Research Article pubs.acs.org/journal/ascecg
Selective Adsorption and Efficient Removal of Phosphate from Aqueous Medium with Graphene−Lanthanum Composite Mingli Chen, Chunbao Huo, Yikun Li, and Jianhua Wang* Research Center for Analytical Sciences, College of Sciences, Northeastern University, Box 332, Shenyang 110819, China S Supporting Information *
ABSTRACT: A three-dimensional adsorbent, i.e., lanthanum oxide decorated graphene composite (3D graphene−La2O3 composite), is prepared. The composite exhibits favorable adsorption performance to phosphate, providing a sorption capacity of 82.6 mg g−1 at pH 6.2. The adsorption behavior for phosphate fits the Langmuir model, and the adsorption kinetics fit a pseudo-second-order model, with rate constants of 0.1847 and 0.007 969 g mg−1 min−1 at phosphate concentrations of 35 and 142 mg L−1, respectively. For the removal of 25 mg L−1 phosphate in 1.0 mL aqueous medium, the commonly encountered anionic species in waters, e.g., Cl−, SO42−, and NO3−, pose no interfering effect at 8000 mg L−1, providing a favorable removal efficiency of 100% by 2.0 mg of composite. When 142 mg L−1 phosphate solution is treated, 100% and >80% adsorption efficiencies are achieved respectively in the presence of 1000 and 8000 mg L−1 of Cl−, SO42−, and NO3−. The high tolerance capacity against coexisting anionic species by the graphene−La2O3 composite makes it suitable for water cleanup by selective and fast adsorption/removal of phosphate. KEYWORDS: Lanthanum oxide, Graphene composite, Phosphate, Adsorption, Removal
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hollow silica spheres,15 lanthanum-modified clay,16 zeolite/ lanthanum hydroxide,10 metal incorporated with lanthanum composites,17 La(III)-modified bentonite,13,18 and lanthanum hydroxides.19 The incorporation of lanthanum improves both the retention capacity and the adsorption kinetics.13 As a biocompatible material, graphene and its derivatives are emerging as promising sorption medium due to their ultrahigh specific surface area20 and abundant binding sites.21 Graphene or graphene oxide (GO) have been recognized as attractive media for efficient adsorption/separation of various species, e.g., graphene oxide for the removal tetramethlammonium hydroxide22 and magnetic citric-acid-functionalized graphene oxide for the removal of methylene blue.23 However, the nonspecific interactions between graphene/GO nanosheets and the adsorbate result in poor selectivity to the adsorption process. Therefore, extensive attentions have been directed to the development of high selective graphene based nanosorbents toward specific species of interest. Among which metal or metal oxide incorporated graphene have been proved to be attractive for the improvement of adsorption selectivity. Nickel cations and Ni nanoparticles decorated graphene were demonstrated to be effective for the selective isolation of specific protein species.24,25 Iron oxide−graphene composite exhibited favorable adsorption performance toward arsenic and was used for arsenic removal,26 whereas a water-dispersible magnetic
INTRODUCTION The presence of high concentration of phosphate anionic species, e.g., PO43−, HPO42−, and H2PO4−, in water bodies is attributed to various sources, including agricultural pollution, municipal/industrial wastewater disposal,1,2 and natural solubilization of rocks.3 The huge amount of wastewater discharge with high level of phosphate not only results in eutrophication for surface water resources but also deteriorates the natural ecosystem and aquatic environment.4,5 Therefore, the effective elimination/removal of phosphate from waters with high level of nutrients is an urgent issue for the purpose of protecting the natural water system. During the last years, a lot of efforts have been directed to the exploitation of various adsorbents for fast removal of phosphate. Such sorption media include natural red mud,6 ash,7 iron oxide,8 and metal based/doped materials.5,9−11 It is known that rhabdophane (LaPO4) is insoluble in aqueous medium,12−14 it is thus imaginable that lanthanum or lanthanum oxide based materials should facilitate the adsorption/removal of phosphate based on the formation of LaPO4. However, the dilemma is that when large amount of lanthanum is introduced to form lanthanum−phosphate precipitate, the excessive free lanthanum cations would cause further problems in the water system. In this respect, the development of materials incorporating lanthanum or lanthanum oxide should be a useful approach for overcoming the above issue, and meanwhile to maintain high efficiency for the removal of phosphate. Some lanthanum incorporating materials were reported for this purpose, including lanthanum-doped ordered mesoporous © XXXX American Chemical Society
Received: October 16, 2015 Revised: December 18, 2015
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DOI: 10.1021/acssuschemeng.5b01324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. SEM images of the 3D graphene−La2O3 composite (A) and 3D graphene−La2O3 composite after the adsorption of 142 mg L−1 phosphate (B). solution. The reaction mixture was stirred at room temperature for 12 h. After centrifugation at 10000 rpm, the product was dispersed in 15 mL of DI water and sealed into a 50 mL reactor with a PTFE inner coating. The graphene oxide was hydrothermally treated and reduced to graphene at 180 °C for 12 h.29 The final product of 3D graphene− La2O3 composite was obtained by freeze-drying. Instrumentation. UV−vis spectra were recorded on a U-3900 UV−vis spectrophotometer (Hitachi, Japan) for phosphate quantification. pH measurement was performed with an Orion Model 868 pH meter (Thermo Electron, USA). A Sartorius BP 211D balance with a precision of ±0.01 mg (Sartorius AG, Germany) was used for weighing. Surface charge analysis was investigated by measuring ζpotential with a Nano Zetasizer ZS/ZEN3690 (Malvern, England). A S-3400 scanning electronic microscopy (SEM, Hitachi High Technologies, Japan) instrument was used for surface imaging and elemental analysis. X-ray diffraction (XRD) patterns were recorded on a X’Pert Pro MPD diffractometer (PW3040/60, PANalytical BV, Holland) with Cu Kα irradiation (40 kV, 35 mA) in the range of 2θ from 10° to 60°. Phosphate Adsorption/Removal by 3D Graphene−La2O3 Composite. For the investigation of phosphate adsorption/removal, 2 mg of the 3D graphene−La2O3 composite were used for the treatment of phosphate solution (1 mL, pH 6.2) at various concentration levels in a 2 mL centrifugal tube. The mixture was shaken in a vortex generator for 5 min to facilitate the adsorption of phosphate. For adsorption kinetics study, 2 mg of each dose of the adsorbent was suspended in 1 mL of phosphate solution (35 or 142 mg L−1, at pH 6.2) in a centrifugal tube. The centrifugal tube was sealed and shaken in a vortex generator for various time intervals. The adsorbent was finally separated by filtering through a PES membrane filter. After separation of the solid phase, the residue phosphate remained in the supernatant was determined by spectrophotometry based on the measurement of molybdenum blue31 at 820 nm with a detection limit of 8.8 μg L−1.
nanoparticle−graphene oxide composite was effective in the removal of selenium.27 In the present work, we investigate the lanthanum oxide decorated 3D graphene composite serves as adsorbent for the selective adsorption/removal of phosphate. Our results have indicated that this composite material provides a high sorption capacity of 82.6 mg g−1 for phosphate. In addition, it exhibits high tolerance capacity to the commonly coexisting anionic species in water bodies, e.g., chloride (Cl−), sulfate (SO42−), and nitrate (NO3−), at a concentration level of 8 g L−1. In general, the present 3D graphene−lanthanum oxide composite provides a suitable sorption medium for water cleanup by selective adsorption/removal of phosphate.
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EXPERIMENTAL SECTION
Materials and Reagents. All the reagents used were at least of analytical reagent grade, and deionized (DI) water of 18 MΩ cm−1 from Heal Force Bio-Meditech Holdings Limited (Shanghai, China) was used throughout. A phosphate solution of 1000 mg L−1 was prepared by dissolving potassium phosphate monobasic in deionized water. Working standard solutions of phosphate at various concentrations were prepared by stepwise dilution of the stock solution. Other chemicals used include ammonium molybdate [(NH 4 ) 6 Mo 7 O 2 4 ·4H 2 O], potassium phosphate monobasic (KH2PO4), ammonium bicarbonate, sulfuric acid, hydrogen peroxide, ascorbic acid, sodium hydroxide, sodium tetrahydroborate, lanthanum nitrate, potassium persulfate, sodium chloride, potassium sulfate, sodium nitrate, and (3-aminopropyl)trimethoxysilane were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Preparation of La2O3 Particles. To produce La2O3 particles, 30 mL of lanthanum nitrate solution (0.30 mol L−1) was injected into 30 mL of ammonium bicarbonate (0.87 mol L−1) at a flow rate of 0.75 mL min−1 by a peristaltic pump with a 0.38 mm i.d. Tygon tubing. The mixture was stirred at room temperature for 6 h followed by a thorough wash with DI water. After centrifugation, the obtained La2O3 particles were dried at 110 °C followed by calcination at 750 °C in a vacuum tube furnace for 6 h. 0.6 g of the La2O3 particles was dispersed into 200 mL of toluene followed by the addition of 2 mL of (3-aminopropyl)trimethoxysilane.28 The mixture was then stirred at room temperature for 24 h under argon atmosphere. Toluene was rotary evaporated, and the modified La2O3 particles were centrifuged and dried at 120 °C for future use. Preparation of the 3D Graphene−La2O3 Composite. Graphene oxide (GO) was prepared by oxidation of graphite following the improved Hummers method.29,30 Then 0.05 g of GO was dispersed in 500 mL of deionized water to prepare a GO solution at 0.1 mg mL−1. 0.14 g of the modified La2O3 particles was dispersed in 400 mL of deionized water under sonication and then mixed with 500 mL of GO
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RESULTS AND DISCUSSION Characterizations of the 3D Graphene−La2O3 Composite. SEM images in Figure 1A illustrate the surface morphology of three-dimensional graphene−La2O3 composites. It reveals smooth and uniform surface for graphene in a 3D structure, and it is clearly seen the decoration of La2O3 particles in the 3D structure of graphene. In addition, Figure 1B indicates the presence of fine crystallines on the surface of graphene−La2O3 composite after the adsorption of 142 mg L−1 of phosphate solution. These crystallines can be identified as lanthanum phosphate. As illustrated in Figure 2, the X-ray diffraction patterns for the 3D graphene−La2O3 composite and for that after the adsorption of phosphate are indexed on the basis of the Joint B
DOI: 10.1021/acssuschemeng.5b01324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 3. Dependence of phosphate adsorption efficiency on the acidity of sample solution and the distribution fractions of various phosphate species in aqueous medium. 3D graphene−La 2 O 3 composite, 2 mg; sample solution (volume), 1.0 mL; phosphate concentration, 142 mg L−1.
Figure 2. XRD patterns for the 3D graphene−La2O3 composite (bottom) and for that after adsorption of phosphate at 142 mg L−1 (top).
Committee on Powder Diffraction Standards (JCPDS). For the graphene−La2O3 composite, the diffraction peaks recorded at around 29.9° and 39.5° are unambiguously assigned to La2O3.32 After adsorption of sufficient amount of phosphate, two additional diffraction peaks are observed at 30.9° and 51.2°, and these peaks can be safely assigned to LaPO4.33 These observations clearly illustrated that phosphate is retained on the surface of the 3D graphene−La2O3 composite and wherein converted to crystalline lanthanum phosphate. Thermogravimetric analysis (TGA) in air provides further information about the structure of the 3D graphene−La2O3 composite, as illustrated in Figure S1. The initial slight weight loss at 800 °C, a constant weight of 62.8% is observed corresponding to La2O3 in the 3D graphene−La2O3 composite. Adsorption/Removal of Phosphate: The Mechanistic Study. Generally, pH value is considered as a most important parameter in the physicochemical reaction at the water−solid interface.11 The adsorption/removal ratio of phosphate as a function of pH value in the initial phosphate solution is illustrated in Figure 3. Within a range of pH 3−9, a favorable removal efficiency of 100% is obtained. A slight decline of the adsorption efficiency to 78% is observed when further increasing to pH 11. Considering the fact that the acidity of natural water or wastewater generally falls into a range of pH 6−9, the present 3D graphene−La2O3 composite possesses obvious advantages in practical applications for the removal of phosphate from aqueous medium. The distribution of phosphate species, e.g., H3PO4, H2PO4−, HPO42− and PO43−, in aqueous medium is highly dependent on the variation of pH value, which associates closely to the dissociation equilibrium constants of these phosphate species, i.e., pK1 = 2.12, pK2 = 7.21, and pK3 = 12.67.11 The distribution fraction of HPO42− is gradually decreased when increasing the pH value to >10; meanwhile, an increase is observed for the distribution fraction of PO43−. The ζ-potentials of 3D graphene−La2O3 composite at various pH values have been investigated, demonstrating an isoelectric point of ca. 5,
corresponding to a ζ-potential of ∼0 (Figure S2). This indicates that electrostatic attraction between phosphate and the composite might among the driving forces for the interaction of phosphate with the composite, as within pH 3−6, the negatively charged H2PO4− is the dominative phosphate species; meanwhile, the composite surface possesses positive charge. At pH > 6, the surface of 3D graphene−La2O3 composite turns to negatively charged, and the dominative phosphate species is HPO42−. In this case, the adsorption of phosphate cannot be explained by electrostatic attraction any more. Starket al.31 proposed that the phosphate binding capability of lanthanum oxide could lead to the formation of acanthous precipitate of lanthanum phosphate through the following equation: La 2O3 + 2HPO4 2 − + H 2O = 2LaPO4 + 4OH−
This has been well confirmed in the present study by XRD diffraction patterns (Figure 2). After adsorption of phosphate by the 3D graphene−La2O3 composite two additional diffraction peaks assigned to crystalline LaPO4 at 30.9° and 51.2° are clearly identified with respect to those observed for the bare composite.31,33 In addition, SEM image illustrated obvious acanthous precipitate on the surface of composite after it has adsorbed phosphate (Figure 1B). The acidity variation for the reaction medium during the adsorption process has been investigated. By using 20 mg of the composite to adsorb phosphate in 5 mL of sample solution (142 mg L−1), we found that the original acidity for the reaction mixture is pH 6.2, whereas after adsorption for 30 min, the acidity of the mixture is changed to pH 7.9. The obvious increase of pH value from neutral to basic well facilitates the formation of LaPO4. The phosphate adsorption can be further elucidated by FTIR spectrum as shown in Figure S3. In comparison with the absorption bands for the bare composite, an additional absorption at 1004 cm−1 is clearly identified after adsorption of phosphate. This band is attributed to the typical characteristic absorption of P−O stretching vibration in the PO4− group.9,34,35 This observation clearly illustrated the binding of C
DOI: 10.1021/acssuschemeng.5b01324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 4. Time-dependent kinetics for phosphate adsorption/removal (A), pseudo-first-order (B) and pseudo-second-order (C) linear plots for the removal of phosphate at different concentrations. 3D graphene−La2O3 composite, 2 mg; sample solution (volume), 1.0 mL; pH, 6.2.
Table 1. Pseudo-First-Order and Pseudo-Second-Order Kinetic Parameters for the Adsorption of Phosphate onto the Graphene−La2O3 Composite with Different Initial Concentrations pseudo-first-order kinetic −1
−1
−1
2
C0 (mg L )
Qe (mg g )
Qe,c (mg g )
R
35 142
17.8 71.3
24.9 57.5
0.9505 0.9709
k1 (g mg
−1
min )
0.3732 0.06113
−1
Qe,c (mg g )
R2
k2 (g mg−1 min−1)
18.3 73.5
0.9972 0.9860
0.1847 0.007969
equilibrium for phosphate adsorption benefit further practical applications on phosphate removal. The adsorption behaviors could be illustrated by plotting the equilibrium adsorption capacity Qe versus equilibrium concentration Ce (as shown in Figure 5). It could be seen that the
phosphate on the surface of the 3D graphene−La 2 O 3 composite. Adsorption/Removal of Phosphate: Kinetics and Isotherms. A series of experiments have been performed to investigate the kinetics for the adsorption/removal of phosphate by the 3D graphene−La2O3 composite (Figure 4). It is obvious that for the adsorption of 35 and 142 mg L−1 phosphate, 100% removal efficiency could be readily achieved within 5 and 25 min, respectively, resulting in experimental adsorption capacities of 17.8 and 71.3 mg g−1, accordingly. For the elucidation of the adsorption process, pseudo-firstorder and pseudo-second-order adsorption models are generally used as expressed in the following.13,36 Where t is time, Qt is the adsorption capacity at a particular time t, Qe is the equilibrium adsorption capacity, k1 and k2 are the rate constants for the pseudo-first- and pseudo-second-order adsorptions, respectively. lg(Q e − Q t ) = lgQ e −
pseudo-second-order kinetic −1
k1 t Figure 5. Dependence of adsorption capacity (Qe) on the equilibrium phosphate concentration(Ce). 3D graphene−La2O3 composite, 2 mg; sample solution (volume), 1.0 mL; pH, 6.2; adsorption time, 25 min.
t 1 t = + Qt Qe Q e2 × k 2
According to the pseudo-first-order kinetics model, lg(Qe − Qt) versus t is plotted in Figure 4B. Although relative straight lines are obtained for linear fitting, a significant deviation of the calculated Qe,c value form the experimental result is observed. This indicates that pseudo-first-order kinetic model cannot fit the present adsorption process. On the other hand, Figure 4C illustrates very good linear fittings for t/Qt versus t at both concentration levels of 35 and 142 mg L−1. Kinetic parameters for the pseudo-first-order and pseudo-second-order models are given in Table 1. The good linear fittings in Figure 4C indicate that the adsorption kinetics follow pseudo-second-order model, and the rate constants are calculated as 0.1847 and 0.007 969 g mg−1 min−1 for 35 and 142 mg L−1 phosphate, and the calculated equilibrium adsorption capacity (Qe,c) are 18.3 and 73.5 mg g−1, respectively. The high capacity and fast
adsorption capacity increases rapidly at a zero equilibrium concentration, that means when a low initial concentration is employed for adsorption, the adsorbent is unsaturated and thus all phosphate in the solution is adsorbed by the 3D graphene− La2O3 composite. After saturation of the adsorbent, the Qe remain unchanged or slightly changed with the increase of the equilibrium concentration. The adsorption efficiency of phosphate by the 3D graphene− La2O3 composite is further investigated by varying the initial phosphate concentration within 14−228 mg L−1 (Figure 6A). It is clearly seen that adsorption efficiency of 100% is achieved as the initial phosphate concentration below 142 mg L−1, corresponding to an adsorption capacity of 71.3 mg g−1. A gradual decrease of the adsorption efficiency is observed with the increase of phosphate concentration, and the adsorption D
DOI: 10.1021/acssuschemeng.5b01324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 6. (A) Dependence of adsorption efficiency on the initial concentration of phosphate (C0). (B) Dependence of adsorption capacity (Qe) on the initial concentration of phosphate (C0). (C) Langmuir model: Ce/Qe vs Ce plot. (D) Freundlich model: ln Qe vs ln Ce plot. 3D graphene−La2O3 composite, 2 mg; sample solution (volume), 1.0 mL; pH, 6.2; adsorption time, 25 min.
capacity is improved to 82.7 mg g−1 with an initial phosphate concentration of 228 mg L−1. The adsorption capacity (Qe) of phosphate is obviously a function of the initial concentration (C0) (Figure 6B), and it becomes a concentration-independent process only after the adsorption saturation is reached. The Langmuir isotherm is expressed in the following, with Qe (mg g−1) as the equilibrium adsorption capacity, Ce (mg L−1) as the equilibrium concentration, Qm (mg g−1) as the maximum adsorption capacity, and K as the Langmuir adsorption constant.17,37
above, k is the Freundlich constant and 1/n is the adsorption intensity. ln Q e = ln k +
1 ln Ce n
The plot of ln Qe versus ln Ce illustrated in Figure 6D deviates significantly from linearship, that is, the adsorption of phosphate by the graphene−La2O3 composite does not follow the Freundlich model. Some lanthanum or lanthanum oxide incorporating materials have been employed for the adsorption/removal of phosphate, as summarized in Table 2. It is clearly seen that the present 3D graphene−La2O3 composite provides a significant improvement on the adsorption capacity with respect to the previously reported lanthanum containing adsorbents.13,17,38,39 Interferences and Phosphate Removal from Environmental Water. The coexisting concomitant anionic species might compete with phosphate for the adsorption by the graphene−La2O3 composite, and their interfering effects on the phosphate adsorption can eventually deteriorate the efficiency for phosphate adsorption/removal. In this respect, potential interferences from some commonly encountered anionic species in environmental water samples, e.g., Cl−, SO42−, and NO3−, are investigated. The experiments are performed with 2 mg of the composite to adsorb phosphate at two concentration levels, e.g., 142 and 25 mg L−1, in the presence of various
Ce C 1 = + e Qe Qm × K Qm
It is apparent that the adsorption of phosphate by graphene− La2O3 composite fits the Langmuir adsorption model, illustrating a perfect linear relationship of Ce/Qe versus Ce (Figure 6C). The maximum adsorption capacity (Qm) is derived to be 82.6 mg g−1. We have further demonstrated that the bare graphene surface exhibits virtually no adsorption to phosphate, i.e., 80% removal is maintained at 8000 mg L−1 of Cl−, SO42−, and NO3−. The favorable performance of the graphene−La2O3 composite on the tolerance of foreign species is most important for its practical use in treating real world water bodies. The capability for phosphate removal by the 3D graphene− La2O3 composite has been demonstrated for the treatment of riverine water samples collected from Hun-He river (Shenyang, China), and the removal ability for the 3D graphene−La2O3 composite, defined as massP(removal)/masscomposite, has been derived. When 2.0 mg of the composite is used for the treatment of various amount of riverine water containing 12 mg L−1 of phosphate (this solution was prepared by adding certain amount of standard phosphate solution in the riverine water), a removal ability of 10 mg g−1 is obtained for the 3D graphene− La2O3 composite. This performance is obviously superior to those previously presented in the literature, where removal ability of 2.35 mg g−1 is reported for the removal of phosphate from seawater,38 and 1.31 mg g−1 for phosphate in polluted water.39
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DOI: 10.1021/acssuschemeng.5b01324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
