Environ. Sci. Technol. 1991, 25, 930-935
Adsorption Kinetics of Vapor-Phase m-Xylene on Coal Fly Ash Simon J. Rothenberg,+Glen Mettzler, Jeff Poliner, William E. Bechtold, Arthur F. Eidson," and George J. Newton
Inhalation Toxicology Research Institute, Lovelace Biomedical and Environmental Research Institute, P.O. Box 5890, Albuquerque, New Mexico 87185 We determined adsorption kinetics for a representative aromatic hydrocarbon, m-xylene, and three types of fly ash, two from commercial power plants and one from an experimental fluidized-bed combustor. Rate constants and adsorption half-times were compared to fly ash residence times in coal combustion power plant stacks. Kinetics of m-xylene adsorption by fly ash resembled kinetics reported for penetration of absorbates into porous adsorbents. No increase in adsorption rates was observed with increased temperature; rate constants decreased with increase in vapor pressure. This suggested that adsorption was diffusion limited. The residence times of fly ash in the stack were similar to the half-times required for monolayer formation, but were much shorter than the half-times characteristic of multilayer formation and pore filling. Introduction The association of polycyclic aromatic hydrocarbons (PAH) with fly ash is of environmental interest. Over lo8 metric tons of fly ash were emitted in the United States in 1980 (1). Some PAH vapors are known experimental animal carcinogens ( 2 , 3 )and possibly carcinogenic (3). Fly ash is an active adsorbent ( 4 ) , which adsorbs PAH (5). Adsorption of vapors by fly ash could result in the subsequent deposition of particles and their associated vapors in the lower respiratory tract. Studies of the metabolism of PAH associated with diesel soot and carbon black have shown that the toxic effects of particle-associated PAH are greater than those of PAH alone (6). The rather limited data on pulverized coal combustors (7) suggest that stack vapor-phase PAH concentrations are usually low (ppb). Most of the PAH eventually becomes particle associated in the cooled diluted plume downwind of the combustor (7). Hanson et al. (8) determined much higher concentrations (ppm) for an experimental fluidized-bed combustor. Less than 10% of the PAH became particle associated after dilution and cooling. Calculations based on nitrogen-specific surface areas demonstrated that the PAH associated with particles was less than a monolayer (8). Power plant engineering data (9) demonstrate that, in a typical power plant, the interval between fly ash particle formation in the furnace and exit of fly ash from the stack is quite short, typically less than 1 min. Several minutes may be required for adsorption equilibrium to be reached. Thus, adsorption kinetics are important in prediction of PAH loading on fly ash. Equilibrium adsorption isotherms may overestimate loading on fly ash. The direct determination of the amount of PAH adsorbed by fly ash in the stack (or plume) is desirable, but requires a major sampling effort (8, 10). Natusch and Tomkins (11) recognized the need to complement data obtained by stack and plume sampling with models of adsorption kinetics. They employed a simple model consistent with the available data, Langmuir adsorption kinetics with activated adsorption. Our experimental work on the adsorption of water by fly ash (12)demonstrated +Present address: Brookhaven National Laboratories, Medical Department, Building 490, Upton, NY 11973-5000. 930
Environ. Sci.
Technol., Vol. 25, No. 5, 1991
that the rate constants actually decreased with increases in water vapor pressure, whereas the Langmuir model predicts an increase in rate constants (13). The experimental data showed that the basic approach adopted by Natusch and Tomkins (11) was sound, but that it required modification to include small (or zero) activation energies and diffusion- or transport-limited adsorption. Kittleson and Barris (14) subsequently developed a model of adsorption by diesel exhaust soot that included diffusion and a wide range of values for the heat of adsorption. The limited data base available for fly ash suggests that less than a monolayer of PAH is usually adsorbed; this is in contrast to the situation for diesel exhaust soot, for which multilayer adsorption is normal (15). We wanted to determined whether the conclusions reached for water adsorption kinetics were applicable to other adsorbates whose modeling was of environmental interest. Because aromatic hydrocarbons are of environmental interest, we decided to determine adsorption kinetics for a representative aromatic hydrocarbon, m-xylene. We first satisfied ourselves that m-xylene adsorption was readily observed at the high temperatures found in power plant exhaust streams. We then carried out more precise measurements at temperatures close to ambient, to determine the kinetics of adsorption. The low-temperature data are directly relevant to modeling of plume adsorption kinetics. The rather small temperature dependence of diffusion-controlled adsorption (13) is the basis of extrapolation to higher temperatures. The limitations caused by thermomolecular flow forces ( 4 , 16, 17) prevented precise measurements at temperatures much above 100 "C. Our results showed that the adsorption kinetics for mxylene are similar to those previously reported for water adsorption (12)and for penetration of nitrogen into porous adsorbents (16). Also, we showed that times of flight of fly ash in power plant stacks are probably adequate to form PAH monolayers, but multilayer formation and pore filling are slow processes that are probably not significant in the stack. Materials and Methods The composition (Table I) of the samples used for this study (Table 11) had been previously determined (18)and the rates of water adsorption measured (12). According to the classification system of Roy and Griffin (19),the fly ash obtained from the baghouse of an experimental, fluidized-bed combustor (FBC) burning Montana rosebud (C) was calcic, that from the baghouse of a stoker-fed power plant burning Colorado coal (D) was ferric, and that from the electrostatic precipitator of a conventional power plant burning pulverized western coal (E) was ferric. Carryover from the limestone in the fluidized bed explains the very high calcium content of sample C (20). T o facilitate cross-reference, we have retained the same notation as that employed in the paper on sample composition (18). The samples labeled A, B (FBC, Texas lignite) were not studied further. m-Xylene (spectral grade, Kodak, Rochester, NY) was determined to be 99.8% pure, by gas chromatography (GC), with the balance (0.2%) being o- and p-xylene.
0013-936X/91/0925-0930$02.50/0
0 1991 American Chemical Society
Table I . Elemental Composition of Fly Ash Samples Determined by ESCA Peak Height Analysis
sample type
0
C
Ca
S
FBC, Montana rosebud ( C ) stoker fed, western (D) conventional, western (E)
45.3 44.0 55.5
0.5 12.7 5.5
24.8 3.4 4.7
6.7 8.9 9.7
element. wt 70 Si AI Fe 4.5 9.9 6.7
6.4 9.6 6.6
2.8 5.3 3.1
Mg
Ti
4.5
Na
K
2.8
1.8 1.2
4.9
5.9
2.4
Table 11. Fly Ash Samples
sample
combustor type
coal type
sample type
specific surface area: m2 g-'
C D E
FBC stoker fed conventional (pulverized coal)
Montana rosebud Colorado western
baghouse baghouse electrostatic
5.2 i 0.4 37.2 f 0.4 5.2 f 0.4
OA range of 2-40 m2 g-' was found ( 1 2 ) . These values are for the specimens on which the adsorption kinetics were done.
The apparatus used for this study was essentially the same as that used in our studies of m-xylene adsorption by diesel soot (21). The procedures were similar to those employed in our studies of the kinetics of adsorption of water by fly ash (12). We used a Lardner-Brinkman recirculating bath (9.4 f 0.02 "C) or an ice-water bath (0 " C ) to maintain the sample at a constant temperature. We also carried out some studies with the samples maintained a t 250 f 5 (after heating overnight) or 150 f 5 "C. The temperature for these experiments was controlled by a furnace and Love proportioning controller. Each sample was loaded in the vacuum microbalance and the pressure gently reduced from atmospheric to Torr or less. The sample was then heated overnight (outgassed) a t 250 "C to remove adsorbed vapors. Nitrogen was then admitted and the base-line weight at room temperature determined. Nitrogen adsorption and desorption isotherms were determined a t 77 K. Sample outgassing was repeated and adsorption/desorption of m-xylene determined a t 0 "C. The highest chart speed available (10 in./min) was used to facilitate analysis of kinetics of adsorption. Outgassing was repeated, and m-xylene adsorption/desorption a t 9.4 "C was determined. The procedures and the precautions required have been described in detail ( 4 ) . The only substantial change to the previously reported procedures was that, prior to the adsorption studies, we determined the saturation vapor pressure (SVP, Po)of m-xylene (0-20 " C ) (22). The pressure gauge used for these experiments was more sensitive than that used in our earlier work (4). The gauge was an MKS type 370, with bakeable head set a t 150 "C, and temperature compensation. The 100.Torr gauge head used provided resolution of Torr. A t the saturation vapor pressure, the observed pressure cycled with the same frequency as the thermostat and an amplitude approximately half that expected from the temperature fluctuation of f0.02 "C. This made the useful resolution for our experiments Torr. Previously reported problems, for example, air leakage causing high apparent saturation vapor pressure values [ranging from 2 to 5 Torr at 0 "C; (21)],were eliminated by minor improvements in apparatus construction. Models and Methods of Data Analysis Our treatment of data to obtain adsorption rate constants was described previously in detail (12); the essentials are repeated here. First-order kinetics may be expressed as In [ ( x , - z)/(x, - xi)] = -kt (1) where X, is the weight of vapor adsorbed a t equilibrium and a t a constant adsorbate vapor pressure ( P ) ,x is the
3001 9 0
]
-0
Equilibrium Value X.
1
2
3
4
TIME IN MINUTES
Figure 1, Microbalance chart recorder trace, following admission of m-xylene (sample E, 0 "C). Note the rapid initial increase in weight, followed by a slow approach to equilibrium.
weight adsorbed a t time t , xi is the initial weight, and k is the rate constant. The time (tl,,) required to adsorb a fraction l / q of the total amount of vapor adsorbed (x, - xi) is readily measured (Figure l). If first-order kinetics are obeyed, then
til, = -In (1- l/g)/k
(2)
which is a generalized form of the equation used by Natusch and Tomkins (11)to predict values for the half-time (tl
2).
Langmuir's theory is a particular case of first-order kinetics, for which k = k,P
+ kd
(3)
where k , and kd are rate constants for adsorption and desorption and P is adsorbate vapor pressure. Values of k may be predicted theoretically from Langmuir's theory (eq 3) or may be determined empirically (eq 1). The values of k presented in this paper were empirically determined from the slopes of log-linear data plots (eq 1). Whereas the use of eq 1 is only possible if the kinetics are first order, half-times may be empirically determined, regardless of the type of kinetics. An upper bound on tl12 may be estimated if over 50% of the adsorption occurs within the instrument response time. We have, therefore, estimated half-times, as well as first-order rate constants, where applicable. The data were evaluated by using linear regression methods and standard statistical tests provided by the RS/i software package (23), including analysis of variance, residual plots, and F tests. Stack residence times, Reynolds numbers, and exit temperatures were calcuated from published data for the Kingsnorth power station [CEGB (24)] and for the power Environ. Sci. Technol., Vol. 25, No. 5, 1991
931
Table 111. Summary of Conditions in the Stacks of Sixty-Nine Power Plants, as Reported by the EPA for Region 8 (Rocky Mountains and Adjacent Areas)"
plant type over 200 MW
50-200 MW
under 50 MW
post-1965
1955-1965 (incl)
pre-1955
all plants
4.4 2.4 20 1.9-9.9
5.6 7.5 27 1.1-40.9
5.4 5.2 69 1.1-40.9
4.8 3.0 19 0.6-12.6
1.9
1.3 27 0.6-6.7
3.7 3.0 69 0.6-12.6
310 97 18 145-600
414 96 25 290-580
344 107 69 130-600
Residence Times, s mean ( t , ) U
N range
6.7 3.1 12 1.9- 10.7
5.2 2.4 21 1.8-9.9
8.1 2.5 11 5.9-12.6
4.6 2.3 21 1.7-8.2
246 75 12 130-385
294 54 17 145-380
5.1 6.9 32 1.1-40.9
6.0 3.6 18 1.6-10.7
Reynolds Numbers (X10-6) mean U
N
range
1.8 1.2 35
6.3 2.9 16 1.8-10.9
0.6-6.7
Stack Exit Temperatures, mean U
N range
396 99 33 240-600
O F
283 86 17 130-500
Plants have been grouped by size (megawatt output) and by age.
stations in EPA region 8 (25). Velocity changes with stack temperature and diameter. No corrections were made for this. Therefore, the tabulated values (Table 111) were correct within 1 order of magnitude, but may be in error by as much as 50%. The data in the report (25) that were not self-consistent, or were obvious typographical errors, were excluded from the data set used to prepare Table 111. Stack exit temperature data could not be checked for reporting errors.
Results Specific surface areas of the samples studied are summarized in Table IT. Figure 1 shows a typical adsorption experiment trace after vapor admission. A fast initial increase in weight after adsorbate admission is followed by a slow approach to equilibrium. Half-times for adsorption and adsorption isotherms are shown in Figures 2 and 3, respectively. Plots of In (1- f) against time, where f is the fractional adsorption, [ ( x - x i ) / & - xi)], are shown in Figure 4. Residual plots and F values were employed to judge the best fit; all the plots (Figure 4) demonstrated some deviation from linearity. The slopes of these plots ( h ) were used to calculate the values of the time constants ( l / k ) for experiments performed a t 0 and 9.4 "C, the results of which are shown in Figure 5.
In
PRESSURE (torr)
?29.2 Inin
The adsorption isotherms (Figure 3) for all of the samples showed a knee-point ( I 7) (monolayer coverage) a t low relative pressure ( P / P " ) ,followed by an almost flat isotherm position, followed by a steep rise as the saturation vapor pressure (P) was approached. Thus, most samples of fly ash would attain monolayer coverage of aromatic hydrocarbons if residence times in the stack were long enough to allow the particles and vapors to equilibrate. For all power plants, temperatures steadily decrease as the power plant gases pass through superheaters, economizers, and rotary air heaters, which extract the useful energy produced by combustion. If stack exit temperatures are sufficiently low to cause saturation by cooling, pore filling would be observed a t equilibrium. This phenomenon is significant for diesel soot (15, 21), but not for fly ash. To explain, we must consider adsorption kinetics, competition, and the differences between bonding to diesel soot and fly ash surfaces. 932
Environ. Sci. Technol., Vo!. 25, No. 5, 1991
ooc.
-
B
u1
al a
.-
E
Y
w
2
5-
I-
LL -I
-
U
I -
0
1 A
'.
A
-
-IA
PRESSURE (torr)
h
Discussion
r'
10-
2or
C
.-
lac.?
;El 5 t
Y
I
0
1
2
3
3.5
PRESSURE ( t o r r )
Figure 2. Adsorption half-times for rn-xylene at 0 and 9.4 ' C as a function of pressure; measured for fly ash from conventional (A), stoker-fed (B), and fluidized-bed combustors (C). Data points indicate duplicate measurements. At low coverages, the adsorption half-times are much less than stack residence times, but the times for significant pore filling greatly exceed typical stack residence times (1-40 s). 0 , first sample, 0 'C. Replicate, 0 ' C (C).
The adsorption kinetics were similar for all samples studied. A fast initial adsorption phase, lasting less than
PRESSURE (torr)
PRESSURE (torr)
Figure 3 . Adsorption isotherms for rn-xylene at 0 (A) and 9.4 OC (6).Note the sharp "knee point", which corresponds to monolayer formation, at relative pressures of less than 0.2, and the sharp increase in adsorption at relative pressures over 0.8, which is probably caused by pore filling. For some samples, the sharp increase occurred at relative pressures as low as 0.6.
0
T M
-5.0
(secs)
I
100
200
300
400
500
TIME (sees)
Figure 4. Plots of In (I - f ) vs time for adsorption at 0 O C . Note the decrease in slope with increase in pressure for both samples. (A) Fluidized-bed combustor: (B) stoker-fed combustor.
10 s, was followed by a slower uptake of adsorbate, to equilibrium. At low relative pressure (P/Po < O . l ) , the fast phase represented over 80% of the weight adsorbed. The time scale of the fast phase was similar to that of
normal stack residence times (Figure 6), which are thus adequate for monolayer formation. The fast phase of m-xylene adsorption was comparable to that for water adsorption (12). Thus, the composition of the monolayer formed on fly ash particles will be determined by kinetic competition between vapors for adsorption sites. Water molecules, which are ~ 4 % by weight in the stack gases, will be the predominant species. Particles that are pure carbon, common in stoker-fed combustor ash, may absorb a layer of PAH because carbon is hydrophobic (17), and PAH will displace water from carbon. Thus, the carbon particles in fly ash may behave somewhat like the carbon particles of diesel soot. At higher relative pressures ( P / P o > 0.6), over 60% of the adsorption takes place during the slow phase. Plots of In (1- f , vs t are close to linearity ( F > 1000, r2 > 0.9), demonstrating that first-order kinetics were a good approximation to the second phase of adsorption. The values of the slope (k) decrease with increasing pressure and are independent of temperature for both types of fly ash. This behavior is similar to that previously reported by us for water adsorption (12),and by Fuller for nitrogen adsorption by a porous catalyst (16). The behavior observed was consistent with rates controlled by transport or diffusion processes (13). The behavior did not agree with that predicted by a Langmuir model for activated adsorption, which predicts an increase of k values with both pressure and temperature (because k , = ae-m/RT). The data on PAH concentrations in stack and plume are sparse (7), but suggest that the vapor-phase concentrations found in both stack and plume are too low for equilibrium multilayer formation. Multilayer formation and pore filling occurred at relative pressures of greater than 0.5. The half-times for multilayer formation and pore filling will decrease ~ 2 - f o l dbetween 0 and 300 "C if the process is diffusion or transport controlled (13). The half-times we observed are in excess of the longest stack residence times reported. Thus, multilayer formation and pore filling are readily observed in a laboratory setting, but are probably not environmentally significant. This finding contrasted sharply with the observations for diesel soot. Environ. Sci. Technol., Vol. 25, No. 5, 1991 933
180
600500 -
140
400 h
h
In
u)
0
0
0,
2 300-
v $100.
r
\ Y
0
200;
RELATIVE PRESSURE (P/Po)
RELATIVE PRESSURE
Figure 5. Time constants for the adsorption of rn-xylene by fly ash from (A) an FBC and (B) a stoker-fed combustor. For both types of fly ash, time constants exceed 30 s, and data obtained at both 0 ( 0 )and at 9.4 OC (0)fall on a single curve.
vapor for adsorption sites is probably the principal reason why PAH loadings on low carbon fly ash are typically less than a monolayer.
Acknowledgments We thank H. McDonald and J. C. Male (CEGB) for copies of CEGB reports, T. L. Thoem for a copy of the EPA report on the region 8 power plant, T. L. Thoem and K. Schwitzgabel for useful discussions of Radian reports, our colleagues for critical review of this manuscript, and E. Goff for illustration. I
100
I
I
I
200 300 400 PLANT CAPACITY (megawatt)
J
500
Figure 6. Mean residence times for fly ash in power plant stacks vs plant capacity (Megawatt output). Data are for EPA region 8.
Diesel soot has organic loadings of 5-5070. The very fine pore network of primary carbon particles (15, 21), or transient supersaturation as the exhaust is simultaneously cooled and diluted (14), may account for this difference in adsorption of organic materials. However, the very wide range of stack exit temperatures suggest that, for fly ash with a high carbon content, loadings greater than a monolayer of PAH might be obtained. Studies on fly ash samples from the plumes of stoker-fed power plants would be required to confirm this prediction. Fly ash from stoker-fed power plants may contain as much as 3070 (w/w) carbon. Such power plants are steadily being phased out, in favor of more efficient combustors. We predict that inefficient coal combustion, accompanied by high PAH and carbon concentrations in the exhaust gases, are the only conditions to produce fly ash loaded with multilayers of PAH. In 1978, inefficient, stoker-fed peaking plants contributed only 10% of the electric power, but almost 50% of the fly ash emitted ( I ) . Our data suggested that, in the future, the elimination of such plants will simultaneously improve the efficiency of coal use and reduce the emissions of particle-associated PAH. Summary
Stack residence times are comparable to adsorption half-times for monolayer formation by organic vapors. Processes of multilayer formation and pore filling are transport controlled, slow, and probably not of environmental significance. Competition with abundant water 934
Environ. Sci. Technol., Vol. 25, No. 5, 1991
Registry No. m-Xylene, 108-38-3.
Literature Cited (1) Seinfeld, J. H. Air Pollution: Physical and Chemical Fundamentals; McGraw-Hill: New York, 1988. (2) Shabad, L. M.; Pylev, L. N. US.Atomic Energy Commission Symposium; 1970; Vol. 21, pp 227-242. (3) Stenback, F. In Experimental Lung Cancer, Carcinogenesis and Bioassays;Karbe, E., Park, J. F., Eds.; Springer-Verlag: New York, 1974; p 161. (4) Rothenberg, S. J. Atmos. Enuiron. 1980, 14, 445-456. (5) Chrisp, C. E.; Fisher, G. L.; Lammert, J . E. Science 1978, 199,73-75. (6) Sun, J. D.; Bond, J. A.; Dahl, A. R. In Air Pollution, the Automobile, and Public Health; National Academy Press: Washington, DC, 1988. (7) Daisy, J. M.; Cheney, J. L.; Livy, P. J. J . Air Pollut. Control. ASSOC. 1986, 36, 17-33. (8) Hanson, R. L.; Carpenter, R. L.; Newton, G. J.; Rothenberg, S. J. J . Environ. Sci. Health 1979, A14, 223-250. (9) Hart, A. B.; Lawn, C. J. In C.E.G.B. Research No. 5; Male, J. C., Ed.; Central Energy Generating Board: London, 1977. (10) McFarland, A. R.; Bertch, R. W.; Fisher, G. L.; Prentice, B. A. Enuiron. Sci. Technol. 1977, 11, 781-784. (11) Natusch, D. F. S.; Tomkins, B. A. In Carcinogenesis-A Comprehensive Survey, Vol. 3, Polynuclear Aromatic Hydrocarbons; Jones, P. W., Freundenthal, R. I., Eds.; Raven Press: New York, 1978; pp 145-153. (12) Rothenberg, S.J.; Cheng, Y. S. J . Phys. Chem. 1980, 84, 1644-1649. (13) Thomas, J. M.; Thomas, W. J. Introduction to the Principles of Heterogenous Catalysis; Academic Press: New York, 1967. (14) Kittleson, D. B.; Barris, M. A. In Aerosols: Science, Technology, and Industrial Application o f Airborne Particles; Liu, B. Y. H., Pui, D. Y. H., Fissan, H. J., Eds.; Elsevier Science Publishing Co., Inc.: New York, 1984; pp 770-774.
Environ. Sci. Technol. 1991,25,935-939
Amman, C. A.; Siegla, D. C. Aerosol Sci. Technol. 1982, 1 , 73-101. Fuller, E. L., Jr. In Microweighing in Vacuum and Controlled Environments; Czanderna, A. W., Ed.; Elsevier: New York, 1979. Gregg, S. (J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1967. Rothenberg, S. J.; DeNee, P . B.; Holloway, P. Appl. Spectrosc. 1980, 34, 549-555. Roy, W. R.; Griffin, R. A. J . Enuiron. Qual. 1982, 11, 563-568. Carpenter, R. L.; Newton, G. J.; Rothenberg, S. J.; DeNee, D. B. Enuiron. Sci. Technol. 1980, 14, 854. Rothenberg, S. J.; Kittelson, D. B.; Cheng, Y. S.; McClellan, R. 0. Aerosol Sci. Technol. 1985, 4 , 383-400.
(22) Rothenberg, S. J.; Seiler, A. F.; Bechtold, W. E.; Eidson, A. F., submitted for publication in J . Chem. Thermodyn. (23) BBN Software Products Corp., Cambridge, MA 02238, R S / 1 Version 3.0, 1988. (24) Kingsnorth Power Station. Central Electricity Generating Board, Bankside House, London, 1975. (25) Parker, G. E.; Boulter, G. Region 8 Power Plant Summary; EPA-908/4-78-002; U.S. EPA, Government Printing Office: Washington, DC, 1977.
Received for review November 13,1990. Accepted December 4 , 1990. This research was supported by the U.S. Department of Energy's Office of Health and Environmental Research under Contract DE-AC04-76EV01013.
Multicomponent Kinetic Analysis of Iron Speciation I n Humic Lake Tjeukemeer: Comparison of Fulvic Acid from the Drainage Basin and Lake Water Samples Luis E. Sojo" CBR International, P.O. Box 2010 9865 West Saanich Road, Sidney, British Columbia, V81-3S3 Canada
Henk De Haan Limnological Institute of The Netherlands, Tjeukemeer Laboratory, De Akkers 47, 8536 VD Oosterzee, The Netherlands
Iron speciation in Lake Tjeukemeer, The Netherlands, was studied by multicomponent kinetic analysis of the ligand-exchange reactions between 2,4,6-tri(2-pyridyl)-striazine (TPTZ) and naturally occurring ligands bound to iron. Comparison with the kinetic behavior of iron in synthetic solutions made of extracted fulvic acids from the Lake drainage basin indicates that iron is distributed in two forms: polymeric hydrous oxides and, possibly, iron fulvates. Introduction The importance of humic substances in the circulation of nutrients in humic lakes has been more often inferred than actually verified. I t is a common practice to extrapolate results from laboratory studies involving isolated humic substances to natural water samples. Although laboratory studies are necessary in order to understand the main features of nutrient (i.e., Fe) interactions with humic substances, studies including actual samples are necessary. A careful comparison of both results will permit a better assessment of the importance of humic substances in nutrient circulation. Evidence for iron-fulvic acid complexes in natural waters is difficult to obtain due to the complexity of iron speciation. While the importance of fulvic acid in controlling the speciation of metals such as copper in natural waters is relatively well established, the same is not true for iron. The strong tendency of iron to form hydrous oxides presents a competitive reaction against the formation of iron-fulvic acid complexes. Most of the studies involving iron and fulvic acids have been carried out with soil-extracted materials. Despite these drawbacks, there is substantial evidence of iron-fulvic acid compounds in natural waters. *Present address: Seakem Analytical Services, P.O. Box 2045 Mills Road, Sidney, BC, Canada, V8L 3S8.
2219,
Shapiro ( I , 2) showed that Fe(II1) can form soluble or particulate complexes with aquatic humic substances. The same author studied the effects of pH on the reduction of ferric iron in natural waters (3) and was the first one to use colorimetric reagents in the characterization of iron fractions in freshwaters (4). Koenings (5) and Koenings and Hopper (6) also demonstrated the presence of ironorganic aggregates with dissolved humic matter and their importance in phosphate cycling. Tipping et al. (7) have presented the only kinetic evidence of iron-organic complexes in oxygenated waters. Further evidence of iron-fulvic acid complexes in natural water systems and their importance in controlling nutrient speciation has been presented by Steinberg and Baltes (8). They found that, to a certain degree, iron causes humic matter to sorb phosphate in significant quantities. Francko and Heath (9) demonstrated that the rate of release of orthophosphate as soluble reactive phosphorus was coupled to the photoreduction of Fe(II1)-humic acid complexes. Iron geochemistry in Lake Tjeukemeer has been previously studied by De Haan and De Boer (IO). On the basis of ultrafiltration studies, they presented circumstantial evidence of colloidal iron-organic complexes in the form of iron-fulvic acid compounds. The same authors calculated that only 10% of the total pool of fulvic acids was involved in possible iron-fulvic acid complexes. This low percentage may be due to the high pH (>7) of the lake, which translates into a high concentration of hydroxyl groups favoring the formation of iron hydroxides.
-
Background and Theory Langford and Khan (11) introduced the technique of kinetic speciation in the study of iron-fulvic acid interaction in laboratory systems. The technique is based on the ligand-exchange reaction between naturally occurring ligands (fulvic acids, OH-, etc.) and a strong iron complexing reagent. The ligand-exchange reaction can be described by eqs 1 and 2 (charges have been omitted for
0013-936X/91/~0925-0935$02.50/0 0 1991 American Chemical Society
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