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. 2026 Jul 14;98(7):e70486. doi: 10.1002/wer.70486

Ultrasensitive Fluorimetric Detection of Trace Se(IV) as 4,5‐Benzopiazselenol on Nonpolar Adsorbents

Vladimir Losev 1, Anastasya Orobyeva 1, Svetlana Didukh‐Shadrina 1, Sergey Metelitsa 1, Elena Borodina 1,✉, Vyacheslav Cherkashin 1, Urana Ondar 2
PMCID: PMC13367454  PMID: 42446934

ABSTRACT

A method for separating Se(IV) from Se(VI) has been proposed. The method is based on the formation of fluorescent 4,5‐benzopiazselenol (a complex of Se(IV) with 2,3‐diaminonaphthalene), which was quantitatively extracted in dynamic mode from aqueous solutions using glass preconcentrating cartridges filled with nonpolar adsorbents: SiO2‐C8, SiO2‐C16, SiO2‐C18, and SiO2‐HPh. The increase in the fluorescence intensity of the adsorbent in the preconcentrating cartridge with increasing concentration of 4,5‐benzopiazselenol on its surface was used to develop a method for the solid‐phase extraction (SPE)‐fluorimetric determination of Se(IV) with a detection limit of 2 ng per 0.025 g of SiO2‐С16 adsorbent. When using 10 mL of solution, the detection limit was 0.2 ng mL−1 (0.2 μg L−1), and when using 100 mL of solution, it was 0.02 ng mL−1 (0.02 μg L−1). The linearity of the calibration graph was maintained up to 2 μg per 0.025 g when using the SiO2‐C16 adsorbent. Se(VI) did not form a complex with diaminonaphthalene and was not extracted by nonpolar adsorbents. Se(VI) passing through the preconcentrating cartridge can be determined in the effluent by ICP‐MS.

Keywords: 4,5‐benzopiazselenol; fluorescent determination; ICP‐MS determination; nonpolar adsorbent; solid‐phase extraction; speciation of inorganic selenium

Summary

  • Selective Se(IV) preconcentration.

  • Fluorietric Se(IV) determination in the adsorbent phase.

  • Inorganic Se speciation.

A highly sensitive fluorimetric method for Se(IV) determination in water is proposed. It uses selective complexation with 2,3‑diaminonaphthalene and dynamic adsorption of 4,5‑benzopiazselenol onto non‑polar adsorbents (SiO2‑C8–C18, SiO2‑HPh), reaching a detection limit of 0.02 μg L−1 (SiO2‑C16). The method separates Se(VI), which can be determined by ICP‑MS.

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1. Introduction

Selenium is a bioactive element that can affect physiological processes in the human body even in small doses (Barchielli et al. 2022). It is a key component of antioxidant enzymes that protect cells from free‐radical damage. However, biochemical studies show that selenium may have a wide range of toxic effects at lower concentrations than previously thought (Jablonska and Vinceti 2015).

The primary source of selenium for humans is dietary intake, with approximately 10% derived from drinking water. Selenium enters natural waters as a result of leaching of soils and rocks. However, clay minerals and iron oxides present in the soil and water systems adsorb selenium, effectively reducing its concentration in natural water. The concentration of selenium in groundwater and surface water is highly variable, primarily due to regional differences and the type of underlying ores present in the area. Typically, selenium levels range from 0.06 μg L−1 to about 400 μg L−1 (Witczak et al. 2023). Selenium concentrations can indeed increase with changes in pH due to the formation of compounds with greater solubility in water.

Among the toxic elements regulated in drinking water, selenium occupies a special place. The maximum permissible concentration of selenium in Russia is: for centralized water supply, as well as for water packaged in bottles—10 μg L−1 (SanPin 2.1.4.1074–01 2002); and for therapeutic‐table mineral water and therapeutic mineral water—50 μgL−1 (GOST 13273‐88 1988). World Health Organization has set their guideline value for selenium at 40 μg L−1 (WHO 2003), and the US EPA upper limit is 50 μg L−1 (US EPA 2015).

Analysis of selenium levels in natural water bodies and regulatory limits for drinking water reveals that detection methods must be highly sensitive, with a minimum detection limit of 0.001 mg L−1 (1 μg L−1). Therefore, to measure total selenium concentration in both surface and groundwater, advanced techniques like electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry are predominantly used, whereas optical emission spectrometry is less commonly applied in these determinations (Ivanenko et al. 2012; Pettine et al. 2015).

In groundwater, selenium primarily occurs in its inorganic forms, particularly as selenate and selenite. However, in surface waters, where organic compounds are abundant, selenium can form various organoelement compounds such as selenomethionine, selenocysteine, and diacetophenonyl selenide (Pettine et al. 2015). Due to their high water solubility, bioavailability, and ability to form stable complexes with sulfur‐containing enzymes, Se(IV) compounds have a greater negative impact on living organisms than Se(VI) compounds (Shahid et al. 2018). Therefore, it is more important to separate and determine the chemical species of selenium in natural and drinking water than to determine its total content.

For the separation and determination of selenium species, both chromatographic and nonchromatographic techniques are employed. Chromatographic methods include capillary electrophoresis (Deng et al. 2007; Miranda et al. 2007), ion chromatography (Stroud et al. 2012; Chandrasekaran et al. 2009), and high‐performance liquid chromatography (Gosetti et al. 2007). Nonchromatographic approaches primarily involve solid‐phase extraction (SPE) and separation of selenium species, followed by their detection using various techniques such as atomic absorption, atomic emission, atomic fluorescence, mass spectrometry, photometry, and fluorimetric methods (Barua et al. 2020; Herrero Latorre et al. 2013; Dauchy et al. 1994; Devi et al. 2017; Santos et al. 2015).

In aqueous solutions, Se(VI) exists in the anionic form as selenate ions HSeO4 − (pKa1 = −0.3) and SeO4 2− (pKa2 = 1.7) over a wide pH range. In acidic media at pH < 2, Se(IV) exists in aqueous solutions in the form of undissociated selenious acid, and at pH > 3 in the form of selenite ions HSeO3 − (pKa1 = 2.7) and SeO3 2− (pKa2 = 6.7) (Casiot et al. 1998). Therefore, various anion exchangers are used to separate Se(VI) from Se(IV) in solutions with pH < 2: anion‐exchange membranes and resins (Lee et al. 2020; Depecker et al. 2009), magnetic adsorbents based on iron oxides (He et al. 2019), aluminum and silicon oxides (Nyaba et al. 2016; Mendil et al. 2017; Losev et al. 2022; Xiong et al. 2008), modified with polyamines (Losev et al. 2022), and cetyltrimethylammonium bromide (Xiong et al. 2008), carbon nanotubes with 3‐(2‐aminoethylamino)propyltrimethoxysilane (Peng et al. 2015). The narrow pH range in which the separation of Se(VI) as SeO4 2− ion by anion exchangers from the molecular form of Se(IV) is achieved does not always allow the effective separation of these selenium species (Losev et al. 2022).

For the selective extraction of Se(IV) from aqueous solutions, aluminum, cerium, and titanium oxides (Xu et al. 2018; Huang et al. 2008) are used, including those applied to nanographene or its oxide (Baranik, Gagor, et al. 2018; Baranik, Sitko, et al. 2018), and a cellulose mini‐membranes modified with TiO2 (Zawisza et al. 2020).

The use of complexing adsorbents is more effective for extracting Se(IV) and separating it from Se(VI) (Losev et al. 2022; Zhao et al. 2024; Mendil et al. 2017; Didukh‐Shadrina et al. 2023; Fındık et al. 2014; Depecker et al. 2009). These include sulfur‐based functionalized activated carbon (Zhao et al. 2024), multiwalled carbon nanotubes modified with bismuthiol II, silica gel modified with 2,6‐diamino‐4‐phenil‐1,3,5‐triazine (Mendil et al. 2017) or thiol groups (Didukh‐Shadrina et al. 2023), poly(1,8‐diaminonaphthalene) obtained by oxidizing the monomer with ammonium persulfate (Fındık et al. 2014), and Amberlite XAD‐4 modified with 2,3‐diaminonaphthalene (Depecker et al. 2009).

2,3‐Diaminonaphthalene (DAN) is a selective reagent used for determination Se(IV) (Parker and Harvey 1962; Lott et al. 1963). The interaction between Se(IV) and DAN results in the formation of a fluorescent compound—4,5‐benzopiazselenol (BPS). This compound can then be extracted from aqueous solutions using organic solvents. In terms of detection limits for Se(IV) at the level of tenths of a microgram per liter, extraction‐fluorimetric techniques are not inferior to the atomic absorption method with electrothermal atomization and the mass spectrometry method with inductively coupled plasma. These techniques have been successfully applied to determine the selenium content in natural waters, vitamins, food products, and biological fluids (Harrison et al. 1996; Rodriguez et al. 1994). SPE of the analytes allows one to avoid the use of organic solvents, to significantly lower the detection limits for selenium due to its preconcentration in the dynamic mode, and to simplify the procedure by enabling fluorimetric determination directly in the preconcentrating cartridge on the adsorbent surface. However, the number of methods for SPE‐fluorimetric determination of Se(IV) with 2,3‐diaminonaphthalene is rather limited. In work (Serra et al. 2010), 4,5‐benzopiazselenol was extracted onto a membrane disk modified with C18 alkane. After elution, Se(IV) in the form of 4,5‐benzopiazselenol was determined in the eluate using a fluorimetric method.

This work is devoted to the development of techniques for separating Se(IV) from Se(VI) as a complex with 2,3‐diaminonaphthalene using nonpolar adsorbents and its fluorimetric determination directly in the adsorbent phase. Se(VI) was determined in the effluent from the preconcentrating cartridge by ICP‐MS.

2. Materials and Methods

2.1. Reagents

Stock solutions of Se(IV) and Se(VI) with a concentration of 1 g L−1 were prepared by dissolving weighed portions of commercial reagents H2SeO3 and H2SeO4 (Vekton, Russia). Solutions with lower concentrations of selenium ions were prepared by diluting the stock solutions.

Calibration solutions for the determination of selenium by the fluorimetric method and ICP‐MS were prepared from a single‐element certified solution of selenium (IV) ions (GSO (State Standard Sample) No. 7340‐96) by diluting them with 2% HNO3.

Solutions of alkali, alkaline earth, nonferrous, and other metal ions for studying the selectivity of preconcentration and fluorimetric determination of selenium were prepared from their corresponding chloride or nitrate salts by dissolving them in deionized water. To study the effect of nitrite, nitrate, sulfite, sulfate, phosphate, and sulfide anions, their solutions were prepared by dissolving the corresponding sodium salts in deionized water. Solutions with lower concentrations of metal ions and anions were prepared by diluting the initial solutions.

Solutions of hydrochloric acid, nitric acid, and sodium hydroxide of special purity grade were used to adjust the pH.

2,3‐Diaminonaphthalene (DAN) (Sigma‐Aldrich, Germany) was used as a reagent. The stock 0.1% DAN solution was prepared by dissolving an exact weighed portion of the reagent in deionized water. Solutions with a lower reagent concentration were prepared by diluting the stock solution with deionized water.

A range of reversed‐phase adsorbents produced by BioKhimMak (Moscow, Russia) was used in the work. These adsorbents were silica‐based materials chemically modified with various alkyl group, specifically octyl (SiO2‐C8), hexadecyl (SiO2‐C16), octadecyl (SiO2‐C18), and hexadecylphenyl (SiO2‐HPh) groups. Silica Diasorb‐130 (fraction 40–160 μm, specific surface area of 250 m2 g−1, average pore diameter of ~13 nm) produced by Luminophor (Russia) was used as a matrix for the synthesis of adsorbents.

Natural waters from the Tyva Republic and bottled mineral waters were used as real samples containing Se(IV) and Se(VI) species.

2.2. Apparatus

The concentration of selenium and metal ions in solutions was determined by inductively coupled plasma mass spectrometry (ICP‐MS) using an XSeriesII instrument (Thermo Scientific, USA). The operating parameters of the instrument are given in Table S1.

The excitation and fluorescence spectra of solutions and adsorbents were recorded using a Cary Eclipse spectrofluorimeter (Varian, Australia). Standard quartz cuvettes (10 × 10 mm) were used to record absorption, excitation, and fluorescence spectra of solutions. The recording of excitation and fluorescence spectra of adsorbents was carried out directly in a glass preconcentrating cartridge in a special holder placed in the cuvette holder of the spectrofluorimeter (Figure 1a).

FIGURE 1.

FIGURE 1

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Glass preconcentrating cartridge (a) with adsorbent, fitted with feed and outlet silicon tubes (b), placed in a standard cuvette holder of a spectrofluorimeter (c) with fluorescing 4,5‐benzopiazselenol adsorbed onto nonpolar SiO2‐С16 adsorbent (d).

Thermal analysis was performed by the simultaneous STA 449 C Jupiter thermal analyzer (Netzsch, Germany) combined with an FT‐IR spectrometer Nicolet 380 (Thermo Scientific, USA) and with the TGA/FT‐IR interface. This setup enables simultaneous acquisition of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) data, and evolved gas composition data. The sample was placed into a platinum crucible and heated within the temperature range of 30°C–760°C at a rate of 20°C min−1, and air was blown at a rate of 50 mL min−1.

The pH of the solutions was controlled using a SevenEasy S20 pH‐Meter (Mettler‐Toledo, Switzerland) with an InLab Expert Pro combined electrode. The solutions were pumped through the preconcentrating cartridge using a multichannel peristaltic pump BT 100‐1F (LongerPump, China). The solutions were heated in a UTU‐2 thermostat (Horyzont, Poland). Deionized water (18.1 MΩ cm−1) was obtained using the Barnstead E‐pure D4642‐33 system (Barnstead International, USA).

2.3. Experimental Procedure

To determine the optimal conditions for the formation of 4,5‐benzopiazselenol, 1 μg of Se(IV) was introduced into a graduated 15‐mL test tube, hydrochloric acid was added to its concentration in the range of 0.01–1 M, DAN solution was added to its concentration in the range of 0.001%–0.1%, and water was added to a total volume of 10 mL. The tubes were closed with stoppers and placed in a thermostat heated to 50 or 95°C and maintained for 5–30 min.

The tubes were removed from the thermostat, cooled to room temperature, and the solution was transferred to a separating funnel, 10 mL of hexane was added and shaken for 10 min. After layering, the aqueous phase was removed, an aliquot of the organic phase was placed in a quartz cuvette, irradiated with light at a wavelength of 377 nm, and the fluorescence intensity was measured at 577 nm.

When studying the preconcentration of Se(IV) in the form of 4,5‐benzopiazselenol in dynamic mode, a glass preconcentrating cartridge (Figure 1a) with an internal diameter of 2.5 mm and a height of 20 mm, filled with 0.025 g of reversed‐phase adsorbent, was used. Solutions, prepared as described above, were passed through it at flow rates of 0.5, 1, and 2 mL min−1. The cartridge was fixed in a special holder (Figure 1c) placed in the cuvette compartment of the spectrofluorimeter. The cartridge was irradiated with light at a wavelength of 377 nm, and the fluorescence intensity of the adsorbent was measured directly in the cartridge at 577 nm (Figure 1d).

To develop a test method for determining the concentration of Se(IV) in the indicator tube format, the height of the fluorescent zone of the adsorbent in a preconcentrating cartridge was measured as a function of its surface content.

To a solution containing Se(IV) and Se(VI) in 0.05 M HCl, 1 mL of 0.1% 2,3‐diaminonaphthalene solution was added. The mixture was then heated at 50°C for 10 min and diluted to 10 mL with water. After cooling to room temperature, the solution was passed through a preconcentrating cartridge containing 0.025 g of SiO2‐C16 adsorbent at a flow rate of 0.5 mL min−1. The preconcentrating cartridge was placed in a special holder within the cuvette compartment of the spectrofluorimeter (Figure 1). The feed and outlet silicone tubes connected to the preconcentrating cartridge routed through the existing process holes in the top cover and the bottom of the cuvette compartment. The end of the feed tube was placed into a test tube containing the analyzed sample via a roller mechanism of a peristaltic pump, whereas the end of the outlet tube was placed into a graduated test tube to collect the effluent (Figure 2). The concentration of Se(VI) in the effluent from the preconcentrating cartridge was determined by the ICP‐MS.

FIGURE 2.

FIGURE 2

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Scheme for determining selenium species in water: Se(IV) by fluorescence (as 4,5‐benzopiazselenol) and Se(VI) by ICP‐MS method in the effluent from a preconcentrating cartridge (1—feed solution, 2—peristaltic pump, 3—preconcentrating cartridge).

3. Results and Discussion

3.1. Spectral‐Luminescent Characteristics of 4,5‐Benzopiazselenol in Organic Solvents

The effect of organic solvent nature on the spectroscopic‐luminescent characteristics of 4,5‐piazselenol in organic solutions has been studied.

Hexane is most commonly used for the extraction of 4,5‐benzopiazselenol. When extracted from aqueous solutions into hexane, 4,5‐benzopiazselenol exhibited yellow‐green fluorescence. The fluorescence spectrum was a broad structured band with pronounced maxima at 517 and 556 nm, and a weakly pronounced maximum at 535 nm (Figure 3). The excitation spectrum was a structured broad band with an intense maximum at 377 nm, as well as less intense maxima at 360, 366, and 300 nm (Figure 3). In heptane and hexane, the shapes of the excitation and fluorescence spectra, the positions of the maxima, and the fluorescence intensity of 4,5‐benzopiazselenol were identical (Figure 3).

FIGURE 3.

FIGURE 3

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Normalized excitation spectra (1–3) and fluorescence spectra (4–7) of 4,5‐benzopiazselenol (1–6) and 2,3‐diaminonaphthalene (7) in hexane (1, 7), toluene (2, 5), and chloroform (3, 6).

When toluene, o‐xylene, and CHCl3 were used as extractants the fluorescence spectrum of 4,5‐benzopiazselenol changed. In toluene and o‐xylene, the spectrum exhibited a broad structureless band with a maximum at 545 nm; in chloroform, the maximum shifted to 560 nm. Additionally, a broad band with maxima at 414, 434, and 460 nm (Figure 3) appeared in the fluorescence spectra of extracts obtained with toluene, benzene, and o‐xylene. This band corresponded to the free reagent, which was coextracted with 4,5‐benzopiazselenol into the organic solvent. The intensity of the band in the fluorescence spectrum region of 400–460 nm decreased in the series toluene > benzene > o‐xylene. Excitation spectra of 4,5‐benzopiazselenol in benzene, toluene, o‐xylene, and chloroform (300–400‐nm region) were identical to those recorded in hexane and heptane (Figure 3).

Despite the changes in the shape of the fluorescence spectra across different solvents, the fluorescence intensity of 4,5‐benzopiazselenol remained approximately the same in hexane, heptane, benzene, toluene, and chloroform.

3.2. Optimal Conditions of 4,5‐Benzopiazselenol Synthesis

4,5‐Benzopiazselenol is formed via the reaction of Se(IV) with 2,3‐diaminonaphthalene (DAN) in aqueous solution, as shown in the following equation:

3.2.

Therefore, the effect of hydrochloric acid and reagent concentration, temperature, and heating time of the reaction mixture on the formation of the Se(IV) complex with 2,3‐diaminonaphthalene was investigated.

In the range of reagent concentrations from 0.005% to 0.2% and hydrochloric acid from 0.01 to 1 M, the maximum intensity of fluorescence of the Se(IV) complex with 2,3‐diaminonaphthalene in the organic phase (hexane, heptane, benzene, toluene, and carbon tetrachloride) was observed, indicating the completeness of 4,5‐benzopiazselenol formation and its quantitative extraction with organic extractants (Figure 4A,B). The extraction of selenium as 4,5‐benzopiazselenol into the organic solvent, according to mass spectrometric analysis, was ≥ 99%.

FIGURE 4.

FIGURE 4

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Fluorescence of 4,5‐benzopiazselenol in hexane vs. 2,3‐diaminonaphthalene concentration (А), HCl concentration (В), temperature and heating time (С) (СSe(IV) = 1 μg mL−1, λ ex  = 377 nm, λ fl  = 517 nm).

The literature provides different times for the formation of 4,5‐benzopiazselenol, ranging from 2 h at room temperature to 15 and 20 min at 60°C and 50°C, respectively (Parker and Harvey 1962; Lott et al. 1963). The effect of temperature and holding time of the reaction mixture on the fluorescence intensity of 4,5‐benzopiazselenol was studied. As can be seen from Figure 4C, to achieve the maximum fluorescence intensity of 4,5‐benzopiazselenol, the holding time of the reaction mixture at 50°C and 90°C was 10 and 5 min, respectively.

An increase in the holding time of the reaction mixture at these temperatures led to a decrease in fluorescence intensity, likely associated with the degradation of 4,5‐benzopiazselenol during its prolonged heating in an aqueous solution.

3.3. Preconcentration of 4,5‐Benzopiazselenol

For SPE of 4,5‐benzopiazselenol, reversed‐phase silica‐based adsorbents chemically modified with octyl (SiO2‐C8), octadecyl (SiO2‐C18), hexadecyl (SiO2‐C16), and hexadecylphenyl (SiO2‐HPh) groups were selected. These adsorbents can be easily regenerated after the extraction of 4,5‐benzopiazselenol.

The amount of organic phase fixed on the silica surface was determined from the TGA data (Figure S1). Based on the amount of fixed organic phase and the molecular weight of the fixed group, the concentration of grafted groups was calculated (Table 1). As can be seen from the Table 1, the maximum concentration of grafted groups was observed for the SiO2‐C8 adsorbent, and the minimum for the SiO2‐C18 adsorbent.

TABLE 1.

Mass loss of adsorbents upon heating and calculated amount of grafted groups.

Adsorbent Mass loss upon heating, wt.% Calculated amount of groups, mmol g−1
25°С–200°С 200°С–450°С
SiO2‐C8 2.7 12.5 1.1
SiO2‐C16 1.0 12.8 0.56
SiO2‐C18 0.2 0.6 0.024
SiO2‐HPh 1.0 (25°С–260°С) 3.9 (250°С–550°С) 0.14
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The adsorption of 4,5‐benzopiazselenol from aqueous solutions using reversed‐phase adsorbents cannot be studied in batch mode due to their hydrophobicity and water repellency. Therefore, the adsorption of 4,5‐benzopiazselenol was studied in dynamic mode.

4,5‐Benzopiazselenol formed at 50°C–90°C in 0.01–1 M HCl in the concentration range of 2,3‐diaminonaphthalene 0.001%–0.2% was quantitatively extracted (extraction ~99%) by all studied reversed‐phase adsorbents (SiO2‐C8, SiO2‐C16, SiO2‐C18, and SiO2‐HPh) in the dynamic mode. The concentration ranges of the reagent and hydrochloric acid in the solutions from which quantitative extraction of 4,5‐benzopiazselenol was achieved coincided with the ranges of solutions (Figure 4) in which the maximum degree of its formation was observed.

The dynamic capacity of SiO2‐C16, SiO2‐C18, and SiO2‐HPh adsorbents for Se(IV) was about 6.5 μmol g−1, whereas the dynamic capacity of SiO2‐C8 adsorbent was 10 μmol g−1.

3.4. Spectral‐Luminescent Characteristics of 4,5‐Benzopiazselenol on the Surface of Reversed‐Phase Adsorbents

The Se(IV) complex with 2,3‐diaminonaphthalene adsorbed on the surface of reversed‐phase adsorbents exhibited intense yellow‐orange fluorescence. The excitation spectrum of 4,5‐benzopiazselenol was a broad structured band with an intense maximum at 378 nm and less intense ones in the region of 365–370 nm. In the fluorescence spectra of the adsorbents SiO2‐C8, SiO2‐C16, SiO2‐C18, and SiO2‐HPh after the adsorption of 4,5‐benzopiazselenol from aqueous solutions in the range of 400–700 nm, a number of bands were observed (Figure 5). The fluorescence spectrum exhibited a band in the 400–450‐nm region with maxima at 430 and 440 nm, as well as low‐intensity bands with maxima at 483, 515, 530, and 548 nm. These bands and their maxima corresponded to those in the fluorescence spectrum of the adsorbed reagent—2,3‐diaminonaphthalene (Figure 5). A wide structureless band in the fluorescence spectra of the adsorbents with a maximum at 555 nm (for SiO2‐C16) and 566 nm (for SiO2‐C8) corresponded to the fluorescence of 4,5‐benzopiazselenol and is similar to the band in the spectrum of its fluorescence in benzene and chloroform. The fluorescence intensity of 4,5‐benzopiazselenol (at constant surface loading) decreased in the series: SiO2‐C16 > SiO2‐C8 > SiO2‐C18 > SiO2‐HPh (Figure 5).

FIGURE 5.

FIGURE 5

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Excitation spectra (1–4) and fluorescence spectra (5–9) of 4,5‐benzopiazselenol (1–8) and 2,3‐diaminonaphthalene (9) in the phase of SiO2‐С16 (1, 5, 9), SiO2‐С8 (2, 6), SiO2‐С18(3, 7), and SiO2‐HPh (4, 8) adsorbents (СSe(IV) = 1 μg; 0,025 g of adsorbent).

The low fluorescence intensity of the SiO2‐C18 and SiO2‐HPh adsorbents after the adsorption of 4,5‐benzopiazselenol is probably associated with the low concentration of grafted nonpolar groups (Table 1), which resulted in significant broadening of the fluorescent zone of the adsorbent in the preconcentrating cartridge.

3.5. SPE‐Fluorimetric Determination of Se(IV) Using SiO2‐C16

As the concentration of 4,5‐benzopiazselenol in the adsorbent phase increased, the length of its fluorescent zone in the preconcentrating cartridge increased, and accordingly, the fluorescence intensity of the adsorbent also increased. An increase in the adsorbent fluorescence intensity in the preconcentrating cartridge with increasing concentration of the adsorbed 4,5‐benzopiazselenol was used as the basis for developing a method for the SPE‐fluorimetric determination of Se(IV) directly in the adsorbent phase within the preconcentrating cartridge. The increase in the length of the fluorescent zone of the adsorbent was used for its determination by a test method in the format of indicator tubes. This approach significantly simplifies the procedure for determining Se(IV) due to the absence of desorption stage or the need to transfer the adsorbent from the preconcentrating cartridge into a special cuvette for measuring the fluorescence intensity of powdered materials.

For developing a SPE‐fluorimetric method of Se(IV) determination, SiO2‐C16 adsorbent was used; the maximum fluorescence intensity of 4,5‐benzopiazselenol was observed on the surface of this adsorbent. The amount of adsorbent was 0.025 g and its height in the preconcentrating cartridge was 15 mm, which was determined by the ability to effectively illuminate the entire adsorbent layer in the preconcentrating cartridge with exciting light in a special holder in the cuvette compartments of spectrofluorimeters (Figure 1b).

The detection limit for the fluorimetric determination of Se(IV) in the preconcentrating cartridge calculated by the 3‐s criterion was 2 ng per 0.025 g of SiO2‐C16 adsorbent, which corresponds to its concentration in the solution of 0.2 ng mL−1 (0.2 μg L−1) when using 10 mL of the solution. The limit of quantification (LOQ) for Se(IV), calculated using the 6‐s criterion, was 0.6 ng mL−1 (0.6 μg L−1). When determining concentrations above 2 ng mL−1, the relative standard deviation of repeatability did not exceed 0.06, and that of reproducibility did not exceed 0.08. Increasing the volume of the passed solution to 100 mL makes it possible to reduce the relative detection limit of Se(IV) to 0.02 ng mL−1 (0.02 μg L−1), which is significantly lower than the hygienic standards for selenium content in drinking water. The linearity of the calibration graph was maintained up to 2 μg per 0.025 g when using the SiO2‐C16 adsorbent (Figure S2).

In the test determination of Se(IV), the visual detection limit was 0.1 μg per 0.025 g of adsorbent, and the linearity of the calibration characteristic for visual determination was maintained up to 2 μg of Se(IV) per 0.025 g of adsorbent. The calibration equation for the test determination of Se(IV) in the indicator tube format is: l = 6,9·c, where: l—the length of the fluorescent zone of the adsorbent in the preconcentrating cartridge (mm); c—the concentration of Se(IV) in μg per 0.025 g of adsorbent.

The fluorescence intensity of 4,5‐benzopiazselenol on the adsorbent surface in the cartridge did not change for at least 3 days.

The possibility of reusing the adsorbent directly in the cartridge in the “adsorption‐regeneration” mode was investigated. The adsorbed 4,5‐benzopiazselenol was quantitatively desorbed from the adsorbent surface by eluting with 1 mL of hexane through the preconcentrating cartridge. To enable reuse of the adsorbent in the cartridge, residual hexane must be removed by air purging, because any hexane remaining on the adsorbent surface would interfere with the subsequent SPE of 4,5‐benzopiazselenol. Complete removal of hexane from the adsorbent surface was achieved by passing an air stream through the preconcentrating cartridge for 30 min at a flow rate of 100 mL min−1, enabling its reuse for preconcentration of 4,5‐benzopiazselenol. The adsorbent retained its performance through at least five adsorption‐regeneration cycles, confirming its reusability.

3.6. Effect of Coexisting Ions

The study of the interfering effect of accompanying metal ions in the SPE‐fluorimetric determination of Se(IV) was carried out in binary systems containing 0.1 μg Se(IV) and increasing amounts of accompanying metal ions in 10 mL of solution. The determination of 0.1 μg Se(IV) was not interfered with by 5000‐fold excesses of K+, Mg2+, Ca2+, 1000‐fold excesses of Ni2+, Co2+, Cr3+, Ni2+, Zn2+, and Fe2+, and up to 300‐fold Fe3+. Additionally, chloride and nitrate ions did not interfere at concentrations up to 2 g L−1; carbonate, sulfate, and phosphate ions—up to 500 mg L−1; and nitrite ions—up to 100 mg L−1. The high selectivity of the SPE‐fluorimetric determination of Se(IV) on the SiO2‐С16 adsorbent arises from the hydrophobic nature of the material, which prevents the uptake of charged solution components such as cations and anions.

3.7. Separation and Determination of Se(IV) and Se(VI)

The possibility of separating Se(IV) and Se(VI) is based on the use of 2,3‐diaminonaphthalene—a selective reagent for Se(IV)—that does not interact with Se(VI) (Parker and Harvey 1962). When passing solutions containing Se(IV) and Se(VI) and treated at a temperature of 50°C–90°C with 0.01%–0.1% solutions of 2,3‐diaminonaphthalene in 0.01–1 M HCl through a preconcentrating cartridge containing a SiO2‐C16 adsorbent, Se(IV) in the form of 4,5‐benzopiazselenol was extracted by a reversed‐phase adsorbent, and Se(VI) in the form of selenic acid anion passed through the preconcentrating cartridge and could be determined in the effluent by ICP‐MS method.

To evaluate the accuracy of the method, the concentrations of Se(IV) and Se(VI) were determined in model solutions (Table 2), assuming that Se(VI) typically exceeds Se(IV) in natural waters. The results demonstrate that the developed method for separation followed by fluorimetric determination of Se(IV) and ICP‐MS determination of Se(VI) yields accurate and reproducible data.

TABLE 2.

Results of SPE‐fluorimetric determination of Se(IV) as 4,5‐benzopiazselenol and ICP‐MS determination of Se(VI) in model solutions (p = 0.95, n = 3).

Se(IV), μg L−1 Se(VI), μg L−1
Added Found Added Found
0.5 0.4 ± 0.2 10 10.7 ± 0.6
1 0.9 ± 0.1 10 9.8 ± 0.5
10 9.8 ± 0.6 100 98 ± 3
50 49 ± 2 100 98 ± 4
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The developed method was tested for determining the content of Se(IV) and Se(VI) in underground mine water of the Tyva Republic (Russia) and bottled mineral water that do not contain humic and fulvic acids, surfactants, or other organic compounds. As water passes through rock formations, organic substances are adsorbed from it. In turn, the treatment of bottled drinking water includes removal of organic compounds.

The cationic and anionic composition of natural and bottled waters was determined by capillary electrophoresis (Table S2). The concentrations of metal cations and anions in the analyzed waters are well below the levels that could interfere with the SPE‐fluorimetric determination of selenium (IV).

For the determination of selenium(IV) in real samples, an aliquot of the analyzed water (20 mL) was placed in a 25 mL volumetric flask, 1 mL of 1 M HCl was added, deionized water was added to the mark, and then proceeded as described in Section 2.3. The results of selenium determination are presented in Table 3. The data presented in Table 3 confirm that the developed method ensures accurate and reproducible results.

TABLE 3.

Results of Se(IV) and Se(VI) determination in natural water and mineral bottled water (p = 0.95, n = 3).

Sample Added, μg L−1 Found, μg L−1
Se(VI) Se(IV) Se(VI) Se(IV)
Mining water, 30 m from the entrance — — 0.65 ± 0.08 0.42 ± 0.07
1 0.5 1.60 ± 0.14 0.9 ± 0.1
Mining water, near the entrance — — 0.84 ± 0.09 0.6 ± 0.1
1 0.5 1.83 ± 0.20 1.1 ± 0.1
Water from Saylyg village well — — 3.6 ± 0.4 0.4 ± 0.1
5 0.5 8.4 ± 0.5 0.9 ± 0.1
Mineral water «Maltinskaya» — — 14 ± 1 0.68 ± 0.08
10 0.5 24 ± 2 1.2 ± 0.1
Mineral water «Stelmas» — — 1.1 ± 0.1 < LOD
1 0.5 2.1 ± 0.2 0.6 ± 0.1
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4. Conclusion

SPE of Se(IV) as a complex with 2,3‐diaminonaphthalene by nonpolar adsorbents allowed not only to quantitatively separate it from Se(VI) but also to determine its content directly in the adsorbent phase in a preconcentrating cartridge placed in the cuvette compartment of a spectrofluorimeter. Registration of fluorescence directly in the adsorbent phase simplified the method, and the possibility of passing large volumes of solutions through the preconcentrating cartridge allowed to proportionally reduce the detection limit of Se(IV). The dependence of the length of the fluorescent zone of the adsorbent on the Se(IV) content can be used for its test‐determination.

In terms of the achieved detection limit for Se(IV), the SPE‐fluorimetric method surpasses the extraction‐fluorimetric method, as well as methods based on selenium preconcentration, its desorption from the adsorbent surface, and subsequent determination in the desorbing solutions by graphite furnace atomic absorption spectrometry, optical emission spectrometry, and inductively coupled plasma mass spectrometry (Table 4).

TABLE 4.

Comparative data from some studies for Se determination.

Method Preconcentration technique Reagent Adsorbent/extractant LOD, μg L−1 Linear range, (μg L−1) RSD, % Ref.
Fluorescence SPE DAN C18 membrane disk 29.6 10–850 5.61 (Serra et al. 2010)
Fluorescence Extraction DAN Cyclohexane 0.82 10–50 3.9–4.4 (Rodriguez et al. 1994)
Fluorescence Extraction DAN 9‐Anthracenemethanola 1.6 5–125 2.08–2.78 (Aimaitiniyazi et al. 2023)
Fluorescence Extraction DAN Hexane 3 — — (Harrison et al. 1996)
Fluorescence Extraction DAN Hexane 0.1 0.1–5 — (GOST 19413–89 1989)
GFAAS VA‐LLME Ammonium pyrrolidine dithiocarbamate Triton X‐114, 1‐butyl‐3‐methylimidazolium hexafluorophosphate 0.07 0.24–1.75 3.51 (Ali et al. 2021)
GC‐ECD DLLME DAN 0.005 0.015–10.0 4.10 (Bidari et al. 2008)
(PVG)‐HPLC‐ICP‐MS Formic acid MIL‐125‐NH2 photocatalyst 0.80 — 3.7–4.6 (Luo et al. 2022)
ICP‐AES SPE — CTAB‐alkyl silica microcolumn 0.10 0.5–1000 3.6 (Xiong et al. 2008)
ICP‐MS DLLME Bismuthiol II Chloroform 0.047 0.50–100 7.20 (Zhang et al. 2013)
UF‐VIS SPE Oxidizes I− ions into I2 species Thiol cotton 4.7 10–8000 — (Xiong et al. 2018)
SPE‐FLU SPE DAN SiO2‐C16 0.02 0.06–20 This work
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Author Contributions

Vladimir Losev: conceptualization, funding acquisition, writing – original draft, writing – review and editing, data curation, validation. Anastasya Orobyeva: investigation, writing – original draft, validation, formal analysis, data curation, resources. Svetlana Didukh‐Shadrina: methodology, writing – original draft, writing – review and editing, data curation, supervision, validation, investigation. Sergey Metelitsa: methodology, investigation, visualization, validation, data curation, formal analysis, resources. Elena Borodina: project administration, writing – original draft, writing – review and editing, formal analysis, supervision. Vyacheslav Cherkashin: investigation, writing – original draft. Urana Ondar: resources, data curation.

Funding

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (FSRZ‐2020‐0013).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Table S1: Operational conditions for ICP‐MS determination of Se.

Table S2: The average content of anions and metals in mine and bottled water.

Figure S1: TGA and DSC of adsorbents.

Figure S2: Calibration graph for SPE‐fluorimetric determination of Se(IV).

WER-98-e70486-s001.docx (454.2KB, docx)

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Table S1: Operational conditions for ICP‐MS determination of Se.

Table S2: The average content of anions and metals in mine and bottled water.

Figure S1: TGA and DSC of adsorbents.

Figure S2: Calibration graph for SPE‐fluorimetric determination of Se(IV).

WER-98-e70486-s001.docx (454.2KB, docx)

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

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