MONITORING LEAD IONS IN ENVIRONMENTAL OBJECTS USING IMMOBILIZED REAGENTS

ABSTRACT

Due to the uncontrolled pollution of environmental objects (particularly aquatic systems) with industrial waste, monitoring the content of lead ions is relevant and necessary, since they accumulate in various human tissues and subsequently exert toxic effects. One of the developing scientific directions today is the detection and monitoring of heavy metals by sorption-spectroscopic methods based on polymer fiber carriers with immobilized organic reagents. In addition, the sorption-spectroscopic method is much cheaper from an economic point of view. The reflectance coefficient (∆R) at 445 and 580 nm of immobilized sulforhazine reagent and of the complex formed by immobilized sulforhazine with Pb (II) ions was taken as the analytical signal. The determination of lead (II) ions by the sorption-spectroscopic method, in terms of accuracy and relative standard deviation, is not inferior to other available alternative methods.

АННОТАЦИЯ

Из-за неконтролируемого загрязнения объектов окружающей среды (особенно водных систем) промышленными отходами мониторинг содержания ионов свинца является актуальным и необходимым, так как они накапливаются в различных тканях человеческого организма и впоследствии оказывают токсическое воздействие. Одним из развивающихся в настоящее время научных направлений является определение и мониторинг тяжёлых металлов сорбционно-спектроскопическим методом на основе полимерных волокнистых носителей с иммобилизованными органическими реагентами. Кроме того, сорбционно-спектроскопический метод является значительно более дешёвым с экономической точки зрения. Отражательный коэффициент (∆R) при 445 и 580 нм для иммобилизованного сульфорсазинового реагента и комплекса иммобилизованного сульфорсазина с ионом Pb (II) принимался в качестве аналитического сигнала. Определение ионов свинца (II) сорбционно-спектроскопическим методом по точности и относительному стандартному отклонению не уступает другим существующим альтернативным методам.

Keywords: analytical reagent, sulforazene, immobilization, sorption spectroscopy, monitoring, lead(II) ions, environmental objects, surface river waters.

Ключевые слова: аналитический реагент, сульфоразен, иммобилизация, сорбционная спектроскопия, мониторинг, ионы свинца (II), объекты окружающей среды, поверхностные речные воды.

Introduction. In recent years, one of the most important problems facing humanity has been environmental protection [1–2]. To monitor environmental objects, many optical and electrochemical methods have been proposed for their determination [3–6]. Among the environmental factors that determine the functioning of ecosystems, pollutants occupy a central place. One group of pollutants of particular concern is heavy metals (elements with an atomic mass greater than 50), largely due to their biological activity [7]. In addition, unlike organic compounds, metals do not undergo transformation and, once they enter the biological cycle, they leave it extremely slowly. As a result of these properties of metals and the uncontrolled pollution of the natural environment (particularly aquatic systems) with industrial waste, cases of mass human poisoning were already reported at the end of the 20th century. Therefore, almost all developed countries of the world have implemented targeted protective measures and established monitoring services to control the quality of environmental objects, especially natural surface waters [8].

Monitoring the content of lead ions, particularly in drinking water, is highly relevant and necessary because of their accumulation in various human tissues and the subsequent toxic effects [9]. Since the concentration of lead in natural environments is very low (ng/m³, μg/L, mg/kg), it is necessary to develop new and improve existing analytical methods with high sensitivity, accuracy, and wide dynamic ranges. One of the most rapidly developing approaches for the determination of metals is sorption methods, which combine preconcentration with subsequent determination of metals on a solid matrix [10]. Recent studies have demonstrated the prospects of immobilizing organic reagents to improve their chemical and analytical performance and to create sensors based on them [11].

At present, the anthropogenic input of lead into the environment significantly exceeds the natural contribution. A marked increase in the lead content of the environment, including surface waters, is due to its widespread use in industry. Global annual lead consumption is about 3 million tons [12]. Major sources of surface water contamination with lead compounds include coal combustion, the use of tetraethyl lead as an anti-knock agent in motor fuels, and wastewater discharges from ore-processing plants, metallurgical enterprises, chemical factories, and mines [13]. The concentration of dissolved lead in unpolluted freshwater ranges from tenths to several micrograms per liter [14-15]. These advantages of sorption–spectroscopic methods explain the growing interest in this field of research. The purpose of this work was to monitor lead ions in various water samples using rapid, selective, and highly sensitive sorption–spectroscopic techniques with immobilized sulforazene. The optimal conditions for the complexation reaction of lead with immobilized reagents were determined, and the dependence of the results on various factors (reagent concentration, immobilization time, pH of the medium, etc.) was established. At the same time, some physicochemical characteristics of the resulting complexes were evaluated. The absorption and reflection intensities were found to be independent of temperature variations over a wide range.

Reagents and equipment. Lead nitrate was recrystallized and dried at (104 ± 1) °C to constant mass. A 1.599 g portion of the dried salt was dissolved in a small volume of double-distilled water and quantitatively transferred into a 1000 cm³ volumetric flask. Then, 5 cm³ of nitric acid with a density of 1.40 g/cm³ was added to the flask, and the volume of the solution was adjusted to the mark with bidi stilled water. The solution was stored for no longer than one year. The lead concentration in the stock solution was 1 mg/ml. Standard solutions of the required concentrations were prepared by successive dilution of the stock solution 10, 100, and 1000 times. A standard solution of lead chloride (0.1 mg/ml) was prepared from a 1.2600 g sample of the salt in a 100 ml volumetric flask and diluted to the mark with distilled water. Solutions of magnesium, cadmium, calcium, iron, zinc, and manganese salts with concentrations of 1 mg/ml were prepared by dissolving the corresponding chemically pure chlorides in water. The titers of the solutions were determined complexometrically according to the method in. Buffer solutions with pH values from 1 to 10 were prepared from chemically pure salts and acids according to the method in the organic reagent sulforazene with a concentration of 0.01 M was prepared by dissolving 0.0572 g of the reagent in a 100 ml volumetric flask. Freshly distilled and purified solvents and bidi stilled water were used according to along with deionized water, which was previously tested for the absence of luminescence.

The reagents used were of analytical grade (chemically pure, Ch.D.A.). Solutions of metal salts (1.0 mg/ml) were prepared by dissolving the corresponding nitrates or chlorides according to the procedure.

As a solid phase, various fibrous materials of the polyacrylonitrile type containing different functional groups were tested. The sorbent was used in the form of disks with a diameter of 20 mm and a mass of 30–40 mg in the wet state. For this, the disks were kept in a 0.1 M solution of hydrochloric acid, washed with distilled water, and then stored in Petri dishes. Analytical balances of the types FA2204N and ACZET PVT LTD CY 224 C were used, as well as a pH meter pH/Mv/TEMP m FiveEasy F20 (Switzerland). A highly efficient energy-dispersive X-ray fluorescence spectrometer (Applied Rigaku Technologies, Inc.) was used to identify various elements.

To record diffuse reflectance spectra from solid surfaces and to study the dependence of the reflectance coefficient (R) and the function of the reflectance coefficient F(R) on various factors, a recording spectrophotocolorimeter Eye One Pro (Switzerland) and a dual-beam recording spectrophotometer UV–Vis SPECORD M-40 were employed. R =100^*A/A₀,

Method of determination. The study was carried out in both static and dynamic modes. In the static mode, 10.0 ml of the reagent solution was introduced into 50.0 ml flasks, a carrier disk was placed into the solution, and the mixture was stirred for 5–8 minutes. Holding the carrier with a glass rod, the solution was drained, the immobilized carrier was washed with distilled water, and then immersed in the solution under analysis. In the dynamic mode, the analyzed solution was passed through the immobilized disk at a flow rate of 10 ml/min.

The degree of retention of sulforsazene (R %) on the carrier was calculated using the formula: R =100^*A/A₀, where A is the optical density of the reagent after immobilization, A₀ is the optical density before immobilization.

Research methodology. The work was carried out in both static and dynamic modes. In the static mode, 10.0 ml of a 0.5% sulforazene solution (pH 8.20) was introduced into 50.0 ml flasks. The PPD sorbent carrier was placed in the flask, and a magnetic stirrer was inserted. The mixture was stirred for 5–8 minutes at a rotation speed of 50 rpm. Holding the carrier with a glass rod, the solution was drained, the immobilized sorbent was washed with distilled water, and then immersed in the analyzed solution containing lead ions. In the dynamic mode, the analyzed lead solution was passed through the immobilized carrier at a flow rate of 10 ml/min, after which the study was conducted.

Results and discussion Optimization of immobilization conditions was performed by determining the maximum analytical signal while varying the acidity, the concentration of sulforazene (R) in solution, and the reagent–carrier contact time. The change in the diffuse reflectance coefficient (∆R) of carriers with immobilized sulforazene in the presence of lead ions at 445 and 580 nm, respectively, was taken as the analytical signal (Table 1). The forms of lead ions in solution were identified using electronic absorption spectra.

Table 1.

Sorbent selection for immobilization

Polyacrylonitrile-type sorbents modified by various groups were considered as carriers, and the best sorbent was chosen with the maximum ∆F (R) value (Fig.1).

Figure 1. Reflectance coefficients of the sulforsazene reagent immobilized on various sorbents 1-5-PAN, modified with various groups, 6-10 – immobilized R. (C_R =0.2 M, λ_R =440 nm, pH=8.2, m_sorb =0.2000 g, V=10 ml, t=15 min)

Studies have shown that the best sorbent for the immobilization of sulfosarzene are polymer carriers based on polyacrylonitrile modified with hexamethylenediamine and phosphorous acid (PPD).

Table 2.

Spectrophotometric characteristics of the sulforsazen reagent immobilized on a polymer carrier PPD

Research was carried out on the composition of the buffer mixture, the nature and content of the organic reagent, and the contact time. As a result, it was found that the maximum analytical signal is observed at pH 8.0–9.0, with the optimal reagent volume being 2.0 ml at a flow rate of 20 ml/min. The calibration curve is linear in the range of 0.01 to 0.1 μg/ml.

A comparison of the analytical characteristics of the complexes shows that the detection limit of lead has decreased by an order of magnitude, a shift in the optimal pH value to a more acidic region is observed, and therefore the selectivity of the reaction improves, since the influence of interfering easily hydrolyzed ions is reduced.

Table 3.

Optimized conditions for the complexation of lead with sulfosarzene immobilized on a carrier

The maxima in the absorption spectra of lead complexes with an immobilized reagent on a fibrous sorbent correspond to their spectra in solution. The absorption maxima of the complexes formed in the polyacrylonitrile matrix and in solution practically coincide (Fig. 2 and 3). To select the optimal concentration of reagents during immobilization, the ‘load’ of the carrier was determined. The ‘load’ of a carrier should be understood as the amount of reagent that can be immobilized on a certain amount of carrier. The ‘load’ of the carrier was determined by the residual concentration of the reagents above the sediment using a spectrophotometric method.

Methodology for determining the optimal ‘load’ of carriers: a selected amount of buffer with the appropriate pH and 10.00-100.00 μg of reagent were added to 0.4 g of carrier. Then the total volume was brought to 5 ml with water, stirred for 5-15 minutes and centrifuged at a speed of 3000 ml/min. After that, an aliquot of the solution ‘above the sediment’ (1.0-2.0 ml) was taken and 4.0 ml of a buffer mixture with pH 2.0 was added for R (when immobilized on PPD). Then the optical density was measured at the maximum absorption of the solutions, in a cuvette with l=1 cm. The concentration was determined using a calibration curve and recalculated per gram of carrier. Analyzing the connections between the organic reagent and the carrier, it was established that the functional analytical groups of the reagents responsible for the complexation of lead do not participate in the formation of a covalent bond with the polymer carrier, they only form complexes with lead ions.

Summarizing the results obtained on the influence of various amounts of metal ions on the accuracy of lead determination and the possibility of masking, the maximum permissible amounts of foreign ions (selectivity factor) were found. The determination of lead is strongly interfered with by iron, aluminum, bismuth, zinc and other metals, the interfering influence of which is eliminated by introducing masking substances and varying the acidity of the medium. The results showed that by using immobilization and masking mixtures, the selectivity of the complexation reaction was significantly improved compared to reactions in solution. The data obtained were used to develop a method for the sorption determination of lead ions in natural waters. We analyzed samples of surface and river water, the results of which are presented in tables 4 and 5.

Table 4.

Results of sorption-spectroscopic determination of lead in river waters (P=0,95; n=5)

Table 5.

Results of sorption-spectroscopic determination of lead in surface waters (“Tashkent city”) (P=0.95; n=4)

Based on the analysis of river and surface waters, it can be concluded that lead is present in them in quantities not exceeding the MPC.

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