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Trace NaCl and Na2EDTA Mediated Synthesis of α‑Calcium Sulfate Hemihydrate in Glycerol−Water Solution Caiyun Jia,† Qiaoshan Chen,† Xu Zhou,† Hao Wang,† Guangming Jiang,‡ and Baohong Guan*,† †

Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China Engineering Research Center for Waste Oil Recovery Technology and Equipment, Chongqing Technology and Business University, Chongqing 400067, China

‡

S Supporting Information *

ABSTRACT: We report a new class of glycerol−water system to transform FGD gypsum into α-calcium sulfate hemihydrate (α-HH) mediated by trace NaCl and Na2EDTA to regulate the phase transformation rate as well as the crystal morphology to achieve crystals with high mechanical strength. NaCl plays a role in accelerating the nucleation and crystal growth process, whereas Na2EDTA regulates the morphology of α-HH from columnar to lamellar. The paste made from columnar α-HH with higher aspect ratio deserves higher dry bending/compressive strength of 13.6/37.6 MPa, which is comparable with those prepared in nitrate− water medium, chloride−water medium, or by autoclave method. This study provides an improved alternative of glycerol−water system to transform FGD gypsum into α-HH with controlled morphology, high mechanical strength, and less corrosion to the equipment. We also establish the relationship between the mechanical strength of paste and the aspect ratio of α-HH crystals. to equipment. Precipitation of α-HH in alcohol water solution is a promising alternative, 11 and DH can be directly transformed into α-HH in glycerol−water solution and the transformation rate is highly dependent on the glycerol concentration and solution temperature.12 In fact, polyhydric alcohol has already been used as stabilizer in α-HH preparation.13 Mechanical strength is the key property of the paste made from α-HH which influences its application fields. Particle size distribution of α-HH has a great impact on the porosity and mechanical strength of the paste.14,15 The hardened paste with a large porosity has a very low mechanical strength.15 By regulating the additives concentration, the aspect ratio of α-HH crystals is ranged in a large scale.16 Controlling of α-HH morphology plays a key role in obtaining paste with low porosity and enhanced mechanical strength.17 However, the relationship between α-HH morphology and the mechanical strength of its paste is still equivocal. In this work, we try to prepare α-HH from FGD gypsum in glycerol−water solution, using trace NaCl and Na2EDTA to modulate phase transformation rate and α-HH morphology respectively, and establish the relationship between the morphology and the mechanical strength of paste.

1. INTRODUCTION Wet calcium-based flue gas desulfurization (FGD) is the dominated technology in flue gas scrubbing market. A large amount of FGD gypsum mainly composed of calcium sulfate dihydrate (DH) is produced synchronously, leading to land occupation, water and soil contamination.1 A promising utilization way is to transform it into α-calcium sulfate hemihydrate (α-HH), a kind of cementitious material widely used in molding, decoration, construction, and binding industry. The transformation follows a dehydration reaction: CaSO4 · 2H 2O (DH) → CaSO4 ·0.5H 2O (α‐HH) + 1.5H 2O

(1)

Since α-HH is a metastable phase, it is inclined to dehydrate to the stable phase of anhydrite (AH) in the crystallization media. CaSO4 ·0.5H 2O (α‐HH) → CaSO4 (AH) + 0.5H 2O

(2)

When blended with water, the crystals of α-HH hydrate into DH, crystals of DH grow and interlock with each other, and then harden into set paste.2,3 The reported transformation system includes commercialized autoclave technology4 and mixed salts solution process.5−7 The former is energy intensive and unavailable when FGD gypsum serves as raw material, while the latter is corrosive to equipment. And more, the salts ions are apt to incorporate into α-HH crystals and thus reduce the purity and lower the mechanical strength of the paste made from α-HH.8−10 Therefore, a trace NaCl crystallization surrounding is more favored to prepare α-HH with higher purity and less corrosion © 2016 American Chemical Society

Received: Revised: Accepted: Published: 9189

May 30, 2016 August 6, 2016 August 6, 2016 August 7, 2016 DOI: 10.1021/acs.iecr.6b02064 Ind. Eng. Chem. Res. 2016, 55, 9189−9194

Article

Industrial & Engineering Chemistry Research Table 1. Chemical Composition of the FGD Gypsum (wt %)7 CaO

Al2O3

Fe2O3

SO3

SiO2

MgO

H2O

Na2O

K2O

total

31.76

0.25

0.14

43.87

2.00

0.03

19.88

0.01

0.03

98.01

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical reagent grade glycerol, NaCl, and Na2EDTA were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The chemical composition of FGD gypsum received from Banshan Power Plant (Zhejiang province, China) was given in Table 1. 2.2. Methods. The preparation of α-HH was performed in a double walled glass reactor equipped with a glass condenser on top of it. The slurry was stirred at a constant rate of 240 rpm. The temperature was kept at 90 °C by circulating oil flowing through the reactor jacket. The procedure was as follows: 27 mol % glycerol−water solutions with trace NaCl and Na2EDTA were added into the reactor, and preheated to 90 °C. Then 50 g FGD gypsum was added into the reactor. During the reaction, 15 mL hot slurry was withdrawn at certain time intervals. The sample was filtered immediately, washed three times with boiling water, rinsed once with acetone, and dried in an oven at 60 °C for 2 h. The dried sample was subjected to metallographic microscope (XJP-6A, Chongqing Optical and Electrical Instrument Co., Ltd., Chongqing, China) to observe the morphology evolution of α-HH crystals. The X-ray diffraction (XRD, D/ Max-2550PC, Rigaku Inc., Tokyo, Japan) analysis was conducted using Cu Kα radiation at a scanning rate of 8°/ min in the 2θ range from 10° to 35°. For thermogravimetry and differential scanning calorimetry (TG-DSC, STA 409PC NETZSCH, Germany) analysis, 20 mg dried sample was put into an Al2O3 crucible with a lid under temperature programming from 35 to 350 °C at a heating rate of 10 °C/ min under nitrogen gas atmosphere. The Fourier transform infrared (FTIR) spectra were tested on a spectrometer (IRAffiinity−1, Shimadzu, Japan) with a resolution of 4 cm−1 over frequency range of 340−4000 cm−1. For mechanical strength analysis, the prepared α-HH crystals were triturated in a ball mill (XQM-2, Nanjing analysis instrument Co., Ltd., Nanjing, China) to 20−30 μm of powder and blended with water at its normal consistency for 30 s to obtain a homogeneous paste. The normal consistency giving mass ratio of water to hemihydrate (W/H) was tested according to GB/T 17669.4-1999 (China national standard for gypsum plasters).18 Then the paste was poured into three 20 mm × 20 mm × 80 mm molds. After maintained for 30 min, the specimens were demolded and cured at 20 ± 2 °C and relative humidity of 90 ± 5%. After being dried completely, the bending and compressive strength of the specimens were tested according to GB/T 17669.3−1999.18

Figure 1. XRD patterns of the solid samples withdrawn at certain residence time during the transformation of FGD gypsum in 27 mol % glycerol−water solution with 0.171 M NaCl and 0.15 mM Na2EDTA at 90 °C.

exhibit a gradual increase in peak intensity at the expense of DH characterized at 2θ = 11.62°, 20.74°, 23.39°, and 29.11°. The transformation is completed at 6.0 h and little AH (diffraction peaks at 2θ = 29.38° and 32.07°) is formed. The DSC curves in Figure 2, which are characterized with the

Figure 2. DSC curves of the solid samples withdrawn at certain residence time during the transformation of FGD gypsum in 27 mol % glycerol−water solution with 0.171 M NaCl and 0.15 mM Na2EDTA at 90 °C.

3. RESULTS AND DISCUSSION 3.1. Phase Transformation from FGD Gypsum to αHH. FGD gypsum composed of 95.6 wt % DH (see Table 1) is inclined to dehydrate and transform into HH in glycerol−water solution under given conditions. Water activity and reaction time are two determinant factors in the phase transformation.12 Under 27 mol % glycerol and 90 °C, the transformation proceeds spontaneously within the thermodynamic preparation window of HH.12 A typical transformation is tracked by XRD analysis in Figure 1. The diffraction peaks for HH at 2θ = 14.70°, 25.62°, 29.71°, and 31.89° are detected at 3.0 h and

endothermic peak at 173 °C (dehydration of α-HH into AHIII) and exothermic peak at 223 °C (phase transition from AHIII to AH-II), confirm the phase of α-HH. The above observation indicates that FGD gypsum can be completely transformed into α-HH in the medium of glycerol−water solution. 3.2. Effects of Sodium Chloride on the PhaseTransformation Rate. Sodium ions, which are reported to be effective to accelerate α-HH nucleation,19,20 were added into the 27 mol % glycerol−water solution to explore its effect on the transformation from FGD gypsum into α-HH. The crystal 9190

DOI: 10.1021/acs.iecr.6b02064 Ind. Eng. Chem. Res. 2016, 55, 9189−9194

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are provided in a uniform concentration by dissolved FGD gypsum. Addition of NaCl facilitates the dissolution of DH and thus provides more available lattice ions (Ca2+ and SO42−),19 creating a much higher supersaturation. All these results confirm that NaCl can promote the nucleation and crystal growth of α-HH crystals. A moderate amount of NaCl is necessary on tuning the phase transformation in glycerol−water solution. 3.3. Effects of Na2EDTA on Morphology Modulation and Phase-Transformation Rate. The α-HH crystals are inclined to grow in acicular shape due to its preferential 1-D growth along the c-axis.22−24 Na2EDTA was added to modify crystal morphology based on its appropriate capping effect to influence the crystal growth habit.25,26 The dependence of αHH morphologies on Na2EDTA concentrations is shown in Figure 4. Without Na2EDTA, α-HH precipitates in acicular

water content (Figure 3) is cited to indicate whether FGD gypsum (20.93 wt %) has been completely transformed into α-

Figure 3. Variation of crystal water content with residence time during the transformation of FGD gypsum in the presence of 0−0.684 M NaCl in 27 mol % glycerol−water solutions with 0.15 mM Na2EDTA at 90 °C.

HH (6.21 wt %). Without NaCl, the phase transformation does not occur within 720 min. At 0.085 M NaCl, the transformation starts at 300 min, and the crystal water content is slowly decreased with time and reaches 6.2 wt % at 600 min, suggesting a complete transformation. When increasing NaCl concentration to 0.171 M, the gypsum begins to dehydrate at 120 min and the crystal water content is quickly decreased with time. Further increasing NaCl concentration from 0.256 to 0.342 M, the phase transformation starts earlier. As the NaCl concentration goes up to 0.513 and 0.684 M, the transformation starts within 30 min and completes in 120 min. NaCl contributes more to promoting the nucleation rate than the crystal growth rate. At a low concentration of NaCl, for example 0.085−0.256 M, the phase transformation is conspicuously accelerated. The induction time is shortened from about 300 to 60 min and the complete transformation time is shortened from 660 to 240 min. As the NaCl concentration increases from 0.256 to 0.684 M, the induction time is decreased from 60 min to less than 15 min and the complete transformation time is decreased from 240 to 120 min. The promoting effect of NaCl is connected to the supersaturation (SHH) of the solution which plays a crucial role in modulating α-HH crystallization rate.21 A higher supersaturation contributes to a larger driving force, generating a higher nucleation rate and crystal growth rate. In glycerol− water solution, SHH can be defined as follows: SHH =

Figure 4. Morphology variations of α-HH crystals obtained in 27 mol % glycerol−water solutions with 0.256 M NaCl and different Na2EDTA concentrations at 90 °C: (a) 0 mM, (b) 0.15 mM, (c) 0.45 mM, (d) 1.80 mM, (e) 9.00 mM.

K sp,DH/K sp,HH·a H2O1.5

=

aCa 2+·aSO4 2−·a H2O2 /K sp,HH·a H2O1.5

=

c Ca 2+·c SO4 2−·γCa 2+·γSO 2−·a H2O0.5/K sp,HH 4

shape (Figure 4a). At 0.15 mM Na2EDTA, α-HH grows in hexagonal prism with a length of ∼230 μm and diameter of ∼40 μm (Figure 4b). An increase in Na2EDTA concentration to 0.45 mM help to acquire fat and short hexagonal prisms with a size of ∼100 μm in length and ∼50 μm in diameter (Figure 4c). As the Na2EDTA concentration goes up to 1.80 mM, the crystals present in hexagonal plate with diameter of ∼50 μm (Figure 4d). Further increasing Na2EDTA concentration to 9.00 mM, α-HH crystals grow into tiny lamellar and granular forms (Figure 4e). Evidently, Na2EDTA can effectively tune the size and morphology of α-HH. The denser distribution of Ca2+

(3)

where a, Ksp, c, and γ are the activity, thermodynamic equilibrium constant, ion concentration, and activity coefficient, respectively. Because the temperature is fixed at 90 °C and NaCl shows a much lower concentration than glycerol, the activity coefficient, water activity, and Ksp can be defined as constant and the supersaturation is a function of Ca2+ and SO42− ions concentration. Without NaCl, Ca2+ and SO42− ions 9191

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Industrial & Engineering Chemistry Research ions on the top facet of α-HH crystals than that on the side facets indicates that the top facet is more positively charged compared with the side facets.27 Therefore, the negative EDTA2− ions tend to chemically absorb on the top facet by chelating Ca2+ and restrict the crystals’ growth along the c-axis. To confirm the interaction between EDTA2− and the facet of α-HH crystals, the FTIR spectra are analyzed as shown in Figure 5. The peaks at 3610, 3550, and 1620 cm−1 can be

relationship between the morphology and the mechanical strength of α-HH paste (see Figure S2 in the Supporting Information). The prepared α-HH crystals are triturated and mixed with water at its normal consistency to form a paste through a hydration process: CaSO4 · 0.5H 2O (α‐HH) + 1.5H 2O → CaSO4 · 2H 2O (DH) (4)

Table 2 shows the mechanical strength of the paste prepared from α-HH crystals with aspect ratio of 8.3. The dry bending Table 2. Mechanical Strength of α-HH Prepared in Glycerol−Water Medium with Comparison to Those Produced in Nitrate−Water Medium, Chloride−Water Medium, and Commercial Autoclave Method

W/H ratio dry bending strength (MPa) dry compressive strength (MPa)

glycerol− water medium

nitrate− water medium32

chloride− water medium33

autoclave methoda

0.37 13.6

11.2

0.36 16.5

0.32 10.1

37.6

37.4

40.5

43.6

α-HH from Jinxin Construction Material Co. Ltd. (Shandong province, China). a

Figure 5. FTIR spectra of α-HH crystals obtained in 27 mol % glycerol−water solutions with 0.256 M NaCl and different Na2EDTA concentrations at 90 °C: (a) 0 mM, (b) 0.15 mM, (c) 1.80 mM.

strength and drying compressive strength reach 13.6 and 37.6 MPa, respectively. The mechanical strength of the pastes made from α-HH produced in nitrate−water medium, chloride− water medium, or by commercial autoclave method are listed for comparison, which implies the glycerol−water solution provides a pretty good medium for α-HH precipitation. The dependence of the mechanical strength on the crystal morphology is presented in Figure 6. The paste made from

assigned to the vibration of O−H. The adsorption peaks at 1008 cm−1 can be assigned to the characteristic frequency of γ1 (SO42−) stretching, 1153, 1116, and 1096 cm−1 of γ3 (SO42−) stretching, 660 and 601 cm−1 of γ4 (SO42−) stretching (Figure 5a). In the presence of 0.15 mM Na2EDTA (Figure 5b), two adsorption peaks at 2926 and 2854 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of CH2, suggesting the adsorption of EDTA2− on the facet of α-HH crystals. The stretching vibrations of CH2 are enhanced in intensity with an increase of Na2EDTA concentration (Figure 5c). The bonds at 660 cm−1 corresponding to γ3 (SO42−) stretching is prominently shrunk as Na2EDTA concentration goes up to 1.80 mM. What’s more, the peaks of γ3 (SO42−) stretching at 1153, 1116, and 1096 cm−1 are broaden. All these changes confirm that the adsorbed EDTA2− ions interact strongly with SO42− groups on the facets of α-HH crystals. Though Na2EDTA can effectively modulate α-HH morphology, it has an inhibition effect on the nucleation and crystal growth of α-HH. The complete transformation time is prolonged from 2.0 to 36.0 h as the Na2EDTA concentration varies from 0 to 9.00 mM (see Figure S1 in the Supporting Information). The precipitation of α-HH is divided into three steps: dissolution of DH, α-HH nucleation, and α-HH crystal growth.28 The nucleation process is the rate-determining step since it needs to surmount a relatively high surface energy barrier.29,30 As a kind of strong chelating agent,31 EDTA2− ions chelate Ca2+ ions in the solution and thus lower the supersaturation for α-HH, retarding the nucleation of α-HH. During α-HH crystal’s growth period, EDTA2− ions are inclined to adsorb chemically on the negatively charged top facet, retarding its growth along c-axis. 3.4. Mechanical Strength of α-HH Pastes. By balance of the concentrations of NaCl and Na2EDTA, a proper transformation time and columnar α-HH crystals with aspect ratios ranging from 8.3 to 2.8 are obtained to investigate the

Figure 6. Mechanical strength of the pastes made from α-HH with different aspect ratios: (a) 20−30, (b) 8.3, (c) 6.0, (d) 4.0, (e) 2.8.

crystals in acicular shape has a W/H ratio of 1.50, which is much higher than the theoretical value of 0.19, leading to a large amount of excess water in the paste. After the paste is dried, a much porous and less dense structure is formed with the evaporation of excess water, and eventually generates the very low dry bending/compressive strength of 0.94/1.50 MPa, whereas the pastes made from crystals in columnar shape have a W/H ratio of approximately 0.38 and much higher mechanical strength. However, as the aspect ratio decreases from 8.3 to 2.8, the dry bending strength and dry compressive strength of the paste decrease markedly from 13.60 to 9.16 9192

DOI: 10.1021/acs.iecr.6b02064 Ind. Eng. Chem. Res. 2016, 55, 9189−9194

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Industrial & Engineering Chemistry Research

(7) Guan, B. H.; Yang, L. C.; Wu, Z. B.; Shen, Z. X.; Ma, X. F.; Ye, Q. Q. Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum in K, Mg-Containing Concentrated CaCl2 Solution under Mild Conditions. Fuel 2009, 88, 1286. (8) Guan, B. H.; Ma, X. F.; Wu, Z. B.; Yang, L. C.; Shen, Z. X. Crystallization Routes and Metastability of α-Calcium Sulfate Hemihydrate in Potassium Chloride Solutions under Atmospheric Pressure. J. Chem. Eng. Data 2009, 54, 719. (9) Ling, Y. B.; Demopoulos, G. P. Preparation of α-Calcium Sulfate Hemihydrate by Reaction of Sulfuric Acid with Lime. Ind. Eng. Chem. Res. 2005, 44, 715. (10) Block, J.; Waters, O. B. The CaSO4-Na2SO4-NaCl-H2O System at 25° to 100 °C. J. Chem. Eng. Data 1968, 13, 336. (11) Guan, B. H.; Jiang, G. M.; Wu, Z. B.; Mao, J. W.; Kong, B. Preparation of α-Calcium Sulfate Hemihydrate from Calcium Sulfate Dihydrate in Methanol-Water Solution under Mild Conditions. J. Am. Ceram. Soc. 2011, 94, 3261. (12) Guan, B. H.; Jiang, G. M.; Fu, H. L.; Yang, L.; Wu, Z. W. Thermodynamic Preparation Window of Alpha Calcium Sulfate Hemihydrate from Calcium Sulfate Dihydrate in Non-Electrolyte Glycerol-Water Solution under Mild Conditions. Ind. Eng. Chem. Res. 2011, 50, 13561. (13) Kirk, D. W.; Tang, S. T. Process for the Production of Alpha Hemihydrate Calcium Sulfate from Flue Gas Sludge. U.S. Patent, US00562892A, October 8, 1996. (14) Ye, Q. Q.; Guan, B. H.; Lou, W. B.; Yang, L.; Kong, B. Effect of Particle Size Distribution on the Hydration and Compressive Strength Development of α-Calcium Sulfate Hemihydrate Paste. Powder Technol. 2011, 207, 208. (15) Tang, M. L.; Shen, X. D.; Huang, H. Influence of α-Calcium Sulfate Hemihydrate Particle Characteristics on the Performance of Calcium Sulfate-Based Medical Materials. Mater. Sci. Eng., C 2010, 30, 1107. (16) Mao, X. L.; Song, X. F.; Lu, G. M.; Xu, Y. X.; Sun, Y. Z.; Yu, J. Effect of Additives on the Morphology of Calcium Sulfate Hemihydrate: Experimental and Molecular Dynamics Simulation Studies. Chem. Eng. J. 2015, 278, 320. (17) Wang, Y. W.; Meldrum, F. C. Additives Stabilize Calcium Sulfate Hemihydrate (Bassanite) in Solution. J. Mater. Chem. 2012, 22, 22055. (18) Guan, B. H.; Ye, Q. Q.; Wu, Z. B.; Lou, W. B.; Yang, L. C. Analysis of the Relationship between Particle Size Distribution of αCalcium Sulfate Hemihydrate and Compressive Strength of Set Plaster-Using Grey Model. Powder Technol. 2010, 200, 136. (19) Jiang, G. M.; Fu, H. L.; Savino, K.; Qian, J. J.; Wu, Z. B.; Guan, B. H. Nonlattice Cation-SO42‑ Ion Pairs in Calcium Sulfate Hemihydrate Nucleation. Cryst. Growth Des. 2013, 13, 5128. (20) Shen, Z. X.; Guan, B. H.; Fu, H. L.; Yang, L. C. Effect of Potassium Sodium Tartrate and Sodium Citrate on the Preparation of α-Calcium Sulfate Hemihydrate from Flue Gas Desulfurization Gypsum in a Concentrated Electrolyte Solution. J. Am. Ceram. Soc. 2009, 92, 2894. (21) Feldmann, T.; Demopoulos, G. P. The Crystal Growth Kinetics of Alpha Calcium Sulfate Hemihydrate in Concentrated CaCl2-HCl Solutions. J. Cryst. Growth 2012, 351, 9. (22) Singh, N. B.; Middendorf, B. Calcium Sulphate Hemihydrate Hydration Leading to Gypsum Crystallization. Prog. Cryst. Growth Charact. Mater. 2007, 53, 57. (23) Zhao, W. P.; Wu, Y. M.; Xu, J.; Gao, C. H. Effect of Ethylene Glycol on Hydrothermal Formation of Calcium Sulfate Hemihydrate Whiskers with High Aspect Ratios. RSC Adv. 2015, 5, 50544. (24) Hou, S. C.; Wang, J.; Wang, X. X.; Chen, H. Y.; Xiang, L. Effect of Mg2+ on Hydrothermal Formation of α-CaSO4·0.5H2O Whiskers with High Aspect Ratios. Langmuir 2014, 30, 9804. (25) Jayaprakash, J.; Srinivasan, N.; Chandrasekaran, P. Surface Modifications of CuO Nanoparticles Using Ethylene Diamine Tetra Acetic Acid as a Capping Agent by Sol-Gel Routine. Spectrochim. Acta, Part A 2014, 123, 363.

MPa and from 37.57 to 25.90 MPa, respectively. Obviously, it is a requisite for α-HH morphology control to gain paste with high mechanical strength. The paste made from α-HH in columnar shape with higher aspect ratio deserves higher mechanical strength.

4. CONCLUSIONS An alternative method is created for the precipitation of α-HH in glycerol−water solution with trace NaCl and Na2EDTA. NaCl plays a role in accelerating the transformation rate while Na2EDTA acts as a capping agent to modify the crystal morphology and inhibit the transformation process. The mechanical strength of the paste has a strong relationship with the morphology and aspect ratio of α-HH. The paste prepared from crystals in columnar shape with higher aspect ratio achieves higher mechanical strength. This method has the advantage to provide α-HH with less chloride and higher purity compared with the mixed salts solution process, and the relationship between the mechanical strength of paste and the aspect ratio of α-HH crystals is established.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02064. Figure S1 showing the complete transformation time from FGD gypsum under different Na2EDTA concentrations; Figure S2 presenting the precipitated columnar α-HH crystals with different aspect ratios under a synergetic effect of NaCl and Na2EDTA (PDF).

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AUTHOR INFORMATION

Corresponding Author

*B. Guan. Tel.: +86 571 88982026. Fax: +86 571 88982026. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully appreciate Project 21176219 supported by National Science Foundation of China and Changjiang Scholar Incentive Program (Ministry of Education, China, 2009AA064002).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02064 Ind. Eng. Chem. Res. 2016, 55, 9189−9194