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Ind. Eng. Chem. Res. 2003, 42, 5900-5904
Selective Concentration of Uranium from Seawater by Nanofiltration Alain Favre-Reguillon, Gerard Lebuzit, Jacques Foos, and Alain Guy Conservatoire National des Arts et Me´ tiers, Laboratoires de Chimie Organique & des Sciences Nucle´ aires (CNRS UMR 7084), 292 rue St. Martin, 75003 Paris, France
Micheline Draye Ecole Nationale Supe´ rieure de Chimie de Paris, Laboratoire d’Electrochimie et de Chimie Analytique (CNRS UMR 7575), 11 rue P. et M. Curie, 75231 Paris Cedex 05, France
Marc Lemaire* Laboratoire de Catalyse et Synthe` se Organique, Universite´ Claude Bernard Lyon 1, CPE Lyon, (CNRS UMR 5181), 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France
A new procedure for the concentration of uranium, dissolved in seawater with extremely low concentration, was studied. Plate module membrane filtration equipment was operated to evaluate the performance and selectivity of four different nanofiltration flat sheet membranes. Experiments were first carried out using different model waters. The membranes were discriminating by the rejection of uranium, calcium, and sodium. Then, a uranium concentration test using a nanofiltration membrane showing the highest selectivity for uranium toward alkaline and alkaline-earth ions has been performed on natural seawater. A nanofiltration membrane shows a high selectivity for U(VI), illustrating the advantageous use of nanofiltration for the concentration of uranium from seawater. Introduction The oceans contain about 4.5 billion metric tons of dissolved uranium, almost a 1000-fold of the reasonably assured and estimated terrestrial uranium resources in the western world.1 The concentration of uranium in seawater appears to be remarkably constant at about 3.3 µg/L.1-3 Seawater is actually a very low grade uranium source,1 however, the advantage of the dissolved state and the almost inexhaustible quantities of uranium should be kept in mind. Furthermore, the extraction of the seawater uranium has no consequences on the environment. There are no mining residues, no radon emissions, and no collective radioactive exposure of the workers. The design of an effective process for the selective concentration of uranyl ion is connected with the economic importance of the selective concentration of uranium from seawater as well as adoption of environmental laws concerning, for example, the removal of uranyl ion from drinking water4. Potentially, extraction of uranium from seawater is the cleaner and more environmentally friendly source of uranium ore. Direct recovery of uranium from seawater has been investigated over the past 4 decades. Extensive efforts have been made in Japan since the early 1960s,2,5 and numerous procedures recommending organic6-8 or inorganic ion exchangers9 have been developed for the extraction of uranium from seawater. Despite significant improvements, the uranium adsorption rate is very low (0.9 mg of U/g of fiber for 20 days).7 * To whom correspondence should be addressed. Tel.: +33 (0)472448209. Fax: +33 (0)4724314 08. E-mail: marc.lemaire@ univ-lyon1.fr.
For both economical and ecological reasons, any process suitable for the concentration of uranium from seawater should be able to operate at the normal chemical composition of seawater. Membrane separation processes have been used for several years to concentrate or fractionate suspended particles and dissolved substances. Reverse osmosis, currently widespread used to produce drinking or irrigation water from briny waters or seawater,10 seriously challenges the distillation process, whereas ultrafiltration constitutes a valuable aid for the fractionation and concentration of colloidal substances contained in seawater.11 In regards to both the size of separated species and pressures involved, nanofiltration (NF), which has an intermediate position between the two previous techniques mentioned above, is a more recent development in the filtration field. The ability of NF membranes for wastewater treatment12 or for fractionation of mono- and multivalent cations13 leads the way to potential applications for the concentration of uranium from seawater without changing phases. The purpose of the present study is to assess the possibilities of NF in selectively concentrated uranium from seawater. In the first step, the ability of various membranes to retain uranium ion at high sodium chloride concentration is evaluated. Next, the membranes displaying the best capabilities are tested on simulated seawater containing sodium, calcium, and uranium. The goal of this first part was to select the most appropriate membrane for the fractionation of mono- and multivalent cations. Then, the membrane that shows the highest uranium retention coefficient and the highest uranium selectivity toward alkaline and alkaline-earth ions is used for the diafiltration of uranium from doped seawater and seawater.
10.1021/ie030157a CCC: $25.00 © 2003 American Chemical Society Published on Web 10/11/2003
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Figure 1. Schematic flow diagram of the laboratory-scale membrane system: 1, feed tank; 2, pH meter; 3, heat exchanger; 4, pump; 5, cell body; 6, membrane; 7, feed pressure gauge; 8, feed flow control valve; 9, retentate flux meter; 10, permeate flux meter; 11, permeate tank; 12 and 13, valves. Table 1. Ions Analyzed and the Detection Limit of the Ions in Seawater Determined by ICP-MS element
detection limit (ng/L)
Na Mg Ca K Sr
0.2 0.3 7 0.2 0.3
element
detection limit (ng/L)
Li Mo Ba U
0.2 2 0.3 0.5
Materials and Methods NF Pilot. Experiments have been performed with an Osmonics Sepa CF laboratory-scale membrane cell. A schematic flow sheet diagram of the laboratory-scale membrane system is presented Figure 1. The membrane cell consists of two elements (cell boby and cell holder). The feed stream is pumped from the feed tank to the cell body. A cross-flow velocity of 0.3 m/s was applied to minimize concentration polarization. The solution flows tangentially across the membrane surface. A heat exchanger permits all filtration experiments to be controlled at the constant temperature of 20 ( 1 °C. Hydraulic parameters are monitored using pressure gauges and flowmeters. For the characterization of the membrane retention coefficient, valve 12 is open and the feed is kept at constant composition during the experiments by totally recycling the permeate and the retentate. For the concentration of uranium, valve 13 is open and the permeate is collected in tank 11 whereas the retentate (or concentrate) is recirculated. Membranes. Five commercial polymeric NF membranes (Osmonics) were studied. Molecular weight cutoffs, as stated by the manufacturer, are 10 000 Da (G80), 8000 Da (G50), 3500 Da (G20), 2500 Da (G10), and 150-300 Da (5DL). Analytical Procedures. Uranium, sodium, and calcium determination in simulated seawater was performed with a simultaneous inductively coupled plasma spectrometer and Varian CCD Vista ICP-OES with a standard deviation of (2%. Analyte concentrations were determined with respect to calibration standards in dilute nitric acid. The absence of interferences was checked during the analysis. Seawater analysis has been performed using an inductively coupled plasma mass spectrometer (Varian, UltraMass700, ICP-MS). Analyte concentrations were determined using matrix-matched calibration. Table 1 summarizes the ions determined and their detection limits.
Figure 2. Distribution of uranium(VI) hydroxy and carbonate complexes as a function of the pH for [CO32-] ) 2 × 10-3 M, [NaCl] ) 11 g/L, and T ) 25 °C built with CHESS software.
Na, K, Ca, and Mg are run under “cool plasma” conditions (plasma power decreased to 0.6-0.7 kW and nebulization gas increased to 0.9-1.0 mL/min) in order to minimize argon polyatomics and eliminate isobaric interferences. The resolution was set at 0.8 amu at 5% of the peak. The pH was determined using an Advantic, Consort C832 pH meter. Sample Preparation. There is an extensive literature on equilibria involved in the uranium carbonate systems.14,15 Uranium(VI) carbonate systems are usually quite complicated in that they consist of several different complex ions in rapid equilibria with one another and with the aquo ion or hydrolyzed species. At a low uranyl concentration and when this concentration does not exceed the carbonate concentration, monomeric uranyl carbonate species UO2(CO3), UO2(CO3)22-, and UO2(CO3)34- are expected to dominate above pH 5. A speciation diagram was built with the aid of the CHESS software.16 Figure 2 shows that predicted species in a solution containing 1 × 10-5 M UO22+ and 2 × 10-3 M CO32- or HCO3- at pH 8.3 are then UO2(CO3)34- and UO2(CO3)22-. The neutral complex UO2(CO3) dominates the system at pH 5.5, but at a seawater pH of 8.3, UO2(CO3)22- and UO2(CO3)34are the major species. The more general feature with NF membranes is separation of salts according to their size and valency. Thus, a high retention coefficient of this bulky and highly charged U(VI) species can be expected. Synthetic seawater was prepared by introducing 1 × 10-5 mol/L uranyl nitrate, UO2(NO3)2, to distilled water containing 2 × 10-3 mol/L Na2CO3. The pH of the solution was then adjusted to 8.3 with diluted sodium hydroxide. To this solution, various concentrations of sodium chloride or calcium chloride are then added. Prefiltered (5 µm) seawater was supplied by the Institut National des Sciences et Techniques de la Mer, Cherbourg-Octeville Cedex, France. Uranium-doped seawater was prepared by adding uranyl nitrate (UO2(NO3)2) up to ICP-OES detection limits (i.e., U(VI) ) 2.38 mg/L). Evaluation of the Membrane. Retention coefficients (R) were determined according to eq 1 where
R (%) ) (1 - Cp/Cf) × 100
(1)
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Figure 4. Dependence of the observed uranium retention coefficient as a function of the sodium concentration in the feed. Conditions: T ) 20 °C, ∆P ) 3 bar, pH 8.3, [UO22+] ) 1 × 10-5 mol/L, [CO32- + HCO3] ) 2 × 10-3 mol/L, [NaCl] ) 0-11 g/L. Figure 3. Dependence of the volume water flux on the transmembrane pressure for NF membranes.
Cp and Cf are the concentrations in mol/L of metal ions in the permeate and feed. Results and Discussion Prior to the determination of transport-separation properties of the membrane, the dependence of the volumetric water flux on the transmembrane pressure was determined with deionized water. In the case of all five membranes, a pressure increase from 1 to 3 bar leads to a linear volumetric water flux increase. Figure 3 shows the volume water flux dependence on the transmembrane pressure for different NF membranes. The results on the transmembrane pressure influence on the volumetric water flux suggest that the G10 to 5DL membranes are the most compact membranes whereas the G80 and G50 membranes have the most open structure. The five membranes show anyway a linear variation of their water flux volume versus transmembrane pressure. Retention Coefficient of U(VI) with Simulated Seawater. Uranium occurs in seawater in its highest oxidation state (+6). Owing to the seawater carbonate content, uranium predominantly exists in this natural environment as tricarbonato uranylate anion [UO2(CO3)34-]. However, there is no experimental evidence for the presence of bulky ion in natural seawater due to its extremely low concentration. The structure of the tricarbonato uranylate anion [UO2(CO3)34-] has been studied.17 In the equatorial plane, the radius of the complex ion amounts to 4.85 Å; it is thus one of the largest inorganic ions existing in seawater.2 In a previous study, uranium carbonate complexes have been removed from drinking water by NF with rejection yields above 90% for the two major negatively charged uranyl carbonate complexes.4 Uranium(VI) carbonate solutions with a sodium chloride concentration ranging from 0 to 11 g/L were used as feed solutions; the membrane performance was measured in terms of the uranium retention coefficient. The measurements for all modules were performed in stable and identical conditions. The variation of the uranium rejection percentage as a function of the feed concentration for the different membranes is shown in Figure 4. In accordance with the Donnan exclusion theory, the uranium rejection percentage decreases with increasing NaCl concentrations for all of the five membranes.18 Predictably, the highest cutoff membranes exhibit the lowest solute rejection for the same concen-
Figure 5. Retention coefficient of sodium, calcium, and uranium as a function of the operating membrane. Experimental conditions: T ) 20 °C, ∆P ) 3 bar, pH 8.3, [UO22+] ) 1 × 10-5 mol/L, [CO32- + HCO3] ) 2 × 10-3 mol/L, [NaCl] ) 11 g/L, [CaCl2] ) 1 g/L.
tration. For the G50 membrane, uranium rejection starts decreasing at a 1 g/L NaCl concentration while G20 and G10 membrane retention coefficients are kept high. The uranium rejection of the 5DL membrane is kept almost constant. For a seawater sodium chloride concentration, three membranes (5DL, G10, and G20) have a U(VI) retention coefficient of greater than 70%. In view of their good performance, the 5DL, G10, and G20 membranes were then further evaluated. Retention Coefficient of Na, Ca, and U(VI) with Simulated Seawater. The selected membranes were then evaluated with simulated seawater containing, in addition to sodium and uranium salts, calcium chloride, and the results are shown in Figure 5. In the presence of 1 g/L calcium chloride, the U(VI) retention coefficient decreases to 50% for the G20 membrane. For G10 and 5DL membranes, no significant decrease of the U(VI) retention coefficient is noticed but U(VI)/Ca2+ selectivity of the 5DL membrane is dramatically low. Under those conditions, the G10 membrane shows a high retention coefficient for U(VI) and a low retention coefficient for sodium and calcium, leading to high U(VI)/Ca2+ and U(VI)/Na+ selectivities. Retention Coefficient of Uranium from Seawater. Because the seawater uranium concentration is very low compared to those of major elements such as Na+, K+, Mg2+, and Ca2+, inductively coupled plasma atomic emission spectroscopy is not sensitive enough for direct uranium determination. The separation of matrix elements and preconcentration of uranium should be performed prior to uranium determination. Separation
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5903
Figure 6. Concentration of Na+, Ca2+, and U(VI) as a function of the nanofiltered volume ratio. Experimental conditions: prefiltered seawater, [UO2(NO3)2] ) 1 × 10-5 mol/L, T ) 20 °C, ∆P ) 3 bar, G10 membrane, pH 8.3.
and/or preconcentration methods such as coprecipitation,19 chelating resin adsorption,20 solid-phase extraction,21 and solvent extraction22 have been widely employed for seawater analysis. However, these techniques show as main drawbacks large sample volumes and lengthy preconcentration times. Because of this, we introduced into the seawater the minimum of U(VI) salt allowing an ICP-OES analysis of the feed composition during the experience duration. In comparison to experiments with synthetic solutions, a similar behavior for uranium, sodium, and calcium is observed when U(VI)-doped seawater was nanofiltered with the G10 NF membrane (Figure 6). The concentrations of calcium and sodium in the feed are not affected by the NF process, although the concentration of U(VI) was 7-fold raised. The results obtained for the U(VI)-doped seawater prompted us to study the concentration of U(VI) from seawater in real samples. The total concentration of uranium present in seawater was determined by ICP-MS. The precise determination of the uranium concentration and 234U/238U ratio in seawater has been described earlier.23 The most commonly used techniques are ICPMS coupled with matrix ion preconcentration21,24 or coprecipitation.19 In our work, the concentration of U(VI) was directly determined by ICP-MS. A value of 3.66 µg/L of uranium was obtained. The concentration of uranium in the retentate was then determined as a function of the nanofiltered volume ratio, and the results are displayed in Figure 7. As expected, the increase of the U(VI) concentration in the retentate is exponential, and the behavior of U(VI) is the same in both seawater (3.66 µg/L) and doped seawater (2.38 mg/L). The concentrations of the major seawater metal ions were determined before and after NF experiments for a nanofiltered volume ratio of 0.96 (Table 2). The five metal ions Na+, Mg2+, Ca2+, K+, and Sr2+ that occur at concentrations down to 1 g/L are only slightly concentrated by the NF process (concentration factor inferior to 1.4). Among the analyzed ions, only Ba2+ and U(VI) are concentrated, and they show concentration factors of 10 and 8.5, respectively. The high factor of concentration observed for U(VI) and Ba2+ can be explained by the NF membrane properties. Indeed, the separation mechanism is normally described in
Figure 7. Concentration of U(VI) in the retentate versus initial concentration of U(VI) in the feed as a function of the nanofiltered volume ratio. Experimental conditions: prefiltered seawater, T ) 20 °C, ∆P ) 3 bar, G10 membrane, pH 8.3. Table 2. Concentration of the Major Seawater Metal Ions before and after NF with the G10 Membrane for a Nanofiltered Volume Ratio of 0.96 Determined by ICP-MS and Related Concentration Factors element
initial concn (mg/L)
final concn (mg/L)
concn factor
Na Mg Ca K Sr Li Mo Ba U
12200 1420 430 350 7.4 0.3 0.1 0.014 3.66a
13600 1800 565 413 9.8 0.26 0.12 0.14 31.12a
1.1 1.3 1.3 1.2 1.3 0.9 1.2 10.0 8.5
a
µg/L.
terms of charge and/or size effects.25 As expected, a sieving mechanism and electrostatic interaction are responsible for the retention of the bulky and highly charged U(VI) species. Rejection of Ba2+ would have been possible to predict because Ba2+ is the heaviest alkaline-earth ion. These encouraging results highlight the great capabilities of NF for the selective extraction of U(VI) from seawater. Conclusion Because of the weak uranium content of the sea (3.6 µg/L), the recovery of this element is difficult. The objective of this study was to show the potentialities of the NF to selectively concentrate the seawater uranium. The variation of the rejection uranium percentage as a function of the feed composition was studied for five different organic NF membranes. The best efficiency was obtained with the Osmonics G10 membrane. With this membrane, seawater uranium can be selectively nanofiltered and then concentrated by a 8.5 factor. The concentration factor for the major metal ions dissolved in seawater was less than 1.5. All of these results do not presently allow an economically viable process for the selective recovery of seawater uranium. However, its weak environmental impact as well as its high suitability in terms of process engineering make the membrane technology a very promising candidate for the selective concentration of seawater uranium. A process combining the use of the membrane process in
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association with fluidized sorbent beds seems to be advantageous because an increase of the concentration of uranium in the feed allows an increase of the uranium absorption rate on adsorbent. It should be emphasized that the most important factor contributing to the total recovery cost is the plant construction cost, and the scale of the plant could be reduced by the development of a new process having a greater absorption rate6 or kinetics. Acknowledgment We thank Y. Abdelnour from Varian France for the determination of the ion concentration in seawater by ICP-MS and J.-L. Nigon, Cogema, 2 rue P. Dautier, BP 4, 78140 Ve´lizy Cedex, France, for the discussions about of the Japanese publications and patents. Literature Cited (1) OECD. Uranium 1999, Resources, Production and Demand; OECD Nuclear Energy Agency and the International Atomic Energy Agency: Paris, 2000. (2) Schwochau, K. Extraction of metals from sea water. Top. Curr. Chem. 1984, 124, 91. (3) Yabutani, T.; Ji, S.; Mouri, F.; Sawatari, H.; Itoh, A.; Chiba, K.; Haraguchi, H. Multielement determination of trace elements in coastal seawater by inductively coupled plasma mass spectrometry with aid of chelating resin preconcentration. Bull. Chem. Soc. Jpn. 1999, 72, 2253. (4) Raff, O.; Wilken, R.-D. Removal of dissolved uranium by nanofiltration. Desalination 1999, 122, 147. (5) Kanno, M. Present status of study on extraction of uranium from sea water. J. Nucl. Sci. Technol. 1984, 21, 1. (6) Kobuke, Y.; Tabushi, I.; Aoki, T.; Kamaishi, T.; Hagiwara, I. Composite fiber adsorbent for rapid uptake of uranyl from seawater. Ind. Eng. Chem. Res. 1988, 27, 1461. (7) Kawai, T.; Saito, K.; Sugita, K.; Katakai, A.; Seko, N.; Sugo, T.; Kanno, J.-I.; Kawakami, T. Comparison of amidoxime adsorbents prepared by cografting methacrylic acid and 2-hydroxyethyl methacrylate with acrylonitrile onto polyethylene. Ind. Eng. Chem. Res. 2000, 39, 2910. (8) Hasegawa, S.; Seko, N.; Tabata, K.; Tamada, M.; Katakai, A.; Kasai, N.; Watanabe, T.; Kawabata, Y.; Nawata, Y.; Sugo, T. Construction and installation of the experimental marine equipment for recovery of rare metals in seawater; Department of Material Development, Japan Atomic Energy Research Institute, JAERI-Tech: 2001; p 1. (9) Ooi, K.; Ashida, K.; Katoh, S.; Sugasaka, K. Rate of uranium adsorption on hydrous titanium(IV) oxide granulated with polyacrylic hydrazide. J. Nucl. Sci. Technol. 1987, 24, 315. (10) Hassan, A. M.; Al-Sofi, M. A. K.; Al-Amoudi, A. S.; Jamaluddin, A. T. M.; Farooque, A. M.; Rowaili, A.; Dalvi, A. G. I.; Kither, N. M.; Mustafa, G. M.; Al-Tisan, I. A. R. A new approach to membrane and thermal seawater desalination processes using nanofiltration membranes (Part 1). Desalination 1998, 118, 35.
(11) Teuler, A.; Glucina, K.; Laine, J. M. Assessment of UF pretreatment prior RO membranes for seawater desalination. Desalination 1999, 125, 89. (12) Van der Bruggen, B.; De Vreese, I.; Vandecasteele, C. Water reclamation in the textile industry: nanofiltration of dye baths for wool dyeing. Ind. Eng. Chem. Res. 2001, 40, 3973. (13) Anne, C. O.; Trebouet, D.; Jaouen, P.; Quemeneur, F. Nanofiltration of seawater: fractionation of mono- and multivalent cations. Desalination 2001, 140, 67. (14) Clark, D. L.; Hobart, D. E.; Neu, M. P. Actinide Carbonate Complexes and Their Importance in Actinide Environmental Chemistry. Chem. Rev. 1995, 95, 25. (15) Grenthe, I.; Fuger, J.; Konings, R. J. M.; Lemire, J. R.; Muller, B. A.; Nguyen-Trung, C.; Wanner, H. Chemical Thermodynamics of Uranium; North-Holland: Amsterdam, The Netherlands, 1992. (16) van der Lee, J. JCHESS 2.0; E Ä cole Nationale Supe´rieure des Mines de Paris: Fontainebleau Cedex, France, 2002. (17) Graziani, R.; Bombieri, G.; Forsellini, E. Crystal structure of tetraammonium uranyl tricarbonate. J. Chem. Soc., Dalton Trans. 1972, 2059. (18) Schaep, J.; Van der Bruggen, B.; Vandecasteele, C.; Wilms, D. Chemistry for the protection of the environment. Retention mechanisms in nanofiltration; Plenum Press: New York, 1998; Vol. 3. (19) Chou, C. L.; Moffatt, J. D. A simple coprecipitation inductively coupled plasma mass spectrometric method for the determination of uranium in seawater. Fresenius’ J. Anal. Chem. 2000, 368, 59. (20) Yabutani, T.; Mouri, F.; Itoh, A.; Haraguchi, H. Multielement monitoring for dissolved and acid-soluble concentrations of trace metals in surface seawater along the ferry track between Osaka and Okinawa as investigated by ICP-MS. Anal. Sci. 2001, 17, 399. (21) Unsworth, E. R.; Cook, J. M.; Hill, S. J. Determination of uranium and thorium in natural waters with a high matrix concentration using solid-phase extraction inductively coupled plasma mass spectrometry. Anal. Chim. Acta 2001, 442, 141. (22) Babu, S. K.; Satyanarayana, D.; Muralikrishna, U. Photometric estimation of uranium in sea water based on extraction. Indian J. Mar. Sci. 1981, 10, 16. (23) Delanghe, D.; Bard, E.; Hamelin, B. New TIMS constraints on the uranium-238 and uranium-234 in seawaters from the main ocean basins and the Mediterranean Sea. Mar. Chem. 2002, 80, 79. (24) Yabutani, T.; Chiba, K.; Haraguchi, H. Multielement determination of trace elements in seawater by inductively coupled plasma mass spectrometry after tandem preconcentration with cooperation of chelating resin adsorption and lanthanum coprecipitation. Bull. Chem. Soc. Jpn. 2001, 74, 31. (25) Schaep, J.; Van Der Bruggen, B.; Vandecasteele, C.; Wilms, D. Influence of ion size and charge in nanofiltration. Sep. Purif. Technol. 1998, 14, 155.
Received for review February 18, 2003 Revised manuscript received July 16, 2003 Accepted July 23, 2003 IE030157A
