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Solvent extraction and adsorptive bubble separation of metal ions from aqueous solution—II
J. inorg, nucl. Chem., 1974, Vol. 36, pp. 1871 1875. Pergamon Press. Printed in Great Britain.
SOLVENT EXTRACTION A N D ADSORPTIVE BUBBLE SEPARATION OF METAL IONS FROM AQUEOUS SOLUTION--II ADSORPTIVE BUBBLE SEPARATION OF NICKEL(II) USING CARBOXYLIC ACIDS A N D THEIR SALTS AS COLLECTORS A L A N D. JONES* and C A R O L R O B I N S O N Department of Chemistry and Biology, Trent Polytechnic, Nottingham
(Received 30 May 1973) Abstract--The removal of nicket(ll) from aqueous solution has been studied by the adsorptive bubble separation processes of solvent sublation and foam fractionation using long-chain carboxync acids and their salts as collectors. The effects of ionic strength, collector:metal ratio and pH on the efficiency of the removal process are reported and a comparison is made with a solvent extraction process using the sodium salt of the tong-chain acid dissolved in octan-l-ol as extractant.
INTRODUCTION
A STUDY of the removal of metal ions from aqueous solution by solvent sublation has been carried out to make a comparison between adsorptive bubble separation and solvent extraction processes. The results of some solvent extraction studies have been reported in part 1[1]. Karger[2] defines solvent sublation as a nonfoaming adsorptive bubble separation process in which enriched material on bubble surfaces is collected in immiscible liquids, rather than in foams. In addition to solvent sublation, removal of metal ions by foam fractionation has been studied. The term foam fractionation is used here to cover the process of foaming-off of dissolved material from a solution via adsorption at bubble surfaces. Nickel has been used as the primary metal ion {colligend) removed by solvent sublation from aqueous solution since considerable interest is being shown in the separation of nickel from cobalt[3] and in the removal and possible reuse of nickel from effluent streams. Sodium and potassium versatate were used as collectors in the adsorptive bubble separation studies and as extractants in solvent extraction, and octan-l-ol was used as immiscible organic solvent in both systems. EXPERIMENTAL
lab. reagent) was used without further purification. Nickel perchlorate was prepared as reported previously[l] and sodium perchlorate (BDH AR grade) was used to adjust the ionic strength of aqueous solutions.
Adsorptit~e bubble separation studies These studies were carried out in a glass cell of 8 cm internal diameter and 1 dm 3 capacity. A glass flit (Pyrex porosity 4; diameter 7 cm) was fused into one end of the cell and two sampling sockets were included in the lower part of the cell. One of these was fitted with a two-way tap' tv enable the concents of the cell to be drained and the other socket could be used either for a pH probe or fitted with a rubber seal to enable samples to be taken from the cell by syringe for rate studies. The top of the cell was fitted with an outlet tube to enable any excess foam produced during the runs to be collected. Nitrogen was used as the source of gas bubbles in the cell. The gas was passed through two bubblers containing water to saturate the gas and through an empty bubbler before being admitted into the cell. Control of the gas flow from a cylinder was by means of a reduction valve fitted with a fine control. Gas flow rates of the order of 60 cm 3 per minute were found to be convenient for solvent sublation studies when 40 cm 3 of octan-l-ol and 501) cm 3 of aqueous solution was used in the cell. Similar flow rates were also used for foam fractionation studies. Rate studies showed that a run time of 60 rain was adequate for the attainment of the steady state condition in all the solvent sublation and loam fractionation studies carried out with versatate as collector.
Soh,ent extraction studies
Reagents Sodium and potassium versatate were prepared and standardized as reported previously[I]. Octan- 1-ol {BDH
The method reported in Part l[1] was used for solvent extraction studies with a run time of 60 min for equilibrium to be attained.
Analysis *Present address: Department of Chemistry and Biology, Preston Polytechnic, Preston, Lancs.
Sodium and potassium were determined by flame photometry (EEL flame photometer) on aqueous solutions
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ALAN D. JONES and CAROL ROBINSON
resulting from the back-extraction of octan-l-ol phases by acid, or on aqueous solutions directly. Nickel was determined by atomic absorption spectrophotometry (EEL 240) on aqueous solutions, oH measurements were made with a Pye Unicam pH meter (model 292) and a combination electrode. RESULTS AND DISCUSSION As indicated in Fig. 1, the versatate anion acts as a collector for nickel by both foam fractionation and solvent sublation processes. For the former process, an increase in the ionic strength of the aqueous solution resulted in a reduction in the removal of nickel at any given pH. In addition, the amount of foam produced in the system decreased as the ionic strength increased. The reduction in the efficiency of foam fractionation with increasing ionic strength probably arises from competition for the versatate collector between nickel and sodium counter ions. In addition, the presence of electrolyte will affect the stability of the foam and will modify the state of ionization of the collector as well as affecting the equilibria between soluble and insoluble nickel species in the system. Solvent sublation was found to lead to a greater removal of nickel at low pH and to a lower removal at high pH compared to the corresponding foam fractionation results (Fig. 1). Removal of nickel by solvent sublation takes place in a three-phase system whereas in foam fractionation only two phases are present. As a result, a number of additional factors have to be con-
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Fig. 1. Nickel extracted (",,) as a function of ionic strength and equilibrium pH. Sodium versatate: Nickel ratio (S) = 2. lnitialnickelconcentration = 1.70 × 104 mol 1 - 1 O, Foam fractionation: no NaCIO4;. ~, Foam ti'actionation: Ionic strength (NaCIO,0 = 5 × 10 2 mol I-1; tD, Foam fractionation: Ionic strength (NaCIO4) = I × 10 -1 mol 1-1; £x, Solvent sublation: Ionic strength (NaCIOJ = 5 × 10 -2mo11-1 .
sidered when comparing solvent sublation with foam fractionation. The transfer of colligend across the octan-l-ol/aqueous solution interface may be a critical factor in determining both the efficiency and the kinetics of the removal process. The flow characteristics of the gas bubbles and hence transfer of nickel to and from the bubbles will also be modified by the presence of the immiscible layer. In addition the redispersal of the colligend complex may be aided by the presence of the octan-l-ol phase, particularly at high pH where the formation of an insoluble complex was noted in the present system. In the absence of a layer of octan-l-ol the foam entrains some of the bulk solution and this leads to a decrease in the total volume of the aqueous phase during the foam fractionation process. Evidence for redispersal in the present work was the variable recovery of nickel from back-extraction of the octan-l-ol phases with acid after the sublation runs had been completed. It was generally observed that less than 70 per cent of the nickel removed from the bulk solution was detected in the octan-l-ol phases, even though the organic phase was back-extracted three times with acid. These low recoveries of nickel can be contrasted with the corresponding values from solvent extraction in which at least 99 per cent recovery was usually achieved[l]. Analysis of the bulk aqueous solution close to the octan-l-ol/water interface in solvent sublation experiments showed that in the runs where low recovery of nickel was obtained from the organic phase, the nickel concentration of the aqueous solution close to the interface was significantly higher than the bulk concentration. In order to overcome the effect of redispersal on the extraction data, the nickel extracted was calculated as the difference between the total nickel present and the nickel concentration of the bulk aqueous solution after sublation as determined on samples taken from the base of the flotation cell where redispersal should be at a minimum. In the case of sodium extraction, where the amount of sodium removed relative to the total sodium was small, the contents of the back-extracted octan-l-ol phases were determined rather than the bulk concentrations. For potassium extraction it was possible to measure the contents of both organic and aqueous phases and since these results gave good mass balances it seems reasonable to assume that the corresponding sodium data are reliable. The removal of the colligend by adsorptive bubble separation methods was found to be appreciably pHdependent (Fig. 1). The pH conditions have a marked effect on both the nature and charge of the collector and the colligend. A consideration of the species distribution plot obtained from published stability constant data[4] at an ionic strength of 0.1 mol 1-1 shows that up to about pH 8-0 the hydrated nickel cation is the main species present. At pH 8.5, 2 per cent of the nickel is present as a sparingly-soluble hydroxy species and at pH 9.5 this has increased to 95 per cent. At an ionic strength of 0.05 mol 1 - ~ the species distri-
1873
S e p a r a t i o n o f n i c k e l ( l l ) f r o m a q u e o u s solution
bution plot is shifted to a slightly lower pH range. A change in pH will lead to a change in the nature of the nickel removal process since at low pH, where only soluble species are present, ion flotation will be the major process whereas at high pH precipitate flotation will predominate. In the intermediate pH range, a c o m b i n a t i o n of these mechanisms will occur. From the data in Fig. 1 the m a x i m u m removal of nickel is found to occur in the pH range where the a m o u n t of insoluble nickel hydroxy species is at a maximum. The pH conditions will considerably affect the state of ionization of the collector, and hence may also drastically alter the properties of a collector in a given system since the charge carried by the ligand may be changed or neutralized depending on pH conditions. In order to calculate the degree of ionization of the collector, the value of the dissociation constant of the collector under the same conditions of temperature and ionic strength used in the sublation system is required. The dissociation constant (K,) of Versatic 911 in aqueous solution has not been published but Ashbrook[5] quotes a value of 7.24 × 10 - s at 25 ° in 50 vol per cent isopropyl alcohol. The effect of the isopropyl alcohol used as solvent by A s h b r o o k makes a prediction of K, in aqueous solution difficult in view of the c o m m e n t s made by Albert and Serjeant[6] about values of dissociation constants in mixed solvent systems. K o r t u m et aL[7] give a value of 1.11 x 10 s for nonoic acid in water and lower chain acids have
values in the range 1.2 1.4 x 10 -5, so Versatic 911 probably has a K. value of about I x 10 - s in aqueous solution. Sodium is extracted as well as nickel by solvent sublation and as Fig. 2 shows sodium is preferentially removed at Io~, pH, but at high pH nickel becomes preferentially removed. Above pH 9.0 an insoluble complex was formed in the octan-l-ol and data above this pH were not obtained. In the absence of nickel, however, no insoluble complexes were formed and the sodium removal was found to reach a maximum at about pH 8.0, after which it decreased. The sodium removed by solvent sublation may result from the ionization of the versatate and/or perchlorate and to investigate this factor further potassium versatate was used as collector and sodium perchlorate to adjust the ionic strength of solutions. Figure 3 shows that the effect of using the potassium salt of the collector rather than the sodium salt is to make the removal of nickel slightly more efficient. Although the removal of sodium was tound to be similar when using either sodium or potassium versatate as collector, the removal of potassium became appreciable ( > 5 per cent) above pH 6-0. In the absence of nickel, however, potassium was not removed by solvent sublation. The removal of nickel by solvent sublation using potassium versatate may result in formation of mixed complexes in the octan-l-ol phase. Shikheeva[8] has reported lhe formation of mixed complexes of cobalt
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Fig. 2. Nickel and sodium extracted (10 ~ mol 1 ') by solvent sublation as a function of equilibrium pH. Sodium versatate: nickel ratio (S) = 2. Initial nickel concentration = 1-70 × 10-'* mol 1 - ' . Ionic strength (NaC104) = 5 x 10 -' t o o l I ~. +~, Sodium extracted in absence of nickel: O, Sodium extracted in presence of nickel; ©, Nickel extracted in presence of sodium.
Fig.
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Nickel,
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pH
potassium
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solvent sublation as a function of e q u i l i b r i u m p H . S o d i u m
versatate: nickel ratio (S) = 2. Initial nickel concentration = 1-70 x 10 "~mol 1 - ~. Ionic strength (NaCIO4) = 5 x 10 -2 tool 1 - 1 ©, Nickel extracted using potassium versatate as collector. &, Sodium extracted using potassium versatate as collector. [11 Potassium extracted using potassium versatate as collector. Q, Nickel extracted using sodium versatate as collector. A, Sodiurn extracted using sodium versatate as collector.
1874
A L A N D . JONES a n d
and sodium in the organic phase in a study of solvent extraction with n a p h t h e n i c acids. The u.v. spectra of octan-l-ol phases after solvent sublation using potassium versatate as collector indicated the formation of different species to those observed when sodium versatate was used. Sodium versatate and also the sodium/ nickel versatate complex in octan-l-ol gave a shoulder at 185-238 n m and strong peaks at 254 and 292 nm, whereas potassium versatate in octan-l-pl absorbed at 185-238 nm, 263 n m and 281 nm. The octan-l-ol phases containing sodium/potassium/nickel versatate in addition to these latter peaks gave a further peak at 299 nm. An additional factor which may account for the difference in extraction behaviour when sodium versatate is replaced by potassium versatate is the difference in basicity between the two collectors. The effect of varying the bulk nickel concentration at constant collector to nickel ratio (S) and also varying S at constant bulk nickel concentration can be seen in Figs. 4 and 5 respectively. At a constant S value of 2, a tenfold decrease in bulk nickel concentration generally resulted in a greater removal of nickel at any given pH (Fig. 4) and the removal of nickel was increased by a b o u t 10 per cent over the whole pH range when S was increased from 2 to 20 at a constant bulk nickel concentration (Fig. 5). Previous workers [9, 10] have also reported that complete removal of a colligend in adsorptive bubble separation methods generally requires an excess of collector to colligend,
.t'C"°~
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CAROL ROBINSON
except where precipitate flotation is involved[11], and that the o p t i m u m S value required depends on the systems under study. A comparison of the results of nickel removal by solvent sublation and solvent extraction (Figs. 4 and 5) using sodium versatate as collector and extractant, respectively, shows that both removal processes lead to sigmoidal extraction curves and to an increase in nickel removed as p H increases. In the range of pH where nickel hydroxide is formed, solvent extraction appears to be more efficient than solvent sublation. In general, the m a x i m u m extraction of metal ions by ,solvent extraction with carboxylic acids has been found to occur around the pH corresponding to metal hydroxide formationE12]. In removal by adsorptive bubble separation methods, however, an ion flotation type of m e c h a n i s m [ l 1] will operate at low pH, where only soluble nickel species are present, whereas at high pH insoluble hydroxy species will be removed by a precipitate flotation type of m e c h a n i s m [ I l l . For the pH range a r o u n d hydroxide formation, a c o m b i n a t i o n of these two mechanisms may be operative. A possible reason for the lower removal of nickel by solvent sublation over the higher pH range may be redispersal of solid sublate from the octan-l-ol/water interface. In addition, octan-1-ol itself may participate in the solvent extraction process as an extractant rather than acting only as a diluent[13], and this possibility is being investigated further[14].
"/{-
AcklTowledgements The authors wish to thank Trent Polytechnic for supporting the work through a research
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Fig. 4. Nickel extracted (%) by solvent sublation and solvent extraction as a function of equilibrium pH and bulk nickel concentration at constant collector ratio. Sodium versatate: nickel (S) = 2. Ionic strength (NaCIO4) = 5 × 10 -2 mol 1- 1 O, Solvent sublation: initial bulk nickel concentration = 1,7 × 10 -5 mol 1-~, A, solvent sublation: initial bulk nickel concentration = 1.7 × 10 -4 mol 1-~: II, solvent extraction: initial bulk nickel concentration = 2.0 × 10 -4 mol 1- i
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Fig. 5. Nickel extracted (%) by solvent sublation and solvent extraction as a function of equilibrium pH and collector ratio. Ionic strength (NaCIO4) = 5 × 10 -z tool I - L Initial bulk nickel concentration = 1.7 × 10 -5 tool 1- l O, solvent sublation: sodium versatate: nickel = 2 0 : A, solvent sublation: sodium versatate: nickel = 2; I . solvent extraction: sodium versatate: nickel = 20.
Separation of nickel(ll) from aqueous solution
studentship to C.R., and also Dr. D. S. Flett, Department of Trade and Industry, Warren Spring Laboratory and Dr. G E P. Elliott of the Chemistry Department, Trent Polytechnic for helpful discussions.
REFERENCES I. "M..I. Jaycock. A. D. Jones and C. Robinson. ,I. im~;'g. nucL Chem. 36, 887 (1974). 2. B. k. Karger, In Adsorptive Bubble Separation 7k'chniques (Edited by R. Lemlich). Academic Press, New York (1972). 3. G. M. Ritcey and B. H. Lucas, 111I. ('o;?ll o;1 Soh'e,l Evtraction, The Hague ( 1971 ). 4. L. G. Sillcn and A. E. Martell, Stability Constants o/ Metal hm Compleves, Special pub. No. 17. Chem. Soc. London (1964). 5. A. W. Ashbrook, J. inorg, nuel. Chem. 34, 1721 (1972).
I.N.C., Vol. 36, No. 8 M
1875
6, A. Albert and E. P. Serjeant. Tile determinatio, O/ ionisation constants, p. 39. C h a p m a n and Hall, London (1971). 7. G. Kortum, W. Vogel and K. Andrussow. Dissociation Con,~tants o/ Organic Acids in Aqueou,~ Sohctio,. Butterworths, London (1971). 8. L. V. Shikheeva, Russ. J. imn'g. Chem. 10, 808 (1965). 9. P. E. Spargo and T. A. Pinfold, Sep. Sci. 5, 619 (1970), 10. B. L. Karger. A. B. Caragay and S. B. Lee, &7~. Sci. 2, 39 (1967). I1. T. A. Pinfold. In Adsorptiz'e Bubble S~Taration Techniques (Edited by R, Lemlich). Academic Press, New York (1972). 12. A. W. Fletcher and D. S. Flett, J. app[. C/lent. 14. 250 (1964). 13. T. E. Moore, R. J. Laran and P, C. Yates. J. phy.s. Chert;. 59, 90 (I 955). 14. G. Ashwell and A. D. Jones, J. inorg, nucl. Chenl. 36, 1877 (1974).
SOLVENT EXTRACTION A N D ADSORPTIVE BUBBLE SEPARATION OF METAL IONS FROM AQUEOUS SOLUTION--II ADSORPTIVE BUBBLE SEPARATION OF NICKEL(II) USING CARBOXYLIC ACIDS A N D THEIR SALTS AS COLLECTORS A L A N D. JONES* and C A R O L R O B I N S O N Department of Chemistry and Biology, Trent Polytechnic, Nottingham
(Received 30 May 1973) Abstract--The removal of nicket(ll) from aqueous solution has been studied by the adsorptive bubble separation processes of solvent sublation and foam fractionation using long-chain carboxync acids and their salts as collectors. The effects of ionic strength, collector:metal ratio and pH on the efficiency of the removal process are reported and a comparison is made with a solvent extraction process using the sodium salt of the tong-chain acid dissolved in octan-l-ol as extractant.
INTRODUCTION
A STUDY of the removal of metal ions from aqueous solution by solvent sublation has been carried out to make a comparison between adsorptive bubble separation and solvent extraction processes. The results of some solvent extraction studies have been reported in part 1[1]. Karger[2] defines solvent sublation as a nonfoaming adsorptive bubble separation process in which enriched material on bubble surfaces is collected in immiscible liquids, rather than in foams. In addition to solvent sublation, removal of metal ions by foam fractionation has been studied. The term foam fractionation is used here to cover the process of foaming-off of dissolved material from a solution via adsorption at bubble surfaces. Nickel has been used as the primary metal ion {colligend) removed by solvent sublation from aqueous solution since considerable interest is being shown in the separation of nickel from cobalt[3] and in the removal and possible reuse of nickel from effluent streams. Sodium and potassium versatate were used as collectors in the adsorptive bubble separation studies and as extractants in solvent extraction, and octan-l-ol was used as immiscible organic solvent in both systems. EXPERIMENTAL
lab. reagent) was used without further purification. Nickel perchlorate was prepared as reported previously[l] and sodium perchlorate (BDH AR grade) was used to adjust the ionic strength of aqueous solutions.
Adsorptit~e bubble separation studies These studies were carried out in a glass cell of 8 cm internal diameter and 1 dm 3 capacity. A glass flit (Pyrex porosity 4; diameter 7 cm) was fused into one end of the cell and two sampling sockets were included in the lower part of the cell. One of these was fitted with a two-way tap' tv enable the concents of the cell to be drained and the other socket could be used either for a pH probe or fitted with a rubber seal to enable samples to be taken from the cell by syringe for rate studies. The top of the cell was fitted with an outlet tube to enable any excess foam produced during the runs to be collected. Nitrogen was used as the source of gas bubbles in the cell. The gas was passed through two bubblers containing water to saturate the gas and through an empty bubbler before being admitted into the cell. Control of the gas flow from a cylinder was by means of a reduction valve fitted with a fine control. Gas flow rates of the order of 60 cm 3 per minute were found to be convenient for solvent sublation studies when 40 cm 3 of octan-l-ol and 501) cm 3 of aqueous solution was used in the cell. Similar flow rates were also used for foam fractionation studies. Rate studies showed that a run time of 60 rain was adequate for the attainment of the steady state condition in all the solvent sublation and loam fractionation studies carried out with versatate as collector.
Soh,ent extraction studies
Reagents Sodium and potassium versatate were prepared and standardized as reported previously[I]. Octan- 1-ol {BDH
The method reported in Part l[1] was used for solvent extraction studies with a run time of 60 min for equilibrium to be attained.
Analysis *Present address: Department of Chemistry and Biology, Preston Polytechnic, Preston, Lancs.
Sodium and potassium were determined by flame photometry (EEL flame photometer) on aqueous solutions
1871
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ALAN D. JONES and CAROL ROBINSON
resulting from the back-extraction of octan-l-ol phases by acid, or on aqueous solutions directly. Nickel was determined by atomic absorption spectrophotometry (EEL 240) on aqueous solutions, oH measurements were made with a Pye Unicam pH meter (model 292) and a combination electrode. RESULTS AND DISCUSSION As indicated in Fig. 1, the versatate anion acts as a collector for nickel by both foam fractionation and solvent sublation processes. For the former process, an increase in the ionic strength of the aqueous solution resulted in a reduction in the removal of nickel at any given pH. In addition, the amount of foam produced in the system decreased as the ionic strength increased. The reduction in the efficiency of foam fractionation with increasing ionic strength probably arises from competition for the versatate collector between nickel and sodium counter ions. In addition, the presence of electrolyte will affect the stability of the foam and will modify the state of ionization of the collector as well as affecting the equilibria between soluble and insoluble nickel species in the system. Solvent sublation was found to lead to a greater removal of nickel at low pH and to a lower removal at high pH compared to the corresponding foam fractionation results (Fig. 1). Removal of nickel by solvent sublation takes place in a three-phase system whereas in foam fractionation only two phases are present. As a result, a number of additional factors have to be con-
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90 80 "tO
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Fig. 1. Nickel extracted (",,) as a function of ionic strength and equilibrium pH. Sodium versatate: Nickel ratio (S) = 2. lnitialnickelconcentration = 1.70 × 104 mol 1 - 1 O, Foam fractionation: no NaCIO4;. ~, Foam ti'actionation: Ionic strength (NaCIO,0 = 5 × 10 2 mol I-1; tD, Foam fractionation: Ionic strength (NaCIO4) = I × 10 -1 mol 1-1; £x, Solvent sublation: Ionic strength (NaCIOJ = 5 × 10 -2mo11-1 .
sidered when comparing solvent sublation with foam fractionation. The transfer of colligend across the octan-l-ol/aqueous solution interface may be a critical factor in determining both the efficiency and the kinetics of the removal process. The flow characteristics of the gas bubbles and hence transfer of nickel to and from the bubbles will also be modified by the presence of the immiscible layer. In addition the redispersal of the colligend complex may be aided by the presence of the octan-l-ol phase, particularly at high pH where the formation of an insoluble complex was noted in the present system. In the absence of a layer of octan-l-ol the foam entrains some of the bulk solution and this leads to a decrease in the total volume of the aqueous phase during the foam fractionation process. Evidence for redispersal in the present work was the variable recovery of nickel from back-extraction of the octan-l-ol phases with acid after the sublation runs had been completed. It was generally observed that less than 70 per cent of the nickel removed from the bulk solution was detected in the octan-l-ol phases, even though the organic phase was back-extracted three times with acid. These low recoveries of nickel can be contrasted with the corresponding values from solvent extraction in which at least 99 per cent recovery was usually achieved[l]. Analysis of the bulk aqueous solution close to the octan-l-ol/water interface in solvent sublation experiments showed that in the runs where low recovery of nickel was obtained from the organic phase, the nickel concentration of the aqueous solution close to the interface was significantly higher than the bulk concentration. In order to overcome the effect of redispersal on the extraction data, the nickel extracted was calculated as the difference between the total nickel present and the nickel concentration of the bulk aqueous solution after sublation as determined on samples taken from the base of the flotation cell where redispersal should be at a minimum. In the case of sodium extraction, where the amount of sodium removed relative to the total sodium was small, the contents of the back-extracted octan-l-ol phases were determined rather than the bulk concentrations. For potassium extraction it was possible to measure the contents of both organic and aqueous phases and since these results gave good mass balances it seems reasonable to assume that the corresponding sodium data are reliable. The removal of the colligend by adsorptive bubble separation methods was found to be appreciably pHdependent (Fig. 1). The pH conditions have a marked effect on both the nature and charge of the collector and the colligend. A consideration of the species distribution plot obtained from published stability constant data[4] at an ionic strength of 0.1 mol 1-1 shows that up to about pH 8-0 the hydrated nickel cation is the main species present. At pH 8.5, 2 per cent of the nickel is present as a sparingly-soluble hydroxy species and at pH 9.5 this has increased to 95 per cent. At an ionic strength of 0.05 mol 1 - ~ the species distri-
1873
S e p a r a t i o n o f n i c k e l ( l l ) f r o m a q u e o u s solution
bution plot is shifted to a slightly lower pH range. A change in pH will lead to a change in the nature of the nickel removal process since at low pH, where only soluble species are present, ion flotation will be the major process whereas at high pH precipitate flotation will predominate. In the intermediate pH range, a c o m b i n a t i o n of these mechanisms will occur. From the data in Fig. 1 the m a x i m u m removal of nickel is found to occur in the pH range where the a m o u n t of insoluble nickel hydroxy species is at a maximum. The pH conditions will considerably affect the state of ionization of the collector, and hence may also drastically alter the properties of a collector in a given system since the charge carried by the ligand may be changed or neutralized depending on pH conditions. In order to calculate the degree of ionization of the collector, the value of the dissociation constant of the collector under the same conditions of temperature and ionic strength used in the sublation system is required. The dissociation constant (K,) of Versatic 911 in aqueous solution has not been published but Ashbrook[5] quotes a value of 7.24 × 10 - s at 25 ° in 50 vol per cent isopropyl alcohol. The effect of the isopropyl alcohol used as solvent by A s h b r o o k makes a prediction of K, in aqueous solution difficult in view of the c o m m e n t s made by Albert and Serjeant[6] about values of dissociation constants in mixed solvent systems. K o r t u m et aL[7] give a value of 1.11 x 10 s for nonoic acid in water and lower chain acids have
values in the range 1.2 1.4 x 10 -5, so Versatic 911 probably has a K. value of about I x 10 - s in aqueous solution. Sodium is extracted as well as nickel by solvent sublation and as Fig. 2 shows sodium is preferentially removed at Io~, pH, but at high pH nickel becomes preferentially removed. Above pH 9.0 an insoluble complex was formed in the octan-l-ol and data above this pH were not obtained. In the absence of nickel, however, no insoluble complexes were formed and the sodium removal was found to reach a maximum at about pH 8.0, after which it decreased. The sodium removed by solvent sublation may result from the ionization of the versatate and/or perchlorate and to investigate this factor further potassium versatate was used as collector and sodium perchlorate to adjust the ionic strength of solutions. Figure 3 shows that the effect of using the potassium salt of the collector rather than the sodium salt is to make the removal of nickel slightly more efficient. Although the removal of sodium was tound to be similar when using either sodium or potassium versatate as collector, the removal of potassium became appreciable ( > 5 per cent) above pH 6-0. In the absence of nickel, however, potassium was not removed by solvent sublation. The removal of nickel by solvent sublation using potassium versatate may result in formation of mixed complexes in the octan-l-ol phase. Shikheeva[8] has reported lhe formation of mixed complexes of cobalt
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Fig. 2. Nickel and sodium extracted (10 ~ mol 1 ') by solvent sublation as a function of equilibrium pH. Sodium versatate: nickel ratio (S) = 2. Initial nickel concentration = 1-70 × 10-'* mol 1 - ' . Ionic strength (NaC104) = 5 x 10 -' t o o l I ~. +~, Sodium extracted in absence of nickel: O, Sodium extracted in presence of nickel; ©, Nickel extracted in presence of sodium.
Fig.
3.
Nickel,
sodium
and
pH
potassium
extracted
(",,)
by
solvent sublation as a function of e q u i l i b r i u m p H . S o d i u m
versatate: nickel ratio (S) = 2. Initial nickel concentration = 1-70 x 10 "~mol 1 - ~. Ionic strength (NaCIO4) = 5 x 10 -2 tool 1 - 1 ©, Nickel extracted using potassium versatate as collector. &, Sodium extracted using potassium versatate as collector. [11 Potassium extracted using potassium versatate as collector. Q, Nickel extracted using sodium versatate as collector. A, Sodiurn extracted using sodium versatate as collector.
1874
A L A N D . JONES a n d
and sodium in the organic phase in a study of solvent extraction with n a p h t h e n i c acids. The u.v. spectra of octan-l-ol phases after solvent sublation using potassium versatate as collector indicated the formation of different species to those observed when sodium versatate was used. Sodium versatate and also the sodium/ nickel versatate complex in octan-l-ol gave a shoulder at 185-238 n m and strong peaks at 254 and 292 nm, whereas potassium versatate in octan-l-pl absorbed at 185-238 nm, 263 n m and 281 nm. The octan-l-ol phases containing sodium/potassium/nickel versatate in addition to these latter peaks gave a further peak at 299 nm. An additional factor which may account for the difference in extraction behaviour when sodium versatate is replaced by potassium versatate is the difference in basicity between the two collectors. The effect of varying the bulk nickel concentration at constant collector to nickel ratio (S) and also varying S at constant bulk nickel concentration can be seen in Figs. 4 and 5 respectively. At a constant S value of 2, a tenfold decrease in bulk nickel concentration generally resulted in a greater removal of nickel at any given pH (Fig. 4) and the removal of nickel was increased by a b o u t 10 per cent over the whole pH range when S was increased from 2 to 20 at a constant bulk nickel concentration (Fig. 5). Previous workers [9, 10] have also reported that complete removal of a colligend in adsorptive bubble separation methods generally requires an excess of collector to colligend,
.t'C"°~
IOC
CAROL ROBINSON
except where precipitate flotation is involved[11], and that the o p t i m u m S value required depends on the systems under study. A comparison of the results of nickel removal by solvent sublation and solvent extraction (Figs. 4 and 5) using sodium versatate as collector and extractant, respectively, shows that both removal processes lead to sigmoidal extraction curves and to an increase in nickel removed as p H increases. In the range of pH where nickel hydroxide is formed, solvent extraction appears to be more efficient than solvent sublation. In general, the m a x i m u m extraction of metal ions by ,solvent extraction with carboxylic acids has been found to occur around the pH corresponding to metal hydroxide formationE12]. In removal by adsorptive bubble separation methods, however, an ion flotation type of m e c h a n i s m [ l 1] will operate at low pH, where only soluble nickel species are present, whereas at high pH insoluble hydroxy species will be removed by a precipitate flotation type of m e c h a n i s m [ I l l . For the pH range a r o u n d hydroxide formation, a c o m b i n a t i o n of these two mechanisms may be operative. A possible reason for the lower removal of nickel by solvent sublation over the higher pH range may be redispersal of solid sublate from the octan-l-ol/water interface. In addition, octan-1-ol itself may participate in the solvent extraction process as an extractant rather than acting only as a diluent[13], and this possibility is being investigated further[14].
"/{-
AcklTowledgements The authors wish to thank Trent Polytechnic for supporting the work through a research
9C IOC 8C
-
/
9C
7G
8C b
~2 ~
60
7C
~ 5Q
6C
z
"o ® u o
4C 30
4o z
2O IO 0
5C
30
iA..~ 3
4
5
7
8
Equilibrium,
6
pH
9
I0
I
fl
Fig. 4. Nickel extracted (%) by solvent sublation and solvent extraction as a function of equilibrium pH and bulk nickel concentration at constant collector ratio. Sodium versatate: nickel (S) = 2. Ionic strength (NaCIO4) = 5 × 10 -2 mol 1- 1 O, Solvent sublation: initial bulk nickel concentration = 1,7 × 10 -5 mol 1-~, A, solvent sublation: initial bulk nickel concentration = 1.7 × 10 -4 mol 1-~: II, solvent extraction: initial bulk nickel concentration = 2.0 × 10 -4 mol 1- i
20 I0 o
3
4
5
t
I
I
I
I
7
8
9
I0
II
12
Equilibrium,
pH
6
Fig. 5. Nickel extracted (%) by solvent sublation and solvent extraction as a function of equilibrium pH and collector ratio. Ionic strength (NaCIO4) = 5 × 10 -z tool I - L Initial bulk nickel concentration = 1.7 × 10 -5 tool 1- l O, solvent sublation: sodium versatate: nickel = 2 0 : A, solvent sublation: sodium versatate: nickel = 2; I . solvent extraction: sodium versatate: nickel = 20.
Separation of nickel(ll) from aqueous solution
studentship to C.R., and also Dr. D. S. Flett, Department of Trade and Industry, Warren Spring Laboratory and Dr. G E P. Elliott of the Chemistry Department, Trent Polytechnic for helpful discussions.
REFERENCES I. "M..I. Jaycock. A. D. Jones and C. Robinson. ,I. im~;'g. nucL Chem. 36, 887 (1974). 2. B. k. Karger, In Adsorptive Bubble Separation 7k'chniques (Edited by R. Lemlich). Academic Press, New York (1972). 3. G. M. Ritcey and B. H. Lucas, 111I. ('o;?ll o;1 Soh'e,l Evtraction, The Hague ( 1971 ). 4. L. G. Sillcn and A. E. Martell, Stability Constants o/ Metal hm Compleves, Special pub. No. 17. Chem. Soc. London (1964). 5. A. W. Ashbrook, J. inorg, nuel. Chem. 34, 1721 (1972).
I.N.C., Vol. 36, No. 8 M
1875
6, A. Albert and E. P. Serjeant. Tile determinatio, O/ ionisation constants, p. 39. C h a p m a n and Hall, London (1971). 7. G. Kortum, W. Vogel and K. Andrussow. Dissociation Con,~tants o/ Organic Acids in Aqueou,~ Sohctio,. Butterworths, London (1971). 8. L. V. Shikheeva, Russ. J. imn'g. Chem. 10, 808 (1965). 9. P. E. Spargo and T. A. Pinfold, Sep. Sci. 5, 619 (1970), 10. B. L. Karger. A. B. Caragay and S. B. Lee, &7~. Sci. 2, 39 (1967). I1. T. A. Pinfold. In Adsorptiz'e Bubble S~Taration Techniques (Edited by R, Lemlich). Academic Press, New York (1972). 12. A. W. Fletcher and D. S. Flett, J. app[. C/lent. 14. 250 (1964). 13. T. E. Moore, R. J. Laran and P, C. Yates. J. phy.s. Chert;. 59, 90 (I 955). 14. G. Ashwell and A. D. Jones, J. inorg, nucl. Chenl. 36, 1877 (1974).