Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 386-390
386
Removal and Recovery of Organic Pollutants from Aquatic Environment. 4. Separation of Carboxylic Acids from Aqueous Solution Using Cross-Linked Poly(4-vinylpyridine) Narlyoshi Kawabata, Jun-lchi Yoshida, and Yukio Tanigawa Department of Chemistry, Faculty of Polytechnic Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
A new method for the separation of carboxylic acids from aqueous solution is described and involves the treatment with cross-linked poly(4-vinylpyridine)followed by elution using methanol and other organic solvents as eluants. Cross-linked poly(viny1pyridine) was found to have an excellent capacity for removing carboxylic acids from aqueous solution and was efficiently regenerated by organic solvents. Carboxylic acids were concentrated into 8-10 wt % organic solution and could be recovered by distillation.
Introduction Removal of organic pollutants from the aquatic environment has received increasing attention. The use of synthetic resins as polymeric adsorbents for this purpose is important not only in the protection of the environment from pollution but also in the effective w e of raw materials, because the resins can be regenerated and the organic pollutants can be recovered without chemical change. Carboxylic acids are important materials in use by chemical industries. They are often obtained as aqueous solutions. Isolation of them is an important process in chemical industry, but it is not always easy due to the high solubility in water, especially in the aliphatic carboxylic acids of low molecular weight. On the other hand, they are important organic pollutants which increases the COD value of industrial waste water. Synthetic resins such as styrene-divinylbenzene resin (Gustafson et al., 1968), strong base anion-exchange resin (Gregory and Semmens, 1972; Semmens and Gregory, 1974; Pochhali et al., 1977,19781,and strong and weak acid cation-exchange resins (Bafna and Govindan, 1956; Reichenberg and Wall, 1956) have been used to adsorb carboxylic acids in aqueous solution. In the first paper of this series (Kawabata and Ohira, 1979),it was pointed out that vinylpyridine-divinylbenzenecopolymer was found to have an excellent capacity for the adsorption of phenol and was efficiently regenerated by an organic solvent as the eluant. Phenol is a weak acid and intermolecular interaction between phenol and the pyridyl group of the copolymer was suggested during the adsorption. In this work, we report an attempt to separate carboxylic acids from aqueous solution using cross-linked poly(4-vinylpyridine). Experimental Section
Resins. Cross-linked poly(4-vinylpyridine) containing 72 mol % 4-vinylpyridine was prepared by a copolymerization of 4-vinylpyridine with divinylbenzene followed by grinding and sifting to 60-80 mesh as was described previously (Kawabata and Ohira, 1979). For comparison, four commercial resins were used in this work. Amberlite IRA-400 supplied by Rohm and Haas Co., Philadelphia, Pa., was used as a strong base anion-exchange resin, which had a styrene-divinylbenzene matrix with a quaternary ammonium group. Amberlite IRA-45 supplied by Rohm and Haas Co. was used as a weak base anion-exchange resin, which had a styrene-divinylbenzene matrix with primary, secondary, and tertiary amino groups. 0196-432 1/8 1/ 1220-0386$01.25/0
Amberlite XAD-2 and XAD-4 supplied by Rohm and Haas Co. were used as porous polymeric adsorbents with no ion-exchange functional group, which had a styrene-divinylbenzene matrix. The supplier claimed that the surface areas of XAD-2 and XAD-4 were 300 and 784 m2/g, respectively. These resins were preconditioned as was described previously (Kawabata and Ohira, 1979). Adsorption Test. Column studies were conducted using a 1.0-cm diameter glass column with a fritted glass filter connected with a 200-mL dropping funnel in a down-flow fashion at room temperature. The preconditioned resin was placed in the column. The bed was carefully backwashed to eliminate entrained air and then tapped to ensure packing of the bed. The dropping funnel was used to contain influent solution. Samples were taken manually and were checked for the concentration of carboxylic acid. The flow rate was 3 bed vol/h throughout this work. The breakthrough point was chosen as the point of the first detection of the carboxylic acid using methyl red as the indicator. In the cases where the adsorption tests were carried out in the presence of hydrochloric acid and sodium hydroxide, the breakthrough point was chosen as the point of the first detection of the carboxylic acid by the total organic carbon (TOC) analysis. The breakthrough capacity, which was based upon the total amount of adsorbed carboxylic acid until the breakthrough point, was calculated by the equation breakthrough capacity (mg/dry g) = (VB - Vo)Co/X where Co is the influent concentration, VB is the total volume of the effluent solution until the breakthrough point, V , is the void column volume, and X is the dry weight of the resin in the column. The void column volume was determined by the standard method of simple volume measurements (InczBdy, 1966). In a typical experiment the void volume of the column packed with cross-linked poly(4-vinylpyridine) was 0.50 bed volume. The total capacity, which was based upon the total amount of adsorbed carboxylic acid until the effluent concentration reached the influent concentration, was calculated as follows
[
total capacity (mg/dry g) = ~
~
~- C,)~ dV]/X ( C
where Cv is the effluent concentration at effluent volume 0 1981 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 387
Table I. Capacities of Resins for Adipic Acid Adsorption from Aqueous Solution" breakthrough capacity resin PVPb IRA-400(OH$c IRA-400(Cl) IRA-45e XAD-2f XAD-4g
mg/ dryg 310 400 40 380 50 120
total mequiv/ capacity, dryg mg/dryg 5.1 4 20 5.4 540 0.5 120 5.2 580 0.6 90 1.6 160
Determined by the continuous flow column method using 15 000 ppm of aqueous adipic acid solution. The flow rate was 3 bed vol/h. Pulverized copolymer of 4vinylpyridine. Strong base anion-exchange resin in the hydroxide form. Strong base anion-exchange resin in the chloride form. e Weak base anion-exchange resin in the free base form. f Porous styrene-divinylbenzene resin with no ion-exchange functional group claimed to have a Porous styrene-divinylsurface area of 300 m'/g. benzene resin with no ion-exchange functional group claimed t o have a surface area of 784 m'/g. a
V and VT is the effluent volume until the effluent concentration reached the influent concentration. Equilibrium adsorption tests were conducted by placing weighed quantities of resin and samples of aqueous solution of carboxylic acid in a 300-mL Erlenmeyer flask maintained at 30 f 1 "C with magnetic stirring. After the mixture reached equilibrium, the concentration of carboxylic acid was determined. Recovery of Adsorbed Carboxylic Acids from Resins. Resin was saturated with carboxylic acid by a continuous flow column method carried out until the effluent concentration became close to the influent concentrations. Elution of the adsorbed carboxylic acid from the resin was conducted by the continuous flow column method using an organic solvent as the eluant with a flow rate of 3 bed vol/h in a down-flow fashion. Carboxylic acid could be recovered by distillation from a three-component system of carboxylic acid, organic solvent, and water. Determination of Concentrations. Quantitative analysis of carboxylic acid in aqueous and/or organic solutions was performed by titration with standard 0.1 N sodium hydroxide using phenolphthalein as the indicator, or was based upon the value of total organic carbon (TOC) determined using a Sumitomo Model GCT-12N TOC & T N apparatus. The pH value of solutions was determined with use of Toa Electronics Model HM-5B pH meter. Results and Discussion Adsorption of Adipic Acid on Cross-Linked Poly(4-vinylpyridine) Studied by t h e Continuous Flow Column Method. Adipic acid was chosen as a representative of carboxylic acid. An aqueous 15000 ppm solution of adipic acid was passed through a column containing cross-linked poly(4-vinylpyridine). Results are given in Table I. A very sharp breakthrough curve was obtained for each experiment, as illustrated in Figure 1, which reflected on the small difference between the breakthrough capacity and the total capacity. For comparison, capacities of commercial resins for the adsorption of adipic acid were also determined, and results are given in Table I. The breakthrough capacity of the cross-linked poly(4-vinylpyridine) was remarkably higher than those observed with porous styrene-divinylbenzene resin with no ion-exchange functional group (XAD-2 and XAD-4) and a strong base anion-exchange resin (IRA-400) in the chloride form, and it compared well with those obtained
Effluent (bed vol)
Figure 1. An example of the removal of adipic acid from aqueous solution using cross-linked poly(4-vinylpyridine) as the polymeric adsorbent by the continuous flow column method: flow rate, 3 bed vol/h; influent concentration of adipic acid, 15 000 ppm. Table 11. Breakthrough Capacities of Resins for Adipic Acid Adsorption in the Presence of Inorganic Salts" breakthrough inorganic capacity, resin salt mol/L mg/dry g PIT none 370 NaCl 1.00 410 Na,SO, 0.10 310 IRA-400(OH) none 400 NaCl 1.00 0 Na,SO, 0.10 20 IRA-400( Cl) none 40 NaCl 1.00 0 Na,SO, 0.10 20 IRA-4 5 none 380 NaCl 1.00 0 Na,SO, 0.10 0 XAD-4 none 120 NaCl 1.00 130 Na,SO, 0.10 110 a Determined by the continuous flow column method using 15 000 ppm of aqueous adipic acid solution containing inorganic salts. The flow rate was 3 bed vol/h.
with a weak base (IRA-45) and a strong base anion-exchange resin (IRA-400) in the hydroxide form. Effect of Some Inorganic Salts. We have investigated the effect of the presence of sodium chloride and sodium sulfate in aqueous solution of adipic acid on the breakthrough capacities of resins for adipic acid adsorption. Results are given in Table 11. The capacities of crosslinked poly(4-vinylpyridine) and porous styrene-divinylbenzene resins with no ion-exchange functional group for the adsorption of adipic acid were scarcely affected by the presence of these inorganic salts. On the contrary, the breakthrough capacities of weak base and strong base anion-exchange resins were conspicuously reduced in the presence of these inorganic salts. This selective adsorption of organic anions by the cross-linked poly(4-vinylpyridine) is very important in the practical application. Effect of Acid and Alkaline Materials. We have also investigated the effect of the presence of hydrochloric acid and sodium hydroxide in aqueous solution of adipic acid on the breakthrough capacities of resins for the adsorption of adipic acid. Results are given in Table 111. The capacity of the styrene-divinylbenzene resin with no ion-exchange functional group for the adsorption of adipic acid was scarcely affected by the presence of hydrochloric acid. The pH value of the effluent solution before the breakthrough point indicated that the resin did not adsorb hydrochloric acid. The capacity of the weak base anion-exchange resin for the adsorption of adipic acid decreased in the presence of
388 Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 Table 111. Capacities of Resins f o r Adipic Acid Adsorption in the Presence of Acid and Alkaline Materialsa pH of the breakthrough total HCl, NaOH, influent effluent capacity, capacity, resin mmol/L mmol/L soln. soln. b mg/dry g mg/dry g PVP 100 1.21 5.9-6.0 360 400 30 1.75 4.5-5.3 380 460 2.72 3.8-6.4 370 4 20 60 4.46 0 270 200 8.14 0 90 IRA-45 100 1.21 6.1-6.6 170 370 30 1.75 4.4-6.2 270 460 2.72 4.3-6.3 380 580 60 4.46 0 54 0 200 8.14 0 40 XAD-4 100 1.21 1.3-2.0 120 260 30 1.75 1.6-2.5 130 180 2.72 5.2-5.7 120 160 60 4.46 0 120 200 8.14 0 40 a Determined by the continuous flow column method using 1 5 000 ppm of aqueous adipic acid solution containing acid or alkaline materials. The flow rate was 3 bed vol/h. Before the breakthrough point.
hydrochloric acid. The pH value of the effluent solution before the breakthrough point indicated that the weak base anion-exchangeresin adsorbed both hydrochloric acid and adipic acid under the condition. The capacity of the cross-linked poly(4-vinylpyridine) for adipic acid adsorption was scarcely affected by the presence of hydrochloric acid. The pH value of effluent solution before the breakthrough point indicated that the poly(viny1pyridine) adsorbed both adipic acid and hydrochloric acid. It would be interesting that the capacity of the poly(viny1pyridine) for adipic acid adsorption did not decrease in the presence of hydrochloric acid even though the polymer adsorbed both adipic acid and hydrochloric acid under the condition. It is quite obvious that hydrochloric acid was coadsorbed with adipic acid by cross-linked poly(4-vinylpyridine). The first run of Table 111 shows that the resin adsorbed 2.40 mmol/dry g of hydrogen chloride and 2.46 mmol/dry g of adipic acid until the breakthrough of adipic acid. The second run of Table I11 shows that the resin adsorbed 0.76 mmol/dry g of hydrogen chloride and 2.60 mmol/dry g of adipic acid until the breakthrough of adipic acid. On the other hand, when 0.1 N hydrochloric acid was passed by the continuous flow column method in a similar manner, the resin adsorbed 4.84 mmol/dry g of hydrochloric acid until the effluent solution became acidic using methyl red as the indicator. This result would indicate that the resin contained a sufficient amount of pyridyl groups for the one to one adsorption of both carboxylic acid and hydrochloric acid in the case where only one of the two carboxyl groups of adipic acid was used in the adsorption. It would be unfavorable that a single pyridyl group was used to adsorb both carboxylic acid and hydrochloric acid. These observations can be understood based upon the followhg two hypotheses. (i) A small pore of the resin may be useful for the adsorption of hydrogen chloride but useless for the adsorption of carboxylic acid. This hypothesis is reasonable because carboxylic acids are much larger than hydrogen chloride. (ii) In the large pore, carboxylic acid is adsorbed more selectively than hydrochloric acid. These two hypotheses lead us to consider that the large pore and small pore are used for the adsorption of carboxylic acid and hydrochloric acid, respectively, in the coadsorption. Thus the resin adsorbed both carboxylic acid and hydrochloric acid, and the capacity for the adsorption of carboxylic acid was not significantly affected by the presence of hydrochloric acid.
Organic Solvent Throughput (bed vol)
Figure 2. Elution of adsorbed adipic acid from resins using organic solvents as the eluants by continuous flow column method: flow rate, 3 bed vol/h, (A) from cross-linked poly(4-vinylpyridine) using methanol as the eluant; (B)from cross-linked poly(4-vinylpyridine) using acetone as the eluant; (C)from cross-linked poly(4-vinylpyridine) using 2-propanol as the eluant; (D) from Amberlite IRA400 in the hydroxide form using methanol as the eluant; (E) from Amberlite IRA-400 in the chloride form using methanol as the eluant; (F)from Amberlite IRA-45 using methanol as the eluant; (G)from Amberlite XAD-4 using methanol as the eluant; (H) from Amberlite XAD-2 using methanol as the eluant.
On the other hand, the capacities of the above three types of polymeric adsorbents considerably decreased in the presence of sodium hydroxide. However, in the cases where the cross-linked poly(4-vinylpyridine) and the porous styrene-divinylbenzene resin with no ion-exchange functional group were used as the polymeric adsorbent, neutralization of the alkaline materials in the influent solution can prevent the leak of adipic acid in the effluent solution, because the adsorption capacity was scarcely affected by the presence of inorganic salts. Recovery of Adipic Acid from the Cross-Linked Poly(4-vinylpyridine). We examined the elution of adipic acid from the cross-linked poly(4-vinylpyridine) by a continuous flow column method using an organic solvent as the eluant. Results are shown in Figure 2A-C. Most of the adsorbed adipic acid was eluted from the poly(vinylpyridine) with only 2 bed volumes of methanol, acetone, and 2-propanol as the eluants, and the poly(viny1pyridine) was efficiently regenerated. Adipic acid was concentrated to 8-10 wt % eluate solution and could be recovered by
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 2, 1981 389
Table IV. Capacities of Cross-Linked Poly(4~inylpyridine)for the Adsorption of Various Carboxylic Acids from Aqueous Solution" breakthrough capacity carboxylic acid formic acid acetic acid propionic acid butyric acid valeric acid acrylic acid methacrylic acid lactic acid glycolic acid adipic acid malic acid maleic acid citric acid
PKa 3.75 ( 2 5 "C) 4.76 ( 2 5 "C) 4.87 ( 2 5 "C) 4.82 ( 2 5 "C) 4.84 ( 2 5 "C) 4.26 (25 "C) 4.66 (18 "C) 3.86 (25°C) 3.83 ( 2 5 "C) 4.41 ( 2 5 "C) 5.30 ( 2 5 "C)
influent concn, ppm 4 600 6 800 7 500 9 300 9 300 7 000 8 200 6 600 7 600 1 5 000
mg/dry g 120 60 130 24 0 430 170 280 160 170 370
mequiv/dry g 2.6 1.0 1.8 2.7 4.2 2.4 3.3 1.8 2.2 5.1
1 3 400 11300
520 730
7.8 12.6
54 0 770
21 000
760
11.9
830
1.92 ( 2 5 "C) 6.23 ( 2 5 "C) 3.13 ( 2 5 "C) 4.76 ( 2 5 "C) 6.40 ( 2 5 "C)
a Determined by the continuous flow column method. The flow rate was 3 bed vol/h. not reach the influent concentration.
distillation of the organic solvents. For comparison, we also examined the elutions of the adsorbed adipic acid from the commercial resins by a continuous flow column method using methanol as the eluant. Results are shown in Figure 2D-H. Most of the adsorbed adipic acid was eluted from the styrene-divinylbenzene resins with no ion-exchange functional group when only 2 bed volumes of methanol was used as the eluant as can be seen in Figure 2G and H. On the other hand, the elution from a weak base anion-exchange resin and that from a strong base anion-exchange resin in the hydroxide form were difficult. About half of the adsorbed adipic acid was eluted from the weak base anion-exchange resin when 5.4 bed volumes of methanol was used as the eluant (Figure 2F). Only about 40% of the adsorbed adipic acid was eluted from the strong base anion-exchange resin in the hydroxide form even when 12.8 bed volumes of methanol was used as the eluant (Figure 2D). Although most of the adsorbed adipic acid was eluted from the strong base anion-exchange resin in the chloride form when 6 bed volumes of methanol was used as the eluant (Figure 2E), the adsorption capacity of the resin was very low as can be seen in Table I. Adsorption Equilibrium for Adipic Acid and Resins. The experimental isotherms for adipic acid adsorption on various resins are shown in Figure 3. In the region of lower concentration of adipic acid, the weak base anion-exchange resin in the free base form (graph D) had a higher capacity than those observed with the cross-linked poly(4-vinylpyridine) (graph A). This result may be ascribable to the weaker basicity of the pyridyl group than aliphatic amino groups. Adsorption of Other Various Carboxylic Acids on Cross-Linked Poly(4-vinylpyridine) Studied by the Continuous Flow Column Method. Capacities of the cross-linked poly(4-vinylpyridine) for the adsorption of other various carboxylic acids in aqueous solutions were determined by the continuous flow column method in a similar manner. Results are given in Table IV. Elution of the adsorbed carboxylic acids was also easily accomplished by the continuous flow columm method using methanol as the eluant. Results are summarized in Table V. Mechanism of the Adsorption of Carboxylic Acids by the Cross-Linked Poly(4-vinylpyridine). In spite
'
total capacity, mg/dry g 140 80 150 260 4 80 -b -b
200 200 420
The effluent concentration did
0
2
1
n
102
16'
Equilibrium Concentration (mol/L)
Figure 3. Adipic acid adsorption isotherms on various resins at 30 "C: (A) cross-linked poly(4-vinylpyridine);(B)Amberlite IRA-400 in the hydroxide form; (C) Amberlite IRA-400 in the chloride form; (D)Amberlite IRA-45; (E) Amberlite XAD-2; (F)Amberlite XAD4.
Table V. Elution of Carboxylic Acids from Cross-Linked Poly( 4-vinylpyridine) Using Methanol as the Eluant" carboxylic acid formic acid acetic acid propionic acid butyric acid valeric acid acrylic acid methacrylic acid lactic acid glycolic acid adipic acid malic acid maleic acid citric acid
methanol, bed vol
recovery, %
2.5 2.3 2.1 2.1 1.9 2.9 2.5 3.1 2.4
100 100 100 100 100 90 100 100 100
2.8
100
5.8 13.6 12.6
100 76 100
a Carried out by the continuous flow column method using methanol as the eluant. The flow rate was 3 bed/h.
of the fact that we used cross-linked poly(4-vinylpyridine) in the pulverized form, it showed a much larger capacity for the adsorption of carboxylic acids than that of porous
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styrene-divinylbenzene resin with no ion-exchange functional group. Thus the capacity of the poly(viny1pyridine) for the adsorption of carboxylic acids may be derived from the intermolecular interaction between the pyridyl group of the polymer and the carboxylic acids. However, the following experimental results appear unfavorable to an ion-exchange mechanism for this adsorption. (i) The capacity of the poly(viny1pyridine) for the adsorption of carboxylic acid was scarcely affected by the presence of hydrochloric acid, similarly to the case of adsorption by the styrene-divinylbenzene resin with no ion-exchange functional group, and contrary to the adsorption by a weak base anion-exchange resin. (ii) Elution of adsorbed carboxylic acids was easily accomplished using an organic solvent as the eluant, similarly to the case of elution from the styrene-divinylbenzene resin with no ion-exchange functional group, and contrary to the elution from a weak base anion-exchange resin. (iii) The capacity of the poly(viny1pyridine) for the adsorption of carboxylic acids was scarcely affected by the presence of inorganic salts, similarly to the adsorption by the styrene-divinylbenzene resin with no ion-exchange functional group. On the contrary, the capacity of anion-exchange resins for the adsorption of carboxylic acids was conspicuously reduced in the presence of inorganic salts. A comparison of the capacities of the cross-linked poly(viny1pyridine) for the adsorption of various carboxylic acids given in Table IV may be helpful for the understanding of the mechanism of the adsorption. The adsorption capacity in the units of mequiv/dry g increased in the series, acetic < propionic < butyric < valeric acids, although the pKa values show the similar acidity of these carboxylic acids. The adsorption capacity thus increased with the length of the carbon chain in the series of aliphatic carboxylic acids of similar pKa. A similar tendency was also observed in the adsorption of carboxylic acids by the strong base anion-exchange resins (Semmens and Gregory, 1974; Pochhali et al., 1977,1978) and that by strong and weak acid cation-exchange resins (Bafna and Govindan, 1956). The increase of affinity with the poly(viny1pyridine) and other resins may arise from the decrease of solubility of carboxylic acids in water. Therefore physical interaction seems to play an important role in the case of the ad-
sorption on the poly(viny1pyridine). However, the large capacity of the poly(viny1pyridine) for the adsorption of formic acid cannot be explained in terms of the physical interaction. The result may be ascribable to the contribution of the acid-base interaction between the pyridyl group of the polymer and the carboxyl group of the acid, since formic acid has much higher acidity than acetic acid. The larger capacity of the poly(viny1pyridine) for the adsorption of acrylic acid than that for propionic acid can also be attributed to the difference in acidity of these carboxylic acids. Since pyridine is an organic base, it is reasonable to consider the following interaction, where the pyridyl group of the polymer uptakes a proton from the carboxylic acid to form a complex with carboxyl anion as the associated counterion
Such acid-base interaction may increase with acidity of the carboxylic acid and may be responsible for the remarkably higher adsorption capacity of the poly(viny1pyridine) than those observed with the nonionic resins. Ion exchange is an electrostatic interaction. Therefore, it can be interfered with salt as well as by acid and base. But acid-base interaction can only be affected by competition of other acids or bases. Further details of the mechanism of adsorption of organic acids on cross-linked poly(4-vinylpyridine) will be discussed somewhere in the near future.
Literature Cited Bafna, S. L.; Govindan, K. P. I d . Eng. Chem. 1958, 48, 310-317. Gregory, J.; Semmens, M. J. J. Chem. SOC. Faradqy Trans. 7 1972, 68, 1045-1052. Gustafson, R.C.; Albrlght, R. L.; Helsler, J.; Lirio, J. A.; ReM, 0. T. Ind. Eng. Chem. Prod. Res. Dev. 1988. 7 . 107-115. Incz6dy, J. "Analytlcal Appllcatlons of Ion Exchangers", translated by PBII, A.; Pergamon Press: Oxford, 1966; Chapter 5. Kawabata, N.; Ohlra, K. Envfron. Sci. Techno/. 1979, 73, 1396-1402. Pochhali, L.; Adhikary, S. K.; Ray, K. C. J. Indkn Chem. SOC. 1977, 54, 859-862; 1978, 55, 366-368. Reichenberg, D.; Wall, W. F. J. Chem. SOC. 1958, 3364-3373. Semmens, M.; Gregory, J. Envlron. Sci. Techno/. 1974, 8 , 834-839.
Receiued for reuiew April 1, 1980 Accepted January 29, 1981
