- Email: [email protected]
Nitrate removal of brackish underground water by chemical adsorption and by electrodialysis
Desalination 227 (2008) 327–333
Nitrate removal of brackish underground water by chemical adsorption and by electrodialysis M.A. Menkouchi Sahlia, S. Annouarb,c, M. Mountadarb, A. Soufianec, A. Elmidaouia* a
Separation Processes Laboratory, Faculty of Sciences, B.P. 1246, Kenitra, Morocco Tel. /Fax: +212 (7) 37 30 33; email: [email protected] b Unit of Analytic Chemistry and Environmental Engineering; cLaboratory of Coordination Chemistry and Analysis, Faculty of Sciences, El Jadida, Morocco Received 14 February 2007; Accepted 30 July 2007
Abstract Nitrate content has increased in Morocco, especially in underground water of agrarian areas. Several processes including degradation processes and separation processes can remove nitrate from water. Two separating processes were studied to remove nitrate from brackish underground water: adsorption on industrial animal waste and electrodialysis equipped by anion monovalent membrane. The results show that a desired product water quality can easily be obtained by electrodialysis contrary to chemical adsorption which requires a great reactional surface. Keywords: Brackish water; Denitration; Adsorption; Chitosan; Electrodialysis
1. Introduction Today the protection of health and the respect of the environment are among man’s first preoccupations. One of the most important concerns is the contamination of underground waters by the nitrates. In Morocco, as in several other countries, this contamination worsens continually following the demographical and economical *Corresponding author.
development which main origin comes from agricultural practices [1,2]. The main sanitary risks to consider during of the consumption of nitrates in quantity passing these norms required by OMS to 50 ppm are methaemoglobinaemia and formation of the nitrosamines [3]. Facing these permanent risks and the difficulty to warn of this contamination, the elimination of these ions becomes a necessity. Various methods have been tested for the denitration of waters, among others: biological
0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.07.021
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M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
denitrification [4,5], ion exchange [6], membrane techniques [7] and chemical adsorption [8]. In this work we were interested in examining two different methods of nitrate removal: adsorption on a natural biopolymer and electrodialysis. Chemical adsorption is a technique based on the fixing of the harmful elements on the material surface; this fixing can be either by catchments on pores or by chemical reaction. The adsorbing material used in this study is a biopolymer produced by some marine animals [9,10] with importing specific surfaces, biodegradable and non-toxic. Electrodialysis is one of the membrane processes that proved reliable and efficient in many applications, especially in desalination of brackish water. The ionic separation in this process takes place under the effect of an electric field [11,12]. The interest of this work resides first in the examination of the reliability of these two methods for the specific nitrate removal of underground waters destined for consumption. Then to the technical comparison between these two processes.
Table 1 Chemical composition of the untreated water
2. Experimental
2.2. Electrodialysis
The study was carried out on underground water in the region of Doukkala (West Centre of Morocco and agrarian region). Table 1 gives the characteristics of the untreated water, which is brackish and contains many excessive ions, especially nitrate.
The electrodialysis operation was carried out on a laboratory cell already described [14]. 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
2.1. Chemical adsorption The support used for the denitration is chitosan, which is extracted from the waste of shrimps. The preparation of this adsorbent requires a sequence of the chemical treatment stages that has been described in detail in a previous work [13].
Content
NO3! F– C– SO42! HCO3! Na+ K+ Ca2+ Mg2+ pH CE, µS/cm
mg/l
meq/l
210 1.9 821 330 97.6 633 48 195 160 7.75 3800
336 1 2315 687 16 2752 123 975 1315
The tests have been achieved under mechanical agitation in batch system of flasks containing 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 potassium nitrate. The adsorbed quantity is expressed per unit of mass of the chitosan.
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
stopped by the cation exchange membrane in the concentrate compartment.
Table 2 Stack design characteristics Membrane active area, cm2 No. of cell pairs No. of cationic membranes No. of anionic membranes Titanium electrodes: Anode Cathode Solution Dilute compartment: Volume, l Flow, l/h Solution Concentrate compartments: Volume, l Flow, l/h Solution (Na2SO4, M Electrode rinse compartments: Volume, l Flow, l/h Imposed current, A
329
36 1 3 1 Platinum-coated Platinum-coated Raw water 0.5 100 Raw water 0.5 100 0.1 0.5 50 0.12
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 nitrate was determined by an ion method using the Elit 8021 nitrate electrode and reference electrode Elit 001N connected to a Hanna GLP microprocessor pH/ion meter. The other parameters were determined following standard methods [15].
3. Results and discussion The specific ion removal (SIR) expressed by the following relation was introduced to follow the denitration operations:
% SIR =
[Ion]t =0 -[Ion]t × 100 [Ion]t =0
3.1. Chemical adsorption 3.1.1. Optimization of the running conditions Good adsorption must obey the law of Freundlich and Langmuir under their linear shape [16,17]: C Langmuir equation: Fig. 1. Principle of denitration by electrodialysis. R, rinse solution; D, diluted solution; C, concentrated solution.
1 1 1 = + q qm qm bCe C Freundlich equation:
electrodialysis cell are given in Table 2, and Fig. 1 gives the principle of the nitrate removal. Under the influence of direct current, nitrates presents in the raw water migrate towards the anode. They leave the dilute compartment, move through the anion exchange membrane and
log q = log K +
1 log Ce n
where q is the quantity of absorbed nitrate at equilibrium (mg/g of adsorbent); Ce the concentration at equilibrium in mg/l; qm the maximum
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Fig. 2. Kinetics of adsorption (m =5 g/l, [NO3!], 0 = 210 mg/l, pH = 6.24, RS = 600 rpm and T = 20°C).
Fig. 3. Effect of agitation on the capacity of adsorption ([NO3!], 0 = 210 mg/l, m = 5 g/l, T = 20°C, RS = 600 rpm and pH= 6.24).
capacity of adsorption (mg of NO3!/g of adsorbent); b the Langmuir constant associated with the energy of adsorption (l/mg); K the Freundlich constant associated with the capacity of adsorption (l/g adsorbent); and 1/n the Freundlich constant associated with the affinity of adsorption. Fig. 2 presents the kinetics of denitration according to the time of the batch. This figure shows a rapid kinetic of nitrates adsorption with a maximal retention at 15 min. Beyond a light desorption of the ions nitrates is observed. Fig. 3 shows the effect of the agitation on the adsorption capacity. The nitrate removal increases practically linearly with the agitation and tends to a level from 600 rpm. Fig. 4 shows the pH dependence of nitrate removal. The adsorption increases with increasing pH until an optimum at about 6, after the denitration decreases in alkaline medium. In 1998
Fig. 4. pH effect on the adsorption capacity ([NO3!]0 = 210 mg/l, m = 5g/l, T = 20°C and RS = 600 rpm).
Fig. 5. Effect of the temperature on the adsorption capacity ([NO3!]0 = 210 mg/l, m =5 g/l, pH = 6 and RS = 600 rpm).
Jaafari et al. [18] found similar results with a commercial chitosan. Fig. 5 shows that in the studied range, the temperature does not have a significant influence on the nitrate removal. The analysis of these results shows that the optimised conditions are: C time of batch: 15 min C sirring rate: RS = 600 rpm C ambient temperature C slightly acidic pH (pH=6) Under these conditions, and as is shown in Figs. 6 and 7, the adsorption of the nitrates 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 nitrates. The constants of Langmuir and Freundlich determined from Figs. 6 and 7 are regrouped in Table 3.
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
Fig. 6. Freundlich isotherm for nitrate adsorption on chitosan.
331
Fig. 8. Variation with time of specific extraction of anions.
obtained in the last part. Fig. 8 shows the variation with time of the specific ion extraction. Specific ions removal increase with time and tends to a level after 10 min for nitrate and after 5 min for the other ions. The selectivity of the chitosan towards various anions and for this composition of the water is as follows: NO3! > HCO3! > F! > Cl! > SO42!. Table 4 gives the composition of the treated water after one batch of 15 min. The nitrate, the nitrate and fluoride contents and the salinity remains higher than the standards. Many successive serial batches or more chitosan are necessary to reach the drinking water standards. Fig. 7. Langmuir isotherm for adsorption on chitosan.
The maximum adsorption capacities (qm) calculated from the Langmuir and Freundlich isotherms are different and not in agreement with the experimental value (19 mg/g). The adsorption phenomenon is not a simplfied phenomenon and it depends not only on the concentration but on many others parameters. 3.1.2. Denitration of an underground water by chitosan Denitration of underground water by chitosan was carried out under the optimised conditions
3.2. Denitration of underground water by electrodialysis Electrodialysis of the nitrated water was carried out with a nitrate selective membrane ACS previously selected [19]. The cationic exchange membrane CMX was a standard membrane. Both were made by Tokuyama Corp. The tests were conducted on underground water under a current density of 3.3 mA/cm2. Figs. 9 and 10 show the variations with time of the TDS and of the specific ions removal. Table 5 gives the water composition for various demineralisation rates.
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Table 3 Langmuir and Freundlich constants Langmuir qm (mg/g) 8.03
Freundlich b (l/mg) 0.007
R 0.97
K (l/g) 99.48
n 0.616
R 0.97
qm (mg/g) 23.85
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!
2740 900
142.8 50
763.5 300
80 —
1.58 1
313.5 200
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)
27 50 70 80
2.1a 1.4a 0.8 0.6
84a 46 29 21
534a 328a 107 25
92 78 59 41
1.7a 1.5a 1.1 0.7
320a 310a 281a 224
a
Greatly exceeds the WHO standards [20].
Fig. 9. Variation with electrodialysis time of TDS.
Fig. 10. Variation with time of the specific ions removal.
These results confirm the satisfactory performances of electrodialysis in the nitrate
removal from brackish water. An additional effect of this process that makes this charged water
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
drinkable with regard to salinity and others contaminant ions. Different qualities can be obtained while exploiting the driving force or electrodialysis time. Decontaminated produced water with regard to nitrate content was obtained at a demineralization rate of 50% but the other parameters exceed the drinkable standards. The good quality with economical treatment was obtained at 70%.
4. Conclusions The two tested processes showed their capacity to remove nitrate from brackish water. With the electrodialysis equipped by ACS monovalent membrane a desired drinking water respecting the standards can be easily produced from brackish water. The principal disadvantage of this technology is the destiny of the removed concentrate nitrate. The reduction of nitrate by adsorption on natural chitosan is possible but requires an important reactional surface which makes difficult its industrial realisation. The kinetics of nitrate removal by chitosan for this case of water composition is 12 mg-NO3 per gramme of adsorbent with a retention time of 10 min. However, this process can serve to reduce the concentrate nitrate for the electrodialysis reject. Moreover, it permits the valorisation of an industrial animal waste by its utilisation in agriculture as a fertilizer.
Acknowledgements This work was supported by the Eurodia Co. France (an affiliate of the Tokuyama Corp.) and ONEP (National Office of Potable Water) in Morocco. The authors express their thanks for this support.
333
References [1] M. Mountadar, Doctorate Thesis, University Chouaib Doukkali, Eljadida, Morocco, 1995. [2] L. Echihabi, A. Outair and L. Laraki, International meeting on fluorides, nitrates and pesticides in the Mediterranean basin, Kenitra, Morocco, 1997, p. 72. [3] P.J. Golden and R. Weinstein, J. Clin. Aphresis, 13(1) (1998) 28–31. [4] G. Blecon, M. Gillet and G. Martin, Science l’Eau, 2(3) (1997) 267–279. [5] F. Hanus and C. Bernard, Tech. Sci. Méthodes de l’Eau., 83(4) (1988) 243–246. [6] A. Deguin, L’Eau., 4 (1988) 213–234. [7] S. Choi, Z. Yun, S. Hong and K. Ahn, Desalination, 133 (2001) 53. [8] C. Milot, Doctorate Thesis, University Montpellier II, 1998, 256. [9] R.A.A. Muzzarelli, Chitin, Pergamon Press, New York, 1977, p. 309. [10] G.A.F. Roberts, Chitin Chemistry, Macmillan, London, 1992. [11] A.T. Cherif, C. Gavach, T. Cohen, P. Dagar and L. Albert, Hydrometallurgy, 21 (1988) 191–201. [12] P. Moatti, Doctorate Thesis, Montpellier II, 1990. [13] S. Annouar, A. Soufiane and M. Mountadar, Phys. Chem. News, 16 (2004) 128–135. [14] M.O. Elkhattabi, A.R. My. Hafidi and A. Elmidaoui, Desalination, 107 (1996) 149. [15] AFNOR, Recueil des Normes Françaises: qualité de l’eau, 3rd ed., 1999. [16] I. Langmuir, Modelisation of adsorption, Phys. Rev., 6 (1915) 79–80. [17] H. Freundlich, Z. Phys. Chem., 57 (1906) 358–471. [18] K. Jaafari, S. Elmaleh, J. Coma and K. Benkhouja, Water S.A., 27(1) (2001) 9–13. [19] A. Elmidaoui, F. Elhannouni, M.A. Menkouchi Sahli, L. Chay, H. Elabbassi, M. Hafsi and D. Largeteau, Desalination, 136 (2001) 325–332. [20] WHO, Guidelines and Criteria for Water Quality Management in Ontario, Ministry of the Environment Resource Branch, 1994.
Nitrate removal of brackish underground water by chemical adsorption and by electrodialysis M.A. Menkouchi Sahlia, S. Annouarb,c, M. Mountadarb, A. Soufianec, A. Elmidaouia* a
Separation Processes Laboratory, Faculty of Sciences, B.P. 1246, Kenitra, Morocco Tel. /Fax: +212 (7) 37 30 33; email: [email protected] b Unit of Analytic Chemistry and Environmental Engineering; cLaboratory of Coordination Chemistry and Analysis, Faculty of Sciences, El Jadida, Morocco Received 14 February 2007; Accepted 30 July 2007
Abstract Nitrate content has increased in Morocco, especially in underground water of agrarian areas. Several processes including degradation processes and separation processes can remove nitrate from water. Two separating processes were studied to remove nitrate from brackish underground water: adsorption on industrial animal waste and electrodialysis equipped by anion monovalent membrane. The results show that a desired product water quality can easily be obtained by electrodialysis contrary to chemical adsorption which requires a great reactional surface. Keywords: Brackish water; Denitration; Adsorption; Chitosan; Electrodialysis
1. Introduction Today the protection of health and the respect of the environment are among man’s first preoccupations. One of the most important concerns is the contamination of underground waters by the nitrates. In Morocco, as in several other countries, this contamination worsens continually following the demographical and economical *Corresponding author.
development which main origin comes from agricultural practices [1,2]. The main sanitary risks to consider during of the consumption of nitrates in quantity passing these norms required by OMS to 50 ppm are methaemoglobinaemia and formation of the nitrosamines [3]. Facing these permanent risks and the difficulty to warn of this contamination, the elimination of these ions becomes a necessity. Various methods have been tested for the denitration of waters, among others: biological
0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.07.021
328
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
denitrification [4,5], ion exchange [6], membrane techniques [7] and chemical adsorption [8]. In this work we were interested in examining two different methods of nitrate removal: adsorption on a natural biopolymer and electrodialysis. Chemical adsorption is a technique based on the fixing of the harmful elements on the material surface; this fixing can be either by catchments on pores or by chemical reaction. The adsorbing material used in this study is a biopolymer produced by some marine animals [9,10] with importing specific surfaces, biodegradable and non-toxic. Electrodialysis is one of the membrane processes that proved reliable and efficient in many applications, especially in desalination of brackish water. The ionic separation in this process takes place under the effect of an electric field [11,12]. The interest of this work resides first in the examination of the reliability of these two methods for the specific nitrate removal of underground waters destined for consumption. Then to the technical comparison between these two processes.
Table 1 Chemical composition of the untreated water
2. Experimental
2.2. Electrodialysis
The study was carried out on underground water in the region of Doukkala (West Centre of Morocco and agrarian region). Table 1 gives the characteristics of the untreated water, which is brackish and contains many excessive ions, especially nitrate.
The electrodialysis operation was carried out on a laboratory cell already described [14]. 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
2.1. Chemical adsorption The support used for the denitration is chitosan, which is extracted from the waste of shrimps. The preparation of this adsorbent requires a sequence of the chemical treatment stages that has been described in detail in a previous work [13].
Content
NO3! F– C– SO42! HCO3! Na+ K+ Ca2+ Mg2+ pH CE, µS/cm
mg/l
meq/l
210 1.9 821 330 97.6 633 48 195 160 7.75 3800
336 1 2315 687 16 2752 123 975 1315
The tests have been achieved under mechanical agitation in batch system of flasks containing 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 potassium nitrate. The adsorbed quantity is expressed per unit of mass of the chitosan.
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
stopped by the cation exchange membrane in the concentrate compartment.
Table 2 Stack design characteristics Membrane active area, cm2 No. of cell pairs No. of cationic membranes No. of anionic membranes Titanium electrodes: Anode Cathode Solution Dilute compartment: Volume, l Flow, l/h Solution Concentrate compartments: Volume, l Flow, l/h Solution (Na2SO4, M Electrode rinse compartments: Volume, l Flow, l/h Imposed current, A
329
36 1 3 1 Platinum-coated Platinum-coated Raw water 0.5 100 Raw water 0.5 100 0.1 0.5 50 0.12
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 nitrate was determined by an ion method using the Elit 8021 nitrate electrode and reference electrode Elit 001N connected to a Hanna GLP microprocessor pH/ion meter. The other parameters were determined following standard methods [15].
3. Results and discussion The specific ion removal (SIR) expressed by the following relation was introduced to follow the denitration operations:
% SIR =
[Ion]t =0 -[Ion]t × 100 [Ion]t =0
3.1. Chemical adsorption 3.1.1. Optimization of the running conditions Good adsorption must obey the law of Freundlich and Langmuir under their linear shape [16,17]: C Langmuir equation: Fig. 1. Principle of denitration by electrodialysis. R, rinse solution; D, diluted solution; C, concentrated solution.
1 1 1 = + q qm qm bCe C Freundlich equation:
electrodialysis cell are given in Table 2, and Fig. 1 gives the principle of the nitrate removal. Under the influence of direct current, nitrates presents in the raw water migrate towards the anode. They leave the dilute compartment, move through the anion exchange membrane and
log q = log K +
1 log Ce n
where q is the quantity of absorbed nitrate at equilibrium (mg/g of adsorbent); Ce the concentration at equilibrium in mg/l; qm the maximum
330
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
Fig. 2. Kinetics of adsorption (m =5 g/l, [NO3!], 0 = 210 mg/l, pH = 6.24, RS = 600 rpm and T = 20°C).
Fig. 3. Effect of agitation on the capacity of adsorption ([NO3!], 0 = 210 mg/l, m = 5 g/l, T = 20°C, RS = 600 rpm and pH= 6.24).
capacity of adsorption (mg of NO3!/g of adsorbent); b the Langmuir constant associated with the energy of adsorption (l/mg); K the Freundlich constant associated with the capacity of adsorption (l/g adsorbent); and 1/n the Freundlich constant associated with the affinity of adsorption. Fig. 2 presents the kinetics of denitration according to the time of the batch. This figure shows a rapid kinetic of nitrates adsorption with a maximal retention at 15 min. Beyond a light desorption of the ions nitrates is observed. Fig. 3 shows the effect of the agitation on the adsorption capacity. The nitrate removal increases practically linearly with the agitation and tends to a level from 600 rpm. Fig. 4 shows the pH dependence of nitrate removal. The adsorption increases with increasing pH until an optimum at about 6, after the denitration decreases in alkaline medium. In 1998
Fig. 4. pH effect on the adsorption capacity ([NO3!]0 = 210 mg/l, m = 5g/l, T = 20°C and RS = 600 rpm).
Fig. 5. Effect of the temperature on the adsorption capacity ([NO3!]0 = 210 mg/l, m =5 g/l, pH = 6 and RS = 600 rpm).
Jaafari et al. [18] found similar results with a commercial chitosan. Fig. 5 shows that in the studied range, the temperature does not have a significant influence on the nitrate removal. The analysis of these results shows that the optimised conditions are: C time of batch: 15 min C sirring rate: RS = 600 rpm C ambient temperature C slightly acidic pH (pH=6) Under these conditions, and as is shown in Figs. 6 and 7, the adsorption of the nitrates 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 nitrates. The constants of Langmuir and Freundlich determined from Figs. 6 and 7 are regrouped in Table 3.
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
Fig. 6. Freundlich isotherm for nitrate adsorption on chitosan.
331
Fig. 8. Variation with time of specific extraction of anions.
obtained in the last part. Fig. 8 shows the variation with time of the specific ion extraction. Specific ions removal increase with time and tends to a level after 10 min for nitrate and after 5 min for the other ions. The selectivity of the chitosan towards various anions and for this composition of the water is as follows: NO3! > HCO3! > F! > Cl! > SO42!. Table 4 gives the composition of the treated water after one batch of 15 min. The nitrate, the nitrate and fluoride contents and the salinity remains higher than the standards. Many successive serial batches or more chitosan are necessary to reach the drinking water standards. Fig. 7. Langmuir isotherm for adsorption on chitosan.
The maximum adsorption capacities (qm) calculated from the Langmuir and Freundlich isotherms are different and not in agreement with the experimental value (19 mg/g). The adsorption phenomenon is not a simplfied phenomenon and it depends not only on the concentration but on many others parameters. 3.1.2. Denitration of an underground water by chitosan Denitration of underground water by chitosan was carried out under the optimised conditions
3.2. Denitration of underground water by electrodialysis Electrodialysis of the nitrated water was carried out with a nitrate selective membrane ACS previously selected [19]. The cationic exchange membrane CMX was a standard membrane. Both were made by Tokuyama Corp. The tests were conducted on underground water under a current density of 3.3 mA/cm2. Figs. 9 and 10 show the variations with time of the TDS and of the specific ions removal. Table 5 gives the water composition for various demineralisation rates.
332
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Table 3 Langmuir and Freundlich constants Langmuir qm (mg/g) 8.03
Freundlich b (l/mg) 0.007
R 0.97
K (l/g) 99.48
n 0.616
R 0.97
qm (mg/g) 23.85
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!
2740 900
142.8 50
763.5 300
80 —
1.58 1
313.5 200
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)
27 50 70 80
2.1a 1.4a 0.8 0.6
84a 46 29 21
534a 328a 107 25
92 78 59 41
1.7a 1.5a 1.1 0.7
320a 310a 281a 224
a
Greatly exceeds the WHO standards [20].
Fig. 9. Variation with electrodialysis time of TDS.
Fig. 10. Variation with time of the specific ions removal.
These results confirm the satisfactory performances of electrodialysis in the nitrate
removal from brackish water. An additional effect of this process that makes this charged water
M.A. Menkouchi Sahli et al. / Desalination 227 (2008) 327–333
drinkable with regard to salinity and others contaminant ions. Different qualities can be obtained while exploiting the driving force or electrodialysis time. Decontaminated produced water with regard to nitrate content was obtained at a demineralization rate of 50% but the other parameters exceed the drinkable standards. The good quality with economical treatment was obtained at 70%.
4. Conclusions The two tested processes showed their capacity to remove nitrate from brackish water. With the electrodialysis equipped by ACS monovalent membrane a desired drinking water respecting the standards can be easily produced from brackish water. The principal disadvantage of this technology is the destiny of the removed concentrate nitrate. The reduction of nitrate by adsorption on natural chitosan is possible but requires an important reactional surface which makes difficult its industrial realisation. The kinetics of nitrate removal by chitosan for this case of water composition is 12 mg-NO3 per gramme of adsorbent with a retention time of 10 min. However, this process can serve to reduce the concentrate nitrate for the electrodialysis reject. Moreover, it permits the valorisation of an industrial animal waste by its utilisation in agriculture as a fertilizer.
Acknowledgements This work was supported by the Eurodia Co. France (an affiliate of the Tokuyama Corp.) and ONEP (National Office of Potable Water) in Morocco. The authors express their thanks for this support.
333
References [1] M. Mountadar, Doctorate Thesis, University Chouaib Doukkali, Eljadida, Morocco, 1995. [2] L. Echihabi, A. Outair and L. Laraki, International meeting on fluorides, nitrates and pesticides in the Mediterranean basin, Kenitra, Morocco, 1997, p. 72. [3] P.J. Golden and R. Weinstein, J. Clin. Aphresis, 13(1) (1998) 28–31. [4] G. Blecon, M. Gillet and G. Martin, Science l’Eau, 2(3) (1997) 267–279. [5] F. Hanus and C. Bernard, Tech. Sci. Méthodes de l’Eau., 83(4) (1988) 243–246. [6] A. Deguin, L’Eau., 4 (1988) 213–234. [7] S. Choi, Z. Yun, S. Hong and K. Ahn, Desalination, 133 (2001) 53. [8] C. Milot, Doctorate Thesis, University Montpellier II, 1998, 256. [9] R.A.A. Muzzarelli, Chitin, Pergamon Press, New York, 1977, p. 309. [10] G.A.F. Roberts, Chitin Chemistry, Macmillan, London, 1992. [11] A.T. Cherif, C. Gavach, T. Cohen, P. Dagar and L. Albert, Hydrometallurgy, 21 (1988) 191–201. [12] P. Moatti, Doctorate Thesis, Montpellier II, 1990. [13] S. Annouar, A. Soufiane and M. Mountadar, Phys. Chem. News, 16 (2004) 128–135. [14] M.O. Elkhattabi, A.R. My. Hafidi and A. Elmidaoui, Desalination, 107 (1996) 149. [15] AFNOR, Recueil des Normes Françaises: qualité de l’eau, 3rd ed., 1999. [16] I. Langmuir, Modelisation of adsorption, Phys. Rev., 6 (1915) 79–80. [17] H. Freundlich, Z. Phys. Chem., 57 (1906) 358–471. [18] K. Jaafari, S. Elmaleh, J. Coma and K. Benkhouja, Water S.A., 27(1) (2001) 9–13. [19] A. Elmidaoui, F. Elhannouni, M.A. Menkouchi Sahli, L. Chay, H. Elabbassi, M. Hafsi and D. Largeteau, Desalination, 136 (2001) 325–332. [20] WHO, Guidelines and Criteria for Water Quality Management in Ontario, Ministry of the Environment Resource Branch, 1994.