Humic Nanoparticles at the Oxide−Water Interface: Interactions with

Nov 5, 2008 - In this work, data for the interactions between humic acid (HA) or fulvic acid (FA) with phosphate ions at the surface of goethite (α-F...
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Environ. Sci. Technol. 2008, 42, 8747–8752

Humic Nanoparticles at the Oxide-Water Interface: Interactions with Phosphate Ion Adsorption LIPING WENG,* WILLEM H. VAN RIEMSDIJK, AND TJISSE HIEMSTRA Department of Soil Quality, Wageningen University P.O. Box 47, 6700 AA, Wageningen, The Netherlands

Received June 13, 2008. Revised manuscript received September 12, 2008. Accepted October 2, 2008.

In this work, data for the interactions between humic acid (HA) or fulvic acid (FA) with phosphate ions at the surface of goethite (R-FeOOH) are presented. The results show very clear differences between HA and FA in their interactions with phosphate at goethite surface. HA is strongly bound to goethite but surprisingly does not strongly affect the phosphate binding, whereas FA is less strongly bound, but these molecules have a very large effect on the phosphate adsorption, and vice versa. Phosphate adsorption to goethite in the presence of adsorbed HA or FA can be well predicted with the LCD model (ligand and charge distribution). According to the model calculations, the nature of the interactions between HA or FA with phosphate at goethite surface is mainly electrostatic. The stronger effects of FA on phosphate adsorption are caused by its spatial location which is closer to the oxide surface, and as a consequence, the electrostatic interactions between adsorbed FA particles and phosphate ions are much stronger than for HA particles. This is the first time that effects of natural organic matter (NOM) on an anion adsorption are predicted successfully using an integrated ion-binding model for oxides and for humics that accounts for chemical heterogeneity of NOM.

Introduction Natural organic matter (NOM) nanoparticles, such as humic acid (HA) and fulvic acid (FA), bind very strongly to mineral surfaces (1-3). These surfaces also strongly interact with well-known oxyanions such as phosphate, arsenate, and selenite. NOM adsorption to minerals contributes to the storage of terrestrial carbon. Phosphate adsorbed to metal oxide surfaces is probably the most important active pool in the phosphorus (P) nutrient cycle in the terrestrial ecosystems. The competition between NOM and phosphate for adsorption to minerals may strongly influence their solubility and mobility, and consequently, their behavior in the environment. Several experimental studies have been carried out that show big effects of NOM on phosphate adsorption to minerals (4-6). However, fundamental research on the interactions between inorganic phosphorus and NOM at metal oxide surfaces is, up till now, not advanced enough to allow for a quantitative description, prediction and understanding of these effects. The difficulty in the model development arises from the chemical complexity of NOM * Corresponding author phone: 31-317-482332; fax: 31-317-419000; e-mail: [email protected]. 10.1021/es801631d CCC: $40.75

Published on Web 11/06/2008

 2008 American Chemical Society

particles and their relatively large size compared to inorganic ions. There is an ongoing debate on the nature of HA molecules and their molecular mass (7, 8). For systems containing the relatively small NOM particles (molecular mass 0.81

0.31

0.68

2.40 2.39 5.88 1.86

-1.0

FA

carboxylic phenolic carboxylic phenolic

-1.0

0.66

0.57

a Qmax, Nmax: site density. p: parameter for the intrinsic heterogeneity of the ligands. K˜I: mean affinity constant. ni: ion non-ideality parameter. b: a constant used to calculate the Donnan volume of humic substances. Subscript “in” refers to the innersphere complex (-Fe1OOCR-0.5). For HA, the NICA parameters except log K˜in and nin have been derived by fitting the acid-base charge curve of the HA used to the NICA-Donnan model (26). For FA, the NICA parameters except log K˜in and nin are taken from (32). For both HA and FA, log K˜in was derived in this study and nin was assumed equal to nH.

phosphate to goethite in the absence of humics has already been intensively studied using a whole series of techniques, and our mechanistic understanding of this adsorption process at the molecular scale is very good. The competition between phosphate and HA/FA at the oxide surface can be due to site competition as well as electrostatic effects. The calculated potential profile depends strongly on the assumed distribution in space of the adsorbed HA/FA and the net charge of the humics. However, it also depends on the charge distribution of the different adsorbed phosphate species. The interplay will determine the overall effect. In this work, the competitive interactions between HA or FA with phosphate ions at the surface of goethite (R-FeOOH) were studied experimentally. The results were compared to predictions using the LCD model. One objective of this study is to test the applicability of the LCD model to predict ion adsorption in ternary systems containing anions, humics and oxides. In the modeling, the distribution in space of adsorbed HA/FA and phosphate and other model parameters were kept the same as those optimized using data of binary systems (FA-goethite, HA-goethite, phosphate-goethite). Another objective of this study is to understand the mechanisms in the humics-phosphate interactions at oxide surfaces and the difference between HA and FA in these interactions. A successful description of these important interactions will be a major step forward to a realistic modeling of surface complexation in natural systems.

Materials and Methods Experiment. The goethite material was prepared based on the procedure described by Hiemstra et al. (14). The specific surface area of the goethite material was determined using 8748

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the BET-N2 adsorption method and equals 94 m2/g. The point of zero charge (PZC) of this goethite material has been determined as pH 9.3 (22) (Table 1). Humic acid (HA) used in these experiments was purified from a forest soil (Tongbersven, The Netherlands) and fulvic acid (FA) from a peat soil (Strichen Soil Association, Scotland) following the IHSS (International Humic Substance Society) procedures (23). The purified HA and FA material contain, respectively, 54 and 43% carbon. The average molar mass of HA and FA used is, respectively, 13.2 and 0.68 kDalton (Table 2), which was measured using size exclusion chromatography (24, 25). Adsorption was measured with two batch experiments, in which goethite suspensions (1 g/L) were prepared in a background electrolyte solution of 10 mM (first batch) or 1 mM NaNO3 (second batch). Each batch contains five series of treatments, i.e. phosphate-goethite, HA-goethite, FAgoethite, phosphate-HA-goethite, and phosphate-FA-goethite. In each series, one blank treatment was added that contains no goethite. The initial phosphate concentration was 0.15 mM, whereas it was 50 mg/L for HA and FA. The stock solutions of phosphate and HA/FA were added to goethite suspension simultaneously. For all series, acid or base solution (0.1 M NaOH and HNO3) were added to the suspension to adjust the pH to selected values in the range of pH 3-7. During the preparation, the gastight vessels used in the experiments were flushed with N2 gas to minimize the influence of CO2. Thus prepared suspensions were shaken at 20 °C for 3 days, and subsequently centrifuged at 18 000g for 30 min. Our preliminary experiments show that pseudoequilibrium can be reached within 3 days when phosphate and humics are added simultaneously. The phosphate concentration in the supernatant was measured with opti-

mized molybdate blue colorimetric method using a segmented flow analyzer (Skalar). The concentration of dissolved organic C (DOC) was measured with a TOC analyzer (Skalar) after the supernatant solutions were acidified to pH 3-4 to remove inorganic C. The final pH in the supernatant was measured using a pH meter. At 1 mM NaNO3 background, the final ionic strength in the treatments containing HA/FA is 2 mM, due to sodium (Na) present in the stock solutions of HA/FA (NaOH solution was used to dissolve HA/FA). Amount of HA/FA adsorbed was derived from the difference of DOC between the treatments and the blank (no goethite). Our previous study shows that at the ionic strengths studied there is no HA precipitation at pH above 3 (26). Modeling. CD-MUSIC model was used to describe the reactivity and electrostatics at goethite surface (13, 14), in combination with the extended Stern model for the compact part of the EDL, which has two Stern layers and two outerelectrostatic planes (1- and 2-plane) (Figure 1) (20). Two types of surface groups on goethite are proton reactive, i.e., the singly (-FeOH-0.5) and triply (-Fe3O-0.5) coordinated surface groups (13). The basic CD-MUSIC parameters have been derived previously (18) (Table 1). Phosphate can form two types of innersphere surface species, i.e. a protonated monodentate (-Fe1OPO2OH-1.5) and a bidentate (-(Fe1O)2PO2-2) surface complex (27), with the charge distributed between the surface plane (0-plane) and the middle plane (1-plane). The model parameters for phosphate adsorption derived by Rahnemaie et al. (27) were used without modification to predict phosphate adsorption to goethite (Table 1). The mean-field approximation was used to link the charge density and the potential in the various electrostatic planes. Adsorption of phosphate to goethite in the presence of adsorbed HA/FA was calculated using the LCD (ligand and charge distribution) model (15-18, 22), which integrated the NICA model (12) and the CD-MUSIC model (13, 14). In addition to proton binding by both the carboxylic and phenolic type of ligands on HA/FA, it was assumed that the carboxylic groups (RCOO-) of adsorbed HA and FA in the first compact layer can form innersphere complexes (-Fe1OOCR-0.5) with the singly coordinated surface sites (-Fe1OH2+0.5) on goethite (17, 18). The corresponding charge is distributed between the surface (0-plane) and 1-plane (Table 1). Both reactions (protonation, innersphere complexation) were calculated with the NICA model. It was assumed that the NICA model parameters for adsorption of protons to adsorbed HA/FA remain the same as for HA/FA in the solution (Table 2). These parameters have been used to describe the acid-base titration data of these HA/FA (26). In our previous work of modeling FA adsorption to goethite in the absence of phosphate with the LCD model, the basic Stern model was used, which has only one Stern layer and all FA charge was located in the compact part of the EDL, at the 0-/1-plane (17). In the present work, we used the extended Stern model for all calculations, in which the compact part is divided into two layers (Figure 1). Model calculations show that by adjusting the affinity constant for the innersphere complex (-Fe1OOCR-0.5), a reasonable description of FA adsorption data in the absence of phosphate can be obtained for a range of spatial distributions (between the 0-/1- and 2-plane) of adsorbed FA. However, to achieve a good description of phosphate adsorption to goethite in the presence of adsorbed FA, the range of spatial distribution is narrower. The optimal value points to an even distribution, i.e. 50% of FA ligands at the 0-/1-plane and 50% at the 2-plane. The charge distribution of the ligands between the 0- and 1-plane depends on the amount of innersphere complex formed (-Fe1OOCR-0.5), which results from the iterative model calculations. With this spatial distribution, and an adjustment of the affinity constant for the formation of the

innersphere complex (-Fe1OOCR-0.5) (logK from -2 to -1) (Table 1), the model can describe the FA adsorption data quite well (see the Supporting Information). In modeling the HA adsorption to goethite in the absence of phosphate using the LCD approach, the distribution of adsorbed HA in space is an adjustable parameter, which is a function of the ionic strength of the solution (18). The fraction of HA attributed to the 0-/1-plane has been optimized previously (18). The fraction found is, respectively, 5 and 1% for the 10 and 2 mM ionic strength. The fraction of HA at the 2-plane was kept constant at 25%. The rest of HA has been located in the DDL, which is 70 and 74% for, respectively, 10 and 2 mM ionic strength. In the present work, the affinity constant for the formation of the innersphere complex (-Fe1OOCR-0.5) was assumed to be the same for HA and FA (logK ) -1) (Table 1). Changing this constant from -2.0, which was used in the previous work (18), to present value (-1.0) did not influence much the predicted HA adsorption for HA-goethite systems (see the Supporting Information) because only a small fraction of HA was located at the 0-/ 1-plane and allowed to form the innersphere complex. The NICA model calculates fractions of reactive ligands on adsorbed HA/FA particles that are protonated or complexed with surface sites. Together with the amount of HA/ FA adsorbed to goethite, the total charge carried by adsorbed HA/FA can be derived. The charge balance and electrostatic potential at each electrostatic plane at the goethite surface was calculated with the CD-MUSIC model by taking into account the charge contribution from adsorbed HA/FA. Adsorption of phosphate to goethite in the presence of adsorbed HA/FA was calculated with the CD-MUSIC model using the same parameters for phosphate adsorption to goethite without HA/FA (27) (Table 1). Although the spatial distribution of FA was optimized in the present work using FA/phosphate competition data, the results for HA/phosphate competition calculations are a pure prediction. In the present study, the measured amount of HA/FA adsorbed was used as an input to the model, to simplify the calculations and discussions. The model calculations were carried out using the computer program ORCHESTRA (28).

Results and Discussion Adsorption of Phosphate. More than 90% of the added phosphate (1.55 µmol/m2) remained adsorbed at changing conditions, but a very large variation in the equilibrium concentration of phosphate in solution was observed (Figure 2). The data in the figure show that the addition of FA led to a very large increase in the equilibrium concentration of phosphate. The presence of FA increased the phosphate concentration by a factor of up to 100 at low pH. It illustrates that FA is in strong competition with phosphate at the surface of goethite. HA molecules have also competition with adsorbed phosphate, but the increase of the phosphate concentration is much smaller. The data show that the effect of the ionic strength is rather limited in contrast to the effect of a change in pH. Adsorption of HA and FA. The competitive effect between HA or FA with phosphate not only changed phosphate concentrations in solution, it also affected the binding of HA and FA molecules. The stronger competition between FA and phosphate than between HA and phosphate found in Figure 2 for phosphate is mutual. The presence of phosphate led to a large decrease of the amount of FA adsorbed over the entire pH range (Figure 3b). This is very different for the systems with HA. First of all, HA was more strongly bound in the absence of phosphate, and it remained also strongly bound in particular in the lower pH range when phosphate was added (Figure 3a). It has been shown previously that this strong binding of HA is related to the larger number of ligands per molecule and its corresponding charge (16, 18). VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Phosphate concentration in solution in the absence and presence of HA or FA in systems of 1 g/L goethite and 10 mM NaNO3 (closed symbols and thick lines) or 1 mM NaNO3 (open symbols and thin lines) with initially 0.15 mM P, 50 mg/L HA or FA. a: Results for HA. b: Results for FA. Symbols: experimental data. Lines: model calculations. Grey lines: model calculations by assuming an even distribution of adsorbed HA between the two compact layers (see text).

FIGURE 3. Fraction of HA and FA adsorbed to goethite in the absence and presence of phosphate in systems of 1 g/L goethite and 10 mM NaNO3 (closed symbols) or 1 mM NaNO3 (open symbols) with initially 0.15 mM P, 50 mg/L HA or FA. Despite the high amount of HA that was bound, a much smaller effect was found in terms of competition with phosphate as follows from Figure 2. It indicates that HA has only a moderate effect on phosphate adsorption. In other words, HA is strongly bound but surprisingly does not very strongly affect the phosphate binding, while FA is less strongly bound, but these molecules have a very large effect on the phosphate adsorption. Mechanisms of Interaction. The LCD model predicted the effects of adsorbed HA on phosphate concentration rather well (lines in Figure 2). Calculations show that the low fraction of the HA that is located by the model in the compact layer is the key factor for the correct prediction of the effect of HA on phosphate adsorption. If all HA ligands are located in the compact part of the EDL as is done for FA, the model would strongly overestimate the effect of HA on phosphate adsorption. As shown by the gray lines in Figure 2-a, using the same spatial distribution as assumed for FA, i.e., an even distribution between the two Stern layers, the model would largely over predict phosphate concentration in solution in the presence of HA. For phosphate adsorption in the presence of FA, the description of the data is equally good (lines in Figure 2). An equal distribution of FA over both compact layers is essential for the description of the interaction with phosphate. Locating all ligands of FA in the first compact layer (0-/1-plane) leads to overestimation of the competition for phosphate adsorption especially at low pH (results not shown). It is important to note that the charge distribution of the adsorbed humics is a strong function of pH and phosphate concentration because these factors affect the electrostatic potentials at goethite surfaces for a given distribution of the reactive ligands in space. The resulting charge distribution of HA/FA and the electrostatic potentials are output of the model calculations. 8750

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The above suggests that the main reason for the much stronger competition effects of FA in comparison with HA for phosphate adsorption to goethite is the difference in the spatial distribution of these types of molecules in the goethite-water interface. According to the model, the nature of the interaction of HA or FA with phosphate is mainly electrostatic. Site binding effects are of secondary importance. The HA and FA have large differences in the number of reactive groups per particle (mol/mol), which are related to the difference in particle size and therefore the molecular mass (Table 2). The smallest species is the phosphate ion (Figure 1). Its charge is divided between the surface plane (0-plane) and the 1-plane in the CD model (29). The potential at the surface (0-plane) is mainly determined by the concentration of the potential determining ion, i.e., by the pH. The electrostatic potential at the 0-plane does not change significantly at a given pH upon adsorption of other ions, in contrast to the potential at the 1-plane. The potential at the 1-plane will decrease as a function of adsorption of phosphate, HA and FA. The strongest effects of the adsorption of humics on the potential at the 1-plane will occur when all the added negative charge is located at the 1-plane itself. In the modeling, 50% of the FA ligands has been located at the 0-/1plane, whereas for HA it is only 1-5%. Although the charge of the humic acid beyond the 1-plane also affects the potential at the 1-plane, the major effect is caused by the charge that is directly present at the 1-plane. The much lower number of ligands of HA at the 1-plane will thus produce much less negative charge at the 1-plane compared to FA, which results in a less strong interaction with the phosphate ions that are located there. In other words, the difference in the degree of overlap of the location of charge of HA or FA with the charge of phosphate is the

FIGURE 4. Schematic representation of goethite surface (about 100 nm2) covered with phosphate and HA (a), or phosphate and FA (b).

FIGURE 5. Fraction of the bidentate phosphate species (-(Fe1O)2PO2-2) over the total amount of phosphate adsorbed to goethite in the absence or presence of HA or FA calculated with the LCD model. Systems contain 1 g/L goethite and 10 mM NaNO3 (thick lines) or 1 mM NaNO3 (thin lines) with initially 0.15 mM P, 50 mg/L HA or FA. major reason for the difference in interaction of HA and FA molecules with phosphate (Figure 1). The difference between HA and FA can also be visualized in the following way. Without HA or FA, the amount of phosphate adsorbed to goethite for the conditions of the experiments is on average 90 phosphate ions per 100 nm2 in the pH range studied. When HA or FA is present, there is 25-50 mg/m2 HA and 10-35 mg/m2 FA adsorbed. Using the average molecular mass of HA and FA (Table 2), the amount of HA and FA adsorbed is equivalent to, respectively, 1-2 HA nanoparticles and 11-26 FA nanoparticles per 100 nm2 (Figure 4). The larger effect of the FA on phosphate adsorption is thus due to a combination of a much larger number of particles adsorbed (although the amount in terms of mass is lower) and a shorter distance to the surface. Phosphate Surface Speciation. Adsorption of HA/FA to goethite not only influences the amount of phosphate adsorbed, but also its speciation. Model calculations show that the presence of adsorbed HA/FA led to a shift from the bidentate (-(Fe1O)2PO2-2) to monodentate (-Fe1OPO2OH-1.5) adsorbed phosphate species especially at low pH (Figure 5). At pH around 3, the percentage of bidentate species decreased from about 60% to about 50% in the presence of HA and in the presence of FA from about 60% to about 30%. In principle, such a change in speciation is experimentally accessible. The shift from the bidentate to monodentate phosphate species is mainly due to the electrostatic potential at the 1-plane,

which is more negative when HA or FA are present. The reduction of available surface sites on goethite due to the formation of innersphere complex with adsorbed HA/FA also favors the formation of monodentate over bidentate phosphate species, but model calculation shows that it is of much less importance compared to the effects of electrostatic potential. This change in potential at the 1-plane is unfavorable for formation of both types of phosphate species. However, because the monodentate species contributes less negative charge, i.e., -1.28 v.u. (valence units) to the 1-plane than the bidentate species (-1.46 v.u.) (Table 1), the reduction in the amount of the monodentate species is less than that of the bidentate. This leads to the relative shift toward monodentate species when HA or FA are present. This shift is stronger in the presence of FA than HA because of the closer location of FA to the surface and the larger number of adsorbed FA molecules, which led to a stronger reduction of the electrostatic potential at the 1-plane. Implications and Relevance. The strong interactions between HA/FA and phosphate that we have studied in this work have practical relevance. In nature, effects of natural organic matter (NOM) on solution concentration of phosphate can be stronger than observed in this study, due to a possible larger solid-solution ratio in natural samples than used in this experiment. For a proper application of the advanced knowledge in ion adsorption to mineral oxides in natural systems, the interaction with humics should be considered explicitly because it will have large effects on the adsorption of many ions. The knowledge on phosphate-NOM interaction at mineral surfaces may contribute to the improvement of our insights in the role of NOM in influencing the availability of inorganic phosphate for organisms in both terrestrial and aquatic ecosystems. NOM may also influence behavior of other anions such as arsenate and selenite in the environment. Competition between arsenic (As) with NOM for sorption may contribute to the mobility of this toxic element as found in Bangladesh (30, 31). The model used in this work is, as far as we are aware, the first attempt to describe or predict this interaction quantitatively using a mechanistic approach. The fact that it can predict the major effects that are observed experimentally is very encouraging.

Acknowledgments This research was partly funded by the EU project, FUNMIG (516514, F16W-2004). We thank Gerlinde Vink for helping with the experiment. VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available Two additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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