Batch Preparation of High Basicity Polyferric Sulfate by Hydroxide

Research Article pubs.acs.org/journal/ascecg

Batch Preparation of High Basicity Polyferric Sulfate by Hydroxide Substitution from Bipolar Membrane Electrodialysis Xu Zhang,† Xiaoyao Wang,† Qianru Chen,† Yan Lv,† Xiaozhao Han,† Yanxin Wei,§ and Tongwen Xu*,‡ †

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China Laboratory of Functional Membranes, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, People’s Republic of China § School of Chemistry and Chemical Engineering, Hefei Normal University, Hefei 230061, People’s Republic of China Downloaded via WASHINGTON UNIV on July 4, 2018 at 19:15:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: An experimental study was carried out on the batch preparation of high basicity polyferric sulfate (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; meanwhile, 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, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) were conducted to determine the structure and morphology analysis of solid PFS. KEYWORDS: Polyferric sulfate, Bipolar membrane, Electrodialysis, Coagulation efficiency, Ion substitution

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INTRODUCTION Coagulation/flocculation, known as a core environmental protection technology,1 is one of the most important wastewater treatment methods.2 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 coagulants.4 Hence, more and more attention has been paid to the development of inorganic polymer coagulants, such as aluminum-based ones and iron-based ones.1,4,5 However, compared to aluminum-based polymeric coagulants which probably have adverse effects on human health and environment,6 iron-based polymeric coagulants have been proved to have higher turbidity removal efficiency, less adverse effect, and comparatively lower dose.7 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+ and Fe3(OH)45+, formed by OH bridges and a large number of inorganic macromolecular compounds.10 PFS exhibits a superior removal efficiency of COD, BOD, turbidity, heavy metal, and color and is less sensitive to temperature and pH.11 Generally, the preparation of a raw feed of PFS includes sulfuric acid and ferrous sulfate with the following three steps: © 2017 American Chemical Society

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

(a)

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

n 2− SO4 2 (b)

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

(c)

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 Received: October 30, 2016 Revised: January 17, 2017 Published: January 26, 2017 2292

DOI: 10.1021/acssuschemeng.6b02625 ACS Sustainable Chem. Eng. 2017, 5, 2292−2301

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Main Properties of the Ion Exchange Membranes Used in the Experimentsa membrane type

thickness (μm)

IEC (meq g−1)

Lab A Lab C BPM-I

∼0.20 ∼0.15 0.16−0.23

0.8−1.0 0.8−1.0 positive side: 1.4−1.8 negative side: 0.7−1.1

a

water area resistance content (%) (Ω.cm2) 35−40 35−40 35−40

0.5−1.5 0.5−1.5

transport number (%)

burst strength (MPa)

>98 >98

>0.35 >0.35 >0.25

company Chemjoy, Hefei, China Tingrun, Beijing, China

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

Figure 1. 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.

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, the PFS coagulation performance in treating kaolin model suspension was studied as well.

been hydrolyzed.12 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 limited.13 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 change.1 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 improvement.14 Hence, further studies are required to increase the PFS basicity. Bipolar membrane electrodialysis (BMED) is a combined technology of ion separation in conventional electrodialysis and water dissociation at the interface of a bipolar membrane under reverse bias in a direct electric field.15 Hence, BMED can realize self-production of OH− and H+ in situ without any chemical reagent consumption and secondary pollution.16−19 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

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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 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. Experiments. Figure 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 2293


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ACS Sustainable Chemistry & Engineering

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. Culture and the Arts Co., Ltd., China, with the a maximum flow rate of 500 L/h) to store and pump the relative solutions. 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 h, the liquid PFS product was obtained. All the operations were conducted at room temperature (25 ± 3 °C). 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 =

VC K2Cr2O7· 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), and 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. 2294


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ACS Sustainable Chemistry & Engineering Simultaneously, a distilled water blank test was conducted. The calculation formula of reductive substance content is shown in eq 2. X2 =

(V − V0)C KMnO4·0.05585 m

× 100%

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=

(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), and 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

density (g/mL)

pH (1%, sol)

basicity (%)

total iron content (%)

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

(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

Table 3. Effect of Current Density on Liquid PFS Properties

Table 2. Effect of Operation Time on Liquid PFS Properties operation time (mins)

[(V0 − V )C NaOH· 0.0170]/17.0 × 100% mX 2 /18.62

current density (mA/cm2)

density (g/mL)

pH (1%, sol)

0 10 20 30

1.49 1.50 1.56 1.43

2.26 2.35 2.37 2.49

basicity (%)



7.68 11.27 14.72 20.13

11.46 12.01 12.34 10.69

0.0047 0.0059 0.0056 0.0061

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. 2295


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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. 4000−400 cm−1. In which solid PFS was mixed with KBr powder to prepare a pellet. Coagulation Experiments. According to the relevant literatures,1,24,25 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 min, and then at 50 rpm for 4 min. Finally, the suspension was left to settle for 20 min, 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.

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

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 h 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 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

A = 338.8T

(R2 = 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. 2296


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

∫0

t

UI t dt (Ct − C0)VM

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.

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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 min. And at time of 60, 120, and 180 min, respectively, the liquid PFS was taken out from the reaction tank for analysis.

(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 2297


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

removal ratio increases from 87.33 to 91.04% (c.f. Figure 2b), 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. Effect of Current Density. Effect of current density was examined using different current densities of 0, 10, 20, and 30 mA/cm2. In this case, operation time and molar feed ratio of FeSO4 to H2SO4 were 120 min 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 3a. It can be observed that voltage drop increases as the current density increases, indicating that most electrical energy is used to overcome the electrical resistance.28 Similar to that in the section Effect of Operation Time, voltage drop changes slightly at the initial 20−25 min and then stays 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 force.29,30 What’s more, with the current density increasing 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 for this is that the ion selectivity of the membrane decreases with the increase in current density.31 It can be also seen in Figure 3b, 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 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

Figure 2a 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 min) may be caused by high initial electric resistance in acid compartment (CH2SO4 = 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 180 min. 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 mechanism.17,26,27 As shown in Figure 2b, energy consumption increases from 2.41 to 3.52 kW·h/(kg H2SO4) as the operation time increasing from 60 to 180 min. Equation 6 indicates that energy consumption is determined by numerator and denominator simultaneously. To numerator, because of the relatively stable voltage drop after 20 min in Figure 2a, 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 2b happens. Effect of operation time on liquid PFS properties is examined in Table 2. First, 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. Second, as operation time increases from 60 to 180 min, 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 grows from 60 to 180 min. So, the turbidity 2298


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

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. Effect of Molar Feed Ratio of FeSO4 to H2SO4. Effect of molar feed ratio of FeSO4 to H2SO4 is investigated at 10 mA/cm2

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 OH− generated 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 unstable.1 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 3b shows the changes in turbidity 2299


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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−.33 Figure 7 shows the five samples’ XRD patterns, which are similar as those reported in the literature.1,4 Overall, the PFS samples A, B, C, D, and E are amorphous with rather obscure traces of crystallinity, which is also supported by the examination of SEM. In addition, comparing with sample A, the other four samples have a broad intense shoulder at 10−30° 2θ, which can be ascribed to iron based compounds.

current density for 120 min. Figure 4a 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 min, there are three main reasons. First, water splitting occurs in the interface region of bipolar membrane after the electrolyte is exhausted, and this would increase the membrane resistance. Second, there is much H2 and O2 generated in the electrode compartments, which decreases the apparent conductivity of the electrode rinsing solution.28,32 Third, the initial solution concentration in acid compartment is low to increase the electric resistance. From the right part of Figure 4a, it can be seen that as the molar feed ratio increases from 2.01 to 4.08, and the final H2SO4 concentration decreases from 0.27 to 0.24 mol/L. This is because more OH− would exist in the 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 4b, 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 the section Effect of Current Density. 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 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 4b. 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. 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 min, molar feed ratio = 2.58. C is the sample under the operation condition of i = 10 mA/cm2, t = 120 min, molar feed ratio = 2.58. D is the sample under the operation condition of i = 10 mA/cm2, t = 60 min, molar feed ratio = 2.58. E is the sample under the operation condition of i = 10 mA/cm2, t = 120 min, 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. First, 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 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

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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 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.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0551-63601587. Fax: +86 0551-63602171. E-mail: [email protected] (T.X.). ORCID

Tongwen Xu: 0000-0001-6000-1791 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

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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Batch Preparation of High Basicity Polyferric Sulfate by Hydroxide

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