AQUEOUS SURFACE CHEMISTRY OF GOETHITE ADSORPTION AND DESORPTION REACTIONS INVOLVING PHOSPHATE AND CARBOXYLIC ACIDS

Transcription

1 AQUEOUS SURFACE CHEMISTRY OF GOETHITE ADSORPTION AND DESORPTION REACTIONS INVOLVING PHOSPHATE AND CARBOXYLIC ACIDS MALIN LINDEGREN DEPARTMENT OF CHEMISTRY UMEÅ UNIVERSITY SWEDEN 2009

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3 Abstract Phosphorus is an essential element for all known forms of life. In the form of phosphate, this nutrient is retained in soils and sediments by sorption on mineral particles, clays and other soil constituents. In addition, phosphate precipitates with metal ions to form a range of phosphate containing minerals, and only a minor part of phosphate is found dissolved in soil solution. One way of releasing sorbed phosphate is through ligand exchange reactions with for example carboxylic acids. This thesis summarizes five papers and focuses on the interactions of phosphate and carboxylates at the water-goethite interface. Quantitative adsorption data and spectroscopic evaluations of the surface complexation were used collectively to gain a better understanding of these processes. In agreement with previous studies, it was found that the number of carboxylic groups is important to the competitive ability of the organic acids towards phosphate. However, it was also shown that the positions of the functional groups are highly relevant to this ability. Furthermore, partially protonated species were because of hydrogen bond interactions - shown to be more competitive than fully deprotonated equivalents. Another central finding in this work is that competitive interactions do not necessarily involve ligand-exchange reactions between inner sphere surface complexes. To study the lability of the complexes, desorption experiments were performed. Among the benzenecarboxylates, the order of increased lability matched that of the decreased ability to compete with phosphate for surface sites on the goethite. Also shown in this thesis is the ability of goethite to increase the dissolution of fluorapatite mainly through the high affinity of phosphate ions for the goethite surface. iii

4 Aqueous Surface Chemistry of Goethite Adsorption and Desorption Reactions Involving Phosphate and Organic Acids Malin Lindegren This thesis contains a summary and a discussion of the following papers, referred to in the text by their Roman numerals I-V. I II III IV V Molecular Structures of Citrate and Tricarballylate Adsorbed on α-feooh Particles in Aqueous Suspensions Malin Lindegren, John S. Loring and Per Persson Langmuir, published online August 14, 2009 Reprinted with permission by ACS Competitive Adsorption between Phosphate and Carboxylic Acids: Quantitative Effects and Molecular Mechanisms Malin Lindegren and Per Persson European Journal of Soil Science, in Press Reprinted with permission by Wiley-Blackwell Competitive Adsorption Involving Phosphate and Benzenecarboxylic Acids on Goethite the Effect of Molecular Structures Malin Lindegren and Per Persson Submitted to Journal of Colloid and Interface Science Desorption of Benzenecarboxylates from the Water- Goethite Interface Malin Lindegren, John S. Loring and Per Persson Manuscript Dissolution, Adsorption and Phase Transformation in the Fluorapatite-Goethite System Åsa Bengtsson, Malin Lindegren, Staffan Sjöberg and Per Persson Applied Geochemistry, 2007, (22) Reprinted with permission by Elsevier iv

5 Contents 1. Introduction 1 2. Aim 2 3. Experimental Procedures and Techniques Minerals Goethite Fluorapatite (FAP) Carboxylates Experimental Procedures Simultaneous Infrared and Potentiometric Titration (SIPT) Batch Experiments in Ligand-Goethite Systems Batch Experiments in the FAP-Goethite System Quantitative Analysis Infrared Spectroscopy Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy Theoretical Frequency Calculations Two-Dimensional (2D) Correlation Analysis 9 4. Adsorption Reactions in the Single Ligand Systems Types of Surface Complexes General Procedure for Characterisation of Complexes Characterisation of Tricarballylate Surface Complexes Aqueous Tricarballylate Tricarballylate Surface Complexes Characterisation of Citrate, BTCA, Hemimellitate, and Trimesoate Surface Complexes Citrate BTCA Hemimellitate and Trimesoate Summary of Carboxylate Surface Complexes Phosphate Surface Complexes Adsorption Reactions in the Competitive Systems Quantitative Effects The Effect of Number of Carboxylic Groups The Effect of Carboxylate Group Positions Molecular Mechanisms Desorption Reactions Involving Carboxylic Acids Adsorption Reactions Involving Phosphate from Fluorapatite Conclusions References 37 v

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7 1. Introduction In 1669, an alchemist named Henning Brand tried to create the Philosopher s Stone, a long sought after alchemical tool that was supposedly capable of turning metals into gold and even achieving immortality of the user. Brand heated the condensed remains of urine with sand in an oxygen free atmosphere, but instead of the infamous stone he found a white substance that glowed faintly in the dark. This substance was named Phosphorus, which roughly translates as light-bringer and was the Ancient Greek name for Venus, the Morning Star. Phosphorus is an essential element for all known forms of life. It is part of vital biomolecules such as DNA, RNA, ATP, and phospholipids, which stores and communicates inherited biological information, transports energy, and builds up cellular membranes, respectively. It is also a central component of the apatite that is the main material of teeth, bones and shells 1. Phosphorus is widely used commercially in detergents, pesticides, toothpaste, fireworks and explosives. One of the most widespread uses is in the production of fertilizers 2. Due to its extensive reactivity, phosphorus, in the form of phosphate, is the least mobile of the major nutrients, and thereby often the key factor limiting plant growth. There are approximately 5.7 billions of hectares worldwide containing less than the needed available phosphorus for optimal plant growth 3. Phosphate is retained in soils and sediments by sorption on mineral particles, clays and other soil constituents. In addition, phosphate precipitates with metal ions, forming a range of phosphate containing minerals. Phosphorus is also found in organic matter in various stages of decay, and a minor part is found dissolved in soil solution. The dissolved fraction is generally considered the most bioavailable 4, whereas the other fractions need to be converted before assimilated. In aquatic systems, primary production is usually phosphorus limited in fresh water and nitrogen limited in estuaries and coastal ecosystems 5. As in soils, a large fraction (around 95%) 6 of the phosphorus carried by rivers is associated with suspended solids and more than half of this fraction can be bound to iron hydroxides 7. This bound phosphorus can be transformed into dissolved phosphate phases due to redox changes in anoxic layers of sediments 6, 8. An essential way of releasing retained phosphate from soil particles is by altering the ph. The ph regulates the speciation of molecules, ions, and surfaces, and thereby controls sorption/desorption processes and precipitation/dissolution reactions that determine the form in which phosphate is found. Another way of releasing sorbed phosphate is through ligand exchange reactions. Indeed, plant roots excrete low molecular weight (LMW) carboxylates such as oxalate and citrate as a response to phosphate depletion As the functional groups of organic anions (i.e., the carboxylate groups) give these substances similar sorption properties to that of inorganic anions 16, these carboxylates are thought to compete with phosphate for the active sites on mineral particles. In addition, the release of these LMW carboxylates would most likely be accompanied by exudation of protons to compensate for the net flux of negatively charged ions 3 and thereby decrease the ph and potentially release phosphate through increased dissolution of calcium phosphates such as apatites. 1

8 2. Aim The primary aim of this thesis is to further the knowledge of the competitive interactions between phosphate and carboxylates at the water/goethite interface. In order to account for the quantitative adsorption effects seen in both single ligand and competitive systems, particular attention has been paid to the molecular level surface speciation. The lability of the ligands has also been studied in desorption experiments and by changing the order of addition in some of the competitive systems. Moreover, the affinity of phosphate for the goethite surface has been investigated in a mixed mineral system. 3. Experimental Procedures and Techniques 3.1 Minerals Goethite In this study, the mineral goethite has been used as the model compound on which the adsorption processes take place. Goethite (α-feooh) is an iron hydroxide commonly found in all types of soils throughout the world 16. It is frequently formed during weathering of other iron-bearing minerals and is often the cause of the yellowish-brown colour of many soils 17, 18. Large surface area goethites were used in all experiments (Papers I- V), ranging between 82.2 and m 2 /g. The needle shaped particles are dominated by the {110} plane which represents roughly 90% of the total surface area 19 (Figure 3.1). This surface contains singly-, doubly-, and triplycoordinated oxygens of which the FeOH 0.5- and the Fe 3 O 0.5- are generally considered the most reactive. The remaining 10% of the particle surface the terminations of the needles have a larger density of these more reactive sites. The point of zero charge of goethite is 9.4; below 9.4 the surface carries a net positive charge and most of the experiments in this work are carried out in this ph region. Figure 3.1. A goethite particle with the active sites and crystal planes. 2

9 3.1.2 Fluorapatite (FAP) Of the phosphate minerals in soils, apatites (Ca 5 (PO 4 ) 3 OH, F, Cl) are the primary natural source of phosphate 20. The morphology of apatite crystals can have a great variety and the crystals range in size between submicron in biological systems to giant metamorphic crystals of many tons 21. The synthetic fluorapatite (FAP) used in this work is described in detail in Paper V. The dissolution of FAP proceeds according to Equation 1 coupled with Equations 2-4 and is thus strongly ph dependent Ca5 ( PO4 ) 3 F 3 PO4 + 5Ca + F (1) PO 3 + nh + (3 n) 4 H npo4 (n=1, 2, 3) (2) ( n 1)+ Ca + PO4 + nh CaH npo4 (n=1, 2) (3) F + H + HF (4) 3.2 Carboxylates Oxalate Citrate Tricarballylate BTCA Benzoate Phthalate Mellitate Hemimellitate Trimellitate Trimesoate Pyromellitate Figure 3.2. Schematic molecular structures of the carboxylates. Oxygen atoms are red, carbon atoms are grey and hydrogen atoms are white. The carboxylic acids of relevance to the work summarized in this thesis are pictured in Figure 3.2. Most of them are found naturally in various environments either as separate molecules or as part of larger soil constituents such as fulvic acids. Tricarballylate and 1,2,3,4-3

10 butanetetracarboxylic acid (BTCA), however, are included in this work for comparative reasons. Tricarballylate was used to elucidate the importance of the hydroxyl group in citrate adsorption and BTCA served as an intermediate in a series of carboxylates with an increasing number of functional groups. Citric acid is involved in almost all metabolic reactions as part of the citric acid cycle. It is found in many fruits and vegetables as a naturally occurring preservative and flavouring, and is used commercially in food and beverages. Citric acid is also used industrially in soaps and detergents, to dissolve rust from steel, in explosives and in the pharmaceutical industry 22. Oxalic acid is also abundant in many plants and is used commercially in the production of ink, bleachers and rust proofing treatments 23. The benzene carboxylic acids are found in soils and sediments as building blocks of fulvic acids 24. The smaller aromatic carboxylates, benzoic acid and phthalic acid in particular are widely used industrially as intermediates in the production of inks, dyes, and plasticizers 25. Benzoic acid is also found in many fruits, berries and vegetables where it serves as a preservative, and (like citrate) this ability is used commercially in food and beverages. 3.3 Experimental Procedures The experimental work in this study was performed either as a series of batches or in a simultaneous infrared and potentiometric titration (SIPT) setup Simultaneous Infrared and Potentiometric Titration (SIPT) In the SIPT experiments (Figure 3.3), a goethite suspension was circulated in a closed loop over an ATR cell coated with a thin layer of the mineral. This set-up provides an excellent signal-to-noise ratio that facilitates investigations of spectral changes induced by variations in ph or surface coverage of the ligands. This method is used and described in more detail in Paper I and IV. 4

11 Propeller Stirrer Combination ph Electrode Titration Vessel Peristaltic Pump Dosimeter with ph Meter Computer Stir Plate FTIR With Flow-Through ATR Cell Figure 3.3. Apparatus for simultaneous infrared and potentiometric titration (SIPT) experiments Batch Experiments in Ligand-Goethite Systems In the batch experiments in Paper I-III (Figure 3.4A), the ligand or ligands were added to the goethite suspension simultaneously before the ph was adjusted to the desired value. In the vast majority of the experiments the subsequent reaction time was 24 hours. Next, the wet pastes and supernatants were separated by centrifugation and then analyzed. The amounts of adsorbed ligands were calculated as the difference between total ligand and ligand in solution. The latter was quantified using the methods specified in section 3.4. The ligand-surface interactions were extensively investigated with infrared spectroscopy Batch Experiments in the FAP-Goethite System In Paper V, a closed dialysis tube with fluorapatite (FAP) was inserted in the goethite suspension (Figure 3.4B). Separate batches were equilibrated for 2 hours up to 129 days. The remaining FAP was removed and the samples were centrifuged and analyzed as described above. 5

12 Phosphate and/or Carboxylic Acid 24 h 5000 rpm 20 min 5000 rpm 20 min A IC UV-Vis AAS IR Figure 3.4. Schematic presentation of the experimental procedures. A) The procedure used in single ligand and competitive ligand systems of Papers I- III, and B) the procedure used in the mixed mineral system of Paper V. B IC AAS IR 3.4 Quantitative Analysis Different techniques were used to measure the concentrations of ligands and metal ions in solution. In Paper I, scintillation counting and ion chromatography (IC) were used to quantify citrate in solution. IC was also used for samples containing tricarballylate, oxalate, citrate and/or phosphate in Papers I and II, for samples containing phthalate, mellitic and/or phosphate in Paper III, for all nine benzenecarboxylates in Paper IV, and for fluoride and phosphate concentrations in Paper V. In Paper II, the samples containing mellitate, FA, and/or phosphate were analyzed with UV-Vis spectrometry. This technique was also used for the benzoate, trimesoate, hemimellitate, trimellitate, pyromellitate and/or phosphate containing samples in Paper III. Atomic absorption spectrometry (AAS) was used to acquire Fe concentrations in Paper I-V and Ca concentrations in Paper V. 3.5 Infrared Spectroscopy Infrared (IR) spectroscopy is used to study the vibrations of molecules. A sample is irradiated with light in the frequency range cm -1. When the incoming radiation matches the frequency of a molecular vibration that induces a change in the dipole moment of the molecule, energy is absorbed. As these frequencies are specific for every combination of atoms and force constants of the bonds, an IR active molecules produce absorption spectra characteristic for that particular molecule. Thus, the peaks found in infrared spectra can give information on inter- and intramolecular coordination. It has been found experimentally that the vibrations of some functional groups always fall in the same frequency region, regardless of the composition of the rest of the molecule. The peaks due to the stretch vibrations of the carboxylates are found between 1200 and 1800 cm -1 (Figure 6

13 3.5). Around 1400 cm -1 s peaks due to the symmetrical stretch vibrations ( ν CO ) of the deprotonated carboxylate groups are found. The corresponding as asymmetric vibrations ( ν CO ) give rise to peaks around 1550 cm -1. At both sides of the carboxylate originating peaks, peaks due to the protonated carboxylic group appear. Peaks derived from the carbonyl stretch vibrations are found between 1700 and 1750 cm -1, and the stretch and bend vibrations of the carboxylic COH group produce peaks around cm Wavenumber (cm -1 ) Figure 3.5. Presentation of the main frequency regions where different carboxylate/ carboxylic stretch vibrations produce peaks Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy Attenuated total reflectance (ATR) spectroscopy is used for samples that are too thick or too absorbing for conventional transmittance measurements. This technique is also ideal for the type of surface investigations made in this work. The ATR cell consists of a crystal with high refractive index and the sample is placed in direct contact with this crystal (Figure 3.6). The infrared radiation is transmitted through the cell and at the interface between the crystal and the sample an evanescent wave is produced that penetrates about a micron into the sample. The penetration depth (d p ) is wavelength dependent and given by Equation 5. The evanescent wave gives the short path length needed to gain information from a strongly absorbing sample. To increase the intensities of the peaks, ATR cells with up to 20 reflections are used. d p = 2π (sin 2 λ1 n2 ( ) n Θ 1 ) 2 1/ 2 where λ 1, is the wavelength and Θ is the angle of incidence of the infrared beam, n 2 and n 1 are the refractive indexes of the sample and the ATR crystal, respectively. (5) 7

14 Sample Crystal Detector Figure 3.6. A schematic illustration showing the function of the ATR cell. In a Fourier transform infrared (FTIR) spectrometer a rapid scan speed is gained by using an interferometer (Figure 3.7). The collimated radiation from an IR source hits the beamsplitter at an angle of 45. Half the radiation is transmitted through the beamsplitter to a moving mirror that is perpendicular to the beam. The other half of the radiation is reflected and strikes a fixed mirror perpendicular to this beam. Both beams are reflected back to the beamsplitter where they recombine and interfere constructively or destructively. As the velocity of the moving mirror is constant the recombined beam comprises a sinusoidal modulation in radiation intensity that travels to the detector via the sample. The signal from the detector is recorded to form an interferogram and the infrared spectrum as we recognise it is mathematically reconstructed via Fourier transformation. All details of the data collection of each study are found in Paper I-V. Fixed mirror Beamsplitter Moving mirror Infrared Source Sample Detector Figure 3.7. Schematic presentation of an interferometer. 8

15 3.5.2 Theoretical Frequency Calculations Theoretical frequency calculations can be used as a tool to assign specific molecular vibrations to peaks seen in infrared spectra. The theoretical spectra in this work are calculated with an electronic structure method known as the density functional theory (DFT). DFT utilizes a set of approximations called the basis set to optimize the geometry of the molecule and calculate the vibrational frequencies. Visualisation software is used to identify the vibrational modes of the peaks in the theoretical spectrum. Figure 3.8 illustrates the theoretical tricarballylate spectrum with the corresponding vibrational modes. In this example, the stretch vibrations of the carboxylic groups are coupled with two different wagging modes of the CH 2 -groups. Theory predicts three separate peaks for each of the symmetric and asymmetric stretch vibrations because the three individual carboxylic groups on tricarballylate do not have identical surroundings. In the experimental spectrum, however, these peaks are not clearly separated (Figure 4.2). Details on the computations are found in Paper I Figure 3.8. A depiction of part of the theoretical spectrum of tricarballylate with illustrations of the corresponding vibrational modes Two-Dimensional (2D) Correlation Analysis The correlations among peaks in a set of spectra from a system that is exposed to an external perturbation ph in this work can be identified by means of two-dimensional (2D) spectroscopy. In addition, by spreading highly overlapping IR peaks onto two dimensions, the resolution is greatly enhanced and the interpretation of the spectra facilitated 26. These qualities have been used in this work to find the number of surface species of a ligand and the infrared peaks belonging to each species. 2D correlation spectroscopy is based on a quantitative comparison of the variations in spectral intensities that are caused by the external perturbation and observed in two spectral variables, ν 1 and ν 2. The 2D 9

16 correlation spectrum can be expressed as the following complex number function: X(ν 1, ν 2 ) = Φ( ν 1, ν 2.) + iψ (ν 1, ν 2 ) = π ( ph max 1 ph min ) ph max 1 ph min ~ ~ Y ( ω) Y 2 ( ω) dω (6) where the real component (Φ) comprises the synchronous 2D correlation intensities and the imaginary (Ψ) component holds the asynchronous 2D correlation intensities, which are explained further below. ph max and ph min defines the ph limits of the experiment and Ỹ and Ỹ* are the forward Fourier transform of the spectral intensity variations in ν 1 and the conjugate of the Fourier transform of the spectral intensities in ν 2, respectively. ω is the Fourier frequency and represents the individual frequency components of the spectral variation. Before correlating a series of spectra in two dimensions, they are baseline corrected and normalised with respect to total peak area. This procedure retains only the features that are due to the changes in surface speciation caused by the external perturbation and not by other factors such as surface coverage. The two components in Equation 6 generate two types of correlation plots: synchronous and asynchronous. The synchronous plot reveals simultaneous or coincidental changes taking place at two separate frequencies. A set of diagonal auto peaks (Figure 3.9) defines the frequencies where the major intensity changes occur because of the external perturbation. Off the diagonal, cross peaks (Figure 3.9) gives the correlation among the auto peaks. A positive cross peak arises when two peaks increase and decrease jointly or in phase; that is, correlation occurs when the two peaks most likely belong to the same surface species. A negative cross peak indicates that one of the peaks increase at the expense of the other. In the asynchronous plot, only cross peaks are found. These cross peaks arise when two peaks of the original spectra vary out of phase, i.e., when they change independently of each other and thus stem from different surface species 26. Z [Wavenumber / cm -1 ] A Negative B Positive Cross Cross peak 1200 peak 1400 Auto peak Positive 1600 Cross peak Z [Wavenumber / cm -1 ] Negative Cross peak X [Wavenumber / cm -1 ] X [Wavenumber / cm -1 ] Figure 3.9. Illustrations of A) a synchronous correlation plot and B) an asynchronous correlation plot. 10

17 4. Adsorption Reactions in the Single Ligand Systems 4.1 Types of Surface Complexes There are two main types of carboxylate surface complexes: inner sphere and outer sphere. In inner sphere complexes there is at least one direct bond between the ligand and a metal ion of the surface, whereas in an outer sphere complex the ligand is held at the surface by electrostatic forces and separated from it by one or more water molecules. Both these types of surface species may or may not be stabilized by hydrogen bonds. Monodentate inner sphere Bidentate binuclear inner sphere Bidentate mononuclear inner sphere Surface hydration-shared ion pair Surface hydrationseparated ion pair Figure 4.1. Schematic representations of examples of inner and outer sphere surface complexes. Examples of carboxylate inner sphere complexes are pictured in Figure 4.1. Figure 4.1 also illustrates two types of outer sphere complexes. In the surface hydration-shared ion pair the ligand binds to the water of the mineral surface with hydrogen bonding, in the hydration-separated ion pair at least one layer of water molecules separate the ligand from the surface. As the hydration sphere of a hydration-separated ion pair is more or less unaffected by sorption, the infrared spectrum of this complex will be very similar to that of the dissolved ligand. On the other hand, because the spectrum of a hydrationshared ion pair can be quite different from the aqueous spectra, it can be rather difficult to distinguish the spectrum of a hydration-shared ion pair from the spectrum of an inner sphere complex. 11

18 4.2 General Procedure for Characterization of Complexes In this work, a general procedure has been followed when assigning complexes to ligands (Figure 4.2). First, theoretical spectra of the deprotonated and protonated ligand in solution are calculated so the peaks are assigned with molecular vibrations according to the description in section These assignments are then transferred to the corresponding experimental spectra. Once the peaks of the aqueous species are identified, comparisons are made with the spectra of the adsorbed ligand. If available, spectra of aqueous metal-ligand complexes are also used. The similarities and dissimilarities between the spectra of aqueous and adsorbed ligands are the basis for identifying the speciation at the mineral surface. At this point, a 2D correlations analysis may be used to find the number of complexes and to determine which peaks that belong to each species. Finally, the peaks of each complex are compared to those of the aqueous ligand and assigned with plausible complex coordinations : (1390, 1550) 2: (1332, 1360, 1410, 1616) Figure 4.2.Outline of the general procedure used in this work to identify surface complexes. 12

19 4.3 Characterisation of Tricarballylate Surface Complexes In the following section, the complexation of tricarballylate on the goethite surface will be discussed and serve as an example of the assignment procedure presented in the previous section. More detail on this system is found in Paper I Aqueous Tricarballylate Figure 4.3 shows a spectrum of completely deprotonated tricarballylate in aqueous solution with the corresponding 2 nd derivative and theoretical spectrum. The 2 nd derivative spectrum can be used to resolve overlapping peaks. Compared to the theoretical spectrum, the two dominating peaks of the experimental spectrum and 1390 cm -1 originate mainly from asymmetric and symmetric stretch vibrations of the deprotonated carboxylate groups, respectively. In addition, a couple of peaks due to bending modes of the CH 2 -groups are found around 1300 cm -1. A summary of the assignments is found in Table 1. a) b) wavenumber/cm -1 Figure 4.3. a) The infrared spectrum of an aqueous solution of tricarballylate at ph 12.4 in black with the corresponding negative second derivative in grey and b) the DFT-calculated frequencies, intensities, and spectrum of the trianionic ligand. 13

20 Table 1. Tentative assignments of the main peaks in the infrared spectra of the aqueous tricarballylate trianion based on the theoretical calculations. Frequency / cm -1 Assignment* as ν CO δ CH 2 s CO ν + ν CC + s ν CO + ν CC + s ν CO + ν CC + δ CH 2 δ CH 2 δ CH δ CH δ CH δ CH 2 *Note that detailed assignments of the bending modes are not made; thus scissoring, wagging, and rocking are collectively denoted δ modes Tricarballylate Surface Complexes Figure 4.4 depicts a series of spectra of tricarballylate on goethite collected at various ph levels. At ph 8.9, the spectrum of adsorbed tricarballylate is almost identical to that of the completely deprotonated ion in solution. This indicates that the hydration sphere of tricarballylate is intact on adsorption and this presents strong evidence of the formation of an outer sphere complex. As ph is decreased, additional features appear in the spectra. One of these features is a shoulder at 1616 cm -1 at the high frequency side of the 1550 cm -1 peak. To establish that this shoulder is due to a separate molecular vibration of tricarballylate and not due to an overlap with the strong bending mode of water (which appears around 1630 cm -1 ), the experiment was performed in D 2 O. When the solvent is changed from H 2 O to D 2 O, the bending mode is shifted to around 1200 cm -1, and the overlap around 1630 cm -1 is no longer an issue. The spectra from the D 2 O-experiments confirm that the 1616 cm -1 peak is indeed caused by a separate ν co mode. Decreasing ph further leads to the discovery of one more feature, a peak at 1708 cm -1. This peak originates from the vibrations of the doublybonded carbon and oxygen of protonated carboxylic groups, the ν c=o mode, and this is evidence of a protonation surface species

21 ph 3.0 ph 3.9 ph 5.2 ph 6.2 ph 7.6 ph 8.6 ph 8.9 pd Wavenumber / cm -1 Figure 4.4. Infrared spectra of tricarballylate on goethite in black at the ph values indicated by the legend together with the corresponding negative second derivative spectra in grey. The bottom spectrum depicts tricarballylate adsorbed onto goethite in D 2 O at pd 4.9. The arrows indicates a peak associated with a protonated surface complex discussed in the text. 15

22 Figure 4.5. A) Synchronous contour plot obtained from the 2D correlation analysis of the tricarballylate-goethite spectra. B) The corresponding asynchronous contour plot. To assign complexes to the additional features appearing on lowering the ph, a 2D correlation analysis was performed on the adsorption spectra. Seven auto peaks were found in the synchronous plot at 1332, 1360, 1390, 1410, 1550, 1616, and 1708 cm-1 (Figure 4.5A). In the same plot, a strong positive cross peak at (1390, 1550) confirms that these two peaks belong to the same surface complex. Negative cross peaks at (1390, 1616), (1390, 1708), (1550, 1616), and (1550, 1708) show that the 1390 and 1550 cm-1 peaks are negatively correlated to the 1616 and 1708 cm-1 peaks; that is, they stem from different species. In addition, positive cross peaks at (1616, 1332), (1616, 1360), and (1616, 1410) relate the shoulder at 1616 cm-1 to the 1332, 1360, and 1410 cm-1 peaks and organize them in a group of peaks originating from the same surface complex. Further information was gained from the asynchronous correlation plot (Figure 4.5B) where cross peaks at (1616, 1708), (1360, 1708), and (1410, 1708) show that the 1708 cm-1 peak is uncorrelated to the 1616 cm-1 group of peaks. In summary, by using the collective information from the synchronous and asynchronous plots, three groups of peaks were identified: (1390, 1550), (1332, 1360, 1410, 1616) and (1708). As already discussed, the (1390, 1550) most likely originate from a deprotonated outer sphere complex. The frequencies of the second group 1332, 1360, 1410, and 1616 cm-1 do not correspond to frequencies found in the solution spectra. This deviation of frequencies indicates significant molecular perturbation of the sorbed ion and from the way the peaks shift and from the experiment performed in D2O (see Paper I for details) we suggest the formation of an inner sphere complex. On account of the negative cross peaks found in the synchronous plot at (1390, 1708) and (1550, 1708) and of the absence of corresponding cross peaks in the asynchronous plot, the protonation is assigned to the outer sphere complex. 16

23 4.4 Characterisation of Citrate, BTCA, Hemimellitate, and Trimesoate Surface Complexes Similar evaluations as the one described in the previous section have been made for all carboxylic acids in this study. Benzoate, oxalate, phthalate, trimellitate, pyromellitate and mellitate have been investigated by previous authors 27, The assignments of citrate, BTCA, hemimellitate, and trimesoate are part of this work and will be described briefly in this section. A summary of all carboxylate complexes is found in Figure Citrate ph 2.9 ph 4.1 ph 5.5 ph 6.6 ph 8.2 ph 9.4 ph Wavenumber / cm -1 pd 3.4 pd 7.4 Figure 4.6. Citrate on goethite in H 2 O suspensions at ph indicated by the legend or in D 2 O suspensions at the pd indicated. The arrows are indicating a peak associated with a protonated surface complex discussed in the text. Four different citrate complexes were found on the goethite surface using the same methodology as the one employed in the previous section on 17

24 tricarballylate. In the 2D analysis, however, the spectra of citrate had to be grouped by more narrow ph-intervals to find the subtle changes in the spectra caused by the complexation and not only the major changes due to the ph variation (Paper I). One inner sphere complex was identified at low ph and another inner sphere complex involving the hydroxyl group at high ph. Around neutral ph an outer sphere species was found to co-exist with both these inner sphere complexes. In addition a protonated species was discovered in small amounts at low ph, signified by the peak at 1710 cm -1 that is marked with an arrow in Figure BTCA ph 8.7 ph 5.2 ph 4.0 ph 3.0 ph 11.8 (aq) ph 4.0 (aq) ph 1.5 (aq) Wavenumber / cm -1 Figure 4.7. ATR-FTIR spectra of BTCA on goethite and in aqueous solution at the ph indicated by the label. The assignment of BTCA complexes was mainly performed by comparing the spectra of BTCA on goethite to those of aqueous BTCA (Paper II). Briefly, the peaks caused by the sorbed and the dissolved species are essentially identical in shapes and positions; therefore, the surface speciation is concluded to be dominated by outer sphere complexes. The slight broadening of the peaks of the surface species is ascribed to inhomogeneous hydrogen bonding to different types of surface sites and solvation molecules. In addition, at ph 4 and below, a protonated complex is formed, identified by the peaks at 1710 cm -1 and 1230 cm

25 4.4.3 Hemimellitate and Trimesoate ph 11.4 (aq) ph 8.3 ph 6.2 ph 4.9 ph 3.0 ph 1.4 (aq) Wavenumber / cm -1 Figure 4.8. ATR-FTIR spectra of hemimellitate on goethite and in aqueous solution at the ph indicated by the label. In Paper III, the infrared spectra of hemimellitate and trimesoate are used to assign surface complexes to these carboxylates. As for BTCA, the interpretations are mainly based on comparisons with spectra of hemimellitate and trimesoate in aqueous solution. In the aqueous spectra of hemimellitate in Figure 4.8, three dominating peaks are seen. The ones around 1550 and 1370 cm -1 are ascribed mainly to asymmetric and symmetric stretch vibrations of the carboxylate groups respectively. Both these peaks split in two when hemimellitate is sorbed to the surface. This sign of significant perturbation indicates like in the tricarballylate discussion inner sphere coordination and is most pronounced at low ph in this system. At ph 8.3, the spectrum of the sorbed species is very similar to the spectrum of the fully deprotonated ligand in solution. This is why we assign the complexation at high ph to an outer sphere species. In the trimesoate system, all peaks in the spectra of the sorbed species can be found in the spectra of the unprotonated and the protonated ligand in solution (Figure 4.9). Therefore, trimesoate most likely forms only outer sphere complexes on goethite, unprotonated around ph 5 and above and 19

26 partially protonated at lower ph. The protonation is seen from the peaks at 1714 and 1279 cm -1 (Figure 4.9). ph 11.4 (aq) ph ph 4.1 ph 3.9 ph 3.1 ph 1.4 (aq) Wavenumber / cm -1 Figure 4.9. ATR-FTIR spectra of trimesoate on goethite and in aqueous solution at the ph indicated by the label. 20

27 4.5 Summary of Carboxylate Surface Complexes A summary of the carboxylate surface complexes used in this work is presented in Figure The arrows indicate the ph direction in which a surface complex becomes more pronounced. The abbreviations used are explained in the caption. Benzoate 32 O.S. 1 O.S. 2 I.S. Oxalate 33 Phthalate 34 O.S. Pyromellitate 35 Tricarballylate Hemimellitate Trimellitate 27 I.S. H + -O.S. O.S. Citrate H + -O.S. I.S. 1 O.S. I.S. 2 BTCA Mellitate 36 H + -O.S. Trimesoate O.S. ph Figure A summary of carboxylate surface complexes. The arrows symbolize an increased importance of the indicated complex in the ph direction noted. Outer sphere complexes are abbreviated O.S, inner sphere complexes I.S., and protonated outer sphere complexes are abbreviated H + -O.S. The numbers 1 and 2 are included to differentiate between structurally different complexes of the same type. 21

28 4.6 Phosphate Surface Complexes Atkinson 37 and Parfitt et al. 38 concluded the formation of phosphate complexes to consist of bidentate, binuclear bridging complexes. Tejedor- Tejedor and Anderson 39 arrived at the same conclusion, with the addition of a protonated complex. According to Nilsson et al. and Persson et al. 40, 41, however, phosphate probably forms three different monodentate complexes of which one is doubly protonated, one is singly protonated and one is fully deprotonated. Ongoing work in our research group confirms these findings. This ongoing work shows that phosphate and arsenate form equivalent surface complexes on the goethite surface, and arsenate has been shown to form un-, singly-, and doubly-protonated monodentate inner sphere surface complexes. All three complexes proved to be significantly strengthened by hydrogen bonding to neighbouring surface sites 42. In the cm -1 region in the IR spectra the various P-O(H) generated peaks give information on the surface speciation of phosphate (Figure 4.11). At low ph, dominating peaks are seen at 1010 and 1125 cm -1. As ph is increased, the 1010 cm -1 peak experiences an upward shift whereas the 1125 cm -1 peak shifts downwards in frequency, resulting in peak positions of 1050 and 1083 cm -1 in the spectrum of the mainly deprotonated phosphate complex at ph ph 8.8 y ph 4.6 ph Wavenumber / cm -1 Figure ATR-FTIR spectra of phosphate adsorbed on goethite at ph values indicated by the legend. 22

29 5. Adsorption Reactions in the Competitive Systems This section summarises the findings from Papers III-IV. 5.1 Quantitative Effects Adsorption data was collected for phosphate and the organic acids in single ligand systems and in competitive systems including phosphate and one of the organic acids. The aims were to determine the quantitative effects on both phosphate and carboxylate adsorption and to study the molecular mechanisms of these surface interactions The Effect of Number of Carboxylic Groups A first series of carboxylates was used to investigate the effect of an increasing number of carboxylic groups on phosphate adsorption. This series of acids include oxalate, citrate, BTCA and mellitate. Adsorption data from the single ligand and the competitive systems are presented in Figure 5.1 and Paper III. µmol ads. / m 2 goethite 2 1 a) Oxalate ph µmol ads. / m 2 goethite 2 1 b) Citrate ph µmol ads. / m 2 goethite 2 1 c) BTCA ph µmol ads. / m 2 goethite 2 1 d) Mellitate ph Figure 5.1. Adsorption of organic acid and phosphate as a function of ph. denotes adsorption of the organic ligand in the single ligand system and in the competitive system. denotes adsorption of phosphate in the phosphategoethite system and in the competitive system. In the single ligand system, oxalate adsorption decreases almost linearly with increasing ph. When phosphate is introduced in the system, oxalate adsorption is greatly reduced and only a small amount is found on the surface 23

30 in the competitive system. Phosphate adsorption, on the other hand, is unaffected by the presence of oxalate. A similar reduction in carboxylate adsorption is seen in the citrate containing systems. In contrast to the oxalatephosphate system, the adsorption of citrate in the competitive system does induce a small reduction in phosphate adsorption, an observation that was also made by Geelhoed et al. 43. BTCA adsorption is also affected by the presence of phosphate, but less so than oxalate and citrate. In addition, it is seen from the more pronounced dip in the adsorption curve of phosphate from the competitive system that BTCA is a stronger competitor towards phosphate than the two smaller ligands. Mellitate with six carboxylic groups is by far the most successful competitor of the investigated carboxylates. This is seen both in mellitate adsorption, which is only slightly reduced in the competitive system, and in the phosphate adsorption, which is suppressed by over 50% in a wide ph range in the presence of mellitate. As with previous studies 44-46, the findings from this series of carboxylates imply that an increased number of carboxylic groups does indeed increase the competitive ability of the organic acid towards phosphate The Effect of Carboxylate Group Positions A second series of carboxylates was used to investigate the importance of the number and the positions of the functional groups. A series of benzenecarboxylates was chosen as they provide the ability to change both the number of carboxylic groups and positions of these without altering the backbone structure of the molecule. From the data presented in Figure 5.2A-B, it is seen that the adsorption of both benzoate and phthalate are considerably reduced in the presence of phosphate, although the difference for benzoate is less pronounced due to low adsorption in the single ligand system as well. Conversely, and in accordance with the conclusions from the previous section, it is evident that none of these ligands have any significant effect on phosphate adsorption, which agrees with previous findings from a similar phosphate-phthalate system

31 µmol benzoate ads./m 2 goethite µmol trimelliatate ads./m 2 goethite ph A ph D µmol phthalate ads./m 2 goethite µmol hemimellitate ads./m 2 goethite ph B ph E µmol pyromellitate ads./m 2 goethite µmol trimesoate ads./m 2 goethite ph Figure 5.2. Adsorption of benzenecarboxylates and phosphate as a function of time, A) benzoate, B) phthalate, C) pyromellitate, D) trimellitate, E) hemimellitate and F) trimesoate. denotes adsorption of the organic ligand in the single ligand system and in the competitive system. denotes adsorption of phosphate in the phosphate-goethite system and in the competitive system. ph C F The very different curvatures of the adsorption graphs of the three tricarboxylates (Figure 5.2D-F) show that the positions of the carboxylic groups are in fact essential to the adsorption behaviour. Trimellitate adsorption decreases almost linearly with increasing ph, hemimellitate adsorption has the same general trend but a softer slope, and the adsorption curves of trimesoate appear as slender, inverted s-shapes. Interestingly, in the competitive systems the shapes of these curves are approximately mirrored by a corresponding decrease in phosphate adsorption. In addition, the overall adsorption patterns of trimellitate are actually more similar to those of pyromellitate than to the other tricarboxylates (Figure 5.2C-F). This indicates that from a surface coordination point of view trimellitate looks more like pyromellitate than trimesoate or hemimellitate. At ph around 4.5 the competitive effectiveness of these tri- and tetracarboxylates is in this order: hemimellitate > trimellitate pyromellitate > trimesoate Trimellitate, pyromellitate and hemimellitate all form inner sphere complexes, probably by using two carboxylic groups in bidentate, mononuclear chelate structures. Moreover, they all have additional carboxylic groups that can 25

32 stabilise a sorbed species by hydrogen bonding to the surface. Therefore, we deduce that it is the close proximity of the three groups in hemimellitate that renders this complex the ability to increase phosphate desorption. We also believe that the over all similarities in adsorption behaviour in both single and competitive systems of trimellitate and pyromellitate, show that there is little or no distinction in terms of competitiveness between surface complexes having one or two groups in a position two phenylic carbons from the mentioned inner sphere complex. Having none, however, affects the competitive ability negatively as seen in the phthalate systems. At ph 3, the order of competitive effectiveness of the tri- and tetra carboxylates is changed, and trimesoate is the most effective competitor towards phosphate followed by hemimellitate, trimellitate and pyromellitate. A possible explanation is found in the infrared spectra (Figure 5.3 and Paper III). The average number of protons per molecule can be estimated from the relative intensities of the COH originating peak at cm -1, depending on ligand, and the peak between 1330 and 1380 cm -1 of the unprotonated carboxylate groups. The estimation for these carboxylates shows that trimesoate has a markedly higher degree of protonation than hemimellitate, trimellitate and pyromellitate at ph 3. Thus, protonated carboxylic groups are advantageous for competitive effectiveness. 5.2 Molecular Mechanisms Representative infrared spectra from the single ligand and the competitive systems are presented in Figure 5.3. Spectra were chosen for each ligand at a ph where that particular ligand has some competitive ability or co-adsorbs with phosphate. Due to the very low adsorption of benzoate in the competitive system, the spectra are too noisy to gain any information. Therefore, they are not included in this figure. For the other carboxylates, some interesting effects are noted. The features ascribed by Persson and Axe 33 to one inner sphere and one outer sphere complex are all seen in the spectrum of oxalate from the single ligand system (Figure 5.3A). In the spectrum from the competitive system however, only the inner sphere originating peaks are clearly discernible. As oxalate is not able to lower phosphate adsorption, this implies that it is the inner sphere complex that co-adsorbs with phosphate on the goethite surface and a plausible explanation is that these inner sphere complexes do not adsorb onto the same surface sites as the phosphate complexes do. One possibility is that phosphate adsorbs to the main {110} surface of goethite and oxalate adsorbs to the {021} plains of the edges of the crystals. As for benzoate and oxalate, small amounts of phthalate adsorb in the competitive system, leading to poor signal-to-noise levels in the upper spectrum in Figure 5.3B. Compared to the lower spectrum in that figure (from the single ligand system) no apparent changes in speciation can be detected. Citrate is the smallest of the investigated carboxylates that shows any competitive ability towards phosphate. A comparison of the spectra from single ligand and competitive systems for this acid reveals a change in 26

33 coordination. In the single ligand system, an inner sphere complex is found to coexist with an outer sphere complex at low ph (Paper I). In the competitive system, only the peaks due to the outer sphere complex are found (Figure 5.3C). Hence, it is the outer sphere complex of citrate that competes with phosphate. Trimellitate and pyromellitate are both weak competitors towards phosphate, and consequently the spectra from these competitive systems, Figure 5.3D-E, display rather poor signal-to-noise depending on low carboxylate adsorption. There are, however, indications that the deprotonated outer sphere complexes dominate in these systems (see Paper III for details). No change in carboxylate surface speciation is detected in the competitive system of hemimellitate compared to the corresponding single ligand system. BTCA, trimesoate, and mellitate all form only outer sphere complexes on the goethite surface (Paper III, 36 ). In the ph range of successful competition towards phosphate, these carboxylates are partially protonated. From the spectra in Figure 5.3G-I, it is seen that the same speciation is found in the competitive systems as in the single ligand systems. In the competitive system of trimesoate and mellitate, a tendency is observed for the formation of more protonated species. This is seen from the higher relative intensities of the COH originating peaks in comparison to the ν s CO - peaks, illustrated by the dotted lines in Figure 5.3H-I. This finding supports the finding from the previous section that protonation is beneficial for the competitive ability of the carboxylates. This increased protonation also shows that an increased surface coverage by negatively charged ligands is compensated for by a larger number of protons. It is likely that the increased competitive effectiveness of the partially protonated complexes is due to the ability of these complexes to act both as proton acceptors and proton donors to neighbouring surface sites in hydrogen bonds. 27