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Size Modulation of Zirconium-Based Metal Organic Frameworks for Highly Efficient Phosphate Remediation Yue Gu, Donghua Xie, Yue Ma, Wenxiu Qin, Haimin Zhang, Guozhong Wang, Yunxia Zhang, and Huijun Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10024 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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ACS Applied Materials & Interfaces

Size Modulation of Zirconium-Based Metal Organic Frameworks for Highly Efficient Phosphate Remediation Yue Gu,†, ‡ Donghua Xie,†, ‡ Yue Ma,†, ‡ Wenxiu Qin,† Haimin Zhang, ∗, † Guozhong Wang,† Yunxia Zhang∗,† and Huijun Zhao†,§ †

Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui

Key Laboratory of Nanomaterials and Nanotechnology, CAS Centre for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China. ‡

§

University of Science and Technology of China, Hefei 230026, P. R. China

Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia.

ABSTRACT: Eutrophication of water bodies caused by the excessive phosphate discharge has constituted a serious threaten on a global scale. It is imperative to exploit new advanced materials featuring abundant binding sites and high affinity to achieve highly efficient and specific capture of phosphate from polluted waters. Herein, water stable Zr-based metal organic frameworks (MOFs, UiO-66) with rational structural design and size modulation have been successfully synthesized based on a simple solvothermal method for effective phosphate remediation. Impressively, the size of the resulting UiO-66 particles can be effectively adjusted by simply altering reaction time and the amount of acetic acid with the purpose of understanding the crucial effect of structural design on the phosphate capture performance. Representatively, UiO-66 particles with small size demonstrates 415 mg/g of phosphate uptake capacity, outperforming most of the previously reported phosphate 1

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adsorbents. Meanwhile, the developed absorbents can rapidly reduce highly concentrated phosphate to below the permitted level in drinking water within a few minutes. More significantly, the current absorbents display remarkable phosphate sorption selectivity against the common interfering ions, which can be attributed to strong affinity between Zr−OH groups in UiO-66 and phosphate species. Furthermore, the spent UiO-66 particles can be readily regenerated and reused for multiple sorption-desorption cycles without obvious decrease in removal performance, rendering them promising sustainable materials. Hence, the developed UiO-66 adsorbents hold significant prospects for phosphate sequestration to mitigate the increasingly eutrophic problems. KEYWORDS: UiO-66, size modulation, phosphate, adsorption, selectivity

1. INTRODUCTION It is widely recognized that phosphorus (P), as an indispensable nutritive substance, is beneficial to the normal growth of all living beings.1 During the past several decades, large-scale production and undue use of pesticides, detergents and manures have resulted in the release of considerable amounts of wastewater containing phosphate into water environmental systems. The excessive levels of phosphate in water bodies are responsible for accelerating the process of eutrophication with the consequences of undesirable blue algae blooming, water quality worsening, abnormal death of fish/invertebrates and eventually destruction of local ecological balance, which has a negative impact on social economy and can cause latent threat to human health.2 Concurrently, as a nonrenewable and limited resource, the tremendous exhaustion of phosphate makes its recovery and reuse significant from contaminated water. In this sense, efficient removal and recovery of phosphate not only eliminates seriously eutrophic problems associated with public healthcare, water quality

2


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security and environmental remediation, but also prevents the loss of nutrient resources from the viewpoint of social economy sustainable development.3,4 In order to satisfy the increasingly stringent environmental regulations and mitigate the occurrence of eutrophication, various physicochemical technologies, including precipitation method, biological remediation, membranes, ion exchange and coagulation, have been developed for efficient sequestrating of phosphate from polluted waters.5-7 However, the established treatment methods usually suffer from some unavoidable limitations, such as slow kinetics, insufficient capture capacity and poor specificity as well as the production of large amount of solid sludge, which needs the subsequent dispose and thus causes secondary environmental risks. Moreover, it is technically difficult for these water treatment technologies mentioned above to reduce phosphates below the acceptable level (0.5∼1.0 ppm) recommended by the World Health Organization (WHO). Taking inherent limitations of the currently employed techniques into consideration, sorption has been regarded to be an excellent candidate for phosphate removal due to its relative simplicity, high efficiency, fast removal rate and facile regeneration together with comparatively low cost, particularly at trace levels.8-10 In response to great challenges pertinent to phosphate remediation and sustainability, the elaborate construction of effective phosphate adsorbents has become a long-term and persistent research focus. Over the past few decades, porous MOFs, constructed by coordination of metal ions with organic bridging ligands, are regarded to be the leading alternatives for phosphate removal and recovery with significant prospects.11 In contrast to traditional adsorbents available, such as zeolite, activated carbon and metal oxides/hydroxides, MOFs feature more virtues, e.g. high specific surface area, abundant adsorption sites, adjustable pore sizes and versatile architectures, leading to enhanced 3



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binding affinity and uptake capacity.11-13 Furthermore, the strong affinity of unsaturated or saturated metal sites of MOFs towards specific adsorbates lays a solid foundation for the effective uptake performance.14 Despite current progress, the inherent drawbacks of MOFs, such as poor water stability and possible structural damage during sorption treatment, significantly restrict their decontamination applications in aqueous environment systems.15 As an intriguing representative of MOFs, UiO-66, constructed by octahedral Zr6O4(OH)4 clusters and 12-connected terephthalate bridging ligands, have stimulated a wide research enthusiasm for phosphate sequestering in view of additional attractive characteristics, involving low toxicity, natural abundance, environment-friendly nature, excellent water stability and high chemical stability against acids/bases together with highly oxophilic nature of zirconium.16,17 It is well documented that Zr-based adsorbent materials, e.g. ZrO2 nanoparticles and zirconia-functionalized graphite oxide, possess multitudinous merits in phosphate remediation, such as high capturing capacity, fast sorption rate and wide operating pH range thanks to abundant hydroxyl functional groups and preferable sorption affinities of the Zr−OH groups towards phosphates via Lewis acid−base interactions.18,19 In this sense, it is justifiably expected that UiO-66 can demonstrate enhanced uptake capacity and endow significant contributions for phosphate sequestration since each Zr6O4(OH)4 cluster contains six-centred Zr cations and four µ3-O bridges as well as four µ3-OH groups, which can provide abundant Zr−OH groups, rendering them very prospective candidates for the decontamination of phosphates.20 Except for the main chemical compositions or crystalline phase of the resulting adsorbents, another efficient way to achieve excellent capturing performance has been focused on purposeful structural design associated with unique morphology and fine size modulation, in which readily 4


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accessible external and internal surfaces will provide more active sites, while small size is beneficial to fast mass-transfer. Nevertheless, very few explorations are made about the elaborate modulation of microstructure and size of MOFs and thus optimize their physicochemical performances of the final product. In the present work, Zr-based MOFs (UiO-66) with a rational structural design and size modulation have been successfully fabricated based on a facile solvothermal method. The basic morphology and physicochemical properties of the resultant UiO-66 materials are then subjected to systematic characterization. Batch adsorption experiments including sorption kinetics, equilibrium sorption isotherm, initial pH and competing effect of coexisting anion are carried out to evaluate phosphate removal performance of the developed absorbents. Moreover, cyclic adsorption and regeneration experiments are explored using the NaHCO3−HNO3 as the regenerants. Finally, the phosphate removal performance is also conducted in practical eutrophic wastewaters to demonstrate the feasibility of the developed absorbents for practical application. Moreover, the possible mechanism about specific phosphate capture is preliminarily elucidated based on the structural changes before and after phosphate uptake via X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy analyses.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Zirconium chloride (ZrCl4), 1,4-benzenedicarboxylic acid (BDC) and L-antimony potassium tartrate (K(SbO)C4H4O6·0.5H2O) were purchased from Shanghai Aladdin Reagent Co. Ltd.

(China).

N,N-dimethylformamide

(DMF),

ammonium

heptamolybda

tetetrahydrate

((NH4)6Mo7O24·.4H2O), ascorbic acid, acetic acid, nitric acid (HNO3), sodium carbonate anhydrous (Na2CO3), sodium nitrate (NaNO3), sodium sulfate anhydrous (Na2SO4), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), and potassium dihydrogen phosphate 5



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(KH2PO4) were supplied by Sinopharm Chemical Reagent Co. Ltd., China. Deionized (DI) water was obtained through a Barnstead MicroPure (Thermo Fisheer). All reagents were directly used as received. 2.2. Synthesis of UiO-66. UiO-66 samples with different sizes were synthesized using the reported solvothermal route with slight modification by modulating the ratio of acetic acid and reaction time.21 In a typical protocol, 0.45 mmol of ZrCl4 and 0.45 mmol of PTA were added to 30 mL of DMF with magnetic stirring, followed by adding 4.5 mmol of acetic acid as modulator. Then, the resulting suspension was put into a Teflon-lined autoclave (50 mL), sealed and heated at 120 oC for 24 h. After cooling down to room temperature, the resultant precipitate (denoted as UiO-66-1) was washed using DMF for several times and then activated in methanol for three days at 60 °C to remove the trapped DMF molecules. At last, the obtained product was dried under vacuum condition at 60 oC overnight for further use. In an effort to examine the influence of synthetic conditions on the size and morphologies of the resulting samples, increasing amount of acetic acid (72 mmol) with different solvothermal durations (7 or 24 h) were also carried out while keeping all other conditions constant and the corresponding products were designated as UiO-66-2 and UiO-66-3, respectively. 2.3. Batch Sorption Experiments. In order to evaluate adsorption kinetics process, 0.5 g of the adsorbent was immersed to 1000 mL of solution containing 15 mg L-1 of phosphate at a constantly controlled temperature (25 oC). About 1.5 mL of suspension was extracted at various time intervals and filtered using 0.45 µm filter membrane. The remaining concentration of phosphate in the filtrate was quantified using the molybdenum blue spectrophotometric method.22 Taking into account the application in the actual environmental water bodies, the following sorption experiments were carried out under pH 7.0 in the present work. And all sorption tests were performed in triplicate. 6


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To examine the effect of phosphate concentration on the sorption capacities, 10 mg of sample was introduced to 100 mL of phosphate solution with various concentration (3∼300 ppm). The mixtures were then shaken for 24 h to reach the sorption equilibrium, kept at 25 oC and pH 7. The remaining concentration of phosphate was determined using the aforementioned method. The sample UiO-66-2 with moderate adsorption capacity was selected to evaluate adsorption preference of the adsorption process in the subsequent experiments. The pH effect experiments were conducted by dispersing 10 mg of adsorbent to 20 mg/L of phosphate solution (20 mL) in a pH range of 2∼11. After sorption equilibrium, the residual phosphate concentration was examined via the aforementioned colorimetric method. Meantime, the final pH value of the suspension at equilibrium was also measured. As for the competing effect of coexisting ions on phosphate sorption, four typical interfering substances (chloride, sulfate, bicarbonate and nitrate) with two different concentrations (0.01 or 0.1 M) were separately added to 20 mg/L of the phosphate solution, in which the dosage of UiO-66-2 was fixed at 0.5 g/L. In an effort to investigate the recyclability of the fabricated absorbents, the spent absorbent was regenerated with 0.01 M NaHCO3 solution (50 mL) for three times to remove loaded phosphate. Then, the desorbed samples were washed to neutral pH with the help of diluted HNO3 and DI water and dried at 60

o

C. Subsequently, the recovered absorbent was utilized for the next

sorption-desorption recycles. During the reuse experiments, the initial phosphate concentration was fixed at 10 ppm, accompanied by 0.5 g L-1 of the adsorbent dosage. 2.4. Characterization. The crystallinity of the as-obtained products before and after phosphate adsorption was identified by X-ray diffraction (XRD, Philips X’pert PRO) with Cu Kα radiation 7



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(1.5478 Å). The morphology and microstructural analyses were characterized via transmission electron microscope (TEM, JEOL-2010) and field emission scanning electron microscope (FESEM, SU8020). The elemental mappings were obtained based on an energy-dispersive X-ray spectroscopy (EDS). The amounts of N, H, and C were analyzed via an Elementar (Vario EL Cube). Zr content in the resulting products was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, ICP-6300 Thermo Fisher Scientific, USA). N2 adsorption–desorption isotherms were measured using an Autosorb-iQ-Cx analyzer for determining the surface area and pore structure parameters. Thermogravimetric analyses (TGA) were conducted on a Perkin-Elmer apparatus (Pyris 1) from room temperature to 700 oC at a rate of 10 °C·min−1 in flowing air. Thermo Nicolet NEXUS FI-IR spectrophotometer was used to FT-IR analysis. Zeta potential was analyzed via a Zetasizer3000HSa Malvern analyzer. XPS spectra before and after loading phosphate were obtained from Thermo Scientific ESCALAB 250, while all of the binding energies were calibrated using C1s peak (284.8 eV). UV-vis absorption spectra were recorded on Shimadzu UV2700 spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structural Characterization. UiO-66 particles with various sizes are prepared via a facile solvothermal method. In our case, it is found that acetic acid has a great impact on the crystallite size of UiO-66, which can compete with linkers (BDC) for the coordination to zirconium and thus regulate the crystal growth of UiO-66. Typically, the addition of 4.5 mmol of acetic acid resulted in irregular, slightly aggregated and smaller intergrown nanocrystals with a size of 50∼70 nm, as evidence by SEM image in Figure 1a. Interestingly, when upgrading the amount of acetic acid up to 72 mmol and shortening solvothermal treatment time to 7 h, the crystal size of the as-obtained UiO-66-2 increases and individual particles with edge lengths around 400∼500 nm can 8


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be clearly observed, as revealed by SEM image in Figure 1b. Upon further prolonging the reaction time to 24 h and keeping the amount of acetic acid unchangeable (72 mmol), the resultant UiO-66-3 product becomes completely isolated and evolves into highly monodisperse octahedral crystals with increasing size (around 1.0∼1.3 µm), as revealed by SEM image in Figure 1c. In addition, TEM images (Figure 1d-f) clearly show that the particle size of UiO-66-1 to 3 increases from dozens of nanometers to micrometer scale, confirming the particle size is highly dependent on the adding amounts of acetic acid and treatment time in solvothermal reaction. The representative TEM image of UiO-66-3 and corresponding EDS element mappings (Figure 1g-j) reveal the existence of zirconium (red zone), carbon (blue zone) and oxygen (green zone), which are uniformly distributed throughout octahedral crystals. Furthermore, elemental analysis of the resulting UiO-66-3 particles is carried out and the weight percentages of C, H and N elements are 29.30%, 3.06%, 0.25%, respectively, in which N is originated from DMF, indicating some solvent molecules are incorporated into the resulting UiO-66 framework. Besides, the exact weight ratio of Zr is determined to be 36.7% based on the ICP-OES measurement. It is noteworthy that higher Zr and lower C content in the as-synthesized UiO-66-3 with acetic acid modulator is consistent with the decreased amount of BDC linkers in comparison with the perfect UiO-66 (Zr6O4(OH)4(BDC)6).11 The crystallinity of the synthesized three UiO-66 materials is also identified by XRD characterizations. From Figure 2, all three products exhibit a well-defined crystal structure, consistent with the simulation pattern of UiO-66, in which very sharp diffraction lines indicate high crystallinity of the resulting UiO-66 materials. Simultaneously, the decreasing widths of the reflections from UiO-66-1 to 3 indicate the formation of larger crystals (the inset in Figure 2), in good accordance with SEM and TEM observations. These results mentioned above fully reveal that UiO-66 samples have successfully 9



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been fabricated. Based on the above results, the increasing amounts of acetic acid can improve the crystallinity of the fabricating UiO-66 crystals and result in the formation of larger monodisperse microcrystals of UiO-66 with uniform shape and size, similar to the literature report by Andreas Schaate et al.23 These findings further highlight the significant effect of acetic acid upon size modulation of UiO-66. The textural properties of the synthesized UiO-66 are examined by nitrogen sorption technique to characterize the porosity of the obtained materials and evaluate their surface area. As displayed in Figure 3, all the three MOFs exhibit similar typical type-I behaviors, in agreement with the previous observation for UiO-66 structures.24 BET surface areas for UiO-66-1 to 3 are calculated to be 754, 863 and 1604 m2/g, respectively. Meanwhile, the pore size and total pore volume also increase in a similar way, the corresponding pore diameter being 14.10, 14.74 and 15.42 Å, along with the pore volume of 0.420, 0.464, and 0.514 cm3/g, respectively. Strikingly, UiO-66-3 with larger particle size (>1µm), obtained by the incorporation of higher modulator concentrations, possesses higher pore volume and surface area in comparison with smaller UiO-66-1 nanocrystals, suggesting the structural “imperfection” for the former caused by the missing-linker defects. That is, the incorporation of more acetic acid modulator results in increasing surface area and pore volume of UiO-66. The findings is in good consistent with the former reports, in which surface area and pore volume of UiO-66 can be effectively tuned by altering the amount of acetic acid and reaction time.23-25 The structural “imperfection” mentioned above, i.e. the part deficiency of bridging ligands linking inorganic units, is further revealed based on the difference of TGA curves. As illustrated in Figure 4, UiO-66-1, 2 and 3 have similar two stage thermal behaviors, in which the weight loss before 320 oC is ascribed to the removal of physically adsorbed H2O, coordinated DMF and entrapped acetic 10


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acid;26-28 whereas a sharp weight decline around 500 oC corresponds to the disappearance of organic bridging ligands (BDC) inside the UiO-66 framework, accompanied by the subsequent formation of ZrO2.21 Strikingly, there exists an obvious weight loss difference among the obtained UiO-66 samples in the temperature range of 320∼550 oC, in which UiO-66 is deemed to be fully dehydrated and desolvated to Zr6O6(BDC)n and thus can be utilized to estimate the amounts of missing linkers (the linker occupancy) presented in the UiO-66 framework.13 Considering the fact that the molecular weight of the perfect Zr6O6(BDC)6 is a factor of 2.2 higher than the residual mass 6ZrO2, if the ordinate axis is normalized to 100% for the solid residue, the plateau should ideally reach 220% (marked by the horizontal orange dashed line).29 It is worth noting that in the combustion fractions of organic linker among all the three samples display much lower weight loss than the theoretical value, indicative of the existence of defects associated with missing-linkers in the resultant UiO-66 samples obtained from the acetate acid modulator-aided synthesis. Besides, the mass loss of linkers for UiO-66-1 to 3 is 44.62%, 41.95% and 37.11%, respectively. That is, UiO-66-3 has the most defects (with half of the organic ligands missing), followed by UiO-66-2 and UiO-66-1 (missing 4 carboxylate per node or 2 BDC linker per formula unit). These results are in agreement well with the aforementioned N2 sorption and morphology analysis, i.e., the more the missing-linkers, the higher the surface area and the larger the crystal size, consistent with that reported in the literature.13, 28 3.2. Sorption Kinetics. A comprehensive understanding of the phosphate sorption kinetics processes on various absorbents is crucial to the potential practical application. In order to evaluate the sorption rate of the obtained three UiO-66 absorbents with different size towards phosphate, time-dependent sorption experiments are carried out and the removal efficiencies of phosphate are recorded at different intervals with 15 mg/L of phosphate and 0.5 g/L of the adsorbent dosage. As 11



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displayed in Figure 5a, the phosphate removal rates on all the absorbents are extremely fast especially in the initial 10 min and then reach sorption equilibrium within around 60 min, which is beneficial for practical application. By contrast, the reported other absorbents, such as lanthanum functionalized aerosol and lanthanum-doped mesoporous SiO2 hollow spheres, need longer exposure time (over 10 h) to achieve the maximum adsorption.30,31 It is noteworthy that UiO-66-3 demonstrates fastest phosphate capture rate under the identical conditions and achieves over 98% of removal efficiency in a very short period (5 min). More importantly, the remaining concentration of phosphate on UiO-66-3 at equilibrium is reduced to 0.03 ppm, far below the acceptable levels of 0.5 ppm recommended by WHO for drinking water, accomplishing the complete phosphate removal from high concentration solution (15 ppm). These results underscore the superiority of UiO-66 absorbents as promising candidates for removing quickly phosphate from polluted waters. To gain more information regarding the sorption experiment, kinetics curves were fitted with pseudo second order model. Based on high coefficient of correlation (R2 > 0.9999, shown in Figure 5b) of the linear relationship between t/qt versus t, phosphate adsorption onto three UiO-66 absorbents are mainly governed by chemisorption process. Similar sorption behaviors were reported on various phosphate adsorbents.32-34 Additionally, the adsorption rate constants (k2) are calculated to be 0.2096, 0.0644, 0.0151 g·mg-1·min-1 for UiO-66-1 to 3, respectively. Obviously, the rate constant value of UiO-66-1 is significantly higher than that of the others. That is, the rate constant of phosphate sorption increased with decreasing the particle size, suggesting that fine size are preferable for the fast diffusion of phosphate to adsorption sites and highlighting the significance of size effect on the resulting phosphate sorption performance. 3.3. Sorption Isotherm. With the aim of exploring the sorption mode and examining the 12


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maximum sorption capacities of phosphate onto three MOFs, the sorption isotherm studies are performed at equilibrium conditions by exposing 0.5 g/L of MOF into various concentrations (5∼300 ppm) of phosphate solution under pH 7.0. As illustrated in Figure 6, phosphate adsorption capacities on three adsorbents increase with increasing phosphate concentrations. Besides, both Freundlich and Langmuir models are also used to fit adsorption isotherm data and corresponding results are given in Table S1. Obviously, Langmuir model exhibits better fitting for phosphate sorption than Freundlich model in view of comparatively higher correlation coefficients (R2 >0.99) values for the former, revealing that the monolayer adsorption mode of phosphate and the homogeneous distribution of the sorption sites on the obtained absorbents. And the saturated adsorption capacities of phosphate on UiO-66-1 to 3 are calculated from the Langmuir model to be 415, 326 and 286 mg/g, respectively; whereas most prevalent phosphate adsorbents summarized in Table 1 can seldom achieve over 100 mg/g of uptake capacities even at optimal pH. It follows that the obtained UiO-66 absorbents in this study are very effective for phosphate decontamination from entropic waters. In addition, the uptake capacities increase substantially with decreasing UiO-66 size, underscoring the importance of size effect in accomplishing high efficient phosphate uptake, consistent with the foregoing results in the adsorption kinetics. Considering possible secondary pollution from the leakage of ultrafine particles into the treated solution, UiO-66-2 with moderate size is selected as the representative for further investigation of phosphate sorption performance. 3.4. Influence of Initial pH on Phosphate Sorption. Considering the fact that solution pH signiﬁcantly affects both surface charges of adsorbents and various species of phosphate anions, batches of sorption experiments are conducted by dispersing 10 mg adsorbent to 20 mg·L-1 of phosphate solution (20 mL) under various pH values (2.0 ∼ 11) to identify the optimal pH of 13



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phosphate uptake. As demonstrated in Figure 7, there is a strong pH dependent on the removal efficiency of phosphate on UiO-66-2, in which the uptake capacity slightly increases with increasing initial pH from 2 to 8; whereas dramatic decline of removal efficiency is observed at higher initial pH (from 8 to 11). Similar phenomenon was also founded in the previously reported absorbents.40-44 It is well known that pH (strictly speaking, final pH) influences the species of phosphate. When the pH is below 2.13, H3PO4 is predominant; under pH 2.13∼7.20, the existence form of phosphate is mainly as HPO42- and H2PO4-; whereas HPO42- and PO43- are the main phosphate species between pH 7.2 and 12.3; meantime, PO43- prevails at pH > 12.4.36 Additionally, the surface charge of the obtained UiO-66 material is also pH-dependent. From the relationship between zeta potentials and pH (the inset in Figure 7), at pH < 2.13, neural H3PO4 cannot be easily captured by the protonated adsorbent and results in unfavorable sorption performances; at acidic or neutral environments, below the zero potential charge (pHzpc=6.4), the surface of UiO-66-2 adsorbent displays highly positive charges, facilitating the uptake of negatively charged HPO42-/H2PO4- species via electrostatic attraction. Upon increasing solution pH to alkaline range, UiO-66-2 is subjected to deprotonation and exhibits negatively charged surface. The strong coulomb repulsive interaction can decrease the phosphate adsorption on UiO-66-2 and the adverse effect increases with increasing pH. Accordingly, there is an obvious decrease about phosphate uptake capacity, especially at higher pH, associated with more negative surface charges on adsorbents. Besides, excessive OH− might vie for the binding sites with phosphate ions, in turn leading to the sharp reduction of phosphate removal efficiency. To better understanding the phosphate sorption behaviors, variation of the final solution pH against initial pH is also investigated after reaching adsorption equilibrium. As seen from Figure 7, the final pH in the suspension is lower than that before sorption, indicative of ligand-exchange mechanism 14


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during the phosphate adsorption process. Based the above analyses, the favorite solution pH for phosphate adsorption on UiO-66-2 is determined at near neutral region. Taken into account the potential application in the actual environmental water, all sorption experiments are conducted under pH 7.0. 3.5. Influence of Coexisting Anion on Phosphate Adsorption. From the standpoint of practical applications, superior adsorbents should possess good specific sorption towards the target substance to resist the interference from the coexisting species in complex real water samples. As we know, the conventional anions such as NO3-, Cl-, SO42- and HCO3- are ubiquitous in natural water and industrial wastewater, which might interfere with the phosphate uptake through competing for the identical sorption sites on the adsorbing materials.45 Hence, it is highly desirable to examine the adsorption preference of the concerned adsorbent toward target phosphate from the viewpoint of practical application. Interference experiments are performed by monitoring variations of phosphate uptake capacities in the presence of the coexisting anions mentioned above at two different concentrations (0.01 and 0.1 M), where the concentration of phosphate is 0.20 mM and the dosage of UiO-66-2 is 0.1 g/L. It is noteworthy that the concentration of interfering ions in the present work is far more than that of phosphate (50-fold as 0.01 M and 500-fold as 0.1 M). As displayed in Figure 8, the addition of interferents (NO3-, Cl- and SO42-) does not produce any significant alteration in removal rate of phosphate, despite quite high concentration, indicative of strong anti-interference ability of UiO-66 adsorbents and preferable affinity for phosphate over these coexisting anions. Similar phenomena were also founded on other phosphate adsorbents,32,33,35,36,46 in which strong inner-sphere complexing between PO43- and UiO-66-2 may contribute to the negligible interference from these coexisting anions. Comparatively, the presence of HCO3- causes a significant decline in the 15



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phosphate uptake efficiency, especially at higher concentration, due to the competitive action between phosphate and carbonate for the identical sorption sites on given adsorbents. The behavior can be attributed to higher pH (8.9) in 0.1 M of HCO3- solution, in which a large amount of OH- may compete with phosphate for the sorption sites and inhibits the occurrence of ligand exchange, as reported in the literature.9 More attractively, UiO-66-2 can still retain high efficiency over 67% even if in the presence of carbonate with high concentration (0.1 M), revealing great potentials of the resultant absorbents in phosphate sequestration. 3.6. Stability and Recyclability of UiO-66. Except for fast adsorption kinetics, high adsorption capacity and preferable affinity for the target contaminant ions, a promising adsorbent should possess excellent structural stability, which is the prerequisite for meeting the future sustainable application. To evaluate the stability of UiO-66, the samples saturated with phosphate are collected and subjected to XRD characterization. As displayed in Figure S1a, the perfect matching with the pristine counterpart indicates the integrity of crystalline structure of UiO-66 samples. Meanwhile, the original octahedral framework morphology of UiO-66 remains intact after phosphate treatment based on SEM observation (Figure S1b). In addition, the water stability of the designed absorbent is further confirmed by immersing them in aqueous solution for several days, without the occurrence of apparent structural collapse or alteration. The invariable chemical composition and surface morphology after phosphate adsorption guarantee the long-term stability and subsequent recyclability. Efficient regeneration and successive adsorption-desorption cycles are conducted to investigate the recyclability of the fabricated UiO-66 materials towards phosphate (Figure 9), in which 0.5 g/L of adsorbent is exposed to 10 mg/L of phosphate solution. Taking into consideration the moderate 16


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interference of bicarbonate on sorption capacity of phosphate, 0.01 M NaHCO3 solution (50 mL) is utilized as the desorbent accompanied by the multiple elution treatments for the recovery of phosphate-loaded UiO-66 materials. It should be noted that dilute HNO3 is employed to regulate solution pH before next recycle to ensure the same reaction condition. Interestingly, the regenerated absorbent can still maintain almost 100% of phosphate removal efficiency even after five sorption-regeneration cycles, indicating its great potential in phosphate sequestering as a sustainable adsorbent. The satisfactory reusability together with excellent chemical stability makes the proposed UiO-66 materials promising alternatives for phosphate removal. 3.7. Phosphate Removal from Real Lake Water. To test the practical treatment ability of the resulting UiO-66 absorbents towards phosphate in real eutrophic waters, lake water samples are collected from Nanfei River (Hefei city, China) and all the characteristics associated with the concentrations of phosphate and coexisting species as well as pH are given in Table S2. In a typical adsorption treatment, the three kinds of UiO-66 absorbents are separately added to the sampled water filtered using a 0.45 µm membrane at three various dosages (0.01, 0.05, 0.1 g/L). As displayed in Figure 10, there exists significant discrepancy about the phosphate uptake among different absorbents. Meanwhile, the phosphate removal efficiencies are strongly associated with the absorbent contents. The phosphate removal efficiencies with 0.01 g/L of UiO-66-1 to 3 are 77.4%, 72.4% and 66.1%, respectively. Upon increasing the dosage of absorbents, the removal efficiencies towards phosphates increase accordingly. Remarkably, when the dosage rises up to 0.1 g/L, all the three adsorbents can thoroughly remove phosphate from the treated water, indicating their vast prospects for mitigating the thorny eutrophic problems. It is noteworthy that UiO-66-1 exhibits the most efficient phorphate uptake performance and can still effectively reduce phosphate concentration 17



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of the treated solution to below the phosphate discharge requirement recommended by WHO even if under lower dosage (0.01 g/L). These results reveal that the developed UiO-66 materials can be utilized as effective adsorbents to remove phosphate from wastewater. 3.8. Sorption Mechanism of Phosphate on UiO-66. To elucidate the possible sorption mechanism, FT-IR spectra of UiO-66-2 before and after PO43- uptake are recorded (Figure 11a). For the pristine UiO-66 samples, the strong peak at 3400 cm-1 corresponds to hydroxyl vibration from physically adsorbed H2O molecules. Two characteristic bands around 1577 and 1389 cm-1 are assigned to O-C=O symmetric and asymmetric stretching vibration of organic ligands; while the small band around 1506 cm-1 are ascribed to C=C vibrational mode of the benzene ring, reflecting the existence of benzene-carboxylates associated with BDC linkers.47 Additionally, two typical bands around 746 and 667 cm−1 are assigned to C-H vibrations of aromatic organic compounds. Moreover, the band near 552 cm-1 represents the Zr-(OC) asymmetric stretching vibration.47 These main peaks are retained after phosphate sorption, indicating the structure integrity of the as-fabricated UiO-66. By contrast, several sharp bands in the region from 900 to 1200 cm-1, originated from -OH binding vibrations involved in the formation of Zr6-octahedron, become greatly weakened after phosphate adsorption. Meanwhile, the broad and intense vibration bands at 1107 and 1016 cm-1 are observed for the phosphate-treated UiO-66-2 sample due to the formation of P=O and P–OH bonds, suggesting the plausible complexation between Zr-OH groups in UiO-66 and phosphate via the Zr-O-P coordination bonds.20,48 Further evidence about the preferable phosphate adsorption by UiO-66-2 is available from the XPS spectra before and after loading phosphate. As compared to the primitive UiO-66-2, after phosphate uptake, the emergence of P 2p peak verifies the successful loading of phosphate onto 18


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UiO-66-2 (Figure 11b). It is noteworthy that there exists ~ 0.2 eV of negative shift in comparison with the basic spectrum of KH2PO4 (ca. 134.0 eV), indicating the formation of strong chemical bonding between phosphates and UiO-66 absorbents. Furthermore, Zr 3d spectrum is displayed in Figure 11c, in which two peaks at 183.5 and 181.3 eV correspond to Zr 3d3/2 and Zr 3d5/2 of the primitive UiO-66-2, respectively; whereas after phosphate sorption, the resulting Zr 3d peaks are shifted slightly to higher binding energy (ca. 0.8 eV), indicating strong affinity of Zr–O linkages in UiO-66 for phosphate groups.20,48,49 It is noteworthy that atomic ratio of Zr/C from XPS data decreases from 1 : 9.83 to 1 : 6.85 after phosphate loading, suggesting that phosphate is remediated at the expense of carboxylic groups in organic bridging ligands. In the meantime, O 1s XPS spectrum can be deconvoluted into four peaks (Figure 11d), which can be assigned to O in Zr−O−Zr (530.3 eV), P=O and Zr−O−P (531.5 eV), P−O−H (532.2 eV), and O−C=O (533.2 eV),20,48 revealing preferable affinity of Zr–O clusters toward phosphate via strong complexing effect, in accordance with the former FT-IR observations.48 On the basis of FT-IR and XPS analyses mentioned above, the possible sorption mechanism may be preliminarily deduced as follows. First, large amount of the inherent bridging µ3−OH in Zr6O4(OH)4 clusters is responsible for the phosphate uptake,49 serving as the first plausible sorption sites. Second, hydroxyl groups originated from missing-linker induced terminal ones also plays a key role in efficient phosphate sequestration via the formation of Zr-O-P complex.20 In view of the phenomenon that UiO-66-1 sample with the least defect sites and acetic acid modulator demonstrate the best phosphate sorption performance, the binding of the imperfect defect sites with phosphate is not dominated. It is assumed that acetic acid, although participating in the framework formation, is unable to produce massively extra sorption sites.13,24 Besides, we cannot neglect the possibility that 19



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some phosphate species are captured at the expense of carboxylic groups in BDC linkers,48 which can be verified via the TGA comparison before and after phosphate. As displayed in Figure S2, the TGA plateau of UiO-66-2 after phosphate adsorption is lower than that of the primitive counterpart, supporting the fact that part of BDC linkers are replaced by the loading phosphate. Considering the best sorption performance of UiO-66-1, it is possible that the exchange of BDC linkers with phosphate and the interaction between phosphate and the bridging Zr−OH of Zr6O4(OH)4 clusters play predominant roles in efficient phosphate removal compared with the abundant adsorption sites from large specific surface area. That is, it is reasonable that UiO-66-1 with the smallest size and the lowest specific surface area as well as the largest number of BDC linkers possesses the best removal performance of phosphate in terms of fastest kinetics and highest sorption capacities.

4. CONCLUSIONS In summary, Zr-based MOFs with tunable sizes have been successfully fabricated for efficient phosphate removal from entropic water. It turns out that Zr-based MOFs with small size exhibit enhanced phosphate sorption performance as for fast uptake kinetics and high capturing capacity. Furthermore, the current absorbents demonstrate excellent phosphate capturing efficiency in complicated real water samples, suggesting their feasibility for practical applications. In view of outstanding characteristics associated with superb uptake capacity, fast kinetics, excellent regeneration/recyclability and high chemical stability as well as easy fabrication with tailor-made sizes, the developed Zr-based MOFs materials will be expected to be the compelling candidate for phosphate decontamination from entropic waters.

ASSOCIATED CONTENTS Supporting Information. 20


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Fitting results of Freundlich and Langmuir models; XRD pattern and SEM of UiO-66-2 after phosphate adsorption; Details of water collected from Nanfei River; TGA curves of UiO-66-2 before and after phosphate adsorption.

AUTHOR INFORMATION Corresponding Authors ∗Email: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This study was financially supported by National Natural Science Foundation of China (Grant No. 51572263,

51772299,

51472246),

Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09030200 ), National Basic Research Program of China (Grant No. 2013CB934302), Science and Technology Major Project of Anhui Province (15czz04125).

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Figure 1. SEM and TEM images of UiO-66-1 (a, d), UiO-66-2 (b, e) and UiO-66-3 (c, f); representative TEM of UiO-66-3 and corresponding element mappings (g-j).

Figure 2. XRD patterns of the simulated UiO-66 and the as-prepared samples. 28


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Figure 3. N2 adsorption-desorption isotherms of the obtained UiO-66 samples (a) and the corresponding pore size distributions (b).

Figure 4. TGA curves of the as-synthesized UiO-66 samples recorded under air flow, in which the ordinate axis was normalized to 100 for the solid residual at high temperature, corresponding to ZrO2 (the horizontal green dashed line).

29



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Figure 5. Phosphate adsorption kinetics (a) and the corresponding pseudo-second-order kinetic ﬁtting curves (b) on the obtained UiO-66 absorbents. Experimental conditions: 15 mg L-1 of initial phosphate concentration, 0.5 g⋅L-1 of sorbent dosage and pH 7.

Figure 6. Sorption isotherms of phosphate on three different UiO-66 absorbents. Experimental conditions: 5~300 mg⋅L-1 of initial phosphate concentration, 0.5 g⋅L-1 of absorbent, 24 h of equilibrium time at 25 ◦C and initial solution pH 7.0. 30


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Table 1. Comparison of the phosphate adsorption capacities on different adsorbent materials from the reported literatures Adsorption capacity Adsorbents

pH

Reference

11.95

-

[33]

99.01

6.2

[16]

112.23

6.2

[34]

146.8

5

[29]

48.3

6.8

[35]

48.97

7.0

[36]

180.80

7.18

[37]

76.19

-

[38]

56.72

-

[32]

UiO-66-1

286.25

7.0

This work

UiO-66-2

326.40

7.0

This work

UiO-66-3

415.02

7.0

This work

(mg ·g-1)

Fe3O4@NH2-MIL-101(Fe) Amorphous zirconium oxide nanoparticles Ce0.8Zr0.2O2 Lanthanum-doped ordered Mesoporous hollow silica spheres Nanostructured Fe-Al-Mn trimetal oxide Superparamagnetic ZrO2@Fe3O4 Fe-Ti bimetal oxides on a Sulfonated polymer La-porous carbon composites Mg2Al-layered double hydroxide nanosheets

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Figure 7. Effect of initial pH on the phosphate removal rate (▲); dependent relationship between initial and final pH (●); zeta potential under different pH (the inset).

Figure 8. Effect of coexisting ions on the phosphate removal rate.

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Figure 9. Removal rate of phosphate on UiO-66-2 under different regeneration cycles.

Figure 10. Uptake capacities of phosphate on different UiO-66 absorbents at various dosages.

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Figure 11. (a) FT-IR spectra of UiO-66-2 before and after phosphate adsorption; (b-d) XPS analysis of UiO-66-2 before and after phosphate adsorption: (b) P 2p, (c) Zr 3d, (d) O 1s.

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Table of Contents Graphic

35


Size Modulation of Zirconium-Based Metal Organic ... - ACS Publications

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