Environ. Sci. Technol. 1982, 16, 195-197
Water-Leachable Boron from Coal Ashes William D. James*
Center for Trace Characterization, Department of Chemistry, Texas A & M University, College Station, Texas 77843 Christopher C. Graham, Mlchael D. Glascock, and Abdul-Saiam G. Hannat
Research Reactor Facility, University of Missouri, Columbia, Missouri 65201
rn Boron concentrations and the quantity of boron available for water leaching from a number of conventional precipitator fly ashes were studied. Leachabilities in these ashes varied from 17% to 64%. One ash from a magnetohydrodynamic (MHD) high-temperature pilot plant was found to contain only 7% leachable boron. Agreement is found which proposed thermal fixation mechanisms. Many plants can only tolerate a very narrow range of boron concentrations in the soil (I). It is essential for plant growth and development. However, treatments of boron to the level of 0.5-5 ppm have resulted in toxicity in various soil types. The mechanisms by which it affects plant physiology are largely unknown (2). It is known that boron is needed for cell division and can regulate the availability of sugars (3-5). Detailed studies on boron nutrition in animals as well as influence on plant development have been hampered by the nonavailability of an expedient, precise, analytical technique capable of determining boron concentrations. Leaching of boron from disposed coal fly ash by ground waters and rainwaters poses a potential hazard to agriculture. Boron is concentrated in the ashes to levels of up to about 2000 ppm during combustion of coal. The final disposal of this material essentially always allows contact with natural waters. Therefore, one can expect soluble species to enter the environment through leaching. Several studies have dealt with the leaching of trace elements from fly ashes (6-9). However, boron leaching has been largely ignored, primarily because of the difficult analytical procedures involved. Of the few existing measurements for boron, published results are seemingly contradictory. The analytical techniques used in this work included neutron capture prompt y-ray activation analysis (PGAA) and inductively coupled argon plasma emission (ICP). PGAA has been proven to be an extremely sensitive, precise tool for the analysis of boron in many matrices. It is especially helpful in the analysis of difficult-to-dissolve materials such as coal ash. ICP is more sensitive and expedient for the analysis of aqueous solutions. Dreesen et al. (IO) compared levels of trace elements extracted from fly ash using a 1:4 ratio of ash to extractant with those in effluent waters of a coal-fired power plant. Water-leachable boron in that ash from a New Mexico power plant was reported to be 1.5%. Cox et al. (11) reported that approximately 50% of boron present in a southern Illinois fly ash was water leachable. They agitated 0.5 g of ash in 200 mL of water. While experimental conditions differed greatly, the discrepancy between these results cannot be explained readily. Cox also found that boron in the bottom ash from the same source was almost totally insoluble. This led the authors to propose and investigate a thermal fixation mechanism as an explanation. Several ashes were heated Present address: Nuclear Research Institute, Box 766, Baghdad, Iraq. 0013-936X/82/0916-0195$01.25/0
from 1 min to 8 h at 1200 "C and for 30 min at 800 "C. While the treatment at the lower temperature had no significant effect on soluble boron, a treatment at 1200 "C of the same duration reduced available boron by 84%. Boron occurs in nature predominantly as borates (borax) and boric acid. These compounds are readily soluble in water at room temperature under a wide range of pH. However, less-soluble species such as borosilicates may be formed under elevated temperatures. Very high temperatures can result in the combination of boron with air to form boron nitrides. However, while ESCA measurements confirmed the existence of a second independent chemical state of boron, qualitative determination of the persistent chemical species by Cox et al. was unsuccessful. Alternate explanations for the reduced level of leachable boron after high-temperature treatments are possible. Formation of impermeable coatings or cells may occur during recrystallization of metallic oxides. Boron species isolated in this way would not be available to leaching waters. In the present work, 19 Midwestern and Western coal ashes have been leached with water under constant laboratory conditions to study variability of total leachable boron with coal source. In addition, one of these ashes was produced at a high temperature (3000 K) in a magnetohydrodynamic (MHD) coal-fired pilot plsnt. Study of this material's leaching characteristics has further explored the possibility of thermal fixation as a significant factor in reducing boron availability. Experimental Section Ashes from 18 different power plants throughout the midwestern United States were chosen for this study. Five were obtained each from Illinois and Indiana, two from Kansas and Wisconsin, and one each from Michigan, Kentucky, Missouri, and Iowa. It is felt that these precipitator ashes are a valid representation of material being produced in the central part of the country. The MHD ash was produced in a pilot plant in Tennessee. Representative portions (2 g) of the ashes were shaken in 800 mL of distilled deionized water for 24 h. The mixtures were filtered through Millipore type AA filters (0.8-pm pore size), and the filtrates were then frozen in polyethylene bottles. The small ash-to-extractant ratio was adopted to ensure against saturation, thereby allowing all water-soluble species in the ashes to be dissolved. The primary analytical technique for these experiments was PGAA. These analyses were carried out at the University of Missouri Research Reactor (MURR). This newly constructed facility utilizes a thermalized neutron beam extracted from a beam port of the reactor. The beam was focused onto the samples suspended in an evacuated chamber. A high-resolution Ge(Li) y-ray detector (18%) enclosed in a NaI(T1) annular Compton suppression shield viewed the sample at 90" to the beam. Data collection and processing were accomplished by using a Nuclear Data ND 6620 pulse height analyzer. All y-ray interactions occurring
0 1982 American Chemical Society
Environ. Sci. Technol., Vol. 16, No. 4, 1982
195
Table I. Spike Recovery
sample spike spike spike spike
0
1 2 3
boron, ppm
79 501 371 236
3500
0 71.4 47.6 23.8
0 72.7 47.0 24.5
1 .:. . . I . :
spike spike deadded, termined, recovery, Pg PP %
3000
2500
101.8 98.7 102.9
...*a
*.
1
B(477)
1500
in the Ge(Li) detector simultaneously with an event in the NaI crystal were rejected by the analyzer, thus significantly reducing general Compton background of the spectra. As this technique is extremely sensitive for boron, counting rates were high. Thus, statistical counting errors were not significant. A more detailed explanation of the PGAA facility is available (12). Ash samples of about 0.2 g were encapsulated in polyethylene vials. Leachates to be analyzed were prepared by evaporating 10 mL to dryness in Teflon bags at a temperature of about 60 "C. Because of the known volatility of boron from acidic solutions, KOH was added to prevent the loss of boron during evaporation (13). Samples to be activated were then suspended in the beam by using a Teflon bag and holding device. Measurement times were typically 1000 s. While this was sufficient for boron because of its very high sensitivity, longer times would be required to quantify many other elements by this technique. A portion of a typical y-ray spectrum obtained, showing the Doppler-broadened boron line at 477 keV, is shown in Figure 1. In addition to the fly ashes and leachates, several spiked samples consisting of varying amounts of standard boric acid solution dispersed on a pulverized limestone were analyzed to confirm linearity of the method. Spike recovery data are tabulated in Table I. Recoveries indicated the technique to be satisfactory for the present study. Errors associated with spiked-sample measurements are less than 5 % relative. The leachates were also analyzed for boron by ICP. A portion of the leachates was acidified with HNOBand aspirated directly into the nebulizer of the Jarrell Ash 975 ICP optical emission spectrometer. Five separate measurements were made and averaged for each sample result. In order to minimize errors due to instrument electronic
Ann (511)
500
..... ______ --_. *
...*;.I-
.. Sm(505).
u.y........~C**'
01
470
480
490
500
,
'.
Ca(519)
** .....t....U ...
510
520
ENERGY (keV)
Flgure 1. Portion of a typical y-ray spectrum obtained by prompt y-ray activation showing the Doppler-broadened boron peak at 477 keV. Samarium, calcium, and annihilation lines are also shown.
drift, we analyzed standard solutions just before and immediately following each leachate and performed a linear correction on the sample data.
Results and Discussion Results of the leaching studies have been tabulated along with power plant location, coal source, and final leachate pH in Table 11. The ash boron concentration varied by almost 1order of magnitude while that portion available to water leaching varied from about 17% to 64%. The boron leached was calculated two ways: boron leached (% of total) = lOO[boron(leachate, ~g/mL)1(800mL)/{[boron(ash, ~ g / g ) l ( 2g)l (1) boron leached (% of total) = 100{1 [boron(residual ash, pg/g)] / [boron(ash, pg/g)]} (2) Calculations by 1 and 2 should agree if no significant sample mass is lost during leaching and no boron is evolved. Equation 1 will hold true in any case and is preferred. For three samples eq 2 resulted in significantly higher values (>30%). It is thought that perhaps boron was lost during the residual ash drying (60 "C). However, it is not clear why these samples (403,405, and 407) would be selectively affected.
Table 11. Boron Leaching Results
a
sample
plant location
coal source location
37 6 377 379 403 404 405 406 407 408 409 410 41 1 412 41 3 414 415 416 417 425
Kentucky Illinois Indiana Illinois Kansas Kansas Michigan Illinois Indiana Wisconsin Ulinois Indiana Wisconsin Indiana Missouri Indiana Illinois Iowa Tennessee (MHD)
W Kentucky Montana Wyoming Montana Wyoming Wyoming Unknown Montana Indiana W Kentucky Illinois Indiana Illinois Indiana uiilcnown Indiana Colorado Illinois Illinois
[boron (ash)], pg/g
679 865 1261 721 289 6 34 656 9 62 172 256 1370 226 7 69 728 413 679 865 1480 5 44
boron leached, % of total
56 30 62 17 41 42 41
18 23 51 34 42 63 42 55 47 47 64 7
final leachate pHa
9.85 10.11 10.51 10.56 10.79 10.34 10.54 10.36 11.35 9.02 4.98 7.19 8.99 8.57 9.55 9.56 9.23 10.05 4.37
pH of water prior to leaching was 4.85. b Mixture of 50%western Kentucky and 50%Pennsylvania coals.
196
Environ. Sci. Technoi., Vol. 16, No. 4, 1982
Environ. Scl. Technol. 1982, 16, 197-202
Values reDorted in Table I1 for boron leached are calculated by ;sing eq 1. Agreement of the PGAA and ICP results for leachate boron concentrations were generally very good. However, for samples 405,406, and 408 variation exceeded 15%. In those cases, the percentage of total boron leached is calculated with eq 2, which does not require leachate results. Generally relative errors were less than 10%. However, somewhat greater errors might occur in the use of eq 2. No definite correlation is observable from the data in Table I1 with respect to coal origin. The seven Western coal ashes averaged 800 pg of B/g of ash with 37% of it leachable with water. This differs from the 11 Midwestern coal ashes (675 pg/g and 47%) but standard deviations overlapped. While no attempt was made to determine variation of boron availability with leachate pH, the data presented suggest a confirmation of the observations of Cox that the leaching of boron from fly ash is not significantly dependent over the range of expected natural conditions.
MHD Ash Discussion It is noteworthy that the MHD ash (sample 425) contained a comparable quantity of boron but only 7% was water leachable. It is suggested that the fact that this sample was produced at a much higher temperature than generally found in typical combustors correlates well with thermal fixation mechanisms that Cox et al. proposed. The potassium from the MHD seed is generally recoverable as a soluble sulfate which would be expected to be present in this ash. The potassium and sulfur contents of the ash were determined to be 33% and 14.5%)respectively, by PGAA. Approximately 80% of this sample dissolved during leaching. Potassium and sulfur were determined to be 6.6% and 0 . 5 %respectively, ~~ in the residual ash. Calculations show that the loss of potassium and sulfur approximates the stoichiometry for K2S04. It would appear that solubilizing potassium in the ash would be a feasible method of recovering MHD seed.
Acknowledgments We are grateful to American Admixtures Co. for supplying most of the samples studied and to Dr. Steve Lundberg of Montana Energy and MHD Research and Development Co. for supplying the rare MHD fly ash sample. Literature Cited (1) Bingham, F. T. J . Am. Chem. SOC.1973, 95, 130-8. (2) Bowen, H. J. M. “Trace Elements in Biochemistry”;Academic Press: London, 1966. (3) Nason, A.; McElroy, W. D. “Plant Physiology”;Academic Press: New York, 1963; Vol. 3. (4) Gauch, G. H.; Duggar, W. M. “The PhysiologicalActivation of Boron in Higher Plants: A Review and Interpretation”; AgriculturalExperimentalStation, University of Maryland, Technical Bulletin A-80, 1954. (5) Schutte, K. H. “The Biology of the Trace Elements”; Lippincott: Philadelphia, 1964; Chapter 2. (6) Theis, T. L., presented at the 2nd National Conference on Complete Water Reuse, Chicago, IL, May 1975. (7) Holland, W.; Wilde, K.; Parr, J.; Lowell, P.; Pohler, R. “Environmental Effects of Trace Elements from Ponded Ash and Scrubber Sludge”,PB-252 090, NTIS, Sept 1975. (8) Helm, R. B.; Keefer, G. B.; Sack, W. A. “Environmental Aspects of Compacted Mixtures of Fly Ash and Wastewater Sludge”, presented at the 48th Conference of the Water Pollution Control Federation, Oct 1975. (9) James, W. D.; Janghorbani, M.; Baxter, T. Anal. Chem. 1977,49, 1994. (10) Dreesen, D. R.; Gladney, E. S.; Owens, J. W.; Perkins, B. L.; Wienke, C. L.; Wangen, L. E. Environ. Sei. Technol. 1977,11, 1017. (11) Cox, J. A.; Lundquist, G. L.; Przyjazny, A.; Schmulbach, C. D. Environ. Sei. Technol. 1978, 12, 722. (12) Hanna, A. G.; Brugger, R. M. Glascock, M. D. Trans. Am. 1980, 34, 178. Nucl. SOC. (13) Feldman, C. Anal. Chem. 1961, 33, 1916. Received for review December 22, 1980. Revised manuscript received July 28, 1981. Accepted November 16, 1981.
Measurement and Analysis of Aerosols along Texas Roadways Jerry A. Buiiln”
Chemical Engineering Department, Texas A & M University, College Station, Texas 77843 Roderick D. Moe
Texas State Department of Highways and Public Transportation D-8, 11th Street and Brazos Avenue, Austin, Texas 78701
A study of aerosol levels along roadways in Texas was performed by using high-volume air samplers and stacked filter units. Horizontal and vertical concentration profiles were drawn for several elements as well as total suspended particulate (TSP)matter. In most cases the concentrations were found to decrease with distance from the roadway, reaching near-background levels at the edge of the right of way. Although the TSP by high-volume sampler was usually in the range of 80-150 pg/m3, the contribution of the roadway to the ambient air loading was usually less than 10-20 pg/m3. The analytical results from samples of dust from the roadway surface at each site were useful in determining the source of the aerosols. Introduction In recent years, there has been increasing concern about the health effects of suspended particulate material in the air. The health effects are dependent upon the size of 0013-936X/82/0916-0197$01.25/0
particles. Large particles (10 km or more in diameter) are essentially all removed in the nasal chamber where respiratory clearance mechanisms remove them within hours. Smaller particles are deposited at varying depths in the respiratory system and may require much longer periods for removal, Vehicular traffic is one source of suspended particles or aerosols. This material comes primarily from tire, brake, and roadway wear, exhaust train erosion, and the actual vehicle emissions. The particles directly emitted by vehicles become airborne immediately and then undergo a settling and dispersion process. Some of these particles will settle on the roadway surface along with most of the materials which blow from open trucks. This deposited material is then resuspended by passing vehicles until it is eventually carried from the roadway by wind or rain. The process by which particles are dispersed in the air is more complex than the gaseous dispersion process. For
@ 1982 American Chemical Society
Environ. Sci. Technoi., Vol. 16, No. 4, 1982
197
