Leaching and Microstructural Analysis of Cement-Based Solidified

In the pozzolanic reaction of PFA, calcium hydroxide, Ca(OH)2, liberated by cement hydration reacts with silicate phases of PFA to produce calcium sil...
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Environ. Sci. Technol. 2000, 34, 5038-5042

Leaching and Microstructural Analysis of Cement-Based Solidified Wastes

Environ. Sci. Technol. 2000.34:5038-5042. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/26/19. For personal use only.

I R E N E M A N - C H I L O , * ,† C H O N G - I . T A N G , † XIANG-DONG LI,‡ AND CHI-SUN POON‡ Department of Civil Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, and Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Leach tests alone cannot determine the actual performance and long-term environmental impacts of solidified/ stabilized waste because they do not provide information on the physical and chemical changes between waste and additives. This research study utilized a combined approach to investigate the leaching behaviors and binding chemistry of a solidified/stabilized industrial waste. The combined approach included leach tests and microstructural analysis. The OPC system showed more satisfactory results in TCLP and DLT than the OPC/PFA system. Because no identifiable hydration products were observed, SEM/ EDS images and XRD of solidified waste suggest that the presence of zinc retards the hydration of OPC and PFA. However, a new crystalline compound, ZnO, is detected by XRD in samples A01 and B02. These are believed to be a result of the addition of cement to zinc sludge, which may cause zinc to change from its amorphous to crystalline form. In addition, a certain amount of zinc was found on the surface of solidified waste by XPS. The use of microstructural analysis has been proved to be essential in providing useful information on the physical and chemical changes within the cement matrix.

Introduction The disposal of heavy metal-bearing sludge is a major problem in many countries. To reduce the harmful effects and environmental risks, certain treatment processes are required prior to disposal. Solidification/stabilization (S/S) is one of the major treatment methods in dealing with hazardous wastes and metal containing sludge (1, 2). In fact, S/S is referred to as the Best Demonstrated Available Technology (BDAT) in the U.S.A. for treating a wide range of characteristic and nonwastewater wastes listed by Resource Conservation and Recovery Act (RCRA) (3). Solidification/stabilization is a process employing additives to reduce the hazardous nature of a waste by converting the waste and its hazardous constituents into a relatively stable form. “Solidification” refers to the conversion of sludge from a fluid state into a solid state, forming a monolith solid. “Stabilization” means the product is insoluble, which in turn reduces the solubility, mobility, and toxicity of the waste (4). * Corresponding author phone: 852 2358 7157; fax: 852 2358 1534; e-mail: [email protected]. † The Hong Kong University of Science and Technology. ‡ The Hong Kong Polytechnic University. 5038

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Thus, S/S can be used to minimize the rate of contaminant migration into the environment and greatly reduce, if not eliminate, levels of toxic pollution caused by the waste. Although this technology has been used for a long time, the physical and chemical changes that take place as a result of interaction of contaminants with cement and other reagents have not been fully characterized. Normally, the success of a S/S process is determined by unconfined compressive strength tests, leach tests, and possibly other physical properties tests (5). These tests provide data that can be compared to the regulatory standards, but because they do not provide information on the interaction of contaminants with the additives, they can only be a shortterm assessment of the solidified waste. Understanding the physical and chemical changes within the cement matrix is a necessary step for assessing the actual long-term behavior of solidified waste. Various researchers have proposed the use of a number of sophisticated techniques to study the microstructure of solidified waste, particularly the physiochemical changes in cement matrices (6, 7). The objective of this study was to investigate the leaching behaviors and binding chemistry of cement-based solidified/ stabilized wastes. The toxicity characterization leaching procedure (TCLP) and dynamic leach test (DLT) were used to evaluate leaching behaviors. Scanning electron microscopy/ energy dispersive spectrometry (SEM/EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to analyze the microstructure of solidified wastes. These newly combined approaches provide useful information on the chemistry and microstructure of heavy metals in the cementitious materials.

Experimental Materials and Methods Source of Sludge and Additives. Zinc sludge was obtained from processed industrial wastewater of a zinc-galvanizing factory. Wastewater generated by the plant was precipitated by sodium hydroxide and then settled in a sedimentation tank followed by a filter press process to produce zinc sludge of about 25% solid content. The heavy metals in the raw sludge consisted of about 40% zinc, 16% iron on a dry weight basis, and small amounts of manganese, copper, lead, potassium, cobalt, nickel, cadmium, silicate, and chloride. The raw sludge was solidified/stabilized by Type I ordinary Portland cement (OPC) and Grade F pulverized fly ash (PFA). The Type I OPC was obtained from a local cement plant. The PFA was from the China Light & Power Plant in Hong Kong. The composition of OPC, as given by the plant, was 65% CaO, 20% SiO2, 6% Al2O3, and trace amounts of other components such as magnesium. In general, the composition of Portland cement consists of predominantly tricalcium silicates (C3S) and dicalcium silicates (C2S), with small amounts of tricalcium aluminate (C3A) and calcium aluminoferrite (C4AF). The hydration of both C3S and C2S produce calcium silicate hydrates (C-S-H) and calcium hydroxide crystals (CH) (8). The composition of Grade F fine PFA, according to the supplier, consisted of 48% SiO2, 25% Al2O3, 6% Fe2O3, 6% CaO, and other components. PFA is a pozzolan, which is a siliceous or siliceous and aluminous material. PFA in itself possesses little or no cementitious properties but will chemically react with calcium hydroxide to form compounds that possess cement-like properties. In the pozzolanic reaction of PFA, calcium hydroxide, Ca(OH)2, liberated by cement hydration reacts with silicate phases of PFA to produce calcium silicate hydrates (9). 10.1021/es991224o CCC: $19.00

 2000 American Chemical Society Published on Web 10/27/2000

TABLE 1. Composition of Control Samples and Solidified Samples composition (by weight) sample

Zn sludge (%)

R01 X01 X02 X06 A01 B02 T01 T02 D01 D02

100

OPC (%)

PFA (%)

TABLE 2. Multiple TCLP and pH Results at Different Extractions sample

extraction no.

pH

Zn concn (mg/L)

Ca concn (mg/L)

T01

1 2 3 4 5 1 2 3 4 5

12.1 11.0 9.4 7.2 6.6 11.9 8.5 6.7 6.5 5.5

0.4 0.1 0.1 284 398 0.2 2.1 272 2384 803

2508 2190 2140 1995 1256 2626 2316 1480 896 586

100 50 50 50 50 50 50

60 50 30 50 30 50 30

100 40 20 20

T02

20

Solidification/Stabilization Process. Sludge was first oven-dried for 24 h at 105 °C to remove moisture and then ground to fine particles. OPC and PFA were then mixed with the ground sludge particles at various ratios. Distilled water was added at a water-to-OPC & PFA ratio of 0.3 to facilitate curing. Finally, the samples were poured into plastic molds and cured for 28 days. After 28 days, the samples were demolded and crushed to particles less than 10 mm for leaching tests. The respective composition ratios of the control samples and solidified/stabilized samples are presented in Table 1. Leach Tests. The TCLP test was conducted generally in accordance with the method outlined in the U.S. Code of Federal Regulations (10). The test procedures, however, were slightly modified in such a way that a total of five, rather than one, progressive extractions were carried out (11). The crushed samples with particle size of less than 10 mm were put into polypropylene bottles, and TCLP extraction fluid of dilute acetic acid was added. The bottles were agitated using a rotary extractor for 18 h. The DLT employed was a modified version of the American Nuclear Society Leach Test. Whole samples were immersed into the TCLP extraction fluid in a polypropylene beaker without agitation and incubated at a constant temperature of 20 °C. The leachant was renewed periodically. Heavy metals and major elements in the leaching solution were determined using Induced Couple Plasma Spectrometry (ICP-AES). Microstructural Analysis. Samples cured for 28 days were demolded and oven dried for 24 h. Fracture specimens were prepared and coated with gold film prior to SEM/EDS examination using Philips XL30+EDS, with LaB6 filament. SEM analyzes the microstructure and morphology of the solidified samples. With the help of an EDS, which determines the elemental composition of the waste and atomic percentages of the elements, different microstructures can be identified. SEM typically has a penetration depth up to a few tens of Å. Powdered samples were used for XRD analysis by a Powder X-ray Diffraction System (Philips PW 1830). The XRD analysis was carried out by scanning the sample between 2 θ angles from 5° to 70°, with a speed of 0.02° per second. XRD is a bulk analytical technique with a penetration of about 1 µm. It is commonly used to identify the crystalline phases in the samples. Fine powder was prepared for XPS analysis using a Physical Electronics, PHI 5600. The X-ray source was from an aluminum monochromatic source with a takeoff angle of 45°. Because it has a penetration of less than 60 Å, XPS is a pure surface analytical tool. Surface characterization is a rather recent but absolutely necessary technique for S/S research since the waste and other compounds might locate on the surface of the cement particles. XPS analysis provides both qualitative and quantitative information by giving the

binding energy and concentration of each element. Using XPS, it is possible to determine the compounds formed by the elements by measuring the binding energies.

Results and Discussion TCLP was performed for a total of five progressive extractions, the extraction results of which are shown in Table 2. For sample T01 with 50% OPC and 50% sludge, zinc leaching was minimal during the first three extractions and dramatically increased at the fourth and fifth extractions. The solution pH was approximately 12 in the first extraction as a large amount of calcium was being leached out. It reached approximately 6.6 by the fifth extraction. The reduction in pH was probably due to the losses of hydroxide ions from Ca(OH)2 during extraction. Because the amount of calcium being leached decreases in the later extractions, the pH decreased as well. Ca(OH)2 contributes to the alkalinity of the system and hence reduces the leaching of heavy metals into the water environment. Thus, with the drop in the pH values, zinc leaching increased. For sample T02 with 30% OPC, 20% PFA, and 50% sludge, zinc leaching was minimal during the first two extractions but increased significantly over the last three extractions. The pH of sample T02 was about 12 during the first extraction and decreased to about 5.5 in the final extraction. The total amount of zinc leaching in sample T02 is much higher than that of T01 in the total extractions. This indicates the adverse effect of adding PFA (or reducing the amount of OPC) on zinc leachability. In addition, as PFA was added, the rise in pH was smaller due to the smaller amount of alkalinity contained in the sample T02. The pH values were, in general, lower in sample T02 than in sample T01. It is noted that heavy metals generally have a lower solubility at higher pH. With the higher pH found in sample T01, zinc leaching was relatively lower as depicted in Table 2. As the pH value decreases in a larger degree, leaching of zinc from sample T02 results in a much higher concentration. DLT was performed for a total of eight progressive extractions. Table 3 shows the DLT results of sample D01 (50% OPC and 50% sludge) and sample D02 (30% OPC, 20% PFA, and 50% sludge). Because the monolithic samples were suspended in the leaching solution without any agitation, the main leaching mechanism for DLT was by diffusion from the surface. In general, zinc and calcium leaching increased with increasing number of extractions in both samples. However, zinc leaching was higher in sample D02 in all eight extractions. This corresponds to the results of TCLP: reducing OPC by adding PFA increased the leachability of zinc from the solidified waste materials. The leaching pattern of DLT was different from that of TCLP. In TCLP, the initial acidic environment of the extraction solution was significantly changed to an alkaline environment by large amounts of calcium and OH- released during agitation. As a result, zinc leaching was minimal during the VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. DLT and pH Results at Different Extractions sample

extraction no.

pH

Zn concn (mg/L)

Ca concn (mg/L)

D01

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

3.2 3.4 3.5 4.2 4.1 4.0 4.8 4.0 3.6 3.8 4.1 4.1 4.0 3.9 5.0 5.1

32 73 93 268 301 242 369 508 221 66 461 397 325 271 736 919

388 504 589 1605 1282 1068 2584 2647 220 241 412 1357 1004 800 2621 2575

D02

FIGURE 2. SEM image of sample X02.

FIGURE 3. SEM image of sample X06. FIGURE 1. SEM image of sample X01. first three extractions when the pH was high. In DLT, little calcium is released to neutralize the low initial pH during extraction. As a result, zinc leaching for the first three extractions was higher than in TCLP. Compared with samples T01 and D01, it was found that the addition of PFA in sample T02 and D02 increased zinc leachability in both leach tests. Although the U.S. EPA has no limit for zinc leaching as measured by TCLP, California has a state standard (12). Applying the California standard, the zinc concentration from the first three extractions of samples T01 and D01 (with only OPC added) met the required soluble threshold limit concentration (STLC) of 250 mg/L. For samples T02 and D02 with the addition of PFA, only the first two extractions met the California standard. However, it should be noted that the U.S. EPA currently only requires one single TCLP extraction result. SEM/EDS provides morphological structure and elemental composition of the material in stabilized wastes. SEM/ EDS images and elemental analysis make it possible to identify the possible materials and roles of OPC and PFA in the presence of zinc sludge. Approximately 10 spot analyses were performed on each sample. SEM of sample R01 (100% sludge) showed some clusters of particles (not shown in this paper). The quantitative analysis from EDS showed that R01 was composed of Na, Zn, Fe, Al, Cu, Si, O, and Cl in which Na, Zn, and Fe are the main elements. The SEM image of the control sample X01 (100% OPC) is shown in Figure 1. The typical morphology of mature cement paste and hydration products, e.g. CH crystals and fibrous C-S-H, can be seen. The EDS spectra showed that some spots contained a high concentration of calcium and some contained both calcium 5040

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and silicate. These results indicate the possible presence of CH crystals and fibrous C-S-H. Figure 2 shows the SEM image of X02, with 100% PFA. Typical glassy spherical particles can be observed, which are believed to be unhydrated PFA. The irregular particles are also believed to be PFA particles since spot analysis by EDS showed a composition similar to that of the glassy ones, which have high concentrations of silicate and aluminate. A SEM image of sample X06 (60% OPC and 40% PFA) is shown in Figure 3. This sample had the same OPC:PFA ratio as the solidified samples B02, T02, and D02. Compared with Figures 1 and 2, it is believed that hydrated PFA, C-S-H, and CH are all present in the sample X06. Figure 4 illustrates that SEM image of solidified sample A01 (50% OPC and 50% sludge) has a strikingly different appearance to the control samples. The micrograph delineates non-well-defined shapes with no particular hydration products that have been found in samples X01 and X02. EDS consists mainly of zinc, calcium, and silicate which suggests the sample A01 might contain a mixture of sludge and some unhydrated calcium silicate. Roy et al. (13) also reported finding discrete particles of unhydrated calcium silicate and sludge in their study. A SEM image of sample B02 (30% OPC, 20% PFA, and 50% sludge) is shown in Figure 5. A pattern of non-well-defined structure is also observed, and EDS detected these spots containing a similar composition as sample A01 and suggested a mixture of sludge and unhydrated calcium silicate. In addition, lots of individual unhydrated PFA spheres were also found, and spot analysis of PFA particles indicated a composition of calcium silicate only with no sludge components. The leach test results showed that samples T02 and D02, both of which had PFA added, were less effective than samples

FIGURE 4. SEM image of sample A01.

FIGURE 5. SEM image of sample B02.

FIGURE 6. XRD pattern of sample R01. Main crystalline compound: o = Na2ZnSiO4‚H2O, JCPDS #01-1099. without PFA in preventing zinc leaching. The reason for the poorer performance of solidified wastes with PFA added is perhaps the fact that PFA does not interact well with zinc sludge. Compared to the control samples, samples A01 and B02 are very different in morphological features. Two typical hydration products, CH crystals and fibrous C-S-H, were not observed when sludge had PFA and OPC present. It is believed that the addition of zinc sludge might have retarded the hydration of OPC and PFA and thus prevented the formation of these hydration products. X-ray diffraction (XRD) was used to identify the crystalline phases of the samples. As shown in Figure 6, although XRD detects crystalline phases, the lumps in the XRD pattern indicate that a large amount of R01 is of the amorphous phase. The crystalline phases, which match well with the peaks of XRD of sample R01, were Mg6Fe2CO3(OH)16‚4H2O, MgAl(OH)14‚H2O, and Na2ZnSiO4‚H2O. However, the presence

of Mg6Fe2CO3(OH)16‚4H2O and MgAl(OH)14‚H2O are unlikely as neither the bulk analysis nor the EDS analysis of raw sludge indicate the presence of Mg. Figure 7 shows the XRD pattern of the control samples X01 (100% OPC), X02 (100% PFA), and X06 (60% OPC and 40% PFA). In sample X01, XRD shows the peak of CH, calcium carbonate (CaCO3), C-S-H, and calcium magnesium aluminum silicate (Ca54MgAl2Si16O90), whereas CH is the main crystalline compound. Sample X02 is dominated by CH, CaCO3, and quartz (SiO2), with some amounts of aluminum silicate hydrate (Al4Si2O10‚H2O) and aluminum silicate (also named as Mullite, Al6Si2O13) present. Quartz is a major component of PFA, and its peak demonstrates the presence of PFA, which is not detected in sample X01. XRD shows peaks of CaCO3, quartz, CH, Al4Si2O10‚H2O, and Al6Si2O13 in sample X06. It seems that CaCO3 is the main crystalline material. The XRD results of the solidified samples A01 (50% OPC and 50% sludge) and B02 (30% OPC, 20% fine PFA, and 50% sludge) are shown in Figure 8. Crystalline compounds such as calcium silicate (Ca3SiO5), CaCO3, Ca54MgAl2Si16O90, zincite (ZnO), and hydrocalumite (Ca4Al2O6Cl2‚10H2O) crystals were found in sample A01, whereas Ca3SiO5 crystals are found to be the dominant compound. In sample B02, Al6Si2O13, CaCO3, Ca4Al2O6Cl2‚10H2O, and ZnO are detected, whereas Al6Si2O13 and CaCO3 are the main crystalline components. Comparing the solidified samples to the control samples, the major difference is the disappearance of CH and C-S-H crystals. This result is supported by the SEM images in which some typical hydration products such as CH crystals and fibrous C-S-H are not observed. Poon et al. (14) studied the mechanisms of metal stabilization by cement-based fixation processes. Their results showed that CH crystals were absent, and Ca(OH)2 played a role in the fixation of zinc. From this study, it is believed that the disappearance of CH implies that Ca(OH)2 is involved in the binding mechanism of zinc. The presence of zinc sludge is likely to retard cement hydration, and thus Ca(OH)2 cannot be formed into CH crystals. If this hypothesis is correct, Ca(OH)2 is free to react with zinc. The formation of ZnO crystals suggest that Ca(OH)2 might be involved in fixing zinc. Because the majority of Ca(OH)2 comes from OPC and less amount of Ca(OH)2 is available in the OPC/PFA system to react with the leaching solutions, it is not surprising that zinc leaching in samples T02 and D02 was higher than in samples T01 and D01. In addition, ZnO crystals were found in both the OPC and OPC/PFA systems, probably due to the replacement of calcium from Ca(OH)2 by zinc, and partially explains the results of TCLP, which in turn shows that the leaching of zinc is relatively small in contrast to the leaching of calcium. Previous studies (15, 16) suggested that zinc could be precipitated as hydroxide or could react with Ca(OH)2 to produce calcium zincate [CaZn2(OH)6‚H2O]. The latter crystalline compound was not found by XRD detection in this study. Since all solidified samples appear to contain a large portion of amorphous sludge, it is believed that ZnO was trapped physically within the sludge. This means that both physical and chemical fixations are involved in this S/S process, but physical fixation is perhaps the key mechanism. XPS is a pure surface technique with a penetration of less than 60 Å. Through the binding energies given by the XPS analysis, it is possible to determine qualitative information about the surface of a test sample. The XPS results of samples A01 and B02 are shown in Figure 9. From the database, ZnO has a binding energy of 1021.8 eV, while Ca3Si3O9 has a binding energy of 347.0 eV. Referring to the results of A01 and B02, Zn falls in the range of 1021-1022, and it is most likely ZnO. Calcium from the samples falls in the range of 346-347.5, and, as a result, calcium has a high probability of being Ca3VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. XRD pattern of samples X01, X02, and X06. Main crystalline compounds: o = Ca(OH)2, JCPDS # 44-1481; a = CaCO3, JCPDS # 05-0586; b = SiO2 (quartz), JCPDS # 33-1161; c = Al6Si2O13, JCPDS # 15-0776; d = Al4Si2O10‚H2O, JCPDS # 10-0478. stabilized sludge wastes when they are disposed in landfills, especially under acidic conditions because of increasing water solubility of Zn compounds at lower pH.

Acknowledgments This research work was supported by a grant from the Competitive Earmarked Research Grant Council, Hong Kong Special Administrative Region.

Literature Cited

FIGURE 8. XRD pattern of samples A01 and B02. Main crystalline compounds: o = Ca(OH)2, JCPDS # 44-1481; a = CaCO3, JCPDS # 05-0586; c = Al6Si2O13, JCPDS # 15-0776; e = Ca4Al2O6Cl2‚10H2O, JCPDS # 31-0245; f = CaSiO5, JCPDS # 42-0551; g = ZnO, JCPDS # 36-1451.

FIGURE 9. XPS of samples A01 and B02. Si3O9 in the sample. Similar findings on the preferential deposited of zinc on the surface of solidified wastes were also reported (6). XPS also provides a quantitative analysis. The relative concentration of zinc in sample A01 is much higher than that of calcium and silicate. This is apparently in contrast to the findings from EDS where calcium and silicate are usually higher in concentration than zinc. XPS data provides useful information that a certain amount of zinc from ZnO either in crystalline or noncrystalline form is located on the surface of the cement grains. This implies that Zn is easier to be removed or leached out from solidified/ 5042

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(1) Lo, I. M.-C. J. Environ. Eng. ASCE 1996, 122(9), 850-855. (2) Roy, A.; Eaton, H. C.; Cartledge, F. K.; Tittlebaum, M. E. Hazard. Waste Hazard. Mater. 1991, 8(1), 33-41. (3) Means, J. L. The application of Solidification/Stabilization to Waste Materials; Lewis Publishers: Boca Raton, 1995. (4) LaGrega, M. D., Buckingham, P. L., Evans, J. C. Hazardous Wastes Management; McGraw-Hill: NJ, 1994. (5) Barth, E. F.; Percin, de P.; Arozarena, M. M.; Zieleniewski, J. L.; Dosani, M.; Maxey, H. R.; Hokanson, S. A.; Pryately, C. A.; Whipple, T.; Kravitz, R.; Cullinane, M. J., Jr.; Jones, L. W.; Malone, P. G. U.S. EPA. Stabilization and Solidification of Hazardous Waste; Noyes Data Corporation: New Jersey, 1990. (6) Spence, R. D. Chemistry and Microstructure of Solidified Waste Forms; Lewis Publishers: Boca Raton, 1993. (7) Lin, C. K.; Chen, J. N.; Lin, C. C. J. Hazard. Mater. 1996, 48, 137-147. (8) Chang, T. T.; Liao, W. P.; Chen, S. Y. Transportation Res. Record 1996, 1546, 41-52. (9) Weshe, K. Fly Ash in Concrete: Properties and Performance; E & FN Spon: New York, 1991. (10) Federal Register (USEPA). Toxicity Characteristic Leaching Procedure (TCLP); 40 CFR, 50(286), Appendix 2, Part 268, 40643, 1986. (11) Li, X. D.; Poon, C. S. Proceedings of the Fourteenth International Conference on Solid Waste Technology and Management; November, Philadelphia, PA, 1998. (12) Conner, J. R. Chemical Fixation and Solidification of Hazardous Wastes; Van Nostrand Reinhold: New York, 1990. (13) Roy, A.; Eaton, H. C.; Cartledge, F. K.; Tittlebaum, M. E. Environ. Sci. Technol. 1992, 26(7), 1349-1353. (14) Poon, C. S.; Peters, C. J.; Perry, R. Sci. Total Environ. 1985a, 41, 55-71. (15) Poon, C. S.; Clark, A. I.; Peters, C. J.; Perry, R. Waste Management Res. 1985b, 3, 127-142. (16) Yousuf, M.; Mollah, A.; Parga, J. R.; Cocke, D. L. J. Environ. Sci. Health 1992, A27(6), 1503-1519.

Received for review October 27, 1999. Revised manuscript received June 1, 2000. Accepted August 29, 2000. ES991224O