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Sorption of cobalt(II) by amorphous ferric hydroxide
ELSEVIER
Colloids and Surfaces A: Physicochemical and Engineering Aspects 104 (1t~95) 265 270
COLLOIDS AND SURFACES
a
Sorption of cobalt(II) by amorphous ferric hydroxide F. Esmadi a,,, j. S i m m b a Chemistry Department, Yarmouk Unit~ersity, l~-bid, Jordan
b h~stitutf~ir Anorganische und Analytische Chemie, Berlin, Germany Received 28 February 1995: accepted 2 June 1995
Abstract
Sorption of Co(II) ions by amorphous ferric hydroxide was investigated as a function of pH and concentration of Co 2~ ions in the supernatant solution. The study was carried out when Co2+ ions were added in the course of formation of the precipitate, and when Cog+ ions were added after the formation of the precipitate. The etl'ect of foreign ions present in solution on the sorption of Co 2+ ions was also investigated. It was found that sorption increases with increasing pH. For each pH sorption isotherm of Co 2+ ions by Fe(II) hydroxide fitted the Langmuir equation. The sorption of Co 2+ ions is considered as a counter ion exchange, the reason of which being a specific adsoption of OH ions.
Keywords: Amorphous ferric hydroxide; Aqueous medium; Co(I l); Coprecipitation; Sorption
1. Introduction Sorptive processes are thought to play an important role in different areas as ion exchange, electrode reactions and heterogeneous catalysts. Recently, interest has increased in sorption as a method of purification of water from different pollutants. Metal hydroxides such as aluminium or iron hydroxides are of special interest since adsorption on naturally occurring aluminium or iron oxides, and oxyhydroxides may be important in the transport, chemical reactions and biotransformations of trace constituents in the environment. In addition, hydroxides of aluminium, iron, magnesium and silicon have proved to be capable of adsorbing different cations and anions [ 1 - 1 2 ] and therefore have been used for water purification [13-14]. Sorption at amorphous surfaces has been less well studied than at crystalline surfaces due to the difficulty in producing surfaces that have reproduc* Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757195)03289-4
ible properties. Amorphous ferric hydroxide is one of the most studied adsorbents with the potential to adsorb different cations and anions [ 15]. Simon et al. [5,7,16,17] reported previously on the sorption of Mn 2+ and Cd 2+ on freshly precipitated AI(OH)3 and Fe(OH)3, and in this work the sorption of Co x+ ions on amorphous Fe(OH)3 is described. The study involves the sorption when cobalt ions are added during the formation of the precipitate, which we refer to as coprecipitation, and after its precipitation, which we refer to as sorption. The process of the uptake of Co 2+ ions in both cases will be referred to as sorption. The effect of the pH and composition of the solution oll the absorptive surface is described.
2. Experimental 2.1. M~terials All chemicals used were of analytical reagent grade. Tridistilled C02 free water was used for
266
F Esmadi, A Simm/Colloids Surjaces A: Physicochem. Eng. Aspects 104 (1995) 265 270
solution preparation. Cobalt chloride standard solutions were prepared by diluting a standard (1000 ppm) solution from Merck. Ferric solutions (FeC13, Fe(NO3)3 and Fe2(SO4)3) were prepared from the corresponding ferric salt (Merck) in 0.01 M acid. The iron concentration was determined by titrimetric titration [-18].
2.2. Apparatus Polarographic measurements were carried out using a Polograph (E261R) and an AC Modulator (E 393) from Metrohm. The pH was measured using a pH meter (Metrohm, ES 10) and a glass electrode (Schott). All experiments were performed in special flasks from Metrohm.
2.3. Procedure The experiments were done when Co 2+ ions were added during precipitation of ferric hydroxide (coprecipitation) and after ferric hydroxide precipitation (sorption). Coprecipitation experiments were carried out by the following method. The required volume of cobalt(II) chloride was added to a 2 ml solution of ferric salt solution in a Metrohm flask which was thermostatted at 23 oC. Tridistilled water was added until the volume was nearly 20ml. The solution was de-aerated by passing through it a slow stream of N 2 for 15 min. Freshly prepared de-aerated N a O H solution was added dropwise until the required pH was reached. The total volume was made to 20 ml. The suspension was left stirring for 20rain, which is the time required to reach equilibrium, as determined in a separate experiment. The suspension was centrifuged and the supernatant liquid was saved for cobalt analysis. in the case of the sorption experiments, a similar procedure to that used in coprecipitation experiments was followed, except that cobalt chloride solution was added after the sodium hydroxide addition. The time needed to reach equilibrium was determined to be 50 min in the case of solutions prepared from FeC13 or Fe(NO3)3 but 30 min when solutions were prepared from Fez(SO4)3. In all experiments, the iron concentration was kept
constant (2 x 1 0 . 2 M), while that of cobalt was varied from 2 x 10 4 to 6 x 10 3 for each pH value. The study was done at different pH values, in the range 5 8. The amount of cobalt sorbed or coprecipitated was calculated as the difference between the starting cobalt concentration and its concentration at equilibrium. Before the polarographic measurement of cobalt, nitrogen gas was passed through the solutions for 15 rain and was kept over solutions during the measurements.
3. Results and discussion The sorption of cobalt ions on amorphous ferric hydroxide was studied as a function of pH, temperature and composition of the solution. To investigate the effect of pH on the uptake of Co 2 + ions, experiments were done in the range 5-8, since ferric hydroxide starts to precipitate at pH 5 and cobalt hydroxide at pH 8. The effect of pH was studied in both systems: coprecipitation and sorption. In both cases, it was found that the uptake
10,
./" //~x lO-3M •
°
/" 6
. ~
•
/j//. /
~o i
& 8
8 x 10-4M
4
/,/ 6 x i0-4N "
•
°/° ~° / "/ 4 x io-4M
////~
/>L/
//./<./ -_.~__/-
.
.
.
.
.j"
.
6
7 pH
8
~,
Fig. 1. V a r i a t i o n of the a d s o r b e d C o 2+ ions with p H v a l u e u n d e r s o r p t i o n c o n d i t i o n s (ferric h y d r o x i d e w a s p r e c i p i t a t e d f r o m Fe(NO3)3).
I( Esmadi, J. Simm/Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 265 270
of cobalt starts slowly and then increases so that at pH 8, 95% of cobalt was sorbed. Fig. 1 shows the relationship between the amount of sorbed cobalt and pH for different cobalt concentrations containing NO~ anions. Similar curves were obtained for other sorption systems containing other anions and for coprecipitation systems. In both systems (coprecipitation and sorption), the hydroxide produced from Fe2(SO4)a solution has a higher ability to take up cobalt ions from solution than other hydroxides prepared from Fe(NO3)a or FeC13. This is due to the remarkable affinity of the sulfate anion toward the surface [1]. Moreover, the removal of cobalt ions from solution is more efficient in coprecipitation than in sorption. To study the effect of foreign anions on the
267
sorptive process, experiments were done under the conditions described above in the presence of excess anions (2mol 1 t). It was found that the uptake of cobalt ions increased at low pH values in both coprecipitation and sorption experiments. However, no effect was observed at high pH values (Fig. 2). The effect of foreign cations on the sorptive process was studied by performing the experiments in the presence of excess foreign cations. Fig. 3 shows the effect of 2 mol l - t of LiCI, KC1 and NaCI on the uptake of cobalt ions under sorption conditions. Similar results were obtained under coprecipitation conditions. In both cases, the presence of excess cations enhanced the removal of cobalt over the whole pH range.
i00
1cc
8c
80
6q
,~ 6o
I
<+ +
2o
2O
?) 6
7 pH
8
9
•
Fig. 2. Effect of foreign a n i o n s on the a d s o r p t i o n of Co 2 + ions (2 x 10 3 M), by ferric h y d r o x i d e u n d e r s o r p t i o n c o n d i t i o n s (Fe(OH)3 was precipitated from Fe(NO3)3 solution). Foreign ion c o n c e n t r a t i o n was 2mol 1 -1 of N a N O 3 (O), N a B r ( x ) and NaCI ( 3 ) .
6
7 pH
)
Fig. 3. Effect of foreign cations on the a d s o r p t i o n of Co 2 ) ions (2 x 10 3 M ) by ferric h y d r o x i d e under s o r p t i o n c o n d i t i o n s ( F e ( O H ) 3 was precipitated from Fe(NO3) 3 solution). Foreign ion c o n c e n t r a t i o n was 2.0 x 10 -~ tool I ~ of each LiCI (,-). N a C I 1 0 ) and KCI ( x ).
268
F Esmadi, J. Simm/Colloids Surfaces A." Physicochem. Eng. Aspects 104 (1995) 265 270
Increase in cobalt sorption or coprecipitation in the presence of foreign ions indicates the physical nature of the bonding of Co 2+ ions to the surface of ferric hydroxide which is a counterion exchange, as was also reported previously [5]. A plausible explanation for the effects produced by foreign anions could be that the specific adsorption of the anions produces additional negative charge which make the suface more able to adsorb cations. Cations in solution will compete for interaction with the surface of the precipitate but cobalt ions which are smaller in size and of higher charge have higher affinity to the surface than the alkali metals. As support for this competition principle, the order of increase in cobalt uptake was found to be highest for potassium and lowest for lithium, which is the order of their radii of hydration: K + = 2.32, N a + = 2 . 7 6 and L i + = 3 . 4 A [19]. The increase in the uptake of cobalt ions in the presence of anions was found to be in the order: N O 3 > Br > C1 . Nitrate anions enhance the sorption more than the halides, which might be due to their
larger size and lower tendency for complexation with iron compared to halides. At higher pH values anions have no effect on the uptake of cobalt due to the high concentration of O H - that displaces the foreign anions from the ferric hydroxide surface during its precipitation. Similar results were found in a previous study [7]. The effect of temperature on the uptake of cobalt ions was also investigated, and it was found that increasing temperature causes an increase in cobalt sorption (Fig. 4). The sorptive process is normally expected to be an exothermic process. What we found of enhancement in sorption upon increasing the temperature is due to the increase in the ion product of water, which leads to more O H - ions, which consequently shifts the equilibrium indicated below to the right, Fe(OH)3 + nOH
+
C o 2+
[Fe(OH)3_, . . . . . . . . . . Co] 2 ", and consequently causes an increase in sorption.
~
3
0
°C 23 °C
6
Z6 °C
!4
5
~+ 0 0
60
120
18()
240
Time (rain) Fig. 4. Variation of the amount of Co2+ adsorbed with temperature at pH= 7.0 (Fe(OH)3 produced from FeCI 3 solution before Co2+ ions were added).
F Esmadi, J. Simm/ColloMs Sudaces A: Physicochem. Eng. Aspects 104 (1995) 265 270
b/(
269
3.1. Adsorption isotherms
Cr/Cad =
At a constant pH value, the relationship between the concentration of the adsorbed Co 2 + ions (Cad) and the concentration of Co 2 + ions in solution at equilibrium (Cr) for a solution containing N O / is shown in Fig. 5. Similar isotherms were obtained for the coprecipitation system, which all show the typical Langmuir isotherm. The mathematical expression of Langmuir isotherm is given by:
If Cr/Cadis plotted against Or, then C~d and h can be calculated, Using the method of minimum squares to obtain the optimum values for the constants, we were able to obtain the theoretical isotherms. The calculated and experimental isotherms for sorption and coprecipitation systems are shown in Figs. 5 and 6, respectively. In the case of sorption, the experimental and theoretical isotherms are in good agreement, but in the case of coprecipitation there is a deviation from the theoretical isotherms at high cobalt concentrations (Fig. 6). This deviation might be due to the possibility of precipitation of cobalt ions with iron ions at high cobalt concentrations. In the sorption experiments where cobalt ions are added after the precipitation of ferric hydroxide, the chance is less for cobalt to precipitate. Similar results were found in coprecipitation experiments with ferric hydroxide [2].
L ~a Cr Cad
-
b + Cr"
-
Where C,d is the amount adsorbed when the equilibrium concentration is Cr, C~a is the saturation amount adsorbed, and b is a constant which is related to the heat of adsorption. Determination of the constants C = and b using the method of error minimizing seems to be more successful if we alter this equation to a linear form such as
ad -[- Cad C r .
14
/ 12
°
pH 7 . 0
/
/
°pH 6 . 8
10-
/ " °
o
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. ~
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. ~
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__°plI 6.0
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.__.-~.--------'"
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o;//.°
o
:1.o
20
3o
e
t,o
50
(M x t o ~)
Fig. 5. Found (O) and calculated (curves) sorption isotherms of Co 2~ ions by FeIOH h produced from FeINO~b solution under sorption conditions.
F Esmadi, J. Simm/Colloids Surfitces A." Physicochem. Eng. Aspects 104 (1995) 265 270
270
25¸
Acknowledgement
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The authors gratefully acknowledge the support of the Deutscher Akademische Austaushdienst (DAAD) for this project.
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References
....' ¢.'
9:"
.~
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"'~" pH6.6
.P':' .
~6 . p2 ~ "H'~"e '"'" '~" pHPH 6.06,4
10
20 30 Cr (Mx I0~)
40
Fig. 6. Found (©) and calculated (curves) sorption isotherms of Co 2+ ions by Fe(m) hydroxide produced from Fe(NO3)3 solution under coprecipitation conditions.
4. Conclusions The removal of some ions from aqueous solution is essential nowadays due to the increase in pollution problems. Therefore, several research papers have appeared that describe the use of cheap and effective substances capable of absorbing the ions using different techniques. Amorphous ferric hydroxide has proved to be one of the most effective substances that can absorb Co 2+ ions if they are added before or after the formation of the precipitate. The presence of foreign anions enhances the uptake of Co 2+ ions at low pH values, whereas foreign cations enhance the process of adsorption over the whole pH range. Moreover, by increasing the temperature the uptake of ions also increases.
[1] G. Micera, G. Gessa, P. Melis, A. Premoli, R. Dallocchio and S. Deiana, Colloids Surfaces, 17 (1986) 389. [2] I. Melikhov, S . A . Orlova and V.M. Poshkova, Radiokhimiya, 6 ( 1976} 817, [3] I.V. Melkhov, S.A. Orlova and U.M. Peshkova, Radiokhimiya, 6 (1976) 822. [4] W. Raense, Z. Anorg. Allg. Chem., 483 (1978) 222. [5] J. Simon, W. Schulze and M. Voeltz, Z. Anorg. Allg. Chem., 394 (1972) 233. [6] R.A. Buyanov, O.P. Krivoruchko and I.A. Ryzhak, Kinet. Catal., 13 (1972) 416. [ 7] J. Simon, W. Schulze and L.W. Freidmann, Z. Anal. Chem., 286 (1974) 185. [8] C.A, Johnson, Geochim. Cosmochim. Acta, 50 (1986) 2433. [9] N. Mikami, M. Sasaki, T. Kikuchi and T. Yasuhaga, J. Phys. Chem., 87 (1983) 5245. [10] V.N. Ananin and A.I. Tokhimetz. React. Kinet., Catal. Lett., 27 (1985} 129. [ i 1 ] M.M. Benjamin and S. Nickolas, in Param H. Tewari (Ed.), Adsorpt. Aqueous Solutions, Plenum, NY, 1981, p. 41 60. [12] V.V. Parshufkin, Z.N. Prozorovskaya and A.B. Yaroslartsev, Zh. Neorg. Khim., 32 (1987) 998. [13] J.A. Gulledge and T.T. O'Conner, Water Tech. J. AWWA, 548 (1973). [14] S. Maeda, A. Ohki, S. Saikoji and K. Naka, Sep. Sci. Technol., 27 (1992) 381. [15] T. Shigematsu, T. Omori, T. Aoki and M. Matsui, Bull. Inst. Chem. Res., 53 (1975} 435. [16] J. Simon, W. Schulze and M. Voeltz, Z. Anal. Chem., 286 (1974) 185. [17] J. Simon, W. Schulze and R. Reinke, Z. Anal. Chem., 264 (1973) 21. [18] R.A. Day, Jr. and A.L. Underwood, Quantitative Analysis, 4th edn., Prentice Hall, Englewood Cliffs, N J, 1980, p. 596. [19] F. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn., Wiley, New York, 1980, p. 255
Colloids and Surfaces A: Physicochemical and Engineering Aspects 104 (1t~95) 265 270
COLLOIDS AND SURFACES
a
Sorption of cobalt(II) by amorphous ferric hydroxide F. Esmadi a,,, j. S i m m b a Chemistry Department, Yarmouk Unit~ersity, l~-bid, Jordan
b h~stitutf~ir Anorganische und Analytische Chemie, Berlin, Germany Received 28 February 1995: accepted 2 June 1995
Abstract
Sorption of Co(II) ions by amorphous ferric hydroxide was investigated as a function of pH and concentration of Co 2~ ions in the supernatant solution. The study was carried out when Co2+ ions were added in the course of formation of the precipitate, and when Cog+ ions were added after the formation of the precipitate. The etl'ect of foreign ions present in solution on the sorption of Co 2+ ions was also investigated. It was found that sorption increases with increasing pH. For each pH sorption isotherm of Co 2+ ions by Fe(II) hydroxide fitted the Langmuir equation. The sorption of Co 2+ ions is considered as a counter ion exchange, the reason of which being a specific adsoption of OH ions.
Keywords: Amorphous ferric hydroxide; Aqueous medium; Co(I l); Coprecipitation; Sorption
1. Introduction Sorptive processes are thought to play an important role in different areas as ion exchange, electrode reactions and heterogeneous catalysts. Recently, interest has increased in sorption as a method of purification of water from different pollutants. Metal hydroxides such as aluminium or iron hydroxides are of special interest since adsorption on naturally occurring aluminium or iron oxides, and oxyhydroxides may be important in the transport, chemical reactions and biotransformations of trace constituents in the environment. In addition, hydroxides of aluminium, iron, magnesium and silicon have proved to be capable of adsorbing different cations and anions [ 1 - 1 2 ] and therefore have been used for water purification [13-14]. Sorption at amorphous surfaces has been less well studied than at crystalline surfaces due to the difficulty in producing surfaces that have reproduc* Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757195)03289-4
ible properties. Amorphous ferric hydroxide is one of the most studied adsorbents with the potential to adsorb different cations and anions [ 15]. Simon et al. [5,7,16,17] reported previously on the sorption of Mn 2+ and Cd 2+ on freshly precipitated AI(OH)3 and Fe(OH)3, and in this work the sorption of Co x+ ions on amorphous Fe(OH)3 is described. The study involves the sorption when cobalt ions are added during the formation of the precipitate, which we refer to as coprecipitation, and after its precipitation, which we refer to as sorption. The process of the uptake of Co 2+ ions in both cases will be referred to as sorption. The effect of the pH and composition of the solution oll the absorptive surface is described.
2. Experimental 2.1. M~terials All chemicals used were of analytical reagent grade. Tridistilled C02 free water was used for
266
F Esmadi, A Simm/Colloids Surjaces A: Physicochem. Eng. Aspects 104 (1995) 265 270
solution preparation. Cobalt chloride standard solutions were prepared by diluting a standard (1000 ppm) solution from Merck. Ferric solutions (FeC13, Fe(NO3)3 and Fe2(SO4)3) were prepared from the corresponding ferric salt (Merck) in 0.01 M acid. The iron concentration was determined by titrimetric titration [-18].
2.2. Apparatus Polarographic measurements were carried out using a Polograph (E261R) and an AC Modulator (E 393) from Metrohm. The pH was measured using a pH meter (Metrohm, ES 10) and a glass electrode (Schott). All experiments were performed in special flasks from Metrohm.
2.3. Procedure The experiments were done when Co 2+ ions were added during precipitation of ferric hydroxide (coprecipitation) and after ferric hydroxide precipitation (sorption). Coprecipitation experiments were carried out by the following method. The required volume of cobalt(II) chloride was added to a 2 ml solution of ferric salt solution in a Metrohm flask which was thermostatted at 23 oC. Tridistilled water was added until the volume was nearly 20ml. The solution was de-aerated by passing through it a slow stream of N 2 for 15 min. Freshly prepared de-aerated N a O H solution was added dropwise until the required pH was reached. The total volume was made to 20 ml. The suspension was left stirring for 20rain, which is the time required to reach equilibrium, as determined in a separate experiment. The suspension was centrifuged and the supernatant liquid was saved for cobalt analysis. in the case of the sorption experiments, a similar procedure to that used in coprecipitation experiments was followed, except that cobalt chloride solution was added after the sodium hydroxide addition. The time needed to reach equilibrium was determined to be 50 min in the case of solutions prepared from FeC13 or Fe(NO3)3 but 30 min when solutions were prepared from Fez(SO4)3. In all experiments, the iron concentration was kept
constant (2 x 1 0 . 2 M), while that of cobalt was varied from 2 x 10 4 to 6 x 10 3 for each pH value. The study was done at different pH values, in the range 5 8. The amount of cobalt sorbed or coprecipitated was calculated as the difference between the starting cobalt concentration and its concentration at equilibrium. Before the polarographic measurement of cobalt, nitrogen gas was passed through the solutions for 15 rain and was kept over solutions during the measurements.
3. Results and discussion The sorption of cobalt ions on amorphous ferric hydroxide was studied as a function of pH, temperature and composition of the solution. To investigate the effect of pH on the uptake of Co 2 + ions, experiments were done in the range 5-8, since ferric hydroxide starts to precipitate at pH 5 and cobalt hydroxide at pH 8. The effect of pH was studied in both systems: coprecipitation and sorption. In both cases, it was found that the uptake
10,
./" //~x lO-3M •
°
/" 6
. ~
•
/j//. /
~o i
& 8
8 x 10-4M
4
/,/ 6 x i0-4N "
•
°/° ~° / "/ 4 x io-4M
////~
/>L/
//./<./ -_.~__/-
.
.
.
.
.j"
.
6
7 pH
8
~,
Fig. 1. V a r i a t i o n of the a d s o r b e d C o 2+ ions with p H v a l u e u n d e r s o r p t i o n c o n d i t i o n s (ferric h y d r o x i d e w a s p r e c i p i t a t e d f r o m Fe(NO3)3).
I( Esmadi, J. Simm/Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 265 270
of cobalt starts slowly and then increases so that at pH 8, 95% of cobalt was sorbed. Fig. 1 shows the relationship between the amount of sorbed cobalt and pH for different cobalt concentrations containing NO~ anions. Similar curves were obtained for other sorption systems containing other anions and for coprecipitation systems. In both systems (coprecipitation and sorption), the hydroxide produced from Fe2(SO4)a solution has a higher ability to take up cobalt ions from solution than other hydroxides prepared from Fe(NO3)a or FeC13. This is due to the remarkable affinity of the sulfate anion toward the surface [1]. Moreover, the removal of cobalt ions from solution is more efficient in coprecipitation than in sorption. To study the effect of foreign anions on the
267
sorptive process, experiments were done under the conditions described above in the presence of excess anions (2mol 1 t). It was found that the uptake of cobalt ions increased at low pH values in both coprecipitation and sorption experiments. However, no effect was observed at high pH values (Fig. 2). The effect of foreign cations on the sorptive process was studied by performing the experiments in the presence of excess foreign cations. Fig. 3 shows the effect of 2 mol l - t of LiCI, KC1 and NaCI on the uptake of cobalt ions under sorption conditions. Similar results were obtained under coprecipitation conditions. In both cases, the presence of excess cations enhanced the removal of cobalt over the whole pH range.
i00
1cc
8c
80
6q
,~ 6o
I
<+ +
2o
2O
?) 6
7 pH
8
9
•
Fig. 2. Effect of foreign a n i o n s on the a d s o r p t i o n of Co 2 + ions (2 x 10 3 M), by ferric h y d r o x i d e u n d e r s o r p t i o n c o n d i t i o n s (Fe(OH)3 was precipitated from Fe(NO3)3 solution). Foreign ion c o n c e n t r a t i o n was 2mol 1 -1 of N a N O 3 (O), N a B r ( x ) and NaCI ( 3 ) .
6
7 pH
)
Fig. 3. Effect of foreign cations on the a d s o r p t i o n of Co 2 ) ions (2 x 10 3 M ) by ferric h y d r o x i d e under s o r p t i o n c o n d i t i o n s ( F e ( O H ) 3 was precipitated from Fe(NO3) 3 solution). Foreign ion c o n c e n t r a t i o n was 2.0 x 10 -~ tool I ~ of each LiCI (,-). N a C I 1 0 ) and KCI ( x ).
268
F Esmadi, J. Simm/Colloids Surfaces A." Physicochem. Eng. Aspects 104 (1995) 265 270
Increase in cobalt sorption or coprecipitation in the presence of foreign ions indicates the physical nature of the bonding of Co 2+ ions to the surface of ferric hydroxide which is a counterion exchange, as was also reported previously [5]. A plausible explanation for the effects produced by foreign anions could be that the specific adsorption of the anions produces additional negative charge which make the suface more able to adsorb cations. Cations in solution will compete for interaction with the surface of the precipitate but cobalt ions which are smaller in size and of higher charge have higher affinity to the surface than the alkali metals. As support for this competition principle, the order of increase in cobalt uptake was found to be highest for potassium and lowest for lithium, which is the order of their radii of hydration: K + = 2.32, N a + = 2 . 7 6 and L i + = 3 . 4 A [19]. The increase in the uptake of cobalt ions in the presence of anions was found to be in the order: N O 3 > Br > C1 . Nitrate anions enhance the sorption more than the halides, which might be due to their
larger size and lower tendency for complexation with iron compared to halides. At higher pH values anions have no effect on the uptake of cobalt due to the high concentration of O H - that displaces the foreign anions from the ferric hydroxide surface during its precipitation. Similar results were found in a previous study [7]. The effect of temperature on the uptake of cobalt ions was also investigated, and it was found that increasing temperature causes an increase in cobalt sorption (Fig. 4). The sorptive process is normally expected to be an exothermic process. What we found of enhancement in sorption upon increasing the temperature is due to the increase in the ion product of water, which leads to more O H - ions, which consequently shifts the equilibrium indicated below to the right, Fe(OH)3 + nOH
+
C o 2+
[Fe(OH)3_, . . . . . . . . . . Co] 2 ", and consequently causes an increase in sorption.
~
3
0
°C 23 °C
6
Z6 °C
!4
5
~+ 0 0
60
120
18()
240
Time (rain) Fig. 4. Variation of the amount of Co2+ adsorbed with temperature at pH= 7.0 (Fe(OH)3 produced from FeCI 3 solution before Co2+ ions were added).
F Esmadi, J. Simm/ColloMs Sudaces A: Physicochem. Eng. Aspects 104 (1995) 265 270
b/(
269
3.1. Adsorption isotherms
Cr/Cad =
At a constant pH value, the relationship between the concentration of the adsorbed Co 2 + ions (Cad) and the concentration of Co 2 + ions in solution at equilibrium (Cr) for a solution containing N O / is shown in Fig. 5. Similar isotherms were obtained for the coprecipitation system, which all show the typical Langmuir isotherm. The mathematical expression of Langmuir isotherm is given by:
If Cr/Cadis plotted against Or, then C~d and h can be calculated, Using the method of minimum squares to obtain the optimum values for the constants, we were able to obtain the theoretical isotherms. The calculated and experimental isotherms for sorption and coprecipitation systems are shown in Figs. 5 and 6, respectively. In the case of sorption, the experimental and theoretical isotherms are in good agreement, but in the case of coprecipitation there is a deviation from the theoretical isotherms at high cobalt concentrations (Fig. 6). This deviation might be due to the possibility of precipitation of cobalt ions with iron ions at high cobalt concentrations. In the sorption experiments where cobalt ions are added after the precipitation of ferric hydroxide, the chance is less for cobalt to precipitate. Similar results were found in coprecipitation experiments with ferric hydroxide [2].
L ~a Cr Cad
-
b + Cr"
-
Where C,d is the amount adsorbed when the equilibrium concentration is Cr, C~a is the saturation amount adsorbed, and b is a constant which is related to the heat of adsorption. Determination of the constants C = and b using the method of error minimizing seems to be more successful if we alter this equation to a linear form such as
ad -[- Cad C r .
14
/ 12
°
pH 7 . 0
/
/
°pH 6 . 8
10-
/ " °
o
•
. ~
.j-
. ~
°pH
6 £)
6.~
r~
__°plI 6.0
1.1. ./" j ' J
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F Esmadi, J. Simm/Colloids Surfitces A." Physicochem. Eng. Aspects 104 (1995) 265 270
270
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Acknowledgement
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The authors gratefully acknowledge the support of the Deutscher Akademische Austaushdienst (DAAD) for this project.
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References
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Fig. 6. Found (©) and calculated (curves) sorption isotherms of Co 2+ ions by Fe(m) hydroxide produced from Fe(NO3)3 solution under coprecipitation conditions.
4. Conclusions The removal of some ions from aqueous solution is essential nowadays due to the increase in pollution problems. Therefore, several research papers have appeared that describe the use of cheap and effective substances capable of absorbing the ions using different techniques. Amorphous ferric hydroxide has proved to be one of the most effective substances that can absorb Co 2+ ions if they are added before or after the formation of the precipitate. The presence of foreign anions enhances the uptake of Co 2+ ions at low pH values, whereas foreign cations enhance the process of adsorption over the whole pH range. Moreover, by increasing the temperature the uptake of ions also increases.
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