Surface Modification of Poly(butyl methacrylate) with Sulfomethylated Resorcinarenes for the Selective Extraction of Dichromate Ion in Aqueous Media

Abstract

The dichromate ion (Cr₂O₇²⁻), a highly toxic chromium VI species, is widely used in industrial processes, generating serious environmental problems when released into water bodies. This investigation proposes the use of a functionalized polymer as an adsorbent material for its removal in the aqueous phase. Poly(butyl methacrylate) (PBMA) was synthesized and modified by impregnation with resorcinarenes derived from long-chain aliphatic aldehydes. To improve the affinity for the dichromate, the resorcinarenes were functionalized with sulfomethyl groups by treatment with Na₂SO₃. The resulting matrices were characterized using IR-ATR, ¹H-NMR, and ¹³C-NMR, and their adsorbent performance was evaluated via UV-Vis spectroscopy in batch extraction assays. The results showed that the functionalized polymer exhibited a higher adsorption capacity than the base polymer, reaching up to 81.1% removal at pH 5.0 in one hour. These results highlight the potential of PBMA as an effective support and raise a promising research perspective for functionalized resorcinarenes in the development of new materials for the treatment of contaminated water.

1. Introduction

Industrial activity, indispensable for the economic and daily life of modern societies, continues to be one of the main sources of environmental pollution, seriously affecting air quality, soil, and, critically, water resources. Among the different forms of contamination, that generated by heavy metals such as chromium [1] constitutes a persistent threat to human health, food security and the integrity of aquatic ecosystems. This problem is aggravated in regions with little environmental regulation, limited application of treatment technologies, and the significant presence of small and medium-sized industries.

Hexavalent chromium (Cr(VI)), present in the effluents of processes such as the tanneries, galvanoplasty, decorative chromakey, and stainless-steel manufacturing, among other industries, has been classified as carcinogenic by the IARC and has been associated with acute and chronic toxic effects that include neurotoxicity, genotoxicity, and alterations of embryonic development. Its high solubility in water and mobility in aqueous media facilitate its dispersion, affecting both biodiversity and human communities that depend on these sources for domestic and agricultural activities [2] (Figure 1).

Hexavalent chromium (Cr(VI)) is commonly found in the environment in two main anionic forms: chromates (CrO₄²⁻) and dichromates (Cr₂O₇²⁻). Both are highly soluble, toxic, and environmentally persistent [3,4]. This study focuses on the removal of dichromate ions, which are especially relevant in acidic conditions.

Table 1. Common Cr(VI) compounds containing the dichromate anion, their industrial applications, and environmental and health impacts.

Compound	Anion Type	Industrial Applications	Environmental and Health Impacts
Potassium dichromate (K2Cr2O7)	Dichromate (Cr2O72−)	Oxidizing agent in organic synthesis, electroplating, analytical chemistry	Highly toxic, carcinogenic, contaminates water and soil
Sodium dichromate (Na2Cr2O7)	Dichromate (Cr2O72−)	Pigments, leather tanning, corrosion inhibitors, metal finishing	Strong oxidizer, damages liver and kidneys, persistent in water
Ammonium dichromate ((NH4)2Cr2O7)	Dichromate (Cr2O72−)	Pyrotechnics, chemical demonstrations, oxidizing agent	Fire hazard, toxic by inhalation and ingestion, soil contaminant
Chromic acid (H2Cr2O7/CrO3 + H2O)	Acidic dichromate form	Metal cleaning, etching, glassware cleaning in labs	Corrosive, harmful to aquatic life, toxic vapors

presents the most common Cr(VI) compounds containing the dichromate anion, highlighting their industrial uses and associated environmental and health risks.

Various conventional methods have been developed for the removal of Cr(VI) from contaminated waters [5], including chemical precipitation, ion exchange [6], chemical reduction [7], reverse osmosis, adsorption with activated carbon [8,9,10] and biosorption [11,12]. However, many of these techniques require expensive infrastructure, specialized expertise, or generate hazardous by-products, which limit their applicability in vulnerable socio-economic contexts.

In search of more accessible and sustainable solutions, adsorbent materials have become a widely studied alternative for water treatment. Traditional adsorbents such as activated carbon, zeolites, and natural clays are valued for their large surface area and availability [13,14], but they often lack selectivity or show limited regeneration. In recent years, synthetic and functionalized polymers have gained attention due to their chemical tunability, structural stability, and ability to host selective binding sites [15].

In this context, the present study proposes a strategy based on functionalized polymers with calix[4]resorcinarenes as adsorbent materials for the removal of Cr(VI) in aqueous media. This approach leverages the structural versatility of the polymeric matrix and the unique interaction capabilities of calix[4]resorcinarenes, offering a promising and innovative alternative for environmental remediation.

For the quantitative monitoring of the dichromate in solution, the UV-visible spectrophotometry technique is used [16], due to its sensitivity, operational simplicity, and ability to make direct measurements without the need for complex pretreatments.

The polymeric support used in this investigation is poly(butyl methacrylate) (PBMA), selected because of its physical properties appropriate for adsorption processes, such as good adhesion, elasticity, and mechanical resistance [17]. These characteristics have favored its incorporation in industrial and scientific applications, especially as a modifier in polymeric mixtures.

The use of polymers as adsorbent surfaces has gained prominence in recent decades due to their ability to efficiently eliminate heavy metals from [18]. The adsorption efficiency in these materials is determined by various factors, including the molecular weight, the vitreous transition temperature (Tg), the chemical structure (homopolymer or copolymer) [12,14], the surface area, the functional groups present, and the accessible active sites. In this context, the development of efficient, selective and low-cost adsorbent materials has become the first-order line in research. Thus, the use of polyphenolic macrocycles such as calix[4]resorcinarenes has become widespread, due to their structural properties and their capacity to interact with various chemical species [15].

The resorcinarenes are macromolecules composed of an equal number of resorcinol and aldehyde units, organized in a cyclic structure of great stability and functional versatility. Their polyphenolic nature, with multiple hydroxyl groups (-OH) bound to aromatic rings, provides them a high capacity to form hydrogen bridges, as well as to participate in anion–π and cation–π interactions and coordinate with metal ions. These characteristics make them especially useful in adsorption applications and molecular recognition and as central elements in supramolecular assemblies. A property that stands out for these macrocycles is their ability to act as hosts, which makes them interesting, useful platforms to capture specific pollutants, such as hexavalent chrome ions, in aqueous media. Several studies have shown that their efficiency can be significantly increased through chemical modifications, aimed at introducing functional groups that favor selective interaction with certain ionic species. These modifications include, for example, the incorporation of sulfonate or sulfomethyl groups, which not only improve affinity towards anions such as dichromate but also increase the solubility and affinity of the system in polymeric matrices.

Continuing with our study of the uses of calix[4]resorcinarenes [16], the present investigation focuses on evaluating the adsorption capacity of the dichromate ion by PBMA modified with resorcinarene, exploring both its structural properties and its implications in the remediation of contaminated water.

2. Materials and Methods

IR spectra were recorded using a Thermo Fisher Scientific iD1 Nicolet iS5 IR spectrometer with a zinc selenide (ZnSe) ATR accessory, and frequencies are expressed in cm⁻¹ (Thermo Scientific, Waltham, MA, USA). Nuclear magnetic resonance (NMR) spectra were recorded using a BRUKER Avance 400 spectrometer with an operating frequency of 400.131 MHz. Similarly, carbon-13 nuclear magnetic resonance (¹³C-NMR) spectra were recorded on a BRUKER Avance 400 spectrometer, operating at 100.263 MHz for the ¹³C nucleus.

The reagents and solvents were purchased from Merck (Darmstadt, Germany). The reagents used were resorcinol, aldehydes (octanal and dodecanal), hydrochloric acid, sodium sulfite, potassium dichromate, silica gel, 1,1′-azobiscyclohexane (carbonitrile), benzoyl peroxide, butyl methacrylate (BMA), toluene, sodium hydroxide, formic acid, and acetic acid. In addition, the following solvents were used: dimethyl sulfoxide, chloroform, acetonitrile, benzene, methanol, ethanol, and isopropyl alcohol. Milli Q water (κ = 5.6 × 10⁻⁸ S·cm⁻¹) was used as a solvent in the extraction process. For the nuclear magnetic resonance studies, the deuterated solvents dimethyl sulfoxide (DMSO d₆) and chloroform (CDCl₃) were used. All reagents used were of analytical grade and were used without further purification.

2.1. Synthesis and Characterization of Resorcinarenes

2.1.1. Synthesis of Resorcinarenes

For the synthesis of resorcinarenes, x mmol of resorcinol was dissolved in 50 mL of absolute ethanol in a round-bottom flask under constant stirring. 4 mL of 37% HCl was subsequently added to the mixture. In parallel, x mmol of the corresponding aldehyde (octanal or dodecanal) was dissolved in 10 mL of absolute ethanol, and this solution was added dropwise to the previously prepared mixture. The reaction was then refluxed for 6 h at the boiling point of the mixture.

After this, the compound was cooled to room temperature. The solid formed was precipitated by adding deionized water, gravity filtered, and washed appropriately, in order to optimize the yield. Finally, the product was dried in an oven at a temperature below the degradation temperature of resorcinarene until constant weight was reached. The synthesis of resorcinarenes was carried out using an adapted version of the protocol described previously [19].

2.1.2. Synthesis of the Sulfometilated Resorcinarenes

The synthesis of sulfonated resorcinarenes was carried out according to what was reported in the literature [19,20]. In this way, x mmol of each resorcinarene (1 or 2) was dissolved in 50 mL of absolute ethanol in a round-bottom flask, forming Solution A. In parallel, Solution B was prepared by dissolving x mmol of Na₂SO₃ in water and adding 0.4 mL of 37% formaldehyde. Once ready, Solution B was slowly added to Solution A, with constant stirring. The resulting mixture was refluxed at 70 °C for 4 h. After this time, it was neutralized with diluted HCl and refrigerated for 14 h to promote precipitation of the product. It was then filtered and dried until the final sample was obtained.

Table 2. Spectroscopic characterization summary for (1), (2), (3) and (4).

		(1)	(2)	(3)	(4)
RI (ATR-ZnSe /cm−1)	(O–H)	3205	3206	3334	3393
	(ArC-H),	3050	3050	3020	3020
	(Aliph. C-H)	2923–2863	2920–2850	2923–2852	2919–2850
	C=C	1618–1446	1617–1452	1610–1472	1608–1469
	C-C	1170	1172	1142	1144
	C-O	1083	1088	1038	1039
	O=S=O	---	---	1308–1177	1307–1182
	C-S	---	---	776	770
1H NMR (ppm)	H1 [8H, OH]	8.86	9.56	9.71	9.72
	H3 [4H, m. OH]	7.11	7.23	7,31	7.23
	H4 [4H, o. OH]	6,14	6.13	---	---
	H2 [4H, CH]	4.22	4.32	4.21	4.20
	H5 [8H, CH2]	---	---	3.86	3.84
	R	1.98, 1.22, 1.18, 0.84	2.23, 1.35, 1.29, 0.90	2.24, 1.24, 1.25, 0.87	2.18, 1.33, 1.25, 0.86
13C NMR (ppm)	(1) C (1–12)	152.2, 125.3, 123.4, 102.8, 34.6, 33.4, 31.8, 29.7, 29.3, 28.2, 22.5, 14.3
	(2) C (1–16)	150.4, 124.9, 108.1, 103.0, 33.3, 31.9, 29.7, 29.4, 28.1, 22.7, 22.6, 14.3
	(3) C (1–13)	150.5, 125.2, 123.3, 109.5, 48.7, 34.6, 34.1, 31.9, 29.9, 28.9, 22.6, 19.9, 14.4
	(4) C (1–17)	150.5, 125.1, 109.6, 99.6, 48.4, 40.5, 40.3, 40.1, 39.9, 39.7, 39.5, 39.3, 31.9, 29.7, 29.3, 22.6, 14.4

shows the spectrometric results obtained for (1), (2), (3) and (4).

2.2. Impregnation of PBMA

For the polymer impregnation process with resorcinarenes, x mmol of C-tetra(alkyl)calix[4]resorcinarene was dissolved in 10 mL of ethanol. Subsequently, x/2 mmol of PBMA was added. The mixture was left under magnetic stirring at 700 rpm for 24 h to promote the interaction between the two compounds. The process was carried out at room temperature.

After 24 h, the solution was removed from the stirring plate and subjected to a sample filtration process. The product obtained was dried in an oven at a temperature below 50 °C to prevent thermal degradation of the resorcinarene. This protocol used for impregnation was adapted from the method reported by previous studies [18]. Finally, the modified polymers poly(butyl methacrylate) + C-tetra(heptyl)calix[4]resorcinarene (5), poly(butyl methacrylate) + C-tetra(undecyl)calix[4]resorcinarene (6), poly(butyl methacrylate) + C-tetra(heptyl)tetrasulfomethylcalix[4]resorcinarene (7), poly(butyl methacrylate) + C-tetra(undecyl) tetrasulfomethylcalix[4]resorcinarene (8) were obtained, which were characterized via IR-ATR, and gravimetric analysis was performed.

Table 3. Infrared spectroscopy results obtained for (5), (6), (7) and (8).

		PMBA	(5)	(6)	(7)	(8)
RI (ATR-ZnSe /cm−1)	(O–H)	---	3326	3320	3427	3320
	(ArC-H),	---	3020	3020	3020	3030
	(Aliph. C-H)	2956–2872	2926–2855	2919–2851	2956–2871	2956–2872
	C=C	---	1618–1494	1608–1468	1618–1445	1633–1465
	C=O	1722	1725	1722	1721	1721
	C-C	1238	1242	1229	1238	1239
	C-O	1142	1153	1144	1143	1142
	O=S=O	---	---	---	1300–1170	1380–1180
	C-S	---	---	---	746	747

shows the infrared spectroscopy results obtained for (5), (6), (7) and (8).

2.3. Dichromate Calibration Curve

Aliquots were prepared from a stock solution of K₂Cr₂O₇ [8.4 × 10⁻⁴ M]. Volumes of 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, 4.5 mL, and 5 mL were then diluted with distilled water to a final volume of 10 mL Each of these solutions was subsequently analyzed in a UV-Vis spectrophotometer, measuring its absorbance in a reading cell to obtain the relationship between the dichromate concentration and its spectral signal.

Figure 2 presents the calibration curve for the dichromate ion at a wavelength of λ = 350 nm, performed at five different concentrations. The calibration curve at 350 nm was constructed to establish the linear relationship between Cr(VI) concentration and absorbance. The resulting regression equation (y = 857.23x + 0.0999) with a coefficient of determination of R² = 0.9985 indicates excellent linearity and reliability of the method within the concentration range studied. This high R² value ensures that the calibration model can be confidently used to determine unknown concentrations with minimal uncertainty. A significantly lower R² would suggest poor correlation between variables, reducing the accuracy and robustness of the analytical method.

This confirms that the system complies with the assumptions of the Beer–Lambert law in the selected concentration range.

3. Results and Discussion

3.1. Synthesis of C-Tetra(alkyl)calix[4]resorcinarene

The calix[4]resorcinarenes were obtained through an acid-catalyzed cyclocondensation reaction between resorcinol and an aliphatic aldehyde (octanal or dodecanal) under controlled conditions of time, temperature and acidity. A set of specific conditions allowed the formation of the crown conformer (Scheme 1). The products thus obtained were characterized via IR, ¹H-NMR, and ¹³C-NMR, which allowed confirming the obtention of resorcinarenes 1 and 2 in crown conformation. As an example, the spectroscopic description of 1 is shown below.

The ¹H-NMR spectrum (Figure 3) shows a signal at 8.86 ppm attributable to the protons of the hydroxyl groups (–OH). In the aromatic region, signals are observed at 7.11 ppm and 6.14 ppm corresponding to the hydrogen in ortho and meta positions with respect to the –OH. A key signal for the structure of resorcinarene appears at 4.21 ppm, assigned to the protons of the methine bridges. The high-field signals correspond to the protons of the aliphatic chain, thus completing the structural characterization of resorcinarene. The crown conformation is confirmed by the signal pattern of protons A, B, and C, which show a single singlet signal for these protons. The twelve signals observed in the ¹³C-NMR spectrum confirm the assigned structure. For resorcinarene 2, the NMR signals confirm that the macrocycle is obtained in crown conformation.

3.2. Sulfomethylation of Resorcinarenes

Resorcinarenes are characterized by their great chemical versatility, as they can be modified at both the upper and lower rims, providing specific functionality and selectivity for different applications. At the upper rim, one functionalization approach involves the introduction of substituents through electrophilic aromatic substitution in the ortho position to the hydroxyl groups (-OH). The sulfomethylation of resorcinarenes is a reaction that allows the introduction of sulfomethyl groups (-CH₂SO₃⁻) at these positions on the aromatic ring (Scheme 2). This modification is key, because it not only enhances the molecule’s solubility but also enlarges the resorcinarene cavity, optimizing its ability to interact with ionic species and compatibility with aqueous media, making it the most suitable alternative for the design of adsorbent materials for the remediation of contaminants in water.

In this context, the sulfomethylation reaction of resorcinarenes 1 and 2 was carried out using the previously reported conditions [19]. Once the sulfomethylation reaction had been carried out, and after carrying out the respective purification processes, the products obtained were characterized using the spectroscopic methods mentioned in the experimental section. Initially, in the case of product 3, the IR spectrum shows the appearance of signals at 1376 cm⁻¹ and 1177 cm⁻¹, which confirms the presence of the sulfonate group.

In the ¹H NMR spectrum of compound 3 (Figure 4), significant differences can be seen compared to the base resorcinarene 1. In the aromatic region, a single signal was detected at 7.31 ppm (proton B in Figure 4), indicating that substitution of the hydrogen at the carbon ortho to the hydroxyl groups was successful. Additionally, a new signal appears at 3.86 ppm (proton E in Figure 4), which is assigned to the hydrogens of the sulfomethyl group (–CH₂SO₃⁻). Furthermore, the presence of singlets confirms that the crown-type structure was preserved. The ¹³C-NMR spectrum confirmed the proposed structure, since a total of 13 signals were observed, of which the signal at 48.7 ppm stands out, corresponding to the methylene bridge between the sulfonate group and the aromatic ring. By performing a similar analysis, it was possible to establish the structure of 4, which is also characterized by presenting the crown-type conformation.

3.3. Obtention of Poly(butyl methacrylate) and Impregnation with Resorcinarenes

The free-radical polymerization of PBMA, which occurs in three stages, initiation, propagation, and termination, is a crucial phase in the process of obtaining the base polymeric support that will subsequently be impregnated with resorcinarenes. The relationship between these two macromolecules is key, since the polymer’s structural characteristics, such as its hydrophobic nature, thermal stability, and the presence of carbonyl (-C=O) and methyl (-CH₃) functional groups, as well as a moderately long chain, directly influence its ability to retain organic molecules such as calix[4]resorcinarenes. Poly(butyl methacrylate) (PBMA) (Figure 5) was selected for this investigation due to its favorable physical properties compared to other methacrylates such as poly(methyl methacrylate) (PMMA) [21]. The low polarity of its alkyl chain contributes to the stability of the polymer matrix in the working medium, preventing its dissolution or degradation. Furthermore, its tendency to form highly porous materials improves interaction and diffusion processes, since it provides a greater surface area, which optimizes mass transfer and promotes more efficient anchoring between the analyte (dichromate ion) and the stationary phase [17]. Likewise, its lower glass transition temperature, compared with poly(methyl methacrylate), allows for room-temperature processes without major complications.

As shown in the experimental section, the impregnation process was carried out for a period of 24 h for the four synthesized resorcinarenes; the characterization was carried out taking into account the IR spectra of the obtained products.

Table 4. PBMA impregnation: physical fixation and characteristic bands in IR.

Polymer	O-H cm−1	C=O cm−1	μmol/g
(5)	3326	1725	1290
(6)	3320	1722	37
(7)	3427	1721	1114
(8)	3320	1721	882

summarizes the signals that indicate successful PBMA impregnation. They are the O-H stretching vibrations from resorcinarene and the carbonyl signals characteristic of the polymer. On the other hand, the gravimetric analysis allowed observing the degree of fixation in each case.

3.4. Determination of Chromium (VI) Removal Conditions

Proper selection of dichromate anion removal conditions is essential in order to prioritize those with the greatest impact on experimental results. Therefore, to make informed decisions for the design of the tests, a Pareto chart (Figure 6) was constructed to visualize which conditions are most frequently used and therefore are considered most relevant in similar studies. Based on the review of related literature, the main conditions used in processes for removing contaminants from water were identified, as well as their frequency of occurrence. Based on this information, the working conditions to be evaluated in the present investigation were established (

Table 5. Frequency table of experimental conditions affecting Cr(VI) removal efficiency.

pH	8	38.10%	38.10%
Time	6	28.57%	66.67%
Concentration	4	19.05%	85.72%
Temperature	2	9.52%	95.24%
Load volume	1	4.76%	100.00%
	21	100.00%

).

The conditions studied and the literature reviewed were:

Time: [6,17,18,22,23,24].

pH: [6,18,24,25,26,27,28,29].

Load volume: [18].

Concentration: [17,26,27,28,29,30,31].

Temperature: [6,18,24,25,26,27,28,29].

The Pareto analysis proposed as a strategy for selecting extraction conditions shows that pH is the most significant variable, accounting for 38.10% of the total effect. This indicates that adjusting pH could have the greatest impact on process optimization. Time follows, with 28.57%, suggesting that process duration is also a relevant factor, accounting for 66.67% along with pH. Concentration ranks third, with 19.05%, adding up to a total of 85.71% when the three main factors are considered. This suggests that these three variables account for most of the effect, with temperature (9.52%) and volume (4.76%) having a lesser influence, which together account for 100% of the analyzed impact. While the Pareto chart allows identifying the most relevant factors in the extraction, the radar chart (Figure 7) provides a visual representation of their relative contribution, showing that pH is the most influential factor, followed by time and concentration.

According to the removal conditions of the target analyte identified as most significant in the Pareto analysis from the previous section, removal tests were conducted on each of the polymeric matrices (PBMA 5, 6, 7, and 8), considering the pH, contact time, and dichromate concentration.

3.5. The Dichromate Anion (Cr₂O₇²⁻) Removal Experiments in Aqueous Media

The removal experiments of the dichromate anion (Cr₂O₇²⁻) in aqueous media were performed using UV-Vis absorbance spectroscopy. For this purpose, K₂Cr₂O₇ solutions of known concentrations were prepared, and the initial and final absorbance values were measured at specific wavelengths.

The relationship (Equation (1)) allows the determination of the efficiency of the removal process.

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{A}}_{\mathrm{o}}-{\mathrm{A}}_{\mathrm{f}}}{{\mathrm{A}}_{\mathrm{o}}}}\times 100$$

(1)

where:

$${\mathrm{A}}_{\mathrm{o}}=\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}$$

$${\mathrm{A}}_{\mathrm{f}}=\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}$$

Considering the above, the main condition of the extraction process was the effect of pH. Absorbance measurements were initiated at pH values of 2.0, 3.0, 4.5, 5.0, and 5.5 and a wavelength of 350 nm. The concentration was managed at [3.35 × 10⁻⁴ M] and [1.68 × 10⁻⁴ M], as shown in

Table 6. Removal percentages at pH 2.0, 3.0, 4.5, 5.0, and 5.5.

				Matrix
pH	Time (min)	Concentration [M]	Initial Absorbance	(5) **	(6) *	(7) **	(8)	(PBMA)
				Absorbance, % Removal
2.0	60	3.36 × 10−4	0.981	0.961	---	1.071	1.507	1.020
				2.00%	---	---	---	---
3.0	60	3.36 × 10−4		0.954	---	1.052	1.114	0.986
				6.00%	---	---	---	0.030
	120	3.36 × 10−4		0.999	---	1.110	1.168	0.988
				2.00%	---	---	---	0.030
4.5	30	3.36 × 10−4	1.084	1.066	---	1.071	1.097	1.066
				1.66%	---	1.20%	---	1.66%
	60	3.36 × 10−4		1.035	---	1.088	1.083	1.034
				4.52%	---	---	0.09%	4.61%
	90	3.36 × 10−4		1.043	---	1.107	1.127	1.054
				3.78%		---	---	2.77%
	120	3.36 × 10−4		1.041	---	1.115	1.149	1.045
				3.97%	---	---	---	3.60%
	150	3.36 × 10−4		1.042		1.128	1.124	1.055
				3.87%	---	---	---	2.68%
5.0	60	1.64 × 10−4	0.111	---	---	---	0.021	0.079
							81.1%	28.8%
5.5	60		0.113	---	---	---	0.111	0.078
							1.8%	30.9%
5.0	120	3.36 × 10−4	0.173	---	---	---	0.091	0.161
							47.4%	6.9%
5.5	120		0.180	---	---	---	0.133	0.163
							23.1%	5.8%

* Matrix 6 was not included in the extraction processes due to filtration problems, resulting in a very low µmol/g ratio; however, its impregnation was confirmed by IR analysis (see Table 3). ** On the other hand, matrices 5 and 7 were not subjected to the extraction processes at pH 5.0 and 5.5, due to solubility problems, overcome by matrix 8 with a longer aliphatic chain.

.

The results at acidic pH (2.0–4.5) showed that the percentage removal values for the ion under study were nonexistent or very low, with the highest value recorded being 6% for matrix (5). This behavior can be explained by the fact that potassium dichromate, in which chromium exists as Cr(VI), is a strong oxidizing agent capable of accepting electrons and being reduced to Cr(III) [26,29]. This redox process can influence the interaction with resorcinarenes, as the hydroxyl groups (-OH) present in their phenolic structure are susceptible to oxidation under acidic conditions.

Therefore, under these conditions, the stability of resorcinarenes may be compromised due to the oxidation of their phenolic groups by dichromate. Phenols can be transformed into quinones, altering the resorcinarene’s structure and negatively affecting its adsorption capacity. This process can be represented by the following general equation:

$$\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{c}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{e}+{\mathrm{C}\mathrm{r}}_2{{\mathrm{O}}_7}^{2-}+14{\mathrm{H}}^{+}\to \mathrm{R}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{n}\mathrm{e}+2{\mathrm{C}\mathrm{r}}^{3+}+7{\mathrm{H}}_2\mathrm{O}$$

(2)

Consequently, if a partial or complete reduction in dichromate occurs on the material’s surface, the efficiency of the removal process may be compromised. Based on this sample, it can be inferred that dichromate adsorption is being limited and likely depends on weak interactions such as Van der Waals forces or hydrogen bonds between the hydroxyl groups of the resorcinarene and the oxygen atoms of the dichromate. Once the effect of pH (2.0–4.5) on the extraction process was established and extraction processes at very acidic pH values discarded, attention was focused on the pH range of 5.0 to 5.5. Under these pH conditions, the best result was obtained for the sorbent (8) at pH 5.0 (See Table 4), while the other sorbents showed significantly low extraction values.

The next parameters to be evaluated were the time and concentration of the dichromate. In this phase, measurements were taken at the same wavelength (λ = 350). First, Table 4 shows the effect of time on the extraction process. The results show significant differences between the materials. While the unmodified polymer (PBMA) showed virtually no adsorption, sorbent (8) retained a considerable percentage of Cr(VI), particularly at pH 5.0 and with a 1-h interaction time. On the other hand, sorbents 5 and 7 showed very low extraction values, and sorbent 6 presented turbidity in the solution, suggesting a desorption process of the short-chain sulfonated resorcinarene. Based on these results, sorbents 5, 6, and 7 were discarded for the dichromate extraction process.

The best result was obtained with material (8) at pH 5.0 and a concentration of 1.68 × 10⁻⁴ mol/L, achieving a removal efficiency of 81.1% at a wavelength of λ = 350 nm. This value was reached after just one hour of contact, highlighting the importance of the time variable in the adsorption process. Finally, the effect of the dichromate ion concentration on the extraction process with sorbent 8 was evaluated. In this way, at the same pH, but with a higher concentration (3.36 × 10⁻⁴ mol/L), the removal was lower, though still relevant, with a maximum of 47.4% at 350 nm. This is likely because at pH 5.0, the functional groups on material (4) are more protonated, which promotes electrostatic interactions with anionic species such as Cr₂O₇²⁻. Additionally, anion–π interactions are expected, given the electron-rich aromatic rings of resorcinarenes. By contrast, the base polymer (PBMA) shows little to no affinity for Cr(VI), confirming that the observed adsorption is attributable to the modification with (PBMA). Therefore, the functionalization of the material successfully introduces active sites capable of interacting with the pollutant.

For a better visualization of the data of the extraction processes between sorbent 8 and PBMA, the adsorption capacity (qe) value was determined using a standard equation commonly applied to quantify the adsorption of a target analyte (adsorbate) onto a specific material (adsorbent) [14,27].

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{(\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}).\mathrm{V}}{\mathrm{m}}}$$

(3)

where:

• ${\mathrm{q}}_{\mathrm{e}}=\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{o}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}(\mathrm{m}\mathrm{g}/\mathrm{g})$
• ${\mathrm{C}}_{\mathrm{i}}=\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{m}\mathrm{g}/\mathrm{L})$
• ${\mathrm{C}}_{\mathrm{f}}=\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\left(\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{b}\mathrm{r}\mathrm{i}\mathrm{u}\mathrm{m}\right)\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{m}\mathrm{g}/\mathrm{L})$
• $\mathrm{V}=\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{L}\right)$
• $\mathrm{m}=\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{t}\left(\mathrm{g}\right)$

The calculation was performed for pH 5.0 and 5.5, at the wavelength studied (λ = 350 nm) and with initial concentrations of 1.68 × 10⁻⁴ mol/L and 3.36 × 10⁻⁴ mol/L (10:25 dilution) of dichromate. The volume used in all cases was V = 0.025 L. The masses of the polymeric material were close to 50 mg in all cases.

The Lambert–Beer concept of direct proportionality was used:

$${\displaystyle \frac{{\mathrm{C}}_{\mathrm{f}}}{{\mathrm{C}}_{\mathrm{i}}}}={\displaystyle \frac{{\mathrm{A}}_{\mathrm{F}}}{{\mathrm{A}}_{\mathrm{I}}}}∆\mathrm{C}={\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}$$

(4)

• ${\mathrm{C}}_{\mathrm{i}}=\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{m}\mathrm{g}/\mathrm{L})$
• ${\mathrm{C}}_{\mathrm{f}}=\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\left(\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{b}\mathrm{r}\mathrm{i}\mathrm{u}\mathrm{m}\right)\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{m}\mathrm{g}/\mathrm{L})$
• ${\mathrm{A}}_{\mathrm{I}}=\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}$
• ${\mathrm{A}}_{\mathrm{F}}=\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}$

And for the calculations, the molecular mass of K₂Cr₂O₇ ≈ 294.19 g/mol was required. In this way, all the adsorbed quantities were found and are illustrated in Figure 8.

This result can be attributed to the crown-like structure of the resorcinarene [31], which possesses a characteristic cavity and functional groups capable of generating specific interactions with the dichromate anion, such as anion–π interactions, hydrogen bonding, and electrostatic attraction.

Likewise, when comparing the removal percentages (Figure 9), the superior efficiency of the functionalized polymeric material over the unmodified polymer is clear. Material (8) achieves a Cr(VI) removal rate close to 81% within 1 h, whereas the unfunctionalized PBMA reaches only 28.8% under the same conditions. This behavior can be attributed to the direct relationship between the initial concentration of the adsorbate and the adsorption capacity of the material [32]. Additionally, this result highlights the importance of distinguishing between removal efficiency and adsorption capacity, since the former refers to the percentage removed from the medium, while the latter considers the amount retained per unit mass of the adsorbent.

Regarding the effect of contact time, it was observed that 1 h was sufficient to achieve high removal under optimal conditions (81.1% removal with material (8), at pH 5.0 and low concentration), which indicates a rapid adsorption process facilitated by the active sites of the resorcinarene. However, in other cases, extending the contact time presented very similar results.

Among the polymers studied, the functionalized (8) polymer, including the base PBMA, proved to be the most efficient for the removal of the Cr₂O₇²⁻ anion at pH 5.0, with an initial concentration of 1.68 × 10⁻⁴ mol/L and a contact time of 1 h. This behavior supports the hypothesis that resorcinarene functionalization improves the polymer’s affinity for dichromate, possibly by introducing additional interaction mechanisms, such as hydrogen bonding, anion–π interactions, or complex formation (Figure 10). These mechanisms are facilitated by the structure of sulfomethylated resorcinarene (4), whose –CH₂SO₃⁻ groups contribute to increased polarity and affinity toward the dichromate anion. These findings highlight the potential of macrocyclic structures to enhance selective adsorption processes. Overall, the observed improvement in adsorption capacity indicates the relevance of chemical functionalization strategies for the design of efficient anion removal materials.

Although this study focused on Cr(VI) removal using dichromate as a model anion, the improved performance of matrix (8) over unmodified PBMA suggests a degree of selectivity. This selectivity can be attributed to the structural and chemical features introduced by the sulfomethylated resorcinarene, which promotes specific interactions such as hydrogen bonding, electrostatic attraction, and anion–π interactions with dichromate. The size, charge distribution, and geometry of Cr₂O₇²⁻ may favor its interaction over other anions commonly present in aqueous media. In a preliminary test, with sulfate, nitrate or phosphate ions, no appreciable changes in the absorbances of the dichromate ion were observed.

4. Conclusions

The functionalization of PBMA with sulfomethylated resorcinarene (4) significantly improved the material’s capacity to adsorb Cr(VI) in aqueous solution, especially at pH 5.0. The optimal condition for dichromate removal was observed at pH 5.0, with a contact time of 1 h and a Cr(VI) concentration of 1.68 × 10⁻⁴ mol/L, under which the functionalized material achieved a removal efficiency of up to 81.1%. Although lower concentrations resulted in higher removal percentages, the adsorbed amount (q_e) increased with higher initial concentrations, confirming a direct relationship between adsorbate concentration and adsorption capacity.

The modified polymer (8) consistently outperformed the unmodified PBMA, both in terms of removal percentage and adsorption capacity, particularly at 350 nm, where the greatest sensitivity and maximum qe values were observed. These results support the hypothesis that the resorcinarene structure, with its crown-like cavity and polar functional groups, enhances dichromate retention through multiple interaction mechanisms, including electrostatic attraction, hydrogen bonding, and anion–π interactions. The incorporation of sulfomethyl groups (-CH₂SO₃⁻) likely increases the material’s polarity and interaction affinity, making (8) a promising candidate for the removal of anionic pollutants such as Cr₂O₇²⁻ from water.

Compared to traditional techniques such as chemical precipitation, ion exchange, and adsorption with activated carbon, this approach offers several advantages: it operates under mild conditions, does not require expensive infrastructure or toxic reagents, and allows for structural tailoring of the adsorbent material. Therefore, this method represents a sustainable and effective alternative for water treatment applications, particularly in contexts with limited resources.