Removal of Metal Ions in Phosphoric Acid by Electro-Electrodialysis with Cross-Linked Anion-Exchange Membranes.

There are numerous metallic impurities in wet phosphoric acid, which causes striking negative effects on industrial phosphoric acid production. In this study, the purification behavior of metallic impurities (Fe, Mg, Ca) from a wet phosphoric acid solution employing the electro-electrodialysis (EED) technology was investigated. The cross-linked polysulfone anion-exchange membranes (AEMs) for EED were prepared using N,N,N',N'-tetramethyl-1,6-hexanediamine (TMHDA) to achieve simultaneous cross-linking and quaternization without any cross-linkers or catalysts. The performance of the resulting membranes can be determined using quaternization reagents. When the molar ratio of trimethylamine/TMHDA/chloromethylated polysulfone is 3:1:1, the cross-linked membrane CQAPSU-3-1 exhibits lower water swelling and membrane area resistance than the non-cross-linked membrane. The low membrane area resistance of CQAPSU-3-1 with long alkyl chains is obtained due to the hydrophilic-hydrophobic microphase separation structure formed by TMHDA. EED experiments with different initial phosphoric acid concentrations of 0.52 and 1.07 M were conducted to evaluate the phosphoric acid purification of different AEMs. The results show that the EED experiments were more suitable for the purification of wet phosphoric acid solution at low concentrations. It was found that the phosphoric acid concentration in the anode compartment could be increased from 0.52 to 1.04 M. Through optimization, with an initial acid concentration of 0.52 M, CQAPSU-3-1 exhibits an enhanced metallic impurity removal ratio of higher than 72.0%, the current efficiency of more than 90%, and energy consumption of 0.48 kWh/kg. Therefore, CQAPSU-3-1 exhibits much higher purification efficiency than other membranes at a low initial phosphoric acid concentration, suggesting its potential in phosphoric acid purification application.

Introduction

There are abundant phosphate rock resources in China but most of them are of mid-and-low grade; the mined mid-and-low grade phosphate rock has aroused considerable interest over the past years. Currently, phosphoric acid produced is manufactured by the wet process and cannot fully meet the industry requirements because of numerous impurities (Fe, Mg, Ca).[1] Several membrane separation techniques, such as reverse osmosis, nanofiltration, and electrodeionization, are used in the wet phosphoric acid purification process for the raw materials.[2,3] Nowadays, among all different types of membrane separation technologies to simply treat wet industrial phosphoric acid, electro-electrodialysis (EED) has received more attention due to its no waste generation and low energy consumption.[4−7] Touaibia et al. selected two perfluorinated anion-exchange membranes (AEMs) in EED experiments for phosphoric acid purification, and the obtained acid possessed a low concentration of metal impurities.[4] Moreover, the heavy metals were removed from the phosphoric acid solution with an EED technique in Ottosen’s group.[6]

As a core component of EED, AEM is particularly indispensable to improve ion transport efficiency.[8,9] For EED applications, an ideal AEM must have low enough membrane area resistance to promote the counter-ion transport. The membrane area resistance can be decreased by increasing the functional groups but it causes excessive water swelling.[10] Moreover, the AEM should have good chemical and mechanism stabilities to prolong its working life.[11,12] Hence, the successful preparation of AEMs with high conductivity and stability is one of the most challenging issues for the EED technique.

The comb-shaped structure in AEMs could promote ion transport to achieve high conductivity, and minimize the membrane swelling at the same time.[13,14] We synthesized the comb-shaped polysulfone AEMs with long pendant side chains, which exhibit high ionic conductivity and low water swelling.[15] Wang et al. successfully prepared the ion-nanochannels AEMs, which had the microphase separation structure to enhance the ion transport for waste acid reclamation.[16] Lee et al. synthesized the comb-shaped polysulfone copolymers for fuel cells; the membranes showed higher stability and proton conductivity by hydrophilic–hydrophobic phase separation morphology.[17] Moreover, other researchers also synthesized the comb-shaped AEMs with a lower membrane area resistance and better swelling resistance than the non-comb-shaped AEMs.[13,18−20] Further, the cross-linked network structure in AEMs has been demonstrated as an effective way to enhance the dimensional and mechanical stabilities.[21] Different strategies, such as thiol-ene click chemistry and olefin metathesis were utilized to cross-link AEMs.[22−28] Tibbits et al. prepared the cross-linked hydroxide exchange membranes by a single step via the thiol-ene reaction, which demonstrated reasonable conductivity, better stability, and lower water uptake than other non-cross-linked membranes.[24] Xu et al. synthesized the cross-linked AEMs for electrodialysis by thermal cross-linking of unsaturated moieties and the resultant membranes had high mechanical stability.[29] The challenge in olefin metathesis-based cross-linking methods is the low ion conductivity due to the decreased amount of water absorbed by the formed rigid three-dimensional (3D) networks.[25]

Hence, the high-performance AEMs were prepared by combining the phase separation structure and cross-linking strategy. Xu and co-workers designed the AEMs with long functional side chains and densely cross-linked networks, which possessed excellent tensile strength, dimensional stability, and conductivity.[30] More recently, the synthesis of a cross-linked AEM with commercially available diamines has been proved to be a simple and effective method. The cross-linked AEM was prepared by Shao’s group with butyl-substituted doubly charged 1,4-diazabicyclo (2.2.2) octane, which exhibits good flexibility, tensile strength, and a low swelling ratio.[31] Especially, N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA) with six methylenes used as a quaternization agent can provide good nano-channels for ion transport and used as a cross-linking agent to improve the mechanical and chemical stabilities.[32−34] Previously, novel double-cross-linked AEMs with good mechanical properties were prepared by Yu and co-workers using TMHDA as a homogeneous quaternization reagent.[35] The cross-linked AEMs were also synthesized with TMHDA in Wu’s group, which greatly enhanced the ion selectivity and mechanical properties.[36] However, all of the AEMs were designed and prepared for fuel cells, without considering the possible performance requirements of the EED applications.

In this study, a series of polysulfone AEMs with different amounts of TMHDA and trimethylamine (TMA) were synthesized. The structures of cross-linked AEMs were investigated by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) curves. The properties of different AEMs were evaluated systematically using various characterization techniques. Lastly, the EED performances for purifying the phosphoric acid solution were investigated in detail.

Experimental Section

Materials

Chloromethylated polysulfone (CMPSU, DCM = 1.23) and wet phosphoric acid solution (CH = 0.52 or 1.07 M) were obtained according to our previous work.[10]N,N,N′,N′-Tetramethyl-1,6-hexanediamine (TMHDA) was provided by J&K Scientific Ltd. Other chemicals, including N,N-dimethylformamide (DMF), trimethylamine (TMA), NaOH, AgNO3, NaCl, and KCl were purchased locally. Deionized (DI) water was used in all experiments.

Membrane Preparation

Dried CMPSU (1 g) was dissolved in DMF (10 mL) at 40 °C. When CMPSU had dissolved completely, different molar ratios of TMA and TMHDA were added to the solution under stirring for 10 min at 25 °C. The obtained solution was degassed and cast on glass plates for 1 h at 25 °C. The solvent was removed by drying at 50 °C for a day and then dried under vacuum at 80 °C to remove the residual solvent completely to get the transparent and flexible cross-linked quaternized ammonium polysulfone membranes (CQAPSU-x-y, where x and y represent the molar ratio of TMA and TMHDA to CMPSU, respectively). All membranes were fully converted to the Cl– form via immersion in 0.5 mol/L NaCl solution for a day, followed by thorough washing with DI water to remove residual ions, and then kept in DI water. Finally, the thickness of the dried membranes was around 60 μm.

Membrane Characterization

Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectra were recorded on a Bruker Equinox 55 spectrometer.

Thermal gravity analysis (TGA) was performed using a Perkin-Elmer TGA-7 instrument with a heating rate of 10 °C/min under the nitrogen atmosphere from 30 to 700 °C.

The X-ray photoelectron spectroscopy (XPS) analysis was obtained on a Shimadzu-Kratos Axis-Ultra DLD-600 W spectrometer using an Al Kα anode.

The surface morphology of the cross-linked membranes was observed with a scanning electron microscope (SEM) in FEI Quanta 200 instrument. The membranes were dried under vacuum and then coated with gold before the measurement.

Transmission electron microscopy (TEM) was performed using a JEOL JEM-2010FEF instrument. The measurements of the membranes morphologies and microstructures were conducted under an accelerating voltage of 200 kV.

Small-angle X-ray scattering (SAXS) was measured on a SAXS LAB ApS system to characterize the membranes’ morphologies at room temperature. The correlated parameters were associated with the scattering angle θ.The cross-linking degree (CD) of cross-linked AEMs can be determined via XPSwhere AxQA, ATA, and AQA represent the fitting peak area of the cross-linked quaternary ammonium groups, tertiary amine groups, and non-cross-linking quaternary ammonium groups, respectively.

The ion-exchange capacity (IEC) of AEM samples was determined by titrating them with a 0.1 mol/L AgNO3 aqueous solution.[10] The Cl–-form AEMs were dried at 60 °C under vacuum for 24 h and weighed (Wdry); then the SO42–-form AEMs were obtained after being fully immersed in 0.5 M Na2SO4 solution. Subsequently, the experimental IEC values were determined by measuring the consumed volume (VAgNO) and concentration (CAgNO) of the AgNO3 solution, and the theoretical IEC values were determined by the degree of chloromethylation (DCM) and the molar mass of the cation (Mc).where Mc = Ma + MCl + MCH; and Ma, MCl, and MCH are the molar masses of the active groups N+(CH3)3 or (CH3)2N+(CH2)6N+(CH3)2, Cl–, and −CH2, respectively.

The water uptake (WU) and swelling ratio (SR) were determined by measuring the weight and dimension before and after hydration of AEMs. The weight and dimension of the wet membranes (Wwet, Llength, Lwidth, Lthick) were quickly measured right after removing the excess water on the surface of membranes with filter paper. The weight and dimension of dry membranes (Wdry, L′length, L′width, L′thick) were obtained after drying at 60 °C under vacuum for 24 h. After that, the WU, swelling ratio in the plane direction (SRp) and in the thick direction (SRt), and the hydration number (λ) were calculated as followsThe membrane area resistance (Rm) was determined by the alternating current impedance method.[37] The Rm was measured using a two-chamber testing device (effective area: 3.14 cm2) with 0.5 M NaCl solution in our previous work, which was determined by the entire cell with (Rm1) and without (Rm2) the membrane.[10]The transport number (t–) was tested using the resistance testing device under a constant current with KCl solution (0.1 M and 0.2 M). The t– of the anion can be calculated by the potential difference between the electrodes (El) and the potential of the standard solution (E0).where ; C1 and C2 are the solution concentration in each compartment; and F, R, and T represent the Faraday constant, gas constant, and temperature, respectively.

The tensile strength (TS) of the hydrated membrane (1 cm × 4 cm) was measured using a CM4104 electric universal tensile machine at a 5 mm/min stretching rate.

For confirming the stability of AEMs at high temperatures and at different concentrations of the phosphoric acid solution, we prepared the phosphoric acid solution at different concentrations (0.5, 1, 2, 3, 5, 10, and 15 M). The IEC values of the CQAPSU-3-0, CQAPSU-3-1, and CQAPSU-3-2 membranes were measured. The AEMs were immersed in each solution at room temperature and in a 15 M phosphoric acid solution at 60 and 80 °C for 24 h. Then, the AEMs were rinsed with DI water.

Electro-Electrodialysis Experiments for Phosphoric Acid Purification

A series of EED experiments for phosphoric acid purification was performed to evaluate the purification efficiency of the resultant cross-linked AEMs. One cell with two chambers was used by EED at room temperature in our previous works.[10,15] Prior to the test, the AEMs were soaked in 0.52 or 1.07 M phosphoric acid for 24 h and then washed with DI water. After that, the two chambers were separated by one AEM sample, whose effective area was 20 cm2 (5 cm × 4 cm). The cathode compartment contained 500 mL of the simulated phosphoric acid solution and 500 mL of 0.52 or 1.07 M pure phosphoric acid solution in the anode compartment. During the test, the applied current and the flow rate were kept constant at 0.8 A and 4.5 L/h during all of the experiments. In this experiment, NaOH titration was used to determine the concentration of phosphoric acid in both compartments, and the concentration of metal ions was analyzed by atomic absorption spectrometry.

The current efficiency (η) and energy consumption (E) were obtained from the change in the concentration of phosphoric acid in the anode compartmentwhere C and V are the concentration and volume at time t, respectively; C0 and V0 are the concentration and volume at time 0, respectively; and U and I are the voltage drop and current across the membrane in the EED.

The removal rates (Re) of metal ions can be estimated as followswhere Cmic and Cmia are the initial concentrations of metal ions in the cathode compartment and the anode compartment, respectively, and Cmfa is the final concentration of metal ions in the anode compartment.

Results and Discussion

Synthesis and Characterization of AEMs

The synthesis procedure of CQAPSU-x-y AEMs is shown in Scheme using TMA and TMHDA. TMHDA could easily react with the benzyl chloride groups on CMPSU and acted as both quaternization and cross-linking agents due to its multinitrogen structure.[38] The gelation should be avoided by controlling the quaternary amination reaction time; therefore, excess TMHDA was added to the CMPSU solution for ensuring the complete reaction with chloromethyl groups.[39−41] CQAPSU-x-y AEMs were fabricated by casting the final reacting solution.

Scheme 1

Synthesis Procedure of CQAPSU-x-y

The successful synthesis of CQAPSU-x-y AEMs was confirmed by ATR-FTIR, XPS, and TGA analysis, as shown in Figure . In the case of CQAPSU-3-0 and CQAPSU-3-1, a new peak that appeared at 975 cm–1 in Figure a shows the presence of C–N stretching, which confirms the quaternary amination of the benzyl chloride groups.

Figure 1

(a) ATR-FTIR spectra of CMPSU, CQAPSU-3-0, and CQAPSU-3-1; (b) TGA and DTG curves of CQAPSU-3-0 and CQAPSU-3-1; and curves of XPS spectra in the N (1s) region of (c) CQAPSU-3-0 and (d) CQAPSU-3-1.

The thermal properties of AEMs depend on the polymer structure and cationic-exchange groups.[32] As seen in Figure b, the decomposition behaviors of CQAPSU-3-0 and CQAPSU-3-1 are divided into three stages. The first degradation occurred at around 100 °C and could be attributed to the weight loss of the residual solvent and water evaporation: in the second stage (200–350 °C), the degradation of the ammonium functional groups occurs, and in the third stage, around 360 °C, degradation of the polymer backbones occurs.[42,43] As mentioned above, the thermal stability of CQAPSU-x-y below 100 °C could meet the requirements of the membrane preparation and EED application.

Figure c,d shows the XPS analysis of CQAPSU-3-0 and CQAPSU-3-1 to identify the presence of three different types of functional groups (quaternary ammonium group, tertiary amine group, and cross-linked quaternary ammonium group). There is only one peak at 401.7 eV, corresponding to N (1s) appearing in the XPS spectrum of CQAPSU-3-0 in Figure c, which could be attributed to the quaternary ammonium groups without any cross-linking. Figure c,d exhibits three different kinds of N (1s) in CQAPSU-3-1: tertiary amine groups at 399.7 eV, cross-linked quaternary ammonium groups at 402.4 eV, and a peak at 401.7 eV corresponding to N (1s) appearing in the XPS spectrum of quaternary ammonium groups like CQAPSU-3-0, respectively.[44] It was confirmed by FTIR, TGA, and XPS spectra that the CQAPSU-x-y AEMs were successfully synthesized.

Microstructure and Morphology of AEMs

The ion migration of AEMs is influenced by the microphase morphology of the membranes.[45−47] The long hydrophobic side chain facilitates the aggregation of ion clusters and the formation of ionic domains.[48] The CQAPSU-x-y with six methylene side chains may exhibit a good microphase separation structure. The morphologies of CQAPSU-3-0 and CQAPSU-3-1 were examined by SEM, TEM, and SAXS analysis as shown in Figure .

Figure 2

(a) SEM cross-sectional image of CQAPSU-3-1; TEM images of (b) CQAPSU-3-0 and (c) CQAPSU-3-1; and (d) SAXS profiles of CQAPSU-3-0 and CQAPSU-3-1.

The SEM image of CQAPSU-3-1 in Figure a shows a dense and homogeneous cross-sectional morphology without any holes. Generally, the dark region in TEM images usually represents the hydrophilic ion channels, and the light region denotes the hydrophobic domains. There is no obvious hydrophilic–hydrophobic phase separation structure in CQAPSU-3-0 (Figure b). However, the CQAPSU-3-1 exhibits the microphase separation structure as shown in Figure c. This is because the structure of the long side chain provides a good phase separation with large ionic domains. The better the microphase separation and larger ion clusters size, and the lower the membrane resistance and faster ion migration.[49] In addition, there is no obvious scattering peak for CQAPSU-3-0 in Figure d, indicating no characteristic phase separation for CQAPSU-3-0; whereas, the CQAPSU-3-1 with a long alkyl side chain shows a scattering peak at 1.43 nm–1, suggesting that better interconnected ion channels are formed. Furthermore, the dimension of the characteristic peak of the corresponding ion clusters is 4.4 nm, which agrees well with the TEM observation. It is noteworthy that the effect of cross-linking on the microphase separation structure of CQAPSU-x-y is small.[50,51]

Characterization of Physicochemical Properties of AEMs

IEC and CD of AEMs

The IEC and CD are the two significant parameters that influence the properties of the cross-linked AEM in the EED process and they play a crucial role in terms of conductivity, selectivity, and mechanical strength, which can be determined by the amount of the quaternary amination reagent.[11,52,53] As shown in Table , six different AEMs with varying molar ratios of TMA and TMHDA were synthesized. The membrane from CQAPSU-3-0 was functionalized with only TMA, whereas the membranes from CQAPSU-1-1 to CQAPSU-3-3 were prepared using both TMA and TMHDA by the quaternary amination reaction.

Table 1

Effects of the Quaternary Amination Reagent Versus IEC and CD of CQAPSU-x-y

AEMs	nCMPSU (mmol)	nTMA (mmol)	nTMHDA (mmol)	IECTa (mmol/g)	IECEb (mmol/g)	CDc (%)
CQAPSU-1-1	1	1	1	2.11	1.57	53.4
CQAPSU-2-1	1	2	1	2.11	1.74	51.5
CQAPSU-3-1	1	3	1	2.11	1.88	49.6
CQAPSU-3-2	1	3	2	2.11	1.82	50.8
CQAPSU-3-3	1	3	3	2.11	1.78	51.7
CQAPSU-3-0	1	3	0	2.14	1.90

Theoretical IEC.

Experimental IEC.

Cross-linking degree of the membranes.

A higher IEC resulted in higher ion transport and membrane selectivity.[32]Table shows the IEC values of CQAPSU-x-y with different amine/polymer ratios. The IEC values increased from CQAPSU-1-1 to CQAPSU-3-1 with an increase in the amount of TMA because the chloromethyl groups on the main chain could be more easily replaced to form sufficient anion exchange sites, and a larger number of quaternary ammonium groups were incorporated into the membrane. Moreover, by increasing the TMHDA amount, the IEC values decreased from CQAPSU-3-1 to CQAPSU-3-3 and can be attributed to the molecular weight of TMHDA being greater than that of TMA. The bulky molecules of TMHDA will result in more steric hindrance in the membrane than that of TMA.[8] The IEC value of CQAPSU-3-1 is closer to that of CQAPSU-3-0 under the same DCM, which indicates that the cross-linked structure of CQAPSU-x-y does not reduce the IEC values by the functional groups.[20] Furthermore, the experimental IEC values are quite close to the theoretical IEC values, which suggested that all of the chloromethyl groups are converted to ammonium groups.

The CD of CQAPSU-x-y calculated from the XPS spectrum decreased from 53.4 to 49.6% with the increase of TMA and increased from 49.6 to 51.7% by an increase of TMHDA. Abundant of TMA grafted to the polymer will reduce the cross-linking between TMHDA and chloromethyl groups. The grafting degree of the tertiary amine groups increased with the increase of TMHDA; however, quaternization of chloromethyl groups was not 100% due to the limited exchange capacity of TMHDA, which implied that the residual chloromethyl groups can be reacted with the tertiary amine groups and obtained a higher CD.[54,55] As presented in Table , CQAPSU-3-1 has the highest IEC value and lowest CD, except CQAPSU-3-0, which has a large steric resistance due to TMHDA with long alkyl chains.

WU, λ, SR, TS, Rm, and t– of AEMs

Generally, the physicochemical and electrochemical properties such as water swelling and ion transport of non-cross-linked AEMs are determined by the IEC values. The higher IEC values increase the area of hydrophilic domains and water swelling of non-cross-linked AEM.[53] However, it was found that the related properties of cross-linked AEMs are dependent not only on IEC values but also on the CD.[11]

As shown in Figure a,b, the WU and SR of CQAPSU-x-y increased by increasing TMA and decreased as TMHDA increased. The higher IEC values correspond to the lower CD as shown in Table ; the TMA effectively reduced the interaction between adjacent polymers and promoted the water molecules into AEMs.[9] Therefore, the WU and SR of CQAPSU-x-y increased as CD decreased. CQAPSU-3-1 exhibits the highest WU and SR. Subsequently, the WU and SR of CQAPSU-x-y decreased as CD and TMHDA increased by impeding the water molecules into the membranes. The SRp values of CQAPSU-x-y are all lower than SRt in Figure b, which suggests that the cross-linked network structure enhanced the membrane swelling resistance in the plane but not enhanced in the thick direction.[33] λ, the number of water molecules per ion, was determined by the polymer structure and WU. Figure a shows that the λ remains invariant with the increase of TMA and TMHDA, and it is believed that the water adsorption capacity has not been improved with the cross-linked structure of CQAPSU-x-y. The WU of CQAPSU-3-1 (30.9%) was lower than CQAPSU-3-0 (44.7%) at a similar IEC value because the cross-linked network of the AEM will adsorb fewer water molecules. The CQAPSU-x-y possesses good swelling resistance and connected ion-conducting channels by cross-linking and microphase separation structures, respectively. All of the results indicated that the prepared CQAPSU-x-y with a strong cross-linked network structure has remarkable dimensional stability, which can run in the EED process for a long time.

Figure 3

(a) WU and λ; (b) the SR in the plane and thick direction, and the TS; and (c) Rm and t– of CQAPSU-x-y at room temperature.

As reported, the cross-linked AEMs have great mechanical strength in EED application because of the rigid and compact cross-linked structure.[8] The TS of CQAPSU-x-y in the fully hydrated state was measured at room temperature. As shown in Figure b, the TS slightly decreased and improved with increasing the amount of TMA and TMHDA for CQAPSU-x-y, respectively. It is found that the TS values are in the range of 18.3–15.0 MPa, which are higher than that of the non-cross-linked CQAPSU-3-0 (6.8 MPa). The TS increased with the increase of CD, which limits the movement of polymer chains by the cross-linked network. Accordingly, the excellent mechanical strengths of CQAPSU-x-y AEMs are efficient for EED application.

The high conductivity and selectivity are the critical performance metrics for AEMs, which is indicative of efficiency for a realistic EED operation.[56−58] Generally, the cross-linked structure is beneficial to enhance the swelling resistance but reduces the conductivity of AEMs.[9] However, the prepared CQAPSU-x-y membranes show low Rm (in Figure c), despite the low WU and SR. It is an increasing trend with the increasing TMHDA content and with decreasing TMA content, respectively. Moreover, the t– values of CQAPSU-x-y are in the range of 0.90–0.98. The CQAPSU-x-y with long alkyl side chains exhibits a good microphase separation structure, which facilitates the ionic clusters formed and ionic conduction.[48] It is believed that increasing the quaternary ammonium groups commonly results in the suppression of co-ions diffusion and improvement of counter-ions passing through the AEM.[11,59,60] The Rm and t– of CQAPSU-x-y are found to decrease from 5.8 to 1.2 Ω cm2 and increase from 0.95 to 0.98 with enhancing charge density by the addition of more TMA or more quaternary ammonium groups, respectively. A good microphase separation structure in AEM will promote the dissociation of charged functional groups and the transportation of ions. CQAPSU-3-1 exhibits the lowest Rm and highest t–, which indicates that the cross-linked membrane can satisfy the basic requirements of EED application.

Figure represents the stability of AEMs under acidic conditions at room temperature, 60, and 80 °C for 24 h. After CQAPSU-3-0, CQAPSU-3-1, and CQAPSU-3-2 were immersed in phosphoric acid solution at concentrations between 0.5 and 15.0 M, their IEC values did not vary greatly from the initial values obtained prior to immersion. In addition, CQAPSU-3-0, CQAPSU-3-1, and CQAPSU-3-2 exhibited similar IEC values before and after they were stored in 15.0 M phosphoric acid solution at 60 °C, while the IEC values of CQAPSU-3-1 and CQAPSU-3-2 stored in 15.0 M phosphoric acid solution at 80 °C slightly decreased from the values obtained prior to immersion; the IECs of CQAPSU-3-0 stored in 15.0 M phosphoric acid solution at 80 °C decreased from 1.90 to 1.61 mmol/g. As a result, we conclude that the CQAPSU-3-0, CQAPSU-3-1, and CQAPSU-3-2 exhibited acid stability in the phosphoric acid concentration range between 0.5 M and 15.0 M. In addition, CQAPSU-3-0, CQAPSU-3-1,, and CQAPSU-3-2 were stable in 15.0 M phosphoric acid solution at 60 °C, while the cross-linked membrane CQAPSU-3-1 and CQAPSU-3-2 exhibited stability in 15.0 M phosphoric acid solution at 80 °C. The working temperature and acid concentration for the AEMs for EED application are below 60 °C and 10 M. All of the results indicated that the prepared CQAPSU-x-y membranes with a strong cross-linked network structure have remarkable acid and thermal stability, which can run in the EED process for a long time.

Figure 4

Acid stability at room temperature and stability in 15.0 M phosphoric acid solution at 60 and 80 °C of CQAPSU-x-y.

Above all, the addition of TMHDA effectively modified the membrane ion transport and dimensional stability by the microphase separation structure and the cross-linked network structure. The effects of the molar ratio of TMA, TMHDA, and CMPSU were found to influence the IEC and membrane area resistance of the CQAPSU-x-y AEMs such that the molar ratio should be controlled. The ideal molar ratio of TMA, TMHDA, and CMPSU was determined here to be 3:1:1.

EED for Phosphoric Acid Purification

The cross-linked membranes CQAPSU-3-1, CQAPSU-3-2, and the non-cross-linked membrane CQAPSU-3-0 were used in a laboratory-scale EED experiment of phosphoric acid purification at room temperature. The physicochemical properties of three AEMs are given in Table . We can see from Table that CQAPSU-3-1 and CQAPSU-3-0 show similar IEC values but different water swelling and membrane selectivities. Furthermore, CQAPSU-3-2 and CQAPSU-3-0 show different water swelling and mechanical stabilities under similar membrane selectivity. The initial phosphoric acid concentration is the key factor that can affect the EED performance.[61] Therefore, the initial phosphoric acid concentrations of 0.52 and 1.07 M were investigated, respectively.

Table 2

Physicochemical Properties of CQAPSU-x-y

AEMs	IEC (mmol/g)	CD (%)	WU (%)	SRp (%)	Rm (Ω cm2)	t–	TS (MPa)
CQAPSU-3-1	1.88	49.6	30.9	7.4	1.2	0.98	15.0
CQAPSU-3-2	1.82	50.8	30.4	7.0	3.1	0.97	15.5
CQAPSU-3-0	1.90		44.7	12.3	3.0	0.97	6.8

Voltage drop is one of the most important parameters affecting the energy consumption of the EED experiment and the working life of AEM.[62,63] As shown in Figure a, the limiting currents of AEMs were 0.9–1.1 A at an initial phosphoric acid concentration of 0.52 M, and the high current uses less operation time when other parameters were kept constant.[15] Also, the limiting currents of AEMs were higher than 1.6 A at an initial phosphoric acid concentration of 1.07 M in Figure b; for comparison, a current of 0.8 A was used in our experiment. Figure c,d shows that the trend of voltage drop curves at different initial phosphoric acid concentrations for different AEMs was similar. With water diffusion and ions transport, the whole membrane stack resistance decreased so that the voltage drop decreased accordingly at the beginning of the experiment. Then, the voltage drop of CQAPSU-3-0 was stable when the conductivity of the solution in the anode and cathode compartments was at high levels (Figure c), which suggests that the resistance of the whole stack remains almost constant;[64] whereas the voltage drop of CQAPSU-3-1 and CQAPSU-3-2 increased dramatically after 100 and 180 min at an initial phosphoric acid concentration of 0.52 M (Figure c). These results can be attributed to the fact that the current density is higher than the limit current density and the proton leakage is reduced by the densely cross-linked network structure. As mentioned above, all of the H2PO4– ions were transported from the cathode compartment to the anode compartment in a shorter period of time, and then the experiment was complete. Therefore, the operation time of CQAPSU-3-1 is shortened at a low initial phosphoric acid concentration. When the initial phosphoric acid concentration increased to 1.07 M at constant current, the voltage drop was higher at the starting point of the operation due to the high electrical resistance of the phosphoric acid solution and then decreased sharply by the first stage dissociation of phosphoric acid accelerating (Figure d).[10] The voltage drop of CQAPSU-3-1 increased statistically until after 550 min at a high initial phosphoric acid concentration, and the other two AEMs remained stable. This can be ascribed to the change in the electrical resistance of the phosphoric acid solution and membrane resistance. As depicted in Figure , the lower the membrane area resistance, the smaller the voltage drop. That is, CQAPSU-3-1 exhibits the lowest voltage drop, which is caused by the minimum membrane area resistance of 1.2 Ω cm2. In addition (compare Figure c,5d), the higher the initial phosphoric acid concentration, the longer the time required for the experiment. This may be due to the fact that when the phosphoric acid concentration is 1.07 M, the H2PO4– ions take longer to transport from the cathode compartment to the anode compartment. In addition, the voltage drop at a high phosphoric acid concentration (Figure d) was slightly less than the low concentration (Figure c), just at the later stage of the experiment. This was because of the lower resistance of the whole stack.

Figure 5

Plots of the current versus voltage at an initial phosphoric acid concentration of (a) 0.52 M and (b) 1.07 M, and the voltage drop at an initial phosphoric acid concentration of (c) 0.52 M and (d) 1.07 M versus operation time for CQAPSU-x-y.

Figure shows the results of treating AEMs at different concentrations of phosphoric acid. The phosphoric acid concentration increased gradually in the anode compartment, whereas it decreased in the cathode compartment with time elapse for all AEMs, indicating the continuous transport of H2PO4– ions from the cathode compartment to the anode compartment. Meanwhile, CQAPSU-3-1 and CQAPSU-3-2 show higher phosphoric acid concentrations in the anode compartment than that of CQAPSU-3-0 and vice versa. This is due to the cross-linked membranes with long alkyl side chains formed a good microphase separation structure for promoting H2PO4– ions transport.[45] It can be seen that in Figure a the phosphoric acid concentration of CQAPSU-3-1 and CQAPSU-3-2 reduced to 0 in the cathode compartment and reached a maximum in the anode compartment at 150 and 240 min, respectively. The CQAPSU-3-2 required more experimental time due to its higher membrane area resistance than CQAPSU-3-1. In addition, CQAPSU-3-0 shows a lower counterion migration rate and the EED experiment needs at least 360 min because of the high membrane area resistance.[64] The phosphoric acid concentration increased more rapidly at the beginning, which is due to less proton leakage. The greater the initial phosphoric acid concentration, the longer the operation time needed due to the more phosphoric acid required to be transported (Figure . Moreover, the phosphoric acid concentration of CQAPSU-3-1 takes at least 600 min to reach the maximum in the anode compartment and was reduced to 0 in the cathode compartment an initial phosphoric acid concentration of 1.07 M, as shown in Figure b, which is 4 times as much as the initial phosphoric acid concentration of 0.52 M. This may be due to the high phosphoric acid concentration resulting in a more amount of H2PO4– needed to be transported from the cathode compartment to the anode compartment. We can see from Figure that the phosphoric acid concentration in the anode compartment with CQPASU-3-1 could be increased from 0.52 M to 1.04 M, and from 1.07 to 1.95 M, respectively. However, it is found that the effective concentration of CQAPSU-3-2 and CQAPSU-3-0 was not very good, as shown in Figure b. As a consequence, the cross-linked AEM can be successfully applied for the phosphoric acid concentration at a relatively low initial phosphoric acid concentration.

Figure 6

Plots of phosphoric acid concentration at an initial phosphoric acid concentration of (a) 0.52 M and (b) 1.07 M versus operation time for CQAPSU-x-y.

As shown in Figure , the current efficiency decreased with time due to proton leakage.[65] Therefore, CQAPSU-3-1 and CQAPSU-3-2 show much higher current efficiency than CQAPSU-3-0, which can be attributed to the great phase separation structure and low proton leakage rate. Before 100 min, the current efficiency of CQAPSU-3-1 was higher than 90% (Figure a) because of the low proton leakage and subsequently decreased with the increasing voltage drop. The current efficiency was decreased slowly for the first 100 min and then decreased faster at a low initial phosphoric acid concentration (Figure a). However, the current efficiency sharply decreased at a high initial phosphoric acid concentration (Figure b). In addition, the greater the initial phosphoric acid concentration, the lower the current efficiency. It is easy to understand that because a high phosphoric acid concentration resulted in a longer transport time and lower membrane selectivity, more co-ions will migrate through the AEM, which leads to a reduction in the loss of the current efficiency.[61] These results indicate that the CQAPSU-3-1 membrane at a low initial phosphoric acid concentration of 0.52 M shows the highest and the most stable current efficiency in EED.

Figure 7

Plots of current efficiency at an initial phosphoric acid concentration of (a) 0.52 M and (b) 1.07 M versus operation time for CQAPSU-x-y.

The final phosphoric acid purification effect and energy consumption of CQAPSU-3-1, CQAPSU-3-2, and CQAPSU-3-0 are shown in Figure . As depicted in Figure a, the purification rates of CQAPSU-3-1 and CQAPSU-3-2 are higher than 72 wt %. The structure compactness of the membrane increased with the increase of the cross-linking degree, which will restrain the metal ions migration through the AEM.[59,60]Figure b shows that the removal rate of metal ions decreases with the increase of the initial phosphoric acid concentration, which is due to the co-ions transport being promoted by the increasing electrolyte concentration. It is worth noting that CQAPSU-3-2 exhibits a slightly higher purification rate than that of CQAPSU-3-1 due to the higher cross-linking degree of the cross-linked AEM. Figure also shows the energy consumption of different AEMs. The energy consumption of CQAPSU-3-1 (0.48 kWh/kg) is far below CQAPSU-3-0 (1.25 kWh/kg) and CQAPSU-3-2 (1.30 kWh/kg) due to the low membrane area resistance and short operation time. It can be found that the energy consumption increased with the increasing initial phosphoric acid concentration, which is in good agreement with the back diffusion of all ions and water transfer at higher concentrations.[66] The low-energy consumption may be beneficial to prolong the AEM lifetime.

Figure 8

Plots of removal rate of metal ions and energy consumption at an initial phosphoric acid concentration of (a) 0.52 M and (b) 1.07 M versus operation time for CQAPSU-x-y.

According to the above results, it is evident that CQAPSU-x-y AEMs, especially the CQAPSU-3-1, possess long side chains that showed good dimensional stability (with a cross-linked network structure) and excellent conductivity (with a microphase separation structure), which lead to great concentration and purification effect of phosphoric acid at a low initial phosphoric acid concentration.

Conclusions

In this study, the cross-linked polysulfone AEMs were successfully synthesized using TMHDA and TMA as cross-linking and quaternization reagents. The cross-linked membranes have great swelling resistance and mechanical properties and high conductivity at room temperature. Especially, CQAPSU-3-1 exhibits the lowest membrane area resistance of 1.2 Ω·cm2 and appropriate mechanical strength of 15.0 MPa. The excellent performances of cross-linked AEMs are obtained by the cross-linked network structure and the microphase separation structure.

A wet phosphoric acid solution containing metallic impurities (Fe, Mg, Ca) is separated by the EED technique. Compared with non-cross-linked membrane CQAPSU-3-0, the cross-linked membrane CQAPSU-3-1 exhibits the best concentration effect from 0.52 to 1.04 M, a shortest operation time of 150 min, the highest metal ion removal rate of more than 72.0%, the greatest current efficiency of more than 90%, and energy consumption of 0.48 kWh/kg at an initial phosphoric acid concentration of 0.52 M. It is believed that the optimized CQAPSU-3-1 can be used in the EED experiment for phosphoric acid purification at a low initial phosphoric acid concentration.