Engineered Photocatalytic Material Membrane Assemblies for

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Engineered photocatalytic material membrane assemblies for removing nitrate from water Hang Xu, Yang Li, Mingmei Ding, Wei Chen, Kang Wang, and Chunhui Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00917 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Sustainable Chemistry & Engineering

Engineered photocatalytic material membrane assemblies for removing nitrate from water Xu Hang1,2, Li Yang1,2, Ding Mingmei1,2* , Chen Wei 1,2, Wang Kang1,2 Lu Chunhui3 1

Key Laboratory of Integrated Regulation and Resource Development on Shallow Lake of

Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China 2Hohai University, College of Environmental Science, Nanjing 210098, China 3State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, China Mailing address of authors: Hang Xu, Yang Li, Mingmei Ding, Wei Chen, Kang Wang, Chunhui Lu Hohai University, College of Environment, 1st Xikang Road, Gulou District, Nanjing 210098, China. Corresponding Authors: *E-mail: [email protected] (M.M Ding). Tel: +86-025-83786971

Abstract LiNbO3, as a nonlinear optical material (NLO), can remove nitrate from water via photocatalytic denitrification (PCDN), which has received increasing attention in the field of water treatment. In this work, efficient denitrification of water was achieved by coating a poly(ether sulfone) (PES) support membrane with a hydrothermally synthesized LiNbO3 powder. The photocatalytic membrane could achieve a water flux of 237 ± 12 L m-2h-1 per bar with a LiNbO3 and polyethylenimine assembly. In addition, The photocatalytic membrane possess inherent and unique advantages over common ultrafiltration membranes, such as separation performance and antifouling properties. In addition, the LiNbO3 has been successfully applied to the membrane materials, endowing the membrane with high photocatalytic activity. The photocatalytic denitrification experiments on the membranes indicated that the low concentration of nitrate(10mg L-1) can be reduced to N2 via CO2•− radicals under conditions of a 365 nm UV light source, with 81.82 % nitrate removal and 98.29 % photocatalytic selectivity toward nitrogen. The above results imply that the prepared photocatalytic membranes have the potential to be used in future separation technologies and for water treatment procedures to remove nitrate. Keywords: membrane; denitrification

nonlinear

optical

material;

LiNbO3;

photocatalytic

INTRODUCTION Nitrate (NO3−) is the chief active ingredient in synthetic nitrogen fertilizers, which are 1

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now widespread in both ground and surface water. It is imperative that nitrate pollution in water source areas should be controlled immediately, because nitrates exert a negative influence on the quality of drinking water.1-4 Unlike reverse osmosis, ion exchange, electrodialysis, and biological treatment procedures, catalytic processes appear to have several advantages as they can be performed under mild conditions (temperature and pressure) without the production of contaminated disposals.5 Bimetallic catalysts were found multiple times to be able to reduce nitrate with catalytic hydrogenation effectively. M. Al Bahri et al and so on prepared some bimetallic catalysts(Pd-Cu, Pd-Sn and Pd-In) with activated carbon and they could be up to the European Standards for drinking water after a 100mg L-1 nitrate was reduced by H2 at a pH value around 6.6 The analogical catalytic efficiency was conducted by other bimetallic systems such as Pd-Cu,7-8 PdAg/SiO2-NH2,9 Pd/TiO2-SnO2,10 and Sn-Pd.11 Additionally, it is likely to complete nitrate-to-nitrite conversion by the electrocatalysis.12 D. Reyter et al and so on showed that copper and Ti/IrO2 coupled electrodes can remove nitrate with a N2 selectivity of 100%.13 On the other hand, Nanoscale zero-valent iron can achieve the reduction of highly concentrated nitrate without requiring pH control.14-15 Photocatalysis is a recent addition to the field of catalytic processes for the removal of nitrates,16-19 with Zhang et al. making the first breakthrough concerning its potential application. The latter reported an N2 selectivity of 100 % and activities of one order of magnitude higher than common bimetallic systems using Ag/TiO2 as the catalyst.20 Particularly, nitrates are likely to be reduced via direct interactions with eCB− or by combining with reductive CO2•− radicals that originate from the reaction between hVB+ and hole scavengers (e.g., formic acid).21 According to the mechanism reported for several materials, such as conventional TiO2, CdS, and ZnS,22-25 CO2•− radicals may hinder rather than help the photocatalytic denitrification process due to several reasons. First, the condition of the CO2•− radicals is complicated since it is too difficult to predict the hole scavenger used. Secondly, the high VB potential(+2.8 V vs NHE) from the formation of •OH radicals may lead to reoxidation of nitrite or ammonium to nitrate. Lastly, photocatalytic oxidation exerts a negative influence on the PCDN performance due to recombination of electron holes. Various nonlinear optical (NLO) materials have been commercially popularized for applications ranging from the ultraviolet to infrared spectral regions, such as β-BaB2O4,26 LiB3O5,27 KTiOPO4,28 Rb3Al3B3O10F,29 Ba4B11O20F,30 and RbMgCO3F.31 NLO materials with the d0 B ion, such as the LiNbO3-type oxides (e.g., LiNbO3, LiTaO3, ATiO3 (A = Mg, Mn, Fe, Zn), and ZnZrO3), have high spontaneous polarization that can mitigate the problem of electron hole recombination.32-33 According to recent literature, NLO materials have unique photocatalysis properties for application in the field of nitrate removal from water. Guoshuai Liu et al and so on gain insight into the photocatalytic denitrification (PCDN) process of LiNbO3 and it was found that LiNbO3 delivered a much higher photocurrent than P25 due to the strong SHG effect.34 Due to several reasons, powdery catalyst may be hard to be applied to the actual field of water treatment. First, the catalytic efficiency of powdery catalyst depends on 2


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the hydraulic conditions. The catalyst, for instance, is likely to be precipitated when the water is static. Besides, it is difficult to retrieve powdery catalyst after the catalytic process is completed because most of the powdery catalysts are nanoparticles. Herein, we report a novel approach for coating LiNbO3 on the plyethersulfone(PES) membrane and it possesses a favorable durability. Photocatalysis experiments results indicated that assembled LiNbO3-coated membranes had favorable results, achieving 80.18 % nitrate removal and 98.29 % nitrogen selectivity using a 365 nm UV light source. This is the first time the NLO material(LiNbO3) has been applied to advanced membrane assemblies, and the reported results manifest the potential of such membranes for future separation technologies and treatment of water for nitrate removal. EXPERIMENTAL SECTION Materials. Niobium(V) oxide (Nb2O5) powder, lithium hydroxide (LiOH), potassium hydroxide (KOH), sodium dodecyl sulfonate (SDS), potassium nitrate (KNO3), N-methyl-2-pyrrolidinone(NMP), poly(acrylic acid) (PAA, Mw 450kDa), polyethylenimine (PEI, Mw 750kDa), polyvinylpyrrolidone (PVP K-30), dopamine, and bovine serum albumin (BSA, Mw = 66.5 kDa) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shang Hai,China). PES (Ultrason E6020P, Mw = 58 kDa) was supplied by BASF (Ludwigshafen,Germany). Photocatalytic Membrane Fabrication. The membrane support was prepared by using a routine phase inversion method, with PVP as a pore-forming agent.35-36 In order to coat the nonlinear materials(LiNbO3, synthesis detailed in Supplementary 1 in the Supporting Information) on the membrane surface stably, the PES support surface should be modified into the polyelectrolytes.37 First, a fresh dopamine solution prepared by 200mg dopamine and 100ml of 10mM Tris-HCl (pH is 8.5) was sprayed onto the PES support, then dopamine on the PES support was exposed to the air for the self-polymerization. After 8h, the resulting solution (100ml, 100mg L-1 LiNbO3 in 2g L-1 PEI) was subsequently filtered in vacuum onto the polydopamine(PDA)-coated PES at a flow rate of 0.5 ml min-1. Then, the PEI-LiNbO3-coated PES was washed by DI water and soaked in PAA solution(2g L-1) for 1h. Finally, the photocatalytic membrane was obtained by washing three times with ethyl alcohol followed by drying at room temperature overnight. Photocatalytic Membrane Characterization. The photocatalytic membrane cross-sectional and surface morphologies were observed by SEM. The surface roughness of the membrane was analyzed by AFM (Multimode 8, Bruker, Germany). Furthermore, the membrane surface functional groups (e.g., -COOH) were studied by Fourier transform infrared spectroscopy (FTIR, TENSORII, Bruker), since PAA coated the PES support membrane. The membrane surface hydrophilicity was detected by a contact angle measurement system (OCA-15EC, Dataphysics, Germany). In addition, the thickness of membrane was calculated by an ellipsometer(SpecEI, Ocean Optics, Germany). For this measurement, the samples were placed in an oven at 50℃ for drying, then each membrane was divided into three 3



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parts via the frozen fracture(-196℃ of liquid nitrogen). For the middle part, three different places of each cross section were measured and three measurements for each trial were carried out with the average values recorded. To quantify the surface chemical compositions and states of the membrane, X-ray photoelectron spectrometry (XPS, ESCALAB 250Xi, USA) was applied. To verify the optical properties of photocatalytic membrane, UV-vis diffuse reflectance spectroscopy (DRS) and PL emission spectroscopy were carried out using a UV-3600 spectrophotometer (Shimadzu, Japan) and Fluorolog3 PL spectrometer (Horiba Jobin Yvon, Japan), respectively. To identify the second harmonic generation(SHG) effect of photocatalytic membrane, a Q-switched 1064 nm Nd:YAG laser was employed to irradiate the membrane with a pulsed infrared beam. To reveal the photocurrents of photocatalytic membrane, an electrochemical workstation(CHI660B, China) equipped with three-electrode and single-compartment quartz cell was used to illuminate the membrane with a 365nm cut-off filter. Membrane Permeability and Rejection Test. The permeability experiments of membrane were conducted using ultrafiltration cups (Millipore 8400, EMD Millipore, USA). With a constant flow and filtration in dead-end mode, the permeated water flux of the PES support and photocatalytic membranes were directly tested. The integrated electronic balance(Mettler Toledo ML1502E) was employed by measuring the membrane permeate and the flux was automatically recorded at 1min intervals. The membrane rejection properties were conducted by a series of tests that filtered the aqueous PEG solutions with different molecular weights(4, 8, 10, 12, 20, 30, 60, 100, and 200 kDa, respectively). During the tests, the feed containing different PEG concentration was carried out evaluating the molecular weight cut off(MWCO) performance of the membrane. The 10, 100, 1000 mgL-1 of PEG were prepared with one PEG molecular weight for each solution, respectively. The permeate flow rate was 200±16 Lm-2h-1 at at a constant TMP of 20±2kPa. The PEG concentration of the permeate solution was characterized by DOC that was measured by TOC-V CPN. The rejection rate R (%) was calculated using Eq. (4):

(1) where Cfiltrate and Cfeed are the PEG concentrations (mg/L) in the filtrate and feed solutions, respectively. Membrane Fouling and durability Test. To make the fouling test close to real environment, the sewage treatment plant(STP) effluent was used to be a fouling solution. The effluent samples collected at different times of the day(6am, 2pm and 10pm) were obtained from JX STP, located in Nanjing, South-East China. The facility has a treatment capacity of 640,000 m3/day, and it adopts the biological treatment process, namely, the A/O process(the detailed Water quality parameters in Supplementary Table 1, Supporting Information). The test could be divided into several identical cycles and each cycle consisted of two phases (fouling, cleaning). Each fouling phase ended when the membrane specific volume reached 200L m-2. 4


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Subsequently, the fouled membranes were consecutively immersed in NaOCl solution(available chlorine is approximately 1000ppm) for ultrasonic washing(30min) and then soaked in water for 30min to remove the residual NaOCl. Photocatalytic membrane was placed in an ultrafiltration cup, which was driven by N2 at a constant trans-membrane pressure(TMP) of 20±2kPa. The fouling resistance(R) value of each filtration cycle was calculated by Eqs (1) , (2) and (3) (2) (3) (4) where J0 is determined before fouling experiment by the feed water at 1bar and J is the instantaneous flux value (L m-2h-1) of feed water before and after the membrane cleaning. ∆P is the trans-membrane pressure(TMP) during the test. µ represents the viscosity of water. The durability of the cross-linked LiNbO3 surface layer could be regarded as the major index to predict the membrane working life. In this section, the variation of the coating layer thickness was observed during a constant flow permeation. The photocatalytic membrane(diameter is 59mm) filtered deionized (DI) water at different constant flux(50, 100 and 150 Lm-2h-1, respectively) and it will be cleaned by 1L of NaOCl solution(~1000ppm available chlorine) for 30min when the operating time reach 7, 14, 21 and 28 day, respectively. The membranes operated for different days were collected to measure the thickness. Membrane PCDN Test. Before the test, the absorption interference for LiNbO3 particles, the PES support and the photocatalytic membrane in the PCDN process has been excluded(Supporting Information, Supplementary Table 2). For better verifying the practical feasibility of the photocatalytic membrane, we tried to explore the impact of ionic strength on the PCDN process. The different dosages of the CaCl2 (100, 200, 400 and 800mgL-1, respectively) were added to the system. For subsequent testing, a translucent quartz module(Supporting Information, Supplementary Figure 1) equipped with UV light (philips, USA), operating at a peak wavelength of 365 nm and 40W, was utilized. The feed solution can not permeate photocatalytic membrane when the system under a normal atmosphere. First, the membrane was irradiated under UV light for 30 min before the feed solution and formic acid was added. Then, N2 was piped into the device and drive the solution with a constant flow(50Lm-2h-1). Subsequently, solution samples were collected and detected. To verify the membrane PCDN practicability, the JX STP effluent was used as a feed solution. Meanwhile, the formic acid was added to the solution with a dosage of 0, 0.5, 1.0, 1.5 and 2.0mmol L-1, respectively. Besides, in order to explore the mechanisms of PCDN, we simulate the nitrate concentration in the STP effluent and analyse its concentration at different PCDN process. According to the 5



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stoichiometry of the nitrate reduction, the initial concentrations of nitrate and formic acid were 0.714 and 1.750 mmol L-1, respectively. The solution (300 ml) was deoxygenated via N2 stripping, and NO3−, NO2−, and NH4+ were collected and detected by ion chromatography (LC-10A, Shimadzu) at every following time point: 15, 30, 45, 60, 75, 90, 105, 120, 150, and 180 min. After the test, the membrane zeta potential was measured by Zeta potentiometer(Zetasizer Nano ZS, Malvern). The photocatalytic selectivity toward N2, nitrite, and ammonium were calculated according to Eq. (5), (6), and (7), respectively: (5)

(6)

(7) where [X]0 and [X]t are concentrations at time 0 and t, respectively. In addition, the concentration of HCOOH and HCOO- were detected by liquid chromatography mass spectrometry (LC-MS, Agilent 1260-6460, Agilent) and ion chromatography (LC-10A, Shimadzu), respectively. The radical species produced in PCDN process was discerned by electron paramagnetic resonance (EPR, Bruker EMX, Germany). RESULTS AND DISCUSSION Characterization of LiNbO3. The size and morphology of the samples were characterized by SEM (shown in Figure 1a and b), which gives information in terms of their unique structure. The samples had a smooth surface and were highly crystalline. From the Figure 1(c) and (d), a crystal lattice corresponding to LiNbO3 (d012 = 0.3749 nm) could be clearly observed. Figure 2a shows the zeta potential results for the LiNbO3 powders dispersed in the solution of different pH and ionic strength values. Due to hydroxide adsorption, the zeta potential of LiNbO3 changes to less-positive or more-negative with the pH value increase. In addition, the rise of the ionic strength can strengthen the shielding effects on the LiNbO3 surface, which decreases the absolute value of the zeta potential. The isoelectric point of the LiNbO3 is approximately 8.5, and as the concentration of KNO3 increases, its isoelectric point continues to move in the direction of acid pH value. The results of BET analysis (Figure 2b) shows a higher adsorption capacity of the LiNbO3 powders, with intense adsorption-adsorbent interactions, since their isotherm displayed reversible type-I adsorption and minor hysteresis upon desorption of nitrogen. According to calculations using the BET theory model, the BET surface area of the samples was 53.41 m2 g-1, which means that the LiNbO3 powders were more likely to provide numerous active sites for photoreactions. Meanwhile, the XRD characterization of samples shows a close match with characteristic peaks of LiNbO3, 6


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indexed to the trigonal structure (PDF 20-631). It can be conducted by the intense and sharp diffraction patterns that the samples had high crystallinity. Photocatalytic membrane Synthesis. Figure 3 shows the SEM surface image, cross-section, and PES support of the pure PES membrane and the photocatalytic membrane. Compared to the pure PES membrane, a clear boundary of PES support and the LiNbO3 coating layer was present in Figure 3(d) . The PES support exhibited a typical asymmetric structure, containing a top skin layer and a dense ‘sponge-like’ base. As shown in Figure 3(e), the morphology of the LiNbO3 particles was obviously changed compared with that of the as-prepared sample (Supporting Information, details in Supplementary 1), and the ‘ball-like’ particles congregated on the LiNbO3 coating layer. As shown in Figure 3(c) and (f), regular pores, which corresponded to the membrane bottom, provided numerous large pore channels for filtration. Figure 4 reveals the peak-valley morphology of the photocatalytic membrane surface, as characterized by AFM measurements. Figures 4(a) and (b) are 3D AFM images of the pure PES support and the photocatalytic membrane, respectively. The surface of pristine PES was smooth and the photocatalytic membrane with some small and rolling ‘peaks’ was tougher than the pristine. In addition, the root mean square roughness (Rq) and average surface roughness (Ra) of the pristine PES, calculated based on a scanning area of 2 µm × 2 µm, were determined to be 4.24 and 8.29 nm, respectively, while those values for the photocatalytic membrane were 9.36 and 11.90 nm, respectively. In brief, the surface roughness of photocatalytic membrane was dependent on the LiNbO3 particles, with rougher membranes when they are coated with more particles. FTIR analysis indicated the variation in functional groups among the pure PES, PDA-coated PES, PEI-LiNbO3-coated PES and photocatalytic membranes. As shown in Figure 5, for the pure PES membrane, the sharp peaks at 1150 and 1340 cm-1 are associated with O＝S＝O stretching vibrations, while the bands at 1070 and 1263 cm-1 are attributed to C―O stretching vibrations. Compared to the pure PES, the PDA-coated PES showed three new bands at 1510cm-1, 1600cm-1 and 3100 cm-1, which correspond to the stretching vibration of C＝C stretching, the deformation vibration of N―H and the stretching vibration of O―H(Phenolic hydroxyl group), respectively. For the PEI-LiNbO3-coated PES, the sulfone peak (O＝S＝O) was weaker than for the pure membrane and the PDA-coated PES. For the photocatalytic membrane, a new bands was located at 1700 cm-1, attributed to the stretching vibration of C＝O (carboxyl group). Contact angles, important for the measurement of membrane hydrophilicity, indicate better hydrophilic properties when water contact angles are smaller. Supplementary Table 3 in the Supporting Information shows the contact angles of the pristine PES, PDA-coated PES, PEI-LiNbO3-coated PES and photocatalytic membrane. The angle of the PDA-coated PES was smaller than that of the pristine one (39.3º and 68.5º, respectively), while that of the photocatalytic membrane was at 45.7º slightly higher than that of the PDA-coated PES. Overall, the hydrophilic performance of the photocatalytic membrane superior to that of the pristine PES.

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Photocatalytic Membrane Performance. The flux measurements of pure water for the pure PES support, the PDA-coated PES and the photocatalytic membrane are shown in Supplementary Figure 2 in the Supporting Information. The flux of the photocatalytic membrane (237.52 L m-2h-1) was much lower than that of the pure PES and PDA-coated PES, due to the cross-linked LiNbO3 surface layer. As shown in Fig.6 a and b, the rejection performance for the membrane was weaker in the lower concentration of feed solution. Comparing the MWCO of the photocatalytic membrane and pristine PES membrane, we found that the photocatalytic membrane could effectively suppress the pollutants with molecular weight over 50 kDa in the low concentration PEG solution (10 mgL-1), while the molecular weight for pristine membrane rejection exceeded 90 kDa. Besides, its properties for rejection will be more prominent than the pristine PES membrane when it comes to the high-strength feed solution. The antifouling behavior of the photocatalytic membrane was assessed, with data presented in Figure 6(c) and (d). It is obvious that the flux of the pristine PES membrane declined during fouling of the membrane, which could result in a decrease of its working life. As the specific volume of membrane grew to 1000Lm-2, the normalized flux of the pristine PES membrane plummeted to 0.725. Compared to the PES, the flux of photocatalytic membrane even showed a more gentle decrease to 0.893. The reversible and irreversible fouling resistances caused by the effluent solution at the end of each cycle were presented in Figure 6(d). It could be seen, a similar change occurred in the fouling tests for the pristine PES and photocatalytic membrane, that is, as the specific volume for membrane increased, irreversible fouling resistance rose while reversible fouling resistance diminished. Furthermore, we found significant improvements in the anti-fouling property of photocatalytic membrane by the coating. The total fouling resistance and irreversible fouling resistance of photocatalytic membrane were much lower than those of the pristine PES membrane after the test. Figure 7 gives the information about the durability test of membrane. The photocatalytic membrane showed a favorable mechanical integrity in the field of the coating layer. It can be seen that the initial thickness of coating layer was about 9.18µm and it only decreased to 7.25µm with the 150Lm-2h-1 of constant flow after 28 days running. Furthermore, more slight decrease was observed in the lower membrane flux such as 50 and 100Lm-2h-1. Optical properties of LiNbO3 and the membrane. UV-vis DRS is an effective method to evaluate the optical absorption performance of LiNbO3 particles and the membrane. As shown in Figure 8(a), the absorption band of the photocatalytic membrane moved to a new wavelength range (from 241 to 398 nm) compared to pure LiNbO3 (from 228 to 390 nm). Moreover, the intensity of the absorption peak for photocatalytic membrane was weaker than that for LiNbO3. According to the Kubelka-Munk equation, the band gap (Eg, eV) of the pure LiNbO3 and the membranes can thus be estimated.38 The band gap of photocatalytic membrane was 3.4 eV, a value lower than that of the LiNbO3 powder (3.5 eV), which means that the membranes can produce eCB− and holes hVB+ easier under the UV light than pure 8


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LiNbO3 particles. Figure 8(b) gives information about the intensity of the PL emission spectra for the pure LiNbO3 particles and photocatalytic membrane from 300 to 500 nm (the wavelength of excitation light was 330 nm). The membrane exhibited photoresponsive behavior similar to the LiNbO3 powder at the band from 350 to 400 nm. Especially their strongest emission peaks were nearly identical at 365 nm. However, the curve of the photocatalytic membrane was less smooth than that of pure LiNbO3 particles, because PEI adhering onto the LiNbO3 surface may affect fluorescence adsorption. Overall, the membranes inherited the good properties of pure LiNbO3. The second harmonic generation (SHG) is an important pathway to speculate the properties of NLO materials. SHG effect is caused by polarization, which can promote the separation of photoelectron/hole pairs.39 In this section, the Kurtz-Perry method was employed for the verification of SHG performance for the membranes. As shown in Figure 9(a), KH2PO4 (KDP, a typical NLO material) was a reference to evaluated SHG properties of P25, LiNbO3(prepared as SI1) and the membranes. The samples were irradiated by a 1064nm Nd:YAG laser. We found that P25 had no SHG response while the SHG intensity of LiNbO3 powder is 3.9 times as strong as KDP and 3.3 times of KDP was observed for the membranes. It implied that the membranes had the significant performance for NLO materials because they were still possessed of high SHG response after LiNbO3 was adhered onto their surface. Furthermore, a strong SHG effect suggests an intense intrinsic polarity of the materials, which can prevent the recombination of photogenerated electrons/holes. It was confirmed by previous study that LiNbO3 possess an unique structure with single 180º-domain, which can exhibit a strong photovoltaic effect along the Z-axis direction.33 For this season, the photocatalytic membrane has an apparent capability for transporting photogenerated carriers to the surface compared to conventional photocatalysis. As described above, NLO materials possess a high efficiency for the separation of photoelectron/hole pairs due to the surface polarization. As Figure 9(b) shown, the membrane exhibited a much higher photoelectron than P25, which means that the membrane will achieve a higher photocatalytic activity than P25 during PCDN process. PCDN Activity. All items of data in Table 1 do not change significantly with the addition of CaCl2 except the zeta potential of the membrane surface. After 180min PCDN process, the NO3- removal efficiency and the N2 selectivity reach 81% and 98%, respectively. The surface charge of the membrane transforms from positive into negative after nitrate removal. Meanwhile, adding the CaCl2 dosage can strengthen the shielding effects on the membrane surface, and therefore reduces the absolute value of the zeta potential. Fig.10(a) reflect the change of DON, NH4+, NO2-, and NO3- after the STP effluent experiencing the PCDN filtration. According to EPA’s maximum contaminant level (MCL) for nitrate, we regarded 10mgL-1 of NO3--N as the target concentration for the effluent treatment. We found that the nitrate concentration in the effluent could be 9



reduced blow MCL when formic acid dosage come to 0.5mmol L-1. Besides, it is apparent that TN in the sample solution diminished and the downward trend was even more significant with the addition of formic acid. When the dosage of Formic acid amounted to 2mmol L-1, the concentration of NO3- in the water samples leveled off(at 1.06mg L-1) after the PCDN filtration. We have noticed, most surprisingly, that the concentration of NO2- soured first and then plunged. In addition, after the PCDN filtration, the decline of DON as well as the growth of NH4+ was slightly. To explore the variation of the PCDN products, we analyzed the filtrate from the simulated water sample at different time. As shown in Figure 10(b), during the photoreaction process, the NO3--N concentration gradually decreased and ultimately was 1.82 mg L-1. NO2--N reached a peak (2.01 mg L-1) at 50 min and then declined to a final concentration of 0.12 mg L-1. The concentration of NH4+-N was invariably quite low. It can be seen from Figure 10(c) that 80.18 % of nitrate could be removed by the membranes, and photocatalytic selectivity toward nitrogen was 98.04 %, whereas NH4+ selectivity were invariably quite low at the end of the photoreaction. In addition, NO2− selectivity reached the maximum value(90.54%) within the first 50 min. It implies that NO2- is a dominating transitional product during the PCDN process. According to the Figure 10(d), the concentrations of formic acid and HCOOdecreased rapidly after 50 min and eventually decreased to 0.38 mg L-1 and 14.25 mg L-1, respectively. In addition, the pH of the solution remained around 3.5 for the first 80 min and then rapidly increased to 7.04, which was followed with a gradual increase to a final value of 8.13. The change of H+ and HCOO- seems to correspond to the variation of NO2-. With the dramatic decrease of NO2- concentration, the concentrations of H+ and HCOO- dropped quickly. It is likely to be that H+ and HCOO- play a role in the reduction of NO2- to N2. Mechanisms of PCDN. It was well known that formic acid is an efficient hole scavenger to produce CO2•− radicals during the photocatalytic process.40 In this section, electron paramagnetic resonance was employed to identify CO2•− radicals under three different conditions that PES support filtered nitrate under the UV light, photocatalytic membrane filtered nitrate without the UV light and it filtered nitrate under the UV light. As shown in Figure 11, (a) and (b) did not show any signal. Simultaneously, a strong signal of DMPO-CO2•− was presented in (c),40 while the signal of DMPO-•OH did not exist. Actually, it is impossible that •OH radicals released by the water splitting could be produced at lower potential of +0.7 V(vs NHE) for LiNbO3 VBM.33 It could be summarized that HCOOH may get h+ to produce H+ and then generate CO2•− through PCDN process of photocatalytic membrane(detailed in eq(8),(9)). During the process, the reduced CO2•− fails to be oxidized by •OH, which leads to reduce photocatalytic energy efficiency. In this case, the reduction of NO3- to NH4+ or N2 during PCDN process can be divided into two stages(detailed in eq(10),(11),(12) and Figure12): (1) NO3- was absorbed to the LiNbO3 surface and reduced to NO2- via obtaining e-. (2) NO2- reduced to N2 or NH4+ by CO2•− at pH-acid condition. Compared to the conventional TiO2-based photocatalysts, LiNbO3 possess a lower potential of the conduction band which from -3.5 to -2.8eV,34 and it remains 10


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sufficient negative to drive the reduction of NO3- to NO2-. Besides, LiNbO3 can produce more controllable and viable h+ via the interior polarization to speed up the process of CO2•− radicals generating. For those reasons, LiNbO3 has a favorable performance of nitrate removal and high photocatalytic selectivity toward nitrogen.

h + + HCOO − = H + + CO2•−

(8)

CO2•− + h + = CO2

(9)

2 H + + NO3− + 2e − = NO2− + H 2O

(10)

6CO2•− + 8H + + NO2− = NH 4+ + 6CO2 + 2 H 2O

(11)

6CO2•− + 8 H + + 2 NO2− = N 2 + 6CO2 + 4 H 2O

(12)

CONCLUSIONS In this study, the photocatalytic membranes possess inherent and unique advantages over common ultrafiltration membranes, such as separation performance and antifouling properties. In addition, the LiNbO3 has been successfully applied to the membrane materials, endowing the membrane with high photocatalytic activity. The photocatalytic denitrification experiments on the membranes indicated that the low concentration of nitrate(10mg L-1) can be reduced to N2 via CO2•− radicals under conditions of a 365 nm UV light source and anaerobic environment, with 81.82 % nitrate removal and 98.29 % photocatalytic selectivity toward nitrogen. The above results imply that the prepared photocatalytic membranes have the potential to be used in future separation technologies and for water treatment procedures to remove nitrate. ACKNOWLEDGMENTS The authors gratefully acknowledges The National Natural Science Foundation of China (NO.51578209 and NO.51678213), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the 111 project under grant number B17015. The Fundamental Research Funds for the Central Universities(2612017B13414). REFERENCES [1] Browne, E.C.; Min, K.E; Wooldridge, P.J.; Apel, E.; Blake, D.R.; Brune, W.H.; Cantrell, C.A.; Cubison, M. J.; Diskin, G. S.; Jimenez, J. L.; Weinheimer, A. J.; Wennberg, P. O.; Wisthaler, A.; Cohen, R. C. Observations of total RONO2 over the boreal forest: NOx sinks and HNO3 sources. Atmos. Chem. Phys. Discuss. 2013, 13, 201-254. DOI 10.5194/acpd-13-201-2013. [2] Baergen, A.M.; Donaldson, D.J. Photochemical Renoxification of Nitric Acid on Real Urban Grime. ENVIRON SCI TECHNOL. 2013, 47, 815-820. DOI 10.1021/es3037862.

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[40] Perissinotti, L.L.; Brusa, M.A.; Grela, M.A. Yield of carboxyl anion radicals in the photocatalytic degradation of formate over TiO2 particles, Langmuir 2001, 17 (1), 8422-8427. DOI 10.1021/la0155348.

Figures and Captions

14


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Figure 1. (a) and (b) SEM image of the LiNbO3 powder; (c) and (d) TEM image of the LiNbO3 powder

Figure 2. (a)Zeta potential of the LiNbO3 powder as a function of pH and ionic strength; (b) Isotherm adsorption/desorption diagram of LiNbO3 and the XRD pattern and the SEM of LiNbO3 powder

15



Figure 3. (a)SEM cross-sectional view of pure PES membrane; (b)SEM top view of pure PES membrane surface; (c)SEM bottom view of pure PES membrane; (d)SEM cross-sectional view of photocatalytic membrane ; (e)SEM bottom view of photocatalytic membrane; (f)SEM top view of photocatalytic membrane surface

Figure 4. (a)AFM of pristine PES; (b)AFM of photocatalytic membrane

Figure 5. FTIR spectra of the pristine PES, PDA-coated PES, PEI-LiNbO3-coated PES and photocatalytic membranes. 16


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Figure 6. Membrane performances: Actual rejection values of poly(ethyleneglycol)(PEG) as a function of PEG molecular weight for the membrane (a) is the pristine PES membrane, and (b) is the photocatalytic membrane; (c) Normalized flux of the STP effluent solution; (d) Fouling resistance of the STP effluent solution

Figure 7. The durability test of membrane: The variations of the coating layer thickness during different constant flow permeation and a series of chemical cleanings.

17



Figure 8. (a) Diffuse reflectance spectroscopy and (ahv)1/2 vs. hv curves (inset) for pure LiNbO3 and the photocatalytic membrane ;(b) Photoluminescence spectra of pure LiNbO3 and the photocatalytic membrane

Figure 9. (a)Oscilloscope traces of the SHG signals of LiNbO3 and the KH2PO4 reference irradiated by a Q-switched 1064 nm Nd:YAG laser; (b) Transient photocurrent responses of photocatalytic membrane and P25 based on on-off cycles

18


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Figure 10. (a) The DON, NH4+, NO2-, and NO3- in the STP effluent after the PCDN process for different formic acid dosage; (b)Time course of NO3--N, NO2−-N, and NH4+-N concentrations during the PCDN process; (c) The nitrate removal efficiency and photocatalytic selectivity toward N2, NO2−-N, and NH4+-N during the PCDN process; (d) Time course of HCOO- concentration and pH during PCDN process.

Figure 11. The EPR spectrum after 30min photocatalytic reaction in presence of 0.1M DMPO. Reaction condition:(a) PES support filtered nitrate under the UV light;(b) photocatalytic membrane filtered nitrate without the UV light; (c) photocatalytic membrane filtered nitrate under the UV light.

19



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Figure 12. Schematic illustrations of the photocatalytic denitrification reduction by photocatalytic membrane

Table 1. The PCDN results with different CaCl2 dosage R(NO3-) S(NO2-) S(NH4+) S(N2) % % % % initial value

pH

HCOO- HCOOH Zeta potential(mV) (mg/L) (mg/L)

0

0

0

0

3.23

5.37

24.91

55.62

0

81.82

0.59

1.34

98.04

8.13

-18.87

14.25

0.38

100

81.77

0.68

1.37

97.88

7.79

-20.74

14.38

0.31

200

81.53

0.61

1.25

98.11

7.95

-19.65

14.33

0.36

400

81.68

0.66

1.48

97.79

8.00

-17.44

14.29

0.46

800

81.24

0.64

1.35

97.86

8.22

-14.61

14.65

0.41

After 180min PCDN process

CaCl2 dosage (mg/L)

Where R(NO3-) is NO3- removal efficiency. S(NO2-), S(NH4+) and S(N2) were selectivity to NO2-, NH4+ and N2, respectively.

20


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Supporting Materials Supporting materials are to exhibit some results to support the main work Table S1 illustrates the water quality parameters of effluent samples in this study. Table S2 illustrates the adsorption test of KNO3 for the LiNbO3, pure PES and photocatalytic membrane. Table S3 illustrates contact angles of the pure PES, PDA-coated PES, PEI-LiNbO3-coated PES and photocatalytic membranes. Figure S1 shows schematic representation of PCDN membrane device. Figure S2 shows water flux of different membranes. Figure S3 shows XPS wide scan spectra for LiNbO3 powders and photocataytic membrane. Figure S4 shows pore size distribution obtained from the rejection experiments. Figure S5 shows the variations of the flux and photocatalytic activity at the different coating thickness. Figure S6 shows the durability test of membrane thickness in this study. Figure

S7

shows

zeta

potential

of

the

pure

PES,

PDA-coated

PEI-LiNbO3-coated PES and photocatalytic membranes as a function of pH.

21


PES,


For Table of Contents Use Only.

This graphic can be used as a summary of photocatalytic denitrification in this report.

22


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Engineered Photocatalytic Material Membrane Assemblies for

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