1、11 Acidic Surfactant Solutions for Tributylphosphate Removal 2 in Nuclear Fuel Reprocessing Plants: A Formulation Study345 J. Causse*, S. Faure67 Advanced Decontamination Processes Laboratory (LPAD), 8 CEA, DEN, Marcoule, BP 17171 - 30207 Bagnols-sur-Cze Cedex France91011 *Corresponding Author. Tel:
2、 +33(0)466 397 421, jeremy.caussecea.fr 21 Abstract2 The removal of tributylphosphate (TBP), an organic solvent widely used as a complexing 3 agent for uranium and plutonium in nuclear plants, was investigated to understand and adapt 4 the mechanisms involved in TBP detachment and solubilization in
3、acidic surfactant solutions. 5 Two well-known degreasing mechanisms, roll-up and emulsification, should be combined for 6 maximum effect. These mechanisms were characterized with a CCD camera to measure 7 contact angles between a solid substrate and a liquid drop. We measured the contact angles of 8
4、 a TBP drop deposited on a stainless steel plate immersed in an acidic surfactant solution, and 9 quantified the amount of TBP solubilized in the micelles by turbidity measurements. 10 Preliminary results of micelle size characterization by dynamic light scattering are presented. 11 We formulated ne
5、w acidic surfactant solutions associating two industrial surfactants, Pluronic 12 P123 and Rewopal X1207L, with improvement factors in various fields (total organic matter 13 content, oil detachment and solubilization efficiency, emulsion stability, etc.).14151617 Keywords: nuclear decontamination;
6、tributylphosphate; surfactant; solubilization; interfacial 18 tension; roll-up; emulsification; degreasing kinetics31 1. Introduction2 In the coming decades the nuclear industry will have to meet the major challenge of 3 dismantling the first-generation nuclear plants. This article focuses on nuclea
7、r fuel 4 reprocessing plants. The contamination possibly present in these plants is caused by an 5 organic solvent tributylphosphate (TBP), used in the Purex process as a uranium and 6 plutonium complexing agent. TBP exhibits high affinity for metal surfaces, and thus adheres 7 to every stainless st
8、eel device, such as the vessels used in the Purex process in the UP2-400 8 plant at La Hague (France).9 In this case, decontamination consists in rinsing the installations to remove the radioactive 10 elements with the effluent. The rinsing solutions should contain surfactants, molecules 11 operatin
9、g at solid/liquid interfaces and able to detach the radioactive TBP from stainless steel 12 surfaces. Surfactant solutions used in nuclear applications are subject to several limitations. 13 As these solutions become nuclear waste, they are subject to special requirements. For 14 example, the total
10、organic concentration (surfactants) is limited to 1 wt%. Moreover, 15 counterions such as Na+, Cl- or Br- are not suitable for the nuclear waste conditioning in a 16 glass matrix. This led us to choose nonionic surfactants as active molecules; otherwise, 17 surfactant solutions must be prepared in a
11、cidic media to avoid plutonium and uranium 18 hydroxide precipitation, which occurs when the pH becomes slightly basic. These 19 requirements are therefore directly driven by the nuclear application.20 The first studies concerning the use of acidic surfactant solutions as decontamination media 21 we
12、re carried out in 2004 1. The authors concluded that nitric acid was the best solution with 22 an optimum concentration of up to 0.5 mol/L. They also showed that the process was 23 practicable at industrial scale. Their solution was an association of two industrial surfactants, 24 Rewopal X1207L and
13、 Antarox FM33 with a total concentration of 1 wt% with a 41 RewopalX1207L / Antarox FM33 ratio of 4:1. This formulation was defined as the reference 2 solution (RS).3 This paper deals with understanding the mechanisms involved in degreasing phenomena and 4 the improvement brought by formulation scie
14、nce.5 2. Degreasing Phenomenon6 A simplified model describes degreasing of liquid contamination in two separate stages. 7 The oil is first detached from the surface, then the organic solvent molecules solubilize 8 throughout the micelles of surfactant solution. The actual process is much more compli
15、cated 9 with the simultaneous progress of these two steps 2. However, in this study each step in the 10 phenomenon was analyzed using different techniques to assess the effect of the solution 11 formulation on both stages.12 Figure 113 Oil detachment can follow different mechanisms. The two main one
16、s, roll-up and 14 emulsification, are well known and depend on the structure of the surfactant used in the 15 formulation. The first occurs when a surfactant with high wetting power is used (Figure 1), 16 and the second is indicative of a surfactant with high emulsifying power. The differences 17 ob
17、served between these mechanisms are attributable to the preferred location of surfactant 18 adsorption in the three-phase system. The equilibrium contact angle of an organic liquid 19 drop deposited on a substrate surface and immersed in an aqueous surfactant solution is given 20 by Youngs equation
18、(1).21 (1)woscs22 where ws, os and wo are the interfacial tensions between water and substrate, oil drop and 23 substrate, water and oil drop, respectively.51 The roll-up process requires surfactants with high wetting power. Such molecules 2 preferentially lower the water/substrate interfacial tensi
19、on ws, ensuring better wetting by the 3 aqueous solution and allowing the surfactant solution to spread over the solid surface. This 4 tends to reduce the contact line between the oil drop and the solid substrate. Consequently, the 5 oil drop dewets the surface and cos decreases, as shown by relatio
20、n (1). In this case, the 6 drop detaches spontaneously and no oil remains on the solid surface.7 Conversely, oil removal by emulsification requires a surfactant with high emulsifying power 8 that preferentially adsorbs on the oil/water interface 3. The combined effect of this 9 adsorption and oil bu
21、oyancy provokes a cohesive break into the drop, removing part of the 10 drop from the surface. In this case, a small amount of oily soil remains on the solid surface. 11 The removal, solubilization and transport of the soil in the surfactant solution is easier than in 12 the roll-up process, but deg
22、reasing is not complete.13 Once the oil has been removed from the solid support, it must be solubilized in the surfactant 14 solution to facilitate elimination. This second degreasing step requires the surfactant to be in 15 its self-associating form. These aggregate surfactant forms, or micelles, a
23、re capable of 16 sequestering organic molecules in aqueous media. The surfactant concentration must be 17 higher than the critical micelle concentration (cmc) for solubilization to occur. Emulsifying 18 surfactants are more efficient for this stage because of their adsorption at the oil/water 19 int
24、erface.20 The surfactant type therefore very significant affects the degreasing efficiency. The active 21 molecules must be chosen with regard to the nature of the oil and solid substrate. Surface 22 cleaning processes require both wetting and emulsifying power to obtain the best results 4. 23 Maxim
25、um degreasing efficiency is obtained with the association of high-wetting and high-24 emulsifying power surfactants. This paper summarizes the studies carried out to identify the 61 surfactants best capable of emulsifying TBP and of wetting stainless steel with the objective 2 of improving the refer
26、ence solution (RS) developed in the past without formulation studies.3 3. Materials and Methods4 Aqueous surfactant solutions were all prepared in 0.5 mol/L nitric acid. The water used for 5 sample dissolution was first deionized. Surfactant materials were industrial samples provided 6 by vendors: P
27、luronics P123 and P84 synthesized by BASF Corp., Antarox FM33 by 7 Rhdia, and Rewopal X1207L or Ifralan B1286 by Ifrachimie (ex-Witco). The trade name 8 of Rewopal X1207L has changed with the name of the company. They are all nonionic 9 polyoxyethylenated surfactants. The hydrophilic part of the mol
28、ecules consists of 10 polyethylene oxide (PEO) polymer chains, and the hydrophobic part comprises alkyl chains 11 for Rewopal X1207L, polypropylene oxide (PPO) chains for Pluronic type surfactants, and 12 both chains for Antarox FM33. Industrial surfactants must be used to ensure availability for 13
29、 large-scale operation.14 In order to make sure that all the stainless steel plates exhibited the same surface state, the 15 metal was corroded in an oxidizing solution (0.11 mol/L CeIV+; 3 mol/L HNO3) for 4 hours 16 and then abundantly rinsed with deionized water.17 3.1. Contact Angle Measurements1
30、8 Figure 219 Figure 2 shows a schematic description of the experimental setup (Digidrop by GBX) 20 allowing us to measure TBP detachment kinetics from a stainless steel plate. The classical 21 sessile drop method was used, with data acquisition by a goniometer to record the contact 22 angles. First,
31、 a TBP drop was deposited on the solid substrate with a microsyringe. The metal 23 plate was then immersed in a surfactant solution to measure the oil detachment kinetics and 24 plot the kinetic curves = f(t). At t = 0 s, the plate was immersed in the solution and = 0 71 because of the affinity of T
32、BP for stainless steel. The TBP/stainless steel contact angle 2 increased with time. Degreasing was considered “effective” when reached a minimum of 3 90.4 This technique was also used to classify the surfactants according to the degreasing 5 mechanism involved. A CCD camera was used to observe the
33、TBP drop shape during removal 6 so that simple image analysis can conclude whether a surfactant is high-wetting or high-7 emulsifying.8 3.2. TBP/“Aqueous Solution” Interfacial Tension9 The interfacial tension between TBP and aqueous surfactant solutions was determined using a 10 Krss K12 tensiometer
34、 equipped with a platinum-iridium Du Noy ring. TBP (14 mL) was 11 added to the surfactant solution (14 mL) 24 hours before the measurement. After 24 h under 12 steady-state conditions, the repeatability of the interfacial tension values indicated that 13 equilibrium had been reached.14 3.3. Turbidit
35、y Measurements15 Light scattering was used to quantify the total amount of TBP sequestered in the micelles and 16 to analyze the stability of the emulsions created when the TBP concentration was much 17 higher. When TBP molecules penetrated into surfactant micelles, the system remained 18 monophasic
36、, optically clear until a concentration limit was reached (C(TBP)MAX). At TBP 19 concentrations exceeding C(TBP)MAX, the system became diphasic, cloudy. From this 20 concentration, the dispersion was an emulsion. The dispersed objects were no longer 21 nanometric swollen micelles, but micrometric oi
37、l drops stabilized by surfactants. Whereas 22 solutions containing swollen micelles are stable, emulsions are unstable and subject to phase 23 separation. The time necessary for the system to reach phase separation depended on the oil 24 drop size and the type of surfactant.81 C(TBP)MAX could be ide
38、ntified by light scattering analysis, because light is fully scattered 2 throughout the solution in a cloudy system. A “spectrode” electrode emitted a beam of light 3 and analyzed the light scattering ratio in the solution as TBP was added in the vessel.4 3.4. Dynamic Light Scattering (DLS)5 This sc
39、attering technique was used to assess the size increase of micelles with different 6 amounts of TBP. DLS was performed at 298 K using a Malvern Zetasizer NanoZS 7 instrument, fitted with a 532 nm laser at a fixed scattering angle of 90. The surfactant 8 micellar solutions were adjusted with a suitab
40、le quantity of TBP and filtered through a 9 0.45 m cellulose membrane filter before analysis. An average micelle size distribution was 10 determined using the program CONTIN (Provencher) 5.11 4. Results and Discussion12 Table I13 Table I presents some physicochemical properties at 298 K and the stru
41、ctures of the 14 surfactants used in this study. The critical micellar concentration and the surface tension at 15 1 wt% and 0.1 wt% are listed for the four industrial surfactants and for the reference solution 16 (80/20 Rewopal X1207L / Antarox FM33 at 1 wt%). In this table, exceptionally, the 17 s
42、urfactants are considered in pure water without nitric acid so that comparison is possible with 18 prior studies, including numerous papers by Alexandridis concerning Pluronics 6-9. Each 19 subsequent solution contained 0.5 mol/L of nitric acid.20 4.1. Reference Solution (RS) Degreasing Performance
43、Evaluation21 Surface tension values provide information about the wetting power of aqueous surfactant 22 solutions. Generally, the more the surfactants decrease the surface tension, the more the 23 aqueous solutions will spread onto the solid surface. Table I shows that the two surfactants in 24 the
44、 reference solution are high-wetting power surfactants. Originally, Antarox FM33 was 91 chosen as a wetting surfactant and Rewopal X1207L as an emulsifying surfactant on the basis 2 of property descriptions on vendor datasheets.3 Figure 34 Figure 3-A shows the TBP degreasing kinetics obtained with t
45、he reference solution at various 5 concentrations. It appears that the long-term angle (eq) is directly related to the oil 6 detachment efficiency of each solution. The time necessary to reach equilibrium is quite short, 7 around 2000 seconds, and is independent of the solution concentration. The tw
46、o more 8 concentrated solutions exhibit better degreasing power with eq exceeding 90, up to 120 and 9 110, respectively, for the 1 wt% and 0.5 wt% solutions. These results are quite positive as 10 experimental conditions driven by an industrial approach are unfavorable. Several studies 11 have exami
47、ned oil detachment from solid surfaces in the past. The solid substrates considered 12 included silica 10, gold 11, or glass 12,13. For a stainless steel surface, the results are 13 different whether the surfactant used is ionic or nonionic. Anionic, zwitterionic and nonionic 14 surfactants proved t
48、o be more effective cleaners at high pH, whereas cationic surfactants are 15 most effective at low pH 14-16. At low pH, the stainless steel surface is positively charged, 16 and the most favorable case to optimize oil drop detachment is to provide an oil/water 17 interface presenting the same charge
49、, using cationic surfactants. Because nonionic 18 polyoxyethylenated surfactants like Triton X-100 14 maintain a slightly negative charge at 19 low pH, optimal oil detachment cannot be obtained with HNO3 concentrations up to 0.5 M.20 Figure 3-B shows the roll-up phenomenon observed with 1 wt% RS. The solution being 21 prepared with two wetting surfactants, “roll-up” is the expected degreasing mechanism. At 22 t = 1830 s, the TBP drop is almost totally detached with a contact angle equal to 120.3. The 23 drop does not detach spontan
