ISOTHERMS OF THE SYNTHESIZED IONITE FOR Al3+ ION ACCORDING TO LANGMUIR AND FREUNDLICH MODELS

ABSTRACT

In this study, the optimal conditions for synthesizing a copolymer based on crotonaldehyde oligomer and urea were investigated. The resulting copolymer was treated with colloidal silicasol to obtain a hybrid composite, and its sorption properties with respect to aluminum ions were examined. To determine the optimal temperature for the sorption of aluminum ions by the ion-exchange material, experiments were conducted at various temperatures (20 °C, 30 °C, and 40 °C). In order to assess whether the sorption process of aluminum ions by the synthesized ion-exchanger is exothermic or endothermic, the Langmuir and Freundlich isotherm constants were calculated.

АННОТАЦИЯ

В данном научном исследовании были изучены оптимальные условия синтеза сополимера, полученного на основе олигомера кротонового альдегида и мочевины. Полученный сополимер подвергался модификации с использованием коллоидного кремнезоля, в результате чего был получен гибридный композит. Сорбционные свойства данного материала по отношению к ионам алюминия были детально исследованы. Для определения оптимальной температуры сорбции ионов алюминия данным ионитом были проведены опыты при различных температурах (20 °C, 30 °C и 40 °C). С целью установления термодинамического характера процесса сорбции (экзотермический или эндотермический) были рассчитаны константы изотерм Ленгмюра и Фрейндлиха.

Keywords: static exchange capacity, adsorption isotherm, urea, croton aldehyde, potassium hydroxide.

Ключевые слова: статическая обменная ёмкость, адсорбционная изотерма, мочевина кротоновый альдегид, гидроксид калия.

INTRODUCTION

According to numerous scientific studies conducted by researchers to date, it has been established that anion exchangers are capable of forming complexes with a number of metal ions. However, the fundamental nature of this process has not yet been sufficiently studied or fully explained. Nevertheless, analysis of various scientific sources suggests the idea that anion exchangers can form complexes with metal ions having the following structures: (RNH₂)₂MeX₂, (R₂NH)₂MeX₂, and (R₃N)₂MeX₂. According to the authors, the instability constant of a metal–ammine complex is inversely proportional to its sorption capacity [1-3].

The uptake of metal ions by the resin occurs not only as a result of complex formation, but also through the formation of poorly soluble compounds (such as salts, oxides, and even free metals, provided that reducing agents are present in the system), as well as via physical sorption [1].

Despite the fact that the nature of the sorption phenomenon has not been fully clarified, some researchers have applied it for practical purposes such as the purification of brine from heavy metal mixtures, the separation of cobalt and nickel, and the separation of chromium, iron, and copper from manganese, cobalt, and nickel [5-6].

MATERIALS AND METHODS

Crotonaldehyde was oligomerized at a temperature range of 10–70 °C for 3 hours, resulting in a brownish powder-like oligomer. The obtained oligomer is well soluble in organic solvents such as pyridine, dimethylformamide, acetone, and dimethyl sulfoxide, but insoluble in water. The synthesized oligomers were reacted with urea at a temperature of 20–30 °C. To enhance the mechanical strength of the resulting product, it was treated with colloidal silica sol. The static exchange capacity of the composite with respect to Al³⁺ ions was determined at various temperatures, and based on the obtained data, adsorption isotherms were established [5-6].

The adsorption isotherms of the ion exchanger (Langmuir and Freundlich) were used to characterize the equilibrium adsorption behavior.

The adsorption capacity (A_S) of the ion exchanger was calculated using formula I.

A_S =                                                                      (I)

Here, C_d is the initial concentration of ions in the solution.

 C_m - the equilibrium concentration of ions in the solution.

m_i – mass of the ion exchanger

V- volume of the solution

The linear form of the Freundlich isotherm was calculated using equation (II).

A= K_F C_m^1/n                                                                      (II)

Here, A_mak- is the maximum comparative adsorption, and C_m - is the equilibrium concentration of ions in the solution.

The linear form of this equation is expressed as equation (III).

logA= log K_F +(1/n)logC_m                                                         (III)

In the equation, K_F – is the Freundlich constant, and 1/n -  is the adsorption intensity. The Langmuir isotherm was calculated using equation (IV).

A=                                                                  (IV)

Here, A- is the comparative adsorption,
A_max- is the maximum comparative adsorption,
C_m -  is the equilibrium concentration of ions in the solution,
K_L -is the Langmuir constant.

The graphical method was used to determine the constants in the Langmuir equation. For this purpose, the Langmuir formula is expressed as follows:

=  +                                                              (V)

Here, A_mak represents the maximum adsorption capacity (mg/g), and K_L (l/mg) is the Langmuir isotherm constant, which indicates the affinity between the ions and the ion exchanger in the experiment.
The separation factor R_L was calculated using the following equation [4].

R_L =                                                                     (VI)

RESULTS AND DISCUSSION

When studying the isotherms of the synthesized ion exchanger according to the Freundlich model, the value of R² at 40 °C was found to be 0,92, which is higher than the values obtained at other temperatures. The higher R² value indicates a stronger correlation between the adsorption process and temperature, suggesting that the adsorption of Al³⁺ ions is more efficient at elevated temperatures. The Freundlich constant K_f  was also found to be 3,89 at 40 °C, indicating that the adsorbent exhibits stronger adsorption properties at this temperature.

Analyzing the results obtained according to the Langmuir model shows that as the temperature increases, the value of  A_max also increases correspondingly, indicating that the adsorption process is not exothermic but rather endothermic. With increasing temperature, the Langmuir constant K_L also increases, suggesting a stronger interaction between the adsorbent and adsorbate, which is consistent with an endothermic adsorption process. The values of the separation factor R_L were found to be in the range of 0.05–0.07, indicating a highly favorable adsorption process.

 Figure 1. Adsorption isotherms of CA-U-CS ion exchanger for Al³⁺ ions according to the Freundlich model at: 1-20 ⁰C, 2-30 ⁰C, 3-40 ⁰C

 Figure 2. Langmuir isotherms of the CA-U-CS  ion exchanger for Al³⁺ ions. 1-20 ⁰C, 2-30 ⁰C, 3-40 ⁰C.

CONCLUSION

In conclusion, the synthesized ion exchanger exhibits the ability to sorb aluminum ions, and it was determined that the sorption process is endothermic in nature. The optimal temperature for aluminum ion sorption was concluded to be 40 °C based on the evaluation of Freundlich and Langmuir isotherm constants.

However, the coefficient of determination R² in the Langmuir model was below 0,7 showing that the Langmuir model does not fit the experimental data well. Therefore, it was concluded that the adsorption of Al³⁺ ions by the synthesized ion exchanger is better described by the Freundlich model.

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