Chemical Durability, Properties and Structural Approach of the Glass Series xFe₂O₃-(45-x) PbO-55P₂O₅ (with 0 ≤ x ≤ 20; mol%)

Radouane Makhlouk¹, Zineb Chabbou¹, Yassine Er-rouissi¹, M’hammed Taibi², Said Aqdim^1,3*

¹Laboratory of Materials Engineering for Environment and Valorization, Faculty of Sciences, Hassan II University Ain Chock, Casablanca, Morocco.
²Physico-Chemistry Laboratory of Inorganic and Organic Materials ENS, University Md V, Rabat, Morocco.
³Mineral Chemistry, Department of Chemistry, Faculty of Science, Hassan II University Ain Chock, Casablanca, Morocco.

DOI: 10.4236/njgc.2023.131001                

Abstract

The synthesis for glasses series xFe₂O₃-(45-x)PbO-55P₂O₅, (with 0 ≤ x ≤ 20; mol%) carried out in a temperature (1050 ± 10)°C, leads to obtaining transparent glasses, brown in color and with a non-hygroscopic appearance. The study of glasses dissolution rate, immersed in distilled water at 90°C for 24 days, indicates a considerable chemical durability. The increase in the Fe₂O₃ content in the vitreous network to the detriment of PbO is a favorable factor for the chemical durability improvement. Different techniques have been used such as X-ray diffraction, infrared spectroscopy, DSC, SEM and density for the study of these glasses. These techniques have led to establish correlations between chemical and structural properties. Thus the results obtained confirmed the creation of P-O-M bonds (M = Pb, Fe) with a strongly covalent nature to the detriment of the hydrated P-O-P bonds and led to the formation, mainly, of pyrophosphate groups. The low melting point of Pb-O makes it possible to play an important role, at the same time, on the viscosity, on the equilibrium between the vitreous bath and the crystallites formed. The dissolution rate obtained is 100 times smaller than that of silicate glasses used as an alternative form for the vitrification of radioactive waste.

Keywords

Phosphate Glasses, Lead, Iron, Density, IR, DRX, MEB, Durabilité Chimique, Vitrification, Nuclear Wastes

1. Introduction

Phosphate glasses have been investigated principally because of their relatively low processing temperatures (1000˚C - 1100˚C) compared with borosilicate glasses (1200˚C - 1500˚C). Their properties, such as a low melting point, high coefficient of thermal expansion, dielectric properties and optical properties, make these glasses potential candidates for many technological applications such as: medical field (biomaterials), solid electrolytes, vitrification of nuclear waste, etc. [1] - [11]. Recently, the glass structure in the PbO-P₂O₅ binary system has been studied. Moreover, the incorporation of Pb-O into the phosphate network would have resulted in the breaking of hydrated P-O-P bonds and the appearance of covalent and rigid P-O-Pb bands [10]. In the previous work we are confirmed that the substitution of Na₂O with lead oxide by more than 28 mol%, with the presence of 2 mol% of Cr₂O₃ in the vitreous network appeared to be an unfavourable factor for chemical durability. The phenomenon has been explained by the approach of the boundary zone between the crystal and the glass by the continuous formation of isolated phosphate groups ${\text{PO}}_{\text{3}}^{\text{4}-}$ [7] [12] [13]. Hence, the crystallites exceed a certain limit, and the equilibrium between the glass bath and these crystallites is no longer maintained; we notice, once, a few decrease in the chemical durability. The aim of the present work is to study the mixed effects of lead and iron on the structure and to improve the chemical durability by reducing the PbO rate by substitution with Fe₂O₃ content which will lead to the decrease of isolated orthophosphate groups by promoting the formation of pyrophosphate groups [7], and consequently the decrease in the crystallites number which prevents the equilibrium formation between these and the vitreous bath. The study of the dissolution rate as a function of iron oxide content in distilled water at 90˚C for the glass series xFe₂O₃-(45-x)PbO-55P₂O₅ with (0 ≤ x ≤ 20; mol%) reveals an important chemical durability. The relatively low melting point of Pb-O makes it possible to play an important role, at the same time, on the viscosity and on the equilibrium between the vitreous bath and the crystallites formed. The dissolution rate (D_R) of the analyzed compounds is 100 times less than the values of borosilicate glasses and 200 times less than BaBal glass which is used as alternative material for the immobilization of nuclear waste substances.

2. Experimental Section

The synthesis of phosphate-based glasses was carried out by direct fusion of mixed reagents (NH₄)₂HPO₄, Pb(NO₃)₂ and Fe₂O₃ in adequate proportions. The reagents were finely ground and then introduced into a porcelain crucible. They were first heated at 300˚C for 1 hour and then at 500˚C for 1 hour to complete their decomposition. The mixture reaction was then heated to (1050 ± 10)˚C for 20 min. The liquid obtained is homogeneous. It was then poured into an aluminum plate previously heated to 200˚C to avoid thermal shocks. The chemical durability of these glasses was evaluated by the weight loss of the sample. The samples were then polished with carbon silica sandpaper (CSI), cleaned with acetone and immersed in Pyrex beakers containing 100 ml of distilled water and brought to 90˚C. The surface of the sample must be constantly immersed in distilled water for 24 days. The glass density was measured at room temperature using Archimedes’ method. The glass was immersed in a solution of diethyl orthophtalate whose density, depending on the temperature, was known. The precision is 0.05 g/cm³. The infrared spectra of the studied phosphate glasses were located in the region between 400 and 1600 cm⁻¹ with a resolution of 2 cm⁻¹. The samples were finely ground and mixed with KBr (potassium bromide), which was transparent in the infrared, and whose role was to serve as a matrix. The microstructures of the sample glasses were characterized by scanning electron microscopy (SEM), equipped with a full system micro-analyser (EDX-EDAX).

3. Results and Discussion

3.1. Location of the Studied Glasses in the Fe₂O₃-PbO-P₂O₅ Ternary Diagram

As can be seen clearly from the ternary diagram (Figure 1) and taking into account the oxygen/phosphate ratios shown in Table 1. The location of S₀, S₅, S₁₀, S₁₅ and S₂₀ indicated that the structure of meta-phosphate chains (S₀ and S₅) evolved to predominant short pyrophosphate groups (S₁₅ to S₂₀).

3.2. Chemical Durability Analysis and pH of Glasses of the xFe₂O₃-(45-x)PbO-55P₂O₅ System (with (0 ≤ x ≤ 20 mol%)

The chemical durability of the studied glasses series was approached using the measurement of the dissolution rate, which was defined as the weight loss of the glass expressed in g·cm⁻²·min⁻¹. The values of the dissolution rate (D_R), as a function of the Fe₂O₃ molar percentage, are grouped in Table 1 and presented in

Figure 1. Localization in the ternary diagram, of system glasses xFe₂O₃-(45-x)PbO-55P₂O₅ (avec 0 ≤ x ≤ 20; mol%).

Table 1. Compositions, chemical durability and pH of the glass series xFe₂O₃-(45-x)PbO-55P₂O₅ system with (0 ≤ x ≤ 20 mol%)

Figure 2. The typical results obtained for the samples, after their immersion in 100 ml of distilled water and heated at 90˚C for 24 consecutive days, showed a remarkable chemical durability as the Fe₂O₃ content increased in the vitreous network to the detriment of PbO content. Figure 3 shows the pH of the aqueous solution after the corrosion tests. Due to the high chemical durability of glasses containing more than 10 mole % Fe₂O₃ the pH decreases slightly from its initial values. However, there is a relatively significant decrease in the pH of solutions for immerged glasses with a less Fe₂O₃ content than 10 mol%. The decrease in pH between 5.2 and 7.2 is attributed to the formation of phosphoric acid whose pH generally, varies between 5 and 11, due to the degradation of glass by water, which plays an amphoteric role [14]. The decrease in pH is consistent with the large values of D_R. The visual appearance of glassy compounds emerged in distilled water at 90˚C for 24 days shows that glasses which have low Fe₂O₃ content such as S0 and S5 are relatively corroded, while S₁₅ and S₂₀ glasses are practically intact and keep their original appearance [15].

3.3. Analysis by Differential Scanning Calorimetry (DSC)

Figure 4 shows the DSC curves of iron-lead phosphate glasses. The results of the glass transition temperatures (T_g) as a function of the Fe₂O₃ content are grouped in Table 2 and presented in Figure 5. The T_g evolution indicated that the substitution of lead oxide by iron oxide led to an almost linear increase in T_g. The analysis of these results indicated a significant improvement in T_g when the Fe₂O₃ content increased, this increase characterizes a strengthening of the vitreous networks [8] [14] [16] [17]. This behavior has been confirmed by previous work [14]. For the S₀ sample (45PbO-55P₂O₅) the two endothermic peaks around 70˚C and 193˚C are due to the dehydration of the sample while the endothermic peaks which appear in the interval 376 - 470˚C are tuned to the glass transition temperatures of different samples.

3.4. Density and Molar Volume of the Glass Series of Composition xFe₂O₃-(45-x) PbO-55P₂O₅ (with Avec (0 ≤ x ≤ 20; mol%)

The density measurements made it possible to follow the evolution of the molar

Figure 2. Chemical durability of the glass series 55P₂O₅-(45-x)PbO-xFe₂O₃ (with (0 ≤ x ≤ 20 mol%) as a function Fe₂O₃ (mol%).

Figure 3. pH curve of phosphate glasses leaching, after 24 days of attack, as a function of Fe₂O₃ content (mol%).

Table 2. Compositions and transition temperature (T_g) of the glass series xFe₂O₃-(45-x)PbO-55P₂O₅ system with (0 ≤ x ≤ 20 mol%)

Figure 4. Thermogram curves of the composition phosphate glass series xFe₂O₃-(45-x) PbO-55P₂O₅.

Figure 5. Variation of T_g versus Fe₂O₃ content for the glass series 55P₂O₅-(45-x)PbO-xFe₂O₃ (with (0 ≤ x ≤ 20 mol%).

volume according to the composition of the glass along the xFe₂O₃-(45-x) PbO-55P₂O₅ system, which was measured at room temperature. As can be deduced from Table 3 and from the curve in Figure 6, we notice an increase in the density of the glass from 3.86 to 4.35 (g/cm³) when the Fe₂O₃ content varies, respectively, from 0 to 20 mol%. This increase leads to reinforce the vitreous network bonds by replacing the hydrated P-O-P bonds by covalent and rigid Fe-O-P bonds. The glass density is given by the follow equation:

$\rho =\frac{m_{\text{glass}}}{\left[m_{\text{glass}}+\left(m_{\text{ortho}}-m_{\text{ortho}+\text{glass}}\right)\right]}{\rho }_{\text{ortho}}$

Table 3. Compositions, density and related molar data of the glass series xFe₂O₃-(45-x)PbO-55P₂O₅.

Figure 6. Variation of density versus Fe₂O₃ content for the glass series xFe₂O₃-(45-x)PbO-55P₂O₅ (with 0 ≤ x ≤ 20; mol%).

with:

ρ = Density

m_glass = Weight of glass measured in air

m_ortho = Weight of diethyl orthophthalate only

m_ortho+glass = Weight of glass immersed in diethyl orthophthalate

ρ_ortho = 1.11422 g/cm³

The molar volume (V_OM) and the oxygen anionic radius r_cal (O²⁻) in the glass were determined from Equations (1) and (2), respectively:

$V_{OM}=M/\rho N_A\cdot N_0$ (1)

$r_{cal}=\sqrt[3]{V_{OM}}/2$ (2)

with M = molar mass.

ρ = density.

N_A = Avogadro’s number.

N₀ = number of oxygen contained in the molar formula.

Data analysis in Table 4 showed that, both, the molar volume and the anionic radius value of relative oxygen r_cal (O²⁻), calculated by Equation (2), decreased when the Fe₂O₃ content increased at the expense of PbO. [13] [16]. It should also be noted that the anionic radius of oxygen r_cal (O²⁻) decreased when the composition of the glass tended to pass from ultra and meta-phosphate domains (S₀, S₅) (O/P = 2.9, 3) to pyrophosphate domain (S₂₀) (O/P = 3.27), such a decrease in r_cal (O²⁻) highlights a strengthening of the Fe-O-P bond in the vitreous network which is interpreted by a shortening of the interatomic distances due to the purely covalent character coordination bonds [14] [17] [18].

3.5. Infrared Spectroscopy (IR) of the Glass Series xFe₂O₃-(45-x)PbO-55P₂O₅ with (0 ≤ x ≤ 20; mol%)

The infrared spectra of the glass series xFe₂O₃-(45-x) PbO-55P₂O₅ are shown in Figure 7. All phosphate vibration bands of the treated samples were situated in the frequency domain between 1600 - 399 cm⁻¹ [17] [18]. The band between 466 - 500 cm⁻¹ was attributed to the δ (P-O-P) skeleton vibration mode [17] [18]. We also noted that the 1210 - 1228 cm⁻¹ band attributed to the ν_as (PO₂) vibration mode of the meta-phosphate groups (Q²) becomes a simple shoulder when the molar content of Fe₂O₃ reached 20 mol%, while the band located between 1000 - 1060 cm⁻¹ moved towards high frequencies giving a wide band attributed to the vibration mode ν_as (PO₃) of pyrophosphate groups (Q¹) [19] [20]. The band observed at 1000 cm⁻¹, when x = 0, can be attributed to the isolated ortho-phosphate group ${\text{PO}}_{\text{4}}^{\text{3}-}$ (Q⁰) [19]. The band located at 755 - 766 cm⁻¹, attributed to the ν_s (P-O-P) vibration mode, moved towards high frequencies when the Fe₂O₃ oxide content increased and indicated the formation of non-bridge oxygens of the di-phosphate groups (Q¹) to the detriment of the meta-phosphate groups (Q²).

3.6. X-ray Diffraction of the Glasses Composition xFe₂O₃-(45-x)PbO-55P₂O₅ with (0 ≤ x ≤ 20; mol%)

To study the amorphous state of the prepared glasses, X-ray diffraction analyses were carried out on all the prepared samples. The results obtained in Figure 8,

Table 4. Variation of the ratio [FeII]/[FeII + FeIII] versus mol% Fe₂O₃ along the glass series xFe₂O₃-(45-x)PbO-55P₂O₅.

Figure 7. IR spectra of phosphate glass series xFe₂O₃-(45-x)PbO-55P₂O₅, with (0 ≤ x ≤ 20; mol%).

Figure 8. X-ray diffraction of the S₀, S₁₀ et S₂₀ samples phosphate glasses.

confirmed the amorphous character which reflects a disordered structure. As expected, the analysis by X-ray diffraction of annealed glasses S₀, S₁₀ and S₂₀ respectively, at 580˚C and 620˚C for 72 hours, are presented in Figure 9. The analysis

Figure 9. X-ray diffraction of S₀, S₁₀ et S₂₀ samples anneled glasses, respectivement at 580˚C and 620˚C pendant 72 h.

of these spectra indicated the appearance of peaks mixture linked to meta-phosphate, ortho-phosphate and pyrophosphate phases. On the other hand, it indicated a radical change in the structure, when the iron oxide content increased in the vitreous network, showing that the studied glasses structure evolved from the meta-phosphate (presumably cyclic) and the isolated ortho-phosphate phases to majorities pyrophosphate phases while we moved from S₀, S₁₀ to S₂₀. The x-ray diffraction spectra obtained from the S₁₀ sample tended to crystallize as a mixture of meta-phosphate, ortho-phosphates and pyrophosphates with majorities of meta-phosphate and/or rings meta-phosphate phases. However, in the S₂₀ sample, we noticed that the meta-phosphate phases disappeared and the main phases called pyrophosphates such as PbFe₂(P₂O₇)₂ and Pb₂P₂O₇ were mostly intense in the sample [21] [22].

3.7. Scanning Electron Microscopy (SEM) Analysis of Glass Series xFe₂O₃-(45-x)PbO-55P₂O₅ with (0 ≤ x ≤ 20; mol%)

The scanning electron microscopy (SEM) micrographs presented in Figure 10 illustrated the morphology of the studied glasses. The micrographs of Figures 10(a)-(c) showed the existence of two phases, a vitreous phase and a crystalline phase. It also indicated the formation of agglomerates in the crystalline phase of different sizes, ranging from a few microns to several tens of microns, as shown by the different images attributed respectively to samples S₀, S₁₀ and S₂₀ [7] [17] [19] [21]. A deep of SEM micrograph analysis of the glasses considered in this work indicated the glass form of S₀ shown in Figure 10(a), exhibited the presence of an overload crystalline phases with different form and size [12] [23] [24]

Figure 10. Scanning electronic micrograph of S₀, S₁₀ and S₂₀ samples phosphate glasses.

[25]. When the Fe₂O₃ content increased in the glass, the number of crystallites decreased as seen in the S₁₀ (Figure 10(b)) and S₂₀ (Figure 10(c)) samples, respectively. Hence, SEM analysis confirmed a relatively large homogenous vitreous phase with the coexistence of crystalline particles in the S₂₀ sample which has the maximum Fe₂O₃ content. Some different crystalline phases were identified by XRD and it seems that a decrease of crystallization tendency was enhanced and PbFe₂(P₂O₇)₂, Pb₂P₂O₇, phases were crystallized in the last sample (S20) with some traces of Pb₃Fe(PO₄)₃ phases. This explains that the structure changes towards more short pyrophosphates at the detriment of metaphosphates and shorter isolated orthophosphate chains as the Fe₂O₃ content increases in the glass network [17] [19] [21] [26] [27].

3.8. Iron(II)/Iron(III) Redox Phenomenon

Like all transition elements, iron was reputed to have at least two oxidation degrees under normal atmospheric conditions. Therefore, as expected the starting iron(III) oxide has been reduced during the elaboration of the title phosphate glasses. Indeed the batch melting is realized in a rather reduced atmosphere which is created by the decomposition of the starting compound (NH₄)₂HPO₄ which gives rise at high temperatures, to a gas mixture of the type (NH₃₍g) +H₂ + H₂O(g)) [16] [17] [22] [28]. Analysis of the results (Figure 11) shows that all the glasses studied underwent a reduction at the time of melting, from the Fe(III) state to the Fe(II) state. The study of this reduction shows that it decreases when the Fe₂O₃ content exceeds 10 mol%. This reduction does not exceed, in general under normal atmospheric conditions, 10%. However, in the case of iron-lead-phosphate glasses, the reduced Fe(III) rate reaches higher values for low iron oxide content, this can be explained by the oxidation in atmospheric air of a part of PbO into PbO₂ leading to a greater reduction of Fe(III) to Fe(II) [7] [14] [17].

Figure 11. Variation of the [Fe²⁺]/[Fe²⁺ + Fe³⁺] ratio versus mol% Fe₂O₃ along the glass series xFe₂O₃-(45-x)PbO-55P₂O₅.

4. Discussions

The glass series xFe₂O₃-(45-x)PbO-55P₂O₅ with (0 ≤ x ≤ 20; mol%) were prepared by direct melting at (1050 ± 10)˚C. The structure and the chemical durability of these glasses have been investigated using various techniques such as dissolution rate (D_R), density, IR spectroscopy, X-ray diffraction, SEM and iron redox phenomena. The dissolution rate for all the glasses studied indicates an improvement in chemical durability when the Fe₂O₃ content increases to the detriment of PbO. The variation of transition temperature versus Fe₂O₃ content indicates an increase of T_g from 376˚C to 501˚C when the Fe₂O₃ content increases from 0 to 20 mol%, elucidating an improvement in the rigidity of the glass [19] [20] [29]. The increase both, of the density and the glass transition temperature reflects the strengthening of the Fe-O-P bonds, the electron density becomes very important and leads to a reinforcement of the bonds between the irons and non-bridge oxygen atoms and thus to a reinforcement of the interconnection of the vitreous network chains [11] [28] [30]. The coordination bonds, translated by the hybridization phenomenon, and to the distorted sites occupied by the iron, which become smaller in the phosphate glass compared to the crystalline sites, leads to a reinforcement of the interconnection of the vitreous network chains [28]. The reduction of Fe(III) to Fe(II) in iron-lead-phosphate glasses, is principally due to the presence of ammonium phosphate (NH₄)₂HPO₄ in a batch composition as a source of P₂O₅. As a matter of fact, reducing conditions were created during the melting process which favours the reduction of Fe(III) into Fe(II) [1] [17] [28] [29]. Vibrational spectroscopy has allowed us to follow the evolution of the glass structure of iron-lead phosphate glasses. The analyses of the IR spectra revealed that the substitution of Fe₂O₃ at the expense of PbO caused a radical change in phosphate matrix structure. The band observed at 1000 cm⁻¹, when x = 0, can be attributed to the isolated orthophosphate group ${\text{PO}}_{\text{4}}^{\text{3}-}$ (Q⁰) [21]. When the Fe₂O₃ oxide content increases above 10 mol%, the bands located at 755 cm⁻¹ and 1000 cm⁻¹ attributed, respectively to the ν_s (P-O-P) and ν_as (P-O-P) vibration modes, moves towards high frequencies and indicates the formation of non-bridge oxygen’s of pyrophosphate groups (Q¹) to the detriment of the meta-phosphate (Q²) and orthophosphate (Q⁰) groups. The X-ray diffraction spectra confirmed this results, hence, it can be explained that the increase of Fe₂O₃ to the detriment of PbO has the effect of phenomenon seems to play an important role, both, in the viscosity, in the equilibrium between the vitreous bath, in depolymerizing the structure from metaphosphate and/or cyclic metaphosphate and polymerizing isolated orthophosphate chains towards the formation of pyrophosphate chains. This behavior leads to the replacement of the hydrated P-O-P bands and possibly Pb-O-P, by the strongly covalent Fe-O-P bonds. The glass structure can be considered as pyrophosphate units connected with Fe(II) and Fe(III) in octahedral or distorted octahedral coordination [14] [15] [17] [19] [28] [30]. The presence, probably, of Pb(II) and Pb(IV) ions (melting point of PbO₂ is lower than that of PbO) leads to the formation of Pb(IV)-O-P band which is more rigid and covalent than the Pb(II)-O-P band. It contributes to the increase in density and eventually to the formation of pyrophosphates units. Analysis by scanning electron microscope confirmed that the increase in the iron oxide content led to a decrease of crystallites and a polymerization of the structure from the shorter isolated orthophosphate groups towards the majority short pyrophosphate groups which ensures an adequate balance between the crystallites formed and the vitreous bath leading to better chemical durability.

5. Conclusion

The study of glass series xFe₂O₃-(45-x) PbO-55P₂O₅, (with 0 ≤ x ≤ 20; mol%), indicated a considerable chemical durability. The results obtained confirmed the creation of P-O-M bonds (M = Pb, Fe) with a strongly covalent nature to the detriment of the hydrated P-O-P bonds and led to the formation of majorities pyrophosphate groups. The SEM Micrograph indicated an obvious decrease in crystallites with the increase in Fe₂O₃ content, causing a large equilibrium between the glass bath and the crystallites, hence leading to better chemical durability. The presence, both, of lead in the ion form Pb(II) and Pb(IV) seems to play an important role as well for the viscosity that for the equilibrium between the vitreous bath and the crystalline phases. The dissolution rate (D_R) of the analyzed compounds is 100 times less than to the values of borosilicate glasses and 200 times less than BaBal glass which are used as alternative materials for the immobilisation of nuclear waste substances.

Acknowledgements

The authors wish to thank National Center for Scientific and Technical Research [Division of Technical Support Unit for Scientific Research (TSUSR) Rabat, Morocco] for their assistance to the realization of this work

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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