Fluoride removal for underground brackish water by ... - Science Direct

Desalination 212 (2007) 37–45

Fluoride removal for underground brackish water by adsorption on the natural chitosan and by electrodialysis M.A. Menkouchi Sahlia, S. Annouarb,c, M.Tahaiktb, M. Mountadarb, A. Soufianec, A. Elmidaouia* Separation Processes Laboratory, Faculty of Sciences, B.P 1246 Kenitra, Morocco Tel./Fax +212 7 37 30 33; email: [email protected] b Unit of Analytical Chemistry and Environmental Engineering, Faculty of Sciences, El Jadida, Morocco c Laboratory of Coordination, Chemistry and Analysis, Faculty of Sciences, El Jadida, Morocco a

Received 15 April 2004; accepted 15 September 2006

Abstract In Morocco the concentration of fluoride in underground water in some phosphoric regions greatly exceeds the WHO standards. Defluoridation operations of brackish underground water have been carried out by two separation processes: adsorption on natural chitosan and electrodialysis using membranes CMX-ACS previously selected. The optimal operating conditions for fluoride removal by the chitosan have been determined firstly on synthetic water. Results indicate that both chemical adsorption on natural biopolymers and electrodialysis using specific anion membranes demonstrated their capacity to remove fluoride from brackish water. The results show also that a coupling of these two processes is an interesting alternative to removal of fluoride for brackish water with a high fluoride content. Keywords: Brackish water; Defluoridation; Adsorption; Chitosan; Electrodialysis

1. Introduction The underground waters that circulate in contact with fluorinated ores are particularly rich in fluoride [1]. The beneficial and the harmful effects of fluoride consumption are well known [2]. The dental and skeletal fluorisis (known in Morocco by Dargmous) are the widespread harmful

effects of a long consumption of fluoride [3–5]. Because of the permanent risks, fluoride removal from waters with a high fluoride content becomes a necessity. Various methods have been tested for defluoridation of waters: coagulation–flocculation and adsorption [6,7], precipitation [8], ion exchange, reverse osmosis, nanofiltration and electrodialysis [9–12].

*Corresponding author. 0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.09.018

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M.A. Menkouchi Sahli et al. / Desalination 212 (2007) 37–45

Chitin and chitosan are two biopolymers produced by some animals. They were first developed for pharmacology, cosmetology and medicine [12]. Recently studies have been carried out on the properties of chitosan for fixing heavy metals [13,14]. Electrodialysis [15] is one of the membrane processes previously developed for desalination of brackish water, but there are many other applications. The aim of this work is to study fluoride removal for brackish underground water by adsorption on chitosan and by electrodialysis. 2. Experimental Study was carried out on underground water in the region of Doukkala (Centre of Morocco). Table 1 gives the characteristics of the untreated water, which is brackish and contains many excessive ions, especially fluoride. 2.1. Chemical adsorption The support used for the defluoridation is chitosan, which is extracted from the waste of shrimps. The preparation of this adsorbent requires a sequence of chemical treatment stages described in detail in a previous work [16]. The tests have been achieved under mechanical agitation in a batch system of flasks containTable 1 Chemical composition of the untreated water Content pH CE, µS/cm F–, mg/l Cl–, mg/l SO42–, mg/l NO3–, mg/l HCO3–, mg/l Na+, mg/l Ca2+, mg/l Mg2+, mg/l

7.87 3900 3.25 1083 215 25 171 448 127 136

ing a determined quantity of the adsorbent. The optimization of the operating conditions has been carried out on a synthetic solution prepared by the distilled water and the sodium fluoride. The adsorbed quantity is expressed per unit of mass of the chitosan. 2.2. Electrodialysis The electrodialysis operation was carried out on a laboratory cell already described [17]. This apparatus was a batch electrodialysis unit composed of five compartments alternatively separated by cationic and anionic exchange membranes. The electrical field was imposed by two platinum-coated titanium electrodes coupled to an electrical generator. The two electrode compartments are separated from the others to prevent a modification of the composition of the solution, which could be caused by electrode reactions. The circulation of water through the dilute, concentrate and electrode rinse compartments was assured by ASTI pumps (Heidolph 110.40 type). The stack design characteristics of the electrodialysis cell are given in Table 2, and Fig. 1 gives the scheme of the process. 2.3. Parameters and method of analysis During the tests of both processes, water samples are taken periodically and the ion concentrations were determined analytically. The content of fluoride was determined by an Ion Method using Elit 8221 fluoride electrode and reference electrode ELIT 001N connected to HANNA GLP Microprocessor pH/Ion Meter. The other parameters were determined following Standard Methods [18].

3. Results and discussion The specific ion removal (SIR) expressed by following relation was introduced to follow the defluoridation operations:


39

Table 2 Stack design characteristics Membrane active area, cm2 Number of cell pairs Number of cationic membrane Number of anionic membrane Electrodes: anode cathode

36 1 3 1 Platinum-coated titanium Platinum-coated titanium

Dilute compartment

Solution Volume, l Flow, l/h

Raw water 0.5 100

Concentrate compartments


Raw water 0.5 100

Electrode rinse compartments


0.1 M Na2SO4 0.5 50 0.12

Imposed current, A

S

Fig. 1. Scheme of the electrodialysis cell process. G: generator; C: concentrate solution; D: dilute solution; R: electrodes rinse solution; P: pumps; S: stack; A: ampermeter; V: voltmeter.

3.1. Chemical adsorption

% SIR =

[Ion]t =0 − [Ion]t × 100 [Ion]t =0

(1)

3.1.1. Optimization of the running conditions A good adsorption must obey the law of Freundlich and Langmuir in their linear shape [19,20]:

40


Langmuir equation:

1 1 1 = + q qm qm bCe

adsorption (mg of NO–3/g of adsorbent); b: Langmuir constant associated with the energy of adsorption (l/mg); K: Freundlich constant associated with the capacity of adsorption (l/g adsorbent); 1/n: Freundlich constant associated with the affinity of adsorption. Fig. 2 presents the kinetics of defluoridation according to the time of the batch. This figure shows a rapid kinetic of fluorides adsorption with a maximal retention at 5 min. Beyond a light desorption of the ions fluorides is observed. Fig. 3 shows the effect of the agitation on the adsorption capacity. The fluoride removal in-

(2)

Freundlich equation:

1 log q = log K + log Ce n

(3)

where q: quantity of absorbed nitrate at equilibrium (mg/g of adsorbent); Ce: concentration at equilibrium in mg/l; qm: maximum capacity of

%RS

35 30 25 20 15 10 5 temps (min) 0 0

5

10

15

20

25

30

35

40

Fig. 2. Kinetics of adsorption (C0= 4mg/l, m = 1 g/l and T = 20°C).

%RS

60 50 40 30 20 10

V (tr/min)

0 0

200

400

600

800

1000

1200

1400

Fig. 3. Effect of agitation on the capacity of adsorption (C0 = 4 mg/l, m = 4 g/l and T = 20°C).


creases practically linearly with the agitation and tends to a level from 750 rpm. Fig. 4 shows that in the studied range, the temperature does not have a significant influence on the fluoride removal. Fig. 5 shows the pH dependence of fluoride removal. The adsorption increases with increasing pH until an optimum at about 6, after the defluoridation decreases in alkaline medium. In 1998 Jaafari et al. [21] found similar results with commercial chitosan. The analysis of these results shows that the optimised conditions are:

• • • •

Time of batch: 5 min Agitation speed: 750 rpm Ambient temperature Slightly acidic pH (pH = 6)

Under these conditions and as is shown in Figs. 6 and 7, the adsorption of the fluorides on the chitosan is described by the model of Freundlich and Langmuir. The capacity of adsorption increases with the increase of the concentration of the fluorides. The constants of Langmuir and Freundlich determined from Figs. 6 and 7 are regrouped in Table 3.

%RS

52 50 48 46 44 42 T(°C) 40 20

40

60

80

100

120

Fig. 4. Effect of the temperature on the adsorption capacity (C0 = 4 mg/l, m = 4 g/l and V = 750 rpm).

%RS

65 60 55 50 45 40 35

pH

30 0

2

4

41

6

8

10

Fig. 5. pH effect on the capacity of adsorption (C0 = 10 mg/l, m = 4 g/l and T = 20°C).

12

42

M.A. Menkouchi Sahli et al. / Desalination 212 (2007) 37–45 2.2 y = -0.7189 + 5.4696x R= 0.99883

2

1/q (g/mg)

1.8 1.6 1.4 1.2 1 0.8

1/C (l/mg) e

0.6 0.25

0.3

0.35

0.4

0.45

0.5

0.55

Fig. 6. Langmuir isotherm for fluoride adsorption on chitosan. 0.6 y = -1.8642 + 1.6378x R= 0.99338

Log(q)

0.4 0.2 0 -0.2 -0.4 -0.6

Log (C ) e

-0.8 0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

Fig. 7. Freundlich isotherm of for fluoride adsorption on chitosan.

Table 3 Langmuir and Freundlich constants

Langmuir qm b (mg/g) (l/mg)

R

K (l/g)

n

R

qm (mg/g)

1.39

0.99

0.155

0.61

0.99

4.67

0.131

Freundlich

3.1.2. Defluoridation of an underground water by chitosan Defluoridation of underground water by chitosan was carried out under the optimised con-

ditions obtained in the last part. Fig. 8 shows the variation with time of the specific ion rejections. Ion removal increases with time and tends to a level after 5 min for all analysed ions. The selectivity of the chitosan towards various anions and for this composition of the water is as follows: F– > HCO–3 > NO–3 > Cl– > SO2– . 4 Table 4 gives the composition of the treated water after one batch of 15 min. The fluoride content and the salinity remain higher than the standards. Many successive serial batches or more chitosan are necessary to reach the drinking water standards.


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Fig. 8. Variation with time of the specific anions removal.

Table 4 Analytical results of treated water after one batch of 15 min

Content, mg/l WHO standards, mg/l

TDS

NO3–

Cl-

HCO3–

F–

SO42 –

2853 900

19 50

1018 300

116 —

1.82 1.5

205 200

A second batch of 15 min on the treated water of the first batch shows that the fluoride decreases under the standards to reach a value of 0.9 mg/l, while the TDS remains relatively high (2728 mg/l). 3.2. Defluoridation of underground water by electrodialysis Electrodialysis of the fluoridised water was carried out with an ACS membrane previously selected in the laboratory for fluoride removal [22]. The cationic exchange membrane CMX was a standard membrane. Both were made by Tokuyama Corp. The tests were conduced on the brackish underground water. Fig. 9 shows the variations with time of the TDS and of the specific ions removal. Table 5 gives the water composition for various demineralisation rates.

These results confirm that electrodialysis is reasonable for desalinating brackish water to drinking standards. With this composition the percentage removal of Cl– was the highest, fol. lowed by NO–3, F–, HCO–3 and SO2– 4 Fluoride content was obtained at a high demineralization rate of 80%. These results show that the defluoridation by electrodialysis of equilibrated water with a high fluoride concentration is not convenient because, on the one hand, it requires high energy consumption and, on the other hand, the primary and secondary polarization problems can occur in both dilute and concentrate compartments. 4. Conclusion Defluoridation of brackish underground water by adsorption on chitosan is very fast and re-

44


Fig. 9. Variation with time of the TDS and specific ions removal.

Table 5 Variation with demineralisation rates of the treated water compositions

*

Demineralisation rates (%)

Salinity (g/l)

Nitrate (mg/l)

Chloride (mg/l)

Bicarbonate (mg/l)

Fluoride (mg/l)

Sulphate (mg/l)

30 50 70 80

2* 1.4* 0.9 0.6

10 5.7 3.8 3

487* 195 107 25

160.6* 133.2 93.9 64.9

2.97* 2.5* 1.69* 0.91

209 202 181 135

Greatly exceed WHO standards [23].

quires a light acidity of the treated medium. The importance of this operation lies in the valorisation of an industrial animal waste in the region. However the efficiency of this support in salt removal remains very weak. Electrodialysis gives good performance for salt and fluoride removal from brackish water. However, the cost of the operation and the polarization problems are the limiting factors for equilibrated water with a high fluoride content. The combination of these two processes appears to be a good economical and alternative solution to remove fluoride from brackish underground water with a high fluoride content: adjust-

ment of the salinity by electrodialysis makes it compatible with the standards for potability in the first stage and elimination of the remaining fluoride ions by adsorption in the second stage. Acknowledgements This works was supported by Eurodia Co., France. The authors express their thanks for this support. References [1] Y. Travi, Rec. Acad. Sci. Paris, 298(7) (1984) 313– 316.

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