Uptake of Palladium Using Liquid Emulsion Membrane Technique

The waste stream generated at the reprocessing facility (first cycle raffinate-Hi level waste) contains Palladium as one of the fission products. The recovery of Palladium from the waste stream reduces the activity burden and lends its use for catalytic applications. This paper deals with its recovery from the waste stream generated at various stages of nuclear fuel cycle using liquid emulsion membrane technique. Complexants chosen were based on the chelating ability with metal ions with high stability constant in the waste stream. Complexants 8-hydroxy quinoline, Thio salicylic acid, dithiazone, cupferron, 1-Niroso-2-naphthol, Quinalizarin were chosen as complexants. The effect of surfactant concentration, complexant concentration, agitating speed, and emulsifcation time on the emulsion stability have been studied. Treat ratio, the ratio of the emulsion phase to the external phase and the impeller speed on the removal of Palladium have been studied. Results show that stable emulsion could be prepared by using 1.5 ml of surfactant span 80 in 50 ml emulsion, with impeller speed 3000 RPM, for duration of 20 minutes using complexants of strength of 70-200 moles/m Effective recovery of Pd could be achieved by using strippant of pH 1-3.7 in the emulsion phase at 250-300 RPM of impeller speed.


Introduction
Conventional liquid extraction suffers from the drawbacks of being equilibrium limited and need for a separate stripping operation.Liquid emulsion technique has been recognized as promising technology as it overcomes these draw backs of the conventional liquid extraction.In general, the liquid emulsion membrane is prepared by forming an emulsion of two immiscible phases and then dispersing the emulsion into a third phase.The transport of dissolved material through the liquid membrane is termed as permeation.Since the liquid film offers much higher diffusivities and area exceeding 10,000 m 2 /m 3 , the fluxes of the liquid membranes are very high.Furthermore liquid membranes can be tailor made for specific applications.
LM technology can carry out extraction and stripping process simultaneously and have benefit of non equilibrium mass transfer and uphill effect where solute can be moved from low to high concentrated solution (Franken, 1997;Gu et al., 1994;Giaikwad, 2004).The main types of liquid membrane systems include Emulsion Liquid Membrane (ELM) (Li et al., 1997), Supported Liquid Membrane (SLM) (Bloch et al., 1967), Bulk Liquid Membrane (BLM), Flowing Liquid Membrane (FLM) (Teramoto et al., 1989), Supported Emulsion Liquid Membrane (SELM) (Fouad et al., 2008;Sonawane et al., 2007), Hollow Fibre Contained Liquid Membrane (HFCLM) (Gabelman et al., 1999) and Supported Liquid Membrane with Stripping Dispersion (SLMSD) (Basualto et al., 2003;Gu et al., 2006) etc.The potential advantages of LM techniques over traditional separation techniques and solid membranes techniques are lower capital and operational costs, lower energy and extractant consumption and higher concentration factors and fluxes.However LM techniques have not been adopted to large scale industrial processes yet (Zhang et al., 2000;Danesi et al., 1987;Gu et al., 2003;Lin et al., 2004;Neplenbroek et al., 1992;Bechiri et al., 2008;Zha et al., 1995) primarily due to lack of emulsification and deemulsification steps in ELM and SELM processes and membrane resistance in BLM and HFCLM processes etc. (Ren et al., 2007).

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Role
Since the the emulsi ml of wat emulsion s Figure 1a 2.      from 60 to 180 minutes from 1000 RPM to 3000 RPM and then it comes down from 180 minutes to 90 minutes from RPM of 3000 to 4000 indicating the optimum agitation time of 3000 RPM for the preparation of stable emulsion.

Complexant Strength
Figure 1d indicates that membrane with high concentration of complexant decreases the stability but with low concentration of complexant decreases the solute transport rate.Hence an optimum complexant concentration was taken in order to maintain stability and solute transport.From Figure .1dit is observed that with the complexants quinalizarin, Thiosalycylic acid, 1-nitroso-2-naphthol the stability of the emulsion was found to be very high at 80 moles/m 3 whereas with the complexants 8-hydroxyquinoline, dithiazone and cupferron the stability of the emulsion was found to be high at 70 moles/m 3 .By repeated trials, a good emulsion could be prepared which contains 6 % (v/V) surfactant, 70-80 moles/m 3 extractant prepared at 3000 rpm at mixing time of 20 minutes.

Effect of pH of the Strippant Solution
From Figure 2, we observe that using dithiazone when the strippant PH was between 1 to 3, the % uptake was 64.5-65.6 whereas from pH 3 to 4.6, the % uptake was 65.6-55.1.Using quinalizarin when the strippant PH was between 1 to 2.5, the % uptake was 54.8-56.9whereas from pH 2.5 to 4.6 the % uptake was 56.9 to 41.Using 8-hydroxy quinoline, when the strippant PH was between 1 to 2.5, the % uptake was 49.3 to 51.6 whereas from pH 2.5 to 4.6, the % uptake was 51.6 to 36.Using 1-nitrso-2-naphthol when the strippant PH was between 1 to 1.5, the % uptake was 46.7 to 47.8 whereas from pH 1.5 to 4.6 the % uptake was 47.8 to 27.8.Using cupferron when the strippant PH was between 1 to 1.5, the % uptake was 38.9 to 44.5 whereas from pH 1.5 to 4.6.the % uptake was 44.5 to 23.3.Using Thiosalycylic acid, when the strippant PH was between 1 to 1.5, the % uptake was 19.8-20.6 whereas from pH 1.5 to 4.6 the % uptake was 20.6-11.8.
Hence the optimum pH of the strippant included in the emulsion for the various complexants are as follows: For dithiazone the pH of the strippant is optimized as 3, for quinalizarin and 8-hydroxy quinoline the pH of the strippant is optimized as is 2.5, for 1-nitroso-2-naphthol, cupferron and thiosalycylic acid the pH of the stripping solutuion was 1.5.

Kinetics of Removal of Palladium
From the Figure 3 on the kinetic studies of removal of Palladium using emulsion technique, we observe that with dithiazone, % removal ranges 34.7-65.6,with quinalizarin it ranges 22.8 to 56.9 with 8-hydroxy quinoline it ranges 21.9 to 51.6, with 1-nitroso-2-naphthol it ranges 19.3 to 44.5 and with thiosalycylic acid it ranges 8.9 to 20.6 in a period of 15 to 60 minutes.

Effect of Strength of Complexant in the Emulsion
From Figure 4, we observe the % removal of Pd from 0.09 mM solution using emulsion containing different complexants of strength 0.07 to 0.2 moles /L at treat ratio of 1:1 at optimum pH of the strippant as function of moles of complexants.It is seen that the % removal of Pd was maximum at 0.15 moles/L for dithiazone 8-hydroxy quionoline and 0.18 moles /L for quinalizarin, 1-nitroso-2-naphthol, cupferron and thiosalycylic acid.

Effect of Treat Ratio
The treat ratio defined as the ratio of the volume of emulsion phase to the aqueous phase containing the metal ion to be recovered plays a dominant role in the uptake of metal since more emulsion phase provides more complexant for the metal to permeate the membrane phase and hence get recovered.It is observed from Figure 5, that as the treat ratio is increased from 1:1 to 1:3 the % recovery increased from 69.6 to 89.1, 60.1 to 81.4, 55.7 to 80.1, 45.9 to 76.7, 42.5 to 73.4, 21.3 to 67.4 for dithiazone, quinalizarin, 8-hydroxy quinoline, 1-nitroso-2-naphthol, cupferron, and thiosalycylic acid due to larger availiblility of the complexant.With further increase in the membrane phase, the % recovery was not improved.Since large interfacial area between external and emulsion phase is required which gets reduced due to the formation of a dense miscelle interfacial layer at the membrane phase which resists solute transport.Hence the treat ratio for the maximum recovery of Pd was fixed at 1:3 in all the cases.

Effect of Agitation Speed
For permeation of the metal ion and subsequent reaction with the complexant present in the emulsion, sufficient agitation of the mixture is needed.If the agitation time and agitation speed is more than the optimum level there is likelihood of the breakage of the membrane and leakage of the metal ion back to the solution phase.It is observed from this Figure 6 that as the speed is increased from 200 to 400 rpm the % solute recovered from the external phase changed.For dithiazone it increased from 65.4 to 69.6 at RPM 200 to 250 and then it dropped to 48.6 at 350 RPM.For Quinalizarin it increased from 55.9 to 60.1 at RPM 200 to 250 and then it dropped to 40.1 at 350 RPM.For 8-hydroxy quinoline it increased from 51.1 to 55.7 at RPM 200 to 250 and then it dropped to 33.3 at 350 RPM.For 1-nitroso-2-naphthol it increased from 42.9 to 45.9 at RPM 200 to 250 and then it dropped to 31.86 at 350 RPM.For cupferron it increased from 39.8 to 42.5 at RPM 200 to 250 and then it dropped to 22.9 at 350 RPM.For Thiosalicylic acid it increased from 19.8 to 21.3 at RPM 200 to 250 and then it dropped to 17.3 at 350 RPM, at a specified treat ratio (1:3) using complexnat at the optimum strength The agitation speed of 250 rpm is found to be the most effective to recover Pd 2+ from external.The drop in % of metal ion recovery beyond 300 RPM is attributed due to either de emulsification induced by higher shear of impeller or due to leakage from internal stripping phase.

Effect of Feed Concentration
As seen from Figure 7, we observe that as the feed strength is increased from 0.08 to 0.1437 mM, the % removal of Pd was found to decrease from 91.2 to 66, 82 to 65.1, 81.3 to 62, 77.6 to 60.2, 74.5 to 59.3, 68.2 to 55.4 using dithiazone, quinalizarin, 8-hydroxy quinoline, 1-nitroso-2naphthol, cupferron and thiosalycylic acid respectively.
In all the cases the order of removal of Pd follows the sequence dithiazone>quinalizarin>8-hydroxy quinoline>1-nitroso-2-naphthol>cupferron>thiosalycylic acid It is observed that increase in feed phase concentration decreases the % solute recovered.This confirms that this technique is more effective to treat dilute streams.

Conclusions
The complexants chosen have Nitrogen, Oxygen and Sulphur for enabling complexation with Palladium.When the complexants are emulsified using span 80 as surfactant the emulsion formed could allow permeation of Palladium by providing good surface area /volume and hence the recovery of Pd was found to be effective.The order of uptake of Palladium by different complexants follow the sequence: dithiazone>quinalizarin>8-hydroxy quinoline>1-nitroso-2-naphthol>cupferron>thiosalycylic acid

Figure
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