1. Introduction
Due to the extensive and rapid development, environmental pollution has increased to alarming levels in nearby industrialized areas, which are generally contaminated with heavy metals. Among many others, hexavalent chromium, Cr(VI), is one of the most serious concerns, as there is credible evidence that through the water medium, it is a major contributor to the global burden of cancer in humans [
1]. Chromium compounds are mainly used in industrial activities, such as corrosion control, oxidation process, leather tanning, electroplating, metallurgy, cement, textile dyeing, papermaking, inks, paints and pigments, and photographic industry [
2]. Due to this wide use, there are locations where chromium compounds have been released to the environment via leakage, poor storage, or improper industrial disposal practices, so chromium pollution of waters and groundwaters represents a serious environmental concern. The USEPA regulates total chromium in drinking water and has set a Maximum Contaminant Level (MCL) of 0.1 mg/dm
3 [
3]. The World Health Organization (WHO) guideline is 0.05 mg/dm
3 for total chromium [
4]. Although this reference value has been questioned, as a practical measure, it has been retained as a provisional guideline value until additional information becomes available and chromium can be re-evaluated [
4]. Chromium discharge limits in water are regulated on a national scale and often vary depending on the different types of industry or receiving water bodies (marine water, lake, river, or sewer system) [
5]. The maximum discharge limit to the aquatic environment in the EU is 1 and 5 mg/dm
3 for Cr(VI) and Cr
total, respectively [
6].
There are currently several technologies for the remediation [
5] and detection [
7] of Cr(VI) from aqueous samples. As this species is usually present at low concentration levels, it is measured by atomic absorption and emission methods, providing high precision and sensitivity measurements. The selection of these techniques will depend largely on the sensitivity, reproducibility, detection and quantification limits, and simplicity of the method. However, environmental scientists are increasingly in need of measurement closer to the sample, in situations outside the laboratory, which restricts the use of such analytical techniques. Alternatively, UV/VIS, FTIR, Raman, and NIR portable spectrometers [
8,
9] are available to meet the growing need driven by the general trend in analytical instrumentation toward smaller size, improved reliability, and greater ease of operation. In such a context, the use of ion-selective optodes (ISOs), ionophore-based optical chemical sensors, represents an excellent alternative for the portable determination of ions [
10]. In this regard, several systems for Cr(VI) monitoring using optical sensors have been developed in recent years [
11,
12,
13,
14].
On the other hand, besides being used for the removal and transport of metals [
15,
16], polymer inclusion membranes (PIMs) have also been employed for the detection of many chemical species [
17]. These membranes are composed of a polymeric matrix, which serves as support of an encapsulated extractant (carrier) that is responsible for binding with the target analyte at the source solution/membrane interface and transporting it across the membrane, which can contain a plasticizer to improve mobility. In the case of Cr(IV), Aliquat 336 (methyltrioctylammonium chloride) has proved to be an effective and selective extractant in solvent extraction (SX) [
18,
19], supported liquid membranes (SLMs) [
20], solvent-impregnated resins (SIR) [
21,
22], and PIM [
20,
23,
24,
25,
26,
27,
28,
29] systems.
In previous work, the capabilities of a PIM sensor to perform cadmium(II) determination in aqueous solutions by in-situ visible (VIS) and mid-Fourier transform infrared spectroscopy (MID-FTIR) analyses of the polymeric films using a partial least squares (PLS) chemometric approach were demonstrated [
30]. One major advantage of the developed MID-FTIR-PLS PIM-based method was that it did not require the presence in the membrane of a chemical reagent with special properties, either a chromophore species that can complex the metal ion, i.e., acting as an ionophore [
14,
31], or a mixture of an ionophore and a chromophore in the same PIM [
32], or a fluorescent reagent [
33]. However, to extend the potentiality of the methodology to low analyte concentration ranges, a careful selection of the dielectric nature of the medium and the dipole moment of the bond associated with IR vibrations of the extracted complex was suggested [
30], so that an increase in the band intensities could be achieved, i.e., an augment in the magnitude of the analytical signal, improving the detection capabilities of these sensors.
Taking advantage of the high dielectric constant values observed in PIMs with Aliquat 336 plasticized with 2-nitrophenyl octyl ether (NPOE) [
34] and the excellent transport properties of Aliquat 336 for Cr(VI) in PIMs [
20,
23,
24,
25,
26,
27,
28,
29], in this work, such membranes were employed for MID-FTIR-PLS analysis of Cr(VI) from aqueous media. Once the system had been characterized and optimized, the calibration performance and the figures of merit (FOM) of the method were determined to show the potentiality of the proposed methodology.
2. Materials and Methods
2.1. Reagents and Apparatus
PIMs were prepared using cellulose triacetate (CTA, Honeywell Fluka, Charlotte, NC, USA) as polymer support, 2-nitrophenyl octyl ether (NPOE, ≥99.0% Honeywell Fluka) as a plasticizer, Aliquat 336 (≥97%, methyltrioctylammonium chloride, Sigma-Aldrich, Chem. Co., St. Louis, MO, USA) as an extracting agent, and dichloromethane (Merck, Kenilworth, NJ, USA) as a casting solvent. Working Cr(VI) solutions were prepared by dissolving the corresponding amounts of (NH4)2Cr2O7 (≥99.5%, Sigma-Aldrich) in deionized water (18 MΩ cm, MilliQ, Merck Millipore, Burlington, MA, USA). A 1000 mg/dm3 Sigma-Aldrich AAS standard solution (≥99.5%, Sigma-Aldrich, 1 g/dm3 Cr in 2% nitric acid, prepared with high purity (NH4)2Cr2O7, HNO3 and water) was diluted using deionized water for the preparation of the standards for flame atomic absorption spectrometry (FAAS) determinations. Tris (hydroxymethyl)aminomethane (TRIS, 99.8% Sigma-Aldrich), sodium acetate (99% Sigma-Aldrich), acetic acid (99.7% Sigma-Aldrich), hydrochloric acid (37% Sigma-Aldrich), and 2-(N-morpholino) ethanesulfonic acid (MES, 99.5% Sigma-Aldrich) were employed to assess the effect of pH on chromium extraction.
A model 75Wrist ActionTM shaker (Burrell Scientific Inc., Pittsburgh, PA, USA) with 10 speeds and a Metrohm 620 pH-meter (Herisau, Switzerland) were used. A 3100-flame atomic absorption spectrometer (Perkin Elmer Waltham, MA, USA) and a Spectrum GX IR spectrometer (Perkin Elmer) were employed to measure metal ion contents and obtain the IR spectra, respectively. A Fowler IP54 micrometer (Fowler High Precision, Newton, MA, USA) was used for measuring PIM thicknesses.
2.2. Preparation of PIMs
Membranes were prepared dissolving 30 mg of CTA, 75 mg of NPOE, and 30 mg of Aliquat 336 in 10 cm
3 of CH
2Cl
2. This mixture was stirred for 1 h in a 50 cm
3 beaker and then the solvent was allowed to evaporate for 24 h. After this time, the membranes were detached from the beakers and their diameter and thickness were measured with a Fowler IP54 micrometer. An average weight of (135.86 ± 2.05) mg, an average thickness of (95 ± 15) µm, and an average diameter of (3.00 ± 0.02) cm were determined (n = 54). The PIM thickness is almost double that previously reported in a similar sensor [
30] to enhance the detectability of the analyte.
2.3. Liquid-Solid Extraction
The obtained membranes were placed in a 50 cm3 polypropylene falcon tube together with 30 cm3 of Cr(VI) solutions at different concentrations. Stirring was performed for 20 min, taking a 1 cm3 aliquot every 2 min. At the end, the membranes were removed from the aqueous phase and the aliquots were brought to a volume of 2 cm3 to be able to measure their concentration by FAAS. Experiments were performed on a duplicate basis with an average RSD of 5%.
2.4. Metal Quantification
The absorbances corresponding to the samples and standards were measured by FAAS according to the conditions established by the manufacturer (354.7 nm wavelength (λ), 7 nm slit, with an air-acetylene flame (99.6%, Praxair, Danbury, CT, USA and an oil-free GS-003—Air compressor, PG Instruments, Loughborough, Leicestershire, UK), from 0 to 7 mg/dm3, using a hollow cathode lamp, and sensitivity check of 5 mg/dm3). The calibration curves were performed at each of the experimental conditions employed, mismatching the standards and experimental samples.
2.5. Measuring Infrared Spectra
The IR spectra were obtained by direct analysis of the PIMs in the transmission mode for the quantitative analysis of chromium in the mid-infrared region. The PIM was mounted on the transmission accessory of the equipment after sandwiching the membrane between two Petri dishes to avoid wrinkles and movement. The spectrum of each sample was acquired in the 400–5000 cm−1 region in triplicate with 30 scans per measurement to reduce the within-sample variation, which was found to be, on average, 1% RSD, and the mean value of the spectra was used in further data treatment. This procedure ensures the reduction in sample differences due to random variations in experimental conditions (e.g., aqueous volume, PIM contact area, PIM composition, etc.).
2.6. Development of the Chemometric Model
The multivariate calibration method was built from the information provided by the IR spectra of the PIMs with a set of 54 samples covering 27 different concentrations each by duplicate with an interval ranging from 1.92 × 10−7 to 1.92 × 10−6 mol/dm3 (10–100 ppb). The spectral range used for quantitative analysis initially was 400–5000 cm−1, so the experimental matrix consisted of 54 rows corresponding to the samples and 4601 wavelengths. Further cross-validation was performed in the calibration set using Venetian blinds with 3 data splits and 2 samples per blind (thickness). A test set consisting of 10 samples acquired as the calibration test, but independently, had dimensions of 10 × 4601. Metal concentrations given along the work refer to the initial concentrations of the metal in the solution. Data processing consisted of baseline correction (automatic weighted least squares, order one) and mean-centering. The PLS-Toolbox 9.0 software (Eigenvector Research, Inc. Wenatchee, WA, USA) was employed for all chemometric analyses. An in-house-made MATHLAB 9.7 R2019b (Natick, Apple Hill Campus, MA, USA) program was used for the evaluation of the FOM.
4. Conclusions
Using infrared spectroscopy and multivariate statistical analysis, a MID-FTIR-PLS PIM-based sensor for the quantitative determination of Cr(VI) from an aqueous medium was developed with satisfactory results. Optimized conditions for the system consisted of (22.2 ± 0.7) w/w%, CTA, (54.4 ± 1.3) w/w% NPOE and (23.4 ± 1.5) w/w% Aliquat 336, pH 6, and 20 min of extraction time, although pH values in the range of 4–7 could also be used. It was observed that chromium extraction decreases as the pH increases, in such a form that at pH 9, the extraction is less than 20%. However, at pH 4, the extraction exceeds 80%; this behavior is mainly due to the change in metal speciation (from HCrO4− to CrO42−) with increasing pH of the solution. Cr(VI) adsorption followed a Langmuir-type isotherm with KL = 2199 cm3/mmol, qmax = 0.188 mmol/g and 0 < RL < 1. At low metal concentrations, D is ca.10,000 units, which indicates a great affinity of the metal towards the membrane; on the other hand, a decay in its value at high concentrations is indicative of lower affinity, as also observed in the E% profile. The highest enrichment factor was obtained at low analyte concentrations, with E ≈ 33, where the extraction percentage is practically constant. On the other hand, the enrichment factor at high concentrations corresponds to E ≈ 15. Characterization of the extraction reaction indicated a 1:1 Cr(VI): Aliquat 336 ratio. The PCA analysis of the PIMs revealed a complex pattern, which was satisfactorily simplified and related to Cr(VI) concentrations through the use of a variable selection method (iPLS) in which bands in the ranges of 3451–3500 cm−1 and 3751–3800 cm−1 were chosen. The final PLS model including the 100 wavelengths selected by iPLS and 10 latent variables shows excellent FOM values. The developed PIM sensor is suitable for in situ analysis of aqueous samples, and it is easy to implement as it requires few manipulations and a reduced number of chemical compounds in comparison to other similar reported systems.