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Facile Synthesis of β‑SrHPO4 with Wide Applications in the Effective Removal of Pb2+ and Methyl Blue Zhenzhen Lu,† Weiwei Chu,† Ruiqin Tan,*,† Shihui Tang,† Feng Xu,‡ Weijie Song,‡ and Junhua Zhao§ †

Faculty of Electrical Engineering and Computer Science, Ningbo University, Ningbo 315211, China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China § College of Chemical and Material Engineering, Quzhou University, Quzhou, 324000, China ‡

S Supporting Information *

ABSTRACT: A facile synthesis routine of β-type strontium hydrogen phosphate (β-SrHPO4) nanosheets was provided by the method of solution growing at temperatures below 80 °C. β-SrHPO4 was utilized for removing lead ions (Pb2+) and adsorbing methyl blue. Within the pH values from 2.0 to 6.0 of lead solutions, the adsorption capacity of β-SrHPO4 could reach above 1000 mg·g−1. The maximum removal capacity was 1409 mg·g−1 for Pb2+ within 15 min at pH = 4.5 and remained stable with the increase of temperatures. Different cations (Zn2+, Cr3+, Cd2+, and Co2+) affected slightly on the adsorption of Pb2+. It is revealed that the adsorption mechanism of Pb2+ was based on dissolution−precipitation theory. The maximum adsorption capacity was 335.68 mg·g−1 for methyl blue. The adsorption data of methyl blue accorded with the Langmuir isotherm model with R2 = 0.994 and the pseudo-second-order kinetic model with R2 = 0.99996, showing that the removal of methyl blue was dominated by a chemical sorption process. The thermodynamic studies demonstrated that the adsorption process toward methyl blue was endothermic and spontaneous. All of the results showed that β-SrHPO4 possessed potential applications in removing Pb2+ and methyl blue. SrHPO4: α-, β-, and γ-SrHPO4. Taher et al.19 reported that β-SrHPO4 was isomorphous with the orthorhombic BaHPO4; α-SrHPO4 was isostructural to the triclinic CaHPO4, and γ-SrHPO4 was a new orthorhombic form. There have been many articles investigating the impact of temperature on SrHPO4 synthesis,15,20,21 but there are few studies on its performance of removing heavy metal ions and dyes. In this work, β-SrHPO4 was obtained using a facile method of solution growing at low temperatures to investigate its removal capacity toward Pb2+ and methyl blue. The factors such as initial lead and methyl blue solution concentrations, reaction time, pH, and coexisting cations were researched. Furthermore, the isotherm and kinetic models were used to analyze the adsorption process of methyl blue.

1. INTRODUCTION With the high development of the modern industrial technologies, the ecosystem crisis caused by heavy metal ions and dye pollution draw people’s great attention. Heavy metal ions even at a low concentration can accumulate in human bodies, causing poisoning, cancer, and damage to the nervous system. Lead, a common nonferrous heavy metal element,1−3 widely exists in soil, water, and other elements of nature. Lead cannot be decomposed easily, which causes the accumulation of lead in living organisms resulting in anemia, kidney disease, and mental retardation.3 Methyl blue has been widely used in printing, the paper industry, and research laboratories.4 Methyl blue can lead to serious consequences such as difficulties in breathing, carcinogenicity, nausea, vomiting, and diarrhea.5,6 Many materials have been investigated to remove heavy metal ions and dyes from wastewater, such as carbon-based materials,7,8 chitosan,9,10 algae,11 and zeolites.12 Yet these materials are confined by various conditions, and the removal capacities still need further improvements. Strontium hydrogen phosphate (SrHPO4) is a kind of industrial chemical and analytical reagent. Numerous studies have now shown that SrHPO4 has lots of uses in many fields such as luminescent materials, all-solid-state lasers, flame proofing, surface conditioner, ceramics batteries, fuel cells, and so on.13,14 Various synthetic methods of SrHPO4 have been reported such as solution growing,15,16 the cathodic reduction method,17 and the hydrothermal method.18 There are three structures for © 2017 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. The characteristics of the chemical regents used in this work is shown in Table S1. All chemicals were used as received without further purification. 2.2. Synthesis of β-strontium Hydrogen Phosphate (β-SrHPO4). β-SrHPO4 was obtained using a facile method of solution growing. First, Sr(NO3)2 (4.8 g) was dissolved in 70 mL of deionized water using magnetic stirrer in a round-bottom flask. Received: June 3, 2017 Accepted: July 24, 2017 Published: August 7, 2017 3501

DOI: 10.1021/acs.jced.7b00499 J. Chem. Eng. Data 2017, 62, 3501−3511

Journal of Chemical & Engineering Data

Article

syringe-driven filters (0.22 μm). The UV−visible−near-infrared light spectrophotometer was used to analyze the rest of the concentration of methyl blue. The removal capacity and efficiency were calculated as follows:

After Sr(NO3)2 was completely dissolved in deionized water, (NH4)2HPO4 (1.8 g) was added into Sr(NO3)2 solution. The mixed solution was heated at different temperatures (45, 60, and 80 °C) for 24 h using an oil bath. In the process, a condense pipe was used to prevent solvent evaporation. The precipitation was filtered and washed with deionized water and ethanol for three times, respectively. The final products were obtained after being dried at 60 °C for 12 h in a vacuum drying oven. The products synthesized at 45, 60, and 80 °C were labeled as sample 1, sample 2, and sample 3, respectively. 2.3. Characterization and Analytical Methods. The phase analysis of β-SrHPO4 was characterized using X-ray diffraction (XRD) (Bruker D8 Advance, Bruker AXS, Germany) with monochromatized CuKα radiation (λ = 1.540 Å). The morphology of β-SrHPO4 was carried out by a field emission scanning electron microscope (FESEM) (Sirion200, FEI, America) and a field emission transmission electron microscope (FETEM) (Tecnai F20, FEI, America). BET analysis was performed on a full-automatic surface area and pore porosity analyzer (ASAP2020HD88, America). The zeta potential was measured at various pH with a zeta potential analyzer (Zetasizer Nano ZS, Malvern, Britain). The X-ray photoelectron spectra (XPS) were carried out on a electron energy spectrometer (PHI Quantera SXM, ULVAC-PHI, Japan). The concentration of the rest of Pb2+ was measured by an inductively coupled plasma-optical emission spectroscopy (ICP-OES) (PE Optima 2100DV, PerkinElmer, America). An acidometer (PB-21, Sartorius, Germany) was used to adjust the pH values of Pb2+ solution. The concentration of methyl blue was obtained by a UV−visible−near-infrared light spectrophotometer (Lambda 950, PerkinElmer, America). 2.4. Batch Experiments. 2.4.1. Adsorption of Pb2+. The removal of Pb2+ on β-SrHPO4 was studied in batch experiments. The influence of different pH values (from 2.0 to 6.0) was investigated at room temperature by adding 0.03 g of β-SrHPO4 into 30 mL of Pb2+ solution with an initial concentration of 2000 mg·L−1. Dilute nitric acid and dilute ammonia solution were used to adjust the pH value. β-SrHPO4 (0.03 g) was put into 30 mL of Pb2+ solution (initial concentration of 2000 mg·L−1) at different temperatures (from 299 to 339 K) to investigate the effect of temperatures on the removal process. The kinetics of Pb2+ adsorption were studied within 60 min by adding β-SrHPO4 (0.03 g) into Pb2+ solution (30 mL, initial concentration of 2000 mg·L−1, the temperature of 299 K) at the pH value of 4.5. The adsorption isotherms were analyzed by adding β-SrHPO4 (0.03 g) into 30 mL of Pb2+ solution (initial concentrations from 100 mg·L−1 to 2500 mg·L−1, pH = 4.5, the temperature of 299 K). To analyze the effect of different cations (Zn2+, Cr3+, Cd2+, and Co2+) on Pb2+ adsorption, corresponding nitrates were dissolved into Pb2+ solution (150 mg·L−1 for all cations and 2000 mg·L−1 for Pb2+). The concentration of residual Pb2+ was analyzed by ICP-OES. 2.4.2. Adsorption of Methyl Blue. The influence of different temperature values on methyl blue adsorption was studied by putting β-SrHPO4 (0.03 g) into methyl blue solution (30 mL, initial concentration of 400 mg·L−1) at different temperatures (from 299 to 339 K). β-SrHPO4 (0.03 g) was put into 30 mL of different methyl blue solutions with different initial concentrations (from 50 mg·L−1 to 500 mg·L−1, 299 K) to study its adsorption isotherms. The adsorption kinetics were obtained by dissolving 0.03 g of β-SrHPO4 into 30 mL of methyl blue solution (400 mg·L−1) within 60 min at the temperature of 299 K. The methyl blue solution after adsorption was collected using

qe =

(C0 − Ce)V M

removal efficiency(%) =

(1)

C0‐Ce × 100% C0

(2)

where qe (mg·g−1) is the adsorption capacity of pollutants at equilibrium time. C0 (mg·L−1) and Ce (mg·L−1) are the initial and equilibrium Pb2+ and methyl blue concentrations, respectively. V (L) is the volume of the pollutant solution, and M (g) is the amount of β-SrHPO4.

3. RESULTS AND DISCUSSION 3.1. Characterization of β-SrHPO4. The XRD patterns of sample 1, sample 2, and sample 3 are shown in Figure 1.

Figure 1. XRD patterns of sample 1, sample 2, and sample 3.

Comparing with the standard data of β-SrHPO4 (JCPDS no. 12-0368), there existed a diffraction peak at 2θ = 25.5° which was contributed to the (210) planes of Sr(NO3)2 at temperatures of 45 and 60 °C. The intensity weakened along with the rise of temperature and eventually disappeared when the temperature raised to 80 °C. Beyond that, no other new peaks or impurities were found. It is suggested that the crystallinity was dependent on the synthesis temperature and sample 3 with good crystallinity was highly pure. The SEM image of sample 1 is shown in Figure 2a. It was a flower-like structure with a diameter of about 8−10 μm and was composed of many nanosheets with the thickness of about 40−50 nm. β-SrHPO4 prepared at the temperature of 60 °C also had a flower-like morphology with a smaller diameter (4−6 μm) as shown in Figure 2b. Figure 2c shows the SEM image of sample 3. It was piled up with long nanosheets, and the morphology was irregular. It is concluded that the morphology of β-SrHPO4 was influenced by the synthesis temperature. Figure 2d shows the TEM image of sample 3 with an average length of 2.5 μm and a width of about 260 nm. The high-resolution transmission electron microscopy (HRTEM) image and the selected area electron diffraction (SAED) pattern (corresponding to Figure 2d) are shown in Figure 2e and f, respectively. It is observed that the 3502



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Figure 3. Zeta potentials of sample 3 at various pH values.

Figure 2. SEM images of β-SrHPO4 synthesized at different temperatures: sample 1 (a); sample 2 (b); sample 3 (c); TEM image (d); HRTEM image (e) and SAED pattern (f) of sample 3.

lattice fringes were indexed to (0−11), (011), and (002) place of β-SrHPO4 with the d-spacings of 0.51, 0.99, and 0.37 nm, respectively. From the diffraction rings shown in the SAED pattern, we can inferred that β-SrHPO4 has polycrystalline properties. According to the BET analysis, the average pore sizes were 10.46 nm (sample 1), 15.25 nm (sample 2), and 17.83 nm (sample 3), and the specific surface areas were 15.79 m2·g−1 for sample 1, 16.89 m2·g−1 for sample 2, and 20.71 m2·g−1 for sample 3 (the nitrogen adsorption−desorption isotherm and the plot of the pore size distribution of sample 3 was shown in Figure S1). To analyze the surface charge properties of β-SrHPO4, the method of zeta potential is used, and the measurement results of sample 3 are shown in Figure 3. The pHzpc, which determines the electrophoretic mobility where the net total particle charge is zero,22 is observed to be 1.6. When the pH value is higher than 1.6, the surface of the adsorbent would be negatively charged, which could promote the adsorption capacities toward cation pollutants in the acid condition. 3.2. Effect of pH on Pb2+ Adsorption. The effects of various pH values on Pb2+ removal are shown in Figure 4. It is observed that the adsorption capacity for Pb2+ on β-SrHPO4 increased with increasing the synthesis temperature. In connection with Figure 2, β-SrHPO4 nanosheets stacked compactly at low temperatures, which may give rise to less reactive sites and Pb2+ in the solution could not contact fully with the adsorbent. β-SrHPO4 nanosheets obtained at 80 °C were much looser than it was at 45 and 60 °C, leading to the exposure of more reactive sites. The maximum ability to remove Pb2+ of sample 3 reached up to 1409 mg·g−1 at pH = 4.5. The adsorption capacity of β-SrHPO4 synthesized in this work was higher than the

Figure 4. Effect of initial lead solution pH on removing Pb2+ on β-SrHPO4 synthesized at different temperatures.

flower-like spherical β-SrHPO4 (1120 ± 22 mg·g−1) synthesized by Zhuang et al.14 using a hydrothermal method. It is concluded that the morphology of β-SrHPO4 could affect the adsorption capacity toward Pb2+. According to the BET analysis results, sample 3 had larger specific surface area, which could further explain why sample 3 possessed higher removal efficiency. It is also observed that the adsorption capacity for Pb2+ could reach and stay above 1000 mg·g−1 in a wide range of pH values (from 2.0 to 6.0), which could be explained by the negatively charged surface of β-SrHPO4 when the pH value was higher than pHzpc. Therefore, lower pHzpc gave β-SrHPO4 (synthesized in this work) an advantage in Pb2+ removal over the reported one (synthesized by Zhuang et al.14). It is supposed that β-SrHPO4 had good application prospects because of its superior performance in Pb2+ removal within a wide range of sewage pH value. The comparison of Pb2+ adsorption capacities with different materials is shown in Table 1. It is evident that β-SrHPO4 was far superior to other reported adsorbents in Pb2+ removal. When the pH value was higher than 5.0, partial lead ions were precipitated in the form of Pb(OH)2 due to the existence of high concentration of hydroxyl ions, so pH = 4.5 was therefore considered to be the best pH value for further Pb2+ removal investigation, and it was in fact the natural pH of Pb2+ solution.23 3503



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Table 1. Comparison of Adsorption Capacities of Various Materials towards Pb2+

Table 2. Comparison of the Equilibrium Time of Various Materials

materials

qmax (mg·g−1)

ref

materials

time (min)

ref

graphene oxide 8-HQ-MWCNTs NSSCAC PLA/HAp n-CaHAP nanohydroxyapatite/chitosan porous materials kaolinite activated alumina paper sludge Na-birnessite chitosan immobilized on bentonite montmorillonite clay modified with tri-n-octylamine β-SrHPO4

125.00 0.07 23.81 140.50 769.23 548.90 12.10 83.32 103.50 173.10 28.00 33.10 1409.00

1 7 24 25 26 27 28 29 30 31 32 33 this work

graphene oxide nanosilversol-coated activated carbon biochars carbon nanotubes paper sludge natural clayey adsorbent caulerpa fastigiata biomass kaolinite Na-birnessite PTh/TiO2 montmorillonite montmorillonite-illite β-SrHPO4

360 60 300 40 60 30 60 180 300 60 60 45 15

1 24 34 35 30 36 37 28 31 38 39 40 this work

The mechanism of Pb2+ adsorption would be discussed in the following sections. 3.3. Kinetics and Isotherms of Pb2+ Adsorption. We mainly studied corresponding adsorption performance of β-SrHPO4 synthesized at the temperature of 80 °C unless noted otherwise. According to the kinetic curve of β-SrHPO4 showed in Figure 5a, it is clear that the adsorption process could achieve the equilibrium quickly within about 15 min. Table 2 lists the comparison of the equilibrium time of β-SrHPO4 with

other materials. This is quite rapid and efficient in the recent reports. At the beginning (within 10 min), the removal rate was quite high due to lots of active sites on β-SrHPO4 and reached the equilibrium eventually. The adsorption isotherm is shown in Figure 5b. It is observed that the balance of adsorption capacity on Pb2+ was 1409 mg·g−1. 3.4. Effect of Temperatures on Pb2+ Adsorption. Temperature is an important factor which could affect the adsorption process of Pb2+. The effect of different temperatures (from 299 to 339 K) on the removal process is shown in Figure 6. It is

Figure 6. Influence of different temperature values on Pb 2+ immobilization.

observed that the adsorption capacity remained stable with the increase of temperatures. This result suggested that the removal process toward Pb2+ was not completely controlled by thermodynamics but controlled by kinetics. 3.5. Effect of Various Cations on Pb2+ Adsorption. The effect of some coexisting cations such as Zn2+, Cr3+, Cd2+, and Co2+ on the removal of Pb2+ were studied. Figure 7 shows the adsorption of Pb2+ under the disturbance of those cations. The removal efficiency for Pb2+ on β-SrHPO4 was nearly up to 100% in the bisystems (Pb2+ + Zn2+, Pb2+ + Cr3+, Pb2+ + Cd2+, and Pb2+ + Co2+). It is also observed that β-SrHPO4 had a weak adsorption capacity for those coexisting cations (1.1% for Zn2+, 4.9% for Cr3+, and 0.6% for Cd2+). In general, β-SrHPO4 shows its excellent performance and selectivity toward Pb2+ in

Figure 5. Kinetic curve (a) and isotherm curve (b) of Pb 2+ immobilization on β-SrHPO4. 3504



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composed of the flake and stick shaped substances. Figure 8b shows the XRD patterns of the solids after the adsorption process (#1), the standard data of PbHPO4 (JCPDS No. 75-0757) (#2), and Pb10(PO4)6(OH)2 (PbHAp) (JCPDS No. 86-0236) (#3). Figure 9a shows the TEM image of the used SrHPO4 after the

Figure 7. Influence of coexisting cations on Pb2+ removal.

purifying wastewater. It is suggested that β-SrHPO4 had such a strong removal capacity and good selectivity to Pb2+ because Pb2+ and Sr2+ have similar ionic radii41 and Pb2+ possesses higher electronegativity than Sr2+.42 3.6. Mechanism of Lead Ion Adsorption. The morphology of the final products after immobilizing Pb2+ is displayed in Figure 8a. It is observed that the morphology of the solids changed tremendously and the precipitation was mainly

Figure 9. TEM image of the used β-SrHPO4 (a), HRTEM images (b and c) taken from corresponding portions from the used β-SrHPO4 (*1 and *2).

adsorption process. The morphology was consisted with the SEM image shown in Figure 8a. To further analyze the structure of the solids after the adsorption process, we have obtained the HRTEM images corresponding to the flake and stick solids. As shown in Figure 9b (corresponding to the stick portion of *1), the lattice fringe was found to be 0.34 nm which corresponds to (−111) plane of PbHPO4, and it was single crystal in nature as shown in the fast Fourier transform (FFT) pattern (inset in Figure 9b). Figure 9c was the HRTEM of the flake shaped solids (*2). It is observed that the lattice fringes of 0.25 and 0.49 nm belonged to the lattice plane of PbHPO4 [(−121) and (100), respectively] and the lattice fringe of 0.29 nm belonged to the lattice plane (112) of PbHAp. According to the XRD results and TEM images, the solids were mainly PbHPO4 and PbHAp. It is concluded that the main mechanism of the immobilization process was based on the dissolution−precipitation theory and it could be expressed by the following equation: Dissolution: SrHPO4(solid) → Sr 2 +(aq) + HPO4 2 −(aq)

(3)

Precipitation: Pb2 +(aq) + HPO4 2 −(aq) → PbHPO4(solid)

(4)

HPO4 2 − + H+ → H 2PO4 −

(5)

6H 2PO−4(aq) + 10Pb2 +(aq) + 2H 2O Figure 8. Feature of β-SrHPO4 after the immobilization process: SEM image (a) and XRD patterns (b).

→ PbHAp(solid) + 14H+(aq) 3505

(6) DOI: 10.1021/acs.jced.7b00499 J. Chem. Eng. Data 2017, 62, 3501−3511


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sites on β-SrHPO4.44 Within the pH values from 4.5 to 5.0, there is a slight decline in adsorption capacity, which is resulted from the existence of abundant Pb(OH)+.45 The ability of removing Pb(OH)+ by β-SrHPO4 was weaker than that of removing Pb2+ and part of Pb(OH)+ was unable to be precipitated, which brought about the downward trend of immobilizing process. With further increase in pH values, the deposit included not only PbHPO4 and Pb10(PO4)6(OH)2 but also the precipitation of Pb(OH)2 on account of numerous hydroxyl ions in solution, which could explain why the adsorption of lead ions increased when the pH value was higher than 5.0. 3.7. Researches on the Performance of Methyl Blue Adsorption. β-SrHPO4 nanosheets showed superior performance in removing Pb2+, but its application in adsorbing dyes is unknown. In this work, the adsorption ability of β-SrHPO4 (sample 3) in removing methyl blue was studied. The adsorption behavior of β-SrHPO4 toward methyl blue was supposed to be attributed to the charged functional groups in methyl blue, such as a sulfonic group (−SO3−) and amido group (=NH+). The ionic interaction between Sr2+ and −SO3− could draw methyl blue closer toward β-SrHPO4. Besides, hydrogen bonding interactions between HPO42− and hydrogen atoms which exist in NH+ of methyl blue could also accelerate the adsorption process.46 3.7.1. Effect of pH on Methyl Blue Adsorption. The adsorption of methyl blue, a kind of cationic dye, could be closely related to the charged surface of an adsorbent. pH affects the surface charge of the adsorbent, and so it could further affect the adsorption capacity of pollutants. The adsorption capacity toward methyl blue increased with increasing pH values over the range of 2.0−6.0 (shown in Figure 11). Due to electrostatic

The XPS spectra of the adsorbent before and after the adsorption process are shown in Figure S2 (the survey scan) and Figure 10 (the narrow scan). The appearance of strong peaks of

Figure 10. XPS spectra with a narrow scan of the pristine and used βSrHPO4.

Pb (Pb 4f, Pb 4d, and Pb 4p) and the vanish of Sr (Sr 3p and Sr 3s) after adsorption further proved the formation of Pb precipitation. In Figure 10, the peak of 133.0 eV consisted of two elements, corresponding to Sr 3d and P 2p. After adsorption, the weak peak of 133.0 eV was mainly attributed to P 2p, and the strong peaks of Pb 4f7/2 (138.5 eV) and Pb 4f5/2 (143.3 eV) appeared. The method of utilizing peak area and peak height sensitivity factors was used to determine the relative concentrations of the elements. The quantitative equation was given as follows:43 Cx =

nx I /S = x x ∑ ni ∑ Ii /Si

(7)

where Cx is the atom fraction of the element x (i represents other elements appeared in the sample) in the sample, n is the number of atoms of the element per cm3 of the sample, I is the intensity of the characteristic peaks, and S is the atomic sensitivity factor. The calculation results of the element proportion are shown in Table 3. It is approved that β-SrHPO4 dissolved and Sr2+ was Table 3. Proportion of Elements Existed in the Solids before and after the Adsorption Process element

wt % before the adsorption

wt % after the adsorption

C O P Sr Pb

6.41 59.60 17.00 16.99 0.00

32.53 42.37 12.34 0.94 11.81

Figure 11. Effect of solution pH on the methyl blue adsorption on β-SrHPO4.

attraction force, β-SrHPO4 with a negative charge surface (when the pH value was higher than 1.6) favored sorption toward methyl blue with a positive charge. 3.7.2. Adsorption Isotherms. Figure 12a shows the variation of adsorption capacity with different concentrations of methyl blue solution. It was clear that the maximum adsorption capacity was 335.68 mg·g−1. Table 4 lists the adsorption capacity of some reported adsorbents, and it is suggested that β-SrHPO4 possessed good performance in methyl blue adsorption. Based on the above experimental results, Freundlich and Langmuir models were used to get the linear fitting.

displaced by Pb2+, resulting in the insoluble products of PbHPO4 and PbHAp. Furthermore, the dramatic decrease in proportion of Sr element indicated that β-SrHPO4 had a high removal efficiency toward Pb2+. It is observed that the adsorption capacity increased along with the increasment of pH values until it reached a greatest value at pH = 4.5 according to Figure 4. In more acid environment, there were lots of positively charged hydronium ions (H3O+), which leads to the competition between H3O+ and Pb2+ for the active 3506



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Table 4. Comparison of Adsorption Capacities of Various Materials for Methyl Blue materials

qmax (mg·g−1)

ref.

magnetic chitosan (MCGO) natural chitosan membranes magnetic chitosan and graphene oxide graphene oxide activated charcoa graphene KGM/GO Mn/MCM-41 AC from coconut shell activated carbon from coconut coir dust β-SrHPO4

60.40 46.23 95.16 43.50 25.25 50.00 92.30 45.38 14.89 14.36 335.68

48 48 48 48 49 50 51 52 53 54 this work

methyl blue solution. KF (indicator of the adsorption capacity) and n (indicator of the adsorption intensity) are the adsorption constant and linearity index of Freundlich model, respectively. KL is the Langmuir constant which indicates the energy of adsorption. The fitting results using the two models are given in Figure 12b,c and Table 5. Comparing the correlation coefficients, the adsorption of methyl blue on β-SrHPO4 fitted the Langmuir model well with R2 = 0.994, which indicated that the removal of methyl blue on β-SrHPO4 belonged to monolayer and homogeneous adsorption. The calculated value of qmax was 320.513 mg·g−1 which was close to the experimental value of 335.68 mg·g−1. 3.7.3. Adsorption Kinetics. The adsorption kinetics of methyl blue on β-SrHPO4 was investigated, and the results are shown in Figure 13a. It is clear that the adsorption equilibrium could be reached within 15 min. The adsorption results were studied through the pseudo-first-order and pseudo-second-order kinetic models. The expressions were respectively listed as follows:55 log(qe − qt ) = log qe − t 1 t = + qt qe K 2qe 2

⎛ K1 ⎞ ⎜ ⎟t ⎝ 2.303 ⎠

(10)

(11)

−1

where qt (mg·g ) is the adsorption capacity at time t (min), qe (mg·g−1) is the adsorption capacity of methyl blue at equilibrium time, K1 (min−1) and K2 (g·mg−1·min−1) are rate constants of pseudo-first-order and pseudo-second-order models, respectively. The fitting curves of the two models are described in Figure 13b and c. The corresponding parameters are shown in Table 6. It is evident that the adsorption process of methyl blue on β-SrHPO4 fitted the pseudo-second-order kinetic model well with R2 = 0.99996. Furthermore, the calculated value of qe was 341.297 mg·g−1, and it was logical compared with the experimental value of 335.68 mg·g−1. The results above suggested that the removal process of methyl blue was a chemical adsorption process.56 3.7.4. Thermodynamic Studies. The effect of different temperature values on methyl blue adsorption is shown in Figure 14a. It is observed that the adsorption capacity was increasing with the increase of temperature (from 299 to 339 K), suggesting that the adsorption process toward methyl blue was endothermic. To further study the effect of temperature on the adsorption process, the thermodynamic parameters were calculated by the following equations:57

Figure 12. Effect of initial methyl blue concentration on methyl blue adsorption (a), Freundlich (b), and Langmuir isotherms (c) of methyl blue adsorption on β-SrHPO4.

Freundlich and Langmuir models could be respectively represented as follows:47 1 log qe = log KF + log Ce (8) n

Ce C 1 = e + qe qmax KLqmax

(9)

−1

where qe (mg·g ) is the adsorption capacity of methyl blue at equilibrium time, qmax (mg·g−1) is the maximum adsorption capacity, and Ce (mg·L−1) is the equilibrium concentration of

ΔG = −RT ln Kc 3507

(12) DOI: 10.1021/acs.jced.7b00499 J. Chem. Eng. Data 2017, 62, 3501−3511


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Table 5. Freundlich and Langmuir Parameters for Methyl Blue Adsorption Freundlich

Langmuir

n

KF (mol1−n·Ln·g−1)

R2

Qmax (mg·g−1)

KL (L·mol−1)

R2

3.101

84.425

0.745

320.513

0.244

0.994

Figure 14. Influence of different temperature values on methyl blue adsorption (a) and thermodynamics fitting for methyl blue adsorption (b) onto β-SrHPO4.

ln Kc = Kc =

ΔS ΔH − R RT

(13)

qe Ce

(14) −1

where R is the gas constant (8.314 J·mol ·K ), ΔG (kJ·mol−1) is the Gibbs free energy change, T (K) is the absolute temperature, Kc is the equilibrium partition coefficient, ΔS (J·mol−1·K−1) is the entropy change, ΔH (kJ·mol−1) is the enthalpy change, qe (mg·g−1) is the adsorption capacity of methyl blue at equilibrium time, and Ce (mg·L−1) is the equilibrium concentration of methyl blue solution. The values of ΔG, ΔS, ΔH, and R2 are listed in Table 7, and the linear fitting of T−1 and ln Kc is shown in Figure 14b. The values of ΔG were negative, which showed that

Figure 13. Influence of contact time on adsorbing methyl blue (a), pseudo-first-order (b), and pseudo-second-order kinetics (c) of methyl blue adsorption on β-SrHPO4.

−1

Table 6. Pseudo-First-Order and Pseudo-Second-Order Parameters for Methyl Blue Adsorption pseudo-first-order −1

−1

pseudo-second-order −1

2

K1 (min )

qe,cal (mg·g )

R

0.561

4325.835

0.929 3508

−1

K2 (g·mg ·min )

qe,cal (mg·g−1)

R2

0.012

341.297

0.999



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Table 7. Thermodynamic Parameters for Methyl Blue Adsorption pollutant

temperature (K)

ΔG (kJ·mol−1)

ΔS (J·mol−1·K−1)

ΔH(kJ·mol−1)

R2

methyl blue

299 304 309 314 319 329 339

−3.853 −3.963 −4.110 −4.528 −4.694 −5.003 −5.552

43.320

9.173

0.977

■

the adsorption process was spontaneous.44 The positive values of ΔS suggested that there was an increased randomness with the increase of temperature. The positive values of ΔH implied that the adsorption process was endothermic.58

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4. CONCLUSIONS In this work, β-SrHPO4 with a perfectly crystalline phase was synthesized using a simple synthesis method of solution growing at a low temperature of 80 °C. The product showed a high removal capacity toward Pb2+ in acidic aqueous solutions. The isotherms of methyl blue adsorption on β-SrHPO4 fitted the Langmuir model well, and the kinetic results accorded with the pseudo-second-order model. The maximum fixation quantity of β-SrHPO4 was 1409 mg·g−1 for Pb2+ at pH = 4.5 and 335.68 mg·g−1 for methyl blue. The thermodynamic studies implied that the removal capacity toward Pb2+ was not influenced by temperature, and the adsorption process toward methyl blue was endothermic and spontaneous. It is worth noting that the removal capacity toward Pb2+ could reach above 1000 mg·g−1 in a wide acidic ambient conditions (from 2.0 to 6.0 of pH values). The experiment results showed that β-SrHPO4 synthesized by solution growth has more superior performance and a broader application scope in pollutant removal than the reported β-SrHPO4 synthesized by the hydrothermal method and other reported adsorption materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00499. Additional table: Properties of the chemical reagents used in the present study. Additional figures: BET analysis result of sample 3; XPS spectra with the survey scan of the pristine and used β-SrHPO4 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel. and fax: 86-574-87913375. ORCID

Ruiqin Tan: 0000-0003-3743-4884 Feng Xu: 0000-0002-4951-8500 Funding

This work was financially supported by the National Natural Science Foundation of China (no. 21377063), the Ningbo Natural Science Foundation (no. 2017A610063), K.C. Wong Magna Fund in Ningbo University, and the Quzhou Science and Technology Bureau Projects (no. 2015Y003). Notes

The authors declare no competing financial interest. 3509



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