Liquid-Liquid Extraction of Zinc with Aliquat 336-S-1 from Aqueous Iodide Solutions Curtis W. McDonald' and Thornton Rhodes Department ot Chemistry, Southern University, Baton Rouge, l a . 708 13
Cadmium has been firmly established as a highly toxic metal (I). Extensive research is currently under way to develop new and better analytical methods for the detection and determination of this toxic material. There has also been considerable interest recently in developing pollution abatement processes to remove cadmium from industrial waste before it is allowed to enter the environment. In nature, this toxic metal is always found in trace quantities in zinc minerals and ores. Any industrial process for the removal of cadmium must take into account the presence of zinc which will usually be in large excess with respect to cadmium. Liquid-liquid extractions using high-molecular-weight amines show great promise for processes for the rapid removal of toxic metal ions from aqueous solutions. The high-molecular-weight quaternary amine, Aliquat 336% has been recommended for the removal of mercuric and cadmium ions from industrial waste water (2-4). Aliquat 336-S is especially attractive in that it extracts mercuric and cadmium ions from both acidic and alkaline solutions. In this communication, we have investigated the extraction behavior of zinc from aqueous iodide solutions with Aliquat 33643. We compared our results with similar investigations with mercury and cadmium to ascertain any periodic trends in the zinc family. In addition, a method was developed for separating cadmium from zinc which shows promise for industrial applications.
EXPERIMENTAL Apparatus. A NaI(T1) well type gamma scintillation counter, 1.75 X 2.0 inches, was used for gamma counting. A Sargent heavy duty electrolytic analyzer was used for the electogravimetric analyses. A Sargent NX digital pH meter was used to make the pH measurements. Reagents. Aliquat 336-S (impure tricaprylammonium chloride) is a quaternary amine chloride available from General Mills, Inc., Kankakee, Ill. A 30% Aliquat 336-S-C1 stock solution was prepared by dilution with reagent grade xylene. The amine chloride was converted to the iodide by the method described by McDonald and Moore (4). Cadmium-109 and zinc-65 tracers were obtained from New England Nuclear Corporation, Boston 18, Mass. All other chemicals were reagent grade. Evaluation Procedure. Five milliliters of the indicated aqueous phase containing approximately 1 x 1V gamma counts per minute of zinc-65 were extracted at room temperature with an equal volume of Aliquat 336-S-I-xylene in 50-ml heavy-duty glass centrifuge tubes for five minutes. High speed motor stirrers, equipped with glass paddles, were used to carry out the extractions. After extraction, the tubes were centrifuged in a clinical centrifuge for two minutes. Each phase was then analyzed for zinc-65 by counting 1-ml aliquots in culture tubes with a welltype gamma scintillation counter. To whom correspondence should be addressed. Present address, Department of Chemistry, Texas Southern University, Houston, Texas 77004. ( 1 ) "Our Chemical Environment." J. Giddings and M . Monroe, Ed., Canfield Press, San Francisco, Calif., 1972. p 158. ( 2 ) F . L. Moore, EnVirOfl. Sci. Techno/.,6, 525 (1972). (3) F . L. Moore, Separ. Sci., 7 , 505 (1972). (4) C . W. McDonald and F. L. Moore, Anal. Chern., 45,983 (1973).
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RESULTS Aliquat 3363-1 and its salt with the iodo complex of zinc are essentially insoluble in aqueous solutions but show very high solubility in most common organic solvents. The pertinent variables of the zinc-Aliquat 33643-1 extraction system were investigated by use of the evaluation procedure previously described. Each solution studied contained 1 mg/ml zinc as zinc iodide unless otherwise specified. A minimum of approximately 4% Aliquat 336-S-I-xylene is required for quantitative removal of the zinc in an equal volume of a solution which is 1 mg/ml zinc 1M in HI. There was no measurable extraction observed when attampts were made to extract zinc from aqueous iodide with pure xylene or xylene which had been treated with lM HI. A 5% Aliquat 336-S-I-xylene solution was used in the later investigations. The extraction of zinc with 5% Aliquat 336-S-I-xylene as a function of HI concentration (Table I) shows that a relatively high HI concentration (approximately 0.8M) is required to remove the zinc essentially quantitatively from aqueous iodide solutions. The effect of p H on the extractability of zinc is closely related to the effect of HI concentration. A solution at pH 1.0, containing approximately 10- 2M iodide added with the carrier and as HI in adjusting the pH, is 64% extracted with Aliquat 336-S-I-xylene. At a pH of 2.0, the per cent extracted is reduced to 27. If, on the other hand, the aqueous solution is made 1.OM in iodide with KI, the per cent extracted is increased to 98.3 for the pH 1.0 solution and 97.5 for the pH 2.0 solution. At a pH 2 4 , precipitation was observed in the solution containing excess iodide. Equilibrium is achieved fairly rapidly; a mixing period of about two minutes proved to be adequate for essentially quantitative extraction. Five-minute extraction periods were used in the evaluation procedure. Several aqueous reagents (Table 11) were evaluated to determine their ability to strip zinc from 5% Aliquat 336S-I-xylene solutions. The organic phase, initially containing a mg/ml zinc as the iodo zinc complex salt of Aliquat 336-S-I was stripped by extracting for five minutes with equal volume portions of the various strippants. All the strippants investigated stripped the zinc substantially with the majority removing more than 99% of the element from the organic phase. Those strippants removing more than 99% of the zinc include 0.5M Na2S, NaZS03 (>0.3M), NaOH (10.05M), NH3 (20.05M), EDA (10.2%),and EDTA (20.05M). SEPARATION OF ZINC FROM CADMIUM Since cadmium always occurs in trace quantities with zinc in nature and neither cadmium or zinc can be extracted selectively with Aliquat 336-S-I-xylene solutions when the two elements occur together in aqueous solution, a rapid, simple, quantitative method for separating the two is of utmost importance. Further studies of the Aliquat 336-S-I-xylene system led to a method of separating
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Table II. Stripping of Zinc from Aliquat 336-S-I-Xylene Solutions
Table I. Extraction of Zinc as a Function of HI Concentrationa HI concentration, M Zinc extracted, % 0.01 17.0 0.02 28.9 0.10 60.0 0.20 78.8 0.40 92.4 0.60 96.8 0.82 >99.5 1.64 >99.5 a Initial aqueous solution contained 1 mg/ml Aliquat 336-S-I-xylene.
Strippant
NaZS, M Na2S03, M
NaOH, M
zinc: solvent, 5%
these elements based on the selective stripping of zinc from the organic phase with an aqueous solution of Na2S03. In a typical experiment, two solutions were used. Each solution contained 0.5 mg/ml cadmium carrier, 0.5 mg/ml zinc carrier and was 1.OM in HI. The first solution contained zinc-65 tracer (5 x lo5 gamma counts per minute) and no radioactive cadmium, whereas the second solution contained cadmium-109 tracer and no radioactive zinc. Ten milliliters of each solution were extracted for 5 minutes with equal volumes of 5% Aliquat 336-S-I-xylene. Both cadmium and zinc extracted quantitatively. The two organic phases from these extractions were stripped for 5 minutes with equal volumes of 1M Na2S03 and each phase was measured for its gamma activity. The organic phase of the solution originating from the solution containing the zinc-65 had no gamma activity and the aqueous phase had the predicted gamma activity. In the second solution, there was no measurable activity in the aqueous phase and the predicted number of counts in the organic phase was evident. The cadmium was stripped from the organic phase with concentrated ammonia ( 4 ) . The carrier cadmium and zinc were determined from the two aqueous solutions by electrogravimetry (5). Better than 99% separation was achieved.
DISCUSSION The proposed mechanism for the extraction of zinc from aqueous iodide solutions with Aliquat 3363-1 is the same as that proposed for mercury (3) and cadmium ( 4 ) . This investigation with zinc completes studies of the elements in the IIB family. Definite periodic trends have emerged. The least difficulty is encountered in extracting mercury, in that this element can be quantitatively removed from aqueous solutions that are highly acidic to solutions at a pH of 12.5. Cadmium can be extracted from acidic solutions, but for solutions with a pH >7 excess iodide is required for quantitative extraction. Zinc can be quantitatively extracted only from fairly acidic solutions, even in the presence of excess iodide. Zinc requires a longer mixing time for quantitative separation than does mercury or cadmium. Very large aque(5) "Handbook of Analytical Chemistry," L. Meites, Ed., McGraw-Hill, New York, N.Y., 1963, pp 5-174and 5-181.
"3,
M
EDA, %
EDTA, M
Zinc stripped, %
0.1 0.5 1 .o 0.1 0.5 1 .o 0.025 0.05 0.50 0.025 0.05 0.50 0.1
0.2 2.5 0.05 0.1 0
63.0 94.7 >99.0 75.1 >99.5 >99.5 88.6 >99.5 >99.5 90.2 >99.5 >99.5 96.1 >99.5 >99.5 >99.5 >99.5
ous phase to organic phase ratios can be used for extracting mercury and cadmium, whereas small ratios are required for zinc. The reverse trend was observed for the stripping studies. Zinc is easily stripped from the organic phase with a wide variety of aqueous solutions; a smaller number can strip cadmium and only alkaline cysteine, of all the solutions tested, stripped mercury. The results indicate that the complex salt formed by the interaction of the tetraiodomercurate(I1) anion with Aliquat 336-S-I was more stable than the salts formed with the iodo complexes of cadmium and zinc. The order of stability of the complex salts is Hg > Cd > Zn. It is noteworthy to point out that the stability of the salts is in the same order as that of the iodo complexes-ie., Hg142> CdId2- > Zn142-. The method of separating cadmium from zinc is rapid, simple, quantitative, and uses very inexpensive chemicals. It therefore shows some promise for large scale industrial use. It can also be used by the analytical chemist to make this separation on the laboratory scale. The Aliquat 336-S-I-zinc extraction system is also of particular value to the analytical chemist since it offers a rapid, simple method for concentrating zinc from aqueous solutions containing only trace quantities of the element. The organic phase containing zinc can be used directly in atomic absorption determinations in which the analyst can benefit from both the solvent concentration and enhanced sensitivity of zinc.
ACKNOWLEDGMENT The authors wish to thank their colleagues J. H. Jefferson and Celestine Tillman for. helpful discussions and technical assistance. Received for review June 25, 1973. Accepted August 20, 1973. Work supported by the United States Atomic Energy Commission, Contract Number AT-(40-1)-1452.
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