Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater

Journal of Cleaner Production xxx (xxxx) xxx

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Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater Xi Lu a, Zhenyu Li a, Yu’an Liu b, Bin Tang a, c, *, Yanchao Zhu d, Joselito M. Razal a, Esfandiar Pakdel a, Jinfeng Wang a, c, **, Xungai Wang a, c a

Deakin University, Institute for Frontier Materials, Geelong, Melbourne, Australia Jiangsu Bohn Environmental Engineering Complete Equipment Co Ltd, Nanjing, China Wuhan Textile University, National Engineering Laboratory for Advanced Yarn and Fabric Formation and Clean Production, Wuhan, China d College of Chemistry, Jilin University, Changchun, 130023, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 February 2019 Received in revised form 30 September 2019 Accepted 17 October 2019 Available online xxx

The reckless discharge of untreated industrial wastewater into the environment has caused serious pollution. Sunlight driven degradation with assistance of catalysts is a promising strategy for removing contaminants from water. TiO2 nanoparticles have been successfully immobilized on melamine foam via a biomimetic polydopamine coating strategy followed by a carbonization treatment. The obtained carbonized TiO2 nanoparticles-coated melamine foam inherited the porous structure from pristine melamine foam and had a high loading density of TiO2 nanoparticles on the foam fibrils. The binding interaction between TiO2 nanoparticles and melamine foam was enhanced by carbonization. The carbonized 3D framework of melamine foam improved the thermal stability of TiO2 nanoparticles and prevented TiO2 nanoparticles from aggregation. The carbonized TiO2-coated melamine foam maintained the porous structures of melamine foam and can act as a microreactor for color removal from the dyeing wastewater. The carbonized TiO2-coated melamine foam demonstrated the prominent photocatalytic activity on Rhodamine B degradation, with 16 times higher than the non-carbonized counterpart, with 98% of Rhodamine B removed after 60 min of irradiation. More importantly, the carbonized TiO2-coated melamine foam removed 90% of dyes in the traditional dyeing wastewater after 120 min under simulated sunlight irradiation, which shows 20 times higher photocatalytic performance than TiO2 nanoparticles for treating traditional dyeing wastewater. The remarkable photocatalytic performance of the carbonized TiO2-coated melamine foam is attributed to the combination of both high light absorption of nitrogen doped carbonized foam and the efficient charge carrier separation promoted by the carbon layer, which inhibited the recombination of electron-hole pairs in TiO2. The TiO2-coated carbon foam can be used as a microreactor, which has promising practical applications in clean production and environmental remediation. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Prof. S Alwi Keywords: Melamine foam Titanium dioxide Photocatalysis Porous structure Microreactor Wastewater treatment

1. Introduction Environmental pollution becomes a growing global concern with the rapid development of industries. It is still a long-standing challenge to effectively remove color from the wastewater from

* Corresponding author. Deakin University, Institute for Frontier Materials, Geelong, Melbourne, Australia. ** Corresponding author. Deakin University, Institute for Frontier Materials, Geelong, Melbourne, Australia. E-mail addresses: [email protected] (B. Tang), [email protected]. au (J. Wang).

textile industries. As the most popular natural fiber, cotton has been widely produced for apparel fabrics. Many types of synthetic dyes are used for cotton dyeing, with assistance of salts and alkali. The additives may impede the removal of dyes in the cotton dyeing wastewater (Dong et al., 2019). There are few relatively inexpensive and individual treatment methods to degrade the dyes in wastewater due to the complex chemicals in the wastewater (Sharma et al., 2019). Among diverse environmental remediation strategies, semiconductor nanomaterials based photocatalysis is of great interest because it can utilize the abundant energy of either natural sunlight or artificial indoor illumination to eliminate a wide range of organic pollutants (Mustapha et al., 2017; Schneider et al., 2014).

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Please cite this article as: Lu, X et al., Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118949

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Traditionally, doping of semiconductor nanoparticles has been developed to enhance the photocatalytic activity. For example, multi-walled carbon nanotube (Shahrezaei et al., 2016), barium (Shahmoradi et al., 2017b), chromium (Shahmoradi et al., 2017a) and nickel (Pirsaheb et al., 2016) were used to dope titanium dioxide nanoparticles (TiO2 NPs) to modify the photocatalytic properties. In addition, to achieve improved photocatalytic performance, photocatalysts with high surface area were fabricated by reducing photocatalyst particle sizes. However, agglomeration of photocatalysts during application could lead to reduced catalytic activity (Tong et al., 2012). Immobilization of nanoscale photocatalysts on supporting substrates with sophisticated structures have been attempted for easy recovery and reuse of photocatalysts (Fan et al., 2015; Saeli et al., 2018). For example, TiO2 NPs were insitu synthesized on carbonized filter paper to obtain TiO2/carbon paper composite, which was used as a catalyst for photodecomposition of methylene blue and showed a high photocatalytic activity (Hu et al., 2014). Different types of three-dimensional porous foam have been used as the supporting materials and loaded with catalyst nanoparticles, including macroporous polyurethane foam (PUF) (Zhang et al., 2016a), super-hydrophobic mesocellular foam (Xing et al., 2012), SiO2 foam (Pan et al., 2018). The obtained hierarchical macro/mesoporous structure facilitated the pollutants adsorption and photon utilization efficiency. Meanwhile, aerogels were also used as supporting materials for loading catalyst, such as graphene aerogel (Fan et al., 2015), graphitic carbon nitride (gC3N4)/graphene oxide (GO) aerogel (Tong et al., 2015), carbon aerogel (Cui et al., 2016). The aerogel acted as 3D framework and provided support to photocatalysts, which also promoted separation and utilization of electrons and holes in the photocatalytic process, leading to improved photocatalytic efficiency (Cui et al., 2016). Although many successes have been achieved to load TiO2 on 3D substrates, most of the works are still limited to rigid substrates (Robert et al., 2013). The obtained photocatalysts are fragile and there is loss of the photocatalyst particles during the recovery process (Wan et al., 2018; Xiao et al., 2016). It is important to develop porous and robust substrates to support photocatalysts for practical use. Melamine foam is commercially available and widely used as abrasive cleaner due to its microporous open-cell structure, flexibility and good strength. After carbonization, the obtained carbonized melamine foam (CMF) exhibited higher flexibility and mechanically stronger than the existing carbon aerogels (Zhang et al., 2016b). CMF with low density inherits the interconnected and reticulated structures of pristine melamine foam. Yuan et al (2018) reported a facile approach to obtain a flexible 3D N-doped carbon foam/carbon nanotubes hybrid using melamine foam as the supporting substrate. An ultrasonic-coating method was also used to load g-C3N4 on melamine sponge skeleton to form a monolithic g-C3N4/melamine sponge (g-C3N4/MS). The obtained g-C3N4/MS possessed a larger specific surface area than the powder counterpart of g-C3N4 and showed photocatalytic performance for nitric oxide (NO) removal and CO2 reduction (Yang et al., 2018). Sunlight driven photocatalysts with easy separation performance could provide a potential solution for the clean production of textile industry. In this study, a facile fabrication strategy was developed to produce carbonized melamine foam (CMF) supported TiO2 NPs for the effective dye removal from industrial dyeing waste water. Melamine foam (MF) was chosen as the supporting substrate to immobilize TiO2 NPs via a biomimetic coating strategy using polydopamine (PDA). Subsequently, carbonization of both melamine foam and the PDA coating layer was achieved through heat treatment of the TiO2 coated melamine foam (TiMF). The photocatalytic activities of the obtained TiO2 NPs loaded CMF for the degradation

of organic pollutants (e.g. Rhodamine B and traditional dye Kayacelon React Red CNe3B) under simulated sunlight irradiation were evaluated and compared with that of TiO2 NPs. 2. Experimental section 2.1. Materials Melamine foam was obtained from SINOYQX Co. Ltd. (Sichuan, China). TiO2 NPs (P25, 20% rutile and 80% anatase), 3,4dihydroxyphenethylamine hydrochloride (DOPA, 98%), tris(hydroxymethyl)-aminomethane (TRIS, 99%) and Rhodamine B (RhB) were purchased from Sigma-Aldrich. Kayacelon React Red CNe3B (C. I. Reactive Red. 221) for dyeing cotton fabrics was obtained from Nippon Kayaku Co. Ltd. All chemicals were used as received without further treatment. 2.2. Characterization Transmission electron microscopy (TEM) images were taken by a JEOL 2100 transmission electron microscope at 200 kV. The surface morphologies of the samples were observed on a Supra 55 VP field emission scanning electron microscope (SEM). The energy dispersive X-ray (EDX) mapping analysis was conducted using A Hitachi SU8010 SEM coupled with an energy disperse X-ray spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos XSAM800 XPS system with Ka source and a charge neutralizer. X-ray diffraction (XRD) analysis was performed on a powder diffractometer (PANalytical/X’Pert Powder) using Cu Ka radiation. Thermogravimetric analysis (TGA) was carried on a TA-Q50 instrument at a heating rate of 10
C/min in air atmosphere. Brunauer-Emmett-Teller (BET) surface area analysis was performed using a Micromeritics ASAP 2020 Surface Area & Porosity Analyzer. Fourier transform infrared (FTIR) spectra were collected on a FTIR spectrometer (Tensor 27, Bruker, Germany) at attenuated total reflectance (ATR) mode. Ultravioletevisible (UVevis) absorption spectra were obtained from a Varian Cary 300 UVevis spectrophotometer. Infrared thermal images were recorded by a FLIR ONE PRO Thermal Camera. 2.3. Modification of melamine foam with polydopamine (PDA) Melamine foam was modified by PDA according to the method reported previously (Huang et al. 2013, 2018). Briefly, a piece of melamine foam (MF) with size of 20 mm  20 mm  20 mm was cut and immersed into petroleum spirit for 10 min under vacuum, followed by squeezing for 5 times to eliminate inside air. The foam was then washed in deionized water for subsequent surface modification. The washed MF was immersed into 40 mL of DOPA aqueous solution (4.0 mg/mL) and incubated under stirring for 60 min at ambient condition. Subsequently, 10 mL of TRIS aqueous solution (3.0 mg/mL) was added into the reaction system and stirred for 24 h. The polydopamine coated melamine foam (DMF) was collected after washing in deionized water. 2.4. Immobilization of TiO2 nanoparticles on polydopamine coated melamine foam (DMF) TiO2 NPs (P25, 50 mg) were dispersed in 100 mL of deionized water in a 250 mL of flask and sonicated for 30 min. The obtained DMF was immersed into the solution containing TiO2 NPs and stirred for 24 h at room temperature. The DMF was taken out from the P25 dispersion by tweezers. Then the extra P25 solution was squeezed out of the foam, followed by rinsing the foam with deionized water. This process was repeated for at least 5 times to

Please cite this article as: Lu, X et al., Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118949

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remove unbound P25 in the foam. TiO2 were grafted onto the DMF with the assistance of polydopamine. Then, the TiMF were placed in a MTI tube furnace and heated to 550
C with a heating rate of 5
C/ min, kept for 2 h under nitrogen protection before cooling down to room temperature. Different heating time including 3 h and 4 h were performed to investigate the impact of heating time on the carbonization of PDA and TiMF. The heating time for the characterized samples was 2 h unless further specified. The weight of 20 mm  20 mm  20 mm pristine melamine foam cube was 188.6 mg with a density of 23.58 kg/m3. After coating and carbonization process, the size of the cube shrunk to 14 mm  15 mm  13 mm with a weight of 70.6 mg. The density of CTiMF cube was calculated to be 25.86 kg/m3. 2.5. Dye degradation under simulated sunlight irradiation The decomposition of dye RhB was implemented under simulated sunlight irradiation using a Suntest instrument (SUNTEST XLSþ from Atlas Material Testing Technology LLC). The test method for photocatalysis was described in our previous work (Wang et al., 2012). In brief, RhB was used as a model dye molecule to evaluate the photocatalytic activity of CTiMF under simulated sunlight irradiation. The characteristic absorption peak of RhB at 554 nm was chosen to indicate the RhB degradation process (Verma et al., 2012). The initial concentration of RhB solution was 12 mg L1. For RhB degradation measurement, the CTiMF cube was immersed into 20 mL RhB aqueous solution and the size of CTiMF cube was determined by maintaining P25 content inside the cube to be 0.1 mg/mL in the RhB aqueous solution. The solution was kept stirring in the dark for 1 h to reach the adsorption and desorption equilibrium of RhB on the CTiMF surface. The RhB solution with CTiMF was then irradiated by simulated sunlight (350 W/m2) under stirring. At a certain time interval, UVevis absorption spectra of the RhB solution were measured. 2.6. Traditional dyeing of cotton and photodegradation of practical dyeing wastewater

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3. Results and discussion 3.1. Preparation of carbonized TiO2 NPs-coated melamine foam (CTiMF) and its microstructure The fabrication process of CTiMF is illustrated in Fig. 1a. Pristine melamine foam (PMF) changed from white to brown in color but maintained its shape after it was coated by polydopamine (PDA). As can be seen from Fig. S1a and b, a dense layer of TiO2 NPs were coated onto the surface of MF indicating that PDA effectively bridged the MF and TiO2 NPs, which may be due to the amino groups from grafting of PDA. After carbonization, the original brown TiMF (20 mm  20 mm  20 mm) reduced in size (14 mm  15 mm  13 mm) and turned black, which indicates the formation of CTiMF (Fig. 1a). This 25% shrinkage is mainly caused by the heat induced deformation of MF framework and the thermal polymerization of the melamine (Liang et al., 2015). To gain insights of the microstructure change, SEM was employed to observe the morphologies of foam during the fabrication process. The PMF exhibited porous structure with fibrils inside (Fig. 1b). Most of the fibrils have a smooth surface and triangle cross-section shape (inset in Fig. 1b). Adjacent melamine fibrils intersected to form reticulated structure by joints. The loading of TiO2 NPs did not affect the porous structure of the foam even when TiO2 NPs densely packed on the surface of the fibrils (Fig. 1c). The cross-section shape of the melamine fibrils maintained during the loading process of TiO2 NPs (inset in Fig. 1c). The carbonization process led to the shrinkage of TiMF and the increase of the pore density (Fig. 1d). The porous structure of the foam remained after carbonization, demonstrating the supporting ability of CMF for TiO2 NPs. The triangular shape of the cross-section fibrils deformed to concave triangle (insert in Fig. 1d) as a result of the collapsing of inter fibrils. Additionally, no obvious change of the TiO2 NPs coating was observed after the carbonization process (Fig. S1c), which indicates that the combination of TiO2 NPs and foam fibril was stable. 3D model was used to illustrate the structural changes of the foam (as shown in Fig. 1e). 3.2. Stability and loading of TiO2 NPs on foam

Photodegradation performance of the CTiMF was further assessed by treating the residual dyeing solution after cotton dyeing. Cotton fabrics were dyed with Kayacelon React Red CNe3B (C. I. Reactive Red. 221) at liquor-to-fabric ratio of 50:1. Cotton dyeing was performed using a laboratory dyeing machine (Ahiba IR Pro, Datacolor International). In brief, cotton fabrics were immersed in the dye solution (1 wt%) in the presence of sodium sulfate (60 g/ L). Then the dyeing pots were rotated at a speed of 20 rpm and heated at a heating rate of 3
C/min to 65
C, and held at this temperature for 30 min. Subsequently, sodium carbonate (25 g/L) was added into the dyeing bath. The cotton fabrics were further heated to 75
C and held for 30 min. Finally, the dyed fabrics were taken out from dyeing solution. The residual dyeing aqueous solution containing dyes and salts was then used for photodegradation tests. The degradation of dyeing wastewater was carried out under simulated sunlight in the presence of pure TiO2 NPs and CTiMFs with different TiO2 loading. In brief, the TiO2 NPs or CTiMF was added into 20 mL of the obtained dyeing wastewater. The concentration of TiO2 NPs in each reaction system was 0.5 mg/ mL. The size of CTiMF cube was determined by maintaining P25 content inside the cube to be 0.5 mg/mL in the dyeing wastewater solution. The dyeing wastewater with catalysts was stirred in the dark for 1 h and then subjected to irradiation by the simulated sunlight (350 W/m2) under stirring. UVevis absorption spectroscopy was used to monitor the photodegradation process of the dye wastewater.

To further study the stability of immobilized TiO2 NPs on the surface of the foam fibrils, the CTiMF was washed with deionized water by stirring vigorously for 30 min. Compared with CTiMF before washing, no obvious change of the TiO2 NPs density was observed after washing (Fig. S1d), revealing the excellent stability of TiO2 NPs on the fibrils of the foam. The UVevis absportion spectrum of pure TiO2 NPs aqueous dispersion dispalyed a broad absorption band below 325 nm (Fig. S2a). No notable TiO2 absorption bands were found in the UVevis absorption spectra of residual water after the CTiMF was washed for 10 and 30 min (Fig. S2a), which further proves that almost no TiO2 NPs peeled off from CTiMF. These results demonstrated the excellent stability of TiO2 NPs anchored to the foam. The stability of the TiO2 on the foam was further evaluated to confirm the effect of the cabonization process. TiMF and CTiMF were immersed into deionized water and followed by being irradiated for 5 h by simulated sunlight under vigorous stirring. UVevis absorption spectroscopy was used to monitor if the TiO2 NPs peeled off from the foam. For TiMF, obvious TiO2 absorption band appreared, which suggested that some TiO2 NPs separated from the foam (Fig. S3). However, no UVevis absorption band was observed for CTiMF under the same conditions. This result indicates that carbonization significantly improved the anchoring stability of TiO2 NPs onto the foam. The content of TiO2 NPs in CTiMF was determined by

Please cite this article as: Lu, X et al., Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118949

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Fig. 1. (a) Schematic process of preparing the carbonized TiO2@melamine foam (CTiMF). SEM images of (b) PMF, (c) TiMF and (d) CTiMF. Inserts: corresponding magnified crosssection SEM images. (e) Three-dimensional modelling structures of PMF, TiMF and CTiMF. (f) SEM images of CTiMF with different loading amount of TiO2.

thermogravimetric analysis (TGA) (Fig. S2b). The first weight loss at around 100
C was attributed to the water vapor inside the CTiMF. The CTiMF started to degrade at 450
C due to the degradation of melamine in air. No further weight loss was observed when the temperature was further increased above 600
C, indicating that only TiO2 NPs left. The maximum loading amount of TiO2 NPs was calculated to be 43 wt%. The SEM images of CTiMF with different loading amounts of TiO2 NPs (12 wt%, 24 wt%, 30 wt% and 43 wt%) are shown in Fig. 1f. It can be clearly seen that the density of TiO2 NPs on the surface of foam fibrils increased with increasing the TiO2 NP loading. The surface of 3D foam framework was fully covered with TiO2 NPs at the maximum loading amount of 43 wt%, which further verifies that the interaction between DMF and TiO2 NPs was effective. 3.3. Photocatalytic activity Dye Rhodamine B (RhB) was used as a model molecule to evaluate the photocatalytic performance of the TiMF and CTiMF with 24 wt% loading of TiO2 NPs. Fig. 2a and b shows the time dependent UVevis absorbance of the RhB solution (12 mg/L) under simulated sunlight irradiation. With the presence of TiMF, 12% of RhB was degraded after 60 min irradiation. This is compared with 98% of RhB degradation with the presence of the CTiMF. The relative dye concentration change as a function of irradiation time is plotted in Fig. 2c. C0 and C denoted the dye concentrations before and during irradiation. It can be seen that CTiMF showed a greatly improved degradation rate of RhB than TiMF. As shown in Fig. 2d,

the ln(C/C0) showed a linear relationship with the irradiation time, which demonstrates that the photocatalysis of RhB follows the pseudo-first-order hypothesis (Qi et al., 2017). The apparent rate constant (Kapp) values of the photo-degradation reactions in the presence of TiMF and CTiMF were calculated to be 4.00  103 and 6.76  102 min1, respectively. These Kapp values suggest that the CTiMF has 16 times higher photocatalytic performance than TiMF. The Kapp of CTiMF is comparable to related literature results for TiO2 NPs combined with carbon materials (Qi et al., 2017). The photocatalytic reaction of RhB solution (12 mg/L) was performed in the presence of different catalysts including pure TiO2 NPs (P25), CTiMF (30 wt% TiO2) and CTiMF (12 wt% TiO2). The concertation of TiO2 NPs in this test was adjusted to be 0.1 mg/mL. Fig. 2e shows the RhB concentration changes with the presence of different catalysts. The TiO2 NPs immobilized on CMF with high density (TiO2 loading of 24 wt% and 30 wt%) exhibited lower catalytic rate than the dispersive TiO2 NPs, which may be due to a decrease in exposure surface to light irradiation after being loaded on CMF. Whereas, the CTiMF with 12 wt% of TiO2 displayed comparable photocatalytic activity as TiO2 NPs. It is suggested that the low loading density of TiO2 on CMF did not greatly reduce the interaction between RhB dye molecules and TiO2 NPs under light irradiation. 3.4. Dyeing wastewater treatment The dyeing wastewater from traditional dyeing process was used for photodegradation to evaluate the performance of CTiMF for treating practical dyeing wastewater. It can be seen that the

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Fig. 2. Evolution of UVevis absorption spectra of RhB solution (12 mg/L) in the presence of (a) TiMF and (b) CTiMF (24 wt% of TiO2 NPs) under the irradiation of simulated sunlight (350 W/m2). (c) Plots of C/Co RhB solution with TiMF and CTiMF as a function of irradiation time. (d) Corresponding plots of ln(C/C0) versus time during the photo-degradation. The lines are linear fitting curves. (e) Plot of C/Co RhB solution in the presence of pure TiO2 NPs (P25) and CTiMF with different loading of TiO2 NPs as a function of irradiation time. The concentration of TiO2 NPs was 0.1 mg/mL in each test.

color of the dyeing wastewater faded after irradiation by simulated sunlight, which reveals that the residual dye in solution was degraded. With the presence of CTiMF (24 wt% TiO2) (Fig. 3a), the characteristic absorption band of dye CNe3B decreased greatly with prolonging the irradiation time, and 60% of degradation was achieved after 120 min irradiation. This is compared with 20% of degradation for TiO2 NPs and 90% for CTiMF (12 wt% TiO2). It can be seen than TiO2 showed the lowest catalytic performance for practical dyeing wastewater among all the three catalysts. Compared with TiO2 NPs, the CTiMF (12 wt% TiO2) significantly improved the dye degradation efficiency. The Kapp values of the photodegradation of dyeing wastewater were calculated to be 7.38  104, 7.58  103 min1 and 1.58  102 min1 for TiO2 NPs, CTiMF (24 wt% TiO2) and CTiMF (12 wt% TiO2). The Kapp values indicate that the CTiMF (24 wt% TiO2) and CTiMF (12 wt% TiO2) showed 9 and 20 times higher photocatalytic activity than TiO2 NPs. The dye degradation performance of the catalysts for practical dyeing wastewater was observed to be different compared with that for RhB solution. This may be caused by the co-existence of

other additives in the practical dyeing wastewater (e.g. sodium sulfate and sodium carbonate in the present study). It was reported that the presence of salts in wastewater could inhibit the dye degradation performance of dispersive TiO2 NPs (P25) (Tang et al., 2018). The inorganic ions (sulfate and sodium ions) was proposed to serve as scavengers of radicals to eliminate the hydroxyl radicals (OH) from TiO2 (Wang et al., 2013). It was also suggested that the anions were adsorbed on the surface of the TiO2 NPs to block the active sites and the adsorption of dyes onto TiO2 was inhibited (Guillard et al., 2003). However, the 3D porous structures of CTiMF could acted as a microreactor for photocalysis, which could eliminate the disturbance of salts in wastewater. After being stirred for 1 h in the dark, the dye concentration in wastewater decreased by 1%, 18% and 28% with the presence of TiO2 NPs, CTiMF (24 wt% TiO2) and CTiMF (12 wt% TiO2). This reveals that CTiMFs processed distinct adsorption ability for dyes. The results indicate that CTiMFs have remarkable advantages than pure TiO2 NPs in treating practical dyeing wastewater and possible other environmental pollutant treatment.

Fig. 3. (a) UVevis absorption spectra of dyeing wastewater in the presence of CTiMF (24 wt% TiO2) irradiated by simulated sunlight (350 W/m2). (b) Plots of C/Co of dyeing wastewater in the presence of pure TiO2 NPs (P25), CTiMF (24 wt% TiO2) and CTiMF (24 wt% TiO2) as a function of time after the CTiMF was added into the dyeing wastewater. (c) Corresponding plots of ln(C/C0) versus time during the photo-degradation. The lines are linear fitting curves. The concentration of TiO2 NPs was 0.5 mg/mL in each solution.

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3.5. Mechanism analysis CTiMF exhibited greatly improved catalytic performance compared to TiMF, which may be attributed to the improved light absorption of CTiMF. Infrared thermal imaging was employed to investigate the ability of light absorption of the obtained CTiMF (Fig. 4a). Compared with TiMF and TiO2 suspension, CTiMF achieved the highest temperature among all the three different samples under simulated sunlight irradiation. This result indicates that the carbonized foam improved the light absorption efficiency for photocatalysts. The coated polydopamine layer converted to a multi-layer graphene-like structure graphitic carbon nitride composite after the heating treatment (He et al., 2015). In our previous research, it was also found that the generated $OH increased after polydopamine carbonization (Huang et al., 2018). The carbon layer from carbonization of polydopamine could suppress the recombination of photo-induced charge carriers and prolong electron lifetime, leading to an improved photocatalytic performance. In addition, the N-doped carbonized foam may also promote the photo-generated charge carrier separation during the photocatalysis (Wen et al., 2017). Carbonization is vital to fabricate highefficiency photocatalysts based on melamine foam and TiO2 NPs. The specific surface area of CTiMF was determined by BET, which was 146.3 m2/g. This demonstrates the high porosity and microreactor feature of carbonized melamine foam. CTiMF has a

high content of N-doped carbon with hydrophilic property. The hydrophilicity/hydrophobicity nature of the prepared foam was measured by water contact angle test. MF, TiMF and CTiMF all showed great hydrophilicity (Fig. S4). The carbonization treatment and nanoparticle coating did not change the wettability of foam samples. These properties enable CTiMF to soak up dye solution easily (Fig. 4b). The extruded solution from the CTiMF was colorless after the simulated sunlight irradiation, revealing that RhB molecules have been photodegraded within the foam microreactor (Zhu et al., 2015). The CTiMF showed high elasticity. After being repeated pressing-releasing for 20 cycles (Fig. 4c), the CTiMF maintained its original morphology without any evidence of crack formation. This elasticity and the durability provide CTiMF with easy recycling for reuse. 3.6. Surface characterization and analysis The surface functionalities of the different foam samples were investigated by FTIR spectroscopy. Fig. 5a shows the FTIR spectra of PMF, TiMF, CMF and CTiMF. The broad FTIR band at 3000 to 3500 cm1 is assigned to the typical stretching vibrations of NeH and OeH. The FTIR spectra of PMF and DMF display two peaks at 1551 and 1471 cm1, which belong to the stretching vibration of C] N. The two peaks at 1328 and 970 cm1 are ascribed to the stretching vibrations of CeH. The peak at 810 cm1 is attributed to

Fig. 4. (a) Optical and infrared thermal images of TiMF, TiO2 NP suspension (3.3 mg TiO2 P25 in 10 mL deionized water), CTiMF immersed in deionized water under the simulated sunlight. (b) Photo-degradation of RhB in the CTiMF microreactors: ①, ② adsorption of dye solution; ③, ④ squeezing out dye solution after photocatalysis. (c) Elastic deformation of CTiMF under repeated pressure.

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Fig. 5. (a) FTIR spectra of pristine melamine foam (PMF), polydopamine coated melamine foam (DMF), carbonized melamine foam (CMF) and carbonized TiO2 coated melamine foam (CTiMF). (b) XRD patterns of original TiO2 NPs (P25), the heat treated TiO2 NPs (P25 heated for 2 h at 600
C) and CTiMF from the 2 h of heating at 550
C. (c) Raman spectra of CTiMF after being heated for different heating time (2e4 h) at 550
C.

the triazine ring bending (Stolz et al., 2016). Although polydopamine was coated on the melamine foam, no obvious difference was observed between the FTIR spectra of PMF and DMF, which may be due to low amount of polydopamine coating. After carbonization, new peaks arose in the FTIR spectra of CMF and CTiMF. The two peaks at 2923 and 2860 cm1 are attributed to the stretching vibration of CeH (Wang et al., 2005). The peak located at 1732 cm1 could originate from the C]N bond (Li et al., 2007). The two peaks at 1243 and 1467 cm1 are ascribed to CeN stretching vibration (Guo et al., 2016). The peak at 810 cm1 from the triazine of melamine disappeared after thermal treatment, implying that 1,3,5-triazine molecular structure converted into carbonized structure after the carbonization. Compared to CMF, CTiMF showed no obvious change in the chemical bonds of carbonized melamine foam, which may be due to very few groups on the surface of TiO2 NPs. Fig. 5b shows the XRD curves of original TiO2 NPs, the TiO2 NPs heated at 600
C and the CTiMF. It can be seen that anatase crystalline phase dominated in the original TiO2 NPs (P25) (Yin et al., 2001). It has been reported that the crystal structure of TiO2 converted from anatase to rutile after heating at 600
C (Inagaki et al., 2003). The XRD peaks assigned to the rutile crystalline phase increased after the TiO2 NPs were heated for 2 h at 600
C (Fig. 5b). CTiMF maintained the crystal structure of original TiO2 NPs, with a relatively high intensity of anatase phase in the corresponding XRD curve. This may attribute to the encapsulation of carbon layers, which inhibited the TiO2 phase transformation from anatase to rutile even at 800
C, which led to a higher thermal stability of TiO2

NPs (Zhang et al., 2008). It is also reported that TiO2 QDs embedded in SiO2 foam can also maintain the anatase phase even after being calcined at 900
C (Pan et al., 2018). In this study, it is suggested that the anchoring effect from foam fibrils could impede the crystalline conversion of TiO2 at high temperature (Czanderna et al., 1958). Raman scattering spectroscopy was used to verify the crystalline structure of CTiMF. The two Raman peaks at 1350 and 1590 cm1 are typical D band and G band (Fig. 5c), which are assigned to the disordered sp3 carbon and graphitic sp2 carbon. The presence of D and G bands reveals that the melamine foam were carbonized by heat treatment (Zhang et al., 2015). To further understand the effect of heating time on the carbonization degree of MF and PDA, three different heating times (2, 3 and 4 h) were chosen to treat TiMF (Fig. 5c). The ID/IG intensity ratio from the Raman spectra was calculated to be 1.01 for the 2 h heat treatment and remained unchanged as the treatment time was increased to 4 h. This result indicates that the carbon structure of the CTiMF did not change with prolonging the heating time. The three Raman peaks at 397, 518 and 639 cm1 are attributed to anatase TiO2 NPs, indicating that TiO2 NPs were predominantly anatase phase, which is consistent with XRD analysis. A small Raman peak at 445 cm1 ascribed to the rutile crystalline of TiO2 was also observed as shown in Fig. 5c, this may attribute to the presence of a small amount of rutile phase in the initial TiO2 NPs (Chen et al., 2005). XPS characterization was carried out to assess the surface composition of the CTiMF (Fig. 6a). The XPS survey spectrum shows the signals of Ti, C, N and O elements. The high-resolution Ti 2p XPS

Fig. 6. (a) XPS spectra of the CTiMF: Survey, Ti 2p, C 1s, N 1s and O 1s. (b) SEM image and corresponding EDX element mapping (Ti, C, O and N) of the CTiMF.

Please cite this article as: Lu, X et al., Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118949

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X. Lu et al. / Journal of Cleaner Production xxx (xxxx) xxx

spectrum identified two peaks at 464.6 and 458.9 eV for the binding energies of Ti 2p1/2 and Ti 2p3/2 (Erdem et al., 2001), which further confirms that TiO2 NPs were integrated into the carbonized melamine foam. The C 1s spectrum displays two peaks at the binding energies of 284.8 and 288.5 eV, which are assigned to adventitious carbon and the tertiary carbon (N]CeN or C]N) (Dong et al., 2013). In the N 1s XPS spectrum, the two peaks located at 398.0 and 400.1 eV are attributed to sp2-hybridized nitrogen CeNeC and tertiary nitrogen N-(C)3 (Wei et al., 2018). The XPS peak located at 530.1 eV and a shoulder peak around 532.1 eV are from the O 1s signal, assigned to hydroxyl (OH) and oxide (O2) species of TiO2 NPs (Erdem et al., 2001). The XPS data suggests that TiO2 NPs were incorporated in the N-doped carbonized foam after the heat treatment. EDX mapping analysis was implemented to study the distribution of elements on the CTiMF. The EDX mapping from a region marked by a box in SEM image reveals that Ti, C, O and N co-existed in the CTiMF (Fig. 6b and Fig. S5). It can be seen that the 3D fibrils of carbonized foam consisted elements of C and N. The elements of Ti and O from TiO2 NPs distributed both on the surface and around the 3D framework, implying that the TiO2 NPs were successfully coated onto the fibrils of carbonized melamine foam. 4. Conclusions TiO2 NPs loaded 3D carbonized melamine foam has been successfully achieved through a biomimetic method followed by carbonization. The 3D framework of melamine foam were preserved after carbonization, providing porous structure for the micoreactors. The immobilization of TiO2 using PDA inhibited TiO2 phase transformation from anatase to rutile during the heat treatment. The carbonization process played a crucial role in improving the anchoring stability of TiO2 on the melamine foam. The CTiMF displayed significant improved photocatalytic activity for Rhodamine B dye degradation under simulated sunlight irradiation, which is up to 16 times higher than TiMF. More importantly, the CTiMF microreactor structures led to significant photocatalytic activity in dye degradation from the practical dyeing wastewater, with 20 times higher catalytic performance than TiO2 NPs. This work demonstrated a facile and practical approach to produce TiO2-loaded photocatalytic foam with easy recycling and high degradation performance, which will have great promising application in color removal from practical dyeing wastewater and other organic pollutant removal from industrial wastewater. Furthermore, the photocatalytic composite carbon foams with high catalytic activity would have a wide range of potential applications, such as purification of urban wastewater, treatment of agricultural water pollution and improvement of indoor air quality. 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. Acknowledgement The authors acknowledge the financial support from the Victorian Government’s Victoria-Jiangsu Program for Technology and Innovation Research and Development. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.118949.

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Please cite this article as: Lu, X et al., Titanium dioxide coated carbon foam as microreactor for improved sunlight driven treatment of cotton dyeing wastewater, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118949