photocatalysts on the surface was almost stable (Fig. 3d and f), while the amount of biofilm loaded in the core was decreased to some extent (Fig. 3c and e). This suggested that the 3
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Fig. 2. SEM images of middle section of BiOCl/Bi2WO6/Bi-coated carrier. BiOCl/Bi2WO6/Bi located in the core (a) and on the surface (b) of the carrier; the insert images are magnified view of red squares. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Enhanced adherence strength of photocatalyst
powder spraying method imparted excellent adhesion between catalysts and carrier and provide high abrasion resistance to the composite of catalyst-coated-carrier. Thus, the powder spraying method led to an ideal case for the VPCB degradation of TCH: BiOCl/Bi2WO6/Bi photocatalysts were coated on the exterior surface of sponge carrier, and the active microorganisms were cultivated inside the carrier. In this case, the biodegradable intermediates produced by photocatalysis could be immediately biodegrade by the internal microorganisms.
To evaluate the adhesion strength of the powder spraying method, the weight of VPC carriers prepared via the powder spraying method and sol-gel method was tested and compared. As shown in Fig. 4, the weight of carriers prepared by the powder spraying method barely reduced throughout the experiment. In contrast, the weight of carrier prepared by the sol-gel method decreased to 73% of initial weight
Fig. 3. SEM images of the BiOCl/Bi2WO6/Bi-coated biofilm carriers of the VPCB during six operating cycles. Samples were collected at the end of the noted cycle. Rad highlights areas of biofilms at different locations. 4
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protocol B was due to adsorption rather than biodegradation. Obviously, TCH was difficult to degrade in the natural environment. In contrast, TCH removal rate of the protocol VPC reached ~70% within 10 h, indicating the high-efficiency photocatalytic degradation of TCH by BiOCl/Bi2WO6/Bi. When protocol VPCB was used, 95.2% of the TCH was rapidly removed within the first 2 h. TCH removal rate reached 97.2% finally within 10 h. The TCH removal of the protocol VPCB was superior to the protocol VPC. This trend supported that the biodegradation of the photocatalytic produced intermediates reduces the competition for photocatalytically generated free radicals, which makes these free radicals more available for TCH degradation [27]. The stability of VPCB with carriers prepared by our strategy and conventional sol-gel method were tested by cycle experiments. The degradation efficiency declined as the recycle time increases likely due to the detachment of biofilms or the decrease in photocatalytic activity. After six cycles experiments, the removal rate of TCH in VPCB prepared our strategy was stabilized at 94.3% (Fig. 5b). The high efficiency and stability of the VPCB confirms that the adhesion between catalysts and carriers introduced by polyurethane waterproof coating is strong enough to acclimate to the moving aquatic environment [11]. On the contrary, the removal rate of TCH in VPCB by sol-gel method decreased more significant with 81.5% removal rate achieved after six cycles (Fig. 5c). This suggests that the carrier prepared by sol-gel method has a worse stability. Apparently, the powder spraying method was superior to the sol-gel method in practical applications. Dissolved organic carbon (DOC) was used to represent the concentration of residual organic in the treated liquid. The decrease in DOC concentration followed a similar trend under the VPC and VPCB protocols (Fig. 6). The DOC removal rate of the VPC was 53.4% after 10 h, while the DOC removal rate of the VPCB eventually reached 70%. The higher DOC removal rate of VPCB indicates that the microorganisms play a significant role in enhancing TCH mineralization, as they can degrade the photocatalytic intermediates and increase TCH degradation and mineralization by lowering competition for free radicals [28]. Although biodegradation could enhance the mineralization of TCH, the complete removal of DOC is still hard to achieve likely because some
Fig. 4. The weight change of carrier by the different coating methods in the protocol VPC over 6 operating cycles (60 h).
(Table S1), indicating a significant catalyst detachment during 60 h VPC experiments. In case of sol-gel prepared carriers, catalysts were loaded on the carrier via weak force of electrostatic adsorption [27], which could lead to the pealing and cracking of catalysts during long-term operation under strong water disturbance. However, the powder spraying coating strategy significantly enhanced the adhesion strength between catalysts and carriers via glue binder, leading to high stability of catalysts during VPCB operation. 3.4. Enhancing TCH and DOC removal using VPCB For the degradation test, the concentration changes for different protocols during 10 h are shown in Fig. 5a. Visible light photolysis (P) did not degrade TCH. For the absorption (AD), the final removal rate of the carriers was 25.9% within 10 h. The concentration reduction trend in protocol B was similar to AD, which means that the TCH loss in
Fig. 5. (a) TCH degradation (points) and pseudo-first-order kinetic simulation (solid lines) during operation. Cycling experiment of TCH degradation for VPCB’ carrier coated catalysts by (b) powder spraying method and (c) sol-gel method. The initial TCH concentration for each experiment was 30.0 mg/L. 5
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TCH-transformed intermediates were non-biodegradable.
3.5. Microbial survival during VPCB operation After the degradation test, the exterior surface of VPCB carrier was still uniformly coated by BiOCl/Bi2WO6/Bi nanoparticles (Fig. 3f). At the same time, the core of carrier was still densely covered by microbes (Fig. 3e). The microbial survival on the catalyst-coated biofilm carrier was visualized by CLSM (Fig. S8). Prior to operating the VPCB, a large amount of active bacteria was present in the catalyst-coated carrier after biofilm cultivation (Fig. S8a and b). After the illumination of VPCB, the death of microbes occurred on the exterior surface of the carrier due to photocatalytic sterilization (Fig. S8d) [29–31]. In contrast, the biofilm inside the carrier was well preserved after irradiation (Fig. S8c). The high survival rate of microbes confirms that the powder spraying method provides a friendly environment for the cultivation and subsistence of microbes. What’s more, the microbes inside carrier showed a gradient from death to live, indicating the self-sacrificing protection mechanism of microbes when photocatalytic products invade the carrier [10]. These trends were typical of ICPB implementation that the biofilm inside the carrier was still biodegradable despite
Fig. 6. The DOC concentrations versus time plots under VPC and VPCB protocols.
Fig. 7. The proposed transformation pathways of TCH degradation. 6
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3.6. TCH degradation pathways Fig. 7 presents a potential TCH degradation pathway proposed on the base of photocatalytic products identified by UPLC-MS (Fig. S8). Briefly, the m/z values of TCH photocatalytic initial products were 461.15, 417.13 and 426.14, formed by hydroxylation, dealkylation and dehydration of TCH, respectively [32–34]. The molecule with m/ z = 461.15 was further attacked by the •OH to form hydroxylated product with m/z = 477.15. Afterwards the 477.15 fragment was then transformed to the product of m/z = 449.12 by dealkylation. Subsequent products with m/z values of 401.11 and 383.14 were produced via deamination and N-C bond cleavage, respectively [27]. Ringopening products with m/z values of 363.13, 375.10, 279.12 and 359.10 were formed through ·OH attack, the further oxidation and dehydration. The TCH of the 4-ring molecular structure was very stable and resistant to biodegradation (Fig. 5a). However, the 3-ring products obtained by VPC degradation were capable of being degraded by microorganisms. Small molecule intermediates with m/z values of 168.12, 120.01, 102.07, and 59.05 were generated, mainly due to biodegradation. Some of these small molecule products were finally degraded by microorganisms into CO2 and H2O [35]. Some maybe non-biodegradable contributing to the residue DOC in the final products. Since the microorganisms in the activated sludge are less tolerant to TCH (EC50 is 2.2 mg/L) [36], photocatalysis occurred on the exterior surface of the carrier effectively protects the internal biofilm. Moreover, the efficient degradation of the TCH by BiOCl/Bi2WO6/Bi allowed a continuous stream of biodegradable intermediates to be obtained for subsequent biodegradation. The intimately coupled of the BiOCl/ Bi2WO6/Bi and biofilm supported the long-term benefits of VPCB in the treatment of TCH degradation and mineralization. 4. Conclusions In summary, we have proposed a new strategy for adhering catalysts to the carriers of the VPCB system: powder spraying method. In this strategy, the BiOCl/Bi2WO6/Bi photocatalyst was firmly coated on the exterior surface of carrier, while the biofilm inside the carrier was excellently protected. The intimately coupled photocatalysis and biodegradation contributed to a high degradation efficiency of TCH, reaching 97.2% throughout the experiment. Even after six cycles, the photocatalyst on the exterior surface and the interior biofilm maintained good stability during experiment. Therefore, our work demonstrated a practical catalysts coating method for ICPB technology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements This work was supported by Doctoral Foundation of Natural Science Foundation of Shandong Province (No. ZR2017BEM013) and National Natural Science Foundation of China (No. 11804180). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123092. 7
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