Efficient Photocatalytic Removal of Methylene Blue Using a

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Energy, Environmental, and Catalysis Applications

Efficient Photocatalytic Removal of Methylene Blue Using a MetalloporphyrinPoly(vinylidene fluoride) Hybrid Membrane in a Flow-Through Reactor Roman Lyubimenko, Dmitry Busko, Bryce Sydney Richards, Andrea I. Schaefer, and Andrey Turshatov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04601 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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ACS Applied Materials & Interfaces

Efficient Photocatalytic Removal of Methylene Blue Using a Metalloporphyrin-Poly(vinylidene fluoride) Hybrid Membrane in a Flow-Through Reactor Roman Lyubimenko†‡, Dmitry Busko†, Bryce S. Richards†§, Andrea I. Schäfer‡, Andrey Turshatov†*

†Institute

of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT),

Hermann von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡Membrane

Institute

of

Technology Department, Institute of Functional Interfaces (IFG-MT), Karlsruhe Technology

(KIT),

Hermann-von-Helmholtz-Platz

1,

76344

Eggenstein-

Leopoldshafen, Germany §Light

Technology Institute (LTI), Karlsruhe Institute of Technology (KIT), Engesserstrasse 13,

76131 Karlsruhe, Germany Keywords: membranes; photocatalysis; photosensitizers; water treatment; solar energy

Abstract A novel combination of a poly(vinylidene fluoride) (PVDF) membrane with pore size 0.2 μm and a photosensitizer 5,10,15,20-tetrakis (pentafluorophenyl)-21H,23H-porphine palladium(II)

ACS Paragon Plus Environment

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(PdTFPP) makes a promising hybrid material for the generation of singlet oxygen (1O2) and, thus, water treatment applications. The fabricated photocatalytic membrane exhibits permeability of 4280±250 L m-2 h-1 bar-1 and stable photocatalytic degradation performance over a 90 hour period, when illuminated with green light (528±20 nm) and operated in a dead-end, single-pass configuration. Methylene blue (MB) degradation of 83% was achieved for MB concentration of 1 mg L-1 under the flow rate of 0.1×10-3 L min-1 (flux of 30 L m-2 h-1), light intensity of 21 mW cm-2 and PdTFPP loading of 25 mol g-1. Due to an enhanced mass transfer, the reaction rate of MB removal (with apparent rate constant of kapp = 6.52 min-1) results in an efficient photodegradation of MB inside PdTFPP-PVDF membrane. The influence of experimental parameters such as catalyst loading, flow rate, light intensity and solute concentration on MB removal was investigated. This research enables the application of photocatalytic PdTFPP-PVDF membranes as a potential technology for water decontamination under visible light illumination.

1. Introduction Despite the tremendous effort and progress achieved in water purification technologies, the lack of clean drinking water still poses a risk to humanity. Hence, innovative and sustainable remediation technologies are required.1 While pressure-driven membrane processes can remove many dissolved water contaminants, some pose challenges. Amongst these are organic micropollutants that are often not completely removed and, if removed, accumulate in concentrates.2,3 A viable solution to both remove and destroy organic micropollutants would be to combine membrane filtration with in situ photodegradation of contaminants. The photocatalytic conversion of water and/or dissolved oxygen to form reactive oxygen species (ROS) – defined as hydroxyl radicals (•OH), hydrogen peroxide (H2O2), superoxide radicals (•O−2), and singlet oxygen (1O2) – is, thus, a possible answer to challenges of micropollutant


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removal.4,5 One of the most prevalent inorganic photocatalysts, undoped titanium dioxide (TiO2), lacks such a visible-light response. Hence, the overall efficiency of any TiO2-driven process will be ultimately limited by the low fraction of ultraviolet (UV) light contained within sunlight (about 2% of sunlight photon flux).6 While much progress in the development of inorganic lowbandgap semiconductors (visible-light activated) for water treatment application has been demonstrated, challenges regarding their cumulative efficiency, stability and costs still remain.7,8 In order to harvest visible light for water treatment processes, another family of prospective materials for photocatalysis is organic photosensitizers (PSs). Largely, organic PSs possess an inherent ability to produce singlet oxygen under visible-light illumination.9 In contrast to other short-lived (ns-range lifetime) radical ROS produced by inorganic semiconductors, 1O2 has a significantly longer lifetime in the μs range.10 The strong electrophilicity of 1O2 leads to a high rate of interaction with electron-rich organic compounds, possessing unsaturated carbon bonds, aromatic units, nitrogen or sulfur atoms, and their decomposition.11 In addition, 1O2 is less affected than hydroxyl radicals by the background inorganic ions that occur in natural waters,12 thus lending itself better to micropollutants removal. The immobilization of PSs on the surface of a suitable substrate (nanoparticles, films or membranes) is a way of fabricating photocatalytic material for dissolved contaminants removal. However, the mass transfer of reactants in heterogeneous systems, with low surface-to-volume ratio in particular, could be inhibited. It affects the overall kinetics of the photocatalytic process and results in diffusion-controlled reactions. This problem, often encountered when performing experiments in batch reactors,13 can be overcome when executing the photocatalytic experiment in a continuous-flow multichannel microreactor. In that case, the diffusion-limited reactions (for example, micropollutant removal) can be significantly accelerated due to enhanced mass


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transfer.14 The same effect can also be achieved via the use of “photocatalytic membranes” in flow-through. These can be considered as a special case of the multichannel reactor, where the in situ generation of ROS is achieved both on the external and internal surface of the photocatalystassisted porous substrate (membrane).15,16 While the application of inorganic semiconductors (such as TiO2 and ZnO) immobilized onto membranes has been demonstrated for dissolved contaminants removal,17,18 there have, to date, been no reports relating to the application of membrane-immobilized PSs for photocatalytic degradation of water-borne pollutants. Although many PSs have been developed in the past, their application for visible-light-activated water treatment remains challenging.19 The lack of research most likely originates from the photobleaching of organic PSs, which is commonly observed under a wide range of light intensities and wavelengths.20 However, the problem of photostability can be addressed via the use of specifically functionalized porphyrinoid PSs.21 Lee et al. reported that, metalloporphyrins with large, rigid electron-withdrawing perfluorophenyl groups achieve PS photostability of 50 hours under excitation at 508 nm.22 In addition, metalloporphyrins exhibit both strong absorption of visible light and high quantum yield of 1O2 generation.9

Among

them,

5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphine

palladium(II) (PdTFPP) has been chosen as the PS in this study to achieve a good combination of high photocatalytic activity and photostability. To summarize, the scientific advances reported in this paper answer the following questions: (i) How can a photocatalytic membrane exhibiting a strong absorption in the visible part of the solar spectrum (~400 - 700 nm) be fabricated? (ii) What are the interaction mechanisms of excited PdTFPP on prepared membrane with dissolved oxygen and a dissolved contaminate? (iii) What is the best photocatalytic performance (pollutant degradation) that can be achieved in a


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single-pass continuous flow membrane reactor via variation of system operation parameters? In order to answer the above questions, the photocatalytic degradation of methylene blue (MB), a frequently-used and water-soluble model contaminant, in the photochemical membrane reactor operated in a single-pass configuration was investigated as illustrated in Figure 1.

Figure 1. Schematic representation of photochemical membrane reactor and photocatalytic PdTFPP-PVDF membrane for visible light-driven single-pass continuous flow pollutant degradation.


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2. Materials and methods Fabrication of PdTFPP-PVDF membrane To guarantee the best possible long-term stability of such a photocatalytic membrane, the membrane material must be carefully chosen such that it should neither self-degrade under continuous light exposure nor chemically interact with ROS. It was reported previously that the electrophilic 1O2 species can oxidize polymers containing electron-rich atoms or unsaturated chains (including common membrane materials such as cellulose and polyvinylchloride).23 Fluoropolymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) would, thus, appear to be a good choice. These are not affected by long-term UV/visible light exposure or ROS-induced oxidation.24 In addition, PVDF is a commercially available and widely employed membrane material.25 Therefore, a PVDF microfiltration (MF) membrane was chosen as PS support for the photocatalytic experiments in this work. Prior to modification, PVDF hydrophobic membranes (GVXAL-PVDF, 0.2 μm, Millipore) were cut into circles (2.5 cm in diameter), cleaned in an ultrasonic bath (VWR) in acetone (Merck, >99.5%) for 10 min and then rinsed in methanol (Merck, >99.8%) several times before leaving to dry for a period of 10 min at ambient air. Once dried, the PVDF membrane samples were immersed in a six-well custom-built plate filled with tetrahydrofuran (THF, Merck, 99.9%) solution of PdTFPP (for chemical formula, see Figure S1a, Frontier Scientific, >94%) with concentrations in the range of 1 – 23 mM and shaken in the dark for 4 hours at ambient temperature (22±1 oC), as shown in Figure S3 of the Supporting Information (SI). The stainless-steel shaker plate was designed such that six membranes could be prepared in one batch (solution volume of 1.5 mL). Subsequently, the membranes were removed, washed and then sonicated in ultrapure water (10 MΩ/cm at 25oC) to remove solvent residuals and any


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PdTFPP molecules that were not adsorbed. The fabricated photocatalytic PdTFPP-PVDF membranes were stored in water in order to keep it moist (at ambient temperature, in dark for period up to one week). As a figure-of-merit of catalyst (here, PS) mass to refer in photocatalytic experiments, the loading (µmol g-1) was chosen. Photosensitizer loading was evaluated via dissolution of the modified membrane in dimethylformamide (DMF) and subsequent assessing PdTFPP concentration with UV/visible (UV/vis) spectroscopy (as described in SI). Material characterization Verification of the chemical structure of adsorbed porphyrin was determined using Fouriertransform infrared (FTIR) spectrometer with an attenuated total reflectance (ATR) module (Vertex 80, Bruker). Zeta potential (potential) of the membrane was determined using an electrokinetic analyzer (SurPASS, Anton Paar) operating in streaming current mode using 1 mM KCl electrolyte solution (VWR chemicals, 99.9%). To investigate the variation of potential with solution pH, the pH values were adjusted using 0.5 M hydrochloric acid (HCl, VWR Chemicals, 99.9%) and 0.5 M potassium hydroxide (KOH, VWR Chemicals, 99.9%). Time-ofFlight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis was performed on a ToF-SIMS 5 machine (ION-TOF) equipped with 25 keV Bi3+ ion beam. The vacuum in the analysis chamber was held at 510-9 mbar. Prior to measurement, the sample was cryo-fractured in liquid nitrogen to allow a cross-sectional view of PdTFPP-PVDF membrane. Images (500 scans, 256×256 pixel on a field of view of 250×250 µm2) of Pd isotopes and PdTFPP molecules were obtained using delayed extraction mode, and charge compensation by low energy electron (21 eV) flooding. In contrast to other imaging modes tested, this mode enabled topographic effects to be minimized.


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Optical characterization UV/vis absorption spectra were recorded on a spectrophotometer (Lambda 950, Perkin-Elmer) equipped with an integrating sphere. The absorption spectra of all solutions were measured in 10 mm quartz cuvette (Hellma Analytics) against the cuvette with respectable solvent (THF or DMF) as a reference. The membrane sample was placed in a water-filled 10 mm quartz cuvette and measured inside the integrating sphere. A specular exclusion port was removed to release the reflected light and, thus, increase the upper measurement limit. For photoluminescence (PL) measurements, the samples were excited with a 405 nm continuous-wave diode laser (LD-51510MG, Roithner), mounted in a temperature-controlled housing (TCLDM9, Thorlabs) with an intensity of 35 mW cm-2. PL detection was measured using a double monochromator (DTMS300, Bentham) and a photomultiplier tube (R928P, Hamamatsu) mounted in cooled housing (CoolOne, Horiba). For the experiment requiring no oxygen, the sample was held under the vacuum at pressures 18.2 MΩ/cm at 25 °C) water solutions, except for the experiments with uric acid (UA) which was performed in phosphate buffer solution (0.044 M Na2HPO4, 0.022 M NaH2PO4, Sigma, pH = 7.4). A time-correlated multichannel scaling board (Timeharp 260, PicoQuant) was used for the time-resolved PL measurements. The modulation of the radiation of exciting diode laser was performed using a laser driver with a built-in function generator (ITC4001, Thorlabs). Photocatalytic membrane reactor set-up A custom-built photocatalytic membrane filtration system was used for testing the performance of the photosensitizer-immobilized membranes. A schematic of the system is given in Figure 2. The main components are: i) the high-performance liquid chromatography (HPLC) pump


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(BlueShadow Pump 80P, Knauer) equipped with 500 mL pump head; ii) a custom-built membrane cell (active membrane area of 2.01 cm2, active illumination area 1.72 cm2); iii) a collimated green light emitting diode (gLED, Thorlabs M530L3) with the peak wavelength of 528±20 nm and a maximum power output of 350 mW and white light emitting diode (WLED, Thorlabs SOLIS-3C) with a maximum power output of 4.6 W. The membrane cell was designed to have a feed inlet (blue) and two outlets (permeate and retentate). More detailed information, including the hydrodynamic conditions can be found elsewhere.26 The system was operated in a dead-end filtration mode, with experimental protocol given in Supporting information. The LED emission was measured with a spectrophotometer (Avantes, AvaSpecULS2048x64TEC). The illumination intensity was controlled using an LED controller (Thorlabs, DC2200). The light beam was collimated to be of the size of 2 x 2 cm. The light intensity was measured using an optical power meter (Thorlabs PM100D) equipped with a thermal power sensor (Thorlabs S175C, active detector area 1.8 cm  1.8 cm) placed after the quartz window of the membrane cell prior to the experiments.


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Figure 2. Schematic of experimental setup for testing photocatalytic membrane. The dissolved oxygen (DO) concentration in the feed solution were measured using a multiparameter meter (MultiLine 3510 IDS, WTW) equipped with an oxygen sensor (FDO-925, WTW). Permeate temperature was measured with a thermocouple (Type T, NI USB-TC01, National Instruments). The transmembrane pressure (TMP) was measured with feed lowpressure (WIKA S-20, 0 – 1 bar) and permeate (WIKA S-20, 0 – 0.4 bar) inline pressure sensors. Additionally, the high-pressure feed (WIKA A-10, 0 – 40 bar) and retentate (WIKA A-10, 0 – 40 bar) pressure sensors were used for high flow rates. All data from the sensors was transferred to a data acquisition card (USB-6000, National Instruments), visualized and saved by the LabVIEW 2014 software (National Instruments). Permeability measurement Milli-Q water was used in all experiments. The permeability (LV, L m-2 h-1 bar-1) was calculated via Equation 1: Lv = QP/(A ∆P),

(1)


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where QP is the permeate flow rate (L min-1); ΔP is the TMP calculated as the difference of feed and permeate pressures (0.610-3 - 910-3 bar) from the sensors (bar); and A is the active filtration area (m2). The flow rate was set by the pump and additionally controlled by weighing the collected permeate on a balance (Ohaus AX622/E). The mean value and error bar of permeability values were calculated by statistical averaging results of five tests, using a new membrane each time. Photocatalytic experiments Photodegradation studies were performed in dead-end filtration mode (valve closed on the retentate side) directly after permeability tests. The two solutes used were UA (≥99%, Aldrich) as water-soluble specific 1O2 quencher and MB (99%, Aldrich) as a model pollutant (for chemical formulas, see Figure S1b,c, SI). The repeatability of the photocatalytic experiments was assessed by conducting at least five measurements using membranes from the same batch for the flow rates of 0.110-3 and 110-3 L min-1 (representing the lowest and highest flow rates used in the experiment). The applied fluxes (30-300 L m-2 h-1) are up to hundred times lower compared to the typical fluxes reported for PVDF MF membranes with pore size 0.1 – 0.65 m (1500 – 41400 L h-1 m-2 respectively) but correspond to the fluxes of nanofiltration membranes (37 – 62 L h-1 m-2 for NF90, NF270 respectively).27 The detailed protocol of the photocatalytic experiments as well as the matrix of tested operating and physical parameters (Table S1) are presented in detail in the SI. Analytical methods The concentrations of solutes were determined with a UV/vis spectrophotometer (Perkin Elmer Lambda 365) equipped with a flow-through cuvette (light path of 10 mm, Hellma Analytics) and


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a reference cuvette filled with Milli-Q water. The absorbance was measured at the wavelength of 664 nm for MB and 292 nm for UA. Products of the PdTFPP transformation and MB degradation were determined using mass spectroscopy (MS), specifically the method of electrospray ionization (ESI-MS) with a mass spectrometer ExpressionL CMS (Advion), in negative and positive mode, respectively. 3. Results and Discussion 3.1. Characterization of PdTFPP-PVDF membrane PdTFPP-PVDF membranes changed the colour from white to reddish upon adsorption of PdTFPP. To qualitatively confirm the successful immobilization of PdTFPP on the PVDF membrane, FTIR spectroscopy was performed. The FTIR spectra of PdTFPP-PVDF membrane (Figure 3a) revealed peaks at 1495 and 1519 cm-1 for pyrrole and phenyl rings C=C vibrations (C=C), respectively; C-H in-plane deformation (C-H) at 991 and 1018 cm-1; Pd-N stretching (PdN)

at 947 cm-1; and C-H out-of-plane deformation (C-H) at 703 cm-1,28,29 which can also be found

in the FTIR spectra of PdTFPP powder. A minor shift of peak positions (14911495 cm-1, 990987 cm-1, 949947 cm-1) observed after the immobilization was attributed to adsorption. The absence of any peak at 3320 cm-1, where the N-H stretching of free-base (no metal coordinated) TFPP reveals itself, signified that Pd metal is still coordinated. Thus, membrane modification method did not result in any change of a chemical structure of PdTFPP upon its immobilization. The presence of a metal, such as Pd, allowed to visualize the distribution of PdTFPP along the PdTFPP-PVDF membrane cross-section using ToF-SIMS. Firstly, the images revealed identical mapping of both the Pd element and the PdTFPP molecules on the membrane, thus further confirming the earlier result that Pd is coordinated in the porphyrin cycle, as shown in Figure


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S4a,b in the Supporting Information (SI). Secondly, the longer data acquisition revealed that the highest Pd signal comes from the top surface of a membrane, followed by the 40 µm region with a reduced Pd content, a broad local maximum, then finally a substantial rise in the Pd signal at the rear surface (see Figure 3c and Figure S4c). The highest PdTFPP loading followed by the steady decline of the signal observed at both external surfaces was due to the hindered access of PdTFPP to the internal surface of a membrane. It is estimated that the central region (bulk of the membrane) contains ~63% of total PdTFPP loaded. More importantly, PdTFPP is adsorbed throughout the whole thickness of a membrane that enables the generation of ROS throughout the depth of the membrane. In order to gain insight into the mechanism of PdTFPP adsorption, the adsorption kinetics and adsorption isotherm were examined. Firstly, the influence of adsorption time on the PdTFPP uptake was explored for a given concentration of PS. Figure 3c indicates a rapid increase of loading of PdTFPP within the first minutes followed by a plateau. Such a high affinity adsorption process can be explained by strong hydrophobic nature of the fluorinated substrate (PVDF) and the adsorbate (PdTFPP molecule).


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(a)

PVDF

PdTFPP-PVDF

1.0 0.8

Intensity (a.u.)

Transmittance

(b)

Pd-N

PdTFPP

0.6 0.4 0.2

C-H

 C=C

4000 3000

1600

1200

C-H

(d)

30 24 18 12 6 0 0

1

2

3

4

5

Q-bands

6

100 80 60 40

Solution Loading, mol g-1 1.6 2.9 6.7 9.6 17.0 25.0

50

100

150

40 30 20 10 0

0

4

8

12

16

20

24

Equilibrium concentration (x10-3 M)

(f)

Solution Loading, mol g-1 2.6 5.3 10.3 14.1 16.9 19.1 25.0

PL intensity

Soret band

0

18.2%

50

Time (h)

(e)

63%

18.8%

Distance (m)

PdTFPP uptake mol g-1)

PdTFPP uptake (mol g-1)

0.0

800

-1

Wavenumber (cm )

(c)

% of absorbed light

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 0

400

500

600

700

600

Wavelength (nm)

700

800

Wavelength (nm)

900

Figure 3. (a) FTIR spectra of the PdTFPP powder, pristine PVDF membrane and PdTFPP PVDF membrane. (b) Distribution of Pd along the thickness of PdTFPP-PVDF membrane obtained from ToF-SIMS images. (c) The adsorption kinetics of PdTFPP (13.8 mM) on PVDF membrane. (d) The adsorption isotherm of PdTFPP on PVDF membrane with a linear fitting,


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contact time - 4 h. (e) Absorbance spectra of PdTFPP - PVDF membrane measured in water for different photosensitizer loading (solid) and absorbance spectrum of 15.5 µM THF solution (dash). (f) Normalized phosphorescence spectra of (solid) PdTFPP - PVDF membrane measured in water and (dashed) PdTFPP in degassed THF solution (excitation at 405 nm) The influence of the equilibrium concentration of PdTFPP solution on the amount of PS loaded onto the membrane was demonstrated in Figure 3d. The adsorption isotherm revealed a linear rise of PS uptake up to 0.025 M with no signs of saturation being reached. Such a behavior can be explained by the formation of a PdTFPP multilayer on the surface and inside the pores, driven by the hydrophobic interactions of PdTFPP molecules. Theoretical membrane coverage of 0.91016 - 1.61016 PdTFPP molecules (with assumption of the monolayer formation for flat and upright orientation of PdTFPP molecules) and the experimentally calculated coverage of 6.41016 PdTFPP molecules (see detailed description with Figure S2 in SI) gave the 6-fold difference that implies the formation of PdTFPP multilayer as well. Given to the multilayers, it was important to check for possible aggregation, as this process may suppress the formation of singlet oxygen due to a decrease of the triplet state lifetime of the PS. Optical spectroscopy studies of the modified membrane, namely absorption and PL measurements were performed in order to reveal the spectral changes (peak shifts or change of peak shape) of photosensitizer after its immobilization. These changes can be caused either due to the significant altering of environment polarity of PS molecules or aggregation of PdTFPP molecules. In this regard, absorption spectra of membranes with different loading of PdTFPP were measured and compared with the spectrum of 15.5 M solution in THF (Figure 3e). The characteristic absorption peaks of the porphyrin – for example, the Soret-band (403 nm) and Qbands (517, 551 nm)30 – were found, which confirms the adsorption of PdTFPP, however it was


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not possible to draw a solid conclusion about the shift of the absorption peak position and PS chromophore aggregation (all modified membranes demonstrate absorption saturation in the broad spectral range (300 – 420 nm and 510 – 560 nm) with > 99.9% of absorbed light). PL spectroscopy is a more sensitive technique analyzing the shifts in the position of the PL peak (due to aggregation) and yielding information about the properties of adsorbed PdTFPP molecules. Figure 3f displays the PL emission spectra where a phosphorescence peak at 671 nm remained almost unchanged after PdTFPP immobilization on PVDF membranes for the entire range of PdTFPP loadings (2.6 – 25 mol·g-1). Only appearance of weak red-shifted PL peak at a maximum of 790 nm can possibly be attributed to minor formation of PS chromophore aggregates. Thus, the chromophore aggregation does not appear to play an important role in photophysical behavior of adsorbed PdTFPP. The inhibited aggregation of PdTFPP is thus can be explained by the structure of the fluorinated porphyrin with the strong repulsion between pentrafluorophenyl rings due to the steric effect.31 In addition to the steric effect of the pentrafluorophenyl rings, the role of the electrostatic interaction potentially affecting the PS and pollutant adsorption can be significant in regard to the reaction rate of the photochemical process.32 To determine the likelihood of such interactions, the membrane surface charge was evaluated by means of the estimation of potential. The pristine PVDF membrane has an inert, hydrophobic surface.33 It exhibits a negative surface charge for pH greater than the isoelectric point – measured to be pHIEP = 3.7 (see Figure 4a). The negative charge of the hydrophobic surface can be explained by higher surface affinity of the hydroxide ions compared to the hydronium ions (H3O+).34 Upon adsorption of PdTFPP molecules, only a slight change in the potential occurs (pHIEP = 3.4).


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Thus, electrostatic properties of the surface remain largely unchanged (the surface remains negatively charged) after PdTFPP adsorption. In order to verify that the filtration properties of the PVDF membranes are retained after PdTFPP adsorption, the permeability of the membrane before and after PdTFPP loading was assessed in the photocatalytic reactor (Figure 4b). The relation between TMP and flux for pristine, photocatalytic membrane before and after permeability test are presented in Figures S5a-c (SI). No significant loss of permeability was observed both after loading of PdTFPP and after completion of the prolonged photodegradation experiment. Thus, immobilization of PdTFPP and the photocatalytic process itself do not affect the membrane permeability. What is more, the permeability (4280±250 L m-2 h-1 bar-1) of PdTFPP-PVDF membrane is not significantly different to that of reported PVDF-GVWP membrane (0.22 µm, 5914 L m-2 h-1 bar1).27

To address the both stability in dark and photostability of the immobilized PdTFPP layer, the photocatalytic membrane was exposed to water filtration (in dark) and simultaneous water filtration and light illumination. The PdTFPP-PVDF membrane in the wetted state preserved its translucent nature. Photographs before and after filtration are provided in Figure S6 (SI). After 6 hours of water filtration (without light irradiation), no visible change of the color was observed (Figure S6b, SI), suggesting that the hydrophobic PdTFPP molecules appear to be firmly attached to the surface of the PVDF membrane due to hydrophobic interaction.35


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20

(b)

PVDF pHIEP = 3.8

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PdTFPP-PVDF2 pHIEP = 3.4

-20 -40 -60 -80 2

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(d)

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Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

 C=O

Wavenumber (cm-1)

PdTFPP-PVDFafter expt PdTFPP-PVDFpristine

4000 3500 3000 2500 2000 1500 1000

Page 18 of 48

Pristine PVDF

1.2

PdPFPP-PVDF PdPFPP-PVDF before expt after expt

PdTFPP-PVDFafter expt PdTFPP-PVDFpristine

1.0 0.8 0.6 0.4 0.2 0.0

Wavenumber (cm-1)

500

600

700

800

Wavelength (nm)

Figure 4. (a) Zeta potential under different pH values of (black) pristine and (pink) modified PdTFPP - PVDF membrane. (b) Permeability measurement of pristine (gray), porphyrin-loaded membrane (pink) before and (green) after a photocatalytic experiment with MB during 14 h light illumination. (c) FTIR spectra porphyrin-loaded membrane (pink) before and (green) after a photocatalytic experiment with MB during 14 h light illumination Inset: Magnification of IR region demonstrating the appearance of new peaks. (d) UV/vis absorption spectra of the photocatalytic membrane (pink) before and (green) after experiment during 14 h illumination -2

(λexc = 528±20 nm, 21 mW cm ) in Milli-Q water.


18

Page 19 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


After 14 hours exposure to green light along with Milli-Q water filtration, the color of PdTFPP changed in some places. The appearance of a greenish color in the central area of the membrane (Figure S6c, SI) could possibly be attributed to the PdTFPP undergoing a photochemical transformation with a possible formation of porphyrinoid products. This photochemical transformation leads to formation of products with modified photocatalytic activity and does not aggravate the photocatalytic activity of the membrane. This effect will be discussed in the part “The extended photocatalytic run experiment” (section 3.4). FTIR spectroscopy was employed to correlate these visual and structural changes. As Figure 4c displays no dramatic changes in the FTIR spectra, except for the marginal peak at 1732 cm-1, attributed to the C=O stretching vibration. PdTFPP from the illuminated membrane was then re-dissolved in THF and analyzed by UV/vis absorption. New absorption peaks at 582 nm and 600 nm were discovered, along with a shoulder at 650 nm (Figure 4d). This finding suggests the formation of oxidized PdTFPP with the chlorin or bacteriochlorin structure. Those compounds are known to result in a red-shifting of the Q-band absorption and be formed from porphyrin due to oxidation of the double bond in the pyrrole ring.36 Mass spectroscopy of the solution obtained from the photobleached membrane indicated a peak corresponding to PdTFPP m/z = 1078.0 (calculated for C44H8F20N4Pd-, m/z = 1078.0) and new products of m/z=1096.1 (calculated for C44H10F20N4OPd- 1096.0), m/z=1110.0 (calculated for C44H8F20N4O2Pd- 1110.0). The latter species, attributed to chlorin and bacteriochlorin derivatives respectively, remained attached to the PVDF membrane and exhibited good photocatalytic activity in agreement with the literature.31, 37


19


Page 20 of 48

3.2. PL studies of singlet oxygen generation with PdTFPP-PVDF membrane Since the goal of the work was to prepare a photocatalytic membrane able to produce 1O2 for degradation of MB in water, it was essential to demonstrate the generation of singlet oxygen in the process (2) – Type II oxidation pathway: 3PdTFPP

+ 3O2  1PdTFPP + 1O2

(2)

In the above reaction, singlet oxygen is formed due to energy transfer between the triplet state of photosensitizer (3PdTFPP) and molecular oxygen (3O2). In addition, 3PdTFPP can interact with a substrate with electron donor properties (e.g. MB) producing a porphyrin radical anion. The radical anion, in turn, interacts with oxygen to produce superoxide, hydrogen peroxide (H2O2) and hydroxyl radicals - Type I oxidation pathway. Type I and Type II pathways can be distinguished via measurements of time-resolved PL decays for PdTFPP phosphorescence. The PL decays at 671 nm – taken from PdTFPP-PVDF membrane under 405 nm laser excitation – are plotted in Figure 5a. The PL decay measured in vacuum (no quenching by molecular oxygen) demonstrated a single-exponential behavior with long PL lifetime of 1.05 ms, indicating a triplet state (T1) nature of PdTFPP emission. When PdTFPP-PVDF membrane was immersed in the aerated MB solution, the multi-exponential decay with shorter PL lifetime ( =0.100 ms) was revealed. This change indicated that strong quenching might be induced by different species, such as molecular oxygen, MB or water molecules. In order to quantify the impact of the Type II pathway, the average PL lifetimes τ for the PdTFPP-PVDF membrane were measured at different conditions as described in Figure 5a. The average PL lifetime was derived from the double exponential fitting of the PL decays and calculated using Equation 3:38


20

Page 21 of 48

𝜏  =

∑𝛼𝑖𝜏𝑖 ∑𝛼𝑖

,

(3)

where τi is the decay time of ith-component, and αi is the decay amplitude of ith-component.

(b)

Under vacuum 2 -1

Degassed H2O

Degassed MB, 1 mg L-12 Air-sat. H2O-1 Air-sat. MB, 1 mg L-1

0

1

2

3

Degassed PBS Degassed PBS-UA Air-sat. PBS-UA

PL intensity @ 670 (a.u.)

(a)

PL intensity @ 670 nm (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

0

1

2

3

4

Time (ms)

Time (ms)

Figure 5. (a) Time-resolved phosphorescence kinetics at 670 nm of PdTFPP-PVDF membrane in vacuum (gray, deoxygenated water (red), deoxygenated MB solution blue), air-saturated water (orange) and air-saturated MB solution (violet). (b) Time-resolved phosphorescence kinetics at 670 nm of PdTFPP-PVDF membrane in degassed phosphate buffer saline (PBS, pH 7.4) (pink), degassed UA (100 M) in PBS (blue and air-saturated UA (100 M) in PBS (gray). Laser excitation was at 405 nm with the intensity of 35 mW cm-2. Black lines represent the results of the double-exponential fitting.

When the membrane sample was exposed to deoxygenated water, a decreased PL lifetime of  = 0.739 ms due to quenching by the solvent molecules was measured. A further reduction of the PL lifetime was observed when the membrane was immersed in the deoxygenated MB solution ( = 0.383 ms). A marked decline of the PL lifetime reflects PdTFPP quenching via interaction with MB. This important interaction can lead to either additional MB photodegradation without the aid of 1O2 or generation of new ROS (e.g. superoxide, H2O2 and


21


Page 22 of 48

hydroxyl radicals) via the Type I oxidation pathway. In general, the quenching induced by MB can be assigned to processes, namely: photoexcited electron transfer, triplet-singlet energy transfer or triplet-triplet energy transfer (Equations 4-6). 3PdTFPP

+ 1MB+  PdTFPP + MB2+

(4)

3PdTFPP

+ 1MB+  1PdTFPP + 1MB*+

(5)

3PdTFPP

+ 3MB+  1PdTFPP + 3MB+

.

(6)

The contribution of each of the above processes to the quenching of PdTFPP triplet states cannot be easily distinguished. Hence, it was assumed that all three processes can potentially be involved in the MB-induced quenching pending further elucidation of mechanisms. Another strong reduction of the PL lifetime was observed in both air-saturated H2O ( =131 µs) and MB ( =0.100 µs) solutions. The ratio between rates of oxygen and MB-induced quenching kO2(Type II)/kMB(Type I) can be calculated using Equation 7: 𝑘𝑂2(𝑇𝑦𝑝𝑒 𝐼𝐼) 𝑘𝑀𝐵(𝑇𝑦𝑝𝑒 𝐼)

( = (

1 1

1

) 〉)

〈𝜏2〉 ― 〈𝜏1〉 〈𝜏3〉

―

1

,

(7)

〈𝜏 1

where 1,  and 3 are the PL lifetimes measured in deoxygenated H2O, aerated H2O, and deoxygenated MB solution, respectively. The value of kO2(Type II)/kMB(Type I) = 4.97 indicated that quenching induced by molecular oxygen is the dominating channel for PdTFPP triplet deactivation, where direct quenching by MB is the minor (but still relevant) channel. In the previous section, it was revealed that the quenching of PdTFPP triplets mainly resulted from collisions with molecular oxygen. However, not every collision might lead to the generation of singlet oxygen. Thus, an additional confirmation of the singlet oxygen generation was needed. The attempt to measure PL of singlet oxygen at specific band of 1270 nm failed due


22

Page 23 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to an extremely low quantum yield of 1O2 PL ( 9.3×10-7 for water).39 Therefore to confirm 1O2 generation, the method of chemical trapping was employed. This experiment was performed directly in the photocatalytic reactor setup in the continuous flow regime to demonstrate in situ 1O

2

generation via UA bleaching. UA is a widely used water-soluble trap for 1O2, exhibiting

excellent selectivity to interaction with the latter.40,41 It should be noted that Bregnhoj et al. reported that UA should be used with caution42 as photoexcited electron transfer might occur and result in degradation of the UA via the process described in (8): 3PdTFPP

+ 1UA  PdTFPP + UA+ .

(8)

To confirm whether the electron transfer took place, the PL lifetimes of 3PdTFPP both with and without UA in the degassed environment were measured. The results displayed similar PL lifetimes (Figure 5b).This observation means that the UA decomposition occurs predominantly via the 1O2 pathway. No direct interaction between PdTFPP and UA takes place. The typical result of UA bleaching is demonstrated in Figure 6. The experiments started from the breakthrough curve (the system response function) exhibiting an “S”-shaped with the saturation reached in a time period of about 1 h. Subsequently, the concentration decay of solute occurred (here, UA) after switching on the light. As UA predominantly reacts via 1O2-induced bleaching, the degradation test was denoted as the specific probe test for 1O2 generation on the surface of membrane pores. Due to expected charge repulsion between negatively charged UA molecules (see Figure S1c) and negatively charged PdTFPP-PVDF membranes, FTIR spectroscopy could not detect UA adsorption to the PdTFPP-PVDF membrane (Figure S7, SI). Taking into account that 1O2 diffusion length (before relaxation of 1O2 to 3O2 ground state) in water of  270 nm43 is higher than the average pore size of the membrane (200 nm), it was


23


assumed that 1O2, generated at the walls of the pore, is able to diffuse into the pore volume and react there with travelling pollutants. Concentration decay

Breakthrough curve

Light on, 528 nm

1.0 0.8 0.6 O

0.4

NH HN

0.2

O

0.0 0.0

0.5

Removal

1.2

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 48

Ceq

O

H N

O

N H

NH N H

O

1.0

O

1.5

2.0

Time (h)

Figure 6. Concentration profile in a typical photocatalytic experiment, using the example of UA (left chemical structure) as a specific quencher of singlet oxygen (product of UA degradation is -1

-1

also presented). The flow rate of 10-4 L·min , PdTFPP loading of 25 mol·g , light intensity of -2

21 mW cm and [UA] =10 M were used. The curve consists of two parts: breakthrough curve with s-response function (gray) and concentration decay (yellow) that follows after the PdTFPPPVDF membrane is exposed to gLED illumination.

3.3. Influence of reaction parameters on the photocatalytic degradation of MB To understand how to control the photochemical reaction in the membrane reactor, the effect of multiple parameters on the degradation of MB was investigated. The four most influential parameters were those affecting the rate of singlet oxygen generation (PdTFPP loading and light intensity) and reactant flux (flow rate and MB concentration). MB removal (R) and rate of disappearance (rdis) were used as the figures-of-merit of photocatalytic degradation. The removal (R) was calculated via Equation 9:


24

Page 25 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝑅=1―

𝐶𝑒𝑞 𝐶0

,

(9)

where Ceq and C0 are the equilibrium and initial concentrations of analyzed compound. Rate of disappearance was calculated using Equation 10: ―1 𝑀𝐵 𝑀𝐵 (𝑐0 ∙ 𝑅) 𝑟𝑑𝑖𝑠 = 𝐽𝑀𝐵 0 ― 𝐽𝑒𝑞 = 𝑄𝑃 ∙ 𝐴

,

(10)

𝑀𝐵 -2 -1 -2 where 𝐽𝑀𝐵 0 is the initial flux MB (mol·m ·min ), 𝐽𝑒𝑞 is the equilibrium flux of MB, mol·m ·min 1;

QP is the permeate flow rate, (L·min-1), A is the active filtration area (m2), 𝑐𝑀𝐵 0 is the initial

concentration of MB (mol·L-1), R is the removal of MB. The gLED (528±20 nm) used for the photodegradation experiments was chosen in order to have an as large as possible overlap of the LED emission with Q-bands of PdTFPP (500-600 nm) and an as small as possible overlap with the MB absorption spectrum (540-700 nm). MB is known to act as a photosensitizer and can undergo photobleaching that occurs both in oxygenated (1O2 – induced degradation) and oxygen-free (degradation via the photoexcited electron transfer mechanism) media.44 In order to separate the self-degradation of MB, control tests with the pristine PVDF membrane were performed for the major part of photocatalytic experiments. Influence of PdTFPP loading It has been reported that the dependence of reaction rate on photocatalyst loading rises linearly until it approaches a plateau, where full absorption of incident photons is reached.45 To verify if such a linear behavior is valid in continuous-flow operation, the influence of PdTFPP loading on removal and rate of disappearance was investigated as indicated in Figure 7a and Figure 7b. Figure 3c indicated that an increase of PdTFPP loading leads to nearly 100% light absorption for loading 25 mol·g-1 in the 500 – 600 nm spectral range. Meantime, both removal and the rate of disappearance gradually rise within this range. Higher catalyst loadings (32 – 42 mol·g-1) do


25


not increase the removal. The increase of the loading is accompanied by less reproducible results for the removal along with a significant rise in the TMP measured during the experiment. While the exact mechanism of this remains unclear, in order to minimize the PS consumption, the PdTFPP loading was fixed for further experiments at 25 mol·g-1. No detailed investigation of the negative effect on the removal in case of extremely high PdTFPP loading was carried out. Photodegradation experiments for different membrane orientations (taking into account that PdTFPP loading is slightly different for the opposite surfaces of the membrane as seen in Figure 3b) demonstrated only minor influence of the membrane orientation on the photocatalytic performance (Figure S8, SI). (a) 100

(b)

80

Rate of disappearance (x10-8 mol m-2 min-1)

MB removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 48

60 40 20 0

5

10

15

20

25

30

PdTFPP loading (mol g-1)

35

2.5 2.0 1.5 1.0 0.5 0.0

5

10

15

20

25

30

PdTFPP loading (mol g-1)

35

Figure 7 Degradation of MB in a single-pass continuous-flow reactor under green LED (528±20 nm) illumination (red symbols).. Influence of PdTFPP photocatalyst loading on degradation kinetics of MB (a) removal and (b) rate of disappearance.

Influence of irradiation intensity With the fixed mass of photocatalyst, the number of absorbed photons influencing the MB degradation can be controlled by varying the light intensity. Here, the increase in the light intensity resulted in both a higher MB removal and rate of disappearance, rising from 25%


26

Page 27 of 48

(0.3910-6 mol·m-2·min-1) at 1 mW·cm-2 up to 75 % (1.1710-6 mol·m-2·min-1) at 21 mW·cm-2 (Figure 8a and Figure 8b). At light intensities greater than 21 mW·cm2, no significant further change in the removal was observed. It is logical to conclude that with increasing light intensity, more photons are to be absorbed. In turn, it results in an increased 1O2 production and, thus, an increased MB removal. The photolysis of MB obtained using a pristine PVDF membrane increased from 1 to 20 % in this range. Thus, these experiments clearly demonstrate the ability of the PS to accelerate the MB photodegradation and limiting performance of phodegradation process at light intensities less than 21 mW·cm2. PdTFPP-PVDF PVDF

80 60 40 20 0

(b)

Rate of disappearance (x108 mol m-2 s-1)

(a) 100

MB removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 2.0 1.5 1.0 0.5 0.0

0

5

10

15

20

25

0

-2

Light intensity (mW cm )

5

10

15

20

25 -2

Light intensity (mW cm )

Figure 8 Degradation of MB in a single-pass continuous-flow reactor under green LED (528±20 nm) illumination (red symbols). The MB self-degradation (photolysis) experiments were performed using pristine PVDF membrane (blue symbols). Influence of illumination intensity on degradation kinetics of MB (a) removal and (b) rate of disappearance.

It has been reported earlier that the reaction rate (r) is related to the light intensity (I) for catalytic reactions in a membrane reactor following Equation (11):46,47

𝑟 𝐼𝑛

(11)


27


Page 28 of 48

At low light intensities, a linear increase of the reaction rate is observed with (n=1); however, the rate becomes proportional to the square root of r (n=0.5) or even follows zero-order kinetics with respect to light intensity (n=0) when light intensity increases due to limited number of photocatalytic centers available at the surface. Figure 8a and Figure 8b indicate that by increasing the light intensity the plateau is reached (with n < 1). Taking into account that photosensitization of singlet oxygen (1O2) occurs after collision of dissolved molecular oxygen (3O2) with the membrane surface covered by PdTFPP, we assumed that the plateau in Figures 8a,b is caused by either limited number of photocatalytic centers (PdTFPP molecules) available at the surface of PS or limited concentration of dissolved molecular oxygen. To verify if the concentration of dissolved oxygen could be a rate-limiting step in the generation of singlet oxygen at intensities greater than 21 mW·cm2, the flux of irreversibly lost (reacted) oxygen and dissolved oxygen were compared. Assuming complete oxidation of MB (12), C16H18N3S+ + 21.51O2  16CO2 + 1.5N2 + SO2 + 9H2O

(12)

one molecule of MB can potentially react only with 21.5 molecule of 1O2. Thus, the flux of irreversibly lost (reacted) singlet oxygen (𝐽1𝑂2, mol·m-2·min-1) was calculated using Equation 13:

𝐽1𝑂2 = 21.5

𝑄𝑃 ∙ (𝑐𝑀𝐵 0 ∙ 𝑅) 𝐴

,

(13)

where 21.5 is the stoichiometric ratio in the full oxidation reaction between 1O2 and MB, QP is -6 the permeate flow rate (10-4 L·min-1), 𝑐𝑀𝐵 0 is the initial molar concentration of MB (3.110 M),

R is the highest achieved removal of MB (80 %). The dissolved oxygen flux (𝐽𝐷𝑂) was obtained using Equation 14:

𝐽𝐷𝑂 =

𝑄𝑃 ∙ [𝑂2] 𝐴 ∙ 𝑀𝑂2

,

(14)


28

Page 29 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where [O2] and MO2 are the measured concentration (8.5·10-3 g·L-1) and molecular mass of dissolved oxygen (32 g·mol-1), respectively. The calculated value for 𝐽𝐷𝑂 = 1.33·10-4 mol m-2 min-1 was 5 times higher than 𝐽1𝑂2= 0.27·10-4 mol·m-2·min-1. Such a difference suggested that depletion of dissolved molecular oxygen (3O2) is unlikely to be a limiting factor hindering 1O2 generation in these experiments. Therefore, it was envisioned that a limited number of photocatalytic centers at the PS surface, where photosensitization of 1O2 occurs, prevents 100% removal of MB under investigated light intensities. Influence of light source Since driving the photocatalytic process with sunlight was one of the motivations of the current paper, a comparison of the gLED and white LED (WLED) emission with terrestrial sunlight was performed. The number of absorbed photons by PdTFPP using gLED excitation (intensity of 21 mW cm-2), WLED (81.8 mW cm-2) and the standard air-mass 1.5 global (AM1.5g, intensity of 100 mW cm-2)6 solar spectrum was calculated via integrating the area under the curve of absorbed photon flux from 300 nm to 800 nm, as illustrated in Figure 9a. The obtained absorbed photon flux of 5.3·1020 photons·s-1·m-2 (0.85 suns) via gLED illumination, 8.9·1020 photons·s1·m-2

(1.43 suns) via WLED illumination and AM1.5g sunlight (absorbed photon flux is 6.2·1020

photons·s-1·m-2) are comparable (Figure 9b). A MB removal of 84 % was achieved using WLED as the excitation source (Figure S9, SI). Thus, it was assumed that solar irradiation could potentially be used to drive the photocatalytic processes of the PdTFPP-PVDF membrane to remove organic pollutants from water.


29


1.6 1.2 0.8

80 60

20

400

500

600

700

(b) 1.6

40

0.4 0.0 300

100

0 800

Abs. photon flux 19 -1 -2 -1 (x10 s m nm )

AM1.5g WLED gLED PdTFPP MB

% of absorbed light

(a)

Photon flux (x1019 s-1 m-2 nm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 48

White LED

Nabs = 8.9x1020 s-1 m-2

AM1.5g

Nabs = 6.2x1020 s-1 m-2

Green LED

Nabs = 5.3x1020 s-1 m-2

1.2 0.8 0.4 0.0 300

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Figure 9 (a) Photon flux of standard air-mass 1.5 global (AM1.5g) solar spectrum, white LED (WLED) and the green LED (gLED, 528±20 nm, 21 mW·cm-2) along with percent of the absorbed light by PdTFPP-PVDF membrane (25 mol·g-1) and MB (1 mg·L-1) solution. (b) Absorbed photon flux of PdTFPP-PVDF membrane with gLED, WLED and AM1.5g sunlight as the light source.

Influence of the flow rate The performance of a photocatalytic process (removal) depends on reaction time. In continuous flow experiments, the reaction time is defined by time of reactant inside the photocatalytic reactor equivalent, namely the residence time. The residence time itself can be controlled via changing the flow rate, porosity or thickness of the membrane.48 The mean residence time of the solute (𝑡) in the membrane for a plug-flow reactor can be calculated using Equation 15:49 𝑉

𝑡=𝑄,

(15)

where V = A·d· is the volume of the reactor (in this case the void volume of membrane) (L); Q is the flow rate (L·min-1), while d and  are the thickness (1.5·10-4 m, see Figure S4c) and


30

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porosity (0.70 was assumed from the value for the similar GVHP membrane)50 of the pristine PVDF membrane . The residence time in photocatalytic experiments was changed via variation of flow rate. Indeed, in Figure 10a, it was demonstrated that the highest MB removal of 7514 % was achieved for the lowest flow rate investigated (10-4 L min-1 or flux 30 L m-2 h-1), corresponding to the longest residence time (0.21 min). In contrast, the highest flow rate of 1 mL min-1 (or flux 300 L m-2 h-1) exhibited the lowest MB removal of 15% (0.02 min). As was mentioned in experimental section, the water fluxes of photocatalytic membrane (30 - 300 L m-2 h-1) are up to hundred times lower for PVDF MF membranes, but correspond to the fluxes of nanofiltration membranes. The feed and permeate temperature as well as feed pressure changes during photocatalytic experiments can be found in Figure S10 (SI). The self-degradation of MB follows the same trend with MB removal of up to 20% and rdis of ~0.510-8 mol m-2 min-1. The rdis is 4-8 times lower than that realized via the PdTFPP-assisted process (210-8 - 410-8 mol m-2 min-1) as seen in Figure 10b.


31


PdTFPP-PVDF PVDF

80 60 40 20 0 0.0

0.2

0.4

0.6

0.8

(b)

Rate of disappearance (x10-8 mol m-2 min-1)

(a) 100

MB removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 48

1.0

4 3 2 1

0.0

-1

Flow rate (mL min )

0.2

0.4

0.6

0.8

1.0

-1

Flow rate (mL min )

Figure 10 Degradation of MB in a single-pass continuous-flow reactor under green LED (528±20 nm) illumination (red symbols). The MB self-degradation (photolysis) experiments were performed using pristine PVDF membrane (blue symbols). Influence of flow rate on degradation kinetics of MB (a) removal and (b) rate of disappearance. To demonstrate that mass transfer of MB in a membrane is very fast and cannot be the ratelimiting factor, the residence time of the solute in the membrane was compared with its molecular diffusion time. The obtained values of the residence time 2.1×10-2 min and 2.1×101

min for flow rates 1 and 0.1 mL min-1, respectively, were compared with a diffusion time of

MB and O2 that is driven in case of the plug flow only by molecular diffusion and can be calculated as a radial diffusion time from Equation 16:51

𝑡𝑑 =

(𝑑𝑝/2)2 𝐷

,

(16)

where dp is the average pore diameter (0.2·10-6 m) and D is the diffusion coefficient in water for oxygen (D = 1.2610-7 m2 min-1)52 and MB (D = 0.4610-7 m2 min-1).53 Since the diffusion time of O2 (0.8×10-7 min) and MB (2.2×10-7 min) from the pore volume to the surface of pore channel calculated via Equation 16 is five to six orders of magnitude less than the residence time in the confined space of the submicrometer membrane pores, the diffusion


32

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limitations were regarded as being negligible (as shown in Figure 11). The diffusion, often the rate-limiting step in batch experiments, is, thus, minimized in the membrane reactor configuration due to the convective mass transfer through microchannel pores. Hence, the kinetics of the process is likely to be governed by the chemical reaction instead, with such parameters as reactant concentrations and temperature playing a more important role.

O2, td = 0.810-7 min MB, td = 2.210-7 min

Pore td

𝒕 0.02 – 0.21 min

𝑡

td Membrane

𝑡

Figure 11. Time scale of diffusion time (td) and residence time (𝒕) of solutes (O2 and MB) inside the PdTFPP-PVDF membrane

To increase the removal at high flow rates (short residence time), concentration of singlet oxygen should be increased via increase of the membrane surface area, irradiation intensity and PdTFPP loading. Meanwhile, the removal is a function of initial concentration that can also be varied.

Effect of MB concentration


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As the removal is a relative measure of concentration change, different amount of MB within the same reaction time (here, residence time) is realized under different initial concentration of reactant.45 An investigation into the effect of MB concentration on the degradation process demonstrates a decrease of removal from 83% at initial concentration of 0.5 mg·L-1 to 20% at 5 mg·L-1, ten times higher concentration (as illustrated in Figure 12a). In turn, the rate of disappearance with different MB concentrations, observed in Figure 12b exhibited a similar behavior to that presented in Figure 10b for the flow rate dependence. Namely, rdis is higher for the solution with higher MB concentration or higher flow rate. It was difficult to draw a conclusion about the degradation efficiency at low MB concentration (< 1 mg L-1), as the concentration was close to the detection limit of the in-line UV/vis spectrophotometer. However, it was assumed that the highest removal was achieved at MB concentration < 1 mg L-1 (a similar effect was discussed in the section “Influence of irradiation intensity” for the light intensity > 21 mW·cm2). High removal at low concentrations could be applied in future for treatment of micropollutants.


34

(a) 100

MB removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PdTFPP-PVDF PVDF

80 60 40 20 0

0

1

2

3

4

Concentration (mg L-1)

5

6

(b)

Rate of disappearance (x108 mol m-2 s-1)

Page 35 of 48

2.5 2.0 1.5 1.0 0.5 0.0

0

1

2

3

4

5

6

Concentration (mg L-1)

Figure 12 Degradation of MB in a single-pass continuous-flow reactor under green LED (528±20 nm) illumination (red symbols). The MB self-degradation (photolysis) experiments were performed using pristine PVDF membrane (blue symbols). Influence of MB concentration on degradation kinetics of MB (a) removal and (b) rate of disappearance.

Assuming that the 1O2 concentration remains constant due to in situ generation, a pseudo-firstorder kinetics was suggested for MB concentration. The slope of the linear fit of Equation 17 yielded the apparent rate constant (𝑘𝑎𝑝𝑝) for the self-degradation and the PdTFPP-PVDF membrane driven degradation of MB (plotted in Figure S11): ― ln

( )=𝑘 𝐶𝑀𝐵 𝑒𝑞 𝐶𝑀𝐵 0

𝑎𝑝𝑝𝑡

,

(17)

where 𝑡 is the mean residence time. The value for 𝑘𝑎𝑝𝑝 = 0.75 min-1 in the MB self-degradation process is around one order of magnitude lower than 𝑘𝑎𝑝𝑝= 6.52 min-1 in case of the PdTFPPPVDF membrane. This observation also indicates the importance of 1O2 generation within the MB degradation via excitation of the PdTFPP. Compared to TiO2-based photocatalytic membranes, the resulted apparent reaction rate is two orders of magnitude higher than those of reported in the literature.54 Such an outcome is likely to originate from the enhanced mass


35


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transfer in the photocatalytic membrane reactors that promises significant advantages of 1O2induced processes and warrant further investigations. 3.4 The extended photocatalytic run experiment The instability of organic photocatalysts is one of potential drawbacks that are typically ascribed to the limiting factor of broader development of organic photocatalyst. The PdTFPPPVDF membrane was tested for a long-term (90 hours) photocatalytic activity (Figure 13). The removal of MB reached R = 60% during the first 20 hours of experiment and then increased slowly over the entire 90 hours period. It should be emphasized that prolonged photocatalytic test was performed in the single pass, continuous regime. The photochemical reactor was continuously fed with fresh solution of MB at each moment of the experiment and the solution after the photodegradation experiment was not reused again. The test conducted in the dark demonstrated no changes of MB concentration throughout the whole experiment. Over this 90 hours performance of the as-prepared PdTFPP-PVDF membrane was received and 540 mL volume treated by an active membrane of 2 cm2. Moreover, PL decays of PdTFPP phosphorescence measured for PdTFPP-PVDF membrane before and after MB degradation (Figure S12, SI) do not display any changes in the PL lifetime that additionally emphasizes photostability of the chosen porphyrin.


36

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1.2 1.0

Dark test Photodegradation

0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

R = 60%

0.4

  

0.2 0.0 0

20

40

60

80

100

Time (h)

Figure 13. Stability of MB (1 mg L-1) photodegradation during 90 hours continuous operation -1

-1

-2

(red). Flow rate: 10-4 L·min ; PdTFPP loading: 25 mol·g ; light intensity: 21 mW cm . Dark test (black) display a baseline drift of spectrophotometer.

3.5.

Irreversible character of MB degradation

In addition to the aforementioned self-degradation of MB, a known issue when using MB dye as the model pollutant is the reversible reduction to leucomethylene blue (LB).55,56 Upon formation of LB, the permanent degradation of MB can be overestimated due to a misinterpretation of changes in the MB absorption spectrum. Confirmation of the permanent degradation of MB was investigated using UV/vis spectroscopy (Figure S13, SI). The peaks at 664 nm and 292 nm diminished greatly compared to the spectrum of the MB feed solution. No specific peak at 256 nm attributed to LB57 could be detected. Thus, it was assumed that no reversible photoreduction of MB to LB occurred. More detailed information about the degradation products was acquired from mass spectroscopy results (see Figure S14, SI). Initially, only peaks related to MB (m/z = 284.1) and its impurity (probably azure B, m/z = 270.1) were found.55 Upon achieving 75% of MB removal, a significant reduction in the MB signal was


37


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observed and numerous residual fragments with molecular weights in the range m/z of 71 – 234 appeared. Though, no full mineralization of MB in our photochemical reactor was demonstrated, this finding confirmed irreversible fragmentation of MB under 1O2-induced photodegradation. The detailed description of degradation pathways of waterborne pollutants will be addressed in detail in future work where photodegradation of actual water pollutants will be examined. 4. Conclusions A novel PdTFPP-PVDF membrane for efficient removal of a model pollutant (MB) in a photochemical membrane reactor operated in a single-pass configuration was fabricated. The membrane is able to generate ROS (singlet oxygen) in situ and under light intensities comparable to the intensity of terrestrial solar radiation. The active role of PdTFPP as PS in the generation of singlet oxygen was assessed via spectroscopic investigations, such as the measurement of the oxygen-induced quenching of PdTFPP phosphorescence and the reaction of singlet oxygen with UA. Model pollutant (MB) removal of up to 83% was achieved at PdTFPP loading of 25 mol·g1,

excitation intensity of 21 mW·cm-2, flow rate of 0.1×10-3 L min-1 and MB concentration of

1 mg·L-1 in the single-pass experiment. Beside of the high removal, the PdTFPP-PVDF membrane demonstrated the stable degradation performance over a 90 hour period. Testing the PdTFPP-PVDF photocatalytic membrane in a photocatalytic membrane reactor ensured an enhanced mass-transport of reactants allowing the measurement of an intrinsic kinetics. The water fluxes applied in photocatalytic experiments were lower than those of the typical PVDF MF membrane, but comparable to the level of nanofiltration membranes. The presented method can potentially offer a new treatment options for water challenges such as removal of micropollutants. In situ removal and degradation would solve water quality and concentrate


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disposal providing no toxic by-products. Despite this initial study demonstrated great potential, further work on this new process is required. ASSOCIATED CONTENT Supporting Information Experimental part including the used chemicals (Figure S1), the evaluation of PS loading, the calculation of theoretical and experimental PdTFPP coverage (Figure S2), the experimental protocol (Table S1), the matrix of experimental parameters and operating parameters (Table S2); schematics of PdTFPP immobilization process (Figure S3); ToF-SIMS mapping images (Figure S4); the relation between TMP and water flux (Figure S5); photographs of as-prepared PdTFPPPVDF membrane, after 6h permeability test and after 14 h exposure to green LED illumination (Figure S6); FTIR spectra of UA powder and PdTFPP-PVDF membrane soaked in UA solution at different pH (Figure S7); photocatalytic performance of PdTFPP-PVDF membrane with different side orientation (Figure S8); photocatalytic removal of MB with WLED illumination (Figure S9); temperature and TMP changes during the photocatalytic experiments (Figure S10); the plot of logarithmic relative concentration over the residence time (Figure S11); PL decays measured before and after degradation experiment (Figure S12); UV/Visible absorbance spectra (Figure S13) and MS spectra (Figure S14) of MB feed and permeate (PDF) AUTHOR INFORMATION Corresponding author *E-mail: [email protected] ACKNOWLEDGMENT


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This work was financially supported by the following funding from the Helmholtz Association: i) the Recruitment Initiative for A.I.S. and B.S.R.; ii) the NanoMembrane strategic initiative of the Science and Technology of Nanosystems (STN) program; iii) the Helmholtz Energy Materials Foundry (HEMF); and iv) the support of the Karlsruhe Nano Micro Facility (KNMF). The Institute for Micro Process Engineering (IMVT, KIT) and Dr. H. Lambach (IMVT, KIT) in person are acknowledged for design and manufacture of the micro crossflow membrane cell. B. Chatillon, C. Onorato and T. Berger are acknowledged for their contributions to the design of micro crossflow system and setting up the LabView software. X. Zhan (IMVT, KIT) is acknowledged for BET measurements. Dr. A. Welle (KIT, IFG) is acknowledged for the ToF-SIMS measurement of the PdTFPP-PVDF membranes. T. Berger and Dr. I.A. Howard (KIT, IMT) are acknowledged for fruitful discussions relating to the results of photocatalytic experiments. Institute of Nanotechnology (INT) at KIT is acknowledged for the provision of lab space. Merck Millipore is thanked for provision of PVDF membrane.

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MerckMillipore

Durapore®

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Filter,

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µm.

Overview.

http://www.merckmillipore.com/DE/en/product/Durapore-Membrane-Filter-0.22m,MM_NFGVHP00010 (accessed 16 November 2018).


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Graphical abstract


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Efficient Photocatalytic Removal of Methylene Blue Using a

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