Theoretical Study on the Effects of Spatial Structure of P Complexation Sites on the Soil Phosphorus Activation in Leonardite Humic Acid Complexes

1. INTRODUCTION

Humic acid is an important active constituent in soil environment^[1-4]. The interactions of humic acid with metal ions and inorganic pollutants are very critical in nature^{[5, 6]}. Humic acid can react with phosphate and metal elements in soil to form complexes by adsorption and complexation. The formed humic acid-metal-phosphate complex (HMP)^{[7, 8]} is a main component of humic acid phosphate fertilizer. Compared with water-soluble phosphate fertilizer, the humic acid phosphate fertilizer can effectively solve some problems, such as huge waste of resources, soil phosphorus pollution, and low utilization rate of phosphate fertilizer in the current season^[9-12].

The effect of humic acid phosphate fertilizer is mainly realized by the interaction between humic acid and phosphate. Soil phosphorus can be activated during this process. In nature, phosphorus activation could reduce the adsorption and fixation of phosphorus in soil, and increase the release of insoluble phosphorus in soil, so as to improve the availability of phosphorus in soil. For the studies of the interactions between humic acid and phosphate, there have been relevant reports^[13-16]. And these reports mainly include the decomposition and double decomposition mechanism, substitution and adsorption mechanism, and complexation mechanism, as shown in Scheme 1.

Scheme 1

Scheme 1.  Interaction mechanisms between humic acid and phosphate

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At present, the rhizosphere controlled fertilizer (RCF)^[17-19] and compound superphosphate fertilizer(CSP)^{[20, 21]} have been hot spots in the field of humic acid phosphate fertilizer. The phosphorus in humic acid phosphate fertilizer has high bioavailability for plants^[22-27], and the synergized action of this fertilizer is obvious.

Previous studies have shown that there are different coordination modes of metal-carboxyl in metal-humic acid complexes (M-HA)^[28]. That means the complexation sites of P have different spatial structures, which are unidentate structure(Ⅰ), bidentate chelating structure(Ⅱ), and bidentate bridging structure(Ⅲ), as shown in Scheme 2. Metal iron is taken as an example in the scheme. It has been found that, the spatial structures of P complexation sites play an important role in soil phosphorus activation and the fertilizer efficiency. But how do these spatial structures affect them? What's the effect? These have not been reported in the literature. Furthermore, for the P complexation sites, there are different opinions on the spatial structures and electronic characteristics^[28-31].

Scheme 2

Scheme 2.  Different modes of coordination for the interaction of Fe-carboxylate in the complex

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In this study, mineral humic acid represented by Leonardite humic acid has been selected as the calculation model, because Leonardite humic acid contains more carboxyl, hydroxyl groups and higher humic acid content. The monomer structure of Leonardite humic acid, which has been proposed by Niederer group^[32], is used to construct the models M-HA and HMP, as shown in Scheme 3. The previous results have shown that^[24], the theoretical stability of iron is the best in HMP complex. Therefore, in this study, the models containing iron are taken as an example to be analyzed. By analyzing the three coordination modes of iron-carboxyl, the effects of spatial structure of P complexation sites on soil phosphorus activation will be revealed. And the structure-activity relationship between the fertilizer efficiency and spatial structure will be also studied. This research could provide a theoretical basis for improving the fertilizer efficiency of humic acid phosphate fertilizer and further optimizing the production process.

Scheme 3

Scheme 3.  Leonardite humic acid model monomer (MHA)

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2. MATERIALS AND METHODS

The monomer structure of leonardite humic acid, proposed by Niederer group^[32], was selected to construct the calculation model (MHA model). And this model contained carboxyl group, benzene ring and its connected phenolic hydroxyl group, p-benzoquinone, methoxy group, and so on. The model chemical formula was C₃₁H₂₆O₁₂, and the total charge was 0. On the basis of model MHA, the iron-leonardite humic acid model (Fe-MHA) and the phosphate-iron-humic acid model (P-Fe-MHA) were constructed. These two models were the key structures in the process of soil phosphorus activation. The three different coordination modes of iron-carboxyl in models were studied, in which the unidentate structure was labelled as UD, the bidentate chelating structure as BD, and the bidentate bridging structure as BD-BG. These structures are shown in Scheme 4 below.

Scheme 4

Scheme 4.  Three different coordination modes of iron-carboxyl in models Fe-MHA and P-Fe-MHA

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All calculations were performed using the Gaussian09 package^[33]. The calculation models were calculated by the ONIOM method, which combined quantum mechanics with molecular mechanics^[34]. In models, one benzene ring and its connected phenolic hydroxyl group, one carboxyl group, metal iron and phosphate were located in the active center of the reaction. And they were classified as QM region and displayed by ball-and-stick model. The other areas were classified as MM region and displayed by linear model.

For the QM region, the B3LYP hybrid density functional available in the package was employed to optimize the configuration^[35]. When the QM region contained transition metal iron, the UB3LYP hybrid density functional was used for configuration optimization, and the different spin states of iron were considered. This functional was known to predict properties of humic acid compounds and complexes containing transition metals with good reproducibility^{[24, 31, 36, 37]}. The polarizable continuum model PCM with water solvent parameters was applied to consider the influence of solvent effects^[38]. In the QM region, the effective core potential with the double-ξquality basis sets (LanL2DZ)^[39] was used to describe Fe atom, and the calculations of the other atoms were carried out with the 6-31+G(d) basis set (Basis Set B1). The frequency calculations were performed on the optimized geometries to ensure that they had no imaginary mode, and the zero-point energy (ZPE) corrections and Gibbs free energy were then made. Subsequent single point calculations with a triple-ξquality basis set (TZVP)^[40] on all atoms were done to correct the energies (Basis set B2). The theoretical natural bond orbital (NBO) analysis was also performed to obtain the natural charges and Wiberg bond indices. And the amount of charge transfer was calculated by the difference value of the natural charges in the two adjacent structures. The basis set superposition error was corrected in the calculation of interaction energy^[41]. According to the frontier orbital theory of quantum chemistry^[42], the molecular orbital energy diagram, electron configuration and frontier orbital occupation of the models were also analyzed. For the MM region, the UFF force field was used for calculation^[43].

3. RESULTS AND DISCUSSION

3.1 Study on model Fe-MHA with three different coordination modes of iron-carboxyl

3.1.1   Optimized structures and the relative energies of model Fe-MHA

The iron-humic acid complex is formed by the reaction of leonardite humic acid with metal iron. To reveal the effects of different spatial structures, the three different coordination modes of ironcarboxyl in Fe-MHA are studied. The optimized geometrical parameters and relative energies of models are listed in Fig. 1. These parameters are based on the Basis set B1, and the relative energies are computed at the UB3LYP/B2//B1+ZPE level. Three spin states of the models are calculated, and they are marked at the upper left in the figure.

Figure 1

Figure 1.  UB3LYP/B1 optimized structures and the relative energies (UB3LYP/B2//B1+ZPE) of Fe-MHA-UD, Fe-MHA-BD and Fe-MHA-BD-BG. Bond lengths are in Å, energies in kcal·mol^-1, and the spin state of the model is marked at the upper left (Only QM region is displayed)

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As can be seen from Fig. 1, the relative energies of high spin state in the models are lower than that of low spin state. In Fe-MHA-UD and Fe-MHA-BD, the energies of the sextet state are the lowest, which are 31.9 and 0.3 kcal·mol^-1, 34.4 and 0.9 kcal·mol^-1 lower than that of the doublet and quartet states, respectively. In Fe-MHA-BD-BG, the energy of the quintet state is the lowest, and it is 31.5 and 12.0 kcal·mol^–1 lower than the singlet and triplet states, respectively. Thus, the high-spin state is the ground state. And the models in high-spin state are analyzed in this part.

As shown in Fig. 1, the bond length of Fe–O(1) is 2.081 Å, and Fe–O(2) is 3.301 Å in ⁶Fe-MHA-UD. This model is a unidentate structure formed by bonding of metal Fe with carboxyl O(1). For ⁶Fe-MHA-BD, the bond length of Fe–O(1) is 2.348 Å, and Fe–O(2) is 2.084 Å. Metal iron forms bonds with carboxyl O(1) and O(2) simultaneously, and this model is a bidentate chelating structure. In ⁵Fe-MHA-BD-BG, the bond length of Fe(1)–O(1) is 2.022 Å, and Fe(2)–O(2) is 2.064 Å. In the model, metal Fe(1) and Fe(2) bind with O(1) and O(2) of carboxyl group respectively to form a bidentate bridging structure.

3.1.2   Electron configurations and bonding descriptions of high spin state model Fe-MHA

The energy-level diagrams and the electronic configurations of three coordination modes in high spin state model Fe-MHA are shown in Fig. 2. The orbital energies are obtained from the spin-down frontier molecular orbital (relative to the highest occupied MO, HOMO).

Figure 2

Figure 2.  Energy-level diagrams and the electronic configurations of the molecular orbitals for ⁶Fe-MHA-UD, ⁶Fe-MHA-BD and ⁵Fe-MHA-BD-BG

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Fig. 2 shows that in ⁶Fe-MHA-UD, the d_yz orbital of Fe occupies the HOMO orbital. Five single spin-up electrons occupy the frontier orbitals, and they occupy the d_xz, d_xy, d_z² and d_x²_–y² of Fe and O(1)2p_y orbitals, respectively. The composition analyses of the frontier molecular orbital show that, there are two types of interactions between Fe and O(1), which are Fe–O(1) σ- and π-interaction. Thed_x²_–y² orbital and the 2p_x orbital develop the Fe–O(1) σ-interaction, while the d_xy and 2p_y orbitals engage in the Fe–O(1) π-interaction. Through these two interaction modes, the carboxyl group in humic acid can transfer charge to Fe as σ and π donor, and the charge transfer amount is 0.091 e. The oxidized state of Fe in ⁶Fe-MHA-UD is Ⅱ valence. The bond length of Fe–O(1) is 2.081 Å, and the Wiberg bond order is 0.24.

In ⁶Fe-MHA-BD, the d_xz orbital of Fe occupies the HOMO orbital. There are five single spin-up electrons in the frontier orbitals, which occupy the d_yz,d_x²_–y², d_xy and d_z² of Fe and O(1) 2p_z orbitals, respectively, as shown in Fig. 2. The composition analyses of the frontier molecular orbital show that, the d_xy orbital and the 2p_x orbital develop the Fe–O(1) and Fe–O(2) σ-interaction, while the d_xz orbital and the 2p_z orbital engage in the Fe–O(1) and Fe–O(2) π-interaction. The carboxyl group can transfer charge to Fe as σ and π donor by these two interaction modes, and the charge transfer amount is 0.089 e. The oxidized state of Fe in ⁶Fe-MHA-BD is Ⅱ valence. The bond length of Fe–O(1) is 2.348 Å, the Wiberg bond order is 0.14, and the bond length of Fe–O(2) is 2.084 Å, the Wiberg bond order is 0.25. In contrast, the bond length of Fe–O(2) is shorter and the bond level is stronger.

For ⁵Fe-MHA-BD-BG, thed_x²_–y² orbital of Fe(1) occupies the HOMO orbital. There are four single spin-up electrons in the frontier orbitals, and they occupy the d_xy, d_yz of Fe(1) andd_x²_–y², d_xy of Fe(2) orbitals, respectively. The composition analyses of the frontier molecular orbital show that the Fe(1)–O(1) σ-interaction includes d_yz and 2p_y orbitals, and π-interaction includes d_yz and 2p_z orbitals. The Fe(2)–O(2) σ-interaction includesd_x²_–y² and 2p_x orbitals, and π-interaction includes d_xy and 2p_y orbitals. Through the above interactions, the carboxyl group can transfer charge to Fe(1) and Fe(2) as σ and π donors, respectively. The charge transfer amount is 0.216 e. In ⁵Fe-MHA-BD-BG, the oxidized states of Fe(1) and Fe(2) both are Ⅱ valence. The bond length of Fe(1)–O(1) is 2.022 Å, the Wiberg bond order is 0.31, and the bond length of Fe(2)–O(2) is 2.064 Å, the Wiberg bond order is 0.35. Compared with ⁶Fe-MHA-UD and ⁶Fe-MHA-BD, the bond order of Fe–O in model ⁵Fe-MHA-BD-BG is stronger, and the amount of charge transfer is greater because of the existence of two charge transfer channels.

3.2 Study on model P-Fe-MHA with three different coordination modes of iron-carboxyl

3.2.1   Optimized structures and the relative energies of model P-Fe-MHA

On the basis of the iron-humic acid complex, phosphate can react with Leonardite humic acid to form the phosphate-iron-humic acid complex through metal bridge. The optimized geometrical parameters and the relative energies of three models are listed in Fig. 3. These parameters are based on the Basis Set B1, and the relative energies are computed at the UB3LYP/B2//B1+ZPE level. Three spin states of the models are calculated, and they are marked at the upper left in the figure.

Figure 3

Figure 3.  UB3LYP/B1 optimized structures and the relative energies (UB3LYP/B2//B1+ZPE) of P-Fe-MHA-UD, P-Fe-MHA-BD and P-Fe-MHA-BD-BG. Bond lengths are in Å, energies in kcal·mol^–1, and the spin state of the model is marked at the upper left (Only QM region is displayed)

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It can be seen from Fig. 3 that the relative energies of high spin state in the models are lower than that of low spin state. In P-Fe-MHA-UD and P-Fe-MHA-BD, the energies of the sextet state are the lowest, which are 18.1 and 9.3 kcal·mol^–1, 22.4 and 16.8 kcal·mol^–1 lower than that of the doublet and quartet states, respectively. In P-Fe-MHA-BD-BG, the energy of the quintet state is the lowest, and it is 68.5 and 36.2 kcal·mol^–1 lower than the singlet and triplet states, respectively. So, the high-spin state is the ground state. In this part, the sextet state of P-Fe-MHA-UD and P-Fe-MHA-BD and quintet state of P-Fe-MHA-BD-BG are analyzed.

In the models of three coordination modes, the phosphate is connected with iron through O, and it can interact with humic acid under the action of metal bridge. The bond length of Fe–O(4) is 1.943 Å in ⁶P-Fe-MHA-UD, and it is 1.881 Å in ⁶P-Fe-MHA-BD. In ⁶P-Fe-MHA-BD-BG, the Fe(1)–O(4) bond is 1.932 Å, and Fe(2)–O(5) is 1.880 Å. Compared with the models with only one phosphate, the bond length of Fe–O(4) in a unidentate structure model ⁶P-Fe-MHA-UD is longer than that of a bidentate chelating structure model ⁶P-Fe-MHA-BD. In the bidentate bridging structure model ⁶P-Fe-MHA-BD-BG, which complexes two phosphates, the bond lengths of two Fe–O bonds differ by 0.052 Å, and it is quite different.

3.2.2   Electron configurations and bonding descriptions of high spin state model P-Fe-MHA

The energy-level diagrams and electronic configurations of three coordination modes in high spin state model P-Fe-MHA are shown in Fig. 4. The orbital energies are obtained from the spin-down frontier molecular orbital (relative to the HOMO).

Figure 4

Figure 4.  Energy-level diagrams and electronic configurations of the molecular orbitals for ⁶P-Fe-MHA-UD, ⁶P-Fe-MHA-BD and ⁵P-Fe-MHA-BD-BG

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As can be seen from Fig. 4, in ⁶P-Fe-MHA-UD, the five d orbitals of Fe are all spin-up halfoccupied MO. The order of orbital energy from low to high is d_xz, d_yz,d_x²_–y², d_z² and d_xy orbitals. The three 2p orbitals of O(4) in phosphate are both low energy doubly occupied MOs. The composition analyses of the frontier molecular orbital show two types of interactions between Fe and O(4), which are Fe–O(4) σ- and π-interactions. The d_z² and 2p_y orbitals develop the Fe–O(4) σ-interaction, whiled_x²_–y² and 2p_y orbitals engage in the Fe–O(4) π-interaction. Through these two interaction modes, metal Fe can transfer charge to phosphate as σ and π donors, and the charge transfer amount is 0.138 e. The oxidized state of Fe in ⁶P-Fe-MHA-UD becomes Ⅲ valence. The bond length of Fe–O(1) is 2.048 Å, and the Wiberg bond order is 0.36. The bond length of Fe–O(4) is 1.943 Å, and the Wiberg bond order is 0.44.

In ⁶P-Fe-MHA-BD, the five d orbitals of Fe are all spin-up half-occupied MO. The order of orbital energy from low to high is d_xy, d_z²,d_x²_–y², d_xz and d_yz orbital. As with ⁶P-Fe-MHA-UD, the three 2p orbitals of O(4) in phosphate are both low energy doubly occupied MOs (Fig. 4). The composition analyses of the frontier molecular orbital show that the d_yz and 2p_z orbitals develop the Fe–O(4) σ-interaction, while d_z² and 2p_z engage in the Fe–O(4) π-interaction. In this model, metal Fe can transfer charge to phosphate as σ and π donors, with the charge transfer amount to be 0.141 e. The oxidized state of Fe becomes Ⅲ valence. The bond length of Fe–O(1) is 2.065 Å, the Wiberg bond order is 0.31; the bond length of Fe–O(2) is 2.018 Å, the Wiberg bond order is 0.35; and the bond length of Fe–O(4) is 1.881 Å, the Wiberg bond order is 0.47.

For the electronic configurations of Fe(1) in ⁵P-Fe-MHA-BD-BG, the d_yz orbital occupies the HOMO orbital, and thed_x²_–y² orbital occupies the HOMO-1 orbital. They are all doubly occupied orbitals. One spin-up single electron occupies the d_z² orbital, and the other two d orbitals are empty. The three 2p orbitals of O(4) in phosphate are both low energy doubly occupied MOs. For the electronic configurations of Fe(2), the three d orbitals of Fe are all spin-up half-occupied MO, and the order of orbital energy from low to high is d_z², d_yz andd_x²_–y² orbital. The three 2p orbitals of O(5) in phosphate are both low energy doubly occupied MOs, as shown in Fig. 4. The composition analyses of the frontier molecular orbital show that, the Fe(1)–O(4) σ-interaction includes d_xy and 2p_x orbitals, and π-interaction includes d_xz and 2p_x orbitals. The Fe(2)–O(5) σ-interaction includes d_z² and 2p_z orbitals, and π-interaction includes d_xz and 2p_x orbitals. In this model, metal Fe(1) and Fe(2) can transfer charge to two phosphates as σ and π donors, respectively. The charge transfer amount is 0.181 e. The oxidized states of Fe(1) and Fe(2) are both Ⅲ valence. The bond length of Fe(1)–O(1) is 2.065 Å, the Wiberg bond order is 0.26; the bond length of Fe(2)–O(2) is 1.936 Å, the Wiberg bond order is 0.42; the bond length of Fe(1)–O(4) is 1.932 Å, the Wiberg bond order is 0.40; and the bond length of Fe(2)–O(5) is 1.860 Å, the Wiberg bond order is 0.53.

In the three coordination modes of HMP complexes, with the binding of phosphate, the interactions between Fe and active functional group carboxyl in models are not weakened (except for the Wiberg bond order of Fe(1)–O(1) in ⁵P-Fe-MHA-BD-BG slightly weakened). This phenomenon has been confirmed by infrared spectroscopy and fluorescence experiments^[28], and these changes are related to molecular dissociation^[31].

3.3 Gibbs free energy of the high spin state model Fe-MHA and P-Fe-MHA

The Gibbs free energies of the high spin state models Fe-MHA and P-Fe-MHA in three different coordination modes are listed in Table 1. It can be seen from Table 1 that these model compounds all can form stable structures in solution. In models, the carboxyl group in Leonardite humic acid is single ionized state, and the calculated model compounds are weak acid.

Table 1

Table 1.  Gibbs Free Energy of the High Spin State Model Fe-MHA and P-Fe-MHA Calculated at the UB3LYP/B1 Level

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Model name	Gibbs free energy (kcal·mol–1)		Model name	Gibbs free energy (kcal·mol–1)
6Fe-MHA-UD	–68.4		6P-Fe-MHA-UD	–74.4
6Fe-MHA-BD	–68.2		6P-Fe-MHA-BD	–75.5
5Fe-MHA-BD-BG	–78.6		5P-Fe-MHA-BD-BG	–74.8

According to the calculated results, it is known that in models Fe-MHA, the thermodynamic stability of ⁵Fe-MHA-BD-BG in a bidentate bridging structure is the best (–78.6 kcal·mol^–1), followed by the ⁶Fe-MHA-UD in a unidentate structure (–68.4 kcal·mol^–1) and ⁶Fe-MHA-BD in a bidentate chelating structure (–68.2 kcal·mol^–1). The Gibbs free energies of the latter two models differ only by 0.2 kcal·mol^–1. Based on experimental studies^[28], in the weak acidic environment with PH value of 6, the coordination structure of Fe-MHA complex detected by infrared spectroscopy is a bidentate bridging structure, which is consistent with the thermodynamic stability of theoretical model ⁵Fe-MHA-BD-BG.

With the complexation of phosphate in P-Fe-MHA, the thermodynamic stabilities of the three models change. The thermodynamic stability of ⁶P-Fe-MHA-BD is the best (–75.5 kcal·mol^–1), followed by ⁵P-Fe-MHA-BD-BG (–74.8 kcal·mol^–1) and ⁶P-Fe-MHA-UD (–74.4k cal·mol^–1). The difference of Gibbs free energy between the latter two models is only 0.4 kcal·mol^–1. This change has also been verified in the experimental study^[28]. In weak acidic environment (PH = 6), with the addition of phosphate, the coordination structure becomes a bidentate chelating structure detected by infrared spectroscopy. And the theoretical calculation results are consistent with the experimental results.

3.4 Interaction energies of phosphate and Fe-MHA complexes in high spin state models with three coordination models

By calculating the interaction energies between phosphate and Fe-MHA complex, the interaction energies of ⁶P-Fe-MHA-UD, ⁶P-Fe-MHA-BD and ⁵P-Fe-MHA-BD-BG are –38.2, –46.2 and –199.5 kcal·mol^–1, respectively. Compared with the energies, the interaction energies of models ⁶P-Fe-MHA-BD and ⁶P-Fe-MHA-UD are smaller, and the energy difference between them is only 8.0 kcal·mol^–1. The adsorption of phosphate is weaker, and it is beneficial for soil phosphorus activation. By comparison, the interaction energy of model ⁵P-Fe-MHA-BD-BG is the largest and the adsorption of phosphate is stronger. It is not conducive to soil phosphorus activation.

4. CONCLUSION

In conclusion, the spatial structures of P complexation sites in HMP complex have an important influence on soil phosphorus activation. The effects of different spatial structures are as follows: the unidentate structure model ⁶P-Fe-MHA-UD, the bidentate chelating structure model ⁶P-Fe-MHA-BD > the bidentate bridging structure model ⁵P-Fe-MHA-BD-BG.

The different spatial structures influence the soil phosphorus activation by affecting the electronic structure, Gibbs free energy and interaction energy of the models. For the electronic structure, metal Fe and phosphate O interact through σ and π interactions to make Fe transfer charge to phosphate as an electron donor. Among the three coordination structures, the charge transfer amount of Fe is smaller and the Wiberg bond order of Fe–O is weaker in ⁶P-Fe-MHA-UD and ⁶P-Fe-MHA-BD. For the Gibbs free energy, the Gibbs free energy of model ⁶P-Fe-MHA-BD is the smallest and the thermodynamic stability is the best. In addition, the interaction energies of phosphate and Fe-MHA complex in ⁶P-Fe-MHA-BD and ⁶P-Fe-MHA-UD models are smaller and the adsorption of phosphate is weaker. Therefore, compared to ⁵P-Fe-MHA-BD-BG, the phosphate is easy to separate and activate from the complexes in ⁶P-Fe-MHA-UD and ⁶P-Fe-MHA-BD.

Through the above research results, it can be used to guide the production process of humic acid phosphate fertilizer. The fertilizer efficiency can be improved through increasing the proportion of unidentate structure and the bidentate chelating structure in the production engineering. And the research provides a theoretical basis for further optimization of the production of humic acid phosphate fertilizer.

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