other anticholinesterase materials in water by determining the relationship between rate constants and inhibition time for the tickets. If we know the inhibition rate constants ( k ) for a given compound, the times required for “partial” and “complete” inhibition of the ticket can be determined for the concentration range desired for given conditions of pH and temperature. For use of these detector tickets in the field and to obtain the most sensitive response. several steps are required. If possible, the water sample should be brought to a temperature of approximately 20°C and adjusted to pH 8. Several detector tickets would then be prepared and immersed in the test water. These would be withdrawn after 5 , 10, 15, and 30 min and substrate added to the ticket. The immersion time required for complete inhibition would be a semiquantitative indication of the amount of anticholinesterase present in the water. If the temperature and/or pH differed from the above values, the semiquantitative relationship would still hold but would be shifted by an amount related to the change. Calibration could be obtained if needed by the field operator.
Literature Cited Epstein, J., “Properties of GB in Water,” J . Amer. Water Works Ass. in press, 1973. Fleisher, H., Spear, S., Pope, E. J., Anal Chem, 27, 1080-3 (1955). Gamson, R. M., Kramer, D., CRDLR 3093, “Development of Improved Detector Kits, ” E27R6 and E28R2, September 1961. Gelman, C., Kramer, D. N., CRLR 541, “Enzymatic Field Test for Anticholinesterases; Assay Method of Cholinesterase,” 3 July 1956. Kramer, D. N., Gamson, R. M., Anal Chem, 30,251-4 (1958). Kramer, D. N., Gamson, R. M., Miller, F. M., J. Org. Chem., 24, 1742-7 (1959). Michel, H. O., Gordon, E., Epstein, J., “The Detection and Estimation of Isopropyl Methylphosphonofluoridate and O-ethylS-diisopropylaminoethylmethylphosphonothioate in Seawater in the Parts-per-Trillion Level,” Enciron. Sci. Techno/., 7, 1045 (1973). Military Specification MIL-D-51083C, Detector Ticket, Anticholinesterase Agent 29, December 1967. Science, 128,19 (1958). White-Stevens, R., Ed. “Pesticides in the Environment,” Vol I, Part I, pp 147-8. Marcel Dekker. New York, N.Y., 1971. Received for recielc June I, 1973. Accepted August 20, 1973.
Assessment of Organic Content of Incinerator Residues Steven E. Hrudey’ and Roger Perry2 Public Health Engineering Section, Imperial College, London, U.K.
The difficulties in evaluating the quality of residues from continuous feed incinerators are discussed. Several different procedures for examining the oxidizable organic materials are proposed and used to analyze samples obtained from several incinerators. The variation in results obtained from these different methods is discussed with reference to X-ray fluorescence and electron microscopy.
In Britain, incineration is currently the method of refuse disposal being most widely adopted for new systems, although sanitary landfill still accounts for the largest proportion of refuse disposed. Almost without exception, the incineration plants adopted are continuous feed, mechanically stoked direct incinerators, producing residues combusted to a higher degree than those associated with the old style batch incinerators. The primary reason for the trend toward incineration, despite its cost of two and one-half to seven times that of sanitary landfill (HMSO, 1971) is the volume reduction achieved. An incinerator residue fill site will require less than one sixth the volume occupied by sanitary landfill of crude refuse followed by compaction (HMSO, 1971). Although volume reduction is the primary concern in any incineration system, the quality of the residue is also a matter of considerable interest. The latter is a primary factor indicative of the superiority of continuous feed direct incineration plants over the older batch processes. Present address, Water Pollution Control Section, North West Region, Environmental Protection Service, Edmonton, Canada. 2 To whom correspondence should be addressed. 1140
Environmental Science & Technology
Both processes may be able to achieve roughly the same volume reduction, but the continuous feed process provides for greater combustion efficiency leading to smaller quantities of oxidizable organic materials remaining in the residue. Although residues are primarily disposed of by landfill or as fill cover material, they are being considered for use in building aggregates and road construction. In all such uses of residues, the amount of incompletely combusted organic matter capable of sustaining biological activity is important. Contrary to some trade views, the actual sterility of the residue as it leaves the plant is irrelevant, since ultimate exposure to microorganisms is guaranteed, in any case, by environmental exposure. With the number of largely unclassified organic materials involved in this residue, the problem of assessment of the biodegradable fraction is complex. Ultimate carbon analysis may provide an indication of the degree of combustion achieved on the refuse, but will not satisfactorily provide an indication of the amount of organic material remaining. The reason for this can be seen from an analysis of the input crude refuse (HMSO, 1971). For example, in Britain in 1968 an average of over 20% of the refuse consisted of dust and cinder, primarily resulting from the domestic use of solid fuel. This type of carbonaceous material is largely biologically inert. When we have been faced with the problem of differentiating organic carbon from “biologically” inert carbon, various chemical analyses have been suggested. Technischer Ueberwaschung Verein Rheinland, Dusseldorf (1972), use a test employing hydrolysis and extraction of the organic matter with caustic soda followed by permanganate oxidation to carbon dioxide as an appropriate esti-
mation of organic matter. An earlier version of this test came to be known as the Dusseldorf test (Bowen and Brealy, 1968) and has been widely used in specifying incinerator performance guarantees. The method itself is cumbersome in operation and the use of potassium permanganate as sole oxidizing agent is unsatisfactory. The Swiss Federal Institute for Water Supply, Sewage Purification and Water Pollution Control (Orsanic, 1966; EAWAG, 1970) developed a test using direct oxidation of the residue samples with a standard potassium dichromate solution in 50% sulfuric acid followed by back titration with ferrous ammonium sulfate solution. They claim the test to be selective for organic carbon compared to carbon as graphite. However, tests in this laboratory on purified charcoal (900OC for 4 hr in oxygen-free nitrogen) averaged 9.6% by weight as oxidizable carbon by this method. Furthermore, the test as specified is vulnerable to interference from inorganic reducing agents. Schoenberger et al. (1968) performed this same test with the addition of a silver catalyst to enhance the oxidation of refractory organics. In addition, they analyzed residue samples for water-soluble, ether-soluble, and volatile fractions; calorific value; and elemental C, H, and N. Some significant correlations between methods of analysis were apparent, but no one method emerged as an obvious choice for the estimation of the organic carbon content. The procedures adopted by the above workers and, in particular, the interpretation of their experimental results have led to a great deal of controversy in the assessment of residue samples. Accordingly, it was decided to examine in detail several alternative procedures that could be used to measure the organic carbon content of residue samples from six different incinerators. This primarily would be aimed at obtaining a method for estimating the potential of the residue to provide a substrate for undesirable microbiological activity. It was recognized that many of the organic materials remaining in the residue would be products of incomplete combustion, and preliminary separation of some of the more volatile aromatic and aliphatic hydrocarbons was carried out by using gas-liquid chroma-
tography. Further background information on the nature of the residues was obtained by X-ray fluorescence analysis and Stereoscan electron microscopy.
Experimental Collection and Preparation of Sample. Samples of residue were collected a t random over one- or two-day periods from different continuous feed, mechanical grate, and direct incineration plants. They were mixed manually to produce a combined sample from each incinerator. Then 10- to 15-kg samples were dried in a laboratory oven a t 105°C for 48 hr. After drying, rough sorting was performed to remove large metallic objects and any other items which could not be crushed. From the samples chosen, this fraction formed less than 5% by weight of the overall sample. The material was then sieved on a mechanical shaker to collect all material passing B.S. mesh 72 (211 p). The remaining material was then crushed and finally ground to pass B.S. mesh 72 with a disk mill. Less than 0.1% by weight could not be crushed to this specification, and this proportion was primarily soft metal. The ground powder so obtained was then recombined with that sieved initially. The total was then agitated and inverted continuously in a covered plastic container for 30 min to ensure complete mixing. Prior to further analysis, samples were dried a t 105°C for 4 hr, and, as the samples were found to be hygroscopic, these were allowed to cool in a desiccator before weighing for analysis. Electron Microscopy. Samples were mounted in the normal way, gold-plated, and studied with an S600 Cambridge Stereoscan electron microscope. X-ray Fluorescence. Semiquantitative analysis of the residues was carried out using a Philips 1220C X-ray fluorescence spectrometer. Elemental Analysis. Carbon, Hydrogen, and nitrogen analyses were carried out in duplicate using a Perkin Elmer Model 240 elemental analyzer. Measurement of Organic Carbon. The following procedures were used: Method 1 (Cl). Samples of residues (500 mg) were oxidized with 0.25N potassium dichromate (50 ml) in 50%
r"'
Sarnp). Water In
-l
-
F I m lonizmtii Detector
-?i Alnplifiw
Air
I" Figure 1. Layout of organic
carbon analyzer Volume 7, N u m b e r 13, December 1973
1141
ereoscan photograi
sulfuric acid for 1 hr and back titrated with 0.25N ferrous ammonium sulfate. The percent carbon oxidized was estimated on the hasis that 1 equivalent of dichromate will oxidize 3 grams of carbon to carbon dioxide (modification of Swiss test-Orsanic, 1966; EAWAG, 1970). Method 2 (Cz). A sample of residue (50 grams) was heated under reflux with 3% NaOH for 3 hr on a boiling water bath. The mixture was cooled and filtered and the alkaline filtrate retained. It was then acidified with 50% sulfuric acid and heated under reflux on a boiling water bath for 30 min. The liquid was then placed in a closed absorption vessel and oxidized with 0.5N potassium per1142
Environmental Science & Technology
manganaLe. i n e evo~vt.ucarvuu dioxide was collected by bubbling into sodium hydroxide solution and estimated by the methyl orange and phenolphthalein end points (modification of the German test-Technischer Ueherwaschung Verein Rheinland, 1972). Method 3 (C3).A sample of residue (50 grams) was heated under reflux with 3% NaOH for 3 hr on a boiling water hath. The mixture was collected and filtered and the alkaline filtrate was retained, and made up to 250 ml with distilled water. A 50-ml aliquot was taken and neutralized with 50% sulfuric acid (15 ml). Oxidation was then performed according to the ASTM COD procedure
values before and after pyrolysis in nitrogen a t 500°C as a weight percent of the dried sample. Preliminary Identification of Organic Matter A fresh sample of residue was obtained and prepared in the manner described previously to produce a powder passing British Standards mesh 72. Approximately 40gram portions were taken and extracted for 24 hr with dichloromethane in a Soxhlet apparatus. The extract obtained at this stage required purification before analytical separation techniques could be effectively carried out. Purifying the extract was approached in two ways. In the first a liquid-liquid purification was performed. The dichloromethane extract was evaporated to dryness and the residue redissolved in cyclohexane. The cyclohexane solution was then shaken with an equal volume of a methanol-water mixture ( 4 : l ) . The aqueous layer was then separated from the cyclohexane layer. This was extracted four times with an equal volume of nitromethane to extract the aromatic hydrocarbons into the nitromethane, leaving the aliphatic hydrocarbons in the cyclohexane. Samples of each were then separated by gas-liquid chromatography to give some indication of the number of volatile compounds present. Two portions of the nitromethane extract were retained for preliminary characterization by mass spectrometry linked to gas-liquid chromatograPhY. The second sample cleanup procedure employed was thin layer chromatography. The dichloromethane extract was evaporated to dryness and the residue redissolved and introduced onto silica gel thin layer chromatographic plates. These were developed in cyclohexane and the fluorescent bands (aromatic compounds) removed, extracted, and the extracts re-run on a thin layer plate with benzene as eluent. Again the fluorescent band was extracted and the extract divided into three. A gas chromatogram was run on the first, while the other two extractions were separated into individual compounds by two-dimensional thin layer chromatography and retained for preliminary characterization by mass spectrometry. Results and Discussion The complexity of structure of the residues involved is shown in the Stereoscan photographs (Figures 2 and 3). Here the magnifications used are indicated by the length of the micron marker shown in the photographs. It is quite clear that the samples are anything but homogenous and that they offer large surface areas for adsorption of organic materials. There were no characteristic types of particles but many fibers and spherical globules were present. The former are likely to be cellulose in one form or another, while the latter are possibly derived from the fly ash from electrostatic precipitators added to the residue, since they resemble pulverized fuel ash (PFA) particles (Gutt, 1972). The larger particles appear to be aggregates of different materials, possibly glass, with various other materials fused inside. The surface structure of these particles indicates that there may well be organic matter of interest isolated in pockets within the porous aggregates. No difference in overall characteristics was apparent between the two samples studied. Although it is not possible firmly to identify any of the particles as being one material or another, the study clearly demonstrates the complexity of the sample being analyzed. The results of the semiquantitative X-ray fluorescence analysis and the C, H, N anaysis are presented in Table I. It should be noted that all the results are reported in 1144
Environmental Science & Technology
Table I. Elemental Combustion and X-ray Fluorescence Analysis (Weight % of dry sample) Element
Sample A
6
0.21-0.24 0.17-0.18 C 13.7-18.2 22.8-22.9 N 0.51-0.56 0.36-0.37 Mg0.1-1.0 0.1-1.0 AI 5-10 5-10 Si 10-20 10-20 P 0.1-0.5 0.1-0.5 S 0.5-1.0 0.5-1.0 CI 0.2-1.0 0.2-0.5 K 0.5-2.0 0.5-2.0 Ca 5-10 5-10 Ti 0.1-0.5 0.1-0.5 Cr 0.01-0.05 0.01-0.05 M n 0.1-0.5 0.1-0.5 Fe 10-20 10-20 N i 0.01-0.05 0.01-0.05 c u 0.1-0.5 0.1-0.5 Z n 0.5-2.0 0.5-1.0 Rb Trace Trace Sr 0.01-0.10 0.01-0.10 H
C
D
0.23-0.32 0.10-0.13 15.9-16.3 10.3-12.0 0.48-0.54 0.20-0.37 0.1-1.0 0.1-1.0 5-15 5-15 10-20 10-20 0.1-0.5 0.1-0.5 0.5-2.0 0.5-1.0 0.2-0.5 0.2-0.5 0.5-2.0 0.5-2.0 5-10 5-10 0.1-0.5 0.1-0.5 0.01-0.05 0.01-0.05 0.1-0.5 0.1-0.5 10-20 10-20 0.01-0.05 0.01-0.05 0.1-0.5 0.1-0.5 1.0-2.0 1.0-2.0
Trace
Trace
E
F
0.16-0.19 15.7-17.0 0.31-0.47 0.1-1.0 5-10 10-20 0.1-0.5 0.5-1.0 0.2-0.5 0.5-2.0 5-10 0.1-0.5 0.01-0.05 0.1-0.5 10-20 0.01-0.05 0.1-0.5 0.7-1.5
0.57-0.63 33.6-34.8 0.71-0.86 0.1-1.0 5-15 10-20 0.1-0.5 0.5-1.0 0.5-1.0 0.5-2.0 5-10 0.1-0.5 0.01-0.05 0.1-0.5 5-10 0.01-0.05 0.1-0.5 0.7-2.0
Trace
Trace
0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10
Zr Trace Trace Trace Trace Trace Trace Sn 0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10 Ba 0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10 0.01-0.10 Pb
0.1-0.6
0.05-0.10
0.1-0.6
0.1-0.3
0.1-0.3
0.1-0.3
terms of weight percentage based on the element concerned rather than on the mineral forms in which they will be present. As would be expected from the high content of glass in incinerator residues, silicon is the most abundant element. However, relatively high concentrations of iron and aluminum are present as well. In general, the composition of the various residues is similar, but semiquantitative X-ray fluorescence would not show minor variations between samples. The C, H, N figures tend to be more characteristic of the individual samples with a spread of 10.3-34.870 for the carbon figures. The need for attempting to differentiate between the total carbon present in the residues and that which is organically combined is shown by comparison with results obtained by Schoenberger et al. (1968). They compared the C/H ratio of crude refuse with those of two residues and found that the C/H ratio of crude refuse corresponds to that of cellulose, while for the residues, higher values of the order of 10 were obtained. These compare with the figures for residues from British refuse which show C/H ratios of from 50-135. Clearly British residues contain a considerable portion of carbon which would not normally be classed as organic. The results for the various methods of estimating organic carbon are presented in Table 11. Method 1, although convenient to perform, utilizes only 500-mg samples and this contributes to the scatter of the results. The method makes no provision for interference from oxidizable inorganic materials. Thus the presence of chloride, for instance, could constitute a potential problem. The results obtained were an order of magnitude higher than those obtained by methods using extraction with sodium hydroxide. However, the finding that purified charcoal itself gave significant values by this method, and the strong correlation between those results and total carbon analysis (the correlation coefficient, r = 0.9359) indicates that this method estimates a fraction of the total
carbon figure. Whether this can be considered to be representative of the organic carbon content is subject to the same doubt as that for the total carbon figure itself. The modified Dusseldorf procedure, Method 2, was particularly tedious. The collection of carbon dioxide under these conditions was difficult to reproduce, and the use of potassium permanganate as a n oxidizing agent is questionable in the light of experience gained in water analysis, where permanganate has been largely superseded by dichromate. Analysis for correlation between Methods 2 and 3 showed a high degree of correlation ( r = 0.9949). However, the least squares best fit regression equation, plotted in Figure 4, indicates that the dichromate oxidation registered approximately 70% higher oxidizable carbon values than obtained by permanganate oxidation. Method 3 is the method recommended for use in a laboratory not equipped for instrumental carbon analysis. It was relatively convenient to perform and has the advantage of using the standard chemical oxygen demand (COD) procedure for the final step. Since crude refuse contains primarily organic matter as cellulose, the 3-hr treatment with sodium hydroxide was considered sufficient to hydrolyze the polysaccharides as well as the proteins and lipids present. The resulting extract contains the majority of the uncombusted organic matter likely to provide a microbiological substrate, and the subsequent determination of total dissolved carbon in this extract is therefore the best estimate of organic carbon in the residue concerned. The possibility of inorganic interference was tested by performing the test on samples which had been heated in oxygen-free nitrogen a t 500°C for 12 hr. When we evaluated the residual organic content of these extracts using the instrument of Method 4, and compared results with results obtained on them by Method 3, in the samples examined, inorganic interference accounted for
0200
0 150
0"
0 100
v-&-
I
0 050
0
- 0. ---0 5
OOSO
0200
0 150
0100
Figure 4. Correlation between %C* and %C3 (C3 = 1.683 X C p 0.002. r = 0.9949)
Table II. Comparative Estimations of Organic Carbon (Results as %C by wt, except as noted) B
A
Sample:
C
F
E
D
Method 1(C,) No. of analyses
Mean value Rel. std. dev., % No. of analyses Mean value Rel. std. dev., %
5 2.13
5
5
2.38
2.82
5.3
4.5
5 0.127 3.7
11.8 Method 2 (C,) 5 5 0.127 0.090 1.5
4.7
5 1.31
5 2.20
5 5.86
7.2
3.4
3.3
5 0.045
5 0.055
5 0.040
9.2
7.6
8.0
Method 3 (C,) No. of analyses 5 Mean value 0.190 Rel. std. dev., % 2.9
5
5 0.194
0.132
5 0.060
5 0.076
5 0.033
1.7
6.8
3.6
3.4
1.4
2 0.065
2 0.081
2 0.044
2
2
2
0.52
0.55
0.14
2 0.040
2 0.210
2 0.005
Method 4 (CJ No. of analyses 2 Mean values 0.232 No. of analyses 2 Mean values,
mg/g
0 2
demand
3.15
2 0.235
2 0.150 Method 5 (C5) 2 2 1.82
0.48
Method 6 (C,) No. of analyses Mean values
2 0.275
2 0.340
2 0.275
0
0.050
QXM
0200
0.150
QZK)
=4
Figure 5. Correlation 0.006. r = 0.9971)
between %OC3 and %C4
(C3
= 0.805
X
C4
+
between 3% and 6%, only, of the full values obtained by dichromate oxidation. Analysis for correlation between Methods 3 and 4 showed a high degree of correlation ( F = 0.9971). The least squares best fit regression equation was plotted and the results are shown in Figure 5. This suggests that Method 3 is a convenient alternative when the instrumental method is not available. Some interesting comparisons with other methods were provided by Method 5 . Although some degree of inhibition was anticipated, the performance of total organic carbon analysis on the aqueous extract used for the biological oxygen demand (BOD) test showed a high degree of correlation ( r = 0.9986). Thus, the lower correlation ( r = 0.8415) which existed between BOD and the chemical procedure (Method 3) was a result of the difference in extraction efficiency between distilled water and sodium hydroxide solution. Carbon analyses on the water extracts were correspondingly less than those obtained for sodium hydroxide extracts. Yet a further problem with. this methVolume 7, Number 13, December 1973
1145
od of analysis is that of obtaining a suitable seeding material for the BOD dilution water. Method 6 was limited primarily by the heterogeneity of the samples and the resulting scatter of the carbon determinations obtained on 500-mg samples. Since the scatter of carbon determinations was of the same order of magnitude as the volatile carbon values themselves, the results obtained were not considered reliable. The order of magnitude of the volatile carbon indicated, however, agrees well with the figures obtained from the extraction techniques.
The work involved in separating and classifying the individual organic compounds present is complex and time consuming. The solvent extraction procedures used led to excellent separations by gas-liquid chromatography of both the aromatic and aliphatic fractions (Figures 6 and 7). Two-dimensional thin layer chromatography indicated the presence of polynuclear aromatic hydrocarbons in the residues and these, together with the other compounds separated, are a t present being characterized by high resolution mass spectrometry.
nme - d n .
Figure 6. Gas chromatogarphic separation of
20
lo
30 Tim-
mh.
Figure 7. Gas chromatographic separation of 1146
Environmental Science & Technology
the aromatic compounds
the aliphatic compounds
40
50
Conclusions
Literature Cited
Extensive difficulty has been encountered in the use of the Dusseldorf Method for oxidizable organic materials in the assessment of incinerator performance. Many of the problems can be attributed to difficulties in standardization arising from the wide interpretation which can be put on the original description of the analytical procedure. If, however, a rigorous procedure is adopted, good correlation is obtained between this and the two new procedures described (Methods 3 and 4). The determinations carried out using the total organic carbon analyzer (Method 4) are undoubtedly the most rigorous, but as this technique is available t o only a limited number of laboratories, the procedure recommended for general use is the modified chemical oxygen demand test (Method 3), which offers many advantages over the Dusseldorf procedure.
Bowen I. G., Brealy L., “Incinerator Ash-Criteria of Performance,’’ Proc., ASME Incin. Conf., New York, N.Y., 18-22, 1968. Croll, B. T., Chem. Ind., 6 May, 386, 1972. Gutt, W., ibid., 3 June 1972. Lahann, H., Technischer Uberwaschung-Verein Rheinland, e.V. Chemical Laboratory, Dusseldorf, personal communication, January 1972. “Methods of Sampling and Analysis of Solid Wastes,” EAWAG, Section for Solid Wastes, pp 70-2, Dubendorf, Switz., 1970. Orsanic, B., Inforrnationsblatt Nr. 26, 5-11 (1966). Report of the Working Party on Refuse Disposal, Refuse Disposal, p 23, HMSO, London, 1971. Schoenberger, R. J., Trieff, N. M., Purdom, P. W., “Special Techniques for Analyzing Solid Wastes or Incinerator Residue,” Proc., ASME Incin. Conf., New York, N.Y., 242-8, 1968. “Standard Method of Test for Carbon Black in Ethylene Plastics -D,” 1603-68, A n n . Book A S T M Std., 27, 541-54 (1969). “Standard Method of Test for Chemical Oxygen Demand (Dichromate Oxygen Demand) of Waste Water-D,” 1252-67, ibid., 23,211-13 (1971).
Acknowledgments The authors wish particularly to acknowledge the help received from Brian Croll of the Water Research Association in determining the total organic carbon values obtained in Method 4. They are also indebted to I. W. Davies and R. A. Wellings for helpful discussion and advice.
Received for review April 12, 1973. Accepted September 4, 1973. The uork descrihed mas sponsored by the Department of the Environment and further financial help was received from the Science Research Council through Grant Number B / S R / 8 9 7 3 to R. Perry.
Thermodynamic Basis for Existing Experimental Data in Mg-SO,-0, and Ca-SO,-0, Systems Klaus Schwitzgebel and Philip S. Lowell’ Radian Corp., Austin, Tex. 78766
Thermodynamic data are presented in equations and predominance area diagrams for the Mg-S02-02 and CaSOz-02 systems. The experimentally observed decompositions of MgSO3 and C a s 0 3 are explained, including the lack of MgS in MgS03 decomposition products. The thermodynamically feasible temperature ranges of SO2 air pollution control processes, using MgO or CaO, are defined. Systems involving the formation and/or decomposition of calcium or magnesium sulfites and sulfates have been of interest for some time. The pulping industry is an example of a large commercial use of SO2 recovery with MgO. Recently, air pollution control schemes for SO2 have been based upon either calcium or magnesium oxides, hydroxides, or carbonates. Since both sorption and, in the case of magnesium, regeneration of the solid are steps in the process, it is important that thermodynamics and reaction paths be investigated. To interpret the results of sulfite and sulfate decomposition experiments, it is necessary to consider the experimental setup. Most results were obtained on batch samples in a flowing, inert gas. A few results are reported for static gas systems, while still others used a flowing air (an oxidizing medium) stream. This paper is presented in two parts. The first part summarizes literature experimental results for the decomTo whom correspondence should be addressed
position of calcium and magnesium sulfite. The second part presents thermodynamic predictions for the decomposition and disproportionation reactions. The reaction for the decomposition of sulfates to SO3 and metal oxides is related to the decomposition of sulfates to SO2 + l / 2 0 2 by the equilibrium:
so, +
‘/,O,
t
so,
This relationship allows plotting the decomposition of sulfates in terms of SO2 and 0 2 partial pressures even when the major decomposition product may be SO3. The thermodynamically stable phases are presented for both systems in three-dimensional predominance area diagrams as a function of temperature and oxygen and sulfur dioxide partial pressure. High-Temperature Behavior of Calcium and Magnesium Sulfites Since the early investigations of the thermal behavior of sulfites (Muspratt, 1844; Rammelsberg, 1846), it has been known that sulfites disproportionate according to Equation 2: 4MeS0,
-
3MeS0,
+
MeS
(2)
This equation described the disproportionation tendency of sodium and calcium sulfite as well as that of lithium, potassium, strontium, barium, zinc, lanthanum, samarium, cadmium, lead, manganese, and thallium sulfites (Castellani and Garrini, 1963; Castellani and Cola, 1961, 1962; and Castellani and Clerici, 1963). Only magnesium Volume 7, Number 13, December 1973
1147
