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DIVISION S-2-SOIL CHEMISTRY

Interaction of Carbonate and Organic Anions with Sulfate and Selenate Adsorption on an Aluminum Oxide

^a Connecticut Agric. Exp. Stn., P.O. Box 1106, New Haven, CT 06504 USA
^b Dep. of Plant Science, U-67, Univ. of Connecticut, Storrs, CT 06269 USA

c.schulthess{at}uconn.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The presence of anionic cosolutes often influences anion adsorption on soil mineral surfaces. The possible effect of common solutes, such as CO₃ species, on the adsorption of other anions on metal (hydr)oxides is poorly understood. Accordingly, we determined the effect of CO₃, formate, acetate, oxalate, and citrate on the adsorption of SO₄ and SeO₄ on Al oxide. Carbonate markedly promotes the adsorption of SO₄ and SeO₄ between pH 6 and 8. The maximum effect occurs at relatively low CO₃ concentrations (0.3 mM) and the promotive effect decreases with further increase in CO₃ adsorption. Formate and acetate also promote adsorption of SO₄ and SeO₄, while oxalate and citrate have a competitive effect, especially at lower pH values. Based on existing Fourier transformed infrared (FTIR) spectroscopic data and new electrophoretic mobility data, CO₃ may promote the adsorption of other oxyanions by forming extra reactive protonated sites that exist with the adsorbed CO₃. Acetate and formate presumably interact through the same mechanism. This promotive mechanism was ineffective in the case of adsorption of a high affinity anion such as PO₄. Finally, the adsorptive properties of Al oxide, as we know it from simple model systems, can significantly be altered in soils, where CO₃ and organic anions are ubiquitous solutes. For example, the adsorption maximum of SO₄ > SeO₄ became in the presence of CO₃.

Abbreviations: CV, coefficient of variance • DRIFT, diffuse reflectance infrared Fourier transformed • EM, electrophoretic mobility • FTIR, Fourier transformed infrared • IC, ion chromatograph • LMW, low molecular weight • TIC, total inorganic C • TOC, total organic C

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
ANION ADSORPTION on soil mineral surfaces is often affected by the interactive adsorption of anionic cosolutes (Kafkafi et al., 1988; Inskeep, 1989; Mesuere and Fish, 1992; Van Geen et al., 1994; Ali and Dzombak, 1996; Manning and Goldberg, 1996; Geelhoed et al., 1997, 1998). In general, interaction effects between ions for adsorption can be either indifferent, competitive, or promotive (Davis and Kent, 1990) and can influence the bioavailability and mobility of nutrients and contaminants in soils and water (Lopez-Hernandez et al., 1986; Melamed et al., 1995; Martinez et al., 1998). Indifferent and competitive effects are commonly observed in multi-anion adsorption studies. A promotive effect between anions is rarely reported (for example Kafkafi et al., 1988), yet it is sometimes observed in published data (Neal et al., 1987).

Dissolved CO₂ and CO₃ species are abundant solutes in soils (Fernandez and Kosian, 1987; Castelle and Galloway, 1990) and are known to adsorb on the mineral surfaces of Al and Fe (hydr)oxides (Parfitt, 1978; Schulthess and McCarthy, 1990; Van Geen et al., 1994). In turn, the adsorbed CO₃ affects surface chemical properties such as surface charge and protonation (Zeltner and Anderson, 1988; Lumsdon and Evans, 1994; Schulthess et al., 1998) and adsorption of other ions (Parfitt, 1978; Hingston, 1981). However, dissolved CO₂ and CO₃ species have mainly been neglected or specifically avoided in sorption studies, and their possible impact on the reactivity of metal oxide surfaces for adsorption of other important anions has not been fully appreciated (Lumsdon and Evans, 1994).

The limited number of studies that have included CO₃ in multisorbate systems indicates that CO₃ can compete with the adsorption of other oxyanions on metal (hydr)oxides when it is present in large excess relative to the other anions. Phosphate adsorption on Al oxide was slightly reduced in the presence of relatively high HCO₃ concentrations (Chen et al., 1973). Selenite adsorption on goethite was reduced only at CO₃ concentrations that were more than 1000 times higher than SeO₃ (Balistrieri and Chao, 1987). Chromate adsorption on Fe (hydr)oxides was reduced in systems with elevated CO₂ pressures (Zachara et al., 1987; Van Geen et al., 1994). Duff and Amrhein (1996) found that, at high CO₃ alkalinity, CO₃ competed effectively with the adsorption of anionic U(VI) species on goethite. The adsorption of a low affinity anion, such as acetate on Al oxide, was depressed when CO₃ was present at an equimolar concentration (Schulthess and McCarthy, 1990). The effect of CO₃ on the adsorption of the moderate affinity anions, such as SO₄ and SeO₄, on metal (hydr)oxides, to our knowledge, has not been documented in the literature.

It is useful to extend this question to the effect of organic compounds, such as low molecular weight (LMW) anions of the organic acids (namely, formic acid, acetic acid, citric acid, and oxalic acid) that are common in soils (Fox and Comerford, 1990; Stevenson, 1994; Lundegard and Kharaka, 1994). Organic anions adsorb to soil mineral surfaces (Sposito, 1989; Filius et al., 1997) and can affect the adsorption of other ions such as PO₄, SO₄, and SeO₃ (Nagarajah et al., 1970; Earl et al., 1979; Kafkafi et al., 1988; Inskeep, 1989; Violante et al., 1991; Violante and Gianfreda, 1993; Dynes and Huang, 1997). These studies indicate that the effect of organic ligands on the adsorption of other oxyanions depends on the type of organic ligand. Multicarboxylic ligands such as oxalate, citrate, malate and tartrate have a relative high adsorption affinity, and can compete with the adsorption of oxyanions on Al hydroxides (Violante et al., 1991; Dynes and Huang, 1997). Monocarboxylic organic ligands, such as formate and acetate, are weaker adsorbates and compete less with oxyanions (Earl et al., 1979; Dynes and Huang, 1997). Accordingly, most studies have focused on the stronger adsorbing multicarboxylic ligands (e.g., oxalate, citrate, malate, and tartrate). The weaker adsorbing monocarboxylic acetate and formate anions have received much less attention and little is known about their possible effect on the adsorption of anions with moderate affinity for adsorption, such as SO₄ and SeO₄.

The extent of the interaction effects between anions for adsorption depends on the relative concentrations, the intrinsic adsorption affinities of the anions, and the pH (Mesuere and Fish, 1992, Geelhoed et al., 1997, 1998; Dynes and Huang, 1997; Karltun, 1998). Formate, acetate, CO₃, SO₄, and SeO₄ possess low to moderate affinity for adsorption on metal (hydr)oxides (Sposito, 1989; Dynes and Huang, 1997) and are quite sensitive to competition from high affinity anions. For example, the presence of high affinity anions such as PO₄, oxalate, and citrate substantially reduce the adsorption of SO₄ on Fe (hydr)oxides (Inskeep, 1989; Geelhoed et al., 1997; Karltun, 1998). Moderate affinity anions such as SO₄ and SeO₄ can also compete with each other for adsorption (Ghosh et al., 1994). Typically, the adsorption of these moderate affinity anions is also sensitive to the concentration of the background salt (Hayes et al., 1988; Schulthess and McCarthy, 1990; He et al., 1997). This has commonly been interpreted as an indication for the formation of an outer-sphere surface complex of these anions (Hayes et al., 1988; He et al., 1996, 1997). Spectroscopic studies have provided contradicting data about the type of surface complexes of SO₄ and SeO₄ on metal (hydr)oxides. Outer-sphere surface complexes were found by Hayes et al. (1987) and Persson and Lövgren (1996), while inner-sphere complexes were suggested by Hug (1997) and Manceau and Charlet (1994). Recently, it has been suggested that a spectrum of intermediate behaviors is likely. Adsorbates classed as outer-sphere may have small subpopulations of inner-sphere complexes at a given point in time (Eggleston et al., 1998).

The FTIR spectra of adsorbed CO₃ species at the hydrated -Al₂O₃ surface show that only the monodentate inner-sphere complexed CO₃ anion species is present at the surface in the pH range of 5.2 to 7.2 (Wijnja and Schulthess, 1999). The spectra also indicated the existence of extra protonated surface groups associated with the adsorbed CO₃. This agrees with the macroscopic titration data presented by Schulthess et al. (1998), where, overall, approximately one proton coadsorbs with each adsorbing HCO₃ anion. Since surface protonation plays an important role in the adsorption of ions on metal (hydr)oxides, enhanced surface protonation due to CO₃ adsorption may affect the reactivity of Al oxide for adsorption of other anions.

The objective of our study was to determine the effect of the presence of CO₃ on the adsorption of SO₄ and SeO₄ on the hydrated -Al₂O₃. Some LMW organic anions (formate, acetate, oxalate, and citrate) were included in this study to compare inorganic C effects with the effects of its organic counterparts. The adsorption edges and isotherms of SO₄ and SeO₄ were determined in batch adsorption experiments in the absence and presence of CO₃ or organic anions. The CO₃ and organic anion adsorption were also measured in order to determine the adsorption affinities of the various different anions in the presence and absence of SO₄ and SeO₄. In addition, the effects of anion adsorption on the surface charge were determined by electrophoresis. Possible mechanisms for the observed interaction effects are discussed based on the knowledge of the effects of CO₃ adsorption on the Al oxide from previous studies (Schulthess et al., 1998; Wijnja and Schulthess, 1999) and the surface charge effects of anion adsorption on Al oxide provided by electrophoretic mobility measurements.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Adsorbent
The Al oxide used in this study was a -Al₂O₃ (Aluminum Oxide-C, DeGussa Corp., Akron, OH). The manufacturer provided a Brunauer-Emmett-Teller surface area of 100 ± 15 m² g^-1 with impurities <0.4%. Several batches of stock suspensions were prepared during this study. The stock suspensions were prepared by adding the Al oxide to deionized water (Milli-Q 50, Millipore Corp., Bedford, MA). The suspension was stirred continuously in a closed glass container at room temperature. The Al oxide suspension was washed according to a procedure similar to the one described by Schulthess and McCarthy (1990). In order to remove anionic impurities, the pH of the suspension was increased to pH 8.5 by adding NaOH. After 2 h, this suspension was centrifuged and the solid was resuspended in deionized water. Then, the pH of the suspension was decreased to pH 3.6 by adding HCl to remove cationic impurities. After 2 h, this suspension was centrifuged, the supernatant was decanted, and the solids resuspended in deionized water. Subsequently, an additional 8 to 10 washings with deionized water were done for 10 to 30 d until the pH in the supernatant remained constant at pH 4.4 ± 0.05 and the electric conductivity was 50 µS cm^-1. Analysis of the anions present in the supernatant (using ion chromatography, described below) showed that Cl was the only detectable anion. The Al oxide was water-suspended for at least 1 mo to let stabilize the transformation process of the Al oxide surface into an Al hydroxide phase (bayerite) (Dyer et al., 1993; Wijnja and Schulthess, 1999). Laiti et al. (1998) reported that the specific surface area did not significantly change with aging in aqueous suspension. Prior to its first use for adsorption experiments, the suspension was purged with pure air (CO₂ free) for at least 7 d to remove autochthonous CO₃ from the suspension.

Adsorption Experiments
The anion adsorption edges and isotherms were determined in batch adsorption experiments. Aliquots of the aged Al oxide suspension (containing 0.6 g of Al oxide) were transferred into a 50-mL (nominal) polyallomer centrifuge tube. To each sample, a specific volume of deionized water was added to achieve a final volume of 35 mL. A specific volume of 0.5 M NaCl was added to achieve a final background salt concentration of 0.011 M NaCl. That is, the concentration of total added Cl (NaCl + HCl) was 0.011 M in all samples. If additions of NaOH were required to achieve a desired pH, specific volumes of 0.192 M NaOH were added at this point in the sample preparation procedure. Specific volumes of either 0.1 M Na₂SO₄ or 0.1 M Na₂SeO₄ were added to achieve desired initial concentrations of SO₄ or SeO₄. Next, if desired, specific volumes of either 0.1 M NaHCO₃ or 0.1 M solutions of the Na salts of formate, acetate, oxalate, or citrate were added to achieve the desired initial concentration. Then, if needed for the pH adjustment, a specific volume of 0.177 M HCl was added. Stock solutions of the Na salts of the anions used as adsorbates in this study were prepared using deionized water and analytical grade reagents. The organic anions studied were Na salts of formate (HCOONa), acetate (CH₃COONa), citrate (NaOOCCH₂C(OH)(COONa)CH₂COONa), and oxalate (NaOOCCOONa). The dissociation constants of these compounds are: for formate; for acetate; , for oxalate; and , , .39 for citrate (Lide, 1995).

Adsorption edges of SO₄ and SeO₄ were determined at an initial concentration of 1 mM. The pH in these samples ranged from 5 to 9.5. The CO₃ or organic anion was concurrently added at the same initial concentration of 1 mM in the binary-anion systems. The effect of varying CO₃ concentrations was determined by measuring SeO₄ adsorption in binary-anion systems of 1 mM SeO₄ and 0.1 to 3 mM CO₃ in the pH range of 6.5 to 7.2. In order to study the effect of SeO₄ on the SO₄ adsorption and vice versa, a series of samples was prepared with initial concentrations of 1 mM of both anions. The adsorption of CO₃ or organic anion in single adsorbate systems was determined in order to evaluate the effect of SO₄ or SeO₄ on the adsorption of CO₃ or organic anion.

Adsorption isotherms of SO₄ and SeO₄ in the presence and absence of CO₃ were determined at a pH value of 6.9. A series of samples was prepared in which the initial concentrations ranged from 0.5 to 3 mM Na₂SO₄ or Na₂SeO₄. In the binary-anion systems, HCO₃ was added at an initial concentration of 1 mM. For each specific initial concentration of either SO₄ or SeO₄, several samples were prepared, which usually resulted in an equilibrium pH value somewhat above or below the target pH of 6.9 ± 0.4. The adsorption and the corresponding equilibrium concentration of SO₄ or SeO₄ at pH 6.9 were then determined by interpolation of the measured data at that particular initial concentration.

The prepared samples were mixed for 19 to 24 h on a hematology mixer at 20°C, and then centrifuged for 20 min at 20000 g. An aliquot of the supernatant was then collected for the anion measurements. The pH was measured in the supernatant using a combination calomel electrode (Accu-pHast, Fisher Scientific, Pittsburgh, PA).

The SO₄ and SeO₄ concentrations in the supernatants were determined using a Dionex-300 ion chromatograph (IC) (Dionex Corp., Sunnyvale, CA) with a Dionex IonPac AS4A SC column connected to an anion self-regenerating suppressor (ASRS) and a conductivity detector (CDM-3). The eluent was 1.8 mM CO₃/1.7 mM HCO₃ with a flow rate of 2 mL min^-1. Peak areas were measured with a Dionex 4400 integrator. The retention times were 3.9 min for SO₄ and 4.7 min for SeO₄. The supernatants were diluted 20 to 40 times for the measurement on the IC. The coefficient of variance (CV) in the SO₄ and SeO₄ measurements was typically <4%.

The concentrations of CO₃ and organic anions were measured using a total organic C (TOC), total inorganic C (TIC) analyzer (TOC-5000, Shimazdu Corp., Braintree, MA) according to the procedure outlined by Schulthess and McCarthy (1990) and Schulthess et al. (1998). The CV value for the TIC measurements was <2% and for the TOC measurements was <1%.

Propagation-of-error analysis indicated that the variance in the IC and TIC–TOC measurements were the major source of experimental error in the obtained adsorption data. The error in the adsorption data was typically ± 0.01 µmol m^-2 for SO₄, SeO₄, and CO₃, and ± 0.006 µmol m^-2 for the organic anions. The data from different Al oxide stock suspensions showed relatively small differences, typically <0.02 µmol m^-2. The uncertainty in the pH measurements of the samples without CO₃ or organic anions was ± 0.05 to 0.10 pH units. The samples with CO₃ or organic anion were more stable, and the uncertainty in pH of these samples was typically less than ± 0.02 pH units.

The Al concentration in the supernatants was measured in order to determine the effect of the anions on the dissolution of Al oxide. The supernatants were filtered (0.1-µm Supor membrane filter, Gelman Sciences, Ann Arbor, MI), acidified with concentrated HNO₃, and analyzed for Al by inductively coupled plasma atomic emission spectroscopy.

The electrophoretic mobility (EM) of Al oxide was measured using a Laser Zee meter (Model 501, Pen Kem, Bedford Hills, NY). In order to perform EM measurements, a 0.25-mL aliquot of a batch adsorption sample was diluted in 30 mL of its own supernatant, resulting in a solid concentration of 0.14 g L^-1.

Diffuse Reflectance Infrared Fourier Transformed Spectroscopy
The diffuse reflectance infrared Fourier transformed (DRIFT) spectra were collected with a Perkin-Elmer FTIR 1600 spectrophotometer (Perkin-Elmer, Norwalk, CT) and a Perkin-Elmer diffuse reflectance sampling cell following the procedure described by Wijnja and Schulthess (1999). After centrifuging the Al oxide suspensions, an aliquot of the oxide paste was spread onto a paper filter (Whatman no. 1) and was allowed to dry 30 to 45 min in a desiccator that was purged with CO₂-free air. A 0.010-g sample of this dried oxide was mixed gently with 0.30 g of KBr, using a pestle and mortar, and analyzed in the diffuse reflectance cell. The KBr powder was used as a background. In addition to each suspension sample containing CO₃, a reference suspension without CO₃ or SO₄ but with similar pH and ionic strength was prepared. The spectrum of the reference Al oxide was subtracted from the spectrum of the Al oxide with adsorbed CO₃ or SO₄. The spectra were the result of 64 scans.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Effect of Carbonate
The SO₄ and SeO₄ adsorption edges and envelopes in the presence and absence of CO₃ are shown in Fig. 1 . These data show a startling effect: in the presence of CO₃, the SO₄ or SeO₄ adsorption are markedly increased in the pH range of 6 to 8. The adsorption edges are clearly shifted to a higher pH in the presence of CO₃. The maximum effect is around pH 7, where the adsorption of SO₄ and SeO₄ was more than doubled. The promotive effect diminished at pH <6, where adsorption was approaching 100%, and at pH >8, where the adsorption of SO₄, SeO₄, and CO₃ were low. In contrast, a slight competitive effect can be observed between the SO₄ and SeO₄ adsorption. Specifically, SO₄ decreased the adsorption of SeO₄, while SeO₄ decreased the adsorption of SO₄.

View larger version (24K): [in this window] [in a new window]   Fig. 1 Adsorption edges of (a) SO4 and (b) SeO4 in the presence and absence of CO3 and in binary-anion systems of SO4 and SeO4

View larger version (17K): [in this window] [in a new window]   Fig. 2 Adsorption of SeO4 at pH 6.8 as a function of the total CO3 concentration

View larger version (19K): [in this window] [in a new window]   Fig. 3 Adsorption isotherms of (a) SO4 and (b) SeO4 in the presence and absence of CO3. Lines are Langmuir fits determined according to the normal nonlinear least squares (NLLS) regression outlined by Schulthess and Dey (1996) with the following parameter values: SO4: and ; SO4 + CO3: and ; SeO4: and ; SeO4 + CO3: and ; . Note that the vertical NLLS regression yields nearly identical results

The effect of the presence of SO₄ and SeO₄ on the adsorption of CO₃ is shown in Fig. 4 . The CO₃ adsorption is lower in the presence of SO₄ or SeO₄ compared with the systems with only CO₃. The CO₃ adsorption further decreased with increasing SO₄ concentrations. Thus, while the presence of CO₃ promoted the adsorption of SO₄ and SeO₄, the effect on CO₃ was competitive. A possible explanation for the difference in these interaction effects will be discussed with the EM data below.

View larger version (23K): [in this window] [in a new window]   Fig. 4 Adsorption of CO3 in the presence and absence of SO4 and SeO4

Effect of Low Molecular Weight Organic Anions
The finding of this intriguing phenomenon of a promotive effect of CO₃ on the adsorption of SO₄ and SeO₄ raises the question if it is unique for the interaction of CO₃ with oxyanions or if its organic counterparts have similar promotive effects. The effect of the LMW organic anions on the adsorption of SO₄ and SeO₄ is shown in Fig. 5 . The presence of acetate and, to a lesser extent, formate promoted the SO₄ and SeO₄ adsorption in the pH range of 6 to 8 for acetate and a somewhat narrower pH range for formate. In contrast, the presence of oxalate decreased the SO₄ adsorption at pH <6.7, while it had no significant effect at pH >6.7. Citrate had the strongest competitive effect and substantially decreased the SO₄ and SeO₄ adsorption across the entire pH range studied.

View larger version (26K): [in this window] [in a new window]   Fig. 5 Effect of the presence of organic anions on the adsorption edges of (a) SO4 and (b) SeO4

The determination of the adsorption of both anions in the binary- and single-anion adsorption experiments can provide useful information about the interaction effects on adsorption and the stoichiometries of interactive adsorption (Violante et al., 1991; Geelhoed et al., 1998). The adsorption data of the organic anions in the absence and presence of SO₄ or SeO₄ are shown in Fig. 6 . The formate and acetate adsorption affinity in the single-sorbate systems was relatively low compared with the adsorption of oxalate and citrate, which is consistent with other published data (Ward and Brady, 1998). Citrate adsorbed almost completely below pH 7, which is similar to what has been observed for citrate adsorption on pseudoboehmite (Cambier and Sposito, 1991). Oxalate showed a more moderate adsorption affinity similar to what has been observed by others (Mesuere and Fish, 1992; Karltun, 1998; Violante et al., 1991).

View larger version (35K): [in this window] [in a new window]   Fig. 6 The adsorption of (a) acetate, (b) formate, (c) oxalate, and (d) citrate in the presence and absence of SO4 and SeO4

The presence of SO₄ or SeO₄ in the binary-solute systems substantially decreased the adsorption of formate and acetate (Fig. 6). The oxalate and citrate adsorption, in contrast, were not much affected by the presence of SO₄ or SeO₄; they were almost completely adsorbed around pH 5.5. Dynes and Huang (1997) also observed much lower adsorption intensities for acetate and formate than oxalate and citrate on Al hydroxide in the presence of SeO₃ at pH 5; however, their fractions of oxalate and citrate that were adsorbed were much lower. This may be the result of the much lower solid concentrations used by Dynes and Huang (1997). Mesuere and Fish (1992) found that the presence of CrO₄, which showed a higher adsorption affinity than oxalate, only resulted in a very small decrease in the oxalate adsorption on goethite at low adsorption densities and equimolar total concentrations. In contrast, the presence of PO₄ can significantly reduce the citrate and oxalate adsorption on metal (hydr)oxides (Violante et al., 1991; Geelhoed et al., 1998). Apparently, SO₄ and SeO₄ have a lower adsorption affinity and are less effective in competing with oxalate and citrate for adsorption relative to oxyanions with high adsorption affinity (such as SeO₃ and PO₄).

The solubility of Al oxide was measured to confirm that our sorption data above were indeed the result of adsorption processes rather then Al–ligand complexation and precipitation reactions. The Al concentrations were 1.3 ± 0.3 µM for the various single and binary anion batch adsorption systems at pH 6. These data indicate that the dissolution of Al oxide is not substantially affected by the presence of various anions in the pH range of 5 to 7. Under the same conditions, He et al. (1996) did not observe an increase in dissolution in the presence of SO₄. These observations indicate that in our systems of Al (hydr)oxide suspensions, adsorption processes were responsible for the observed anion partitioning phenomena.

Mechanistic Considerations
Competitive interaction between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on surface charge and protonation (Mesuere and Fish, 1992; Geelhoed et al., 1997, 1998; Dynes and Huang, 1997). Promotive interaction between different anionic adsorbates can only be caused by indirect effects; that is, the promoter anion must cause an alteration of surface protonation or surface charge that favors the adsorption of the coadsorbing anion.

The FTIR data presented by Wijnja and Schulthess (1999) indicated that CO₃ adsorption on Al oxide results in monodentate CO₃ surface species and additional protonated surface groups associated with it. The following concurrent adsorption reaction equations were proposed:

(1)

(2)

Assuming no competition of these reactions with the natural surface protonation reactions observed in the absence of CO₃ adsorption, this set of adsorption reactions suggests no change in surface charge upon CO₃ adsorption. In the presence of competition, the net surface charge is reduced. Therefore, it is useful to evaluate the surface charge effect of CO₃ adsorption on Al oxide provided by EM measurements.

The EM data for Al oxide in the presence and absence of adsorbed carbonate are shown in Fig. 7A . These data show that at pH <7, where CO₃ adsorption reaches its maximum (Fig. 4), the EM value of Al oxide with adsorbed CO₃ is indeed very similar compared with the system with only NaCl. In the pH range of 7 to 9, there is a small decrease in EM upon CO₃ adsorption. In this pH range, only a relatively small fraction of surface sites can protonate. Carbonate occupies surface sites that otherwise could have been protonated and, thereby, decreases the surface charge to some extent. At pH 7, the effect of CO₃ is compensated by further protonation of surface groups. Carbonate adsorption at pH >9 is very low (Schulthess et al., 1998) and, consequently, there is only a very small decrease in isoelectric point upon CO₃ adsorption.

View larger version (35K): [in this window] [in a new window]   Fig. 7 The effect of adsorbed anions on the electrophoretic mobility (EM) of Al oxide. (a) Effect of CO3, SO4, and SeO4. (b) Effect of SO4 in single-anion systems and in binary-anion systems with CO3. (c) Effect of organic anions. (d) Effect of organic anions in binary-anion systems with SO4. The blank samples contain only the background salt NaCl. All systems contained a background salt concentration of 0.011 M NaCl

Surface protonation plays an important role in the adsorption of oxyanions on metal (hydr)oxides. Outer-sphere complexation with a protonated surface site (S-OH⁺₂) has been proposed as a plausible adsorption mechanism for both SO₄ and SeO₄ (Zhang and Sparks, 1990a, 1990b; He et al., 1996). Inner-sphere complexation through exchange reaction with a S-OH⁺₂ group has also been suggested for both anions (Hug, 1997; Manceau and Charlet, 1994). Moderate affinity anions such as SO₄ and SeO₄, therefore, only adsorb at pH values below the point of zero charge of the metal (hydr)oxides, where the surface becomes protonated (Geelhoed et al., 1997). This is also observed in our systems, where SO₄ and SeO₄ have adsorption edges below pH 7 in the absence of CO₃ (Fig. 1). At pH >6.5, where the SO₄ and SeO₄ adsorption intensity are relatively low, significant CO₃ adsorption occurs (Fig. 4). The additional protonated surface groups that coexist with adsorbed CO₃ can, in this case, provide additional complexation sites and result in the enhanced adsorption of SO₄ and SeO₄ in the binary-solute systems. At pH <6, the increased surface protonation that occurs solely due to the lower pH becomes significant and provides sufficient sorption sites for SO₄ and SeO₄. The extra protonation due to adsorbed CO₃ is not significant under these low pH conditions.

DRIFT spectra of Al oxide with adsorbed SO₄ and CO₃ (Fig. 8) also indicate that the extra protonated surface groups that coexist with adsorbed CO₃ play a role in the adsorption enhancement of SO₄. The sharp OH-stretching bands at 3514 and 3446 cm^-1 in the spectrum of Al oxide with adsorbed CO₃, which were attributed to extra protonated surface groups by Wijnja and Schulthess (1999), are substantially decreased in intensity in the presence of coadsorbed SO₄. New OH-stretching bands appear in the 3600 to 3750 cm^-1 region in the presence of SO₄ and SO₄ with CO₃. These bands may be due to protonation of surface groups that coexists with adsorbed SO₄ on Al oxide. Presumably, each conjugate anion (CO₃ and SO₄) adsorbed on Al oxide influences the H-stretching vibrations of the protonated surface groups to various different degrees.

View larger version (24K): [in this window] [in a new window]   Fig. 8 Diffuse reflectance infrared Fourier transformed (DRIFT) spectra of Al oxide with adsorbed CO3 and/or SO4. These difference spectra are the result of subtraction of a spectrum of Al oxide with only NaCl from a spectrum of Al oxide with adsorbed CO3 and/or SO4. The pHs of the Al oxide suspensions were 6.5 ± 0.1

The promotive effect of formate and acetate on SO₄ and SeO₄ adsorption could also be the result of increased surface protonation. Formate and acetate are known to be relatively weak adsorbing anions, forming mainly outer-sphere surface complexes with mineral surfaces (Ward and Brady, 1998; Persson et al., 1999). The EM data of Al oxide in the presence of formate and acetate in Fig. 7C and 7D show that the EM is slightly increased compared with the system with only NaCl. These data suggest a slightly more positive Al oxide in the presence of these organic anions, which, in turn, promotes the adsorption of SO₄ and SeO₄. Spectroscopic data are needed to obtain direct evidence of extra protonated surface groups for these LMW organic anion systems.

In contrast to SO₄ and SeO₄, high affinity anions such as PO₄ show adsorption edges at higher pH values, and adsorption occurs even at pH values above the point of zero charge (Geelhoed et al., 1997; He et al., 1997). This indicates that the adsorption of PO₄ is less dependent on surface protonation and that the chemical energy of adsorption is relatively high. Adsorption experiments with PO₄ in our systems under the same conditions as with SO₄ and SeO₄ showed an adsorption edge similar to that observed by He et al. (1997) with more than 90% adsorption at pH <8.5 (data not shown). Phosphate is a strong competitor with CO₃ for adsorption at this pH. The promotive interaction of adsorbed CO₃ becomes ineffective in this case. The adsorption data indeed showed that the presence of CO₃ did not have a significant effect on the PO₄ adsorption (data not shown).

A similar interaction effect could explain why the presence of CO₃ did not promote the adsorption of CrO₄ on Fe (hydr)oxides (Zachara et al., 1987; Van Geen et al., 1994). Chromate also shows adsorption edges at higher pH than CO₃ and, consequently, the promotive effect of adsorbed CO₃ was ineffective. In addition, the total CO₃ concentration in their systems was higher than in our systems. The magnitude of the promotive effect decreased with higher CO₃ concentrations (Fig. 2). In addition, adsorption experiments in systems with goethite showed that the promotive effect of CO₃ on SO₄ and SeO₄ was smaller than on Al oxide (Wijnja, 1999); in systems with 3 mM CO₃ even a slight competitive effect was observed on goethite. These observations indicate that, relative to the zero CO₃ condition, the interaction effect of CO₃ becomes more neutral or perhaps even competitive with increasing CO₃ concentration. Relative to initial concentrations of CO₃ >0.3 mM, all larger CO₃ concentrations yield results that will be erroneously perceived as exclusively competitive.

The presence of oxalate did not significantly affect the SO₄ and SeO₄ adsorption at pH >6.7 (Fig. 5). In this pH range, the oxalate adsorption substantially decreased with increasing pH (Fig. 6C). The oxalate may not be adsorbed enough to compete with the other anions. On the other hand, the oxalate adsorption density was still much higher than SO₄ or SeO₄ in this pH range. Alternatively, the absence of significant competition effects may suggest that oxalate adsorption is site-specific to some extent. Violante and Gianfreda (1993) also suggested that the interaction effects of oxalate with PO₄ adsorption on an Al hydroxide montmorillonite complex were affected to some extent by site-specific adsorption of oxalate anions. At pH <6.7, the adsorption of SO₄ or SeO₄ and oxalate further increases. At these higher adsorption densities, however, SO₄ and SeO₄ are more affected by the presence of adsorbed oxalate (Fig. 5). The EM data in Fig. 7C indicate that the presence of oxalate decreased the EM to an extent similar to SO₄ and SeO₄ (Fig. 7A). The EM data in the binary anion systems (Fig. 7D) show clearly the further decrease in EM at lower pH values associated with the higher total anion adsorption density.

Citrate had the highest adsorption affinity of all of the anions studied and almost completely adsorbed at pH <7 (see Fig. 6D). Citrate adsorption had also the largest effects on the EM data as shown in Fig. 7C and 7D. In addition, citrate is the largest molecule and, consequently, had the largest surface coverage. These properties resulted in a strong competitive effect with SO₄ and SeO₄ across the entire pH range studied (Fig. 5).

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
A novel observation in this adsorption study is that, besides the commonly observed competitive or indifferent interaction effects, certain anions can also promote the adsorption of other anions. The presence of CO₃ enhanced the adsorption of SO₄ and SeO₄ on Al oxide between pH 6 and 8. The maximum effect was in the pH range of 6.8 to 7 at relatively low CO₃ concentrations and decreased at higher CO₃ concentrations. The small monocarboxylic organic formate and acetate anions also promoted the SO₄ and SeO₄ adsorption in the pH range of 6 to 8, but the effect was somewhat smaller than the effect of CO₃. In contrast, the more bulky polycarboxylic organic anions, oxalate and citrate, adsorbed strongly and competed with SO₄ and SeO₄ for adsorption. In binary-solute systems, SO₄ and SeO₄ also showed a competitive effect toward each other.

A possible mechanism for the promotive effect of CO₃ is the generation of extra adsorption sites by extra protonated surface groups that coexist with adsorbed CO₃. Formate and acetate may also enhance the surface protonation and, thereby, enhance the adsorption of SO₄ and SeO₄ through a similar mechanism as CO₃. This mechanism (of promotive interaction through extra coprotonated surface sites) is only effective in the pH range where the fraction of surface protonation would be otherwise low in the absence of enhancing anions. The relative adsorption affinity of the interacting anions, therefore, is critical in the interaction between the different anions. The adsorption of moderate affinity anions, such as SO₄ and SeO₄ that have adsorption edges at lower pH than CO₃, are enhanced in the pH range where surface protonation is limited. In the same pH range, a high affinity anion, such as PO₄, adsorbs strongly and, consequently, is not affected by the presence of CO₃. With increasing total anion adsorption densities, competitive interaction also begins to play a role and counteracts with the promotive effect.

The effects of CO₃ on the adsorptive properties of Al oxide highlight the importance of taking into account the effects of dissolved CO₃ species in sorption studies with metal (hydr)oxides. Previous studies already showed that the presence of adsorbed CO₃ affects the surface charge and protonation of these minerals (Zeltner and Anderson, 1988; Lumsdon and Evans, 1994; Schulthess et al., 1998). Our study shows that these effects of adsorbed CO₃, in turn, can significantly affect the adsorption of moderate affinity anions such as SO₄ and SeO₄ on Al oxide. Furthermore, the promotive interaction effect of CO₃ diminished the difference in adsorption intensities between SO₄ and SeO₄. This indicates that affinity sequences for anions as observed in single-solute systems can easily be skewed in natural environments due to the presence of CO₃. It can also cause much variability in laboratory results when the CO₃ in the prepared samples is allowed to vary depending on the degree of exposure to the atmosphere.

This study indicates that the adsorptive properties of Al oxide, as we know it from single-solute systems, can significantly be altered in natural soils, where CO₃ and organic anions are ubiquitous solutes. These effects should be taken into account in the transfer of knowledge from simple model systems to the more complex field scenarios. The surface of Al oxides present in natural environments may have substantial amounts of CO₃ adsorbed. This may enhance the adsorption of other moderate affinity anions such as SO₄ and SeO₄. The presence of organic anions may enhance or decrease the adsorption of these oxyanions depending on the type of organic anion that is dominant in the system.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Storrs Agric. Exp. Stn. Scientific Contribution no. 1853.

Received for publication December 15, 1998.

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