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Removal of Sr2+, Ca2+, and Mg2+ from aqueous and aqueous-alcoholic saline media
ELSEVIER
Removal of Sr2+, Ca2’, and Mg2’ aqueous and aqueous-alcoholic saline media S.A. Sayed Department of Chemistq,
University,
Helwan
from
Cairo, E&d
Synthetic inorganic ion exchangers have special uses and can be valuable in some trace separations. Zeolite NaA is used in an ion exchange study with Sr”, Ca*+, and Mg2+ in aqueous NaCl media of ionic strengths, 1, 2, 3, 4, and 5, and in the oversaturated range of 7 and 10 M. A mixed aqueous methanol, ethanol, n-propyl alcohol, and n-butyl alcohol, 1 M NaCl medium has been investigated with the aim of studying the effect of changing the dielectric constant of the medium on the uptake of the studied cations, which are present in trace concentrations in caustic soda manufactured by the selective membrane technique. The temperature effect on uptake shows an endothermic character for the exchange reactions under investigation. Keywords:
Zeolite
NaA; Nacl media;
Sr”;
Mg*’
INTRODUCTION
EXPERIMENTAL
from BDH and were used without further purification Zeolite NaA (Degussa HAB A40), with the technical data, had the general formula Na,, (AlO,),, ion exchange capacity (SQ) 12 * 27H,O. Its theoretical was 5.62 Val/g air dried; average particle size 4 pm, and ignition loss, 20. It was 14% w/w after 20-h heating at 400°C. It can be used as a commercial substance after 2-h air drying at 2OO”C, to be simply applied in the industrial process of caustic soda. Ion exchange kinetics were obtained by shaking 20 ppm of Sr’+, Ca’+, or Mg” with 0.1-1.0 g of zeolite in 100 ml of aqueous 1 M ionic strength medium (NaCl), pH 9, at 35°C. Shaking was done in a thermostated shaker water bath at 200 rpm. The equilibrium time thus established is used in studying the ion exchange isotherms (IEI) by measuring the fraction of the metal ion absorbed at different ionic strengths, which range up to saturation with NaCl. IEI were studied at temperatures of 35, 45, 55, and 65°C at an ionic strength of 1 M NaCl, pH 9. IEI were also investigated for the three metal ions in four different alcohols using alcohol to water ratios of 0.1, 0.2, 0.3, 0.4, 0.6, and 0.8. The alcohol-water systems were all investigated at 1 M ionic strength NaCl at a temperature of 35°C. Concentration measurements were made using a 2380 Perkin-Elmer atomic absorption spectrophotometer.
MgC1,:6H2):CaC1,:2h,0:SrC1,:2 H,O was obtained from Merck. NaCl was from the El-Naser company, Egypt. Methyl, ethyl, propyl, and butyl alcohols were
RESULTS
Address reprint requests to Dr. Sayed at the Chemistry Depart-
Kinetic
ment, 11792,
To establish the equilibrium time of the system, zeolite A, Sr2+, Ca”, or Mg’+ in a 1 R?Iionic strength medium (NaCl), pH 9, at a temperature of 35”C, 20 ppm
The efficiency of separation processes using selective permeable membranes which are applicable to metal recovery and the manufacture of caustic soda from a saline solution depends mainly on the elimination of insoluble impurities and soluble alkaline earth metals that may block the membrane surface.‘.2 Ion exchange is considered an attractive technique for the removal of contaminant metal cations because of the relative simplicity of application. Synthetic zeolites have been used in the removal of both NH4+, alkali, alkaline earths, and hea\? metals from waste waters”-” and as a demineralizer ‘in the reverse osmosis and ion exchange processes. 5,s In this work we report on the removal of Sr2+, Ca’+, and Mg ‘+ in trace level concentrations from aqueous media of different ionic strengths, Na Cl solution. The effect of the dielectric constant of the medium on the uptake of the mentioned ions is made by studying the effect of using various alcohols in 1 M NaCl solutions, since the ion exchange in water solvent and pure solvent is less often investigated.”
Faculty Cairo,
Received March
27
of Science, Egypt.
Helwan
University,
Helwan
P. No.
June 1995; revised 30 January 1996; accepted 28
1996
Zeolites 17:361-364, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas,
New
York,
NY 10010
AND DISCUSSIONS studies
0144-2449/96/$15.00 PII SO144-2449(96)00068-l
Removal
of ions
from
saline
media:
S.A. Sa yed
of each of these metal ions was added to 100 ml of the aqueous solution and shaken in a thermostated water bath for different time intervals. Aliquots were withdrawn after 0.5, 1, 1.5, 2, 3, and 4 h for analysis of the fraction of the ion absorbed Q. initial concentration of metal equilibrium concentration of metal Q= The
initial distribution
concentration ratio
of metal.
Kd is thus
where v is the volume of solution in ml and w stands for the weight of zeolite in g. It is noted that with the zeolite loading increase from 0.1 to 1.0 g/100 ml of aqueous solution, 1.0 g/100 ml and 3-4 h is sufficient to remove about 75% of the metal ions concentration. Figure 2 represents a plot of log Kd versus shaking time. All equilibrium experiments thereafter are made by shaking the studied systems for 3 h. The initial rates of uptake of the various ions on zeolite as well as the equilibrium u take capacity decrease in the order Sr** > Ca2+ > Mg t +. The decrease is in the order of decreased radii and increased standard molar Gibbs free energies of hydration of ions, [ r(nm) Sr’+, 0.113; Ca*+, 0.100; [-bG”,,Jd (kJ molli) Sr*+, 1,386; Ca*+, Mg*+, 0.072] 1,515; Mg’+, 1,838] (see Ref. 10). The more strongly hydrated divalent ions pass through the S-ring Zeolite lattice structure with more difficulty, so it must lose many of the strongly coordinated water molecules to enter the structure.
Figure 2. Effect of ionic Symbols as in Figure 7.
strength
on distribution
coefficient.
slopes of the plot of log Kd versus I’/? are very similar, with an approximate slope of unity. It is quite important here to calculate the log Kd values at the extrapolated zero ionic strength where the activity coefficient is truly 1. These values for Sr*+, Ca*‘, and Mg2+ are 2.87, 2.84, and 2.83, respectively.
Effect
of temperature
Increased salinity of the aqueous phase decreases the uptake of the studied ions by zeolite. These results are shown in Figure 2 where log Kd is plotted versus the square root of the ionic strength I’/P. The decrease of the uptake as the ionic strength increases is due to the increased interionic interactions, which would lead to increased ion pairing. “,i* The pairing of Cll with S?+, Ca’+, or Mg” increases in a similar manner as the
3 is a plot of log Kd versus T for the exchange reaction of Sr*+, Ca*+, and Mg’+ in a 1.0 M ionic strength medium (NaCl), pH 9, with zeolite. The log Kd values increase as the temperature is increased, indicating an endothermic character that is to be expected from the large dehydration energies of the Sr’+, Ca’+, and Mg*+ which precede the exchange with the Na+ of the zeolite. The enthalpy changes in zeolite ion exchange reactions are generally small, and in many cases complete exchange is not obtained because of cation
Figure 1 Effect of shaking (Sr”, 01, (Ca”, o), (Mg”, A).
time
Figure 3 Effect of temperature bols as in Figure 7.
362
1996
Effect
Figure
of ionic strength
Zeolites
17:361-364,
on
distribution
coefficient.
on distribution
coefficient.
Sym-
Removal
of ions
Figure 6 Effect of butanol cient. Symbols as in Figure Figure 4 Effect of ethanol cient. Symbols as in Figure
sieve effects, investigation.
Equilibria
especially
percentage 7.
during
in waterdcohol
on distribution
the 3-h contact
from
saline
concentration I.
media:
S.A. Sayed
on distribution
coeffi-
coeffi-
in our
systems
Factors that may affect the behavior of the uptake of metal ions in mixed solvents are the dielectric constant, ion solvation, solvent-solvent interactions, and solventzeolite interactions. ‘us To study this point, an aqueous phase was made of mixed water:alcohol in the ratio of 10, 20, 30, 40, 60, and 80% alcohol. Four alcohols were used separately. These were methanol, ethanol, npropyl alcohol, and n-butyl alcohol. In all cases the ionic strength of the aqueous phase was 1 M (NaCl), and the temperature was 35°C. Figures 4, 5 and 6 represent plots of log Kd versus water:alcohol ratios for ethanol, n-propyl alcohol, and n-butyl alcohol, respectively. It is seen that the log Kd values decrease as the alcohol content increases. The addition of alcohol to water would have the net effect of decreasing the dielectric constant of the me-
dium. This will have the net effect of increasing the interionic interactions. This, however, is not expected to make a major contribution to the observed changes in the cation affinity. If one discards the increased coordination with Cll and the changes in the interzeolitic solvents as reasons for the observed changes in the zeolite affinity, then the most probable cause for the decreased affinity as the alcohol percent increases would be the change in the outer hydration coordination sphere of the metal ion as the alcohol replaces the water molecules. Apparently it becomes energetically more favorable for ion exchange mixtures containing increased amounts of alcohol, to solvate one Sr’+, Ca’+, or Mg2+ ion instead of two Na*+ ions. The dielectric constants of the alcohols studied decrease in the order: methanol > ethanol > n-propyl alcohol > n-butyl alcohol; the order of decreased log Kc,is in the same sequence. This confirms our previous assumption that a decrease in the dielectric constant would increase the ion pairing and the interionic interactions, which would have the main effect of decreasing the log Kd values, as the experimental data indicate. In conclusion, from the data obtained one could use the commercial zeolite NaA even with its 60% efficiency or in its nonactual equilibrium time in the removal of these alkaline earth metals that contaminate the saline solution used for the caustic soda production by the selective membrane technique. Zeolite Na A could be regenerated by NaCl solution and reused.
REFERENCES
Figure cient.
5 Effect of propanol Symbols as in Figure
percentage 7.
on distribution
coeffi-
1
Applegate,
2
Babcock, W.C. Am. ence on Separation 1986, paper 171
L.E. Chem.
Eng.
1984,
64, 4
Chem. Sot. First international Science and Technology,
3
Coleman,
R.T. EPA-600/2-80-074,
4
Blanchard 18, 1501
G, Maunaye
M and
Zeolites
NTIS Martin,
New
ConferYork,
PB 80-196918,
1980
G. Water
1984,
17:361-364,
Res.
1996
363
Removal 5 6 7 8
9
364
of ions
from
saline
media:
S.A.
Sayed
Naden D. and Streat, S. /on Exchange Technology, Society of Chemical Industry, Ellis Horwood Limited, London, U.K., 1984 Zamzow M.J. and Murphy, J.E. Sep. Sci Technol. 1992, 27, 1969 Maliou E, Malamis E. and Sakellarides, M. Water Sci. Techno/. 1992, 25, 133 Ghassan. A., Judges. D.J.H., Tao, P.D.. Chia-hwai. Fourth World Congress on Desalination and Water Reuse, Kuwait, 1989, 76, 27-37 Ebaid, A.R., Barakat, A.K., Khalifa, N.A., Kandil, A.T., Mae-
Zeolites
17:361-364,
1996
10
11 12 13
sen, T.L.M. and Van Bekkum, H. Rec. Trav. Chim. Pays-Bas 1991,110,374 Rydberg, J., Musikas C. and Choppin, G.R. Principles and Practices of Solvent Extraction, Marcel Dekker. New York, 1992, p. 38 Pearson, P.G. J. Inorg. Chem. 1988,27, 734 Bockris, J.O.M. and Reddy, A.K.N. Modern Electrochemistry, Vol. 1, Plenum, New York, 1970, p. 251 Barret, R.B. and Marinsky, J.A. J. Phys. Chem. 1971, 75, 85
Removal of Sr2+, Ca2’, and Mg2’ aqueous and aqueous-alcoholic saline media S.A. Sayed Department of Chemistq,
University,
Helwan
from
Cairo, E&d
Synthetic inorganic ion exchangers have special uses and can be valuable in some trace separations. Zeolite NaA is used in an ion exchange study with Sr”, Ca*+, and Mg2+ in aqueous NaCl media of ionic strengths, 1, 2, 3, 4, and 5, and in the oversaturated range of 7 and 10 M. A mixed aqueous methanol, ethanol, n-propyl alcohol, and n-butyl alcohol, 1 M NaCl medium has been investigated with the aim of studying the effect of changing the dielectric constant of the medium on the uptake of the studied cations, which are present in trace concentrations in caustic soda manufactured by the selective membrane technique. The temperature effect on uptake shows an endothermic character for the exchange reactions under investigation. Keywords:
Zeolite
NaA; Nacl media;
Sr”;
Mg*’
INTRODUCTION
EXPERIMENTAL
from BDH and were used without further purification Zeolite NaA (Degussa HAB A40), with the technical data, had the general formula Na,, (AlO,),, ion exchange capacity (SQ) 12 * 27H,O. Its theoretical was 5.62 Val/g air dried; average particle size 4 pm, and ignition loss, 20. It was 14% w/w after 20-h heating at 400°C. It can be used as a commercial substance after 2-h air drying at 2OO”C, to be simply applied in the industrial process of caustic soda. Ion exchange kinetics were obtained by shaking 20 ppm of Sr’+, Ca’+, or Mg” with 0.1-1.0 g of zeolite in 100 ml of aqueous 1 M ionic strength medium (NaCl), pH 9, at 35°C. Shaking was done in a thermostated shaker water bath at 200 rpm. The equilibrium time thus established is used in studying the ion exchange isotherms (IEI) by measuring the fraction of the metal ion absorbed at different ionic strengths, which range up to saturation with NaCl. IEI were studied at temperatures of 35, 45, 55, and 65°C at an ionic strength of 1 M NaCl, pH 9. IEI were also investigated for the three metal ions in four different alcohols using alcohol to water ratios of 0.1, 0.2, 0.3, 0.4, 0.6, and 0.8. The alcohol-water systems were all investigated at 1 M ionic strength NaCl at a temperature of 35°C. Concentration measurements were made using a 2380 Perkin-Elmer atomic absorption spectrophotometer.
MgC1,:6H2):CaC1,:2h,0:SrC1,:2 H,O was obtained from Merck. NaCl was from the El-Naser company, Egypt. Methyl, ethyl, propyl, and butyl alcohols were
RESULTS
Address reprint requests to Dr. Sayed at the Chemistry Depart-
Kinetic
ment, 11792,
To establish the equilibrium time of the system, zeolite A, Sr2+, Ca”, or Mg’+ in a 1 R?Iionic strength medium (NaCl), pH 9, at a temperature of 35”C, 20 ppm
The efficiency of separation processes using selective permeable membranes which are applicable to metal recovery and the manufacture of caustic soda from a saline solution depends mainly on the elimination of insoluble impurities and soluble alkaline earth metals that may block the membrane surface.‘.2 Ion exchange is considered an attractive technique for the removal of contaminant metal cations because of the relative simplicity of application. Synthetic zeolites have been used in the removal of both NH4+, alkali, alkaline earths, and hea\? metals from waste waters”-” and as a demineralizer ‘in the reverse osmosis and ion exchange processes. 5,s In this work we report on the removal of Sr2+, Ca’+, and Mg ‘+ in trace level concentrations from aqueous media of different ionic strengths, Na Cl solution. The effect of the dielectric constant of the medium on the uptake of the mentioned ions is made by studying the effect of using various alcohols in 1 M NaCl solutions, since the ion exchange in water solvent and pure solvent is less often investigated.”
Faculty Cairo,
Received March
27
of Science, Egypt.
Helwan
University,
Helwan
P. No.
June 1995; revised 30 January 1996; accepted 28
1996
Zeolites 17:361-364, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas,
New
York,
NY 10010
AND DISCUSSIONS studies
0144-2449/96/$15.00 PII SO144-2449(96)00068-l
Removal
of ions
from
saline
media:
S.A. Sa yed
of each of these metal ions was added to 100 ml of the aqueous solution and shaken in a thermostated water bath for different time intervals. Aliquots were withdrawn after 0.5, 1, 1.5, 2, 3, and 4 h for analysis of the fraction of the ion absorbed Q. initial concentration of metal equilibrium concentration of metal Q= The
initial distribution
concentration ratio
of metal.
Kd is thus
where v is the volume of solution in ml and w stands for the weight of zeolite in g. It is noted that with the zeolite loading increase from 0.1 to 1.0 g/100 ml of aqueous solution, 1.0 g/100 ml and 3-4 h is sufficient to remove about 75% of the metal ions concentration. Figure 2 represents a plot of log Kd versus shaking time. All equilibrium experiments thereafter are made by shaking the studied systems for 3 h. The initial rates of uptake of the various ions on zeolite as well as the equilibrium u take capacity decrease in the order Sr** > Ca2+ > Mg t +. The decrease is in the order of decreased radii and increased standard molar Gibbs free energies of hydration of ions, [ r(nm) Sr’+, 0.113; Ca*+, 0.100; [-bG”,,Jd (kJ molli) Sr*+, 1,386; Ca*+, Mg*+, 0.072] 1,515; Mg’+, 1,838] (see Ref. 10). The more strongly hydrated divalent ions pass through the S-ring Zeolite lattice structure with more difficulty, so it must lose many of the strongly coordinated water molecules to enter the structure.
Figure 2. Effect of ionic Symbols as in Figure 7.
strength
on distribution
coefficient.
slopes of the plot of log Kd versus I’/? are very similar, with an approximate slope of unity. It is quite important here to calculate the log Kd values at the extrapolated zero ionic strength where the activity coefficient is truly 1. These values for Sr*+, Ca*‘, and Mg2+ are 2.87, 2.84, and 2.83, respectively.
Effect
of temperature
Increased salinity of the aqueous phase decreases the uptake of the studied ions by zeolite. These results are shown in Figure 2 where log Kd is plotted versus the square root of the ionic strength I’/P. The decrease of the uptake as the ionic strength increases is due to the increased interionic interactions, which would lead to increased ion pairing. “,i* The pairing of Cll with S?+, Ca’+, or Mg” increases in a similar manner as the
3 is a plot of log Kd versus T for the exchange reaction of Sr*+, Ca*+, and Mg’+ in a 1.0 M ionic strength medium (NaCl), pH 9, with zeolite. The log Kd values increase as the temperature is increased, indicating an endothermic character that is to be expected from the large dehydration energies of the Sr’+, Ca’+, and Mg*+ which precede the exchange with the Na+ of the zeolite. The enthalpy changes in zeolite ion exchange reactions are generally small, and in many cases complete exchange is not obtained because of cation
Figure 1 Effect of shaking (Sr”, 01, (Ca”, o), (Mg”, A).
time
Figure 3 Effect of temperature bols as in Figure 7.
362
1996
Effect
Figure
of ionic strength
Zeolites
17:361-364,
on
distribution
coefficient.
on distribution
coefficient.
Sym-
Removal
of ions
Figure 6 Effect of butanol cient. Symbols as in Figure Figure 4 Effect of ethanol cient. Symbols as in Figure
sieve effects, investigation.
Equilibria
especially
percentage 7.
during
in waterdcohol
on distribution
the 3-h contact
from
saline
concentration I.
media:
S.A. Sayed
on distribution
coeffi-
coeffi-
in our
systems
Factors that may affect the behavior of the uptake of metal ions in mixed solvents are the dielectric constant, ion solvation, solvent-solvent interactions, and solventzeolite interactions. ‘us To study this point, an aqueous phase was made of mixed water:alcohol in the ratio of 10, 20, 30, 40, 60, and 80% alcohol. Four alcohols were used separately. These were methanol, ethanol, npropyl alcohol, and n-butyl alcohol. In all cases the ionic strength of the aqueous phase was 1 M (NaCl), and the temperature was 35°C. Figures 4, 5 and 6 represent plots of log Kd versus water:alcohol ratios for ethanol, n-propyl alcohol, and n-butyl alcohol, respectively. It is seen that the log Kd values decrease as the alcohol content increases. The addition of alcohol to water would have the net effect of decreasing the dielectric constant of the me-
dium. This will have the net effect of increasing the interionic interactions. This, however, is not expected to make a major contribution to the observed changes in the cation affinity. If one discards the increased coordination with Cll and the changes in the interzeolitic solvents as reasons for the observed changes in the zeolite affinity, then the most probable cause for the decreased affinity as the alcohol percent increases would be the change in the outer hydration coordination sphere of the metal ion as the alcohol replaces the water molecules. Apparently it becomes energetically more favorable for ion exchange mixtures containing increased amounts of alcohol, to solvate one Sr’+, Ca’+, or Mg2+ ion instead of two Na*+ ions. The dielectric constants of the alcohols studied decrease in the order: methanol > ethanol > n-propyl alcohol > n-butyl alcohol; the order of decreased log Kc,is in the same sequence. This confirms our previous assumption that a decrease in the dielectric constant would increase the ion pairing and the interionic interactions, which would have the main effect of decreasing the log Kd values, as the experimental data indicate. In conclusion, from the data obtained one could use the commercial zeolite NaA even with its 60% efficiency or in its nonactual equilibrium time in the removal of these alkaline earth metals that contaminate the saline solution used for the caustic soda production by the selective membrane technique. Zeolite Na A could be regenerated by NaCl solution and reused.
REFERENCES
Figure cient.
5 Effect of propanol Symbols as in Figure
percentage 7.
on distribution
coeffi-
1
Applegate,
2
Babcock, W.C. Am. ence on Separation 1986, paper 171
L.E. Chem.
Eng.
1984,
64, 4
Chem. Sot. First international Science and Technology,
3
Coleman,
R.T. EPA-600/2-80-074,
4
Blanchard 18, 1501
G, Maunaye
M and
Zeolites
NTIS Martin,
New
ConferYork,
PB 80-196918,
1980
G. Water
1984,
17:361-364,
Res.
1996
363
Removal 5 6 7 8
9
364
of ions
from
saline
media:
S.A.
Sayed
Naden D. and Streat, S. /on Exchange Technology, Society of Chemical Industry, Ellis Horwood Limited, London, U.K., 1984 Zamzow M.J. and Murphy, J.E. Sep. Sci Technol. 1992, 27, 1969 Maliou E, Malamis E. and Sakellarides, M. Water Sci. Techno/. 1992, 25, 133 Ghassan. A., Judges. D.J.H., Tao, P.D.. Chia-hwai. Fourth World Congress on Desalination and Water Reuse, Kuwait, 1989, 76, 27-37 Ebaid, A.R., Barakat, A.K., Khalifa, N.A., Kandil, A.T., Mae-
Zeolites
17:361-364,
1996
10
11 12 13
sen, T.L.M. and Van Bekkum, H. Rec. Trav. Chim. Pays-Bas 1991,110,374 Rydberg, J., Musikas C. and Choppin, G.R. Principles and Practices of Solvent Extraction, Marcel Dekker. New York, 1992, p. 38 Pearson, P.G. J. Inorg. Chem. 1988,27, 734 Bockris, J.O.M. and Reddy, A.K.N. Modern Electrochemistry, Vol. 1, Plenum, New York, 1970, p. 251 Barret, R.B. and Marinsky, J.A. J. Phys. Chem. 1971, 75, 85