Ind. Eng. Chem. Res. 1998, 37, 367-373
367
Ozonation of Monocrotophos in Aqueous Solution Young Ku,* Wen Wang, and Yung-Shuen Shen Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 106, Republic of China
The decomposition of monocrotophos (cis-3-((dimethoxyphosphinyl)oxy)-N-methylcrotonamide) in aqueous solution by ozonation was studied under various solution pH values, gaseous ozone dosages, and alkalinity levels. The decomposition rate of monocrotophos was increased with decreasing solution pH and increasing ozone dosage. The presence of alkalinity (HCO3- and CO32- species) inhibited the decomposition of monocrotophos to certain extents, but severely retarded the mineralization of the organic intermediates. The presence of Fe2+ and Mn2+ ions interfered with the decomposition of monocrotophos in acidic solutions. The decomposition pathway of monocrotophos was proposed and the breakage of the carbon-carbon double bond by ozonation was found to occur at first to form various nitrogen- and phosphorus-containing compounds, and subsequently decomposed to be H2O, CO2, NO3-, and PO43- species. The quasiglobal kinetics based on a simplified consecutive-parallel reaction scheme was developed to describe the temporal behavior of monocrotophos decomposition in an aqueous solution by ozonation. Introduction Many pesticides in the aquatic environment are potentially toxic and are hard to be removed by conventional treatment processes. Organic phosphates, such as ethylparathion, malathion, and monocrotophos, are widely used as pesticides because of their rapid rate of decomposition and therefore are not likely to be bioaccumulated. Despite their benefits, organic phosphates are still of great concern because they are more acutely toxic to birds and mammals than chlorinated hydrocarbons, and are also more likely to contaminate surface water and groundwater because they are more soluble in water than chlorinated hydrocarbons. Monocrotophos, a foliar insecticide mainly used for cotton, is active against a wide range of insects and mites and has contact, systemic and residual activity. Dureja (1989) evaluated the environmental fate of monocrotophos under sunlight and ultraviolet irradiation in soil and water and on plant foliage, and the experimental results indicated that the effect of various environmental factors have to be considered. Lee et al. (1990) examined the fate of monocrotophos in the aqueous and soil environment. The half-lives of monocrotophos in pH 3 and 9 buffer solution at 25 °C were found to be 131 and 26 days, respectively. Hua et al. (1995) studied the titanium dioxide mediated photocatalytic oxidation of monocrotophos in aqueous solution. About 51% of monocrotophos was found to degrade after an hour with an initial concentration of 50 mM. By the literature investigation, the biodegradation and photocatalysis of monocrotophos do not seem to be feasible in realistic treatment conditions due to the slow decomposition rate. Because of the powerful oxidizing capability of the ozone molecule, ozonation has attracted attention as a potential treatment for various organic compounds in waters and as part of an advanced wastewater treatment process in the last few decades. Gunther et al. * Author to whom all correspondence should be addressed.
(1970) observed the formation of the sulfate ion during the ozonation of parathion and proposed the rupture of the sulfur group from the aromatic ring to form paraxon as a major organic intermediate. Laplanche et al. (1984) reported the ozonation of parathion to form phosphate and nitrophenols. Although the ozonation of organic phosphates has been discussed phenomenologically by the previous investigators, detailed information on the reaction kinetics is scarce. The objective of this study was to investigate the effect of various reaction parameters on the decomposition of monocrotophos in aqueous solution by the ozonation process. The reaction rates and temporal behaviors of monocrotophos and various reacting species by ozonation were characterized using an empirical kinetic model. Experimental Section The reactor used in this research was made entirely of Pyrex glass with an effective volume of 2.5 L and was water-jacketed to keep solution temperature at 25 °C for all experiments. The solution pH value was kept constant at desired levels by the addition of NaOH and HCl solutions using a Kyoto APB-118-20B autotitrator. The reaction solution in the batch reactor was mixed by a magnetic stirrer with 300 rpm of stirring speed. Ozone content of the feed gas was controlled by adjusting the power input to a Fischer model 500 ozone generator at an inlet oxygen gas flow rate of 33.6 L/h. The influent concentration of ozone was ranged from 0.05 to 0.47% (540-4660 ppmv). The monocrotophos and other chemicals used for analysis were reagent grade, and all experimental solutions were prepared with deionized water. Two liters of monocrotophos solution with or without the presence of Fe2+ or Mn2+ ions was added into the reactor, and the ozone-containing gas was continuously added into the reactor through a 100-meshed diffuser during the course of the reaction. Typical reaction runs lasted 3-6 h. At desired time intervals, aliquots of a 15 mL solution were withdrawn from the sampling port which was located at the bottom
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368 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 1. Pseudo-First-Order Hydrolysis Rate Constant of Monocrotophos in Aqueous Solution under Various Solution pH pH
hydrolysis rate constant (min-1)
3 5 7 9
0.0 8.8 × 10-6 6.0 × 10-5 7.0 × 10-4
of the reactor. The total sample volume was kept below 10% of the total reactor volume. Each run of the experiments in this work was replicated at least twice. Monocrotophos was analyzed by a Spectra-Physics P1000 HPLC equipped with a UV detector and a Spherisorb ODS2 5 µm column. The total organic carbon (TOC) was analyzed by an O.I.C. model 700 TOC analyzer. To avoid the origination of CO2 from air, the sample has been adjusted to acidic conditions before the analysis of the amount of total organic compounds by the TOC analyzer. The anion concentrations, including nitrite, nitrate, sulfate, and phosphate, were analyzed by a Dionex model DX-100 ion chromatograph. The concentrations of ozone in the aqueous and gaseous phases were determined by the Indigo blue method (Hoigne and Bader, 1982) and the Seki SQZ-6000 ozone analyzer, respectively. For several experiments, the reaction intermediates were determined using a HPG1800A GCD equipped with an electron ionization detector (EID). The concentrations of iron and manganese were analyzed by a GBC 904 atomic absorption spectrophotometer.
Figure 1. The temporal distribution of reacting species of the decomposition of monocrotophos in aqueous solution by ozonation at pH 9.
Results and Discussion Because most of the experiments carried out in this research were performed in a semibatch reactor with ozone-containing feed gas flow into the reactor, the volatilization of monocrotophos in aqueous solution was examined at various pH and the loss by volatization was found to be negligible. The hydrolysis of monocrotophos under various pH values was studied, and the hydrolysis rate constants are shown in Table 1. The hydrolysis rate was found to increase with increasing solution pH and followed first-order kinetics with respect to the concentration of monocrotophos, similar to the experimental results reported by Lee et al. (1990). But the reaction rates for hydrolysis were found to be much slower than those for the ozonation of monocrotophos in aqueous solutions. For instance, the pseudo-firstorder hydrolysis rate constant at pH 9 was determined to be about 7.0 × 10-4 min-1 while the pseudo-first-order ozonation rate constant was about 4.5 × 10-1 min-1. In the ozonation system, the monocrotophos molecule can be destroyed and sequentially degraded into some smaller molecules by an ozone molecule or hydroxyl radicals. Theoretically, the yields of various final productss7 mmol of CO2, 1 mmol of phosphate ion, and 1 mmol of the nitrate ionscan be generated from the mineralization of 1 mmol of the monocrotophos molecule. The temporal distribution of reaction species during the decomposition of monocrotophos in aqueous solution by ozonation at pH 9 is shown in Figure 1. Monocrotophos could be almost completely (>95%) decomposed within 15 min. No indication of any anions, except the carbonate ion, were formed during the early stage of the decomposition of monocrotophos. After about 30 min of reaction time, phosphate ions displayed a very fast formation rate and were almost stoichiomet-
Figure 2. The temporal distribution of reacting species for the decomposition of monocrotophos in aqueous solution by ozonation at pH 5.
rically released from the monocrotophos molecule before the first 50 min. On other hand, the nitrate ions showed a relatively slow formation rate. Even though the concentration of CO2 increased between 50 min and 240 min, the formation of nitrate ions was still low (about 30% of the total amount of the nitrate atom on the monocrotophos molecule). Similar experiments were conducted at various solution pH. The results are shown in Figure 2 for pH 5; no indication of the formation of phosphate, nitrate, and
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 369
carbonate were detected. These findings can be explained by observing the structure of monocrotophos: O P
O
H
CH3
C
C
C O
H
N CH3
(CH3O)2
The CdC double bond is supposed to be easily detached by ozone attack from the monocrotophos molecule to form phosphorus- and nitrogen-containing organic intermediates. Because of the breakage of the CH3O-P bond, the phosphorus was converted almost entirely to phosphate, indicating that the organophosphate intermediates were effectively destroyed by OH• which can be generated from the reaction of O3 and OH- at alkaline conditions rather than at acidic conditions. The nitrogen-containing organic intermediates were postulated to be very refractory to the ozone molecule or OH• attack possibly because of the slow and sequential oxidation of the nitrogen-containing species (Buchel, 1983): OH•
OH•
OH•
[Interme]n 9 8 NH3 9 8 NO2- 9 8 NO3O O O 3
3
3
(1)
where the [Interme]n is the nitrogen-containing intermediate. On the basis of the experimental observations discussed above, the simplified decomposition pathway of monocrotophos by ozonation in an aqueous solution is suggested as follows: On the basis of the simplified
pathway described above, each step of the reaction was assumed to be irreversible and first-order with respect to the organic species, consistent with other studies of ozonation of various organic compounds (Beltran et al., 1990; Hayashi et al., 1993; Grosjean et al., 1993; Shen et al., 1995). The reaction rate equations for various species can be derived on the basis of elemental mass balances of carbon, phosphorus, and nitrogen:
[M] ) [M]0e-kmt
(2)
[Interme]p ) [M]0km(e-kmt - e-kpt)/(kp - km) (3) [Interme]n ) [M]0km(e-kmt - e-knt)/(kn - km) (4) [Interme]c ) [M]0km(e-kmt - e-kct)/(kc - km)
(5)
[PO43-]p ) [M]0,p - [M]p - [Interme]p
(6)
[NO3-]n ) [M]0,n - [M]n - [Interme]n
(7)
[CO2]c ) [M]0,c - [M]c - [Interme]c
(8)
where [M] is the concentration of monocrotophos (mM); [M]o is the initial concentration of monocrotophos (mM); [Interme]p is the concentration of phosphorus-containing intermediates (mM); [Interme]n is the concentration of nitrogen- containing intermediates (mM); [Interme]c is the concentration of organic intermediates (mM); km is the pseudo-first-order decomposition rate constant of monocrotophos (min-1); kp is the pseudo-first-order formation rate constant of phosphate (min-1); kn is the pseudo-first-order formation rate constant of nitrate (min-1); and kc is the pseudo-first-order formation rate constant of carbonate (min-1). Using the reaction rate equations proposed above, the rate constants were determined by regressing the experimental data. The temporal behavior of the reacting species during the decomposition of monocrotophos by ozonation was reasonably consistent with the model, and the results are shown in Figure 1 for pH 9. Similar satisfactory results were also obtained for other solution pH conditions. The pseudo-first-order reaction rate constants for monocrotophos decomposition and for the release of constituent anions at various pH values are summarized in Table 2. The decomposition rate constants of monocrotophos by ozonation was decreased linearly with increasing solution pH, and the relationship can be regressed as follows:
km ) 0.2800[OH-]-0.051
r2 ) 0.99
(9)
This result differs from the experimental findings reported by several researchers on the ozonation of several phenols (Singer and Gurol, 1983; Beltran et al., l992; Ku et al., l996) and of alkylamines (Hoigne and Bader, 1983) which stated that the decomposition rates were increased with solution pH due to the formation of the dissociation of phenols since the dissociating form of phenol directly reacts faster with ozone than the nondissociating form or the formation of OH• radicals. However, the experiments reported by Yocum et al. (1978) and Baillod et al. (1982) also show some controversial results. They studied the ozonation of toluene2,4-diisocyanate and 2-chlorophenol, respectively, in aqueous solutions and found that the oxidation rates decrease with increasing solution pH. Thus, it was postulated that the pH effect on the ozonation of organics might also depend on the molecular structure of target organic compounds and needs more detailed research, conclusions based solely on the formation of OH• radicals may not be concrete. The pKa (dissociation equilibrium constant) of monocrotophos was not reported in the literature and was then determined to be 4.42 at 25 °C in the study by the potential titration method; thus, the decomposition of monocrotophos (MCP) species (protonated and unprotonated) by ozonation might vary with solution pH conditions as follows: pKa 4.42
MCP-H+ {\} MCP
(10)
For pH lower than pKa, the positively-charged protonated monocrotophos predominates in aqueous solu-
370 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 2. Pseudo-First-Order Rate Constant of Monocrotophos in Aqueous Solution by Ozonation at Various Solution pH rate constants solution pH 3 5 7 9
removal rate constant of monocrotophos km (min-1) 1.032 0.815 0.638 0.451
r2 ) 0.98 r2 ) 0.97 r2 ) 0.99 r2 ) 0.98
formation rate constants of PO43- kp (min-1) 0.000 0.000 0.028 0.049
r2 ) 0.92 r2 ) 0.90
Figure 3. The effect of ozone mass flow rate on the pseudo-firstorder decomposition rate constant of monocrotophos by ozonation at pH 9.
tions and could be more favorably decomposed into organic intermediates by ozone molecules, but further mineralization of organic intermediates was not proceeded by ozone molecule as shown in Table 2 where kp, kn, and kc were zero for pH 3 and 5. For higher pH conditions, the neutral unprotonated monocrotophos gradually predominates in aqueous solution and is attacked by the OH• free radicals formed by the reaction between the ozone molecule and hydroxide ion in alkaline solutions. Even though the decomposition rate of monocrotophos is lowered with increasing solution pH, the mineralization of organic intermediates could further proceed by OH• free radicals as shown in Table 2. Thus, the solution pH altered not only the distribution of monocrotophos species but also the distribution of less-oxidative ozone molecules and more-oxidative OH• free radicals. The dependence of reaction rate constants of the ozonation of monocrotophos in an aqueous solution at pH 9 on the gaseous ozone mass flow rate and constant volumetric ozone-containing gas flow rate are shown in Figures 3 and 4, respectively. The rate constants for the decomposition of monocrotophos, km, kp, kn, and kc were found to increase with increasing gaseous ozone mass flow rate, similar to the positive influence of ozone mass flow rate or ozone partial pressure on the ozonation rate in experimental results reported by previous researchers on the ozonation of various organic compounds (Joshi and Shambaugh, 1982; Gurol and Nekouinaini, 1984; Hoigne et al., 1985; Sotelo et al., 1989;
formation rate constants of NO3- kn (min-1) 0.0000 0.0000 0.0000 0.0011
r2 ) 0.88
formation rate constants of CO2 kc (min-1) 0.0000 0.0000 0.0095 0.0175
r2 ) 0.84 r2 ) 0.92
Figure 4. The effect of ozone mass flow rate on the pseudo-firstorder formation rate constants of various anions for the ozonation of monocrotophos at pH 9.
Beltran et al., 1994a). The reaction rate constants illustrated in Table 3 were linearly related to the gaseous ozone mass flow rate:
km ) 0.12031[O3]g
r2 ) 0.97
(11)
kp ) 0.03023[O3]g
r2 ) 0.94
(12)
kn ) 0.00189[O3]g
r2 ) 0.96
(13)
kc ) 0.00335[O3]g
r2 ) 0.97
(14)
Increasing the gaseous ozone mass flow rate by increasing the ozone content of feed gas, based on Henry’s law, would linearly increase the equilibrium ozone concentration in aqueous solution. These dissolved ozone molecules would be transformed into OH• radical is in alkaline solutions. The linear dependence of the reaction rate on the gaseous ozone mass flow rate coupled with the fact that the residual ozone concentration in an aqueous solution was always found to be near zero suggests that the mass transfer of ozone from the gaseous phase to the aqueous phase is possibly the ratedetermining step. The formation rate constant of nitrate was much smaller than those of other anions, possibly because of the sequential oxidation of the amine group on the molecular structure to form nitrate being
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 371 Table 3. Pseudo-First-Order Rate Constant of Monocrotophos in Aqueous Solution by Ozonation at Various Ozone Mass Flow Rates rate constants ozone mass flow rate (mg/min) 0.66 1.00 2.38 3.54 5.60
removal rate constant of monocrotophos km (min-1) 0.192 0.273 0.439 0.452 0.757
r2 ) 1.00 r2 ) 0.96 r2 ) 0.99 r2 ) 0.94 r2 ) 0.96
formation rate constants of PO43- kp (min-1) 0.028 0.069 0.121 0.149 0.167
r2 ) 0.97 r2 ) 0.91 r2 ) 0.89 r2 ) 0.91 r2 ) 0.93
formation rate constants of NO3- kn (min-1) 0.0031 0.0059 0.0068 0.0106 0.0112
r2 ) 0.95 r2 ) 0.88 r2 ) 0.89 r2 ) 0.84 r2 ) 0.89
formation rate constants of CO2 kc (min-1) 0.0098 0.0103 0.0113 0.0175 0.0202
r2 ) 0.89 r2 ) 0.93 r2 ) 0.91 r2 ) 0.87 r2 ) 0.89
advantageous because the decomposition rate is linearly increased with ozone dosages while the utilization of ozone is relatively constant. The presence of HCO3-/CO32- ions was found to decrease the reaction rates (km, kp, kn, and kc) of monocrotophos by ozonation at pH 9 as shown in Table 4. Similar results were found by previous researchers (Betlran et al., l994a,b) for the ozonation of some other organic compounds and was usually attributed to the scavenging effect of bicarbonate and carbonate ions to capture OH• free radicals (Weeks and Rabani, 1966):
Figure 5. The utilizations of ozone during the oxidation of monocrotophos to form final oxidized species at various ozone dosages.
a slow process; similar findings were reported by previous researchers that ammonia is refractory to ozonation (Singer and Zilli, 1975; Hoigne and Bader, 1978). The total amount of final oxidized species formed during the ozonation of monocrotophos, which is defined as the sum of the amounts of phosphate, nitrate, and carbonate ions was slightly dependent on the amount of ozone used, which was determined by the reaction time multiplied by the difference between the ozone contents of feed and effluent gas during the oxidation of monocrotophos as shown in Figure 5. The mineralization reaction between ozone molecules and monocrotophos can be stoichiometrically shown as below:
C7H14O5PN + 22/3O3 f 7H2O + 7CO2 + PO43- + NO3- (15) The ozone used for semibatch reaction systems is considered to be sufficient to completely mineralize monocrotophos because the necessary amount of ozone can be continuously supplied to the reaction system. For a gaseous input ozone mass flow rate from 0.65 to 5.6 mg/min, the amount of ozone consumed was determined to be about 14-120 times that of the initial concentration of monocrotophos based on the mass balance of ozone, indicating that the ozone supplied to the reaction system was far beyond the required amount based on eq 15. The amount of final oxidized species was found to increase with increasing the amount of ozone used until the amount of the final oxidized species reached about 0.09 mmol/L possibly because the excessive ozone dosage was consumed by various side reactions. Thus, ozonation at reasonably elevated ozone dosages is
HCO3- + OH• f CO3•- + H2O
(16)
CO3- + OH• f CO3•- + OH-
(17)
The presence of Fe2+ and Mn2+ ions in aqueous solutions significantly retarded the decomposition of monocrotophos by ozonation for acidic solutions as shown in Table 5, compared to those in Table 2, possibly because the oxidation of Fe2+ and Mn2+ ions caused excessive ozone consumption in acidic solutions (Hoigne et al., 1985). Thus, the role of Fe2+ and Mn2+ ions in the oxidation system could be as scavengers to ozone molecules to impede the decomposition of monocrotophos. In this research, the formation of Fe3+ and Mn7+ was observed by noticing the color change of the reaction solutions, and the presence of tiny precipitates was also found in the alkaline reaction solutions. The precipitates generated during the ozonation of monocrotophos in the presence of Fe2+ were collected and further identified to be Fe(OH)3 particles by X-ray diffraction (XRD) analysis. The reaction solutions sampled sequentially at predetermined sampling times were examined by a GCDEID to identify the organic intermediated for the ozonation of monocrotophos in an aqueous solution at pH 9. Combining the analytical results by GCD-EID with those by HPLC, IC, and TOC, the more detailed decomposition pathway of monocrotophos by ozone is proposed and shown in Figure 6. The breakage of the carbon-carbon double bond by ozonation was found to occur at first to form various nitrogen- and phosphoruscontaining compounds, and subsequently decomposed to be H2O, CO2, NO3-, and PO43- species. Conclusion Ozonation has been shown to be feasible for achieving a high degree of monocrotophos decomposition in aqueous solutions. The decomposition rates of monocrotophos by ozonation were increased with decreasing solution pH, possibly due to the species distribution of monocrotophos and ozone in aqueous solution, but the mineralization of organic intermediates was significantly promoted in alkaline solutions, possibly due to
372 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 4. Pseudo-First-Order Rate Constant of Monocrotophos in Aqueous Solution by Ozonation Presence of Alkalinity at pH 9 rate constants CO32-/HCO3(mg/L) 0.0 1.0 10.0
removal rate constant of monocrotophos km (min-1) 0.452 0.453 0.160
r2 ) 0.99 r2 ) 0.95 r2 ) 0.98
formation rate constants of PO43- kp (min-1) 0.049 0.003 0.000
r2 ) 0.90 r2 ) 0.96 0.0000
formation rate constants of NO3- kn (min-1) 0.0011 0.0005 0.0000
r2 ) 0.90 r2 ) 0.84
formation rate constants of CO2 kc (min-1) 0.0175 0.0000 0.0000
r2 ) 0.85
Table 5. Pseudo-First-Order Rate Constant of Monocrotophos in Aqueous Solution by Ozonation Presence of Fe2+ and Mn2+ Ions at Various Solution pH rate constants removal rate constant of formation rate constants of formation rate constants of formation rate constants of monocrotophos km (min-1) PO43- kp (min-1) NO3- kn (min-1) CO2 kc (min-1)
metal ions pH 3 pH 5 pH 9
0.073 0.122 0.555
r2 ) 0.96 r2 ) 0.98 r2 ) 0.97
0.000 0.011 0.052
Mn2+ 2.0 mg/L pH 3 pH 5 pH 9
0.198 0.260 0.326
r2 ) 0.97 r2 ) 0.99 r2 ) 0.94
0.000 0.000 0.030
Fe2+ 10 mg/L
r2 ) 0.91 r2 ) 0.89
0.0000 0.0006 0.0010
r2 ) 0.95
0.0000 0.0000 0.0000
r2 ) 0.87 r2 ) 0.85
0.0000 0.0010 0.0235
r2 ) 0.86 r2 ) 0.85
0.0000 0.0246 0.0319
r2 ) 0.83 r2 ) 0.88
Figure 6. Organic intermediates identified by GCD-EID for the ozonation of monocrotophos and the proposed decomposition pathway of monocrotophos in aqueous solution at pH 9 by ozonation.
the formation of OH• free radicals. The decomposition rates of monocrotophos by ozonation increased with input gaseous ozone mass flow rate, but the ultiliation of ozone was relatively independent of ozone mass flow rate. The presence of HCO3-/CO32- ions inhibited the ozonation of monocrotophos due to the scavenging of OH• free radicals by carbonate ions. The decomposition pathway of monocrotophos by ozone was proposed, and the breakage of the carbon-carbon double bond was found to occur at an early stage of the decomposition of monocrotophos to form various nitrogen- and phosphoruscontaining intermediates. The temporal behavior of various species could be globally described by a simplified consecutive-parallel reaction kinetics.
Acknowledgment This research was supported in part by Grant EPA85-E3J1-09-03 from the Environment Protection Administration, Republic of China. Abbreviations [Interme]c: the concentration of organic intermediates, mM [Interme]n: the concentration of nitrogen-containing intermediates, mM [Interme]p: the concentration of phosphate-containing intermediates, mM kc: the pseudo-first-order formation rate constant of carbonate, min-1
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 373 km: the pseudo-first-order decomposition rate constant of monocrotophos, min-1 kn: the pseudo-first-order formation rate constant of nitrate, min-1 kp: the pseudo-first-order formation rate constant of phosphate, min-1 [M]: the concentration of monocrotophos, mM [M]0: the initial concentration of monocrotophos, mM [O3]gas: the concentration of ozone in the gas region
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Received for review March 17, 1997 Revised manuscript received November 20, 1997 Accepted November 20, 1997 IE970219V
