Batch Preparation of High Basicity Polyferric Sulfate by Hydroxide

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Batch preparation of high basicity polyferric sulphate by hydroxide substitution from bipolar membrane electrodialysis Xu Zhang, Xiaoyao Wang, Qianru Chen, Yan Lv, Xiaozhao Han, Yanxin Wei, and Tongwen Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02625 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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ACS Sustainable Chemistry & Engineering

Batch preparation of high basicity polyferric sulphate by hydroxide substitution from bipolar membrane electrodialysis List of Authors

Mailing Address

Xu Zhang

No.193 Tunxi Road, Baohe district, Hefei, Anhui Province, China

Xiaoyao Wang


Qianru Chen


Yan Lv


Xiaozhao Han


Yanxin Wei

No.327 Jinzhai Road, Baohe district, Hefei, Anhui Province, China

Tongwen Xu*

No.96 Jinzhai Road, Baohe district, Hefei, Anhui Province, China

1

ACS Paragon Plus Environment


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Batch preparation of high basicity polyferric sulphate by hydroxide substitution from bipolar membrane electrodialysis Xu Zhang1, Xiaoyao Wang1, Qianru Chen1, Yan Lv1, Xiaozhao Han1, Yanxin Wei3, Tongwen Xu2* 1

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China 2

Laboratory of Functional Membranes, School of Chemistry and Materials

Science, University of Science and Technology of China, Hefei 230026, People’s Republic of China 3 School of Chemistry and Chemical Engineering, Hefei Normal University, Hefei 230061, People’s Republic of China ∗

Corresponding author.

Tel.: +86-551-6360-1587. E-mail address: [email protected] (T. W. Xu). ABSTRACT: An experimental study was carried out on the batch preparation of high basicity polyferric sulphate (PFS) by hydroxide substitution from bipolar membrane electrodialysis (BMED) with BP-A configuration. Effects of operation time, current density and molar feed ratio of FeSO4 to H2SO4 on PFS properties were investigated. Results show that increasing operation time and molar feed ratio may increase the PFS basicity, turbidity removal ratio and energy consumption simultaneously; while increasing current density could increase the basicity and turbidity removal ratio, but decrease the energy consumption. When the current density is 20 mA/cm2, the basicity attains to 14.72%, turbidity removal ratio increases to 93.43%, and energy consumption decreases to 2.72 kW·h/(kg H2SO4). Moreover, XRD, SEM and FT-IR 2


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were conducted to determine the structure and morphology analysis of solid PFS. Keywords: Polyferric sulphate; Bipolar membrane; Electrodialysis; Coagulation efficiency; Ion substitution.

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■ INTRODUCTION Coagulation/flocculation, known as a core environmental protection technology1, is one of the most important wastewater treatment methods2. Generally, the used coagulants include inorganic coagulants, inorganic polymeric coagulants, organic polymeric coagulants, microbial coagulants and composite coagulants etc. Among them, inorganic polymer coagulants have been confirmed to be much cheaper than organic polymeric coagulants3, and more effective than inorganic coagulants4. Hence, more and more attention has been paid to the development of inorganic polymer coagulants, such as aluminum-based ones and iron-based ones1,

4-5

. However,

compared to aluminum-based polymeric coagulants which probably have adverse effects on human health and environment6, iron-based polymeric coagulants have been proved to have higher turbidity removal efficiency, less adverse effect, comparatively lower dose7. Hence, the iron-based polymeric coagulants have been widely used in drinking water and wastewater treatment. Polyferric sulfate (PFS), which has the molecular weight as high as 105, is a kind of iron-based polymeric coagulants. As described by the chemical formula [Fe2(OH)n(SO4)3-n/2]m 8-9, it contains polynuclear complex ions, such as Fe2(OH)24+, Fe3(OH)45+, formed by OH bridges and large number of inorganic macromolecular compounds10. PFS exhibits a superior removal efficiency of COD, BOD, turbidity, heavy metal and color, and is less sensitive to temperature and pH11. Generally, the preparation raw feed of PFS includes sulfuric acid and ferrous sulfate with the following three steps: 4


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Oxidation: 6FeSO4 + 3H 2 SO4 + KClO3 (oxidizing agent ) → 3Fe2 (SO4 )3 + KCl + 3H 2O

(a)

Hydrolysis: Fe2 ( SO4 )3 + nOH − → Fe2 ( OH )n (SO4 )3−n/2 + n SO42−

(b)

Polymerization: mFe2 ( OH )n (SO4 )3−n/2 →[Fe2 ( OH )n (SO4 )3−n/2 ]m

(c)

2

The oxidizing agents used in the first step include NaClO3, HNO3, KClO3 and H2O2 etc., and under the action of oxidizing agents, Fe2+ is oxidized to Fe3+. The second step (hydrolysis reaction) requires that the molar feed ratio of Fe2+ to SO42- is strictly above 3/21, at which OH- would replace SO42- to form the basic salt Fe2(OH)n(SO4)3-n/2. Subsequently, polymerization reaction would proceed to generate the PFS. Among the three reaction steps, the first step reaction rate is the slowest. For PFS, basicity (B, OH/Fe molar ratio) is a more important index than others, such as density, pH (1%, Sol.), the total iron content etc. It indicates the degree that the iron has been hydrolyzed12. Usually, the higher the basicity, the better performance the flocculation. Hence, how to increase the PFS basicity has become one of the hot research topics in the PFS field. For example, Zouboulis et al. investigated the effect of hydrolysis duration, base concentration and oxidation duration on PFS properties (mainly the basicity), and the improvement was very limited13. Besides, they regulated the PFS basicity through altering the reaction temperature and the addition rate of base solution, but results show that the basicity had no obvious change1. Hu et al. managed the PFS basicity by changing the molar feed ratio of Fe2+ to SO42- and the dosage of oxidizing agents, but received no appreciable improvement14. Hence, further studies are required to increase the PFS basicity. Bipolar membrane electrodialysis (BMED) is a combined technology of ion 5



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separation in conventional electrodialysis and water dissociation at the interface of a bipolar membrane under reverse bias in a direct electric field15. Hence, BMED can realize self-production of OH- and H+ in situ without any chemical reagents consumption and secondary pollution16-19. Due to its low environmental impact and high energy efficiency, BMED has been drawing more and more attention from all over the world, and already had some investigations or applications in food processing industry, environmental protection, and chemical syntheses and so on 20-23

. In allusion to the features of PFS preparation process and BMED technology,

theoretically, if BMED is integrated with PFS preparation process, there are sufficient OH- ions to replace SO42- ions to increase the PFS basicity to a great extent without adding any other reagents. Therefore, to confirm the feasibility of high basicity PFS generated by hydroxide substitution from BMED, in this study, PFS preparation process integrated with BMED was investigated. The generated liquid PFS was characterized in terms of typical properties, such as pH (1%, Sol.), density, basicity, the total iron content, as well as the reductive substance content. The solid PFS was conducted structure and morphology measurements such as XRD, FT-IR and SEM. In addition, PFS coagulation performance in treating kaolin model suspension was studied as well. ■ EXPERIMENTAL SECTION Materials. The main properties of ion exchange membranes used in this study are listed in Table 1. The chemicals, such as FeSO4, H2SO4, and Na2SO4 etc., are of 6


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analytical pure grades. They were purchased from a domestic chemical reagents company. Commercial liquid PFS were provided by Laiwu Hongshen Water Purification Materials Co., Ltd., China. Distilled water was used throughout. Table 1 The main properties of the ion exchange membranes used in the experiments*. Membrane type

Thickness (µm)

LabA

~0.20

LabC

~0.15

BPM-I

0.16~0.23

Water content (%)

Area resistance (Ω.cm2)

Transport number (%)

Burst Strength (Mpa)

0.8~1.0

35~40

0.5~1.5

>98

>0.35

0.8~1.0

35~40

0.5~1.5

>98

>0.35

Positive side: 1.4~1.8 Negative side: 0.7~1.1

35~40

—

—

>0.25

IEC (meq.g-1)

Company Chemjoy, Hefei, China Tingrun, Beijing, China

*The data were referred to the relative company websites: www.cj-membrane.com, www.tingrun.com.

Experiments. Fig.1 shows the integrated experimental setup, which comprises of: (a) BMED membrane stack (1) (BP-A configuration); (b) DC power supply (2) (RLD-3005D1,

Shanghai

Huanzhen

electronics

Co.,

Ltd.,

China);

(c)

Temperature-regulated magnetic stirring apparatus (3) (Changzhou GuoYu Instrument Manufacturing Co., Ltd., China) and temperature sensor (4); (d) tanks (5-7) and peristaltic pumps (8) (JY-PG70, Hangzhou Jiyin Culture and the Arts Co., Ltd., China, with the a maximum flow rate of 500 L/h) to store and pump the relative solutions.

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Figure 1 The schematic diagram of integrated experimental setup. (1) BMED membrane stack; (2) DC power supply; (3) Temperature-regulated magnetic stirring apparatus; (4) Temperature sensor; (5) Reaction tank; (6) Electrode rinse tank; (7) Acid solution tank; (8) Peristaltic pump. BMED membrane stack contained three pieces of anion exchange membranes and three pieces of bipolar membranes, constituting two electrode compartments, three reaction compartments and two acid compartments. Compartments were separated by anion/bipolar membranes, plastic partition nets (thickness ≈ 1.0 mm), and silicone gaskets (thickness ≈ 0.8 mm). The effective area of each membrane was 0.002 m2. The electrodes were made of titanium coated with ruthenium. Na2SO4 solution (200 mL, 0.3 mol/L) was added into electrode rinse tank (6). Acid solution tank (7) was filled with a dilute H2SO4 solution (200 mL, about 0.18 mol/L). Reaction tank (5) was filled with about 65 mL reaction raw feed (1.11-2.22 mol/L H2SO4, 4.30 mol/L FeSO4 and 0.75 mol/L KClO3). Constant current and batch operation mode was adopted in this study. During the operation, voltage drop across the membrane stack was recorded and samples from the acid solution tank were taken out and analyzed at predetermined time intervals. After the operation was stopped and aging for over 24 8


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hours, the liquid PFS product was obtained. All the operations were conducted at room temperature (25±3 oC). Characterizations of PFS Characterizations of liquid PFS. Liquid PFS characterizations include pH (1%, Sol.), density, the total iron content, the reductive substance content and basicity. pH (1%, Sol.) was acquired by detecting the pH of the diluted PFS solution (1%, Sol.) using the pH meter (FE20, Mettler Toledo Instruments (Shanghai) Co., Ltd.). PFS density was obtained by the weighing 1 mL liquid PFS using electronic balance (AUY120, Shimadzu Corporation, Japan). The total iron content was measured according to Chinese National Standards (GB 14591-2006). The process was briefly described here. TiCl3 solutions were used to reduce all the Fe3+ in samples, and standard K2Cr2O7 solution was used to titrate samples with diphenylamine sodium sulfonate as an indicator. The total iron content can be calculated as follows (GB 14591-2006). X1 =

VCK2Cr2O7 ⋅ 0.05585 m

× 100%

(1)

where X1 is the total iron concentration (wt.%), V is the consumed K2Cr2O7 solution volume (mL), CK2Cr2O7 is the standard K2Cr2O7 solution concentration (mol/L), m is the liquid PFS sample mass (g), 0.05585 is the 0.001 mol Fe mass (g/mol). The reductive substance content was also determined according to Chinese National Standards (GB 14591-2006). H3PO4 buffered solution was used to provide an acidic solution environment for samples, and then standard KMnO4 solution was used to titrate samples. Simultaneously, a distilled water blank test was conducted. 9



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The calculation formula of reductive substance content is shown in Eq. (2). X2 =

(V − V0 )CKMnO4 ⋅ 0.05585 m

×100%

(2)

where X2 is the reductive substance content (wt.%), V is the consumed KMnO4 solution volume (mL), V0 is the consumed KMnO4 solution volume (mL) of distilled water blank test, CKMnO4 is the standard KMnO4 solution concentration (mol/L), 0.05585 is the 0.001 mol Fe mass (g/mol), m is the liquid PFS sample mass (g). Basicity was measured according to Chinese National Standards (GB 14591-2006). KF solution was introduced to cover the iron ions in samples, which were then titrated with standard NaOH solution with phenolphthalein as an indicator. Simultaneously, a distilled water blank test was conducted. The basicity is calculated as B=

[(V0 − V )CNaOH ⋅ 0.0170] /17.0 ×100% m ⋅ X 2 /18.62

(3)

where B is the basicity (wt.%), V is the consumed NaOH solution volume (mL), V0 is the consumed NaOH solution volume (mL) of distilled water blank test, CNaOH is the standard NaOH solution concentration (mol/L), m is the liquid PFS sample mass (g),

X2 is the reductive substance content (wt.%), 0.017 is the mass (g) of 0.001 mol OH-, 17.0 is the mass (g) of 1 mol OH-, 18.62 is the 1/3 mol Fe mass (g/mol). Characterizations of solid PFS. Solid PFS products were obtained by freeze-drying the liquid products for over 36 hours using a freezer dryer (LGJ-10C, Four-Ring Science Instrument Plant Beijing Co., LTD, China.). Then, the solid PFS was ground using a mortar and pestle for further analysis, such as SEM, XRD and FT-IR. The morphology of the solid PFS was detected by SEM using field emission 10


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scanning electron microscope (SU8020, Hitachi, Co.). The crystalline phase of the solid PFS was analyzed through X-ray diffraction (X’Pert PRO MPD, PANalytical Co., The Netherlands). FT-IR spectroscopy was recorded with a Spectrophotometer (Thermo Nicolet, Nicolet 67) and the spectra were recorded in the range of 4000-400 cm−1. In which solid PFS was mixed with KBr powder to prepare a pellet. Coagulation experiments. According to the relevant literatures1, 24-25, the coagulation experiments were conducted with Kaolin model suspension. After adding the coagulants, 250 mL suspension (83 NTU, 0.1 g/L) was stirred rapidly at 250 rpm for 2 mins, and then at 50 rpm for 4 mins. Finally, the suspension was left to settle for 20 mins, and samples were collected at 2 cm below the solution surface to determine the turbidity. Turbidity was measured through determining the sample absorbance by ultraviolet spectrophotometer (VIS-7220G, Beijing Beifen-Ruili analytical instrument (Group) Co., Ltd., China), and the standard equation was shown in Eq. (4).

A = 338.8T ( R 2 = 0.9997)

(4)

where A is the suspension absorbance, and T is the suspension turbidity. The turbidity removal ratio (R) was calculated by the followed equation:

R=

T0 − Tt ×100% T0

(5)

where T0 (NTU) is the initial suspension turbidity before coagulation experiments, Tt (NTU) is the suspension turbidity at time t after the coagulation runs. Energy consumption. Generally, the total energy consumption was calculated based on target product generated by a bipolar membrane. But, it is difficult to determine the amount of OH- in reaction tank because of the formation of inorganic polymer. As we 11



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know, the amount of H+ generated by bipolar membrane is equal to that of OH-, theoretically. Therefore, in this study, the total energy consumption E (kW·h/(kg H2SO4)) was calculated based on the amount of H+ in acid tank, and as shown in Eq. (6). E=∫

t

0

U t Idt (Ct -C0 )VM

(6)

where Ut (V) is the voltage drop across the membrane stack at time t, I (A) is the current, C0 and Ct (mol/L) are the H2SO4 concentrations at time 0 and t in acid tank, V (L) is the acid tank volume, and M (g/mol) is H2SO4 molar molecular weight. The acid concentration was determined by titrating with a standard NaOH solution with phenolphthalein as an indicator.

■ RESULTS AND DISCUSSION Effect of operation time. In this case, current density and the molar feed ratio of FeSO4 to H2SO4 were at 10 mA/cm2 and 2.58, respectively. The total operation time was 180 mins. And at time of 60, 120 and 180 min respectively, the liquid PFS was taken out from the reaction tank for analysis. Figure 2(a) illustrates the changes in voltage drop across the membrane stack and H2SO4 concentrations in acid compartment with time elapsing. The initial variation of voltage drop (within 15-20 mins) may be caused by high initial electric resistance in acid compartment ( CH2 SO4 = 0.16 − 0.18 mol/L ) and instability of solution flow in compartments. Besides, there is an around linear relationship between H2SO4 concentration and operation time on the whole. The final H2SO4 concentration increases from 0.22 to 0.31 mol/L, with the increment of operation time from 60 to 12


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180 mins. As we all know, when the BMED operation time increases, the amount of H+ produced by bipolar membrane increases proportionally, so the final H2SO4 concentration increases with the same multiple. But as a matter of fact, the increase range of final H2SO4 concentration is not strictly proportional to that of operation time, and the main reason is that the strong proton leakage of anion exchange membrane by the so-called tunnel transported mechanism17, 26-27. As shown in Figure 2(b), energy consumption increases from 2.41 to 3.52 kW·h/(kg H2SO4) as the operation time increasing from 60 to 180 mins. Equation (6) indicates that energy consumption is determined by numerator and denominator simultaneously. To numerator, because of the relatively stable voltage drop after 20 mins in Fig. 2(a), it is easy to understand that the value increases with operation time proportionally. But as for the denominator, as discussed above, the increasing range of H2SO4 concentration is smaller than that of operation time. Hence, when numerator is divided by the denominator, the variation trend of energy consumption in Figure 2(b) happens. Effect of operation time on liquid PFS properties is examined in Table 2. Firstly, according to the Chinese National Standards (GB 14591-2006), density, pH, basicity, the total iron content and the reductive substance content of all these cases are consistent with the requirements. Secondly, as operation time increases from 60 to 180 mins, the density and total iron content decrease slightly, while pH and the reductive substance content change in a small range of 2.27-2.33 and 0.0057-0.0059%, respectively. However, basicity increases from 8.84 to 13.70%, as the operation time 13



grows from 60 to 180 mins. So, turbidity removal ratio increases from 87.33 to 91.04% (c.f. Figure 2(b)), subsequently. The reason for the increasing basicity is that the amount of OH- generated by bipolar membrane increases over the operation time. From the above, it could be concluded that increasing the operation time could correspondingly increase the PFS basicity and turbidity removal ratio, but increase the energy consumption as well. (a) 10

0.36

9

0.32

Voltage drop (V)

8 0.28 7 0.24 6 0.20 5 0.16

4

3

CH2SO4 in acid compartment(mol/L)

0.12 0

30

60

90

120 150 180

0

30

60

90

120 150 180

Time (min)

Energy consumption (kw.h/kgH2SO4)

(b)

4

100

95 3 90

2

85

80 1

Turbidity removal ratio (%)

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75

0

70 60

120

180

60

120

180

Operation Time (mins)

Figure 2 Effect of operation time on (a) Voltage drop across the membrane stack and H2SO4 concentration in acid solution tank; (b) Energy consumption and turbidity removal ratio. Table 2 Effect of operation time on liquid PFS properties. 14


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Operation time (mins)

Density (g/mL)

pH (1%, Sol.)

Basicity (%)

The total iron content (%)

The reductive substance content (Fe2+) (%)

60 120 180

1.54 1.50 1.49

2.27 2.35 2.33

8.84 11.27 13.70

12.05 12.01 11.08

0.0057 0.0059 0.0059

Effect of current density. Effect of current density was examined using different current densities of 0, 10, 20, 30 mA/cm2. In this case, operation time and molar feed ratio of FeSO4 to H2SO4 were 120 mins and 2.58, respectively. The changes in voltage drops across the membrane stack and H2SO4 concentrations in acid compartment over time are shown in Figure 3(a). It can be observed that voltage drop increases as the current density increases, indicating that most electrical energy is used to overcome the electrical resistance28. Similar to that in

Section 3.1, voltage drop changes slightly at the initial 20-25 mins and then keeps stable. From the right part of this figure, H2SO4 concentration increases with the increment of current density, since a higher current density could result in a higher driving force29-30. What’s more, with the current density increases from 10 to 30 mA/cm2, the final H2SO4 concentration increases from 0.27 to 0.42 mol/L. Also, the increasing range of H2SO4 concentration is not proportional to that of current density, and the reason is that the ion selectivity of membrane decreases with the increase in current density31. It can be also seen in Figure 3(b), with the increasing current density, the energy consumption decreases from 3.29 to 1.72 kW·h/(kg H2SO4). Though an increase in current density tends to increase the total energy consumption as indicated in Eq. (6), due to the rapid increase in product quantity, the energy consumption for per kg 15



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product decreases correspondingly. Table 3 demonstrates the effect of current density on liquid PFS properties. Within the investigated current density range, the density, total iron content and reductive substance content of the liquid PFS vary in a certain range of 1.43-1.56 g/mL, 10.69-12.34% and 0.0047-0.0061%, respectively. Besides, the pH increases from 2.26 to 2.49 slightly, while basicity increases from 7.68 to 20.13% drastically. The main reason is that the higher the current density, the larger the amount of OHgenerated by bipolar membrane. Here, it is important to note that when the basicity gets to 20.13%, there are some yellow precipitates at the bottom of reaction tank, signifying that the product is unstable1. The generation of yellow precipitates also leads to that the total iron content (10.69%, I=30 mA/cm2) is lower than the relative datum (11%) in GB 14591-2006. The right part of Figure 3(b) shows the changes in turbidity removal ratio over current density, and as current density varies from 0 to 20 mA/cm2, turbidity removal ratio increases from 87.16 to 93.43%. In short summary, increasing current density could not only increase the PFS basicity and turbidity removal ratio drastically, but also decrease the energy consumption synchronously. However, excessive high current density could lead to the unstable product.

16


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(a) 16

2

0.48

10 mA/cm

2

20 mA/cm

0.44

2

30 mA/cm 14

Voltage drop (V)

0.40 12

0.36 0.32

10 0.28 8

0.24 0.20

6


0.16 4 0

20

40

60

80

100 120

0

20

40

60

80

100 120

Time (min)


(b) 4.2

100

3.6

95

3.0 90 2.4 85 1.8 80 1.2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75

0.6

0.0

70 0

10

20

0

30

2

10

20

Current density (mA/cm )

Figure 3 Effect of current density on (a) Voltage drop across the membrane stack and H2SO4 concentration in acid solution tank; (b) Energy consumption and turbidity removal ratio. Table 3 Effect of current density on liquid PFS properties. Current density (mA/cm2)

Density (g/mL)

pH (1%, Sol.)

Basicity (%)



0 10 20 30

1.49 1.50 1.56 1.43

2.26 2.35 2.37 2.49

7.68 11.27 14.72 20.13

11.46 12.01 12.34 10.69

0.0047 0.0059 0.0056 0.0061

Effect of molar feed ratio of FeSO4 to H2SO4. Effect of molar feed ratio of FeSO4 to 17



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H2SO4 is investigated at 10 mA/cm2 current density for 120 mins. Figure 4(a) shows the influence of molar feed ratio of FeSO4 to H2SO4 on the voltage drop across the membrane stack and H2SO4 concentration in acid compartment. It can be seen that the higher the molar feed ratio is, the lower the voltage drop would be. When it comes to the variations of voltage drops within 15-20 mins, there are three main reasons. Firstly, water splitting occurs in the interface region of bipolar membrane after the electrolyte is exhausted, and this would increase the membrane resistance. Secondly, there is much H2 and O2 generated in the electrode compartments, which decreases the apparent conductivity of the electrode rinsing solution26, 30. Thirdly, the initial solution concentration in acid compartment is low to increase the electric resistance. From the right part of Figure 4(a), it can be seen that as the molar feed ratio increases from 2.01 to 4.08, the final H2SO4 concentration decreases from 0.27 to 0.24 mol/L. This is because that more OH- would exist in reaction compartment at the case of higher molar feed ratio, and redundant OH- would migrate through the anion exchange membrane to neutralize the H+ in acid compartment under the action of electric field. From the left part of Figure 4(b), with an increase in the molar ratio of FeSO4 to H2SO4, energy consumption increases from 3.42 to 5.21 kW·h/(kg H2SO4). The reason is similar to that stated in Section 3.2. Table 4 shows the effect of molar feed ratio of FeSO4 to H2SO4 on liquid PFS properties. As molar feed ratio increases from 2.01 to 4.08, density decreases from 1.56 to 1.44, while pH and the reductive substance content increase from 2.23 to 2.37, from 0.0056 to 0.0061%, respectively. The total iron content varies in a little range of 18


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12.01-12.57%. Effect of molar feed ratio on basicity is relatively more obvious, i.e. basicity grows from 8.02 to 14.77% with the increment of molar feed ratio. This suggests that more OH- generated would improve the OH/Fe molar ratio in PFS under the condition of higher molar feed ratio. Accordingly, turbidity removal ratio increases from 86.65 to 95.62% in the right part of Figure 4(b). To sum up, increasing the molar feed ratio of FeSO4 to H2SO4 could improve PFS basicity and turbidity removal ratio, but meanwhile, increase the energy consumption slightly. molar ratio=4.08 molar ratio=2.58 molar ratio=2.01

(a) 10

0.30

Voltage drop (V)

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0.27

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7 0.21 6 0.18


5 0.15 0

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Mole ratio of Fe2SO4 and H2SO4 in reaction raw feed

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Figure 4 Effect of molar feed ratio of FeSO4 to H2SO4 on (a) Voltage drop across the membrane stack and H2SO4 concentration in acid solution tank; (b) Energy consumption and turbidity removal ratio. Table 4 Effect of the molar feed ratio of FeSO4 to H2SO4 on liquid PFS properties. Molar ratio

Density (g/mL)

pH (1%, Sol.)

Basicity (%)



2.01 2.58 4.08

1.56 1.50 1.44

2.23 2.35 2.37

8.02 11.27 14.77

12.23 12.01 12.57

0.0056 0.0059 0.0061

Characterizations of solid PFS. Solid PFS powder was obtained after being frozen-dried and ground. Here, five samples, which named A, B, C, D and E were selected as representatives for further characterizations. A represents the sample of commercial PFS. B is the sample under the operation condition of i=0 mA/cm2, t=120 mins, molar feed ratio=2.58. C is the sample under the operation condition of i=10 mA/cm2, t=120 mins, molar feed ratio=2.58. D is the sample under the operation condition of i=10 mA/cm2, t=60 mins, molar feed ratio=2.58. E is the sample under the operation condition of i=10 mA/cm2, t=120 mins, molar feed ratio=2.01. As shown in Figure 5, the microphotographs of the samples in this study are similar to those of the commercial PFS. Also, it can be seen that the PFS samples behave as amorphous materials, forming many aggregates of various sizes and shapes. Figure 6 illustrates the FT-IR spectra of PFS samples. Generally speaking, the five spectra are quite similar. Firstly, the characteristics bond at the 3250-3000 cm-1 can be attributed to the stretching vibration of –OH, while that at the about 1628 cm-1 can be attributed to the stretching vibration of water absorbed or complexed. The 20


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peaks in the region of 1160-1120 cm-1 can be assigned to the symmetric stretch of the S=O and/or the O=S=O bonds. In addition, there exhibits a characteristic peak at 1000-980 cm-1 which is assigned to the bending vibration band of Fe-OH. A considerably weaker band in the region 680-610 cm-1 can be attributed to the SO42- 31. Figure 7 shows the five samples’ XRD patterns, which are similar as those reported in literatures 1, 4. Overall, the PFS samples A, B, C, D, E are amorphous with rather obscure traces of crystallinity, which is also supported by the examination of SEM. In addition, comparing with sample A, other four samples have a broad intense shoulder at 10-30o 2θ, which can be ascribed to iron based compounds. A

B

C

D

21



E

Figure 5 SEM microphotographs of PFS samples. A: commercial PFS; B: i=0 mA/cm2, t=120 mins, molar ratio=2.58; C: i=10 mA/cm2, t=120 mins, molar ratio=2.58; D: i=10 mA/cm2, t=60 mins, molar ratio=2.58; E: i=10 mA/cm2, t=120 mins, molar ratio=2.01. A C E

100

B D

80

Transmittance (T%)

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Figure 6 FT-IR spectra of PFS samples. A: commercial PFS; B: i=0 mA/cm2, t=120 mins, molar ratio=2.58; C: i=10 mA/cm2, t=120 mins, molar ratio=2.58; D: i=10 mA/cm2, t=60 mins, molar ratio=2.58; E: i=10 mA/cm2, t=120 mins, molar ratio=2.01.

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(A)

6000

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400

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0 20

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2-Theta (degree)

Figure 7 X-ray diffraction patterns of PFS samples. A: commercial PFS; B: i=0 mA/cm2, t=120 mins, molar ratio=2.58; C: i=10 mA/cm2, t=120 mins, molar ratio=2.58; D: i=10 mA/cm2, t=60 mins, molar ratio=2.58; E: i=10 mA/cm2, t=120 mins, molar ratio=2.01.

■ CONCLUSIONS An experimental study was conducted on batch preparation of high basicity PFS by hydroxide substitution from BMED. Several operation variables, such as operation time, current density and molar feed ratio of FeSO4 to H2SO4 were investigated. Results show that increasing 23



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operation time and molar feed ratio of FeSO4 to H2SO4 could increase the PFS basicity and turbidity removal ratio, and increase the energy consumption simultaneously. However, increasing current density could not only increase the basicity and turbidity removal ratio, but also decrease the energy consumption. When the current density is 20 mA/cm2, the basicity attains to 14.72%, turbidity removal ratio increases to 93.43%, and the energy consumption decreases to 2.72 kW·h/(kg H2SO4). Nonetheless, excessive high current density could lead to the unstable PFS. XRD, SEM and FT-IR were conducted to determine the structure and morphology analysis of solid PFS. Results show that the generated PFS samples are amorphous with rather obscure traces of crystallinity. In summary, BMED is a feasible technology to increase the PFS basicity. Due to its common feature, such technology can be also used in the production of other inorganic polymer coagulants.

■ AUTHOR INFORMATION Corresponding Author *Tel.: +86 0551-63601587. Fax: +86 0551-63602171. E-mail: [email protected] (T. W. Xu). Notes The authors declare no competing financial interest.

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■ ACKNOWLEDGMENT This research is supported by the Fundamental Research Funds for the Central Universities, the Natural Science Foundation of Anhui Province (No. 1608085QB41) and the National Natural Science Foundation of China (No. 21606063).

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■ Table of Contents

Batch preparation of high basicity polyferric sulphate by hydroxide substitution from bipolar membrane electrodialysis Xu Zhang1, Xiaoyao Wang1, Qianru Chen1, Yan Lv1, Xiaozhao Han1, Yanxin Wei3, Tongwen Xu2* Synopsis A bipolar membrane electrodialysis-based process was proposed to prepare the high basicity polyferric sulphate (PFS) by hydroxide substitution. TOC Graphic

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Batch Preparation of High Basicity Polyferric Sulfate by Hydroxide

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