Environ. Sci. Technol. 1983, 17, 139-145
South Coast Air Quality Management District: El Monte, CA; Technical Services Division Air Quality Report No. 86, 1979. (16) Wadley, M. W.; Schoenemann, K. H.; MacPhee, R. D. “Suspended Particulate Matter in the Atmosphere of the South Coast Air Basin and Southeast Desert Areas 1977“; South Coast Air Quality Management District: El Monte, CA; Technical Services Division Air Quality Report No. 87, 1979. (17) U.S.Environmental Protection Agency, ’Air Quality Data for 1968 from the National Air Survelliance Networks and Contribution State and Local Networks”; U.S. Environmental Protection Agency APTD-0978,1972 (similar reporta are available for other years). (18) Faoro, R. B.; McMullen, T. B. “National Trends in Trace Metals in Ambient Air 1965-1974”; U.S.Environmental Protection Agency, EPA-450/1-77-003, 1977. (19) Taback, H. J.; Brienza, A. R.; Macko, J.; Brunetz, N. “Fine Particle Emissions from Stationary and Miscellaneous Sources in the South Coast Air Basin”; KVB Inc.: Tustin, CA; Report KVB 5806-783 (profiles are contained in Appendix A). (20) Cass, G. R.; McRae, G. J. “Source-Receptor Reconciliation of South Coast Air Basin Particulate Air Quality Data”; Final report to the California Air Resources Board under Agreement A9-014-031, 1982; NTIS PB-82-250093. (21) Huntzicker, J. J.; Friedlander, S. K.; Davidson, C. I. Environ. Sei. Technol. 1975, 9, 448-457. (22) Radke, N. M.; Cherniack, I.; Witz, S.; MacPhee, R. D. “The Effect of Type of Air Sampler on Composition of Collected Particulates: A Comparison of Brushless and Brush-type Hi-Vol Air Samplers”;South Coast Air Quality Management District El Monte, CA; Technical Services Division Report, 1977. (23) White, W. H.; Roberts, P. T. Atmos. Environ. 1977, 11, 803-812. (24) Hidy, G. M.; et al. “Characterization of Aerosols in California (ACHEX)”;Science Center, Rockwell International,
1974. Prepared under California Air Resources Board Contract 358. (25) Pierson, W. R.; Brachaczek, W. W. “Particulate Matter Associated with Vehicles on the Road”; Society of Automotive Engineers Paper 760039, 1976. (26) Cam, G. R.; Boone, P. M.; Macias, E. S. In “Particulate Carbon: Atmospheric Life Cycle”; Wolff, G. T., Klimisch, R. L., Eds.; Plenum Press: New York, 1982; pp 207-243. (27) U.S.Bureau of Mines, “Motor Gasolines”;U.S.Department of the Interior, Mineral Industry Surveys, Petroleum Products Surveys 75 et seq., 1966-1980. Series continues semiannually. Later editions in this series are published by the U.S.Department of Energy. (28) Habibi, K. Environ. Sei. Technol. 1973, 7, 223-234. (29) Pierson, W. R. Ford Motor Company, personal communication, Aug 1977. (30) Laresgoiti, A.; Springer, G. S. Environ. Sei. Technol. 1977, 11, 285-292. (31) Muhlbaier, J. L.; Williams, R. L. In “Particulate Carbon: Atmospheric Life Cycle”;Wolff, G. T., Klimisch, R. L., Eds.; Plenum Press: New York, 1982. (32) Morton, M., Ed. “Rubber Technology”;Litton Educational Publishing: New York, 1973. (33) Pierson, W. R.; Brachaczek W. W. Rubber Chem. Technol. 1974,47,1275-1299. (34) Lynch, J. R. J. Air Pollut. Control Assoc. 1968,18,824-826. (35) nillard, C. R.; Goldberg, D. E. “Chemistry-Reactions, Structure and Properties”; Collier-MacMillian: London, 1971. (36) Ondov, J. M. Ph.D. Thesis, University of Maryland, 1974. As cited by Kowalczyk et al. (4). (37) Kneip, T. J.; Eisenbud, M.; Strehlow, C. D.; Freudenthal, P. C. J . Air Pollut. Control Assoc. 1970,20, 144-149.
Received for review June 7,1982. Revised manuscript received October 19,1982. Accepted November 8,1982. This work was supported by the California Air Resources Board Under Contract A9-014-31.
Time-Dependent Leaching of Coal Fly Ash by Chelating Agents Wesley R. Harris“ and David Sllberman Laboratory for Energy-Related Health Research, University of California, Davis, California 956 16
w The rates of leaching of several transition-metal ions from coal fly ash by pH 7.4 solutions of the chelating agents citric acid, EDTA, histidine, and glycine have been measured. These results are compared to leaching of the same fly ash by 0.5 M HC1,O.lO M pH 7.4 Tris buffer, 0.5 M “,OH, and canine serum. The general order of leaching ability is HCl>> EDTA citric acid > histidine > glycine Tris. Canine serum is more effective as a leaching agent than one would predict on the basis of its concentrations of citrate and histidine, so that other biological chelators, possibly cysteine, appear to be important leaching agents. For the trace elements Zn, Mn, Cr, Ni; and Cu, the initial leaching rates with 0.5 M HC1 range from 350 to 850 pug of metal per gram of ash per day (ppm/day). The rates drop by 1-2 orders of magnitude within 24 h and then level off at 1-10 ppm/day. The initial rates with EDTA and citric acid are also high, 100-400 ppm/day, but they fall off even more rapidly than the HCl leaching rates. The leaching of vanadium is exceptionally rapid, with initial rates of 1000-3000 ppm/day. In addition, EDTA and citric acid leach over 50% of the acidsoluble vanadium compared to only 10-35% of the other transition metals,
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N
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Introduction Because of dwindling supplies of other fossil fuels, the use of coal in this country is likely to increase dramatically 0013-936X/83/0917-0139$01.50/0
in the near future. Even with modern pollution abatement equipment, the general population in many areas will be exposed to higher levels of particulate emissions from coal combustion. Since coal fly ash contains significant concentrations of a wide variety of heavy metals, there are potential health problems associated with the inhalation of larger quantities of fly ash. Coal fly ash consists primarily of amorphous aluminosilicates, with lower levels of heavy metals and organic compounds (1-3). However, previous leaching (4) and spectroscopic studies ( 5 , 6 )have shown that many of the metals are concentrated on the surface of the aluminosilicate core, rather than evenly distributed throughout the particle. Many elements, including As, Cd, Mo, Sb, Se, W, and Zn, are present almost exclusively in this outer layer of heavy metals ( 4 ) . There are still other elements such as Co, Cr, Cu, V, and Pb for which more than 50% of the total particle concentration is present in this outer layer ( 4 ) . This enhanced surface concentration has two implications with respect to possible health effects. First, these metals are readily available for leaching without the necessity of dissolving the relatively inert aluminosilicate core of the particle. More importantly, this enhancement results in higher heavy-metal concentrations in the smaller particles, due in part to their larger surface-to-mass ratio. These smaller particles are more likely to pass through the
0 1983 American Chemical Society
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Environ. Sci. Technol., Vol. 17, No. 3, 1983 139
pollution control devices and to stay airborne longer than the larger particles. In addition, these smaller particles are carried into the deep lung for long-term deposition. Because of this laboratory's interest in the potential health effects of inhaled fly ash, we are primarily concerned with trace-metal leaching a t physiological pH. However, most previous leaching studies have involved either acidic (4,7-9)or uncontrolled pHs (9-11). Many transition-metal ions are essentially insoluble at neutral pH. This is especially true of Fe and Al, which are major components of coal fly ash (1-3). Accordingly, Theis and Wirth (8) showed that trace-metal leachability dropped sharply upon going from pH 3 to pH 6. Thus the only effective mechanism for removing significant amounts of the transition metals from the fly ash in vivo is through chelation by biological ligands. The alveolar fluid, which would be the initial solvent for inhaled fly ash, contains basically the same set of complexing agents found in serum (12).Therefore, we have conducted in vitro studies of the leaching of heavy metals from coal fly ash by solutions of the biological ligands glycine, histidine, cysteine, and citrate, all buffered at physiological pH. Comparative data were also collected by using solutions of EDTA, HC1, and ammonium hydroxide. The results show that chelation is not clearly as effective as acid leaching and that citrate is the most effective biological ligand for most of the transition-metal ions included in this study. Experimental Methods
The fly ash was leached with 10 mM solutions of sodium citrate, glycine, L-histidine, and EDTA, all in 0.05 M Tris (2-amino-2-(hydroxymethyl)-l,3-propanediol) buffer at pH 7.4. Ash was also leached with 0.5 M HCl, 0.5 M ",OH, pH 7.4 0.05 M Tris buffer, a simulated serum solution described in the text and in ref 13,and pooled canine blood serum collected from dogs in this laboratory's beagle colony. The ash was a fine cut (volume median diameter = 2.3 pm) of ESP hopper ash from a western conventional coal-fired power plant. Samples of 1.0 g of fly ash were suspended in 100 mL of the appropriate solvent and mechanically shaken for 1 h. The suspension was then vacuum filtered through 47mm 0.1-pm Nuclepore filters. The solutions were stored in polyethylene bottles, and the ash was immediately resuspended in 100 mL of fresh solvent. This filtration step was then repeated after 1, 2, 3, 5, and 7 days. When NHIOH was used to leach the ash, acetone-wetted 0.2-pm Fluoropore filters (Millipore Corp.) were used, since strongly basic solutions destory Nuclepore filters. After the 7-day leaching period, all the samples were analyzed for Fe, Al, Ca, K, Zn, V, Cu, Ni, and Cr by flame atomic absorption spectroscopy with a Perkin-Elmer Model 306 spectrometer. Analog data from a strip-chart recorder was transformed by using a quadratic standarization function as described by Limbeck et al. (14). The reported concentrations are the average of three replicate samples for each leaching solvent for each time period. Results
HCI Leaching. HC1 is commonly used in leaching studies, since at acid concentrations greater than -0.01 M, the protonation and dissolution of most metal oxides as the corresponding aquo ions are thermodynamically favored. In previous studies, it has been assumed that 0.5 M HC1 would strip metal ions from the surface of the fly ash particle within 24 h without dissolving significant amounts of the aluminosilicate core ( 4 ) . 140
Environ. Sci. Technol., Vol. 17, No. 3, I983
5.5
,
I
I
1
I
I
I
-I
1.54 \-\'..
0
1
1
2
3
4
5
6
7
DAYS
Flgure 1. Rates of leaching of trace elements from coal fly ash by 0.5 M HCI.
Coal fly ash was leached for 7 days in 0.5 M HC1. The solution was changed at 1 h, and 1, 2, 3, 5,and 7 days. Figure 1 shows the leaching rates, expressed as ppm/day (pg metal/(g of ash per day)) for seven metal ions. The leaching rates are characterized by an initial burst in the dissolution of all elements. For the major components such as Al, Fe, and Ca, this initial rate is 80000-180000 ppm/day for the initial hour of leaching. These rates drop very sharply but at 24 h still range from 1000 to 10000 ppm/day. The leaching rates for the core elements remain high over the entire 7-day period, indicating the continuous dissolution of the aluminosilicate core and the iron oxide phases. Even after 7 days, the rates are still quite high (Ca, K 140 ppm/day; Fe = 800 ppm/day; A1 = 2400 PPm/daY)* The trace components (V, Zn, Mn, Cu, Ni, and Cr) are characterized by initial leaching rates ranging from 400 to 3100 ppm/day, some 2-3 orders of magnitude slower than the leaching of the major components. Within 24 h, the leaching rates drop to 3-30 ppm/day. The leaching of Zn, Mn, and Cu continues in the 3-10 ppm/day range for the entire 7 days. The leaching rates for Cr, V, and Ni drop off more sharply as shown in Figure 2, falling below 1 ppm/day at the end of the 7-day period. The chromium leaching is essentially complete within the first 24 h. Table I lists the bulk analysis of this fly ash (15),along with the amounts of metal leached by HC1 in 24 h and the total for 7 days of leaching. The last column indicates the fraction of each element that dissolved in the HC1 in 7 days on the basis of the bulk analyses of Fisher et al. on this sample of fly ash (15). The 7-day values typically range from 40% to 60% of the bulk total. The values of A1 and K are somewhat lower, which is consistent with previous reports that these elements are largely associated with the core material (3-5). Hansen and Fisher have reported that 84-98% of the potassium in coal fly ash is associated with the aluminosilicate core of the particles (4). Our data show that 12% of the potassium is leached from the particle within 24 h. This is easily within the reported range of nonmatrix po-
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49 1
4.0
Table 11. Leaching (24 h ) of Metal Ions from Fly Ash b y Selected Chelating Agents" histiele- Tris ment only EDTA citrate dine glycine NH, 0 3.7 1194 71.3 1348 Fe 0 29.0 81.7 20.0 13.3 97.7 v 19.1 5.3 2.7 8.7 7.3 12.3 cu 1.1 7.0 0 19.0 11.3 20.0 Mn 5.1 1.0 0 12.7 12.0 14.3 Zn 1.6 0.33 3.7 18.7 3.0 20.0 Cr 0.4 4.0 3.7 6.3 3.7 5.3 Ni 2.8 a
I
0
'
I
I
'
I
'
2
1
'
3
1
4
-,
All values reported in pg leached/g of ash.
1
5
DAYS
Flgure 2. Rates of leaching of V, Ni, and Cr from coal fly ash by 0.5 M HCI.
Table I. Leaching of Metal Ions from Coal Fly Ash b y 0.5 M HCl 24 h / 7 24-h 7-day 7 day/ eleleaching, leaching, day, bulk, ment PPm ppm % % 58 36 26800 46700 A1 129000 16000 61 41 39000 9750 Fe 11400 89 64 Ca 17800 10200 K 9500 2300 49 24 1100 750 59 45 Zn 89 330 Mn 76 110 70 46 24 0 V 260 170 94 66 160 65 52 34 52 cu 100 Ni 65 34 73 53 25 40 90 49 Cr 82 37 a Values from ref 15. bulk" analysis, ppm
tassium. For the entire set of elements, there is a loose correlation between the fraction leached by HC1 in 7 days and the fraction of nonmatrix material reported by Hansen and Fisher. Subsequent leaching of 2-4% of the potassium per day is taken as an index of aluminosilicate dissolution. Thus it appears that a 24-h HC1 leaching will not dissolve a significant portion of the aluminosilicate core. The data in Figures 1and 2 indicate the leaching of most surface-bound trace elements is not completed within 24 h. The rates for the leaching of Zn, Cu, Mn, and Ni do not level off until after 2.5-3 days of leaching. Table I lists the percent of the 7-day leaching total that occurs in the initial 24 h. For these four elements, about 60-70% leaches in the initial 24 h. Significant quantities of material, in excess of what might arise from dissolution of the aluminosilicate core, continue to leach off the particles for several days. This is particularly true of Zn and Mn, so that a 24-h HC1 leaching would probably underestimate the surface concentration of these elements by 20-30%. The elements V and Cr are exceptional in that almost 70% of the total HC1-leachable metal is removed from the particle within 1h, and 90% is removed within the first 24 h. This implies the existence of soluble salts, such as Cr042-,rather than insoluble metal oxides such as Cr203. However, the chromium solubility drops tremendously at
DAYS
Flgure 3. Rates of leaching of trace elements from coal fly ash by 0.05 M EDTA at pH 7.4.
neutral pH, so the actual speciation is still not clear. Ligand Leaching (24 h). The amount of material that dissolves in the presence of any chelating agent can be partitioned into two components. Some species such as salts of Mn04-, CrOt-, and vo43are soluble at pH 7.4 in the absence of any chelating agents. Therefore, fly ash samples were leached for 24 h in 0.05 M Tris buffer at pH 7.4, as well as in 10 mM ligand solutions in 0.05 M Tris buffer at the same pH. The results are shown in Table 11. Vanadium is by far the most soluble transition element in the absence of chelating agents. Very little of the other metal ions dissolve in this solvent, and as expected, the iron is quantitatively insoluble at neutral pH. Citrate and EDTA are clearly the most effective chelating agents, especially for solubilizing large amounts of ferric ion. Histidine can solubilize small amounts of iron, copper, manganese, and zinc in 24 h, but glycine appears to be ineffective for all metals except copper. Ligand Leaching Rates. EDTA was selected as the prototypical strong chelating agent, and the leaching rates for 10 mM EDTA are shown in Figure 3. As in the HC1 study, there is an initial burst in dissolution, which ranges from about 50000 ppm/day for Al, Fe, and Ca to about 300 ppm/day for Cr and Zn. The rates for the major components (Al, Fe, and Ca) level off at 80-200 ppm/day, compared to rates of 140-2400 ppm/day with HCl leaching. However, the most dramatic change in leaching rate is for potassium, which drops from a final value of 140 ppm/day with HC1 to only 6 ppm/day with EDTA. Envlron. Scl. Technol., Vol. 17, No. 3, 1983
141
Table 111. Leaching Rates of Trace Elements by 10 mM EDTA (ppm/day) element
V Mn cu Ni
1h
1 day
2 days
1900 430 250 150
57 2 2
6 0.7 1
3 days
5 days
1
1
Table IV. Leaching Rates of Trace Elements by 10 mM Citrate (ppm/day) element
1h
1 day
Zn Mn Cr Cu Ni
216 360 280 140 140
4 4 7 2 1
2 3 days days 5 days 7 days 2 1 1 1
1 1 1 1
1 0.5 0.5 0.5
1 1 0.5 0.5
Table V. Leaching (24 h ) of Coal Fly Ash by Selected Chelating Agents 4.0
4\
0.5 M HCl, ppm A1 Cr cu Fe Mn Ni V Zn
13420 37 34 9753 76 25 164 89
% of HC1-soluble material
ci- histi- glyTris EDTA trate dine cine ",OH
1 3 0 7 11 12 2
16.9 54 36 14 26 21 60 16
12.3 50 25 12 25 27 50 14
0 8
22 1 15 15 12 13
0 0 16 0 9 16 8
2
18 10 8
0.01 0 15 18 0
0.51
0
1
2
3
4
5
6
7
DAYS
Flgure 4. Rates of leaching of metal ions from coal fly ash by 0.05 M citrate at pH 7.4.
Clearly the EDTA is relatively ineffective at solubilizing the aluminosilicate core material which contains the potassium. The continued leaching of -100 and 200 ppm/day of iron and aluminum suggest rather large surface deposits of the oxides of these two metals, which may act as a sorbent for other trace metals (8). Leaching data for additional elements are listed in Table 111. Vanadium leaches very rapidly in the EDTA solvent, with an initial rate of about 2000 ppm/day. However, the rates for vanadium and the other transition-metal ions drop to less than 3 ppm/day after about 1 day. The leaching rates of Mn, Cu, and Ni all drop to less than 1 ppm/day within 24 h. The leaching rates of selected metals in 10 mM citric acid are shown in Figure 4. Given the huge difference in binding affinity between citric acid and EDTA, the similarities in their leaching rates is striking. There is a further reduction in the rate of potassium leaching to less than 1 ppm/day, so that citrate is almost totally ineffective at dissolving the aluminosilicate core. However, the leaching rates for Fe and A1 are almost identical with the EDTA values, providing further evidence for surface oxide deposits of these metals. The leaching rates for the other transition metals fall off very rapidly in the first 24 h. The V rates, shown in Figure 3, start at over 1000 ppm/day. The rates for Cr, Ni, Cu, and Mn, listed in Table IV, start at 150-450 ppm/day and drop to about 1 ppm/day after 36 h. Only the vanadium leaching lags significantly behind the rate observed in EDTA solution. Histidine is a much poorer leaching agent than EDTA and citrate. Leaching rates for several elements are shown 142
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DAYS
Figure 5. Rates of leaching of metal ions from coal fly ash by 0.05 M histidlne at pH 7.4.
in Figure 5. Substantial amounts of Ca and Fe leach from the particles, but the other elements have rather low initial rates which fall rather rapidly. The leaching of nickel and copper is essentially complete within 24 h. In addition, the histidine leaches no detectable amounts of aluminum from the particles. Glycine is a very poor leaching agent, and for most of the transition metals, the leaching occurs only within the initial 24-48 h. Iron was anomalous, in that the initial leaching rate was essentially zero, but leaching began after about 3 days and accelerated to about 10 ppm/day at the end of the experiment. Leaching Effectiveness. Since variable fractions of each element are buried in the relatively inert aluminosilicate core, the effectiveness of any chelating agent is best judged by comparing it to HC1. We will refer to material leached by HC1 within 24 h as the surface component of the fly ash, as opposed to the acid-insoluble aluminosilicate
Table VI. Total Concentration of Metal Ions Leached in 7 Days from Fly Ash by Selected Chelating Agents bulka analysis Fe 39 000 A1 129 000 Mn 237 V 262 cu 100 Ni 65 Zn 332 Cr 82 Ca 17 800 K 9 540 a Values from ref lant.
HCl EDTA citrate histidine ssc glycine "3 13 ( 4 ) 35 ( 3 ) 16 010 (300)& 2 0 3 0 (26) 1 8 6 0 (200) 340 ( 3 ) 82 ( 3 ) 5 240 (30) 0 46 700 (700) 4 050 ( 3 5 ) 3 100 (600) 0 370 (20) 12 ( 1 ) 0 14 (1) 108 (2) 23 (1) 15 (1) 22 (1) 108 ( 4 ) 174(2) 114 ( 1 ) 44 ( 5 ) 31 ( 6 ) 31 ( 3 ) 16 ( 2 ) 7 (1) 6 (1) 9 (2) 7 (3) 52 ( 2 ) 15 ( 1 ) 12 ( 4 ) 5 (1) 12 ( 2 ) 4 (1) 4 (1) 34 ( 2 ) 6 (2) 7 (3) 8 (3) 4 (2) 154 ( 4 ) 20 (1) 8 (3) 19 ( 3 ) 1 8 (3) 1(1) 7 (1) 6 (1) 40 ( 2 ) 5 (1) 28 (2) 22 ( 2 ) 1 1 400 (350) 4 6 3 0 (70) 5 280 (330) 3 230 (250) 2 960 (220) 3 180 (110) 1 5 1 0 (70) 2 290 (40) 175 (1) 87 (40) 69 ( 2 2 ) 7 7 (16) 114 ( 6 ) 277 ( 5 ) Synthetic serum simu15. Numbers in parentheses are the standard deviation based on three runs.
Table VII. Composition of Serum and Synthetic Serum Simulant serum chemical concn, mM synthetic soln, mM Na K Ca Mg "3
protein
sot-
c1-
HC0,-
Po:-
citrate organic acid
142 5 2.5 1.5
145 0.2
10 6.6 0.5 103 27 1.2 0.2 6
0.5 126 27 1.2 0.2 5 (glycine) 1 (cysteine)
core. Table V lists the fraction of surface material removed by each of the chelating agents. With the exception of Cr and V, even EDTA and citrate dissolve only 10-36% of the surface material. Histidine dissolves 15% of most of the elements, but it is very poor at solubilizing iron and aluminum. Glycine is a very poor leaching agent. It appears to dissolve small amounts of Ni and Cu and is totally ineffective for Al, Fe, V, Mn, Zn, and Cr. We initially included cysteine as another biological chelating agent. However, during the course of the experiments, a white precipitate (presumably cystine) formed in the cysteine leachates. Since this precipitate has not been characterized, no analyses for cysteine leaching are reported. The total amounts of each element leached over the 7-day experiment are listed in Table VI. Obviously the largest concentrations correspond to the major components such as Fe, Al, Ca, and K. Of the minor components, vanadium is by far the most soluble. Both EDTA and citrate leach over 100 ppm of vanadium. Leaching of the other metal ions is much less effective, ranging up to only 20-30 ppm for the leaching of Mn, Zn, and Cr by EDTA and citrate. Serum and Serum Simulant. Although the composition of the alveolar fluid is not well established (12),we have prepared a solution based on the composition of serum that should reasonably approximate the lung fluid responsible for in vivo leaching (13). A comparison between this synthetic solution and serum is shown in Table VII. The major difference between these solutions is the absence of proteins from the synthetic solution. The most important chelating agents are obviously citrate, amino acids, and phosphate. Leaching data for the synthetic serum simulant is shown in Table VI. Because of the low concentrations of chelating
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Table VIII. Metals Leached by Serum and Serum Simulant (ppm) after 72 h element serum simulant dog serum A1 240 1124 Fe 62 515 Ca 2585 K 76 V 29 61 Zn 8 236 Mn 11 10 Ni 12 6 cu 8 43 Cr 5 12
agents used in this solution, it is likely that the complexing capacity of the leaching solution may be saturated by components such as Al, Ca, and Fe. Therefore, the interpretation of these data must remain cautious. Compared to the individual chelating agents, this material is reasonably effective for solubilizing Ca, Cu, V, Zn, and especially Ni. The leaching of Fe and A1 is low, which is expected since the citrate concentration is only 2 X M. The percentages of dissolved chromium and vanadium are also low, but in absolute terms, more vanadium is leached from the ash than any other transition metal except Fe. The relatively high effectiveness for Cu, Ni, and Zn, compared to single chelating agents, may be due to the cysteine. These metal ions all coordinate very strongly to thiol groups, so that cysteine is a very good complexing agent. In the case of copper, reduction of Cu2+to Cu+ may also be a factor, since thiol coordination strongly favors the cuprous state. A sample of fly ash was also leached for 72 h in normal pooled canine serum. The results are compared with the data on the synthetic serum simulant in Table VIII. There are some very important differences in the results from use of these two solvents. The actual serum leaches over 200 ppm of Zn, as opposed to only 8 ppm for the synthetic solution. The serum also extracts much larger concentrations of vanadium, copper, and iron. Thus the use of the synthetic fluid appears to drastically underestimate the amount of material that would likely be removed from the fly ash particles in vivo.
Discussion These experiments were designed to avoid thermodynamic equilibrium between the solid and solution phases. Relatively large excesses of chelating agents were used, and the solutions were replaced at regular intervals. The EDTA was included, even though it is obviously nonbiological, to give an estimate of the total amount of material Environ. Sci. Technol., Vol. 17, No. 3, 1983
143
available for chelation. In all cases, this value is far short of the amount of material accessible to HC1, even though the relatively low potassium values indicate that little of the “core” material is dissolved by the HC1. In some cases, the concentration of metal ion dissolved by citric acid exceeds that solubilized by EDTA, even though the formation constants for EDTA are several orders of magnitude greater than those for the citrate complexes. Thus the results do appear to reflect kinetic parameters rather than thermodynamics. This is further supported by the fact that citrate leaches more Ni and Ca than does histidine, even though histidine forms the more stable complexes with these metals. Very little potassium is leached by the chelating agents, indicating that these ligands are not able to dissolve the aluminosilicate core material of the coal fly ash particles. Even though both EDTA and citric acid continue to leach substantial amounts of iron and aluminum over the entire 7-day period, the total chelated iron and aluminum is only 1520% of the amount removed by the 24-h HC1 leaching. Thus, even after 7 days, the chelating agents are still leaching only surface material. The leaching rates with either HC1 or chelating agents are characterized by very rapid initial leaching, followed by a slower long-term leach rate. This initial burst in the leach rate is presumably a reflection of the enrichment of the surface in the various metals. However, the speciation of these metals will also have a strong effect on leaching rates. Metal ions present as relatively soluble salts such as the sulfates, halides, and phosphates would leach more quickly than the corresponding oxides. Thus the drop in leaching rates should also reflect the depletion of the surface concentration of these soluble compounds. Work by Theis and Wirth (8) and by Hansen et al. (16) has shown that transition metals are selectively concentrated in the oxides of iron, aluminum, and manganese. In this case manganese is probably not important, simply because of its relatively low concentration (Fe, 39OOO ppm; Al, 129000 ppm; Mn, 237 pprn). However, there is clearly enough iron and aluminum to serve as adsorbents for the other transition elements. Therefore, it is interesting to compare the data for 24-h leaching with EDTA and ammonium hydroxide. Both EDTA and NH40H solubilize -2300 ppm of Al, but EDTA also solubilizes 1350 ppm of Fe, while the ammonia dissolves essentially no iron. Thus the difference between the leaching efficiency of EDTA and NH40H should reflect in part the amounts of material associated with the iron and aluminum oxides. Since the dissolution of 2300 ppm of aluminum by ammonia leads to undetectable amounts of manganese and zinc, it appears that very little of these two metal ions is associated with the aluminum oxides. The ammonium hydroxide also leaches much less V, Cu, and Cr compared to EDTA, which suggests that these elements may also be more concentrated in the iron oxides than in the aluminum oxides. Such leaching studies do not differentiate between crystalline forms of iron oxide, so that these results refer to total iron, both crystalline and amorphous. These results are consistent with the qualitative results of Theis and Wirth (8) for Cr, Cu, and Zn. These studies show that citrate is a very effective biological chelating agent. Since the absolute magnitudes of the leaching rates, in ppmfday, depend on the concentration of ligand and the volume of leaching solvent per gram of fly ash, the observed rates are unlikely to reflect the actual rates of in vivo dissolution. However, they do indicate certain trends. First, the leaching rates start very high and fall off very rapidly. Thus, toxicological studies 144
Environ. Sci. Technol., Vol. 17, No. 3, 1983
Table IX. Fraction of Total Metal Content Leached by Citrate this study, 1 h, % ref 9, % V 21 16 13 Cr 14 cu 6.3 4.6 Zn 2.7 1.8
should consider that cells adjacent to recently inhaled particles may be subject to high trace-metal concentrations for brief periods of time. The actual dose will depend both on the rate of leaching and the rate of clearance from the lung into the blood stream. Second, chelation in all cases is a much less efficient process than HC1 leaching. Thus HC1 leaches more total metal, and at much higher initial rates, than any of the chelating agents. Dreesen et al. have also studied the leaching of coal fly ash by citrate, but under a much different set of conditions: 0.1 M citrate, 1:4 ashxolvent, final pH 3.6, 3-h leaching time (9). Despite these differences in experimental conditions, there is a very close agreement between their values for 3 h of leaching and our values for 1h of leaching as shown in Table IX. EDTA has also been used in previous fly ash leaching studies (In,which described the leaching of Cr from coal fly ash over a 15-day period. However, because these authors were concerned with reactions in landfill and holding ponds, they used very large amounts of ash (- 1g/5 mL), relatively low concentrations of EDTA M), and they allowed the pH to vary. Thus it is difficult to compare their results with the data reported here. They did observe the same pattern of very high initial leaching rates, which then sharply decreased over the initial 10-20 h for one type of fly ash. However, for a second fly ash sample from low-sulfur coal, the leaching gradually increased over the first 60 h, then declined very slightly to a fairly constant value. The leaching studies reported here indicate that vanadium is the most soluble trace element of those studied. Every ligand that we considered removed more vanadium than any other trace metal, and this was not merely a reflection of an exceptionally high concentration of vanadium in the particle. Zinc and manganese had equal or greater total concentrations. Nor is it a reflection of exceptional surface enrichment, since the chelating agents remove a much higher fraction of the acid-soluble vanadium component. Following vanadium, chromium displayed the highest solubility, although the low bulk analysis of Cr results in smaller absolute amounts of chromium leaching from the particle. Copper, manganese, and nickel show moderate solubility, while zinc has the lowest solubility in chelating solvents. Although this a study could not be used to predict the actual in vivo rate of dissolution, it does predict a trend in relative leach rates. However, the expected trend in solubilities is not found in the studies using canine serum as the solvent. The serum is particularly effective at leaching zinc and copper. The trends in the leaching by the synthetic serum simulant agree reasonably well with those for citrate and EDTA, so that the dilution factor between serum and the more concentrated ligand solutions is probably not responsible for the unique results from the canine serum study. Rather it appears that other components in serum control the leach rate for copper and zinc. Since these metals are transported in serum in relatively large concentration as complexes with albumin, it may be that albumin and other serum proteins are enhancing the leaching effectiveness of serum.
Environ. Scl. Technol. 1983, 17, 145-149
(9) Dressen, D. R.; Gladney, E. S.; Owens, J. W.; Perkins, B.
Registry No. ",OH, 1336-21-6; HC1, 7647-01-0; EDTA, 60-00-4;TI%,77-86-1; Zn, 7440-66-6; Mn, 7439-96-5; Cr, 7440-47-3; Ni, 7440-02-0; Cu, 7440-50-8; V, 7440-62-2; Al, 7429-90-5; Fe,
L.; Wienke, C. L.; Wanger, L. E. Environ. Sci. Technol.
1977,11, 1017-1019. (10) Dudas, M. J. Environ. Sci. Technol. 1981, 15, 840-843. (11) Shannon, D. G.; Fine, L. 0. Environ. Sci. Technol. 1974, 8, 1026-1028. (12) Kanapilly, G. M. Health Phys. 1977, 32, 89-100. (13) Kanapilly, G. M.; Raabe, 0. G.; Goh, C. H. T.; Chimenti, R. A. Health Phys. 1973,24, 497-507. (14) Limbek, B. E.; Rowe, C. J.; Wilkinson, J.; Routh, M. W. Am. Lab. 1978,10,89-100. (15) Fisher, G. L.; Raabe, 0. G.; Prentice, B. A,; Silberman, D.,
7439-89-6; Ca, 7440-70-2; K, 7440-09-7; citric acid, 77-92-9; histidine, 71-00-1; glycine, 56-40-6.
Literature Cited Coles, D. G.; Ragaini, R. C.; Ondov, J. M.; Fisher, G. L.; Silberman, D.; Prentice,B. A. Environ. Sci. Technol. 1979, 13,455-459.
Davison, R. L.; Natusch, D. F. S.; Wallace, J. R.; Evans, C. A. Environ. Sci. Technol. 1974,8, 1107-1113. Campbell, J. A,; Laul, J. C.; Nielson, K. K.; Smith, R. D.
Annual Report, Laboratory for Energy Related Health Research, UCD 472-125, 1978; pp 26-32. (16) Hansen, L. D.; Silberman,D.; Fisher, G. L. Environ. Sci.
Anal. Chem. 1978,50, 1032-1040. Hansen, L. D.; Fisher, G. L. Environ. Sci. Technol. 1980, 14,1111-1117.
Technol. 1981, 15, 1057-1062. (17) Eggett, J. M.; Thorpe,T. M. J . Environ. Sci. Health 1978, A13, 295-313.
Linton, R. W.; Loh, A,; Natusch, D. F. S.; Evans, C. A.; Williams, P. Science (Washington,D.C.) 1976,191,852-854. Campbell, J. A.; Smith, R. D.; Davis, L. E. Appl. Spectrosc. 1978,32, 316-319.
James, W. D.; Janghorbani, M.; Baxter, T. Anal. Chem. 1977,49, 1994-1997.
Theis, T. L.; Wirth, J. L. Environ. Sci. Technol. 1977,11, 1096-1110.
Received for review June 23,1982. Accepted November 15,1982. This work was supported by the U.S. Department of Energy under Contract DE-AM03-76SF00472.
Solubility of Ozone in Aqueous Solutions of 0-0.6 M Ionic Strength at 5-30 OC Lynn F. Kosak-Channlng and Oeorge R. Helz"
Department of Chemistry, University of Maryland, College Park, Maryland 20742
rn The equilibrium constant of the reaction 03(1) + 03(g)
has been measured between 5 and 30 "C by analyzing equilibrated liquid and gas phases and has also been bracketed on both the high and low sides by interpreting the dynamics of transfer of ozone between the liquid and gas phases. Mildly acidic solutions were used in order to minimize ozone decay. The effect of ionic strength was determined by adding sodium sulfate to the solution. The following equation predicts all the measured values of Kh with a root mean square deviation in KhoM/Kh*d of 2.3% (2' = temperature in Kelvin; I.L = molar ionic strength): ln Kh = -2297T1 + 2 . 6 5 9 ~- 688.OpT' 12.19. Experience gained during this work with dynamic equilibrium methods of measuring Kh suggests that proof of equilibrium may be more difficult to attain than previously suggested. Particularly a t the upper end of the temperature range studied, the ionic strength effect on the value of Kh is large enough to be significant in natural masstransfer processes such as absorption of tropospheric ozone into seawater or into aqueous aerosols.
+
Introduction
Knowledge of the solubility of ozone in water and salt solutions is needed to design efficient ozonation systems for water treatment (I) and to predict rates of ozone removal from the troposphere into the oceans (2) or into aqueous aerosols (3). Published measurements of the Henry's law constant for ozone in pure water disagree by as much as a factor of 2 (4-12).Many of the older values were obtained in neutral or alkaline solutions and may be questionable because ozone decomposes at appreciable rates under these conditions (9). Nevertheless, the nineteenth century measurements of Mailfert (lo),which are systematically lower than most subsequent ones, have found their way into standard reference books (11)and therefore continue to be widely used. A particular defi0013-936X/83/0917-0145$01.50/0
ciency of the data in the literature is their inability to predict the effect of dissolved salts on ozone solubility. This effect can be important in the case of ozone dissolution into the ocean or into sulfate-laden aerosols. In this paper, we report values of ozone's Henry's law constant, measured in a continuously flowing bubble column after the liquid and gas phases had come to equilibrium. Dissolved ozone was stabilized by conducting the measurements in mildly acidic solutions (pH 3.4 f 0.1). We also attempted to confirm these data by an independent, dynamic equilibrium method similar to that described by Mackay et al. (13). However, because mass transfer was too slow to maintain equilibrium when ozone was being added to or removed from the column, the dynamic equilibrium measurements serve only to bracket the true value of the Henry's law constant. Experimental Section
A diagram of the apparatus is shown in Figure 1. Ozone gas from a Welsbach ozone generator (Model T-408) in a stream of dried O2 (A) was bubbled through a fine, cylindrical frit at a typical flow rate of 60 mL/min into a thermostated column containing 500 mL of an aqueous solution. The solution circulated at 23 mL/min by a peristaltic pump (B) through a flow cell for measuring ozone's optical absorbance at 260 nm (S). The ozone gas was introduced until a steady-state O3 concentration in the liquid was observed on the recorder (C). The exit gas above the solution was partly exhausted (D) and partly recirculated at approximately 250 mL/min by a bellows pump (E). Recirculating the exit gas was intended to facilitate the system's attaining equilibrium by increasing the contact of the gas and liquid. When a steady-state ozone concentration was observed in the liquid, the exhaust gas was diverted for 2 min to a trap (F) containing 500 mL of 2% KI. The flow of O3to
0 1983 American Chemical Society
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