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Nitrate removal from ternary ionic solutions by a tight nanofiltration membrane
Desalination 204 (2007) 63–71
Nitrate removal from ternary ionic solutions by a tight nanofiltration membrane A. Santafé-Moros*, J.M. Gozálvez-Zafrilla, J. Lora-García Dpto. de Ingeniería Química y Nuclear, Universidad Politécnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain Tel. +34 (96) 387-7633; Fax. +34 (96) 387-7639; email: [email protected] Received 13 February 2006; Accepted 4 April 2006
Abstract Groundwater with 50–150 mg·L!1 of nitrate ion and low salinity is a very common type of water in the Mediterranean area of Spain where nitrate-polluted water is associated with agricultural use. This study focused on the influence of the most common ions in groundwater on the nitrate rejection of the NF90 membrane (Dow-Filmtec). The effect of sulphate and chloride was studied using Na2SO4/NaNO3 and NaCl/NaNO3 solutions. The influence of different types of counter-ions on nitrate rejection was studied for sodium, calcium and magnesium using solutions containing Ca(NO3)2/NaNO3 and Mg(NO3)2/NaNO3. Both studies were performed at two levels of pH and nitrate concentration. It must be pointed out the high ionic rejection obtained with this membrane, not only for divalent ions but also for the monovalent ones, demonstrated that this membrane can have significant rejection rates at the studied concentration levels. In ternary ionic mixtures, the increase in co-ion concentration always produced a decrease in nitrate rejection. Nitrate rejection in cationic mixtures decreased when the concentration ratio of the divalent cations and the monovalent sodium was varied. A pH increase from 7 to 9 improved the nitrate rejection whatever the concentration and the composition were. The results show that it is possible to diminish nitrate concentration from polluted groundwater reaching concentrations under the maximum allowed limit for human consumption of 50 mg·L!1 of nitrate ion. Keywords: Nanofiltration; Nitrates; pH; Concentration; Drinking-water
*Corresponding author.
Presented at the EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.04.024
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A. Santafé-Moros et al. / Desalination 204 (2007) 63–71
1. Introduction Groundwater that slightly exceeds the legal limit of nitrate concentration for human consumption (50 mg·L!1) is becoming very common in the Mediterranean area of Spain. As a consequence, many drinking water resources are polluted by nitrates representing a risk for human health [1]. This type of groundwater is associated with agricultural irrigation, so its salinity is not very high. Typically, reverse osmosis and electrodialysis are used to treat this type of water, specially the former technique [2–4]. As an alternative, some nanofiltration (NF) membranes which exhibit enough rejection to monovalent ions can be used, especially at moderately high concentration. NF and reverse osmosis, unlike electrodialysis, can assure the permeate quality when treating groundwater polluted with organics or micro-organisms. However, NF processes can be more energetically efficient than the reverse osmosis ones, because of the lower operating pressures. NF has shown its effectiveness in the removal of a great variety of undesirable components from water. Its separation mechanisms combine sieving effect, differences in diffusivity and solubility of solutes and electrostatic interactions between the membrane surface groups and the ions. In the case of negatively charged membranes, anions like nitrates can be effectively rejected. However, ionic rejection is not only influenced by interaction between the membrane and a specific ion. It is known that for ionic solutions the solute– solute interactions and the solute–membrane interactions are dependent on the concentration, the composition and pH value of the feed solution [5–10]. In the case of ion mixtures, electrostatic interactions between co-ions may cause, according to the Donnan exclusion, a decrease in nitrate rejection, especially if less permeable co-ions are present in the solution. Moreover, shielding effect, as a function of ionic concentration and the counter-ion type, can modify anion rejection.
As a consequence, NF treatment of multicomponent solutions is complicated and still not completely understood, being necessary to obtain experimental data to know the performance of a particular NF membrane with a specific ionic solution. A suitable NF membrane for nitrate removal must have tight porous structures and be negatively charged. The Donnan effect has significant importance in membranes with looser structure [11], so the presence of more rejected species of the same charge would produce a remarkable decrease in nitrate rejection. Rejection decrease caused by the addition of sulphate and chloride ions had been referred by several authors, being even possible negative values [12–17]. There are also many authors that had observed variations in monovalent anion rejection caused by the type of cation in the solutions. Nevertheless, this variation in anion rejection as well as cation rejection seems to depend on ionic concentration and on the membrane itself [5,7,9,12,18,19]. Previous studies [20] using NaNO3 solutions and natural polluted groundwater showed that the NF90 membrane (Dow-Filmtec) can be suitable for the removal of nitrates from groundwater. The NF90 membrane was designed to remove a high percentage of salts, nitrate, iron and organic compounds such as pesticides, herbicides and THM precursors. Its high rejection has been confirmed by several authors [11,21–23]. It can be mainly explained by its tight porous structure. Pontié et al. [24] determined, using uncharged solutes, a pore radius of 0.54 nm, and Krieg et al. [11] obtained a MWCO ~200 Da, indicating that this membrane has a narrow pore size distribution. The present work studied the nitrate rejection of the NF90, as a representative NF membrane of tight porous structure. The influence of pH, concentration and ionic composition was studied using the main ions commonly present in groundwater. The following points were taken into account:
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1. a preliminary study of the performance of the NF90 with single salt solutions (binary solutions); 2. the effect of other anions at different pH and concentration. In order to do this, experiments with the ternary ionic systems chloride/ nitrate/sodium and sulphate/nitrate/sodium were performed at different concentration ratios; 3. the effect of cation interaction. The following ternary ionic systems were used: nitrate/sodium/calcium and nitrate/sodium/ magnesium. It was also studied the effect of pH and ionic concentration.
2. Materials and methods The experiments were carried out at a pilot plant equipped with a spiral-wound membrane module NF90-2540. This membrane is a thin-film composite polyamide manufactured by DowFilmTec. The plant was operated in the batch recirculation mode. The schematic diagram of the membrane test set-up for the laboratory study is shown in Fig. 1. All influent variables different from feed composition were kept constant. Transmembrane pressure was maintained at 5.00 ± 0.02 bar and temperature at 20.0 ± 0.2EC. The feed flow rate was varied to operate at a constant recovery of 15% in all experiments. Pure grade nitrate salts NaNO3, Na2SO4· 10H2O, NaCl, Ca(NO3)2·4H2O and Mg(NO3)2· 6H2O (Panreac) and deionized water were used to prepare the binary and ternary mixtures. Binary solutions were prepared at two levels of anion concentration, 1.61 and 4.84 meq·L!1 (that corresponds to 100 and 300 mg·L!1 of nitrate ion, respectively, in case of NaNO3 solutions). Two types of experiments were carried out with ternary solutions. In the experiments to study the influence of co-ions, the molar ratios sulphate/nitrate and chloride/nitrate were 1/1, 3/1 and 6/1, being sodium the counter-ion. To study the influence of cation type the equivalent ratios
Fig. 1. Schematic diagram of the NF pilot plant. 1 feed tank with temperature control, 2 20 µm filter, 3 speed variable pump, 4 2.5" spiral-wound membrane module, 5,6 pressure regulation valves, 7 DAQ of permeate flow, conductivity and temperature.
of divalent cation (calcium or magnesium) to sodium were 1/3, 1/1 and 3/1 for a constant nitrate content. All compositions were tested at two nitrate concentration levels (100 and 300 mg·L!1). The pH was adjusted using sodium hydroxide to work at pH 7 and 9. All experiments were performed during at least 4 h. A data acquisition system was used to monitor the permeate flow, conductivity and temperature in order to assure the stabilization of the membrane results in every experiment. Anion and cation concentration in both feed and permeate were analyzed using a 790P Metrohm ion chromatograph. Metrosep A Supp 5 and Metrosep C 2 (Metrohm) columns were used for anions and cations, respectively.
3. Results and discussion 3.1. Rejection in binary systems: Influence of anion type Fig. 2 shows the anionic rejection obtained when treating sodium salts as a function of feed pH and concentration. From these results, it must
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Fig. 2. Anionic rejection for sodium salts as a function of anionic concentration (C1 = 1.61 meq·L!1 and C2 = 4.84 meq·L!1) and feed pH (ΔP = 0.5 MPa, T = 20EC).
Fig. 3. Nitrate rejection for nitrate salts as a function of anionic concentration (C1 = 1.61 meq·L!1 and C2 = 4.84 meq·L!1) and feed pH. (ΔP = 0.5 MPa, T = 20EC).
be pointed out the high rejection reached with this membrane, not only for the divalent anion but also for the monovalent ones. The rejection follows the order: R(SO42!) > R(Cl!) > R(NO3!). The high sulphate rejection is mainly explained by its size and charge. The higher rejection of chloride compared with that of the nitrate ion is explained by many authors by differences in hydration [5,12,16] or by differences in their affinity with the membrane [17]. Nevertheless, the nitrate rejection for this membrane (>97%) is higher than those obtained with other NF membranes [12,13,17,25–28]. The pH increase caused an increase in the anion rejection in all cases, especially for the nitrate ion. This result could be the effect of an increase in the negative charge of the membrane. The concentration did not have a significant effect, except in the case of sodium nitrate at pH 7. At this pH the membrane charge is weaker than at pH 9 and consequently would be affected by ionic concentration.
treating the cationic binary solutions as a function of feed pH and concentration. It can be seen that nitrate ion is highly rejected for the three studied cations. The pH increase caused a significant increase in nitrate rejection, irrespective of the counterion. However, the difference in nitrate rejection as a function of pH for divalent cations is smaller than that obtained for sodium salts. The concentration did not have a significant effect on rejection. Nevertheless, it must be noticed that the feed concentration effect is different for the different salts. A higher ionic concentration caused a small increase in the rejection of salts with divalent cations but a decrease in the case of the sodium salt. This effect has also been observed in other tight NF membranes [25]. Fig. 4 shows that only at pH 7 and the highest concentration the rejection sequence derived from the Hofmeister’s series was observed [R(Mg2+) > R(Ca2+) > R(Na+)]. At pH 9, the order changed to R(Mg2+) > R(Na+) > R(Ca2+).
3.2. Rejection in binary systems: Influence of cation type Fig. 3 shows nitrate rejection obtained when
3.3. Effect of co-ions on nitrate rejection of ternary systems. Experiments were performed using sodium as cation and varying the molar ratio of sulphate or
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Fig. 4. Cationic rejection for nitrate salts as a function of anionic concentration (C1 = 1.61 meq·L!1 and C2 = 4.84 meq·L!1) and feed pH. (ΔP = 0.5 MPa, T = 20EC).
chloride to nitrate in order to study how these anions influence nitrate rejection. Rejection results corresponding to the ternary system NO3!/SO42!/Na+ are shown in Figs 5. and 6. Fig. 5 shows the nitrate rejection as a function of the concentration ratio of sulphate to nitrate at fixed nitrate concentrations of 100 and 300 mg· L!1 and for both feed pH studied. The results obtained when filtrating NaNO3 solutions have also been included as a reference. As can be seen, a decrease in nitrate rejection was obtained due to the increase of sulphate concentration. This behaviour is similar to that obtained by other authors with NF membranes [12–15]. The strong rejection of sulphate ion caused that many nitrates are forced to pass through the membrane in order to counterbalance the transfer of sodium ions (Fig. 6). However, it can be said that the decrease in nitrate rejection caused by sulphate addition in this membrane is not as pronounced as those found in the literature for other membranes. The rejection decrease was higher for solutions with the highest nitrate concentration. This could be caused by the highest total ionic concentration in regard to the solutions with 100 mg·L!1. As a consequence, superficial membrane charge would be more shielded by the
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Fig. 5. Nitrate rejection vs. molar ratio SO42!/NO3! for the ionic system NO3!/SO42!/Na+ as a function of nitrate ion concentration and feed pH. (ΔP = 0.5 MPa, T = 20EC).
Fig. 6. Ionic rejection vs. molar ratio SO42!/NO3! for the ionic system NO3!/SO42!/Na+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
sodium ions and the driving force diminishes due to a higher osmotic pressure. Results at pH 9 showed higher rejection than those reached at pH 7. Moreover, the rejection decrease caused by both sulphate and total ionic concentration were slightly less sharp at pH 9. Nitrate rejection and ionic rejection corresponding to the ternary system NO3!/Cl!/Na+ are shown in Figs. 7 and 8, respectively. The addition of chloride ion also caused a decrease in nitrate
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Fig. 7. Nitrate rejection vs. molar ratio Cl!/NO3! for the ionic system NO3!/Cl!/Na+ as a function of nitrate ion concentration and feed pH (ΔP = 0.5 MPa, T = 20EC).
Fig. 8. Ionic rejection vs. molar ratio Cl!/NO3! for the ionic system NO3!/Cl!/Na+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
rejection, as it has been previously observed by other authors [12,14,15,17]. As seen in Fig. 8, the rejection of both monovalent anions was influenced by the total ionic concentration and by the increase in chloride addition. However, the decrease in rejection was less sharp for the chloride ion, less permeable and thus more rejected, than for the nitrate ion. As expected, the decrease in nitrate rejection by chloride addition was lower than that obtained by sulphate addition (Figs. 7 and 5, respectively). This effect is caused by the influence of the characteristics of the co-ion. Comparing Figs. 5 and 7, results obtained for solutions with the same total ionic concentration (e.g., solutions with a molar ratio of SO42!/NO3! = 3 and molar ratio Cl!/NO3! = 6), it can be seen that the effect is stronger when sulphate ion is present in the mixtures. Therefore, the Donnan effect has an important role in the mechanism transport of this membrane when treating ionic solutions.
two levels of nitrate concentration, 100 and 300 mg·L!1, and the study was developed as a function of the equivalent fractions of divalent and monovalent cations. The influence of pH on rejection at the two studied levels was also assessed. In general, nitrate rejection was slightly influenced by the feed concentration of all studied compositions, with feed pH being the more significant influence. Fig. 9 shows the nitrate rejection reached for the ternary mixtures containing the system NO3!/ Ca2+/Na+ in solutions. The results obtained when filtrating the binary nitrate solutions of both cations have also been included as a reference. It can be seen that the mixtures solutions with calcium and sodium ions exhibited lower nitrate rejection than the solutions containing a single type of cation. It must be pointed out that the total ionic concentration, for each level of nitrate concentration in the feed, was the same irrespective of the proportion of each cation in the solution, and equal to that of the binary solutions. The highest nitrate rejections were reached for solutions at pH 9, and the effect of feed pH was similar for the two levels of feed concentration studied. For all the feed characteristics analyzed
3.4. Effect of cations on nitrate rejection of ternary systems The experiments were performed for feeds at
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Fig. 9. Nitrate rejection vs. calcium fraction for the ionic system NO3!/Na+/Ca2+ as a function of nitrate ion concentration and feed pH. (ΔP = 0.5 MPa, T = 20EC).
Fig. 10. Ionic rejection vs. calcium fraction for the ionic system NO3!/Na+/Ca2+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
Fig. 11. Nitrate rejection vs. magnesium fraction for the ionic system NO3!/Na+/Mg2+ as a function of nitrate ion concentration and feed pH (ΔP = 0.5 MPa, T = 20EC).
Fig. 12. Ionic rejection versus magnesium fraction for the ionic system NO3!/Na+/Mg2+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
the differenced in nitrate rejection were smaller for the compositions where calcium ion predominated. Fig. 10 shows that with the increase in calcium concentration the sodium rejection decreased and the difference in its rejection as a function of the feed concentration was greater. Calcium rejection looks to reach a maximum for a fraction of calcium equivalents respect to the total cationic equivalents between 0.5 and 0.7. When there were enough sodium ions in solutions
to permeate with nitrate ions to maintain electroneutrality, the greater the quantity of calcium ions the greater its own retention. However, when calcium ion was the predominant cation in the feed solution, its rejection diminished. On the other hand, calcium rejection was higher for the solutions with higher concentration. Comparing the latter results with those obtained for the system NO3!/Mg2+/Na+ (Fig. 11), it can be seen that nitrate rejection was slightly higher when the divalent cation was magnesium.
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In this case, magnesium rejection reached a maximum near a magnesium fraction of 0.5 (Fig. 12). The effects of feed pH and concentration were similar to those obtained with calcium mixtures. The higher rejection of divalent ions respect to sodium can be explained by its divalent charge, larger size and strong hydration. The small pore size of this membrane would contribute to make these effects stronger [11].
However, the extent of these effects is smaller, approaching the behaviour of the NF90 to that of a reverse osmosis membrane. The relatively high nitrate rejection and its small dependence on feed conditions make this membrane suitable to treat types of water that slightly exceed the legal limit of nitrate concentration for drinking purposes.
Acknowledgments The authors wish to thank the Dow-Filmtec Co. for the membranes supplied for this research.
4. Conclusions It was shown that the NF90 membrane has a high rejection even to monovalent ions. This fact can be mainly explained by its tight pore structure and the negative charge of the membrane. Sulphate ions were highly rejected, having great influence over nitrate rejection. Chloride ions were also highly rejected but their effect over nitrate rejec-tion was smaller. The filtration of ternary mixtures with sodium ion and bivalent cations exhibited smaller nitrate rejections than those reached for binary mixtures with a single type of cation in solution. The different separation of ions could be mainly explained by their size. The experiments with binary and ternary mixtures showed that rejection is influenced by pH and ionic concentration. In general, the increase on pH yielded to higher nitrate rejection because of the increase in the negative charge of the membrane, whilst the concentration increase caused, in general, a decrease in ion rejection. It can be concluded that for the NF90 membrane the transfer mechanism involving sieving effect and electrostatic interaction effects seems to play an important role. The feed pH, concentration and the ion size determine which mechanism predominates. The NF90 membrane presents, from a qualitative point of view, similar effects to those observed in a conventional NF membrane.
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Nitrate removal from ternary ionic solutions by a tight nanofiltration membrane A. Santafé-Moros*, J.M. Gozálvez-Zafrilla, J. Lora-García Dpto. de Ingeniería Química y Nuclear, Universidad Politécnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain Tel. +34 (96) 387-7633; Fax. +34 (96) 387-7639; email: [email protected] Received 13 February 2006; Accepted 4 April 2006
Abstract Groundwater with 50–150 mg·L!1 of nitrate ion and low salinity is a very common type of water in the Mediterranean area of Spain where nitrate-polluted water is associated with agricultural use. This study focused on the influence of the most common ions in groundwater on the nitrate rejection of the NF90 membrane (Dow-Filmtec). The effect of sulphate and chloride was studied using Na2SO4/NaNO3 and NaCl/NaNO3 solutions. The influence of different types of counter-ions on nitrate rejection was studied for sodium, calcium and magnesium using solutions containing Ca(NO3)2/NaNO3 and Mg(NO3)2/NaNO3. Both studies were performed at two levels of pH and nitrate concentration. It must be pointed out the high ionic rejection obtained with this membrane, not only for divalent ions but also for the monovalent ones, demonstrated that this membrane can have significant rejection rates at the studied concentration levels. In ternary ionic mixtures, the increase in co-ion concentration always produced a decrease in nitrate rejection. Nitrate rejection in cationic mixtures decreased when the concentration ratio of the divalent cations and the monovalent sodium was varied. A pH increase from 7 to 9 improved the nitrate rejection whatever the concentration and the composition were. The results show that it is possible to diminish nitrate concentration from polluted groundwater reaching concentrations under the maximum allowed limit for human consumption of 50 mg·L!1 of nitrate ion. Keywords: Nanofiltration; Nitrates; pH; Concentration; Drinking-water
*Corresponding author.
Presented at the EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.04.024
64
A. Santafé-Moros et al. / Desalination 204 (2007) 63–71
1. Introduction Groundwater that slightly exceeds the legal limit of nitrate concentration for human consumption (50 mg·L!1) is becoming very common in the Mediterranean area of Spain. As a consequence, many drinking water resources are polluted by nitrates representing a risk for human health [1]. This type of groundwater is associated with agricultural irrigation, so its salinity is not very high. Typically, reverse osmosis and electrodialysis are used to treat this type of water, specially the former technique [2–4]. As an alternative, some nanofiltration (NF) membranes which exhibit enough rejection to monovalent ions can be used, especially at moderately high concentration. NF and reverse osmosis, unlike electrodialysis, can assure the permeate quality when treating groundwater polluted with organics or micro-organisms. However, NF processes can be more energetically efficient than the reverse osmosis ones, because of the lower operating pressures. NF has shown its effectiveness in the removal of a great variety of undesirable components from water. Its separation mechanisms combine sieving effect, differences in diffusivity and solubility of solutes and electrostatic interactions between the membrane surface groups and the ions. In the case of negatively charged membranes, anions like nitrates can be effectively rejected. However, ionic rejection is not only influenced by interaction between the membrane and a specific ion. It is known that for ionic solutions the solute– solute interactions and the solute–membrane interactions are dependent on the concentration, the composition and pH value of the feed solution [5–10]. In the case of ion mixtures, electrostatic interactions between co-ions may cause, according to the Donnan exclusion, a decrease in nitrate rejection, especially if less permeable co-ions are present in the solution. Moreover, shielding effect, as a function of ionic concentration and the counter-ion type, can modify anion rejection.
As a consequence, NF treatment of multicomponent solutions is complicated and still not completely understood, being necessary to obtain experimental data to know the performance of a particular NF membrane with a specific ionic solution. A suitable NF membrane for nitrate removal must have tight porous structures and be negatively charged. The Donnan effect has significant importance in membranes with looser structure [11], so the presence of more rejected species of the same charge would produce a remarkable decrease in nitrate rejection. Rejection decrease caused by the addition of sulphate and chloride ions had been referred by several authors, being even possible negative values [12–17]. There are also many authors that had observed variations in monovalent anion rejection caused by the type of cation in the solutions. Nevertheless, this variation in anion rejection as well as cation rejection seems to depend on ionic concentration and on the membrane itself [5,7,9,12,18,19]. Previous studies [20] using NaNO3 solutions and natural polluted groundwater showed that the NF90 membrane (Dow-Filmtec) can be suitable for the removal of nitrates from groundwater. The NF90 membrane was designed to remove a high percentage of salts, nitrate, iron and organic compounds such as pesticides, herbicides and THM precursors. Its high rejection has been confirmed by several authors [11,21–23]. It can be mainly explained by its tight porous structure. Pontié et al. [24] determined, using uncharged solutes, a pore radius of 0.54 nm, and Krieg et al. [11] obtained a MWCO ~200 Da, indicating that this membrane has a narrow pore size distribution. The present work studied the nitrate rejection of the NF90, as a representative NF membrane of tight porous structure. The influence of pH, concentration and ionic composition was studied using the main ions commonly present in groundwater. The following points were taken into account:
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1. a preliminary study of the performance of the NF90 with single salt solutions (binary solutions); 2. the effect of other anions at different pH and concentration. In order to do this, experiments with the ternary ionic systems chloride/ nitrate/sodium and sulphate/nitrate/sodium were performed at different concentration ratios; 3. the effect of cation interaction. The following ternary ionic systems were used: nitrate/sodium/calcium and nitrate/sodium/ magnesium. It was also studied the effect of pH and ionic concentration.
2. Materials and methods The experiments were carried out at a pilot plant equipped with a spiral-wound membrane module NF90-2540. This membrane is a thin-film composite polyamide manufactured by DowFilmTec. The plant was operated in the batch recirculation mode. The schematic diagram of the membrane test set-up for the laboratory study is shown in Fig. 1. All influent variables different from feed composition were kept constant. Transmembrane pressure was maintained at 5.00 ± 0.02 bar and temperature at 20.0 ± 0.2EC. The feed flow rate was varied to operate at a constant recovery of 15% in all experiments. Pure grade nitrate salts NaNO3, Na2SO4· 10H2O, NaCl, Ca(NO3)2·4H2O and Mg(NO3)2· 6H2O (Panreac) and deionized water were used to prepare the binary and ternary mixtures. Binary solutions were prepared at two levels of anion concentration, 1.61 and 4.84 meq·L!1 (that corresponds to 100 and 300 mg·L!1 of nitrate ion, respectively, in case of NaNO3 solutions). Two types of experiments were carried out with ternary solutions. In the experiments to study the influence of co-ions, the molar ratios sulphate/nitrate and chloride/nitrate were 1/1, 3/1 and 6/1, being sodium the counter-ion. To study the influence of cation type the equivalent ratios
Fig. 1. Schematic diagram of the NF pilot plant. 1 feed tank with temperature control, 2 20 µm filter, 3 speed variable pump, 4 2.5" spiral-wound membrane module, 5,6 pressure regulation valves, 7 DAQ of permeate flow, conductivity and temperature.
of divalent cation (calcium or magnesium) to sodium were 1/3, 1/1 and 3/1 for a constant nitrate content. All compositions were tested at two nitrate concentration levels (100 and 300 mg·L!1). The pH was adjusted using sodium hydroxide to work at pH 7 and 9. All experiments were performed during at least 4 h. A data acquisition system was used to monitor the permeate flow, conductivity and temperature in order to assure the stabilization of the membrane results in every experiment. Anion and cation concentration in both feed and permeate were analyzed using a 790P Metrohm ion chromatograph. Metrosep A Supp 5 and Metrosep C 2 (Metrohm) columns were used for anions and cations, respectively.
3. Results and discussion 3.1. Rejection in binary systems: Influence of anion type Fig. 2 shows the anionic rejection obtained when treating sodium salts as a function of feed pH and concentration. From these results, it must
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Fig. 2. Anionic rejection for sodium salts as a function of anionic concentration (C1 = 1.61 meq·L!1 and C2 = 4.84 meq·L!1) and feed pH (ΔP = 0.5 MPa, T = 20EC).
Fig. 3. Nitrate rejection for nitrate salts as a function of anionic concentration (C1 = 1.61 meq·L!1 and C2 = 4.84 meq·L!1) and feed pH. (ΔP = 0.5 MPa, T = 20EC).
be pointed out the high rejection reached with this membrane, not only for the divalent anion but also for the monovalent ones. The rejection follows the order: R(SO42!) > R(Cl!) > R(NO3!). The high sulphate rejection is mainly explained by its size and charge. The higher rejection of chloride compared with that of the nitrate ion is explained by many authors by differences in hydration [5,12,16] or by differences in their affinity with the membrane [17]. Nevertheless, the nitrate rejection for this membrane (>97%) is higher than those obtained with other NF membranes [12,13,17,25–28]. The pH increase caused an increase in the anion rejection in all cases, especially for the nitrate ion. This result could be the effect of an increase in the negative charge of the membrane. The concentration did not have a significant effect, except in the case of sodium nitrate at pH 7. At this pH the membrane charge is weaker than at pH 9 and consequently would be affected by ionic concentration.
treating the cationic binary solutions as a function of feed pH and concentration. It can be seen that nitrate ion is highly rejected for the three studied cations. The pH increase caused a significant increase in nitrate rejection, irrespective of the counterion. However, the difference in nitrate rejection as a function of pH for divalent cations is smaller than that obtained for sodium salts. The concentration did not have a significant effect on rejection. Nevertheless, it must be noticed that the feed concentration effect is different for the different salts. A higher ionic concentration caused a small increase in the rejection of salts with divalent cations but a decrease in the case of the sodium salt. This effect has also been observed in other tight NF membranes [25]. Fig. 4 shows that only at pH 7 and the highest concentration the rejection sequence derived from the Hofmeister’s series was observed [R(Mg2+) > R(Ca2+) > R(Na+)]. At pH 9, the order changed to R(Mg2+) > R(Na+) > R(Ca2+).
3.2. Rejection in binary systems: Influence of cation type Fig. 3 shows nitrate rejection obtained when
3.3. Effect of co-ions on nitrate rejection of ternary systems. Experiments were performed using sodium as cation and varying the molar ratio of sulphate or
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Fig. 4. Cationic rejection for nitrate salts as a function of anionic concentration (C1 = 1.61 meq·L!1 and C2 = 4.84 meq·L!1) and feed pH. (ΔP = 0.5 MPa, T = 20EC).
chloride to nitrate in order to study how these anions influence nitrate rejection. Rejection results corresponding to the ternary system NO3!/SO42!/Na+ are shown in Figs 5. and 6. Fig. 5 shows the nitrate rejection as a function of the concentration ratio of sulphate to nitrate at fixed nitrate concentrations of 100 and 300 mg· L!1 and for both feed pH studied. The results obtained when filtrating NaNO3 solutions have also been included as a reference. As can be seen, a decrease in nitrate rejection was obtained due to the increase of sulphate concentration. This behaviour is similar to that obtained by other authors with NF membranes [12–15]. The strong rejection of sulphate ion caused that many nitrates are forced to pass through the membrane in order to counterbalance the transfer of sodium ions (Fig. 6). However, it can be said that the decrease in nitrate rejection caused by sulphate addition in this membrane is not as pronounced as those found in the literature for other membranes. The rejection decrease was higher for solutions with the highest nitrate concentration. This could be caused by the highest total ionic concentration in regard to the solutions with 100 mg·L!1. As a consequence, superficial membrane charge would be more shielded by the
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Fig. 5. Nitrate rejection vs. molar ratio SO42!/NO3! for the ionic system NO3!/SO42!/Na+ as a function of nitrate ion concentration and feed pH. (ΔP = 0.5 MPa, T = 20EC).
Fig. 6. Ionic rejection vs. molar ratio SO42!/NO3! for the ionic system NO3!/SO42!/Na+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
sodium ions and the driving force diminishes due to a higher osmotic pressure. Results at pH 9 showed higher rejection than those reached at pH 7. Moreover, the rejection decrease caused by both sulphate and total ionic concentration were slightly less sharp at pH 9. Nitrate rejection and ionic rejection corresponding to the ternary system NO3!/Cl!/Na+ are shown in Figs. 7 and 8, respectively. The addition of chloride ion also caused a decrease in nitrate
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Fig. 7. Nitrate rejection vs. molar ratio Cl!/NO3! for the ionic system NO3!/Cl!/Na+ as a function of nitrate ion concentration and feed pH (ΔP = 0.5 MPa, T = 20EC).
Fig. 8. Ionic rejection vs. molar ratio Cl!/NO3! for the ionic system NO3!/Cl!/Na+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
rejection, as it has been previously observed by other authors [12,14,15,17]. As seen in Fig. 8, the rejection of both monovalent anions was influenced by the total ionic concentration and by the increase in chloride addition. However, the decrease in rejection was less sharp for the chloride ion, less permeable and thus more rejected, than for the nitrate ion. As expected, the decrease in nitrate rejection by chloride addition was lower than that obtained by sulphate addition (Figs. 7 and 5, respectively). This effect is caused by the influence of the characteristics of the co-ion. Comparing Figs. 5 and 7, results obtained for solutions with the same total ionic concentration (e.g., solutions with a molar ratio of SO42!/NO3! = 3 and molar ratio Cl!/NO3! = 6), it can be seen that the effect is stronger when sulphate ion is present in the mixtures. Therefore, the Donnan effect has an important role in the mechanism transport of this membrane when treating ionic solutions.
two levels of nitrate concentration, 100 and 300 mg·L!1, and the study was developed as a function of the equivalent fractions of divalent and monovalent cations. The influence of pH on rejection at the two studied levels was also assessed. In general, nitrate rejection was slightly influenced by the feed concentration of all studied compositions, with feed pH being the more significant influence. Fig. 9 shows the nitrate rejection reached for the ternary mixtures containing the system NO3!/ Ca2+/Na+ in solutions. The results obtained when filtrating the binary nitrate solutions of both cations have also been included as a reference. It can be seen that the mixtures solutions with calcium and sodium ions exhibited lower nitrate rejection than the solutions containing a single type of cation. It must be pointed out that the total ionic concentration, for each level of nitrate concentration in the feed, was the same irrespective of the proportion of each cation in the solution, and equal to that of the binary solutions. The highest nitrate rejections were reached for solutions at pH 9, and the effect of feed pH was similar for the two levels of feed concentration studied. For all the feed characteristics analyzed
3.4. Effect of cations on nitrate rejection of ternary systems The experiments were performed for feeds at
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Fig. 9. Nitrate rejection vs. calcium fraction for the ionic system NO3!/Na+/Ca2+ as a function of nitrate ion concentration and feed pH. (ΔP = 0.5 MPa, T = 20EC).
Fig. 10. Ionic rejection vs. calcium fraction for the ionic system NO3!/Na+/Ca2+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
Fig. 11. Nitrate rejection vs. magnesium fraction for the ionic system NO3!/Na+/Mg2+ as a function of nitrate ion concentration and feed pH (ΔP = 0.5 MPa, T = 20EC).
Fig. 12. Ionic rejection versus magnesium fraction for the ionic system NO3!/Na+/Mg2+ as a function of nitrate ion concentration at feed pH 7 (ΔP = 0.5 MPa, T = 20EC).
the differenced in nitrate rejection were smaller for the compositions where calcium ion predominated. Fig. 10 shows that with the increase in calcium concentration the sodium rejection decreased and the difference in its rejection as a function of the feed concentration was greater. Calcium rejection looks to reach a maximum for a fraction of calcium equivalents respect to the total cationic equivalents between 0.5 and 0.7. When there were enough sodium ions in solutions
to permeate with nitrate ions to maintain electroneutrality, the greater the quantity of calcium ions the greater its own retention. However, when calcium ion was the predominant cation in the feed solution, its rejection diminished. On the other hand, calcium rejection was higher for the solutions with higher concentration. Comparing the latter results with those obtained for the system NO3!/Mg2+/Na+ (Fig. 11), it can be seen that nitrate rejection was slightly higher when the divalent cation was magnesium.
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In this case, magnesium rejection reached a maximum near a magnesium fraction of 0.5 (Fig. 12). The effects of feed pH and concentration were similar to those obtained with calcium mixtures. The higher rejection of divalent ions respect to sodium can be explained by its divalent charge, larger size and strong hydration. The small pore size of this membrane would contribute to make these effects stronger [11].
However, the extent of these effects is smaller, approaching the behaviour of the NF90 to that of a reverse osmosis membrane. The relatively high nitrate rejection and its small dependence on feed conditions make this membrane suitable to treat types of water that slightly exceed the legal limit of nitrate concentration for drinking purposes.
Acknowledgments The authors wish to thank the Dow-Filmtec Co. for the membranes supplied for this research.
4. Conclusions It was shown that the NF90 membrane has a high rejection even to monovalent ions. This fact can be mainly explained by its tight pore structure and the negative charge of the membrane. Sulphate ions were highly rejected, having great influence over nitrate rejection. Chloride ions were also highly rejected but their effect over nitrate rejec-tion was smaller. The filtration of ternary mixtures with sodium ion and bivalent cations exhibited smaller nitrate rejections than those reached for binary mixtures with a single type of cation in solution. The different separation of ions could be mainly explained by their size. The experiments with binary and ternary mixtures showed that rejection is influenced by pH and ionic concentration. In general, the increase on pH yielded to higher nitrate rejection because of the increase in the negative charge of the membrane, whilst the concentration increase caused, in general, a decrease in ion rejection. It can be concluded that for the NF90 membrane the transfer mechanism involving sieving effect and electrostatic interaction effects seems to play an important role. The feed pH, concentration and the ion size determine which mechanism predominates. The NF90 membrane presents, from a qualitative point of view, similar effects to those observed in a conventional NF membrane.
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