Investigation of Copper Adsorption Isotherm Models of Organo-MMT Clays

In this study, Montmorillonite (MMT) clay, which is cheap and abundant in nature, was modified using two different modifiers. In the modification of MMT clay, 1-Butyl-3-Methylimidazolium Bromide (BMIM-Br) was used as quaternary ammonium salt and Tetraethylammonium Bromide (TEABr) was used as ionic liquid modifiers. OMMT-1, which was obtained by modifying MMT clay with BMIM-Br modifier, and OMMT-2, which was synthesized using TEABr modifier, were investigated for their use as adsorbents in the removal of Cu(II) ions from aqueous solutions. At a Cu(II) concentration of 635mg/L, OMMT-1 organoclay adsorbed 20.31% and OMMT-2 organoclay adsorbed 10.94% of Cu(II) ions. The data obtained to determine the adsorption mechanism were evaluated for their suitability according to the Langmuir, Freundlich, Temkin, Dubinin-Radushkevitch and Harkins-Jura isotherms. It was determined that the OMMT-1 organoclay obeyed the Langmuir isotherm and its maximum adsorption capacity was 38.91mg/g, while the OMMT-2 organoclay obeyed the Freundlich isotherm and its adsorption capacity, KF, was found to be 0.036. It was determined that the OMMT-1 organoclay was a better sorbent than the OMMT-2 organoclay in removing Cu(II) ions from aqueous

Keywords:Keywords: Organo-montmorillonite; Cu(II) adsorption; Adsorption isotherms; Langmuir isotherm; Freundlich isotherm; RMSE

Uncontrolled consumption, which has increased with industrialization and urbanization, is causing serious pollution of water resources. One of the pollutants commonly found in wastewater is the heavy metal copper Cu(II). Many industries, such as metal processing, electronics, chemicals, fertilizers, paper, and plastics, contain high levels of copper in their wastewater [1,2]. Copper can accumulate in living organisms without degrading and cause toxic effects. Long-term and high-level exposure to copper causes kidney dysfunction, liver disease, neurotoxicity, eye irritation, stress, depression, memory problems, muscle and joint pain, and the development of brain, skin, pancreas, and heart diseases [3,4]. Although copper is an essential element for the body, the World Health Organization has set the maximum limit for Cu(II) in drinking water at 4.0mg/L. The effective removal of biologically non-degradable copper from water is of great importance for both human health and the environment [5].

Montmorillonite (MMT) clay is a layered silicate mineral first discovered in 1696 in the Montmorillon region of France. It is usually found together with impurities such as gravel, quartz, and feldspar, and this mixture is called bentonite. MMT is obtained from bentonite using wet separation methods [6]. Structurally, it consists of layers approximately 1nm thick and 200-300nm in horizontal dimensions. The Na⁺ and Ca²⁺ ions between the layers are exchanged with organic cations, making the clay organophilic. Modification enhances the compatibility of MMT with hydrophobic polymers. [7,8]. MMT is an important type of clay that attracts the interest of researchers in many fields such as adsorption, catalysis, and composite material development due to its high surface area, cation exchange capacity, and excellent thermal stability. Owing to these properties, MMT has a wide range of applications in the design of sensors, filtration membranes, and membranes for lithium-ion batteries [9,10]. One of the commonly used methods for removing toxic metals from wastewater is adsorption. Although various adsorbent materials are used in this process, clay minerals stand out due to their large surface areas, high ion exchange capacities, and ability to effectively retain metal ions thanks to the negative charge in their structures. For this reason, clay minerals are among the most widely used adsorbents in wastewater treatment worldwide [11- 13]. Adsorption isotherms describe the behavior and mechanism of a substance in a liquid medium adhering to a solid surface at a constant temperature and pH. These isotherms help us understand the interaction between the adsorbent and the adsorbed substance, the surface properties, and how adsorption occurs in a single layer, multiple layers, physically, or chemically. Each model has been developed to explain different adsorption conditions and characteristics [14]. Table 1 shows the isotherm models and equations used in this study [15-23].

Table 1:Adsorption isotherm models and equations.
C_o: Initial solution concentration (mg/L)
C_e: Concentration of the substance remaining in the solution after adsorption (mg/L)
q_e: Amount of substance adsorbed onto a unit adsorbent at equilibrium (mg/g)
b: A constant relating to the net enthalpy of adsorbed molecules or ions (L/mg)
q_m: The amount of adsorbed substance per unit weight of adsorbent required to form a single layer on the surface depends on the adsorption energy (mg/g)
K_F: Freundlich constant related to adsorption capacity (mg/g)
n: A constant that provides information about the shape of an isotherm.
A_T: Equilibrium binding constant corresponding to maximum binding energy (L/mg)
b_T: Temkin constant
B_T: Temkin isotherm constant related to the heat of adsorption
R: Universal gas constant (kJ/mol K)
T: Absolute temperature (K)
E: Average adsorption free energy (kJ/mol)
β : A constant related to adsorption energy
ɛ : D-R isotherm constant
A_H ve B_H: Harkins-Jurassic isotherm constants.

In this study, OMMT-1 organoclay was obtained by subjecting MMT clay to a cation exchange reaction with BMIM-Br. Subsequently, OMMT-2 organoclay was synthesized by modifying MMT with TEABr. The adsorption capacities of both organoclays towards Cu(II) ions were determined, and the obtained experimental data were evaluated using various adsorption isotherm models.

In this study, Montmorillonite (MMT, <63μ) clay obtained from Esan-Eczacıbaşı was used as clay [24]. 1-Butyl-3-Methylimidazolium Bromide (BMIM-Br), Tetraethylammonium Bromide (TEABr), Ethanol (C₂H₅OH), Sodium Hydroxide (NaOH), and Hydrochloric Acid (HCl), Copper(II) Sulfate Pentahydrate (CuSO₄·5H₂O), were commercially obtained from Aldrich and Fluka. All chemical reagents used in organoclay synthesis were used directly without undergoing any purification process. A UV-Vis Spectrophotometer (Shimadzu UV-1800) was used for Cu(II) determination, and no buffer or reagent was used during absorbance measurement. Weighing’s were performed using an OHAUS analytical balance with a sensitivity of 0.1mg. pH measurements were performed using a Mettler Toledo (FiveEasy F_{2sub>0) model digital pH meter analyzer.}

Synthesis of OMMT-1 and OMMT-2 organoclays

MMT clay (10g), dried in an oven at 110 °C for 48 hours, was stirred in a mixture of ethanol (30mL) and pure water (40mL) at room temperature for 1 hour. In addition, solutions of 1.97g (0.89mmol) BMIM-Br and 1.89g (0.89mmol) TEABr modifiers were prepared separately in ethanol (3mL). To obtain OMMT-1 organoclay, BMIM-Br solution was added dropwise to the MMT clay solution, and to obtain OMMT-2 organoclay, TEABr solution was added dropwise to the MMT clay solution, and the mixture was stirred at 60 °C for 24 hours [25,26]. The precipitates formed as a result of both processes were filtered, purified by washing with a hot ethanol-water mixture, and dried in an oven at 120 °C for 48 hours [25,27]. Figure 1 shows the chemical synthesis of OMMT-1 and OMMT-2 organoclays.

Figure 1:Chemical synthesis of OMMT-1 and OMMT-2 organoclays.

Characterization of organoclays

The spectroscopic characterization of OMMT-1 and OMMT- 2 organoclays was carried out according to the literature. In the FTIR spectrum of MMT clay, O-H stretching vibrations at 3619cm^- 1, Si-O stretching vibrations at 1003cm^-1, and Al-O bond vibrations at 795cm^-1 were observed [28]. In the FTIR spectrum of OMMT-1 organoclay, ring-bound C-H stretching vibrations were observed in the 3200-3000cm^-1 range. Additionally, aliphatic C-H stretching at 2800cm^-1 and vibrations belonging to C≡N groups at 2191 and 2130cm^-1 were detected. Vibrations at 1570cm^-1 for CH₂-N and CH₃-N bonds, at 1460cm^-1 for CH₃-CN groups in the ring, at 1427cm^- 1 for C-C stretching vibrations, and at 1380cm^-1 for C-C vibrations in the ring were observed [29,30]. The bands observed in the MMT vibration were also observed in the OMMT-2 vibration, with the aliphatic symmetric C-H vibration at 2894-2853cm^-1, the aliphatic asymmetric C-H vibration at 2986-2927cm^-1, and the CH3 vibration at 1443cm^-1, as reported in the literatüre [25]. XRD analysis has determined that when MMT clay is modified with both ionic liquid and quaternary ammonium salt, the distance between clay layers increases [31,32]. Since these data are available in the literature, they have not been repeated in this study.

Preparation of the calibration curve

For adsorption studies, Cu(II) solutions were prepared as fresh solutions from a stock solution containing 6355mg/L Cu(II) using CuSO₄.5H₂O. 1.0, 2.5, 6.0, 8.0, and 10.0mL were taken from the stock solution and diluted to 10mL with water. Thus, solutions of 635, 1588, 2541, 3812, 5083, and 6355mg/L were obtained. The obtained solutions were used to prepare a calibration graph at a wavelength of 790nm using a UV-Vis spectrophotometer (Figure 2) [24].

Figure 2:Calibration graph of Cu(II) solutions.

In this study, adsorption experiments were conducted at room temperature (25 °C). The effect of temperature variation on adsorption was not investigated, and parameters such as pH, adsorbent amount, and initial concentration were evaluated. Adsorption studies on the removal of Cu(II) ions using OMMT-1 and OMMT-2 organoclay materials were conducted at pH=1-3. However, it has been observed that the adsorption capacity of both organo- MMT type adsorbents for Cu(II) ions is very low. Nevertheless, the highest adsorption capacity was achieved at pH=4.5. In contact time studies conducted using a UV-Vis spectrophotometer, adsorption efficiency (%Ads) was found to be quite low. Measurements taken at different time intervals (maximum 3 hours) showed that the system did not reach equilibrium, and the optimum adsorption time was determined to be 24 hours. In this regard, measurements were taken at the end of the 24-hour contact period for each amount of adsorbent used, and evaluations were made. In determining the amount of adsorbate, pH=4.5 and room temperature were kept constant, and the value of 5083mg/L given in the calibration graph in Figure 2 was taken into account in the studies. The effects of OMMT organoclays on Cu(II) adsorption studies were evaluated using the following equations. %Ads [33,34] and the amount of Cu(II) ions retained per unit mass (qe) [35] were calculated using the mathematical expressions given in Equations 8 and 9. Solutions containing various amounts of adsorbent material were left to stand at room temperature and pH 4.5 for 24 hours under the same experimental conditions to reach equilibrium, and the solution was decanted. Measurements were performed using the calibration graph provided in Figure 2 for the Cu(II) concentration.

A₀: Absorbance value read under UV before the solution comes into contact with the adsorbent
A_e: Absorbance value read under UV after the solution comes into contact with the adsorbent
C₀: Initial concentration of Cu(II) solutions (mg/L)
C_e: Cu(II) concentration remaining in the solution after contact with the adsorbent (mg/L)
q_e: Amount of adsorbed substance in equilibrium on the adsorbent (mg/g)
V: Volume of the solution (L)
m: The mass of adsorbent (g)

Adsorption studies

All experiments were performed in triplicate

Figure 3:Effect of adsorbent amount on Cu(II) adsorption by OMMT-1 organoclay

Effect of adsorbent amount: To investigate the effect of initial amounts of organoclays on Cu(II) adsorption, 0.1, 0.2, 0.3, 0.4 and 0.5g of organoclays were weighed, respectively, and their adsorbent properties were studied using 5083mg/L Cu(II) solution (15mL) for 24 hours at 25 °C pH 4.5 (Figure 3 & 4). The data obtained are shown in Table 2.

Figure 4:Effect of adsorbent amount on Cu(II) adsorption by OMMT-2 organoclay.

Table 2:Effect of adsorbent amount on Cu(II) adsorption by OMMT-1 and OMMT-2 Organoclays.

Effect of initial concentration on Cu(II) adsorption: After determining that the best adsorption effect was achieved using 0.1g of organoclay, the effect of different Cu(II) concentrations on adsorption was investigated. For this purpose, 15mL of Cu(II) solutions at different concentrations, as given in Table 3, were each supplemented with 0.1g of OMMT-1 and OMMT-2 organoclays and the solutions were left to stand at pH 4.5 for 24 hours. The highest adsorption capacity was determined to be 5083mg/L (Table 3). The graphs for these results are presented in Figure 4 for OMMT-1 and Figure 5 for OMMT-2.

Figure 5:Effect of initial concentration on Cu(II) adsorption by OMMT-1 organoclay (A:635; B:1270; C:2541; D:3812; E:5083).

Table 3:Effect of initial concentration on Cu(II) adsorption by OMMT-1 and OMMT-2 organoclays.

Adsorption isotherm results: Adsorption experiments were carried out at a constant adsorbent dosage of 0.1g, room temperature (25 °C), contact time of 24 hours, and pH=4.5. Cu(II) solutions with different initial concentrations of 635mg/L, 1270mg/L, 2541mg/L, 3812mg/L, and 5083mg/L were prepared (Figure 6). The Cu(II) adsorption behavior of OMMT-1 and OMMT-2 organoclays was evaluated using the Langmuir, Freundlich, Temkin, D-R, and Harkins-Jura isotherm models, and the corresponding plots are presented in Figure 7. The constants obtained from these adsorption isotherms are given in Table 4. The RMSE values calculated to evaluate the fit of different isotherm models are given in Table 5.

Figure 6:Effect of initial concentration on Cu(II) adsorption by OMMT-2 organoclay (A:635; B:1270; C:2541; D:3812; E:5083).

Table 4:Cu(II) adsorption isotherm graphs of OMMT-1 and OMMT-2 organoclays.

Figure 7:Cu(II) adsorption isotherm graphs of OMMT-1 and OMMT-2 organoclays.

Table 5:RMSE values of OMMT-1 and OMMT-2 organoclays in different isotherm models..

In adsorption studies, pH affects adsorption efficiency by altering the surface properties of the adsorbent. Ma et al. [36] stated that the low amount of Cu(II) ions adsorbed by the adsorbent at low pH could be explained by the adsorption competition between H+ and Cu(II) ions. They reported that when the pH of the solution is increased, the competition between H+ and Cu(II) ions decreases, leading to an increase in the number of negatively charged regions on the adsorbent, which in turn increases the Cu(II) adsorption efficiency. In this study, pH=4.5 was preferred for adsorption studies because it was observed that Cu(II) ions precipitated as Cu(OH)₂ when pH>5. In the literature [5], [37] it has been reported that copper metal precipitates as Cu(OH)₂ at pH values above 6.

When OMMT-1 and OMMT-2 were used, the highest adsorption capacity (q_e) for Cu(II) adsorption was observed when 0.1g of organoclay was used (Table 2). These values were found to be 38.12mg/g for 0.1g of OMMT-1 and 66.72mg/g for OMMT-2. Additionally, it was observed that q_e values decreased with increasing adsorbent amount in both types of organoclay. In accordance with the literature [38-40] it was determined that there was no significant change in %Ads values at the end of 24 hours when the system reached equilibrium. In this study, adsorption experiments conducted with both OMMT-1 and OMMT-2 organoclays revealed a decrease in q_e values, which represent the amount of metal ions absorbed per unit mass, as the amount of adsorbent increased. This situation has been previously reported in the literature by Ekinci & İlter [41] and is consistent with the explanation that an increase in adsorbent quantity may lead to the active regions on the adsorbent surface not being fully saturated, thereby causing a decrease in adsorption capacity [41]. Similarly, Mellouk et al. [38] stated in their study using different amounts of organoclay that the use of high amounts of adsorbent did not significantly change the adsorption capacity of Cu(II) ions and preferred 2.5g as the optimal adsorbent amount in their subsequent experiments. These literature findings are consistent with our experimental data and indicate that increasing the adsorbent amount may limit adsorption efficiency under certain conditions.

The effect of the initial concentration of Cu(II) ions on the adsorption capacity and %Ads was investigated. The highest adsorption capacity was observed at an initial concentration of 5083mg/L (Table 3), and the adsorption capacity corresponding to this value was determined to be 37.17mg/g for OMMT-1 and 67.35mg/g for OMMT-2. Although a significant increase in adsorption capacity was observed with increasing initial concentration, a decrease in %Ads values was observed due to the limited number of active sites on the surface. As the initial Cu(II) ion concentration increases, the amount adsorbed (qe) increases due to the presence of more metal ions in the solution. The observed decrease in the adsorption percentage (%Ads) is due to the limitation of active adsorption areas on the adsorbent surface over time and the approach to surface saturation [41]

Cu(II) adsorption of OMMT-1 and OMMT-2 organoclays was evaluated according to Langmuir, Freundlich, Temkin, D-R and Harkins-Jura isotherms. Table 4 shows the Cu(II) adsorption isotherm constants of OMMT-1 and OMMT-2 organoclays. The Langmuir isotherm was found to be the most suitable isotherm for the OMMT-1 organoclay due to its R² value of 0.9595, the small difference between the experimental q_e and the calculated q_m, and the RMSE value of 3.92, which is a small value when comparing the adsorption isotherm models given in Table 5. In the case of OMMT-2 organoclay, the most suitable model was found to be the Freundlich model, as evidenced by the R² value of 0.9995, the n value representing the adsorption capacity of 0.036, and the RMSE value of 0.85 given in Table 5, [42]. When the isotherm constants given in Table 4 & 5 are ranked from most suitable to least suitable, the Langmuir>Freundlich>Temkin>Harkins- Jura>D-R isotherm was determined for OMMT-1. The most suitable isotherm model ranking for OMMT-2 has been determined as Freundlich>Langmuir>Temkin>D-R>Harkins-Jura. The fact that different isotherm models fit experimental data to varying degrees stems from each model being based on different fundamental assumptions regarding the adsorption process. The findings from this study show that the adsorption of Cu(II) ions onto the organoclay surface is based on electrostatic interactions resulting from increased surface negativity under pH=4.5 conditions [43-45].

It is considered that weak physical interactions such as Van der Waals forces contribute to the adsorption process at a secondary level in interlayer regions. The adsorption mechanism is based on pH-dependent electrostatic attractions and weak Van der Waals interactions originating from the modified clay surface. However, due to the pH dependence of electrostatic interactions, this mechanism is only effective within certain pH ranges and cannot be considered the primary determinant of adsorption under all pH conditions. This situation reveals that adsorption efficiency varies depending on surface charge and environmental pH values. In studies on Cu(II) adsorption in MMT clays modified with different modifiers in the literature, it has been observed that the adsorption isotherms most closely follow the Langmuir and Freundlich isotherms (Table 6).

Table 6:Adsorption of Cu(II) by different types of clay adsorbents.

In this study, two different types of organoclays, OMMT-1 and OMMT-2, were synthesized using commercially available MMT clay. The adsorption of Cu(II) ions was investigated using these synthesized organoclays in different amounts, and the adsorption effect against different Cu(II) ion concentrations was examined with the amount of the best adsorbent material determined. In adsorption studies, the fit of synthesized organoclays to the Langmuir, D-R, Freundlich, Temkin, and Harkins-Jura adsorption isotherm models was investigated [46-49]. The OMMT-1 organoclay follows the Langmuir isotherm due to its high R² and low RMSE value compared to other models. Accordingly, it was determined that OMMT-1 conforms to the Langmuir model, possessing equal energy regions and a homogeneous adsorbent surface, and that Cu ions adhere to the adsorbent surface in a single layer. Furthermore, since the n value is in the range 1< n< 10 and the R² value is 0.9970, which is high compared to other models, it has been observed that the surface of OMMT-2 has a heterogeneous and multi-layered structure in accordance with Freundlich. Due to the hydrophobic nature of the synthesized OMMT-1 and OMMT-2 organoclays and their increased adsorption capacity when used in composite form, it is predicted that they can be used for the effective removal of organic and metallic pollutants in wastewater.

We would like to thank the Uşak University Scientific Research Projects Unit (Project No: 2017/TP018) for their support of this thesis study. This study is derived from Özlem Sökmen’s doctoral thesis at Uşak University, Graduate Education Institute-Department of Chemistry.