Environ. Sci. Technol. 1992,26, 593-598
duction Emphasizing Chemical Equilibria i n Natural Waters, 1st ed.; Wiley Interscience: New York, 1970; pp
Hull, M.; Kitchener, J. A. Trans. Faraday SOC.1969,65, 3093. Ives, K. J.;Gregory, J. Proc. SOC. Water Treat. Exam. 1966, 15, 93. Pashley, R. M.; Israelachvili, J. N. J. Colloid. Interface Sci. 1984, 97, 446. McDowell-Boyer, L. M. Ph.D. Dissertation, University of California a t Berkeley, 1989. Yao, K. M.; Habibian, M. T.; O’Melia, C. R. Environ. Sci. Technol. 1971, 5, 1105. Fitzpatrick, J. A.; Spielman, L. A. J. Colloid Interface Sci. 1973,43, 350-369. Ghosh, M. M.; Jordan, T. A.; Porter, R. L. J. Environ. Eng. Diu. (Am. SOC.Civ. Eng.) 1975, 100, 81. Sakthivadivel, R. Report HEL 15-5. Hydraulic Engineering Laboratory, University of California, Berkeley, 1966. Traut, G. R.; Fitch, R. M. J . Colloid Interface Sci. 1985, 104, 216. Tobiason, J. E. Colloids S u r f . 1989, 39, 53.
454-457. Hunter, R. J. Zeta Potential i n Colloid Science: Principles and Applications; Academic Press: London, 1981;pp 11-55. Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday SOC.1966, 66, 1638. Hamaker, H. C. Physica 1937,4, 1058.
Received for review April 19,1991. Revised manuscript received October 17, 1991. Accepted November 5, 1991. M u c h o f the research was funded by t h e University of California’s W a t e r Resources Center.
Tobiason, J. E. Ph.D. Dissertation, John Hopkins University, 1987. Dahneke, B. J . Colloid Interface Sci. 1975, 50, 89. Ruckenstein, E.; Prieve, D. C. A I C h E J . 1976, 22, 276. Bowen, B. D.; Epstein, N. J . Colloid Interface Sci. 1979, 72, 81. Yoshida, H.; Tien, C. J. Colloid Interface Sci. 1986,11,189. Kallay, N. E. Barouch; Matijevic, E. Adv. Colloid Interface Sci. i987, 27, 1. Israelachvili, J. N.; McGuiggen, P. M. Science 1988,241, 795. Faraday Ruckenstein, E.; Prieve, D. C. J . Chem. SOC., Trans. 2 1973, 69, 1522. Spielman, L. A.; Friedlander, S. K. J. Colloid Interface Sci. 1974, 46, 135. Gregory, J.; Wishart, A. J. Colloid Surf. 1980, 1 , 313. Stumm, W.; Morgan, J. J. Aquatic Chemistry, An Intro-
Dissociation Pathways and Species Distribution of Aluminum Bound to an Aquatic Fulvic Acid Mark S. Shuman Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7400
-
A ligand-exchange reaction between Al-fulvic acid (FA)
complexes and lumogallion (Lum), A1-FA + Lum AlLum + FA, was investigated by following the appearance of the fluorescent complex AlLum. Rate data fit a model of simultaneous first-order reactions and indicated that the concentration of the slowest reacting component increased with increasing fulvic acid concentration. Small changes in pH (5.0-5.5) and in ionic strength ( I = 0.02-0.11) had no effect on the observed rates. The data were also analyzed in terms of two parallel mechanisms, A1-FA dissociation (disjunctive mechanism) and direct attack by Lum (adjunctive mechanism). Direct attack was the more important pathway when the incoming ligand, Lum, was in great excess of the A1-FA concentration or when free FA concentrations were much larger than bound FA concentrations. It was concluded that in natural freshwaters, where the incoming ligand is likely not to be in large excess, most AI-FA complexes undergo ligand exchange via the disjunctive pathway. Conditionalbinding constants were also estimated, and an A1 species distribution was calculated.
appears to increase with DOM content and represents a greater fraction of the total A1 as pH increases from 4 to 6. These analytical procedures are based essentially on differential kinetic methods in which separation into inorganic and organic Al is judged using the assumption that organic A1 undergoes ion exchange or color development much more slowly than inorganic Al. Evaluation by Backes and Tipping (5) and Hodges (6) of the procedures has stressed their operational nature. Recent work has investigated the kinetics of A1-DOM association (7) and dissociation (8) and has indicated multireacting components, multimechanisticpathways, and a wide range of rate constants. Studies of A1-DOM complexation have generated several empirical binding models ( 4 , 9, 10). The work described here uses data on the exchange rates of ligands to investigate mechanisms of Al-fulvic acid (FA) dissociation. The data are also used to estimate distribution and binding parameters of reaction species. The objectives are to provide information about ligand-exchange reactions of A1 in natural waters and to exploit kinetic methods for estimating A1 speciation in natural waters.
Introduction
Soluble A1 in acidic natural waters is complexed with hydroxide, sulfate, and fluoride, and with dissolved organic matter (DOM) macromolecules. DOM appears to be very important to A1 transport; in southeastern U S . rivers, for example, total A1 concentrations correlate with DOM (1). In the widely used method of Driscoll and others (2, 3), cation-exchange/colorimetricprocedures combined with empirical/thermodynamic models are used to estimate the distribution among these numerous A1 species in streams and lakes ( 4 ) . The “organic Al” identified in this way 0013-936X/92/0926-0593$03.00/0
Experimental Section
Aluminum solutions of 2.0 and 4.0 FM Al at pH 5.0 and 5.5 were prepared by diluting an AlK(SO& stock with buffer solutions at the desired pH and ionic strength. Sodium acetate/NaCl buffer solutions (0.011 M) were prepared by diluting electrolytically cleaned sodium acetate with distilled-deionized water and adjusting the ionic strength with Aldrich gold label reagent NaC1. A 2.5 mM stock lumogallion [[3-(2,4-dihydroxyphenyl)azo]-2hydroxy-5-chlorobenzenesulfonic acid] (Pfalz & Bauer)
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 3, 1992 593
solution was prepared by dissolving 0.086 g (fw = 344.0) in 100.0 mL of distilled-deionized water. Lumogallion working solutions of pH 5.0 or 5.5 were prepared by diluting the stock solution with the buffer to give 50.0, 250.0, and 500.0 pM solutions. Lumogallion was chosen because of its low detection limit for A1 (-2 nM) and because it reacts at a reasonable rate between pH 4.5 and 5.5. The FA used in this study was from Lake Drummond, near Suffolk, VA, and was isolated by using an XAD-8 resin procedure. This Lake Drummond fulvic acid (LDFA) is described by Thompson (11). A stock solution was prepared by dissolving 0.205 g of solid LDFA in 100.0 mL of water, resulting in a 1000.0 mg of C/L solution, which was refrigerated before use. LDFA working solutions were prepared by diluting the stock solution in 10.0 mL of buffer to give 2.0, 10.0, 20.0,40.0, and 160.0 mg of C/L solutions. Fluorescence measurements were performed with a SLM-Aminco Instruments Inc. Model SPF 500-C spectrofluorometer controlled by an IBM-PC microcomputer and software. All experiments were conducted a t 25 OC. Most kinetic runs were carried out at an aluminum concentration of 1.0 pM and fulvic acid concentrations of 1.0, 5.0, 10.0, 20.0, and 80.0 mg of C/L. The A1-FA solution and the lumogallion (Lurn) solution were allowed to stand about 12-18 h in a covered beaker a t room temperature. The lumogallion concentration used for most runs was 125.0 pM, a considerable excess over Al(II1). The pH of the A1-FA and Lum solutions were rechecked before kinetic analysis and found to be within 0.04 pH unit of the initial values. Kinetic runs were performed by mixing and following the fluorescence intensity of Al-Lum formation in 1.0-cm cuvettes. A pipet delivered 1.0 mL of Lum working solution into the cuvette, and the solution was allowed to reach 25 “Cover the course of about 10-15 min. Data acquisition was initiated -2 s after another pipet transferred 1.0 mL of Al-FA working solution into this same cuvette, and data were recorded every 5 min up to 150 min. A final data point was collected at 24 h. Two additional solutions were measured concurrently: (i) a blank of FA and Lum without Al; (ii) a 1.0 pM solution of A1 containing 125.0 pM of Lum, a solution which was used to correct for variations in instrument performance between uses. A linear standard curve of fluorescence intensity vs Al-Lum concentration was obtained for aluminum concentrations from 0.2 to 2.0 pM a t pH 5.5. Aluminum contamination of the buffer solutions was estimated to be -90.0 nM. Noise was about 2.0-8.0% of the signal. The concentrations of aluminum and fulvic acid were below those that experience Al-FA coagulation (13). Preliminary kinetic runs in buffered solutions containing no FA showed that Al-Lum formation was complete within 2 min at 1.0 pM of aluminum and 125 pM Lum; thus, it was assumed that all inorganic aluminum species reacted with Lum before the first data point at 5 min. Data Treatment Mechanistic Analysis. Hering and Morel (12) have introduced the nomenclature “disjunctive” and “adjunctive”for describing mechanisms of ligand-exchange reactions. For a single A1-FA reaction site and exchange with Lum to form the fluorescent complex Al-Lum, the overall reaction A1-FA + Lum Al-Lum + FA (1) can follow either a disjunctive pathway, where the A1-FA bond is first broken in a rate-determining step followed by a fast reaction to form Al-Lum, or an adjunctive
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pathway, where there is direct attack of A1-FA by Lum and a ternary intermediate A1-FA-Lum formed with subsequent breaking of the A1-FA bond. The disjunctive mechanism is described thus: A1-FA
A1
kaw
-
+ FA
(2)
kAL,
Al-Lum (3) A1 + Lum the reverse reaction is ignored when [Lum] is in great excess of total Al. The rate of the reaction is defined by the appearance of Al-Lum d[Al-Lum] /dt = kAILum[Al] [Lum] (4) which, assuming [All to be at steady state, becomes (in terms of the original reactants) d[Al-Lum] /dt = ~ ~ A 1 L u m ( ~ d i s s o c / ( ~ a s s o c+[ ~ ~ l AIL^^ [Lum]))1[Al-FAI [Luml (5) For conditions where [FA] >> [Lum] (assuming that k,,, and kAILumare of equal magnitude), the rate becomes d[Al-Lum]/dt = (kA1Lum~dissoc/~assoc) (I/ [FA])[Al-FA] [Luml (6) When [Lum] >> [FA], the steady-state concentration of AI is insignificant and “trapping conditions”exist for which d[Al-Lum]/dt = kdiss0,[A1-FA] (7) For the adjunctive pathway, the mechanism is expressed A1-FA + Lum A1-FA-Lum Al-Lum FA (8) whose rate is expressed as
-
-
+
d[Al-Lum] /dt = kadj[A1-FA][Lum]
(9)
where kadj is the adjunctive, overall rate constant (that is, without resolution of the individual steps in eq 8). For the case [FA] >> [Lum], the rate can be expressed by an overall rate constant d[Al-Lum] /dt = kove,al~[A1-FA] [Lurn] =(kdis(l/[FAI) + kadjJ[Al-FAI [Luml (10) where the disjunctive rate constant is kdis
=
(kAILumkdissoc/kassoc)
(11)
The two pathways can be distinguished by the effect of FA concentration on the experimentally observed rate constant; the disjunctive path is influenced by [FA] and the [FA]/[Lum] ratio, whereas the adjunctive pathway is independent of [FA]. For conditions where [FA] > [Lum], plotting the observed overall second-order rate constant as a function of l/[FA] gives kdisas the slope and kadj as the intercept (eq 10). Under trapping conditions, the observed overall pseudo-first-order rate constant is koverall
=
kdissoc
+ kadjbm]
(12)
Values of kadj are obtained from the slope and kdissoc from the intercept of koverall plotted against [Lum]. With regard to AI-FA and its reaction with Lum, the following situations are anticipated. For saturated sites, the overall rate may be determined predominantly by either the adjunctive or disjunctive pathway, depending on the ratio of [Lum] to [Al-FA]. When [Lum] >> [AlFA], the adjunctive pathway will likely predominante (eq 12), whereas if [Lum] is the same magnitude or smaller than [&FA], the disjunctive mechanism may be the major path. At small loadings (i.e., a small percentage of binding sites occupied by Al), where there is a large excess of free
0.0 0
:
.
A
401
A
i
A
I
Y.0
2 0
3.0
111
AIIFA
i
0 AIIFA
i
A
= 1\20
AI/FA
115
4.0
5.0
-L.v
Figure 1. Percent AI reacted with lumogallion as a function of time for three concentrations of FA: 1, 5, and 20 mg of C/L.
over bound concentration of the leaving ligand, the reverse reaction suppresses the disjunctive pathway and the adjunctive mechanism will predominate even if [Lum] is not in excess. Kinetic Model. The overall reaction of a mixture of A1 complexes consisting of n components designated AlL, is AlL, + Lum Al-Lum L, (13)
-
+
where L, represents the ith binding site on an organic macromolecule or the ith inorganic ligand, Lum is in excess, and Al-Lum is the fluorescent product, which is monitored to follow reaction progress. Each component, AIL,, is assumed to undergo a pseudo-first-order reaction simultaneously with all other components. It is also assumed that no coagulation or precipitation occurs. For reactant AlL,, the first-order rate law is d[AlL,]/dt = -k,t or expressed in integrated form, [AlL,], = [AIL,joe(-kJ), where [AIL,],is the concentration of unreacted aluminum at time t, [AlL,], is the original concentration of complex AlL,, t is the reaction time, and k, is the apparent first-order rate constant. The concentraion of all complexes in the mixture at time t is C[AlL,], = CIAILl]oe(-k~t)
(14)
and the observed fluorescence intensity is It = a[Al-Lum], (15) where a is a proportionality constant. Combining eqs 14 and 15 gives
It = aCIAIL,lo(l - e(-kit))
(16)
When there is a small number of components, simple graphical methods can be used to extract rate constants and component concentrations. The so-called infinite time method was selected, which utilizes the integrated form of eq 16 and the long-time asymptotic limit of fluorescence intensity, I,, as the estimate of C[A1LJ0. The Guggenheim and time lag methods, which use data at a constant time interval and do not require estimates of I, were also considered, but the noise level precluded their use. When the number of reacting species is not known, as in the present study, the data can be analyzed by nonlinear regression analysis. The statistical package SYSTAT (14) was used, and the data were found to fit best a threecomponent, four-parameter model
I, = I,(1- e(+,))
'
50
0
In(minutes)
+ Iz(l- e(-kZt))+ X
(17)
100
150
Time, min.
Figure 2. Infinite time curves for 1 FM AI and 1, 5, 10,20, and 80 mg of C/L FA, and for 2 FM AI and 5 mg of C/L. Lines are the slopes of the long-time data.
-
'0 Q
m
rr"
. !
20
4
1 .o
125)1MLum
A
0
40 -
I 250uMLum
I
O
2.0
3.0
4.0
5.0
I
In (minutes)
Figure 3. Percent AI reacted as a function of time for three concentrations of lumogallion, 1 pM AI, and 5 mg of C/L FA.
where It is the measured fluorescence intensity, Itl and k z are the apparent first-order rate constants for components 1 and 2, X is a time-independent term which includes a blank and fast reactions, Il and I2 are a[AlL,] and a[AlL2], respectively, and t is the reaction time, the first data point recorded a t 5 min after mixing. This model neither requires nor computes Imm.Initial estimates for kl,kz,and 11, I z were obtained graphically. Results and Discussion Infinite Time Plots. Figure 1 illustrates the course of the reaction of 1 pM A1 and 125 pM Lum at three concentrations of FA: 1, 5, and 20 mg of C/L. The overall reaction rate is seen to decrease as Al/FA decreases. The reaction is 70-90% complete within 150 min for these conditions and 100% complete within 24 h. Figure 2 illustrates data analysis by the infinite time method. Curvature suggests the presence of more than one reacting component. The long-time slope of each curve is independent of time after -75 min (In t = 4.3 in Figure 1)and the extrapolated intercepts, which are related to the initial concentration of the slow-reacting components, increase with increasing FA concentration (decreasing Al/FA ratio). The apparent rate constant of the fast reactions, as judged from the dope of the curve at short time decreases with increasing [FA]. Figure 3 shows the reaction for three concentrations of Lum (25,125, and 250 pM), at a single FA concentration Environ. Sci. Technol., '301. 26, No. 3, 1992
595
Table I. Values of Fitted Parameters"
experimental conditions [LDFA], mg of DOC/L
[All, pM
[Lum], pM
x,70
70
28.2 28.9 34.9 43.1 37.3
1.0
1.0
1.0
5.0 5.0 10.0 20.0
125.0 125.0 125.0 125.0 125.0
54.1 47.4 35.7 23.4 11.3
1.0 1.0 LOb
80.0 5.0 5.0 5.0
125.0 25.0 250.0 125.0
10.5 18.0 44.3 33.6
1.0'
5.0
125.0
35.8
1.0
1.0 1.0 1.0
oarameter estimates A1-FA, k , min-'
"Numbers in parentheses are standard errors; %, percent of total noted. b0.02 M. e Ionic strength 0.02 M.
AI-FA, k , min-'
70
0.1722 17.7 0.0309 (0.0390) (0.00622) 0.0924 23.8 0.0094 (0.0076) (0.00120) 0.0703 29.4 0.0074 (0.0087) (0.00075) 0.0614 33.5 0.0064 CO.0081) (0.00080) 0.0754 51.4 0.0086 (0.0069) (0.00032) 26.9 0.0973 62.6 0.0119 (0.0110) (0.00056) 44.0 0.0659 38.0 0.0060 (0.0031) (0.00026) 31.8 0.1010 23.8 0.0194 (0.0360) (0.01052) 35.7 0.0864 30.7 0.0076 (0.0134) (0.00058) 35.4 0.0682 28.8 0.0072 (0.0026) (0.00031) Al. Conditions: pH 5.5, 25 "C, ionic strength 0.11 M except where 100.0%
80.0% --C
AIFAl+AIFPS
-1 60.0%
I 40.096
25bMLUM
3
125uMLUM
20.0%
~ U M L U M
-3
0.0%
0
100
50
150
111
115
1/10
1/20
1/80
AllFA ratio
Time, min.
Flgure 4. Infinite time curves for three concentrations of lumogallion, 1 pM AI, and 5 mg of C/L FA.
(5 mg of C/L). The overall reaction speeds up with [Lum], at least a t the beginning. Apparently the long-time reaction is also affected. Figure 4 illustrates that the longtime slope of the infiiite time curves increases with [Lum], indicating an increase in the rate with [Lum]. Infinite time plots gave no evidence of a pH effect on rate constants (no change in slope) between pH 5 and 5.5, although a small shift in concentration to the slower reacting component was observed. No ionic strength effect on rate constants or concentration distribution was seen when the ionic strength was increased from 0.02 to 0.11. These observations are consistent with the results of regression analysis (see next section and Table I). Regression Analysis. Results of fitting the threecomponent, four-parameter model are compiled in Table I. The model estimates the concentration of a fast-dissociating component, designated X in the model, which includes the blank and kinetically unidentified components assumed to be both inorganic and organic species, and two more slowly reacting components designated A1-FAI and A1-FA2. The rate constant for component A1-FA1 is 10 times that for A1-FA,; thus, the size order of dissociation rate constants is X > .U-FA1 > A1-FA,. The distribution of these species is displayed in Figure 5, which illustrates predominance of slower reacting species at Al/FA ratios smaller than 1/1. Mechanistic Analysis: Long-Time Data. The infinite time plots of Figure 4 show a dependence of the
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Environ. Sci. Technol., Vol. 26, No. 3, 1992
Flgure 5. Distribution of components as a function of AVFA ratio in pM AVmg of C.
r
.-C
E
3-
/ 0.00 0
100
200
300
LUM, uM
Flgure 8. Long-time rate constants as a function of lumogallion concentration. Line is the least-squares fit.
long-time slope on [Lum]. When the apparent rate constants estimated from these slopes are plotted against [Lum], the result is Figure 6. Since [Lum] >> [FA] (see below for an estimate of the molar site density of FA in pmol of Al/mg of C), trapping conditions are assumed and the slope of Figure 6 can be interpreted as the rate constant for the [Luml-dependent adjunctive pathway for the long-time reaction and the intercept as the [Luml-independent rate of the disjunctive pathway. As noted above, the disjunctive rate constant for trapping conditions is also
0,141
.
400
300
0.06
200
0.04 100
0
20
40
60
80
FA, mgCll Flgure 8. Mole ratio plot of concentration of long-time reacting components.
a
2 E!
. .
801
A
AI AlOH
0
AIfOHIZ
0
601 0
0
5
10
I
15
20
FA, mg Cil
Flgure 9. Computed AI species distribution as a function of FA concentration: pH 5.5, i KM AI.
Figure 2 are plotted as a function of FA concentration, they describe a relationship which can be analyzed by the mole ratio method, as is done in Figure 8. Extrapolating the first data point to the maximum bound A1 concentration of 424 nM Al results in a site density of 121 nmol of Al/mg of C and a conditional stability constant, K = [Al-FA]/ [Al'][FA'], of approximately where [Al'] and [FA'] are the concentrations of A1 and FA not associated with A1-FA. This estimate is considered approximate due to the limited data base. The value of 121 nmol/mg of C is approximately the concentration of the strongest site for Cu binding and FA (190 nmol/mg of C) found in earlier work (18). Computation of A1 Species Distribution. Species distribution wm computed using the conditional formation constant for the long-time reactants, K = [Al-FA]/[Al][FA'] = 107.75, which is the conditional constant given above increased by the A1 hydrolysis side reaction coefficient of 101.25 at pH 5.5. Formation constants used for AlOH and Al(OH), wre and 1017.90, respectively, derived from hydrolysis constants given in ref 4. The relative concentrations of Al, AlOH, Al(OH),, and A1-FA are shown in Figure 9 as a function of FA concentration. The figure shows that AI-FA is the predominant species above -4 mg of C/L when total A1 is 1 p M . This distribution of organic species of A1 is not greatly affected by inclusion of the short-time reactions or by fluoride concentrations below -2 pM. Comparison with Previous Work. Hering and Morel (12)investigated the ligand-exchange reaction between calcein and a Cu-HA (HA was Aldrich humic acid) that Environ. Sci. Technol., Vol. 26, No. 3, 1992
597
had an observed site density of 200 pmol of Cu/g of C. Total Cu concentration was 50 nM and calcein concentrations were 20 and 50 nM. Experimental Cu/HA ratios were between 9 and 20 pmol of Cu/g of C or about 5-10% loading of the site. Their results indicated that for ratios below -8 pmol of Cu/g of C, that is, for conditions of
