Environ. Sci. Techno/. 1993, 27, 915-922
pH-Dependent Binding of Aluminum by a Fulvic Acid 6. A. Browne'
School of the Environment, Duke University, Durham, North Carolina 27706 C. T. Drlscoll
Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York 13244 In this study, we use the Suwannee fulvic acid to investigate the chemistry of organic aluminum complexes in natural waters. We show that formation of organically bound A1 (AlORG) is systematically pH-dependent and can be modeled using a macroscopic binding parameter, Korg, comprised of three concentration quantities that can be measured in natural water (AlORG, A13+,and dissolved organic carbon). We demonstrate that AlORG behaves effectively as a polyprotic acid, and we provide an analysis framework from which the H+-dissociation constants of AlORG can readily be determined. A diprotic model of AlORG acidity is presented for the Suwannee fulvic acid (pK,, = 4.23, pK,, = 4.43). Our results suggest that aluminum organic complexes may contribute significantly to the acid-base chemistry of natural waters. We also show that fulvic acid combines with and/or facilitates the formation of polynuclear hydroxy-A1 ions, depending on the pH and total concentration of Al.
Introduction Organically complexed A1 (AlORG) is thought to contribute significantly to the acid and base neutralizing capacity of acidic soils and natural waters. Potentiometric titrations of AlORG (1, 2 ) suggest that H+ and OHsorption/exchange may account for a significant component of pH buffering in acidic soils and their drainage waters. However,the featureless titration curves obtained for natural organic matter in the presence and absence of A1 have precluded development of pH-dependent A1 binding models from potentiometric data. Speciation measurements of AlORG have produced conflicting data on A1 binding and A1 acidity in current studies. For example, Pott et al. (3)reported littlevariation in the conditional stability of Al-humate complexes between pH 3 and 5 using a column cation-exchange method to distinguish free and humate-bound Al. In contrast, Backes and Tipping (4) reported enhanced formation of AlORG with increasing pH using a ferronreactive speciation method and were able to fit their data to both a linear logarithmic expression and an empirical polyelectrolyte-type model. Such discrepancies may originate from the differences in operational definitions of AlORG and the use of A1 speciation techniques which have not been calibrated and verified using model ligands. An additional factor contributing to the discrepancies between current studies may be differences in conceptual models of Al-fulvate binding (i.e., reaction models). In this study, we apply the morin fluorescence technique ( 5 , 6 )to characterize the pH-dependent binding of A1 to Suwannee fulvic acid. We examine the theoretical basis for the pH-dependent binding of A1 by dissolved organic matter, develop a pH-dependent binding model, and apply this model to compare and contrast the effective acidity 0013-936X/93/0927-0915$04.00/0
0 1993 Amerlcan Chemical Society
Table I. -Characteristics of Suwannee River Fulvic Acida functional group content moles of sites/mol of C carboxylic 0.126 phenolic 0.056 alcohols 0.028 ketone 0.056 elemental composition % carbon 53.75 hydrogen 4.29 oxygen 40.48 nitrogen 0.68 sulfur 0.50 0.01 phosphorous total 99.71 % ash 0.82
-
Personal communication, Jerry Leenheer, USGS, Denver, CO. Number of average molecular weight: 800 gimol.
of fulvate-bound A1 to that of inorganic mononuclear Al. To facilitate the application of our results to acidic natural waters, the model employs a theoretically based macroscopic binding parameter which is comprised of directly measurable quantities (AlORG, AP+, and dissolved organic carbon, DOC).
Materials and Methods Reagents. Analytical-grade reagents were used in all experiments. Distilled-deionized H20 was used in the preparation of all stock and working solutions. Aluminum standards were prepared from Al(N03)3*9H20crystals. Working morin solutions (1pM)were prepared from fresh morin reagent (100 pM)as described in ref 6. Quinine sulfate fluorescence standards (1 and 10 mg/L) were prepared in 0.01 M H2S04 and stored in the dark. Fulvic Acid. Suwannee fulvic acid (SFA)was obtained from the International Humic Substance Society. Elemental analysis and other characteristics of the SFA are given in Table I. The SFA was dissolved in distilleddeionized water to produce a 1000 mg/L stock solution. This stock solution was#storedin a Teflon bottle at 4 "C. A1 Speciation Analysis. We applied a fluorescence probe technique which has been used successfully to characterize binding between A1 and various ligands including silicate (7),fluoride, sulfate, ethylenediaminetetraacetic acid (EDTA) and others (46). Aluminum and SFA were combined in acidic solutions of the aqueous fluorescent reagent morin (1.00 pM total morin). Concentrations of free A1 (A1kee)and fulvate-bound A1 (AlORG) were measured by comparing the fluorescence signal to solutions lacking SFA. The concentration of A13+ (with an estimated coefficient of variation of 110%, 6) was calculated from Aheeusing thermodynamic data for the mononuclear Al-hydroxy (Al-OH), A1 acetate (AlOAcZ+), and Al-morin (Al-MOR) complexes (46). Formation of A10Ac2+was maintained below 20% during the binding Envlron. Scl. Technol., Vol. 27, No. 5, 1993 915
experiments by employing a low concentration of acetate (0.001M CH,COO-) to buffer pH (6). This step was taken to minimizethe influence of uncertainty in thermodynamic data for A10Ac2+on calculation of AV+. Measurements of fluorescent intensity (If) were obtained with a Turner Model 10-005 or Turner Model 111 filter fluorometer using a quartz bulb as the radiation source. The excitation and emission filters were Corning color specifications 5-60 and 16, respectively. Instrument response was set relative to the Ifvalues of quinine sulfate standards and periodically checked for drift. Stable response was obtained throughout the experiments (drift was less than 1% of full-scaleper hour for the most sensitive instrument setting employed in the experiments). Total fluorescence was corrected for the fluorescence contributed by the fulvic acid without morin to obtain the fluorescence attributable to A1-MOR alone. Absorbance readings for the fulvic acid without morin (in the presence and absence of Al) were less than 0.05 unit at the excitation maximum (420 nm). Under this condition, the excitation radiation received by A1-MOR was largely unaffected by the presence of the fulvic acid, and additivity of the fulvic acid fluorescence and A1-MOR fluorescence could be assumed (6). After correction for the fluorescence signal of fulvic acid, the excitation and emission spectra of AlMOR did not reveal qualitative evidence of interactions between morin and the fulvic acid. A1 and Fulvic Acid Titrations. Static and continuous titrations were performed at constant pH (f0.02 pH unit), SFA or Alconcentration, and ionic strength. Acetate (Na+/ H+) buffers were used to control pH. (As necessary, fine pH adjustments to offset the acidifying effect of A1 were made with 0.1 M NaZC03.) Solution temperatures were maintained at 25 f 0.1 "C throughout the experiments using a thermostated water bath. In the static titrations, the SFA or A1 was added at different concentrations in individual 20-mL solutions. Aluminum speciation was measured 1-2-h after solution preparation. In the continuous titrations, A1 was added incrementally to each individual 50-mL SFA aliquot. Speciation was measured 0.75-h after each incremental addition of Al. All A1-SFA solutions were prepared and maintained in Teflon bottles.
Calculations. The formation of Al-fulvate complexes was examined using a macroscopic binding parameter comprised of measurable terms
Korg= [A1ORGI/[Al3+1[DOC] (1) where [DOC], the concentration of dissolved organic carbon, is used as a surrogate for the total concentration of fulvate-A1 complexing sites L, = [DOC16 (2) The constant 6 expresses the moles of fulvate-A1 complexing sites per mole of carbon. (In natural waters, fulvic acid may represent only a fraction of the total DOC, and 6 is best defined as the moles of all organic-A1complexing sites per mole of carbon.) At constant pH, K,,,is analogous to the Scatchard parameter (8)for metal-ligand binding and can be interpreted using the conventions of discrete ligand and continuous distribution theories presented in the literature (9, IO; see Results and Discussion). 916
Environ. Sci. Technol., Vol. 27, No. 5, 1993
25
20
3
5 15 0
e
-c
6
10
5
0 0
5
10
15
20
25
AI, ( P M ) Figure 1. Measured [Aie,,] as a function of [Ai,] for 10 mglL [SFA]. (Ionic strength = 0.01 M, [CH3COO-] = 0.001 M, [morin] = 1 X M). The solid line represents the case of no binding with SFA.
Results and Discussion Formation of A1-SFA Complexes. The influence of 10 mg/L of SFA on the concentration of free A1 ([Alf,,,l = Alt - [AlORGI) at five pH values between 3.6 and 5.5 is shown in Figure 1. The 1:l line represents the case of no binding with SFA (i.e., [AlORGI = 0). Measured values of [Alae,l were below the 1:l line over the range of experimental Alt values, indicating appreciable formation of AlORG. Systematically lower values of [AlfrJ occurred as the pH value of the A1 titration increased between 3.6 and 5.5. From these data, we were able to calculate the amount of A1 bound with SFA in each solution (Figure 2). The ~ ) highest at low values fraction of Alt as AlORG ( a ~ 1 - s ~was of Alt. This result can be attributed to two factors: (1) low binding site coverage (i.e., a remaining high concentration of free A1 binding sites to drive the reaction toward A1-SFA formation) and (2) the influence of a small population of binding sites with high affinity for Al. As Alt was increased to approximately 5 pM, a steep decline in (YAl-SFA was evident due to complexation of A1 to the binding sites with high affinity for Al. Above concentrations of approximately 5 pM Alt, the slopes of the plots of aA1-SFA as a function of Alt shifted noticeably from steep to shallow. The section of the titration curve with shallow negative slopes can be attributed to complexation of A1 to alarger pool of binding sites with lower affinity. [At higher concentrations of A1 (>5 pM), binding site coverage did not increase markedly with additions of Al.] These trends are reflected in plots of Korgas a function of [Al3+1(Figure 3).
Evidence of Polynuclear AI-SFA Complexes. A positive slope was evident in the plots of ffA1-SFA as a function of Alt (Figure 2) in the upper range of Alt concentration. This pattern emerged above approximately 15 pM Alt at pH 3.6, pH 4.0, and pH 4.5 but was most prominent at pH 5.0 and pH 5.5 at concentrations above 5 pM. To illustrate this trend more clearly, the pH 5 and pH 5.5 data were replotted with an expanded ordinate scale using [AlORG] as the abscissa (Figure 2, panels B
pH pH V pH pH 0 DH 0
5.5 5.0 4.5 4.0
3.6 1 .oo
1 .o n
2
?-
0.8
aa
v
-x 0 . 6 -+
Lc
0
c
.-0
0.4
-e
0
2
IL
0.2
0.0 0
5
10
15
AI, ( P W
20
25
o'800
5
io
15
20
25
A l O R G (pM)
M). (Panel A) Flgure 2. Fractlon of [Al,]Al-sFnbound with 10 mg/L SFA (ionic strength = 0.01 M, [CH3COO-] = 0.001 M, [morin] = 1 X Ail data as a function of [AI,]. (Panels B and C) Highlight of polynucleation evidence. Expanded ordinate scales for pH 5.0 and pH 5.5 data, respectively, recast as a function of AIORG.
and C). Enhanced formation of AlORG with increasing Alt was particularly evident at pH 5.5 and noticeable at pH 5. This result casts light on stoichiometric relations between A1 and SFA. The transition from negative slope to positive slope in the A1 titrations signals a shift in the stoichiometric coefficient of A1 from p = 1to p > 1. At least two explanations can be offered for the apparent shift in reaction stoichiometry. The first and most obvious explanation involves a polynuclear A1-SFA complex. For example, assuming a maximum stoichiometric coefficient for A1 of p = 2, two sets of reactions can describe the equilibrium status of the solutions, allowing for the formation of dihydroxy bridges between A1 ions: Ai3++ SFA = A1-SFA AI-SFA + Al(OH),+ = Al,(OH),-SFA
(3)
(4) where charge and proton displacement reactions have been omitted for simplicity. Inclusion of the mononuclear dihydroxy-A1affords the mechanism for hydroxy bridge formation, resulting in chainlike structures with at least two A1 per chain. This mechanism is consistent with the observed pH dependence of the polynuclear stoichiometry. The second mechanism of polynucleation is an extension of the first. However, the fulvic acid is now visualized as a template for the formation of polynuclear hydroxy A1 ions:
Al,OH,-SFA = Al,(OH);+ + SFA (5) On this basis, the polynuclear forms responsible for the increase in slope evident in A1 titrations (Figure 2, panels B and C)would be a mixture of polynuclear hydroxy-A1 ions and polynuclear hydroxy-A1 fulvates. Based on the
apparent absence of polynuclear forms in our experimental solutions without SFA, this pattern suggests that fulvic acid may facilitate the formation of polynuclear hydroxyA1 ions by overcoming the activation energy necessary for hydroxy-bridge bonding between A1 ions: pA13++ qH,O = A1,(OH)y-q'
+ qH+
(6)
(Implicit here is the assumption that the experimental solutions without SFA were metastable with respect to polynuclear hydroxy-A1ions.) An important implication of this model is that current estimates of the stability of polynuclear hydroxy-A1 ions may be low by several orders of magnitude. [We have noted similar polynucleation reactions in the presence of phosphate (11). In these experiments, A1 was in stoichiometric excew to phosphate such that the phosphate complexes alone could not account for the majority (>90 74 of polynuclear AI in the solutions. Preliminary estimates of the equilibrium constant for the dinuclear hydroxy-Al ion based on this data suggest this species is 3-4 orders of magnitude more stable than previously reported.] Studies of the role of AI in the coagulation of fulvic acids suggest that Al-fulvate interactions promote polynucleation reactions leading to Al(OH)s(s) formation (12). Continuous Distribution Model of AI-SFA Binding. A conditional average stability constant (K') for the binding of A1 by SFA at constant pH can be defined in a form similar to that used by Perdue and Lytle (9) to describe metal binding by a hypothetical complex ligand mixture:
K' = e/ (1- 0) [Al3+1
(7)
Environ. Sci. Technol., Vol. 27, No. 5, 1993 917
pH 3 . 6
1
0.4
0.0
11 .o .5
v-
I
I
I
t
0.5
0.0 0
3
6
9
I
I
I
4 3
d-
0
1
p H 4.0
3
B
12
pH 4.5
-I
1
Table 11. Parameters of Gaussian log Ki Distributions Shown in Figure 4 PH
mean
SD
3.6 4.0 4.5 5.0 5.5
5.368 6.015 6.394 7.011 7.583
1.072 1.137 0.877 0.649 0.458
declines. This trend is the motivation for continuous distribution models that essentially attempt to capture the moments (e.g., mean and standard deviation) of mole fraction-weighted estimates of Ki values, harvested from successive positions along Scatchard-type curves eq 10. At each experimental pH, a set of Gaussian distribution curves (Figure 4) was generated from Korgdata using the numericalapproach described by Stevenson and Chen (13). A set of successive values of log K‘ were generated from the instantaneous slope values at successivepositions along plots of Korgas a function of [A1ORGl/[DOCl (eq 10). These data were then fit to polynomials of the form log K’ = a + bx + c x 2 + dx3 + ... where x = [AlORGl/[DOC16 and 6 = 0.182. (The value of 6 is the sum of phenolic and carboxylic groups in the SFA reported in Table I. In natural waters, values of 6 may differ significantly depending on the source and composition of DOC.) The polynomials were used to generate a series of log Ki values at equal increments of x . These data were then fit to the Gaussian distribution function:
&
24 “0.0
16
1.5
4.5
3.0
k‘
p H 5.0
i
8
0 ’ 0.0
I
I
I
0.3
0.6
120
6o
lLooo@o
0.00
I
I
0.9
I
4
1.2
I
pH 5.5
ai = 6 ( l / u ( 2 ~ ) 0 . 5 ) e - 0 . 5 ( ( ~ - l o g ( K , ) / ~ )d2 log(Ki)
0 0.12
Flgure 3. Korg(eq 2) as a function of [AI3+] and the fit of the KO,*data to the two-site model (eq 29). The symbols represent measuredvalues. The solld line represents the best-fit line generated by a nonlinear Marquardt regression algorithm.
where 6 = [sites boundl/[l,l (8) (To estimate the concentration of sites bound, we used the measured values of [AlORG] under the assumption that bound A1 is mononuclear.) In terms of [DOC] and [AIORG], eq 7 becomes
K’ = [A10RG]/[A13+l([DOC16- [AlORG]) (9) Rearranging eq 9, we obtain a Scatchard-type (8)equation which relates Korgto K’: Korg= K’6 - K’( [AlORG]/[DOC]) (10) In its application to fulvic/humic acids, eq 10 is curvilinear because K’ is an implicit function rather than an explicit function of [AlORGI/ [DOC] (i-e.,binding site coverage). A number of explanations can be offered for the implicit character of eq 10 and similar expressions used by other workers (13),including electrostatic effects and formation of polynuclear A1 complexes with SFA. Foremost, however, is the concept that fulvic acid is an assemblage of binding sites of varying affinity for A1 (9, IO). As the most prominent ligands (where prominence is defined as the product of an A1 binding site’s conditional stability constant, Ki, and mole fraction &/a) become occupied as [AlORG]/ [DOC] increases, the value of K’ necessarily 918
Environ. Sci. Technol., Vol. 27, No. 5, 1993
(11)
where 6i is the moles of binding sites per mole of carbon in the interval d log(Ki), and u is the standard deviation of the distribution of log(Ki) values about the mean (p). (Note that the distribution described by eq 11 can be generated in the absence of a value for 6. This can be accomplished by (1)generating log K’ values using the ratio [AlORGI/[DOCl in place of x = [AlORGI/[DOC16 and (2) dividing both sides of eq 11 by 6. This approach is particularly useful for natural water samples where 6 is difficult to define and values are frequently unavailable.) The Gaussian distribution curve of log Ki values for each pH is shown in Figure 4, and the parameters of the distributions are reported in Table 11. The affinities of SFA complexing sites for A1 range nearly 4 orders of magnitude at pH 4 and more than 2 orders of magnitude at pH 5.5. Note that the variance of the log Ki distribution curves declines with increasing pH. This pattern may reflect more uniform binding characteristics at higher pH due to the deprotonation of a majority of the binding sites, and/or an increasing influence of hydroxy-A1 binding as pH increases (i.e., binding becomes controlled more by the characteristics of A1 and less controlled by the variance of the characteristics of SFA binding sites). Most notable about the Gaussian distributions is the systematic pH trend. The trend is evident across the central tendencies, as well as the tails of the distributions (Figure 4). It is perhaps significant that the pH range spanned by the lower tails of the distributions (3 orders of magnitude) is significantly larger than the range of the upper tails of the distribution ( (H+}will be true for a large portion of q = 1binding sites (eq 19); for these sites, the value of n will be 2. The statement Kail < (H+}will be true for a second large portion of the q = 1 binding sites; for these sites, the value of n will be 3. Similar assignments can be made for binding sites with q = 2 (eq 20). Possible values of n will be 4,3, and 2, respectively, depending on which of the following conditions apply for a specific site: {H+}2 >> (Kial(H+}+ KialKiad, Kial(H+}>> ((H+}2+ KialKiad, or KialKia2>> ((H+j2+ Kial(H+}).Thus, after grouping terms of like-degree and summing over all sites, we obtain an approximate expression for Korgthat is functionally equivalent to a polynomial of the fourth degree: l d
...I
I
Dividing both sides of eq 14 by [Al3+1and [DOC] gives a pH-dependent expression for the conditional stability of AI-Li I
Korg(i) = r K i p r ( S -i [A1-Lil/[DOCl)ai,(H’}-q(H’)-r (15) 0
where the relation to Korgis given by the summation over 1:
(16) To simplify the discussion for the general pH-dependence of Korg,we will specify that the term (Si - [Al-Lil/[DOC]) is invariant over the pH range of interest. (This case is approached when the fulvic acid concentration is substantially in excess of A1 and nearly 100% of A1 is bound.) Hence, we have R
where K’igr= Ki,,(Si - [Al-Li]/[DOC]). This expression can take on three forms depending on the number of protons (q = 0, 1, or 2) that are displaced from a particular complexing site by Al. For q = 0, aiq= 1and eq 17 become a simple second order polynomial of (H+).
Korg(i) =
+ K’iol/(H+}+ K’i02/(H’}2
(18)
Slightly more complicated expressions arise when q = 1 and q = 2.
-
Korg(i)- ((H’}
+ KiJ1Wil0 + K’ill/(H+)+ K’i12/IH+)2) (19)
Korg(i) = ((H’)’ + KiaI(H+)+ KialKiaJ1(K’i2o + K’i21/(Hf)+ K’i22/(H+)2)(20) The leading parenthetical phrases on the right-hand side of eqs 19 and 20 represent the form of aiq(H+)-qfor monoprotic (q = 1)and diprotic (q = 2) A1 binding sites, respectively. For values of {H+Jmuch above or below the values of Kial and Kiaz, eqs 19 and 20 become polynomials of (H+) that can be represented by a general function of the form: 920
C,Z+
Environ. Scl. Technol., Vol. 27, No. 5, 1993
c ~ ( H + J(22) -~ where the constants, CO, ..., c4, represent summations of the coefficients of like-degree terms. It is apparent, therefore, that Korgmust be at least a second degree polynomial and that a number of sites may also contribute third degree terms. However, in our experimental solutions, fewer sites are likely to contribute fourth degree terms because the pH (3.6-5.5) likely exceeds the PKial of most sites. [Perdue and Rueter (14)estimated a mean pK, for carboxylic groups in a Satilla River fulvic acid of 3.7.1 The second-order dependence of KO%on (H+)-1 exhibited in Figure 5 is not surprising given these considerations. This pattern can be attributed to a mixture of proton displacement and A1 hydrolysis reactions in the experimental solutions. It is possible that a third and fourth order dependence of Korgon (H+}-lemerges at pH values above those employed in our experiments. Polyprotic Acid Model of Al-Fulvate Binding. Based on the second-order dependence of Korgon (H+}-’, an analogy can now be drawn between fulvate-bound A1 and a diprotic acid. The distribution of acid species between protonated and deprotonated forms for a diprotic acid is given by: A,/ [H,AI = 1+ Kal/(H+)+ Ka1Ka2/1H+}2 (23) where AT = [H2A] + HA- + A2- and Ka1 and Ka2 are the first and second proton dissociation constants. The diprotic form of Korg(WOrg) is obtained after dividing the second degree polynomial reported in Table I11 by the intercept value:
+
K’org= 1 5.94 X 10-5/{H)+ 2.37 X lO-’/{H+)’ (24)
where the coefficients of the first and second degree terms correspond now to estimates of Kal and Ka1Ma2. These data suggest that fulvate-bound A1 is effectively a stronger acid than that of inorganic mononuclear A1 (Table IV). pH-DependentMultisite Model of A1 Binding. The macroscopic binding parameter Korg,in the multisite form described by eq 15 and 16, can be reexpressed as function of [AP+]. The distribution of free and Al-bound forms of the ith complexing site is given by: [Li(T)l = [Li(nI + [Al-LiI
(25)
~
~~
Table IV. Acid Dissociation Reactions of Inorganic and Organic Forms of Mononuclear Ala reaction
PKa1
PKa2
+ Hz0 = Al(0H)" + H+ A10H2++ H20 = Al(OH)'+ + H+ 4.23 A10RG2+ H20 HOAIORGt + H+ HOAIORGt + HzO = (H0)2A10RGo+ Ht
5.13
4.99
AIS+
+
4.39
For illustrative purposes, the proton dissociation reactions of AlORG are depicted as hydrolysis reactions. Protons may also be generated by a displacement of H+ by A1 from SFA. The pKa values for inorganicA1 were obtained from ref 17. The pKavalues for organic A1 were estimated from eq 13 and data in Table 111.
Recast in terms of DOC we obtain [DOC]6, = ([DOC16,- [Al-LJ)
0 4.67 0 4.86
+
v 5.04
R
CKiQr[A13+] ([DOC]ai - [A1-Li])aiQ(H')-Q(H')-' (26)
A
5.22 5.33
0
Solving for ([DOC]6i - [AI-Lil) gives ([DOC]6, - [Al-L,]) = [DOC]6,/(1+ R
~KiQr[A13+]aiQ(H+)-Q{H+)'r) (27) 0
This expression can be substituted into eq 14 and then divided by [AI3+] and [DOC] to give a revised form of eq 15: R
Measured AI (pM) R
Figure 6. Measured AI concentrations and values predicted using the pHdependent two-site model (eq. 29). (10 mglL [SFA], [CH3COO-] [CH&OOH] = 0.01 M.)
+
Multisite models are usually simplified by assuming a small number of sites (9). The two-site form of eq 28 is, therefore, given by:
pH 4.87
Korg= K16,/(1 + Kl[Al3+1)+ KZ6,/(1+ Kz[Al3+1) (29) where the K1 and Kz are functions of (H+)similar in form to eq 17: R
K, =
Predicted
I
CK,~~~~~{H+)-Q{H+J-~ (30) j5
0
The solid lines in Figure 3 depict the best fit of the Korg data to a two-site model at each pH. (The tail of the pH 5.5 data was not included in the model fit because the behavior in this region of the plot significantly violates the implicit assumption of a simple 1:l Al-ligand binding in the Scatchard model.) Model runs were constrained to a maximum binding capacity, at, of 0.182 binding sites/ mol of C, given by the sum of carboxylic and phenolic group contents of SFA (Table I). In an initial set of model calculations, values of 61 and 62 did not vary greatly as a function of pH. In subsequent calculations, the model was constrained to the 61 and 62 values obtained from the fit at pH 4 in order to facilitate analysis of the influence of pH on the parameters K1 and Kz. In accordance with eq 30 and the discussion above, both K1 and KZbehaved as second degree polynomials similar to Korg (Table 111, Figure 5). We extended the two-site Korgmodel to a pH-dependent form by inserting the second degree polynomials reported in Table I11 in place of K1 and KZin eq 29. To verify the
0
0
5
10
15
I
I
20
25
30
AI, ( P W Figure 7. Measured AI concentration and values predicted by the pHdependent two-site model (eq 29). (pH 4.87, 10 mg/L [SFA], [CH&OO-] [CH&OOH] = 0.01 M.)
+
model, predictions were generated of the species distribution of A1 for a set of SFA solutions (independent of those used in calibrating the model). These solutions represented a range of pH (Figure 61, A1 (Figures 6 and 7), and SFA (Figure 8) concentrations. Best agreement was obtained at low A1 concentrations. The systematic divergence at high AI concentrations suggests that the KZ values are biased. Environ. Sci. Technol., Vol. 27, No. 5, l9Q3 921
- Predicted
6
0 0.0
0.5
1.0
1.5
2.0
2.5
DOC (mM) Flgure 8. Measured AI concentrations and values predicted by the pHdependent two-site model versus the concentration of SFA-C. (pH 3.96, ionic strength = 0.001 M, [AI,] = 7.35 X
Summary and Conclusions An assumption of 1:l binding between A1 and fulvic acid binding sites has been frequently employed to model and analyze Al-fulvate binding data. This assumption appears to be valid at low pH and low site coverage but may result in error in predictions of AlORG at higher pH and A1 concentrations that may be relevant to natural waters. Polynuclear A1 formed at pH 5.5 in the presence of SFA. Possible explanations include the formation of polynuclear Al-fulvate complexes and the formation of polynuclear hydroxy-A1ions. However, a distinction between the two mechanisms could not be made from the results presented in this study. The latter scenario suggests that fulvic acid may behave as a template for bridge bonding between hydroxy-A1ions. This observation suggests that equilibrium between polynuclear hydroxy-A1 ions and Al(OH)3(s) may be facilitated in natural waters by inorganic-A1 binding ligands. These reactions may be important in facilitating or directing the path of secondary mineral formation (7). The conditional average stability (K') of A1-SFA complexes, the macroscopic binding parameter Korg,and the stability constants (K1,KP) for individual sites in a twosite binding model increased systematically with pH. We attribute these patterns to two types of reactions: (1) the displacement of H+ from SFA complexing sites by Al, and (2) the release of H+ from water within the coordination sphere of Al. In agreement with this reaction model, the macroscopic binding parameter Korgand the stability constants for individual binding sites (K1,Kz, ...,K,) were shown to be second to fourth degree polynomials of (H+) on a theoretical basis. The polynomial form of Korgwas shown to be analogous to a polyprotic acid (H,A where n = 2, 3, or 4) when normalized to its zeroth degree term. On this basis, the effective acidity of SFA-bound Al, as measured by the
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Envlron. Scl. Technol., Vol. 27, No. 5. 1993
first and second proton dissociation constants of a diprotic form of Korg,is significantly greater than predicted by the hydrolysis constants of inorganic mononuclear Al. Similar pH-dependent behavior has been observed for organically bound A1 in natural waters (11). This type and strength of A1 acidity has not been accounted for in studies of A1 buffering in low ionic strength natural waters (e.g., ref 15). Such studies have consistently found a systematic divergence between calculated (base cations minus strong acid anions) and measured Gran acidity (16), which suggests the presence of an unidentified weak acid. We suggest that a significant component of the ANC and BNC of such waters may be accounted for by the weak acidity of organically complexed Al. Further study is needed to evaluate this hypothesis. The pH-dependent two-site model of A1-SFA binding successfully predicted the species distribution of A1 over a wide range of pH, SFA, and A1 concentrations. Extension of this model to natural waters (11) can be achieved by fitting measured values of Korg (based on [AlORGI, [Al3+1, and [DOC] measurements) over wide ranges of pH, [DOC], and [Al3+1values to eq 29.
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Received for review August 3, 1992. Revised manuscript received January 5, 1993. Accepted January 8, 1993.
