670
Dyrkacz et al.
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
Extraction of Pertechnetate with Tri(alkyl)methylammonium Nitrates. Kinetics and Mechanism in the System o-Xylene-Nitric Acid Gary R. Dyrkacz,* G. F. Vandegrlft, M. W. Thomsen, and E. P. Horwitz Chemistry Division, Argonne National Laboratory, Argonne, Iiiinois 60439 (Received July 27, 1978; Revised Manuscript Received December 13, 1978) Publication costs assisted by Argonne National Laboratory
The extraction kinetics of pertechnetate (Tc04-)in the system trioctylmethylammonium nitrate (TOMAN) or tridodecylmethylammonium nitrate (TDMAN) in o-xylenelaqueous nitric acid has been investigated. Procedures for the synthesis and purification of the TOMAN and TDMAN were developed. The kinetics were performed by stirring the two phases while maintaining a quiescent and constant area interface. Interfacial mass transfer coefficients were measured as a function of concentration of the quaternary ammonium salt, nitric acid, lithium nitrate, and stirring rates. The results of the kinetic experiments show that mass transfer across the interface is rate limiting. Aggregation data were obtained for TOMAN in o-xylene at 25 "C. An extraction mechanism is proposed and discussed in terms of aggregation. Both quaternary ammonium salts follow the same kinetic behavior.
Introduction Quaternary ammonium salts have been used for many years as extractants in liquid-liquid extraction of metal ions and complexes1s2and more recently have received attention as phase transfer catalysts for various reaction^.^ However, the extraction kinetics of quaternary ammonium salts have not been investigated except indirectly as phase transfer catalysts. Recently, some extraction kinetics of tertiary amines have been r e p ~ r t e d . ~ In conjunction with previous work from our group on the extraction kinetics with a phosphorus based extractant5 and on the use of quaternary ammonium salts for nuclear waste pro~essing,~ we decided to expand detailed kinetic studies to the quaternary ammonium nitrate salts. Two tetraalkylammonium salts have been widely used in extraction studies: a commercial preparation of tricaprylmethylammonium chloride (Aliquat-336 chloride) which is a mixture of C8-CI2 chains with C8 and Clo predominating, and tridodecylmethylammonium nitrate, which is available as a relatively pure material (Fluka). In our study we decided that Aliquat-336 chloride was unsatisfactory for our purpose, because of its mixed composition and, therefore, we synthesized and purified trioctylmethylammonium nitrate (TOMAN). In addition, we synthesized and purified tridodecylmethylammonium nitrate (TDMAN) to determine the effect of alkyl chain length. We chose to study the extraction of pertechnetate (TcOJ because of its stability in nitric acid and its simple oneto-one stoichiometry with quaternary ammonium salts.2 The extraction of pertechnetate by quaternary ammonium nitrates may be described by the equation: - Tc04- + ANOB + ATc04 + NO3(1) where the bar represents a species in the organic phase and A = RB(CH3)N.
Experimental Section Reagents. All acid solutions were prepared using U1trex-grade acids (J. T. Baker, Phillipsburg, N.J.) and ultra-pure water obtained from a Milli-Q2 system water purifier (Millipore Corp., Bedford, Mass.). All organic solvents were distilled-in-glass grade (Burdick and Jackson, Muskegon, Mich.). The technetium-99 was obtained from 0022-3654/Z9/2083-0670$0 1.OO/O
ANL stock and purified by crystallization of its ammonium salt. Synthesis and Purification of T O M A N and T D M A N . Trioctylmethylammonium chloride (TOMAC1) was synthesized by allowing purified trioctylamine (TOA) to react with methyl chloride. The tertiary amine was purified by recrystallization of its hydrochloride salt from 10% benzene in hexane followed by distillation of the free TOA under reduced pressure (1mm) at 154-156 "C. To prepare TOMAN, 45.6 g (0.13 mol) of TOA, 2.3 g of sodium bicarbonate, 15 mL of 2-propanol, and 9.7 g (0.18 mol) of methyl chloride were allowed to react in a Parr minireactor (300 mL capacity) at 100 OC. The mixture was stirred for 2 h and then allowed to cool. The resultant straw-yellow liquid was then filtered to remove excess NaHC03. The TOMACl was separated from any TOA by extracting a cyclohexane solution (200 mL) of the mixture with an equal volume of a 1:l water-acetonitrile solution. The TOMACl reports primarily to the acetonitrile-H20 phase whereas the tertiary amine remains primarily in the cyclohexane phase. Acetonitrile-H20 was removed from the TOMACl by evaporation under reduced pressure. The resultant residue was a sticky viscous liquid weighing 44.5 g. This represents a yield of 85% assuming that the residue is all TOMAC1. The crude TOMACl was converted to the nitrate salt by contacting a benzene solution with 1 M "0,. Final purification of the TOMAN was performed by recrystallization from 300 mL of 7 % ethanol in ether a t 5 -40 "C (dry ice-acetone). The TOMAN appears to crystallize as an ethanol solvate which dissolves in the ethanol of solvation on warming above -40 "C forming a viscous solution. The crystalline precipitate was separated from supernatant liquid by filtration at
