Mechanism of influence of Organic Impurity on Crystallization of

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Mechanism of influence of Organic Impurity on Crystallization of Sodium Sulfate Nannan Su, Yongli Wang, Yan Xiao, Haijiao Lu, Yajing Lou, Jingjing Huang, Meng He, Yang Li, and Hongxun Hao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04625 • Publication Date (Web): 13 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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Industrial & Engineering Chemistry Research

Mechanism of influence of Organic Impurity on Crystallization of Sodium Sulfate Nannan Su a, Yongli Wang a,b, Yan Xiao a, Haijiao Lu a, Yajing Lou a, Jingjing Huang a, Meng He a, Yang Li a, Hongxun Hao a,b,* a

National Engineering Research Center of Industrial Crystallization Technology,

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin 300072, China. ABSTRACT To promote the development of crystallization technology for recovering salt from high salinity wastewater, the effect of organic impurity on crystallization of sodium sulfate was investigated by using phenol as representative organic impurity. The effect of phenol on crystallization thermodynamics of sodium sulfate was evaluated by measuring solubility data of sodium sulfate in water in presence of phenol. It was found that the existence of phenol could suppress the solubility of sodium sulfate in water. The effect of organic impurity on crystal nucleation was performed by measuring the metastable zone width (MSZW) and induction time of sodium sulfate. Two models (Self-consistent Nývlt-like equation and Classical 3D nucleation theory) were used to analyze the experimental data. It was found that Classical 3D nucleation theory (3D CNT) can better explain the effect of phenol on nucleation. From both MSZW data and induction time data, it was found that the 1

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existence of phenol will apparently increase the interfacial energy γ, which will result in higher nucleation Gibbs energy barrier and thus lower nucleation rate. Furthermore, the existence of phenol will increase the critical nucleus radius r* and the critical Gibbs energy ∆G*, which means that the formation of the nuclei will be more difficult in the presence of phenol. According to the above analysis, the possible mechanism of influence of organic impurity on crystallization of sodium sulfate was proposed. 1. INTRODUCTION As we all know, for natural resources, China is rich in coal while poor in oil and gas. Therefore, coal is widely used in China to produce necessary energy and raw materials for different industries. However, with the fast development of coal industry, huge amount of wastewater is generated every year. With the putting forward of the concept of "zero emissions" and stricter laws and regulations on environmental protection in China, wastewater from coal industry is forbidden to be directly discharged. As a result, many technologies, such as advanced oxidization, membrane separation and nanotechnology, have been developed to treat wastewater. Generally, after membrane treatment, high salinity wastewater with high concentration of sodium sulfate and sodium chloride will be obtained. To solve the pollution issue of high salinity wastewater and to recover salt from it, crystallization technology can be used to reclaim sodium sulfate and sodium chloride from high salinity wastewater. Generally, the existence of organic impurity in wastewater, which is inevitable, will affect the crystallization process and the quality of the final product. Some 2


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studies have been conducted to investigate the influence of impurities on the crystallization of organic compounds. Lu et al.1 studied the effect of organic additives (peptone, phenol and heptanediacid) on solubility, supersolubility and MSZW of sodium chloride in wastewater. They found that organic additives will decrease the solubility and increase the MSZW of sodium chloride. Peng et al.2 investigated the influence of anions on the nucleation of borax decahydrate and found out that the addition of impurity will result in higher solubility, wider MSZW and longer induction time. Rajesh et al.3 and Dhanaraj et al.4 demonstrated that the MSZW and the induction time of ammonium dihydrogen orthophosphate (ADP) will be enhanced by urea and amino acid. Becheleni et al.5 investigated the influence of phenol ( 0.2wt % ) on CSD, purity and morphology of sodium sulfate decahydrate and found out that the crystal will be larger but the morphology will not change in the presence of phenol. However, no comprehensive research on the effect of typical organic compounds on nucleation of sodium sulfate could be found, although it is very important for the development and optimization of crystallization technology for extracting sodium sulfate from high salinity wastewater. Under this background, in this work, the effect of organic impurity on crystallization of sodium sulfate was investigated in detail by using phenol, which is one of the typical organic impurity in high salinity wastewater, as the representative impurity. The effect of phenol on crystallization thermodynamics was investigated by measuring the solubility of sodium sulfate in water in the presence of phenol. The effect of phenol on crystallization kinetics (mainly nucleation) 3



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was evaluated by measuring the MSZW and induction time of sodium sulfate in the presence of phenol. To investigate the possible mechanism of influence of organic impurity on crystallization of sodium sulfate, two different theoretical models were used to analyze the experimental MSZW data and induction time data. The MSZW refers to the maximum supercooling ∆Tmax ----the difference between the saturation temperature T0 and the temperature Tlim when visible crystals appear in the solution.6 The induction time is defined as the time interval between the formation of a given supersaturation and the detection of the crystals under the given supersaturation. Many techniques, such as turbidity,7-9 conductivity,10 focused beam reflectance measurements (FBRM) and attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy,11-12 can be used to measure the MSZW and induction time. In this work, FBRM was used to measure the MSZW and induction time. 2. THEORETICAL MODELS 2.1 Self-consistent Nývlt-like model13,14 The MSZW of a substance relies on various factors such as saturation temperature, impurities, seeds, stirring speed and cooling rate. Nývlt's equation which can be expressed by Eq. (1) has been widely used because of its simplicity.

ln∆Tmax =

1- m dc 1 1 ln( ) ln k + ln R m dT m m

(1)

where ∆Tmax is the MSZW. T and c refer to temperature and the mass fraction solubility, and R, m and k denote the cooling rate, apparent nucleation order and nucleation constant, respectively.13 Generally, Eq. (1) can be used to correlate the 4


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measured MSZW. However, the units of the calculated nucleation order and nucleation constant by Eq. (1) are complex13 and have no physical significance. To overcome this disadvantage, Sangwal recently put forward the self-consistent Nývlt-like equation. In this approach, the nucleation rate J was redefined as13 J = K (lnS max ) m

(2)

According to regular solution theory, the correlation between the supersaturation ratio and the maximum temperature difference can be written as13

ln S max = ln(

∆Hs ∆Tmax c0 )=( ) clim RGT0 Tlim

(3)

where c0 and clim are the solution concentrations corresponding to saturation temperature, T0, and the nucleation temperature, Tlim, respectively. Smax denotes the supersaturation ratio when primary nucleation occurs.13 RG and ∆HS refer to the universal gas constant and the dissolution enthalpy, respectively. In addition, the nucleation rate J is proportional to the rate of change of solution supersaturation ∆c/c0 with time t,

J= f

∆H S R ∆c ∆c ∆T = f = f( )( ) clim ∆t clim ∆T ∆t RGTlim T0

(4)

where f is a constant and its unit is number of entities per unit volume.13 Then, the following equations can be deduced from Eqs.(2), (3) and (4) ln

∆Tmax 1 - m ∆H s 1 f 1 = ln( ) + ln( ) + ln R T0 m RGTlim m KT0 m

(5)

Therefore, the nucleation order, m, and ln (f/KT0), can be gotten from the slope and intercept of linear line of ln (∆Tmax/T0) versus ln R because ∆HS/RG can be calculated from the solubility data. 5



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2.2 Classical 3D nucleation theory model According to the classical nucleation theory, the nucleation rate can be written as following: J = Aexp（

- 16πγ 3VS 2 1 ) 3k B 3Tlim 3 ln 2 S

(6)

where J is the rate of nucleation, A is the pre-exponential factor, γ is the solid–liquid interfacial energy, VS is the molecular volume, Tlim is the nucleation temperature, and S is the supersaturation. Combining Eqs. (4) and (6), the following equation can be derived: exp[(

16πγ 3VS 2 RGTlim 2 T0 2 ∆H S R ) ( ) = f 3 3 )( ∆H S ∆Tmax RGTlim AT0 3k B Tlim

(7)

Taking logarithm on both sides of Eq. (7) gives15 (

T0 2 ) = F1 (Χ + ln T0 - ln R) = F - F1 ln R ∆Tmax

(8)

With F=F1(X+lnT0), F1 =

3 k B 3Tlim 3 ∆H S 2 ( ) 16π γ 3VS 2 RGTlim

Χ = ln(

(9)

A RGTlim ) f ∆H S

(10)

where the constant A is related to the kinetics of nuclei formation in the growth medium, kB is the Boltzmann constant equal to RG/NA (NA is the Avogadro number).14 We can obtain A and γ from the intercept and slope of the linear line of ln (∆Tmax/T0) versus ln R. Similar to the Arrhenius reaction rate equation, according to the classical nucleation theory, the nucleation rate J can be written as follows: 6


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J = A exp(

∆G ) k BTlim

(11)

where Gibb's energy, ∆G, relies on nucleus radius r and can be given as following:

4 ∆G = 4πr 2γ + πr 3∆GV 3

(12)

Where ∆Gv is the Gibbs energy change of the transformation per unit volume; and γ is the effective interfacial energy. When d (∆G)/dr=0, the nucleus reaches the critical size, r* and Gibbs energy ∆G reaches maximum, the critical radius can be obtained as following equation: 2γ ∆GV

r* =

(13)

According to Gibb's - Thompson equation, ln S =

2γVS k BTlim r

(14)

Combining Eqs. (12), (13), (14), the following equation can be obtained 16πγ 3VS 2 ∆G = 3(k BTlim ln S ) 2 *

(15)

Theoretically, the induction time has an inverse ratio to the nucleation rate:

tind ∝

1 J

(16)

Taking logarithm on both sides of Eq. (16) can give15 ln tind = Υ +

16πγ 3VS 2 3k B 3Tlim 3 ln 2 S

(17)

From the slope of the straight line of ln tind versus ln2S, the following value can be obtained:

α=

16πγ 3VS 2 3k B 3Tlim 3

(18)

7



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Therefore, the interfacial energy can be obtained by:

γ =（

3αk B 3Tlim 3 13 ) 16πVS 2

(19)

3. EXPERIMENTAL SECTION 3.1. Materials and instruments Sodium sulfate decahydrate (analytical grade, ≥99.5% mass fraction purity) was purchased from Wind Boat Chemical Reagent Co. Ltd. Sodium sulfate anhydrous (guaranteed grade, ≥99.8% mass fraction purity) was purchased from Commie Chemical Reagent Co. Ltd. Phenol (analytical grade, ≥99.5% mass fraction purity) was purchased from Yuanli Chemical Reagent Co. Ltd. Water (resistivity=18.25 MΩ·cm) was deionized from a water purification system. The prepared solutions were filtered through nylon membranes (pore size=0.45 µm).16 The solubility measurement experiments were carried out in immersion oscillator (WE-1, Tianjin Honour Instrument Co. Ltd., China). The experiments of MSZW and induction time were carried out in a 100 mL jacketed glass crystallizer with agitation speed of 300 rpm by a magnetic stirrer (EMS-9A, Tianjin Honour Instrument Co. Ltd., China) to ensure ideal mixing of solution. Programmed heating and cooling of solution was fulfilled by a thermostat water bath (XOYS-2006, Nanjing Xianou Technology Co. Ltd., China).7 A thermocouple thermometer was used to record the temperature of the solution with a precision of ±0.1 °C. The nucleation was monitored by using FBRM (G400, Mettler Toledo Instruments Co. Ltd., Switzerland). All measurements were repeated at least three times to ensure the reliability of the experimental data.16 Measurements of MSZW and induction time were confirmed to 8


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be reliable with no big deviations. 3.2. Solubility measurements The solubilities of sodium sulfate in water in the presence of phenol were measured by using isothermal method.6 The concentrations of phenol in water are 0.05%, 0.1%, 0.5%, 1%, 2%, 3% (mass fraction), respectively. A known amount of water with phenol and excess sodium sulfate decahydrate were added into the conical flask. First, the flask was maintained at a certain temperature and was shaken for 12 hours to make the solution reach solid-liquid equilibrium. It has been confirmed by our pre-experiments that 8h is long enough for the system to reach equilibrium. Then, the oscillation was stopped and the solution was kept still for 1h to ensure the undissolved particles to settle down. Later, samples were taken with 2 mL syringe and filtered through nylon membranes into 50 mL pre-weighed volumetric flasks, weighed, and diluted. Finally, the concentration of sodium sulfate in solutions was obtained by automatic potentiometric titration (KH-100J, Taizhou Datang Analytical Instrument Co. Ltd., China). Consequently, solubility data of sodium sulfate in the presence of phenol at different temperatures were obtained. 3.3. MSZW and induction time measurements The MSZW experiments were conducted by using the conventional polythermal method.6,17 Before measurements, to ensure the complete dissolution of the solute, saturated solutions were maintained at temperatures which are 5 °C higher than the saturation temperatures for 30 min under agitation of 300 rpm.16 Then, solutions were cooled down to saturation temperature and then further cooled down at constant rates 9



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until nucleation was detected by FBRM whose detecting frequency was set at 10s. The difference between saturation temperature and the temperature corresponding to the onset of nucleation is defined as MSZW.16 The cooling rates are set at 12 K/h, 30 K/h and 60K/h, respectively. The induction time experiments were carried out with different supersaturation ratios from 1.2 to 1.36 at two different temperatures (293.15K and 298.15K). Induction time was measured by the isothermal method.17 Firstly, saturated solutions were prepared according to solubility data. Then, they were maintained at temperatures which are 5℃ higher than the saturation temperature for 30min under agitation of 300rpm,16 to ensure the complete dissolution of the solute. Then, the solutions were rapidly cooled down to preset temperature and were kept at constant temperature until nucleation was detected. The induction time tind was the elapsing time from the reach of target supersaturation to the detecting of the nucleation phenomenon.

4. RESULTS AND DISCUSSION 4.1. Effect of impurity on solubility To investigate the effect of impurity on thermodynamics of sodium sulfate, solubilities of sodium sulfate in water with different concentrations of phenol at different temperatures from 278.15K-303.15K were measured. The results are shown in Table 1. It can be seen that the solubility data decrease with the increasing concentration of phenol and increase with the increasing of temperature. The experimental solubility data were correlated using empirical equations. The results are 10


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shown in Table S1. The experimental and correlated solubility data of sodium sulfate in pure water and water with 3% phenol are shown in Figure 1. As shown in Table S1 and Figure 1, the experimental results are well correlated as a function of temperature with the exponential function.15 The solubility can also be described in mole fraction x, which can be calculated by using Eq. (20) x=

m1 M 1 m1 M 1 + m2 M 2 + m3 M 3

(20)

where m1, m2, m3 refer to the mass of sodium sulfate, water and phenol, respectively. Similarly, M1, M2, M3 represent the molar mass of sodium sulfate, water and phenol, respectively.

Table 1. Solubility data of Na2SO4 in water with different concentrations of phenol at different temperatures from (278.15K - 303.15K) w(phenol) (%) 0 0.05 0.1 0.5 1 2 3

c(g/100g) 278.15K 6.383 6.268 6.149 5.998 5.871 5.542 5.175

283.15K 9.111 8.774 8.670 8.563 8.528 8.039 7.676

288.15K 13.12 12.71 12.55 12.41 12.32 11.71 11.00

293.15K 19.50 18.30 18.21 17.96 17.67 17.11 16.58

298.15K 27.88 26.50 26.48 26.29 25.91 25.16 24.93

303.15K 40.80 38.69 38.56 38.20 37.91 37.69 36.98

11



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Figure 1. Experimental and correlated solubility data of sodium sulfate in pure water and water with 3% phenol. Solid lines are correlated values by the exponential function.

From literature,15 the data of ∆HS can be obtained from the Van’t Hoff equation as following ln x =

∆H S ∆S + RGT RG

(21)

where x is mole fraction, T is the temperature in Kelvin, ∆HS is the dissolution enthalpy, ∆S is the dissolution entropy and RG is the gas constant (8.314 J K-1 mol-1). According to the Eq. (21), the dissolution enthalpy can be obtained from the slope of the straight line of ln x versus 1/T. As an example, the plot of ln x versus 1/T in water with 3% phenol is shown in Figure 2. The obtained ∆HS / RG are shown in Table S2.

12


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Figure 2. Experimental and correlated mole fraction solubility (ln x) of sodium sulfate in water with 3% phenol versus 1/T. Solid line is correlated values by the Van’t Hoff model.

4.2 Effect of impurity on MSZW To investigate the effect of impurity on nucleation behavior of sodium sulfate, the MSZW of sodium sulfate in presence of phenol were measured under different saturation temperatures and different cooling rates. The MSZW of sodium sulfate in water with different concentrations of phenol at saturation temperature 283.15 K are shown in Figure 3 and all the MSZW data are listed in Table S3. And the mean standard deviations of MSZW are given in Table S5. It can be found that the MSZW increases with the increasing of saturation temperature and cooling rate. This result is different from some other nucleation investigations. Generally, at higher temperature, the concentration of salt will be higher, which will increase the frequency of ions collision frequency and hence promotes the crystal nucleation rate. But on the other hand, with the increasing of the concentration, the viscosity of solution will also increase significantly, which will suppress the ion diffusion process and hence delays 13



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the crystal nucleation process. Meanwhile, at higher temperatures, the ions will possess higher energy, meaning that more work is needed to fix them into the crystal lattice, which will also delay nucleation process. Therefore, the nucleation process is controlled by many factors. As for sodium sulfate, the restraining effect of higher viscosity and higher energy of ions at higher temperature might be dominant and thus will result in a wider MSZW. More importantly, it can be seen from Figure 3 and Table S3 that the MSZW increase apparently with the increasing of phenol concentration. This phenomenon is in accordance with the result that the MSZW of ammonium dihydrogen orthophosphate (ADP) was enhanced by the addition of the organic compounds (Urea, L-arginine monohydrochloride and L-alanine)3-4.

Figure 3.MSZW of sodium sulfate in water with different concentrations of phenol at saturation temperature (283.15K).

To investigate the affecting mechanism of impurity on nucleation of sodium sulfate, Self-consistent Nývlt-like model and Classical 3D nucleation theory model were used to analyze the experimental MSZW data. According to Self-consistent 14


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Nývlt-like model, the plots of ln (∆Tmax/T0) versus ln R are shown in Figure 4. It can be clearly seen that all the plots are straight lines. The model parameters (listed in Table 2) can be obtained by using these plots. According to Classical 3D nucleation theory, plots of (T0/∆Tmax)2 versus ln R are plotted. The results are shown in Figure 5. Again, it can be clearly seen that all the plots are also straight lines. The model parameters (listed in Table 3) for Classical 3D nucleation theory were also obtained by using these plots. From Table 2 and Table 3, it can be seen that there is no regular relationship between the concentration of phenol with the nucleation order m, the constant K and A of sodium sulfate. From Table 3, it can be seen that, for all tested temperatures and concentrations of impurity, the interfacial energy γ increases with the increasing of concentrations of phenol. Although both of the two models can well correlate the MSZW data with no significant deviations, the model parameters of Nývlt-like model had no apparent physical significance and hence it is difficult to use Nývlt-like model to interpret the influence of impurity. However, the model parameters in CNT model are associated with the kinetic and thermodynamic of formation of nuclei and γ shows a good correlation with the impurity concentrations. Therefore, in this work, the CNT model was used to explain the influence of impurity. According to CNT model, the increasing value of interfacial energy γ will increase the nucleation Gibbs energy barrier and thus makes the nucleation more difficult. That means higher concentration of phenol will make the nucleation of sodium sulfate more difficult, which will result in wider MSZW. 15



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Table 2(a). Calculated m for sodium sulfate in both pure and impure systems m T0 (K) 278.15 283.15 293.15 303.15

0 8.347 4.361 5.000 5.562

0.05 7.347 4.868 4.854 8.052

0.1 7.163 9.033 9.302 5.203

w(phenol) (%) 0.5 1 7.849 8.258 7.199 8.482 8.826 8.203 7.513 6.188

2 9.132 9.950 7.949 6.631

3 11.27 10.49 7.806 7.037

Table 2(b). Calculated K for sodium sulfate in both pure and impure systems

T0 (K) 278.15 283.15 293.15 303.15

0 4218 0.9991 2.213 1.123

K(×1031m-3•h-1) w(phenol) (%) 0.1 0.5 1 39.31 57.91 44.78 373.7 15.22 51.94 545.4 59.36 38.62 0.5001 8.714 1.080

0.05 365.6 1.638 1.026 6.781

2 67.84 153.4 11.57 1.157

3 458.9 166.0 5.433 1.149

Table 3(a). Calculated γ for sodium sulfate in both pure and impure systems

T0 (K) 278.15 283.15 293.15 303.15

0 7.844 7.021 7.929 10.21

0.05 8.131 7.698 8.427 10.82

0.1 8.352 10.89 11.13 11.64

γ (mJ•m-2) w(phenol) (%) 0.5 1 10.04 10.05 11.11 11.40 11.48 11.78 12.34 13.00

2 10.98 12.03 13.39 14.15

3 11.84 12.49 14.34 16.04

Table 3(b). Calculated A for sodium sulfate in both pure and impure systems

T0 (K) 278.15 283.15 293.15 303.15

0 27.76 3.780 4.910 5.892

0.05 24.37 4.762 4.605 8.059

A(×1028m-3•h-1) w(phenol) (%) 0.1 0.5 1 5.985 18.98 9.582 19.43 17.03 31.60 21.55 14.24 29.37 14.70 18.97 23.30

2 12.72 26.03 59.83 35.95

3 15.95 22.98 80.30 113.9

16


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(a)

(b)

(c)

17



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(d)

Figure 4. The plots of ln (∆Tmax/T0) versus ln R at saturation temperatures (a)278.15K (b) 283.15K (c) 293.15K (d)303.15K. Lines are fitted according to Self-consistent Nývlt-like model.

(a)

(b)

18


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(c)

(d)

Figure 5. The plots of (T0/∆Tmax)2 versus ln R at saturation temperatures (a) 278.15K (b) 283.15K (c) 293.15K (d) 303.15K. Lines are fitted according to 3D CNT model. 4.3 Effect of impurity on induction time The induction time tind measured at different supersaturations and different concentrations of phenol are graphically shown in Figure 6 and listed in Table S4. The mean standard deviations of induction time are also given in Table S6. It can be seen that the induction time decreases dramatically with the increasing of supersaturation ratio S while it increases obviously with the increasing of temperature. The effect of temperature on induction time could be explained by the fact that the increase of solubility of sodium sulfate with temperature will causes apparent 19



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viscosity increase in the system, which will delay the crystal nucleation process. It can also be seen that the induction time increases with the increasing of concentrations of phenol. That means the existence of phenol will delay the nucleation of sodium sulfate. This result is in accordance with the effect of amino acid materials (L-arginine monohydrochloride and L-alanine) on the induction time of ADP: the induction time increases with the increasing concentrations of impurity. To further investigate the effect of impurity on nucleation of sodium sulfate, the relationships between ln tind versus 1/ (ln2S) in pure and impure systems are displayed in Figure 7. It can be seen that all lines are linear, which is consistent with classical nucleation theory. From Figure 7, the interfacial energy γ can be obtained from the slopes of the straight lines according to Eq. (19). The results are listed in Table 4, it can be seen that the interfacial energy γ increases with the increasing temperature and the increasing concentration of phenol. The higher interfacial energy at higher temperature could also be explained by the higher viscosity of the system at higher temperature since higher viscosity will generally result in higher interfacial energy. These results are not unique in the nucleation studies. For example, Maheswata and Debasis18 also reported increasing of the interfacial energy of L-asparagine monohydrate with the increasing temperature. Meanwhile, Zhou et al.19 also reported the same results with analgin. The effect of impurity on interfacial energy confirms again that the existence of impurity will make the nucleation more difficult. The critical nucleus radius r* and the critical Gibbs energy ∆G* were also obtained according to Eq (13) and Eq (15), and listed in Table 5 and Table 6. It can be seen that 20


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the critical nucleus radius r* and the critical Gibbs free energy ∆G* all increase with the addition of phenol. According to classical nucleation theory, the increasing critical nucleus radius and the increasing critical Gibbs free energy will increase the nucleation Gibbs energy barrier, which will make the formation of new nuclei more difficult. Therefore, all these data confirm that the existence of phenol will restrain the nucleation of sodium sulfate.

(a)

(b)

Figure 6. The plots of induction time tind versus supersaturation ratio S at saturation temperatures (a) 293.15K (b) 298.15K. The lines are guides to the eye.

21



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(a)

(b)

Figure 7. The plots of ln tind versus 1/ (ln2S) at saturation temperatures (a) 293.15K (b) 298.15K in water with different concentrations of phenol. Lines are fitted according to CNT model.

Table 4. Calculated γ for sodium sulfate in both pure and impure systems by tind T0 (K) 293.15

298.15

γ (mJ•m-2)

S 1.356 1.305 1.257 1.211 1.356 1.305 1.256 1.210

0 7.117 7.129 7.141 7.154 8.192 8.206 8.220 8.234

0.05% 7.455 7.468 7.480 7.493 8.700 8.715 8.730 8.744

0.1% 7.889 7.902 7.916 7.930 8.734 8.749 8.764 8.779

0.5% 8.281 8.295 8.309 8.324 8.775 8.790 8.805 8.820

1% 8.304 8.318 8.333 8.347 8.784 8.800 8.814 8.829

2% 8.320 8.335 8.349 8.364 8.795 8.810 8.825 8.840

3% 8.468 8.483 8.497 8.512 8.808 8.823 8.838 8.853 22


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Table 5. Calculated r* for sodium sulfate in both pure and impure systems T0 (K) 293.15

298.15

r* (×10-10m)

S 1.356 1.305 1.257 1.211 1.356 1.305 1.256 1.210

0 2.863 3.266 3.802 4.550 3.238 3.700 4.317 5.181

0.05% 2.999 3.421 3.983 4.766 3.439 3.930 4.584 5.502

0.1% 3.174 3.620 4.214 5.043 3.452 3.945 4.603 5.524

0.5% 3.331 3.800 4.424 5.294 3.469 3.964 4.624 5.550

1% 3.341 3.811 4.436 5.309 3.472 3.968 4.629 5.555

2% 3.347 3.818 4.445 5.319 3.476 3.972 4.634 5.562

3% 3.407 3.886 4.524 5.414 3.482 3.978 4.641 5.571

Table 6. Calculated ∆G* for sodium sulfate in both pure and impure systems T0 (K) 293.15

298.15

∆G* (×10-14 J)

S 1.356 1.305 1.257 1.211 1.356 1.305 1.256 1.210

0

0.05%

0.1%

0.5%

1%

2%

3%

0.6467 0.8430 1.144 1.642 0.9521 1.245 1.698 2.450

0.7432 0.9689 1.315 1.887 1.141 1.492 2.034 2.935

0.8808 1.148 1.559 2.236 1.154 1.510 2.058 2.970

1.019 1.328 1.803 2.586 1.170 1.531 2.087 3.011

1.027 1.339 1.818 2.608 1.174 1.535 2.093 3.020

1.033 1.347 1.829 2.623 1.178 1.541 2.101 3.032

1.089 1.420 1.928 2.765 1.184 1.548 2.111 3.045

It is worth noting that the interfacial energy γ calculated by isothermal method is slightly lower than the interfacial energy γ obtained by the polythermal method. This might be caused by the underestimated value of ln S. The chemical potential difference ∆µ for crystallization is given by20,21 ∆µ = RGTC ln (α 0 / α C）= RGT Cln S = RGT C[ln(c0 / cC ) + ln( f 0 / f C )

(22)

where α0 and αc are the activities of the solution at T0 and Tc, respectively. f0 and fc are the corresponding activity coefficients, and ∆µ = (∆H S / T0 )∆T

(23)

In our calculations of ln S, we used the values of the concentrations c0 and cc alone. Since the ratio f0/fc > 1 for aqueous solutions of fairly soluble electrolytes, this 23



Page 24 of 31

will make the value of ln S lower than the real values obtained from the activities of the solution. Therefore, the interfacial energy γ calculated using ln S in the isothermal method will be lower than the values obtained from ∆Tmax (MSZW) by the polythermal method.2 4.4 Possible mechanism of the effect of impurities on crystallization Crystallization in multicomponent systems will be influenced by various factors, such as solutes, foreign molecules, additives and other components. Generally, impurities will affect both thermodynamics and kinetics of the crystallization. In this work, obviously, the solubility is significantly affected by the existence of phenol. According to the data of MSZW and tind, the interfacial energy γ which is a key parameter for the nucleation of crystals was also affected by the existence of phenol. Therefore, the existence of phenol will affect both the thermodynamics and kinetics of sodium sulfate. The effect of the phenol may be explained by the interaction between phenol and sodium sulfate. There is double bond of sulfur-oxygen in sodium sulfate while there is π bond in phenol molecules. Therefore, hydrogen bond could be formed between phenol and sodium sulfate and it will affect the formation of sodium sulfate decahydrate. To investigate the effect of hydrogen bond, molecular dynamic simulation was carried out. The results showed that the energy of crystallization system changed from -61390.7 Kcal/mol to - 61672.1 Kcal/mol with the addition of phenol, which reveals that the system is more stable in the presence of phenol. Meanwhile, the bond distance between phenol and sodium sulfate is 1.386 Å while the average bond distance between water and sodium sulfate is 1.7395 Å. These data 24


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indicate that the nucleation of sodium sulfate will become more difficult in the presence of phenol.

5. CONCLUSIONS The effect of phenol on crystallization thermodynamics of sodium sulfate was performed by measuring solubility data of sodium sulfate in water in the presence of phenol. It was found that the existence of phenol will decrease the solubility of sodium sulfate in water. Higher concentration of phenol will result in lower solubility of sodium sulfate. The effect of organic impurity on crystal nucleation was investigated by measuring the MSZW of sodium sulfate. Two models (Nývlt-like equation and 3D CNT) were used to analyze the experimental data. It was found that 3D CNT theory can better explain the effect of phenol on nucleation. The addition of phenol will apparently increase the interfacial energy γ, which will result in higher nucleation Gibbs energy barrier and thus lower nucleation rate. Furthermore, the effect of existence of phenol on nucleation of sodium sulfate was also investigated by measuring the induction time. It was found that the induction time became longer with the increasing concentration of phenol. According to classical nucleation theory, it was confirmed again by the induction time that the existence of phenol will increase the interfacial energy γ, the critical nucleus radius r* and the critical Gibbs energy ∆G*, which means that the formation of the nuclei will be more difficult with the addition of phenol. This phenomenon could be explained by the strong interaction between phenol and sodium sulfate molecules. The results presented in this work could help us to develop and optimize the 25



Page 26 of 31

crystallization technology for extracting sodium sulfate with ideal properties from high salinity wastewater.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: 86-22-27405754. Fax: +86-22-27374971.

ORCID Hongxun Hao: 0000-0001-6445-7737

NOTE The authors declare no competing financial interest.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed information of solubility equations, correlated dissolution enthalpy, the data of MSZW and induction time, the mean standard deviations in MSZW and induction time for sodium sulfate in pure and impure systems.

ACKNOWLEDGMENTS This work is financially supported by the National Key Research and Development (No. 2016YFB0600504)

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REFERENCES (1) Lu, S.; Zhao, Y.; Yuan, J. The Influence of Different Organic Impurity on the Solubility and MSZW of Sodium Chloride. Chem. Ind. Eng. Pro. 2017, 36 (09), 3210. (2) Peng, J.; Dong, Y.; Wang, L.; Li, L.; Li, W.; Feng, H. Effect of Impurities on the Solubility, Metastable Zone Width, and Nucleation Kinetics of Borax Decahydrate. Ind. Eng. Chem. Res. 2014, 53 (30), 12170. (3) Rajesh, N. P.; Lakshmana Perumal, C. K.; Santhana Raghavan, P.; Ramasamy, P. Effect of Urea on Metastable Zone Width, Induction Time and Nucleation Parameters of Ammonium Dihydrogen Orthophosphate. Cryst. Res. Technol. 2015, 36 (1), 55. (4) Dhanaraj, P.V.; Bhagavannarayana, G.; Rajesh, N.P. Effect of Amino Acid Additives on Crystal Growth Parameters and Properties of Ammonium Dihydrogen Orthophosphate Crystals. Mater. Chem. Phys. 2008, 112 (2), 490. (5) Becheleni, E. M. A.; Rodriguez-Pascual, M.; Lewis, A. E.; Rocha, S. D. F. Influence of Phenol on the Crystallization Kinetics and Quality of Ice and Sodium Sulfate Decahydrate during Eutectic Freeze Crystallization. Ind. Eng. Chem. Res.

2017, 56 (41), 11926. (6) Mullin, J.W. Crystallization; 4thed, Butterworth-Heinemann: Oxford, 2001. (7) Kuldipkumar, A.; Kwon, G. S.; Zhang, G. G. Z. Determining the Growth Mechanism of Tolazamide by Induction Time Measurement. Cryst. Growth Des.

2007, 7 (2), 234 (8) Mitchell, N. A.; Frawley, P. J. Nucleation Kinetics of Paracetamol–Ethanol 27



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Solutions from Metastable Zone Widths. J. Cryst. Growth 2010, 312 (19), 2740. (9) Ni, X.; Liao, A. Effects of Mixing Seeding, Material of Baffles and Final Temperature on Solution Crystallization of L-glutamic Acid in an Oscillatory Baffled Crystallizer. Chem. Eng. J. 2010, 156 (1), 226. (10) Xu, C.; Liu, D.; Chen, W. Effects of Operating Variables and Additive on the Induction Period of MgSO4–NaOH System. J. Cryst. Growth 2008, 310 (18), 4138. (11) Gherras, N.; Fevotte, G. Comparison between Approaches for the Experimental Determination of Metastable Zone Width: a case Study of the Batch Cooling Crystallization of Ammonium Oxalate in Water. J. Cryst. Growth 2012, 342 (1), 88. (12) Nagy, Z. K.; Fujiwara, M.; Woo, X. Y.; Braatz R. D. Determination of the Kinetic Parameters for the Crystallization of Paracetamol from Water using Metastable Zone Width Experiments. Ind. Eng. Chem. Res. 2008, 47 (4), 1245. (13) Zhang, X.; Yang, Z.; Chai, J.; Xu, J.; Zhang, L.; Qian, G.; Zhou, X. Nucleation Kinetics of Lovastatin in Different Solvents from Metastable Zone Widths. Chem. Eng. Sci. 2015, 133, 62. (14) Sangwal, K. A Novel Self-consistent Nývlt-like Equation for Metastable Zone Width Determined by the Polythermal Method. Cryst. Res. Technol. 2009, 44, 231. (15) Xu, S.; Wang, J.; Zhang, K.; Wu, S.; Liu, S.; Li, K.; Yu, B.; Gong, J. Nucleation Behavior of Eszopiclone-Butyl Acetate Solutions from Metastable Zone Widths. Chem. Eng. Sci. 2016, 155, 248. (16) You, S.; Zhang, Y.; Zhang, Y. Nucleation of Ammonium Aluminum Sulfate Dodecahydrate from Unseeded Aqueous Solution. J. Cryst. Growth 2015, 411, 24. 28


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(17) Garside, J.; Mersmann, A.; Nývlt, J. Measurement of Crystal Growth and Nucleation Rates; IChemE: London, 2002. (18) Lenka, M.; Sarkar, D. Determination of Metastable Zone Width, Induction Period

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List of figure captions Figure 1. Experimental and correlated solubility data of sodium sulfate in pure water and water with 3% phenol. Solid lines are correlated values by the exponential function.

Figure 2. Experimental and correlated mole fraction solubility (ln x) of sodium sulfate in water with 3% phenol versus 1/T. Solid line is correlated values by the Van’t Hoff model.

Figure 3.MSZW of sodium sulfate in water with different concentrations of phenol at saturation temperature (283.15K).

Figure 4. The plots of ln (∆Tmax/T0) versus ln R at saturation temperatures (a)278.15K (b) 283.15K (c) 293.15K (d)303.15K. Lines are fitted according to Self-consistent Nývlt-like model.

Figure 5. The plots of (T0/∆Tmax)2 versus ln R at saturation temperatures (a) 278.15K (b) 283.15K (c) 293.15K (d) 303.15K. Lines are fitted according to 3D CNT model.

Figure 6. The plots of induction time tind versus supersaturation ratio S at saturation temperatures (a) 293.15K (b) 298.15K.

Figure 7. The plots of ln tind versus 1/ (ln2S) at saturation temperatures (a) 293.15K (b) 298.15K in water with different concentrations of phenol. Lines are fitted according to CNT model.

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Mechanism of influence of Organic Impurity on Crystallization of

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