Treatment of fluoride concentrates from membrane unit using salt solutions

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JWPE-19; No. of Pages 6 Journal of Water Process Engineering xxx (2014) xxx–xxx

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Treatment of fluoride concentrates from membrane unit using salt solutions S.V. Jadhav, C.R. Gadipelly, K.V. Marathe, V.K. Rathod * Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 40019, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 November 2013 Received in revised form 10 February 2014 Accepted 10 April 2014 Available online xxx

A detailed study has been carried out for the removal of fluoride from concentrated retentate stream by precipitation overcoming the drawback of membranes. Calcium hydroxide (Ca(OH)2), magnesium hydroxide (Mg(OH2)) and calcium chloride (CaCl2) used for fluoride precipitation, have shown effective results in the pH range varying from 4 to 14. Kinetic studies have shown that the reaction between F and Ca ions in aqueous phase is fast and completed within 20 min at stoichiometric ratio. Reactions carried out with method of excess (keeping calcium concentration very high) showed pseudo-first order kinetics for both Ca(OH)2 and CaCl2. Parameters such as effect of reactant loading, reactant ratio and temperature which affect the particle size were studied. The maximum particle size of calcium fluoride (CaF2) was observed to be around 1 mm and 0.5 mm for Ca(OH)2 and CaCl2 respectively. The reactant ratio studies showed that an increase in the calcium dose decreases final fluoride concentration upto USEPA and WHO standards. Temperature showed an accelerating effect on reaction kinetics for both the reactions. The CaF2 particle size essentially remained unaltered with an increase in temperature for both the reactions. The study showed a successful reduction of fluoride concentration in the concentrated stream of the membrane and can be applied as relatively inexpensive assisting treatment to isolate large amount of fluoride precipitate. ß 2014 Elsevier Ltd. All rights reserved.

Keywords: Fluoride Lime Precipitation Defluoridation Membrane retentate

1. Introduction Although almost 70% of total earth is occupied by water, however, there is limited water available which can be used for domestic and industrial purposes. Groundwater has appropriately been described as the ‘hidden sea’ because groundwater constituting 97% of global freshwater is used for drinking by more than 50% of the world population and serves as the only economically viable option for many communities. This is true to a greater extent in the case of developing countries like India where an estimated 80% of domestic consumption in rural and 50% in urban areas are met by groundwater sources alone. Groundwater has become a source of drinking water since last few decades, due to the scarcity, non-availability and bacteriological pollution of surface waters in many developing and transition countries. As per UNICEF and WHO assessments, it has been concluded that a large proportion of the world’s population does not have access to adequate or microbiologically safe

* Corresponding author. Tel.: +91 22 33612020; fax: +91 22 33611020. E-mail addresses: [email protected], [email protected] (V.K. Rathod).

sources of water for drinking and other essential purposes [1]. Thus, the supply of qualitatively and quantitatively safe water is regarded as a ‘‘human right’’ rather than a ‘‘human need’’. The presence of various hazardous contaminants such as fluoride, arsenic, nitrate, sulfate, pesticides, other heavy metals etc. in ground water has been reported from different parts of world [2– 10]. These contaminants are introduced into the water through (i) erosion and dissolution of rocks, minerals, and ores, and, (ii) anthropogenic processes such as infiltration or runoff from mining, groundwater abstraction etc. Fluoride is one of the constituents of drinking water and the disease fluorosis is caused by an element known as fluorine, the 13th most abundant element available in the earth crust. In contrast to arsenic, fluoride contamination of drinking water receives much less attention. However, fluoride, as a dissolved constituent of drinking water, is perhaps the only substance causing contrary health effects on the consumer depending upon its relative extent. Excessive fluoride consumption leads to the loss of calcium from the tooth matrix, aggravating cavity formation throughout life. The dental or skeletal fluorosis is irreversible and no treatment exists. In India, about 17 states have been identified as epidemic for fluorosis [11]. Table 1 shows the percentage of districts affected by fluorosis in India.

http://dx.doi.org/10.1016/j.jwpe.2014.04.004 2214-7144/ß 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: S.V. Jadhav, et al., Treatment of fluoride concentrates from membrane unit using salt solutions, J. Water Process Eng. (2014), http://dx.doi.org/10.1016/j.jwpe.2014.04.004

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Table 1 Percentage of districts affected by fluorosis in India. % Affected District 50–100 30–50 <30

Andhra Pradesh, Tamil Nadu, Uttar Pradesh, Gujarat, Rajasthan Bihar, Haryana, Karnataka, Maharashtra, Madhya Pradesh, Punjab, Orissa, West Bengal J & K, Delhi, Kerala

Fluoride in groundwater is one of the most harmful pollutants to the human health, recognized worldwide by the World Health Organization (WHO). The guideline value of WHO standards and BIS: 10500 permits the optimum concentration of fluoride ion in drinking water ranged from 0.5 to 1.5 mg/L for the good health of teeth and bones of mammals [12,13]. USEPA established the effluent standard of 4 mg/L for fluoride from the wastewater treatment plant [14]. Various techniques such as coagulation and electrocoagulation [15,16], adsorption [17–20], ion exchange [21– 25], and membrane processes [26–29] have been reported in literature for the removal of fluoride from groundwater. However, membrane technology has found to be the best suitable treatment and presently, membrane units are in operation in villages at domestic level, which generates fluoride free water and concentrated fluoride retentate. The major problem associated with membrane technology is the treatment of retentate stream containing very high concentration of fluoride (typically in the range of 150 mg/L) which again contaminating the ground water [30]. Tahaikt et al. [30] managed to remove nearly 99% fluoride using NF90 and NF270 membranes on Moroccan ground waters where initial fluoride concentrations were as high as 20 mg/L. As the large part of fluoride was discarded to recycling retentate stream, it is safe to predict that at given permeate flow rate (700 L/ h) and pressures (10–40 bar), the feed and retentate will reach extremely high fluoride concentration very quickly. The membrane units are designed to work up to certain operating conditions. Thus, in order to avoid frequent maintenance activities and to ensure longer membrane life it is needed to treat this retentate stream and develop a process which will operate at village places. Also, previously reported reviews of Mohapatra et al. [31], Emamjomeh and Sivakumar [32], and Meenakshi and Maheshwari [33] do not mention the data available on the precipitation reaction kinetics as well as particle size of the fluoride precipitate which is required for design of thickeners. The objectives of this work are to study the removal of fluoride from concentrated retentate stream using calcium hydroxide (Ca(OH)2) and calcium chloride (CaCl2) overcoming the drawbacks of membrane. Additionally, the focus of this work is to produce better knowledge on reaction progression and particle size at a range of operating conditions. Comparison between above two stated materials with Mg(OH)2 is also provided for better removal of fluoride based on experimental results.

2.2. Experimental A series of laboratory scale jar experiments were executed on simulated water samples. All the tests were conducted in 1 L volume of aqueous phase and at ambient temperature (30  2 8C) except temperature studies. The temperature of the system was controlled using temperature controlled bath. At each experimental run, aqueous solution of 500 mL of desired concentration (150 mg/L) of fluoride and calcium/magnesium was prepared. A rapid stirring (400 rpm) was applied for initial 10 min followed by slow stirring (100 rpm) for 5 min. The parameter studies were carried out by varying the pH, reactants ratio, reactants concentration (from 20 mg/ L to 120 mg/L), and temperature to see effect on particle size and final fluoride concentration. Kinetics studies have also been carried out with method of excess (keeping calcium concentration very high) to find out the order of reaction. All experiments were repeated in thrice for their reproducibility and the average values have been reported in figures along with standard deviation. 2.3. Analysis The fluoride concentration in water was analyzed by using the USEPA ion selective electrode method. This method electrochemically determines the concentration of fluoride in drinking water in the range of 0.1–1000 mg/L [34]. The electrode used was an Orion 9609BNWP ionplus1 Sure-Flow fluoride electrode, coupled to an Orion Dual Star pH/ISE benchtop electrometer which also measures pH values of the solution. Standards solutions (1– 100 mg/L) were prepared from a stock solution (100 ppm F ions) of sodium fluoride. Total ionic strength adjusting buffer (Orion ionplus1 application solution TISAB-III) solution was added to samples and standards in the ratio 1:10. TISAB-III solutions regulate the ionic strength of samples and standard solutions adjusting the pH (5–5.5) and also avoid interferences by polyvalent cations such as Al, Fe, Ca and Si, which form complex or precipitate with fluoride and reduce the free fluoride concentration in the solution. Therefore, the electrode is selective for the fluoride ion over other common anions by several orders of magnitude. Particle size analysis of residual calcium fluoride precipitate (CaF2) was carried out on the Malvern Zetasizer nano series ZS90. The Zetasizer Nano series of particle characterization systems can measure particle size in the range from 0.3 nm to 5 mm at a 90 degree scattering angle using dynamic light scattering, also with the ability to measure zeta potential. 3. Results and discussion 3.1. Reaction feasibility

2. Materials and methods

Addition of lime compounds to fluoride rich water at ambient temperature (30  2 8C) initiates the reaction. Calcium/magnesium reacts with fluoride impurities such as NaF, HF, etc. to form insoluble calcium/magnesium fluoride.

2.1. Chemicals and reagents

Caþþ þ 2F ! CaF2

All reagents were pure and analytical grade and used as received. Sodium fluoride (NaF) (M. W. 41.9881) and anhydrous granular calcium chloride (M. W. 110.99) and magnesium hydroxide (M. W. 58.32) LR grade were procured from s.d. Fine Chem. Ltd., Mumbai, India. Calcium hydroxide (M. W. 74.093) was obtained from Central Drug House (P) Ltd., New Delhi, India. Ultra-pure Deionized (DI) water of conductivity 0.054 mS/m was used for all the washing purpose and to prepare the solutions of desired concentration. DI water was prepared by purification of distilled water of conductivity 18 mS/m using sartorius stedim arium1 water purifier system.

(1)

For calcium hydroxide, CaðOHÞ2 ðaqÞ þ 2NaFðaqÞ ! CaF2 ðsÞ þ 2NaOHðaqÞ

(2)

Alternatively, for calcium chloride, CaCl2 ðaqÞ þ 2NaFðaqÞ ! CaF2 ðsÞ þ 2NaClðaqÞ

(3)

If the ionic product of the reactants is greater than the solubility product (Ksp) of product, then precipitation of product occurs [35]. Since, the solubility product calculated for CaF2 (3.9  1011) was found to be smaller than its ionic product (6.275  106), the

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(a)

30

1.6

Table 2 Available calcium and magnesium species from calcium hydroxide, calcium chloride and magnesium hydroxide.

1.4 1.2 1 15

0.8 0.6

10

Particle size, μm

Final F conc., mg/L & pH

25

20

0.4 5 0.2 0

0 0.5

1

1.5

2

2.5

3

3

3.5

Compound

Stoichiometrically available Ca/Mg (wt%)

Water solubility (g/100 ml)

CaCl2 Ca(OH)2 Mg(OH)2

36.36 54.05 41.61

74.5 0.185 0.0014

lower than calcium chloride (0.185 g/100 mL vs. 74.5 g/100 mL). The solubility of magnesium salt is also very low as compared to both calcium salts. Hence, when calcium source is added in the aqueous, there is a significantly lower concentration of calcium species in the system that is available for interaction with the fluoride ions [36].

Ca(OH)2 dose in stoichiometry with 2 moles of F ion, moles/L Final F conc. mg/L

Particle size, micron

3.2. Influence of reactants ratio (X:F, where X is Ca or Mg ions)

20

0.7

18 0.6

0.5

14 12

0.4

10 0.3

8 6

Particle size, μm

Final F conc., mg/L & pH

16

0.2

4 0.1 2 0

0 0.5

1

1.5

2

2.5

3

3.5

CaCl2 dose in stoichiometry with 2 moles of F ion, moles/L Final F conc. mg/L

Particle size, micron

160

1.4

140

1.2

120

1

100 0.8 80 0.6 60

3.3. Kinetics study An attempt has been made to find out the order for the above stated reactions. Kinetic studies have shown that the reaction

0.4

40

160

0.2

20 0

Particle size, μm

Final F conc., mg/L & pH

(c)

Final pH

In order to study the activity of salts, the effect of reactant ratio was studied by increasing the calcium/magnesium dose in stoichiometric ratio with fluoride i.e. 1:2, 1.5:2, 2:2, 2.5:2 and 3:2. In case of Ca(OH)2, by increasing the calcium concentration, the final fluoride concentration decreased from 25 to less than 5 ppm (Fig. 1(a)) but the final pH increases to a value above 11. The pH value increased due to excess OH concentration and the particle size essentially remained less than 1.5 mm for all the ratios. In case of CaCl2, the final fluoride concentration decreased from 18 to 2 ppm and a marginal decrease in pH was observed due to the slight acidity (Fig. 1(b)). The particle size showed lesser value for the initial 1:2 ratio (around 0.3 mm) but increased to around 0.55 mm for the rest of the ratios. It has been reported that the fluoride from groundwater can be removed from 109 mg/L to 4 mg/L using lime [37]. The lowest efficiency was observed in case of Mg(OH)2 reacting with NaF (Fig. 1(c)). Here the maximum reduction in fluoride concentration was observed to be nearly 80 mg/L at Mg:F ratio of 3:2 with particle size remaining nearly 1 mm. A comparative removal of fluoride by the three reactants can be observed in Fig. 2. Hence, calcium hydroxide and calcium chloride were chosen for further studies.

0 1 1.5 2 2.5 3 3.5 0.5 Ca(OH)2 dose in stoichiometry with 2 moles of F ion, moles/L Final F conc., mg/L

Final pH

Particle size, micron

Fig. 1. (a) Effect of reactants ratio for Ca(OH)2 reacting with NaF at initial F ion concentration 150 mg/L, temperature 30  2 8C, and Ca to F ion ratio 1:2, 1.5:2, 2:2, 2.5:2 and 3:2. (b) Effect of reactants ratio for CaCl2 reacting with NaF at initial F ion concentration 150 mg/L, temperature 30  2 8C, and Ca to F ion ratio 1:2, 1.5:2, 2:2, 2.5:2 and 3:2. (c) Effect of reactants ratio for Mg(OH)2 reacting with NaF at initial F ion concentration 150 mg/L, temperature 30  2 8C, and Ca to F ion ratio 1:2, 1.5:2, 2:2, 2.5:2 and 3:2.

precipitation of CaF2 is possible. Addition of coagulating agents such as aluminium salts, ferric salts, etc. facilitates settling faster. A chemical comparison between calcium/magnesium hydroxide and calcium chloride is given in Table 2. Calcium hydroxide contains higher concentrations of calcium species than calcium chloride which is 54.05% against 36.36%, but its solubility is much

140 120

Final F conc, mg/L

(b)

Final pH

100 80 60 40 20 0 0.5

1

1.5

2

2.5

3

3.5

Molar Ratio F:X

Calcium Hydroxide

Calcium Chloride

Magnesium Hydroxide

Fig. 2. Comparative fluoride removal by various reactants (X is for Ca or Mg ions) at initial F ion concentration 150 mg/L and temperature = 30  2 8C.

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(a) 14

140

12

120

10

100

140

pH after reaction

F conc, mg/L

120 100 80 60 40

8

80

6

60

4

40

2

20

20

F conc. after reaction, mg/L

4

0 5

10

15

20

25

0 0

between F and Ca ions in aqueous phase is very fast and completes within 20 min at stoichiometric ratio. Here, a method of excess has been used to find out the rate law of reaction as two reactants are present. Experiments were performed at very high concentration of one reactant so as to make other reactant as a limiting reactant. According to Eq. (1), the theory can be represented as follows [38]: dC F n ¼ kCFm CCa dt

Simplifying,   dC F ¼ lnk þ mlnC F þ nlnC Ca ln  dt

8

10

12

F conc. mg/L

(b) 8

160

7

140

6

120

5

100

4

80

3

60

2

40

1

20

(4)

(5)

For excess Ca (method of excess), Eq. (4) reduces to,

This again simplifies to,   dC F ¼ lnk þ mlnC F ln  dt

6

pH after reaction

0

0 0

dC F n ¼ kCFm CCa  kCFm dt

4

Adjusted pH before reaction

Fig. 3. Reaction progression with time at initial F ion concentration 150 mg/L and temperature 30  2 8C; where Ca ion concentration  F ion concentration.

r F ¼ 

2

Calcium Chloride

pH after reaction

Calcium Hydroxide

r s ¼ 

0

Time, min

F conc. after reaction, mg/L

0 -20

2

4

6

8

10

12

Adjusted pH before reaction

(6)

(7)

where, rF is rate of reaction, CF and CCa are concentrations of F and Ca ions respectively, m and n are orders with respect to F and Ca ions and k is specific rate constant. Reactions carried out with method of excess by keeping calcium concentration 20 times excess than the stoichiometric ratio, have shown that the reaction followed first order kinetics for both Ca(OH)2 and CaCl2. Fig. 3 shows that F concentration reduces from 150 mg/L to around 10 mg/L within first minute of reaction. Then the reaction followed minor declining path due to the lack of F ions. This can be considered as a pseudo-first order nature as one of the reactants concentrations was kept practically infinite.

pH after reaction

F conc. mg/L

Fig. 4. (a) Effect of pH (NaF reaction with Ca(OH)2). Initial pH adjusted using 0.1 N HCl and 0.1 N NaOH from 2, 4, 6, 8 and 10. Initial F ion concentration = 150 mg/L and temperature = 30  2 8C. (b) Effect of pH (NaF reaction with CaCl2). Initial pH adjusted using 0.1 N HCl and 0.1 N NaOH from 2, 4, 6, 8 and 10. Initial F ion concentration = 150 mg/L and temperature = 30  2 8C.

However, the final F ion concentration obtained was above 120 mg/L at pH 2 for both Ca(OH)2 and CaCl2 indicating that at extreme acidic pH levels, the reaction takes place at very low rate. This can be contributed to the fact that, at pH values less than 5, fluoride associates with hydrogen ion (H+) to form HF or HF2 molecule which is very stable in nature and does not react with calcium ion present [36]. Hence, very low F ion conversion was obtained below pH 4; however, the best fluoride removal was accomplished at pH range of 5.5–7.5 for CaCl2 [39]. 3.5. Influence of reactant loading

3.4. Influence of pH The pH of the medium is one of the important parameters, which significantly affects the fluoride conversion. Hence it is necessary to study the effect of pH on removal of fluoride from water. The pH of the solution was adjusted by using 0.1 N HCl or 0.1 N NaOH. Reactants chosen to react with NaF namely Ca(OH)2 and CaCl2 have worked effectively in the pH range of 4–14 (Fig. 4(a and b)). The final pH of the solution is stabilized at 11.4 and 6.7 for Ca(OH)2 and CaCl2 respectively, except for the initial values.

The particle size of the material has always been an important design parameter in research and industrial scale operations as it decides the settling rate of particle and thus helpful in design of the thickener. Keeping this in view, the effect of reactants loading, stoichiometry and temperature on particle size were studied. Two sets of experiments were carried out with calcium hydroxide and calcium chloride by varying the fluoride concentration from 20 to 120 mg/L in stoichiometric ratio (1:2). Fig. 5 shows that at the particle size of calcium fluoride (CaF2) was

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1.2

0.5 mm at lower fluoride concentrations using Ca(OH)2, but after 40 ppm the particle size is increased up to 1 mm. For CaCl2 also, at lower fluoride concentrations the particle size was about 0.3 mm which increased to around 0.5 mm after 80 ppm. Here, it is to be noted that the particle size obtained with Ca(OH)2 is greater than obtained with CaCl2. This is due to the fact that, lime itself acts as a coagulant when dissolved in water. These results are in good agreement with the literature results, where particle size of the precipitate formed is reported less than 2 mm [40].

1

Particle size, μm

5

0.8

0.6

0.4

3.6. Influence of temperature 0.2

0 0

20

40

60

80

100

120

140

Initial F conc. mg/L Calcium hydroxide

Calcium chloride

Fig. 5. Effect of reactant loading for reaction with NaF at F ion concentration ranging from 20, 40, 60, 80, 100 and 120 mg/L, temperature = 30  2 8C and Ca to F ion ratio at stoichiometric value 1:2.

(a)

1.5

50 45

1.3

40

F conc, mg/L

0.9

30 25

0.7

20

0.5

Particle Size, μm

1.1 35

15

Temperature has a great effect on chemical reaction kinetics and usually is a deciding factor for desired product as well as byproduct conversion. Hence, an attempt has been made to carry out reactions at different temperatures ranging from 10 to 50 8C. Ca to F ion ratio was kept maintained at stoichiometric value i.e. 1:2, while keeping F ion concentration at 150 mg/L. Samples were taken at 15 min, 30 min and 24 h intervals. The sample taken after 24 h is used for particle size measurement. The studies showed that there is an accelerating effect of temperature on the reaction kinetics for both reactions (Fig. 6(a and b)). A substantial decrease in F ion concentration was observed from 150 mg/L to less than 45 mg/L within the first 15 min followed by gradual decrease in concentration with further increase in reaction time. However, the reaction can be carried out at ambient temperature (30  28 C) due to very fast nature of reaction. It was expected that with an increase in temperature the viscosity of the solution decreases, resulting in reduced binding energy between the molecules leading to reduction in particle size. In this case, CaF2 particle size essentially remained unaltered with an increase in temperature for both the reactions which was found to be less than 1.5 mm for Ca(OH)2 and less than 0.6 mm for CaCl2. 4. Conclusions

0.3 10 0.1

5 0

-0.1 0

10

20

30

40

50

60

Temperature, 0C 15min

(b)

30min

24hr

Particle Size, micron 1

40

0.9

35

0.8

F conc, mg/L

0.7 25

0.6 0.5

20

0.4

15

Particle Size, μm

30

0.3 10 0.2 5

0.1

Amongst the three salts studied, calcium compounds proved to be successful in removing fluoride impurities from groundwater under the given set of conditions. Kinetic studies have shown that reaction between F and Ca ions in aqueous medium is very fast and followed pseudo-first order kinetics in excess. Both Ca(OH)2 and CaCl2 can work effectively in the pH range 4–14. However, best fluoride removal was achieved between pH 5.5 and 6.5. Particle size of calcium fluoride formed was found to be nearly 1 mm and 0.5 mm for Ca(OH)2 and CaCl2 respectively. Temperature variations showed significant effect on reaction kinetics; but not the particle size. Calcium chloride was found to be more effective in fluoride removal than calcium hydroxide (fluoride concentration reduced to around 2 mg/L). Optimum Ca to F ion ratio was found to be 3:2 and 2.5:2 for Ca(OH)2 and CaCl2 respectively. USEPA and WHO standards of fluoride concentration in water can be achieved by adding optimum dose of calcium source. This data can be further used to design a precipitation process in combination with membrane separation to solve the problem associated with concentrated retentate. List of symbols

0

0 0

10

20

30

40

50

60

Temperature, 0C 15min

30min

24hr

Particle Size , Micron

Fig. 6. (a) Effect of temperature on NaF reaction with Ca(OH)2 at initial F ion concentration = 150 mg/L and Ca to F ion ratio at stoichiometric value 1:2. (b) Effect of temperature on NaF reaction with CaCl2 at initial F ion concentration = 150 mg/L and Ca to F ion ratio at stoichiometric value 1:2.

Ca(OH)2 CaCl2 Mg(OH)2 NaF CaF2 WHO USEPA

calcium hydroxide calcium chloride magnesium hydroxide sodium fluoride calcium fluoride World Health Organization United States Environmental Protection Agency

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BIS UNICEF M. W. DI water TISAB pH ISE HF NaOH NaCl aq s Ksp rF CF CCa m n k

Bureau of Indian Standard United Nations International Children’s Emergency Fund molecular weight deionized water total ionic strength adjusting buffer potential of hydrogen ion selective electrode hydrofluoric acid sodium hydroxide sodium chloride aqueous solid solubility product rate of reaction concentration of F ion concentrations of Ca++ ion order with respect to F order with respect to Ca++ specific rate constant

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Please cite this article in press as: S.V. Jadhav, et al., Treatment of fluoride concentrates from membrane unit using salt solutions, J. Water Process Eng. (2014), http://dx.doi.org/10.1016/j.jwpe.2014.04.004