Envlron. Sci. Technol. 1982, 16, 635-637
values for Whatman 41 filter, Whatman GF/A filter, glass AE filter and ceramic filter is poor. Also the (\kW)l/'/ (\klo)1/2 ratios for these filters are all greater than 1.3.
Conclusions On the basis of results obtained with oleic acid aerosols, Millipore AA and VC filters, Selas silver membrane filter, and stainless steel substrates appear to be the most suitable collection substrates of those tested. Fiber-filter substrates exhibited poor collection efficiency curves when (\kw)1/2/(\klo)1/2 and (\k60)1/2values were used as the determining factors. In using any substrate it is important that the cascade impactor be carefully calibrated with the specific substrate. Acknowledgments We gratefully acknowledge the assistance of R. Henderson, G. M. Kanapilly, B. V. Mokler, Y.S. Cheng, and
S. J. Rothenberg for technical review of the manuscript and E. E. Goff for illustrations. Literature Cited (1) Rao, A. K.; Whitby, K. T. J . Aerosol Sci. 1978, 9, 87. (2) Willeke, K.; McFeters, J. J. J. Colloid Interface Sci. 1975,
53, 121. ( 3 ) Newton, G. J.; Carpenter, R. L.; Cheng, Y. S.; Barr, E. B.; Yeh, H. C. J . Colloid Interface Sci. 1982, 87, 279. (4) Berglund, R. N.; Liu, B. Y. H. Environ. Sci. Technol. 1973, 7, 147. ( 5 ) Ranz,W. E.; Wong, J. B. Ind. Eng. Chem. 1952,44,1371. (6) May, K. R. J. Aerosol Sci. 1975, 6 , 403. (7) Marple, V. A.; Liu, B. Y. H. Environ. Sci. Technol. 1974, 8 , 648. Received for review November 30,1981. Accepted April 30,1982. Research performed under U.S. Department of Energy Contract Number DE-AC04-76EV01013.
Progress toward a Solution to the Nitrate Problem Maurice M. Kreevoy" and Carmen I. Nltsche
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
rn The equilibrium constant for the extraction of nitric acid from water by a fatty amine in a water-immiscible liquid (eq 1) is increased by a factor of lo2 when the solvent is changed from benzene to trioctyl phosphate. This permits the H+ and NO, to be coextracted from a near-neutral solution. The amine solution in trioctyl phosphate can be carried on a thin porous plastic support and used as a membrane. Such a membrane has been mounted in a modified dialysis cell, with loading carried out on one side and stripping with base on the other. This device is effective in removing nitrate from water.
Introduction Excessive nitrate ion in drinking water is a widespread problem resulting from agricultural and industrial activity, domestic sewage, and automobile exhausts (1). One possible line of attack on this problem is through solvent extraction. If the contaminated water is equilibrated with a water-immiscible solution of a fatty amine (chosen so that even its salts are organic soluble and water insoluble), the elements of nitric acid can be extracted by the process shown in eq 1. (The overbar indicates a solution in the
NO3-
+ H+ + B + BH+N03-
(1)
water-immiscible phase. B is an aliphatic amine with 24 or more carbon atoms. Ion pair dissociation is not expected in solutions as nonpolar as these.) The organic solution can then be stripped by equilibration with a much smaller volume of aqueous base and reused (2). This system has been proposed for the decontamination of very acidic waste streams (3), with kerosine or an aromatic hydrocarbon as solvent and conventional solvent-extraction technology. For application to potable water this system has two drawbacks: the equilibrium constant for reaction 1,KO,,, is so small that the water must be quite acidic before a substantial fraction of the nitrate is removed, and the product water is likely to be contaminated with a significant quantity of the extraction solvent ( 4 ) . We now suggest modifications that go a considerable way toward removing these drawbacks. First we change the solvent 0013-936X/82/0916-0635$01.25/0
to a strong hydrogen-bond acceptor, trioctyl phosphate, substantially increasing KO,,.Then we conduct the extraction and stripping in a solid-supported liquid membrane, eliminating oil-in-water emulsification and dramatically reducing the total volume of organic phase required.
Experimental Section A water-jacketed separatory funnel was used for the determination of KO,,.Water at 25.0 "C was pumped through the jacket. After equilibration the last traces of emulsified organic were removed from the aqueous phase, before analysis, by treating it with paraffin shavings. Nitrate was determined in the aqueous phase before and after equilibration with an Orion Research Co. nitratesensitive electrode, Model 93-07, previously calibrated with known compounds and shown to give the theoretical response. The pH was controlled with an appropriate phosphate buffer and checked with a glass-electrode pH meter before and after equilibration. Membrane experiments were carried out in a modified dialysis cell, patterned after one described by Brandlein (5), shown in Figure 1. Loading and stripping solutions were stored in glass reservoirs of about 100 cm3 capacity and pumped through the cell by a Gilson Minipuls 2 peristaltic pump at the rate of 0.45 cm3 s-l. The liquid membrane was supported on Celgard (Celanese Corp.) 2400 porous polypropylene film, 3 X cm thick, about 40% void space. In membrane experiments nitrate was monitored by periodically withdrawing samples and determining their optical absorbance at 228 nm (6). Both the loading and stripping reservoirs were monitored, and the sum of the two absorbances was shown to be constant. The pH of the loading reservoir was established with a dilute phosphate buffer and was monitored as the the experiment proceeded. Small amounts of 6 M phosphoric acid were added from time to time to maintain a constant PH. Didodecylamine was a gift of the Armack Co. It was recrystallized from ethanol before use. It had appropriate NMR and infrared spectra and a mp of 42-48 OC. Am-
0 1982 Amerlcan Chemical Society
Environ. Sci. Technol., Vol. 16,No. 9, 1982 635
I
Membrane
-
Feed in
Feed out I
I
I
-
U (Vexav
)
~~
I
I----
ri I I
Strippant -A
1 J I I I
I
4Strippawt
out Figure 1. Exploded view of the membrane cell. in
berlite (Rohm and Haas Co.) LA-2 is a mixture of variously branched secondary amines. I t was found by acid-base titration to have an equivalent weight of 360. It therefore has about 24 carbons, which would give it an equivalent weight of 353. Trioctyl phosphate was purchased from the Ventron Division of the Thiokol Co. (Alpha Products) and was distilled under vacuum before use. It has appropriate spectral properties.
Results and Discussion With didodecylamine as B, NaN03 as the nitrate source, and aqueous solution pH values around 6, six determinations of KO,,gave an average value of (1.2 f 0.1) X lo8. This may be compared with KO,, values of 5.4 X lo6 and 3.8 X lo6 determined for nitric acid with Amberlite LA-1 as base in benzene and carbon tetrachloride, respectively, as solvents (7). It has been known for some time that the phosphate esters are effective solvents for the extraction of ionic species from aqueous solution (8), but this particular combination, an amine dissolved in a phosphate ester to extract the elements of an acid, does not appear to have been previously explored. It may have other applications; for example, for U02(S04)22extraction from leach liquors (9,lO). In the present case trioctyl phosphate was used in the place of the more common tributyl phosphate because of the excessive solubility of the latter in water (8). The exaltation of KO,,in trioctyl phosphate is probably due, at least in part, to the effectiveness of the P=O group in forming hydrogen bonds (8,11,12),in this case to the ammonium ion. It has been demonstrated that water-immiscible extractants such as our amine-trioctyl phosphate solutions, can be supported on thin, porous, plastic films, with aqueous solutions on both sides. The organic layer can then act as a membrane for active transport of the anion, with extraction taking place on one side and stripping by aqueous alkali on the other (13,14). The driving force for the transfer of the nitrate ion is provided by the transfer of H+ into a basic solution (2).This result has been realized. In a typical experiment the initial NaN03 conM (20 ppm of nitrogen) in 75 mL centration was 1.4 X of loading solution. The stripping solution was 25 mL of 0.1 M NaOH also containing 1.4 X lo4 M NaN03to guard against the possibility of false positive results due to leakage. The membrane liquid was a 1.0 M solution of Amberlite LA-2 in trioctyl phosphate. The pH of the 636 Envlron. Scl. Technol., Vol. 16, No. 9, 1982
I 0
I
I
IO
20
I
I
30 40 t (hours)
I
I
I
50
60
?O
Figure 2. Successful test of eq 2 for the experiment described in the text. A values were corrected for the effective dilution by the small volumes of aqueous H,PO, that were added to maintain the pH. The value of k is 6 X io4 s-’. The irregularity of the plot near the beginning of the experiment is due to the inadequacy of the pH control during thls period.
loading solution was maintained at 6.1. This apparatus worked continuously for 72 h and was still working when it was interrupted. In this time it removed 78% of the original NO3-from the loading solution, producing a final nitrate level of - 4 ppm of nitrogen; well within standards (15).At the end of the experiment the loading aqueous solution was tested for trioctyl phosphate by extracting it with 6.0 cm3of tetrachloroethylene, then examining the infrared spectrum of the resulting tetrachloroethylene solution. After calibration with known compounds, the solution from which nitrate had been removed was estiM) of trioctylmated to contain 0.06 g dmP3(1.4 X phosphate. This estimate may be high due to the presence of small amounts of impurities, related to the trioctyl phosphate but more water soluble than it. In an apparatus such as that described, the time course of the nitrate-related absorbance, A , in the loading solution should be represented by eq 2 if the small increase in k u / a = On (A,,/Ad/(t - t o ) (2) volume due to the addition of aqueous H3P04is ignored (16). The time is t and the subscript zero indicates a value pertaining to the initial conditions. The volume of the loading reservoir is u and the area of membrane is a. The rate constant, k, should be independent of the concentration of nitrate and the physical dimension of the apparatus, as long as it has hydrodynamic characteristics similar to those of the apparatus used. Figure 2 shows the success of eq 2. Other work with this and similar apparatus (17,18) indicates that k is largely determined by the flux of BH+N03-through the membrane and depends linearly on the organic base concentration and the H+ concentration of the loading aqueous solution. The pH tends to rise during these experiments, possibly because phosphate is extracted as an amine salt of H3P04 (19). This would be a less important effect in practical water treatment because ambient water has a much lower phosphate concentration than our experimental solutions. In addition to nitrate and phosphate an apparatus of this type would remove detergent anion ( 4 ) ,humic acids, and many organic materials that might be present in the water’at trace levels. These removals would provide a better quality of water, but as some of these things would not be stripped off of the membrane, they would tend to accelerate its deterioration.
Conclusion It seems quite possible that Kolacan be increased by a further 1-2 powers of ten by varying the amine structure
(20) and/or by further variation of the solvent structure. This would increase k by at least a factor of 10. A decrease in the water solubility of the solvent would also be desirable. With such a k, a device with an active surface of 1 m2, which could be of modest size and cost (21),would provide about 100 dm2/day of acceptable water, starting with water containing about 20 ppm of nitrate nitrogen. Such a device would contain only about 10 cm3 of active liquid if the present membrane support was used. The small volume of the active liquid required results from ita continuous regeneration in use and is one of the main reasons for the potential economy of such devices.
Literature Cited (1) Brezonik, P. L.; Harris, W. F.; Harriss, R. C.; Johnston, H. S.; Keeney, D. R.; Mar, B. W.; Schulze, W. D.; Shark, R. C. “Nitrate: An Environmental Assessment”; National Academy of Sciences: Washington, D.C., 1978; pp 225-336. (2) Thelander, P. F.; Hasledalen, L. A.; Kreevoy, M. M. J. Chem. Educ. 1980,57, 509-511. (3) Mattila, T.; Lokio, A. Environ. Pollut. Manage. 1979,9,68, 70-71. (4) Dunning, H. N.; Kreevoy, M. M.; White, J. M. U.S.Patent 3215622, 1965; Chem. Abstr. 1966, 6 4 , 4 3 6 ~ . (5) Brandlein, L. S. M.S. Thesis, Columbia University, New York, 1976; pp 18-20. (6) Brezonik, P. L.; Harris, W. F.; Harriss, R. C.; Johnston, H. S.; Keeney, D. R.; Mar, B. W.; Schulze, W. D.; Shark, R. C. “Nitrate: An environmental Assessment”; National Academy of Sciences: Washington, D.C., 1978; p 148. (7) Kertes, A. S.; Platzner, I. T. J. Inorg. Nucl. Chem. 1962, 24, 1417-1428.
(8) Marcus, Y.; Kertes, A. S. “Ion Exchange and Solvent Extraction of Metal Complexes”; Wiley-Interscience: New York, 1969; pp 650-716. (9) Chem. Eng. News. 1956,34,2590-2592. (10) Jenkins, I. L. Hydrometallurgy 1979, 4, 1-20. (11) Arnett, E. M.; Jones, L.; Mitchell, E.; Murty, T. S. S. R.; Gorrie, T. M.; Schleyer, P. v. R. J. Am. Chem. SOC.1970, 92, 2365-2377. (12) Hadzi, D.; Rajnvajn, J. J. Chem. Soc., Faraday Trans. 1 1973,69, 151-155. (13) Cussler, E. L. Am. Inst. Chem. Eng. J. 1971,17,1330-1303. (14) Block, R. In “Membrane Science and Technology”; Flinn, J. E., Ed.; Plenum Press: New York, 1970; pp 171-187. (15) Brezonik, P. L.; Harris, W. F.; Harriss, R. C.; Johnston, H. S.; Keeney, D. R.; Mar, B. W.; Schulze, W. D.; Shark, R. C. “Nitrate: An Environmental Assessment”; National Academy of Sciences: Washington, D.C., 1978; p 598. (16) Albery, W. J.; Burke, J. F.; Leffler, E. B.; Hadgraft, J. J . Chem. Soc., Faraday Trans. 1 1976, 72, 1618-1626. (17) Hasledalen, L. A.; Kreevoy, M. M. “Proceedings of the 2nd Canterbury Conference on the Kinetics of Reactions in Solution”, Canterbury, England, July, 1979;The Chemical Society: London, 1979; SP5. (18) Ulrick, L. A.; Lokkesmoe, K. D.; Kreevoy, M. M., submitted for publication. (19) Moore, F. L. Anal. Chem. 1957,29, 1660-1662. (20) Reyes, A.; Scott, R. M. J. Phys. Chem. 1980,84g3600-3603. (21) Baker, R. W., U. K. Patent Application 2047564, 1980; Chem. Abstr. 1981,94, 195691~.
Received for review January 11, 1982. Accepted April 23, 1982. This work was supported by the US.National Science Foundation through Grant No. CHE79-25990 to the University of Minnesota.
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