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Separation study of cadmium through an emulsion liquid membrane using triisooctylamine as mobile carrier
Talanta 46 (1998) 927 – 932
Separation study of cadmium through an emulsion liquid membrane using triisooctylamine as mobile carrier Quan-Min Li *, Qi Liu, Qing-Fen Zhang, Xian-Jun Wei, Jin-Zhi Guo Department of Chemistry, Henan Normal Uni6ersity, Xinxiang 453002, People’s Republic of China Received 14 March 1997; received in revised form 15 September 1997; accepted 1 October 1997
Abstract A study of the transport of Cd2 + ions through a triisooctylamine (TIOA) — sorbitan monooleate (Span 80)— dimethylbenzene liquid membrane has been performed with varying concentrations of HCl, KI, TIOA, Span 80 and NaOH in the feed, membrane and stripping solutions. Maximum transport was observed with 0.025 M HCl, 0.01 M KI, 0.02 M TIOA, 3% (w/v) Span 80 and 0.05 M NaOH. With this system cadmium could be completely separated with Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + . The transport mechanism of this metal ions through the membrane has been discussed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Ions; Transport; Mechanism
1. Introduction As a new method, the emulsion liquid membrane (ELM) has been studied for the preconcentration and separation of metal ions [1–3], because of its advantage of high efficiency and low expense. Tri-n-octylamine (TOA) has been studied as an ELM mobile carrier to separate metal ions [4,5], but triisooctylamine (TIOA) has not been applied for the separation of cadmium from coexistence ions. As an important extractant, TIOA can be combined with CdI24 − ions after its protonation [3]. * Corresponding author.
In this paper, an ELM with TIOA as mobile carrier is studied for the transport of cadmium. Various parameters influencing the transport of cadmium across the membrane have been optimized to separate cadmium from Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + , and the transport mechanism of this metal ions through the membrane has been discussed.
2. Experimental
2.1. Reagents A standard (1 mg ml − 1) solution of cadmium was prepared from metal cadmium (99.99%) dis-
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(97)00357-3
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Q.-M. Li et al. / Talanta 46 (1998) 927–932
solved in nitric acid and distilled, deionized water. TIOA (analytical grade) was obtained from Aldrich. Sorbitan monooleate (Span 80; chemical grade) was obtained from Shanghai Dazhong Medical Company (Shanghai, People’s Republic of China). A 0.10 M solution of TIOA and a 3% (w/v) solution of Span 80 in dimethylbenzene were used in this work.
2.2. Apparatus The following instruments were used: a motordriven emulsifier (range 0 – 6000 rpm); motordriven stirrers (range 0 – 600 rpm); and a model 722 spectrophotometer (Shanghai Analytical Instrument Factory, People’s Republic of China).
2.3. Procedure 2.3.1. Preparation of ELM A 20 ml portion of solution TIOA and Span 80 in dimethylbenzene are emulsified at a stripping speed of 2000 rpm. Stripping solution was added at a rate of 20 ml min − 1 until the volume ratio of organic membrane solution to stripping solution was 1:1. The solution was then stirred continuously for 15 min to obtain a stable white ELM. 2.3.2. Transport of metal ions To small beakers containing 10 ml metal ion feed solutions was added 2 ml of ELM and the contents stirred at 200 rpm for a given transfer time; the phases were allowed to separate, clear feed solution was pipetted into a 25 ml volumetric flask and analyzed for the amount of cation remaining. 2.3.3. Determination of cadmium [6] Three millilitres of 5×10 − 4 M 4-(2-pyridylazo) resorcinol (PAR) alcohol solution was added to 5 ml of 0.1 M Na2B4O7 · 10H2O buffer solution (pH=9.2) in a 25 ml volumetric flask containing cadmium ions, after diluting to the mark the absorbance was read at 495 nm against the reagent blank. In separation experiment, the concentration of cadmium as well as other cations was determined by inductively-coupled plasma atomic emission spectrometry (ICP-AES).
3. Results and discussion
3.1. The effect of hydrochloric acid concentration in the feed The relationship between the concentration of hydrochloric acid in the feed solution and extraction of cadmium is shown in Fig. 1. An acid concentration of 0.025 M was found to be best for transport of Cd2 + through ELM. In Fig. 1, it also is found that the transport of cadmium is completed in 5 min, this is because extraction and back-extraction occur simultaneously, and the degree of dispersion of the ELM in the feed solution is high [7]. In the presence of HCl and KI in the feed solution, the transport process of cadmium through an ELM is illustrated by the following equations: 1. TIOA (shown as R3N) in the membrane phase reacts with hydrochloric acid in the feed phase. R N + H + + Cl − = R3NH + Cl −
3 (membrane)
(feed)
(membrane)
2. In the feed, CdI24 − exchanges with Cl − of R3NH + Cl − in the membrane phase. CdI24 − + 2R3NH + Cl − = (R3N)2CdI4 + 2Cl − (feed)
(membrane)
(membrane)
(feed)
3. NaOH in the stripping solution reacts with (R3NH)2CdI24 − to strip cadmium into the stripping solution.
Fig. 1. Effect of HCl conc. in the feed on the percent extraction of Cd2 + membrane phase: 0.02 M TIOA +3% (w/v) Span 80+ dimethlybenzene; stripping phase: 0.05 M NaOH; feed phase: 50 mg ml − 1 Cd2 + +0.01 M KI+HCl (M): 1– 0.01 – 0.10, 2 – 0.005, 3 – 1.0.
Q.-M. Li et al. / Talanta 46 (1998) 927–932
929
abruptly decreases with time. This is because less H + is available to protonate R3N, and precipitation of Cd2 + as Cd(OH)2 blocks the membrane. These can be proved by finding white Cd(OH)2 precipitation in the stripping solution after demulsification. When the concentration of H + \ 0.1 M, the extraction of cadmium decreased too, this is because a great amount of H + transport into the stripping where OH − is neutralized immediately, then the original transport direction of cadmium is changed (the cadmium transport from the stripping to the feed) [8]. This process is shown in Fig. 2b.
3.2. The effect of the concentration of surfactant
Fig. 2. (a) Co-transport of CdI24 − with H + . (b) Countertransport of CdI24 − with Cl − co-transport or counter-transport of CdI24 − with protonated triisooctylmine as carrier. − (R3NH)2CdI4 +2OH (strip) (membrane)
= R3N + CdI (membrane)
2− 4
+ 2H2O
(strip)
The expected mechanism of cadmium transport in the present case is shown in Fig. 2a. Cd2 + and H + were transported from the feed solution to the stripping solution where H + was neutralized by NaOH. The transport of H + along a concentration gradient supplied the energy for the transport of cadmium against a concentration gradient. It was discovered that the decrease of H + concentration in the feed solution was more considerable than that of cadmium. This was due to the formation of R3NH + Cl − -type species and their transport into the stripping phase. When the concentration of HCl was B 0.005 M, transport of cadmium
Span 80 is better than other surfactant when used in ELM [2]. Both the stability of the emission and the viscosity of the liquid membrane were altered by the proportion of surfactant in the organic phase. An increase in the concentration of Span 80 increased the stability of the emulsion, however as follows from Table 1, the extraction of cadmium decrease. When the concentration of Span 80 was B 2% by weight in the organic phase, the ELM was easy to break with transfer time. A concentration of 3% (w/v) in the organic phase resulted in good extraction and stability.
3.3. The effect of the concentration of TIOA The effect of the concentration of TIOA (mobile carrier) in the organic phase on the extraction of cadmium is shown in Fig. 3. The optimum concentration range of carrier was 0.02–0.03 M. When the concentration of TIOA was 0.01 M, less H + entered the stripping, Cd2 + precipitated as Cd(OH)2 and clogged the membrane, the extraction of Cd2 + decreased. When the concentration of TIOA was \ 0.05 M, H + transported into the stripping and neutralized OH − immediately, then the cadmium transported inversely from the stripping to the feed [8] (see Fig. 2b). In the experiment 0.02 M concentration of TIOA was selected for a rapid and complete transport of cadmium.
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Table 1 The effect of concentration of surfactant Conc. of Span 80 [%(w/v)] Percent extraction
1 93.3
2 94.7
3 99.0
4 96.5
5 96.8
Membrane phase: 0.02 M TIOA+Span 80; feed phase: 0.025 M HCl+50 mg ml−1 Cd(II)+0.01 M KI. Stripping phase: 0.05 M NaOH; transfer time: 10 min.
3.4. The effect of the concentration of NaOH in the stripping solution The NaOH in the stripping phase functioned as a back-extracting solution. When the composition of emulsion was fixed, the maximum extraction was obtained in different stripping solution at 0.01–0.05 M NaOH. It has been observed that above a concentration of 0.2 M NaOH, the transport decreased markedly (see Fig. 4). This may be because more H + transported the stripping, formed more water, made the membrane swell and break. When the concentration of NaOH increased to 1.0 M, CdI24 − precipitated Cd(OH)2 and the membrane was clogged as soon as the transport began. In the experiment, 0.05 M NaOH was selected. When the concentrations of NaOH were 0.025 and 0.5 M, by changing the transfer time (from 0.5 to 2.5 min) measured the natural logarithm of the ratio of initial concentration (C0) to give time
Fig. 3. Effect of TIOA conc. in the membrane phase on the percent extraction of Cd2 + feed phase: 50 mg ml − 1 Cd2 + + 0.01 M KI+ 0.025 M HCl; stripping phase: 0.05 M NaOH; membrane phase: 3% (w/v) Span 80 + TIOA (M): 1 – 0.02 0.03, 2 – 0.01, 3–0.05.
concentration (C) of cadmium the kinetic curve can be obtained (see Fig. 5). ln C0/C−t curve line is a good straight line, they are (1) ln C0/C= 1.479× 10 − 2t+ 0.6386, r= 0.9908; (2) ln C0/C= 1.285× 10 − 2t+ 0.2556, r=0.9985, respectively. This shows that transport of cadmium is near kinetic pseudo-first-order reaction. The transfer rate constant of Cd2 + was 0.0148 and 0.0128 s − 1, which are similar. This shows that the transfer rate of Cd2 + was controlled only by (R3NH)2CdI4 concentration on the feed-side interface. Its kinetic equation is illustrated by the following equation − d[Cd(II)]= k[(R3NH)2CdI4] dt
3.5. The effect of other reagents in the feed solution With a suitable acid concentration of the feed solution, the concentration of KI in the feed was
Fig. 4. Effect of concentration of NaOH on the percent extraction of Cd2 + membrane phase: 0.02 M TIOA + 3% (w/v) Span 80 + dimethylbenzene; feed phase: 50 mg ml − 1 Cd2 + +0.025 M HCl+0.01 M KI; stripping phase: NaOH (M): 1 – 0.01 – 0.05, 2 – 0.10, 3 – 0.20.
Q.-M. Li et al. / Talanta 46 (1998) 927–932
Fig. 5. Kinetic curve line of transport of Cd2 + membrane phase: 0.02 M TIOA +3% (w/v) Span 80 + dimethylbenzene; feed phase: 50 mg ml − 1 Cd2 + + 0.025 M HCl+ 0.01 M KI; stripping phase: NaOH (M): 1–0.025, 2–0.05.
\ 0.01 M, the extraction efficiency of cadmium was always \98%. If KI was replaced by KBr or KSCN in turn, the extraction of cadmium decreased greatly, it is because the logarithm of their cumulative stability constants (log b4) of CdI24 − , CdBr24 − and Cd(SCN)24 − are respectively 5.35, 2.93 and 2.91 [9], the log b4 of CdI24 − is the highest. If KCl, KNO3, K2SO4 or KClO4 were added to above systems in turn, the results were shown in Fig. 6, ClO4− is the biggest anion and so its association with TIOA resulted in the most significant decrease in the transport of cadmium (see curve 4 in Fig. 6). The smallest anion Cl − did not interfere the transport of cadmium (see curve 1 in Fig. 6).
3.6. Separation of cadmium from other metal ions 2+
Under suitable conditions, transport of Cu , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + were studied. Results for the competitive transport of cadmium and other common cations in mixed solution are shown in Table 2. (Note that the data of Table 2 comes from ICP-ASE measurements. All other data are from colorimetric measurements.) Selectivity of separation of cadmium was excellent and the transport of Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + was found to be negligible.
931
Fig. 6. Effect of different salts in the feed solution on the percent extraction of Cd2 + membrane phase: 0.02 M TIOA + 3% (w/v) Span 80 + dimethylbenzene; stripping phase: 0.05 M NaOH: feed phase: 50 mg ml − 1 Cd2 + +0.05 M HCl + salt: 1 – 0.01 M KI+KCL, 2 – 0.01 M KI+ K2SO4, 3 – 0.01 M KI +KNO3, 4 – 0.01 M KI+HClO4, 5 – 0.01 M Br, 6 – 0.01 M KSCN; transfer time: 10 min.
The TIOA-Span 80-dimethylbenzene liquid membrane is feasible to separate cadmium from other common cations, specifically from Zn2 + , because the character of Zn2 + is significant.
4. Conclusion In this paper, transport of cadmium through a TIOA-Span 80-dimethylbenzene ELM was studied. The mechanism of transport of cadmium was discussed and is presented in Fig. 2a, b. The optimum conditions of transport have been found to be 0.01 M KI and 0.025 M HCl in the feed Table 2 The percent extraction of Cd2+ in a mixed solution Initial amount of each co-ions (Fe2+, Zn2+, Co2+, Ni2+, Al3+, Mn2+, Cr3+
The percent extraction of Cd2+
10 mg ml−1 50 mg ml−1 100 mg ml−1
99.3 97.6 95.8
Initial amount of Cd2+: 50 mg ml−1; membrane phase: 0.02 M TIOA+3% (w/v) Span 80+dimethylbenzene; feed phase: 0.05 M HCl+0.025 M KI; stripping phase: 0.05 M NaOH; transfer time: 10 min.
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solution, 0.02 M TIOA and 3% (w/v) Span 80 in the liquid membrane, and 0.05 M NaOH in the stripping solution. It is concluded that this method can be applied for the selective separation of cadmium from a mixed solution of Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + or as a preconcentrating step of measuring cadmium. Small amount of carriers are involved and the extraction efficiency is high.
References [1] R.M. Izatt, G.C. Lindh, R.L. Bruening, Anal. Chem.
.
60 (1988) 1694. [2] Y. Li, A. Wang, J.C. Van Loon, R.R. Barefoot, Talanta 39 (1992) 1337. [3] C. Malik, A. Li, Sep. Sci. Technol. 25 (3) (1990) 263. [4] Q. Li, K. Li, C. Zhou, S. Tong, Chem. J. Chin. Univ. 14 (2) (1993) 171. [5] Q. Cai, Z. Yan, H. Chao, Sci. Technol. Membr. 9 (4) (1989) 33. [6] F. Liu, K. Li, J. Lu, Y. Shun, S. Tong, Ion-Exch. Adsorpt. 8 (5) (1992) 400. [7] T.L. Largman, S. Sifiniades, Hydrometallurgy 3 (1978) 153. [8] W.C. Babcock, J. Membr. Sci. 7 (1980) 71. [9] Analytical Chemistry Studio of Zhongnan Mineral College, Handbook of Chemical Analysis, Science Press, Beijing, People’s Republic of China, 1984, pp. 624– 630.
Separation study of cadmium through an emulsion liquid membrane using triisooctylamine as mobile carrier Quan-Min Li *, Qi Liu, Qing-Fen Zhang, Xian-Jun Wei, Jin-Zhi Guo Department of Chemistry, Henan Normal Uni6ersity, Xinxiang 453002, People’s Republic of China Received 14 March 1997; received in revised form 15 September 1997; accepted 1 October 1997
Abstract A study of the transport of Cd2 + ions through a triisooctylamine (TIOA) — sorbitan monooleate (Span 80)— dimethylbenzene liquid membrane has been performed with varying concentrations of HCl, KI, TIOA, Span 80 and NaOH in the feed, membrane and stripping solutions. Maximum transport was observed with 0.025 M HCl, 0.01 M KI, 0.02 M TIOA, 3% (w/v) Span 80 and 0.05 M NaOH. With this system cadmium could be completely separated with Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + . The transport mechanism of this metal ions through the membrane has been discussed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Ions; Transport; Mechanism
1. Introduction As a new method, the emulsion liquid membrane (ELM) has been studied for the preconcentration and separation of metal ions [1–3], because of its advantage of high efficiency and low expense. Tri-n-octylamine (TOA) has been studied as an ELM mobile carrier to separate metal ions [4,5], but triisooctylamine (TIOA) has not been applied for the separation of cadmium from coexistence ions. As an important extractant, TIOA can be combined with CdI24 − ions after its protonation [3]. * Corresponding author.
In this paper, an ELM with TIOA as mobile carrier is studied for the transport of cadmium. Various parameters influencing the transport of cadmium across the membrane have been optimized to separate cadmium from Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + , and the transport mechanism of this metal ions through the membrane has been discussed.
2. Experimental
2.1. Reagents A standard (1 mg ml − 1) solution of cadmium was prepared from metal cadmium (99.99%) dis-
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(97)00357-3
928
Q.-M. Li et al. / Talanta 46 (1998) 927–932
solved in nitric acid and distilled, deionized water. TIOA (analytical grade) was obtained from Aldrich. Sorbitan monooleate (Span 80; chemical grade) was obtained from Shanghai Dazhong Medical Company (Shanghai, People’s Republic of China). A 0.10 M solution of TIOA and a 3% (w/v) solution of Span 80 in dimethylbenzene were used in this work.
2.2. Apparatus The following instruments were used: a motordriven emulsifier (range 0 – 6000 rpm); motordriven stirrers (range 0 – 600 rpm); and a model 722 spectrophotometer (Shanghai Analytical Instrument Factory, People’s Republic of China).
2.3. Procedure 2.3.1. Preparation of ELM A 20 ml portion of solution TIOA and Span 80 in dimethylbenzene are emulsified at a stripping speed of 2000 rpm. Stripping solution was added at a rate of 20 ml min − 1 until the volume ratio of organic membrane solution to stripping solution was 1:1. The solution was then stirred continuously for 15 min to obtain a stable white ELM. 2.3.2. Transport of metal ions To small beakers containing 10 ml metal ion feed solutions was added 2 ml of ELM and the contents stirred at 200 rpm for a given transfer time; the phases were allowed to separate, clear feed solution was pipetted into a 25 ml volumetric flask and analyzed for the amount of cation remaining. 2.3.3. Determination of cadmium [6] Three millilitres of 5×10 − 4 M 4-(2-pyridylazo) resorcinol (PAR) alcohol solution was added to 5 ml of 0.1 M Na2B4O7 · 10H2O buffer solution (pH=9.2) in a 25 ml volumetric flask containing cadmium ions, after diluting to the mark the absorbance was read at 495 nm against the reagent blank. In separation experiment, the concentration of cadmium as well as other cations was determined by inductively-coupled plasma atomic emission spectrometry (ICP-AES).
3. Results and discussion
3.1. The effect of hydrochloric acid concentration in the feed The relationship between the concentration of hydrochloric acid in the feed solution and extraction of cadmium is shown in Fig. 1. An acid concentration of 0.025 M was found to be best for transport of Cd2 + through ELM. In Fig. 1, it also is found that the transport of cadmium is completed in 5 min, this is because extraction and back-extraction occur simultaneously, and the degree of dispersion of the ELM in the feed solution is high [7]. In the presence of HCl and KI in the feed solution, the transport process of cadmium through an ELM is illustrated by the following equations: 1. TIOA (shown as R3N) in the membrane phase reacts with hydrochloric acid in the feed phase. R N + H + + Cl − = R3NH + Cl −
3 (membrane)
(feed)
(membrane)
2. In the feed, CdI24 − exchanges with Cl − of R3NH + Cl − in the membrane phase. CdI24 − + 2R3NH + Cl − = (R3N)2CdI4 + 2Cl − (feed)
(membrane)
(membrane)
(feed)
3. NaOH in the stripping solution reacts with (R3NH)2CdI24 − to strip cadmium into the stripping solution.
Fig. 1. Effect of HCl conc. in the feed on the percent extraction of Cd2 + membrane phase: 0.02 M TIOA +3% (w/v) Span 80+ dimethlybenzene; stripping phase: 0.05 M NaOH; feed phase: 50 mg ml − 1 Cd2 + +0.01 M KI+HCl (M): 1– 0.01 – 0.10, 2 – 0.005, 3 – 1.0.
Q.-M. Li et al. / Talanta 46 (1998) 927–932
929
abruptly decreases with time. This is because less H + is available to protonate R3N, and precipitation of Cd2 + as Cd(OH)2 blocks the membrane. These can be proved by finding white Cd(OH)2 precipitation in the stripping solution after demulsification. When the concentration of H + \ 0.1 M, the extraction of cadmium decreased too, this is because a great amount of H + transport into the stripping where OH − is neutralized immediately, then the original transport direction of cadmium is changed (the cadmium transport from the stripping to the feed) [8]. This process is shown in Fig. 2b.
3.2. The effect of the concentration of surfactant
Fig. 2. (a) Co-transport of CdI24 − with H + . (b) Countertransport of CdI24 − with Cl − co-transport or counter-transport of CdI24 − with protonated triisooctylmine as carrier. − (R3NH)2CdI4 +2OH (strip) (membrane)
= R3N + CdI (membrane)
2− 4
+ 2H2O
(strip)
The expected mechanism of cadmium transport in the present case is shown in Fig. 2a. Cd2 + and H + were transported from the feed solution to the stripping solution where H + was neutralized by NaOH. The transport of H + along a concentration gradient supplied the energy for the transport of cadmium against a concentration gradient. It was discovered that the decrease of H + concentration in the feed solution was more considerable than that of cadmium. This was due to the formation of R3NH + Cl − -type species and their transport into the stripping phase. When the concentration of HCl was B 0.005 M, transport of cadmium
Span 80 is better than other surfactant when used in ELM [2]. Both the stability of the emission and the viscosity of the liquid membrane were altered by the proportion of surfactant in the organic phase. An increase in the concentration of Span 80 increased the stability of the emulsion, however as follows from Table 1, the extraction of cadmium decrease. When the concentration of Span 80 was B 2% by weight in the organic phase, the ELM was easy to break with transfer time. A concentration of 3% (w/v) in the organic phase resulted in good extraction and stability.
3.3. The effect of the concentration of TIOA The effect of the concentration of TIOA (mobile carrier) in the organic phase on the extraction of cadmium is shown in Fig. 3. The optimum concentration range of carrier was 0.02–0.03 M. When the concentration of TIOA was 0.01 M, less H + entered the stripping, Cd2 + precipitated as Cd(OH)2 and clogged the membrane, the extraction of Cd2 + decreased. When the concentration of TIOA was \ 0.05 M, H + transported into the stripping and neutralized OH − immediately, then the cadmium transported inversely from the stripping to the feed [8] (see Fig. 2b). In the experiment 0.02 M concentration of TIOA was selected for a rapid and complete transport of cadmium.
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Q.-M. Li et al. / Talanta 46 (1998) 927–932
Table 1 The effect of concentration of surfactant Conc. of Span 80 [%(w/v)] Percent extraction
1 93.3
2 94.7
3 99.0
4 96.5
5 96.8
Membrane phase: 0.02 M TIOA+Span 80; feed phase: 0.025 M HCl+50 mg ml−1 Cd(II)+0.01 M KI. Stripping phase: 0.05 M NaOH; transfer time: 10 min.
3.4. The effect of the concentration of NaOH in the stripping solution The NaOH in the stripping phase functioned as a back-extracting solution. When the composition of emulsion was fixed, the maximum extraction was obtained in different stripping solution at 0.01–0.05 M NaOH. It has been observed that above a concentration of 0.2 M NaOH, the transport decreased markedly (see Fig. 4). This may be because more H + transported the stripping, formed more water, made the membrane swell and break. When the concentration of NaOH increased to 1.0 M, CdI24 − precipitated Cd(OH)2 and the membrane was clogged as soon as the transport began. In the experiment, 0.05 M NaOH was selected. When the concentrations of NaOH were 0.025 and 0.5 M, by changing the transfer time (from 0.5 to 2.5 min) measured the natural logarithm of the ratio of initial concentration (C0) to give time
Fig. 3. Effect of TIOA conc. in the membrane phase on the percent extraction of Cd2 + feed phase: 50 mg ml − 1 Cd2 + + 0.01 M KI+ 0.025 M HCl; stripping phase: 0.05 M NaOH; membrane phase: 3% (w/v) Span 80 + TIOA (M): 1 – 0.02 0.03, 2 – 0.01, 3–0.05.
concentration (C) of cadmium the kinetic curve can be obtained (see Fig. 5). ln C0/C−t curve line is a good straight line, they are (1) ln C0/C= 1.479× 10 − 2t+ 0.6386, r= 0.9908; (2) ln C0/C= 1.285× 10 − 2t+ 0.2556, r=0.9985, respectively. This shows that transport of cadmium is near kinetic pseudo-first-order reaction. The transfer rate constant of Cd2 + was 0.0148 and 0.0128 s − 1, which are similar. This shows that the transfer rate of Cd2 + was controlled only by (R3NH)2CdI4 concentration on the feed-side interface. Its kinetic equation is illustrated by the following equation − d[Cd(II)]= k[(R3NH)2CdI4] dt
3.5. The effect of other reagents in the feed solution With a suitable acid concentration of the feed solution, the concentration of KI in the feed was
Fig. 4. Effect of concentration of NaOH on the percent extraction of Cd2 + membrane phase: 0.02 M TIOA + 3% (w/v) Span 80 + dimethylbenzene; feed phase: 50 mg ml − 1 Cd2 + +0.025 M HCl+0.01 M KI; stripping phase: NaOH (M): 1 – 0.01 – 0.05, 2 – 0.10, 3 – 0.20.
Q.-M. Li et al. / Talanta 46 (1998) 927–932
Fig. 5. Kinetic curve line of transport of Cd2 + membrane phase: 0.02 M TIOA +3% (w/v) Span 80 + dimethylbenzene; feed phase: 50 mg ml − 1 Cd2 + + 0.025 M HCl+ 0.01 M KI; stripping phase: NaOH (M): 1–0.025, 2–0.05.
\ 0.01 M, the extraction efficiency of cadmium was always \98%. If KI was replaced by KBr or KSCN in turn, the extraction of cadmium decreased greatly, it is because the logarithm of their cumulative stability constants (log b4) of CdI24 − , CdBr24 − and Cd(SCN)24 − are respectively 5.35, 2.93 and 2.91 [9], the log b4 of CdI24 − is the highest. If KCl, KNO3, K2SO4 or KClO4 were added to above systems in turn, the results were shown in Fig. 6, ClO4− is the biggest anion and so its association with TIOA resulted in the most significant decrease in the transport of cadmium (see curve 4 in Fig. 6). The smallest anion Cl − did not interfere the transport of cadmium (see curve 1 in Fig. 6).
3.6. Separation of cadmium from other metal ions 2+
Under suitable conditions, transport of Cu , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + were studied. Results for the competitive transport of cadmium and other common cations in mixed solution are shown in Table 2. (Note that the data of Table 2 comes from ICP-ASE measurements. All other data are from colorimetric measurements.) Selectivity of separation of cadmium was excellent and the transport of Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + was found to be negligible.
931
Fig. 6. Effect of different salts in the feed solution on the percent extraction of Cd2 + membrane phase: 0.02 M TIOA + 3% (w/v) Span 80 + dimethylbenzene; stripping phase: 0.05 M NaOH: feed phase: 50 mg ml − 1 Cd2 + +0.05 M HCl + salt: 1 – 0.01 M KI+KCL, 2 – 0.01 M KI+ K2SO4, 3 – 0.01 M KI +KNO3, 4 – 0.01 M KI+HClO4, 5 – 0.01 M Br, 6 – 0.01 M KSCN; transfer time: 10 min.
The TIOA-Span 80-dimethylbenzene liquid membrane is feasible to separate cadmium from other common cations, specifically from Zn2 + , because the character of Zn2 + is significant.
4. Conclusion In this paper, transport of cadmium through a TIOA-Span 80-dimethylbenzene ELM was studied. The mechanism of transport of cadmium was discussed and is presented in Fig. 2a, b. The optimum conditions of transport have been found to be 0.01 M KI and 0.025 M HCl in the feed Table 2 The percent extraction of Cd2+ in a mixed solution Initial amount of each co-ions (Fe2+, Zn2+, Co2+, Ni2+, Al3+, Mn2+, Cr3+
The percent extraction of Cd2+
10 mg ml−1 50 mg ml−1 100 mg ml−1
99.3 97.6 95.8
Initial amount of Cd2+: 50 mg ml−1; membrane phase: 0.02 M TIOA+3% (w/v) Span 80+dimethylbenzene; feed phase: 0.05 M HCl+0.025 M KI; stripping phase: 0.05 M NaOH; transfer time: 10 min.
932
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solution, 0.02 M TIOA and 3% (w/v) Span 80 in the liquid membrane, and 0.05 M NaOH in the stripping solution. It is concluded that this method can be applied for the selective separation of cadmium from a mixed solution of Cu2 + , Zn2 + , Fe2 + , Co2 + , Ni2 + , Mn2 + , Cr3 + and Al3 + or as a preconcentrating step of measuring cadmium. Small amount of carriers are involved and the extraction efficiency is high.
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