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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Structure Determination of Phosphoric Acid and Phosphate Ions in Aqueous Solution Using EXAFS Spectroscopy and Large Angle X-ray Scattering Ingmar Persson, Mylène Trublet, and Wantana Klysubun J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05641 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on September 4, 2018
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The Journal of Physical Chemistry
Structure Determination of Phosphoric Acid and Phosphate Ions in Aqueous Solution Using EXAFS Spectroscopy and Large Angle X-ray Scattering Ingmar Persson,*,† Mylene Trublet,‡ and Wantana Klysubun§ †
Department of Molecular Sciences, Swedish University of Agricultural Sciences, P.O.Box 7015, SE-750 07 Uppsala, Sweden, ‡
Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden,
§
Synchrotron Light Research Institute, 111 Moo 6, University Ave., Muang, Nakhon Ratchasima, 30000, Thailand.
Phosphoric acid, phosphate ions, hydration, structure in solution, EXAFS, large angle X-ray scattering
ABSTRACT: The structures of hydrated phosphoric acid and phosphate ions (H2PO4-, HPO42- and PO43-) in aqueous solution have been determined by P K-edge EXAFS and large angle X-ray scattering (LAXS). The P-O bond distance in all phosphate species studied is close to 1.53 Å. The P-(O)⋅⋅⋅Oaq distances have been refined to ca. 3.6 Å from the LAXS data giving a P-O⋅⋅⋅Oaq bond angle close to tetrahedral suggesting that each oxygen or OH group of phosphoric acid and dihydrogenphosphate, on average, hydrogen bind three water molecules. The (P-)O(-H)···Oaq and (P-)O···(H-)Oaq hydrogen bonds in hydrated phosphoric acid and the H2PO4- ion are shorter than the hydrogen bonds in neat water. This supports previous infrared spectroscopic studies claiming that the hydrogen bonds in hydrated phosphoric acid and phosphate ions are stronger than the hydrogen bonds in neat water. Phosphoric acid and phosphate ions can therefore be regarded as structure making solutes. This is the first study applying transmission mode X-ray absorption spectroscopy (XAS) data collection on the P K-edge. It shows that XAS spectra collected in transmission mode have a much better S/N ratio than data collected in fluorescence mode, allowing accurate determination of P-O bond distances. Furthermore, P K-edge EXAFS data collected in fluorescence mode display a higher amplitude at high k than expected due to increasing radiated volume of the sample with increasing energy as the total absorption decreases sharply with increasing energy of the X-rays. As a result, the fluorescence signal becomes non-proportional to the intensity of the X-ray beam over the EXAFS spectrum. This results in an increasing amplitude of the EXAFS function with increasing energy of the X-ray beam resulting in too small Debye-Waller coefficients.
The number of X-ray absorption spectroscopy (XAS) studies using the phosphorus K-edge has increased sharply in recent years in order to get insight and to identify phosphate compounds in soil and mineral systems.2-10 These studies have mainly used the XANES region with data collection in fluorescence mode. It has been shown in several studies that the XANES spectra of sulfur compounds using the sulfur K-edge, 2472 eV, may differ significantly depending on concentration and total absorption of the sample.11,12 This is most likely also true for phosphorus compounds at the phosphorus K-edge which has even lower absorption edge energy and a larger total absorption than for corresponding sulfur compounds at the same concentration. A study of sulfur K-edge EXAFS data of potassium sulfate and hydrogensulfate in solid state and aqueous solution, based on data collected in fluorescence mode, reports S-O bond distances shorter12 than expected compared to literature data of solid state structures.13 A more recent study on 0.1 mol·dm-3 aqueous solutions of sodium sulfate at pH 0.7, 2.0 and 5.5 with data collection in fluores-
INTRODUCTION Phosphorus is one of the key elements necessary for the growth of plants and animals as it is part of the Kreb’s Cycle and DNA. In fresh-water ecosystems it tends to be the growthlimiting nutrient for plants and algea causing eutrophication. Unlike nitrate, phosphate is retained in soil by a complex system of biological uptake, absorption, and mineralization. Adenosine triphosphate, ATP, is responsible for the storage and use of energy and a key stage in the Kreb’s Cycle. The phosphate sources can be divided in (i) non-point ones such as agricultural runoff, stormwater runoff, natural decomposition of rocks and minerals, erosion and sedimentation, atmospheric deposition, direct input by animals/wildlife, and (ii) point sources such as wastewater treatment plants and permitted industrial discharges. In general, the non-point sources are significantly larger than the point ones, with agricultural runoff being the most important one.1
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The Journal of Physical Chemistry cence mode and refinement of the EXAFS data in the k-range 4-13 Å-1 is reported.14 The S/N ratio was sufficient, but it is striking that the envelope has a higher amplitude at high k values than expected for such a light back-scatterer as oxygen which resulted in unrealistically small Debye-Waller coefficients.14 This is a pattern we have observed in own unreported data collected in fluorescence mode at the sulfur K-edge. This pattern with increasing amplitude with increasing energy using data collected in fluorescence mode is most likely caused by an increasing radiated volume of the sample with increasing energy as the total absorbance decrease sharply with increasing energy over the range scanned at EXAFS data collection at the P and S K absorption edges. Consequently, the intensity of the formed fluorescence radiation reaching the detector increases non-proportionally in relation to the intensity of the incoming beam with increasing energy.
mean bond distance. The plot is a summary of the data reported in Table S1. ion, apart from alkali ions, are summarized in Table S1. A very clear pattern of the P-O bond distances is observed where the oxygens binding a hydrogen atom are significantly longer than those not protonated. The difference in bond distance between P-O(-H) and P-O increases with decreasing number of protonated oxygen atoms, Figure 1 and Table S2. However, the mean P-O bond distance of the phosphate ions appears to be independent of the degree of protonation, 1.536 Å within the statistical errors, while the mean P-O bond distance in phosphoric acid is slightly shorter, 1.530 Å, Table S1. This means that the P-O bond distance distribution is expected to be larger in phosphoric acid and in protonated phosphate ions than in the regularly tetrahedral phosphate ion, PO43-. A very similar pattern with different As-O and As-O(-H) bond distances has been observed for arsenic acid and arsenate ions with different degree of protonation, Figure 1 and Table S2.18
The aim of this study is to determine the structure of hydrated phosphate ions in aqueous solution. The first step was to determine the kind of data collection mode which is the most suitable one. It is expected that data collected in transmission mode shall not have any envelop problems as the same volume of sample is studied throughout the experiment, but no X-ray absorption spectroscopy data recorded on the P K-edge in transmission mode have been reported. In order to get a deeper insight of the phenomenon of increasing amplitude at data collection in fluorescence mode for light elements, phosphorus K-edge EXAFS data on solid sodium dihydrogenphosphate monohydrate have been collected in both transmission and fluorescence mode for comparison. Only very few studies using phosphorus K-edge EXAFS attempting to determine the structure around the phosphorus atom have been reported.12,14-16
The structures of phosphoric acid and phosphate ions in aqueous solution have only been studied experimentally in a few studies. A P K-edge EXAFS study of solid and 0.1 mol·dm-3 aqueous solutions of KH2PO4, K2HPO4 and K3PO4, with data collected in fluorescence mode, reported a P-O bond distance of 1.55 Å for all phosphates in aqueous solution and slightly shorter P-O bond distances in the solids with multiple scattering paths within the PO4 tetrahedron at ca. 3.2 Å.19 The data are unfortunately noisy already at low k values, and therefore the structure parameters have large errors. A neutron scattering study of concentrated aqueous solutions of KH2PO4, K2HPO4 and K3PO4, 1.77, 1.18 and 0.89 mol·(kg water)-1, respectively, shows a mean P(-O)···Oaq distance of 3.7 Å, and mean (P-)O···Oaq distances of ca. 2.5 Å.20 A neutron scattering study on a very concentrated aqueous phosphoric acid solution, 53 mol%, reported a mean P-O bond distance of 1.540(3) Å and (P-)O···Oaq distance of 2.73 Å, showing shorter hydrogen bonds between phosphoric acid and water than between water molecules in neat water.21 Caminiti reported from a LAXS study that water molecules were hydrogen bound to phosphoric acid through the oxygen and OH groups at a mean P(-O)···Oaq distance of 3.6-3.7 Å, depending on the concentration.22 An infrared spectroscopic study has also shown that the hydrogen bonds between phosphoric acid and water are shorter and stronger than between water molecules in neat water. It was also demonstrated that the hydrogen bond strength increases with decreasing number of protons and increasing negative charge of the phosphate ion.23
The structure of phosphoric acid, H3PO4, and of phosphate ions (H2PO4-, HPO42- and PO43-) have been reported in a large number of solid compounds.13,17 The solid state structures containing phosphoric acid or phosphate ions not bound to a metal
P/As-O bond distance in Å
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1.75
H3AsO4 H2AsO4- HAsO42- AsO43-
1.70 1.65 1.60 1.55
A simulation study of the hydrated phosphate ion in aqueous solution using ab initio QMCF MD methodology showed that, on average, 13 water molecules are bound in the first hydration sphere with a mean P-(O)···Oaq distance of 3.8 Å, a P-O bond distance of 1.58 Å, and a mean (P-)O···Oaq distance of 2.57 Å.24 Simulation studies on DFT level have obtained similar results with overestimated P-O bond distances.25,26 Two large angle X-ray scattering (LAXS) studies have reported the structures of magnesium, nickel and cadmium dihydrogenphosphate complexes in aqueous solution, but no detailed structure of the phosphate ion in the hydrated complexes was mentionned.27,28
1.50 1.45
-
2-
0.5H3PO4 1.5H2PO4 2.5HPO4 3.5
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PO43- 4.5
Figure 1. P-O and As-O bond distances to protonated and non-pronated oxygen atoms in phosphate and arsenate ions in the solid state in compounds not binding to a metal except the alkali metal ions. Filled triangles – arsenate data, filled circles – phosphate data, red – mean P/As-O distance of all bonds, blue – P/As-O bond mean distance, and green – P/As-O(-H)
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The Journal of Physical Chemistry
A couple crystal structures where the phosphate or hydrogenphosphate ions are completely surrounded by water molecules, even though they are shared with the hydrated counter metal ions, showed that the phosphate ions hydrogen bind 12 water molecules, three per oxygen or OH group.29-31
worked surprisingly well even though there is room for development. Also in this case it is important to aim for an absorption edge step in the range 0.5-1.0. The sample preparation for fluorescence measurements of solids was the same as for the transmission experiments, though the sample thickness was less crucial. For solutions, the samples were placed in sample holders made of Plexi glass with one side made of 5 µm polypropylene X-ray film (Nitto Denko Co., Japan). A summary of the compositions of the studied solutions can be found in Table 1.
The aim of this study is to determine the structure of phosphoric acid and the phosphate ions with different degree of protonation in aqueous solution using P K-edge EXAFS and LAXS, and to evaluate the EXAFS data collection mode to prefer for elements with low absorption energy as phosphorus and sulfur.
It is extremely hard to fully avoid pin-hole effects at long wavelengths as for phosphorus K-edge absorption spectroscopy data. However, we have made our best to minimize these effects by extensive grinding of the solids and by soaking paper to get as thin and even samples as possible.
EXPERIMENTAL SECTION Chemicals. Phosphoric acid, H3PO4 (Merck, 97 weight%), sodium dihydrogenphosphate monohydrate, NaH2PO4·H2O, sodium hydrogenphosphate heptahydrate, Na2HPO4·7H2O, sodium phosphate dodecahydrate, Na3PO4·12H2O (all Merck, analytical grade), were used as purchased. Millipore filtered deionized water was used for the preparation of the aqueous solutions.
EXAFS data treatment. The EXAFS oscillations were extracted from averaged raw data using standard procedures for pre-edge subtraction, spline removal and data normalization. The data sets have to be terminated at k=11 Å-1 due to a severe glitch which could not be treated in a proper way. In order to obtain quantitative information of the coordination structure of the phosphate ions, the experimental k3-weighted EXAFS oscillations were analyzed by least-squares fits of the data to the EXAFS equation. The model parameters number of backscattering atoms, N, mean interatomic distances R, Debye-Waller factor coefficients, σ2, and relative ionization energy, ∆Eo, were refined. The data analysis was performed using the EXAFSPAK program package.36 The theoretical phases and amplitudes used in the refinements were calculated by the use of the FEFF7 program.37 The standard deviations reported for the obtained refined parameters listed in Table 2 are those related to the least-squares refinements and do not include any systematic errors. The errors given in the text have been increased to include the estimated systematic errors as well. Variations in the refined parameters obtained using different models and data ranges indicate that the accuracy of the distances given for the separate complexes is within ± 0.005– 0.02 Å, which is typical for well-defined interactions.
X-ray absorption data collection. X-ray absorption data were collected at the Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand, using the bending magnet beam line 8.32,33 SLRI operated at 1.2 GeV and a ring current of 80-150 mA. The radiation was monochromatized by an InSb[111] double crystal monochromator, d111=3.7405 Å.34 The data were collected in transmission mode using ion chambers filled with a mixture of nitrogen and helium or in fluorescence mode using 13 element Ge array detector (Canberra, Ultra LegE Detector (GUL)). For each sample three scans were collected in transmission mode, while 8 scans were collected when data were collected in fluorescence mode. Despite fewer scans, the S/N ratio was significantly higher for data collected in transmission mode. The spectrum of red phosphorus, Pn, was recorded after beam fill and thereafter at regular intervals assigning the first inflection point of the absorption edge to 2145.5 eV.35 The solid samples for transmission experiments were prepared by putting a thin layer of a finely ground powder on Nitto polypropylene tape (Nitto Denko Co., Japan) put on a plastic frame. The powder was smeared out to a very thin layer with a spatula. If it is difficult to get sufficiently thin samples the powder can be diluted with e.g. boron nitride, BN. The goal was to get an absorption edge of 0.5-1.0 absorption units. The change in total absorption over the EXAFS region was very large, ca. 1.5 absorption units excluding the absorption edge, Figure S1. It is therefore important to tune the absorption edge step in the range 0.5-1.0 as a larger absorption step will cause an even larger change in total absorption over the EXAFS range, and a smaller step may decrease the S/N ratio of the data. The XANES data for the H2PO4- ion collected in transmission mode as solid salt and aqueous solution, and in fluorescence mode as solid salt are given in Figure S2.
Large angle X-ray scattering (LAXS). A large-angle θ-θ diffractometer was used to measure the scattering of Mo Kα radiation (λ=0.7107 Å) on the free surface of aqueous solutions of phosphoric acid and sodium dihydrogenphosphate, Table 1. The solution was contained in a Teflon cuvette inside a radiation shield with beryllium windows. The scattered radiation was monochromatized by means of a focusing LiF [200] crystal. The Table 1. Compositions (in mol·dm-3), densities (ρ), and linear absorption coefficients (µ) of the aqueous solutions of phosphoric acid and sodium dihydrogenphosphate studies by LAXS, and aqueous solutions of sodium dihydrogenphosphate, hydrogenphosphate and phosphate studied by EXAFS. Sample [HnPO4(3-n)-] [Na+] a H3PO4 (aq) 2.002 H3PO4 (aq)a 3.024 NaH2PO4a 2.000 2.000 NaH2PO4b 1.000 1.000
It turned out that it was extremely difficult to prepare sufficiently thin cells for transmission experiments of solutions on the P K-edge. Instead, a drop of the aqueous solution was laid on a thin filter paper or Kleenex tissue. This method
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[H2O] ρ/g⋅cm-3 50.063 1.098 47.037 1.158 51.354 1.165
µ/cm-1 1.747 2.035 1.921
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Na2HPO4b 1.000 2.000 a LAXS b EXAFS intensity was measured at 450 discrete points in the angle range 1
