the residence time in the catalyst bed was only about 0.003 s, about 270 times less than the 0.8 s in a typical SO2 converter for sulfuric acid manufacture. The results and considerations reported above suggest that in the future it may be possible to avoid adding sulfur oxides from external sources to condition fly ash. The use of internal, in situ catalytic conditioning appears to offer considerable promise. Acknowledgment
N. J. Weinstein and H. Hall have shown great interest in this project and contributed many helpful suggestions. L i t e r a t u r e Cited (1) White, H. J., Air Repair, 3,79 (Nov. 1953). (2) Schrader, K., Combustion, 22 (Oct. 1970). (3) Electr. World, 22 (June 29, 1970). (4) White, H. J., J. Air Pollut. Control Assoc., 24 (4), 314 (April 1974).
(, 5,) Test Code for Determining the Prouerties of Fine Particulate Matter, ASME, PTC 28,196y5. ( 6 ) Method 8, Fed. Reaist.. 36 (247) (Dec. 23.1971). (7) Fritz, J. S., Yamamura, S. S., Anal. Chem., 27 (9), 1461 (1955).
(8) Corbett, P. F., J . Inst. Fuel, 247 (Nov. 1957). (9) Terraglio, F. P., Manganelli, R. M., Anal. Chem., 34 (6), 675 (1962). (10) Roberts, L. M., U.S. Patent 3,581,463 (June 1,1971). (11) “A Study of Resistivity and Conditioning of Fly Ash”, p 113,
Final Report to EPA Office of Air Programs Under Contract CPA-70-149,Southern Research Institute, Birmingham, Ala., Feb. 1972.
Received for review February 12,1976. Accepted J u l y 28,1976. Financial support provided by the Middle Atlantic Power Research Committee and the National Science Foundation under Grant No. GK-38188.
Supplementary Material Available. Experimental details, ( 4 pages), Table I ( 1 page, details of almost 35 experiments),and Figures 5 and 6 ( 2pages, conditioning behaviors of different fly ashes) will appear following these pages i n the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 m m , 24X reduction, negatiues) containing all of the supplementary material for the papers in this issue may be obtained from the Business Operations Office, Journals Department, American Chemical Society, 1155 16th S t . , N . W., Washington, D.C. 20036. Remit check O F money order for $4.00 for photocopy or $2.50 for microfiche, referring to code number ES&T- 77-67.
Potential Hazards Associated with Spray Drying Operations Joseph Epstein’, George T. Davis, Leslie Eng, and Mary M. Demek Environmental Research Division, Headquarters, Edgewood Arsenal, Aberdeen Proving Grounds, Md. 21010
To illustrate the potential hazards in spray drying of salt solutions, the mechanism of formation of the organophosphorus nerve gas, GB, from the spray drying of solutions of GB fully decontaminated by reaction with sodium hydroxide is described. GB is formed in two phases of the spray drying operation: the distillation of the aqueous phase and the thermal decomposition of the salts remaining after all water has been removed. In both phases the formation of GB takes place according to the equation:
of the mechanism could form the bases for sound engineering practices which would ensure that only concentrations a t permissible levels were being emitted into the atmosphere. In this paper we present the mechanism of GB formation from completely detoxified solutions. The mechanistic studies suggest that there is a potential of formation of relatively large quantities of physiologically active materials such as anhydrides and acyl halides in the spray drying of some salts. T h e Process U n d e r S t u d y
/O P-OH /
H3c\ H,C,
+ HF
GB
+
H20
:HCO’ H,C’ The acidity is provided by carbon dioxide from the burning of the natural gas and/or fuel oil used to provide heat for spray drying, In the case of the fuel oil, sulfur dioxide is also emitted. This provides even more acidity to the solution and produces more GB. The mechanistic studies provide the bases for minimizing the production of GB. Spray drying of solutions of salts for the removal of water is a common engineering practice. The possible hazard associated with formation of physiologically active materials in spray drying operations has not, to the writers’ knowledge, been cited. In studies on the spray drying of completely detoxified solutions of the toxic nerve gas, isopropyl methylphosphonofluoridate (GB), it was discovered that GB was being resynthesized in extremely small, but still detectable, amounts. Although the quantities formed were extremely low and only emitted into the atmosphere in permissible concentrations, the mechanism of formation was of interest, since a knowledge 70
Environmental Science & Technology
The demilitarization of GB consists of a decontamination of the toxic material by hydrolysis in strong aqueous alkali followed by spray drying of the resulting brine solution. The decontaminated brine contains a precipitate consisting mainly of sodium fluoride with some of the sodium salt of isopropyl methylphosphonic acid (IMPA). The solution contains, in addition to the sodium salts of IMPA and hydrogen fluoride, unused sodium hydroxide (1.5-4%), some dissolved tributyl amine (which had been added previously to prevent GB deterioration), and diisopropyl methylphosphonate, a common impurity in the manufacture of GB. The effluent from the spray drying operation, after water scrubbing, is emitted into the air, which is continuously sampled and monitored for anticholinesterase activity (1).The concentration of an anticholinesterase in the air stream must never exceed 0.0003 pg/l. (0.3 ng/l.). It was established by mass spectral analysis of air samples, mass spectral analyses of material isolated from aqueous collection bubbler samples, and hydrolysis rate characteristics of the anticholinesterase collected and isolated from the bubbler that the anticholinesterase was GB ( 2 ) . Suggestions that the GB expelled into the atmosphere was from residual GB dissolved in the “decontaminated” solution or occluded in the solids were rejected by the authors in that concentrations as high as 0.25 pg/l. in the brine solution (analyses of the GB brine showed less than 0.25 pg/l.; in our
opinion, the equilibrium concentration of GB in a solution 1.5-4% NaOH would be less than 0.25 pg/l. by many orders of magnitude) would result in an atmospheric concentration of only 0.000003 pg/l. if all the GB were transported into the atmosphere, and in that the kinetics of the reaction of GB with sodium hydroxide under the conditions of the demilitarization operation are so rapid that all the GB is destroyed (a 10l2 reduction in concentration in less than 2 s) before nucleation and agglomeration take place ( 3 ) . On the other hand, anhydrides are produced from acids under severe dehydrating conditions ( 4 ) . It would be anticipated that, for example, GB could be formed as in Equation 1:
;HCO'
H,C 'HCO'
/
'F
H3C'
under conditions where there was removal of the water to displace the equilibrium to the right and there were sufficient quantities of the acids to produce measurable amounts of GB. The hypothesis of GB formation via Reaction 1 became more attractive when i t was learned that overwhelming quantities of carbon dioxide, provided by the combustion of natural gas, were fed directly into material being spray dried and that higher concentrations of GB were being found in the effluent gases when fuel oil (which produced, in addition to C o p , quantities of the more acidic sulfur dioxide) was the source of heat. Moreover, calculation of the quantities of GB expected assuming the displacement of the equilibrium of Equation 1 a t a pH of 8.3 and 7.75 (the p H values of sodium bicarbonate and sodium bisulfite solutions, respectively) was within a factor of two of the analytical values. The agreement between the calculated and the found values was considered corroborative evidence for the mechanism inasmuch as the yield of GB is of the order of lo-% (based upon the quantity of salt).
Distillation Studies Further confirmation for the mechanism of GB formation as given by Equation 1 was provided by distillation of sodium hydroxide decontaminated GB solutions as well as solutions made up from pure sodium fluoride and sodium isopropyl methylphosphonate. The solutions were adjusted to pH 7.0 (by bubbling in SOZ), 8.3 (by addition of Cot), or left alkaline. Where GB decontaminated solutions were used, the precipitated solids were redissolyed before distillation so that analysis for GB content could be made on a homogeneous sample. The solutions were distilled to dryness and beyond. Aliquots of the distillate were analyzed for GB by gas-liquid chromatography (GLC) as well as enzyme inhibition ( I ) . The results of these studies were as follows: GB is distilled out of solutions of both decontaminated GB and solutions of sodium salts of hydrofluoric acid and IMPA adjusted to pH 7.0 and 8.3. The GB must be formed since the quantities in the distillate are orders of magnitude larger than in the distillation pot. For both synthetic and decontaminated GB solutions, the quantity of GB produced is related to the pH of pot solutions: approximately 20-30 times as much GB is found in the distillates from the pH 7.0 as from pH 8.3. No GB could be
detected in distillation from solutions a t alkaline pH levels, i.e., > pH 12. In all solutions at pH 7.0 and 8.3, the concentration of GB in the distillate increases as the distillation progresses. The continued heating of the dried salts from all solutions, even those at extremely high pH levels, produced GB and in larger quantities than was produced by the distillation. The pot temperatures in some cases were higher than 300 "C. Other observations were: the pH of all distillates including those from the high pH pot samples were originally close to neutrality and became more acidic as the distillation progressed; the pH levels on the last fractions distilled over, especially a t the very end, were often very acidic (pH ca. 2.5), and analysis of fluoride ion indicated that the acidity was almost totally due to hydrofluoric acid; and aqueous solutions of the residues after heat and obvious decomposition were acidic. That 20-30 times the quantity of GB was produced at pH 7.0 as a t 8.3 indicates that the yield of GB is dependent upon the quantity of one of the two reactants, viz., H F or IMPA, but not both. If the yield were dependent upon both, the GB concentration would be 400 times greater at pH 7 than at 8.3. A consequence of the yield-reactant relationship is that although the equilibrium as shown in Equation l lies far to the left in dilute solution, it is completely displaced to the right upon separation of the water molecules from GB during distillation. The mechanism of formation that emerges from the preceding observation is that the hydrofluoric acid and IMPA which are in equilibrium with their respective salts volatilize with the volatilization of the water molecules from the surface of the solution. The quantities of the two acids, now in the vapor phase, are in relation to their respective quantities in the aqueous phase which has distilled. The synthesis of GB takes place in the vapor above the solution. The yield of the GB will be dependent upon the concentration of IMPA rather than of H F since the IMPA is present in much lower concentration. (The ionization constants of the IMPA and H F at 25 "C are 1.1 x IO-* and 3.5 x 10-4, respectively.) Two other pieces of information are in accord with the proposed mechanism: these are the yields of GB on distillation and the effect of fluoride ion concentration on the yield of GB. According to the model, stoichiometric quantities of GB will be formed from the IMPA present at the pH and distillation temperature. In one experiment the concentration of GB was determined in the distillate from the distillation of only a small fraction (to minimize concentration changes) of a solution at pH 7.4 and 0.42 M with respect to each of the sodium salts of hydrofluoric acid and IMPA. An apparent acid ionization constant for IMPA at 100 "C was calculated from these data using the formula:
K, =
[H+][IMPA-] [IMPA]
where [IMPA] was taken as [GB]. K , was determined to be 4.2 X a reasonable value inasmuch as K , at 25 "C is 1.1 X Using this value of K,, the theoretical yield of GB that would be distilled from a solution containing the salts of hydrofluoric acid and IMPA can be calculated from the equation:
where A0 is the initial concentration of the sodium salt of IMPA, V Ois the initial volume in liters, and V1 is the final volume (in liters) in the pot after distillation. The solubility of NaIMPA a t 100 "C corresponds to a 12.4 M solution; the value of VI should be such that saturation has not been Volume 11, Number 1, January 1977
71
reached. In the distillation experiments described herein, where the initial concentration of NaIMPA was 1.0 M, the volume distilled could be at least 90% before saturation of the solution with NaIMPA occurs; where the initial concentration was 0.33 M, the volume distilled could be 97.5% before saturation occurs. Table I shows the good agreement between the calculated and analytically determined values for GB for three distillation experiments. With respect to the effect of (F-] on the yield of GB, according to the proposed model, the GB yield should be dependent upon the acid present in the lower concentration. The relative concentrations of the acids will be related to the concentrations of their conjugate anions as well as their pK, values. Assuming that the pK, values of the two acids a t 100 "C are in the same ratio as a t 25 "C, the ratio of fluoride ion to IMPA would have to be less than 0.03 before one would observe a significant reduction in the yield of GB. Table I1 shows the yields of GB from the distillation of pH 7.0 solutions containing 1 M IMPA- and different amounts of F-. A decrease in the yield of GB is not observed even at a molar ratio of F-/IMPA- of 0.01: a marked decrease is observed at a molar ratio of F-/IMPA- of 0.001. This result is qualitatively in accord with the proposed mechanism; a more quantitative correlation would be obtained if the ratio of the pK, values at 100 "C is 0.005. Data for the pK, of HF at 100 "C are not available. Extrapolation of pK, values for H F a t 0 and 25 "C ( 5 )to 100 "C gives a K , value for HF of 3.9 X Using this value and the gives a ratio estimated K , for IMPA a t 100 "C of 4.2 X of approximately 0.009, which is in fair agreement with the 0.005 necessary for quantitative correspondence of the data to the proposed mechanism.
Thermal Decomposition of Salts Preliminary observations on the heating of residues of the distillation experiments had suggested that both volatile and nonvolatile acids were formed upon thermal decomposition of the residue salts. Moreover, there was evidence that hydrogen fluoride was among (or perhaps the only one of) the volatile acids. Further studies were made on NaIMPA; mixtures of NaIMPA and NaF and NaIMPA, NaF, and NaOH; and on intermediates of the decomposition of NaIMPA and the mixtures. These studies included thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and isothermal pyrolysis studies. The results of these studies are as follows: Pure NaIMPA (mp ca. 260 "C) showed a weight loss of approximately 36% over a range of 260-330 "C by TGA. There appeared to be two maxima in the curve of weight loss with
Table 1. Quantities of GB Found and Calculated from Equation 2 for Three Distillation Runs
[i2/:
yo. rnl
v1 rnl
pH
Calcd, rng OB
Found, rng OB
0.33 0.33 1.o
2487 2487 1000
217 60 500
7.4 8.3 7.0
Ql8 0.043 0.23
0.16 0.049 0.17
Table II. Yields of GB from Distillation of pH 7.0 Solutions of (NaO)P( :O)(CH3)0CH(CH3)2](Initial Volume = 1000 ml; Flnal Volume = 500 ml) [IMPA-1. MII.
1 1 1 72
iF-1. M/I.
0.1 0.01
0.001
Environmental Science & Technology
mg GB
0.18 0.20 0.04
temperature (suggesting different reactions) at 260-270 and 320-330 "C. Thereafter, there was another significant weight loss over the range 500-570 "C (maximum, ca. 540 "C), amounting to an additional ca. 16%of the original weight. No weight loss was observed for NaF up to 700 "C. The percent weight loss of a 1:l mixture (molar ratio) of NaF and NaIMPA was equal to that expected for NaIMPA alone. The temperatures at which reactions occurred resulting in weight losses were the same in the mixtures and the NaIMPA alone. Isothermal studies have been made of the weight loss with time of pure NaIMPA at 240,260,280,300,310, and 330 "C. Under all conditions of temperatures, the weight loss was approximately 36%. First order kinetics of loss of weight with time was found over the whole loss of weight for the runs at 280 "C and above: a t 240 and 260 "C, the kinetics were first order for only a portion of the runs. At 240 "C, following the first order portion, there was an increasing percent loss of weight with time; at 260 "C, a decreasing percent loss of weight with time. A plot of the logarithm of the first order rate constants with the reciprocal of the absolute temperature showed good linearity for the runs at 260,280,300, and 320 "C. The point on the graph for the run at 340 "C was high but not excessively so, whereas the point a t 240 "C was extremely low. It is considered that with more data points, two straight lines would exist with an intersection point a t approximately 260 "C. The equation for the line covering the temperature 260 "C and above is Equation 3. -8330 log k = - 13.405 (3) T where K is in units of min-l and T is K. Half-lives of decomposition of NaIMPA at different temperatures are given in Table 111. The Arrhenius activation energy, calculated from the slope of the line log k = AE/RT C is 38.0 kcal/mol. The line connecting the points between 260 and 240 "C is much steeper so that it would appear that the rate of decomposition is considerably slowed by decreasing the temperature only slightly below the melting point of the NaIMPA. It is probable that the kinetics of melting or mass transfer in the liquid-solid mixed phases are responsible for the different rates. Differential scanning calorimetry (DSC) for pure NaIMPA showed the existence of an endothermic reaction at 260 "C (the melting point of NaIMPA is ca. 260 "C; the absorption of heat at this temperature would be expected in the melting process), followed by two exothermic reactions occurring in the region ca. 290-340 "C. When NaF was added to NaIMPA, the DSC showed the same endotherm at 260 "C, followed by a new exotherm a t 275 "C and a single exotherm at 290-340 "C. Effluents from the isothermal pyrolysis of NaIMPA at 270 "C were separated by gas chromatography and identified by their mass spectra as follows: carbon dioxide, water, propylene, isopropanol, trimethylphosphine oxide, and diisopropyl methylphosphonate (in order of increasing retention times). The isopropanol peak was most intense and appeared to include an, as yet, unidentified component. It was established that the propylene did not result from the pyrolysis of isopropanol. It has not been established whether or not isopropanol could have been formed by the acid catalyzed reaction
+
+
Table 111. Half-Lives of NalMPA at Different Temperatures OC
1112, rnln
37 1 343 315 288
0.23 0.81 4.1 19.2
of propylene and water. Pyrolysis of NaIMPA a t 350 "C gave the same products as at 270 "C; larger quantities of propylene were formed. An equimolar mixture of NaIMPA and NaF on pyrolysis at 425 "C generated the same materials as those of neat IMPA, plus two new compounds: methylphosphonic acid and GB. GB was also detected in a similar pyrolysis run a t 270 "C. Pyrolysis of a mixture of NaF, NaIMPA, NaOH (6%) a t 425 "C also produced GB.
Interpretation of the Data The data are interpreted as follows: Loss of propylene occurs according to Equation 4a (6):
H,C'
It is possible that all the above reactions take place in addition to several others. The overall occurrence at 270 "C can be summarized by Equation 5:
'
H3C\PH0 NaO \o/
P@CH, 'ONa
(5)
The equation accounts for the appearance of propylene and isopropanol; smallamounts of water would escape from Reaction 4b; the pyro compound whose structure has been established by IR, has been found in the melt of the NaIMPA samples following TGA analysis. Occurring simultaneously with Reactions 4a-c is the formation of DIMP as shown in Equation 6.
-
H3C 2 The monosodium salt of methylphosphonic acid (NaMPA) rapidly condenses with itself to form a pyrophosphonate (pyro) as shown in Equation 4b (6).
+ i-C3HiOH + C,H,
H3c\ H,C
/HCo
\p/o
/ \
ONa
The formation of trimethylphosphine oxide (TMPO) is accounted for by Equation 7
N">'I
r
+ H20
(4b)
NaO
0-P-CH:, O \
Na
At the same time, the ejection of superheated steam (water formed at 260 "C) cleaves some unreacted NaIMPA (Equation 4c) to NaMPA and isopropanol (7). (It is not necessary to speculate that the only source of water is via Equation 4b. Analyses of salts after having been spray-dried at temperatures as high as 700 "F inlet temperature still contained water in appreciable quantities.) fast
HL!'
Reactions 4b and 4c are very rapid as compared with 4a. Equation 4c shows the formation of NaMPA which can react according to Equation 4b to produce more water and continue the process until the supply of NaIMPA is exhausted. Isopropanol could also be formed via Reaction 4d.
HJC\ NaO/
/o
P-OH
+
/o
H3C,
p
,P-OCH NaO
'CH,
3
-
-
0Na 2(CH3)sP=0
+ Na,P,O,,
(7)
where NasP4013 could be mixtures of meta-, ortho-, and pyrophosphates. In support of the mechanism of formation as TMPO as given in the above equation, GLC showed that TMPO was formed on heating NaMPA. (NaMPA is shown as the precursor to pyro (Equation 4b), which in turn forms TMPO via Equation 7 . ) Reaction 7 has been verified also by preparative runs on the pure salt as well as TGA studies on the pure salt. Also, the disodium salt of methylphosphonic acid, the product of Reaction 6, failed to produce TMPO on pyrolysis. If the total reaction path were to take place according to Equation 5, then one would expect a weight loss of 31.9%. On the other hand, if the total reaction path is given by Equation 6, then there would be a weight loss of 56.25%. The actual weight loss was 36%, which could be accounted for by assuming that the reactions shown by Equation 5 took place to the extent of 83.1% whereas that shown by Equation 6,16.9%. It can also be shown that if 83.1% of the reaction occurred via Equation 6, then a second loss in weight equivalent to 15.95% of the original would be expected as a result of Equation 7 , in excellent agreement with the 16% found experimentally. Formation of GB occurs as a result of reactions shown in Equations 8a-c. Although small quantitites of reacting acids, viz..
0
PH3
1I
HF and HO-P-OCH
I
CH,
'
CH3
Volume 11, Number 1, January 1977
73
are formed by these reactions, they are in sufficient quantities to produce GB.
NaF
+
HJC\
//O P-,OH e HF \ ONa
0 H3C-PyON>H,
+
+
this is the only mechanism by which GB can be formed. It is thought possible, for example, that GB can be formed according to the following equations:
H3C \ H0 PrONa
Bo H,C-P-OH
HJC HF
+
\
//O H/COP\OC\H
HJC’
\
@
GB
+
l’-C,H,OH
CH, ‘CH, (10)
e
\ONa OCH
\
CH,
+
0 H3C-PtOH,cH,
//O H,C-P-ONa \ONa
(8b)
\
CH,
CH,
’OCH
\
(11)
CH3
The reactions shown in the series 4a-c provide a continuous and constant acidic milieu for Reactions 8a and 8b. That proton transfer of this nature can take place is strongly indicated by the identification of methyl phosphonic acid (MPA) from isothermal pyrolysis studies of a mixture of NaF and NaIMPA. MPA is most logically formed by the proton exchange reaction:
Hydrogen fluoride and IMPA can also be formed from the hydrolysis of the salts. The similarity in TGA’s of IMPA alone and mixtures of NaF and IMPA is attributed to the fact that the quantities of HF and IMPA are extremely small as compared with the quantities of products shown in Equations 4a-c, so that the reaction of the two acids produces, t o all intents and purposes, a negligible weight loss although these quantities may be significant from a toxicological viewpoint. In the DSC analyses, positive indications were obtained for a new reaction when NaF was mixed with NaIMPA. From the composition of the effluent in the pyrolysis studies at 270 “C of NaIMPA and the mixture of NaF and NaIMPA, it is concluded that the new peak seen in DSC is due to GB formation.
Discussion The mechanism of GB formation is shown as a reaction between HF and IMPA. That the free acids can be formed, even in the presence of strong base, has been shown by identification of methylphosphonic acid in pyrolysis experiments and of H F in distillation experiments. The results of the distillation experiments suggest that GB can be formed in the vapor phase from the two acids. It is not meant to imply that 74
Environmental Science & Technology
ONa ‘CH, In all reactions that we think plausible for GB formation, however, there must be an acidic molecule involved. In Equation 10 above, it is HF; in Equation 11, the pyroester would form as a result of an interaction of two moles of IMPA. The production of acid molecules is possible via at least two routes: hydrolysis of the salts and decomposition of NaIMPA via Equation 4a. The mechanistic studies show the avenues for minimizing the reformation of GB in spray drying operations. These are: use of heat which does not provide acid gases to the material being spray dried, Le., indirect heating; use of as low a temperature as is compatible with engineering practice to minimize thermal decomposition of the salts and formation of acids; and distillation only to the point where the salts can be handled effectively since the largest quantities of GB percentagewise.are distilled in the last phases of the distillation. A low temperature of operation is also advantageous where combusted fuel gases are fed directly into the material being sprayed. Smaller quantities of fuel gas would need to be combusted if lower temperatures are satisfactory; the burning of smaller quantities of fuel gas would result in the formation of smaller quantities of acidic gases. The studies indicate that an intuitively attractive avenue for minimization of GB formation, viz., separation of salts by filtration, will not provide a practical solution to the problem. There are several ancillary benefits of the present study. The first is the realization that environmental problems can be precipitated by chemical reactions where yields are of the order of lo-%. Seemingly unfavorable chemical reactions should not be discarded from one’s thinking in the analysis of environmental problems. The second is an obvious corollary to the first, namely, that conclusions as to the safety of a process by analysis of the destruction of the reactant (even to greater than 99.9 . . . %) in lieu of the formation of the product can be very dangerous. Then, too, the finding that the toxic nerve gas GB can be synthesized from salts brings to light the possibility that many other substances (perhaps not quite as toxic but still physiologically active and hence which should be monitored if they are to be introduced into the atmosphere) can be synthesized in a spray drying operation. One can visualize the formation of, for example, acyl fluorides from mixtures of salts of organic acids and hydrofluoric acid, or the formation of anhydrides from salts of organic acids. These materials are highly toxic themselves. In general, however, the formation of anhydrides will not be a serious problem in spray drying operations unless
the quantities of salts being dried and the concentrations of the conjugate acids are relatively high. For the latter to occur, the pH of the aqueous solution must be low and the pK, of the conjugate acid, high. Acids having the proper pK, for high anhydride formation (as well as the correct chemistry) are carboxylic acids (e.g., acetic, propionic, benzoic) or possibly substituted phenols (e.g., nitro and dinitrophenols). The ratio of the anion to free acid for a salt whose conjugate acid has a pK, of 5 at a p H 8.3 is 2 X lo3. In the study herein, the ratio was close to IO7. In a spray drying operation involving a salt whose conjugate acid has a pK, of 5, one might expect, as a first approximation for the same molar quantity of salt under drying, more than lo3 times the molar quantity of an anhydride as was found in these studies. The effluent concentration might then contain 1 ppm. On the other hand, inorganic acids such as hydrochloric, hydrobromic, and nitric are too acidic to yield appreciable concentrations of free acids upon hydrolysis of their alkali salts. Other acids (such as sulfurous acid) are too unstable to form anhydrides. Nevertheless, there are a number of inorganic acids (e.g., arsenic and phosphoric) which possess pK, values of the proper range for pyroacid formation. If the pyroacids (or mixed anhydrides) are volatile, then there could be a problem. It is recommended that the effluents from spray drying of salt systems be examined carefully prior to production runs.
Acknowledgment The authors acknowledge the valuable contributions made to this study by Leon J. Schiff, Emory W. Sarver, Harold Z. Sommer, Lester W. Daasch, Joseph V. Pistritto, James A. Richmond, Frank Block, Johnnie T. Jones, James Bouck, Ronald Piffath, H. Klapper, and Linda J. Szafraniec.
Literature Cited (1) Michel, H., Gordon, E. C., Epstein, J., Environ. Sci. Technol., 7,
1045 (1973). (2) Young, J., private communication. (3) Epstein, J., Bauer, V., Agent GB Neutralization Data in Support of M34 Demil Program, July 5 , 1973 (report on file a t Edgewood Arsenal). (4) Kirk-Othmer, “Encyclopedia of Chemical Technology”, Vol I, 2nd ed., p 211, Wiley-Interscience, New York, N.Y., 1963. (5) Washburn, E. W., Ed., “International Critical Tables of Numerical Data, Physics, Chemistry and Technology”, 1st ed., National Research Council, McGraw-Hill, New York, N.Y., 1929. (6) Karayannis, M. M., Mukulski, C. M., Strocko, M. J.,Pytlewski, L. L., Labes, M. M.,lnorg. Chim. Acta, 5,118 (1971). (7) Feigl, F., “Spot Tests in Organic Analysis”, 6th ed., Chap. 1,p 18, Elsevier, 1960.
Received f o r review March 9,1976. Accepted July 28,1976.
Dynamics of Mercury at Coal-Fired Power Plant and Adjacent Cooling Lake William L. Anderson* and Kenneth E. Smith Illinois Natural History Survey, Urbana, 111. 61801
The dynamics of mercury at the Kincaid Power Plant-Lake Sangchris complex in central Illinois were investigated by collecting and analyzing samples of coal, slag, fly ash, airborne particulate matter, soil, lake sediment, fish, macrophytes, and ducks. Of 546 kg of mercury calculated to be in the 2.7 million metric tons of coal burned by the power plant from September 1973 to August 1974, an estimated 97% was vaporized and emitted into the air. Mean concentrations of total mercury in soil were 0.022 ppm to the northeast of the power plant, the direction of prevailing winds, and 0.015 ppm to the southwest. Mean concentrations in sediment were 0.049 ppm in deposits that occurred after the power plant began operating in 1967, and 0.037 ppm in earlier years. Seven species of fishes from Lake Sangchris contained low amounts of mercury: the mean value for largemouth bass was 0.07 ppm, as compared with means of 0.16-0.56 ppm for bass from three other Illinois lakes. Some unidentified factor at Lake Sangchris has apparently suppressed mercury accumulation in the fishes.
Coal-fired electric power plants have come under scrutiny as potential sources of mercury contamination because they consume enormous quantities of coal that contains mercury, as well as many other trace metals, vaporizing the mercury in the process of combustion. A total of 352 million metric tons of coal was used by power plants in the United States in 1974 ( I ) . In Illinois, 33 power plants burned coal in 1972 and collectively consumed 26.8 million metric tons of this fossil fuel (2). Coal currently being mined in this midwestern state contains an average of 0.21 ppm (range 0.03-1.6 ppm) of mercury ( 3 ) ,and mercury values for coal from other states
range as high as 33 ppm ( 4 ) .Billings and Matson have shown that when coal is burned, more than 95% of the mercury present is vaporized and emitted into the air ( 5 ) .Thus, the coal-fired plants in Illinois could have discharged in excess of 4576 kg of mercury into their local environments in 1972. This paper summarizes an investigation of the physical and biological dynamics of mercury in the vicinity of the Kincaid Power Plant-Lake Sangchris complex. The study involved collecting and analyzing samples of airborne particulate matter, soil, lake sediment, fishes, macrophytes, and ducks. Samples of the coal burned by the power plant and samples of the slag (bottom ash) and fly ash produced during combustion were also analyzed. This study was conducted from September 1973 through August 1974. The Kincaid Power Plant is located in northwest Christian County of central Illinois and is owned and operated by Commonwealth Edison Co. This 1 200-MW facility began generating electricity for commercial consumption in 1967 and consumed 2.71 million metric tons of coal in 1972 (2). Upon combustion in this plant, coal is reduced to approximately 15% of its original mass, according to plant officials. About half of this residue is slag, which is slurried into a diked area (containing 32 ha of settling pond) located immediately northeast of the plant (Figure 1).The remainder is primarily fly ash, 98% of which is removed from the smoke by electrostatic precipitators on each of the plant’s two, 152-m-high smokestacks. The coal, which contains approximately 4% sulfur, is provided by a shaft (underground) mine located 1.6 km west of the Kincaid Plant (Figure 1). The Kincaid Power Plant is positioned between the west and middle arms of Lake Sangchris (Figure 1)and uses water in this impoundment for cooling purposes. The lake, constructed in 1964, currently has a surface area of 872 ha and Volume 11, Number 1, January 1977
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