IMMOBILIZATION MECHANISM AND SPECTROSCOPIC ANALYSIS OF THE ORGANIC REAGENT NITROSO-R SALT ON A POLYMER SUPPORT

ABSTRACT

This article analyzes the optimal conditions for the immobilization of Nitroso-R salt onto PPA synthetic fiber, the chemical bonding mechanism, IR analysis, and its reflectance and absorbance spectroscopic properties. It was determined that 0.2 g of the PPA fiber could immobilize 275.73 mg of the reagent within 30 minutes under optimal conditions: a pH range of 4.09-5.10 and a temperature of 25-30 °C. By immobilizing Nitroso-R salt onto the PPA fiber, a chemical sensor was developed for the detection of metal ions in wastewater. This chemical sensor has been created for the first time.

АННОТАЦИЯ

В данной статье анализируются оптимальные условия иммобилизации нитрозо-R-соли на синтетическое волокно ППА, механизм химического связывания, данные ИК-анализа, а также его спектроскопические характеристики отражения и поглощения. Установлено, что 0,2 г волокна ППА иммобилизует 275,73 мг реагента в течение 30 минут в оптимальных условиях: диапазон рН 4,09–5,10 и температура 25–30 °C. Путем иммобилизации нитрозо-R-соли на волокно ППА разработан химический сенсор для определения ионов металлов в сточных водах. Данный химический сенсор создан впервые.

Keywords: Nitroso-R salt, IR analysis, reflectance spectroscopy, absorbance spectroscopy, PPA-1 fiber, wastewater.

Ключевые слова: нитрозо-R-соль, ИК-анализ, спектроскопия отражения, спектроскопия поглощения, волокно ППА-1, сточные воды.

Introduction

Currently, test systems—simple and rapid methods for the qualitative and quantitative analysis of substances without using complex equipment or requiring labor-intensive sample preparation—are continuously improving. Typically, test systems are either solutions of organic reagents (OR) or hybrid systems formed by these reagents immobilized onto solid supports [1]. There is a need today to develop hybrid methods for detecting ecotoxicants by immobilizing complex-forming organic reagents onto various polymer carriers that possess high operational and metrological characteristics. Detecting trace metal ions in solutions within various samples often requires pre-concentration. In this context, adsorption pre-concentration is highly effective because, unlike solvent extraction, it allows for the achievement of lower detection limits without using toxic organic compounds. Among adsorbents used for metal pre-concentration, inorganic adsorbents, particularly those based on silica, have been widely employed. Characteristic features of inorganic adsorbents include the rapid establishment of adsorption equilibrium and the frequent ease with which the adsorbed metal can be eluted [2]. In chemical analysis, alongside analytical reagents immobilized on silica, synthetic polymers featuring covalently bonded functional groups of various natures are also increasingly being used [3].

Some studies [4] have utilized various sorbents modified with complex-forming organic reagents, where immobilization on the sorbent surface occurs through adsorption, electrostatic interaction, ionic bonding, dipole-dipole interactions, hydrogen bond formation, and other types of interactions.

Research involving complexes formed between immobilized organic reagents and metal ions on such sorbent surfaces yields effective results, potentially due to the nature of binding. However, a drawback exists in such processes, as a portion of the immobilized organic reagent can leach into the solution upon interaction.

Organic polymer sorbents are distinguished from most inorganic sorbents by their high sorption capacity; however, the arrangement of functional groups within their structure often leads to relatively slow diffusion rates of metal ions on the sorbent surface, requiring a certain amount of time to establish sorption equilibrium. This, in turn, prolongs the analysis time and can significantly affect metal sorption dynamics.

Sorbents with strong sorption properties are formed by modifying the external surface of organic polymer sorbents with monomers containing various functional groups, positioning these functional groups on the surface of the solid matrix. Representatives of this class include highly cross-linked polystyrene, polyurethane foam, and polymer fiber materials modified with various functional groups. The location of functional groups on the polymer surface leads to high rates of the sorption process. Thus, when separating trace and micro amounts of metals from solution, the absolute sorption capacity value is less crucial, because when concentrating metals from large solution volumes using a small mass of sorbent, achieving high rates of sorption equilibrium establishment is very important. The nature of the functional groups in the sorbent significantly influences the pre-concentration efficiency, which becomes particularly evident when using complex-forming sorbents. The higher the stability constants of the metal complexes bound to functional groups on the sorbent surface, the higher the acidity of the solutions in which quantitative separation of metal ions can be achieved.

The main advantage of complex-forming sorbents synthesized by covalently bonding organic reagents to polymer sorbents is their chemical stability and the possibility of designing them with maximum conformational mobility of the functional groups [5].

Various chelate-forming polymer sorbents, used for separating metal ions from samples under investigation, are typically high molecular weight compounds possessing various functional groups. During their synthesis, factors such as the reactivity of intermediate products, incomplete conversion of functional groups, and the nature of active groups present in the initial polymer or incorporated into the matrix can influence the final properties.

The sorption capacity value of fibrous sorbents depends on the number of functional groups they contain, the nature of the ion being adsorbed, and the sorption conditions, varying over a wide range. High sorption ability is particularly pronounced in sorbents possessing functional groups such as thioamide, hydroxamic acid, amidoxime, and the like.

Polymers, alongside known porous materials, can be used as robust supports for the immobilization of organic reagents (OR) [6-8]. The main requirements for polymer support materials include optical transparency, high sorption characteristics, simple synthesis, ease of use, inertness towards reagents, mechanical strength, stability in acidic and alkaline solutions, and enabling high sensitivity towards analytes in the final analytical system. Most known polymer sorbents meet only some of these requirements. Therefore, the search for and synthesis of new porous polymer supports for sorption-spectrophotometric analytical test systems holds promise for expanding the capabilities of analytical chemistry. Based on these desired properties, research is ongoing involving polyacrylonitrile-based fibers, synthesized by scientists at the Department of Polymer Chemistry, National University of Uzbekistan, which are treated with various modifying monomers to enhance their sorption properties for application as solid carriers in analytical test systems.

It is known from the literature that treating polyacrylonitrile (PAN) with polyfunctional amines allows to produce strong base anion exchangers. For example, chemical modification of Nitron fiber (a type of PAN fiber) has previously been carried out using hexamethylenediamine and ethylenediamine. Fibrous sorbents designated SMA-1 and SMA-5 were obtained, which possess high sorption capacity towards hexavalent chromium ions [9]. The sorbents had a cross-linked structure and contained strong basic groups, which allowed them to be used for purifying wastewater from electroplating workshops of hexavalent chromium ions. Moreover, the degree of purification of wastewater treated according to the proposed method in a continuous flow regime reached quantitative levels. The developed method allows for the rapid and effective purification of chromium-containing wastewater from electroplating and other industrial enterprises [10]. In this context, the use of polyethylene polyamine (PEPA) for PAN modification was of interest, as its structure contains polyethylene polyamine moieties which, upon interaction with PAN, can form both weakly basic and strongly basic functional groups.

The kinetics of the reaction of PAN modification with polyethylene polyamine were studied, and an equation for the rate of this process was derived. For modification with PEPA, hydroxylamine (HA) activated PAN fiber was used [11]. The ion exchange capacity of the HA-activated fiber towards HCl was 1 mg-eq/g. The PEPA modification reactions were carried out at 353-373 K for 1-5 hours; the maximum static exchange capacity (SEC) of the resulting PEPA-modified fiber towards HCl was 5.4 mg-eq/g [9]. The functional  groups on the organic polymer sorbent, activated with HCl acid, form a strong bond (likely an ionic interaction) with the  functional groups of the organic reagent (Nitroso-R salt). Such chemical sensors important for detecting metal ions in solution.

The aim of this work is to study the optimal conditions and mechanism for the immobilization of Nitroso-R salt, a reagent used for detecting metal ions, onto PPA-1 organic fiber, and to analyze the spectra obtained using IR and reflectance spectroscopy.

EXPERIMENTAL

Materials and methods

Standard Solutions of Analytical Reagents: Nitroso-R salt (systematic name: 3-hydroxy-4-nitroso-2,7-naphthalenedisulfonic acid disodium salt) was selected as the analytical reagent for immobilization. It was purchased from ‘Khimreaktivinvest’ LLC, Tashkent, Uzbekistan (CAS number: 525-05-3). A 1×10⁻⁴ M standard solution of Nitroso-R salt was prepared using distilled water. This standard solution was used in the immobilization processes. The molecular structure of Nitroso-R salt is shown in Figure 1.

Figure 1. Molecular structure of the analytical reagent, Nitroso-R salt

Buffer Solutions: Universal buffer solutions (prepared from 0.04 M H₃PO₄, 0.04 M H₃BO₃, and 0.04 M CH₃COOH stock solutions, initial pH ≈ 1.31, subsequently adjusted) were used to control the pH of the investigated systems.

Selection of the Polymer Matrix: In this study, the selected analytical reagent, Nitroso-R salt, was immobilized onto a polyethylene polyamine-activated polyacrylonitrile (PPA) matrix. It was synthesized by scientists at the Department of Polymer Chemistry, National University of Uzbekistan. The purpose of immobilization is to increase the analytical efficiency of Nitroso-R salt for the detection of Cu²⁺, Co²⁺, and Fe²⁺ metal ions in solution. PPA was prepared by the modification of polyacrylonitrile (PAN) with polyethylene polyamine (PEPA) and dichloroethane. The structure of the polymer fiber is shown in Figure 2. The PPA polymer is a suitable matrix for Nitroso-R salt analytical reagents.

Figure 2. Reaction procedure and molecular structure of the PPA polymer matrix

Methods and Equipment

The wavelength of light absorption, optical density, and optimal conditions for immobilization were studied using various methods. Specord 50 and UV-1900i spectrophotometers were used to accurately measure the absorption spectra of the selected compounds. An I-160 MI ionometer was used to measure the concentration of ions in the solution. Diffuse reflectance spectra were recorded in the 380-730 nm range using an Eye-One Pro (i1 Pro) mini-spectrophotometer – monitor calibrator (X-Rite, Switzerland). Additionally, a magnetic stirrer, shaker, and laboratory heater were also used. The results obtained from the PerkinElmer FT-IR/NIR Spectrum 3 infrared spectrometer were important for confirming the binding of functional groups to the PPA-1 fiber.

RESULTS AND DISCUSSION

Spectrophotometric Analysis

The spectrophotometric characteristics of Nitroso-R salt immobilized on a polyphosphazene (PPA) matrix were investigated. The efficiency of immobilization of the Nitroso-R salt onto the selected matrix was evaluated. Figure 1 shows the absorption spectra of the Nitroso-R salt reagent solution before (1) and after (2) immobilization on the PPA matrix. The obtained results confirmed that the optical density of the Nitroso-R salt reagent solution was around 1.626. After immobilization on the PPA matrix, the optical density sharply decreased to 0.019. The reason for this decrease in optical density is the effective immobilization of Nitroso-R salt on the PPA matrix. The immobilization efficiency of Nitroso-R salt on the PPA matrix was evaluated using the following equation [12-15]:

Where A₀ is the optical density of the Nitroso-R salt reagent solution before immobilization, and A is the optical density of the Nitroso-R salt reagent solution after immobilization [16-18]. The immobilization efficiency of Nitroso-R salt on the PPA matrix was determined to be 98.7%, which means that 98.7% of the Nitroso-R salt was immobilized on the matrix. Nitroso-R salt interacted with the polymer PPA matrix, likely by forming bonds between the amino groups of PPA and the oxygen atoms of the sulfonate group of the analytical reagent. The resulting immobilized reagent is thermodynamically stable under aggressive conditions.

Effect of pH on the immobilization of Nitroso-R salt onto PPA-1 fiber. It was determined that the immobilization of the Nitroso-R salt analytical reagent onto the PPA polymer matrix occurred maximally at pH = (4.09-5.10). The optimal pH medium is of great importance in the immobilization of organic reagents onto the fiber, as it converts the reagent and the fiber into their ionic forms, which in turn affects the immobilization process [19]. The concentration of hydrogen ions in the solution is one of the important factors. Most organic reagents and chelate-forming sorbents used for the separation and determination of metal ions in solution are weak acids. The results regarding the effect of pH on immobilization onto the fiber are presented in Table 1.

Table 1.

Optimal pH results for the immobilization of organic reagents on fibrous carriers [λ_max = 370 nm, l = 1.0 cm, t = 20 ± 5 °C]

As can be seen from the table, the effect of pH on immobilization is significantly important. We can conclude that in a weakly acidic medium, the Nitroso-R salt organic reagent was effectively bound to the PPA-1 fiber with 98.7% efficiency.

One of the modern analytical methods, IR spectrometry, was employed to confirm the binding of the immobilized organic reagent to the PPA-1 fiber. This is shown in Figure 3.

Figure 3. IR spectrum of PPA-1 fiber (1), IR spectrum of Nitroso-R salt immobilized on PPA-1 fiber (2)

From the infrared spectra, we can observe that chemical binding occurred between the immobilized Nitroso-R salt and the fiber.

In the IR spectra of the PPA-1 fiber, an absorption band characteristic of the nitrile (-C≡N) groups of polyacrylonitrile is visible in the 2243 cm^-1 region. Absorption bands in the 3213 cm^-1 region correspond to the stretching and deformation vibrations of -N-H groups. The broad absorption in the 3000-3400 cm^-1 region indicates the presence of hydrated water molecules in the polymer. Absorption bands at 1560 cm^-1 correspond to the deformation vibrations of NH groups, while the absorption band at 1662 cm^-1 is attributed to -C=N- bonds. In the fiber, the protonated amine group (R-NH²⁺- R') formed an absorption band in the 2926 cm^-1 region.

During the immobilization process of the Nitroso-R organic reagent onto the PPA-1 fiber, it was observed that the absorption band of the R-NH²⁺-R' group remained unchanged at 2926 cm^-1, while the absorption band associated with the -C=N-H group shifted from 1633 cm^-1 to 1643 cm^-1. Changes were also indicated in the absorption regions characteristic of the immobilized Nitroso-R organic reagent: the two sodium sulfonate (-SO₃Na) groups (in the 1278 - 1027 cm^-1 range), and the band associated with ionic interactions involving the -O-Na moiety near 640 cm^-1.

Based on this infrared analysis, it was further confirmed/demonstrated that the Nitroso-R salt organic reagent is chemically bound to the PPA-1 fiber.

Conclusion

The optimal conditions for immobilization onto PPA synthetic fiber were determined, and the chemical bonding mechanism, IR analysis, reflectance spectroscopy, and absorbance properties were analyzed. The binding of Nitroso-R salt immobilized onto PPA fiber in the pH range of 4.09-5.10 was analyzed using an infrared spectrometer; reflectance and absorbance spectroscopic data were obtained. It was demonstrated that at a temperature of 25-30 °C, 0.2 g of fiber immobilizes 275.73 mg of the reagent in 30 minutes. By immobilizing this Nitroso-R salt onto PPA fiber, a chemical sensor applicable for detecting metal ions in wastewater was developed for the first time.

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