ORGANIC
INTERFERENCE
SOLIDIFIED/STABILIZED
OF
HAZARDOUS
WASTES
H. C. E A T O N , M. B. W A L S H , M. E. T I T T L E B A U M , F. K. C A R T L E D G E , and D. C H A L A S A N I
Hazardous Waste Research Center, College of Engineering and College of Basic Sciences, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
(Received May 1986) Abstract. Liquid hazardous waste disposal in landfills is usually allowed only after solidification/stabilization. Although various procedures are commonly practiced, little is known about the mechanism(s) of the processes. A particular problem is the interference of organics. Small amounts of organics can interfere with the reaction between inorganic sludges and cementitous matrices. The present communication reports studies of the interaction between selected organic hazardous wastes and Type I Portland cement. Microscopic studies of the structural differences between cements set with water and those set with water plus organic liquids are discussed. In these studies the scanning electron microscope is used to observe samples fractured at 78K. The results provide technical background data on the ultimate stability of critical waste constituents solidified by various binding agents.
1. Introduction
The disposal of hazardous industrial wastes is a major concern of environmental scientists. Methods which produce stable, nontoxic products which do not expose the public to major health risks like those at Love Canal in the United States must be developed. A widely used method for the disposal of liquids or sludges is solidification/stabilization. In this method, the liquid is mixed with a suitable second component and a stable, solidified product results. Although the method is not as attractive for organic wastes as incineration appears to be, it could still be an economically viable approach in some cases. In addition, it is certain that solidification/stabilization will be practiced, at least to some extent, since incineration processes produce hazardous residues which must ultimately be safely contained. The present document describes research which seeks information about the waste bonding mechanisms operative in certain solidification/stabilization schemes. There has been very little consideration of the mechanisms in the past because it was not demanded. There is concern, however, for the destabilization of sites which were previously thought to be safe. But, there is also the distinct possibility that by elucidating the mechanisms of solidification/stabilization, safer, more stable solidification schemes can be devised. The focus o f the present research project has been on the mechanisms of solidification/stabilization o f organic hazardous wastes as opposed to inorganic wastes. There are a number o f reasons for this research direction. It is well known that the presence of small amounts of organics in inorganic wastes can complicate the normal Environmental Monitoring and Assessment 9 (1987) 133-142. 9 1987 by D. Reidel Publishing Company.
134
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EATON ET AL.
procedures for solidification/stabilization of the uncontaminated inorganic. In fact, interference phenomena have been well known to workers in the cement industry who, for example, intentionally use certain additives to retard the set and certain others to accelerate the set of Portland cement. These additives are often salts such as calcium chloride and sodium chloride. Hydrocarbons also have a strong effect. For example, a one percent solution of sugar can completely inhibit the set. There would be a definite economic advantage to the use of solidification/stabilization techniques for the disposal of organic wastes since the basic technology is already widely practiced, but there have been few attempts to understand the micromechanisms of interferences. One reason for the lack of understanding is the difficulty of analyzing the complex microstructures of the solidified products. Cement alone is rather poorly understood due to the complexity of the reactions which occur and due to the wide variety of microstructures which develop with time. With the addition of interfering substances, the system becomes even more complex. Since the solidification of hazardous inorganics has been a widely used technology and since there is the future possibility of disposal of organics by these methods, a clear understanding of the effectiveness of these processes is most important to long term public health and safety. The present paper reviews the current understanding of the structure of cement and stabilized/solidified organics and new results are presented. These results are obtained using microchemical methods and scanning electron microscopy (SEM).
2. Experimental Method The procedures used in the present study were in all cases developed in order to give qualitative information about the chemical and physical interactions occurring in solidified/stabilized mixtures. Samples for each task were prepared separately, and each task performed in duplicate in order to assure that representative samples were investigated. Specimens were prepared using Type I Portland cement as the binder. Two organic wastes were employed, ethylene glycol and para-bromophenol. Ethylene glycol was selected because of its relative molecular simplicity and para-bromophenol because it could be detected using energy dispersive x-ray analysis (EDX). The organic was mixed with 10 g of Type Portland cement and 4 ml of deionized water is a screw-cap vial. The contents were stirred thoroughly with a glass stirring rod. The samples were allowed to set in the vials for a prescribed length of time and were then broken. Each sample was cooled to liquid nitrogen temperature and then fractured into several small pieces, each approximately 1 mm in diameter. A representative piece was mounted onto an aluminium stub for scanning electron microscopy in a an ISI 60-A Scanning Electron Microscope. Prior to examination, the mounted sample was coated with a 20 nm film of Au-Pd in a Hummer VI Sputter Coater. The SEM was operated at 15-30 keV accelerating voltage with the vacuum chamber
ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
135
at a p r e s s u r e o f less t h a n 5 x 1 0 - s t o r r . W h e n fine surface textures were o b s e r v e d at high m a g n i f i c a t i o n s , the lower voltages were necessary in o r d e r to reduce b e a m p e n e t r a t i o n . M i c r o - c h e m i c a l analyses were m a d e with an E D A X - E C O N 2 E n e r g y Dispersive X - R a y A n a l y z e r a n d x - r a y p o w d e r d i f f r a c t i o n analysis b y a G E X - R a y D i f f r a c t o m e t e r with a m a g n e s i u m target.
3. Results and Discussions 3.1. MORPHOLOGY AND PHASE STRUCTURES T h e m o s t c o m m o n b i n d e r in s t a b i l i z a t i o n processes is P o r t l a n d C e m e n t . T h e a p p r o x i m a t e c h e m i c a l c o m p o s i t i o n o f P o r t l a n d c e m e n t clinker is s h o w n in T a b l e I, expressed as m e t a l oxides. B i n a r y a n d t e r n a r y c o m b i n a t i o n s o f these oxides are f o r m e d d u r i n g c e m e n t m a n u f a c t u r e at 1400 to 1600 ~ leading to a very c o m p l e x m i x t u r e , the p r i n c i p a l c o m p o n e n t s o f which are c a l c i u m silicates. TABLE I The composition of Portland cement Constituent
Percent
AI203 CaO (lime) Fe203 SiO2 Balance (SO3, MgO, K 2 0 , and Na20 )
7 63 3 20 5.5
W h e n P o r t l a n d c e m e n t is m i x e d with w a t e r , the c o n s t i t u e n t c o m p o u n d s u n d e r g o a series o f c h e m i c a l r e a c t i o n s which are r e s p o n s i b l e for the eventual h a r d e n i n g o f the paste. Several h y d r a t i o n p r o d u c t s are p r o d u c e d b u t t w o m a j o r classes o f h y d r a t e s are c o m m o n : c a l c i u m silicate h y d r a t e a n d c o m p l e x c a l c i u m a l u m i n a t e h y d r a t e salts. These, t o g e t h e r with c a l c i u m h y d r o x i d e a n d u n h y d r a t e d c e m e n t m i n e r a l s , f o r m set
TABLE It Typical mineral composition of ordinary Portland cement Mineral
Chemical Formula
Shortland Notation
Weight percent
Tricalcium Silicate Dicalcium Silicate Tricalcium Aluminate Tetracalcium Aluminoferite Calcium Sulfate Dihydrate Amorphous Hydrated Calcium Silicate Gel
3CaO - SiO2 2CaO 9SiO2 3CaO 9AI203 4CaO 2 - Al203 9Fe203 CaSO4" 2H20
C3S C:S
50 25 12 8 3.5
2CaO
C-S-H
" S i O 2 9 4H20
C3A
C4AF
136
ft. c. EATON ET AL.
Fig. I. Scanning electron micrographs showing the three principal phases observed in plain hydrated cement. The phases are labeled: CH=calcium hydroxide, CSH=calcium silicate hydrate gel, and AFm=monosulfate aluminate. The cement was 21 days old when examined. (a) 5000 x , (b) 2000 x , (c) 2500 x , and (d) 3100 x magnification.
Portland cement [1]. Reactions of the tricalcium silicate and dicalcium silicate upon hydration are very similar, with both forming calcium silicate hydrated cement is shown in Table II. The difference between the silicates is apparent when it is noted that
ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
137
tricalcium silicates produce three times as much calcium hydroxide in a hydration reaction going to completion. This is shown in the following chemical formulae: 2(3CAO 9 SiO2) + 6 H z O ~ 3 C a O 9 2SIO2 9 3H2 + 3Ca(OH)2(D 2(2CAO 9 SiO2) + 4H20--*3CaO 9 2SiO 2 3H20 + Ca(OH)~." A calcium silicate (C-S-H) gel phase constitutes approximately one-half to two-thirds o f the volume of hydrated pastes, and consequently, strongly influences their
138
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EATON ET AL.
behavior. The gel is believed to consist of a mass of extremely small irregular particles, of indefinite morphology, which are usually smaller than the resolution of the optical microscope. Mills [2] reported the formation of rosettes of calcium silicate hydrates in set Portland cement pastes and his micrographs also revealed indistinct masses which were assumed to be calcium silicate hydrate. Chatterji and Jeffery [3, 4] reported splines of C-S-H protruding in random directions from a fractured tricalcium silicate paste. Midgley [5] found both plates and splines. In the present study, gel phase was observed in the plain cement samples and in those containing organic waste. Figure l shows examples of the ill-defined, nearly amorphous structure of the gel. Calcium hydroxide crystals constitute 20-25 ~ of the paste volume. As the paste ages, the calcium hydroxide becomes an increasingly prominent constituent of the microstructure. The crystals can vary in morphology being found as small equiaxed crystals; large, flat, plate-like crystals; large, thin elongated crystals; or variations between these extremes. The characteristic striated appearance of the crystals is a consequence of the way in which they fracture within the paste. While the calcium hydroxide is well crystallized with a definite stoichiometry, the crystals are not uniform in morphology since they will only grow only where there is space in the matrix [6]. Researchers utilizing the electron microscope to characterize Ca(OH) 2 commonly observe hexagonal plates. The scanning electron micrographs (Figure 1) reveal the presence of this phase in both plain and waste containing cements. Calcium sulfoaluminates initially form in set cement as the mineral ettringite, if there is sufficient sulfate for reaction. Ettringite also crystallizes as hexagonal prisms but in a much more elongated form than in crystalline calcium hydroxide. The exact morphology depends on the available space and supply of ions for crystal growth. Commonly, ettringite forms as long slender needles [6], but Chatterjij and Jeffery [3, 4] found that it was difficult to distinguish between ettringite crystals and the C-S-H which had formed less than three days after hydration. After this time, however, the ettringite crystals lengthened and thickened and were therefore longer and easily distinguishable from the C-S-H gel. If sulfate ions are consumed before the tricalcium silicate has completely hydrated, ettringite transforms to another form of calcium sulfoaluminate hydrate containing less sulfate. This is simply called monosulfate aluminate and generally forms clusters of "rosettes" of irregular plates. They grow into well developed, but very thin, hexagonal plates. In pure systems, the structures are well crystallized, but the incorporation of impurities descreases the degree of crystallinity. It is evident that more than one microstructure may be characteristic of set Portland cement paste. The environment during crystal growth helps to determine the final microstructure. In addition, influences such as the water to solids ratio, and the presence of minor components such as alkali, sulphate, and aluminium ions may also have an effect [5]. However, the present study shows that basically the same major phases appear in both plain hydrated cement and hydrated cement containing either ethylene glycol or para-bromophenol. However, the results also show that the relative amounts of these major phases will change when the organic is added.
ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
3.2.
139
C H E M I S T R Y OF S O L I D I F I E D - S T A B I L I Z E D SYSTEMS
Although the crystalline phases formed during hydration of pure calcium silicates are reasonably well characterized, the crystallization processes in Portland cement are much more complex. Solid solutions involving the various components play an important role [7]. It is well known that additions of certain materials, including CaSO4- 2H20 (gypsum) and polyhydroxyorganic compounds, retard the set of Portland cement. There have been speculations about the mechanisms [7], but the available data on retardation effects is sometimes conflicting. Gypsum is the best characterized retarder, and it has been suggested that the presence of extra sulfate ions promotes the formation of insoluble sulfoaluminates (probably ettringite). This reduces the aluminate concentration in solution and hinders the formation of a alumina-silica gel, which is responsible for the initial rapid set. However, there is not agreement that the initial set of Portland cement is mainly due to alumina-silica gel formation. Clearly, adding extra ions to the Portland cement mixture which .is undergoing hydration can result in (a) differences in the relative rates of formation of pure crystalline phases, (b) substitution of one ion for another in either pure crystalline
(o)
(b)
(c) Fig. 2.
Space-filfing models of (a) H20, (b) ethylene glycol, and (r 1r
nm).
para-bromopheno! (scale:
140
H. C. EATON
ET AL.
phases or solid solutions, and (c) formation of totally new crystalline phases or solid solutions. Many of the possibilities can be determined by relative ionic sizes [8]. The retarding effects of organics have been suggested to be due to hydrogen bonding attachment of organics to the surfaces of cement particles. The fact that retardation of set can occur, and the fact that ionic substitution can upset the usual crystallization phenomena that take place in the cements, suggest that chemical effects may well be the dominant mechanism for the solidification/stabilization of inorganic sludges by Portland cement. Little mechanistic work has been reported, however, except for studies that were performed in Holland by the Institute for Waste Research [9]. They reported a comparative investigation of four immobilization techniques in a laboratory study of chemical aspects of leachability. Unfortunately, for proprietary reasons, the nature of the immobilization material was not revealed. The synthetic metal hydroxide sludge contained the metals Hg, As, Cd, Cu, and Pd. In their scanning electron microscope studies of the structure of the solidified product, there was some evidence (in two of the samples) for the occurrence of specific combinations of metal ions and microscopically separable phases. The two organics considered in the present investigation are ethylene glycol and para-bromophenol. The former is water soluble and capable of extensive hydrogen
I
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I I CEMENT
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",
~~ mD~m
~ "--..,,'" %
CH
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W F-Z
" "~''"'"l":" "~" "" "~''''': ~ CEMENT 8~ ETHYLENE GLYCOL
",.,.]'.. 9.,.. ,,., ,.... -% .,....".;." ,
..." ;...-.~." i II
I 13
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15
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19
'
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21
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23
27
28 Fig. 3. X-ray powder diffractionspectra from a plain hydrated cement and from a hydrated cement containing ethyleneglycol.
ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
141
bonding which is one of the mechanisms suggested to be important in set retardation. The phenol is a weak acid (pKa=9.6), and under the highly basic conditions of setting cement ( p H - 13, but depending to some extent on the amount of water used) will be essentially completely converted into phenoxide anion. In principle at least, both of these compounds could be incorporated into ionic crystal lattices, ethylene glycol in place of water of hydration and phenoxide as the anionic ion. The participation should be dependent on size relationships, however, and both ethylene glycol and para-bromophenol are probably too large to substitute effectively without major deformations of the crystalline lattices. The dimensions of H20, ethylene glycol and para-bromophenol are illustrated in Figure 2. For comparison with para-bromophenol, sulfate and silicate ions are essentially spheres with diameters of 0.46 and 0.48 nm, respectively [10]. Each sample was analyzed by x-ray powder diffraction prior to each microstructural analysis. This has been found to be very useful since changes in the crystalline components of the waste/binder sample can be determined if the quantities of the phases are sufficient, Figure 3 is an example of two x-ray spectra. The spectrum on top was obtained from a plain cement sample and the spectrum on the bottom from a sample containing ethylene glycol. It is clear that there have been changes in the spectra as shown by the vertical lines drawn through the major peaks. These major peaks are identified as being produced by either C-S-H gel or by Ca(OH)2 and there has been a noticeable change in the relative intensities of the C-S-H reflections suggesting chemical changes in this phase. In addition to powder diffraction analysis, EDX analysis was performed on the para-bromophenol samples that were studied by scanning electron microscopy. EDX allows detection of the organic (actually the halogen) and a determination of its
~0 z 0
II
I S I
C A
F E
A U
B R
ENERGY
Fig. 4. An energy dispersive x-ray spectrum from a specimen containing para-bromophenol. The bromide peak is marked.
142
H. C. EATON ET AL.
location in the complex microstructure of the waste/binder system. The spectrum shown in Figure 4 is an example of the results. In the figure, it can be seen that the peak marked by the arrow is the Br Ka peak occurring at 11.9 keV. Finally, it was possible to show, using electron dot maps, that the majority of the phenol was contained in the gel phase of the cement [11]. This is believed the first evidence that the organic is identifiable with a specific mineral phase rather than generally distributed through the binder or exclusively in large pore or interstitial spaces.
4. Summary The present work reports the use of scanning electron microscopy to illustrate the complex microstructures in the binder of stabilized/solidified organic wastes. The microstructure evolves with age and consequently it is important that the kinetic behavior is understood if the long-term stability of the systems are to be assured. It is shown that, for the waste concentrations studied, the major constituent phases present in hydrated Portland cement remain upon the addition of either ethylene glycol or para-bromophenol. Since these major phases (Ca(OH)2, ettringite, and calcium silicate hydrate gel) are stable, attention was focused on the concentration of organic in those phases. Energy dispersive x-ray analysis detected the presence of the brominated phenol in the gel phase. No detectable amounts of the organic were observed in either the ettringite or in the Ca(OH)z. X-ray diffraction analyses revealed changes in the relative peak heights for several C-S-H gel peaks for cement containing ethylene glycol. These results support the suggestion that the large organic molecules locate themselves in the complex gel phase. It is unlikely that the organic can directly substitute into the crystal lattice of either the ettringite or the Ca(OH)2 due to molecular size differences. These findings represent important steps toward an understanding of the mechanisms of organic waste solidification/stabilization.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Midgley, H. G.: 1969, Structure, Solids Mechanics and Engineering Design 1, 275. Mills, R. H.: 1968, 'Conference on Scanning Electron Microscopy', Cambridge, England, p. 1. Chatterji, S. and Jeffery, J. W.: 1966, Nature 209, 1233. Chatterji, S. and Jeffery, J. W.: 1967, Nature 214, 559. Midgley, H. G.: 1968, Bulletin o f the British Ceramic Society, p. 89. Mindess, S. and Young, J. F.: 1981, Concrete, Prentice Hall, Inc., Englewood Cliffs, NJ. Lea, F. M.: 1971, The Chemistry o f Cement and Concrete, 3rd ed., Chemistry Publishing Co., Inc., New York, NY. Hume-Rothery, W. and Raynor, G. V.: 1954, The Structure o f Metals and Alloys, Institute of Metals, London. 'Comparative Investigation on Four Immobilisation Techniques', Publication No. 39, Institute for Waste Research, Amersfoort, The Netherlands, 1979. Waddington, T. C.: 1958, Adv. Inorg. Chem. Radiochem. t, 180. Eaton, H. C., Tittlebaum, M. E., Cartledge, F. K., Walsh, M. B., and Chalasani, D.: (to be published).
INTERFERENCE
SOLIDIFIED/STABILIZED
OF
HAZARDOUS
WASTES
H. C. E A T O N , M. B. W A L S H , M. E. T I T T L E B A U M , F. K. C A R T L E D G E , and D. C H A L A S A N I
Hazardous Waste Research Center, College of Engineering and College of Basic Sciences, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
(Received May 1986) Abstract. Liquid hazardous waste disposal in landfills is usually allowed only after solidification/stabilization. Although various procedures are commonly practiced, little is known about the mechanism(s) of the processes. A particular problem is the interference of organics. Small amounts of organics can interfere with the reaction between inorganic sludges and cementitous matrices. The present communication reports studies of the interaction between selected organic hazardous wastes and Type I Portland cement. Microscopic studies of the structural differences between cements set with water and those set with water plus organic liquids are discussed. In these studies the scanning electron microscope is used to observe samples fractured at 78K. The results provide technical background data on the ultimate stability of critical waste constituents solidified by various binding agents.
1. Introduction
The disposal of hazardous industrial wastes is a major concern of environmental scientists. Methods which produce stable, nontoxic products which do not expose the public to major health risks like those at Love Canal in the United States must be developed. A widely used method for the disposal of liquids or sludges is solidification/stabilization. In this method, the liquid is mixed with a suitable second component and a stable, solidified product results. Although the method is not as attractive for organic wastes as incineration appears to be, it could still be an economically viable approach in some cases. In addition, it is certain that solidification/stabilization will be practiced, at least to some extent, since incineration processes produce hazardous residues which must ultimately be safely contained. The present document describes research which seeks information about the waste bonding mechanisms operative in certain solidification/stabilization schemes. There has been very little consideration of the mechanisms in the past because it was not demanded. There is concern, however, for the destabilization of sites which were previously thought to be safe. But, there is also the distinct possibility that by elucidating the mechanisms of solidification/stabilization, safer, more stable solidification schemes can be devised. The focus o f the present research project has been on the mechanisms of solidification/stabilization o f organic hazardous wastes as opposed to inorganic wastes. There are a number o f reasons for this research direction. It is well known that the presence of small amounts of organics in inorganic wastes can complicate the normal Environmental Monitoring and Assessment 9 (1987) 133-142. 9 1987 by D. Reidel Publishing Company.
134
H.C.
EATON ET AL.
procedures for solidification/stabilization of the uncontaminated inorganic. In fact, interference phenomena have been well known to workers in the cement industry who, for example, intentionally use certain additives to retard the set and certain others to accelerate the set of Portland cement. These additives are often salts such as calcium chloride and sodium chloride. Hydrocarbons also have a strong effect. For example, a one percent solution of sugar can completely inhibit the set. There would be a definite economic advantage to the use of solidification/stabilization techniques for the disposal of organic wastes since the basic technology is already widely practiced, but there have been few attempts to understand the micromechanisms of interferences. One reason for the lack of understanding is the difficulty of analyzing the complex microstructures of the solidified products. Cement alone is rather poorly understood due to the complexity of the reactions which occur and due to the wide variety of microstructures which develop with time. With the addition of interfering substances, the system becomes even more complex. Since the solidification of hazardous inorganics has been a widely used technology and since there is the future possibility of disposal of organics by these methods, a clear understanding of the effectiveness of these processes is most important to long term public health and safety. The present paper reviews the current understanding of the structure of cement and stabilized/solidified organics and new results are presented. These results are obtained using microchemical methods and scanning electron microscopy (SEM).
2. Experimental Method The procedures used in the present study were in all cases developed in order to give qualitative information about the chemical and physical interactions occurring in solidified/stabilized mixtures. Samples for each task were prepared separately, and each task performed in duplicate in order to assure that representative samples were investigated. Specimens were prepared using Type I Portland cement as the binder. Two organic wastes were employed, ethylene glycol and para-bromophenol. Ethylene glycol was selected because of its relative molecular simplicity and para-bromophenol because it could be detected using energy dispersive x-ray analysis (EDX). The organic was mixed with 10 g of Type Portland cement and 4 ml of deionized water is a screw-cap vial. The contents were stirred thoroughly with a glass stirring rod. The samples were allowed to set in the vials for a prescribed length of time and were then broken. Each sample was cooled to liquid nitrogen temperature and then fractured into several small pieces, each approximately 1 mm in diameter. A representative piece was mounted onto an aluminium stub for scanning electron microscopy in a an ISI 60-A Scanning Electron Microscope. Prior to examination, the mounted sample was coated with a 20 nm film of Au-Pd in a Hummer VI Sputter Coater. The SEM was operated at 15-30 keV accelerating voltage with the vacuum chamber
ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
135
at a p r e s s u r e o f less t h a n 5 x 1 0 - s t o r r . W h e n fine surface textures were o b s e r v e d at high m a g n i f i c a t i o n s , the lower voltages were necessary in o r d e r to reduce b e a m p e n e t r a t i o n . M i c r o - c h e m i c a l analyses were m a d e with an E D A X - E C O N 2 E n e r g y Dispersive X - R a y A n a l y z e r a n d x - r a y p o w d e r d i f f r a c t i o n analysis b y a G E X - R a y D i f f r a c t o m e t e r with a m a g n e s i u m target.
3. Results and Discussions 3.1. MORPHOLOGY AND PHASE STRUCTURES T h e m o s t c o m m o n b i n d e r in s t a b i l i z a t i o n processes is P o r t l a n d C e m e n t . T h e a p p r o x i m a t e c h e m i c a l c o m p o s i t i o n o f P o r t l a n d c e m e n t clinker is s h o w n in T a b l e I, expressed as m e t a l oxides. B i n a r y a n d t e r n a r y c o m b i n a t i o n s o f these oxides are f o r m e d d u r i n g c e m e n t m a n u f a c t u r e at 1400 to 1600 ~ leading to a very c o m p l e x m i x t u r e , the p r i n c i p a l c o m p o n e n t s o f which are c a l c i u m silicates. TABLE I The composition of Portland cement Constituent
Percent
AI203 CaO (lime) Fe203 SiO2 Balance (SO3, MgO, K 2 0 , and Na20 )
7 63 3 20 5.5
W h e n P o r t l a n d c e m e n t is m i x e d with w a t e r , the c o n s t i t u e n t c o m p o u n d s u n d e r g o a series o f c h e m i c a l r e a c t i o n s which are r e s p o n s i b l e for the eventual h a r d e n i n g o f the paste. Several h y d r a t i o n p r o d u c t s are p r o d u c e d b u t t w o m a j o r classes o f h y d r a t e s are c o m m o n : c a l c i u m silicate h y d r a t e a n d c o m p l e x c a l c i u m a l u m i n a t e h y d r a t e salts. These, t o g e t h e r with c a l c i u m h y d r o x i d e a n d u n h y d r a t e d c e m e n t m i n e r a l s , f o r m set
TABLE It Typical mineral composition of ordinary Portland cement Mineral
Chemical Formula
Shortland Notation
Weight percent
Tricalcium Silicate Dicalcium Silicate Tricalcium Aluminate Tetracalcium Aluminoferite Calcium Sulfate Dihydrate Amorphous Hydrated Calcium Silicate Gel
3CaO - SiO2 2CaO 9SiO2 3CaO 9AI203 4CaO 2 - Al203 9Fe203 CaSO4" 2H20
C3S C:S
50 25 12 8 3.5
2CaO
C-S-H
" S i O 2 9 4H20
C3A
C4AF
136
ft. c. EATON ET AL.
Fig. I. Scanning electron micrographs showing the three principal phases observed in plain hydrated cement. The phases are labeled: CH=calcium hydroxide, CSH=calcium silicate hydrate gel, and AFm=monosulfate aluminate. The cement was 21 days old when examined. (a) 5000 x , (b) 2000 x , (c) 2500 x , and (d) 3100 x magnification.
Portland cement [1]. Reactions of the tricalcium silicate and dicalcium silicate upon hydration are very similar, with both forming calcium silicate hydrated cement is shown in Table II. The difference between the silicates is apparent when it is noted that
ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
137
tricalcium silicates produce three times as much calcium hydroxide in a hydration reaction going to completion. This is shown in the following chemical formulae: 2(3CAO 9 SiO2) + 6 H z O ~ 3 C a O 9 2SIO2 9 3H2 + 3Ca(OH)2(D 2(2CAO 9 SiO2) + 4H20--*3CaO 9 2SiO 2 3H20 + Ca(OH)~." A calcium silicate (C-S-H) gel phase constitutes approximately one-half to two-thirds o f the volume of hydrated pastes, and consequently, strongly influences their
138
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behavior. The gel is believed to consist of a mass of extremely small irregular particles, of indefinite morphology, which are usually smaller than the resolution of the optical microscope. Mills [2] reported the formation of rosettes of calcium silicate hydrates in set Portland cement pastes and his micrographs also revealed indistinct masses which were assumed to be calcium silicate hydrate. Chatterji and Jeffery [3, 4] reported splines of C-S-H protruding in random directions from a fractured tricalcium silicate paste. Midgley [5] found both plates and splines. In the present study, gel phase was observed in the plain cement samples and in those containing organic waste. Figure l shows examples of the ill-defined, nearly amorphous structure of the gel. Calcium hydroxide crystals constitute 20-25 ~ of the paste volume. As the paste ages, the calcium hydroxide becomes an increasingly prominent constituent of the microstructure. The crystals can vary in morphology being found as small equiaxed crystals; large, flat, plate-like crystals; large, thin elongated crystals; or variations between these extremes. The characteristic striated appearance of the crystals is a consequence of the way in which they fracture within the paste. While the calcium hydroxide is well crystallized with a definite stoichiometry, the crystals are not uniform in morphology since they will only grow only where there is space in the matrix [6]. Researchers utilizing the electron microscope to characterize Ca(OH) 2 commonly observe hexagonal plates. The scanning electron micrographs (Figure 1) reveal the presence of this phase in both plain and waste containing cements. Calcium sulfoaluminates initially form in set cement as the mineral ettringite, if there is sufficient sulfate for reaction. Ettringite also crystallizes as hexagonal prisms but in a much more elongated form than in crystalline calcium hydroxide. The exact morphology depends on the available space and supply of ions for crystal growth. Commonly, ettringite forms as long slender needles [6], but Chatterjij and Jeffery [3, 4] found that it was difficult to distinguish between ettringite crystals and the C-S-H which had formed less than three days after hydration. After this time, however, the ettringite crystals lengthened and thickened and were therefore longer and easily distinguishable from the C-S-H gel. If sulfate ions are consumed before the tricalcium silicate has completely hydrated, ettringite transforms to another form of calcium sulfoaluminate hydrate containing less sulfate. This is simply called monosulfate aluminate and generally forms clusters of "rosettes" of irregular plates. They grow into well developed, but very thin, hexagonal plates. In pure systems, the structures are well crystallized, but the incorporation of impurities descreases the degree of crystallinity. It is evident that more than one microstructure may be characteristic of set Portland cement paste. The environment during crystal growth helps to determine the final microstructure. In addition, influences such as the water to solids ratio, and the presence of minor components such as alkali, sulphate, and aluminium ions may also have an effect [5]. However, the present study shows that basically the same major phases appear in both plain hydrated cement and hydrated cement containing either ethylene glycol or para-bromophenol. However, the results also show that the relative amounts of these major phases will change when the organic is added.
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C H E M I S T R Y OF S O L I D I F I E D - S T A B I L I Z E D SYSTEMS
Although the crystalline phases formed during hydration of pure calcium silicates are reasonably well characterized, the crystallization processes in Portland cement are much more complex. Solid solutions involving the various components play an important role [7]. It is well known that additions of certain materials, including CaSO4- 2H20 (gypsum) and polyhydroxyorganic compounds, retard the set of Portland cement. There have been speculations about the mechanisms [7], but the available data on retardation effects is sometimes conflicting. Gypsum is the best characterized retarder, and it has been suggested that the presence of extra sulfate ions promotes the formation of insoluble sulfoaluminates (probably ettringite). This reduces the aluminate concentration in solution and hinders the formation of a alumina-silica gel, which is responsible for the initial rapid set. However, there is not agreement that the initial set of Portland cement is mainly due to alumina-silica gel formation. Clearly, adding extra ions to the Portland cement mixture which .is undergoing hydration can result in (a) differences in the relative rates of formation of pure crystalline phases, (b) substitution of one ion for another in either pure crystalline
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phases or solid solutions, and (c) formation of totally new crystalline phases or solid solutions. Many of the possibilities can be determined by relative ionic sizes [8]. The retarding effects of organics have been suggested to be due to hydrogen bonding attachment of organics to the surfaces of cement particles. The fact that retardation of set can occur, and the fact that ionic substitution can upset the usual crystallization phenomena that take place in the cements, suggest that chemical effects may well be the dominant mechanism for the solidification/stabilization of inorganic sludges by Portland cement. Little mechanistic work has been reported, however, except for studies that were performed in Holland by the Institute for Waste Research [9]. They reported a comparative investigation of four immobilization techniques in a laboratory study of chemical aspects of leachability. Unfortunately, for proprietary reasons, the nature of the immobilization material was not revealed. The synthetic metal hydroxide sludge contained the metals Hg, As, Cd, Cu, and Pd. In their scanning electron microscope studies of the structure of the solidified product, there was some evidence (in two of the samples) for the occurrence of specific combinations of metal ions and microscopically separable phases. The two organics considered in the present investigation are ethylene glycol and para-bromophenol. The former is water soluble and capable of extensive hydrogen
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ORGANIC INTERFERENCE OF SOLIDIFIED/STABILIZED HAZARDOUS WASTES
141
bonding which is one of the mechanisms suggested to be important in set retardation. The phenol is a weak acid (pKa=9.6), and under the highly basic conditions of setting cement ( p H - 13, but depending to some extent on the amount of water used) will be essentially completely converted into phenoxide anion. In principle at least, both of these compounds could be incorporated into ionic crystal lattices, ethylene glycol in place of water of hydration and phenoxide as the anionic ion. The participation should be dependent on size relationships, however, and both ethylene glycol and para-bromophenol are probably too large to substitute effectively without major deformations of the crystalline lattices. The dimensions of H20, ethylene glycol and para-bromophenol are illustrated in Figure 2. For comparison with para-bromophenol, sulfate and silicate ions are essentially spheres with diameters of 0.46 and 0.48 nm, respectively [10]. Each sample was analyzed by x-ray powder diffraction prior to each microstructural analysis. This has been found to be very useful since changes in the crystalline components of the waste/binder sample can be determined if the quantities of the phases are sufficient, Figure 3 is an example of two x-ray spectra. The spectrum on top was obtained from a plain cement sample and the spectrum on the bottom from a sample containing ethylene glycol. It is clear that there have been changes in the spectra as shown by the vertical lines drawn through the major peaks. These major peaks are identified as being produced by either C-S-H gel or by Ca(OH)2 and there has been a noticeable change in the relative intensities of the C-S-H reflections suggesting chemical changes in this phase. In addition to powder diffraction analysis, EDX analysis was performed on the para-bromophenol samples that were studied by scanning electron microscopy. EDX allows detection of the organic (actually the halogen) and a determination of its
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H. C. EATON ET AL.
location in the complex microstructure of the waste/binder system. The spectrum shown in Figure 4 is an example of the results. In the figure, it can be seen that the peak marked by the arrow is the Br Ka peak occurring at 11.9 keV. Finally, it was possible to show, using electron dot maps, that the majority of the phenol was contained in the gel phase of the cement [11]. This is believed the first evidence that the organic is identifiable with a specific mineral phase rather than generally distributed through the binder or exclusively in large pore or interstitial spaces.
4. Summary The present work reports the use of scanning electron microscopy to illustrate the complex microstructures in the binder of stabilized/solidified organic wastes. The microstructure evolves with age and consequently it is important that the kinetic behavior is understood if the long-term stability of the systems are to be assured. It is shown that, for the waste concentrations studied, the major constituent phases present in hydrated Portland cement remain upon the addition of either ethylene glycol or para-bromophenol. Since these major phases (Ca(OH)2, ettringite, and calcium silicate hydrate gel) are stable, attention was focused on the concentration of organic in those phases. Energy dispersive x-ray analysis detected the presence of the brominated phenol in the gel phase. No detectable amounts of the organic were observed in either the ettringite or in the Ca(OH)z. X-ray diffraction analyses revealed changes in the relative peak heights for several C-S-H gel peaks for cement containing ethylene glycol. These results support the suggestion that the large organic molecules locate themselves in the complex gel phase. It is unlikely that the organic can directly substitute into the crystal lattice of either the ettringite or the Ca(OH)2 due to molecular size differences. These findings represent important steps toward an understanding of the mechanisms of organic waste solidification/stabilization.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Midgley, H. G.: 1969, Structure, Solids Mechanics and Engineering Design 1, 275. Mills, R. H.: 1968, 'Conference on Scanning Electron Microscopy', Cambridge, England, p. 1. Chatterji, S. and Jeffery, J. W.: 1966, Nature 209, 1233. Chatterji, S. and Jeffery, J. W.: 1967, Nature 214, 559. Midgley, H. G.: 1968, Bulletin o f the British Ceramic Society, p. 89. Mindess, S. and Young, J. F.: 1981, Concrete, Prentice Hall, Inc., Englewood Cliffs, NJ. Lea, F. M.: 1971, The Chemistry o f Cement and Concrete, 3rd ed., Chemistry Publishing Co., Inc., New York, NY. Hume-Rothery, W. and Raynor, G. V.: 1954, The Structure o f Metals and Alloys, Institute of Metals, London. 'Comparative Investigation on Four Immobilisation Techniques', Publication No. 39, Institute for Waste Research, Amersfoort, The Netherlands, 1979. Waddington, T. C.: 1958, Adv. Inorg. Chem. Radiochem. t, 180. Eaton, H. C., Tittlebaum, M. E., Cartledge, F. K., Walsh, M. B., and Chalasani, D.: (to be published).
