Adsorption properties of Merrifield-bCCA chelating resins: a new alternative for Pb²⁺ removal from water

Abstract

In this work, five new chelating resins (MR_nBz) functionalized with N-benzyl bis(carbamoyl)carboxylic acid molecules (BzbCCA) on their surface were prepared to study the metal ion (Mⁿ⁺) adsorption properties in water. MR_nBz resins were characterized by FTIR, TGA, FESEM and EDS. The surface charge as a function of pH and the chemical adsorption of Mⁿ⁺ on the surface were evaluated through zeta potential (ζ) measurements. The Mⁿ⁺ adsorption capacity of MR_nBz resins was evaluated using Pb²⁺, Cu²⁺, Cd²⁺, and Ni²⁺ mixture model solutions at low concentrations. MR_nBz resins displayed selective adsorption of Pb²⁺ even in the presence of a molar excess of other cations, due to the intrinsic affinity and selectivity of BzbCCA molecules for this metal ion. The adsorption isotherms of Pb²⁺ showed that the adsorption capacity of MR_nBz resins was influenced by the spacer chain length. In addition, the resins were characterized by FTIR, TGA, FESEM and EDS after the Mⁿ⁺ adsorption process, confirming the Mⁿ⁺ loading on the resin surface.

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1. Introduction

Heavy metal pollution in water has increased dramatically due to its direct association with the growth of industrial activities, such as electroplating, batteries, pesticides, mining, rayon production, metal rinsing processes, tanning, fluidized bed bioreactors, textiles, metal casting, petrochemicals, paper manufacturing and electrolysis applications.¹ This has caused serious damage to the environment and the organisms in ecosystems, as heavy metals are toxic, non-biodegradable and lead to bioaccumulation, posing a risk to human health.^2,3 Each country establishes specific regulations and national or international standards that regulate the permissible concentration limits of heavy metals in aqueous systems, such as the EPA (Environmental Protection Agency) in the USA and SEMARNAT (Secretaría del Medio Ambiente y Recursos Naturales) in Mexico.⁴ Heavy metals are considered chemical elements with high density (greater than 5 g cm⁻³), with mass and atomic weight above 20 units, and they are toxic even in low concentrations. Some of these elements include aluminum (Al), beryllium (Be), copper (Cu), iron (Fe), manganese (Mn), cadmium (Cd), mercury (Hg), and lead (Pb), among others.^5–7 In Mexico, the Official Mexican Standard NOM-001-SEMARNAT-2021 establishes the maximum permissible limits for on-site measurements of Pb²⁺ (2.0 mg L⁻¹), Cd²⁺ (1.0 mg L⁻¹), Cu²⁺ (20.0 mg L⁻¹) and Ni²⁺ (8.0 mg L⁻¹) in wastewater discharges to sewage systems. These regulations aim to prevent and control the contamination of national waters and resources, as well as protect the infrastructure of these systems.⁸

There are various processes for heavy metal treatment (physical-chemical techniques), such as chemical precipitation, coagulation–flocculation, flotation, membrane filtration (ultrafiltration, nanofiltration, reverse osmosis), ion exchange, electrochemical treatment techniques (electrodialysis, membrane electrolysis, electrochemical precipitation), adsorption and the use of biomass and microbial processes, which have proven to be of interest.⁹

Compared with other methods, adsorption processes are the most widely used due to their low operating cost, high adsorption capacities, reusability, and high efficiency in removing various contaminants.¹⁰ Among the most common functional chemical groups used to achieve adsorption properties are oxygen (alcohols, crown ethers) and nitrogen (amines, amides) donors, which have proven to be the most effective for achieving efficient adsorption.¹¹ Several natural and synthetic adsorbents have been tested for removing contaminants from aqueous media.^12–19 Chelating resins containing ionizable residues (carboxylic, phosphonic and sulfonic acids) have also proved to be very useful in the treatment of heavy metals.²⁰

Considering the increase in environmental awareness, both academia and industry have focused enormous efforts to develop more efficient and selective methods in chemical processes. The Merrifield's resin was introduced in 1963 as a solid support for peptide synthesis,²¹ but different modifications have been performed for various purposes.^22–27 The synthesis of carbamates using chelated iron supported on organic molecules has also been reported.²⁸ Various synthetic strategies were used to modulate the selectivity of reducing agents, such as varying the interconnection chain length of alkyldiamines in the reduction of imines for the formation of chelating amines.²⁹ Modified resins are useful synthetic platforms because these materials are easily recovered by filtration. Therefore, they can be reused instead of been disposed as waste.³⁰

On the other hand, the anchoring of the flavonoid morin to the Merrifield's resin gave the formation of morin-metal complexes in the removal of metals from an aqueous solution.³¹ Chemo-sorbents based on porous materials treated with amines have also gained attention recently due to their numerous basic sites that can interact with acidic SO₂.³² Furthermore, the covalent bonding of Merrifield's resin with primary, secondary and tertiary amines allowed for SO₂ adsorption at low concentration.³³ The support of chelating compounds such as heterofluorenone³⁴ and tri-pyridine aniline³⁵ on Merrifield's resin was achieved with the aim of capturing metal cations such as Cu²⁺, Pb²⁺ and Ni²⁺ from aqueous solutions. A supported dithiocarbamate has been used for the determination of Pb²⁺ in samples of contaminated aqueous effluents and soils, demonstrating the reusability capacity of the functionalized resin for up to 100 cycles.³⁶ The removal of transition metal ions was reported with supported calix[4]arene-type ligands,^37–39 showing the best removal results with Pb²⁺ and Cr⁶⁺ ions. Supported bis(azoimidazole) ligands⁴⁰ were efficient in the removal of Cu²⁺ and Ag⁺. Furthermore, supported quercetin was efficient in the removal of Pb²⁺.^41,42 Lanthanide ions such as Eu³⁺ have also been removed using supporting calix[4]arene-tetraphosphonate chelating resins.⁴³ Polymeric resins based on Merrifield's resin grafted with polyvinyltetrazol have also been used in the removal of Pb²⁺, Ni²⁺ and Cu²⁺.⁴⁴ Natural products such as chitosan have been supported in Merrifield's resin and used to remove Cu²⁺.⁴⁵ Some chelating resins with dual adsorption capability for a variety of cations were prepared, simultaneously supporting glycidylmethacrylate, 5-aminosalicylicacid and 2-(4-imidazolyl)ethylamine, and evaluated in the removal of harmful metals, such as Cu²⁺ and Pb²⁺, Fe³⁺ and Cr³⁺, as well as Cd²⁺ and Ag⁺.⁴⁶

The design of new compounds with low structural complexity for the detection and removal of metal ions has attracted attention in the area of environmental chemistry due to the biological and environmental importance of metal ions. One example is the bis(carbamoylcarboxylic) acids (bCCAs), which have been studied as chelating agents for metal ions to achieve the formation of complexes in organic and aqueous media.^47,48 Previously, it was reported by Lingaiah et al. that monotopic carbamoylcarboxylic acids form coordination complexes with metal ions such as Cu²⁺ through a metal-heteroatom ionic bond with the carboxylate group and a coordination bond with the amide carbonyl group.⁴⁹ On the other hand, N,N-dibenzyl, N,N-dinaphthyl and N,N-dinaphthylmethyl bCCAs interact with Pb²⁺, Cu²⁺, Hg²⁺ at specific 1 : 1 ligand–metal ratios. It is noteworthy that Pb²⁺ complexes precipitate in aqueous media, reaching removal efficiencies up to 95% and an average removal capacity of 3.3 mg of Pb²⁺/mg of ligand at pH 2.5.⁵⁰ The simultaneous removal of seven metal ions was also achieved with aniline- and pyridine-functionalized bCCAs, showing good average removal efficiencies and capacities of 1.08 to 19.3 mg of Mⁿ⁺/mg of ligand at pH 4.0.⁵¹ Due to the ditopic nature of bCCAs, the formation of coordination polymers or aggregates with these ligands has been proposed. Once the precipitates are formed in the aqueous medium, the cations are separated from water.

Furthermore, chitosan grafted with N-benzyl bCCA achieved the simultaneous removal of a mixture of cations with good efficiency (66% to 99%),⁵² but a high dose of grafted polymer was required (1.12 to 1.15 mg of Mⁿ⁺/mg of grafted chitosan). The performance parameters were improved by grafting the chitosan with aniline- and pyridine-functionalized bCCAs. The grafted chitosan showed good removal efficiencies (73% to 99%) and excellent removal capacities of metal ions (43.5 to 50 mg of Mⁿ⁺/mg of grafted chitosan) at pH 4.4.⁵³

In this work, we propose the immobilization of the chelating compound N-benzyl mCCA on the Merrifield's resin to synthetize new Merrifield-bCCA chelating resins (MR_nBz) in order to study their metal ion adsorption properties and their potential as new alternatives for the removal of Pb²⁺ ions from water, taking advantage of the outstanding bCCAs coordination/chelation properties and affinity, as well as the characteristics of Merrifield's resin (Fig. 1). Based on previous reports with this type of ligand, it is proposed that complexation is the main mechanism of cation adsorption on the resin (Fig. 1). These resins have high affinity and selectivity in the adsorption of Pb²⁺ even in the presence of other excess metal cations. Removal efficiencies greater than 90% in a concentration range from 3.5 to 74 mg L⁻¹ in the experiments were carried out in batches, and they can also be reused at least four times. All these characteristics demonstrate the potential of these chelating resins for applications in the treatment of wastewater contaminated with metals, particularly lead.

Fig. 1 Merrifield-bCCA (MR_nBz) chelating resins for the removal of metal ions.

2. Experimental

2.1 Apparatus

Fourier transform infrared spectra were recorded in the frequency range of 400–4000 cm⁻¹ using a Perkin FTIR Spectrum Two spectrophotometer. The microstructure of samples was detected by field emission scanning electron microscopy (FESEM), and were carried out with a JEOL 7800 PRIME microscope with a 5 keV electron beam. Zeta potential analysis was conducted using a Meinsberger Zeta Stabino Particle Metrix, GmbH A0207 Titration System instrument and a porcelain cuvette. The thermal stability of the adsorbents was detected through thermogravimetric analysis (TGA), recorded via the TA instrument SDT 2960 simultaneous DSC-TGA model by raising the temperature from 30 °C to 800 °C using ceramic pans at a rate of 20 °C min⁻¹ under N₂ atmosphere. The metal ion concentration was determined using a PerkinElmer induced coupled plasma atomic emission spectrometer (ICP-AES) Optima 8300 model; the Argon plasma, auxiliary and nebulizer flow were 15.00 L min⁻¹, 0.20 L min⁻¹ and 0.55 L min⁻¹, respectively, with a radio frequency of 1300 W and a sample flow rate of 1.5 L min⁻¹.

2.2 Reagents

Merrifield's resin (MR) was used with 200–400 mesh, and the following extent of labeling: 3.5–4.5 mmol g⁻¹ of Cl loading, 1% cross-linked. All compounds were purchased from Merck and Sigma companies, and used as received without further purification. All aqueous metal ion solutions were prepared using deionized water.

2.3 Preparation of chelating resins

2.3.1 Preparation of aminoalkyl-functionalized Merrifield's resins (MR_n). Merrifield's resin (2.5 g, 10 mmol) was placed into a 250 mL round-bottom flask. It was expanded into 60 mL of THF : DMF (1 : 1), then K₂CO₃ (4.1463 g, 30 mmol) and KI (0.8300 g, 5 mmol) were added. The mixture was stirred in an orbital oscillator shaker for 30 min at 250 rpm, then the corresponding alkyldiamine (30 mmol) was added and the mixture was stirred for 12 h at 250 rpm. Finally, water (30 mL) was added and stirred for 30 min. Then, the product resin was filtered and washed with water (3 × 5 mL), MeOH (3 × 5 mL), THF (3 × 5 mL), DCM (3 × 5 mL) and Et₂O (3 × 5 mL), and dried under vacuum. This procedure was repeated twice to ensure the complete functionalization of the resin.

Merrifield's ethylenediamine resin (MR₂). 2.65 g, 97% recovery yield. FTIR (ATR): 3300, 3022, 2914, 1447, 695 cm⁻¹. TGA (°C, Δw): 166.9 °C (1.9), 441.3 (81.1), 528.8 (5.5).

Merrifield's propylenediamine resin (MR₃). 2.75 g, 96% recovery yield. FTIR (ATR): 3289, 3024, 2918, 1450, 700 cm⁻¹. TGA (°C, Δw): 141.2 (3.8), (4.9), 418.2 (71.3), 533.2 (4.6).

Merrifield's butylenediamine resin (MR₄). 2.83 g, 94% recovery yield. FTIR (ATR): 3290, 3018, 2915, 1445, 694 cm⁻¹. TGA (°C, Δw): 154.43 (0.99), 328.4 (8.8), 444.9 (71.8), 533.6 (4.6).

Merrifield's hexylenediamine resin (MR₆). 2.93 g, 89% recovery yield. FTIR (ATR): 3308, 3020, 2917, 1449, 697 cm⁻¹. TGA (°C, Δw): 155.3 (0.49), 355.0 (11.5), 446.7 (76.4), 559.9 (8.6).

Merrifield's octylenediamine resin (MR₈). 2.78 g, 78% recovery yield. FTIR (ATR): 3278, 3020, 2920, 1449, 699 cm⁻¹. TGA (°C, Δw): 151.0 °C (1.8), 366.3 (12.0), 451.9 (79.5).

2.3.2 Preparation of aminoalkyl-carbamoylcarboxylic acid Merrifield's resins (MR_nBz). The corresponding aminoalkyl-functionalized Merrifield's resin (6 mmol) was placed into a 250 mL round-bottom flask under an inert [Ar] atmosphere, then dry DCM (40 mL) was added to expand the resin, and the mixture was stirred for 30 min. The benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (5.80 g, 18 mmol in 20 mL of dry THF) was added via cannula, and the mixture was stirred for 6 h at 250 rpm. Then, benzylamine (5.1437 g, 5.24 mL, 48 mmol) was added and the reaction was stirred continuously for 6 h. Finally, 30 mL of water at 50 °C was added to the reaction, and it was stirred for 15 min. The mixture was vacuum filtered and washed with water (3 × 20 mL), MeOH (3 × 20 mL), THF (3 × 20 mL), DCM (3 × 20 mL) and Et₂O (3 × 20 mL). This procedure was repeated twice to insure the complete functionalization of the resin.

Merrifield's ethylenediamine carbamoylcarboxylic acid resin (MR₂Bz). 2.45 g, 64% recovery yield. FTIR (ATR): 3390, 3022, 2920, 1658, 1603, 1549, 1359, 699 cm⁻¹. TGA (°C, Δw): 189.2 (9.1), 324.5 (3.7), 527.1 (54.4), 570.7 (6.2).

Merrifield's propylenediamine carbamoylcarboxylic acid resin (MR₃Bz). 2.58 g, 69% recovery yield. FTIR (ATR): 3404, 3021, 2925, 1773, 1604, 1554, 1357, 696 cm⁻¹. TGA (°C, Δw): 191 (11.9), 439.1 (53.9), 573.1 (7.2).

Merrifield's butylenediamine carbamoylcarboxylic acid resin (MR₄Bz). 2.42 g, 67% recovery yield. FTIR (ATR): 3410, 3022, 2925, 1765, 1645, 1605, 1553, 1355, 696 cm⁻¹. TGA (°C, Δw): 190.0 (8.8), 314.8 (2.4), 444.2 (69.6).

Merrifield's hexylenediamine carbamoylcarboxylic acid resin (MR₆Bz). 2.60 g, 75% recovery yield. FTIR (ATR): 3404, 3026, 2922, 1761, 1700, 1558, 1354, 695 cm⁻¹. TGA (°C, Δw): 198.7 (8.5), 323.7 (3.2), 441.1 (58.6), 564.4 (5.2).

Merrifield's octylenediamine carbamoylcarboxylic acid resin (MR₈Bz). 2.13 g, 65% recovery yield. FTIR (ATR): 3370, 3026, 2923, 1769, 1708, 1563, 1354, 697 cm⁻¹. TGA (°C, Δw): 201.0 (9.6), 446.2 (74.9).

2.4 Zeta potential measurements

For the measurements of the ζ variation as function of pH, 20 mg of a chelating resin was suspended in 10 mL of deionized water. Then, 200 μL of a 1.1 M HCl solution was added, and the suspension was obtained at a final volume of 20 mL. The initial pH was around 2.3. Then, 20 μL aliquots of a 0.1 M NaOH solution were added, and the changes in pH and ζ were registered.

The measurements of the ζ variations as a function of Mⁿ⁺ concentration were performed using 20 mg of a different chelate resin in 20 mL of deionized water, adjusting the pH at 5.0 with a 0.1 M HCl solution. Each material was titrated with a single metal ion solution at 0.16 M (Cu²⁺, Pb²⁺, Ni²⁺ and Cd²⁺), adding 10 μL aliquots until the isoelectric point was reached or the ζ potential value remained constant.

2.5 Metal ion adsorption of chelating resins MR_nBz

The metal removal test was performed using a batch modified micro jar technique with a Pb(NO₃)₂ solution at 28.2 mg L⁻¹ of Pb²⁺. The dosage tests were carried out in 10 mL of metal solution with the addition of a progressive addition of 10, 20, 30, 40 and 50 mg of the chelating resin to the solution. The vials were shaken for 1 h, and allowed to stand before being filtered. Finally, the supernatant was evaluated to determine the residual concentration of the metal using the ICP-IOEs atomic emission technique. Three parallel experiments were performed, and the final concentration values correspond to the average of the three measurements.

A second metal removal test was performed using a batch modified micro jar technique with Ni(NO₃)₂, Pb(NO₃)₂, Cd(NO₃)₂ and Cu(NO₃)₂, equivalent to 23.75 mg L⁻¹ of Ni²⁺, 33.51 mg L⁻¹ of Pb²⁺, 21.46 mg L⁻¹ of Cd²⁺ and 27.49 mg L⁻¹ of Cu²⁺. The dosage tests were carried out in 10 mL of metal solution with the progressive addition of 10, 20, 30, 40 and 50 mg of the chelating resin to the solution. The vials were shaken for 1 h, and allowed to stand to be filtered. Finally, the supernatant was evaluated to determine the residual concentration of the metal using the ICP-IOEs atomic emission technique. Three parallel experiments were performed, and the final concentration values correspond to the average of the three measurements.

The removal percentage, E (%), was calculated using eqn (1):

where: C_i = initial concentration of metal ion (mg L⁻¹). C_f= final concentration of metal ion (mg L⁻¹).

2.6 Equilibrium binding experiments for the study of metal adsorption isotherms

For the adsorption isotherms, 10 mg of MR_nBz (n = 3, 4, 6) were dispersed in 10 mL of a Pb²⁺ solutions at different concentrations (20 to 423 mg L⁻¹). The mixture was kept in contact for 1 h to ensure adsorption equilibrium. Then, the MR_nBz resins were separated from the solution by filtration. The amount of Pb²⁺ adsorbed onto the resins was determined by the difference between the initial Pb²⁺ concentration and the concentration remaining in the supernatant. Three parallel experiments were performed, and the final concentration values correspond to the average of the three measurements.

The equilibrium adsorption amounts of Pb²⁺ were calculated according to eqn (2):⁵⁴

where Q_e (mg g⁻¹) is the amount of Pb²⁺ adsorbed at equilibrium, C₀ and C_e (mg L⁻¹) are the concentrations of Pb²⁺ at the initial state and equilibrium, respectively, V is the volume of Pb²⁺ solution, and m is the mass of the resin.

3. Results and discussion

3.1 Synthesis and characterization of the Merrifield-bCCA chelating resins

Five Merrifield-bCCA chelating resins MR_nBz were prepared using Merrifield's resin (MR) as the starting material (Scheme 1). MR was selected due to its robustness and chemical resistance in acidic media. Furthermore, it does not expand in water, so its separation by filtration is quick and simple.⁵⁵ It was proposed to immobilize the compound N-benzyl-bCCA because the free N,N′-dibenzyl-bCCA has a great affinity towards Pb²⁺, and exerts selective and efficient removal at low pH. Then, MR resin was reacted with five different length alkyldiamines in order to introduce the required amino-functionality to support the chelating molecule, as well as a spacer between the polymer matrix and the bCCA molecule. The amino-functionalized Merrifield's resins MR_n were obtained with good recovery yields (78% to 97%). Then, the benzophenone-3,3′,4,4′-tetracarboxylic dianhydride reacted with the MR_n resins to form a mono-carbamoylcarboxylic acid supported on solid phase through the reaction of the amino-resin group with one anhydride of the reactant. Then, benzylamine (Bz) was added in situ to react with the available anhydride group, forming the corresponding bCCA molecule supported on the polymer. MR_nBz chelating resins were obtained in 64 to 75% recovery yields.

Scheme 1 Synthetic route for the chelating Merrifield-bCCA (MR_nBz) resins.

3.1.1 FTIR characterization. Fig. 2a shows the FTIR spectra for Merrifield's resin and the resins obtained in each synthetic step. The spectrum of MR (red) has the characteristic bending vibration at 1262 cm⁻¹ of the methylene group linked to chlorine and the vibration at 662 cm⁻¹ corresponding to the carbon–chlorine bond stretching. The spectrum of the aminoalkyl-functionalized MR₄ resin (blue) does not show the vibrations at 1262 and 662 cm⁻¹, indicating that the substitution of chlorine in the resin by the diamines was successful. Finally, in the IR spectrum of MR₄Bz resin (magenta), a peak is observed at 3453 cm⁻¹ that corresponds to the amide N–H bond stretching, and a wide band extending up to 2400 cm⁻¹ corresponds to the O–H bonds of the carboxylic acids. From 1800 to 1650 cm⁻¹, a series of vibrations corresponding to carbonyl groups are also observed. These vibrations confirm the presence of the N-benzyl-bCCA molecule attached to the polymer resin. Fig. 2b compares the spectra of the MR₄Bz chelating resin with the free N,N-dibenzyl carbamoylcarboxylic acid (bBzCCA). It is observed that when the vibrational bands at 1738, 1724 and 1657 cm⁻¹ of the free bCCA are weaker than that expected for a common carbonyl group, then the vibrations observed in the chelating resin are also weak. Similar results were obtained in the FTIR characterization of the rest of the prepared resins (Fig. S1 to S4†).

Fig. 2 (A) FTIR spectra of Merrifield's resin (red), MR₄ resin (blue) and MR₄Bz (magenta). (B) FTIR spectra of MR₄Bz (magenta) and BzCCA (black).

3.1.2 Thermogravimetric analysis. The thermogravimetric analysis of the MR, MR_n and MR_nBz resins was performed at 20 °C min⁻¹ under a nitrogen atmosphere (Fig. 3 and S5–S8†). The thermogram of MR (red) shows three thermal weight loss events; the first occurs at a maximum temperature of 331.7 °C with a weight loss of 16.55%, which may be attributed to the pyrolysis of the chloromethylene group. Chlorine has a molar mass of 35.45 g mol⁻¹ and corresponds theoretically to the 14.18% of Merrifield's resin mass, considering the average load of 4.0 mmol of Cl reported by the supplier. Based on these results, a loading of 4.72 mmol of Cl/g of resin was calculated from the experimental data. The second weight loss event is held at 459 °C with a weight loss of 43.48%. The last event occurs at 526.5 °C, having a weight loss of 14.00%, and the final residue was 23.20%. The thermogram of MR₄ resin (blue) shows a noticeable change in the thermal decomposition pattern. The thermal event in MR at 331.7 °C is not observed in this resin, confirming the substitution of chlorine by the alkyldiamine. The first thermal event corresponds to a weight loss of 8.85% at 328 °C. The second event was found at 444 °C with a weight loss of 71.8%. The third thermal event occurs at 533.5 °C with a weight loss of 4.59%. Finally, the residue of this functionalized resin was 8.15%. These results indicate that the functionalization of the resin with the alkylenediamine changes its thermal properties.

Fig. 3 Thermograms of Merrifield's resin (red), MR₄ resin (blue) and MR₄Bz resin (black) obtained at 20 °C min⁻¹ under a N₂ atmosphere.

Finally, the thermogram of the chelating resin MR₄Bz (black) shows three main thermal decomposition events. The first thermal event is observed at 190.0 °C with a weight loss of 8.83%. The second event with a weight loss of 2.40% occurs at a temperature of 314 °C, and the third event is observed at 445 °C with a 69.68% weight loss.

The thermogravimetric analysis of the free mono- and bis(carbamoylcarboxylic) acids, mBzCCA and bBzCCA, was useful in the interpretation of the thermal properties of the chelating resins (Fig. 4, S9 and S10†). These compounds present two well defined thermal events, the first at 187.6 °C and 193.0 °C, which corresponds to a weight loss of 13.5% and 16.12%, respectively. These events are attributed to the pyrolysis of one and two carboxylic acids present in their molecules, since they theoretically represent 10.2% and 16.4% of their molecular weight. The second thermal event occurs at 470.6 °C and 490.0 °C with 72.98% and 74.82% weight losses, respectively, corresponding to the thermal decomposition of the rest of the molecule.

Fig. 4 Thermograms of MR₄Bz resin (black), mBzCCA (red) and bBzCCA (blue) obtained at 20 °C min⁻¹ under a N₂ atmosphere.

It can be seen that the thermogram of the chelating resin MR₄Bz also presents a thermal event at 190 °C, which by comparison may be attributed to pyrolysis of the carboxylic acid groups in the molecules attached to the polymer resin. Based on the weight loss of this thermal event, a loading of 1.00 mmol of bCCA/g of resin was calculated, which agrees with the theoretical calculated loading of 1.37 mmol of bCCA/g of resin. The thermograms for the other MR_nBz resins show the same thermal event (Fig. S11 to S14†), and the calculated loading values were 1.03 mmol g⁻¹ for MR₂Bz, 1.35 mmol g⁻¹ for MR₃Bz, 0.97 mmol g⁻¹ for MR₆Bz and 1.09 mmol g⁻¹ for MR₈Bz.

3.1.3 Morphology and elemental analysis of the chelating resins by FESEM and EDS. The morphology of the MR and MR_nBz resins was observed by FESEM for multiple beads (Fig. 5). As expected, the FESEM images of the MR resin show a smooth and flat microsphere surface (A), which is maintained after its functionalization with the alkylenediamines and the bCCA (B). It can be clearly observed in the magnification (C) that the resin beads preserve their surface, size and spherical shape along the synthesis steps. The morphological characteristics observed in these materials were like those reported for other polystyrene supported ligands.⁵⁶ For its part, the elemental analysis by energy dispersion spectroscopy (EDS) of MR shows the presence of carbon. However, in the functionalized resins MR₃Bz and MR₆Bz, nitrogen and oxygen also are detected, which are present in the alkylenediamine spacer chains and the bCCA molecules (Fig. S15–S17†).

Fig. 5 FESEM images of Merrifield's resin (MR) (A) and MR₃Bz resin (B and C).

3.2 Surface analysis of chelating resins by ζ

Zeta potential measurements were performed to qualitatively and quantitatively analyze the interaction of the free bCCAs and bCCA-functionalized chitosan with metal ions, and to determinate the affinity and metal ion removal capacity of these materials in a homogeneous phase.^41–44 Then, ζ measurements were performed to investigate the suitability of this technique to analyze the surface chemistry of the chelating resins and the adsorption of metal ions in the heterogeneous phase. Subsequently, the effect of pH over ζ in the resins MR, MR_n and MR_nBz was investigated, suspending 20 mg of each resin in 20 mL of deionized water. The variation of ζ with respect to the pH of pure water was evaluated to determine the existence of a background response. Fig. 6 shows that water gives a background response with an electropositive character at low pH in the range of 2.5 to 5.2 (IEP of water). Subsequently, it becomes electronegative, reaching a limit of ζ (≈−12 mV) at a pH of 6. The MR resin shows a ζ profile similar to water at low pH (2.5 to 4). It then decreases, having an IEP at 4.5 until it reaches an electronegative limit of −39.6 mV at pH 6.2. These differences in the response profile with respect to water validate that the zeta potential measurement corresponds to the surface of the resin particles. In general, the shape of the ζ profiles obtained with the MR_n and MR_nBz resins is similar to that of the MR resin, with differences in IEPs and ζ limits caused by the chemical modifications on the resin surface (Fig. S18–S20†). For example, the amino-functionalized resin MR₃ presents a maximum ζ limit (ζ_max) of 36.6 mV, a minimum ζ limit (ζ_min) of −44.0 mV and IEP at 4.65, while bCCA-functionalized resin MR₃Bz has a ζ_max of 25.5 mV, a ζ_min of −42.8 mV and IEP at 4.05 (Fig. 6A). The same effect is observed in MR₆, which presents a ζ_max of 34.6 mV, a ζ_min of −45.2 mV and IEP at 5.02, but the MR₆Bz resin has ζ_max of 21.1 mV, a ζ_min of −44.4 mV and IEP at 3.91 (Fig. 6B). As can be seen in Table 1, the most notable change in zeta potential between MR_n and MR_nBz resins occurs in the ζ_max at low pH, which becomes more negative. The free bBzCCA has a negative ζ at low pH. The IEP for this ligand is observed at 2.5, and then the binding of this ligand to the resins causes a decrease in the ζ_max at low pH. Also, a slight increase in the IEP of the MR_n resins with respect to MR is observed, in addition to a slight decrease in the MR_nBz resins with respect to MR_n. The IEP corresponds to the pH at which the surface of the resins is neutral, and it is proposed that the negative potential at a pH value higher than the IEP is given by the formation of negative charges on the surface, in this case given by the deprotonation of the carboxylic groups of the bCCA molecules that are anchored to the resin (Fig. 6C).

Fig. 6 ζ = f[pH] profiles for (A) MR, MR₃ and MR₃Bz and (B) MR, MR₆ and MR₆Bz. (C) Schematic of the deprotonation of bCCA molecules attached to the resins, forming negative charges. Mass of resin: 20 mg. Volume: 20 mL. Solvent: deionized water.

Table 1 Zeta potential limits and IEP values for MR, MR_n and MR_nBz resins

Resin	ζmax (mV)	ζmin (mV)	Δζ (mV)	IEP
MR	41.5	−40.0	81.5	4.54
MR2	36.9	−39.0	78.9	4.40
MR2Bz	22.1	−46.2	68.3	4.20
MR3	36.6	−44.0	80.6	4.65
MR3Bz	25.5	−42.8	68.3	4.05
MR4	34.0	−44.3	78.3	4.64
MR4Bz	23.8	−44.3	68.1	4.42
MR6	34.6	−45.2	79.8	5.02
MR6Bz	21.1	−44.4	65.5	3.91
MR8	33.2	−48.1	81.3	4.69
MR8Bz	19.5	−35.0	54.5	4.23

---

3.3 Surface analysis of chelating resins in the adsorption of metal ions by ζ

Once the superficial characteristics of the MR_nBz chelating resins were analyzed by ζ, the effect of the adsorption of metal ions over ζ was evaluated as an indicator of the chemical interaction with the chelating molecules on the resin surface. For these experiments, 10 mg of chelating resin was suspended in 20 mL of deionized water at pH 5.0 into a zeta potential cell, then aliquots of individual metal ion (Pb²⁺, Cu²⁺, Cd²⁺ and Ni²⁺) solutions (0.16 M) were automatically added to the suspension, and the ζ changes were registered. At this pH, the resin surface has a negative surface charge, as indicated in Fig. 6. The ζ study of the MR_nBz resins provided stable measurements with Ni²⁺ and Pb²⁺ metal ions in the MR₃Bz resin and with Pb²⁺ in MR₆Bz and no other metal ions. Fig. 7 shows that in MR₃Bz (A) and MR₆Bz (B) resins, the ζ increases dramatically in the range of 0 to 0.001 M (0.010 mmol) of added Pb²⁺, which agrees with the calculated bCCA loading on resins by TGA, 0.0097 and 0.0109 mmol, respectively. The IEP is reached in both resins with Pb²⁺, indicating that these resins have great affinity towards this metal ion by the bCCA functionalization. In addition, the titrations of the MR₃Bz resin with Pb²⁺ and Ni²⁺ begin at similar potential limits but different profiles are observed, indicating that the response depends on the affinity towards the metal ion. The rest of the MR_nBz resins have a similar behavior with some of the metal ions (Fig. S21–S23†).

Fig. 7 ζ = f[Mⁿ⁺] profiles of MR₃Bz (A) and MR₆Bz (B) with different metal ions. Mass of resin: 20 mg. Volume: 20 mL. Solvent: deionized water. pH: 5.0.

An additional experiment was performed to evaluate the ζ response by titrating with a solution containing a mixture of Pb²⁺, Ni²⁺, Cd²⁺ and Cu²⁺ in order to determine differences according to their chemical structure at the surface. As can be seen in Fig. 8, the ζ of the bBzCCA ligand increases with the concentration of Mⁿ⁺ due to the metal–ligand interaction,⁴¹ but it does not reach the IEP. On the other hand, the surface of the MR resin is not only susceptible to pH, but also to the metal ions present in the solution and the ζ increases gradually with the Mⁿ⁺ concentration, but the IEP is not reached either. Taking the asymptotic profile of ζ for this resin as a basis, it is now observed that by modifying the surface of the resin, the curvature of the asymptote is modified, showing a more abrupt neutralization effect in the amino-functionalized MR₈ resin and even more marked response in the bCCA-functionalized MR₈Bz resin. Also, it is observed that the MR₈ resin almost reaches the IEP, while MR₈Bz exceeds the IEP at low Mⁿ⁺ concentration and stabilizes at an electropositive potential. Then, it is more than evident that the bCCA molecules attached to the resin enhance the interaction with the metal ions on the resin surface. A similar behavior is observed with the rest of the resins (Fig. S24–S27†).

Fig. 8 ζ = f[Mⁿ⁺] profiles of bBzCCA, MR, MR₈ and MR₈Bz obtained by titration with a mixture of metal ions. Mass of resin: 20 mg. Volume: 20 mL. Solvent: deionized water. pH: 5.0.

3.4 Metal ion adsorption in MR_nBz chelating resins

Previous studies have demonstrated that bCCAs have a great affinity and removal efficiency over the Pb²⁺ ion.^41,42 Then, the performance of the MR_nBz resins was first evaluated using 10 mL of a 28.2 mg Pb²⁺ per L (0.136 mmol L⁻¹) aqueous solution by varying the amount of MR_nBz resin at pH 5, in order to find the optimal dose (lower resin amount and maximum cation removal) in the removal of Pb²⁺ from the aqueous solution for 1 h of contact. At this pH, the surface of the resins has a negative character and lead is in the cationic form.^57,58 Table 2 and S28–S32† indicate that the optimal dose of chelating resin under these experimental conditions was 10 mg. The removal% values were greater than 83% and the adsorptions were in the range of 23.4 to 27.4 mg Pb²⁺ per g (0.114 to 0.132 mmol Pb²⁺ per g) under these experimental conditions. Also, beyond this dose, the removal% slightly decreases. This is probably due to a re-dispersion process, which is very common in a coagulation–flocculation process. As seen in Table 3, the final concentration of Pb²⁺ achieved with most of the MR_nBz resins is below to the limit allowed by the Official Mexican Standard, meaning that treated water would have the necessary quality for discharge into the municipal sewage system.

Table 2 Performance parameters for the Pb²⁺ removal process using the MR_nBz resins

Resin	Dose (mg)	Removal (%)	[Pb^2+]o (mg L^−1)	[Pb^2+]f (mg L^−1)	Qe (mg Pb^2+ per g)	Qe (mmol Pb^2+ per g)
MR2Bz	10	96	28.2	1.13	27.1 ± 1.02	0.131 ± 0.005
MR3Bz	10	97	28.2	0.84	27.4 ± 0.79	0.132 ± 0.004
MR4Bz	10	83	28.2	4.79	23.4 ± 1.33	0.113 ± 0.006
MR6Bz	10	93	28.2	1.97	26.2 ± 2.37	0.126 ± 0.011
MR8Bz	10	93	28.2	1.97	26.2 ± 1.17	0.126 ± 0.006

---

Table 3 Performance parameters of the M²⁺ mixture adsorption process using MR_nBz resins

Resin	Optimal dose	M^n+	Removal (%)	Qe (mg M^n+ per g)	Qe (mmol M^n+ per g)	Molar ratio
a Molar ratio Pb^2+ : Cd^2+ : Ni^2+.b Molar ratio Pb^2+ : Cu^2+.
MR2Bz	20 mg	Pb^2+	91 ± 0.26	17.6	0.11	67 : 18 : 15^a
		Cd^2+	18 ± 0.51
		Ni^2+	12 ± 0.63
MR3Bz	20 mg	Pb^2+	85 ± 4.92	17.2	0.10	67 : 16 : 17^a
		Cd^2+	17 ± 0.72
		Ni^2+	9 ± 0.29
MR4Bz	50 mg	Pb^2+	67 ± 6.84	5.8	0.04	58 : 19 : 23^a
		Cd^2+	19 ± 0.12
		Ni^2+	11 ± 0.41
MR6Bz	30 mg	Pb^2+	90 ± 0.12	11.4	0.07	70 : 30^b
		Cu^2+	15 ± 2.06
MR8Bz	20 mg	Pb^2+	91 ± 0.13	16.0	0.09	86 : 14^b
		Cu^2+	6 ± 0.67

---

Once the optimal resin mass was determined, we proceeded with the optimization of the pH value and contact time. The study of the variation of the Pb²⁺ adsorption of the chelating resins with respect to pH (2.5 to 7.0) shows that the pH value of 5 is precisely optimal for the adsorption of this cation; this is due to the higher amount of cation that is adsorbed. At a lower pH, the ligand is in the neutral form, which decreases the chelating capacity of the resins. At higher values, other lead species such as PbOH⁺ are formed and no longer complex with the ligand (Fig. S33†). However, the partial adsorption of Pb²⁺ at lower pH may indicate that the physical adsorption mechanism could also be involved, in addition to chemisorption by complexation with the ligand.^59,60

Having already obtained the optimal resin mass and pH, it was proceeded with the optimization of the contact time, evaluating the adsorption of Pb²⁺ in the MR₃Bz and MR₆Bz resins from 5 to 360 min. Fig. S34† shows that the MR₃Bz resin reached 70% adsorption at 5 min of contact and 96% at 60 min, while the MR₆Bz resin presented 62% adsorption at 5 min and reached 100% at 60 min of contact. Based on these results, a contact time of 60 min was established in subsequent experiments.

Another factor which influences the Pb²⁺ adsorption on the resin is the initial concentration of the metal ion. Then, the adsorption of MR₃Bz, MR₄Bz and MR₆Bz resins was evaluated by varying the initial concentration of Pb²⁺ in the solution from 3.5 to 422.6 mg L⁻¹, as shown in Fig. 9 and Tables S1–S3.† The adsorption of Pb²⁺ at the equilibrium Q_e was calculated with eqn (2), and the isotherms show that MR₃Bz and MR₆Bz reached saturation at 164 mg L⁻¹ of Pb²⁺, while MR₄Bz reached saturation at 239 mg L⁻¹. The experimental maximum adsorption (Q_max) of Pb²⁺ on MR₃Bz, MR₄Bz and MR₆Bz are 67.1, 44.6 and 74.8 mg Pb²⁺ per g or 0.324, 0.215 and 0.361 mmol Pb²⁺ per g, respectively. It is noteworthy that resins MR₃Bz and MR₆Bz display removal efficiencies higher than 90% in the concentration interval of 3.5 to 74 mg Pb²⁺ per L, which would allow their use in the treatment of contaminated water with low and high concentrations of Pb²⁺ (Tables S1–S3†).

Fig. 9 Non-linear adsorption isotherms of Pb²⁺ in (A) MR₃Bz, (B) MR₄Bz and (C) MR₆Bz resins. Mass of resin: 10 mg. Volume: 10 mL. Solvent: water. pH: 5.0.

The Q_e data were fitted using a linearized form of the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherm models (Table S4, eqn (S1)–(S6)†) to determine different parameters that describe the adsorption properties of the MR_nBz resins (Tables S5–S7†).⁶¹ The Langmuir model states that there is only a layer of absorbate, a uniform adsorbent surface, and there are no interactions between the adsorbed ions. The Langmuir isotherm non-linear model shows an increase in the adsorption capacity as the initial concentration of the adsorbate increases. The separation factor (R_L) indicates the nature of the adsorption process, such as linear (R_L = 1), irreversible (R_L = 0), unfavorable (R_L > 1), or favorable (0 < R_L < 1). The surface coating (θ) represents the fraction of sites occupied by the dissolved adsorbate in the adsorbent. The Freundlich isotherm model proposes that the adsorption process occurs on an inhomogeneous surface, while in the Dubinin–Radushkevich (D–R) isotherm model, no fixed adsorption potential or homogeneous adsorbent surface is considered. This model can be applied to both homogeneous and inhomogeneous surfaces. Finally, in the Temkin isotherm model, the increase in the thickness of the absorbable coating layer on the adsorbent surface causes an increase in the heat adsorption (bT) of all the molecules on the adsorbed layer.⁶²

Based on the results of the Langmuir model, the K_L values indicate that Pb²⁺ adsorption is approximately ten times stronger in the MR₃Bz resin than in MR₄Bz and MR₆Bz. The separation factor R_L of the three resins is in the range of 0 to 1, which indicates that the adsorption process is favorable in the three resins. Furthermore, the decrease in R_L with increasing concentration indicates that the interaction becomes stronger with increasing Pb²⁺ concentration, and values close to zero indicate that the adsorption is irreversible and extremely strong, particularly in MR₃Bz (Fig. S35†). The surface coating θ values indicate a gradual increase in the occupation of complexation sites in MR₄Bz and MR₆Bz resins as the concentration increases, reaching up to 91% and 94% of occupied sites at the maximum concentration of Pb²⁺. In the MR₃Bz resin, the occupation of the sites is more abrupt; that is, an occupation of 95% of the adsorption sites is observed at 32 mg L⁻¹ of Pb²⁺ and at the maximum concentration, the occupation is 99.6% (Fig. S36†).

From the Freundlich model, it is observed that the adsorption capacity (K_F) is greater in MR₃Bz, followed by MR₆Bz. The inhomogeneity parameters 1/n, which are also related to the adsorption intensity, indicate that the inhomogeneity is greater in MR₃Bz and MR₆Bz; these have the highest Q_e. The parameters of the Temkin model indicate that the Pb²⁺ adsorption process is much more exothermic in the MR₃Bz resin than in MR₆Bz, and that it is rather endothermic in the MR₃Bz resin. The b_T value also indicates that the Pb²⁺ interaction in MR₃Bz is stronger than that in MR₆Bz. For its part, the K_T data indicate a greater amount of Pb²⁺ adsorbed on MR₆Bz compared to the other two resins. The adsorption energy (E_a) values obtained with the D–R model are very low, and indicate that physical adsorption occurs instead of chemical adsorption. However, this point is debatable since, as it is described later, there is a selectivity towards the adsorption of Pb²⁺ in the presence of a molar excess of other cations. This selectivity is given by the chelating properties of the bCCA molecules anchored in the resin. Both models, Langmuir and D–R, determine the maximum Pb²⁺ adsorption capacities close to the experimental values. However, considering the R² values obtained, the Langmuir model reports the best values with the three resins. So, in general, this model best describes the Pb²⁺ adsorption process in the MR_nBz resins (Fig. S37–S39†).⁶²

The ΔG^o values for the adsorption of Pb²⁺ in the MR₃Bz, MR₄Bz and MR₆Bz resins were calculated using eqn (3) and (4):⁶¹

ΔG⁰ = −RT ln K_c (4)

The MR₃Bz resin has negative ΔG⁰ values in the Pb²⁺ concentration interval of 7.7 to 74 mg L⁻¹ with a minimum value of −7.54 kJ mol⁻¹ at 20.5 mg L⁻¹. From 164 mg L⁻¹, the values are positive (Table S4†). The MR₄Bz resin has negative ΔG⁰ at the two lowest concentrations, 3.5 and 7.7 mg L⁻¹ (−1.54 and −1.08, respectively). From 14.0 mg L⁻¹, the ΔG⁰ values are positive (Table S5†). For its part, the MR₆Bz resin presents negative ΔG⁰ values in the range of 3.5 to 164 mg L⁻¹ with a minimum value of −8.91 kJ mol⁻¹ at 16.9 mg L⁻¹. These results show that at low concentrations, the adsorption process is spontaneous, but significant differences are observed between the resins. In fact, the MR₆Bz resin is the one with the widest concentration range and the lowest ΔG⁰ value compared to the other resins. This difference can be attributed structurally to the length of the spacer that joins the chelating molecule to the polymer matrix.

Subsequently, the adsorption–desorption cycles of the MR₃Bz and MR₆Bz resins were performed using 10 mg of resin in 10 mL of a Pb²⁺ solution at 290 mg L⁻¹ and pH = 5.0. Based on the isotherms, the complexation sites in both resins are saturated at this concentration. The desorption was carried out by washing with 10 mL of a 0.5 M HNO₃ aqueous solution, recovering the resin by filtration. Fig. 10 shows that the MR₃Bz resin loses approximately 15% of its adsorption capacity and remains constant until the fourth cycle. However, in the fifth cycle, the adsorption decreases by 40%. The MR₆Bz resin maintained its adsorption capacity in the first two cycles. In the third and fourth cycles, it decreased by 15%. Finally, in the fifth cycle, a drop of 40% was observed. These results indicate that these resins can be reused four times while maintaining their adsorption capacity. This gradual decrease in the adsorption capacity of Pb²⁺ may be attributed to the fact that the metal–ligand interaction is so strong and possibly the presence of unexposed complexation sites, which generates the retention of the Pb²⁺ in the resin.

Fig. 10 Relative Pb²⁺ adsorption in several adsorption–desorption cycles for (A) MR₃Bz and (B) MR₆Bz resins. Mass of resin: 10 mg. Volume: 10 mL. Solvent: water. pH: 5.0.

Finally, the adsorption of a metal ions mixture on these resins was evaluated using 10 mL of a solution containing Ni²⁺ (23.7 mg L⁻¹, 0.4038 mmol L⁻¹), Pb²⁺ (33.5 mg L⁻¹, 0.1616 mmol L⁻¹), Cd²⁺ (21.5 mg L⁻¹, 0.1913 mmol L⁻¹) and Cu²⁺ (27.5 mg L⁻¹, 0.4328 mmol L⁻¹) to determine the selectivity in the adsorption of metal ions. As can be seen, there is a molar excess of the other metal ions with respect to Pb²⁺. The resins with a short spacer remove Pb²⁺, Ni²⁺ and Cd²⁺ from the solution (Fig. 11 and S40–S44†), while the resins with a long spacer remove Pb²⁺ and Cu²⁺ and no other cation. Independent of the spacer length, all resins predominantly remove Pb²⁺, despite there being a molar excess of the other cations in the solution. The affinity or selectivity in these resins is consistent with the selectivity if the bBzCCA ligand towards Pb²⁺.^50,51 Thus, the behavior of the chelating resins confirms the presence of chelating molecules grafted on the polymer.

Fig. 11 Metal ion removal percentage of MR_nBz resins at their optimal dose. Mass of resin: 10 mg. Volume: 10 mL. Solvent: water. pH: 5.0.

Table 3 shows that the optimal dose required in the removal of a mixture of metal ions increases compared to the exclusive removal of Pb²⁺, which indicates that metal ions compete for the chelating sites in the resin and a higher amount is required to achieve the maximum removal of cations. As the adsorption capacity depends on the optimal dose, their values are also lower than that obtained in the Pb²⁺ removal experiment. At their optimal doses, the absorption capacity of these resins is in the range of 5.8 to 17.2 mg of M²⁺/g or 0.04 to 0.11 mmol of M²⁺/g of resin. The removal% and adsorption are different in each resin, and the molar ratios indicate a greater adsorption of Pb²⁺ compared to the other metal ions. However, the short spacer resins exhibit significant adsorption of Cd²⁺ and Ni²⁺. The chelating resins with larger spacer MR₆Bz and MR₈Bz achieved the removal of Pb²⁺ (90% and 91%, respectively) and Cu²⁺(15% and 6%, respectively) and no other metal ion, showing the effect of the spacer length over the removal capacity and selectivity. Interestingly, the ζ analysis of these resins predicted or indicated the selectivity in the adsorption capacity of these resins.

3.5 Analysis of chelating resins after the metal ion adsorption process

Once chelating resins were used to adsorb metal ions from aqueous solutions, they were dried and characterized by the different available techniques to evaluate the loading of the metal ions. The FTIR spectra of the metal-loaded MR₃Bz resins are shown in Fig. 12 and in S45† for MR₆Bz. Significant changes are observed in the vibrations; the wide band corresponding to the O–H vibration disappears, and a strong vibration for carbonyls is observed at higher frequency (∼1740 cm⁻¹) compared to the pristine resin (∼1645 cm⁻¹). Previous reports have indicated that the complexation of carbamoylcarboxylic acids with metal ions such Cu²⁺ and Hg²⁺ causes the O–H vibration to disappear because a carboxylate is formed and changes the energy of the carbonyl vibrations due to both amide and carboxylate groups being coordinated to the metal ion.^48,49

Fig. 12 FTIR spectra of pristine MR₃Bz (magenta), Pb²⁺-loaded MR₃Bz (green) and M²⁺-loaded MR₃Bz (purple) resins obtained under a N₂ atmosphere. M²⁺ denotes a mixture of divalent cations.

The thermogravimetric analysis of metal-loaded chelating MR_nBz resins (Fig. 13 and S46–S49†) shows changes in the thermal decomposition pattern and thermal stability. For the MR₆Bz resin, a weight loss stage is observed at around 100 °C, which may correspond to the loss of water adsorbed in the resin or coordinated to the metal complex. Also, the degradation temperature of the polymer increases at around 10 °C, and the percentage of refractory residue increases because of the loading of the metallic components.

Fig. 13 Thermograms of pristine MR₆Bz resin (black), Pb²⁺-loaded MR₆Bz (red) and M²⁺-loaded MR₆Bz (blue) obtained under a N₂ atmosphere. M²⁺ denotes a mixture of divalent cations.

Fig. 14 shows the FESEM images at a different scale of M²⁺-loaded MR₃Bz and MR₆Bz resins isolated after the adsorption process with a metal ions mixture solution at low concentration. Here, it can be seen that the resin microspheres preserve their size and shape but they are semi-covered by a crust or shell of a different material, which contrasts with the resin surface. The elemental analysis on the surface for the M²⁺-loaded MR₃Bz and MR₆Bz resins by EDS shows a high atomic% for carbon and oxygen. Meanwhile, for cadmium, copper, lead and nickel, the atomic% is lower but there is evidence of the presence of these metal ions on the surface (Fig. S50 and S51†). These results evidence the loading of the metallic salts on the resins.

Fig. 14 FESEM images of M²⁺-loaded MR₃Bz (A and B) and MR₆Bz (C and D) resins.

The morphology of the Pb²⁺-loaded MR₃Bz and MR₆Bz resins obtained in the isotherm experiments at the highest concentration of Pb²⁺ was observed by FESEM (Fig. 15). The MR₃Bz (A and B) and MR₆Bz (C and D) resins preserve their spherical shape and size, but their surface is completely covered by the shell of a different material and the surface looks very rough. The elemental analysis by EDS for the Pb²⁺-loaded resins obtained after the treatment of Pb²⁺ solutions at low and high concentrations shows that the atomic% of lead on the surface increases considerably as the initial concentration in the solution increases. The resins immersed into a Pb²⁺ solution at 28 mg L⁻¹ show 0.16 and 1.74 atomic% of lead, respectively (Fig S52 and S53†), while those immersed into a solution at 340 mg L⁻¹ show 27.86 and 29.77 atomic% of lead, respectively (Fig. S54 and S55†). These results are consistent with the adsorption ratios found in lead adsorption experiments on these resins at low and high concentrations of the metal ion.

Fig. 15 FESEM images of Pb²⁺-loaded MR₃Bz (A and B) and MR₆Bz (C and D) resins.

3.6 Comparison of the adsorption performance of MR_nBz resins with other chelating resins

The literature describes the performance of several functionalized Merrifield's resins with different ligands in the removal of metal ions showing different adsorption capacities. Table 4 shows the adsorption capacity of several MR-chelating resins, indicating the initial concentration of metal ions, volume of treated water, amount of resin and the experimental method (batch or packed column). The adsorption efficiency of the MR_nBz chelating resins developed in this work is slightly lower compared to other materials. This difference in the adsorption capacity is directly related to the loading of the resin, the affinity of the chelating molecule incorporated into the polymer matrix, and also with the expansion capacity in the aqueous medium, since the expansion allows the diffusion of the metal cation towards the matrix. Also, the design of the experiment, batch or column, influences the adsorption. However, there are some main aspects that distinguishes the performance of the MR_nBz resins: (1) the chelating molecules attached to the resins have low structural complexity, facilitating their preparation at large scale; (2) they works well in the adsorption of metal ions present in water at low and high concentration. Meanwhile, in some of the listed resins, the initial concentration of metals is high (100 and 350 ppm); (3) their performance was successful in the individual removal of Pb²⁺ ion, and in the removal of a mixture of metal ions (Pb²⁺ and Cu²⁺ or Pb²⁺, Cd²⁺ and Ni²⁺), showing a significant adsorption and selectivity towards Pb²⁺ ions. The previous reported resins were evaluated for individual metal ions, and do not include results for a mixture of metal ions.

Table 4 Comparison of the adsorption capacity of various MR-functionalized adsorbents for heavy metal ions

Adsorbent	Adsorption capacity (mmol g^−1)	Method	Reference
CPS-DI	1.35 (Pb^2+), 0.94 (Ni^2+), 2.55 (Cu^2+)	Batch, 250 rpm, 0.1 g/0.1 L (100 ppm)	34
Thiourea calixarene	84.80 (Pb^2+)	Batch, 160 rpm, 0.1 g/0.01 L (5 × 10^−5 M ≈ 10.4 ppm)	37
Piperidine calixarene	2.1 (Cr^6+)	Batch, 0.025 g/0.02 L (1 × 10^−4 M ≈ 5.2 ppm)	38
CMPDB	0.21 (Cr^6+)	Batch, 1.0 g/0.1 L (100 ppm)	39
MR-Q	1.65 (Pb^2+)	Batch, 0.01 g/0.02 L (350 ppm)	41
MR6Q	1.21 (Pb^2+)	Column, 0.04 g/1.0 mL min^−1 (0.11 ppm)	42
CPS-TA	1.11 (Pb^2+), 2.97 (Ni^2+), 4.23 (Cu^2+)	Batch, 0.1 g/0.1 L (100 ppm)	43
Thiacalixarene	5.5 (Eu^3+)	Batch, 0.1 g/0.02 L (10 ppm)	44
PVT-g-PS	1.52 (Pb^2+), 2.65 (Cu^2+), 3.36 (Cr^6+)	Batch, 0.1 g/0.1 L (4 mM)	45
PS-CS	0.14 (Cu^2+)	Batch, 0.0135 g/0.1 L (10 ppm)	46
ASA/IMEA-PGMA@PS	1.57 (Pb^2+), 0.70 (Cd^2+), 0.87 (Cu^2+), 1.19 (Cr^6+)	Batch, 0.1 g/0.025 L (4.0 mM)	47
MRnBz	0.11 to 0.13 (Pb^2+)	Batch, 0.01 g/0.01 L (28 ppm)	This work
MR3Bz	0.322 (Pb^2+)	Batch, 0.01 g/0.01 L (164 ppm)	This work
MR6Bz	0.392 (Pb^2+)	Batch, 0.01 g/0.01 L (164 ppm)	This work
MR2Bz	0.10–0.11 (Pb^2+, Cd^2+ and Ni^2+ mixture)	Batch, 0.02 g/0.01 L (33.5 ppm, 21.5 ppm, 24.7 ppm)	This work
MR3Bz
MR8Bz	0.09 (Pb^2+ and Cu^2+ mixture)	Batch, 0.02 g/0.01 L (33.5 ppm, and 27.5 ppm)	This work

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4. Conclusions

The functionalization of Merrifield's resin with chelating bCCA molecules provides specificity to the ion adsorption capacity of this material. The functionality and spacer length influence the adsorption capacity and selectivity. MR_nBz resins display a predominant or selective Pb²⁺ adsorption. However, the spacer length plays an important role in the adsorption of other metal ions. Short spacers improve the adsorption of Cd²⁺ and Ni²⁺, while larger spacers favor the adsorption of Cu²⁺. Based on the adsorption capacity and the parameters of the isotherm models that describe the adsorption properties, it was possible to select the MR₃Bz resin as the best among this mini-library of chelating resins, although they all have good characteristics. The adsorption properties of these resins are comparable with other reported resins considering the conditions used in these experiments. On the other hand, FTIR and TGA analyses demonstrated the functionalization of the Merrifield's resin in each synthetic step, as well as M²⁺-loading in the resin after the adsorption process. FESEM images provide unequivocal evidence of the loading of metallic materials on the resin surface and the preservation of its structure along the synthesis steps and after the removal process. Finally, the ζ analysis of these resins indicated the selectivity in the adsorption capacity. Currently, we are conducting further research to support the use of Merrifield's resin and other bCCA molecules having aniline, pyridine or carboxylic substituents on the amide, with the aim to improve its adsorption capacity and/or to modulate it towards the adsorption of different cations.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge Tecnológico Nacional de México (TECNM) for supporting this research study through the research project “Evaluación de resinas quelantes funcionalizadas con ácidos carbamoilcarboxílicos en el tratamiento de aguas contaminadas con metales” (Grant No. 13763.22-P). José García-Elías is also thankful to CONAHCYT for the scholarship granted to carry out his doctoral studies.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra06929c

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