Electrochemical removal of fluoride ions from industrial wastewater

Chemical Engineering Science 58 (2003) 987 – 993

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Electrochemical removal of uoride ions from industrial wastewater Feng Shen, Xueming Chen, Ping Gao, Guohua Chen∗ Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Abstract Fluoride ions were removed electrochemically from a solution using a combined electrocoagulation and electro otation process. For an in uent uoride concentration of 15 mg=l, a value after lime precipitation, the e,uent uoride concentration can be lower than 2 mg=l when the pH in the coagulation cell is around 6, charge loading is at 4:97 F=m3 water, and the residence time is 20 min. Even lower e,uent concentration can be achieved if 50 mg=l of Fe3+ or Mg2+ are added into the coagulation unit. The anions generally reduce the uoride removal e3ciency except Cl− whose corrosion pitting of the electrode can result in 130% current e3ciency. The composition of the sludge produced from the operation was analyzed using X-ray photoelectron spectroscopy (XPS) and time-of- ight secondary ion mass spectroscope system (ToF-SIMS). The characterization results show that the de uorination is a chemical adsorption process with F− replacing the –OH group from the Aln (OH)3n ocs. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Adsorption; Electrocoagulation; Environment; Flotation; Pollution; Sludge

1. Introduction Fluoride ions can be found from wastewaters derived from semiconductor, metal processing, fertilizers, and glass-manufacturing industries (Liu et al., 1983; Cheng, 1985; Chaturvedi, Yadava, Pathak, & Singh, 1990; Sujana, Thakur, & Rao, 1998; Toyoda & Taira, 2000). The discharge of such wastewater into the surface water would lead to the contamination of groundwater. Long-term drinking of water containing high uoride content can result in mottling of teeth, softening of bones, and ossiBcation of tendons and ligaments. Many people in the world are affected by the uorosis, especially in China, India, Pakistan, and Thailand (Reardon & Wang, 2000). According to the guidelines of World Health Organization (WHO) in 1984, the excessive limitation of uoride is 1:5 mg=l for drinking water quality (WHO, 1993). Frequently, however, the concentration found in groundwater is higher than the standards. For example, as high as 5 mg=l in Cang Zhou city, China, was reported (Liu et al., 1983). Due to its high toxicity, industrial wastewater containing uoride is strictly regulated although not necessarily enforced. US EPA recently established a discharge standard of 4 mg=l for uoride from wastewater treatment plant. Beijing is ∗

Corresponding author. Fax: +852-2358-0054. E-mail address: [email protected] (G. Chen).

working toward this standard value although the national uoride discharge standard for industrial wastewater is 10 mg=l in China. There are several de uorination processes tested or employed globally, such as adsorption (Suzuki, Chida, Kanesato, & Yokoyama, 1989; Chaturvedi et al., 1990; Zhang & Liang, 1992; Azbar & Turkman, 2000), chemical precipitation (Bu,e, Parthasarathy, & Haerdi, 1985; Parthasarathy, Bu,e, & Haerdi, 1986; Nawlakhe & Paramasivam, 1993; Malhotra, Kulkarni, & Pande, 1997; Sujana et al., 1998; Huang & Liu, 1999; Azbar & Turkman, 2000; Reardon & Wang, 2000; Toyoda & Taira, 2000), and electrochemical method (Liu et al., 1983; Cheng, 1985; Mameri et al., 1998). When the uoride concentration is high, lime precipitation is commonly used to form CaF2 precipitate. Although the theoretical solubility of uorite (CaF2 ) is 17 mg=l at 25◦ C in water, the CaF2 precipitation can only reduce the F− concentration to 10 –20 mg=l in practice (Toyoda & Taira, 2000). Depending on the discharge standards of industrial wastewaters, a polishing step may be necessary. In this step, the water containing 10 –20 mg=l F− was usually passed through columns of diMerent adsorbents, such as activated alumina, metal oxide, aluminum phosphate, ion exchanging resins, poly-aluminum chloride, or y ash (Bu,e et al., 1985; Suzuki et al., 1989; Chaturvedi et al., 1990; Azbar & Turkman, 2000; Toyoda & Taira, 2000). However, the recovery of adsorption column signiBcantly aMects this application.

0009-2509/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0009-2509(02)00639-5

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F. Shen et al. / Chemical Engineering Science 58 (2003) 987 – 993

Some researchers (Liu et al., 1983; Cheng, 1985; Mameri et al., 1998) have demonstrated that electrocoagulation (EC) using aluminum anodes is eMective in de uorination. In the EC cell, the aluminum electrodes sacriBce themselves to form aluminum ions Brst. Afterwards, the aluminum ions are transformed to Al(OH)3 before polymerized to Aln (OH)3n (Liu et al., 1983; Mameri et al., 1998; Chen, Chen, & Yue, 2000a, b). The Al(OH)3 oc is believed to adsorb F− strongly as shown by Reaction (1). −

−

Al(OH)3 + xF ↔ Al(OH)3−x Fx + xOH :

_

+

b

g

f

a

c

(1)

At aluminum cathode, hydrogen gas is released according to the following reaction: 2H2 O + 2e− → H2 + 2OH− :

h

e d

(2)

Unfortunately, up to date, no solid evidence was reported to support the hypothesis of the above adsorption mechanism. Moreover, the hydrogen gas produced at the EC cathodes prevents the ocs from settling properly on leaving the electrolyzer (Pouet, Persin, & Rumeau, 1992). In order to overcome this problem, an EC process followed by an electro otation (EF) operation developed for restaurant wastewaters treatment (Chen et al., 2000a, b) was modiBed and applied in uoride removal in this study. In this combined process, the EC unit is primarily for the production of aluminum hydroxide ocs. The EF unit would undertake the responsibility of separating the formed ocs from water by oating them to the surface of the cell. Advanced analytical instruments were used in analyzing the sludge formed so as to conBrm the uoride ion removal mechanism.

Fig. 1. Schematic diagram of the experimental setup (a) in uent; (b) pump; (c) electrocoagulation cell; (d) electro otation cell; (e) separator; (f) sludge collector; (g) e,uent; (h) DC power supply.

Electrodes

2. Experimental methods 2.1. Materials and reagent preparation

Cathode

Anode

Substrate Fig. 2. Fork-like interlocking electrodes in EF cell.

Desired concentrations of F− solution were prepared by mixing proper amount of sodium uoride with D.I. water. In order to increase the conductivity of the solution, sodium chloride was added to the solution before pumping it into the apparatus. The chloride salt added to the solution can prevent the formation of the alumina layer on the anode and therefore reduce the passivation problem of the electrode (Liu et al., 1983; Cheng, 1985; Mameri et al., 1998). The in uence of diMerent parameters such as pH value of the solution, cations (Ca2+ , Mg2+ , Fe3+ ) and anions (Cl− , Br − , 3− SO2− 4 , PO4 ) was studied under a constant charge loading of around 5 F=m3 water. The in uent pH was adjusted by using 1:0 M sodium hydroxide and 1:5 (volume) sulfuric acid solution.

and an EF cell. The EC cell was a bipolar electrode having three aluminum electrodes, each with a dimension of 55 mm × 100 mm × 3 mm and an eMective area of 50 cm2 . The net spacing between the aluminum electrodes was 4 mm. The novel EF cell developed in our laboratory was employed (Chen, Chen, & Yue, 2002). Its major diMerence from that reported previously (Chen et al., 2000a) is the material and also the geometry of the electrodes. The novel EF cell has two interlocking electrodes, Fig. 2. The anode was prepared using thermal decomposition method. Details have been reported elsewhere (Chen, Chen, & Yue, 2001). A PD110-5AD Kenwood Regulator supplied DC power to the system.

2.2. Experimental setup

2.3. Chemical analysis

The experimental setup is schematically shown in Fig. 1. The system is primarily consisted of an EC cell

An ion selective electrode (Orion Research Inc., ion plus uoride 9609BN, USA) was used to determine the

F. Shen et al. / Chemical Engineering Science 58 (2003) 987 – 993

3. Results and discussion 3.1. E2ect of pH The in uent pH is one of the important factors in aMecting the performance of electrochemical process (Liu et al., 1983; Chen et al., 2000a). As expected, the pH of the coagulation cell increases with the in uent value initially, as shown in Fig. 3. After the coagulation pH reaches 9 at an in uent pH of 5, it does not vary very much. So does the residual F− concentration. Apparently, low pH is favorable for uoride removal according to the adsorption reaction (Reaction 1). When the in uent pH was 3, the residual F− concentration

30 Residual F- Concentration (mg/L)

F− concentration according to the standard method given by American Public Health Association (Greenberg, Clesceri, & Eaton, 1992). To prevent the interference from other ions (Al3+ , Fe3+ , etc.), TISAB II buMer solution containing CDTA (trans-1,2-Diaminocyclohexane-N; N; N  ; N  tetraacetic acid, Orion Research Inc.) was added to samples. This solution can form complex with interfering cations and release F− . The sludge in the collector was dried at 80◦ C overnight and ground to powders before being analyzed by XPS (PHI 5600, Physical Electronics, USA) equipped with an Al monochromatic X-ray source. A pass energy of 187:85 eV was used for XPS survey spectra. The spectra were excited using a monochromatic AlK  X-ray source at a take-oM angle of 45◦ . After XPS analysis, the samples of sludge powder were analyzed by ToF-SIMS (Model PHI 7200, Physical Electronics, USA). The high-resolution mass spectra were obtained using a Cs+ primary ion source operating at 8 keV. The scanned area was 200 × 200 m2 and the total ion dose for the acquisition of one spectrum is about 1012 ions=cm2 , ensuring static conditions. As the samples are non-conductive, a low-energy electron ood gun was utilized for charge compensation.

989

Initial 10 mg/L F-

25

Initial 15 mg/L FInitial 20 mg/L F-

20

Initial 25 mg/L F

-

15 10 5 0

0

1

2

3

4

5

6

7

Charge Loading (Faradays/m3) Fig. 4. The residual F− concentration for diMerent charge loading at diMerent initial F− concentrations: pH = 6 ± 0:5; retention time = 32 min.

was 1:74 mg=l, well below the wastewater discharge standard of 4 mg=l. The increase of pH between in uent and coagulation unit was due to the hydrogen generated at the EC cathode (Vik, Carlson, Eikum, & Gjessing, 1984). In addition, Reaction (1) shows that part of the pH increase was contributed by the release of OH− by F− . Since aluminum hydroxide is an amphoteric hydroxide, pH is a sensitive factor for the formation of the Al(OH)3 ocs. In our experiments, no formation of Al(OH)3 oc was observed when the in uent pH was 2. Thus no F− removal was noted. An optimal pH for Al(OH)3 formation is in the range of 5 –7 (Liu & Liptak, 2000). The coagulation pH around 6 was also found to be preferred in this process. Too high pH will lead to the formation of AlO− 2 , which is soluble and useless for de uorination. No Al(OH)3 oc was observed when pH is beyond 10.

16

10

14

9

12

8

10

F- Concentration pH

8

7 6

6

5

4

4

2

3

0

pH in Coagulation Unit

-

Residual F Concentration (mg/L)

3.2. E2ect of charge loading and initial F− concentration

2 0

2

4

6

8

10

Influent pH Fig. 3. EMect of in uent pH: initial F− = 15 mg=l, NaCl = 400 mg=l, retention time = 32 min, charge loading = 4:97 F=m3 water.

In the EC process, the amount of the aluminum ion produced is proportional to the charge loading supplied. Therefore, it aMects the F− removal signiBcantly, Fig. 4. The drop of the residual F− concentration is expected when charge loading increases. Moreover, high charge loading produces more bubbles in the EF cell, helpful for the sludge to separate from the treated water. One can tell from Fig. 4 that the residual F− concentration depends on its incoming value. Although at charge loading of over 6 F=m3 , the residual F− concentration is still very high for the situation of high in uent F− concentrations, 20 and 25 mg=l. Obviously, for these conditions, even longer residence time is necessary. The results shown in Fig. 4 reveal that a charge loading of 5 F=m3 is su3cient to have the residual F− concentration below 4 mg=l if the wastewater has undergone the CaF2 precipitation to have the incoming uoride concentration of 15 mg=l.

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F. Shen et al. / Chemical Engineering Science 58 (2003) 987 – 993

3.5

7.0

Residual F Concentration (mg/L)

6.5 BrCl-

6.0 5.5

2-

SO4

PO43-

5.0

-

Residual F- Concentration (mg/L)

7.5

4.5 4.0 0

2

4

6

8

3.0 2.5 Ca2+

2.0

Mg2+ Fe3+

1.5 1.0 0.5

10

0

Concentration of Anions (mmol/L) Fig. 5. EMect of anions: initial F− = 15 mg=l, charge loading = 4:97 F=m3 water.

3.3. E2ect of other chemicals The industrial wastewater usually contains many other chemicals that may have eMects on the de uorination process. In order to quantify their eMect, diMerent anions and cations were added into the F− containing solution before EC and EF. The anions tested include Cl− (NaCl), 3− Br − (NaBr), SO2− 4 (Na 2 SO4 ), and PO4 (Na 3 PO4 ). Theoretically, the competitive adsorption between the anions and F− should reduce the uoride removal e3ciency as shown 3− by the behavior of Br − , SO2− 4 and PO4 in the experiments, 3− Fig. 5. At high PO4 concentration, the F− removal e3ciency decreased signiBcantly. The hydrolysis of PO3− 4 resulted in a pH increase of the in uent solution. For example, the in uent pH became 11.5 when PO3− 4 concentration was 4 mmol=l. No Al(OH)3 oc was formed at such a high pH. That explains the low F− removal at high PO3− 4 . The insigniBcant Cl− eMect may be explained by the “corrosion pitting” phenomenon at the EC electrode, Reactions (3) and (4) (Mameri et al., 1998). Al + 3HCl = AlCl3 + H2 ;

(3)

AlCl3 + 3H2 O = Al(OH)3 + 3HCl:

(4)

The “corrosion pitting” can produce more aluminum hydroxide ocs than the equivalent current supplied which counter balance the competitive adsorption eMect. This is consistent with the 130% current e3ciency obtained. This current efBciency value is similar to the value reported by Mameri et al. (1998). The cations tested include Ca2+ (CaCl2 ), Fe3+ (FeCl3 ), and Mg2+ (MgCl2 ). Calcium is widely used for high uoride concentration removal. As shown in Fig. 6, better uoride removal result was obtained with the increasing of the calcium concentration. This is probably because of the formation of CaF2 . The solubility product of uorite is 1:46 × 10−10 at 20◦ C. For 200 mg=l (5 mmol=l) Ca2+ , if there is no Al(OH)3

50

100 150

200

250 300 350

Concentration of Cations (mg/L) Fig. 6. EMect of cations: initial F− =15 mg=l, charge loading =4:97 F=m3 water.

in the solution, the equilibrium soluble F− concentration should be 3:25 mg=l. But in fact, two aspects aMect the formation of CaF2 . Firstly, the formation of the solid phase was very slow. Secondly, the settling down of the particles formed in the solution is not complete (Parthasarathy et al., 1986). In our case, Al(OH)3 particle was not only an adsorbent for F− removal, but also acted as nuclei for the formation of calcium uoride precipitate. In addition, Al(OH)3 helps the oc grow much larger so as to be oated up and separated easily. Both Fe3+ and Mg2+ are good coagulants. They are frequently used as co-coagulant with aluminum salt. As shown in Fig. 6, 50 mg=l of iron or magnesium ion gives the best result. Too high Fe3+ or Mg2+ concentration may lead to the precipitation of Fe(OH)3 and Mg(OH)2 onto the Al(OH)3 ocs surface. Because the a3nity between uoride and Fe(OH)3 or Mg(OH)2 is much smaller than that of Al(OH)3 , the Fe(OH)3 and Mg(OH)2 precipitates blind the Al(OH)3 ocs (Hicyilmaz, Bilgen, & Ozbas, 1997). 3.4. Characteristics of sludge In order to explore the mechanism of F− removal, the composition of the sludge was studied by using XPS Brst. The result showed that, in addition to aluminum and oxygen, there were other elements in the sludge such as Na, F, Cl, S, P, see Table 1. At high in uent uoride concentration, according to the equilibrium, more uoride was found. The negative ToF-SIMS spectra show that the negative charged groups in the sludge were complicated but very informative. Within the atomic mass unit range from 0 to 50, some strong peaks appeared at 16, 17, 19 and 35.5, apparently correspond, respectively, to O, OH, F and Cl (Fig. 7). When the atomic mass unit is in the range of 50 –100, there appear many types of negative groups. The strongest peak in this range appeared at 59, which was identiBed to be AlO2 . Other peaks, 61, 62, 63, 77, 79 and 81

F. Shen et al. / Chemical Engineering Science 58 (2003) 987 – 993

991

Table 1 The surface composition of dried sludge analyzed by XPS

Atomic concentration, % F−

Initial 15 mg=l Initial F− 25 mg=l 1 mmol=l SO2− 4 1 mmol=l PO3− 4

C

O

F

Na

Al

Cl

P

S

5.01 5.84 8.71 11.09

61.11 61.07 61.80 54.09

4.40 5.39 3.93 1.32

2.64 1.26 0.31 10.17

25.41 25.75 25.21 13.82

1.42 0.69 — —

— — — 9.51

— — 1.03 —

7

4.0

6

3.5

AlO2

3.0 Counts ( x 10 -4 )

Counts ( ×10-5 )

5 4

3

2.5 2.0 Al(OH)2O 1.5

AlFO2H

AlFO Al(OH)2

2

1.0

1

0.5

Al(OH)F

Al(OH)3

AlF2

AlF2O

0 0

50 0

10

20

30

40

60

50

90

100

Fig. 8. Negative ToF-SIMS spectra of the dried sludge: 8:0 keV Cs+ , AcqTime = 2:72 min, mass unit: 50 –100.

9 8 7 6 Counts (x 10 -3 )

represent Al(OH)2 , AlFO, Al(OH)F, Al(OH)2 O, AlFO2 H and AlF2 , respectively (Fig. 8). Within the atomic mass unit range of 100 –200, the strongest peak appeared at 119 (Fig. 9). According to the elemental analysis of the sludge, this peak should be due to (AlO2 )2 H. A relatively weak peak appeared at the atomic mass unit of 121 should be due to the replacement of OH by F. Considering the atomic mass unit range from 200 to 1000, there are some very regular series of peaks appearing, such as 221, 325, 427, 529 and 241, 343, 445, 547, showing a typical property of polymeric material (Fig. 10). The typical mass unit diMerence between the major peaks is 102, which should be due to the structure of Al2 O3 . It is worthwhile to point out that for every regular interval of separation peak of 102 atomic mass units, there are some relatively weak peaks around it. Interestingly, the atomic mass unit diMerence between the relatively weak peak and the strong peak all are 2 atomic mass units. As discussed above, the weak peaks around the strong peak should be due to the replacement of the OH group by F, which is similar to the peaks appeared at the atomic mass unit of 119 and 121. The diMerent series of the peaks are attributed to the diMerent main chain of the polymer. Based on above analysis,

80

Atomic Mass unit

Atomic Mass unit Fig. 7. Negative ToF-SIMS spectra of the dried sludge: 8:0 keV Cs+ , AcqTime = 2:72 min, mass unit: 0 –50.

70

5 4 3 2 1 0 100

120

140

160

180

200

Atomic Mass Unit Fig. 9. Negative ToF-SIMS spectra of dried sludge: 8:0 keV Cs+ , AcqTime = 2:72 min, atomic mass unit: 100 –200.

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F. Shen et al. / Chemical Engineering Science 58 (2003) 987 – 993

helpful to remove uoride. In general, anions give a negative eMect on uoride removal. This may be due to the competitive adsorption between uoride ion and other anions. The mechanism of the removal process was conBrmed based on instrumental analysis, the mechanism of the removal process was conBrmed to be a competitive adsorption between hydroxide group and F− .

6

Counts (x10-3)

5

4

3

Acknowledgements 2

The Materials Characterization and Preparation Facility (MCPF) of HKUST is gratefully acknowledged for their help in doing some of the analysis.

1

0 200

300

400

500

600

700

800

900 1000

Atomic Mass unit Fig. 10. Negative ToF-SIMS spectra of the dried sludge: 8:0 keV Cs+ , AcqTime = 2:72 min, mass unit: 200 –1000.

F

Al OH

O

Al

O

O

O

O Al

Al

O

Al

O O

F

Al O

O Al

Al

O

Al OH

O

Al F

Fig. 11. Microstructure of dried sludge calculated based on ToF-SIMS graph.

the microstructure of the sludge material can be illustrated in Fig. 11. At the edge of the network structure, some of the –OH groups were replaced by F. 4. Conclusion The combined EC–EF process was successfully applied in treating wastewater containing uoride. The experimental results showed that weakly acidic condition is favored in this treatment, while too high or too low pH can aMect the formation of the Al(OH)3 ocs. The optimal retention time in our case was 20 min. Higher charge loading resulted in good uoride removal e3ciency. A feasible charge loading for 15 mg=l uoride wastewater was 5 F=m3 water. Cations and anions can aMect the removal process. Ca2+ is helpful in precipitating F− and reducing the residual F− concentration. A proper solution of 50 mg=l Fe3+ or Mg2+ was also

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