Freundlich, Langmuir, Temkin, Elovich, Redlich-Peterson, and Halsey Models of Fenitrothion Adsorption-Desorption Isotherm onto Two Different Soil Textures

Introduction

The adsorption-desorption behavior of pesticides in soils significantly influences their mobility, bioavailability, and environmental fate. Understanding the sorption characteristics of compounds like fenitrothion (an organophosphorus insecticide) is essential for predicting its persistence and potential risks in agricultural ecosystems. Various isotherm models, including Freundlich, Langmuir, Temkin, Elovich, Redlich-Peterson, and Halsey, have been widely employed to describe and interpret the sorption processes of pesticides on different soil types. However, the applicability and accuracy of these models can vary depending on soil texture and composition, necessitating a comparative study to determine which models best fit the sorption data for fenitrothion in distinct soil textures.

Gas or solute adsorption isotherms are usually described by isotherms, in which the amount of adsorbate on the surface of the adsorbent is a function of the gas pressure or solute concentration at a constant temperature. To date, exactly 15 isotherm models have been developed and used [1-4]. Adsorption involves the mass transport of an adsorbate from the liquid phase, bulk solution, film solution and intraparticle transportation to the surface of the adsorbent. No further adsorption occurs after thermodynamic equilibrium is established between the solution and the adsorbent. The most fundamental property of adsorbate-adsorbent interactions is the adsorption equilibrium. Accordingly, theoretical and empirical models that describe desorption have been developed based on thermodynamic equilibrium [5, 6]. The most common adsorption isotherm models include two, three, four, and five parameters. Adsorption can occur as a monolayer or multilayer. Seven error functions were determined to investigate the fitness quality of the isotherm models with the experimental data [7].

Nowadays, the Freundlich isotherm is a very useful tool for adsorption research. Numerous experimental adsorption data have been found to be described by the original empirical Freundlich isotherm equation [8, 9]. Van der Waals adsorption, chemisorption, multilayer adsorption, and monolayer adsorption can all be described by the empirical Freundlich equation [10]. For many years, the commercial utility of adsorbents was demonstrated using the Freundlich isotherm. The adsorption process is governed by a number of intricate interactions. These interactions are summarized by the isotherm, and examining them is essential to comprehending the physicochemical elements involved in the adsorption process. Understanding a process as a multiple-component adsorption system can be accomplished through statistical modeling and experimental measurements [5]. Assuming that each component has an exponential distribution of energies, a multicomponent adsorption isotherm is obtained. The isotherm demonstrates that each component complies with the Freundlich adsorption isotherm on its own [11, 12].

For decades, the Langmuir isotherm model has been widely used in adsorption-desorption processes. It can be theoretically derived based on certain fundamental assumptions [13]. The literature shows that when the Langmuir equation is applied at equilibrium to fit adsorption data, in most cases, the adsorbate concentration at equilibrium is given as its volumetric concentration (such as mg L^-1) [14]. The Langmuir isotherm model, which was originally derived for gas adsorption on solid surfaces, was modified to fit the solute adsorption isotherm on solid surfaces in solution. The level of data fit to the Langmuir isotherm equation in the literature can be improved by simple modification by using a concentration-dependent factor (X) because the solute concentration affects both the adsorption and desorption processes. The modified Langmuir isotherm more accurately describes the experimental data. Moreover, the concentration-dependent factor could be related to the surface heterogeneity factor [15].

The characteristics of 4-chloro-2-methylphenoxyacetic acid adsorption on bituminous shale was evaluated. The adsorption process was studied under various conditions using a batch technique. The adsorption ability of this adsorbent increases with decreasing pH and increasing temperature in a concentration series of 0.6 × 10^-4 to 4.0 × 10^-4 M. Theoretical curves obtained from the Freundlich, Langmuir and Temkin equations show a two-step isotherm [16]. Mesoporous phenolic resin and mesoporous carbon were used for the removal of pesticides (bentazon and s-metolachlor). Pesticide adsorption kinetics and isotherms were studied at pH 2 and 4. Variable isotherm models such as the Freundlich, Langmuir and Temkin models were used to describe the adsorption process [17]. The use of corn cob, an agricultural solid waste material, as a biosorbent for removing metribuzin from water solution was investigated. The maximum sorption capacity of the herbicide was found at pH 5. The sorption process was analyzed by the Freundlich, Langmuir and Temkin isotherm models [18]. The equilibrium adsorption of atrazine on activated carbon prepared from sheanut shell acid was fitted to the Temkin model. The estimated value of energy sorption was < 20 KJ/mol, and the free energy was < 8, indicating that physisorption was nonspecific [19].

Originally proposed in 1939, the Elovich equation is appropriate for systems with heterogeneous adsorbing surfaces and is satisfied in chemical adsorption processes [20]. The Elovich equation has been used to describe numerous adsorption systems including pesticides and metals [20-21]. The Elovich isotherm model was fit to the adsorption data depending on the soil type as occurred for the adsorption of simazine, imidacloprid and boscalid [21]. The adsorption of nonylphenol followed Elovich kinetics and the Elovich isotherm, which indicated multilayer adsorption [22].

A three-parameter adsorption isotherm equation (the Redlich–Peterson isotherm model) was suggested by Redlich and Peterson in 1959. This equation avoids the inaccuracies of the Freundlich and Langmuir isotherm models in some sorption systems [23]. According to a literature survey, approximately 30 papers indicated that the Redlich-Peterson model was more accurate than the Freundlich and Langmuir models in describing the adsorption process [23]; approximately 12 papers indicated that both the Redlich−Peterson isotherm equation and Langmuir isotherm equation had equally high accuracy [24, 25], and the forms of both the Redlich−Peterson equation and Langmuir equation were the same when the α value was equal to 1. In addition, 3 papers indicated that both the Redlich–Peterson and Freundlich equations had equally high accuracy because when the Redlich–Peterson constant is large enough, its form is the same as that of the Freundlich isotherm equation [26]. The results are doubtful because when the constant value is adjusted to 1, both the Freundlich and Redlich–Peterson equations are the same; thus, both accuracies must be the same. However, many papers have indicated that the accuracy of the Redlich−Peterson equation is greater than that of the Freundlich and Langmuir equations [23]. The Redlich-Peterson isotherm model had the best correlation with imidacloprid adsorption on soil. The maximum adsorption capacities were 4.14 mg/g for banana field soil and 5.74 mg/g for cocoa field soil [27]. The adsorption of methomyl on natural clay was evaluated as a possible alternative method for removing methomyl from aqueous solutions. The experimental adsorption data were fitted to the Freundlich, Langmuir and Redlich-Peterson isotherm models [28]. The Redlich−Peterson isotherm model best fit isotherm adsorption data of activated carbon prepared from pistachio shell for many dyes [23].

The Halsey equation describes the relationships among various types of materials under a broad range of relative humidities and temperatures [29]. The Halsey isotherm model is used to study multilayer adsorption at a relatively large distance from a solid surface [30]. The Halsey isotherm model was used to describe the adsorption of pesticides such as bispyribac-sodium and metribuzin and heavy metals such as Cd and Pb [31]. Additionally, this model was used to describe the adsorption of different dyes onto activated carbon [32, 33].

Materials and Methods

Fenitrothion

IUPAC (International Union of Pure and Applied Chemistry) name: O,O-dimethyl O-4-nitro-m-tolyl phosphorothioate. Chemical formula: C₉H₁₂NO₅PS. Solubility in water: 0.038 g/L. Pesticide type: insecticide, miticide. Group: organophosphate. Product: technical 97% a.i.

Tested soils

The soil samples were collected from the surface layer from different locations without pesticides [34, 35]. The physical and chemical properties were determined and are presented in Table 1.

Sorption isotherm

Soil sorption isotherms were quantified using the batch equilibration technique [36]. The experiments were carried out in duplicate with a sorbent mass to fenitrothion solution ratio of 1:5 for the soil. Initial solutions with concentrations in the 10-50 μg/mL range were prepared in 0.01 M CaCl₂. The fenitrothion solutions were equilibrated with soil and in polypropylene centrifuge tubes. The tubes were shaken mechanically at 100 rpm for a time period to achieve equilibrium based on the kinetic study and centrifuged at 5000 rpm for 10 min. The fenitrothion concentration in the supernatants was determined by a spectrophotometer at 266 nm (λ_max) [37].

Desorption isotherm experiments were conducted immediately after the sorption experiments for all concentrations using a parallel system. Five milliliters of fresh 0.01 M CaCl₂ background solution was added to each tube. The tubes were shaken mechanically at 100 rpm to establish a new desorption equilibrium. After centrifugation, the liquid phase containing desorbed fenitrothion was analyzed [2, 38].

Mathematical isotherm models

The empirical formula of the equation is presented in Table 2. Where K_f is a constant indicative of the adsorbent (mg^1-(1/n) L^-1/n g^-1) and 1/n is a constant indicative of the adsorption intensity. The maximum adsorption capacity q_m (mg g^-1) could be theoretically determined, K_f = q_m/C_o^1/n, and it is necessary to operate with a constant initial concentration (C_o); thus, log q_m is the extrapolated value of log q for C = C_o [39].

The Langmuir equation may be written in the form shown in Table 2, where q_e is the amount of solute adsorbed per unit weight of adsorbent at equilibrium (μg g^-1), C_e is the equilibrium concentration of the solute in the bulk solution (mg L^-1), q_m is the maximum adsorption capacity (μg g^-1) and is a constant related to the free energy of adsorption (L mg^-1) [40].

The model is given in Table 2, where β_T and A_T are the Temkin equilibrium constants, β_T = R_T/b and R are the universal gas constants (kJ mol^-1 K^-1), T is the temperature (K), and b is a constant [39].

The Elovich model equation, which implies multilayer adsorption, can be expressed as shown in Table 2. Where K_E is the Elovich equilibrium constant (L mg^-1) and q_m is the Elovich maximum adsorption capacity (mg g^-1). The parameters can be calculated from the slope and the intercept of the plot (q_e/C_e) vs. q_e using the linear form of the equation [41].

The nonlinear and linear forms of this model are represented in Table 2, where α_R (mg⁻¹) and KR (L g⁻¹) are the Redlich-Peterson constants and β is the Redlich-Peterson exponent. The parameters can be calculated from the slope and the intercept of the plot ln C_e/q_e vs. ln C_e using the linear form of the equation [42].

The empirical equation and the linear form are presented in Table 2. K_H and n_H are Halsey isotherm constants and can be obtained from the slope and intercept of the plot of ln q_e versus ln 1/C_e [29, 30].

Validation of the mathematical models

The validity of the adsorption models was tested by the correlation coefficient (R²) [36, 43], which compares the experimental and calculated data [36], the normalized standard deviation (Δqe %) and the summed squared error (SSE) [44].

Results and Discussion

Adsorption and desorption isotherms

Adsorption-desorption isotherms of fenitrothion on clay soil and sandy clay loam soil were examined in the range of 10-50 μg/ml as initial concentrations. The results of the equilibrium adsorption and desorption tests on the two tested soils are presented in Table 3. The equilibrium concentrations and the adsorption of fenitrothion on clay soil and sandy clay loam soil increased as the initial concentrations increased, and the adsorption of fenitrothion ranged from 32.776-167.081 and 9.129-127.259 μg/g on clay soil and sandy clay loam soil, respectively. The amount of adsorbed fenitrothion on the adsorbent has a positive correlation with the initial pesticide concentration [45]. The adsorption of fenitrothion on clay soil was significantly greater than that on sandy clay loam soil at all initial concentrations and all equilibrium concentrations. The low adsorption of fenitrothion on sandy clay loam soil could be due to the physicochemical parameters of the soil surface and the properties of the compound. This observation is in agreement with those of Kovacević et al. [46], who reported that the low adsorption of fenitrothion was due to the hydrated mineral surface and the hydrophobicity of the fenitrothion molecules. The adsorption of fenitrothion is due to the interaction of the hydrophobic molecules of fenitrothion with the organic cations on the surfaces [47]. It was found that fenitrothion is adsorbed on soils in amounts that increase with increasing organic matter content of the adsorbate [48]. In addition, a desorption experiment was performed to determine the reversibility of the adsorption process of fenitrothion to predict the release and movement of the pesticide in the soil profile. The largest proportion of fenitrothion, in the range of 1.007-101.956 μg/g, was released from sandy clay loam soil, while the proportion of fenitrothion released from clay soil ranged from 5.455-47.754 μg/g. Moreover, the maximum equilibrium concentrations for the adsorption and desorption of fenitrothion in clay soil (12.652 and 10.674 μg/mL, respectively) were lower than those in sandy clay loam soil (26.571 and 18.788 μg/mL, respectively). The desorption experiment demonstrated that a strong binding mechanism dominates the pesticide and clay soil interaction. The fact that the adsorption of fenitrothion was largely irreversible from clay soil but largely reversible from sandy clay loam soil must be considered to minimize soil and groundwater contamination [49-51]. The adsorption of fenitrothion increased with increasing concentration, yielding a specific isotherm shape.

The distribution coefficient, Kd, represents the adsorption at equilibrium and is defined as the ratio between the adsorbate concentration in the soil and that in the solution at equilibrium. Cs = Kd Ce (partition of solute between solvent and adsorbent), where Cs is the concentration of the adsorbed pesticide (μg/g soil), Ce is the equilibrium insecticide concentration (μg/mL solution), and Kd is the linear partition constant (mL/g) of fenitrothion between the soil and the solvent. Kd values can be obtained graphically by plotting Cs vs. Ce or mathematically as the mean corresponding to different concentrations [52]. The value of Kd is related to soil organic matter by the following equation K_om = (Kd/%OM) × 100, where K_om is the soil OM partition coefficient [53], which is related to soil OC by the relation K_oc = (Kd/%OC) × 100, where K_oc is the soil OC partition coefficient and is related to the soil clay content as K_clay = (Kd/% clay) × 100, where K_clay is the clay partition coefficient [54]. As shown in Table 4, the average adsorbed quantities of fenitrothion were 89.613 and 67.135 μg/g in the clay soil and sandy clay loam soil, respectively. Additionally, the Kd value of fenitrothion was 10.415 in clay soil and 3.327 in sandy clay loam soil. Interestingly, the average adsorption and values of Kd, log K_om (which is frequently used to measure the adsorption of pesticides), log K_oc and log K_clay were greater in clay soil than in sandy clay loam soil.

Modeling of adsorption and desorption isotherms

The isotherms of fenitrothion in sandy clay loam soil were generally confirmed by the Freundlich adsorption equation (Fig. 1), with a correlation coefficient R² > 0.97, and the values of Δqe (%) and SSE were lower than those in clay soil. As shown in Table 5, for clay soil, K_F = 16.474 and 1/n = 0.755, and for sandy clay loam soil, K_F = 0.061 and 1/n = 2.382. The Freundlich model agreed with the experimental data for fenitrothion desorption in clay soil and sandy clay loam soil (Fig. 2), as indicated by higher values of the determination coefficient (R² > 0.99) and low values of Δqe (%) and SSE (Table 5). The exponent l/n accounts for nonlinearity in the adsorption isotherm. The 1/n value was 0.996 in clay soil and 1.020 in sandy clay loam soil. The Freundlich isotherm model indicates that the adsorption process occurs through multilayer and multisite interactions among neighboring pesticide molecules adsorbed on free active sorption sites [42]. Additionally, the isotherms of the experimental and Freundlich models calculated for fenitrothion desorption in the tested soils were almost identical, whereas that of adsorption decreased, particularly in the clay soil (Fig. 3, 4).

Notably, the K_F values of fenitrothion isotherms were greater in clay soil than in sandy clay loam soil, indicating the highest sorption capacity of clay soil [55]. The greater adsorption (larger K_F value) of fenitrothion by clay soil has been attributed to a higher soil OM content. These results are in agreement with those obtained by Koskinen and Harper [56], El-Aswad et al. [57], and Fouad [58] who reported that, compared with sandy loam soil, which contains a large amount of calcium carbonate, clay loam soil is more susceptible to pesticides, which might block the adsorption sites available for pesticides. The experimental and Freundlich data of fenitrothion in the two soil types showed greater desorption convergence than adsorption, based on the low values of qe (%) and SSE.

The slope (1/n) of the adsorption isotherm for fenitrothion on sandy clay loam soil was greater than unity, suggesting an S-type isotherm (which reflects a low adsorbent-adsorbate affinity at low concentrations, and the solid has a greater affinity for the solvent than the solute at low concentrations) [59]. Additionally, the literature shows that S-type isotherms are often recorded in clays and soils with low OM contents [60, 61]. The 1/n value of the fenitrothion isotherm in clay soil was less than unity, suggesting an L-type isotherm, which is characterized by a reduction in fenitrothion adsorption at higher compound concentrations in solution. This showed greater competition for pesticide adsorption sites, which became limited as the concentration of the solute solution increased. A 1/n value less than one indicates that the soil surface is favorable for pesticide adsorption [62].

The Langmuir adsorption-desorption isotherms of fenitrothion in the two studied soils are shown in Figs. 1 and 2. The adsorption and desorption of fenitrothion by isotherms were well described using the Langmuir model, with an R² value of approximately 0.9 for adsorption in clay soil and ranging from 0.98 to 1.00 for adsorption in sandy clay loam soil and desorption in the two tested soil types. The values of Δqe(%) and SSE were lower for desorption than for adsorption in clay soil and sandy clay loam soil. The constant values of the Langmuir model for fenitrothion are listed in Table 5. Low values of the Langmuir constants were obtained. This result is in agreement with the results obtained by Pandiarajan et al. [62], who stated that the low intercept values obtained correspond to the binding energy of the adsorption, which reflects the binding affinity of the sorbent toward the sorbate. The experimental and Langmuir isotherms calculated for the adsorption and desorption of fenitrothion in the two tested soils are shown in Figs. 3 and 4, respectively. The experimental and calculated isotherms disagree, particularly at high concentrations, suggesting that the Langmuir model is not suitable for this type of linearization. The Langmuir isotherm is applied for monolayer sorption on a homogeneous solid surface [18].

In addition, the essential Langmuir isotherm characteristics can be written by a dimensionless constant called the separation factor (R_L) [30].

R_L = 1 / (1 + b C_o)

where Co is initial concentration of adsorbate (mg/g). R_L the separation factor which used to determine whether the sorption process is favourable or unfavourable for the Langmuir adsorption [62]. R_L values illustrate the adsorption process to be unfavourable (R_L > 1), linear (R_L = 1), favourable (0 < R_L < 1), and irreversible (R_L = 0) [63]. The results indicated that the R_L value was >1 for adsorption fenitrothion in sandy clay loam soil. While the adsorption of fenitrothion in clay soil was favourable, 0 < R_L < 1. The R_L values for desorption of fenitrothion in the two soil types were almost equal 1 thus, the desorption process was linear.

The Temkin adsorption and desorption isotherms of fenitrothion in the two studied soils are shown in Figs. 1 and 2. The points corresponding to the data predicted by the Temkin model are located far from the trend line of the calculated data. Additionally, a large gap between the predicted and experimental adsorption and desorption data was observed (Figs. 3, 4) for fenitrothion in clay soil and sandy clay loam soil. The Temkin equilibrium constants were calculated from the linear equation and are presented in Table 5. The values of the correlation coefficient (R²) were low, and the values of Δqe% and SSE were high. Accordingly, the Temkin isotherm model is not appropriate for describing the experimental data of the adsorption and desorption of the insecticide fenitrothion in clay soil and sandy clay loam soil.

The Temkin model does not provide a close fit to the adsorption data of metribuzin [18], s-metolachlor and bentazon [17]. However, the best model for describing the adsorption of ibuprofen is the Temkin model [39]. The Temkin isotherm is fit only for an intermediate range of pesticide concentrations [64]. The application of the Temkin model is chemical adsorption [65]. Thus, this model was applied in the chemisorption process for cadmium [66], and methylene blue [67]. The Temkin isotherm model concerns the effects of indirect sorbate/sorbate interactions on the adsorption phenomenon and is related to the heat of adsorption [65]. The binding energies (ATs) were calculated from the Temkin plot of fenitrothion. During physisorption, the adsorbate adheres to the adsorbent only by weak van der Waals interactions; thus, physisorption processes have relatively low adsorption energies (< 8 kJ/mol). However, chemisorption processes have relatively high adsorption energies (8 and 16 kJ/mol) because the adsorbate adheres to the surface via chemical bonds [68].

The simulated isotherm curves determined using the Elovich model for the adsorption and desorption of fenitrothion in clay soil and sandy clay loam soil are given in Figs. 1 and 2. Additionally, the Elovich isotherm constants K_E and q_m, as well as the statistical parameters R², Δqe% and SSE for the adsorption and desorption of the tested pesticides in soils, are presented in Table 5. The values of Δqe and SSE were low, ranging from 3.587 to 32.937 and from 0.092 to 31.208, respectively. However, the correlation coefficient values were negligible (< 0.02) for the adsorption and desorption of fenitrothion in clay soil, very low (< 0.4) for the desorption of fenitrothion in sandy clay loam soil, and moderate (approximately 0.9) for the adsorption of fenitrothion in sandy clay loam soil. Thus, the Elovich model is unable to describe the adsorption and desorption isotherms in clay soil and sandy clay loam soil. This result was supported by the comparison of qe (experimental) and qe (calculated) by the Elovich isotherm model, which illustrated that the patterns for the adsorption of fenitrothion in the tested soils were unavailable, as shown in Figs. 3 and 4, respectively. In addition, the values of the maximum adsorption capacity of fenitrothion determined through the linear form of the Elovich equation (Table 5) were lower than the experimental adsorbed quantities at equilibrium corresponding to the plateaus of the sorption isotherms. This means that the assumption of the exponential covering of adsorption sites that indicates multilayer adsorption is not in agreement with the experimental results in the tested concentration range. The Elovich isotherm model was unsuitable for describing the adsorption of various pesticides, such as chlorantranilprole and dinotefuran [3, 4], as well as phenol and chlorophenols [69]. However, the adsorption of copper (II) onto chitin has been described [70].

Figs. 1 and 2 show the Redlich–Peterson isotherm plots for the adsorption and desorption of fenitrothion onto clay soil and sandy clay loam soil. The calculated isotherm parameters and the R² values are given in Table 5. The lower R² values and the incompatibility between qe (experimental) and qe (calculated) by this model (Figs. 3, 4) suggest that the Redlich-Peterson isotherm does not fit the isotherm for the adsorption and desorption of fenitrothion, particularly in clay loam soil. Other studies indicated that the Redlich–Peterson isotherm was the best fit isotherm for sorption on activated carbon for N2 [71], dichloromethane [72], and methyl acetate [73].

The Halsey adsorption and desorption isotherms of fenitrothion in clay soil and sandy clay loam soil are presented in Figs. 1 and 2. The points corresponding to the data modeled by the Halsey model are located in a straight line. Additionally, the experimental and calculated desorption isotherms for fenitrothion were almost identical (Figs. 3, 4). In addition, the data in Table 5 indicate high correlation coefficients and low values of Δqe% and SSE. Therefore, the Halsey isotherm fits the experimental sorption data well for the above reasons, which may be attributed to multilayer adsorption on surfaces and the heterogeneous distribution of active sites. The constant values of the Halsey isotherm model for fenitrothion are included in Table 5. The Halsey isotherm model can be used to describe the adsorption of the herbicides bispyribac-sodium and metribuzin [74], and it can be used to investigate multilayer adsorption systems for metal ions at relatively large distances from adsorbent surfaces [31, 75]. The Halsey model was used to describe the adsorption of a mono azo dye (methyl-orange) on granular pinecone-derived activated carbon [32], and acid dye sorption on activated carbon [33].

Modeling of experimental adsorption data is a very important means to predict the mechanisms of different adsorption systems [63]. Six mathematical isotherm models were examined to describe the experimental data of the adsorption and desorption isotherms for fenitrothion in clay soil and sandy clay loam soil. Linear regression analysis has been one of the most commonly used tools for defining the best fitting sorption models [76]. The validity of the isotherm models was evaluated by the correlation coefficient (R²), which represents the fitting of experimental data with linearized forms of the model. The R² may vary from 0 to 1 when comparing the isotherms of the experimental and calculated data, the normalized standard deviation (Δqe %) and the summed squared error (SSE) [25, 36, 44, 77-79]. The models were compared not only on the basis of one statistical parameter, such as R² but also of the Δqe(%) and SSE values [55]. In fact, a high R² value alone is generally not sufficient for selecting the most suitable model, as it tends to increase, and a number of variables are considered. The Freundlich and Halsey isotherm models agree with the experimental data for the adsorption and desorption of fenitrothion in clay soil and sandy clay loam soil, and the points corresponding to the data predicted by the two models are located almost consistent with the experimental data (Tables 6, 7 and Figs. 5, 6). Additionally, Table 5 shows high correlation coefficients and low values of Δqe% and SSE for fenitrothion. Therefore, both the Freundlich and Halsey isotherm models fit the experimental data. However, the high values of Δqe (%) and SSE corresponding to the high concentrations and disagreement of isotherms for experimental data and Langmuir calculated data at high concentrations of fenitrothion in the two tested soils illustrated that the Langmuir model is limited to describing the experimental data of adsorption and desorption for low concentrations of fenitrothion. This result is in agreement with those of Shanavas et al. [80] and Fouad et al. [37, 81], who stated that the Langmuir isotherm has only limited applicability. Moreover, large gaps between the experimental data and the data predicted by the Temkin model, Elovich model or Redlich-Peterson model for the adsorption and desorption of fenitrothion in clay soil and sandy clay loam soil were identified (Figs. 5, 6 and Tables 6, 7). Additionally, the correlation coefficient (R²), the Temkin and Elovich models were lower, and the values of Δqe% and SSE were greater than those of the other isotherm models (Table 5). Therefore, the Temkin, Elovich and Redlich-Peterson isotherm models are unable to describe the experimental data of the adsorption and desorption of fenitrothion in clay soil and sandy clay loam soil. It can be concluded that the Freundlich and Halsey isotherm models provided much better fits than did the other tested isotherm models. Consequently, the adsorption and desorption behaviors of the insecticide fenitrothion on clay soil and sandy clay loam soil can be well described using these two isotherm models. The literature shows that the Freundlich isotherm model is suitable for describing the experimental data for different compounds, such as metolachlor [60], fenitrothion [50, 59, 82-83], spinosad [59], clothianidin [84], imidacloprid [85], 2,4-D [42, 62], and metribuzin [18, 74]. Additionally, the Halsey isotherm model was used to describe the adsorption of different compounds, such as dyes [32, 33]; heavy metals [75]; and chlorantraniliprole, dimethoate, dinotefuran, bispyribac-sodium, and metribuzin [86-89].

Conclusion

The amounts of adsorbed fenitrothion on soils are in positive correlation with the initial concentration of the pesticide. The adsorption of fenitrothion (average 89.613 μg/g) in clay soil was more than the adsorption in sandy clay loam soil (67.135 μg/g). The adsorption of fenitrothion was largely irreversible from clay soil whereas, largely reversible from sandy clay loam soil, Thus, the amount of pesticide desorbed from clay soil was lower than desorbed from sandy clay loam soil. The desorbed amounts of fenitrothion were lower than their adsorbate amounts, due to the hysteresis phenomenon. The adsorption and desorption behaviour of insecticide fenitrothion on clay soil and sandy clay loam soil can be well described using the Freundilch and Halsey isotherm models. The Langmuir model is limited to describe the experimental adsorption and desorption data at low concentrations. The Temkin, Elovich and Redlich-Peterson isotherm models are unable to describe the experimental data of adsorption and desorption of fenitrothion.

Data Availability: All data are available in the main text or in the Supplementary Information.

Notes: The authors declare no conflict of interest.

Additional Information:

Supplementary information The online version contains supplementary material available at https://doi.org/10.5338/KJEA.2025.44.34

Correspondence and requests for materials should be addressed to Mohamed R. Fouad.

Peer review information Agricultural and Environmental Sciences thanks the anonymous reviewers for their contribution to the peer review of this work.

Reprints and permissions information is available at http://www.korseaj.org

Tables & Figures

Table 1. Physical and chemical properties of the tested soils

Electrical conductivity (EC)

Table 2. Adsorption and desorption isotherm models and their linear forms

Table 3. Adsorption and desorption (± SE) of fenitrothion in soils

Table 4. Partition parameters for the adsorption of fenitrothion in soils

Fig. 1. The linear fit of the models to the fenitrothion adsorption isotherms in soil.

Fig. 2. The linear fit of the models to the fenitrothion desorption isotherm in soil.

Table 5. Adsorption and desorption isotherm parameters of fenitrothion in soil

Fig. 3. Isotherm curves of experimental and calculated data from models for the adsorption of fenitrothion in soil.

Fig. 4. Isotherm curves of experimental and calculated data from models for the desorption of fenitrothion in soil.

Table 6. Comparison of the experimental (exp) and calculated (cal) adsorption quantities of fenitrothion by different isotherm models in clay and sandy clay loam soils

exp: experimental, cal: calculated, *: odd values.

Table 7. Comparison of the experimental (exp) and calculated (cal) desorption quantities by different isotherm models for fenitrothion in clay and sandy clay loam soils

exp: experimental, cal: calculated

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