Environ. Sci. Technol. 1993,27,310-315 Hoign6, J.; Zuo, Y.; Nowell, L. Preprints of Extended Abstracts, 203rd National Meeting of the American Chemical Society, Division of Environmental Chemistry, San Francisco, CA; American Chemical Society: Washington, DC, 1992; Paper 92, pp 147-149. Zepp, R. G.; Faust, B. C.; Hoign6, J. Environ. Sei. Technol. 1992, 26, 313-319. Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; John Wiley and Sons: New York, 1966; pp 737-786. Appleby, A. J.; Mayne, J. E. 0. J. Gas Chromotogr. 1967, 5, 266-268. Stabler, R. N.; Chesnick, J. Int. J . Chem. Kinet. 1978, I O , 461-469. Barb, W. G.; Baxendale, J. H.; George, P.; Hargrave, K. R. Trans. Faraday SOC. 1951, 47, 462-500. Walling, C.; Goosen, A. J. Am. Chem. Soc. 1973, 95, 2987-2991. Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L. J. Phys. Chem. Ref. Data 1985, 14, 1041-1100. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988,17, 513-886. Neta, P.; Hule, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data 1990, 19, 413-513. Metelitsa, D. I. Russ. Chem. Rev. (Engl. Transl.) 1971,40, 563-580.
Langford, C. H.; Carey, J. H. Can. J. Chem. 1975, 53, 2430-2435. Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic Press: London, 1970; Chapter 10, pp 145-192. Evans, M. G.; George, P.; Uri, N. Trans. Faraday SOC.1949, 44, 230-236. Behar, B.; Stein, G. Science 1966, 154, 1012. Walling, C. H.; Humphreys, W. R. J. Org. Chem. 1981,46, 1260-1263. Dufek, P.; Cernohorsky, I.; PacBkovB, V. J. Chromatogr. 1982,241, 19-28. Carey, J. H.; Cosgrove, E. G.; Oliver, B. G. Can. J. Chem. 1977, 55, 625-629. Kunai, A.; Hata, S.; Ito, S.; Sasaki, K. J. Am. Chem. Soc. 1986, 108, 6012-6016. Dorfman, L. M.; Taub, I. A.; Buhler, R. E. J. Chem. Phys. 1962,36, 3051. George, P. Trans. Faraday Soc. 1954,50,4349-4359.
Received for review May 28, 1992. Revised manuscript received October 5, 1992. Accepted October 19, 1992. Support was received from US.Department of Agriculture Pesticide Impact and Water Quality Special Grants Programs.
Reduction of Trace Element Concentrations in Alkaline Waste Porewaters by Dedolomitkation Eric J. Reardon,” C. James Warren, and Monlque Y. Hobbs
Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Dolomite [CaMg(C03)2]addition to alkaline waste materials, such as high lime content fly ash, is proposed as a method to reduce the concentrations of undesirable elements in leachate waters. The results of this study indicate that dolomite, in the presence of portlandite [Ca(OH)2],undergoes conversion to an assemblage of brucite [Mg(OH),] and calcite (CaCOJ in unstirred pastes reacted for 8 months. Calcite coated the surfaces of reacted dolomite fragments while brucite occurred as individual acicular crystals. The generation of calcite can reduce porewater concentrations of certain trace elements through coprecipitation mechanisms. The results of experiments conducted in this study showed moderate uptake of B and Se by precipitating calcite. Arsenic was included in this evaluation, but the high pH of the reaction solution resulted in its immediate precipitation as basic calcium arsenate [Ca4(As04)2(OH)z.4Hz0]. The potential of dedolomitization to reduce the concentrations of other undesirable elements in alkaline waste porewaters needs to be investigated. Dolomite is readily available in many areas of North America and can be easily processed to yield a fineness that is reactive when mixed with alkaline waste material. 1. Introduction
Fly ash, a byproduct of coal-fired electrical generating stations, poses a major disposal problem. With the placement of more stringent limits by many countries on the trace element content and leachability of materials used as lakefill, landfill, and construction material, it appears that ever-increasing volumes of fly ash will be relegated to monitored disposal sites. Contamination of local groundwaters by fly ash leachates at these sites is of environmental concern. Although ash leaching characteristics 310
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vary widely, the elements most often observed to be above drinking water standards include B, As, Se, Mo, Cd, Cr, Zn, and S as SO4 (1). The precipitation of calcite (CaC03)may be a means to reduce trace element concentrations in fly ash leachates. Most natural groundwaters are at, or near, calcite saturation. If calcite could be induced to precipitate in fly ash waste and entrain undesirable trace elements through the process of coprecipitation, then these elements would be relatively secure from rerelease because of the unaggressiveness of background groundwaters to dissolve calcite. Calcite precipitation in fly ash can occur, or be induced to occur, by various mechanisms. Calcite may precipitate through reaction with natural sources of carbon dioxide due to the intrinsic carbonation capacity of fly ash. If the carbonation capacity of the ash is very low, it could be artificially increased through the addition of lime or cement. Some flue gas desulfurization technology involves the addition of slurried limestone into the stack, which would generate high lime contents in the ash, and thus high carbonation capacities (2). If trace element concentrations in ash leachates can be reduced by coprecipitation with calcite, a practical method should be developed to induce carbonate precipitation at ash disposal sites. Below the water table, influx of background groundwater would contribute carbonate. However, influx of groundwater into fly ash would simply displace the existing trace element-enriched porewater into the surrounding aquifer. Carbonation by background groundwater could encapsulate elements, such as Se, which are not heavily partitioned into the aqueous phase to begin with, but readily soluble elements such as B would escape encapsulation. Above the water table, fly ash carbonation would occur as a result of diffusion of atmospheric or soil
0013-936X/93/0927-0310$04.00/0
0 1993 American Chemical Society
carbon dioxide into the material. However, carbon dioxide concentrations in soil air are usually low (9) typical of many fly ash leachates. Consequently, there is the potential for these oxyanions to proxy for CO?during the precipitation of calcite, thereby reducing their solution concentrations in ash leachates if calcite precipitation can be induced. Other trace elements, such as Mo and Cr, which are also oxyanions at high pH values, might similarly proxy for C03,- in the calcite crystal lattice. Dedolomitization thus can serve as a mechansim to remove these elements from the pore solution as well. The concentrations of undesirable cationic elements, such as Cd or Zn, may be reduced by substitution for Ca2+in calcite. Of course, any dolomitic material considered for this purpose must be analyzed to ensure that dedolomitization does not contribute a greater concentration of a specific trace element to the porewater than it removes. 2. Experimental Section 2.1. Evaluating the Coprecipitation of B, As, and Se with Calcite. A stock solution was prepared containing 50 mg/L B, 12 mg/L As, and 15 mg/L §e. Reagent-grade sodium salts of borate, arsenate, and selenate (J. T. Baker) were used in this preparation. Twenty grams of Ca(OH), and 500 mL of the stock solution were placed in each of two 500-mL flasks. Magnetic stirring bars were added and the flasks stoppered with rubber bungs. The stirring bars were used to constantly stir the solutions during the 8 days of reaction. Both vessels were kept in a constant-temperature bath at 25 f 0.1 "C. During each day of the experiment, a portion of the Ca(OH), in one of the flasks was reacted with CO,, ) to form calcite. This was accomplished by bubbling into the vessel until the total mass of the vessel had increased by 1.5 g. The bubbling procedure usually took 2-3 h. The 1.5-g increase in mass represented -1/8 of the carbonation capacity of the total 20 g of Ca(OH),, and conversion was complete within the 8 days of the experiment. Carbon dioxide was not added to the other reaction vessel, which served as a control during the experiment. Each day, before the addition of CO,, a 10-mL sample of solution was withdrawn from each reaction vessel, filtered through 0.2-rm cellulose acetate filters, and acidified with 100 pL of 1:l HC1. All samples were then refrigerated. The experiment was terminated after 8 days when no mass increase was observed in the reactant vessel after 18 h of Environ. Sci. Technol., Vol. 27, No. 2, 1993 311
continuous bubbling with CO,,,,. The concentration of Se in the extracted solutions was determined by flame atomic absorption spectrometry (AAS). Arsenic occurred at very low concentrations in most samples and was determined as a hydride using AAS (9). Boron was analyzed by the carmine method (10) using a Spectronics 20 spectrophotometer. Carmine red, dissolved in concentrated sulfuric acid, was used as the complexing agent for boron. 2.2. Inducement of Carbonation through Dedolomitization Reactions. Three specimens of dolomite were used for this study. Two of the samples were Middle Silurian in age from the Eramosa Formation. These samples were collected in midwestern Ontario, one from the Bruce Peninsula (ONl), and the other from Manitoulin Island (ON2). The third sample was from the St. George group in western Newfoundland (NF1) and is Lower Ordovician in age. The dolomite samples were broken into pieces, -5 cm in diameter, using a rock splitter. The pieces were then crushed with a jaw crusher in three successive runs. The width of the jaws was decreased with each run, until the rock fragments were -0.5 cm in diameter. The samples were then placed in a shatter box, where they were ground until all material passed through a No. 230 mesh sieve ( c
.-0
W c
c
.v) C
m
C a, C
-
C -
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j
20
O
Q
25
30
35
40
15
20
Degrees 20 (Cu Ka) NFI
F
25
II
IY 35
30
‘ 40
Degrees 20 (Cu Ka) ’ortlandite
C
P
P
.0
m C
W
e
C -
15
20
25
30
35
Jh..
I
,
40
15
Degrees 20 (Cu Ka)
20
25
-,
30
35
40
Degrees 20 (Cu Ka)
Flgurr 2. X-ray diffraction patterns for material recovered from the dedolomitization experiments. Results are presented for samples ONI, ON2, and NF1 including traces for each nontreateddolomite below traces for the leached portlandite/dolomite pastes reacted for 8 months. A control sample containing only portlandite is Included for comparison. Labeled peaks In the diagrams refer to diffraction peaks for the following minerals: D, dolomlte; C, calclte; P, portlandlte; Q , quartz, F, feldspar, and B, bruclte.
senate phase [Ca3(As04)2].This program calculated a log IAP of -22.8 in the test solutions, compared to the log KBp of -18.9, indicating that the solutions were decidedly undersaturated with respect to this phase. Put another way, the WATEQ4F-predicted solubility for As was over 200 pg/L as compared to the measured value of
