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IR SPECTROSCOPY OF SURFACE PHOSPHATE COMPLEXES ON (HYDRO) OXIDE PHASES OF IRON (REVIEW).

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IR SPECTROSCOPY OF SURFACE PHOSPHATE COMPLEXES ON (HYDRO) OXIDE PHASES OF IRON (REVIEW).

1,2Kuzin A.V., 3Samadov A.S., 4Shelontsev V.A., 1Eliseeva E.A.,

5Gerasimova I.V.

1Moscow State Technical University named after Bauman. 2Moscow State. Pedagogical University.3Tajik National University. 4Omsk Humanitarian Academy. 5Omsk State Pedagogical University

Iron and aluminum oxyhydroxides can have a significant impact on the processes of formation, distribution, distribution of environmental pollutants, nutrients and various other substances dissolved in water, water systems and "mineral surfaces of the soil" through adsorption and co-adsorption processes and, thus, control the geochemical cycles of many elements.

The interaction between various types of ions of orthophosphoric acid and "mineral surfaces of the soil" such as, as previously reported, - (AlO(OH), FeO(OH)) plays a very important and decisive role in the immobilization of such an important element as phosphorus and, therefore, can affect its availability for the flora.

A huge number of studies have been devoted to identifying the types of adsorbed "phosphate-containing particles" on the surface of iron (hydr)oxide phases [1-29].

The cited works note that during the adsorption of various types of orthophosphate ions, the most important role is almost always played by hydrolysis processes, concentrations of various types of sorption particles in the solution, adsorption and desorption of hydrogen ions, the qualitative and quantitative composition of the background electrolyte or background electrolytes and the features of the processes of surface complexation of absorbed particles, their selectivity, "affinity" for the adsorbent, competition with other particles for "surface space".

The adsorption value of inorganic anions, including orthophosphate ions, also depends on such factors as the surface characteristics of the adsorbent, namely the surface area, density, porosity, volume and pore size distribution, the value of the hydrogen index of the zero charge point of the adsorbent surface, the characteristics of the sample synthesis, and its purity.

The physicochemical properties of inorganic anions, such as solubility, ionic and "hydrated" radii, volume diffusion coefficient, and hydration energy also significantly affect the process and characteristics of adsorption. These factors also probably affect the adsorption patterns of other cations and anions that are concentrated in the surface layer of oxide phases, including iron.

Infrared spectroscopy (IR) has been most frequently used to study phosphate surface complexes on various (hydr)oxides, including iron (hydr)oxide phases. It is “very sensitive” to the coordination environment and protonation states of the phosphate complexes, as illustrated well in Table 1 [3,4]. The success of this method is based on the fact that phosphate ions have strong mid-infrared bands originating from PO(H) vibrations, which can be detected even at low phosphate ion concentrations on the surface. Furthermore, the infrared features of phosphate ions are characteristically dependent on the symmetry of the ion, and therefore allow the coordination geometry of the complex to be inferred from the IR spectrum [26].

Infrared spectroscopy (IR) has been most commonly used to study phosphate surface complexes on various (hydr)oxides, including iron (hydr)oxide phases.

It is “very sensitive” to the coordination environment and protonated states of phosphate complexes, which is well illustrated in Table 1 [3,4].

The success of this method is based on the fact that phosphate ions have strong mid-infrared bands originating from PO(H) vibrations, which can be detected even at low surface concentrations of phosphate ions.

There are various modifications of the IR spectroscopy method, which are used to analyze surface complexes, for example, ATR (attenuated total internal reflection) and DR (diffuse reflection).

There are various modifications of the IR spectroscopy method, which are used to analyze surface complexes, for example, ATR (attenuated total internal reflection) and DR (diffuse reflection). The big advantage of the DO method compared to ATR is the low detection limit, as well as the possibility of “precise

The disadvantage of the DR method is that it requires samples with a lower water content [28].

Table. 1. Frequencies, symmetry of aqueous solutions of  H3PO4 and its anions.

Particle type

Frequency, cm-1

Symmetrical view

Description

, Td

1010

F2

P-O

934

A1

P-O

567

F2

O-P-O

420

E

O-P-O

, C3v

1077

E

P-O

989

A1

P-O

847

A1

P-OH

580

E

O-P-O

535

A1

O-P-O

394

E

P-O3

, C2v

1155

A1

P-O

1075

В1

P-O

940

В2

P-OH

874

A1

P-OH

567

A1

O-P-O

540

В2

P-O2

527

В1

P-O2

510

А2

P-O2

380

A1

НО-Р-ОН

, C3v

1174

A1

Р=О

1006

E

P-OH

890

A1

P-OH

500

E

НО-Р-ОН

394

A1

НО-Р-ОН

345

E

P-(OH)3

 

IR spectroscopy has been applied to the study of phosphate ion complexes on the surfaces of iron (hydr)oxides, namely goethite [3,4,30], ferrihydrite [5], hematite [4,22,24,27] and magnetite [10, 12]. The results of the studies differed in some initial positions relating, for example, to drying the sample, reaction conditions, synthesis conditions and the complexity of the object (iron oxide hydroxide or composition)).

Work [3] studied the formation of phosphate complexes on the surface of goethite depending on pH (3.6-8.0) and surface concentration of phosphate ions (100-190 µmol/g) and identified three different phosphate complexes

In the pH range (3.6-6.5) with a decrease in pH and an increase in the surface concentration of phosphate ions, surface complexes with IR bands of approximately 1123, 1006 and 982 сm-1 are formed, while at higher pH values ​​and lower concentrations of phosphate ions ions on the surface, in the same pH range, phosphate complexes with bands at 1096 and 1044 сm-1 are formed.  The first complex is very similar to the phosphate complex that forms on hematite [27] at low pH values ​​and high surface concentration of phosphate ion in the pH range 3.5-7.5 with bands at 1117, 1007 and 964  сm-1

The study [3] identified this complex as monoprotonated, bridged, based mainly on the P = O position of the 1125 cm-1 band. Two bands of the second complex observed in [3] at high pH values ​​and low surface concentration of the ion under study in the pH range (3.5-6.5), are very similar to the frequency bands of 1086, 1034 and 966 сm-1, which were observed for the phosphate complex formed on hematite at high pH values ​​and low surface concentration [27].

 The absence of the 966 сm-1 ν3 band in the research data [3] may indicate the formation of various complexes on the surface of goethite and hematite under these conditions. However, the similarity in position and relative intensity of the other two ν3 bands in studies [3] and [27] is remarkable and suggests that the complexes formed on both surfaces are similar.

The 966 cm-1 band may be difficult to detect in the “phosphate-goethite system” due to its relatively low intensity and proximity to the strong IR band at 900 cm-1 of goethite, as well as its overlap due to the presence of other complexes formed on goethite.

Study [6] confirms the presence of phosphate surface complexes on goethite at pH>7.5 with ν3 bands at 945, 1044 and 1089 сm-1, similar to what was observed for phosphate complexes on hematite at high pH in work [27]. This phosphate complex was defined in [3] and [6] as unprotonated, bridging.

The third phosphate complex, defined in [3], has ν3 bands located at 1025 and 1001 сm-1. This complex is observed only at pH values ​​greater than six and coexists with the second complex discussed above and increases its dominance on the surface with increasing pH. The study [3] defined this complex as unprotonated, monodentate.

The work [27] also identified three phosphate complexes on hematite at similar pH values ​​and surface concentration of the adsorbed phosphate ion, as in the works [3,4], but the interpretation of the nature of the complexes is somewhat different: monoprotonated bridged or monoprotonated monodentate with a strong hydrogen bond into the adjacent surface hydroxide ion, monoprotonated monodentate and unprotonated monodentate complexes.

In [4], DR was used to characterize the complexation of phosphate ions on the surface of goethite and hematite as a function of pH (3.1 and 12.8). Three different assemblages were also discovered on goethite. At low pH values ​​(3-4), a complex with ν3 bands at 1178 and 1001 сm-1 was observed. With increasing pH, the second complex with ν3 bands at 1122, 1049 and 939 cm-1 became increasingly prominent and dominated the surface in the pH range 8-11.

With increasing pH, the second complex with ν3 bands at 1122, 1049 and 939 cm-1 became increasingly noticeable and dominated the surface in the pH range 8-11. At pH 12.8, the third phosphate complex with bands at 1057 and 966 сm-1  “made itself felt.”

            The overall pH trend observed in the “phosphate-goethite data” [4] is consistent with the results of [3] and [27], but there are differences in the positions of the ν3 bands for the complexes. This is probably due to the preliminary drying of the samples [3,4,27]. These differences can affect the extent of hydrogen bonding between phosphate complexes and water molecules.

The use of drying and stirring with KBr can also lead to structural changes in surface complexes, as has been noted for sulfate complexes with hematite and goethite [31,32]. In addition to the spectral differences observed for phosphate complexes formed on goethite, [4] presents a different interpretation in the identification of complexes than in the complexes are monodentate and mononuclear with varying degrees of protonation: diprotonated and monoprotonated at low and medium pH values ​​and unprotonated at high pH values.

            The results in the study [5] are consistent with the results in the works [3,4,27] in that the phosphate sorption mechanism varies with pH changes and protonated phosphate complexes are observed at low pH values. The ν3 bands defined in [5] are somewhat different from the bands formed on hematite and goethite [3,4,27], which may be due to differences in the speciation of adsorbed phosphate on ferrihydrite compared to hematite and goethite.

            An important difference is that phosphate complexes formed on ferrihydrite at high pH values ​​≈7.5 are not protonated [5], while the results of [4,27] show the presence of protonated surface complexes of phosphate ions on hematite and goethite at pH data.

            As noted above, the study [27] suggests that adsorption of phosphate ions on hematite results in the formation of a mixture of different phosphate complexes as a function of both pH and surface concentration. Experiments using D2O and H2O indicate the presence of two monoprotonated phosphate surface complexes at pH/pD = 3.5-7.0. The difference in the IR spectra of these complexes is interpreted by different surface coordination: in one case, these are bridged forms, while in the other, they are monodentate mononuclear complexes. In addition, the bridged complex can form hydrogen bonds with neighboring surface atoms. The formation of bridged complexes predominates at lower pH values and high surface loading in the range of pH = 3.5-7.0. At pH 8.5-9.0, a third complex is formed, which is considered to be unprotonated, monodentate, mononuclear, present on the surface along with the monodentate monoprotonated complex.

            The data of relatively recent studies, also based on the use of IR spectroscopy [2,29] to study surface phosphate complexes on (hydr)oxide phases of iron, do not seriously contradict the results of the above-described works [1,3-6,8,9,13-28]. In the work [29] the presence of a bidentate monoprotonated complex - [( FeO ) 2 PO ( OH )] was determined (at 4.5<pH<8), and in the alkaline region (8<pH<9) - a monodentate monoprotonated complex - [( FeO ) PO 2 ( OH )] on the surface of hematite.

            In work [2] the formation of surface complexes on hematite and goethite in neutral solutions containing phosphate ions was studied. The "adsorption behavior" of phosphate ions in this study differs slightly for the studied iron phases [2]. A mixture of two inner-sphere complexes is formed on both phases; the dominant one for the studied phases is the formation of a monodentate monoprotonated surface complex - [( FeO ) PO 2 ( OH )].

             The second "in importance" for hematite is - [( FeO ) 2 PO ( OH )]; it was not possible to finally determine the type of the second complex for goethite in the study [2].

            The studies [33,34,35] are also devoted to the study of complex formation on the surfaces of α-FeOOH, β-FeOOH, γ-FeOOH. They note that bidentate, binuclear complexes are predominantly formed on these surfaces.

            The study [36] is also devoted to the study of the types of phosphate complexes on the goethite surface in a weakly acidic medium (pH=6.30). In this article, the mechanism of molecular binding of adsorbed phosphorus at the goethite/water interface was investigated by conducting experiments on the kinetics of phosphorus sorption and characterizing the adsorbed particles using IR Fourier spectroscopy. The authors of the study concluded that in a weakly acidic medium, the bidentate, monoprotonated complex dominates on the surface, and the monodentate, monoprotonated complex is present to a small extent.       Moreover, they found that water plays an important role in controlling the process of adsorption of orthophosphate ions on the goethite surface. Surface water molecules form hydrogen bonds with orthophosphate ions as well as with surface goethite atoms, with a proportion of water molecules forming covalent bonds with surface iron atoms, while others dissociate at the surface [36,37].

            In work [38], studies were conducted on the adsorption of orthophosphate ions on weakly crystalline ferrihydrite and amorphous mixtures consisting of iron ( III ) hydroxides and aluminum hydroxides of different molar compositions. The kinetics of adsorption and desorption were studied at a pH of six and different initial concentrations of orthophosphate ions.

For ferrihydrite, the formation of surface complexes - FeHPO 4 or Fe 2 PO 4 was suggested . Surface complexes Fe 2 HPO 4 or Fe 2 PO 4 were identified for Fe-hydroxides, the surface complex AlH 2 PO 4 was identified for Al-hydroxide. The formed complexes had hydrogen bonds with neighboring hydroxyl groups of the surface, or hydrogen bonds with outer-sphere complexes [38].

            In the study [39], the adsorption properties of micro- and nanosized goethite with respect to orthophosphate ions were studied, the synthesis methods of which were given in this work. The results of the study showed that microsized goethite has a greater ability to adsorb orthophosphate ions than nanosized goethite.

            Three types of monodentate mononuclear phosphate complexes in different protonation states were also identified, as well as an unprotonated bidentate complex. Protonated monodentate complexes formed at relatively low pH and “high surface loads”, while unprotonated complexes were the predominant species at medium and high pH. It should be noted that microsized goethite had a higher percentage of monodentate complexes than the nanosized sample.

            In [40], a new spectroscopic method, “infrared surface titration”, was presented, which was used to study the surface complexation of orthophosphate ions on goethite in the pH range of 4.5–9.5 at different surface coverages of the goethite sample. All complexes were found to be monodentate, with mononuclear and monoprotonated species predominating when pH > 5.5 and mononuclear diprotonated species predominating when pH < 5.5 at a surface coverage of 2.0 μmol m -2 . In contrast, at a low surface coverage of 0.7 μmol m -2, mononuclear monoprotonated species were found to predominate at all pH values.

            Thus, the adsorption of orthophosphate ions on the surface of iron (hydr)oxides is in most cases pH dependent; protonation of surface inner-sphere complexes is assumed in almost all studies, especially in the case of complexes formed at low pH values.

            The nature of surface phosphate complexes on iron (hydr)oxides is determined by the hydrophosphate and dihydrophosphate anions located on the surface, which interact with iron ( II ) and ( III ) cations.

            Other physical and physicochemical methods that have been used to characterize iron (hydr)oxide phases and describe the surface “iron-phosphorus” interaction are given in the review [41].

REVIEWER: Ruziev J.R.,

Doctor of Technical Sciences

Professor,

 

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IR SPECTROSCOPY OF SURFACE PHOSPHATE COMPLEXES ON (HYDR) OXIDE PHASES OF IRON (REVIEW)

A review is given on the study of the nature of surface phosphate complexes on iron (hydr)oxide phases using IR spectroscopy. IR spectroscopy is very popular for studying surface phosphate complexes on various iron (hydr)oxides. This research method is “very sensitive” to the coordination environment and protonated states of phosphate complexes. The adsorption of phosphate ions on the surface of (hydr)iron oxides depends on pH. Protonation of surface inner-sphere complexes is assumed in almost all studies, especially in the case of complexes formed at low pH values. The nature of surface phosphate complexes on iron (hydr)oxides depends on the hydrogen phosphate and dihydrogen phosphate ions that interact with Fe (II) and Fe (III) ions on the surface.

Key words: IR spectroscopy, iron (hydr)oxide phases, phosphate ions, adsorption, pH, surface, complexes.

 

Information about authors:  Alexander Vasilyevich Kuzin - Moscow Pedagogical State University, Senior Lecturer, Department of General Chemistry. Address: 119991, Moscow, Russia, Malaya Pirogovskaya Street, 1. Phone: (+7) 977-154-49-42. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..

Samadov Abdurasul Saidovich - Tajik National University, Candidate of Chemical Sciences, Senior Lecturer, Department of Physical and Colloidal Chemistry. Address: 734025, Dushanbe, Tajikistan, Rudaki Avenue, 17. Phone: (+992) 111-50-38-38. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Shelontsev Vladimir Alexandrovich - Omsk Humanitarian Academy, candidate of chemical sciences, associate professor, associate professor of the department of pedagogy, psychology and social work of private institution of educational organisation of higher education. Address: 644105, Chelyuskintsev St., Omsk. Phone: (+7) 9139617129. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Elena Anatolievna Eliseeva - Bauman Moscow State Technical University, Associate Professor, Department of General Chemistry. Address: 105005, Moscow, Russia, Baumanskaya str. 5. Phone: (+7) 903-964-48-64. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Gerasimova Irina Vladimirovna - Omsk State Pedagogical University, Associate Professor of the Department of Chemistry and Chemistry Teaching Methods. Address: 644099, Russia, Tukhachevskiy Embankment, Omsk.  E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Article received 06.05.2024

Approved after review 30.08.2024

Accepted for publication 07.10.2024

   
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