ИССЛЕДОВАНИЕ СНИЖЕНИЯ АДСОРБЦИОННОЙ ПОРИСТОСТИ АММИАЧНОЙ СЕЛИТРЫ ПРИ ДОБАВЛЕНИИ СЕРПЕНТИНИТА В СООТНОШЕНИИ 100:2.5

УДК 546

Abstract

In this study, the effect of serpentinite addition on the adsorptive porosity and textural properties of ammonium nitrate was investigated. Ammonium nitrate was modified with serpentinite at a mass ratio of 100:2.5, and the adsorption characteristics of the obtained samples were studied using benzene vapor adsorption on a McBain adsorption apparatus within a relative pressure range of 0.1-1.0. The adsorption isotherms and structural parameters demonstrated a significant influence of serpentinite on the porous structure of ammonium nitrate granules. The initial ammonium nitrate exhibited a monolayer adsorption capacity of 0.038 mol/kg, a specific surface area of 9.26 m²/g, a micropore volume of 0.006494 cm³/g, and a saturation volume of 0.0085977 cm³/g. After modification with serpentinite, these values decreased to 0.012 mol/kg, 2.97 m²/g, 0.0022872 cm³/g, and 0.0041659 cm³/g, respectively. The average pore radius increased from 1.86 to 2.80 nm, indicating structural densification and partial blocking of fine capillary pores. In both samples, the mesopore volume remained close to zero.

Аннотация

В данном исследовании было изучено влияние добавки серпентинита на адсорбционную пористость и текстурные свойства нитрата аммония. Нитрат аммония модифицировали серпентинитом в массовом соотношении 100:2,5, а адсорбционные характеристики полученных образцов изучали методом адсорбции паров бензола на адсорбционной установке Мак-Бена в диапазоне относительных давлений 0,1-1,0. Изотермы адсорбции и структурные параметры продемонстрировали значительное влияние серпентинита на пористую структуру гранул нитрата аммония. Исходный нитрат аммония имел ёмкость монослоя 0,038 моль/кг, удельную площадь поверхности 9,26 м²/г, объём микропор 0,006494 см³/г и объём насыщения 0,0085977 см³/г. После модификации серпентинитом эти значения снизились до 0,012 моль/кг, 2,97 м²/г, 0,0022872 см³/г и 0,0041659 см³/г соответственно. Средний радиус пор увеличился с 1,86 до 2,80 нм, что свидетельствует о структурном уплотнении и частичной блокировке мелких капиллярных пор. В обоих образцах объём мезопор оставался близким к нулю.

Ключевые слова: исследование, снижение, адсорбционная пористость, аммиачная селитра, добавление, серпентинит, соотношение.

Keywords: research, reduction, adsorption porosity, ammonium nitrate, addition, serpentinite, ratio.

Introduction

Ammonium nitrate is one of the most widely used nitrogen-containing mineral fertilizers in agriculture [1]. Due to its high nitrogen content, high water solubility, and rapid assimilation by plants, this compound plays an important role in increasing agricultural productivity [2]. However, the physicochemical properties of ammonium nitrate, particularly its high hygroscopicity and adsorptive porosity, create significant challenges during storage and transportation. A highly porous structure promotes intensive moisture absorption, reduces granule strength, increases caking tendency, and decreases the physical stability of the product. As a result, the quality and operational performance of ammonium nitrate deteriorate [3-4].

Currently, numerous scientific studies are focused on improving the physicomechanical properties of ammonium nitrate through the use of various mineral and organic additives [5]. In particular, modifiers based on natural silicate minerals attract considerable attention due to their environmental safety, economic efficiency, and high adsorption activity [6]. One such mineral is serpentinite, which is characterized by the presence of magnesium silicates, a developed dispersed structure, and surface-active properties. Serpentinite-based additives can modify the surface structure of fertilizer granules, reduce capillary porosity, and decrease moisture adsorption [7].

The incorporation of serpentinite into ammonium nitrate contributes to the reduction of adsorptive porosity by increasing the structural density of the granules [8]. Consequently, the hygroscopicity of the product decreases, mechanical strength improves, and storage stability is enhanced [9]. In addition, mineral additives may increase thermal stability and reduce the tendency of granules to degrade. Therefore, determining the optimal amount of serpentinite in ammonium nitrate is of significant scientific and practical importance [10].

In the present study, the reduction of the adsorptive porosity of ammonium nitrate modified with serpentinite at a ratio of 100:2.5 was investigated. The influence of the serpentinite additive on the physicochemical properties, porous structure, and adsorption characteristics of the granules was analyzed. The obtained results demonstrate that modification with serpentinite is a promising approach for improving the storage stability and quality characteristics of ammonium nitrate.

Materials and Methods

Commercial ammonium nitrate was used as the primary raw material in this study. Natural serpentinite was applied as a mineral modifying additive. Prior to use, the serpentinite was dried at 105 °C to constant weight, mechanically ground, and sieved in order to obtain a homogeneous dispersed fraction (0.1 mm sieve mesh). The modification of ammonium nitrate was carried out by introducing serpentinite in a mass ratio of 100:2.5 (ammonium nitrate-manufactured by Asl Kimyo, purity 99.9%). The prepared mixture was thoroughly homogenized under laboratory conditions to ensure uniform distribution of the mineral additive throughout the fertilizer granules.

The adsorption characteristics and adsorptive porosity of the obtained samples were investigated using benzene vapor adsorption. Adsorption measurements were carried out on a McBain (McBain–Bakr quartz spring adsorption apparatus, fused quartz spiral spring gravimetric adsorption system; measurement accuracy: ±0.1–0.01 mg with a sensitivity of 10⁻⁵–10⁻⁶ g; adsorption calibration performed using standard reference weights and quartz spring elongation calibration, temperature control accuracy typically ±0.1 K, vacuum operating range up to 10⁻⁵–10⁻⁶  Torr, used for gravimetric determination of gas and vapor adsorption isotherms) adsorption apparatus under equilibrium conditions. Before the adsorption experiments, the samples were preliminarily dried in order to remove physically adsorbed moisture and volatile impurities that could affect the adsorption results. The samples were then cooled in a desiccator and transferred to the adsorption system.

Benzene (C₆H₆, manufactured by Bizkim, purity 99.9%) was selected as the adsorbate due to its stable physicochemical properties and widespread application in pore structure analysis. The adsorption process was conducted over a relative pressure range (P/P0) from 0.1 to 1.0. Equilibrium adsorption values were recorded at each pressure interval after stabilization of the adsorption system. Based on the obtained adsorption isotherms, the adsorptive porosity and pore structure characteristics of the samples were evaluated.

The adsorption capacity of the modified ammonium nitrate samples was compared with that of the unmodified ammonium nitrate. The influence of serpentinite addition on the reduction of adsorptive porosity was determined from changes in adsorption volume and pore filling behavior. A decrease in benzene adsorption was considered as evidence of reduced porosity and lower accessibility of capillary pores within the granule structure.

The obtained experimental data were processed using standard physicochemical analysis methods. Adsorption isotherms were constructed, and the relationship between relative pressure and adsorption capacity was analyzed in order to determine the effect of serpentinite on the porous structure and adsorption properties of ammonium nitrate.

Results and Discussion

The adsorption properties and adsorptive porosity of ammonium nitrate modified with serpentinite were evaluated by benzene vapor adsorption measurements using the McBain adsorption apparatus. The adsorption behavior of the samples within the relative pressure range of 0.1–1.0 made it possible to assess the influence of the mineral additive on the porous structure of the fertilizer granules. The obtained benzene adsorption isotherms for the initial and modified ammonium nitrate samples are presented in Figure 1.

Figure 1. Benzene adsorption isotherm of ammonium nitrate modified with serpentinite at a ratio of 100:2.5

As shown in Figure 1, the benzene adsorption isotherm of ammonium nitrate modified with serpentinite is characterized by relatively low adsorption values in the initial region of relative pressure. In the range of low relative pressures, the adsorption values remain almost constant at approximately 0.01869, indicating limited accessibility of active adsorption centers and partial reduction of the capillary-porous structure of the granules. This behavior suggests that the introduction of serpentinite contributes to the densification of the ammonium nitrate structure and decreases the number of open adsorption pores.

With further increase in relative pressure, a noticeable rise in benzene adsorption is observed. At higher relative pressures, the adsorption value increases sharply from 0.02803 to 0.04672, which is associated with the filling of larger capillary pores and polymolecular adsorption processes. The observed increase in adsorption at elevated relative pressures indicates the presence of residual porous regions within the modified structure; however, the overall adsorption capacity remains limited compared to highly porous systems.

The shape of the adsorption isotherm demonstrates that serpentinite addition significantly affects the porous structure of ammonium nitrate granules. The reduction in adsorption intensity at low relative pressures confirms the decrease in adsorptive porosity and suggests improved structural compactness of the modified material. Such structural changes are expected to positively influence the hygroscopic stability and storage properties of ammonium nitrate.

Table 1.

Textural and Adsorption Characteristics of Serpentinite-Modified Ammonium Nitrate

The textural and adsorption characteristics presented in Table 1 demonstrate the influence of serpentinite addition on the porous structure of ammonium nitrate. The monolayer adsorption capacity (am) of 0.012 mol/kg indicates a relatively low amount of benzene required to form a monolayer on the surface of the modified material. This result suggests a decrease in the number of energetically active adsorption sites, which is associated with the partial blocking and densification of pores after modification with serpentinite.

The specific surface area of the sample was determined to be 2.97 m²/g, confirming the presence of a limited adsorption surface. Compared with highly porous adsorbent materials, this value is relatively low and indicates that the serpentinite additive reduces the accessible surface area of ammonium nitrate granules. Such a decrease in surface area is favorable for lowering moisture adsorption and improving storage stability.

The micropore volume (W0) was found to be 0.0022872 cm³/g, which indicates the presence of only a small amount of microporous structure in the modified sample. In addition, the mesopore volume (Wme) was practically absent, suggesting that the modification process significantly reduced the development of mesocapillary pores. The reduction of both micro- and mesoporous regions confirms the decrease in adsorptive porosity of ammonium nitrate after serpentinite incorporation.

The saturation volume (Vs) of 0.0041659 cm³/g reflects the total adsorption capacity of the pore structure at saturation conditions. The relatively low value further supports the conclusion that the modified material possesses a compact and less accessible porous network.

The average pore radius was determined to be 28.0 Å (2.80 nm), corresponding to the transition region between microporous and mesoporous structures. This indicates that the remaining pores in the modified ammonium nitrate are predominantly fine capillary pores with limited adsorption accessibility. The obtained results confirm that the addition of serpentinite contributes to structural densification and reduction of adsorptive porosity, which can positively affect the hygroscopic resistance and physicomechanical stability of ammonium nitrate during storage and transportation.

Table 2. Textural and Adsorption Characteristics of Initial Ammonium Nitrate

The textural and adsorption characteristics presented in Table 2 indicate that the initial ammonium nitrate possesses a more developed porous structure compared with the serpentinite-modified sample. The monolayer adsorption capacity (am) of 0.038 mol/kg is significantly higher than that of the modified ammonium nitrate, demonstrating the presence of a larger number of active adsorption centers on the granule surface. This result confirms the higher adsorption ability of the initial sample toward benzene vapor.

The specific surface area of the initial ammonium nitrate was determined to be 9.26 m²/g, which is considerably higher than the value obtained for the serpentinite-modified sample (2.97 m²/g). The larger surface area indicates a more accessible and highly developed porous structure, contributing to increased adsorption capacity and higher hygroscopicity.

The micropore volume (W0) of 0.006494 cm³/g also exceeds the corresponding value for the modified sample. This demonstrates that the unmodified ammonium nitrate contains a greater amount of microporous regions capable of adsorbing benzene molecules. Similarly, the saturation volume (Vs) of 0.0085977 cm³/g is noticeably higher, indicating a larger total pore volume and greater adsorption potential.

Although the mesopore volume (Wme) remained practically absent, the average pore radius of 18.6 Å (1.86 nm) confirms the predominance of fine microporous structures in the initial ammonium nitrate. Compared with the modified sample, the smaller pore radius together with the higher pore volume suggests a more developed capillary network and increased adsorptive accessibility.

Overall, the obtained results demonstrate that the incorporation of serpentinite significantly reduces the adsorption capacity, specific surface area, and micropore volume of ammonium nitrate. The modified sample exhibits a denser and less porous structure, confirming the effectiveness of serpentinite in decreasing the adsorptive porosity and improving the structural stability of ammonium nitrate granules.

Conclusion

In this study, the effect of serpentinite addition on the adsorptive porosity and textural characteristics of ammonium nitrate was investigated using benzene vapor adsorption. The obtained adsorption isotherms and calculated structural parameters demonstrated that modification with serpentinite at a ratio of 100:2.5 leads to a significant decrease in adsorption capacity and pore development of ammonium nitrate granules.

The modified sample exhibited lower monolayer adsorption capacity, specific surface area, micropore volume, and saturation volume compared with the initial ammonium nitrate. In particular, the specific surface area decreased from 9.26 to 2.97 m²/g, while the micropore volume decreased from 0.006494 to 0.0022872 cm³/g. These changes confirm the reduction of adsorptive porosity and indicate structural densification of the granules after serpentinite incorporation.

The adsorption analysis also showed that serpentinite partially blocks and stabilizes the capillary-porous structure of ammonium nitrate, resulting in reduced adsorption accessibility toward benzene vapor. The decrease in porous development is expected to lower hygroscopicity, improve physicomechanical stability, and enhance the storage properties of ammonium nitrate.

Therefore, serpentinite can be considered an effective mineral additive for reducing the adsorptive porosity and improving the structural stability of ammonium nitrate fertilizers.

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