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 2219, 2045 Mills Road, Sidney, BC, Canada, V8L 3S8.
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 T h e o r y 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
Environ. Sci. Technol., Vol. 25, No. 5, 1991
935
kl
Fe-L, F= Fe k-l
+ L,
(1)
Fe+C-P (2) simplicity), where Fe-L, stands for any iron complex present in the sample, including iron-organic complexes. C represents the complexing agent, which forms a colored complex P with iron. Reaction 2 is irreversible and pseudo first order. Equations 1 and 2 represent a disjunctive ligand-exchange mechanism as described by Hering and Morel (12) based on the imposed reaction conditions. In our study, the complexing agent employed (2,4,6tri(2-pyridyl)-s-triazine,TPTZ) is an Fe(I1) reagent, so that the introduction of an iron reducing agent is required in order to detect total iron. This requirement introduces reaction 3. In the absence of reaction 3 only Fe(I1) species, NH,OH.HCI
Fe3+
-
Fez+
(3)
if present, can be detected. Reaction 3 is also pseudo first order and almost instantaneous with respect to the time scale of the experiments. Under these circumstances, the rate of formation of complex P represents the rate of dissociation of complex Fe-L, under the solution conditions imposed by the reagent mixture. The buildup of complex P with time can be mathematically described by eq 4, n
[P](t)= CA,(1- exp(-h,t)) r=l
+X
(4)
where A, is the initial concentration of each iron complex (Fe-L,), h, the rate constant for the dissociation of each complex, and X is the concentration of free iron and any other complexes that can react “instantaneously”with the complexing agent C after blank correction. The resolution of A,’s and k,’s from the [P](t) function has been discussed by Mak and Langford (13)and Lavigne et al. (14). Estimates of A,’s and hi’s are obtained from applying Guggenheim’s method to the data. These estimates are used in a nonlinear regression routine to fit the data to the form of eq 4. The final values of A,’s and h,’s are obtained from the best fit parameters. It is important to mention that the numerical values obtained for the rate constants reflect the solution conditions imposed by the reagent mixture. Solution pH will change drastically from the value at equilibrium to the value maintained by the buffer system of the reagent. The result of this is that the rates constants are no longer the same as those under natural conditions; nevertheless, they still represent iron dissociation from those ligands initially occupied under those conditions. The main goal of this methodology is to distinguish species and their respective concentrations and to provide estimates of their relative dissociation rates. Materials and M e t h o d s
Reagents. All reagents were analytical reagent grade. and 1 x M were Standard Fe(II1) solutions of 1 X prepared by diluting a Fe(II1) stock solution (1 X M). The stock solution was made by dissolving 0.4277 g of Fe(NO,),.9H2O (Baker) in 100 mL of 1 : l O HC104. TPTZ-Fe(I1) reagent solution was prepared by dissolving 0.1 g of 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) in a minimum volume of concentrated HC1. This solution was stirred while a solution containing sodium acetate (40 g), glacial acetic acid (29 mL), and water (40 mL) was added. TPTZ-total Fe reagent was prepared exactly as for the TPTZ-Fe(I1) reagent with the addition of 3 g of hydroxylamine hydrochloride. 936
Environ. Sci. Technol., Vol. 25, No. 5, 1991
Soil fulvic acid (FA) from Lake Tjeukemeer basin was extracted as described by Schnitzer and Skin (15). Briefly, the soil sample was homogenized and extracted in 0.1 N NaOH by shaking overnight. Humic acids were precipitated by addition of concentrated HCl to the soil extract and separated by centrifugation. The supernatant contained FA. This solution was extensively dialyzed until no more C1- was detected in the external solution by the AgNO, test. Dialysis tubing of a nominal 24-A pore size was employed. Titration of FA with standard base was carried out as described by Gamble (16). The carboxyl content was found to be 5.72 mequiv/g. Lake Tjeukemeer water samples were collected in 2-L polyethylene containers during the fall season. Samples were immediately filtered through 0.2-pm filters and stored a t 4 “C until further analysis. Tjeukemeer fulvic acid (TjFA) was extracted as described by Thurman and Malcolm (17). Two liters of lake water, previously, filtered through a 0 . 2 - ~ mfilter, were acidified and passed through a column of XAD-2 resin in the chloride form. Fulvic acids were stripped off the column with 0.1 N NaOH. The alkaline extract was passed through a Dowex-50 ion-exchange resin in the H form. Fulvic acids were collected in the effluent, which was then dialyzed to remove excess salt. Sample Preparation for Kinetic Studies of Iron Species. A solution of known concentration of fulvic acid (soil or lake) was equilibrated with a known total amount of iron [Fe(III)]. pH was adjusted to either 4 or 6 by titrating this solution with HC10, or sodium acetate, depending on the pH. The volume was adjusted to 100 mL and the solution was allowed to equilibrate a t 22 “C for 48 h in the dark. The 48-h “aging” period was used as comparison point. Blanks containing fulvic acid only were prepared as described for the samples. A control solution was also prepared containing iron only at the required pH. After the “equilibration time”, 1 mL of solution was transferred to a 1-cm cuvette already placed in the sample compartment of a Philips 8500 UV-vis double-beam spectrophotometer. One milliliter of TPTZ [Fe(II) reagent] containing hydroxylamine hydrochloride was injected and the absorbance produced by the formation of FeTPTZ complex was immediately monitored a t 590 nm. The time between the TPTZ injection and the first absorbance reading was kept constant a t 15 s. This time lapse was taken into account when the data were analyzed. A blank solution consisting of 1 mL of H,O and 1 mL of TPTZ solution was used as reference. Absorbance data were collected every 30 or 60 s, depending on the reaction rate, by using a data acquisition program provided by Philips. Kinetic analysis of Lake Tjeukemeer water was carried out as described above, by mixing 1 mL of previously filtered water (0.2-pm filter) with 1 mL of TPTZ reagent solution. Samples filtered through AC62 and AC64 Schleicher and Schull (S&S)ultrafilters were also analyzed. Data Handling and Analysis. Guggenheim’s method of kinetic analysis was used to provide estimates of rate constants and initial species concentrations. Guggenheim plots [In ( A , - A,) versus time] of averaged kinetics runs were prepared for each experiment. The absorbance reading a t infinity was determined by analyzing each sample for total iron after acid hydrolysis. In all cases, the plateau in the absorbance versus time curves coincided with the absorbance reading after acid hydrolysis. Absorbance versus time curves were fitted to eq 4 by an iterative nonlinear regression program written by one of the authors using the rate constants and initial concen-
Table I . Summary of Dissociation Rate Constants for Iron Hydrous Oxides and Iron Fulvates in Laboratory Solutionsg
rate const, min-’ kA1( ~ 1 0 % ) ~ kAz ( ~ 1 0 % ) ~
Instantaneous component. Fast component. Slow component. dPercent error. e Solution containing fulvic acid extracted from Lake Tjeukemeer‘s drainage basin. f Solution containing fulvic acid extracted from Lake Tjeukemeer. g Total Fe concentration in all runs, 1.00 X M. FA and TjFA concentrations as DOC, 90.0 and 8.95 mg/L, respectively. Values in parentheses indicate standard deviations. 0.3 7
I
+
I 0.26
0.2 0.16
0.1 -8
t
0.06
I 0
60
100
160
200
Timehln)
Figure 1. Formation curve for iron-TPTZ complex in fulvic acid solutions at pH 6.23. ).( experimental values; (+), nonlinear regression
fitted curve.
trations obtained from the Guggenheim plots as initial estimates of these parameters. Error estimates for the fitted parameters were calculated as described by Caceci (19).
Results Kinetic Behavior of Iron in the Absence and Presence of Fulvic Acids. Table I summarizes the results of experiments involving Fe(II1) in the presence and absence of fulvic acids. In these experiments all solutions were allowed to “age” for 48 h prior to any kinetic measurement. No settling of precipitate was observed during this process and during subsequent kinetic measurements. This observation was corroborated by filtering some of these solutions through a 0.2-pm filter after the 48-h period and performing the kinetic analysis. No significant difference was detected when compared with unfiltered solutions. The formation curve for the iron-TPTZ complex is shown in Figure 1, for the case of the Fe(II1) solution a t p H 6.23. The first three entries in Table I correspond to Fe(II1) solutions in the absence of fulvic acids. Two kinetically distinguishable components were resolved. The dissociation rate constant of component ( A , ) is very close to the hydrolysis rate of ferric hydroxide colloids. Sommer et al. (20) obtained a value of 0.03 min-l; a similar value was obtained by Lmgford et al. (18) and Tipping et al. (7). The difference between the average hydrolysis rate constant obtained in our experiments and the reported values is not statistically significant, based on our experimental error. Analysis of the Guggenheim plots for the “ferri-hydroxide” solutions indicates a linear kinetic behavior (Figure 2 ) , as was also found by Tipping et al. (7) for Fe(II1) gels. This finding is in contrast with the results obtained by Langford et al. (181,in which more than two components were
-8’
0
20
40
80
80
100
120
140
Tlmehln)
Figure 2. Comparison of kinetic behavior of iron in Lake Tjeukemeer
and fulvic acid solutions. Guggenheim plot for the formation curve of the iron-TPTZ complex.).( Fe(II1) solution at pH 6.00; (”) Fe(II1)-FA solution at pH 6.23; (+) Lake Tjeukemeer at pH 7.21.
postulated. In the present case two major Fe(II1) species can be postulated: a small, probably monomeric species and a hydrous oxide polymeric colloid, with particle size less than 0.2 pm. The contribution of “polymeriecolloida1” hydrous oxide species (component A,) to the total iron content increases with increasing p H as expected, due to the reaction of OH- with Fe(II1). The effects of fulvic acid on the kinetic behavior of Fe(II1) can be inferred from the last three entries in Table I and can be clearly seen in Figure 2. The first observation is the existence of a third component (component A&. The dissociation rate constant for component A, for the fulvic acid experiments is 1 order of magnitude larger than in the absence of fulvic acid. This indicates the formation of a Fe(II1) species that dissociates faster than any hydrous oxide colloid. This component has been assigned to a fulvic acid-iron complex. Component A2 has a dissociation rate constant statistically ( a = 0.05) similar to the hydrolysis rate of ferric hydroxide colloids; therefore, it has been assigned to hydrous oxide colloids not bound to fulvic acid. The effects of p H on iron speciation can be clearly seen in Figure 3. Displacement of iron from component A, by protons results in an increase of the “instantaneous” component. Control experiments involving iron-fulvate solutions and TPTZ in the absence of hydroxylamine hydrochloride in the dark resulted in a very slow formation of Fe-TPTZ complex, with less than 5% of total iron detected after 2 h. No Fe-TPTZ was detected in control solutions containing Fe(II1) only. Lake Tjeukemeer Water. The formation curve for the Fe(I1)-TPTZ complex for Lake Tjeukemeer water samples at various pH levels is shown in Figure 4. The results are summarized in Table 11. The first two entries demonstrate Environ. Sci. Technol., Vol. 25, No. 5, 1991 937
Table 11. Summary of Dissociation Rate Constants for Iron in Lake Tjeukemeer Water Sample# total iron, 90 sample
PH
TjW-02 TjW-0.2' TjW-AC641 TjW-O.Z* TjW-0.2. TjW-0.2*
6.4 6.4 6.4 7.21 3.31 1.82
xo
(+8%)d
75.0'e.o'
A," (+5%Id
A*' ( * 6 % ) d
60.0'3.0' 63.6'3.2)
40.0'z,')
62.5'3.''
37.5'2.3' 513.0'~.~'
36.4'2.2'
25.0"-"
44.0'3.5' 76.0'"')
rate constant, m i d k,, (*10%)d
k,, (+10%)d 0.95~0.011 0.~2'0.011
0.03"") 0.0310~1)
0,2610.03)
0.0~'0.001)
0.~4'0.01'
0,01Io.Wl) 0,01"."l~
24.0".4'
DInstantaneouscomponent. "Fast component.
