Recognition of Heavy Metals by Using Resorcin[4]arenes Soluble in Water

Abstract

The complexing properties of two water-soluble resorcin[4]arenes (tetrasodium 5,11,17,23-tetrakissulfonatemethylen 2,8,14,20-tetra(butyl)resorcin[4]arene, Na₄BRA, and tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(2-(methylthio)ethyl)resorcin[4]arene, Na₄SRA) with polluting heavy metals such as Cu²⁺, Pb²⁺, Cd²⁺ and Hg²⁺ were studied by conductivity, and the findings were confirmed by using other techniques to try to apply this knowledge to removing them. The results indicate that Na₄BRA is able to complex Cu²⁺ in a 1:1 ratio and Pb²⁺ in a 1:2 ratio, while Na₄SRA complexes Hg²⁺ in a 1:1 ratio. On the contrary, no indications have been observed that either of the resorcin[4]arenes studied complexes the Cd²⁺ ions. The results suggest that the bonds established between the sulfur atoms located at the lower edge of the SRA⁴⁻ and the solvent hydrogens could prevent the entry of the guest into the host cavity. However, in the case of Hg²⁺ ions, the entry is favoured by the interactions between the sulfur donor atoms present on the lower edge of Na₄SRA and the Hg²⁺ ions. Therefore, it can be said that Na₄BRA is selective for Cu²⁺ and Pb²⁺ ions and Na₄SRA is selective for Hg²⁺ ions.

1. Introduction

Soil contamination by heavy metals poses a great risk to human health, animals, plants, microorganisms and their interactions [1]. In humans, for example, prolonged exposure to contaminated soils has negative effects on the central nervous system, the gastric system and the respiratory system [2,3]. Additionally, heavy metals can cause great ecological risk when they are absorbed by different aquatic organisms, thus entering the food chain [4]. They also affect the functioning of soil enzymes and microbial biomass (affecting microorganisms is crucial since they play an important role in the cycle of nutrients and in the decomposition of matter), hindering their growth and consequently degrading the quality of the soil. Examples of the above are those produced by Pb, which has effects on bacteria, and by Cd, which impacts the fungal population [5]. The fate and effect of these pollutants depend on the geographical and environmental conditions, the nature of the soil and the type of human activities, including the use of agrochemicals [6,7], mining [8,9,10,11] and industrial activities [12,13].

Aware of the problem, this research seeks to show progress in the recognition of certain polluting heavy metals in solution, such as Cu²⁺, Pb²⁺, Cd²⁺ and Hg²⁺, using complexing macrocycles such as the resorcin[4]arenes, to try to apply it in their removal. Although, when Cu is present in the human body in amounts that do not exceed 75–100 mg, it is vital for health, when the concentration of this metal is higher, various disorders occur such as nausea, vomiting, abdominal pain and cramps, headache, dizziness, weakness and diarrhea, among others; such disorders occur mainly in the case of individuals with liver diseases and other pathologies in which the excretion of bile is compromised [14]. The effect of lead (Pb) on humans is also important, since it acts on the calcium and potassium channels in the cell membrane, affecting coordinated cellular functioning and giving rise to neuropsychological disorders [13,15]. Likewise, cadmium (Cd) is classified within the first group of carcinogens by the International Agency for Research on Cancer (IARC) as a highly toxic metal [16,17]. It also causes decreased bone density and kidney damage [18]. Finally, mercury is a toxic metallic element that affects human health, wildlife, aquatic ecosystems (severe and persistent toxicity has been documented from the contaminated fish consumption from fresh and marine waters [4]) and, in general, soils, water and air [19,20,21,22,23,24].

Resorcin[4]arenes (Figure 1) and their functionalized derivatives have been widely used for complexation studies. Both the selectivity of these ligands towards a given guest [25,26,27,28] and the stability of the complexes formed are determined by factors such as geometric complementarity, the types of interactions [25,29] and the nature of the substituents in the macrocycle [30,31,32,33]. The stability of the complex is determined by the magnitude of the complex formation constant, β_ƒ [34,35]. Information about host-guest interactions can be derived from its value. The macrocycles evaluated were tetrasodium 5,11,17,23-tetrakissulfonatemethylen 2,8,14,20-tetra(butyl)resorcin[4]arene (Na₄BRA) and tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(2-(methylthio)ethyl) resorcin[4]arene (Na₄SRA), whose structures are shown in Figure 2.

There are several simple experimental techniques that can be used to determine the stoichiometry and stability constants of complex species. Among the most widely used are conductometry, sonometry, ion-selective potentiometry (ISE), acid-base potentiometry, nuclear magnetic resonance (NMR), UV-VIS spectroscopy and isothermal titration calorimetry (ITC). In the present case, the techniques used to study the complexation of Na₄BRA and Na₄SRA with the heavy metals of interest are shown in

Table 1. Techniques used to study the complexation of heavy metals with Na₄BRA and Na₄SRA.

Host	Cu2+	Pb2+	Cd2+	Hg2+
Na4BRA	Conductometry Ion-selective potentiometry (ISE)	Conductometry Atomic absorption	·Conductometry	Conductometry
Na4SRA	Conductometry	Conductometry	·Conductometry	Conductometry Atomic absorption

.

The process of recognition and evaluation of the selectivity of the resorcin[4]arenes was carried out in water by using the conductivity technique, and the positive findings were confirmed by other techniques such as ion-selective potentiometry (ISE) and atomic absorption. The results are discussed in terms of the structure and three-dimensional arrangement of each macrocycle.

2. Materials and Methods

2.1. Materials

Na₄BRA and Na₄SRA were synthesized, purified and characterized according to the procedures described by Sanabria et al. [36]. Heavy metal perchlorates used as guests were used as supplied without further purification; they were stored in amber flasks and dried in vacuo over activated silica gel; their purities are shown in

Table 2. Heavy metal salts used for the complexation study with Na₄BRA and Na₄SRA.

Substance	Source	Purity
Cu(ClO4)2·6H2O	Alfa Aesar	>98%
Pb(ClO4)2·3H2O	Alfa Aesar	97% min.
Cd(ClO4)2·6H2O	Alfa Aesar	>99%
Hg(ClO4)2·3H2O	Alfa Aesar	>99%

. Water used to prepare the solutions for the conductivity complexation, potentiometry and atomic absorption tests was obtained from a Milli-Q purifier, and it was degassed before use; its conductivity was always <0.1 μS·cm⁻¹. The solutions were prepared by weight and corrected to vacuo using an OHAUS Analytical Plus balance (AP250D) (Ohaus Corporation, Florham Park, NJ, USA) that has an accuracy of 1 × 10⁻⁵ g in the range of 80 g and 1 × 10⁻⁴ g in the range from 80 g up to 250 g.

2.2. Equipments and Experimental Techniques

2.2.1. Conductometry

Conductometry has been commonly used in complexation studies between macrocycle ligands and different types of ions [37,38]. The stability constants found by conductometry have been reported in the literature for various macrocycles, among which are: α and β-cyclodextrins [39,40], alkylcalix[4]arenes [41] and crown ethers [41,42].

Conductivity measurements were carried out using a VEB hydromat Bannewitz LM 3000 cell (Bannewitz, Germany) with a cell constant of K_cell = 1.04 cm⁻¹. It is made of borosilicate glass and provided with two platinum electrodes. The cell was placed inside a 120 mL jacketed glass vessel and kept at a constant temperature of (298.15 ± 0.01) K, by circulating water through the outer jacket, using a Julabo LC6 temperature controller (Julabo Labortechnick, Seelbach, Germany). The cell was carefully washed, purged with deionized water and dried at 105 °C before use. To avoid the presence of CO₂, nitrogen gas was passed through the cell before performing each determination. Resistance measurements were carried out with a Stanford SR720 (US) LCR meter (Stanford Research Systems, Sunnyvale, CA, USA) whose accuracy is 0.05%. A voltage of 1.0 V, at a frequency of 1 kHz, was used for all measurements. In a typical complex formation experiment, 40 mL of approximately 1 × 10⁻³ M Na₄RA aqueous solution were placed into a titration cell and thermostated at 298.15 K. Once the solution reached thermal equilibrium, the resistance of the solution was measured with the LCR meter, and then it was titrated with a ten-times more concentrated guest solution (approximately 1 × 10⁻² M). This titration solution was added in 0.1 mL portions, using a metrohm (Metrohm Ltd., Herisau, Switzerland) digital burette, until a ratio of 5/1 (guest/host) was reached. After each addition, the solution was stirred for 20 s, allowed to stand for another 20 s, and its resistance was recorded. The above procedure was carried out with all the proposed metals. A blank was run for each experiment using water instead of the Na₄RA solution [25].

2.2.2. Ion-Selective Potentiometry (ISE)

Ion-selective electrodes have been successfully used for the determination of complex stability constants [43]. The general procedure consists of carrying out titration, at a constant ionic strength of a solution of the guest with the host, while measuring the potential. The formation of 1:1 complexes of Na₄RA with a M²⁺ cation can be represented by the equilibrium:

H₈L⁴⁻ + M²⁺ ↔ H₈LM²⁻

(1)

whose stability constant, at pH 7, can be obtained using the hyperquad software (Wiley Organics, Inc., Coshocton, OH, USA) [44].

Potentiometric titration was carried out only with the Na₄BRA-Cu²⁺ system. For this, a Cu²⁺ ion-selective electrode (Cu-ISE, Orion model 94-29, (ThermoFisher Scientific, Waltham, MA, USA) filled with 0.1 N potassium nitrate solution) was used. As in the case of conductivity titrations, it was carried out in a 120 mL jacketed glass cell, and the temperature was kept constant at 298.15 K by circulating water, through the said jacket, from a bath equipped with a Julabo LC6 thermostat (Julabo Labortechnick, Seelbach, Germany), that ensures a temperature control of ±0.001 degrees. The calibration curve was prepared with solutions of 0, 1, 10, 100, 300, 600, 800 and 1000 ppm and the ionic strength was controlled with 0.01M NaClO₄. For the sample analysis, 20 mL of 2.9 × 10⁻⁴ M Cu(ClO₄)₂ solution were placed inside the titration vessel thermostated at 298.15 K; once equilibrium was reached, the sample was titrated using a Metrohm burette (Metrohm Ltd., Herisau, Switzerland) with an 8.3 × 10⁻⁴ M Na₄BRA solution [25]. The sample voltage was recorded after each addition, and subsequently, the value obtained was interpolated in the calibration curve to determine the Cu²⁺ concentration.

2.2.3. Atomic Absorption

This is not a technique used to find the formation constant of a complex, but it allows to determine in a simple, fast and selective way the percentage of metal present in a complex and thus verify the stoichiometric ratio determined by other techniques.

The conductometric titration of both Na₄BRA with Pb(ClO₄)₂ and Na₄SRA with Hg(ClO₄)₂ produced precipitates three hours after the end of the titration. In both cases, the precipitate was filtered, washed with plenty of water, dried and analyzed by atomic absorption. For this analysis, a Perkin Elmer Analyst 300 spectrophotometer (Perkin Elmer Ltd., Wembley, UK) with an air-acetylene flame in a 10:2 ratio, 0.70 slot, and AAwinlab software was used. The corresponding calibration curve for each metal ion was created by dilution from a concentrated solution of 1000 ppm of Me(NO₃)₂; the final concentrations obtained were: 1, 4, 8, 12, 16 and 20 ppm of Me²⁺. The complex with either Pb or Hg was digested with nitric acid and afterwards analyzed. The absorbance of the sample was determined in triplicate and the concentration was obtained by interpolation in the corresponding calibration curve [25].

3. Results and Discussion

The complexation measurements were carried out in water at 298.15 K and pH = 7. According to the species distribution diagram, under these conditions, the predominant species is H₈L⁴⁻ for both Na₄BRA and Na₄SRA [25].

3.1. Complexation of Na₄BRA with Cu²⁺

Figure 3 shows the dependence of the specific conductivity against the [Cu²⁺]/[Na₄BRA] ratio.

As it can be ascertained, a slight change in slope is observed for a [Cu²⁺]/[Na₄BRA] ratio of approximately 1.2, which could suggest the presence of a complex of 1:1 stoichiometry. Both the presence of the complex and its stoichiometry were confirmed by potentiometric titration with a copper selective electrode (Cu-ISE). The dependence between the potential and the [Cu²⁺]/[Na₄BRA] ratio is shown in Figure 4.

A value of Log β_ƒ = 5.38, at 0.01 M ionic strength was determined for the stability constant of the [BRA-Cu]²⁻ complex using the Hyperquad software [44,45]. In this sense, higher values are reported in the literature for the stability constants of complexes formed by various metals with p-sulfonatocalixarenes [46].

3.2. Complexation of Na₄BRA with Pb²⁺

Figure 5 shows the dependence of the specific conductivity against the [Pb²⁺]/[Na₄BRA] ratio.

As it can be seen, an abrupt change in slope appears at a value of 1.9 in the [Pb²⁺]/[Na₄BRA] ratio, approximately. According to Ashram [47] and Jalali and colbs. the abrupt changes in the slopes of these specific conductivity plots are related to the formation of stable complexes [41]. Therefore, this suggests the presence of a strong and stable complex between Na₄BRA and Pb²⁺, BRA-Pb, of 1:2 stoichiometry. On the other hand, since this complex precipitates approximately three hours after titration as a fine pink powder [25], it was isolated, purified and analyzed by atomic absorption as indicated in the previous section. The lead content determined in the sample was 25%, which corresponds to a 1:2 stoichiometry for the BRA-Pb complex. This result agrees perfectly with that obtained by conductometric titration and can be explained as the result of the electrostatic interactions between the four sulfonate groups of BRA⁴⁻ and two Pb⁺² cations.

3.3. Complexation of Na₄BRA with either Cd²⁺ or Hg²⁺

Figure 6 shows the dependence between the specific conductivity and the [Hg²⁺]/[Na₄BRA] and [Cd²⁺]/[Na₄BRA] ratios.

As it can be seen in both cases, the appearance of a change in slope is not observed in these plots, so the formation of Cd²⁺ or Hg²⁺ complexes with Na₄BRA must be ruled out.

3.4. Complexation of Na₄SRA with Hg²⁺

Figure 7 shows the dependence of the specific conductivity with the [Hg²⁺]/[Na₄SRA] ratio.

As it can be seen, there is a significant change in the slope of the plot for a ratio [Hg²⁺]/[Na₄SRA] equal to 1, which clearly indicates the formation of a stable complex of 1:1 stoichiometry [41,47]. As in the case of [BRA-Pb], this [SRA-Hg] complex precipitates approximately three hours after the end of the titration, appearing as a fine pale-yellow powder [25]. This precipitate was isolated, purified and analysed by atomic absorption to confirm its 1:1 stoichiometry.

The affinity for mercury of sulfur-containing compounds has already been reported by several authors [48,49]. In the case of Na₄SRA, this can be explained as a consequence of the interactions between the Hg²⁺ ions and the sulfur donor atoms present at the lower rim of the Na₄SRA.

3.5. Complexation of Na₄SRA with either Cu²⁺, Pb²⁺, or Cd²⁺

Figure 8 shows the dependence of specific conductivity against the [guest]/[Na₄SRA] ratio.

As it is ascertained, in no case a change in the slope is observed, so it must be deduced that none of the Na₄SRA-Pb²⁺, Na₄SRA-Cu²⁺ and Na₄SRA-Cd²⁺ systems show complexation.

Table 3. Stoichiometry of the complexes formed with the hosts Na₄BRA and Na₄SRA.

Host/Guest	Cu2+	Pb2+	Cd2+	Hg2+
Na4BRA	1:1	1:2	-	-
Na4SRA	-	-	-	1:1

summarizes what has been seen so far, indicating which guest metals form complexes with the hosts Na₄BRA and Na₄SRA.

According to the above table, Na₄SRA selectively complexes Hg²⁺, while Na₄BRA complexes Cu²⁺ and Pb²⁺. The fact that Na₄BRA complexes two guests and Na₄SRA only one can be explained on the basis of the greater conformational stability of Na₄BRA compared to Na₄SRA, caused by Van der Waals-type interactions between the lower edge chains of Na₄BRA, whereas in the case of Na₄SRA, conformational mobility can prevent the entry or facilitate the exit of the guest through the upper edge of the host. Additionally, due to the presence of sulfur at the lower edge of Na₄SRA, this can form bonds with hydrogen from water, which could prevent the inclusion of some guests within the host. In the case of the [SRA-Hg]²⁺ complex, this situation does not occur since mercury can interact with the sulfur donor atoms of Na₄SRA.

4. Conclusions

The complexing properties of Na₄BRA and Na₄SRA were studied by conductometry, and the positive results were confirmed by other techniques such as ion-selective potentiometry (ISE) and atomic absorption. The results indicate that Na₄BRA is able to complex Cu²⁺ in a 1:1 ratio and Pb²⁺ in a 1:2 ratio. Regarding the complex formed by Na₄SRA with Hg²⁺, the ratio found was 1:1. In this latter case, the results suggest that the interaction between the Hg²⁺ ions and the sulfur atoms present at the lower edge of the SRA⁴⁻ is greater than that established between these latter atoms and the hydrogens of the solvent (an interaction that would prevent the entry of the guest into the cavity of the SRA⁴⁻), which favors the entry of Hg²⁺ and means that the Na₄SRA are selective for these ions.