Vol.25, No. (201) Article Effect of Interfacial Activity on Silver Extraction Rate with Calix[]arene Derivatives in Nitrate Media Jee Young KIM, Shintaro MORISADA, Hidetaka KAWAKITA and Keisuke OHTO * Department of Chemistry and Applied Chemistry, Saga University, 1-Honjo, Saga 80-8502, Japan (Manuscript received May 21, 201; Accepted June 27, 201) Abstract p-t-octylcalix[]arene derivatives as extraction reagents have been prepared to investigate silver extraction rate in nitrate media by batch-wise method. The interfacial tension was also measured by the drop volume method to investigate the effect of interfacial activity on silver extraction rate with the calix[]arene derivatives in chloroform / nitric acid media by varying the concentrations of the extraction reagent and nitric acid. Interfacial activities of calix[]arene derivatives were evaluated by comparing the obtained interfacial tension values. The interfacial excess and the average area per the adsorbed extractant molecule at chloroform / nitric acid interface were estimated from the Gibbs adsorption equation and were used to investigate the relationship with silver extraction by using calix[]arene derivatives in nitrate media. From the result, silver extraction rate with calix[]arene derivatives in nitrate media was significantly related to their interfacial activities. Keywords: p-t-octylcalix[]arene derivatives, Silver extraction rate, Interfacial activity 1. Introduction The solvent extraction is one of major separating operation technology in hydrometallurgical processes for the separation and purification of metals. 1-) In such processes, the liquidliquid interface plays a key role since the transport of metal ions occurs through the interface. 5) However, the interfacial chemistry of extraction systems is still incompletely understood. Generally, liquid-liquid extraction reagents (extractants) are interface-active reagents. Hence, they adsorb at liquid-liquid interface and lower the interfacial tension (γ) of the system. 6-9) The drop weight method improved from drop volume method is one of the preferred methods for surface and interfacial tension measurement due to its simplicity of handling, easy temperature control, small sample size, good reproducibility and its applicability for liquid-liquid system. 10-12) The lowering extent of interfacial tension depends upon the chemical composition and structure of the extractant and diluents as well as extractant concentration in the bulk phase. Inorganic electrolytes in the aqueous phase can also significantly influence the interfacial tension. The lowering of the interfacial tension in some systems may lead to finer dispersion of droplets, 13) stable emulsions, 1) and interfacial turbulence 15) and may also enhance the extraction rate. 16,17) Thus, study on the interfacial tension which is the most easily measured interfacial property of liquid-liquid systems has * Corresponding author E-mail: ohtok@cc.saga-u.ac.jp been required, because the kinetics of the metal extraction process depends upon the concentration of the reactant species at the reaction sites and it should supports to fundamentally understand the mechanism of metal extraction. 3) The interfacial activity of extractants has been used to explain kinetic and equilibrium data of systems. 18-22) Since the structures of the extractants significantly influence such physicochemical property and extraction behavior, macrocyclic compound such as calixarene derivatives are attractive. 23) In our previous work by the batchwise method, methyl ketonic 2) and 2-pyridyl 25) derivatives of p- t-octylcalix[]arene as extractants exhibited high extraction selectivity for silver among precious metals in nitrate media. However, silver extraction rates between two derivatives exhibited large difference. 26) In the present work, the further study was investigated to evaluate interfacial tension of 2-pyridyl, methyl ketonic and phenyl ketonic derivatives of p-t-octylcalix[]arene and to further understand the relationship with extraction rate in nitrate media. 2. Experimental 2.1 Reagents Chemical structures of p-t-octylcalix[]arene type extracta nts and their abbreviations.are shown in Fig. 1. 25,26,27,28-Tetrakis(acetomethoxy)-5,11,17,23-tetrakis 136
ION EXCHANGE OR R = H (TOC) N 1 : R = (2-PyTOC) 2 : R = COCH 3 (MKTOC) 3 : R = COC 6 H 5 (PKTOC) Fig. 1 Chemical structures and the abbreviations of calix[]arene type extractants. (1,1,3,3-tetramethylbutyl)calix[]arene (MKTOC), 2) and 25,26,27,28-tetrakis(2-pyridylmethoxy)-5,11,17,23-tetrakis(1,1, 3,3-tetramethylbutyl)calix[]arene (2-PyTOC) 25) were prepared by both single steps from 5,11,17,23-tetrakis(1,1,3,3- tetramethylbutyl)calix[]arene-25,26,27,28-tetrol (TOC) in similar manners to previously described procedures. 25,26,27,28- Tetrakis(2-oxo-2-phenylethoxy)-5,11,17,23-tetrakis (1,1,3,3-tetramethylbutyl)-calix[]arene (PKTOC) 18) has been newly prepared followed by the synthetic scheme shown in Scheme 1. That is; Under a nitrogen stream, to 5 cm 3 of dry acetone, sodium iodide (1.35 g, 9.00 mmol) and phenacyl chloride (1.39 g, 9.00 mmol) were added and stirred for 20 min at room temperature. To the solution TOC (1.00 g, 1.1 mmol), potassium carbonate (1.30 g, 9.00 mmol) and 50 cm 3 of dry acetone were added and refluxed for more than 2 h. Then, the solvent was evaporated to obtain the crude compound and the residue was dissolved in chloroform (150 cm 3 ). The organic phase was washed twice with saturated sodium thiosulfate solution, once with distilled water, twice with 1 M (M = mol dm -3 ) hydrochloric acid solution and twice distilled water (30 cm 3 ). The solution was dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated in vacuo. To the residue, acetonitrile was added to reprecipitate the desired compound. It was filtered and a yellow powder was obtained (yield 31 %, TLC (SiO 2, chloroform : methanol = 5 : 1 v/v, R f =0.69) OR Cl COPh K 2 CO 3, NaI acetone O C O Scheme 1 Synthetic scheme of PKTOC. Other reagents were of analytical grade and employed without further purification. 2.2 Preparation of apparatus 10-13) A dripping tip, glass pipette and reservoir and temperature controller were equipped for the apparatus of interfacial tension measurement by drop weight method. 2.3 Distribution study An organic solution was prepared by diluting each extractant to analytical grade chloroform to a desired concentration. An aqueous solution was prepared by dissolving silver nitrate to a desired concentration into aqueous HNO 3. Equal volumes (3 cm 3 ) of both phases were mixed and vigorously shaken at 303 K for desired time, which was sufficient to attain equilibrium. After phase separation, concentration of silver ion was measured by ICP-AES (Shimadzu, ICPS-8100). The amount of extracted Ag was calculated according to the following equation (1); %E C C i e 100 (1), C i where, C i and C e represent initial and equilibrium metal concentrations in the aqueous phase (M), respectively. 2. Measurement of interfacial tension by drop volume method Drop volume measurement as a modified drop weight measurement was carried out by slowly adding 10 drops of chloroform phase containing extractant into aqueous phase to determine the average volume per drop. Extractant in chloroform to a desired concentration was added in the glass pipette. In the reservoir added 25 cm 3 of a desired concentration of nitric acid solution. To maintain temperature of nitric acid solution constant, the apparatus was equipped in a thermostatted water-bath. Interfacial tension γ was determined by using the following equation (2). r V g 2 r f (2), V 1 3 where V, Δρ, g, r, and f are volume of drop [m 3 ], density difference between aqueous and organic phases [kg/m 3 ], gravity [m/s 2 ], glass edge radius [m], and correction factor [-]. 3. Results and Discussion 3.1 Silver extraction rate Time dependency of silver extraction with three derivatives was investigated. Effects of shaking time on extraction percentage of silver ion with 2-PyTOC, MKTOC and PKTOC are shown in Figs 2(a)-(c), respectively. Silver extraction equilibrium with 2-PyTOC was reached within 30 min in both 0.1 M and 1.0 M nitric acid. The result of Fig. 2(a) indicated that silver extraction rate with 2-PyTOC was remarkably fast at both nitric acid concentrations. Thus, the extremely fast extraction rate may be attributed to the interfacial activity. The effect of shaking time on extraction percentage of silver ion with MKTOC in 0.1 M nitric acid was already investigated by the batch-wise method, 26) and the data in 1 M nitric acid was added as shown in Fig.2(b). The silver extraction percentage was gradually increased with increasing shaking time in both nitric acid concentrations. It took for 72 h to reach the extraction equilibrium. It indicated that silver extraction rate of MKTOC was extremely slow and extraction rate was not drastically changed with increasing nitric acid concentration. As 137
Vol.25, No. (201) shown in Fig. 3(c), it took for more than 96 h to reach the extraction equilibrium with PKTOC in 0.1 M nitric acid, whereas silver extraction equilibrium was reached within 12 h in 1.0 M nitric acid. It was found that nitric acid concentration significantly affected silver extraction rate for PKTOC. Fig. 2 Effect of shaking time on extraction percentage of silver ion with (a)2-pytoc, (b)mktoc, and (c)pktoc. [Extractant] = 5 mm, [Ag + ] = 0.1 mm, : 0.1 M HNO 3, : 1.0 M HNO 3. 3.2 Interfacial tension study Then, interfacial tensions (γ) of three derivatives at chloroform / nitric acid interface were investigated. Effect of extractant concentration on interfacial tension of three calix[]arene derivatives at chloroform / nitric acid interface are shown in Figs. 3(a)-(c), respectively. The interfacial tensions for three derivatives were decreased with increasing extractant concentration at both nitric acid concetrations. The interfacial tension in 1.0 M nitric Fig. 3 Effect of extractant concentration on interfacial tension of (a)2-pytoc, (b)mktoc, and (c)pktoc at chloroform / nitric acid interface. [Extractant] = 5 mm, [Ag + ] = 0.1 mm, : 0.1 M HNO 3, : 1.0 M HNO 3. acid for all derivatives were lower than those in 0.1 M nitric acid. It indicated that the extractants in 1.0 M nitric acid were interfacially more active than those in 0.1 M nitric acid. That is, the interfacial tension of the extractants in 1.0 M nitric acid was relieved more quickly than that in 0.1 M nitric acid. However, the values of interfacial tension were significantly dependent on the structure and the nature of the extractant in Figs. 3(a)-(c). The interfacial tension of 2-PyTOC was much lower than those of the other ketonic derivatives. It indicated that 2-PyTOC was interfaciallay more active than the other extractants in both nitric acid concentrations. Interfacial tension of 2-PyTOC would be relieved quickly due to protonizable pyridyl functionality. 138
ION EXCHANGE Significant difference of interfacial tension values between 0.1 and 1.0 M nitric acid concentration was not observed for MKTOC as shown in Fig. 3(b). It meant that MKTOC was not interfacially active in nitrate media due to neutral ketonic functionality. It would cause slow extraction rate of MKTOC. As seen from Fig.3(c), the interfacial tension in 1.0 M nitric acid was similar to that in 0.1 M nitric acid in the extractant concentration from 10-8 to 10-3 M, while it was remarkably lower at the extractant concentration higher than 5 mm compared with that in 0.1 M HNO 3. It indicated that PKTOC in 1.0 M HNO 3 was interfacially more active than that in 0.1 M HNO 3. Although the extractant concentration of the present work condition was not completely consistent with that at the suddenly falling point of the interfacial tension, the interfacial activity may be one of some factors to slow silver extraction rate for PKTOC. From the comparison of the interfacial tensions for MKTOC and PKTOC, it was found that slight difference was observed. It was notable that a slight structure difference with or without the extended aromatic ring caused such a significant difference in the interfacial activity in high acidic condition. It also may lead to the different silver extraction behavior between MKTOC and PKTOC. 3.3 The average area per adsorbed extractant molecule (A) The average area per the adsorbed extractant molecule (A) is related to the interfacial excess (Г) which is the difference between the extractant concentration at the interface and that in the bulk phase. The value of the interfacial excess, Г is obtained from the Gibbs s adsorption equation (3), 1 d (3), 2.303RT d log a where R is the gas constant, T is the absolute temperature, and a is the activity of the extractant. Г can be related to A shown by eq.(), A 1 N A (), where N A is the Avogadro number. Since Г was evaluated under the conditions (just before aggregation) where extractants concentration in the chloroform phase was typically in the vicinity of 10-3 F, the systems were assumed to be ideal, and the activity was replaced by concentration (C) as shown in Eq. (5), Table 1 Interfacial excess and the average area per adsorbed extractants at chloroform /nitric acid interface. The concentration of extractant is expressed in terms of formality (F). The obtained values are summarized in Table 1. From a thermodynamic point of view, it was likely that MKTOC adsorbed at the chloroform / nitric acid interface rather than PKTOC, although both extractants have the similar ketonic functionality. Consequently, MKTOC was less interfacially activity than that of PKTOC. It is attributed to the high electron density of the phenyl group as a side chain of PKTOC. This behavior is expected the reduction in the repulsive forces between the head groups with the addition of silver ion.. Conclusions Silver extraction rate for 2-pyridyl, methyl ketonic and phenyl ketonic derivatives of p-t-octylcalix[]arene was investigated together with their interfacial tension in chloroform / nitric acid media. Silver extraction rate for 2-pyridyl derivative is much faster than those for ketonic derivatives. It is attributed to the different structure, that is, the protonizable pyridyl functionality of 2-pyridyl derivative. The slight difference was also observed in the extraction rate and the interfacial tension between two ketonic derivatives. Even slight, the structural difference affects both of the extraction rate and the interfacial tension. For further understanding, the diffusion rate of chemical species will be investigated whether it will be related to the extraction rate or not in this system. Acknowledgement This work was financially supported by The Environment Research and Technology Development Fund, No. 3K- 123022, from the Ministry of the Environment, Government of Japan. 1 2.303 RT d (5), d log C References 1) Y. Marcus, J. Rydberg, M. Cox, C. Musikas, R. G. Choppin, Solvent extraction principles and practice, Marcel Dekker Inc., New York, (1992). 139
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