Article Cite This: Langmuir 2019, 35, 8699−8708
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Titanium Dioxide−Layered Double Hydroxide Composite Material for Adsorption−Photocatalysis of Water Pollutants Min-Jeong Suh,†,‡ Yi Shen,†,‡ Candace K. Chan,†,§ and Jae-Hong Kim*,†,‡
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Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT) 6100 Main Street, MS 6398, Houston, Texas 77005, United States ‡ Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Avenue, New Haven, Connecticut 06511, United States § Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 501 E Tyler Mall, ECG 301, Tempe, Arizona 85287, United States S Supporting Information *
ABSTRACT: Although adsorption has gained favor among numerous water treatment technologies as an effective pollutant removal method, its application is often hindered by challenges with its resource- and energy-intensive regeneration procedure once the available adsorption sites are exhausted. Herein, we present adsorption−photocatalysis composite materials combining layered double hydroxides (LDHs) and titanium dioxide (TiO2) for water treatment. Incorporation of the photocatalyst into the material opens opportunities to harness light from the sun or lamps for oxidative degradation of the adsorbed contaminants on the material surface, to free adsorption sites for material reuse. In addition to allowing photocatalytic regeneration, the addition of TiO2 to colloidal suspensions of delaminated LDH enabled the formation of TiO2− LDH composites with far superior adsorptive performances compared to their parent LDH compounds. During the material synthesis, positively charged LDH layers and negatively charged TiO2 particles combine through electrostatic attraction to yield composites with dramatically enhanced adsorption capacities toward model contaminants, methyl orange and 2,4-dichlorophenoxyacetic acid, by 16.0 and 76.7 times, respectively. Combining delaminated LDH with TiO2 allowed us to maximize the exposure of positively charged surfaces to the contaminants, in a form that can be used as a solid adsorbent. After regeneration, the material regained up to 92% of its adsorption efficiency toward model contaminants. In light of our findings showing significantly different kinetics of adsorption and photocatalytic regeneration, we propose a new scheme to utilize adsorption−photocatalysis systems, in which the two processes are separated to better utilize their unique strengths.
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INTRODUCTION Adsorption is a widely used water treatment process that has been proven, both in the lab and in the field, to effectively remove a broad range of organic pollutants.1−5 Although the process is well established in large scale applications including municipal water treatment,6 uses in smaller scales for point-ofuse or point-of-entry water treatment are often challenged by the difficulty of adsorbent regeneration, which is commonly achieved via high temperature treatment or acid addition, or costly replacement of adsorbent after disposal.7 Consequently, strategies to combine adsorption with other treatment methods to regenerate the adsorbents concurrently with adsorption have been explored. Combining adsorption with photocatalysis in particular has gained considerable interest, as it provides means for oxidative degradation, possibly mineralization, of pollutants without addition of any harsh chemicals, but by harnessing light from lamps or even the sun.8,9 © 2019 American Chemical Society
This goal has been pursued by engineering composite materials to gain dual functionality, that is, adsorption and photocatalysis. The majority of past studies attempted to combine carbon-based adsorbent (e.g., predominantly activated carbon,10 but also other advanced materials such as multiwalled carbon nanotube11) and benchmark photocatalysts, such as TiO 2, for contaminant removal by simultaneous adsorption and photocatalysis. In addition to the low contaminant removal capacities, these composite materials have demonstrated (e.g., 12.1 mg/g for phenanthrene removal by activated carbon supported titania nanotubes10), a major concern in using carbon-based materials is gradual oxidation of the adsorbent materials12 and consequential reduction in their adsorption capacities. Light absorption and Received: February 22, 2019 Revised: May 8, 2019 Published: June 21, 2019 8699
DOI: 10.1021/acs.langmuir.9b00539 Langmuir 2019, 35, 8699−8708
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Figure 1. Schematic illustration of materials synthesized and experimental steps for synthesis of the composite materials. Figures of the materials are not to scale for clarity. The positive charge results from partial isomorphic substitution of divalent cations in a brucite lattice by trivalent cations. LDHs are represented by the general formula [M(1−x)IIMxIII(OH)2]Ax/nn−·mH2O, where MII and MIII are di- and trivalent cation in the brucite-like layers and An− may be any organic or inorganic anions.1
adsorption−photocatalysis treatment of two select pollutants: MO as a representative organic anionic azo dye25 and 2,4-D, an anionic herbicide and a representative endocrine disrputor.26,27 We explore two different approaches in TiO2LDH composite material architecture (Figure 1). The first composite material consists of TiO2 particles attached to the surface of LDH materials, wherein the original layered structure of LDH is preserved (TiO2/CO3-LDH). The second composite material is prepared by delaminating LDH to yield single, individual layers, and compositing with TiO2 particles, such that the composite material consists of randomly stacked layers of LDH along with TiO2 dispersed across the surface of these layers (F-TiO2/NO3-LDH). Comparing the structure of the synthesized TiO2/LDH materials and their adsorptive and photocatalytic performances provides insights into potential strategies for optimum design of LDH-based adsorption− photocatalysis composite materials. We further discuss the large disparity in time scales of adsorption versus photocatalysis, and propose a new way to better exploit the dual functionality in realistic scenarios.
scattering by the adsorbent material present additional challenges that have not yet been well characterized. Considering such limitations presented by the choice in adsorbent, we expect that a careful selection of the base adsorbent with transparency in the wavelength window of incident irradiation, high resistance to oxidation by reactive oxygen species (ROS), high adsorption capacity, and fast adsorption kinetics would help realize the proposed approach. Layered double hydroxides (LDHs) are appealing adsorbent materials for adsorption/photocatalysis systems. They represent a class of lamellar clays that consist of positively charged brucite-like layers and hydrated exchangeable anions in the interlayer spaces that compensate for the charge.1 The large surface area, high anion-exchange capacities, and flexibility in interlayer spaces of LDHs result in efficient uptakes of anionic contaminants via adsorption and ion-exchange,13,14 especially for those that are inorganic, such as arsenate, chromate, and iodate.15,16 Although efficient removal of certain anionic organic molecules, including methyl orange (MO, qe = 148 mg/gNO3‑LDH),13,17 2,4 dichlorophenoxyacetic acid (2,4-D, qe = 395 mg/gMgZnAl‑LDH),18 and 2-methyl-4-chlorophenoxyacetic acid (MCPA, qe = 243 mg/gNO3‑LDH),19 has been demonstrated using sophisticated LDH materials, adsorption of organic anions has been in general more challenging due to the strong hydrophilicity of the brucite-like layers and aggregation of LDH crystals.20 Strategies taken to overcome such difficulties include incorporation of surfactants in the LDH interlayer space,21 as well as formation of ordered LDH structures22 and composite materials23 from colloidal suspensions of LDH nanocrystals. Composite materials containing TiO2 have also been synthesized for enhanced photocatalytic reduction of carbon dioxide,24 taking advantage of LDH materials’ chemical robustness against ROS attack and transparency in the wavelength of light that photocatalysts utilize; however, few studies have yet attempted to utilize TiO2-LDH composite materials for adsorption−photocatalysis of organic pollutants in water. We herein present a composite material combining magnesium aluminum−LDH (MgAl-LDH) and TiO2 for
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EXPERIMENTAL SECTION
Materials. All reagents for material synthesis and analysis were purchased from Sigma-Aldrich. Degussa P25, a mixture of anatase and rutile (8:2) TiO2 with a bandgap of 3.1−3.3 eV,8,28 was selected in this study for its known performance and popularity as a standard photocatalyst. Synthesis of MgAlCO3-LDH and TiO2/CO3-LDH. Hydrothermal synthesis of CO3-LDH (i.e., LDH with CO32− as the intercalated anion) was achieved using a typical urea hydrolysis method adapted from previous literature.29,30 Briefly, Mg(NO3)2·6H2O (1.20 g), Al(NO3)3·9H2O (0.88 g), and urea (1.31 g; the amount that results in monodisperse LDH crystals29) were dissolved in DI water to form a clear solution with a total volume of 70 mL, then placed in a 100 mL Teflon inner vessel with a stainless steel outer vessel. To synthesize TiO2/CO3-LDH, 0.09 g of TiO2 was dispersed in the precursor solution by ultrasonication to yield a suspension. The vessels were placed in an oven heated at 90, 120, 140, and 170 °C, and the mixture was allowed to react under airtight conditions. After 24 h, the vessels were removed and cooled to room temperature. As-synthesized 8700
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Langmuir and Freundlich models were used to fit the adsorption isotherms (see SI Text S1 for details). Regeneration Experiments. Spent composite material (i.e., the material that has reached its maximum adsorption capacity) was prepared by subjecting 0.2 g of composite material to 200 mL of 5 or 50 mg/L MO solution. After reaching equilibrium, the material was collected by centrifugation, and washed with DI water to remove any excess MO. For regeneration, 15 mg of spent composite material was dispersed in 15 mL of DI water using ultrasonication in a quartz beaker and illuminated by UVA lamp (intensity 16 mW/cm2) or an ABET industries solar simulator. The solar simulator generates light using a short arc xenon lamp directed through an AM1.5G filter, and the distance between the sample and the light source was aligned to correspond to 1 sun intensity at 100 mW/cm2, as measured with a PMA2140 global radiometer. This was also conducted with the spent composite material dispersed in 100 mM H2O2 instead of DI water, to test regeneration with an increased ROS production. After the fixed amount of exposure under stirring, the suspension was removed from the light source, and 15 mL of 1 M hydrochloric acid was added to the aqueous suspension. The acid desorption method was adopted to evaluate regeneration kinetics, through gaining quantitative estimates of the amount of MO that remains on the adsorbent surface after a fixed time of photocatalytic regeneration. When acid is added, protonated MO is released from the composite material surface into the solution. The pH of the acidified solution was also sufficiently low for LDH dissolution, which also contributed in releasing all MO that remains adsorbed on the material surface after regeneration. The concentration of the supernatant solution was analyzed to quantify the relative amount of regeneration after illumination (see UV−vis spectra of MO and protonated MO in SI Figure S1a).
material was washed twice by repeated centrifugation at 6000 rpm for 40 min and then dried at 80 °C. Decarbonation and Delamination of MgAlCO3-LDH. The deintercalation of carbonate ions (i.e., removal of carbonate ions from LDH interlayer space) from the as-prepared CO3-LDH was carried out using the salt-acid method, as previously reported.31,32 Typically, 0.5 g of the LDH sample was dispersed into 500 mL of an aqueous solution containing 1 M NaNO3 and 3.3 mM HNO3. The vessel was sealed after purging with nitrogen gas, and then was stirred for 48 h at ambient temperature. The product, MgAlNO3-LDH (NO3-LDH), was isolated and washed three times by centrifugation at 10 000 rpm for 10 min. Delamination of NO3-LDH was conducted using a method modified from those previously reported.32 NO3-LDH (0.6 g) was mixed with 300 mL of formamide, which was tightly capped after purging with nitrogen gas. Then, the mixture was sonicated for 22 h, to yield a colloidal suspension. Surface Fluorination of TiO2. To better ensure robust binding with the LDH layers during the formation of the composite material, surface of TiO2 was fluorinated by simple ligand exchange between surface hydroxyl groups on TiO2 and fluoride anions.33−35 Sodium fluoride (10 g) was added to an aqueous TiO2 suspensions (0.1 g/L), and the pH was adjusted to 3.5 using HNO3. The resulting suspension was stirred at 500 rpm for 30 min. The particles were washed once by centrifugation at 2000 rpm for 10 min, then dried in air at 100 °C to obtain fluorinated TiO2 (F-TiO2). Synthesis of F-TiO2/NO3-LDH Composite Material. F-TiO2 (0.18 g) was dispersed in 90 mL of formamide by ultrasonication to yield a suspension. The resulting suspension was added to 300 mL of colloidal suspension of delaminated NO3-LDH in formamide, and stirred for 24 h at room temperature. The suspension was centrifuged at 2000 rpm for 15 min. The supernatant was discarded, and the sediment was washed three times by centrifugation at 6000 rpm for 45 min, then dried in air at 100 °C. Material Characterizations. The shape and structure of LDH and composite materials were characterized using a Hitachi SU-70 scanning electron microscope (SEM) and an FEI Tecnai Osiris 200 kV tunneling electron microscope (TEM). High-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) was used for STEM images with elemental mapping. The absorption spectra of MO solutions were determined by measuring absorption at λmax = 462 nm (Supporting Information (SI) Figure S1b) using a Varian Cary 50 Bio UV−visible spectrophotometer. The 2,4-D concentration was analyzed using a high-pressure liquid chromatography (HPLC) with an Eclipse XDB-C18 column and a photodiode array detector (Agilent Technologies 1260 Infinity HPLC). Zeta potential was determined using dynamic light scattering (DLS) with a NanoBrook omni particle sizer and phase-alternative light scattering (PALS) with a zeta potential analyzer, respectively. X-ray diffraction (XRD) was performed using Rigaku SmartLab X-ray diffractometer, to evaluate the change in structure after the composite material synthesis. X-ray fluorescence (XRF) was performed using the pressed powder method using the wavelength dispersive X-ray fluorescence (WDXRF) system with high resolution (20 eV) and minimal spectral overlaps. The Brunauer−Emmett−Teller (BET) nitrogen-specific surface areas were estimated by nitrogen adsorption−desorption at −196 °C with a Micrometrics ASAP 2460 surface area and porosity analyzer. Adsorption Experiments. Batch adsorption experiments of MO and 2,4-D were performed in 100 mL glass bottles at 25 ± 1 °C. The sorption kinetics studies were conducted at solid-to-water ratio of 0.1 mg per 100 mL, using initial MO and 2,4-D concentrations of 100 mg/L and 10 mg/L respectively. The remaining concentrations in a series of independent samples were measured from 5 to 3600 s. Isotherm experiments were performed using a batch equilibration method. To reach equilibrium, the bottles were placed on magnetic stirrer plates and stirred in the dark at 500 rpm for 24 h. The initial concentrations were controlled to obtain equilibrium concentrations from 1 to 1000 mg/L for MO, and 1 to 100 mg/L for 2,4-D. The samples were filtered using 0.45 μm syringe filters and the concentrations of MO and 2,4-D in the supernatants were analyzed.
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RESULTS AND DISCUSSION CO3-LDH Composite with TiO2. We first optimized the synthesis conditions for CO3-LDH, an LDH material that is environmentally benign, robust against ROS, and does not absorb light in the same wavelength range as TiO2 (SI Figure S1). CO3-LDH also serves as the starting material for the later synthesis of NO3-LDH and its composite with TiO2. We found that changing the synthesis temperature from 90 to 120, 140, and 170 °C significantly impacted the uniformity and morphology of produced LDH, while the yield and crystal size did not change significantly as the reaction time was increased beyond 24 h.29 The effect of reaction temperature on the size of synthesized LDH was previously explored,36,37 but its impact on LDH morphology and adsorption efficiency has not been well-defined. LDHs synthesized below 170 °C exhibited hexagonal structure typical of MgAl-LDH (Figures 2a−c),36 but the LDH synthesized at 170 °C (Figure 2d) displayed ring-like and spiral crystals. The proposed LDH crystal formation follows a circular path from the central aluminum hydroxide seed, during which the growth at the center is retarded due to the presence of the seed.29 The imperfect morphology of LDH synthesized at 170 °C may therefore suggest that the crystal growth occurs too rapidly at the elevated temperature. LDH crystals synthesized at 140 °C have particle size of 2.9 ± 0.7 μm, whereas those synthesized at 90, 120, and 170 °C have particle sizes of 2.1 ± 1.0 μm, 2.5 ± 1.0 μm, and 2.6 ± 1.3 μm, respectively (Figures 2a−d), as determined by analyzing TEM images, averaged over 15 individual particle measurements. Considering more clearly defined hexagonal structures with greater size uniformity at 140 °C, all CO3-LDH used hereafter were synthesized at this temperature. XRD analysis confirmed the successful synthesis of CO3LDH (Figure 3). Strong diffraction peaks, assigned to basal planes (003), (006), (110), and (113) of MgAl-LDH, were 8701
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Figure 2. Characterization of synthesized materials: TEM images of CO3-LDH synthesized at (a) 90 °C, (b) 120 °C, (c) 140 °C, (d) 170 °C; (e) TEM image, and (f) EDX mapped STEM image of TiO2/CO3-LDH.
of Ti using EDX in STEM images of TiO2/CO3-LDH revealed that Ti, and consequently TiO2, distributed across the base CO3-LDH material (Figure 2f); most of them appeared in aggregates. XRD analysis also confirmed the presence of TiO2 (SI Figure S2). Reflections at 25.3°, 27.4°, and 48.0° are attributed to anatase (101), rutile (110), and anatase (200) peaks for TiO2, respectively,39 and the ratio between anatase and rutile peak intensities is consistent with the anatase−rutile composition of P25. However, characteristic XRD peaks for LDH (SI Figure S2) as well as apparent hexagonal crystal structures (Figure 2e,f) did not change during TiO2 addition. This suggests that TiO2 particles attached primarily to the outer surface of CO3-LDH instead of occupying interlayer spaces and modifying the LDH layered structure. Even when TiO2 is added during the formation of LDH, the strong affinity of CO32− toward the MgAl−OH layers, binding the layers together through a dense network of hydrogen bonds,31,40 is likely to leave little room for TiO2 to interfere with the layer formation process. NO3-LDH Composite with TiO2. The second type of TiO2-LDH composite was prepared via the delamination of LDH, followed by reconstitution with TiO2, with the goal of maximizing access to surfaces of the LDH layers. We initially sought to delaminate the above-synthesized CO3-LDH directly, but XRD and TEM analysis of the product from the attempted exfoliation of CO3-LDH revealed incomplete delamination, despite severe fracture of crystals (SI Figure S3). The incomplete exfoliation is attributed to the strong, dense network of hydrogen bonds binding the LDH layers through the interlayer CO32− ions,31,40 the reason similar to why the layered structure of TiO2/CO3-LDH was undisturbed throughout TiO2 addition. Hence, a complete decarbonation (i.e., removal of CO32− from interlayer spaces) would be necessary to achieve complete delamination.41
similar to those observed in previous literature for MgAlLDH.32,38 Diffraction peaks at 11.7o (003) and 23.5o (006) result from diffraction by parallel basal planes, indicative of an ordered, layered structure.36 These basal reflections occurred at d spacings of 0.754 and 0.378 nm (003 and 006 reflections, respectively), confirming the presence of CO32− ions in the interlayer spaces.38 The (110) and (113) reflection peaks were attributed to the atom arrangement within the MgAl−OH layers.38 We then synthesized the TiO2-LDH composite material via a one-pot hydrothermal synthesis (Figures 2e,f) by simply adding TiO2 to the CO3-LDH precursor solution: the resulting material is referred to as TiO2/CO3-LDH. Elemental mapping
Figure 3. XRD patterns of CO3-LDH, NO3-LDH, and F-TiO2/NO3LDH. 8702
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Langmuir Therefore, we replaced interlayer CO32− with anions of weaker affinity toward the MgAl−OH layers, NO3−, prior to delamination. The resulting NO3-LDH (Figure 4a) exhibited
transverse sliding force is applied on the swollen phase, and the layers are forced to fall apart resulting in exfoliation. Combining TiO2 with delaminated NO3-LDH was achieved by adding TiO2 to the suspension of delaminated LDH in formamide. This reconstitution of the LDH structure around TiO2 particles was primarily driven by electrostatic interactions between positively charged LDH layers and negatively charged TiO2. To ensure a more robust binding between these materials, we modified TiO2 using surface fluorination to yield F-TiO2, which increased both the magnitude of the negative surface charge, and the pH range for which the surface charge was below zero, without significant structural and morphological changes (Figure 5a,b and SI Figure S5). F-
Figure 4. TEM images of (a) NO3-LDH and (b) delaminated NO3LDH.
almost the same morphology and size as those of CO3-LDH. This substitution was performed at pH 6.5−7.0 so that CO32− in the interlayer space would be protonated to form HCO3− which sufficiently weakened the interlayer hydrogen bonding network.31 Under lower pH values, acid corrosion (i.e., LDH crystal dissolution) has been observed due to the susceptibility of the MgAl−OH layers to proton attack.42 An attempt to alternatively use only salt (NaNO3) did not lead to facile substitution. XRD analysis confirmed the ordered, layered structure of the LDH, and proved the successful structural change from CO32− to NO3− intercalation (Figure 3). The (003) and (006) peaks for CO3-LDH shifted to yield a new series of intense basal reflections at lower 2θ angles, suggesting a complete removal of CO32− ions from the interlayer spaces and the formation of NO3-LDH. The basal spacing of LDH increased from 0.754 to 0.892 nm, further confirming the incorporation of NO3− ions, in agreement with previous literature.32 The (110) and (113) reflection peaks remain unchanged, consistent with the expected unchanged chemical composition and fixation of the MgAl−OH layers despite the replacement of the intercalated anions.38 TEM and XRD analyses corroborated to confirm that we had achieved complete deintercalation of CO32− without significant damage to the MgAl−OH layers from acid corrosion. Successful delamination of NO3-LDH in formamide to yield individual layers was confirmed using TEM analysis (Figure 4b). TEM images of delaminated NO3-LDH portrayed thin, morphologically irregular sheets with a very faint, but even contrast that stems from their ultrathin nature and uniform thickness, as previously reported.32 The sheets are dimensionally diminished (particle size 0.65 ± 0.13 μm, as determined by analyzing TEM images, averaged over 15 individual particle measurements) in comparison with the parent LDH crystals, reflecting the substantial fracture of sheets during the exfoliation process. This is anticipated from the proposed pathway by which delamination occurs, through rapid swelling and subsequent slow exfoliation (SI Figure S4).43 During the suggested mechanism, a large influx of formamide into the interlayer space results in the disruption of the hydrogen bonding network with water, and yields a swollen phase in which the layers are held together by hydrogen bonding between the carbonyl group of formamide and MgAl− hydroxide layers. Upon mechanical shaking or ultrasonication,
Figure 5. (a) TEM image of fluorinated P25 (F-TiO2); (b) PALS measurements of zeta-potential of TiO2 and F-TiO2; EDX mapped STEM images of F-TiO2/NO3-LDH, showing (c) Ti in blue, and (d) Mg in red, Al in green, and Ti in blue.
TiO2/NO3-LDH exhibited drastic structural changes, in contrast to TiO2/CO3-LDH synthesized in the one-pot hydrothermal synthesis. In the STEM images of F-TiO2/ NO3-LDH (Figure 5c,d), the morphologically irregular nature of delaminated NO3-LDH is apparent, and hexagonal crystals of NO3-LDH can no longer be seen. This verifies that the nanosheets remain delaminated throughout the composite material synthesis, instead of recombining to revert back to the ordered, layered LDH structure, which has been shown to possibly occur previously.44 Elemental mapping show an even distribution of Mg and Al across the nanosheets, validating that the nanosheets are derived from MgAl-LDH, despite the morphological and structural differences from their parent crystals. XRD patterns of F-TiO2/NO3-LDH further confirmed the complete separation of the LDH layers and successful combination with F-TiO2 (Figure 3). The loss of the sharp NO3-LDH basal peaks in the XRD pattern of F-TiO2/NO3LDH is apparent, which corresponds to a complete delamination of NO3-LDH. The LDH (110) reflection in the F-TiO2/NO3-LDH spectrum at 60.8° indicates that the two-dimensional crystalline order of LDH layers are still preserved, showing that the layers remain intact despite the breakdown of the layered structure. Elemental mapping of Ti, 8703
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Langmuir XRD, and XRF (SI Table S1) confirmed the presence of Ti, and correspondingly TiO2, in the synthesized F-TiO2/NO3LDH. Compared to TiO2/CO3-LDH, aggregation of TiO2 particles appeared less severe in F-TiO2/NO3-LDH: surface fluorination imparting a more negative surface charge on TiO2 most likely enhanced the distribution of the photocatalyst in FTiO2/NO3-LDH. Adsorption Isotherm and Kinetics. The adsorption isotherm and saturated adsorption capacity (Q0) of LDH and TiO2-LDH composite materials for the adsorption of select organic pollutants are listed in SI Table S2. The isotherms were in general better fitted by the Langmuir than by the Freundlich model, indicating that the adsorption of pollutants to all LDH-based materials tends toward monolayer rather than multilayer. This is likely due to the dominant role of electrostatic interaction between positively charged LDH surface and negatively charged pollutants (e.g., pKa = 3.5 for MO and 2.7 for 2,4-D), which becomes less effective beyond monolayer adsorption. Consistently, the adsorption of positively charged pollutant such as methylene blue was minimal (SI Table S2). Nevertheless, the adsorption capacities of unmodified LDHs synthesized under various reaction conditions (e.g., CO3-LDH synthesized at different temperatures) overall showed very low adsorption capacities, demonstrating their limited potential for adsorption of even strongly acidic molecules.22,23 According to XRD analysis, prior to delamination, the interlayer spacing for CO3-LDH and NO3-LDH is 0.27 and 0.41 nm, respectively (the respective d003 spacings minus the host layer thickness, 0.48 nm),45 which is too small compared to the size of the target contaminants; for instance, MO molecules are estimated to be 1.5 nm by 0.55 nm.17 This resulted in adsorption primarily to the outer surface of undelaminated LDH, underutilizing its large surface area that stems from its layered structure. It is noteworthy that NO3-LDH showed slightly increased adsorption capacity toward both contaminants (SI Table S2). This may be due to interlayer adsorption of contaminants, made possible through the weakened interlayer forces through anion substitution.41 As NO3− has a weaker affinity toward MgAl−OH layers, previous work has shown that, using highly concentrated MO solutions, contaminants are able to access the interlayer spaces, giving rise to a new set of (003) and (006) peaks with corresponding enlarged d spacings.17 XRD spectrum of NO3-LDH postadsorption showed that NO3-LDH formed CO3-LDH after adsorption (SI Figure S6), revealing that once the contaminant replaces NO3− ions in the interlayer space, CO32− ions present in solution from dissolved carbon dioxide enters the interlayer space to form CO3-LDH. Consequently, a strategy to enlarge the interlayer spacing by intercalation of NO3− would not be a viable option to enhance the adsorption capacity of LDH, due to the rather irreversible formation of CO3-LDH. On the contrary, dramatic enhancement of the adsorption capacity through LDH delamination and reconstitution was observed, as shown in Figure 6 for MO and 2,4-D adsorption by F-TiO2/NO3-LDH compared to that by CO3-LDH (Figure 6 inset). This enhancement can be attributed to the exposure of previously unused sites within LDH, thereby maximizing the number of adsorption sites the contaminants can access. The BET specific surface area of F-TiO2/NO3-LDH was measured to be 108 m2/g, which was significantly higher than 62.4 and 67.1 m2/g of CO3-LDH and NO3-LDH, respectively. It is noteworthy that the BET specific surface area of TiO2/CO3-
Figure 6. Adsorption isotherms of (a) MO, and (b) 2,4-D onto FTiO2/NO3-LDH and CO3-LDH (inset). Batch adsorption experiments of MO and 2,4-D were performed at 25 ± 1 °C, in the pH range 6.5−8.0; pH was observed to vary with the contaminant concentration.
LDH was as low as 63.5 m2/g, which suggests that compositing with TiO2 alone without modifying the LDH structure did not result in surface area augmentation. A significant increase in the adsorption kinetics, nearly 6.6 times for MO, was also observed for F-TiO2/NO3-LDH relative to CO3-LDH (Figure 7 and SI Figure S7). The regression kinetic parameters are presented in SI Table S3 and adsorption kinetics data for CO3-LDH in SI Figure S7. Adsorption of both contaminants onto F-TiO2/NO3-LDH was extremely fast, with most of adsorption occurring within the first 10 min and equilibrium being reached within 30 min. For both MO and 2,4-D, the adsorption kinetics curves are better fitted by the pseudo second-order model, which implies that the adsorption mechanisms of both contaminants are likely the same. Adsorption kinetics was faster for MO than for 2,4-D, although it is important to recognize that the experiments were performed at a higher pollutant concentration for MO. Since adsorption rate is proportional to the concentration gradient between the solution and adsorbent surface, the faster kinetics observed with MO is consistent with the increase in solution concentration used from 10 mg/L (2,4-D) to 100 mg/L (MO). The significantly enhanced kinetics with F-TiO2/NO3LDH also corroborate well with the above premise that LDH delamination allows the LDH layer surface to be exposed and readily accessible to adsorbates in the bulk phase. Regeneration. The goal of combining adsorbent materials with photocatalysts such as TiO2, in both previous reports and 8704
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Figure 8. Regeneration kinetics of F-TiO2/NO3-LDH under the solar simulator, and UVA light with and without H2O2 addition. Regeneration was conducted at 25 ± 1 °C, in the absence of H2O2 unless otherwise stated. Spent composite material used was made using 5 or 50 mg/L solutions, as specified. C is the concentration of the supernatant recovered after acid addition to a sample after a fixed amount of regeneration, and C0 is the concentration of the supernatant recovered after acid addition to unregenerated, spent composite material.
even slower under simulated sunlight. When F-TiO2/NO3LDH was saturated with 5 mg/L of MO, regeneration was much faster, with over 60% regeneration achieved within 24 h of illumination. However, the significantly slower kinetics of regeneration (destruction of adsorbed MO by photocatalysis), relative to that of adsorption, emphasizes the distinct disparities between the kinetics of adsorption and photocatalysis, and questions the long-pursued idea of utilizing simultaneous adsorption and photocatalysis for contaminant degradation. This is also evident when past reports are examined. For instance, the degree of 2,4-D mineralization using photocatalysis has been gauged by analysis of the change in total organic content (TOC) throughout the degradation process. Using a 1 mM 2,4-D solution, 94% decrease in TOC was observed after 4 h of irradiation using TiO2, and 95% decrease was observed upon 5 h of irradiation using Pt/TiO2.46 It is noteworthy that in this research, 2,4-D degradation is conducted under optimized conditions, with TiO2 particles suspended and well-mixed within a solution of 2,4-D. In our proposed material, the photocatalysis kinetics is anticipated to be slower, as the contaminant is adsorbed onto the material, and its degradation is likely to be limited by various factors, including, but not restricted to, the diffusion of ROS to come in contact with the contaminant and decreased light absorption of TiO2 due to the presence of adsorbed contaminants blocking the light. With adsorption equilibrium attained within 30 min in our system, the disparity between the kinetics of the two processes is evident, and highlights the need to develop an alternative strategy to take advantage of the strengths of both adsorption and photocatalysis, instead of utilizing them simultaneously. We therefore propose that photocatalytic regeneration is performed separately and independently from adsorption. The adsorption process benefits from fast kinetics, which allows large volumes of water to be treated within shorter exposure times, enabling the development of a flow-through process (e.g., suspension or packed-bed of adsorbents). Once the adsorbents are spent and adsorption sites are exhausted, the adsorbents can be exposed to either UVA lamps or sunlight for a prolonged period, as long as the time to reach adsorption
Figure 7. Adsorption kinetics of (a) MO, and (b) 2,4-D onto FTiO2/NO3-LDH. Error bars are standard deviation values obtained from experimental duplicates. The sorption kinetics studies were conducted at 25 ± 1 °C, using solid-to-water ratio of 0.1 mg per 100 mL, with initial MO and 2,4-D concentrations of 100 mg/L and 10 mg/L respectively.
this study, has been to achieve destruction of pollutants at the same time as adsorption, such that a separate adsorbent regeneration step is obviated. We tested the photocatalytic destruction of adsorbed MO under UVA-emitting lamp and solar simulator irradiation, and the results are shown in Figure 8. Percentage regenerated here is defined as the percentage of the MO that had been adsorbed onto F-TiO2/NO3-LDH (spent material), that has been removed from the material surface through photocatalytic degradation. The aim of using the acid desorption method was to gain an estimate of the percentage of adsorption sites that became available, after a fixed time of material regeneration through photocatalytic degradation of MO that is adsorbed on the spent material. Spent material was prepared by exposing F-TiO2/NO3-LDH to 5 and 50 mg/L MO solutions until equilibrium, to investigate the effect of the amount of MO present on the material surface on the regeneration kinetics. Upon UV−vis analysis of the supernatant solution after the preparation of the spent composite material, 0.97 mg and 9 mg of MO was estimated to be adsorbed on 0.2 g of composite material, for samples made using 5 and 50 mg/L MO solutions, respectively. The regeneration kinetics (Figure 8) were orders of magnitude slower than the adsorption kinetics (Figure 7). When F-TiO2/NO3-LDH was pre-equilibriated with 50 mg/L of MO, approximately 40% regeneration was achieved under 72 h illumination using the UVA reactor, and regeneration was 8705
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Figure 9. Conceptual diagram of the proposed adsorption−photocatalysis composite material system. Contaminated water passes through a packed bed of composite material, and contaminant is removed through fast adsorption. Once the material reaches its maximum adsorption capacity, it is removed from the reactor and placed under the sunlight for slower, photocatalytic regeneration.
those of carbon-based adsorbents.50,52 Achieving higher adsorption capacity, presumably through rendering more positive charges on the LDH surface and better strategies to expose all the surface areas of each layer, while retaining the fast adsorption kinetics and the capability to anchor photocatalysts, remains as a priority research challenge. Despite the improved yet still low adsorption capacity and long regeneration times in the present study, we believe there exist ample opportunities for further optimization of both adsorbent and photocatalyst materials. The results of this study clearly suggests that adsorption and photocatalysis should be separately operated, considering the large disparity in time scale. When the two processes are isolated, optimization of each material can be also pursued in a separate manner; for example, increased adsorption efficiencies could be achieved through a selection of a base adsorbent material with a higher adsorption capacity, in addition to mechanical and chemical robustness, especially toward ROS. An enhanced loading and distribution of photocatalyst across the composite material will likely increase the regeneration efficiency of the material. A careful material design to optimize both adsorption and regeneration performances has the potential to provide a novel material with large treatment capacities that eliminates resource- and energy-intensive regeneration methods by harnessing sunlight.
breakthrough is long enough for photocatalytic regeneration, such that the flow-through adsorption can be performed in a continuous fashion. Separating the photocatalytic process from adsorption will not only allow an easier operation of the system by eliminating the need for a continuous light source, but also enable utilization of sunlight as a light source, since the slow kinetics would not necessarily limit the overall water treatment efficiency. This idea of separating adsorption from photocatalytic regeneration, to be demonstrated in future study, is schematically illustrated in Figure 9. The reusability of the synthesized material is crucial for the proposed scheme, and when the material was tested for its readsorption efficiency after a regeneration cycle, it was observed to regain up to 92% of its initial adsorption efficiency (SI Figure S8). In the proposed scheme, photocatalytic regeneration could be expedited by augmenting •OH generation through hydrogen peroxide (H2O2) addition. The enhanced activity and production of •OH with the addition of H2O2 in the presence of a photocatalyst has been well-documented.47−49 H2O2 is an attractive alternative to acid addition for adsorbent regeneration, as it eliminates the need for large amounts of highly acidic waste that would need to be safely disposed postregeneration. With the addition of H2O2 at 100 mM, we were able to achieve over 90% regeneration within 24 h under UVA irradiation (Figure 8).
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CONCLUSIONS Compared with past reports on LDH, the adsorption rate of FTiO2/NO3-LDH was found to be substantially higher, by 1 to 2 orders of magnitude. For example, the pseudo-first order rate of MO on hydrothermally synthesized MgAl-LDH with 18.7 mg/L MO solution was 8.03 × 10−4 s−1.13 Similarly, the results stand strong when compared with MO adsorption of carbonbased adsorbents; for instance, the pseudo-first order and pseudo-second order rates of activated carbon derived from Phragmites australis using 25 mg/L MO solutions were 7.02 × 10−2 min−1 and 1.60 × 10−5 g/mg·min, respectively,50 and the pseudo-first order rate of multiwalled carbon nanotubes was 0.0523 min−1 using 20 mg/L MO solution.51 Such pronounced improvement in adsorption kinetics from previous research lays great potential for the application of these adsorption− photocatalysis composite material. Low adsorption capacity, however, remains as a drawback, especially when compared to
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00539. Additional information as noted in the text including three tables and eight figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Candace K. Chan: 0000-0003-4329-4865 Jae-Hong Kim: 0000-0003-2224-3516 Notes
The authors declare no competing financial interest. 8706
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ACKNOWLEDGMENTS This work was partly supported by the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500). Support for M.-J.S. was provided by Hyosung Global Scholars program.
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