uranium(vi) complexation with citric, humic and fulvic acids

Radiochim. Acta88, 3452353 (2000) by Oldenbourg Wissenschaftsverlag, München

Uranium(VI) complexation with citric, humic and fulvic acids

By John J. Lenhart1,* , †, Stephen E. Cabaniss3, Patrick MacCarthy2 and Bruce D. Honeyman1

1 Colorado School of Mines, Division of Environmental Science and Engineering, Golden, CO 80401, USA2 Colorado School of Mines, Chemistry and Geochemistry Department, Golden, CO 80401, USA3 Kent State University, Chemistry Department, Kent, OH 44242, USA

(Received August19, 1999; accepted in final form February 25, 2000)

Uranium / Humic / Fulvic / Citrate / Binding /Discrete-ligand

Summary. The binding of uranium(VI) by Suwannee Riverhumic and fulvic acids was studied at pH values of 4.0 and5.0 in 0.10 M NaClO4 using an ion-exchange technique. Fewdata sets currently exist for metal binding to different molecu-lar weight fractions from the same source. The complexationof U(VI) by citric acid was also studied under the same exper-imental conditions in order to “calibrate” the experimental andmodeling approaches. For the citric acid system, the exper-imental results were analyzed using Schubert’s ion-exchangemethod, which indicated the formation of only a1 :1 uranyl-citrate complex. Close agreement was found for the values oflog β1,1 (6.6960.03 at I 5 0.10) determined from nonlinearregression of data collected at pH values of 4.0 and 5.0. Thisvalue represents a more direct measurement of the bindingconstant for the1 :1 uranyl-citrate complex than do other exist-ing literature values derived from experimental data requiringthe simultaneous consideration of1:1 and 2 :2 species.

Both humic and fulvic acids were demonstrated to stronglybind U(VI), with humic acid forming slightly stronger com-plexes and exhibiting greater pH dependence. Analyses of thedata for the humic and fulvic acid systems using the Schubert’sequation previously applied to the citrate system result in anapparent nonintegral number of ligands binding the uranyl ion.Schubert’s method is only appropriate for interpreting mono-nuclear complexes with integral moles of binding ligands.Thus, a more elaborate binding model was required and thedata were interpreted assuming either : (1) a mixture of1 :1and 1 :2 uranyl-ligand complexes or (2) a limited number ofhigh affinity sites forming a1:1 complex. While both of thesemodeling approaches are shown to provide excellent fits to thedata, the second is deemed more appropriate given the largesize of humic and fulvic acid molecules as well as previousresults obtained with other metal cations, such as Cu(II).

1. Introduction

In most aquatic systems, species of natural organic matter(NOM), such as humic and fulvic acids, constitute an im-portant pool of ligands for complexing metals. NOM is apolyfunctional, polyelectrolytic, heterogeneous amalgam oforganic molecules of varying molecular weight and size. Its

* Author for correspondence (E-mail: [email protected]).† Present address: Yale University, School of Forestry and En-

vironmental Studies, New Haven, CT 06511, USA.

physical and chemical properties can be a function of thenominal molecular weight (e.g., [1]); properties will alsovary from one source to the next [2]. Although the chemicaland physical properties of NOM have been extensivelystudied and its metal binding capability is undisputed [2],there still remain many questions regarding its role in metalbinding in heterogeneous systems.

With regard to interpreting metal ion complexation byNOM, there have been two broad issues that have beenexplored. The first consideration is the depiction of metal-NOM complexation in terms of the number of NOM sitetypes (e.g., strong versus weak). In studies of metal/NOMsystems, for which there was significant variation in theratio of metal ion to NOM concentration, multiple NOMsites have been posited to explain the results [3, 4]. Typi-cally, the concentration of the strongest binding sites is low,accounting for less than10 percent of the total carboxylacidity [3, 5].

The second consideration in data interpretation has beenestablishing the stoichiometry of the metal-ion/ligand ratio,where ligand refers to individual NOM functional groupsthat are sufficiently separated from one another and whichdo not interact. For example, model reactions using stoi-chiometries with greater than one ligand per metal ion maybe consistent with limited sets of experimental results ofCu(II) binding by NOM [6]; however, simulations of moreextensive sets of Cu(II) binding data have been ac-complished with model complexes that are strictly1 :1 [7].Although these results are specific to copper, they suggestthat NOM binding by other metal cations (e.g., U(VI))could be treated in a similar manner.

Existing in the literature are several experimental andmodeling studies that have examined U(VI) binding byNOM [8214]. In general, each of these studies [8214]concluded that NOM has a strong affinity for uranium(VI);the affinity was shown to vary as a function of the exper-imental method and the NOM source. In interpreting theirdata, most of these researchers employ a single site-typemodel for NOM, although a variety of U(VI)/ligand reac-tion stoichiometries have been postulated, including : (a)both 1 :1 and1 :2 uranyl-ligand complexes [10212], (b) a1 :1 complex between the uranyl ion and a doubly depro-tonated NOM site [8], or (c) a1 :1 complex between theuranyl ion and a singly deprotonated NOM site [12, 13].No explicit description of binding site types other than

J. J. Lenhart, S. E. Cabaniss, P. MacCarthyet al.346

“carboxylic” or “phenolic” was implied in these approach-es. Of those not using a single site model, Higgoet al. [13]used an empirical heterogeneous site binding model and Liet al. [9] used a Scatchard plot to describe their data.

Each of these approaches [8214] has been reasonablysuccessful in simulating the target system, making it diffi-cult to decide which modeling approach seems the mostreasonable and appropriate. A decision on a model’s appro-priateness hinges on both its ability to accurately describethe data and the number of simplifications required toachieve the fit.

In this paper, we present experimental results of ion-exchange experiments examining the binding of U(VI) withSuwannee river humic and fulvic acid. To test the exper-imental protocol we also examined the complexation ofU(VI) by citric acid over the same experimental conditions.Since the properties of NOM can vary as a function ofmolecular size, one goal was to evaluate U(VI) binding bya humic (HA) and fulvic acid (FA) isolated from the samesource. Second, based on those approaches historically used[8214], we evaluate simple binding models for describingthese interactions. Three different discrete-ligand modelsare presented and used to fit the data:1. a model positinga single1 :n U(VI)-NOM complex (where n refers to thenumber of complexing ligands) with a site concentrationequal to the literature-reported carboxyl acidity of HA orFA (Schubert’s method); 2. a model positing both1 : 1 and1 : 2 U(VI)-NOM complexes with a site concentration equalto the reported carboxyl acidity of HA and FA; 3. a modelpositing a single1 : 1 U(VI)-NOM complex with a site con-centration which is a fraction of the reported carboxylacidity. Each approach used two model constants and thesame dependent and independent variables, thereby al-lowing us to compare model fits and recommend the mostplausible and reasonable modeling approach.

2. Experimental

2.1 Materials

Unless stated otherwise, all chemicals used were reagentgrade. Water for all experiments was supplied from aBarnstead/Nanopure (Easy-Pure UV) ultra-low carbonwater system (UV-water). All labware was cleaned byscrubbing with detergent (Micro, Cole-Parmer InstrumentCompany) followed by sequential base (1% NaOH) andacid (5210% HCl) baths, after which the labware was rins-ed at least 3 times with UV-water and allowed to dry.

Suwannee River humic acid (HA) and fulvic acid (FA)were purchased from the International Humic SubstancesSociety (IHSS) and were used without further treatment.The Suwannee River HA and FA were selected to representtwo chemically different components of NOM from thesame source. Based on the observed differences in the num-ber average molecular weight (1100 daltons and 800 dal-tons for HA and FA, respectively [15]) and percent aro-matic carbon (42% for HA vs. 28% for FA [15]), HA isconsidered to be more hydrophobic than FA. All HA andFA solutions were stored in amber glass bottles in the darkat 4.060.1°C. For experiments with citric acid, a0.10460.001 M stock solution was prepared by dissolving

reagent grade citric acid, HOC(CH2CO2H)2CO2H, in UV-water. This solution was standardized potentiometricallyagainst CO2-free NaOH of known concentration and storedin an amber glass bottle in the dark at 4.060.1°C.

Stock solutions of uranium as U(VI) were preparedusing a uranyl nitrate (UO2(NO3)2) ICP standard purchasedfrom Anderson Laboratories, Inc. To facilitate analysis byliquid scintillation counting on a Packard 2500TR usingUltima Gold scintillation cocktail (Packard Instrument Co.,Inc.), samples were spiked with233U purchased from Iso-tope Products Laboratory. Chemical and physical quench-ing of the measuredA-particles from the decay of233U wasnegligible for those systems studied. These solutions wereprepared in dilute HNO3 and were added as needed to eachbatch system to give the required total uranium concen-tration of 1.0 µM. The relative standard deviation of repli-cate uranium measurements in the absence or presence ofNOM was62%.

Dowex 50WX8 cation exchange resin was used for allexperiments and was cleaned and prepared in the sodiumform [16]. After preparation, the resin was stored in anairtight polyethylene bottle to prohibit dehydration whichcould lead to deviations in the resin mass. The exchangecapacity of the resin employed in this study was measuredtitrimetrically to be 2.96 0.1meq/g.

2.2 Ion-exchange measurements

A series of batch ion-exchange experiments was performedto determine the uranium distribution coefficient,λo, in theabsence of the organic complexant (citric, humic or fulvicacid) and the distribution ratio,λ, in the presence of theorganic complexant. All experiments were conducted at apH of 4.0 or 5.0 and a total uranium concentration of1.0 µM. An ionic strength of 0.10 was maintained usingNaClO4 and pH was adjusted with dilute NaOH or HCl.The mass of resin in each sample was varied from 0.005 to0.066 0.001 g. The concentration of HA or FA was variedfrom 1 to 100 ppm. For the determination ofλo, both 5.0and10.0 ml samples were prepared. All other samples were10.0 ml. A sodium acetate/acetic acid buffer of 0.006 Mand 0.0033 M for pH 4.0 and 5.0 experiments, respectively,was used to maintain the pH at the desired value. Polycar-bonate Oakridge centrifuge tubes were used to hinder urani-um sorption to the container walls. Prior to use, the interiorsurfaces of the tubes were equilibrated with a dilute sodiumperchlorate solution (,0.1M), rinsed with UV-water andallowed to dry. Experiments were equilibrated in the darkat ambient pressure, atmospheric composition and tempera-ture (2162°C).

The equilibration time of 48 hours for all ion-exchangeexperiments was based on the results of time-study experi-ments. These experiments indicated that system equilibriumoccurred in a two-step process: a rapid initial process oc-curring within the first 324 hours, followed by a second,gradual process. Overall system equilibrium was reachedwithin 48 hours, with only slight changes (,2%) occurringover times up to168 hours. This 48-hour equilibration timewas used for all ion-exchange experiments, regardless ofthe absence or presence of a competing organic acid.

Uranium(VI) complexation with citric, humic and fulvic acids 347

The aqueous uranium concentration was determinedusing liquid scintillation counting of the233U tracer, and theuranium bound to the resin was calculated from the differ-ence between the U concentration in these samples and theU concentration in samples prepared without resin. Afterseparating the resin from the aqueous phase, the boundU(VI) was exchanged using1.0 N HCl and uranium recov-ery (as233U) averaged 96.561.6% for an arbitrary selec-tion of samples. Identical samples prepared without urani-um were also analyzed with each suite of experiments. Aseparate suite of experiments verified that interactions be-tween the resin and uranyl-acetate species were negligible(data not shown). Thus, over the experimental conditionsstudied, the binding of citric, fulvic, or humic acids and/ortheir complexes with U(VI) onto the resin were not expect-ed. The experimental standard deviation (σexp) was deter-mined by pooling the errors from replicate experiments[17]. These values are 0.018 and 0.021 µmol/l uranium atpH 4.0 and 0.018 and 0.014 µmol/l uranium at pH 5.0 forexperiments with FA and HA, respectively.

2.3 Schubert’s ion-exchange method

One method of determining an average stability constantfor a metal-ligand system uses a linearization technique toanalyze appropriately-measured ion-exchange data col-lected at a specific ionic strength and pH. This method wasdescribed by Schubert [18] and is commonly called Schu-bert’s method. While originally intended for systems con-taining known ligands (e.g., citrate), the approach has beenapplied with reasonable success to systems with NOM [13,19221]. The technique is based on measuring the distri-bution of total metal between the solution phase and a cat-ion exchange resin, in the presence and absence of a metal-complexing ligand. For proper application of the methodthe metal ion concentration must be negligible compared tothe concentration of the complexing ligand. In addition, theratio of free metal ion concentration to resin sites must bemaintained at those observed on the linear portion of themetal resin ion-exchange isotherm and neither the com-plexing ligand nor the metal-ligand complex should bind tothe resin.

In the presence of a ligand, L, such as citrate or thosefunctional groups present in humic or fulvic acids, the equi-librium expression for m moles of uranium (as uranyl,UO2

21) reacting with i moles of ligand is:

m UO221 1 i L 5 (UO2)m(L)i (1)

with a conditional stability constant,βm,i:

βm,i 5[(UO2)m(L)i]

[L] i[UO221]m

(2)

Due to the relatively complicated solution chemistry ofuranium, the Schubert’s approach employed herein is modi-fied to account for complexes with hydroxide, carbonateand acetate, after Maeset al.[20]. The termΠ is introducedto account for the concentrations of all dissolved uraniumspecies with the exception of those with the target ligand(e.g., citrate).

Π 5 11 βUO2OH[OH2] 1 . . . (3)

To calculateΠ thermodynamic constants and reaction stoi-

Table 1. Equilibrium constants for citric acid (H3Cit), acetic acid(HAc), uranium solution hydrolysis and complexation reactions (fromMartell et al. [22] unless noted otherwise).

Reaction logK (I50) logK (I50.10)

UO221 1 H2O 5 UO2OH1 1 H1 25.2 [31] 25.4a

UO221 1 2 H2O 5 UO2(OH)2 1 2 H1 211.5 [32] 211.7a

UO221 1 H2CO3 5 UO2CO3 1 2 H1 26.9 [31] 27.11a

HAc 5 H1 1 Ac2 24.56Ac2 1 UO2

21 5 UO2Ac1 2.612 Ac2 1 UO2

21 5 UO2Ac2 4.93 Ac2 1 UO2

21 5 UO2Ac32 6.3

H3Cit 5 H2Cit2 1 H1 23.13 22.81 [33]H2Cit2 5 HCit22 1 H1 24.76 24.36 [33]HCit22 5 Cit32 1 H1 26.4 25.7 [33]

a: Calculated from I5 0 value using the Davies equation.

chiometry for complexes that are important at pH 4.0 and5.0 are listed in Table1.

To implement Schubert’s method, ion-exchange data iscollected to determineλo, which is the resin/solution distri-bution coefficient for U(VI) in theabsenceof the organiccomplexant, L.

λo 5[U] resin

[UO221]{ 1 1 βUO2OH[OH2] 1 . . .}

5[U] resin

[UO221]Π

(4)

[U] resin has units of moles U(VI) per gram resin and thesolution U(VI) concentration ([UO221]Π) has units of mo-les U(VI) per liter of solution. Thus,λo has units of l/g.Over a limited range of metal/resin ratios,λo is constant invalue. Note thatΠ is constant for a given set of solutionconditions (pH, ionic strength, [Ac2] and PCO2) and is inde-pendent of total uranium concentration, U(VI)T. Values ofΠ at pH values of 4.0 and 5.0 are1.5660.05 and2.686 0.21, respectively.

A corresponding set of experiments is conducted to de-termine λ, the uranium resin/solution distribution ratio inthe presenceof the target ligand. Assuming that the ligand,L, forms a series of complexes, UO2Li, that are mononucle-ar with respect to uranium (m5 1), λ is defined as follows(for i 5 1 to n) :

λ 5[U] resin

[U] sol

(5)5[U] resin

[UO221]{ Π 1 β1,1[L] 1 1 β1,2[L] 2 1 . . . β1,n[L] n}

where [U]sol is the sum of all dissolved uranium species insolution, including those with the target ligand. CombiningEqs. (4) and (5) gives

Sλo

λ21D 5

β1,1[L] 1 β1,2[L] 2 1 β1,3[L] 3 1. . .1 β1,n[L] n

Π(6)

In the case where only one metal-ligand complex of1 : nstoichiometry is formed, Eq. (6) simplifies to

Sλo

λ21D 5

β1,n[L] n

Π(7)

Log β1,n can be determined from a plot of log {(λo/λ)21}


Fig. 1. Ion-exchange isotherms at pH5 4.0 or 5.0, I 5 0.10 andU(VI)T 5 1.0 µM. Data point at zero represents experimental blank.

versus log [L], which is commonly called a Schubert’s plot.The conditions necessary for the application of Schubert’smethod include a large excess of complexing ligand. Thus,in all equations the uncomplexed ligand concentration, [L],is essentially equal to the total ligand concentration [L]T. Inthis example, the slope, n, of the plot for a1 :1 complexshould be unity.

3. Results and discussion

3.1 Determining λo from an ion-exchange isotherm

Experimental results of ion-exchange experiments con-ducted to determineλo at an ionic strength of 0.10, 1.0 µMU(VI)T and pH values of 4.0 and 5.0 are plotted in Fig.1.In this example, the isotherms for the data at pH 4.0 and5.0 are highly linear, with R2 values of 0.990 and 0.955,respectively. The linearity of an isotherm is a function ofthe fraction of resin sites occupied by the metal: deviationsfrom linearity are commonly observed as a larger fractionof the resin sites is bound by metal ions. In these experi-ments, the maximum percentage of resin sites occupied byuranium was calculated to be less than 0.2%. This value isbased on the measured resin exchange capacity of2.9 mmol/g and carries the assumption of an exchange oftwo sodium ions for each uranyl ion.

The slopes of the regression lines in Fig.1 yield λo, asdefined by Eq. (4), which is equal in value to1.416 0.03and 0.8560.05 l/g at pH 4.0 and 5.0, respectively. Thevalue forλo decreases as the pH increases from 4.0 to 5.0due to increased competition with the exchange sites forthe uranyl ions by increasing concentrations of carbonate,hydroxide and acetate. It should be noted that the concen-tration of the acetate ion increased even though the totalbuffer concentration (HAc1 NaAc) for the pH 5.0 experi-ments is 0.0033 M versus 0.006 M for the experiments atpH 4. The intercepts of the isotherms are 0.016 0.01 and0.060.03µmol/g, which are indistinguishable from zero.

3.2 Uranium complexation by citric acid

Prior to studying uranium complexation with humic orfulvic acids, experiments were performed to test the exper-

Fig. 2.Schubert plots for the exchange of uranium (1.0 µM) with resinat I 5 0.10 and pH5 4.0 or 5.0 in the presence of citric acid.

imental protocol using citric acid. The resulting bindingconstant from these experiments was compared to valuesobtained from the literature based on the following reac-tion :

UO221 1 Cit32 5 UO2Cit2 β1,1 5

[UO2Cit2]

[Cit32][UO221]

(8)

Samples with 0.0406 0.001 g of ion-exchange resin,0.006 M total acetate buffer, 0.60 to 0.002 mM citric acidand1.0 µM total U(VI) were prepared as before at an ionicstrength of 0.10 and a pH of 4.0. Identical samples wereprepared at pH 5.0 with 0.04560.001 g of resin and a totalbuffer concentration of 0.0033 M. The results are plotted inSchubert’s form as log {(λo/λ)21} versus log citrate (Cit32)concentration (mol/l) in Fig. 2. The citrate concentrationwas calculated based on total citric acid added using theconditional acidity constants at I5 0.10 from Table1.Based on the slopes of the regression lines, 0.996 0.05 and0.986 0.03, the predominant complex is concluded to be a1 :1 uranyl-citrate complex. The intercepts of the lines givevalues for log (β1,1/Π) as 6.416 0.27 and 6.136 0.2 at pH4.0 and 5.0, respectively.

To improve the precision of logβ1,1, the data were reana-lyzed per Eq. (7) with the assumption of an exact1 : 1 com-plex; nonlinear regression was performed using the soft-ware Scientist (Micromath, Inc). Setting n in Eq. (7) to1.0results in slight changes in the values of log (β1,1/Π) from6.41 to 6.50, at pH 4, and 6.13 to 6.27, for pH 5. However,the analyses resulted in marked improvements in the errorof log (β1,1/Π) from 60.27 to60.02 and6 0.20 to6 0.01at pH 4.0 and 5.0, respectively. Using the thermodynamicdata in Table1, Π was calculated to be1.5660.05 at pH4.0 and 2.686 0.21 at pH 5.0. These values are based onthe total concentration of acetate buffer (NaAc1 HAc)of 0.006 and 0.0033 for experiments at pH 4.0 and 5.0,respectively. Accounting for these values, logβ1,1 is6.696 0.03 at pH 4.0 and 6.7060.03 at pH 5.0 for I50.10. Thus, excellent agreement is found between the twodata sets further reinforcing the utility of the experimentalmethod.

The available literature values for uranium-citrate com-plexation, as compiled by Martellet al. [22], were obtained


using potentiometric titrations conducted with rather hightotal uranium and citrate concentrations (U(VI)T and ci-trateT 5 17 mM to 1.7 mM) [23]. For these conditions, thevalues for logβ1,1 were determined to be 7.46 0.2 and6.960.1 at ionic strengths of 0.10 and 1.0, respectively.Because of the high metal and ligand concentrations, theformation of a 2 :2 uranyl-citrate species was also evaluated[23]. In a similar study, Markovitset al. [24] analyzed ura-nium binding by citrate and recalculated a value for logβ1,1of 5.7860.18 at an ionic strength of1.0. This logβ1,1 valueis a full log unit lower than that value reported by Rajanand Martell [23] for the same ionic strength. Markovitsetal. [24] attributed this discrepancy to uncertainties of “atleast an order of magnitude” in the slope-intercept methodthat Rajan and Martell [23] applied.

Assuming that the one log unit difference in logβ1,1presented by Rajan and Martell [23] and Markovitset al.[24] at I 5 1.0 translates to a similar difference in the dataat I 5 0.1, we estimate that the Markovitset al. approachwould give logβ1,1 of 6.36 0.4. This value is only slightlylower than our values at the same ionic strength and, aspreviously noted, is an order of magnitude lower than theRajan and Martell [23] value (7.460.2). The methods ofRajan and Martell [23] and Markovitset al. [24] requiredthe simultaneous determination of stability constants for1 : 1 and 2 :2 species. While the determination of stabilityconstants for mononuclear species can be routine, the deter-mination of stability constants for polynuclear species canbe quite difficult, even in the most rigorously studied anddefined systems [25]. Thus, we attribute the lack of strictagreement between our values and either of the literaturevalues [23, 24] to our ability to directly evaluate the con-stant for the1 : 1 species, thereby avoiding complicationsassociated with determining constants for the 2 :2 species.

3.3 Analysis of uranium binding by fulvic acid (FA)using Schubert’s method

Using the same experimental conditions as employed withcitric acid as the target ligand, U(VI) binding with fulvicacid was examined at I5 0.10 and pH5 4.0 or 5.0. Basedon the increased precision seen with the nonlinear analysisof the citric acid data only nonlinear regression techniqueswere employed in the analyses of the FA. This allows adirect fit of the data and eliminates the need for extra datamanipulation. Fulvic acid concentrations were convertedfrom mg/l to eq/l using a FA carboxyl concentration of6.45 meq/g, which is the average of the reported values of6.1 and 6.8 meq/g [15]. It is recognized that other acidicfunctional groups may contribute to the overall FA and HAacidity; however, we assume that the carboxyl groups willdominate uranium binding at pH 4 to 5. A nonlinear simu-lation can be performed using the following Eq. (9), whichresults from Eq. (5) assuming the existence of only one1 :nuranyl-ligand complex:

Sλo [U] sol

[U] resin

21D 5β1,n [L] n

Π(9)

In this simulation, [U]sol is the dependent variable and[U] resin and the fulvic acid concentration [L] are the inde-pendent variables. Values for n and logβ1,n resulting from

Fig. 3. Measured concentration of uranium, [U]sol, versus (a) fulvicacid concentration and (b) humic acid concentration. Experimentaldata was collected in samples with1.0 µM uranium at I5 0.10 andpH 5 4.0 or 5.0. Lines represent best-fit model simulations using theindicated models summarized in Tables 2 and their respective parame-ter values from Tables 3 and 4.

this fit are1.28 and 5.37, respectively, with a coefficient ofdetermination (R2) of 0.965 and a fairly narrow relativeerror (RE) range of212% to 111%. A similar result fromsimulating data at pH 5.0 produces a value for n of1.14and logβ1,n of 5.13. Since the experimental conditions areidentical to those used to study the uranyl-citrate system,Π is 1.5660.05 and 2.686 0.21 at pH 4.0 and 5.0, respec-tively. Table 3 summarizes model parameters and statisticsfor the FA fits at pH 4.0 and 5.0. With an increase in pHthe best-fit value for n decreases from1.28 to1.14; conse-quently the best-fit value for logβ1,n changes from 5.37 to5.13. This decrease in logβ1,n at pH 5.0 correlates to thedecrease in n. For example, fixing n at1.14 and reanalyzingthe pH 4.0 data results in a logβ1,n of 4.83, which is 0.3log units less than the value at pH 5.0. Correspondingly, ifn is fixed at1.28 and the data at pH 5.0 are reanalyzed, logβ1,n equals 5.63, 0.26 log units higher than that at pH 4.0.Fig. 3a shows that, at equivalent FA concentrations, moreU(VI) will be in solution at pH 5.0 than at pH 4.0. Basedon the small increase in the value for logβ1,n with increas-ing pH, much of this result can be attributed to the increasein the concentration of aqueous U(VI)-acetate species, notnecessarily to greater binding of U(VI) by FA.


Unlike the Schubert analysis of the citrate system, whichhad values of n near unity, the value of n for the fulvic aciddata is significantly different from1.0 for both pH valuesstudied. Slopes, n, with other than integral values havecommonly been found when examining metal binding insystems with NOM [26], including U(VI) [13]. However,Clark and Turner [27] have indicated that unless n inEq. (7) is an integer, the stability constantβ1,n is not a validthermodynamic representation of the mononuclear reactionfrom Eq. (1). Thus, the1 :n approach is demonstrated tobe inappropriate for systems with non-integral slopes andadditional methods must be applied. Again, using those ap-proaches historically presented, we chose binding modelsthat either account for the ability of NOM to form com-plexes with mixed stoichiometries or its site-heterogeneity.

3.4 Model for uranium binding by fulvic acid (FA)assuming the formation of 1:1 and 1:2uranyl-ligand complexes

As previously stated, a number of studies have used amodeling approach that is based on the assumption of theexistence of a mixture of1 :1 and1 :2 uranyl-ligand com-plexes, regardless of its physical or chemical meaning [10,11]. This approach can be easily tested and integrated intothe current modeling framework through Eq. (6) for n5 2.As before, the fulvic acid and [U]resin concentrations wereinput as the independent variables and [U]sol was the depen-dent variable. The best-fit values for logβ1,1 and log β1,2are 4.23 and 7.31 at pH 4.0 and 4.54 and 7.45 at pH 5.0(see Table 3). At both pH values studied, the R2, RE andmodel selection criteria (MSC) values from the resultingmodel fit are an improvement over comparable terms fromthe 1 : n approach (Table 3). This modeling approach alsoindicates that the binding of uranium(VI) by FA at pH 5.0will be slightly stronger that that at pH 4.0, a result notreadily apparent from the results of the1 :n model.

3.5 Model for uranium binding by fulvic acidassuming the existence of high affinity sites

In typical applications of Schubert’s method, the ligandconcentration must be knowna priori, which is not reallypractical with natural ligand mixtures like humic and fulvicacids. Previous work examining metal binding with NOMover extended metal and NOM concentration ranges indi-cates the presence of a number of binding sites with differ-ent degrees of affinity for metal ions [4]. Included amongthese sites will be a limited number of high affinity sites

Table 2.Model regression equations, indepen-dent and dependent variables.Model Regression equation Independent Dependent

variables variables

1 : n (Schubert) [L], [U]resin [U] solSλo [U]sol

[U]resin

21D5β1,n [L] n

Π

1 :1/1 : 2 [L], [U] resin [U] solSλo [U]sol

[U]resin

21D5β1,1 [L]

Π1

β1,2 [L] 2

Π

High Affinity [L], [U] resin [U] sol[U]sol 5[U]resin

λo

1A[L][UO 2

21]β* 1,11 1 [UO2

21]β* 1,1

which have been demonstrated to dominate the binding ofselect metals in marine [28, 29] and fresh water systems[30]. Even in the presence of a large excess of weaker affin-ity ligands, these strong sites could dominate metal binding.

The previous modeling approach after [10, 11] showedthat it is possible to interpret U(VI)-NOM binding in termsof a mixture of1 :1 and1 :2 uranyl-NOM complexes. How-ever, this is not the only possible interpretation. For ex-ample, Higgoet al. [13] and Li et al. [9] demonstratedthat models that account for sites of different metal bindingaffinity will also accurately fit experimental data. Hence,we may postulate that complex formation is dominated bysites with high uranium-binding affinity but low concen-tration (with respect to the carboxyl group concentration)that form strictly1 : 1 complexes. In this case, neither theconditional binding constant for a1 :1 uranyl-ligand com-plex (β* 1,1)

β* 1,1 5[UO2Ls]

[UO221][L s]

(10)

nor the total ligand concentration for the relevant site, [Ls]T,are known (where [Ls]T is the total concentration of thestrong binding NOM ligand and [L]T is still the total ligandconcentration, i.e., the carboxyl acidity). However, [Ls]T isproportional to the total NOM concentration, i.e., [Ls]T 5A[L] T, where A is the ratio of high affinity sites to totalNOM sites. Best fit values forA andβ* 1,1 can then be esti-mated using non-linear regression.

Employing mass balance equations for [Ls]T ([L s]T 5[L s] 1 [UO2Ls]) and [U]sol ([U] sol 5 [UO2

21]Π 1 [UO2Ls]),Eq. (10), Eq. (4) and substitutingA[L] for [L s]T yields theregression equation

[U]sol 5[U] resin

λo

1A[L][UO 2

21]β* 1,111 [UO2

21]β* 1,1(11)

Using the ion-exchange results from the uranyl-FA sys-tems, with [L] and [U]resin as the independent variables and[U]sol as the dependent variable, best fit estimates at pH 4.0for A and logβ* 1,1 are 0.00246 0.00029 and 7.3560.09,respectively. At pH 5.0, the best-fit value forA increasesto 0.005360.0012; correspondingly the value for logβ* 1,1decreases to 7.086 0.12. Again, note that there is a strongcorrelation between the values of logβ* 1,1 andA. The R2,RE and MSC values summarized in Table 3 indicate thatthe fit of this model at both pH values is a slight improve-ment over the previous models. Thus, while the concen-tration of total ligands is in excess with respect to [UO2

21],the concentration of the high-affinity U(VI) binding ligands


Table 3. Model parameters and statistics for the uranyl-fulvic acid system.

Model pH σmodela MSCb R2c REd Parameters

1 : n 4 0.030 (µM) 2.98 0.965 212% to 111% log β1,1 5 5.376 0.15; n 5 1.2860.055 0.027 (µM) 3.45 0.975 243% to143% logβ1,1 5 5.136 0.12; n 5 1.1460.04

1 :1/1 : 2 4 0.027 (µM) 3.21 0.972 210% to 17% log β1,1 5 4.236 0.04; logβ1,2 5 7.316 0.065 0.026 3.55 0.977 224 to112 log β1,1 5 4.546 0.03; logβ1,2 5 7.456 0.07

High affinity 4 0.021 (µM) 3.69 0.983 28% to 16% A 5 0.002460.00029; logβ* 1,1 5 7.3560.095 0.024 3.69 0.980 221 to 111 A 5 0.005360.0012; log β* 1,1 5 7.086 0.12

a: σmodel is the standard deviation in calculated Usol. The experimental error in Usol, σexp, is 0.018 µM at pH 4 and 5.b: The model selection criteria (MSC) relates the coefficient of determination to the number of model parameters in order to evaluate theappropriateness of the model. The higher values indicate a more appropriate model.c: In this paper R2 refers to the coefficient of determination.d: The relative error (RE) is the difference between the value of the experimental and calculated dependent variables (e.g., [U]sol in Eq. (9))divided by the experimental value. Ranges are given here.

Table 4. Model parameters and statistics for the uranyl-humic acid system.

Model pH σmodel (µM) MSC R2 RE Parameters

1 : n 4 0.061 2.75 0.954 219% to 18% log β1,1 5 6.756 0.15; n 5 1.4660.055 0.046 3.02 0.963 213% to 113% log β1,1 5 7.576 0.14; n 5 1.4460.04

1 :1/1 : 2 4 0.075 2.34 0.931 226% to113% log β1,1 5 4.756 0.08; logβ1,2 5 8.396 0.085 0.045 3.09 0.965 213% to 110% log β1,1 5 5.386 0.08; logβ1,2 5 9.596 0.06

High Affinity 4 0.042 3.48 0.978 219% to 17% A 5 0.006460.00057; logβ* 1,1 5 7.5960.065 0.032 3.78 0.983 28% to 18% A 5 0.022760.0015; log β* 1,1 5 7.646 0.05

The experimental error in Usol, σexp, is 0.021 and 0.014 µM at pH 4 and 5, respectively.

is not (i.e., [Ls]T ? [Ls]) explaining the nonintegral valuesdetermined for the Schubert’s slope, n.

3.6 Modeling the binding of uraniumby humic acid (HA)

Experimental results at identical system conditions as in theFA system were obtained using HA as the complexant andwere analyzed using the models described previously andsummarized in Table 2. For HA, the ligand concentrationin eq/l is based on the reported carboxyl concentration of4.9 meq/g [15]. The fit of the HA data using Schubert’sapproach results in a non-integral value for n of1.466 0.04and logβ1,n equals 6.7560.15 at pH 4.0. At pH 5.0, n islittle changed at1.4460.04 and logβ1,n increases by almostan order of magnitude to 7.576 0.14. Table 4 summarizesparameter values and statistics for model fits to data at pH4.0 and 5.0. At both pH values the fits to the1 :n modelare good with high R2 values (0.954 and 0.963) and rela-tively small ranges in RE (219% to 18% and6 13%).However, as discussed previously with the U(VI)-FA sys-tem, a non-integral value for n as defined by Eqs. (1), (2)and (5) is not thermodynamically reasonable, invalidatinga direct interpretation of the data using the1 :n Schubertapproach.

Using the same modeling approach that was applied tothe uranyl-fulvic acid system summarized in Table 2, thehumic acid data were next fit assuming the simultaneousformation of 1 :1 and 1 :2 uranyl-ligand complexes. Theoptimal values for logβ1,1 and logβ1,2 were determined tobe 4.75 and 8.39, respectively, at pH 4.0 and 5.38 and 9.59,respectively, at pH 5.0 (see Table 4). Based on the statistics

from Table 4 the fit from this model for the pH 4.0 datawas worse than the fit from the1 : n Schubert’s equation,whereas the fit for the pH 5.0 data was essentially the samefor both modeling approaches. The pH dependence of thismodel fit is also apparent, with the model parametersdescribing the binding of U(VI) at pH 5.0 suggestinggreater complexation than those from the fit at pH 4.0.

The final model applied to the HA data is the high affin-ity model summarized in Table 2. The best fit estimates atpH 4.0 for A and logβ* 1,1 for the HA system were deter-mined to be 0.006460.00057 and 7.5960.06, respective-ly. The resulting fit is quite good, and is a marked improve-ment over the previous methods based on comparing thevalues for R2, RE and MSC from Table 4. The fit for thepH 5.0 data is also a large improvement over the previousmodels and results in an increase in the value forA to0.0236 0.0015 with only a slight increase in logβ* 1,1 to7.646 0.06. The values ofA and logβ* 1,1 are highly corre-lated and the increase in U(VI) binding by HA at pH 5.0 isaccounted for by an increase inA, not by an increase in logβ* 1,1.

These results suggest that the binding of U(VI) by HAhas a greater pH dependence than was observed in similarsystems with FA. Comparing the experimental data for thetwo systems in Fig. 3a and b clearly shows the same result.To some extent, this result is counterintuitive because theFA has a greater concentration of reactive sites/mass (e.g.,carboxyl content of 6.45 meq/g vs., 4.9 meq/g for HA). Forexample, Liet al. [9] saw the reverse effect: their FA hadgreater pH dependence, and also bound U(VI) more strong-ly. However, it is not clear from Liet al. [9] if their FA andHA were extracted from same source. Higgoet al. [13]


Table 5. Model parameters and statistics for the uranyl-Drigg fulvic acid system (from Higgoet al. [13]).

Model σmodel (µM) MSC R2 RE Parameters

1 : n 0.030 3.41 0.9764 28% to 129% logβ1,1 5 6.316 0.11; n 5 1.2660.04

1 :1/1 : 2 0.028 3.55 0.9795 210% to 110% log β1,1 5 5.276 0.07; logβ1,2 5 8.1760.09

High affinity 0.021 3.79 0.9838 27% to 116% A 5 0.002460.0003 ; logβ* 1,1 5 8.3460.07

For these experimentsλo 5 6.84 l/g,Π 5 1.34 and the site density of DFA is 8.89 meq/g.

studied U(VI) uptake for six different humic substances(both FA and HA) isolated from three groundwater systemsand did not see significant differences. Finally, Munier-Lamy et al. [11] saw greater U(VI) binding by HA vs. FAisolated from the same system (marine sediment), but therewas little difference in the pH dependence for the two sys-tems. Thus, it seems that for NOM isolated from differentsystems that the binding of U(VI) is not systematically re-lated to physical differences between HA and FA (e.g., mo-lecular size or hydrophobicity).

3.7 Estimating the effect of NOMon U(VI) speciation in natural systems

A further check of the appropriateness of a model is itsability to accurately describe data from different exper-imental systems. As previously mentioned, there have beenseveral studies of U(VI)/NOM binding. Of these, the studyby Higgo et al. [13] can be directly simulated with thebinding models presented herein. Higgoet al. [13] usedSchubert’s method to analyze U(VI) binding by a fulvicacid extracted from a sandy aquifer (Drigg fulvic acid,DFA) at pH 5 and 0.01M NaCl. Using the binding modelsfrom Table 2, nonlinear regression analyses was applied tofit the data from Higgoet al. [13] and the results are sum-marized in Table 5. In each instance, the model fits arequite good, with high R2 values and relatively low rangesin RE. Like the Suwannee river FA and HA the best fit tothe DFA data is accomplished using the high affinity bind-ing model, although the fit to the1 :1/1 :2 model has asmaller range of relative error. Overall, the binding ofU(VI) by DFA is comparable to that observed for Su-wannee river HA and greater than Suwannee river FA.

Regardless of which modeling approach is used, the dataindicate that uranium will be complexed by NOM in en-vironmentally relevant settings. For example, in a systemat pH 4 to 5, with10 ppm HA (,5 ppm total organic car-bon, TOC, based on a carbon mass percent value suppliedby the IHSS of 54%),1.0 µmol/l U(VI) T and an ionicstrength of 0.10, the models described herein for Suwanneeriver HA predict that 0.77 to 0.93µmol/l U(VI) will becomplexed by humic acid at pH 425. In a correspondingsystem with FA, 0.53 to 0.64µmol/l U(VI) will be bound.In both instances the data indicate that with an increase inpH the amount of U(VI) bound will increase, with the pHdependence of U(VI) binding by HA being more pro-nounced. Calculations using the models in Table 5 for asystem with DFA at similar conditions, except a lower ionicstrength (0.01), indicate that approximately 93 percent ofthe U(VI) will be complexed. The final environmental spe-ciation is a function, of course, of the other ions that are

present. However, these results indicate that in slightlyacidic waters the presence of NOM will significantly influ-ence uranium speciation and must be accounted for in aproper assessment of U(VI) behavior in environmental set-tings.

4. Conclusions

Based on the analysis of ion-exchange data using Schu-bert’s method, citric acid was determined to complex urani-um as a1 :1 uranyl-citrate complex. The calculated valuefor log β1,1 of 6.9960.02 at I5 0.1 for this reaction wasbetween the previously measured values from Markovitsetal. [24] and Rajan and Martell [23]. In these previous stud-ies [23, 24], the value for logβ1,1 was simultaneously calcu-lated with constants for dimeric uranyl-citrate species. Oneof the benefits in using Schubert’s ion-exchange method isthat it can be performed in more dilute systems; thus, theconcentration of the dimeric species was not an issue inthis study (#1%). Hence, our value for logβ1,1 representsa more direct measurement of the1 :1 uranyl-citrate com-plex.

In systems studied, the binding of U(VI) by HA wasfound to be stronger than that for FA. In addition, HA bind-ing of U(VI) exhibited a larger pH dependence than ob-served in identical systems using FA as the complexant.However, comparison of this data to that reported in theliterature suggests that simple relationships between U(VI)binding to NOM, as a function of pH or NOM source, cannot be generalized.

Unlike the citric acid system, the application of Schu-bert’s method to the complexation of uranium by FA or HAindicated that the moles of ligand in an average reactionstoichiometry was not an integral number. Two additionalbinding models assuming either mixed-stoichiometry orheterogeneous-binding, as summarized in Table 2, werealso evaluated. For the FA system, in Fig. 3a, all threemodel fits are very good (Table 3), and the model standarddeviations,σmodel, of 0.030, 0.027 and 0.021 compare fa-vorably with the experimental value,σexp, of 0.018 µM.Similar agreement is also found betweenσmodel andσexp forthe data at pH 5. For the HA system in Fig. 3b, however,the model fit supplied by the high affinity model (see Ta-bles 2 and 4) is clearly superior at both pH 4.0 and 5.0.Furthermore, the model (0.042 and 0.032µM) and exper-imental (0.021 and 0.014 µM) standard deviations are inbetter agreement than with the other models.

Over the experimental conditions studied the1 : nmodeling approach, even though it fits the data, is an inap-propriate choice because the formation of U(VI)-NOM


complexes with nonintegral stoichiometry is neither reason-able nor thermodynamically correct. Because both the highaffinity and the1 :1/1 :2 modeling approaches appear to fitand explain the data equally well, it is difficult to determinewhich model is more appropriate. However, the high affin-ity modeling approach with two fitting parameters (A andβ* 1,1) results in a better fit than the approach positing both1 : 1 and1 :2 complexes, based on best-fit regression terms.In addition, the values for the model error (σmodel) fromTables 3 and 4 and the experimental error (σexp) are moreclosely matched for the high affinity model. Consequently,the experimental data are at least as consistent with strictly1 : 1 stoichiometry as they are with mixed1 :1 and 1 :2binding. In fact, given the large average molecular weightof HA and FA molecules and previous results seen withCu(II) [3] and other metals [28230], a model based on1 :1 stoichiometry and [Ls] ! carboxyl acidity seems moreplausible than a model with1 :2 stoichiometry.

Acknowledgments.The authors would also like to acknowledge thefinancial support provided by the National Science Foundation (OCE-9416088), U.S. Geological Survey and Colorado School of Mines.

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uranium(vi) complexation with citric, humic and fulvic acids

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