1. Introduction
With industrialization in modern society, the problem of carbon dioxide emissions and global warming of the planet has increased [
1,
2]. As a result, other sources of clean energy have been used to minimize these harmful effects on the environment. An example of this is atomic energy [
2,
3].
Uranium, which is primarily used as fuel in nuclear power plants, is found in soil and water. It is known to cause serious toxicological effects to humans and its compounds are carcinogenic. It reaches the environment by leaching from natural deposits, releasing waste from nuclear power plants, and the combustion of coal and other fuels [
4,
5].
Currently, the separation and determination of uranium are gaining more and more importance due to its increasing applications in different fields, its use goes beyond the generation of nuclear energy, also being applied in the industry of anti-tank ammunition, nuclear weapons, catalysts, and pigments of dyes [
6]. According to the World Nuclear Association (WNA), it is estimated that there are 6.14 million tons of uranium in mines around the world. From this amount, Brazil represents about 5% [
5,
7].
There are several reported techniques capable of detecting uranium in environmental samples, such as: fluorometry [
8,
9], X-ray fluorescence [
10,
11], potentiometry [
12,
13], inductively coupled plasma emission spectrometry (ICP OES) [
14,
15,
16], inductively coupled plasma mass spectrometry (ICP-MS) [
17,
18], and spectrophotometry [
5,
19,
20,
21,
22,
23]. Among all these techniques, spectrophotometry has a lot of applications. In recent years, many conventional analytical methods based on computer vision related to the colorimetric analysis of digital images are becoming promising, allowing the development of fast, accurate, and low-cost analytical methodologies [
24]. Some detectors such as scanners [
25,
26,
27], digital cameras [
28,
29], smartphones [
30,
31,
32], tablet computers, and webcams [
33,
34] can be used.
Given this infinity of cited techniques, interference from other ions and low sensitivity are the most common problems to be overcome by the proposed methodologies. For this, it is necessary to create methods that allow the pre-concentration of uranium ions. To date, there are no reports in the literature about methodologies that involve the preconcentration and determination of uranium by digital images. Ion-imprinted polymers (IIPs) have advantageous characteristics to overcome these disadvantages, such as high selectivity, greater stability in aggressive environments, such as high temperatures and pressures, and ease of synthesis [
5,
35,
36].
This work proposes in an unprecedented way, so far, the application of a pre-concentration method using IIP-U as a new sorbent and the quantification using digital images of uranium in natural water samples from the city of Caetité (Bahia, Brazil).
3. Experimental
3.1. Reagents and Solutions
All reagents used in the experiments are analytical grade, and ultra-pure water was obtained using a Milli-Q Plus system (Bedford, MA, USA). Ethylene glycol dimethacrylate (EGDMA), 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP), methacrylic acid (AMA), 2,2′-azobisisobutyronitrile (AIBN), and uranyl acetate (UO2(CH3COO)2·2H2O) were supplied by (Merck, Darmstadt, Germany). Uranium solutions were prepared daily by diluting a 1000 mg L−1 stock solution (Merck, Darmstadt, Germany). Nitric acid solutions were prepared by diluting 67% nitric acid (HNO3) (MERCK, Darmstadt, Germany). Dihydrogen potassium orthophosphate/sodium hydroxide and sodium acetate/acetic acid buffer solutions were prepared with a concentration of 0.1 mol L−1, both (NEON, Suzano, Brazil). The glasses and containers used were washed and kept in a nitric acid solution (10% v/v) for 24 h.
Water samples from rivers and lakes were collected in the city of Caetité, Bahia, filtered through a cellulose acetate membrane with a diameter of 47.0 mm and porosity of 0.45 μm, acidified, and stored in a refrigerator at 4 °C until analysis.
3.2. RGB Data Acquisition and Evaluation
The determination of U(VI) was performed using a closed system consisting of a closed white wooden box (to avoid reflections), with the following dimensions: 35 cm × 40 cm × 25 cm (height × width × depth). The box also had a light control system in the internal compartment to ensure reproducibility during image capture. Attached to this box was a Lifecam Cinema 720 p 5 MB webcam, model H5d-00013 from Microsoft, as can be seen in
Figure 8.
For all experiments, digital images were stored in JPG format and a square region of interest (ROI) was defined at a fixed position using the ImageJ computer program treatment. This program allows the acquisition of RGB data for all pixels in the ROI. This information is organized into a color histogram, and the average value of each color channel, red (R), green (G), and blue (B), is calculated. The definition of the analytical signals based on the color value in accordance with Bee’s law was defined as: (Equation (1))
where P is the R, G, and B value (mean or mode) of the standard solution or sample and P
0 is the R, G, and B value for the analytical blank.
In the batch pre-concentration system, an MS Tecnopon LDP 305-4 peristaltic pump equipped with silicone tubes of various diameters was used to propel the solutions.
3.3. Synthesis of Polymers with Printed Ions (IIP-U(VI))
The polymer synthesis with imprinted ions was based on the work of Dhruv K. Singh [
40]. It was carried out using 0.25 mmol of uranyl acetate (UO
2(CH
3COO)
2·2H
2O) and 0.25 mmol of 5-Br-PADAP which were solubilized in 10 mL of a 1:1 mixture of dimethylsulfoxide/acetonitrile with continuous stirring for one hour. After this period, the resulting solution was mixed with 4.0 mmol of AMA, 20 mmol of EGDMA, and 0.100 g of the radical initiator AIBN. With the addition of such reagents, the polymerization was continued by heating in an oil bath at 60 °C for a period of 24 h. The material at the end of the synthesis reaction was dried, crushed, sieved, and leached in consecutive steps using ethanol and 2.0 mol L
−1 hydrochloric acid. The unprinted polymer (NIP) was prepared in a similar way but without the U(VI) ions. Finally, the materials obtained were characterized by SEM, FTIR, and TG.
Figure 9 represents the proposed synthesis route of the IIP-U(VI).
3.4. Characterization of the Ion-Imprinted Polymer (IIP)
The morphology of the polymers was analyzed by scanning electron microscopy (SEM), using a JEOL microscope, Carry Scopy JSM-6510LV model (JEOL, Ltd., Tokyo, Japan), with an acceleration voltage of 20 kV. The samples were mounted on aluminum sample holders and coated with gold using evaporative plating equipment, specifically a DESK V model (Denton Vacuum, NJ, USA). Thermogravimetric analysis (TGA) was conducted using equipment from TA Instruments, Q50 model (TA Instruments, New Castle, DE, USA) with a heating rate of 10 °C/min in a nitrogen atmosphere, over a temperature range of 27 to 800 °C. Fourier transform infrared (FTIR) analysis was performed using a Nicolet iS 10 instrument from Thermo Scientific (Thermo Scientific, Waltham, MA, USA) with a spectral range from 4000 to 400 cm−1, a resolution of 4 cm−1, and 32 scans.
3.5. Preconcentration System and Determination of U(VI)
The procedure for pre-concentration of U(VI) ions using IIP-U(VI) was carried out in batches and consisted of two steps. In step 1 (pre-concentration), 25 mL of the sample was passed through a mini-column containing 0.1 g of the sorbent. In the second step, 2.0 mL of the eluent (nitric acid) was percolated through the mini-column to carry all the ions of interest that were previously absorbed in step 1.
To the eluted volume (2.0 mL) was added 1.0 mL of the chromogenic reagent (Arsenazo III) with a definite concentration of 0.05% (
w/
v). This final solution was taken to the detection system (
Figure 1). A schematic of the pre-concentration and elution system is illustrated in
Figure 10.
3.6. Multivariate Optimization Strategy
The optimization of the online system was performed using a full two-level factorial design and a Doehlert matrix. All experiments were performed in a random order using uranium solution with final concentrations of 2.0 and 1.0 mg L−1, respectively. The variables pH, eluent concentration (nitric acid), and sampling flow were chosen for the optimization and absorption as a response. Triplicates of the central point were used to calculate the experimental error.
3.7. Sample Preparation
Samples of water from lakes in the region of the city of Caetité (Bahia, Brazil) were collected and stored in polyethylene bottles. The samples were filtered using a filter with a cellulose acetate membrane of 47.0 mm in diameter and 0.45 μm in porosity. With subsequent filtration, the samples were acidified and stored in a refrigerator at 4 °C until analysis. In
Table 7 are found the geographic coordinates.
This region of the municipality of Caetité (Bahia, Brazil), which is located 645 km from the capital of the state of Salvador, Ba, has an approximate population of 51,000 inhabitants, according to IBGE 2019 [
35]. It was chosen for the collection of lake water samples. This region is subject of many studies aimed at the analysis of possible uranium contamination since it is naturally located in a uranium mine, with reserves of 100,000 tons of the ore being explored by the state-owned Indústrias Nucleares do Brasil SA (INB) since 1999, which is capable of producing around 400 tons of uranium/year [
36].
4. Conclusions
The proposed method proved to be efficient in the determination of U(VI) in lake water samples, presenting excellent precision and accuracy, as well as adequate detection and quantification limits for the concentrations frequently found in these samples.
The chemometric tools used to study the optimal conditions of the factors studied proved to be efficient in the face of all variations in the experimental domains.
The methodology developed for the determination of U(VI) ions through digital images presented itself as an excellent alternative for the quantification of these ions, mainly because of the ease of assembling the system, in addition to data processing (image decomposition into RGB signals) being possible through free software such as Chemostat. Finally, the synthesized polymer allowed the extraction and pre-concentration of U(VI) ions from highly complex matrices such as lake waters.