SORPTION-SPECTROSCOPIC DETERMINATION OF CADMIUM (II) IONS IN SOIL WITH AN IMMOBILIZED REAGENTS

ABSTRACT

In the research work, a sorption-spectroscopic method for determining cadmium (II) ion in environmental samples was proposed, and the organic reagent cadion IREA was selected as the reagent. Silk fibroin natural fiber was selected as the carrier. The optimal environment, buffer, and the required time for complex formation were found. The soils of the industrial zone in the Kashkadarya region were monitored, and the dynamic change in the amount of cadmium in the areas around the plant was determined. The amount of cadmium in the soil was analyzed and determined by the developed sorption-spectroscopic method. Conclusions were made on the amount of cadmium (II) in the soils in the plant area and its surroundings.

АННОТАЦИЯ

В научно-исследовательской работе предложен сорбционно-спектроскопический метод определения иона кадмия (II) в объектах окружающей среды, в качестве которого выбран органический реагент кадион ИРЕА. В качестве носителя выбрано натуральное волокно фиброин шелка. Найдены оптимальная среда, буфер и необходимое время для комплексообразования. Проведен мониторинг почв промышленной зоны Кашкадарьинской области, определено динамическое изменение количества кадмия на территориях вокруг предприятия. Разработанным сорбционно-спектроскопическим методом проведен анализ и определено количество кадмия в почве. Сделаны выводы о количестве кадмия (II) в почвах на территории предприятия и его окрестностях.

Keywords: cadmium (II), reagent, cadmium IREA, silk fibroin, soil, monitoring, spectroscopic.

Ключевые слова: кадмий (II), реагент, кадмий-ИРЕА, фиброин шелка, почва, мониторинг, спектроскопический.

Introduction

Heavy metals are released into the environment due to natural and anthropogenic factors and accumulate in soil and groundwater. Although heavy metal pollution is localized, it can occur anywhere due to the release of these metals into groundwater and the movement of water [1-3]. Cadmium (Cd), a white metal widely distributed in the earth's crust, is usually found in zinc deposits [4]. Cadmium is classified as a Class 1 highly toxic substance. Cd is eventually absorbed by crops and gradually accumulated in livestock and poultry in the livestock industry, posing a threat to human health and aggravating chronic diseases. [5] Cadmium is a highly toxic heavy metal. Depending on its chemical composition, cadmium accumulates in the liver, kidneys and other organs or tissues. Cadmium (Cd) accumulates in the human body through two main routes: inhalation and ingestion. [6;7]. Cd causes liver and intestinal damage through multiple mechanisms such as oxidative stress, autophagy, apoptosis, DNA damage, and metabolic disorders [8;9].

Heavy metals not only affect living organisms, but also have a significant impact on plants. Due to the introduction of heavy metals into the environment, human activities pose a greater threat to the ecosystem than natural phenomena. Cadmium concentrations above 0.3 mg/kg in soil significantly affect plant growth and development [10]. Cadmium (Cd) inhibits root and shoot development, as well as photosynthetic activity, stomatal permeability, and total biomass of plants [11]. When the effect of Cd on the photosynthesis process of wheat plants was studied, it was found that molybdenum (Mo), an essential nutrient for plants, was significantly reduced [12].

Studies have been conducted to determine lead, cadmium, and mercury in food products. Up to 96% of the milk samples analyzed were contaminated with at least one of the three metals. Concentrations for lead, cadmium, and mercury ranged from 0.003 to 0.045 mg/kg, 0.0007 to 0.38 mg/kg, and 0.002 to 0.32 mg/kg, respectively [13-14].

Small amounts of metals remain hazardous contaminants that are receiving significant attention. Their detection can help prevent potential hazards[15].

Materials and methods

In the study, the cadmium IREA reagent was proposed for the determination of cadmium (II) ions. The selected reagent is highly selective for cadmium (II) ions. To increase the sensitivity of the method, the selected reagent was used immobilized on a carrier. Silk fibroin fiber was selected as the carrier. Initially, the cadmium IREA reagent was immobilized on the silk fibroin fiber (SF).  To carry out the immobilization process, a 10-3 M solution of the cadmium IREA reagent was prepared. For this, 0.55208 g of the reagent was weighed on an analytical balance (AND HR-250AZG DAVST 24104-88YE (Japan)) and placed in a 100 ml volumetric flask, and double-distilled water was added to the flask line. 0.2 g of the silk fibroin fiber was taken and immersed in the cadmium IREA solution until it was completely submerged. The reflectance spectra of the immobilized reagent and its complex with Cd (II) ions were measured on a Spectrophotometer X-Rite Eye one pro (380-730 nm) and were found to be λ_R=410 nm, λ_com=490 nm. 15 minutes were enough for the reagent to immobilize on the fiber. It was at the 15^th minute that the intensity of the analytical signal reached a high value.

Table 1.

Time dependence of immobilization

Selection of optimal environment and buffer solution for complex formation: The solution environment is one of the main factors in complex formation. To determine the pH environment in which Cd (II) ions react completely with the immobilized cadion IREA reagent, solutions with different pH values ​​were used and analytical signals corresponding to the complex were obtained.

Table 2.

Dependence of the optical density of the formed complex on the solution environment

The results show that at pH 9.5 the analytical signal corresponding to the complex had a maximum value, and various buffer solutions were used to create this environment.

Table 3.

Choice of the optimal buffer for the complexation process of Cd (II) ion with cadion IREA

To provide the necessary environment, a solution of borate buffer solution with a pH of 9.5 was selected and in subsequent works, the pH value of borate buffer solution with a pH of 9.5 was used. The pH values ​​of all solutions were measured using a pH meter “Mettler-Toledo AG” (Switzerland).

Results and discussions

 To determine whether Cd²⁺ ions formed a complex with immobilized cadmium ion IREA, an analysis was performed in the X-ray fluorescence spectrum. It is not possible to determine the areas related to the reagent in the X-ray fluorescence spectrum analysis. Since this analysis method can mainly analyze inorganic substances. The complex formed by methyl thymol blue immobilized on silk fibroin fiber with its cadmium (II) ion was analyzed in an X-ray fluorescence spectrometer at 9825 Spectrum Drive, Austin, TX-78717 (USA).

Table 4.

X-ray fluorescence results before and after immobilization (SF+cadion IREA+Cd²⁺)

In the spectrum of the IF + cadion IREA sample (uncomplexed state), no peak of the Cd element was detected, but when analyzing the IF + cadion IREA +Cd²⁺ complex sample, peaks characteristic of Cd²⁺ ions appeared, their concentration was C% = 0.272, which indicates the binding of cadmium ions on the surface of the sorbent. At the same time, the decrease in Cl and S elements indicates the binding of the Cd ion and the formation of a new phase. These changes are physicochemical evidence of the transition to the complexed state.

Application of the method: To determine the amount of cadmium (II) in the soil, the composition of the soil belonging to the settlement around the industrial area was studied. Methodology: a soil sample was taken from a depth of 20 cm above the ground, 1.0 g of soil was dissolved in water, nitric acid (1:1) was added under the thimble to dissolve additional salts insoluble in water, and poured into a 100 ml flask, and brought to the mark with distilled water. An aliquot part was taken from the analyzed mixture in 50 ml beakers, then 5 μg of cadmium solution, 5 ml of covering agent and 10 ml of cadion IREA reagent, sorbent and 5 ml of borate buffer solution were added and sorbed for 10 min. The determination was carried out by the ‘insert-find’ method. The results are presented in the table below.

Table 5.

Determination of cadmium from soil composition

The results showed that the amount of cadmium in the soil of the residential area did not exceed the permissible limit. The standard deviation did not exceed 0.013, and the relative standard deviation did not exceed 0.005, which confirms the accuracy of the method.

Conclusion: in the proposed method, the cadmium (II) ion reagent IREA was recommended for the determination of cadmium (II) ions, and the parameters of the complex formation of the reagent and the metal were studied and found. The effectiveness of the developed method was analyzed. In addition, the short immobilization time and the time spent on complex formation ensure the speed of the method. The fact that the selected fiber is natural and does not cause any harm to the environment during its synthesis indicates that the method supports the principles of green chemistry. The amount of cadmium (II) ions in environmental objects was determined and analyzed using the developed sorption-spectroscopic method. Soil samples were taken from residential areas around industrial zones and analyzed using the developed method. The selectivity of the method and the simplicity of the implementation process can expand the scope of applications.

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