Electrodialytic Removal of Cu, Cr, and As from Chromated Copper

Jan 22, 2000 - According to ref 1, nearly 5 million tons of preservative treated wood is disposed of ..... the Cu, Cr, and As removal efficiencies are...
1 downloads 0 Views 83KB Size
Download PDF
Environ. Sci. Technol. 2000, 34, 784-788

Electrodialytic Removal of Cu, Cr, and As from Chromated Copper Arsenate-Treated Timber Waste A L E X A N D R A B . R I B E I R O , * ,† EDUARDO P. MATEUS,† LISBETH M. OTTOSEN,‡ AND GREGERS BECH-NIELSEN§ Departamento de Cieˆncias e Engenharia do Ambiente, Faculdade de Cieˆncias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Caparica, Portugal, and Department of Geology and Geotechnical Engineering and Department of Chemistry, Technical University of Denmark, 2800 Lyngby, Denmark

Waste of wood treated with chromated copper arsenate (CCA) is expected to increase in volume over the next decades. Alternative disposal options to landfilling are becoming more attractive to study, especially those that promote reuse. The authors have studied the electrodialytic removal of Cu, Cr, and As from CCA-treated timber waste. The method uses a low-level direct current as the “cleaning agent”, combining the electrokinetic movement of ions in the matrix with the principle of electrodialysis. The technique was tested in four experiments using a laboratory cell on sawdust of an out-of-service CCA-treated Pinus pinaster Ait. pole. The duration of all the experiments was 30 days, and the current density was kept constant at 0.2 mA/ cm2. The experiments differ because in one the sawdust was saturated with water (experiment 1) and in the rest it was saturated with oxalic acid, 2.5, 5, and 7.5% (w/w), respectively, in experiments 2-4. The highest removal rates obtained were 93% of Cu, 95% of Cr, and 99% of As in experiment 2. Other experimental conditions might possibly optimize the removal rates.

Introduction An increase in the amount of waste of wood treated with chromated copper arsenate (CCA) is expected over the next decades. According to ref 1, nearly 5 million tons of preservative treated wood is disposed of annually in the United States into landfills, and only in the Federal Republic of Germany, approximately 3.3-4.7 million m3 timber is impregnated every year (2). This raises a growing concern about the environmental issue of treated wood waste management. The disposal of preservative treated wood into landfills is considered to be the least preferred method, particularly from a long-term perspective: It is becoming increasingly expensive, approved landfill sites are more scarce, and in some years time, it will even be forbidden in some countries (e.g., Germany: 2005). Moreover, potential soil and groundwater contamination may arise (3). * Corresponding author phone: +351 212948300; fax: +351 212948554; e-mail: [email protected]. † Universidade Nova de Lisboa. ‡ Department of Geology and Geotechnical Engineering, Technical University of Denmark. § Department of Chemistry, Technical University of Denmark. 784

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 5, 2000

On the basis of the EU regulations, treated wood waste can be classified according to two different regulatory status: “recycled product or fuel” or “waste”. Then, in the waste status, two categories are possible: “domestic waste and assimilated” or “hazardous waste”. Up to now, no treated wood waste has been quoted as hazardous waste. However, by the classification criteria defined by different EC directives, CCA-treated wood waste could be considered that way (4). If so, these will request elimination or treatment (incineration) under specific conditions (concerning continuous assessment of atmospheric pollutants), with an estimated cost of a destruction process such as incineration amounting to 2000 FF per ton (5) (1$ U.S. ≈ 6 FF). There are studies on the recovery of metals from the ashes of incinerated CCA-treated residues for preservative makeup (6), on the recycling of treated timber by a copper smelting process (7), on the use of cement kilns for the incineration of CCA-treated timber (8), and on the reuse of spent CCAtreated wood to make composites such as wood-cement bonded products or OSB boards (9). However, other alternative options are becoming more attractive to study, particularly those that, after extraction of CCA below a certain level, promote reuse (e.g., for the manufacturing of woodbased composites). Felton and De Groot (10) reviewed processes to extract preservatives from spent treated timber. Within the biological processes, there have been some encouraging results using both fungi and bacteria to release Cu, Cr, and As from treated wood samples (3, 11-15). However, the biological approach is still laboratory based, and scale-up issues as well as the economics of the whole process will require much more attention (16). Other approaches include chemical extraction (17, 18), and recently, the authors of this paper started to study the efficiency of the use of the electrodialytic process. The electrodialytic process is an emerging remediation technique for removal of contaminants from polluted sites (19-24). The method uses a low-level direct current as the “cleaning agent”, combining the electrokinetic movement of ions in the matrix with the principle of electrodialysis. The present study reports results from the application of the electrodialytic process to an out-of-service Portuguese CCA-treated pole. The main goal is to assess whether the method can be applied to extract Cu, Cr, and As from the treated timber waste so that it can be further reused, e.g., to produce cardboard, fiberboard, or particle boards. The efficiency of the process is calculated for the experimental conditions used, and several parameters of importance that are relevant to the process are identified.

Experimental Section CCA-Treated Timber Waste. Four different laboratory experiments of similar duration were carried out using sawdust (particles of size 20 mesh diameter) prepared from an 8-yr out-of-service CCA-treated Pinus pinaster Ait. pole. The pole came from Leiria, in the middle of Portugal. The CCA formulation as well as the treatment scheme used for the pole is unknown. The “total” Cu, Cr, and As content was determined according to Method 1 in ref 25. All the reagents used were pro analysis. All sawdust weights given in the results are dry weights. Laboratory Cell. All experiments were carried out in a cell recently developed at the Technical University of Denmark (26) that is described elsewhere (e.g., refs 27 and 22-24). The cell is divided into three compartments, consisting of two electrode compartments and a central one (L ) 3 cm, i.d. ) 8 cm), in which the sawdust is placed (Figure 10.1021/es990442e CCC: $19.00

 2000 American Chemical Society Published on Web 01/22/2000

FIGURE 1. Schematic representation of the cell used in experiments. AN, anion-exchange membrane. CAT, cation-exchange membrane. 1). The electrode compartments and the sawdust were separated by ion-exchange membranes [cation-exchange membrane (CAT): IC1-61CZL386; anion-exchange membrane (AN): IA1-204SXZL386, both from Ionics Inc., Massachusetts]. Each electrode compartment contained 1000 mL of 10-2 M NaNO3, pH 2, as an electrolyte solution and was equipped with a circulation system. A power supply (Hewlett-Packard E3612A) was used to maintain a constant dc current, and the voltage drop was monitored (Fluke 37 multimeter). The electrodes were platinized titanium bars, with a diameter of 3 mm and a length of 5 cm (Bergsøe Anti Corrosion A/S, Denmark). When a voltage drop was applied between the two electrodes, the ions in the three compartments moved in the electric field, but the AN placed between anode and sawdust prevented cations from passing into the sawdust. In a similar way, the CAT placed between cathode and sawdust prevented anions from passing into the sawdust (Figure 1). The catholyte pH was maintained at 2 with HNO3, thus neutralizing the hydroxyl ions as they were generated at the cathode. The following experimental conditions were used: Before it was put in the cell, the sawdust was saturated with distilled water (experiment 1) and with oxalic acid 2.5, 5.0, and 7.5% (w/w), in experiments 2-4, respectively. In all the experiments, the current density was 0.2 mA/cm2, and duration of treatment was 30 days. During each experiment, samples of the electrolyte solutions (catholyte and anolyte) were collected and analyzed for Cu, Cr, and As. At the end of each experiment, the “total” Cu, Cr, and As content of the sawdust (central compartment of the cell) was also analyzed in accordance with ref 25. Copper and chromium were determined by atomic absorption spectrophotometry (Perkin-Elmer 5000-AAS), and arsenic was determined by inductively coupled plasma (ISA Jobin-Yvon 24-ICP).

Results and Discussion The initial estimated concentration of Cu, Cr, and As in the sawdust was 3149 ( 114 mg of Cu/kg, 8281 ( 420 mg of Cr/kg, and 8111 ( 871 mg of As/kg. These results fall in the wide range reported in the literature for CCA-treated sawdusts (mg/kg): 1176-4285 for Cu, 2280-11909 for Cr, and 118610506 for As (28-30, 11, 3), where the lower limits concern the average concentrations found in stakes after 41 years in a field trial (28), and the upper data are a limiting case for CCA-treated wood waste (29). Figure 2 presents the voltage drop between working electrodes during the experiments 1-4. Figure 3 presents Cu, Cr and As measured in the catholytes (1-, 2-, 3-, and 4-) and in the anolytes (1+, 2+, 3+, and 4+) collected during the experiments 1-4. Table 1 presents the Cu, Cr, and As removal efficiencies obtained at the end of experiments 1-4, and Table 2 presents the Cu, Cr, and As amounts found in the electrode cell compartment sections (in the anode compartment: in the anolyte and in the anion-exchange

FIGURE 2. Voltage drop between working electrodes during the experiment 1 (sawdust saturated with water) and experiments 2-4 [sawdust saturated with 2.5, 5, and 7.5% (w/w) oxalic acid, respectively]. membrane; in the cathode compartment: cation-exchange membrane, in the catholyte, and in the cathode) at the end of experiments 1-4. The electrical resistance measured in the cell of experiments 2-4 shows lower values when compared with experiment 1 (Figure 2). In this first experiment, the voltage drop was nearly always more than two times the highest value registered during experiment 2. These results are in accordance with what was expected. With the oxalic acid incubation (experiments 2-4), an excess of ions was added to the sawdust, the conductivity was kept high, and the voltage drop between working electrodes was low. First, the dissociation of oxalic acid occurred in the sawdust. Second, other ions than the contaminant ones (the interesting ones), either in simple forms or complexed, may also have been mobilized due to the acid condition. In addition, the lower cell voltage meant a lower energy consumption. The increased number of non-heavy metal ions could have resulted in a lower current yield but was totally compensated by the increase in the process efficiency. At the end of the experiment 2, 93% of Cu, 95% of Cr, and 99% of As came out of the sawdust into one of the electrode cell compartments (Table 1). These were the highest overall removal efficiencies obtained in the four experiments, decreasing them in the following order: exp 2 > exp 3 > exp 4 > exp 1 (Table 1). The efficiency decrease from experiment 2 to experiment 4 is due to the decay in Cu removal. The lowest efficiency was found in experiment 1 where, despite high Cu removal (91%), the system fails to remove Cr and shows the lowest efficiency for As (27%), meaning that for concentrations of oxalic acid far below 2.5% a decrease in the removal of Cr and As could be expected. Copper. Copper was mobilized in the sawdust in all experiments. In Figure 3a,b and Table 2, there is a clear indication that Cu electromigrates as a cation in experiment 1 (sawdust saturated with water), because the cathode compartment presents an accumulation of this metal. VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

785

FIGURE 3. Copper, chromium, and arsenic measured in the electrolyte solutions collected during the experiments 1-4: (panel a) Cu concentration in the anolytes 0 1+, ] 2+, O 3+, 4 4+; (panel b) Cu concentration in the catholytes 0 1-, ] 2-, O 3-, 4 4-; (panel c) Cr concentration in the anolytes 0 1+, ] 2+, O 3+, 4 4+; (panel d) Cr concentration in the catholytes 0 1-, ] 2-, O 3-, 4 4-; (panel e) As concentration in the anolytes 0 1+, ] 2+, O 3+, 4 4+; and (panel f) As concentration in the catholytes 0 1-, ] 2-, O 3-, 4 4-.

TABLE 1. Copper, Chromium, and Arsenic Removal Efficiencies (%) Obtained at the End of Experiments 1-4 % removed

exp 1

exp 2

exp 3

exp 4

Cu Cr As

91.4 26.7

93.1 94.8 98.7

39.6 93.4 96.6

22.5 95.6 92.2

However, in experiments 2-4 (saturated with oxalic acid), Cu was removed to both electrode compartments partly as a cation and partly as an anionic complex. The anionic complex is most likely to be CuOx22-, which is known to be rather stable (31). In addition, Cu also reacts with oxalic acid to form copper oxalate (CuOx), which has a limited water solubility and therefore precipitates in the wood (32), which may prevent Cu removal by the electrodialytic process. In experiments 2-4, Cu mobilization toward the anode compartment shows a similar profile (Figure 3a). However, Figure 3b suggests that the efficiency of Cu removal is dependent on Cu mobilization toward the cathode compartment. The beginning of this mobilization is linked with the oxalic acid initial concentration (Figure 3b). During experiments 2-4, the action of the electric field (e.g., electromigration and/or electroosmosis) carries out the oxalic acid from the central cell compartment toward the electrode compartments, where it will remain, due to the ion-exchange membranes role (electrodialysis). The consequence of this mobilization is a continuous decay in its concentration in the central cell compartment. At a certain time, dependent on the initial oxalic acid concentration, there will be no more free oxalic acid in the sawdust compartment. If by then some copper 786

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 5, 2000

TABLE 2. Copper, Chromium, and Arsenic Amounts Found in the Electrode Cell Compartment Sections at the End of Experiments 1-4a cell section Cu (mg)

Cr (mg)

As (mg)

exp 1 exp 2 exp 3 exp 4 exp 1 exp 2 exp 3 exp 4 exp 1 exp 2 exp 3 exp 4

anolyte

ANb

CATb

catholyte

cathodec

0.1 22.8 16.8 15.0 0.9 134.8 108.0 111.4 54.5 248.7 210.3 235.3

0.2 1.2 1.3 1.3 3.9 9.1 13.1 12.7 4.1 7.7 8.3 6.0

19.8 8.1 4.7 0.5 0.2 2.3 6.2 9.3 0.2 1.4 2.3 3.0

5.8 42.7 15.6 2.5 0.1 11.7 8.2 4.6 nd 9.1 9.4 14.1

20.2 6.3 2.6 0.8 0.1 3.5 6.3 1.3 nd 1.4 2.2 1.0

a AN, anion-exchange membrane; CAT, cation-exchange membrane; nd, not detected. b Membranes immersed in 100 mL of 1 M HCl during 48 h and filtered (0.45 µm), and the filtrate was analyzed. c Recovery of the deposited Cu at the cathode by reversal of potential in 100 mL of 1 M HCl, which was filtered (0.45 µm), and the filtrate was analyzed.

still remains, it will be in the form of copper oxalate. Even if this compound has a low solubility, there will be copper cations and oxalate anions in equilibrium with the solid phase, and since these ions will move in opposite directions, the ultimate removal of Cu will take place with Cu moving toward the cathode compartment. In fact, at the end of 30 days of experiments 3 and 4, removal was still occurring (Figure 3b). In Table 1, the decrease of Cu removal in experiments 3 and 4 is probably just apparent, and for longer experiments (>30 days), Cu will be removed at levels similar

to the ones obtained in experiments 1 and 2. Chromium. Chromium mobilization in the cell only occurred in experiments 2-4, being dominated by the flux toward the anode compartment (Figure 3c,d and Table 2). Until approximately day 10 (Figure 3c), there is an exponential removal of Cr as an anion, slowing a bit down after this time. The negatively charged Cr species seem to be easily mobilized from the sawdust and also to migrate easily. Hexavalent chromium forms a number of oxyacids or anions. The dissolved species of Cr(VI) are the hydrogen chromate (HCrO4-) ion, the dichromate (Cr2O72-) ion, and the chromate (CrO42-) ion (24). All the anionic forms are quite soluble (in the absence of Pb2+ and Ba2+) and thus quite mobile (33). CrO42- is known to adsorb onto soil colloids as outer-sphere complexes (34) and can thus be readily desorb. As longer times of the experiments it seems that cationic Cr species are available for migration toward the cathode compartment (Figure 3d). The presence of oxalic acid could have two effects: (i) Cr(III) is known to form complexes with oxalate (31), and the rather stable CrOx33- will, as an anionic complex, move in the direction of the anode. (ii) A reduction of chromate by oxalate is possible in acid conditions. The Cr(III) ions formed in this way may react to form the oxalate complex. However, some of the Cr(III) ions will exist as positively charged ions and move toward the cathode. According to ref 35, the most mobile Cr(III) species are soluble organic complexes (in the present case: CrOx33-) and dispersible colloidal sized inorganic hydroxy polymers, silicates, carbonates, and clay minerals. Arsenic. Arsenic moves in the electrodialytic cell mainly toward the anode compartment (Figure 3e,f and Table 2). Some low As concentrations were also obtained in the cathode compartment. As refered in ref 23, the species most stable over the pH ranges 4-8 are expected to be H3AsO3 (up to pH 9), H2AsO4- (approximately pH 2-7), and HAsO42(above pH 7). It was then expected that arsenic would move toward the anode compartment as anions. However, AsO+ or As(OH)+, and in even more acid solutions As3+, ions may exist, capable of moving in the direction of the cathode (Figure 3f). Further research should be carried out to estimate the point of minimum oxalic acid addition for which the Cu, Cr, and As removal efficiencies are maximized (somewhere between 0 and 2.5%, for this matrix). This estimated point should also be confirmed for wood wastes containing different amounts of CCA as well as the evaluation of other assisting agents for the electrodialytic process. Summary. The authors have studied the removal of copper, chromium, and arsenic from a CCA-treated timber waste using electrodialysis. Three aspects of the study contribute to its originality: (i) the use of the electrodialytic process to this kind of matrix; (ii) the use of an actual contaminated timber waste rather than a spiked sample, and (iii) as the process proved successful, this study opens the opportunity for the reuse of the sawdust, namely, to produce cardboard, fiberboard, or particle boards, or even to recycle both the wood fiber and the metals separately. We succeeded in removing 93% of Cu, 95% of Cr, and 99% of As by the electrodialytic process, using 2.5% oxalic acid as an assisting agent (conditions of experiment 2). It should be stressed that variation of experimental conditions might contribute to the optimization of the removal rates and efficiencies.

Acknowledgments A.B.R. is grateful to D. Reima˜o, Estac¸ a˜o Florestal Nacional, for supplying the CCA-treated timber sawdust; to Instituto de Tecnologia Quı´mica e Biolo´gica for ICP facility; and to Departamento de Pedologia, Estac¸ a˜o Agrono´mica Nacional,

for support, particularly to O. Monteiro and A. M. Mendes for their help with analytical work.

Literature Cited (1) Falk, B. For. Prod. J. 1997, 47 (6), 17-22. (2) Peek, R.-D. Proceedings of the 4th International Symposium on the Challenge of Safety and Environment in Wood Preservation, Cannes-Mandelieu, France, February 1998; IRG/WP 98-50101; International Research Group on Wood Preservation: 1998; pp 55-66. (3) Clausen, C. A.; Smith, R. L. J. Ind. Microbiol. Biotechnol. 1998, 20, 251-257. (4) Deroubaix, G. Proceedings of the 4th International Symposium on the Challenge of Safety and Environment in Wood Preservation, Cannes-Mandelieu, France, February 1998; IRG/WP 9850101; International Research Group on Wood Preservation: 1998; pp 67-76. (5) Labat, G. Proceedings of the 4th International Symposium on the Challenge of Safety and Environment in Wood Preservation, Cannes-Mandelieu, France, February 1998; IRG/WP 98-50101; International Research Group on Wood Preservation: 1998; pp 205-220. (6) Pasek, E. A.; McIntyre, C. R. 24th Annual Meeting of the International Research Group on Wood Preservation, Orlando, FL, 1993; Paper IRG/WP 93-50007. (7) Nurmi, A. J.; Lindroos, L. 25th Annual Meeting of the International Research Group on Wood Preservation, Nusa Dua, Japan, 1994; Paper IRG/WP 94-50030. (8) Stephens, R. W. Proceedings of the 16th Annual Meeting of Canadian Wood Preservation Association; 1995; pp 89-93. (9) Vick, C. B.; Geimer, R. L.; Wood, J. E., Jr. For. Prod. J. 1996, 46 (11/12), 89-91. (10) Felton, C. C.; De Groot, R. C. For. Prod. J. 1996, 46 (7/8), 37-46. (11) Clausen, C. A. 28th Annual Meeting of the International Research Group on Wood Preservation, Whistler, Canada, 1997; Paper IRG/WP 97-50083. (12) Legay, S.; Labat, G. 28th Annual Meeting of the International Research Group on Wood Preservation, Whistler, Canada, 1997; Paper IRG/WP 97-50090. (13) Leithoff, H.; Peek, R.-D. 28th Annual Meeting of the International Research Group on Wood Preservation, Whistler, Canada, 1997; Paper IRG/WP 97-50095. (14) Burgstaller, W.; Schinner, F. J. Biotechnol. 1993, 27, 91-116. (15) Stephan, I.; Peek, R.-D. 23rd Annual Meeting of the International Research Group on Wood Preservation, Harrogate, U.K., 1992; Paper IRG/WP 92-3717. (16) Greaves, H.; Norton, J. Proceedings of the 4th International Symposium on the Challenge of Safety and Environment in Wood Preservation, Cannes-Mandelieu, France, February 1998; IRG/ WP 98-50101; International Research Group on Wood Preservation: 1998; pp 1-13. (17) Honda, A.; Kanjo, Y.; Kimoto, A.; Koshii, K.; Kashiwazaki, S. 22nd Annual Meeting of the International Research Group on Wood Preservation, Kioto, Japan, 1991; Paper IRG/WP 91-3651. (18) Kazi, K. M. F.; Cooper, P. A. 29th Annual Meeting of the International Research Group on Wood Preservation, Maastricht, The Netherlands, 1998; Paper IRG/WP 98-50107. (19) Lageman, R.; Pool, W.; Seffinga, G. Chem. Ind. 1989, 9, 585590. (20) Pamukcu, S.; Wittle, J. K. Environ. Prog. 1992, 11 (3), 241-250. (21) Probstein, R. F.; Hicks, R. E. Science 1993, 260, 498-503. (22) Ottosen, L. M.; Hansen, H. K.; Laursen, S.; Villumsen, A. Environ. Sci. Technol. 1997, 31, 1711-1715. (23) Ribeiro, A. B. Ph.D. Dissertation, Technical University of Denmark, Denmark, 1998 (in English). (24) Ribeiro, A. B.; Villumsen, A.; Bech-Nielsen, G.; Re´fega, A.; Silva, J. V. Proceedings of the 4th International Symposium on the Challenge of Safety and Environment in Wood Preservation, Cannes-Mandelieu, France, February 1998; IRG/WP 98-50101; International Research Group on Wood Preservation: pp 149160. (25) BS 5666 British Standard Methods of analysis of Wood preservatives and treated timber, Part 3, Quantitative analysis of preservatives and treated timber containing copper/chromium/ arsenic formulations; British Standard Institution: London, 1979. (26) Ottosen, L. M.; Hansen, H. K. Electrokinetic cleaning of heavy metal polluted soil; Internal Report, Fysisk-Kemisk Institut & Institut for Geologi og Geoteknik; Technical University of Denmark: Denmark, 1992 (in English). (27) Ribeiro, A. B.; Mexia, J. T. J. Hazard. Mater. 1997, 56, 257-271. VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

787

(28) Hudson, N. J.; Murphy, R. J. 28th Annual Meeting of the International Research Group on Wood Preservation, Whistler, Canada, 1997; Paper IRG/WP 97-50098. (29) Helsen, L.; Van den Bulck, E.; Van den Broeck, K.; Vandecasteele, C. Waste Manage. 1997, 17 (1), 79-86. (30) Kamdem, D. P.; Ma, W.; Zhang, J.; Zyskowski, J. 29th Annual Meeting of the International Research Group on Wood Preservation, Maastricht, The Netherlands, 1998; Paper IRG/WP 9850118. (31) Martell A. E. Stability Constants; Special Publication 17, Section II: Organic Ligands; Martell, A. E., Compiler; The Chemical Society: London, 1964. (32) Stephan, I.; Leithoff, H.; Peek, R.-D. Mater. Org. 1996, 30 (3), 179-199.

788

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 5, 2000

(33) Saleh, F. Y.; Parkerton, T. F.; Lewis, R. V.; Huang, J. H.; Dickson, K. L. Sci. Total Environ. 1989, 86, 25-41. (34) Charlet, L.; Manceau, A. A. J. Colloid Interface Sci. 1992, 148, 443-458. (35) Bartlett, R. J.; James, B. R. Chromium in the natural and human environments; Nriagu, J. O., Nieboer, E., Eds.; Advances in Environmental Science & Technology 20; Wiley-Interscience: New York, 1988; pp 267-304.

Received for review April 20, 1999. Revised manuscript received September 14, 1999. Accepted November 24, 1999. ES990442E