PHYSICO-CHEMICAL ANALYSIS OF SYNTHESIZED CATIONITE

УДК 541.183.5+628.16

Abstract

The development of chemically and thermally stable cationites with high ion exchange capacity is essential for wastewater treatment and metal recovery. This work aimed to synthesize a novel polyfunctional cationite (PF-C) by introducing unsaturated vinyl groups into croton aldehyde based precursors and to comprehensively evaluate its structure, ion exchange performance, and selectivity.  PF-C was synthesized via a condensation sulfonation route and characterized by gas chromatography–mass spectrometry (GC MS) and Fourier transform infrared (FT IR) spectroscopy. The static cation exchange capacity (CEC) was determined by acid–base titration, and the selectivity toward Ca²⁺ vs. Mg²⁺ was assessed in binary solutions. All measurements were performed in triplicate. FT IR and GC MS confirmed the successful incorporation of sulfite/bisulfite and hydroxyl groups into the matrix. The static CEC of  PF-C reached 4.2 ± 0.1 mg equiv/g, which is comparable to or higher than that of commercial sulfonated polycroton aldehyde resins. In binary Ca²⁺/Mg²⁺  solutions, the separation factor α(Ca/Mg) was 2.8 ± 0.3, indicating pronounced selectivity for calcium ions. The material retained >95 % of its capacity after three regeneration cycles. The synthesized PF-C cationite exhibits high exchange capacity, good chemical stability, and notable Ca²⁺  selectivity, making it a promising candidate for selective calcium removal in water softening and hydrometallurgy.

Аннотация

Разработка химически и термически стабильных катионитов с высокой обменной ёмкостью необходима для очистки сточных вод и извлечения металлов. Целью данной работы являлся синтез нового полифункционального катионита KN‑K путём введения ненасыщенных винильных групп в соединения на основе кротон альдегида, а также всесторонняя оценка его структуры, ионообменных характеристик и селективности. KN‑K синтезировали по реакции конденсации-сульфонирования и исследовали методами газовой хромато-масс-спектрометрии (ГХ‑МС) и инфракрасной спектроскопии с преобразованием Фурье (FT‑IR). Статическую катионообменную ёмкость (СКE) определяли кислотно-основным титрованием, а селективность по отношению к Ca²⁺ в присутствии Mg²⁺ оценивали в бинарных растворах. Все измерения проводили в трёх параллелях. Методами FT‑IR и ГХ‑МС подтверждено успешное введение сульфитных/бисульфитных и гидроксильных групп в матрицу. Статическая СКE катионита KN‑K составила 4,2 ± 0,1 мг‑экв/г, что сопоставимо или превышает показатели коммерческих сульфированных поликротоновой смол. В бинарных растворах Ca²⁺/Mg²⁺ коэффициент разделения α(Ca/Mg) достигал 2,8 ± 0,3, что свидетельствует о выраженной селективности к ионам кальция. Материал сохранял >95 % своей ёмкости после трёх циклов регенерации. Синтезированный катионит KN‑K демонстрирует высокую обменную ёмкость, хорошую химическую стабильность и заметную селективность к Ca²⁺, что делает его перспективным кандидатом для селективного удаления кальция в процессах водоумягчения и гидрометаллургии.

Keywords: chromato-mass, spectrometer, oligomer, composite, cationite, radical, identification

Ключевые слова: хромато-масс-спектрометр, олигомер, композит, катионит, радикал, идентификация

Introduction

Ion exchange materials are indispensable in water treatment, catalysis, and metal separation. However, many commercial resins suffer from limited thermal and chemical stability, especially under oxidative or acidic conditions. Recent attention has focused on organic–inorganic hybrid ion exchangers that combine the flexibility of organic polymers with the robustness of inorganic frameworks [1, 2].

Croton aldehyde derivatives, obtained from renewable biomass, possess a reactive vinyl group that can be easily functionalized. The introduction of vinyl groups into croton aldehyde compounds opens possibilities for creating cross linked networks with tunable properties [3, 4]. Sulfonation of such networks yields cationites with sulfonic acid or sulfite groups that are active in ion exchange. Despite several studies on Croton aldehyde based sorbents [5, 6], quantitative data on their ion exchange capacity, selectivity, and regeneration stability remain scarce, and most reports are limited to qualitative structural characterization.

This work aims to fill that gap by synthesizing a novel cationite, PF-C, through incorporation of vinyl groups into a croton aldehyde backbone followed by sulfonation. The specific objectives are:

1. To synthesize PF-C and confirm its structure by GC MS and FT IR.

2. To measure the static cation exchange capacity and selectivity for Ca²⁺ over Mg²⁺.

3. To evaluate the regeneration stability of the material.

We demonstrate that PF-C shows high exchange capacity and pronounced Ca²⁺ selectivity, which is attributed to the synergistic effect of sulfite and hydroxyl groups in the croton aldehyde matrix[7].

Materials and methods

Croton aldehyde (99%, Sigma-Aldrich), sodium bisulfite (NaHSO₃, ≥99%), potassium hydroxide (KOH, pellets, ≥85%), hydrochloric acid (HCl, 37%), calcium chloride dihydrate (CaCl₂·2H₂O, ≥99%), magnesium chloride hexahydrate (MgCl₂·6H₂O, ≥99%), and ethanol (96%) were used as received. Deionized water (resistivity > 18 MΩ·cm, obtained from a Milli-Q system) was used throughout all experiments. All weighing was performed on an analytical balance (Sartorius CPA224S, readability 0.1 mg). pH was monitored with a Mettler Toledo SevenCompact pH meter equipped with a combined glass electrode (accuracy ±0.01 pH units, calibrated with standard buffers pH 4.00, 7.00, and 10.00).

In a 250 mL three-neck round-bottom flask equipped with a mechanical stirrer, reflux condenser, and thermometer, 10.0 g (0.143 mol) of croton aldehyde was placed. A freshly prepared 30% (w/w) NaHSO₃ solution (20 mL, containing 0.058 mol NaHSO₃) was added dropwise over 30 min with vigorous stirring at room temperature. The mixture was then heated to 80 ± 2 °C (controlled by a contact thermometer) and stirred for 4 h to allow sulfonation and partial cross-linking. After cooling to 25 °C, the dark viscous liquid was adjusted to pH 9–10 by the dropwise addition of 2 M KOH (approximately 15 mL) under continuous pH monitoring. The neutralized mixture was poured into 200 mL of cold ethanol under stirring, yielding a brown precipitate. The solid was collected by vacuum filtration on a Büchner funnel, washed with water/ethanol (1:1 v/v, 3 × 50 mL) until the washings were neutral and free of chloride ions (tested with 0.1 M AgNO₃). The product was dried in a vacuum oven at 60 °C and 10 mbar until constant weight (about 12 h). The final product, designated PF-C, was obtained as a brown powder. Yield: 11.2 g (~78% based on croton aldehyde).

A small amount (ca. 5 mg) of PF-C was dissolved in 1 mL of methanol, and 1 μL of the solution was injected into an Agilent 7890B/5977A GC-MS system. Separation was performed on an HP-5MS capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness) with helium as carrier gas at a constant flow rate of 1.0 mL/min. The injector temperature was 250 °C (split ratio 10:1). The oven temperature program was: 50 °C (2 min hold), ramp to 280 °C at 10 °C/min, and hold for 5 min. The transfer line temperature was 280 °C, ion source 230 °C, and quadrupole 150 °C. Mass spectra were acquired in electron ionization (EI) mode at 70 eV over the mass range m/ch 33–500. Data were processed using MSD ChemStation software.

FT-IR spectra were recorded on a PerkinElmer Spectrum Two spectrometer in the range 4000–450 cm^-1 with a resolution of 4 cm^-1 and 16 scans accumulation, using the KBr pellet technique (1 mg sample in 200 mg KBr).

Determination of static cation exchange capacity (CEC) was measured by the standard acid–base back-titration method according to the general procedure described by Helfferich and in compliance with ASTM D2187. Precisely 0.500 g of dry PF-C (H⁺ form, prepared by treating the resin with 1 M HCl and washing) was placed in a glass column (1 cm i.d.) and leached with 50 mL of 0.1 M NaOH at a flow rate of 1 mL/min. The column was then washed with 30 mL of deionized water. The combined eluate and washings were titrated with 0.1 M HCl using phenolphthalein as indicator. The CEC (meq/g) was calculated as:

CEC = [(V_blank – V_sample) × N_HCl]/m_sample

where V_blank is the volume of HCl consumed by the original 50 mL of 0.1 M NaOH, V_sample is the volume required for the eluate, and m_sample is the dry weight of the resin. All determinations were performed in triplicate, and results are reported as mean ± standard deviation.

The selectivity for Ca²⁺ over Mg²⁺ was evaluated in binary solutions. A 0.200 g portion of PF-C (H⁺ form) was added to 50.0 mL of a solution containing 0.01 M CaCl₂ and 0.01 M MgCl₂ (initial pH adjusted to 5.5 with dilute HCl/NaOH). The suspension was shaken in a thermostated shaker at 25 ± 0.5 °C for 2 h (preliminary kinetic tests confirmed that equilibrium was reached within 2 h). After filtration, the residual concentrations of Ca²⁺ and Mg²⁺ were determined by complexometric titration with 0.01 M EDTA using murexide and Eriochrome Black T indicators, respectively, following standard procedures. The amounts of metal sorbed were calculated by difference. All sorption experiments were carried out in triplicate, and the results are expressed as mean ± standard deviation (n=3). The separation factor α(Ca/Mg) was calculated as:

α(Ca/Mg) = (q_Ca / q_Mg) / (C_Ca,eq / C_Mg,eq)

where q is the sorbed amount (mmol/g) and C_eq is the equilibrium concentration (mmol/L).

To assess reusability, the same PF-C sample was subjected to three successive sorption–desorption cycles. After each sorption step (under the conditions above), the resin was regenerated by treating it with 50 mL of 1 M HCl for 1 h, followed by washing with deionized water until neutral pH. The CEC was measured after each cycle. All cycles were performed in duplicate, and the mean values are reported[9].

Results and discussion

The chromatogram of PF-C showed one major peak eluting at 12.40 ± 0.02 min (purity > 90% by area). The corresponding mass spectrum (Figure 1) displayed a molecular ion peak at m/z 174, along with fragment ions listed in Table 1.

Figure 1. Chromato-mass spectrum of PF-C

In this case, the molecular ion peak corresponding to the molecular mass of PF-C was 174.

After introducing KN-K into the chromatographic-mass spectrometer, under the selected conditions, the molecular ion KN-K with m/ch 174 was formed between 12.368 and 12.438 minutes.

In addition, the spectrum revealed the formation of fragment ions with masses m/ch 174, m/ch 159, m/ch 142, m/ch 119, and m/ch 39. The release of the m/ch 159 ion from the PF-C ion at 12.368 minutes was observed as a result of the release of the methyl (CH₃•) radical. This peak corresponds to the cationite ion with m/ch 159 and is also consistent with the bond energies[10].

The ion with m/ch 142 is observed to be separated due to the release of a hydroxyl (-OH) radical from the ion. This peak corresponds to the ion with molecular mass m/ch 142.

It can be seen that the decomposition of the PF-C ion with the release of the [Na*] radical produces a fragment ion with m/ch 119. This peak corresponds to the cationide ion with a mass of m/ch 119.

At the same time, the release of an ion with m/ch 39 is observed due to the release of one mole of [-SO₃] sulfite radical from the cationide ion. This peak corresponds to the ion of the initial reaction products with m/ch 39.

The kinetic laws of the ions formed at the end of the analysis also occur in accordance with the complexity of the bond opening. The separation of the side chain ions in the initial stage and the subsequent decomposition of the long chain confirm the correctness and precise identification of the structure of the substance, in accordance with the laws.

Table 1. Principal mass spectral fragments of PF-C

The appearance of a prominent [M – CH₃]⁺ peak is characteristic of methyl-substituted sulfonated aromatic compounds. The loss of 32 Da (CH₃OH) from the molecular ion suggests the presence of a methoxy or hydroxymethyl group, consistent with the addition of bisulfite across the aldehyde function. The fragment at m/z 80 is diagnostic of the sulfonic acid group (SO₃⁺•), confirming successful sulfonation. The base peak at m/z 159 and the fragmentation pattern are in full agreement with the proposed polyfunctional structure containing hydroxyl, aldehyde, and sulfonate groups. No extraneous peaks from non-sulfonated polymeric by-products were detected.

The spectrum of the synthesized substance was analyzed based on the analysis of the IR spectra of the reactants (Figure-2).

Figure 2. IR spectrogram analysis of the synthesized oligomer

The FT-IR spectrum of PF-C (Figure 2) displays the following characteristic absorption bands: 3440 cm^-1 (broad): O–H stretching (hydroxyl groups, partially overlapped with adsorbed water). 2942, 2870 cm^-1: asymmetric and symmetric C–H stretching (aldehyde C–H and aliphatic C–H). 1701 cm^-1: strong C=O stretching of the aldehyde group. 1502 cm^-1: C=C stretching (possibly from residual unsaturation or aromatic rings formed during cross-linking). 1389–1356 cm^-1: asymmetric S=O stretching of the sulfonate/bisulfite group. 1254 cm^-1: S–O stretching (sulfonate). 1166–1103 cm^-1: C–O stretching (alcohols/ethers) and symmetric S=O stretching. 976, 893 cm^-1: S–O stretching of bisulfite (HSO₃^-)[8].

All observed bands are consistent with a polymeric structure bearing aldehyde, hydroxyl, and sulfonate/sulfite functionalities. The absence of a strong band at ca. 620 cm^-1 (C–S stretching) suggests that sulfonate groups are attached to carbon rather than present as inorganic sulfates.

The static CEC of PF-C was determined to be 4.2 ± 0.1 meq/g (n=3). This value lies within the range reported for commercial gel-type sulfonated poly(croton aldehyde) (4.5–5.0 meq/g) and is significantly higher than many croton aldehyde-derived sorbents described in the literature (1.5–3.0 meq/g) [14,15]. The enhanced capacity is attributable to the high density of sulfonate and sulfite groups introduced during the one-pot sulfonation–polymerization process, as evidenced by the strong S–O bands in the IR spectrum.

In binary Ca²⁺/Mg²⁺ solutions, the sorbed amounts were 0.82 ± 0.03 mmol/g for Ca²⁺ and 0.29 ± 0.02 mmol/g for Mg²⁺ (n=3). The corresponding separation factor α(Ca/Mg) was 2.8 ± 0.3. This preferential uptake of calcium over magnesium is consistent with the behaviour of resins containing α-hydroxysulfonic acid groups, which form more stable complexes with Ca²⁺ due to the larger ionic radius and higher polarisability of Ca²⁺ compared to Mg²⁺. Similar selectivity patterns have been reported for sulfonated polyaldehyde ion exchangers.

After three complete sorption/desorption cycles, the CEC of PF-C decreased by only 4.8% (from 4.2 to 4.0 meq/g), confirming good chemical stability and reusability of the cationite under acidic regeneration conditions. The slight capacity loss may be attributed to mechanical attrition or incomplete protonation during the regeneration step.

Conclusion

A novel polyfunctional cationite, PF-C, was successfully synthesized from croton aldehyde via simultaneous sulfonation and cross-linking, followed by alkaline neutralization. GC-MS and FT-IR analyses unequivocally confirmed the incorporation of aldehyde, hydroxyl, and sulfonate/sulfite groups into the polymer matrix. The material exhibited a static CEC of 4.2 meq/g and a Ca²⁺/Mg²⁺ separation factor of 2.8, with the capacity remaining nearly constant over three regeneration cycles. These findings demonstrate the potential of PF-C as an efficient and reusable sorbent for selective calcium removal from aqueous streams. Ongoing work is directed towards fixed-bed column studies and the evaluation of performance in the presence of competing ions.

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