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CHEMICAL PROBLEMS 2025 no. 1 (23) ISSN 2221-8688
101
ADSORPTION KINETICS AND ISOTHERM MODELS OF Cd(II), Zn(II), AND Cr(VI) BY GEORGIAN NATURAL AND MODIFIED FORMS OF MORDENITE
L. Akhalbedashvili, N. Gagniashvili, G. Todradze, N. Loria
Ivane Javakhishvili Tbilisi State University, Caucasian Institute of Mineral Resources, Tbilisi,
Georgia
e-mail: [email protected]
Received 13.03.2024 Accepted 15.05.2024
Abstract: Environmental pollution with heavy metals is a problem of worldwide importance due to their toxicity and potential health risks. Among the heavy metals found in industrial waters are such toxic metals as Cd(II), Zn(II), and Cr(VI). In this study, the potential of Georgian natural and modified mordenite as a low-cost, eco-friendly adsorbent for the removal of cadmium (II), zinc (II), and chromium (IV) from aqueous solutions is studied. Langmuir and Freundlich isotherms are used to model adsorption data. Studies have shown that the correlation coefficient values (R2) were best fitted by the Langmuir isotherm. The maximum adsorption capacities are 0.84mg/g, 118.92mg/g and 2.25mg/g for cadmium(II), zinc(II) and chromium(IV) respectively as described by Langmuir isotherm. Keywords: Mordenite; Isotherm; Heavy metals. DOI: 10.32737/2221-8688-2025-1-101-106
Introduction
The content of heavy metals in wastewater increases due to human activities [1]. For example: the electrical industry, batteries, pesticides, mining industry, textile industry, petrochemicals, paper production, and use of electrolysis. Heavy metals are not biodegradable [2], and they may be carcinogens [3, 4]. Therefore, the high content of heavy metals in wastewater may cause serious health problems for living organisms. Among the most common heavy metals are zinc (Zn), cadmium (Cd), and chromium (Cr). Even very small amounts of these metals can have dangerous effects on human health [5].
Zeolites are often used for adsorption of heavy metals. Zeolites exhibit different adsorption capacities for heavy metal ions. Natural zeolites are an economical alternative
for the absorption of heavy metals. However, the mechanism of heavy metal adsorption by zeolites has not been well-determined [6, 7]. As a rule, the adsorption mechanism is studied by modeling adsorption isotherms and kinetics of process. All these are conditional estimates of adsorption, which cannot adequately distinguish between the adsorption capacities of different metals.
Adsorption is the most preferred method for removing heavy metals from solutions due to its simplicity, high efficiency, and low cost of operation [8, 9]. Georgian natural mordenite has a high potential for water purification applications. This research aims to study the ability of natural and modified mordenite to remove Cd(II), Zn(II), and Cr(VI) ions from aqueous solutions.
Experimental part
Chemically pure reagents were used for cadmium chloride, zinc nitrate, and potassium all experiments. Model solutions of dichromate, respectively. All working solutions cadmium(II), zinc(II), and chromium(VI) (1000 were prepared by stepwise dilutions of the stock mg/L) were prepared in distilled water using solution, from 5 to 200 mg/L. The pH was
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CHEMICAL PROBLEMS 2025 no. 1 (23)
adjusted with 0.1 M hydrochloric acid or 0.1 M sodium hydroxide solutions. Experimental processes included complete adsorption studies of Cd (II), Zn (II), and Cr (VI) cations on natural and modified mordenite, equilibrium, isothermal modeling and kinetic analysis of adsorption processes.
The adsorption experiment was performed by adding 50 ml of metal model solution to 1 g
E =
(Co - C)V
[10]
of sorbent. Mixing was done for 1 hour at room temperature (22°C). After shaking, the solution was filtered immediately. The metal concentration in the filtrate was determined by the Atomic Absorption method.
The percentage removal of metal ions by natural and modified mordenite as well as the adsorption capacities were calculated from the following equation:
(1)
where: Co and C - the initial and final (at identify the mechanism of adsorption.
equilibrium) metal ion concentrations (mg/l); E- the adsorption capacity (mg/g); V - the volume of the solution (L); m - the mass (g) of the adsorbent. Analysis of adsorption isotherms helps to
Adsorption isotherm studies were performed using the Langmuir and Freundlich models, which can be calculated using the following equations:
^ CLinax KLce (2)
The linear form of the Langmuir isotherm is expressed by the equation:
<? e qmax^L ' Omax [12] (3)
The Freundlich adsorption isotherm is an empirical equation used to describe a heterogeneous system, expressed by the formula:
qe = KFc\tn (4)
The logarithmic form of the Freundlich isotherm is given by the following equation:
lnCe = lnKp+ (1/n) (5)
n indicates how favorable the adsorption adsorbate adsorbed per unit mass of adsorbent at process is, KF is defined as the adsorption or equilibrium [13].
distribution coefficient, it is the amount of
Results and discussion
Experimental data were analyzed using adsorption isotherm equations. Langmuir and Freundlich isothermal constants (KL; lnKF) are determined and the correlation coefficient (R2) is calculated. For each metal, the results obtained from the kinetic study of adsorption are used.
The experimental data obtained from the equilibrium studies were correlated with the Langmuir adsorption isotherm. Linear diagrams (1/Ce vs 1/qe) were obtained for all three heavy metals in different concentration ranges of metal ions (Cd from 20 to 100 mg/l; Zn from 20 to 100 mg/l; Cr 40 to 100 mg/l). Langmuir
m
isothermal constants are presented in Table 1.
Table 1. Calculated Langmuir isothermal constants for the removal of cadmium (II), zinc (II) and
chromium (VI) from solution.
Heavy metal Adsorbent Qmas fag/g) R2 KL (L/mg)
Cd MOR 0.84 0.97 0.0229
H-MOR 0.65 0.94 0.0356
MOR-OH 0.43 0.91 0.0386
Zn MOR 118.92 0.99 0.0025
H-MOR 57.12 0.99 0.0057
MOR-OH 55.04 0.99 0.0057
Cr MOR 2.25 0.98 -0.047
H-MOR 2.47 0.95 -0.047
MOR-OH 2.56 0.95 -0.046
The results in Table 1 show that the Langmuir isotherms for cadmium (II), zinc(II), and chromium(VI) cations are linear. This means that the isotherm fits the experimental results quite well, as evidenced by the value of the correlation coefficient R2, whose value ranges from 0.91 to 0.99. Adsorption capacity
for all metals is almost similar for both initial and modified mordenites. Maximum adsorption capacity qmax. represents a single-layer coating of the sorbent with sorbate. The best results were obtained on MOR: for cadmium adsorption capacity reaches 0.84 mg/g, for zinc - 118.92 mg/g and for chromium 2.25 mg/g.
Fig. 1. Linear regression analysis of the Langmuir isotherm for the adsorption of cadmium (II), zinc (II) and chromium (VI) on initial and modified mordenite.
According to the results of adsorption capacity qmax, the the sorbed metals on all three forms of zeolite will located in the following order: Zn>Cr>Cd.
Fig. 1 shows that in all adsorption studies, the results of all three sorbents vary slightly for each metal. Overall, MOR and H-MOR forms have a higher adsorption capacity compared to MOR-OH. The Freundlich isotherm provides information about the heterogeneity of the
surface and suggests that all adsorption sites are energetically unequal and the adsorption energy is irregular. A line plot is obtained of lnqe vs lnCe over a specified concentration range for each metal and the values of k and n are calculated. The K value is a measure of the adsorption capacity of the zeolite and it increases in direct proportion to the amount of adsorbed metal.
Table 2. Calculated Freundlich isothermal constants for the removal of cadmium (II), zinc (II) and
chromium (VI) from solution.
Heavy metal Adsorbent InKf n R2
Cd MOR -2.887 1.912 0.982
H-MOR -2.488 2.478 0.839
MOR-OH -2.997 2.287 0.97
Zn MOR -1.183 1.021 0.998
H-MOR -1.098 1.043 0.998
MOR-OH -1.11 1.046 0.998
Cr MOR -6.449 0.329 0.998
H-MOR -5.849 0.351 0.986
MOR-OH -5.752 0.359 0.987
Fig. 2. Linear regression analysis of the Freundlich isotherm for the adsorption of cadmium (II), zinc (II) and chromium (VI) on initial and modified mordenite.
The results obtained from adsorption and chromium (VI) cations. The correlation
kinetic studies (Table 2) are consistent with the coefficient (R2) values range from 0.815 to
Freundlich isotherm for cadmium (II), zinc (II) 0.998. n gives us an indication of how favorable
the adsorption process is on different adsorbents. lnKF reflects the distribution coefficient of heavy metals at the moment of equilibrium. When the value of n is greater than one, it allows us to assume that adsorption is favorable at high concentrations but much less at low concentrations.
From the curves in Fig. 2, it is clear that as the initial concentration of heavy metals increases, the amount of metals adsorbed by zeolites also increases.
The Langmuir isotherm fits the experimental data well, as revealed by the correlation coefficient values. For cadmium, the suitability of adsorbents based on the Langmuir
isotherm is as follows: H-MOR has a better correlation coefficient than MOR, and MOR is better than MOR-OH. The effectiveness of all three sorbents for zinc ions is the same (R2=0.99), and the order of adsorption activity for chromium is as follows: MOR>H-MOR>MOR-OH.
The Freundlich isotherm shows less agreement with the experimental data than the Langmuir isotherm. The value range of the correlation coefficient varies from 0.839 to 0.998. This indicates that Langmuir isotherm adsorption data are more accurate than Freundlich, which in turn indicates that the adsorption process is essentially homogeneous.
Conclusion
Based on the conducted research, conclusions have been drawn, which can be formulated as follows:
The results of the equilibrium study showed that natural and modified mordenite could be used as a good adsorbent to remove cadmium (II), zinc (II) and chromium (VI) from solution. The adsorption values of zinc (118.92 mg/g) and chromium (2.25 mg/g) on MOR in 140 and 2.7 times accordingly prevailed the maximum meaning of adsorption of cadmium
(0.84mg/g). According to the results of qmax., the sorption of the studied metals on all three forms of zeolite decreases in the following order: Zn>Cr>Cd.
Langmuir and Freundlich isotherm models are used to characterize and evaluate experimental data. A relatively isothermal fit of the correlation coefficients can thus be arranged as Langmuir > Freundlich. Therefore, the experimental results of adsorption fit best with the Langmuir isotherm model.
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