Research Note pubs.acs.org/IECR
High Precision Droplet-Based Microfluidic Determination of Americium(III) and Lanthanide(III) Solvent Extraction Separation Kinetics C. A. Launiere and A. V. Gelis* Nuclear Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States S Supporting Information *
ABSTRACT: A new method for studying solvent extraction kinetics has been applied for measuring americium and lanthanide extraction rate constants. This droplet-based microfluidic method uses commercially available components and provides rapid, high throughput and accurate determination of absolute interfacial mass transfer rate constants. Reported for the first time are americium extraction rates relevant to TALSPEAK-type process conditions, including the americium and lanthanide rate dependencies on pH and extractant power.
T
Detailed reviews of the classical solvent extraction kinetic methods (such as the highly stirred tank (HST) and Lewis cell (LC)) are available in the literature,3 while recent developments include the laminar flow Y-channel microfluidic chip (Ychip)4 and the work by J. P. Simonin et al. on the rotating membrane cell (RMC).5 These kinetic methods have several disadvantages such as (1) slow mixing in one or both phases, which results in the formation of a diffusion zone near the interface, and consequently affects the values of the rate constants (e.g., LC, Y-chip, RDC, and RMC); (2) small interfacial areas (e.g., LC, Y-chip, and moving drop method); (3) unknown interfacial areas (e.g., HST); (4) ability to analyze only one or a few nuclides (e.g., short time phase contact method and LC); (5) sophisticated data treatment (e.g., RDC and RMC); and (6) large liquid volumes, resulting in generation of significant amounts of waste (e.g., moving drop, HST). In this work, a microscale liquid−liquid extraction system was fabricated from commercially available components and was used to derive kinetic information. This type of microsystem is becoming increasingly popular in research and industrial applications because of advantages such as large interfacial areas, rapid mixing, precise temperature control, and reduced risks with hazardous materials. Since a seminal study in the authors’ laboratory,6 which used a custom fabricated microsystem to perform kinetics measurements, multiple
o close the nuclear fuel cycle, it is necessary to chemically treat used nuclear fuel (UNF) to separate its constituents. The different recovered fractions can be recycled into fresh nuclear fuel, transmuted in a reactor, or processed into waste forms for disposal in a geologic repository. The work described here aims to address some of the technical challenges in implementing a closed nuclear fuel cycle by using a microtechnique to measure interfacial mass transfer rate constants. The work also has implications in the field of microanalytical separations and in the lanthanide separations industry. Two key challenges in advanced fuel cycle development are (1) the accurate modeling of processes and (2) cost reduction in fission product and transuranic element separations. Separation costs can be lowered through the design of improved separation schemes, and through improved process modeling, which can increase process efficiency and material recovery. The method of choice for industrial scale separations for both UNF and lanthanides from natural sources is solvent extraction in combination with chelation chemistries.1 Active research programs in several countries are engaged in efforts to improve existing solvent extraction schemes and to develop new schemes.1 One bottleneck in the design and implementation of nextgeneration solvent extraction schemes is a lack of kinetic data. Because many relevant extraction schemes are somewhat slow2 (kinetically limited), solvent extraction kinetics data are needed for modeling the nonequilibrium processes in the latest generation of phase contact equipmentcentrifugal contactors. Kinetics data can also provide insight into the mechanisms in complex solvent extraction chemistries, which will aid in the design of new extraction chemistries. © 2016 American Chemical Society
Received: Revised: Accepted: Published: 2272
December 9, 2015 February 8, 2016 February 10, 2016 February 10, 2016 DOI: 10.1021/acs.iecr.5b04691 Ind. Eng. Chem. Res. 2016, 55, 2272−2276
Research Note
Industrial & Engineering Chemistry Research
Figure 3. Observed rates for Am back extraction from spiked ndodecane organic phase into aqueous phase with 10 mM DTPA, 0.6 M ammonium malonate, pH 2.78, and T = 22.5 ± 0.7 °C. Constant specific interfacial areas were observed in these tests.
Figure 1. Microfluidic solvent extraction system.
phases traverse a channel, often a length of capillary tubing. An in-line phase separator is used to achieve high temporal resolution in the contact times by providing rapid phase separation at well-defined time points. To determine rate constants using a microsystem, the interfacial area available for mass transfer must be measured. A slug flow regime was used in these studies to ensure sufficient internal mixing in both phases. In slug flow, the droplets (slugs) span the entire width of the channel, except for a thin film of the carrier phase (generally the organic phase), which completely wets the channel wall. The surface area of this film may be excluded in the total interfacial area measurement in cases where the mass transfer occurring at this film is insignificant compared to mass transfer occurring at the caps of the slugs. Because the film is very thin and somewhat stagnant, the film can quickly become saturated or depleted of analyte, and as a result contribute little to the overall mass transfer.12,13 The contribution of the thin film to the overall mass transfer is strongly dependent on the parameters of flow rate and tubing diameter, because these parameters determine the length of the slugs and the thickness of the thin film. The thickness of the film is difficult to determine with photographic methods; however, computational modeling in combination with optical methods have resulted in a number of empirical equations to estimate film thickness in two-phase slug flow. Film thickness has been shown to increase with increasing channel diameter and with increasing flow rate.12 If the thin-film is sufficiently thin (as is the case with small tubing diameters and low flow rates) and the length of the slug is not many times larger than the channel diameter (to maintain a small thin film area relative to the slug cap area), mass transfer at the film will be negligible. Some good examples of these phenomena are reported by Ghaini et al., who used a chemical method to determine the effective specific interfacial area available for mass transfer in microchannel liquid−liquid slug flow.14 The method, which uses extraction accompanied by a fast pseudo-first-order reaction, is also commonly used to estimate specific interfacial areas in bulk scale extractors. The measured effective specific interfacial area was compared to the specific interfacial areas determined photographically. Photographically determined interfacial areas included either the slug caps only or included both the slug caps and the cylindrical area of the slug that is in
Figure 2. pH (pCH) dependence of (a) aqueous to organic and (b) organic to aqueous interfacial mass transfer rate constants for the 15 lanthanides and americium. Error bars represent the 95% confidence interval of the linear regression.
studies have been carried out on microscale liquid−liquid extractions relevant to UNF reprocessing.7−11 However, none of these groups reported absolute interfacial mass transfer rate constants. In the experiments reported herein, absolute interfacial mass transfer rate constants were determined for trivalent lanthanides and Am in several UNF separation schemes using a droplet microfluidic extraction system. Microscale extraction systems rely on pressure driven flow and generally consist of a flow junction, extraction channel, and phase separator (Figure 1). The flow junction combines the aqueous and organic phases into well-defined flow patterns, generally slug flow. Extraction is carried out as the combined 2273
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Research Note
Industrial & Engineering Chemistry Research Table 1. Americium and Promethium Interfacial Mass Transfer Rate Data series
organic phase
kao (mm/s)
aqueous phase
1 2 3 4 5 6 7 8 9 10 11
0.75 M BEPA 0.5 M HEH[EHP] 0.75 M HEH[EHP] 1.0 M HEH[EHP] 0.75 M HDEHP 0.75 M HEH[EHP] 0.75 M HEH[EHP] 0.75 M HEH[EHP] 0.75 M HDEHP 0.75 M HDEHP 0.75 M HDEHP
10 mM DTPA 10 mM DTPA 10 mM DTPA 10 mM DTPA 10 mM DTPA 200 mM HEDTA 25 mM DTPA 50 mM DTPA 200 mM HEDTA 25 mM DTPA 50 mM DTPA
8.0 1.9 1.4 1.5 3.8 6.1 5.7 3.4 1.9 1.8 1.2
1 2 3 4 5 6 7 8 9 10 11
0.75 M BEPA 0.5 M HEH[EHP] 0.75 M HEH[EHP] 1.0 M HEH[EHP] 0.75 M HDEHP 0.75 M HEH[EHP] 0.75 M HEH[EHP] 0.75 M HEH[EHP] 0.75 M HDEHP 0.75 M HDEHP 0.75 M HDEHP
10 mM DTPA 10 mM DTPA 10 mM DTPA 10 mM DTPA 10 mM DTPA 200 mM HEDTA 25 mM DTPA 50 mM DTPA 200 mM HEDTA 25 mM DTPA 50 mM DTPA
4.9 1.3 7.7 1.1 1.9 2.9 4.5 2.5 2.7 9.2 5.7
Americium × 10−6 ± 6.0 × × 10−4 ± 8.3 × × 10−4 ± 1.9 × × 10−4 ± 2.4 × × 10−6 ± 3.4 × × 10−4 ± 1.1 × × 10−5 ± 1.2 × × 10−5 ± 4.1 × × 10−3 ± 1.2 × × 10−4 ± 1.2 × × 10−4 ± 1.6 × Promethium × 10−6 ± 3.2 × × 10−3 ± 6.7 × × 10−4 ± 1.1 × × 10−3 ± 1.8 × × 10−4 ± 1.6 × × 10−3 ± 7.6 × × 10−4 ± 8.8 × × 10−4 ± 7.1 × × 10−3 ± 5.6 × × 10−4 ± 1.8 × × 10−4 ± 1.5 ×
koa (mm/s)
kobs (1/s)
t1/2 (s)
10−7 10−6 10−6 10−6 10−7 10−5 10−6 10−7 10−5 10−6 10−6
6.7 6.7 2.1 1.2 3.1 2.9 2.7 2.9 1.9 3.0 5.2
× × × × × × × × × × ×
10−3 10−4 10−3 10−3 10−5 10−3 10−3 10−3 10−4 10−4 10−4
± ± ± ± ± ± ± ± ± ± ±
5.0 3.0 2.7 1.9 2.8 5.4 5.5 3.5 1.2 2.1 6.6
× × × × × × × × × × ×
10−4 10−4 10−5 10−5 10−6 10−5 10−5 10−5 10−6 10−6 10−6
7.6 3.8 2.4 1.4 3.3 4.5 3.8 3.9 2.9 6.6 8.6
× × × × × × × × × × ×
10−2 10−2 10−2 10−2 10−4 10−2 10−2 10−2 10−2 10−3 10−3
9.11 18.4 29.3 48.7 2080 15.4 18.3 17.8 24.2 106 80.7
10−6 10−5 10−5 10−5 10−5 10−5 10−6 10−7 10−5 10−5 10−5
8.3 2.1 3.8 2.7 4.4 4.8 5.0 5.3 2.0 5.2 7.8
× × × × × × × × × × ×
10−1 10−3 10−4 10−4 10−5 10−4 10−4 10−4 10−5 10−5 10−5
± ± ± ± ± ± ± ± ± ± ±
4.1 5.9 4.0 3.6 3.1 6.5 4.2 2.3 4.3 1.7 1.3
× × × × × × × × × × ×
10−4 10−5 10−6 10−6 10−6 10−5 10−6 10−7 10−5 10−5 10−5
9.4 1.8 1.2 1.5 2.2 4.3 1.3 1.1 3.7 1.3 8.6
× × × × × × × × × × ×
10° 10−2 10−2 10−2 10−4 10−2 10−2 10−2 10−2 10−2 10−3
9.63 53.1 61.4 63.8 320 16.0 52.4 65.5 19.0 52.0 81.0
†
Experiment Series 1−5: Back extraction from loaded dodecane organic phase into aqueous phase with 10 mM DTPA, 0.6 M ammonium malonate, pH 2.78, and T = 22.5 ± 0.7 °C. Experiment Series 6−11: Back extraction from loaded dodecane organic phase into 1 M Na/H lactate buffered aqueous phase at pCH 3.6 ± 0.05 and T = 22.0 ± 0.5 °C. kobs = A/V (kao+ koa). t1/2 is the half time of the overall mass transfer reaction. Error represents the 95% confidence interval on the linear regression.
contact with the thin film. In all of the experiments the measured effective interfacial area was less than the photographically determined interfacial area that included both the slug caps and the thin film. This indicates that, even in larger diameter tubing at high flow rates, the participation of the thin film in mass transfer is limited. Ghaini reported that, in 1 mm ID PTFE capillaries, the measured effective specific interfacial area (interfacial area divided by droplet volume) was greater than the cap-only specific interfacial area only for slug velocities greater than approximately 20 mm/s. An effective specific interfacial area (estimated using the chemical method) greater than the caponly specific interfacial area (measured photographically) indicates that the thin-film is engaged in non-negligible mass transfer. In 750 and 500 μm ID capillaries, slug velocities greater than 40 and 80 mm/s, respectively, were required to observe a contribution to mass transfer from the thin film (as determined based on the effective specific interfacial area measurement). As another example, Darekar et al.15 studied uranium extraction from a 3 M nitric acid aqueous phase into 30% TBP in dodecane in capillary slug flow for PTFE capillaries with diameters between 300 and 800 μm and flow velocities around 50 mm/s. Their results suggest that mass transfer occurs predominantly at the slug caps under these conditions. These two studies demonstrate how mass transfer in the film is negligible in smaller capillaries at low flow rates, but must be considered for larger diameter capillaries and high flow rates. For the industrial application of microscale extraction systems, high throughputs are desired and so channel diameters smaller than 500 μm are usually not feasible. In these larger-
diameter systems, it is essential to understand the effects of channel size and the role of the wall film in mass transfer (for a thorough discussion of channel size effect on mass transfer in liquid−liquid slug flow in capillaries, refer to Tsaoulidis and Angeli16). However, for microextractor research that is focused on elucidating intrinsic kinetic parameters, high throughput is not a concern and channel diameters should be as small as possible to minimize thin film contributions to mass transfer. The resulting low throughputs will be beneficial for expensive or hazardous material research. Using small diameter tubing will simplify interfacial area determination, as only mass transfer in the slug caps needs to be considered. The feasible lower limit on channel diameter is determined by system kinetics, pumping power and resolution, channel entrance effects, and phase separator properties. The internal volume of the extraction channel should be large enough that mass transfer during slug formation and in the phase separator are negligible. To ensure negligible thin film contributions to mass transfer in this study, 250 μm i.d. FEP capillaries were used as extraction channels, and flow velocities were in the tens of millimeters per second range. Slug generation was achieved using a commercially available t-junction droplet generator (Droplet Junction Chip, 190 μm etch depth, Dolomite Microfluidics) and syringe pumps (Mitos Duo XS Syringe Pump, Dolomite Microfluidics). It is important to match as close as possible the diameters of the droplet generator channel and the tubing in order to accomplish uniform slug formation. Rapid and complete phase separation was achieved using a commercially available phase separator (part no. 3000135, Dolomite Microfluidics). This phase separator contains a hydrophobic membrane which allows the organic phase to pass through 2274
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are given in the SI. The two key differences between the two microsystems are that the original microsystem was custom fabricated (versus fabrication from commercially available components for the current system) and its associated protocols required the assumption of a constant specific interfacial area across trials (versus measuring the specific interfacial area for each trial with the current system). TALSPEAK (trivalent actinide−lanthanide separation by phosphorus reagent extraction from aqueous komplexes) is one of the earliest and most successful minor actinide/ lanthanide separation schemes. However, it is known to suffer from slow kinetics,2 and so further investigation is required to understand the kinetics and underlying mechanisms in order to improve TALSPEAK extraction systems. The mechanism of TALSPEAK-type extraction systems has been studied in detail by Kolařiḱ et al.,19 and Danesi and Cianetti,20 although somewhat contradictory conclusions were drawn. Using an HST, Kolařiḱ concluded that Ln−DTPA decomplexation is the rate limiting step, and the Ln−lactate complex is the species that delivers Ln to the interface. In contrast, a formation of ternary Ln−DTPA−Lact was suggested by Danesi (who used a constant interfacial area slowly stirred cell), thus proposing a different extraction mechanism. No pH rate dependencies or Am rate constants were previously reported.2,19,20 The study of the extraction mechanism in such a complex system is outside the scope of this initial study. However, here we provide several examples of extraction kinetics data that were unavailable to date and that provide a starting point for elucidating the extraction mechanisms for these systems. The new microsystem was used to characterize the kinetics of Am and lanthanide back-extraction under conditions that are relevant to citrate-based TALSPEAK,21 malonate-based advanced TALSPEAK,22 and ALSEP23 (actinide lanthanide separation process) processes. The microsystem was also used to characterize the kinetics of forward extraction in TALSPEAK-type systems, and those results are reported in the SI. Of particular interest was the pH-dependence of lanthanide and Am stripping into citrate-buffered DTPA aqueous phase from spiked HEH[EHP] organic phase (0.75 M 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester in dodecane). To study this, four different DTPA aqueous phases (25 mM DTPA, 0.25 M sodium citrate, 1 M Na+, balanced with sodium nitrate) were prepared with pH (calibrated for hydrogen concentration scale) values ranging from 2 to 4.3. Figure 2 shows the measured interfacial mass transfer rate constants for the 15 lanthanides and 241Am at 21.3 ± 1.7 °C. The measured forward rate constants show the pH dependence expected in the kinetic regime, decreasing with increasing pH. The rate constants also show dependence on ionic radius. Lanthanum has the largest ionic radius and also the fastest forward and backward extraction rate constants. Americium, however, demonstrates different behavior. Its aqueous to organic rates are generally slower than those for the Ln in the tested pH range. In contrast, the organic to aqueous mass transfer rate constants depend on the pH. At pH ≤ 2.5, the koa of Am is close to the La value, but at pH > 2.5, where DTPA is deprotonated and the Ln/Am separation occurs through the selective complexation of Am over the Ln2, americium has about the same rate constant as neodymium and praseodymium, which have very similar ionic radii as americium.
upon the application of a negative pressure (applied using another syringe pump), while capillary forces prevent the aqueous phase from exiting through the membrane pores. Before starting experiments, the tubing was flushed with dilute nitric acid solution and then filled with organic phase. Flow rates were set to achieve desired contact times and were chosen to be within a range to generate sufficient internal mixing within each phase so that extraction occurred in the kinetic regime. Operation in the kinetic regime is ideal for simplifying calculations because in this regime the effects of diffusion rates are negligible (refer to Nichols et al.6 for additional details on mixing rates and operation in the kinetic regime). The organic waste pump was started first, followed by the aqueous and organic supply pumps. The pumps were run for three residence times (residence time equals the combined internal volume of the extraction tubing and phase separator divided by the flow rate), after which aqueous outflow was collected for off-line analysis. Flow rate ratios of 1:1 were used for all flow studies. Different phase contact times were achieved by varying the flow rate and the channel length. Off-line analysis of the aqueous outflow was used to determine the extent of interfacial transfer at each contact time. 241Am and 147Pm (or 152Eu) radioactivity were measured by liquid scintillation counting using alpha/beta discrimination, while stable lanthanide (Ln) concentrations were determined by ICP−MS. Equilibrium points were acquired using test tube experiments. To determine the forward and backward interfacial mass transfer rate constants of the overall partitioning Maq↔Morg, the data were fit to a pseudo-first-order mass transfer rate equation derived from the mass balance:17,18 dCaq dt
=
⎛A⎞ ⎜ ⎟(k C − kaoCaq) ⎝ V ⎠ oa org
(1)
where the flow rate ratio is 1:1, Caq and Corg are the concentrations or activities of the metal ions, M, in the aqueous and organic phases, respectively, A is the interfacial area of a droplet (including the caps of the slug only and not the area in contact with the thin film), V is the volume of an aqueous slug, and kao and koa are the forward and backward interfacial mass transfer rate constants, respectively. At equilibrium, the distribution ratio KD = kao/koa = Corg.eq/Caq.eq. Integrating eq 1 gives the equation: ⎛ ⎛ Caq ⎞⎞ ⎛ ⎞ ⎟⎟⎟ = − A ⎜1 + 1 ⎟kaot ln⎜⎜1 − ⎜⎜ ⎟ V⎝ Kd ⎠ ⎝ Caq.eq ⎠⎠ ⎝
(2)
The specific interfacial area (A/V) was measured photographically for each trial as described in the Supporting Information (SI). A linear regression of a plot of A/V·t versus ln(1 − (Caq/Caq.eq)) for data points collected at various flow rates (i.e., contact times) has a slope of kao*(1 + (1/KD)), from which the forward and backward rate constants can be calculated (SI Figure 2). A significant deviation from linearity in this plot could be an indication that the thin film contribution to mass transfer is not negligible for the given flow conditions. In this case the flow conditions should be adjusted as discussed in the SI. To validate the new microsystem used in this study, tests were conducted to confirm that it produced results that were consistent with results obtained by traditional methods and by a previous iteration of the microsystem. Details of those studies 2275
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A series of investigations was also carried out to look at the effect of extractant variants on americium and lanthanide extraction. In these studies a malonate buffered DTPA aqueous phase (0.6 M ammonium malonate, 10 mM DTPA, pH 2.78) was contacted with spiked (241Am and 147Pm) n-dodecane phases containing phosphinic acid (bis(2-ethylhexyl)phosphinic acid, BEPA, supplied by S. Friese at Salisbury University), phosphonic acid (HEH[HEP]), or phosphoric acid (HDEHP) extractants. The results from the test tube equilibrium experiments (equilibrium D-values and separation factors) can be found in the SI, and the results from the microfluidic kinetics experiments are given in Figure 3 and Table 1 (experiment series 1−5). The data indicate that the observed rates are inversely proportional to the extractant power as the extraction power among the three extractants, at 0.75 M, decreases with phosphorus oxidation state from HDEHP to BEPA (see SI Figure 1). The same effect is observed as the concentration of HEH[EHP] decreases from 1 to 0.5 M. Finally, the microfluidic system was used to directly compare HEH[EHP] and HDEHP extractants with either HEDTA (N(hydroxyethyl)-ethylenediaminetriacetic acid) or DTPA as aqueous phase complexing reagents in a sodium lactate buffer. As these two complexing agents have completely different stability constants with the studied cations,19,20 their concentrations were chosen to provide sufficient Am stripping efficiency for the process conditions.21−23 The results of these studies are reported in Table 1 (experiment series 6−11). The data show that HEDTA has higher observed rates of Am back-extraction than does DTPA, particularly for HDEHP. In summary, a microfluidic system and associated protocols were developed and used to measure the kinetics of metal ion extraction under conditions that are in development for the separation of streams of UNF. The system, which is composed of commercially available components, can be used to determine absolute interfacial mass transfer rate constants for a range of elements extracted from a mixture while surface area, mixing rate, and other parameters are under total control. Furthermore, the high specific interfacial area and strong mixing in this system enable rapid rates of reaction, which is valuable for measuring true rate constants that are not obscured by mixing or diffusion rates. Experiments were completed to investigate the effects of pH, extractant power, and solution composition on the kinetics of Am and Ln extraction. This is the first report of lanthanide and americium mass transfer rate constant dependencies on pH and extractant power. The results highlight the importance of studying the kinetics of potential reprocessing chemistries as the formulations that perform best under equilibrium conditions do not perform best on the shorter time-scales of practical operating conditions.
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Research Note
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The submitted manuscript was created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC0206CH11357.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04691. Measurement of specific interfacial area; microsystem validation; equilibrium data; mass transfer rate constant determination; forward extraction rate constant data (PDF) 2276
DOI: 10.1021/acs.iecr.5b04691 Ind. Eng. Chem. Res. 2016, 55, 2272−2276
