Lead Binding to Soil Fulvic and Humic Acids: NICA-Donnan Modeling

Sep 16, 2013 - Binding of lead (Pb) to soil fulvic acid (JGFA), soil humic acids (JGHA, JLHA), and lignite-based humic acid (PAHA) was investigated th...
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Lead Binding to Soil Fulvic and Humic Acids: NICA-Donnan Modeling and XAFS Spectroscopy Juan Xiong,† Luuk K. Koopal,†,‡ WenFeng Tan,*,†,§ LinChuan Fang,§ MingXia Wang,† Wei Zhao,§ Fan Liu,† Jing Zhang,∥ and LiPing Weng⊥ †

College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, P.R. China Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands § State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling, Shaanxi 712100, P.R. China ∥ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, P.R. China ⊥ Department of Soil Quality, Wageningen University, P.O. Box 8005, 6700 EC, Wagneningen, The Netherlands ‡

S Supporting Information *

ABSTRACT: Binding of lead (Pb) to soil fulvic acid (JGFA), soil humic acids (JGHA, JLHA), and lignite-based humic acid (PAHA) was investigated through binding isotherms and XAFS. Pb binding to humic substances (HS) increased with increasing pH and decreasing ionic strength. The NICADonnan model described Pb binding to the HS satisfactorily. The comparison of the model parameters showed substantial differences in median Pb affinity constants among JGFA, PAHA, and the soil HAs. Milne’s “generic” parameters did not provide an adequate prediction for the soil samples. The Pb binding prediction with generic parameters for the soil HAs was improved significantly by using the value nPb1 = 0.92 instead of the generic value nPb1 = 0.60. The nPb1/nH1 ratios obtained were relatively high, indicating monodentate Pb binding to the carboxylic-type groups. The nPb2/nH2 ratios depended somewhat on the method of optimization, but the values were distinctly lower than the nPb1/nH1 ratios, especially when the optimization was based on Pb bound vs log [Pb2+]. These low values indicate bidentate binding to the phenolic-type groups at high Pb concentration. The NICA-Donnan model does not consider bidentate binding of Pb to a carboxylic- and a phenolic-type group. The EXAFS results at high Pb loading testified that Pb was bound in bidentate complexes of one carboxylic and one phenolic group (salicylate-type) or two phenolic groups (catechol-type) in ortho position.

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INTRODUCTION Humic substances (HS) are ubiquitous in soil, sediment, and aquatic environments. Among HS, fulvic acid (FA) and humic acid (HA) are operationally defined natural organic particles differing in solubility, which contain different types of acidic functional groups that can interact with protons and metal ions. FA and HA are generally believed to play an important role in controlling the mobility, bioavailability, and toxicity of metal ions in nature.1 Therefore, accurate prediction of the binding properties of metal ions to HS is of great importance for risk assessment. In nature, binding of metal ions to HS is a complex phenomenon due to the variability of environmental systems and the chemical heterogeneity of HS. Metal ion binding to HS is strongly affected by solution conditions, such as pH, ionic strength, free metal ion concentration, and presence of competing ions.2−5 Moreover, HS extracted from diverse materials exhibit source dependency in a variety of properties. Consequently, it is a challenge to reliably predict metal ion © 2013 American Chemical Society

complexation to HS, and only a few models have been successful.6−8 Among these the NICA-Donnan model is most popular as it is based on a relatively simple analytical binding equation.2,8−11 In NICA-Donnan model the chemical heterogeneity of HS is represented with two types of acidic groups, low proton affinity (“carboxylic” groups) and high proton affinity (“phenolic” groups), each with a continuous affinity distribution.7 Based on a large collection of data, Milne et al.10,11 have derived “generic” parameters for NICA-Donnan model to describe proton and metal ions binding to FA and HA, respectively. These generic parameters make application of the model convenient as they can be used to make predictions Received: Revised: Accepted: Published: 11634

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without further measurements, or serve as guide for fitting model parameters to a new data set. However, one should keep in mind the variations of HS in applying the model with the generic parameters; studies on HS using the NICA-Donnan model2−5,10−14 have shown that the range in parameter values is fairly wide. When validating the ability of NICA-Donnan model in making predictions on metal ion binding in the field, it was found that the model with generic parameters succeeded in giving reasonable predictions over a wide range of environmental conditions for metal ions such as Cu and Cd, but it failed in predicting Pb binding to soil HS.15−18 In the environment, lead is a widespread metal pollutant which can be toxic to plants, animals, and humans. Therefore, a good understanding of Pb binding to soil HS and confidence in the NICA-Donnan model to describe Pb bound by soil HS is important. The discrepancies with Pb might be due to the uncertainties of the model parameters for Pb binding to soil HS. A previous study19 has revealed that the proton binding to a series of soil HS differed substantially from predictions based on Milne’s generic parameters for HA and FA. However, the binding could be described adequately by the NICA-Donnan model using the generic parameters with adjusted values of site densities of carboxylic and phenolic groups and a relatively simple method was proposed to obtain these site densities. With respect to Pb binding, the database used by Milne et al.10 to obtain the generic Pb parameters was mainly based on the peat HA (PPHA) and podzol soil FA. Due to this limitation the generic Pb parameters are somewhat material-specific, so that Milne’s generic Pb parameters may not be sufficiently adequate for Pb binding to soil HS. To improve the prediction of Pb binding in the field, more information on Pb binding isotherms to soil HS is required. Such information gives further insight in Pb−HS interaction that can be parametrized by the NICA-Donnan model and be used to improve the generic Pb parameters. The NICA-Donnan model with calibrated Pb parameters provides information on the Pb affinities and binding stoichiometries, and the speciation of bound Pb as a function of the equilibrium Pb concentration, pH, and ionic strength also can be calculated. However, direct spectroscopic evidence of the Pb binding mechanisms is also highly relevant. Recently, X-ray absorption fine structure (XAFS) spectroscopy has become a promising technique to unravel the coordination modes of ions to functional groups of particles in air, soil, and water.20−22 Combining NICA-Donnan model with XAFS results on Pb binding to soil HS leads to a better understanding of the molecular structure of Pb−HS complexes and the characteristics of Pb binding to soil HS. The objectives of the present study therefore are (1) to obtain high quality data on Pb binding to soil HS and a reference HA; (2) to investigate the coordination environment of Pb binding to HS by XAFS; (3) to fit the Pb binding data to the NICA-Donnan model to derive material-specific parameters and to compare the model results with spectroscopic results; and (4) to apply the NICA-Donnan model with generic parameters for Pb binding and to improve the generic parameters to obtain a better description of Pb bound by soil HS.

meadow soil (semihydromorphic soil) of Jiugong Mountain, in Hubei province (N 29°27′, E 114°42′) in China. HA and FA were extracted and purified following the standard procedure recommended by International Humic Substances Society (IHSS).23 The purified samples will be referred to as JGFA and JGHA for those extracted from the site in Jiugong, and JLHA for that from Jilin. For comparison, Purified Aldrich humic acid (PAHA) derived from Aldrich humic acid (CAS 6813-04-4) was included as a well studied reference sample.24,25 The purified FA and HA samples were freeze-dried and stored in a closed desiccator with drying agent. HS stock solutions were prepared at an initial pH of 10 to make sure that complete dissolution was achieved.26 The chemical characterization of samples and the proton binding, including NICA-Donnan modeling with both material-specific and (adjusted) generic parameters, have been discussed in Tan et al.19 It should be remarked that JGFA is a somewhat typical FA, as both carboxylic and phenolic site densities are exceptionally high. Lead Binding to FA and HA. The binding of Pb to JGFA, PAHA, JGHA, and JLHA was measured at two KNO3 concentrations (0.05 M, 0.1 M) and three pH values (4.0, 6.0, and 7.0) using the Pb titration method.5,27 With this method Pb(NO3)2 is stepwise added to a HS solution under N2 atmosphere of given ionic strength that is kept at a constant pH. After each step the equilibrium Pb2+ concentration was measured potentiometrically with a Pb ion selective electrode (Pb-ISE) in combination with an Ag/AgCl reference electrode. When using the Pb-ISE to measure the free Pb2+ concentration in the solution, the readings of cell EMF (mV) were measured every 2 s; when after 4 min the potential drift was less than 0.2 mV min−1, or after a maximum waiting time of 20 min,5 it was assumed that equilibrium was achieved and the final EMF was recorded. The total Pb concentration after the first Pb addition was 10−5 M and the titrations were stopped before Pb concentrations reached the level at which Pb-(hydr-) oxide precipitation could take place. A detailed description of the calibration of Pb−ref electrode system and of the Pb titration of HS can be found in the Supporting Information (SI). The Pb binding at each point of the titration was obtained from the total Pb concentration, the equilibrium Pb2+ concentration, given pH and ionic strength, and concentrations of major inorganic Pb species in solution. The latter were obtained from the speciation diagram obtained with the chemical speciation program ECOSAT28 at the given pH, ionic strength, and equilibrium Pb2+concentration. The amount of Pb bound by HS was calculated by subtracting the amount of Pb2+ and other inorganic Pb species in the equilibrium solution from the known total amount of Pb. The differential proton-to-lead exchange ratios at each titration step were calculated from the KOH addition to keep the pH constant divided by the increment in Pb binding (see SI). The Pb binding data were fitted to the NICA-Donnan model using the computer codes ECOSAT28 and FIT;29 detailed descriptions of this model can be found in the literature7,9 and in the SI. Parameters for proton binding to HS were taken from our previous paper.19 The general procedure of parameter optimization was in accordance with the approach of Kinniburgh et al.,30 but the method was modified somewhat by adopting a stepwise procedure to obtain reliable parameter values. The detailed procedure has been described in the SI. X-ray Absorption Fine Structure (XAFS) Spectroscopy. The HS−Pb complexes were prepared at pH 5.0. The final equilibrium concentration of Pb2+ was about 0.1 mM and the

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MATERIALS AND METHODS Preparation of FA and HA. Soil samples were collected from the upper horizon of a brown soil (Alfisols) in Tonghua, Jilin province (N 41°30′, E 125°55′) and a mountainous 11635

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Figure 1. Lead binding to FA and HA at pH 4.0, 6.0, and 7.0 in 0.05 and 0.10 M KNO3 plotted against the equilibrium concentration of Pb2+. Symbols: experimental data. Curves: results calculated with the NICA-Donnan model using the material-specific parameters of the HS.

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RESULTS AND DISCUSSION Lead Binding Isotherms. The binding isotherms of Pb to JGFA, PAHA, JGHA, and JLHA are presented in Figure 1. Double logarithmic plots of the measured binding isotherms are linear at sufficiently low Pb concentrations, and the slope of the plots is close to unity. The Pb isotherms of JGFA are slightly steeper than those of the HAs. As expected, the amount of Pb bound by HS, at a given Pb equilibrium concentration, decreases with decreasing pH due to H+−Pb2+ competition for binding sites and the weaker electrostatic attraction. The pH effects are slightly smaller for JGFA than for HAs. The ionic strength effect is measured at pH 4.0. The amounts of Pb bound by HS increase with decreasing ionic strength due to the lesser screening of electrostatic attraction. At high Pb concentrations the amounts of Pb bound by HS level off due to the decrease of the electrostatic attraction and the site occupancy by Pb, and consequently the effects of pH and ionic strength become smaller. At low Pb concentrations (< 10−8 M), which are environmentally relevant, Pb is more strongly bound by the HAs than by JGFA; this implies that the initial affinity of Pb is larger for HA than for FA. The proton binding results have shown that the binding capacity of JGFA is considerably larger than that of the HAs;19 therefore, at high Pb concentrations the Pb binding becomes somewhat larger for JGFA than for the HAs. In accordance with the literature34 these results indicate that the linkage of acidic functional groups to carbon backbone and the total amount of acidic functional groups of FA and HA both affect the Pb binding behavior of HS. The shapes of the adsorption isotherms and the pH effects on Pb binding to JGHA and JLHA are very similar. At pH 4.0

amount of Pb bound was about 1.3 mol/kg for the HAs and 1.8 mol/kg for JGFA. High binding values were selected in order to obtain reliable XAFS information. According to the calculations with ECOSAT, there is no surface precipitation at these conditions. The XAFS spectra of HS loaded with Pb and reference materials (PbO, PbCO3, and 0.1 M aqueous Pb(NO3)2) were measured at room temperature on the 1W1B beamline at the Beijing Synchrotron Radiation Facility (BSRF). Spectra for Pb−HS samples were collected with the fluorescence mode; the reference compounds PbO (ortho), PbCO3, and 0.1 M aqueous Pb(NO3)2 were measured in the transmission mode. All samples were recorded at the Pb L3edge (E = 13035 eV). Detailed descriptions of the preparation of HS−Pb complexes and XAFS measurements are presented in the SI. Data reduction steps were performed using the IFEFFIT program.31 In extracting the χ(k) function, the XAFS signal was isolated from the absorption edge background by using a fit to a cubic spline function. The k3-weighted χ(k) function was then Fourier transformed over k = 2.5−10.5 Å−1 to yield the radial structure function (RSF), see SI Figure S2. Data fitting was done in R space with a multishell fitting routine and with an amplitude reduction factor (S02) fixed at 0.88, which was derived from PbO (ortho) spectral data fitting by use of the FEFF7 code with single-scattering paths. The FEFF7 code reference32 was utilized to calculate single scattering theoretical spectra and phase shifts for Pb−O and Pb−C backscatterers using an input file based on a structural model of PbO (orho),33 with O atoms at 0.336 nm replaced by C. 11636

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at low binding amounts of Pb obtains a relatively high weight, and that at high binding amounts obtains a relatively low weight. This largely explains the fact that the fit at high Pb concentrations is not so accurate. The median affinity constants of Pb are always lower than the corresponding values for protons for all HS (Table 1). This indicates a strong H+−Pb2+ competition. The median Pb affinity constant for carboxylic groups, logKPb1, decreases in the order PAHA > JGHA > JLHA > JGFA. Note that the values of logKPb1 of the HAs tend to be higher than logKPb1 of JGFA by about 1−2 orders of magnitude. This difference is caused by the fact that carboxylic groups of FA are more likely to be linked to aliphatic structures, whereas in HAs salicylate and phthalate groups are generally recognized to be primary Pb binding sites.34,35 For HAs the observed values of logKPb2 (phenolic groups) are within a narrow range of 5.9 ± 0.2 and they are higher than logKPb2 of JGFA by about an order of magnitude. The differences between the logK Pbi values with the corresponding logKHi values for JGFA are much greater than those of the HAs, especially for the phenolic-type sites. This indicates that (i) the proton−lead competition for binding sites of HAs is stronger than for those of JGFA, and (ii) the overlap of the Pb affinities for the carboxylic group and the phenolic group of the HAs are larger than that of JGFA. These differences lead to significant differences in speciation of bound Pb between JGFA and the HAs (see SI Figure S5). The derived values of p1 and p2, which are related to the width of the intrinsic heterogeneity distribution of, respectively, the carboxylic and phenolic groups,9 indicate that the distributions are rather sample specific. The presently observed values of stoichiometry or nonideality parameters nij are in the range of 0.64−0.90 for proton and 0.70−0.95 for Pb, with the value of nPb2 for PAHA (0.53) as an exception. The nHj values correspond well to values reported for FA and HA.5,7,10,27,36 All nPb1 values are greater than 0.9; this is in accordance with the high initial slopes of the Pb isotherms. The present values of nPb1 agree well with results reported by Christl et al.27 for soil HS and by Gondar et al.5 for peat HS, but they are much greater than the values reported in other studies.7,10,30,36,37 The values of nPb2 of soil HS are smaller (0.72−0.85) than those of nPb1, but also at the high end of the literature range.10,27,36,37 Based on the material-specific model parameters it is possible to calculate the differential exchange ratios (ΔH/ΔPb) for each point and the speciation of bound Pb (distribution of Pb over the carboxylic and phenolic groups). For the HAs, the predicted ΔH/ΔPb values agree roughly with the experimental results and with the results of Kinniburgh et al. and Christl et al.7,27 In judging the results it should be realized that the exchange ratio is a differential quantity that is rather sensitive to experimental errors38 and to the values of the ion-specific parameters. The systematic differences between the experimental results and the predictions are likely due to the fact that the ion-specific parameters are not optimal. The predicted values at low Pb concentration show a less pronounced behavior than the experimental results and the gradual increase of the calculated exchange ratios at higher Pb concentrations starts at substantially lower Pb concentrations than in practice. Yet, the latter increase is in accordance with the increasing H− Pb competition for the phenolic sites at high Pb concentrations. For JGFA the agreement between the predicted ΔH/ΔPb exchange ratios and the experimental results is poor (see SI Figure S4). This poor result may be due to the fact that the Donnan model is not very appropriate for FA,39 and as a result

the amounts of Pb bound by PAHA (lignite based) and soil HAs are also quite similar, but at pH 6.0 and 7.0 the Pb binding to PAHA is somewhat larger than that to JGHA and JLHA. Moreover, at low Pb concentrations the binding amount of Pb by PAHA is greater than that by JGHA and JLHA. These results point to a higher affinity of Pb2+ for PAHA than for the soil HAs. Comparing the binding isotherms at pH 6.0 and 7.0 reveals that the Pb binding to PAHA is slightly less affected by variations in pH than that to soil HAs. The measured proton-to-lead ion exchange ratios (ΔH/ ΔPb) of JGFA and the HAs are comparable, as the values ranged from 1.3 to 0.5, except for JGFA at pH 4.0 where ratios up to 2.0 were observed. Figures of the exchange ratios for HS can be found in the SI (Figure S4). The range from 1.3 to 0.5 agrees well with the values reported in literature.5,7,27 The exchange ratios vary with the equilibrium Pb concentration and depend on the pH and ionic strength (see SI Figure S4). At low Pb concentration relatively high exchange ratios are observed, which indicates that functional groups with a high proton affinity (phenolic type) also have a high Pb affinity. However, at high Pb loading, when the amounts of Pb bound start to level off, the ΔH/ΔPb exchange ratios tend to increase with increasing Pb concentrations. This increase is likely attributed to participation of initially protonated phenolic-type groups in the Pb binding. With increasing pH the values of the ΔH/ΔPb exchange ratios decrease because the number of dissociated functional groups increases. NICA-Donnan Modeling and Parameters of Lead Binding. The double logarithmic Pb binding isotherms to JGFA and the HAs were fitted with the NICA-Donnan model. The optimized NICA-Donnan parameters for Pb binding, together with the parameters for proton binding, are presented in Table 1. The R2 values of the fitting are greater than 0.984 Table 1. NICA-Donnan Model Parameters of FA and HAs b mH1 logKH1 Qmax,H1 mH2 logKH2 Qmax,H2 p1 nH1 nPb1 logKPb1 p2 nH2 nPb2 logKPb2 R2

JGFA

GFAa

PAHA

JGHA

JLHA

GHAb

0.81 0.53 2.18 8.72 0.59 8.19 3.90 0.80 0.66 0.95 0.78 0.66 0.88 0.79 4.55 0.993

0.57 0.38 2.34 5.88 0.53 8.60 1.86 0.59 0.64 0.60 −1.16 0.70 0.76 0.69 6.92

0.60 0.65 3.52 2.97 0.26 7.99 2.86 0.74 0.88 0.92 3.13 0.40 0.64 0.53 5.78 0.997

0.48 0.70 3.27 2.35 0.31 8.33 3.48 0.87 0.80 0.91 2.26 0.38 0.83 0.85 5.90 0.989

0.47 0.41 2.95 4.46 0.67 7.52 1.38 0.61 0.67 0.94 1.40 0.75 0.90 0.72 6.09 0.984

0.49 0.50 2.93 3.15 0.26 8.00 2.55 0.62 0.81 0.60 1.25 0.41 0.63 0.69 4.84

a Generic FA. bGeneric HA. The parameters of GFA and GHA were taken from Milne et al.10

and the goodness of fit can be observed in Figure 1 where the curves represent the NICA-Donnan fits to data. In general, the NICA-Donnan model calculations agree well with the experimental data for Pb binding, but small deviations between experimental points and the calculated curves still exist. Note that the present fit is based on a comparison of log [bound Pb] vs log [Pb2+]. A consequence of this way of fitting is that the fit 11637

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Pb2+(aq) spectra. The results indicated that Pb is bound to HS by inner-sphere complexation with a C−O−Pb structure. To gain more insight into the molecular structure of the binding of Pb on HS, the EXAFS spectra were fitted to a C− O−Pb structure (see SI Figure S2). The resulting coordination numbers (CN), atomic separation distances (R), and Debye− Waller factors (σ2) obtained for the different Pb−HS complexes are listed in Table 2. Only average Pb−O distances were obtained from the fitted EXAFS results. The results revealed that Pb was coordinated by 3.73−4.73 O atoms at a distance of 2.37−2.40 Å in the first shell, and by 2.49−3.36 C atoms at a distance of 3.17−3.27 Å for the second shell. These results are in good agreement with the study of Xia et al.21 about the nature of Pb binding to soil HS at pH 4.0, 5.0, and 6.0 using XAFS spectroscopy. Although the values of the various parameters differ for different HS, no clear distinction could be made between soil HAs and PAHA. The distances of Pb−Owater (from water) and Pb−Ogroup (from HS) reported in the literature21,41 are 2.33 and 2.41 Å, respectively. The average distances of Pb−O for Pb−HS complexes were 2.37−2.40 Å, i.e., in the range of Pb−Owater and Pb−Ogroup, suggesting that there were also oxygens from water involved in the Pb−O shell. This explains that the location of the first peak is different for Pb(NO3)2 and Pb−HS complexes (see Figure 2b). It is generally accepted that carboxylic and phenolic groups of HS are quantitatively the most important binding sites for heavy metals ions.42 Pb complexation to carboxylic and/or phenolic groups located at different positions in the HS structure will lead to inner-sphere complexes with different numbers of coordinated O and C atoms (see SI Figure S3). The results of Table 2 are in best agreement with innersphere structures that involve bidentate Pb complexes. Taking into account that five-membered and six-membered rings containing a C−O−Pb−O−C structure are stable,43 the conclusion emerges that, at high Pb loading, the Pb ions tend to be predominantly complexed by phenolic and/or carboxylic groups located in ortho-position. Two oxygens from water molecules further coordinate the Pb ions in the rings. According to the literature,40 salicylate- and catechol-type groups are the major groups for Pb binding to HS. Therefore, the structures based on one carboxylic and one phenolic group (salicylate-type) or two phenolic groups (catechol-type) in ortho position (Figure S3 structures B and D) are the most probable configurations of Pb binding to HS (at Pb equilibrium concentrations of about 10−4 M). Pb Binding Structure Derived from the NICA-Donnan Model and from XAFS. The ion-specific nonideality parameter, nij, of the NICA-Donnan model is according to the derivation of the NICA model related to the binding stoichiometry, but in practice nij is also affected by ion-specific nonideality caused by heterogeneity effects that are not accounted for by the intrinsic heterogeneity distribution.9,44 Because of this complication the ratios nPbj/nHj are best suited to obtain information on binding stoichiometry. The observed high nPb1 values also lead to the ratios nPb1/nH1 that are high (1.05−1.44). The model interpretation of high nPb1/nH1 ratios is that the average number of Pb ions bound by a carboxylic group is relatively high; therefore the ratios are an indication for monodentate Pb binding to carboxylic groups. The nPb2/nH2 ratios of soil HS are substantially smaller (0.80−1.02) and indicate that part of the phenolic groups is involved in bidentate Pb binding. The results regarding the Pb complex structures inferred from the above nPbi/nHi ratios present a

the entire calculated speciation of bound Pb to FA will be artificial. The calculated speciation of bound Pb to the HAs reveals that Pb bound to the carboxylic groups as a function of the equilibrium Pb2+ concentration goes through a maximum, and Pb binding to phenolic groups through a minimum (see SI Figure S5). The latter trend corresponds with the trend in the proton-to-lead exchange ratios. Coordination Environment of Pb. The Pb L3-edge XANES spectra of all HS−Pb complexes were very similar to that of Pb2+ model compounds PbO, PbCO3, and Pb(NO3)2 (see SI Figure S1) indicating the oxidation state of Pb did not change upon binding to HS. The EXAFS spectra for Pb binding to HS at pH 5.0 and of aqueous Pb(NO3)2 are depicted in Figure 2a. The possible inner-sphere complex structures of Pb

Figure 2. The k3-weighted (a) and Fourier transformed (b) EXAFS data (uncorrected for phase shifts) for 0.1 M aqueous Pb(NO3)2 and for Pb binding to FA and HAs at pH 5.0. Vertical dashed lines in (b) indicate the peak positions for the first and second oscillations.

binding to HS are manifold. Likely the EXAFS spectra are a linear combination of different complex structures of Pb, such as Pb−catechol, Pb−phthalate, and Pb−salicylate; they are therefore different from the spectra of these individual structures as obtained by Manceau et al.40 The present spectra very similar to the EXAFS spectra of Xia et al.21 of Pb2+ complexed by soil HS. The k3-weighted Fourier transforms of EXAFS spectra (uncorrected for phase shifts) of 0.1 M aqueous Pb(NO3)2 and of Pb binding to HS at pH 5.0 are depicted in Figure 2b. The vertical dashed lines indicate the peak positions for the first and second oscillation, corresponding with, respectively, Pb−O and Pb−C coordination.21 The EXAFS spectra of the samples were distinctly different from the 11638

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Table 2. EXAFS Fit Results of Pb Binding to FA and HAs sample JGFA-Pb PAHA-Pb JGHA-Pb JLHA-Pb

shell shell shell shell shell shell shell shell

1 2 1 2 1 2 1 2

atomic backscatter

R (Å)a

σ2 (Å2)b

CNc

ΔE0 (ev)d

Rfe

Pb−O Pb−C Pb−O Pb−C Pb−O Pb−C Pb−O Pb−C

2.40 3.25 2.37 3.17 2.38 3.21 2.40 3.27

0.0190 0.0167 0.0196 0.0193 0.0197 0.0226 0.0237 0.0149

3.73 2.49 4.17 3.36 4.01 2.85 4.73 2.77

−11.40 −11.40 −13.504 −13.504 −10.75 −10.75 −8.95 −8.95

0.0072 0.0057 0.0076 0.0098 0.0076 0.0091 0.0237 0.0117

Atomic separation distance. bDebye−Waller factor. cCoordination number. dEnergy shift. eResidual fraction = Σk(k3xexp − k3xcalc)/Σk(k3xcalc), which measures the quality of the Fourier-filtered model contribution (xcalc) with respect to the experimental contribution (xexp). The amplitude reduction factor (S02) was set to 0.88. a

Figure 3. Predictions of the NICA-Donnan model for Pb binding to FA and HA at 0.05 M KNO3 using different parameter sets. Solid symbols: experimental data; open symbols: results calculated using the material-specific parameters. Black curves: predictions of NICA-Donnan model using the full generic parameter set for FA or HA; red curves: predictions of NICA-Donnan model using the generic parameter set of HA with the parameter nPb1 adjusted to 0.92.

Pb binding and before final conclusion can be drawn this aspect need attention as well. Although somewhat unusual because low Pb concentrations are most relevant in practice, the NICA-Donnan model can also be fitted to the binding data using a linear scale for the bound amounts (i.e., similar to fitting the H binding data). In this fitting mode, data at low loading have a relatively low weight and those at high Pb loading a relatively high weight and this may lead to different parameter values than those obtained with the classical fitting procedure. With the new fitting procedure the parameters based on the proton binding were kept the same; the other parameters were reoptimized. The newly

different picture with the spectroscopic results that indicated bidentate structures with a carboxylic and a phenolic group or with two phenolic groups. However, in the NICA-Donnan model only multidentate binding of metal ions with a few carboxylic groups or with a few phenolic groups are considered; mixed complexes involving both carboxylic and phenolic groups, which occur in practice, are not included.42 Apart from the model limitations that do not allow mixed complexes, it should be realized that with fitting the data to the model a relatively high weight has been given to very low Pb loadings, whereas the spectroscopic results are obtained at relatively high 11639

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concentration. Moreover, the values of nPb1 and nPb2 were set equal to those of generic HA. Comparison of the generic parameter values for FA with those obtained for JGFA (Table 1) shows that logKPb1 and Qmax,H1 of generic FA are much lower than the present values, whereas logKH1 is higher than that of JGFA. If other model parameters had been about equal, the generic model parameters would have underestimated Pb binding to JGFA. As the opposite is found, the main reason for the large deviation of the generic prediction and the measurements should be due to either or both the parameters b and nPb1. In view of the too low slopes of the predicted curves and the limited concentration range considered by Milne et al., the generic nPb1 value should be suspected. For HA, the HA-Pb data sets used by Milne et al.10 were mainly based on PPHA, purified HA extracted from Irish moss peat. According to Kinniburgh et al.30 the derived values of logKPb1 (0.73) and nPb1 (0.60) for PPHA agreed well with the values of generic HA parameters (1.25 and 0.60, respectively). Undoubtedly, the generic parameters of Pb binding to HA are constrained by the properties of PPHA. The lignite-derived PAHA shows Pb binding properties similar to those of PPHA. For JGHA and JLHA the parameter values differ from each other and scatter around the generic parameter values, except for nPb1, which is similar for both soil samples but much higher than the generic value. In view of the wrong initial slope also here the relatively low generic nPb1 value is the first parameter that should be suspected. To test the sensitivity of the results for a given parameter, one parameter of a given set of parameters can be changed to see how much the calculated result is affected. By doing this for different parameters it is possible to find the parameter that most strongly affects the outcome of model calculation. To limit the options, the generic value of a parameter was replaced by the material-specific value. A series of sensitivity tests showed that for JGHA and JLHA the model prediction with generic parameters could be strongly improved only when the generic nPb1 was replaced by the specific value of nPb1 observed of JGHA and JLHA. Replacing the generic value of Qmax,H1 by the material-specific value, which did lead to a substantial improvement of the prediction of proton binding to JGHA and JLHA,19 led for the Pb binding to a minor improvement as compared to that by replacing nPb1. Changing nPb1 and Qmax,H1 simultaneously gave somewhat better results at high loading than changing nPb1 only (SI Figure S6). Although for PAHA also the material-specific nPb1 value is much higher than the generic nPb1 value, the model predictions became poorer by using the material-specific value of nPb1. This is due to the fact that the NICA-Donnan model with generic parameters already describes the Pb binding to PAHA reasonably well. For JGFA it was difficult to improve the prediction significantly by replacing one or two generic parameters with material-specific parameters; only when nPb1, logKPb1, and logKPb2 of the generic parameters were replaced by JGFA material-specific values was an improvement achieved. This is probably due to the fact that JGFA is a rather exceptional FA, also its proton binding behavior deviates strongly from GFA.19 The conclusion of this section is that Milne’s generic parameters for HA can be strongly improved with respect to the predictions for soil HA by using a higher value for nPb1. Based on the presently observed values for nPb1 and those of Christl et al.,27 a value of nPb1 = 0.92 can be recommended for future use. A comparison of the model predictions using the generic HA parameters with nPb1 = 0.92 with the measured and

derived parameters are presented in SI Table S1. The obtained values differ significantly from those obtained with the classical fitting procedure. This result indicates that the data presentation matters for fitting the NICA-Donnan model to the data and that for high loadings the double logarithmic data presentation is not very adequate. For the comparison with the spectroscopic results the ratios nPbi/nHi are most relevant. The new nPb1/nH1 ratios (carboxylic groups) were still somewhat larger than unity and only slightly smaller than the ratios obtained from the optimization using the log bound amounts Pb vs log [Pb2+]. Therefore, both optimization methods lead to nPb1/nH1 ratios that indicate monodentate Pb binding to the carboxylic groups. The new nPb2/nH2 ratios (phenolic groups) were in the range of 0.47−0.69. These values are substantially smaller than those observed with the first optimization, and they clearly indicate that the average number of Pb bound by the phenolic groups is relatively low. Therefore, the NICADonnan model predicts that, at high Pb loading, Pb is bound by the phenolic groups in a bidentate structure. Concluding, the NICA-Donnan modeling results indicate that Pb is bound to the carboxylic groups in monodentate complexes for all bound amounts of Pb. The binding to the phenolic groups might change from monodentate to bidentate when the Pb binding increases; at high Pb loading Pb is bound by the phenolic groups in bidentate complexes. In view of the fact that the options for bidentate complexes are restricted in the model, the partial agreement of the model results with the spectroscopic results, which indicated that at high loading Pb was bound in bidentate complexes of one carboxylic and one phenolic group or two phenolic groups, is satisfying. At low Pb loading no direct spectroscopic evidence is available to verify, so the Pb binding mechanism obtained with the NICA-Donnan model. Comparison with Predictions Using the Generic NICADonnan Parameters. Using a large collection of data sets, Milne et al.10 have derived a set of generic NICA-Donnan parameters for both FA and HA; the values are presented in Table 1 as, respectively, GFA and GHA parameters. In Figure 3 a comparison is made between the measured Pb binding to JGFA and HAs at 0.05 M KNO3 (solid symbols), and the predictions of the NICA-Donnan model using the generic parameters and a ionic strength of 0.05 M are presented (black curves). The open symbols depict the model predictions based on the material-specific parameters; they serve to extrapolate the data to very low Pb concentrations. The predicted Pb binding to PAHA corresponds relatively well to the measured and extrapolated data, but at pH 4.0 the binding at high bound amounts is poorly predicted. The predicted values for soil HS differ systematically from the measured and extrapolated results: only at high concentrations is a reasonable agreement observed, at low concentrations the Pb binding is overestimated and the slope of the curves is too low. The present observation is in agreement with the results of Christl et al.27 who showed that at pH 4.0 and 6.0 and low Pb loading the NICA-Donnan predictions using the generic parameters for HA overestimated the Pb binding to soil HA. For FA, the data sets that Milne et al.10 have used to optimize the generic parameters contained eight data sets, of which six were limited to relative high Pb concentrations (> 10−7 M). Five of the FAs were extracted from podzol soils, two from river water and the last one extracted from Gleysol. This implies that the generic parameters of FA obtained by Milne et al. were dominated by the binding behavior of podzol soil FA at high Pb 11640

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extrapolated results of JGHA and JLHA are depicted in Figure 3 by the red curves. For both soil HAs the predictions with the generic set with the new value of nPb1 are much better than the predictions with the original parameter set. The main aim of studying metal ion binding to model HS and to apply the NICA-Donnan model is to use the obtained information for making accurate predictions in the field. Investigations in the field revealed that the model description by the NICA-Donnan model with Milne’s generic parameters was not very satisfactory for Pb binding to soil HAs. Based on the present results of Pb binding to soil HAs, the generic description of the Pb binding to soil HAs could be improved considerably by adjusting only Milne’s nPb1 value to 0.92. For field predictions of Pb binding to soil HAs with the NICADonnan model it is therefore recommended to use Milne’s generic parameter set, with nPb1 adjusting to 0.92. The XAFS spectra obtained at high Pb loading to soil HS provided direct evidence for bidentate Pb complexes with Pb bound to one carboxylic and one phenolic group (salicylate-type) or to two phenolic groups (catechol-type). This result was in fair agreement with the low nPb2/nH2 ratio for the phenolic groups that was obtained from the NICA-Donnan parameter optimization based on Pb bound vs log [Pb2+]. The comparison of results between the NICA-Donnan model and spectroscopic results is very useful for a better understanding of the binding behavior. The spectroscopic results provide direct evidence for binding modes and put the model interpretation in the right perspective. A weakness of the XAFS method is that it is rather difficult to obtain reliable data at low metal loadings.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

(i) Methodology of Pb titration to obtain the binding isotherms, (ii) methodology of the XAFS measurement, (iii) description of NICA-Donnan model, (iv) modifications of Kinniburgh approach for optimization of model parameters, (v) results of molar ΔH/ΔPb exchange ratio, (vi) speciation of bound Pb, (vii) NICA-Donnan model parameters obtained by using Pb bound vs log [Pb] data to fit the model, (viii) Pb binding predictions to soil HAs based on Milne’s generic parameters with adjusted values of Qmax,H1 and nPb1, and (ix) the result of a typical duplicate experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 27 8728 7508; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Science Foundation of China (41201231 and 40971144), the One Hundred Elitist Program of the Chinese Academy of Sciences, the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau and the Institute of Soil and Water Conservation, Chinese Academy of Science, and Fundamental Research Funds for the Central Universities (2011JQ013). We also thank BSRF for the valuable beamtime. 11641

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