Znd. Eng. Chem. Res. 1988,27, 1681-1685
1681
Kinetics of the Coupled Transport of Uranium(V1) across Supported Liquid Membranes Containing Bis(2-ethylhexy1)phosphoric Acid as a Mobile Carrier Ching-Tsven Huangt and Ting-Chia Huang* Department of Chemical Engineering, National Cheng Rung University, Tainan, Taiwan 70101, R.O.C,
Kinetics of the coupled transport of uranium(V1) from nitric acid solutions containing 1.52 X to 1.8 X mol/dm3 of uranium(V1) through a supported liquid membrane (SLM) containing 0.0025-0.025 mol/dm3 of dimeric bis(2-ethylhexy1)phosphoricacid in kerosene as a mobile carrier was investigated. Rate equations describing the transport of uranium(V1) were derived taking into account the resistances of aqueous film diffusion, chemical reaction, and membrane phase diffusion. The relative resistance of each transfer step was evaluated. The transport rate of uranium(V1) was found to be cptrolled by chemical reaction and aqueous film diffusion, both of which occur a t the source-side interface, and is in agreement with that estimated by the derived rate equation including a membrane factor. Liquid membrane extraction (LME) is a novel technique for the selective separation and concentration of the solute of interest from dilute aqueous solutions. A solid-supported liquid membrane (SLM), which uses a porous polymer membrane soaked with complexing liquid to separate source solution and receiver solution, represents one of the feasible types of LME. The solute of interest is extracted from the source solution by the mobile carrier in the membrane liquid, with a very high concentration distribution at the source side of the membrane phase. The species thus formed permeates through the membrane phase and is then stripped by the receiver solution. Uranium transports through SLMs have been discussed elsewhere. For example, Moskvin et al. (1976) used an SLM containing tributyl phosphate (TBP) to separate uranium from fission products; Matsuoka et al. (1980) used TBP entrapped in a cellulose triacetate membrane to demonstrate uphill transport of uranium; Huang and Huang (1986) investigated the transport mechanism of uranyl nitrate across an SLM containing TBP as a mobile carrier and found that the rate of transport is controlled by the source-side aqueous film and the membrane phase diffusions. The mass-transfer resistance of the aqueous film diffusion was also determined in their study. Other extractants of uranium, such as Alamine 336 (Babcock et al., 1980a-c) and LIX 63 (Akiba and Kanno, 1983), were also used as mobile carriers of SLM's in the transport of uranium. Being the commercially powerful extractant of uranium, bis(2-ethylhexy1)phosphoricacid (HDEHP) has been often used as the mobile carrier of the SLM for transporting uranium. Sifniades et al. (1981) used an SLM containing the kerosene-diluted HDEHP/TOPO (trioctylphosphine oxide) mixture as a membrane liquid to recover uranium from phosphoric acid solutions, and Akiba et al. (1984) used the kerosene-diluted HDEHP/l-octanol mixture to investigate the transport of uranium from sulfate media of low acidity to sulfuric acid solutions of high acidity. Due to the lack of available information on the kinetic behavior of the interfacial chemical reaction, as indicated by Danesi et al. (1981),most of the authors assumed that the interfacial reaction is fast. This assumption may result in considerable error since that HDEHP usually exhibits a slow to moderately fast interfacial reaction with metal ions.
* To whom correspondence should be addressed. +Currentaddress: Institute of Nuclear Energy Research, P.O. Box 3-22, Lung-Tan, Taiwan 32500, R.O.C. 0S88-5885/88/2627-1681$01.50/0
In this study, the mechanism and kinetics of the transport of uranium(V1) through SLM's containing the kerosene-diluted HDEHP as the membrane liquid were described. Rate equations were derived taking into account the resistances of aqueous film diffusion, chemical reaction, and membrane phase diffusion. The permeability of the uranium(V1)-HDEHP complex in the membrane phase was evaluated. Transport fluxes of uranium(V1) through the SLM were measured with varying concentrations of uranium(V1)and nitric acid in source solutions and of HDEHP in membrane liquids. The relative resistance of each transfer step was evaluated with the kinetic information obtained in our previous study (Huang and Huanng, 1988), and the rate-controlling step was determined. This study enables us to predict the transport rate of uranium(V1) through SLM's containing HDEHP as a mobile carrier with sufficient precision. Theory Chemical Reaction between Uranium(V1) and HDEHP. The reaction between HDEHP and uranium(VI) in nitric acid solutions at low acidity, e.g., [H'] < 1 mol/dm3, can be represented as follows (Huang and Huang, 1987):
- -
+
UOz2++ HzRz = U02R2 2H+
-UOzR2
(1)
+ HZRZ = UO,R,(HR),
(2) where the solid upper bar denotes the species in the organic phase. Uranyl cations react with HDEHP species at the interface to form UO2R2,which is solvated by a dimeric HDEHP in the bulk organic phase, provided that free HDEHP is available. The overall reaction given by the above equations is
UO;+
+2 m = U02R2(HR)2+ 2H+
(3)
According to the law of mass action, the distribution ratio (O/A) of uranium(V1) is d, = [UO&(HR)zI / [U022+l=
&[=I2/
W+I2
(4)
Equation 4shows that the value of d, is directly proportional to [ H2R2I2and inversely proportional to [H+l2. Thus, when HDEHP is used as a mobile carrier in the transport of uranium(VI), the "pumping effect" becomes greater as the acidity of the source solution decreases. The stripping of uranium(V1) from HDEHP solutions can be successfully achieved with sodium carbonate solu0 1988 American Chemical Society
1682 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 I
I
(Huang and Huang, 1986); hence, eq 8 is simplified to
[H2R21t,IIs 1
Thus, under steady state, Le., J = J, = Jm= N , the transport flux of uranium(V1) is
J = (3.19 X 104G[U022+]f[~2]t,Is1/2/[H+]Iwo~1)/
+--
aqueous trim' L 1iqueous film Figure 1. Schematic representation of the transport mechanism.
tions; however, the sodium salt of HDEHP formed in the stripping exhibits low solubility in kerosene. Concentrated phosphoric acid solution has strong stripping power too; it was used as the receiver solution in this study. Transport Mechanism. The mechanism of the transport of uranium(V1) in the present study can be schematically shown in Figure 1. The transport steps are as follows: (1)uranyl cations in the bulk source solution diffuse through the stagnant aqueous film to the sourceside interface (interface I), (2) uranyl cations are extracted by HDEHP at interface I, (3) uranium(V1) complexes formed at interface I permeate through the membrane phase to the receiver-side interface (interface 11), and (4) uranyl cations are stripped by the receiver solution at interface 11. The aqueous film diffusion flux of uranium(VI) in step 1 is Jw
= kw([U022+lf - [U022+l~w)
Depending on the relative values of the resistances, eq 10 can be simplified for three limiting cases: (i) aqueous film diffusion control
J = k,[U022+]f
(11)
(ii) membrane diffusion control
3.19
J=
X
10-6MP[H,R2]t,~s1/2[ U022']f L[H+IIw2.'
(12)
(iii) chemical reaction control
J = 3.19
X
10+G[~]t,Is1J2[U022+]f/[H+]Iwo.1 (13)
Measurements of the Uranium(V1)-HDEHP Complex Permeability. The permeability of the uranium(VI)-HDEHP complex in the membrane phase is evaluated with Fick's first law:
(5)
In the ordinary diaphragm cell method for measuring permeability, both the source and the receiver solut_ions are the same phase as the membrane liquid, and AC, is taken as the difference of the uranium(V1) concentration between the source and the receiver solutions, ignoring the effect of the liquid-solid boundary layers, since the thickness of the layer is negligible compared with that of the membrane. In the present study, the method is modified by using an aqueous stripping solution as the N (mol/(cm2.s)) = receiver solution, such that [ U02R2]t,11sis negligible, and (33.19 X lo4[ U O , ~ + ] I , [t,Is1/2/ ~~] the operation is more stable because of interfacial tension. [H+II,'.' - [ ~ ~ , R ~ I ~ , I ~ [ H ' I I W(6) ~ / M I Neglecting the liquid-solid boundary layer effect again, we have [ U02Rz = [ UO2R2If,and eq 14 becomes where
where subscript w represents locations on the water side, k , is the mass-transfer coefficient of uranyl cations in the aqueous film, and [U022+]f and [UO?+]Iware the concentrations of uranium(V1) in the bulk source solution and the aqueous side of interface I, respectively. According to our previous study (Huang and Huang, 1988), the formation rate of the uranium(V1)-HDEHP complex in step 2 is
M = 3.55 X 1O6[H+]IW + 3.15
X
1 0 7 [ m ] t , ~ s ' / 2( 7 )
where the subscript s represents locations on the organic solvent side, [ UOzR2],is the total concentration of the uranium(V1) complex, [ HzR2Itis the total concentration of the H2R2species involving the free one and that solvated with U02R2,the subscripts Iw and Is represent the aqueous side and the organic side of interface I, respectively, and G is a correction factor which takes care of the dissimilarity of the transfer behaviors occurring in the SLM and that in the kinetic study. The permeation flux of uranium(V1) in step 3 is where P is the permeability of the uranium(V1) complex in the membrane phase, L is the thickness of the membrane, and the subscript 11s denotes the organic side of interface 11. When an effective stripping solution is used as the receiver solution, the value of [ U02R2]t,IIs is negligible compared with that of [ U02R2]t,Is,and the mass-transfer resistance of the receiver-side interface is also negligible
Remember that the source solution is in no case an organic phase, except in the measurement of the permeability of the uranium(V1)-HDEHP complex.
Experimental Section Apparatus and Reagents. The apparatus used in this study is similar to a diaphragm cell as shown in Figure 2. The source chamber (A) and the receiver chamber (B) are agitated with a magnetic stirrer and a dc motor, respectively. The stirrer (h) in chamber A is a Teflon-coated rodlike stirrer that is 0.75 cm in diameter and 2 cm in length. The stirrer (i) in chamber B is a blade that is 0.75 cm wide and 2 cm long. The volume of chamber A is 120 mL, and chamber B can be freely chosen. The membrane (e) is fixed with a Teflon support (0 and a Teflon gasket (g). The porous membranes used are Durapore membranes, a product of Millipore Co., made of poly(vinylidene fluoride) with a mean pore size of 0.45 pm, a typical porosity of 70%, and a nominal mean thickness of 125 pm. The thickness of the membranes was measured before use, and only those with a thickness deviation of
