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(2) it meets ICBO codes and ita equivalents; (3) it be flame proof; (4) it be durable under all expected conditions indefinitely. At the time of this writing, property owners and the wood industry are waiting for a flame retardant to meet the above requirements before treating wood shingles. This study is addressed to those who may like to do something instead of nothing. It is not always that a full grown burning tree falls on a roof accompanied by 100 mph wind. Frequently it is a branch or a fire cracker. The cost of raw materials for THP-prepolymer flame retardant at current prices is $2 to $6 per square meter of surface depending on quantity of purchase and one-side or both-side treatment. Additional research could be performed for durability to environmental conditions. Registry No. THPS, 55566-30-8; (THPS).(homopolymer), 65257-05-8. Literature Cited Amerlcan Society for Testing and Materials, 1977. ASTM D2863-76, Phiiadelphia, PA.
Carey, James and Nancy, Sllver Tree Treating Company, Gates, OR 97346. Supply of shingles for testing, 1982. Elgal, G. M.; Perklns, R. M.; Knoepfler, N. B. ACS Symp. Ser. 1977a, No. 58, 249. Elgal, G. M.; Perklns, R. M.; Knoepfler, N. B. U S . Patent 4246031, 1977b. Knight, Clyde, Western Red Cedar Shake Manufacturers Association, Edmonds, WA, 98020, 1982, communication. LeVan, S. L., Forest Products Laboratory, USDA, Madison, WI 53705, 1982, communlcatlon. MKM Company, Jeffersonvllle, IN 47130. Ramani, C. P., Internal Conference of Bullding Offlciais, Whittier, CA 90601, 1982, communication and ICBO Publlcation. Vandersaii, H. L. “Useof a Small Flame Tunnel in the Laboratory Evaluation of the Flame Spread Rating”, Monsanto Special Report No. 6090, May 5, 1964. Whlte, R. H. WoodSci. 1979, 72(2), 113-121. Yeadon, D. A.; Verberg, G. B.; Rayner, E. T.; Doiiear, F. G.; Hopper, L. L., Jr.; Dupuy, H. P. Roc. Annual Tung Ind. Assoc. 1065, 32, 18.
Received for review January 11, 1984 Accepted March 21, 1984 Names of companies or commercial products are given solely for the purpose of providing specificinformation; their mention does not imply recommendation or endorsement by the U.S.Department of Agriculture over others not mentioned.
Chelating Resin Functionalized with Dithiocarbamate for the Recovery of Uranium from Seawater Iwao Tabushi, Yoohlakl Kobuke, NorHake Nakayama, Takao Aokl, and Atsushl Yoshlzawa Department of Synthetic Chemistry, Faculty of Engineering, Kyoto Universlty, YoshMa, Kyoto 606, Japan
Chelating resins having dithiocarbamate groups were examined for the recovery of uranium directly from seawater. The resins studied adsorbed uranyl ions with sufficiently high rates, 50 pg of uraniumlg of resin per day from natural seawater. The uranyl ion adsorbed on the resin was liberated quantitatively by treatment with a 10% ammonium carbonate solution. Repeated adsorption and desorption did not cause any appreciable deterioration of the resins. The maximum adsorption was estimated to be 5.1 mg/g of adsorbent. This resin was tested with a contact system that utilized sea current directly. Immersion of the resin in the Kuroshlo current gave 40 pg of uranium/g of resin after one day.
Introduction The amount of uranium dissolved in the oceans of the world is estimated to be ca. 4 billion tons. Even a small percentage, e.g., 10% of the total amount, could provide huge energy resources that would be enough to fuel 2.4 million nuclear power plants X years, based on the assumption that a plant of average size requires 170 tons of uranium per year. Thus the discovery of an efficient way of recovering uranium directly from seawater would alter that aspect of the world-wide energy problem dramatically. A recovery program was begun in Great Britain in the early 1960’s. This was followed by projects in various countries in universities and by private companies. Several methods have been proposed coprecipitation, adsorption, flotation, solvent extraction, biological accumulation, and others. Of all of these, the adsorption method has been accepted as the most promising in recent years. The development of the use of hydrous titania opened the way for direct recovery (Davies et al., 1964; Keen, 1968). Its success stimulated the study of various organic chelating resins resulting in the preparation of various new types of useful resins (Tabushi et al., 1979). Even so, the process still needs improvement in many areas, e.g., adsorption rate, equilibrium adsorption ca-
pacity, mechanical as well as chemical stabilities, and so on. We have proposed the use of macrocyclic compounds, which have extraordinarily large stability constants, as well as high selectivity toward uranyl ions (Tabushi et al., 1979, 1980). There are still a variety of approaches remaining for the use of other chelating u n i k Here we have exploited one possibility of using a polymer having dithiocarbamate as a chelating unit. The resin successfully recovered uranyl ion a t a high adsorption rate, 40-50 pg of U/(g of resin day), directly from natural sewater or natural sea current. Experimental Section Adsorbent. Dithiocarbamate resin was donated by Professor Fujio Mashio (see Acknowledgment). The detailed preparation was reported in Tokkyo Koho (in Japanese; Mashio, 1983). The following description is from the English abstract of the original patent: An equimolar mixture of butane-1,2,3,4-tetracarboxylic acid, (meso form, 117 g) and tetraethylenepentamine (94.5 g) was kneaded well with 100 mL of water. The viscous material obtained was heated at 145-155 OC for 2 h. The light-brown transparent resin was then suspended in water and water-soluble material was filtered off. A light-yellow resin (3-A) was obtained as fine particles (solid fraction: 41.3%, yield on the dry resin: 181.7 g). The resin (3-A) was
0196-4321f84f1223-0445$0l.50/0 0 1984 American Chemical Soclety
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Table 1. Experimental Conditions for Evaluating Adsorption Capacity run 1 run 2 6.8 0.134 [Ulm PPm 5.0 0.118 [UL, ppm 1000 5000 vol, mL adsorbent, mg 74 7.4 [uledsorbed/ p o l p e r , mg/ g 24.3 10.8
dehydrated further at 140-145 "C for 90 min to give a yellow resin of particle forms (3-B). The resin (3-B) was treated with carbon disulfide in an alkaline aqueous ethanol initially at 20 "C and then at 48 "C. After filtration and washing, a light yellow resin (3-C) functionalized with dithiocarbamate was obtained. (Solid fraction: 30.8%, yield on the dry resin: 154 g). Analysis of Uranium. The amount of uranium was determined through the Arsenazo I11 method developed by Ohnishi and Koide (1965). Natural Seawater. Sewater was sampled at 3 miles WSW from Sandanheki, Shirahama Town, in Wakayama prefecture. The uranium concentration was determined, according to the establihsed method (Ohnishi et al., 1977), to be 2.80 ppb. Adsorption from Uranyl Enriched Seawater. U02(OAc)2.2H20(Merck, GR) was added to natural seawater to produce the desired concentrations; 60 mg of the adsorbent was added to 10 mL of uranyl enriched (3.1 ppm) solution, then stirred with a magnetic stirrer for 23 h. After this, the adsorbent was separated from the reaction mixture. The uranyl concentration was determined for the remaining solution. The polymer, collected on filter paper, was then treated three times with 10 mL each of 10% ammonium carbonate for 1 h to liberate uranyl ion from the polymer. After removal of the polymer, the solution was concentrated to a small volume and the amount of uranium in the concentrated solution was determined. Durability Test. The adsorbent (DTC-1) used in the above experiment was washed with a small amount of distilled water to remove ammonium carbonate. The adsorbent was then added to U02 enriched (ca. 3 ppm) seawater and the mixture was stirred with a magnetic stirring bar. A small portion (1mL) of the initial solution was removed for [U], determination at time zero. After appropriate time intervals, the adsorbent was separated and treated with ammonium carbonate as described above. The recovered adsorbent, free from uranium, was recycled for further tests. Adsorption Capacity. The U02-enriched seawater was stirred with the adsorbent under conditions listed in Table I. Uranium concentration, after an appropriate time interval ([UIJ, was determined for the sample solutions. The sample sizes were 2 mL and 100 mL for the 6.8 and 0.13 ppm [U], solutions, respectively. For each solution, the saturation point [U], was reached, as confirmed by the fact that no subsequent decrease was observable for three successive determinations. The amount of uranium adsorbed on the polymer was calculated by ([U], - [VI,) X (volume of the solution). For the trial using 6.8 ppm initial uranyl concentration, uranyl ion was eluted from the adsorbent by treatment with 10% (NH4)2C03. The total amount of uranium in combined eluents was found to be equal to that obtained from the decrease of uranium in solution. Recovery from Natural Seawater. A specified amount of adsorbent was added to 5 L of natural seawater at room temperature. Then, the suspension was stirred slowly with a mechanical stirrer. After specific time intervals, the polymer was collected on filter paper and
washed with distilled water. The polymer was stirred with three successive aliquots (10 mL) of 10% ammonium carbonate solution. The combined eluents and washings were concentrated to a small volume and subjected to uranium determination. Thus all of the time-recovery plots originate from independent runs stopped at different contact periods. Adsorption in Sea Current. The adsorbent was held between two sheets of stainless steel net (40 mesh, 8 cm X 8 cm, 1-2 mm thickness). This apparatus was attached to a frame made of angle iron. The frame was suspended 1-2 m below the water's surface, held up by a set of three buoys. This was held in place by an anchor at a depth of ca. 10 m, using a rope 50 m in length. A rudder was attached behind the frame in order to keep the net perpendicular to the direction of sea current, thus receiving maximum exposure. The whole system was held in seawater just like a kite in the sky. The adsorbent was in contact with seawater for two days. Uranyl ion was then liberated from the adsorbent by treatment with three 10-mL portions of 10% ammonium carbonate. The combined eluents were concentrated to a small volume and subjected to uranium determination. Results and Discussion Dithiocarbamate as a Chelating Unit. Dithiocarbamate (Carbodithioate) has been used as a reagent for the spectrophotometric determination of uranyl ion. For example, 1-pyrrolidine dithiocarbamate gives a yelloworange complex having an absorption maximum at 385 nm, which has been used for quantitative analysis of UO?+ (0-50 ppm uranium) (Traub and Boltz, 1969). It has been claimed that the measurement was satisfactorily insensitive to the presence of alkali or alkaline earth metal ions. The equilibrium constant of diethyl dithiocarbamate with uranyl ion has been determined (Zingaro, 1956) to be in the range of 1.4 X 1017to 6.7 X 1017M-4 (eq 1) K =
[UOZ(E@JCSZ)~~-I [U022+][EhNCS2-I4
(1)
The constant is so large that it is reasonable to expect the dithiocarbamate anion will replace the carbonate in the anionic complex of [U02(C03)3]4-, the assumed species in seawater (Ogata, 1971). Their pK, values of DTC (2.95-5.2) (Hulanicki, 1967) are low enough to ensure complete dissociation in natural seawater (pH 8.0-8.3). This fact and the high association constant for U02-DTC complex formation tell us that a polymer having dithiocarbamate functionality may be a good candidate for an efficient uranyl adsorbent for recovery of uranium from seawater. Synthesis of Dithiocarbamate Polymer. Butanetetracarboxylic acid was condensed with tetraethylenepentamine. Condensation under heat gave water-soluble linear polyamide at first, and further application of heat gave rise to the formation of a three-dimensional framework by cross-linking. The resulting polymer was then treated with carbon disulfide in an alkaline medium to give the dithiocarbamate modified polymer. Two different samples (DTC 1and 2) were used for various adsorptivity tests. (See Scheme I.) Uranyl Adsorption from Enriched Seawater. Before testing the polymer with natural seawater, the polymer obtained as per the above procedure was tested using seawater enriched with uranyl acetate (3 ppm total concentration), as a screening test. Polymer samples 1 and 2 (60 mg each) were stirred in the above solution (10 mL) for 23 h. The uranium concentration of the remaining solution was determined, and the results are listed in Table
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 447
Scheme I CH2-
CH-
t HzN(CHzCHzNH),CH2CHzNH2
CH -CH2
I I I I COOH COOH COOH COOH tCHCH2CONH(CH2CH2NH),CH2CH2NHCOCHt;;l
I
COOH
-
CSP
I
&OH
COO H +CHCH&ON
I COO H
H (CHzCHz NInCH 2C HzNHCOC H& H %
I
CSZNa
I
COOH
Table IV. The Effect of Repeated Adsorption and Desorption on the % Recovery' no. of repeated contact treatment time, h [U],, ppm U,,,,, pg recovery, % 1 23 3.1 23.8 77 2 25 2.8 28.9 102 3 10 2.8 25.6 90 4 10 2.8 22.5 79 5 9 3.0 26.2 88 Solution: uranium-enriched seawater; adsorbent: DTC-1, 60 mg of the sample used in the runs in Tables I and 11.
n = 3-4
Table 11. Adsomtion from Enriched Seawater uranvl solution
DTC-1 60 10 60 10 2 "After 23 h contact.
3.1 3.1
0.6 0.4
81 87
Table 111. Elution of Uranium from Adsorbent elution, pg polymer Uab, p g 1st (%) 2nd (%) 3rd (%) total (%) DTC-1 25 23 (92) 0.8 (3) 0 (0) 23.8 (95) 2 31 23 (85) 1.5 (6) 0 (0) 24.5 (91) Polymer samples adsorbing uranium were obtained in the runs in Table 11. ~~
I1
/
I I
0.001
II. It is apparent from Table I1 that an excellent recovery of uranium was obtained from seawater containing uranium at a concentration of ca. 1000-fold of the natural. Next the polymer separated from the above solution was subjected to the elution procedure, and the content of adsorbed uranium was determined. Table I11 lists the amount of uranium liberated after three successive treatments with 10 mL of a 10% ammonium carbonate solution. Elution of uranyl ion from the adsorbent gave 90% recovery from the first contact with the eluent. Repetition assures almost quantitative elution. The durability of the adsorbent during repeated adsorption and desorption treatment is an important criterion for its applicability. In order to test the durability of the adsorbent, DTC-1 (60 mg) was stirred with 10 mL of uranyl solution ([VIo= 3 ppm in seawater) for specific time intervals. The polymer was separated and immersed in an ammonium carbonate solution. The polymer recovered was subjected again to the adsorption-desorption cycle. The amount of uranium obtained at each cycle is shown in Table IV. The results show no appreciable loss of the initial adsorption capacity by repeated treatments, the recovery remaining at 87.2 f 6.0%. The dependence of percent recovery on the initial uranium concentration was also studied. Thus DTC-1 sample (200 mg) was stirred with U02-enriched seawater ([VI, = 0.1 ppm, 500 mL) for 62 h. The polymer was separated and uranium was liberated as described above to give 42.5 pg of U, which corresponds to 85% recovery. The comparison of this result with the one above suggests that the percent recovery does not drop significantly with a decrease of the uranium concentration. Adsorption Capacity on Saturation. The equilibrium amount was measured for uranyl enriched seawater of adsorption. The adsorbent was stirred in two solutions of enriched seawater containing two different concentrations of uranyl ion ([VI,, 6.8 and 0.1 ppm). The uranium concentration was determined periodically in aliquots sampled from the mother liquor, until there was no observable change for three successive determinations. In
I
I
0.01
0.1
I
1 .o
I
10
[U] i n S e a W a t e r ( p p m )
Figure 1. Estimation of adsorption capacity. 0
m
-
m
0
w
N
3 0
0 a l u 0
al > 0
v
P al I 0 N
0
24
48
72
C o n t a c t Time ( h r s )
Figure 2. Time plot for the recovery of uranium from natural sea300 mg,).( 150 mg. water: seawater 5 L, [U], = 2.8 ppb; DTC-l; (0)
Figure 1, the relative amount of uranium adsorbed onto the polymer is plotted against the concentration of uranium in the liquid phase. Extrapolating this adsorption isotherm plot to the concentration of uranium in natural seawater (3.3 ppb) should give the equilibrium amount of uranium adsorbed from natural seawater. Thus, the adsorbent examined here was estimated to have an adsorption capacity of 5.1 mg/g of polymer under the conditions. Recovery from Natural Seawater. In all of the preliminary experiments described above, the DTC-modified resin seems to exhibit satisfactory characteristics for practical application as a uranium adsorbent from natural seawater. The resin was then tested by using natural seawater without addition of uranyl ion from an external source. Figure 2 illustrates the time dependence of the amount of uranium adsorbed from 5 L of seawater, [VIo = 2.8 ppb. The adsorption rate is quite rapid, giving 40% and 56% after 12 and 24 h contact, respectively, when 300 mg of the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 135"
140"
145'
40"
40"
+\ \+
\ Current
L 5 L o f natural sea water
7 0
0
100
200
300
400
c
I 135"
500
Polymer (mg)
Figure 3. Dependence of uranium recovery on the ratio of seawater to adsorbent.
polymer was used. A similar experiment using only 150 mg of polymer gave 40% recovery after 24 h contact. This value is significantly higher than 28% recovery, which is half of the amount recovered by 300 mg of the resin under identical conditions. A similar phenomenon is observed when two time-percent recovery plots were compared to each other. In these experiments, the total amount of seawater was maintained constant at 5 L. Thus the above results suggest that a larger recovery per weight of resin is obtainable by using larger amounts of seawater. In order to ascertain this relationship a few more recovery experiments were undertaken with different amounts of the adsorbent in a constant volume (5 L) of seawater. Figure 3 shows a plot of the log of the amount (pg) of uranium recovered per gram of adsorbent vs. the grams of adsorbent employed. A straight line was obtained for this plot, indicating that the use of a higher ratio than 5 L of seawater/20 mg of resin, Le., 250 L of seawater/g of resin, affords at least 50 pg of U/g of resin from one day's contact. Adsorption in Sea Current. On a practical scale, enormous amounts of seawater must be processed in a uranium recovery plant. Two different approaches could be used to ensure efficient contact of adsorbents with seawater: pumping-up and direct use of sea current. Some preliminary discussion has taken place concerning which system would be more advantageous. A tentative conclusion has been reached, but it is based on limited information (Harrington et al., 1974; Kanno, 1977; Campbell et al., 1979). Experimental data relevant to adsorbent performance, especially in a rapid sea stream, have never been reported. It is absolutely necessary to accumulate experimental data on such items. We have perfomed some preliminary performance tests in sea current. The adsorbent was immersed in a rapid section of the Kuroshio current near Mikura Island (33' 53' N, 1 3 9 O 34' E), which is located along the central axis of the Kuroshio current (Figure 4). The adsorbent was held between two sheets of meshed (no. 40) stainless steel nets floating in a sea stream ca. 1.5 m below the sea surface by a combination of floating buoys and an anchor. The adsorbent was recovered and treated with ammonium carbonate. The amount of uranium recovered was determined spectrophotometrically. The results are shown
140"
Figure 4. Flow axis of Kuroshio Current and Mikura Island where the experiments were undertaken. Table V. Recovery of Uranium by Direct Use of Sea Current rate, pg of U/(g of resin adsorbent,a g contact, day U,,,,, pg day) 1.14 1.69 a
1.6
1.1
77.6 68.1
43
37
Adsorbent was held in a no. 40 net of stainless steel.
in Table V. The adsorbent successfully recovered uranium at a rate as high as ea. 40 pg of U/(g of resin day). It is noteworthy that such a high value was obtained without either any pretreatment of the seawater, e.g., filtration, or application of any external energy sources, e.g., pumping. Acknowledgment This research was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan. I.T. gratefully acknowledges receipt of the Grant from the Asahi Glass Foundation for Industrial Technology. The authors wish to thank Professor F. Mashio (Research Institute for Production Development) for his generous gift of the dithiocarbamate sample. Special thanks should also be extended to Seto Marine Biological Laboratory, Kyoto University, for their help in sampling natural seawater. Registry No. Uranium, 7440-61-1.
Literature Cited Campbell, M. H., et al. Emon Report XN-RT- 15 1979, 1 . Davles. R. V.; Kennedy, J.; McIloy, R. W.;Hill, K. M. Nature (London) 1964, 203, 1110. Harrington, F. E.; Salmon, R.; Unger, W. E.; Brown, K. 8.; Collman, C. F.; Crouse, D. J. ORNL-TM-4757, 1974. Hulanlcki, A. Talenta 1967, 14, 1371. Kanno, M. J. At. Energy SOC. Jpn. 1977b, 19, 586. Keen, N. J. J. Br. Nucl. Energy SOC. 1968, 7, 178. Mashio, F.; Kltamura, S. Instltute for Production and Development Science, Jpn Kokai Tokkyo Koho, JP 58, 61834; Chem. Abstr. 1984, 99, 141003j. Ogata, N. J. At. Energy SOC.Jpn. 1971, 13, 560. Ohnkhi, H.; KoMe, Y. EunsekiKagaku 1965, 74, 1141. Ohnlshl, K.; Hori, Y . : Tomarl, Y. BunsekiKagaku 1977, 26. 74. Tabushi. 1.; Kobuke, Y.; Nishiya, T.; Nature (London) 1979, 280, 665. Tabushi, I.; Kobuke, Y.; Ando, K.; Kishimoto, M.; Ohara, E. J. Am. Chem. SOC. 1980, 102, 5947. Traub, A.; Bok, D. F. Mikrochim. Acta 1969, 749. Zlngaro, R. A. J. Am. Chem. SOC. 1956, 78, 3588.
Received for review December 12, 1983 Revised manuscript received February 8, 1984 Accepted March 4, 1984
