SYNTHESIS AND CHARACTERIZATION OF A UREA, FORMALDEHYDE AND SUCCINIC ACID POLYMERIC LIGAND AND ITS COORDINATION COMPOUNDS WITH Fe(II) AND Mn(II) IONS

ABSTRACT

This study reports the synthesis of a novel nitrogen- and oxygen-containing polymeric ligand obtained via multi-stage polycondensation of urea, formaldehyde, and succinic acid (UFS). The synthesis was optimized at 60-130 °C and pH 8.0-9.0, yielding a porous granular sorbent with a high yield (92%). The coordination behavior of the UFS ligand was investigated in 1 N aqueous solutions of Fe(II) and Mn(II) salts for 48 h. The obtained complexes were characterized by FT-IR spectroscopy and thermogravimetric analysis (TGA/DTA). The spectral shifts confirmed a chelation mechanism involving carbonyl and amino donor groups. Thermal analysis showed that the ligand retains structural stability up to 200 °C, with a total mass loss of 78.09% at 600 °C. The results indicate that the synthesized UFS polymer is a thermally stable and effective sorbent for immobilization of transition metal ions from aqueous media.

АННОТАЦИЯ

В настоящем исследовании представлен синтез нового полимерного лиганда, содержащего азот и кислород, полученного методом многоступенчатой поликонденсации карбамида, формальдегида и янтарной кислоты (UFS). Синтез был оптимизирован при температуре 60–130 °C и pH 8,0–9,0, что позволило получить пористый гранулированный сорбент с высоким выходом (92 %). Координационное поведение лиганда UFS изучалось в 1 N водных растворах солей Fe(II) и Mn(II) в течение 48 часов. Полученные комплексы охарактеризованы с помощью Фурье-спектроскопии (FT-IR) и термогравиметрического анализа (TGA/DTA). Спектральные сдвиги подтвердили механизм хелатирования с участием карбонильных и аминных донорных групп. Термический анализ показал, что лиганд сохраняет структурную стабильность до 200 °C, а общая потеря массы составляет 78,09 % при 600 °C. Результаты указывают на то, что синтезированный полимер UFS является термически стабильным и эффективным сорбентом для иммобилизации ионов переходных металлов из водных сред.

Keywords: Urea-formaldehyde resin, succinic acid, polymeric ligand, chelation, Fe(II) complex, FT-IR spectroscopy, Thermogravimetric analysis (TGA), adsorption.

Ключевые слова: смола карбамид–формальдегид, янтарная кислота, полимерный лиганд, хелатирование, комплекс Fe(II), FT-IR спектроскопия, термогравиметрический анализ (TGA), адсорбция.

Introduction

The contamination of water resources by toxic heavy metal ions remains a critical environmental challenge due to their non-biodegradability and adverse effects on human health and ecosystem stability [1, 2]. Industrial effluents often contain a complex mixture of organic pollutants and heavy metals such as Hg, Cd, Cr, As, and Pb, which exhibit high toxicity even at trace concentrations [3]. Various methodologies, including chemical precipitation, ion-exchange, membrane filtration, and flotation, have been developed to mitigate these risks [4]. Among these, adsorption and ion-exchange based on functionalized polymeric ligands are considered highly reliable and cost-effective strategies for wastewater treatment [5].

Recent advancements in coordination chemistry have focused on the synthesis of polyfunctional immobilized ligands capable of forming stable complexes with d-metal ions via nitrogen- and oxygen-donor atoms [6]. Following this scientific direction, our research group has previously reported the synthesis of polyfunctional polymer sorbents through polycondensation reactions involving urea and formaldehyde with various modifiers, such as diphenylcarbazone, 2,4-dinitrophenylhydrazine [7], 2-aminopentadienoic acid [8], phenolsulfophthaleic acid [9], and hydrazine hydrate [10]. The physicochemical and sorption properties of these chelating ligands, as well as their complexation processes with Cu(II), Zn(II), Ni(II), and other 3d-metal ions, have been systematically evaluated [11,12].

Building upon these foundations, the present study describes the synthesis of a novel polymeric ligand through the polycondensation of urea, formaldehyde, and succinic acid. The incorporation of succinic acid introduces additional carboxylate functional groups, thereby enhancing the coordination capacity of the polymer matrix. This paper focuses on the structural characterization and thermal stability of the synthesized ligand and its coordination compounds with Fe(II) and Mn(II) ions. The bonding mechanisms and thermal degradation kinetics were investigated using FT-IR spectroscopy and Thermal Analysis TGA/DTA.

Materials and Methods

Synthesis of the UFS Polymeric Ligand. The polyfunctional chelating ligand containing nitrogen and oxygen donor atoms was synthesized via a multi-stage polycondensation method. 1.2 g (0.02 mol) of urea was dissolved in 4 ml (0.05 mol) of formaldehyde (formalin) in a three-necked flask equipped with a reflux condenser and an automatic stirrer. The mixture was stirred at 60°C until the onset of polymerization. The pH of the medium was adjusted to 8.0–9.0 by adding a 20% ammonium hydroxide (NH₄OH) solution. The reaction temperature was subsequently increased to 70–80°C and maintained until a viscous homogeneous mass was formed. A solution of 1.18 g (0.01 mol) of succinic acid dissolved in 3 ml of ammonium hydroxide was added dropwise to the viscous mixture under constant stirring. The temperature was further elevated to 110–130°C, and stirring continued until a resinous solid mass was obtained. The resulting product was transferred to a porcelain crucible and dried in a vacuum oven at 100°C for 20 hours to ensure complete cross-linking. The solidified polymer was crushed and washed thoroughly with a 5% sodium hydroxide (NaOH) solution to remove unreacted monomers and low-molecular-weight oligomers. The product was then washed repeatedly with distilled water until a neutral pH was achieved. The final product was a white, granular, porous material with a synthesis yield of 92%.

To prepare the metal complexes, the granular UFS sorbent was immersed in 1 N aqueous solutions of Fe(II) and Mn(II) salts at 25°C for 48 hours to reach coordination equilibrium. The resulting coordination compounds were filtered, washed, and dried at 60°C. The structural and thermal properties were characterized using FT-IR spectroscopy  and thermal analysis (TGA/DTA).

Results and Discussion

Figure 1. The FT-IR spectrum of the synthesized UFS polymeric ligand

The FT-IR spectrum of the synthesized UFS (Urea-Formaldehyde-Succinic acid) polymeric ligand exhibits several characteristic absorption bands that confirm its polyfunctional structure and the presence of nitrogen and oxygen donor atoms. A broad and intense absorption band in the region of 3180.62 cm^-1 is observed, which corresponds to the stretching vibrations of the -NH groups from the urea fragments and the -OH groups from the succinic acid moieties. The broadening of this peak indicates the formation of extensive intermolecular hydrogen bonds within the polymer matrix. The bands at 3037.89 cm^-1-2945.30 cm^-1 correspond to asymmetric and symmetric stretching vibrations of the -CH₂- methylene groups, confirming the cross-linking formed by formaldehyde. The intense absorption at 1622.13 cm^-1 is assigned to the Amide I (C=O stretching) and potentially the carboxylate groups from the modified succinic acid. The shift to a lower wavenumber (compared to pure urea or succinic acid) suggests the involvement of these groups in the polymeric framework. The bands at 1556.55 cm^-1 and 1504.48 cm^-1 correspond to N-H bending and C-N stretching vibrations, respectively. The series of peaks between 1454.33 cm^-1 and 1226.73 cm^-1 (specifically 1423.47, 1377.17, and 1292.31 cm^-1) represent C-H bending and C-O stretching vibrations of the carboxylic acid fragments, further confirming the successful integration of succinic acid into the urea-formaldehyde structure.

Figure 2. FT-IR spectrum of the Fe(II) complex of the synthesized UFS polymeric ligand

The coordination mechanism between the synthesized UFS (Urea-Formaldehyde-Succinic acid) polymeric ligand and Fe(II) ions was elucidated through a comparative analysis of their FT-IR spectra. The interaction of the metal ion with the polymer’s donor sites (N and O) leads to significant shifts in characteristic absorption bands. The most critical evidence of complex formation is the bathochromic shift of the Amide I (C=O) band from 1622.13cm^-1 to 1604.77cm^-1. This decrease in frequency indicates that the carbonyl oxygen acts as a primary donor site, transferring electron density to the vacant d-orbitals of the Fe(II) ion.In the high-frequency region (3180.62 cm^-1), the broad band corresponding to -NH and -OH vibrations undergoes flattening and intensity reduction in the Fe(II) complex. This suggests that the internal hydrogen-bonding network of the polymer is disrupted to accommodate the metal ion coordination. The minor shift in the 1292.31 cm^-1 region suggests that the succinic acid fragments participate as auxiliary coordination sites, enhancing the stability of the chelate system. The appearance of a new absorption peak at 617.22 cm^-1 in the complex spectrum, which is entirely absent in the free ligand, provides definitive proof of the formation of Fe-O and Fe-N coordination bonds. The Mn(II) complex shows smaller spectral shifts compared to the Fe(II) complex, suggesting relatively weaker ligand–metal interactions. This behavior may be attributed to the high-spin d⁵ electronic configuration of Mn(II), which generally results in lower ligand field stabilization energy.

Table 1.

Characteristic FT-IR absorption bands of the ligand and complexes (cm^-1) |

Figure 3. Thermal Analysis (TGA/DTA) of the UFS Ligand

The thermal stability and decomposition behavior of the synthesized UFS polymeric ligand were evaluated using TGA and DTA techniques. The thermogram exhibits a well-defined three-stage weight loss process in the temperature range of 25–600°C. Stage I: An initial weight loss of 30.31% is observed between 27.81°C and 209.18°C. This stage is attributed to the evaporation of physically adsorbed water and the release of chemically bound moisture trapped within the polymer matrix. Stage II: The most intensive thermal degradation occurs from 210.50°C to 395.03°C, resulting in a significant mass loss of 47.78%. This corresponds to the structural cleavage of the polymer backbone, specifically the decarboxylation of succinic acid fragments and the breakdown of methylene bridges. Stage III: Above 400°C, the rate of mass loss decreases, leading to the formation of a stable carbonaceous residue. The TGA/DTA results demonstrate that the UFS ligand maintains its structural integrity up to approximately 200°C. This thermal robustness confirms that the sorbent is highly suitable for industrial wastewater treatment, where elevated temperatures may be encountered.

Conclusion

The study demonstrates that the synthesized UFS polymeric ligand possesses a polyfunctional structure capable of coordinating Fe(II) and Mn(II) ions through oxygen and nitrogen donor atoms. FT-IR analysis confirmed the formation of metal–ligand bonds, while TGA/DTA results proved sufficient thermal stability up to 200 °C. The incorporation of succinic acid significantly enhances the chelating ability of the polymer matrix. Therefore, the developed material represents a promising sorbent for the removal and immobilization of transition metal ions from industrial wastewater systems.

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