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Competitive adsorption involving phosphate and benzenecarboxylic acids on goethite—Effects of molecular structures
Journal of Colloid and Interface Science 343 (2010) 263–270
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Competitive adsorption involving phosphate and benzenecarboxylic acids on goethite—Effects of molecular structures Malin Lindegren, Per Persson * Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden
a r t i c l e
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Article history: Received 19 September 2009 Accepted 18 November 2009 Available online 23 November 2009 Keywords: Phosphate Benzenecarboxylic acid Competitive adsorption Infrared spectroscopy
a b s t r a c t The competitive adsorption between phosphate and either one of seven benzenecarboxylates (benzoate, phthalate, trimellitate, trimesoate, hemimellitate, pyromellitate, and mellitate) on the surfaces of fine-particulate goethite (a-FeOOH) was investigated as a function of pH. The series of ligands contained molecules with an increasing number of functional groups as well as three structural isomers of the tricarboxylates. Thus, the effects of both the number of carboxylate groups and the relative positions of these groups on the competitive efficiency toward phosphate were probed in this study. Quantitative adsorption experiments in batch mode and infrared spectroscopy were collectively used to evaluate the competitive adsorption reactions. Under the conditions probed, mono- and dicarboxylates had no detectable effect on phosphate adsorption whereas the ligands containing three or more carboxylate groups were able to partially outcompete phosphate. However, the pH dependency and the extent of these competitive effects were strongly dependent on the structure and composition of the benzenecarboxylate. The collective results showed that it was the competition for hydrogen-bonding surface sites rather than inner sphere surface sites that primarily determined the outcome of the competitive adsorption experiments, and it was the ability of the organic ligand to act as hydrogen-bonding acceptor and/or donor in various parts of the pH range that also determined the competitive pH dependency. The importance of H-bonding for the competitive adsorption between phosphate and benzenecarboxylic acids suggested that H-bonding interactions contributed substantially to the stabilities of both the adsorbed benzenecarboxylates and the phosphate ions and that these interactions were structurally specific; i.e., they were sensitive to the locations and the directional properties of the H-acceptor and H-donor surface sites. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Phosphorus is often the growth-limiting factor for biomass production as it is the least available of the major nutrients [1–3]. The low availability is due to the large reactivity of phosphate ions with numerous soil constituents such as mineral surfaces, clays, dissolved metal ions, and organic material. However, organic molecules, e.g., humic and fulvic acids are also able to specifically adsorb to mineral particles and thus have a competitive effect on phosphate adsorption [4]. These competitive interactions may affect the bioavailability of phosphorus. Several previous studies have been devoted to the competitive adsorption between phosphate and carboxylic acids on mineral surfaces [5–12], and in some of these studies the number of carboxylic groups has been identified as an important factor influencing the competitive ability of organic acids toward phosphate [9–12]. However, as other molecular features were also different in the previ* Corresponding author. Fax: +46 90 786 7655. E-mail addresses: [email protected] (M. Lindegren), per.persson@ chem.umu.se (P. Persson). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.11.040
ously investigated series of organic acids it has been difficult to draw a general conclusion concerning the precise effect of the number of functional groups. The present work was designed to counteract this problem. This was accomplished by investigating a series of benzenecarboxylic acids in which the number of functional groups could be increased without altering the molecular backbone structure or including other functional groups (Fig. 1). In addition, three structural isomers of benzenetricarboxylic acid were studied to evaluate the effect of the relative position of the carboxyl groups on competitive adsorption. These competitive reactions were studied at the water–goethite (a-FeOOH) interface. Goethite was chosen as this mineral is known to strongly adsorb both phosphate and carboxylic acids, and this mineral has also been included in several previous studies of competitive reactions between phosphate and carboxylic acids [9–12]. The aims of this study were to determine the quantitative effects on both phosphate and carboxylic acid adsorption, and the molecular mechanisms behind the competitive interactions. To accomplish this, the adsorption of the carboxylic acids and phosphate at the goethite–water interface was examined in both the single-ligand and in the seven competitive systems, containing phosphate and one organic acid. Quantitative adsorption
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Fig. 1. Schematic molecular structures of benzoate, phthalate, hemimellitate (benzene-1,2,3-tricarboxylate), trimellitate (benzene-1,2,4-tricarboxylate), trimesoate (benzene-1,3,5-tricarboxylate), pyromellitate (benzene-1,2,4,5-tetracarboxylate), and mellitate (benze-1,2,3,4,5,6-hexacarboxylate). pKas for benzoate, phthalate, trimellitate, and hemimellitate are from [13], pKas for pyromellitate and mellitate are recalculated for I = 0.1 and T = 25 °C from [13], and pKas for trimesoate are from [14].
data and infrared spectra were collectively used to evaluate the competitive adsorption reactions. 2. Experimental 2.1. Chemicals and solutions Boiled Milli-Q plus 185 water (resistance = 18.2 MX) and a constant ionic medium of 0.1 M NaCl (Merck p.a., dried at 180 °C) were used in all suspensions and solutions. The phosphate solution was made from NaH2PO4H2O (Merck, p.a.). All organic ligand solutions were prepared from the acidic form of the molecules and pH adjusted to above 10.5 to increase solubility. The chemicals used were benzoic acid (Merck, p.a.), phthalic acid (Merck, p.a.), benzene-1,3,5-tricarboxylic acid (trimesic acid) (Fluka, P97%), benzene-1,2,3-tricarboxylic acid (hemimellitic acid), (Aldrich, 98%), benzene-1,2,4-tricarboxylic acid (trimellitic acid), (>98%. Sigma), benzene-1,2,4,5-tetracarboxylic acid (pyromellitic acid), (Aldrich, 98%), and mellitic acid (Aldrich, 99%). A combination glass electrode was used for pH measurements in all samples. The electrode was externally calibrated using commercial buffer solutions (JT Baker, pH 3, 7, and 9). 2.2. Goethite preparation Goethite was prepared and characterized as described by Boily et al. [15]. Briefly, a solution of 2.5 M NaOH (EKA Chemicals) was slowly pumped into a solution of 0.5 M Fe(NO3)39H2O (Merck) under stirring and bubbling with Ar(g) until the pH reached 12. The product was aged at 60 °C for 1 week prior to ca. 2 months dialyzing in Millipore 12–14,000 D tubes and subsequently stored in polyethene bottles. The precipitate was identified as goethite by X-ray diffraction and the needle-like particle morphology was confirmed by electron microscopy. The surface area was determined to 82.5 m2/g by the BET N2(g) single point method using a Micrometrics Flowsorb II 2300. All goethite suspensions were thoroughly purged with nitrogen gas to remove any carbonate species in solution or at the goethite surface. 2.3. Adsorption and dissolution studies Adsorption and dissolution studies were carried out as series of batch experiments at 25 °C. A 6–8 g/L goethite suspension was
purged overnight with nitrogen gas to remove carbon dioxide before use. Five-milliliter aliquots of the suspension were poured into 15-mL polystyrene centrifuge tubes, 2.3 ± 0.1 lmol/m2 of either benzenecarboxylate or phosphate or of each ligand simultaneously were added, and pH was adjusted using 0.1 M HCl or 0.1 M NaOH in 0.1 M NaCl, while the tubes were purged with N2(g). All tubes were left on an end-over-end rotator for 24 h, pH was recorded, and the samples were centrifuged at 5000 rpm for 20 min. A small fraction of the supernatant was left on the paste for IR measurements; the rest of the supernatant was filtered through a 22-lm mixed cellulose ester filter and 3 mL from each sample was acidified to pH 6 2 and stored in a refrigerator for analysis. The phthalate- and mellitate-containing samples were analyzed for total solution concentrations of organic ligand using ion chromatography (Metrohm), and the samples containing benzoate, trimesoate, hemimellitate, trimellitate, pyromellitate, and/or phosphate using UV–Vis spectrometry (Shimadzu). To monitor goethite dissolution, the supernatants were analyzed for total Fe in solution using flame atomic absorption spectrometry (Perkin Elmer 3110). No or negligible concentrations of Fe were found in all samples.
2.4. UV–Vis spectroscopy The total phosphate concentrations in solution were determined by the molybdenum blue method [16] at 880 nm using a Shimadzu UV-2100 UV–visible recording spectrophotometer. Concentrations of benzoate, trimesoate, hemimellitate, trimellitate, and pyromellitate were obtained at 282 nm at pH 6 1 using the same instrument. All samples were diluted before analysis to fit the calibration limits.
2.5. Ion chromatography An ion chromatograph (Metrohm) equipped with a guard column (Metrosep RP, 6.1011.020) was used for analyzes of phthalate and mellitate concentrations in solution. The separation was made under acidic conditions using isocratic acetonitrile/NaCl/HCl, at different ratios depending on the analytes, as the mobile phase, and the flow rate was 0.7 mL/min. The concentrations were determined with a UV detector after injecting 20 lL of the sample and the detection limit was 50 lM for both ions. All samples were diluted before analysis to fit the calibration limits.
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2.6. Infrared spectroscopy The infrared spectra were acquired using a Bruker Equinox 55 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector. Data were collected, while the samples were purged with dried air to remove water vapor, with a horizontal attenuated total reflectance (ATR) accessory and a diamond crystal as the reflection element (SensIR Technologies). The angle of incidence for this setup is approximately 45°, which is far from the critical angle (32.8°). This and the fact that the analyzed peaks are weak (<0.05 absorbance units) and do not overlap with any strong peaks (except for the asymmetric C–O stretch which overlaps the intense H2O bend) indicate that the possible distortions known to occur in ATR spectra are minimized [17]. The samples were applied to the diamond crystal surface directly and sealed from the atmosphere by a lid. For each sample 500 scans were recorded in about 7 min in the range 370–7500 cm 1 with a resolution of 4 cm 1. The samples were prepared according to the description above. Spectra of the empty cell, supernatant, and the wet paste were collected for each sample. The spectrum of the empty cell was automatically subtracted from the two latter spectra by the OPUS software (Bruker Inc.), which was used to control the spectrometer and in all data treatments. Subsequently, the strong contribution from the water and any ligand remaining in solution and from pure goethite was removed from the paste spectrum by subtraction with the spectra of the supernatant and a paste of pure goethite, respectively. The remaining spectrum shows the IR spectral features of the water–mineral interface and any adsorbed species. 3. Results and discussion 3.1. Mono- and dicarboxylates
a 2
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3.2. Tricarboxylates The data presented in Fig. 3 shows that the adsorption of trimellitate, hemimellitate, and trimesoate is significantly reduced in the presence of phosphate as compared to the single ligand systems. The amounts adsorbed and the shapes of the adsorption curves, however, are markedly different for the three carboxylates, both in the absence and in the presence of phosphate. This indicates substantial effects of the relative positions of the three carboxyl groups. In the competitive systems, clearly tricarboxylate adsorption is causing reduction in the phosphate adsorption. However, the adsorbed fractions of trimellitate and hemimellitate seem to lose competitive ability at the most acidic pH values investigated as the adsorption of these ligands does not decrease in the pH region below the maximum competitive effect (Fig. 3). In contrast, the shape of the adsorption curve of trimesoate more closely matches the decrease of adsorbed phosphate and the maximum of trimesoate adsorption coincides with the maximum competitive effect. Note though that no 1:1 ratio exists between adsorbed trimesoate and desorbed phosphate. According to Boily et al. [21] trimellitate coordinates to the goethite surface as one outer sphere, one inner sphere, and one protonated outer sphere complex. These complexes are identified by peaks at 1375, 1407, and 1252 cm 1, respectively, and all appear in the IR spectra from the single ligand system at pH 4.3 (Fig. 4). Both the 1375 and the 1407 cm 1 peaks are also detected in the spectra from the competitive system. At pH 4.3 the former peak is predominant while at pH 3.1 the latter is relatively more intense. Hence, both inner sphere and outer sphere complexes are able to coexist with phosphate on goethite. Furthermore, it follows that the competitive effect, which is most pronounced around pH 4, arises primarily from the deprotonated outer sphere trimellitate surface complexes whereas the increased coadsorption at low pH seems to be caused by the formation of inner sphere surface com-
µmol phthalate ads./m2 goethite
µmol benzoate ads./m2 goethite
Results of adsorption measurements from the benzoate and phthalate systems in the absence and presence of phosphate are presented in Fig. 2. Clearly the adsorption of both organic acids is considerably reduced by phosphate. In the case of benzoate the relative effect is less significant due to low adsorption in the absence of phosphate as well. It is evident, however, that despite the fact that both benzoate and phthalate adsorb somewhat in the presence of phosphate, none of these ligands have any significant effect on phosphate adsorption (Fig. 2). Norén and Persson [18] found that benzoate adsorbs as two structurally different outer sphere complexes at the water–goethite interface, and Persson et al. [19] identified one inner sphere and one outer sphere phthalate surface complex. However, in the competitive systems the low adsorption prevents identification
of the prevailing surface complexes. Previously inner sphere oxalate has been shown to coadsorb with phosphate on goethite [12]. Thus, the fact that more phthalate coadsorbs in comparison with benzoate may be related to that the former ion forms chelating inner sphere surface complexes. In the case of oxalate two hypotheses to coadsorption have been suggested: (1) Oxalate and phosphate form inner sphere complexes at two structurally different surface sites. (2) Oxalate causes partial goethite dissolution and readsorbs as Fe(III)-oxalate surface complexes, as proposed in a recent study on ligand-promoted dissolution [20]. Both scenarios are possible also for phthalate; however, with the present experimental data we cannot distinguish between them.
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Fig. 2. Adsorption of phosphate and (a) benzoate and (b) phthalate on the goethite surface as a function of pH. s represents adsorption of benzoate or phthalate in the single ligand system and in the competitive system. j denotes adsorption of phosphate in the single ligand system and h in the competitive systems. The error bars are based on the standard deviation of three independent replicates.
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Fig. 3. Adsorption of phosphate and (a) trimellitate, (b) hemimellitate, and (c) trimesoate on the surface of goethite as a function of pH. s denotes adsorption of the tricarboxylate in the single ligand system and in the competitive system. j denotes adsorption of phosphate in the single system and h in the competitive system. The error bars are based on the standard deviation of three independent replicates.
1395 1375
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Fig. 4. Infrared spectra of trimellitate adsorbed on goethite at pH: (a) 4.3 in the absence of phosphate, (b) 4.3 in the presence of phosphate, (c) 2.9 in the absence of phosphate, and (d) 3.1 in the presence of phosphate.
Fig. 5. Infrared spectra of hemimellitate adsorbed on goethite at pH: (a) 4.2 in the absence of phosphate, (b) 4.1 in the presence of phosphate, (c) 3.1 in the absence of phosphate, and (d) 3.2 in the presence of phosphate.
plexes. These results are in agreement with our previous study [12] where it was shown that oxalate, which forms predominantly chelating inner sphere complexes, was coadsorbed with but unable to outcompete phosphate while carboxylates capable of forming multiple hydrogen bonds caused phosphate desorption. The structural assignment of the hemimellitate surface complexes on goethite is provided in the Supplementary material, where it is shown that hemimellitate forms three predominant species: one outer sphere, one inner sphere, and one protonated outer sphere complex. These are identified by peaks at (1370, 1555), (1395, 1583), and (1274) cm-1, respectively. In the spectra from the competitive system of hemimellitate the features of deprotonated outer sphere and inner sphere surface complexes are detected, but some minor spectral differences are also observed. In particular, the resolution between the 1555 and 1583 cm 1 peaks and the 1370 and 1395 cm 1 peaks is reduced, which indicates peak broadening effects (Fig. 5). This shows that the molecular state of the hemimellitate surface complexes is affected by the presence of coadsorbed phosphate ions. As a result of the peak broadening, comparison with the distribution of surface complexes in the single ligand system becomes more difficult.
However, both in the absence and in the presence of phosphate it is obvious that the 1395 cm 1 peak is more intense relative to the 1370 cm-1 peak at pH 3.1–3.2 as compared to at pH 4.1–4.2. Thus, the fraction of inner sphere hemimellitate surface complexes is increased at low pH where the competitive efficiency is reduced. This fractionation is in agreement with the observations in the trimellitate system. Analogous to hemimellitate, a more thorough structural description of trimesoate on goethite is provided in the Supplementary material. The larger separation between the carboxyl groups and thus a decreased tendency to form mononuclear chelate structures as compared to trimellitate and hemimellitate results in formation of only outer sphere surface complexes on goethite between pH 3 and pH 8. One deprotonated complex is identified by peaks at 1363 and 1555 cm 1, and a partially protonated complex is formed around pH 4.8 and below and is characterized by peaks at 1714 and 1279 cm 1. In the infrared spectra from the competitive system trimesoate peaks are only visible at pH 3.5 and below (Fig. 6, all spectra not shown) which is in agreement with the adsorption data in Fig. 3c. The general features of these spectra are similar to those of the single ligand system at low pH, i.e., the spectra mainly show
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Absorbance
1714 a
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Wavenumber / cm -1 Fig. 6. Infrared spectra of trimesoate adsorbed on goethite at pH: (a) 3.5 in the presence of phosphate, (b) 3.1 in the absence of phosphate, and (c) 2.9 in the presence of phosphate.
the existence of partially protonated outer sphere complexes (Fig. 6). However, the relative intensities of the –C–O–H peaks around 1279 cm 1 versus the ones originating from the symmetric stretch vibration of the unprotonated carboxylates at 1363 cm 1 are stronger in the competitive system, indicating a higher degree of protonation of trimesoate. This is illustrated by the solid lines in Fig. 6 where the slopes in the spectra from the competitive system are steeper than in the spectrum collected in the absence of phosphate even though pH of the latter sample (3.1) is in between pH of the two competitive samples (2.9 and 3.5). Hence, a more protonated form of trimesoate is favored in the presence of phosphate which indicates that H-bonding interactions by the –C–O–H groups are involved in the competitive reactions. 3.3. Tetra- and hexacarboxylates
µmol pyromellitate ads./m2 goethite
The adsorption curve of pyromellitate on goethite is in agreement with previously published results [21], but in the presence of phosphate the curve shifts substantially to lower pH and the slope of the curve becomes less steep (Fig. 7). Thus, pyromellitate—like the smaller benzenecarboxylates—experiences a strong competition from phosphate ions for goethite surfaces sites. In the presence of phosphate pyromellitate adsorption commences at pH 6 and increases almost linearly as pH is decreased to 3. In this pH region only a very small amount of phosphate is desorbed, and the ratio between desorbed phosphate and adsorbed pyromellitate is markedly smaller
2.4
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than one. Accordingly, pyromellitate is capable of coadsorbing with phosphate without causing any substantial desorption. The overall behavior of pyromellitate is similar to that of trimellitate; however, the competitive effects seem to be smaller in the former system. Hence, this comparison indicates that the number of carboxylate groups of the organic acids cannot be used as the sole criterion for assessing the competitive efficiency toward phosphate. Boily et al. [22] have previously identified IR peaks at 1377, 1254, and 1364 cm 1 as originating from one nonprotonated outer sphere, one protonated outer sphere, and one inner sphere complex of pyromellitate, respectively. It was shown that the nonprotonated outer sphere complex, although important in the whole investigated pH range, predominates at high pH. The protonated outer sphere complex and the inner sphere complex begin to form around pH 5 and pH 6, respectively, and both complexes increase in importance as pH is decreased. As expected, the above-noted peaks are also detected in the spectra collected in the absence of phosphate in the present work (Fig. 8). The 1377 and 1254 cm 1 peaks are readily detected at both pH 4.1 and pH 3.1 whereas the 1364 cm 1 peak is resolved in the second derivative spectra (not shown). The poor signal-to-noise ratios in the spectra collected in the presence of phosphate result in corresponding second derivative spectra dominated by noise, which prevent resolution of the 1377 and 1364 cm 1 peaks. However, at pH 4.6 the 1377 cm 1 peak of the nonprotonated outer sphere species is readily detectable in the regular IR spectrum. At pH 3.0 this peak is broadened toward the low-frequency side and that may indicate an increased intensity of the 1364 cm 1 peak, and thus the appearance of pyromellitate inner sphere complexes. Accordingly, also in this system the coadsorption seems to be caused, at least partially, by the formation of inner sphere pyromellitate surface complexes. Finally, although the 1254 cm 1 peak is difficult to detect, the appearance of the mC=O mode at 1710 cm 1 indicates the presence of protonated surface complexes at pH 3.0. The strong competitive ability by mellitate toward phosphate shown in Fig. 7b has already been documented in a recent study by Lindegren and Persson [12]. Furthermore, according to Johnson et al. [23], mellitate adsorbs as hydrogen-bonded outer sphere complexes throughout the investigated pH range (3–10). These are partially protonated around pH 6 and below, which is indicated by peaks at 1710 and 1279 cm 1 in the IR spectra (Fig. 9c). The same observations can be made in the presence of phosphate, and the predominance of deprotonated outer sphere complexes at pH 6.4 and partially protonated complexes at pH 3.0 (Figs. 9b and d) indicate that both types of complexes are able to compete with phosphate (Fig. 7b). In agreement with the trimesoate system and recently published results [12], the new data presented herein
1279
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Fig. 7. Adsorption of phosphate and (a) pyromellitate, and (b) mellitate on the goethite surface as a function of pH. s denotes adsorption of the carboxylate in the single ligand system and in the competitive system. j denotes adsorption of phosphate in the single system and h in the competitive system. The error bars are based on the standard deviation of three independent replicates.
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1377
1254
Absorbance
a 1710 b
c d
1800
1600
1200
1400
Wavenumber / cm-1 Fig. 8. Infrared spectra of pyromellitate adsorbed on goethite at pH: (a) 4.2 in the absence of phosphate, (b) 4.6 in the presence of phosphate, (c) 3.1 in the absence of phosphate, and (d) 3.0 in the presence of phosphate.
1329
Absorbance
a
b
1279 1710
c
d
1800
1600
1400
1200
Wavenumber / cm -1 Fig. 9. Infrared spectra of mellitate adsorbed on goethite at pH: (a) 6.2 in the absence of phosphate, (b) 6.4 in the presence of phosphate, (c) 3.0 in the absence of phosphate, and (d) 3.0 in the presence of phosphate.
show that there is a higher degree of protonation of mellitate in the presence of phosphate at pH 3 than in the single ligand system at the same pH. This is indicated by the change in relative intensity of the 1279 cm 1 peak of the –C–O–H groups and 1329 cm 1 peak of the deprotonated carboxylate groups, illustrated by the solid lines in Fig. 9c and d. Hence, the mellitate results emphasize the importance of hydrogen-bonded outer sphere surface complexes, both deprotonated and protonated, depending on pH, for the competitive efficiency toward phosphate. 3.4. Overall trends and molecular mechanisms As already noted the number of carboxylic groups has been identified as an important factor determining the ability of organic acids
to compete with phosphate at water–mineral interfaces [9–12]. Accordingly, due to the low number of carboxylic groups of benzoate and phthalate, these ligands were unable to lower phosphate adsorption under the experimental conditions probed in this study. It is clear, however, from the results of the remaining benzenecarboxylates and from those presented by Lindegren and Persson [12] that additional molecular properties are needed to explain the influence on phosphate adsorption by carboxylic acids (Figs. 3 and 7). The point of zero charge of goethite is 9.4; thus, the surface is dominated by positive surface sites in all investigated samples. At pH 3, the efficiency of the benzenecarboxylates to compete with phosphate is in the order pyromellitate trimellitate < hemimellitate < trimesoate < mellitate. At this pH, the IR spectra discussed above show that the surface speciation of pyro-, tri-, and hemimellitate has a substantial contribution from inner sphere surface complexes whereas trimesoate and mellitate exist as partially protonated outer sphere complexes. Furthermore, using the relative intensities of the peaks originating from –C–O–H and symmetric C–O stretching vibrations at 1250–1275 and 1330–1380 cm 1, respectively, the average number of protons per molecule may be estimated. This analysis shows that at pH values around 3, trimesoate and mellitate have a markedly higher degree of protonation than the other three ligands in the series (Figs. 4–6, 8 and 9). Therefore we conclude that the presence of protonated carboxyl groups in a specific combination with deprotonated carboxylate groups is another important factor governing the competitive ability toward phosphate in this pH range. This is especially apparent in the trimesoate spectra where the degree of protonation is drastically decreased from pH 3.9 to pH 4.8 (Fig. S2), i.e., the same pH region where the competitive ability is also abruptly reduced (Fig. 3b). These results are in accordance with our previous study [12] showing that H-accepting carboxylate groups together with H-donating carboxylic acid groups provide a successful combination that increased competitive ability significantly. The reason why protonated carboxyl groups that are capable of H-donor interactions contribute to the destabilization of and the competition with phosphate is indicated by recent results concerning the adsorption of oxoanions on mineral surfaces [24]. It was shown that arsenate, in many aspects a phosphate analogue, forms un-, singly, and doubly protonated monodentate inner sphere complexes at the goethite surface. In addition to the monodentate interaction with surface Fe(III), all three complexes were indicated to be significantly stabilized by hydrogen-bonding to neighboring surface sites. Ongoing work in our group has demonstrated that phosphate forms equivalent surface complexes, and these can be distinguished by IR spectroscopy. The most diagnostic region in the IR spectra is that between 1000 and 1200 cm 1 where the various P–O(H) peaks appear [25–27]. At low pH the main peaks are detected at 1010 and 1125 cm 1 (Fig. 10k), which originate from a protonated phosphate complex [26–28]. In analogy with the recent arsenate results this is most likely a monodentate doubly protonated species. As pH is increased, new peaks appear at approximately 1050 and 1083 cm 1 (Fig. 10b and h) and this is indicative of deprotonation of the phosphate surface complexes [26–28]. Around pH 3, where strong competitive effects are observed from partially protonated trimesoate and mellitate ions, the phosphate surface speciation in the absence of carboxylates is completely dominated by the doubly protonated complex (Fig. 10k). The same peaks also appear in the presence of trimesoate and mellitate but the spectra display lower signal-to-noise ratios due to the lower surface concentrations of phosphate (Fig. 10i and j). Thus, these two benzenecarboxylates partially outcompete the doubly protonated phosphate complex. A complex that—in analogy with the arsenate system—is most likely stabilized by hydrogen bond interactions in which the P–OH groups act as H-donors and
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1050
1010
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b
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c
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j k
1210
1130
1050
970
Wavenumber / cm-1 Fig. 10. Infrared spectra of phosphate adsorbed on the surface of goethite at pH: (a) 5.8 in the presence of mellitate, (b) in the absence of carboxylates, (c) 4.5 in the presence of mellitate, (d) 4.6 in the presence of pyromellitate, (e) 4.3 in the presence trimesoate, (f) 4.3 in the presence of trimellitate, (g) 4.4 in the presence of hemimellitate, (h) 4.6 in the absence of carboxylates, (i) 2.9 in the presence of mellitate, (j) 2.9 in the presence of trimesoate, and (k) 2.8 in the absence of carboxylates.
the surface sites as acceptors. As protonated carboxyl groups can compete for these H-accepting surface sites and thereby destabilize the phosphate complexes, this would explain the competitive effect by trimesoate and mellitate at pH 3. The fact that the partially protonated trimesoate and mellitate outer sphere complexes are able to compete successfully with phosphate indirectly suggests that the contribution of the H-bonding interactions to the overall stability of the phosphate surface complexes most probably is substantial. The ranking of the benzenecarboxylates with respect to the competitive effect toward phosphate at pH 3 is perturbed by an increase in pH. Between pH 4 and pH 4.5 the competitive ability of trimesoate is significantly reduced and completely lost at pH 5 while hemimellitate under these conditions is more efficient and displays a maximum in its competitive ability (Fig. 3). Furthermore, in the same pH range the effect by trimesoate is now comparable to that of trimellitate (Fig. 3). In addition mellitate is able to influence phosphate adsorption over the entire pH interval 3–8 (Fig. 7). Referring to the IR discussion in the previous section, these observations indicate that competition also occurs under condi-
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tions where the surface speciation of the benzenecarboxylates is dominated either by deprotonated outer sphere complexes alone or by a combination of these complexes with inner sphere species. As shown in Fig. 10h the competition above pH 4.5 occurs under conditions coinciding with the onset of predominance of partially deprotonated phosphate surface complexes, i.e., surface complexes that most likely gain stability from H-bonding interactions that involve P–O groups as H-bond acceptors. As mellitate forms solely deprotonated outer sphere complexes above pH 6 we infer that the mellitate-induced desorption of phosphate is caused by competition between P–O and C–O groups for H-donor surface sites. Thus, depending on pH mellitate is apparently capable of competing with phosphate for both H-donor and H-acceptor sites and we believe that it this property together with the large number of functional groups that makes mellitate an efficient phosphate competitor over a relatively wide pH range. This dual effect is further corroborated by the IR spectra collected at pH 5.8 (Fig. 10a and b). Under these conditions mellitate is almost completely deprotonated (Fig. 9) and thus mainly a H-bond acceptor. Accordingly, the primary competition at this pH occurs between mellitate and the partially deprotonated phosphate surface complexes. As the deprotonated complex of phosphate is outcompeted in the presence of mellitate a larger fraction of the remaining phosphate is expected to consist of the protonated species. This is also detected experimentally as a change in relative intensities of the 1010 and 1050 cm 1 peaks (Fig. 10a and b). In contrast, when mellitate is sufficiently protonated it outcompetes the fully protonated phosphate complex as discussed above. Note that the effect at pH 5.8 cannot be due to a change in phosphate coverage as lowering the surface coverage of phosphate is known to affect the peak positions in the opposite direction; i.e., decreasing the surface coverage has the same effect on peak positions as increasing the pH [24, 25]. The competitive trends of the other benzenecarboxylates in the pH region 4–5 is more difficult to explain as the surface speciation of these ligands is a mixture of inner sphere and predominantly deprotonated outer sphere complexes (see discussion above). Nonetheless based on the results presented herein and in our previous study [12] we may conclude that at least three carboxylate groups are needed to induce any substantial phosphate desorption under the conditions probed in these studies. Furthermore, the trimellitate, pyromellitate, and hemimellitate results show that the structure of the ligands (i.e., the relative positions of the carboxylate groups) is also a decisive factor and in fact this may overrule the trend of the increasing competitive effect by an increasing number of carboxylate groups. This is demonstrated by the hemimellitate structure, with three carboxylate groups in close proximity, which is significantly more efficient than pyromellitate where the two pairs of carboxylate groups are separated by CH groups. As both ligands form similar inner sphere complexes consisting of mononuclear seven-membered ring chelate structures it is H-bonding either through the additional carboxylate groups and/ or through the carboxylate groups of the outer sphere complexes that makes the difference between hemimellitate and pyromellitate. This underlines the structural specificity of the H-bonding arrangements that govern the stabilities of the benzenecarboxylate and the phosphate surface complexes. Finally, our conceptual model for the competitive adsorption reactions studied herein based on the H-acceptor and H-donor properties of the surface complexes involved also explains the small differences observed between trimellitate and pyromellitate, and the unexpected larger effect by the former ligand. In a previous work the detailed surface speciation of trimellitate and pyromellitate on goethite was presented [21]. It was shown that that the inner sphere complexes of theses ligands behaved almost identically whereas the deprotonated outer sphere complex of trimellitate seemed to be more predominant in the pH region 3–5 as compared
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to the corresponding pyromellitate complex. From these results we infer that the competitive differences toward phosphate are not due to the inner sphere complexes but instead caused by the slightly larger surface concentration of deprotonated outer sphere complexes of trimellitate that partially outcompetes phosphate via H-acceptor interactions with protonated surface sites.
Appendix A. Supplementary material
4. Conclusions
[1] P. Hinsinger, Plant Soil 237 (2001) 173. [2] I.C.R. Holford, Aust. J. Soil Res. 35 (1997) 227. [3] H. Marschner, Mineral Nutrition of Higher Plants, Academic Press, London, 1995. [4] R.J. Haynes, M.S. Mokolobate, Nutr. Cycl. Agroecosyst. 59 (2001) 47. [5] U. Kafkafi, B. Bar-Yosef, R. Rosenberg, G. Sposito, Soil Sci. Soc. Am. J. 52 (1988) 1585. [6] A. Violante, L. Gianfreda, Soil Sci. Soc. Am. J. 57 (1993) 1235. [7] A. Violante, C. Colombo, A. Buondonno, Soil Sci. Soc. Am. J. 55 (1991) 65. [8] J.S. Geelhoed, T. Hiemstra, W.H. Van Riemsdijk, Environ. Sci. Technol. 32 (1998) 2119. [9] R.L. Parfitt, Plant Soil 53 (1979) 55. [10] H.Q. Hu, J.Z. He, X.Y. Li, F. Liu, Environ. Int. 26 (2001) 353. [11] S.E. Johnson, R.H. Loeppert, Soil Sci. Soc. Am. J. 70 (2006) 222. [12] M. Lindegren, P. Persson, Eur. J. Soil Sci. 60 (2009) 982. [13] A.E. Martell, R.M. Smith, Critical Stability Constants Other Organic Ligands, 3, Plenum Press, New York, 1977. [14] A.E. Martell, R.M. Smith, Critical Stability Constants First Supplement, 5, Plenum Press, New York, 1982. [15] J.F. Boily, J. Lutzenkirchen, O. Balmes, J. Beattie, S. Sjöberg, Colloids Surf. A 179 (2001) 11. [16] J. Murphy, J.P. Riley, Anal. Chim. Acta 27 (1962) 31. [17] M.W. Urban, Attenuated Total Reflection Spectroscopy of Polymers: Theory and Practice, American Chemical Society, Washington, DC, 1996. [18] K. Norén, P. Persson, Geochim. Cosmochim. Acta 71 (2007) 5717. [19] P. Persson, J. Nordin, J. Rosenqvist, L. Lövgren, S. Sjöberg, J. Colloid Interface Sci. 206 (1998) 252. [20] J.S. Loring, A.A. Simanova, P. Persson, Langmuir 24 (2008) 7054–7057. [21] J.F. Boily, P. Persson, S. Sjöberg, Geochim. Cosmochim. Acta 64 (2000) 3453. [22] J.F. Boily, N. Nilsson, P. Persson, S. Sjoberg, Langmuir 16 (2000) 5719. [23] B.B. Johnson, S. Sjoberg, P. Persson, Langmuir 20 (2004) 823. [24] J.S. Loring, M.H. Sandström, K. Norén, P. Persson, Chem. Eur. J. 15 (2009) 5063. [25] A.C. Chapman, L.E. Thirwell, Spectrochim. Acta 20 (1964) 937. [26] M.I. Tejedor-Tejedor, M.A. Anderson, Langmuir 6 (1990) 602. [27] P. Persson, N. Nilsson, S. Sjoberg, J. Colloid Interface Sci. 177 (1996) 263. [28] R.L. Parfitt, J. D Russel, V.C. Farmer, J. Chem. Soc., Faraday Trans. I 72 (1976) 1082.
The comparison of the competitive adsorption on goethite between phosphate and seven benzenecarboxylic acids has corroborated the importance of the number of carboxyl groups. However, the systematic study of different benzenecarboxylate structures has also revealed that the relative positions of the carboxyl groups play a decisive role for the competitive efficiencies. The competitive effect is particularly favored by the existence of three or more closely spaced functional groups. Finally, the collective results show that it is competition for hydrogen-bonding surface sites rather than inner sphere surface sites that primarily determines the outcome of the competitive adsorption experiments. This conclusion suggests that H-bonding makes an important contribution to the stabilities of both the adsorbed benzenecarboxylates and the phosphate ions. Furthermore, the effects by the different benzenecarboxylate structures indicate that the hydrogen-bonding interactions are structurally specific; i.e., they are sensitive to the locations and the directional properties of the H-acceptor and Hdonor surface sites. Acknowledgments We thank Dr. John S. Loring for help with the theoretical frequency calculations and Ingegärd Andersson for help with the spectra collection. This work was supported by the Swedish Research Council. The Kempe foundation, Sweden, is acknowledged for funding of the FTIR spectrometer.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2009.11.040. References
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Competitive adsorption involving phosphate and benzenecarboxylic acids on goethite—Effects of molecular structures Malin Lindegren, Per Persson * Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden
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Article history: Received 19 September 2009 Accepted 18 November 2009 Available online 23 November 2009 Keywords: Phosphate Benzenecarboxylic acid Competitive adsorption Infrared spectroscopy
a b s t r a c t The competitive adsorption between phosphate and either one of seven benzenecarboxylates (benzoate, phthalate, trimellitate, trimesoate, hemimellitate, pyromellitate, and mellitate) on the surfaces of fine-particulate goethite (a-FeOOH) was investigated as a function of pH. The series of ligands contained molecules with an increasing number of functional groups as well as three structural isomers of the tricarboxylates. Thus, the effects of both the number of carboxylate groups and the relative positions of these groups on the competitive efficiency toward phosphate were probed in this study. Quantitative adsorption experiments in batch mode and infrared spectroscopy were collectively used to evaluate the competitive adsorption reactions. Under the conditions probed, mono- and dicarboxylates had no detectable effect on phosphate adsorption whereas the ligands containing three or more carboxylate groups were able to partially outcompete phosphate. However, the pH dependency and the extent of these competitive effects were strongly dependent on the structure and composition of the benzenecarboxylate. The collective results showed that it was the competition for hydrogen-bonding surface sites rather than inner sphere surface sites that primarily determined the outcome of the competitive adsorption experiments, and it was the ability of the organic ligand to act as hydrogen-bonding acceptor and/or donor in various parts of the pH range that also determined the competitive pH dependency. The importance of H-bonding for the competitive adsorption between phosphate and benzenecarboxylic acids suggested that H-bonding interactions contributed substantially to the stabilities of both the adsorbed benzenecarboxylates and the phosphate ions and that these interactions were structurally specific; i.e., they were sensitive to the locations and the directional properties of the H-acceptor and H-donor surface sites. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Phosphorus is often the growth-limiting factor for biomass production as it is the least available of the major nutrients [1–3]. The low availability is due to the large reactivity of phosphate ions with numerous soil constituents such as mineral surfaces, clays, dissolved metal ions, and organic material. However, organic molecules, e.g., humic and fulvic acids are also able to specifically adsorb to mineral particles and thus have a competitive effect on phosphate adsorption [4]. These competitive interactions may affect the bioavailability of phosphorus. Several previous studies have been devoted to the competitive adsorption between phosphate and carboxylic acids on mineral surfaces [5–12], and in some of these studies the number of carboxylic groups has been identified as an important factor influencing the competitive ability of organic acids toward phosphate [9–12]. However, as other molecular features were also different in the previ* Corresponding author. Fax: +46 90 786 7655. E-mail addresses: [email protected] (M. Lindegren), per.persson@ chem.umu.se (P. Persson). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.11.040
ously investigated series of organic acids it has been difficult to draw a general conclusion concerning the precise effect of the number of functional groups. The present work was designed to counteract this problem. This was accomplished by investigating a series of benzenecarboxylic acids in which the number of functional groups could be increased without altering the molecular backbone structure or including other functional groups (Fig. 1). In addition, three structural isomers of benzenetricarboxylic acid were studied to evaluate the effect of the relative position of the carboxyl groups on competitive adsorption. These competitive reactions were studied at the water–goethite (a-FeOOH) interface. Goethite was chosen as this mineral is known to strongly adsorb both phosphate and carboxylic acids, and this mineral has also been included in several previous studies of competitive reactions between phosphate and carboxylic acids [9–12]. The aims of this study were to determine the quantitative effects on both phosphate and carboxylic acid adsorption, and the molecular mechanisms behind the competitive interactions. To accomplish this, the adsorption of the carboxylic acids and phosphate at the goethite–water interface was examined in both the single-ligand and in the seven competitive systems, containing phosphate and one organic acid. Quantitative adsorption
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Fig. 1. Schematic molecular structures of benzoate, phthalate, hemimellitate (benzene-1,2,3-tricarboxylate), trimellitate (benzene-1,2,4-tricarboxylate), trimesoate (benzene-1,3,5-tricarboxylate), pyromellitate (benzene-1,2,4,5-tetracarboxylate), and mellitate (benze-1,2,3,4,5,6-hexacarboxylate). pKas for benzoate, phthalate, trimellitate, and hemimellitate are from [13], pKas for pyromellitate and mellitate are recalculated for I = 0.1 and T = 25 °C from [13], and pKas for trimesoate are from [14].
data and infrared spectra were collectively used to evaluate the competitive adsorption reactions. 2. Experimental 2.1. Chemicals and solutions Boiled Milli-Q plus 185 water (resistance = 18.2 MX) and a constant ionic medium of 0.1 M NaCl (Merck p.a., dried at 180 °C) were used in all suspensions and solutions. The phosphate solution was made from NaH2PO4H2O (Merck, p.a.). All organic ligand solutions were prepared from the acidic form of the molecules and pH adjusted to above 10.5 to increase solubility. The chemicals used were benzoic acid (Merck, p.a.), phthalic acid (Merck, p.a.), benzene-1,3,5-tricarboxylic acid (trimesic acid) (Fluka, P97%), benzene-1,2,3-tricarboxylic acid (hemimellitic acid), (Aldrich, 98%), benzene-1,2,4-tricarboxylic acid (trimellitic acid), (>98%. Sigma), benzene-1,2,4,5-tetracarboxylic acid (pyromellitic acid), (Aldrich, 98%), and mellitic acid (Aldrich, 99%). A combination glass electrode was used for pH measurements in all samples. The electrode was externally calibrated using commercial buffer solutions (JT Baker, pH 3, 7, and 9). 2.2. Goethite preparation Goethite was prepared and characterized as described by Boily et al. [15]. Briefly, a solution of 2.5 M NaOH (EKA Chemicals) was slowly pumped into a solution of 0.5 M Fe(NO3)39H2O (Merck) under stirring and bubbling with Ar(g) until the pH reached 12. The product was aged at 60 °C for 1 week prior to ca. 2 months dialyzing in Millipore 12–14,000 D tubes and subsequently stored in polyethene bottles. The precipitate was identified as goethite by X-ray diffraction and the needle-like particle morphology was confirmed by electron microscopy. The surface area was determined to 82.5 m2/g by the BET N2(g) single point method using a Micrometrics Flowsorb II 2300. All goethite suspensions were thoroughly purged with nitrogen gas to remove any carbonate species in solution or at the goethite surface. 2.3. Adsorption and dissolution studies Adsorption and dissolution studies were carried out as series of batch experiments at 25 °C. A 6–8 g/L goethite suspension was
purged overnight with nitrogen gas to remove carbon dioxide before use. Five-milliliter aliquots of the suspension were poured into 15-mL polystyrene centrifuge tubes, 2.3 ± 0.1 lmol/m2 of either benzenecarboxylate or phosphate or of each ligand simultaneously were added, and pH was adjusted using 0.1 M HCl or 0.1 M NaOH in 0.1 M NaCl, while the tubes were purged with N2(g). All tubes were left on an end-over-end rotator for 24 h, pH was recorded, and the samples were centrifuged at 5000 rpm for 20 min. A small fraction of the supernatant was left on the paste for IR measurements; the rest of the supernatant was filtered through a 22-lm mixed cellulose ester filter and 3 mL from each sample was acidified to pH 6 2 and stored in a refrigerator for analysis. The phthalate- and mellitate-containing samples were analyzed for total solution concentrations of organic ligand using ion chromatography (Metrohm), and the samples containing benzoate, trimesoate, hemimellitate, trimellitate, pyromellitate, and/or phosphate using UV–Vis spectrometry (Shimadzu). To monitor goethite dissolution, the supernatants were analyzed for total Fe in solution using flame atomic absorption spectrometry (Perkin Elmer 3110). No or negligible concentrations of Fe were found in all samples.
2.4. UV–Vis spectroscopy The total phosphate concentrations in solution were determined by the molybdenum blue method [16] at 880 nm using a Shimadzu UV-2100 UV–visible recording spectrophotometer. Concentrations of benzoate, trimesoate, hemimellitate, trimellitate, and pyromellitate were obtained at 282 nm at pH 6 1 using the same instrument. All samples were diluted before analysis to fit the calibration limits.
2.5. Ion chromatography An ion chromatograph (Metrohm) equipped with a guard column (Metrosep RP, 6.1011.020) was used for analyzes of phthalate and mellitate concentrations in solution. The separation was made under acidic conditions using isocratic acetonitrile/NaCl/HCl, at different ratios depending on the analytes, as the mobile phase, and the flow rate was 0.7 mL/min. The concentrations were determined with a UV detector after injecting 20 lL of the sample and the detection limit was 50 lM for both ions. All samples were diluted before analysis to fit the calibration limits.
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2.6. Infrared spectroscopy The infrared spectra were acquired using a Bruker Equinox 55 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector. Data were collected, while the samples were purged with dried air to remove water vapor, with a horizontal attenuated total reflectance (ATR) accessory and a diamond crystal as the reflection element (SensIR Technologies). The angle of incidence for this setup is approximately 45°, which is far from the critical angle (32.8°). This and the fact that the analyzed peaks are weak (<0.05 absorbance units) and do not overlap with any strong peaks (except for the asymmetric C–O stretch which overlaps the intense H2O bend) indicate that the possible distortions known to occur in ATR spectra are minimized [17]. The samples were applied to the diamond crystal surface directly and sealed from the atmosphere by a lid. For each sample 500 scans were recorded in about 7 min in the range 370–7500 cm 1 with a resolution of 4 cm 1. The samples were prepared according to the description above. Spectra of the empty cell, supernatant, and the wet paste were collected for each sample. The spectrum of the empty cell was automatically subtracted from the two latter spectra by the OPUS software (Bruker Inc.), which was used to control the spectrometer and in all data treatments. Subsequently, the strong contribution from the water and any ligand remaining in solution and from pure goethite was removed from the paste spectrum by subtraction with the spectra of the supernatant and a paste of pure goethite, respectively. The remaining spectrum shows the IR spectral features of the water–mineral interface and any adsorbed species. 3. Results and discussion 3.1. Mono- and dicarboxylates
a 2
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6
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3.2. Tricarboxylates The data presented in Fig. 3 shows that the adsorption of trimellitate, hemimellitate, and trimesoate is significantly reduced in the presence of phosphate as compared to the single ligand systems. The amounts adsorbed and the shapes of the adsorption curves, however, are markedly different for the three carboxylates, both in the absence and in the presence of phosphate. This indicates substantial effects of the relative positions of the three carboxyl groups. In the competitive systems, clearly tricarboxylate adsorption is causing reduction in the phosphate adsorption. However, the adsorbed fractions of trimellitate and hemimellitate seem to lose competitive ability at the most acidic pH values investigated as the adsorption of these ligands does not decrease in the pH region below the maximum competitive effect (Fig. 3). In contrast, the shape of the adsorption curve of trimesoate more closely matches the decrease of adsorbed phosphate and the maximum of trimesoate adsorption coincides with the maximum competitive effect. Note though that no 1:1 ratio exists between adsorbed trimesoate and desorbed phosphate. According to Boily et al. [21] trimellitate coordinates to the goethite surface as one outer sphere, one inner sphere, and one protonated outer sphere complex. These complexes are identified by peaks at 1375, 1407, and 1252 cm 1, respectively, and all appear in the IR spectra from the single ligand system at pH 4.3 (Fig. 4). Both the 1375 and the 1407 cm 1 peaks are also detected in the spectra from the competitive system. At pH 4.3 the former peak is predominant while at pH 3.1 the latter is relatively more intense. Hence, both inner sphere and outer sphere complexes are able to coexist with phosphate on goethite. Furthermore, it follows that the competitive effect, which is most pronounced around pH 4, arises primarily from the deprotonated outer sphere trimellitate surface complexes whereas the increased coadsorption at low pH seems to be caused by the formation of inner sphere surface com-
µmol phthalate ads./m2 goethite
µmol benzoate ads./m2 goethite
Results of adsorption measurements from the benzoate and phthalate systems in the absence and presence of phosphate are presented in Fig. 2. Clearly the adsorption of both organic acids is considerably reduced by phosphate. In the case of benzoate the relative effect is less significant due to low adsorption in the absence of phosphate as well. It is evident, however, that despite the fact that both benzoate and phthalate adsorb somewhat in the presence of phosphate, none of these ligands have any significant effect on phosphate adsorption (Fig. 2). Norén and Persson [18] found that benzoate adsorbs as two structurally different outer sphere complexes at the water–goethite interface, and Persson et al. [19] identified one inner sphere and one outer sphere phthalate surface complex. However, in the competitive systems the low adsorption prevents identification
of the prevailing surface complexes. Previously inner sphere oxalate has been shown to coadsorb with phosphate on goethite [12]. Thus, the fact that more phthalate coadsorbs in comparison with benzoate may be related to that the former ion forms chelating inner sphere surface complexes. In the case of oxalate two hypotheses to coadsorption have been suggested: (1) Oxalate and phosphate form inner sphere complexes at two structurally different surface sites. (2) Oxalate causes partial goethite dissolution and readsorbs as Fe(III)-oxalate surface complexes, as proposed in a recent study on ligand-promoted dissolution [20]. Both scenarios are possible also for phthalate; however, with the present experimental data we cannot distinguish between them.
10
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Fig. 2. Adsorption of phosphate and (a) benzoate and (b) phthalate on the goethite surface as a function of pH. s represents adsorption of benzoate or phthalate in the single ligand system and in the competitive system. j denotes adsorption of phosphate in the single ligand system and h in the competitive systems. The error bars are based on the standard deviation of three independent replicates.
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µmol trimelliatate ads./m2 goethite
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b 2
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Fig. 3. Adsorption of phosphate and (a) trimellitate, (b) hemimellitate, and (c) trimesoate on the surface of goethite as a function of pH. s denotes adsorption of the tricarboxylate in the single ligand system and in the competitive system. j denotes adsorption of phosphate in the single system and h in the competitive system. The error bars are based on the standard deviation of three independent replicates.
1395 1375
1370
1407
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1252
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Absorbance
a
b
b
c
c d 1800
d 1600
1400
1200
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1800
1600
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Fig. 4. Infrared spectra of trimellitate adsorbed on goethite at pH: (a) 4.3 in the absence of phosphate, (b) 4.3 in the presence of phosphate, (c) 2.9 in the absence of phosphate, and (d) 3.1 in the presence of phosphate.
Fig. 5. Infrared spectra of hemimellitate adsorbed on goethite at pH: (a) 4.2 in the absence of phosphate, (b) 4.1 in the presence of phosphate, (c) 3.1 in the absence of phosphate, and (d) 3.2 in the presence of phosphate.
plexes. These results are in agreement with our previous study [12] where it was shown that oxalate, which forms predominantly chelating inner sphere complexes, was coadsorbed with but unable to outcompete phosphate while carboxylates capable of forming multiple hydrogen bonds caused phosphate desorption. The structural assignment of the hemimellitate surface complexes on goethite is provided in the Supplementary material, where it is shown that hemimellitate forms three predominant species: one outer sphere, one inner sphere, and one protonated outer sphere complex. These are identified by peaks at (1370, 1555), (1395, 1583), and (1274) cm-1, respectively. In the spectra from the competitive system of hemimellitate the features of deprotonated outer sphere and inner sphere surface complexes are detected, but some minor spectral differences are also observed. In particular, the resolution between the 1555 and 1583 cm 1 peaks and the 1370 and 1395 cm 1 peaks is reduced, which indicates peak broadening effects (Fig. 5). This shows that the molecular state of the hemimellitate surface complexes is affected by the presence of coadsorbed phosphate ions. As a result of the peak broadening, comparison with the distribution of surface complexes in the single ligand system becomes more difficult.
However, both in the absence and in the presence of phosphate it is obvious that the 1395 cm 1 peak is more intense relative to the 1370 cm-1 peak at pH 3.1–3.2 as compared to at pH 4.1–4.2. Thus, the fraction of inner sphere hemimellitate surface complexes is increased at low pH where the competitive efficiency is reduced. This fractionation is in agreement with the observations in the trimellitate system. Analogous to hemimellitate, a more thorough structural description of trimesoate on goethite is provided in the Supplementary material. The larger separation between the carboxyl groups and thus a decreased tendency to form mononuclear chelate structures as compared to trimellitate and hemimellitate results in formation of only outer sphere surface complexes on goethite between pH 3 and pH 8. One deprotonated complex is identified by peaks at 1363 and 1555 cm 1, and a partially protonated complex is formed around pH 4.8 and below and is characterized by peaks at 1714 and 1279 cm 1. In the infrared spectra from the competitive system trimesoate peaks are only visible at pH 3.5 and below (Fig. 6, all spectra not shown) which is in agreement with the adsorption data in Fig. 3c. The general features of these spectra are similar to those of the single ligand system at low pH, i.e., the spectra mainly show
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Absorbance
1714 a
b c
1800
1600
1400
1200
Wavenumber / cm -1 Fig. 6. Infrared spectra of trimesoate adsorbed on goethite at pH: (a) 3.5 in the presence of phosphate, (b) 3.1 in the absence of phosphate, and (c) 2.9 in the presence of phosphate.
the existence of partially protonated outer sphere complexes (Fig. 6). However, the relative intensities of the –C–O–H peaks around 1279 cm 1 versus the ones originating from the symmetric stretch vibration of the unprotonated carboxylates at 1363 cm 1 are stronger in the competitive system, indicating a higher degree of protonation of trimesoate. This is illustrated by the solid lines in Fig. 6 where the slopes in the spectra from the competitive system are steeper than in the spectrum collected in the absence of phosphate even though pH of the latter sample (3.1) is in between pH of the two competitive samples (2.9 and 3.5). Hence, a more protonated form of trimesoate is favored in the presence of phosphate which indicates that H-bonding interactions by the –C–O–H groups are involved in the competitive reactions. 3.3. Tetra- and hexacarboxylates
µmol pyromellitate ads./m2 goethite
The adsorption curve of pyromellitate on goethite is in agreement with previously published results [21], but in the presence of phosphate the curve shifts substantially to lower pH and the slope of the curve becomes less steep (Fig. 7). Thus, pyromellitate—like the smaller benzenecarboxylates—experiences a strong competition from phosphate ions for goethite surfaces sites. In the presence of phosphate pyromellitate adsorption commences at pH 6 and increases almost linearly as pH is decreased to 3. In this pH region only a very small amount of phosphate is desorbed, and the ratio between desorbed phosphate and adsorbed pyromellitate is markedly smaller
2.4
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6
µmol mellitate ads./m2 goethite
than one. Accordingly, pyromellitate is capable of coadsorbing with phosphate without causing any substantial desorption. The overall behavior of pyromellitate is similar to that of trimellitate; however, the competitive effects seem to be smaller in the former system. Hence, this comparison indicates that the number of carboxylate groups of the organic acids cannot be used as the sole criterion for assessing the competitive efficiency toward phosphate. Boily et al. [22] have previously identified IR peaks at 1377, 1254, and 1364 cm 1 as originating from one nonprotonated outer sphere, one protonated outer sphere, and one inner sphere complex of pyromellitate, respectively. It was shown that the nonprotonated outer sphere complex, although important in the whole investigated pH range, predominates at high pH. The protonated outer sphere complex and the inner sphere complex begin to form around pH 5 and pH 6, respectively, and both complexes increase in importance as pH is decreased. As expected, the above-noted peaks are also detected in the spectra collected in the absence of phosphate in the present work (Fig. 8). The 1377 and 1254 cm 1 peaks are readily detected at both pH 4.1 and pH 3.1 whereas the 1364 cm 1 peak is resolved in the second derivative spectra (not shown). The poor signal-to-noise ratios in the spectra collected in the presence of phosphate result in corresponding second derivative spectra dominated by noise, which prevent resolution of the 1377 and 1364 cm 1 peaks. However, at pH 4.6 the 1377 cm 1 peak of the nonprotonated outer sphere species is readily detectable in the regular IR spectrum. At pH 3.0 this peak is broadened toward the low-frequency side and that may indicate an increased intensity of the 1364 cm 1 peak, and thus the appearance of pyromellitate inner sphere complexes. Accordingly, also in this system the coadsorption seems to be caused, at least partially, by the formation of inner sphere pyromellitate surface complexes. Finally, although the 1254 cm 1 peak is difficult to detect, the appearance of the mC=O mode at 1710 cm 1 indicates the presence of protonated surface complexes at pH 3.0. The strong competitive ability by mellitate toward phosphate shown in Fig. 7b has already been documented in a recent study by Lindegren and Persson [12]. Furthermore, according to Johnson et al. [23], mellitate adsorbs as hydrogen-bonded outer sphere complexes throughout the investigated pH range (3–10). These are partially protonated around pH 6 and below, which is indicated by peaks at 1710 and 1279 cm 1 in the IR spectra (Fig. 9c). The same observations can be made in the presence of phosphate, and the predominance of deprotonated outer sphere complexes at pH 6.4 and partially protonated complexes at pH 3.0 (Figs. 9b and d) indicate that both types of complexes are able to compete with phosphate (Fig. 7b). In agreement with the trimesoate system and recently published results [12], the new data presented herein
1279
1363
1555
b 2
1
0 2
4
6
8
10
pH
Fig. 7. Adsorption of phosphate and (a) pyromellitate, and (b) mellitate on the goethite surface as a function of pH. s denotes adsorption of the carboxylate in the single ligand system and in the competitive system. j denotes adsorption of phosphate in the single system and h in the competitive system. The error bars are based on the standard deviation of three independent replicates.
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1377
1254
Absorbance
a 1710 b
c d
1800
1600
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Wavenumber / cm-1 Fig. 8. Infrared spectra of pyromellitate adsorbed on goethite at pH: (a) 4.2 in the absence of phosphate, (b) 4.6 in the presence of phosphate, (c) 3.1 in the absence of phosphate, and (d) 3.0 in the presence of phosphate.
1329
Absorbance
a
b
1279 1710
c
d
1800
1600
1400
1200
Wavenumber / cm -1 Fig. 9. Infrared spectra of mellitate adsorbed on goethite at pH: (a) 6.2 in the absence of phosphate, (b) 6.4 in the presence of phosphate, (c) 3.0 in the absence of phosphate, and (d) 3.0 in the presence of phosphate.
show that there is a higher degree of protonation of mellitate in the presence of phosphate at pH 3 than in the single ligand system at the same pH. This is indicated by the change in relative intensity of the 1279 cm 1 peak of the –C–O–H groups and 1329 cm 1 peak of the deprotonated carboxylate groups, illustrated by the solid lines in Fig. 9c and d. Hence, the mellitate results emphasize the importance of hydrogen-bonded outer sphere surface complexes, both deprotonated and protonated, depending on pH, for the competitive efficiency toward phosphate. 3.4. Overall trends and molecular mechanisms As already noted the number of carboxylic groups has been identified as an important factor determining the ability of organic acids
to compete with phosphate at water–mineral interfaces [9–12]. Accordingly, due to the low number of carboxylic groups of benzoate and phthalate, these ligands were unable to lower phosphate adsorption under the experimental conditions probed in this study. It is clear, however, from the results of the remaining benzenecarboxylates and from those presented by Lindegren and Persson [12] that additional molecular properties are needed to explain the influence on phosphate adsorption by carboxylic acids (Figs. 3 and 7). The point of zero charge of goethite is 9.4; thus, the surface is dominated by positive surface sites in all investigated samples. At pH 3, the efficiency of the benzenecarboxylates to compete with phosphate is in the order pyromellitate trimellitate < hemimellitate < trimesoate < mellitate. At this pH, the IR spectra discussed above show that the surface speciation of pyro-, tri-, and hemimellitate has a substantial contribution from inner sphere surface complexes whereas trimesoate and mellitate exist as partially protonated outer sphere complexes. Furthermore, using the relative intensities of the peaks originating from –C–O–H and symmetric C–O stretching vibrations at 1250–1275 and 1330–1380 cm 1, respectively, the average number of protons per molecule may be estimated. This analysis shows that at pH values around 3, trimesoate and mellitate have a markedly higher degree of protonation than the other three ligands in the series (Figs. 4–6, 8 and 9). Therefore we conclude that the presence of protonated carboxyl groups in a specific combination with deprotonated carboxylate groups is another important factor governing the competitive ability toward phosphate in this pH range. This is especially apparent in the trimesoate spectra where the degree of protonation is drastically decreased from pH 3.9 to pH 4.8 (Fig. S2), i.e., the same pH region where the competitive ability is also abruptly reduced (Fig. 3b). These results are in accordance with our previous study [12] showing that H-accepting carboxylate groups together with H-donating carboxylic acid groups provide a successful combination that increased competitive ability significantly. The reason why protonated carboxyl groups that are capable of H-donor interactions contribute to the destabilization of and the competition with phosphate is indicated by recent results concerning the adsorption of oxoanions on mineral surfaces [24]. It was shown that arsenate, in many aspects a phosphate analogue, forms un-, singly, and doubly protonated monodentate inner sphere complexes at the goethite surface. In addition to the monodentate interaction with surface Fe(III), all three complexes were indicated to be significantly stabilized by hydrogen-bonding to neighboring surface sites. Ongoing work in our group has demonstrated that phosphate forms equivalent surface complexes, and these can be distinguished by IR spectroscopy. The most diagnostic region in the IR spectra is that between 1000 and 1200 cm 1 where the various P–O(H) peaks appear [25–27]. At low pH the main peaks are detected at 1010 and 1125 cm 1 (Fig. 10k), which originate from a protonated phosphate complex [26–28]. In analogy with the recent arsenate results this is most likely a monodentate doubly protonated species. As pH is increased, new peaks appear at approximately 1050 and 1083 cm 1 (Fig. 10b and h) and this is indicative of deprotonation of the phosphate surface complexes [26–28]. Around pH 3, where strong competitive effects are observed from partially protonated trimesoate and mellitate ions, the phosphate surface speciation in the absence of carboxylates is completely dominated by the doubly protonated complex (Fig. 10k). The same peaks also appear in the presence of trimesoate and mellitate but the spectra display lower signal-to-noise ratios due to the lower surface concentrations of phosphate (Fig. 10i and j). Thus, these two benzenecarboxylates partially outcompete the doubly protonated phosphate complex. A complex that—in analogy with the arsenate system—is most likely stabilized by hydrogen bond interactions in which the P–OH groups act as H-donors and
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1050
1010
a
b
Absorbance
c
d e f g h i
j k
1210
1130
1050
970
Wavenumber / cm-1 Fig. 10. Infrared spectra of phosphate adsorbed on the surface of goethite at pH: (a) 5.8 in the presence of mellitate, (b) in the absence of carboxylates, (c) 4.5 in the presence of mellitate, (d) 4.6 in the presence of pyromellitate, (e) 4.3 in the presence trimesoate, (f) 4.3 in the presence of trimellitate, (g) 4.4 in the presence of hemimellitate, (h) 4.6 in the absence of carboxylates, (i) 2.9 in the presence of mellitate, (j) 2.9 in the presence of trimesoate, and (k) 2.8 in the absence of carboxylates.
the surface sites as acceptors. As protonated carboxyl groups can compete for these H-accepting surface sites and thereby destabilize the phosphate complexes, this would explain the competitive effect by trimesoate and mellitate at pH 3. The fact that the partially protonated trimesoate and mellitate outer sphere complexes are able to compete successfully with phosphate indirectly suggests that the contribution of the H-bonding interactions to the overall stability of the phosphate surface complexes most probably is substantial. The ranking of the benzenecarboxylates with respect to the competitive effect toward phosphate at pH 3 is perturbed by an increase in pH. Between pH 4 and pH 4.5 the competitive ability of trimesoate is significantly reduced and completely lost at pH 5 while hemimellitate under these conditions is more efficient and displays a maximum in its competitive ability (Fig. 3). Furthermore, in the same pH range the effect by trimesoate is now comparable to that of trimellitate (Fig. 3). In addition mellitate is able to influence phosphate adsorption over the entire pH interval 3–8 (Fig. 7). Referring to the IR discussion in the previous section, these observations indicate that competition also occurs under condi-
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tions where the surface speciation of the benzenecarboxylates is dominated either by deprotonated outer sphere complexes alone or by a combination of these complexes with inner sphere species. As shown in Fig. 10h the competition above pH 4.5 occurs under conditions coinciding with the onset of predominance of partially deprotonated phosphate surface complexes, i.e., surface complexes that most likely gain stability from H-bonding interactions that involve P–O groups as H-bond acceptors. As mellitate forms solely deprotonated outer sphere complexes above pH 6 we infer that the mellitate-induced desorption of phosphate is caused by competition between P–O and C–O groups for H-donor surface sites. Thus, depending on pH mellitate is apparently capable of competing with phosphate for both H-donor and H-acceptor sites and we believe that it this property together with the large number of functional groups that makes mellitate an efficient phosphate competitor over a relatively wide pH range. This dual effect is further corroborated by the IR spectra collected at pH 5.8 (Fig. 10a and b). Under these conditions mellitate is almost completely deprotonated (Fig. 9) and thus mainly a H-bond acceptor. Accordingly, the primary competition at this pH occurs between mellitate and the partially deprotonated phosphate surface complexes. As the deprotonated complex of phosphate is outcompeted in the presence of mellitate a larger fraction of the remaining phosphate is expected to consist of the protonated species. This is also detected experimentally as a change in relative intensities of the 1010 and 1050 cm 1 peaks (Fig. 10a and b). In contrast, when mellitate is sufficiently protonated it outcompetes the fully protonated phosphate complex as discussed above. Note that the effect at pH 5.8 cannot be due to a change in phosphate coverage as lowering the surface coverage of phosphate is known to affect the peak positions in the opposite direction; i.e., decreasing the surface coverage has the same effect on peak positions as increasing the pH [24, 25]. The competitive trends of the other benzenecarboxylates in the pH region 4–5 is more difficult to explain as the surface speciation of these ligands is a mixture of inner sphere and predominantly deprotonated outer sphere complexes (see discussion above). Nonetheless based on the results presented herein and in our previous study [12] we may conclude that at least three carboxylate groups are needed to induce any substantial phosphate desorption under the conditions probed in these studies. Furthermore, the trimellitate, pyromellitate, and hemimellitate results show that the structure of the ligands (i.e., the relative positions of the carboxylate groups) is also a decisive factor and in fact this may overrule the trend of the increasing competitive effect by an increasing number of carboxylate groups. This is demonstrated by the hemimellitate structure, with three carboxylate groups in close proximity, which is significantly more efficient than pyromellitate where the two pairs of carboxylate groups are separated by CH groups. As both ligands form similar inner sphere complexes consisting of mononuclear seven-membered ring chelate structures it is H-bonding either through the additional carboxylate groups and/ or through the carboxylate groups of the outer sphere complexes that makes the difference between hemimellitate and pyromellitate. This underlines the structural specificity of the H-bonding arrangements that govern the stabilities of the benzenecarboxylate and the phosphate surface complexes. Finally, our conceptual model for the competitive adsorption reactions studied herein based on the H-acceptor and H-donor properties of the surface complexes involved also explains the small differences observed between trimellitate and pyromellitate, and the unexpected larger effect by the former ligand. In a previous work the detailed surface speciation of trimellitate and pyromellitate on goethite was presented [21]. It was shown that that the inner sphere complexes of theses ligands behaved almost identically whereas the deprotonated outer sphere complex of trimellitate seemed to be more predominant in the pH region 3–5 as compared
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to the corresponding pyromellitate complex. From these results we infer that the competitive differences toward phosphate are not due to the inner sphere complexes but instead caused by the slightly larger surface concentration of deprotonated outer sphere complexes of trimellitate that partially outcompetes phosphate via H-acceptor interactions with protonated surface sites.
Appendix A. Supplementary material
4. Conclusions
[1] P. Hinsinger, Plant Soil 237 (2001) 173. [2] I.C.R. Holford, Aust. J. Soil Res. 35 (1997) 227. [3] H. Marschner, Mineral Nutrition of Higher Plants, Academic Press, London, 1995. [4] R.J. Haynes, M.S. Mokolobate, Nutr. Cycl. Agroecosyst. 59 (2001) 47. [5] U. Kafkafi, B. Bar-Yosef, R. Rosenberg, G. Sposito, Soil Sci. Soc. Am. J. 52 (1988) 1585. [6] A. Violante, L. Gianfreda, Soil Sci. Soc. Am. J. 57 (1993) 1235. [7] A. Violante, C. Colombo, A. Buondonno, Soil Sci. Soc. Am. J. 55 (1991) 65. [8] J.S. Geelhoed, T. Hiemstra, W.H. Van Riemsdijk, Environ. Sci. Technol. 32 (1998) 2119. [9] R.L. Parfitt, Plant Soil 53 (1979) 55. [10] H.Q. Hu, J.Z. He, X.Y. Li, F. Liu, Environ. Int. 26 (2001) 353. [11] S.E. Johnson, R.H. Loeppert, Soil Sci. Soc. Am. J. 70 (2006) 222. [12] M. Lindegren, P. Persson, Eur. J. Soil Sci. 60 (2009) 982. [13] A.E. Martell, R.M. Smith, Critical Stability Constants Other Organic Ligands, 3, Plenum Press, New York, 1977. [14] A.E. Martell, R.M. Smith, Critical Stability Constants First Supplement, 5, Plenum Press, New York, 1982. [15] J.F. Boily, J. Lutzenkirchen, O. Balmes, J. Beattie, S. Sjöberg, Colloids Surf. A 179 (2001) 11. [16] J. Murphy, J.P. Riley, Anal. Chim. Acta 27 (1962) 31. [17] M.W. Urban, Attenuated Total Reflection Spectroscopy of Polymers: Theory and Practice, American Chemical Society, Washington, DC, 1996. [18] K. Norén, P. Persson, Geochim. Cosmochim. Acta 71 (2007) 5717. [19] P. Persson, J. Nordin, J. Rosenqvist, L. Lövgren, S. Sjöberg, J. Colloid Interface Sci. 206 (1998) 252. [20] J.S. Loring, A.A. Simanova, P. Persson, Langmuir 24 (2008) 7054–7057. [21] J.F. Boily, P. Persson, S. Sjöberg, Geochim. Cosmochim. Acta 64 (2000) 3453. [22] J.F. Boily, N. Nilsson, P. Persson, S. Sjoberg, Langmuir 16 (2000) 5719. [23] B.B. Johnson, S. Sjoberg, P. Persson, Langmuir 20 (2004) 823. [24] J.S. Loring, M.H. Sandström, K. Norén, P. Persson, Chem. Eur. J. 15 (2009) 5063. [25] A.C. Chapman, L.E. Thirwell, Spectrochim. Acta 20 (1964) 937. [26] M.I. Tejedor-Tejedor, M.A. Anderson, Langmuir 6 (1990) 602. [27] P. Persson, N. Nilsson, S. Sjoberg, J. Colloid Interface Sci. 177 (1996) 263. [28] R.L. Parfitt, J. D Russel, V.C. Farmer, J. Chem. Soc., Faraday Trans. I 72 (1976) 1082.
The comparison of the competitive adsorption on goethite between phosphate and seven benzenecarboxylic acids has corroborated the importance of the number of carboxyl groups. However, the systematic study of different benzenecarboxylate structures has also revealed that the relative positions of the carboxyl groups play a decisive role for the competitive efficiencies. The competitive effect is particularly favored by the existence of three or more closely spaced functional groups. Finally, the collective results show that it is competition for hydrogen-bonding surface sites rather than inner sphere surface sites that primarily determines the outcome of the competitive adsorption experiments. This conclusion suggests that H-bonding makes an important contribution to the stabilities of both the adsorbed benzenecarboxylates and the phosphate ions. Furthermore, the effects by the different benzenecarboxylate structures indicate that the hydrogen-bonding interactions are structurally specific; i.e., they are sensitive to the locations and the directional properties of the H-acceptor and Hdonor surface sites. Acknowledgments We thank Dr. John S. Loring for help with the theoretical frequency calculations and Ingegärd Andersson for help with the spectra collection. This work was supported by the Swedish Research Council. The Kempe foundation, Sweden, is acknowledged for funding of the FTIR spectrometer.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2009.11.040. References