The Controlling Factors and Ion Exclusion Mechanism of Hydrate

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The controlling factors and ion exclusion mechanism of hydrate-based pollutant removal Hongsheng Dong, Lunxiang Zhang, Zheng Ling, Jiafei Zhao, and Yongchen Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00651 • Publication Date (Web): 09 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Sustainable Chemistry & Engineering

The controlling factors and ion exclusion mechanism of hydrate-based pollutant removal Hongsheng Donga,b,1, Lunxiang Zhanga,1, Zheng Linga, Jiafei Zhaoa, Yongchen Songa* a

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of

Education, School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, China. b

The University of Edinburgh, School of Engineering, Institute for Materials and

Processes, Sanderson Building, The King's Buildings, Mayfield Road, EH9 3BF Edinburgh, Scotland, UK

Mailing Address: No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province, P.R.C., 116024.

1 

These authors contributed equally to this work and should be regarded as co-first authors. Corresponding author: Email address: [email protected]

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Abstract Theoretically, impurities can be excluded from hydrate structure and pure water can be extracted during hydrate formation. We achieved the heavy metal (Cu2+ in this study) removal from aqueous solution by hydrate process. The hydrate samples were analyzed by X-ray computed tomography (CT). Boundary layer analysis was conducted to reveal the mechanism of ion exclusion. Moreover, the effect of initial concentrations on removal characteristics was analyzed and the solid–liquid separations were optimized. The main findings were as follows: 1) Solution pockets and trapped solution were identified as the limiting and controlling factors for the removal efficiency; 2) nonguest molecule exclusion and directed substance migration occur during hydrate formation; and 3) this method was suitable for relatively high concentration wastewater treatment application, and a removal efficiency of 96.63% could be achieved. We then proposed a theory in regard to the concentration gradient and directed substance migration during hydrate formation. Additionally, the concentrations near the hydrate front were calculated, which were used to further understand the mechanism of the process. Finally, an optimized heavy metal removal process was proposed. The results provide guidance for wastewater treatment, substance enrichment, and hydrate application improvements.

Keywords: hydrate; heavy metal; wastewater; ion exclusion; water treatment

Introduction


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Heavy metal-bearing wastewater can cause water and soil pollution and should therefore be treated prior to discharge.1 Although chemical precipitation and coagulation–flocculation techniques have been widely used to treat electroplating wastewater, these methods have several drawbacks.2-5 To apply these technologies, certain parameters (such as pH, pollutant type and quantity, temperature) of the raw wastewater should meet rigid requirements. Consequently, there is a growing need for alternative methods of treating wastewater containing heavy metals. Hydrates are crystalline solids composed of water and small molecules.6-7 The small molecules are trapped in water cavities that are composed of hydrogen-bonded water molecules.8-10 Their unique structure includes only water and guest molecules: hydrate formation excludes most organic compounds and all inorganic substances from the crystalline structure, and these materials are left behind in the residual water.9,

11-13

During geological exploration in a number of deep sea drilling projects and at offshore drilling platform sites, hydrates were found to cause pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections.14 During these processes, dissolved ions (Na+ and C1–) are excluded from hydrate structure in a process called ion exclusion.15 Molecular dynamics simulations have revealed that Na+, Mg2+, can be excluded from hydrate structure.16-17 In experiment, Wilson et al.18 measured the Workman Reynolds Freezing Potential between tetrahydrofuran hydrate and water. They found that when tetrahydrofuran hydrate crystals are grown into salt solutions, all ion species are excluded equally. Therefore, it can be concluded that all salt ions could be removed by hydrate technology. However, little research on wastewater treatment



via hydrate process has been published to date. Even so, other hydrate-based technologies have been developed in widely industrial fields. Li’s group performed a lot of valuable studies on CO2 separation and capture via hydrate formation,19-22 this gives useful strategies to deal with the challenge of greenhouse effect and global warming. More creatively, gas hydrates are also applied to cold storage refrigeration processes.23-24 So far, however, there is no report on the research on hydrate-based pollutant removal. Thus, in this work, hydrate samples were scanned with CT to identify the controlling factors of the removal efficiency. Five solid–liquid separation methods were performed to remove the trapped solution. Real-time electrical conductivity measurements during hydrate formation were conducted to reveal the mechanism of ion exclusion. On the basis of the findings, we performed calculations with mass transfer equations for hydrate formation in aqueous solution. In addition, we analyzed the impact of initial ions concentrations on the removal characteristics and optimized the centrifugation process.

Methods We established an experimental system for the removal of heavy metal ions via hydrate process, as shown in Figure S1 (Supplementary Information). The experimental system involved four main parts, namely, hydrate formation, solid–liquid separation, hydrate dissociation, and analysis. HCFC R141b and copper sulfate solution were employed to form hydrate.25 R141b can react with water to form a hydrate of structure II at


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temperatures < 8.4 °C and pressures > 42 kPa.26-27 After the hydrate formation process was complete, five solid–liquid separation methods were respectively applied. All of the above operations were conducted in a refrigeration house. Next, dewatered hydrate was dissociated. R141b was separated from the produced mixture of R141b and dissociated water based on its immiscibility with water. Cu2+ concentrations were measured using inductively coupled plasma optical spectrometer (ICP). Subsequently, an electrical conductivity meter (ECM) was applied in situ to record electrical conductivity changes. Finally, a CT was used for visual observation of hydrate formation. Removal efficiency (Re), the water yield (Wy), and the enrichment factor (Ef) are defined and calculated by concentration ratio and volume ratio,

Re =

C 0 - Cd C0

Wy =

Vd V0

(2)

Cre C0

(3)

Ef =

(1)

respectively. Details of the data and methodology are given in the Supplementary Information.

Results and discussion Analysis of the limiting and controlling factors To analyze the limiting and controlling factors of the removal efficiency, the hydrate samples were scanned by CT, and the spatial distribution of various phases was



identified. Given that the process of hydrate growth is random, repeated trials were conducted. Figure 1 shows the analysis of hydrate samples before solid–liquid separation. From an analysis of the reconstructed three-dimensional (3D) images (Figure 1a), it was determined that the hydrate slurry was composed of 58.13 vol.% hydrate, 23.07 vol.% air, 16.03 vol.% aqueous solution, and 2.77 vol.% R141b. Assuming that the volume change of water converted to hydrate could be ignored, it can be concluded that 78.38 vol.% of the water was converted to hydrate, which is consistent basically with theoretical calculations according to the volume ratio of aqueous solution and hydrate former. Moreover, it was noted that the hydrate, air, aqueous solution, and R141b were randomly distributed (Figure 1b and 1c). The spaces throughout the hydrate were made of air, aqueous solution, and R141b. Aqueous solution and hydrate were segmented, and then, the volumes of aqueous solution and hydrate were computed (Figure 1d). It was found that the volumes of aqueous solution varied from 1 voxel to 10,595 voxels with a super drop (78,886 voxels), and volumes of hydrate pores varied from 1 voxel to 250 voxels with a super pore (526,594 voxels). Therefore, these large aqueous solution drops greater than 250 voxel were filled in the super pore, which were interconnected by air and R141b, and other aqueous solution drops whose volumes equaled the volumes of hydrate pores may have filled in the corresponding hydrate pores exclusively; this may have induced the formation of solution pockets. Further, the hydrate pore space was isolated and partitioned into pores and throats. The pore-throat equivalent radius distributions of hydrate pores were obtained based on pore-network modeling (Figure 1e and 1f). The equivalent radius of


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pores and throats had similar distributions, but the equivalent radiuses of throats were always smaller than those of pores. Because some throats were small (as small as 0.3 pixels), this may have induced a capillary effect in which aqueous solution was trapped more tightly. According to this analysis, some spaces were interconnected and formed channels inside the hydrate, and the majority of the aqueous solution (i.e., trapped solution) and R141b was connected to the channels; this material can be excluded from the hydrate slurry by proper solid–liquid separation techniques. Meanwhile, a small portion of the aqueous solution was trapped in the hydrate within isolated solution pockets. These solution pockets were very tiny and were trapped in hydrate slurry tightly, and thus, it was difficult to separate them from the hydrate slurry. When the hydrate samples decomposed, these solution pockets fractured and aqueous solution was released into the dissociated water; consequently, this phenomenon was found to have decreased the removal efficiency. So solution pockets and trapped solution represent controlling and limiting factors for the removal efficiency, and the amounts will determine the maximum removal efficiency. Thus, separation of the hydrates and trapped solution from the hydrate slurries was the key to increasing the removal efficiency. Five methods of trapped solution removal were performed, and these methods are referred to here as no operation, vacuum filtration (VF), washing with a hydrate former and vacuum filtration (WHVF), washing with fresh water and vacuum filtration (WRVF), and vacuum filtration and centrifugation (VFC). Figure 2 shows a comparison of the effect of different solid– liquid separation techniques on the removal efficiency. If the hydrate slurry was



transferred into a Buchner funnel and only gravity was relied on (no operation) to achieve the separation between the hydrate and the trapped solution, the removal efficiency was only about 44.79%; such a removal efficiency would not be sufficient for meeting wastewater discharge standards. Compared to no operation, the removal efficiency displayed a significant increase of 71.87% when using vacuum filtration for 1 min. However, vacuum filtration was based on a pressure difference of about 1 atmospheric pressure, which was largely inadequate for overcoming the adhesion forces between the hydrate particles and the residual concentrated aqueous solution. To further improve the removal efficiency, a washing operation was performed whereby massive hydrate former (since R141b is recyclable for washing and forming hydrate) or fresh water (one-tenth of the initial aqueous solution volume) were used to sweep away the ions trapped between the hydrate phases. If the hydrate was washed with hydrate former during vacuum filtration, the removal efficiency could feasibly increase to 87.69%; nevertheless, R141b is insoluble in water, which reduced its performance to some extent. Alternatively, only small amounts (one-tenth of the initial solution volume) of fresh water could be used to rinse ions adhering on the hydrate surface. The removal efficiency showed increases of went up to 90.82% when using this method. Compared to washing with hydrate former, ions dissolve well in water, and so this method can be used to achieve excellent removal efficiencies. However, as this process essentially consumes fresh water, it decreases the actual water yield. In addition, the increased removal might be partly attributable to the dilution from fresh water during the washing operation. Finally, to avoid wasting fresh water, vacuum filtration and centrifugation


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were combined to actualize solid–liquid separation. The centrifugal conditions were 1580 N of relative centrifugal force (RCF) and 8 min of centrifugal time. After undergoing vacuum filtration for 1 min and centrifugation for 3 min, 90.46% of the ions were removed from the bulk. This result is comparable to the results obtained when using the washing with fresh water and vacuum filtration method. This method is ideal because it can save fresh water, reduce costs, and achieve high removal efficiencies. Taking these findings into consideration, we can conclude that solid–liquid separation methods directly determine the performance of hydrate-based ion removal technology and that the combination of vacuum filtration and centrifugation is the most effective solid–liquid separation method among the methods tested. To examine the phase distribution and composition change in the hydrate slurry, the hydrate samples after solid–liquid separation were scanned by CT. Figure 3 illustrates the analysis results for the hydrates sample after solid–liquid separation using vacuum filtration and centrifugation. From an analysis of the reconstructed 3D images, it was determined that the system was composed of 82.36 vol.% hydrate, 13.89 vol.% air, and 3.75 vol.% aqueous solution. R141b was almost entirely separated from the system, and the air and aqueous solution had decreased sharply compared with the hydrate slurry composition before solid–liquid separation. In addition, the air and aqueous solution in the hydrate were discontinuous; the residual solution in the hydrate was also isolated. Under such a situation, the aqueous solution cannot be separated from the hydrate using solid–liquid separation techniques, and this will decrease the removal efficiency when the hydrate decomposes and the aqueous solution is released from



solution pockets. On the basis of the above analysis, the trapped solution was found to be the controlling factor for the removal efficiency. Solid–liquid separation was identified as an effective way for increasing the removal efficiency, and the combination of vacuum filtration and centrifugation was the most effective method. Analysis of the ion exclusion mechanism and boundary layer during hydrate formation in an aqueous solution The real-time electrical conductivity changes were recorded during hydrate formation and converted to Cu2+ concentration. The conversion process was showed in Supplementary Information. According to the variation trend of Cu2+ concentration, the hydrate formation process can be divided into four stages. There was a revulsive period (to form the crystal core) in the initial stage in which the Cu2+ concentration was retained at 80.26 mg/L up to 172 min; however, the hydrate would have begun to form even earlier (due to some time needed for the ions near the hydrate front to diffuse to the electrical conductivity electrode). At this stage, there were not obvious signs of hydrate formation, and all of the ions were distributed uniformly in aqueous solution; therefore, the Cu2+ concentration did not show any changes. At the second stage, a hydrate phase appeared in the form of a thin film. The hydrate formed slowly from 172 min to 392 min during this second stage, and correspondingly, the Cu2+ concentration showed a slight increase from 80.26 mg/L to 80.71 mg/L; this proved that ions were excluded from the hydrate structure. During the third stage, large amounts of hydrate were formed rapidly until 1840 min, with a thick hydrate shell intervening between the hydrate-former and liquid-water phases. Because of the rapid hydrate formation,


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massive number of ions were excluded from the hydrate structure and these ions entered into the aqueous solution; this corresponded to drastic growth in the Cu2+ concentration by an increment of 12.57 mg/L. Cu2+ concentration increased sharply by 1.76 mg/L from 1840 min to 1844 min, which was caused by hydrate formation near the conductivity electrode. During the final stage, the thick hydrate shell hindered further hydrate-former-to-water contact, which led to the successive growth of the hydrate phase. This ultimately led to incomplete converted hydrate, and small amounts of residual concentrated solution were wrapped into the hydrate particles. At this stage, hydrate formation stopped, and so the Cu2+ concentration remained stable at around 94.85 mg/L. These changes proved that hydrate formation led to ion exclusion and then drove substance migration, which led to the formation of concentration gradients in this system. To clarify the change rule of Cu2+ concentration, it is necessary to understand this hydrate-based process. Figure 4b shows a schematic diagram for the removal of heavy metals from aqueous solution using the hydrate process. When aqueous solution and hydrate former (taking R141b as an example) were injected into the reactor, there was an obvious interface between them because of their immiscibility. The salt ionized in solution and interacted with the dipoles of the water molecules with a much stronger Coulombic bond than either the hydrogen bonds or the van der Waals forces that caused clustering around the apolar solute molecule.28-30 If the temperature and pressure of this system were controlled to meet the requirements of forming hydrate, H2O and hydrate former diffused towards their interface at Us and UR, a small quantity of hydrate formed



at the interface during the initial stage;31 then, the hydrate front occurred. The stronger bonds of water with salt ions inhibited hydrate formation; water was attracted to the ions more than water was attracted to the hydrate structure.32 During the process of hydrate formation, non-guest impurities were excluded from the hydrate and entered into solution where they diffused away from the hydrate front to the bulk solution at Ung. If hydrate formed quickly, some non-guest impurities did not have enough time to diffuse into the bulk solution, and then, they were trapped in the hydrate. At regular intervals, the concentration of the bulk solution (Cs) was measured. When hydrate formed completely, the constitution contained hydrate, residual solution, and slight amount of R141b (as shown in Figure 1(a)). After solid–liquid separation, interstitial water and residual R141b were separated, and then dry hydrate was obtained. After the dry hydrate was dissociated, pure water could be collected. Because non-guest impurities were concentrated in the residual solution, they could be recovered conveniently and economically by other methods. To illustrate the internal mechanism further, we proposed the following theory in regard to the concentration gradient and directed substance migration during hydrate formation in aqueous solution. Because this hydrate-based process is very similar with ice formation,33 we made improvement with the mass transfer equations of directional ice formation by introducing a third phase (hydrate guest molecule) and replacing ice with hydrate.34 As a result, we obtained the mass transfer equations for hydrate formation. In the ideal case, assuming that the solution layer is homogeneous (uniform distribution) in concentration and that the hydrate layer is heterogeneous in the concentration of


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retentive ions in hydrate, the concentration of hydrate (Ch) can be calculated by the following equation:

Ch 

 s (C0V0  CsVs )  hVh

(4)

The effective partition constant (K) is defined as follows:35 K

Ch Cs

(5)

After hydrate formation for a period, at two given time t1 and t2, based on the mass balance of non-guest impurities, the following equation can be obtained: Cs , t 2Vs , t 2  Cs , t1Vs , t1  Ch Vs , t 2  Vs , t1

(6)

Equation (6) can be expressed in differential form as follow: d(CsVs )  ChdVs

(7)

Equation (7) can be expanded as follow: (8)

CsdVs +VsdCs = ChdVs

By using Equations (5) and (8), the following equation can be introduced: dCs dVs  ( K  1) Cs Vs

(9)

By integrating both sides of Equation (9), the data can be transformed into the relation between the volume ratio against the initial volume ( Vs / V 0 ) and the ratio of the concentration against the initial concentration ( Cs / Co ).

 C0   Vs  ln    (1  K ) ln    Cs   V0 

(10)

In actual situations, aqueous solution phase is heterogeneous, where a concentration gradient exists, and the intrinsic partition constant (K0) can be introduced and defined as follows:36



K0 

Ch Ci

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(11)

where Ch can be obtained by experiments and K0 can be obtained by following the calculations; finally, Ci can be calculated by the above equation, and these results can confirm the presence of ion exclusion and migration as well as the boundary layer. Figure 5 shows the concentration gradient near the hydrate front. The y-axis expresses the depth from the hydrate front. The concentration in the solution phase gradually decreases from the hydrate front and reaches an equilibrium (C=Cs) at y = δ. During the hydrate formation process, there is a non-guest impurity flux of UhCh from the solution to the hydrate phase. Then, the mass balance equation will be:

 dC  D    UhC  UhCh  dy 

(12)

The diffuse velocity is positive along the y axis. Equation (12) is combined with the following two boundary conditions: C=Ci at y=0

(13)

C=Cs at y=δ

(14)

With these boundary conditions, Equation (12) can be written as follows:

 Ci  Ch  Uh ln    Cs  Ch  D

(15)

From Equations (5), (11), and (15), the following expression can be obtained for the effective partition constant:

 Uh   K  K 0 /  K 0  (1  K 0) exp( ) D  

(16)

By taking the logarithm of both sides, Equation (16) can be transformed into the following form:


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1   1   Uh ln   1  ln   1  K   K0  D

(17)

K0 can be calculated from this equation, which is a significant parameter for calculating the concentration of the hydrate front and obtaining a deeper understanding of the ion exclusion and diffusion mechanism. Based on Equations (10), (11), and (17), Ci can be calculated to elucidate the effect of the boundary layer on hydrate formation and salt exclusion according to our experiments. In our calculation, D = 4.428 × 10-6 m2/h,37 Uh = 0.03506 m/h, and δ = 2.5 × 10-4 m.38 We calculated Ci and plotted in Figure 6. As shown in Figure 6, the concentration at the boundary layer increased with the increase of the initial concentration, but the relative deviation decreased with the increase of the initial concentration. When smaller rates were used for the Ci calculation of high initial concentrations, the difference of relative deviations among the different initial concentrations increased. If hydrate forms slowly, the ions excluded out from the hydrate structure will have more time to diffuse into the bulk solution. According to this analysis, Equation (17) can be regarded as a valuable mass transfer equation for hydrate formation and can be used as a guide for making improvements to this method. We took the device characteristics, operation conditions, and mass transfer into account systematically. Obviously, it will be necessary to accelerate the diffusion of ions into the bulk solution by using agitation or other methods to increase D and slower the hydrate formation rate and the movement of the hydrate front so as to decrease Uh. By such measures, one can achieve a high removal efficiency and purer water collection. A proposed heavy metal ion removal process following the optimization of



relevant procedures. The initial concentration determines the application range of this method, and so it is essential to explore the changes in the removal efficiency at different initial concentrations. Figure 7 shows the effect of the initial concentration on the removal characteristics. As shown in Figure 7, the efficiency initially increases with increases in the Cu2+ concentrations, but then, it levels off; more specifically, when Cu2+ concentrations increased from 16.75 mg/L to 907.1 mg/L, the removal efficiencies increased from 72.12% to 89.64%. The theory clarified in the previous section can be used to explain this phenomenon. Electrolytes in aqueous solution, such as CuSO4, lower the water activity and hinder the linking of water molecules with hydrogen bonds;39 as a result, the ion-dipole interactions between ionized mineral salts and water molecules become stronger than the hydrogen bonds between water molecules or the van der Waals forces for hydrate formation.40 This means that lower temperatures and higher-pressure conditions than those needed in the absence of electrolytes are required for hydrates to be formed.41 The driving force (the difference of temperature or pressure between operational conditions and phase equilibrium)42 and speed of hydrate formation diminished as Cu2+ concentrations increased, and higher advance speeds of the hydrate front were associated with more impurities being transferred into the hydrate. This was caused by increased concentrations of the hydrate front. When the advance speed of the hydrate front was high, there was not enough time for the liquid phase to diffuse to a uniform concentration, which resulted in a high concentration of the hydrate front and the easy transfer of impurities into the hydrate; this is consistent


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with our boundary layer theory. The removal efficiency is therefore lower at lower concentrations. As shown in Figure 7, with a change from 16.75 mg/L to 907.1 mg/L, water yields changed correspondingly from 80.83% to 72.50%, which was indicative of a gradual decline. Theoretical calculations according to the hydration number showed that the water yield was 80.33% when water–R141b was 5. The water yield at a Cu2+ concentration of 16.75 mg/L was 80.83%, which was greater than the theoretically calculated value; this may be because small amounts were wrapped in the hydrate particles and the residual solution in the hydrate slurry was not fully separated during solid–liquid separation. In other cases, when the water yield was lower than the theoretical yield, this was due to the fact that formation of hydrates can be limited by the addition of inhibitors like electrolytes.43 As a result of the significant impact of industrial wastes on nature, the requirements for wastewater treatment are high; there are also demanding targets for wastewater minimization and zero liquid discharge (ZLD).44 The enrichment factor, as a critical parameter of ZLD, characterizes the reduction magnitude of wastewater and the difficulty degree of subsequent process for recovering valuable substances in the residual aqueous solution. As shown in Figure 7, the enrichment factor gradually decreased as Cu2+ concentration increased, and it reached a maximum value of 2.80. This was the result of less hydrate formation under higher concentration solution conditions. Conversely, more hydrate formation led to less residual water. This technology can thus reduce liquid discharges and be used to achieve wastewater



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minimization. As discussed earlier, the trapped solution was the controlling factor of the removal efficiency and the combination of vacuum filtration and centrifugation was the most effective method for the removal of the trapped solution; therefore, it is essential to optimize the solid–liquid separation conditions. Revolutions per minute (RPM) and time were considered to be two critical factors that could affect the separation. To study their effects on the removal efficiency, two series of tests were performed. Because the RPM is related to the structure of the centrifuge, RPM was converted into the relative centrifugal force (RCF) according to the following equation:45

RCF =1.118  10-5gn 2 R

(18)

where RCF is the relative centrifugal force, n is the RPM, R is the turning radius, and the gravitational acceleration is g = 9.8 N/kg. Figures 8 and 9 illustrate the effect of the RCF and time on the removal efficiency. As shown in Figure 8, the removal efficiency initially increased sharply with the RCF, but then, it leveled off; the unit RCF trends were the opposite. When RCF was 2809 N, the removal efficiency reached a maximum of 90.04%; however, the removal efficiency of the unit RCF was only 0.0321%/N. When the RCF was 1580 N, the curve of the removal efficiency versus the unit RCF and that of the removal efficiency versus the RCF started to flatten. Therefore, 1580 N could be regarded as the applicable RCF. As shown in Figure 9, as the centrifugal time increased, the removal efficiency gradually increased, and then, it slowed down. In comparison, the removal efficiency of the unit time first decreased sharply, and then, it tended to vary gently. When the centrifugal


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time was 9 min, the removal efficiency reached a maximum value of 96.63%, the removal efficiency of unit time was at 9.663%/min, and the curve of the removal efficiency versus the unit time and that of the removal efficiency versus the centrifugal time started to flatten. So 9 min could be regarded as the applicable centrifugal time. Overall, applicable centrifugal parameters were 1580 N for the RCF and 9 min for the centrifugal time, which achieved a removal efficiency of 96.63% under optimum conditions. Figure 9 shows a comparison of the images of the hydrate before and after solid–liquid separation obtained under these parameters. As is apparent, the hydrate is pure white and dry after solid–liquid separation. Hydrate is a solid substance free from salt and is formed by ordered crystallinity, but crushed hydrate is a muddy mixture of very tiny hydrate crystals and solution. When the centrifuge is operated at a rate of a few thousand RPM, centrifugal forces will tear the solution from the hydrate slurry. If the centrifugal force of the solution is larger than the surface tension and the adhesive force of the solution to hydrate, the hydrate and solution will be separated by a continuous force for a period of time. By combining our findings here with those of our previous study, we have developed a proposal for a heavy metal ion removal process. In short, hydrate will form at a solution–R141b volume ratio of 5, and a temperature of 4 °C and ambient pressure are used. After hydrate forms completely, the hydrate should be vacuum filtered for 1 min and centrifuged for 9 min at 1580N of the RCF. Then, the hydrate after solid–liquid separation will dissociate at ambient temperature and pressure. Conclusions



A proposed method for hydrate-based pollutant removal was investigated. The results confirmed that the trapped solution was the controlling factor of the removal efficiency, and a 3D distribution analysis of the hydrate samples before and after solid–liquid separation was conducted. The combined vacuum filtration and centrifugation method was found to be the most efficient method. A theory in regard to the concentration gradient and directed substance migration during hydrate formation was proposed. According to this theory, a mass transfer equation for pollutant exclusion and hydrate formation was developed. We finally proposed an operating procedure for the removal of heavy metal ions using hydrate-based technology. Overall, the results show the feasibility of the technology and give a good idea of wastewater treatment. Future attempts should be done for wastewater with complex composition because the hydratebased technology may show greater potential for the simultaneous removal of inorganic and organic matters.

Associated content Supporting Information

Experimental procedures and instruments for heavy metal ion removal using the hydrate-based method; Real-time electricity conductivity measurements during hydrate formation; Microfocus X-ray computed tomography (CT) scanning of hydrate samples.

Acknowledgments


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This study was supported by the Major Program of the National Natural Science Foundation of China (Grant No. 51436003) and the National Natural Science Foundation of China (Grant No. 51622603).

Abbreviations C0 The concentration of the initial solution, kg/m3 Cd The concentration of the dissociated water, kg/m3 Vd The volume of the dissociated water, m3 V0 The volume of the initial solution, m3 Cre The concentration of the residual solution, kg/m3 Us The diffuse velocity of H2O in solution towards hydrate, m/h UR The diffuse velocity of R141b, m/h Ung The diffuse velocity of a non-guest impurity towards the bulk solution, m/h Cs The concentration of the bulk solution, kg/m3 Ch The concentration of hydrate, kg/m3 ρs

The density of the residue solution, kg/m3

Vs

The volume of the residue solution, m3

ρh

The density of hydrate, kg/m3

Vh The volume of hydrate, m3 K

Effective partition constant

K0 Intrinsic partition constant C

The concentration of the solution, kg/m3



δ

Gradient layer thickness, m

D

The diffusion coefficient of the non-guest impurity, m2/h

Uh The advance velocity of the hydrate front, m/h Ci

The concentration of the hydrate front, kg/m3

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For Table of Contents Use Only

Ions are excluded from hydrate structure and then simulated industrial wastewater can be purified by hydrate process.



List of figure captions Figure 1. Analysis of the hydrate sample formed in a 96.55 mg/L Cu2+ solution before solid–liquid separation. (a) 3D reconstructed CT image of the hydrate sample; (b) another perspective of the hydrate sample; (c)extracted aqueous solution; (d) volume distributions of the aqueous solution and hydrate; (e) pore-network modeling results; (f) pore-throat distributions. Figure 2. The effect of solid–liquid separation on the Re. Figure 3. Analysis of the hydrate sample formed in a 96.55 mg/L Cu2+ solution after solid–liquid separation. The solid–liquid separation conditions included vacuum filtration for 1 min and centrifugation at 3000 r/min and for 8 min. (a) 3D reconstructed CT image of the hydrate sample; (b) another perspective of the hydrate sample; (c)extracted aqueous solution. Figure 4. (a) Electrical conductivity as a function of time during hydrate formation. (b) Schematic diagram for this hydrate process. Figure 5. The directed substance migration and concentration gradient during hydrate formation. Figure 6. The concentration at the boundary layer and relative deviation with the initial concentration. Figure 7. The effect of the initial concentration on the removal characteristics. Figure 8. Re and Re of unit RCF as a function of the RCF. Centrifugal time was 3 min. Figure 9. Re and Re of unit time as a function of the centrifugal time. The RCF was


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1580 N.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Figure 8



Figure 9


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The Controlling Factors and Ion Exclusion Mechanism of Hydrate

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