The textile industry is one of the most important industries in the developing countries of the global economy, and is considered as a contributor to environmental pollution and health risks, because the wastewater of this industry contains a variety of pollutants from toxic metals, dissolved solids and residual dyes that pose threats to the water system and human and animal health . Dyes are chemicals that appear in the effluents of the textile industries, so they are strictly removed due to their complex composition and synthetic origins . To remove these different dyes from wastewater, many methods are used, the most important of which are flocculation, nano filtration, ozone, reverse osmosis, electrochemical and biological decomposition, photolysis, chemical oxidation, and adsorption [3,4]. Adsorption is one of the most powerful methods for removing dyes from their aqueous solutions due to its efficiency, high speed, low cost, flexibility and simplicity. Adsorption techniques have been successful in reducing the concentration of dye from industrial effluents by using adsorbent materials such as polymeric materials , activated carbon, clay , chitin, peat, leather  and others. Adsorption process means the retention of ions, molecules or atoms on the surfaces of solid materials through physical or chemical bonding. Cationic dyes are widely used in industry and the most dangerous type of dye. It is reported that 12% of the annual production of cationic dyes is lost through industrial waterways that pollute the environment .
Methyl violet 10B (MV-10B) is a synthetic cationic dye also known as gentian violet, basic violet 3, and it belongs to the triarylmethane group. Its IUPAC name is: [4- [bis [4-(dimethylamino) phenyl] methylidene] cyclohexa-2,5-dien-1-ylidene]-dimethylazanium;chloride. Figure 1 shows the chemical formula for MV10B dye. This dye is widely used in the textile industries for dyeing cotton, wool, silk and nylon, in the manufacture of printing inks as well as biological stains, and is a dermatological agent in veterinary medicine. It is soluble in water and alcohol, and its C.I. number is 42555 .
In this work, the vegetable tanned leather waste of cow (VTLW-C) was used as a low cost adsorbent to remove (MV-10B) dye from its aqueous solutions. The properties of the adsorbent were evaluated using FTIR and SEM. To evaluate the properties of the adsorption process, batch adsorption experiments were conducted. Different isothermal models were studied and thermodynamic parameters were calculated for the adsorption process. Adsorption kinetic carried out using first-order pseudo and the second-order pseudo models to estimate the rat constants for the adsorption of MV-10B dye onto VTLW-C.
Adsorbent: Leather waste sample were obtained from a local tannery, which was supplied by Al-Wafa Leather Tannery Company (Baghdad city). The waste leather sample was cut into small pieces, and then initially stirred with distilled water for two hours to remove the excess tannin given the potential form absorbent interference in UV spectrophotometer. It was dried in an oven at 50 °C. The shrinkage temperature of vegetable tanned hides is between 75-85 °C in the humid state. The drying process of the leather sample is continued until it reaches a constant weight. Leather waste pieces are ground into granules using a laboratory mill into 7-14 mesh size particles. The samples are stored in desiccator to keep it dry in order to use these sample as adsorbent in our experiments after characterization of these sample using different technique (FTIR and SEM) to study surface properties.
Adsorbate: A stock solution related to (MV-10B) dye of 1000 mg/L was prepared by dissolution of dye (1 g) into distilled water (1000 mL). The stock solution was diluted to prepare a different concentration of (MV-10B) dye (5,15,25,35 and 45) mg/L.
Batch adsorption experiment: Batch adsorption experiments were performed to study the optimal state of adsorption of dye (MV-10B) on leather waste (VTLW-C) as an adsorbent at 298 K, using an electrostatic shaker at 150 rpm. The effect of cutaneous residue dose was studied by changing the weight (VTLW-C) from 0.05 g to 0.3 g at a dye concentration of 25 mg/L, and solution volume 25 mL, for 45 minutes. The effect of the shaking time was studied by taking different times from 5 to 90 minutes with an adsorbent weight of 0.2 g/50 mL of dye solution and a concentration of 25 mg/L. To study the effect of MV-10B dye concentrations, different concentrations from 5 mg/L to 45 mg/L with a weight of 0.1 g/25 mL of MV-10B dye were prepared for 45 minutes. The concentration of residual MV-10B dye was determined by a UV-Vis-Spectrophotometer (Shimadzu UV 1800, Japan). The removal efficiency R% and amount of adsorbed dye at equilibrium (mg/g) for the leather waste were calculated by the following equation :
Where and are the concentration of MV-10B dye at starting and at the equilibrium (mg/L), m is the weight of leather waste (VTLW-C) (g) and V is the volume of the solution (L).
Langmuir adsorption isotherm: In aqueous solution Langmuir equation written as :
Which: Ce is adsorbate’s equilibrium concentration (mg/L), Ce is the amount of the material adsorbed per gram of the adsorbent at equilibrium (mg.g⁻¹), Qm is the maximum capacity of the mono-layer coverage (mg/g) and KL is Langmuir isotherm constant (L. mg-1. The values of Qm and KLare calculated from the slope as well as intercept of the line obtained when drawing Ce/Ce versus Ce. The fundamental property can also be expressed by Langmuir's equation in terms of a separation factor Rs or dimensionless constant, using the following equation:
Where Ci is the starting concentration (mg. L⁻¹) and KL Langmuir constant (L/mg).
The values Rs between 0 and 1 indicate that the adsorption is adequate, favorable (0 <Rs< 1), linear (Rs= 1), unfavorable (Rs> 1) and Irreversible (Rs= 0).
Freundlich adsorption isotherm: The Freundlich isotherm linear equation is expressed as :
Where Ce is the concentration at equilibrium (mg/L), KFr is Freundlich constant in (mg.g-1(mg.L-1)-1/n related to adsorption capacity and is the heterogeneity factor. The values of KFr and 1/nf are calculated from the intercept and the slope of the linear relationship between ln qe against ln Ce. The adsorption curve is linear when it (1/nf=1) indicating that there is no interaction between the adsorbed types and the adsorption sites are of the homogeneous type . While <1), it indicates a favorable or normal adsorption, when the value of (1/nf>1), indicates cooperative adsorption [14 ].
Temkin isotherm: The linear relationship of the Temkin isotherm is described by the following equation:
The values of AT and BT can be calculated from the intercept and the slope when plotting qe against ln Ce . which was calculated from the following equation:
Which: is Temkin isotherm constant, T is absolute temperature (K) and R is the gas constant (8.314 J/mol.K).
Dubinin-Radushkevich (D-R) isotherm: The linear D-R equation was obtained by the following way:
Which: qe is the amount of adsorbate in the adsorbent at equilibrium (mg.g⁻¹), KD-R is theoretical adsorption saturation capacity (mg.g⁻¹), B is D-R isotherm constant (mol².kJ²) and ε is Polanyi potential which described as :
Which: R is the gas constant (8.314 J/mol.K), is equilibrium concentration of adsorbate (mg/L) and T is the absolute temperature (K).
KD-R and B values of the intercept and slope can be calculated from equation (1.15) when plotting versus ε². The adsorption means free energy (E), which is calculated from the following equation:
When the E value < 8kJ/mol the adsorption process can be classified as physical adsorption, while if the E value is in the range of 8kJ/mol to 16kJ/mol, then the adsorption process is a chemical adsorption process .
Thermodynamic parameters such as standard gibbs free energy (ΔG0), standard enthalpy change (ΔH0), and standard entropy change (ΔS0) are important to check the spontaneity and randomness of the adsorption process. These parameters can be calculated by the following equations :
Where R is the gas constant (8.314 J/mol.K), Keq is the equilibrium constant of the adsorption, T is the absolute temperature (K), Ci is the starting concentration (mg/L) of the adsorbate, Ce is the equilibrium concentration (mg/L) of the adsorbate, V the volume of the adsorbate solution (L) and m is the weight of the adsorbent (g). The values of the standard entropy change ΔS0 and standard enthalpy change ΔH0 can be obtained from the intercept and slope of the linear plot of lnKeq against 1/T through to the Van’t Hoff equation :
Pseudo-first-order (PFO) model or called Lagergren equation for liquid/solid adsorption systems based on solid capacitance has been widely used to describe adsorption kinetics [18,19]. It was expressed as :
Pseudo-second-order (PSO) model of adsorption kinetics is described as :
Which qe is equilibrium adsorption capacity (mg.), qt is the adsorbed amount of adsorbate onto adsorbent at time t (mg.) and K1 is rate constant for the adsorption process per min (min-1) for the (PFO) model. K2 is the equilibrium rate constant for a (PSO) model (g/mg.min). The slop and intercept of linear plots of ln (qe-qt) against t and t/qt against t were applied to estimate the values of k1, qe for the pseudo- first -order and k2 and qe for pseudo-second-order.
SEM analysis was carried out using JTYS-1000, China. Figure 3 shows the SEM images of VTLW-C sample surface. SEM was considered as a good technique to estimate the shape surface, morphology and structure of solids. Figure 3 shows that the surface of the VTLW-C sample is mostly fiber-like and rod-like shapes. These resulting micrographs reveals that the adsorbent is in the form of fibers with its highly organized structure [26,27]. The collagen structure is preserved during tanning to ensure the stability of the leather waste .
Effect of adsorbent amount:
The impact of VTLW-C amount on the removal percentage was evaluated at 298K. From Figure 4, we notice that the R% values increase with increasing amount of VTLW-C and reach nearly a constant value from 0.15 g to 0.3 g adsorbent. This increases could be attributed to the introduction of several active sites for adsorption . The optimum weight chosen for all experiments is 0.1 g.
Effect of shaking time: Figure 5 shows the impact of adsorption time on the adsorption of MV-10B dye. It turns out that the removal percentage increases in a time of 5 minutes and then reaches a stable state in a time of 45 minutes and remains stable. Therefore, 45 minutes were chosen as the best time for the adsorption experiments.
Effect of starting MV-10B dye concentrations: Figure 6a shows the impact of starting concentration of MV-10B dye on to the removal percentage. In this Figure, the removal percentage decreases as the R% values change from 96.2% to 84.8% for VTLW-C, when the starting concentration of MV-10B dye is increased from 5 to 45 mg/L. This trend is due to the saturation of the adsorption sites on the surface of the adsorbent material and the lower adsorption . Also, Figure 6b shows that the adsorption capacity changes from (1.2025 mg/g) to (9.8854mg/g) for VTLW-C. Therefore, 15 mg/L was selected as the optimal concentration for all experiments.
Langmuir adsorption isotherm: Figure 7 shows the Langmuir isotherm for the adsorption of MV-10B dye onto VTLW-C at different temperatures. Table 1 represents the values of Langmuir constants, correlation coefficients and the separation factor.
From this Table, the values of R2 lies between 0.2038 and 0.9694 were found. This indicates that the Langmuir isothermal model is not suitable, and the values of separation factor Rs for the adsorption of 45 mg/L MV-10B dye onto leather sample at different temperatures is 0< Rs <1, indicated that the adsorption process is favorable .
Freundlich isotherm model: Figure 8 shows the Freundlich isotherm of MV-10B dye onto VTLW-C, at different temperatures. Table 2 shows the experimental Freundlich isotherm parameters for the adsorption of MV-10B dye onto leather waste sample.
In this Table, the values of R2 lies between 0.8816- 0.9729. These values are reasonable with Freundlich model isotherm than Langmuir isotherm. The values (1/nf) range between (0 and 1) which gives a measure of the intensity of adsorption and surface heterogeneity . This means that the adsorption process was performed on a heterogeneous surface.
Temkin isotherm model: Figure 9 shows Temkin adsorption isotherm plots for the adsorption of MV-10B dye onto VTLW-C at different temperatures. Table 3 shows the Temkin isotherm for this process.
In this Table, increasing temperature leads to decrease in the values of AT indicating that the adsorption process is an exothermic . The small value of BT <8 KJ.mol-1 indicates a weak interaction between the MV-10B ions and VTLW-C surface, indicating that the adsorption is most likely physisorption [34, 35].
Dubinin–Radushkevich (D-R) isotherm: Figure 10 shows the D-R isotherm plots for the adsorption of MV-10B dye onto VTLW-C at different temperatures. Table 4 shows the D-R isotherm constants for this process.
From Table 4, it is revealed that mean free energy (E) values were less than (8kJ/mol) which suggested that the adsorption of MV-10B dye onto VTLW-C sample is of physisorption process .
Adsorption thermodynamics: Figure 11 refers to the Van’t Hoff plots for the adsorption of MV-10B dye onto VTLW-C. Table 5 shows thermodynamic parameters ΔG0,ΔS0 and ΔH0 for the adsorption of MV-10B dye onto leather waste at different temperatures.
In this table, the negative values of ΔG0 show that the adsorption process is favourable and spontaneous at decreasing temperatures, and no external energy needs to be introduced from outside the system . The negative values of ΔH0 indicate that the adsorption of MV-10B dye onto VTLW-C occurs with releasing heat and exothermic process. The negative values of ΔS0 indicated that dye in the solid phase is arranged in an orderly manner, where adsorption causes a decrease disorder in system .
Kinetic study: The pseudo-first-order (PFO) and pseudo-second-order (PSO) were used to study the mechanisms of adsorption of MV-10B dye onto the leather waste sample as adsorbent to find the appropriate kinetic model for this process. Kinetic experiments were performed at starting concentration of MV-10B dye (5, 15, 25, 35 and 45) mg/L, adsorbent dosage 0.2g/50mL and at different temperatures in the range of 288 to 328 K.
Pseudo-First-Order (PFO) Model: The values k1 and (qe cal) can be estimated from the slope and the intercept of plots, ln(qe-qt) versus t, of Figure 12. Table 6 shows the kinetic constants for this model.
Table 6 displays the highest value of R2 is 0.9984 and the lowest value of R2 is 0.8886. Also, for all MV-10B dye concentration and at 298K, the calculated values of equilibrium adsorption capacity (qe cal) differ from the experimental (qe exp) values. This means that the (PFO) kinetic model is adequately not applicable.
Pseudo-second-order(PSO) Model: The values of qe cal and k2 are calculated from the slope and the intercept of the plot between t/qt versus t, as shown in Figure 13. Table 7 summarizes the corresponding model parameters, with values of the correlation coefficient at 298K.
It was noticed from this table that values of R2 lie between (0.9998 and 1.0000); also, these values are higher than that for (PFO) kinetic model. Moreover, the values of qe calculated (qe cal) agree with the values of qe experimental (qe exp). These obtained results indicated that the adsorption of MV-10B dye onto leather waste sample is more representable by Pseudo-second-order(PSO) kinetic model.
Desorption: The regeneration of VTLW-C adsorbent loaded with MV-10B dye was carried out at a concentration of 15mg/L and 0.1g leather waste weight. Ethanol at a concentration of 75% was used for this adsorption process volume of 25 mL in 100 mL conical flasks with a time taken 4 hours and a shaking speed of 150rpm at 298K that occurs during three cycles. Figure 14 shows desorption of MV-10B dye from VTLW-C surface at 298K.
In this figure, we notice that the removal percentage decreases from 99.1% to 89.1%. This decrease in the adsorption of MV-10B dye from the surface of leather waste is due to the possibility of a large adsorption energy due to the multiple contact points between the adsorbent material and the large dye molecules .
It was clear that these adsorbents can be repeatedly renewed and reused in the process of removing dyes from wastewater. The high desorption removal percent mainly indicates that the adsorption of the MV-10B dye onto the VTLW-C surface is a physisorption type, with weak vander waal's forces conjugated between the adsorbent surface and the molecules of dye.
From this research, we can conclude that VTLW-C was a promise material removal MV-10B dye from aqueous solution. The adsorbent was characterized by FTIR and SEM analysis shows that the surface of VTLW-C sample is mostly fiber-like and rod-like shapes. Studying the optimum conditions for the adsorption process predicted that the best amount of VTLW-C is 0.1 g with 15 mg/L starting concentration dye and for 45 min as adsorption time. Among different adsorption isotherm models used, Freundlich model is better suited the experimental data. From the value E (mean free energy) that calculated form Dubinin–Radushkevich isotherm, we have estimated that the adsorption of MV-10B dye is physisorption type. Thermodynamic study indicated that the adsorption process occurs spontaneous with releasing heat. Kinetic results are compatible with Pseudo-second-order(PSO) model. Desorption process for the MV-10B dye from the VTLW-C surface occurs easily due to the weak vander waal's force that accompanied the physical adsorption.
The authors thank everyone who helped to complete this research.
Noor Hussein AL-Shammari: https://orcid.org/0000-0002-4062-6628
How to cite this article: Noor Hussein AL-Shammari*, Dunya Edan AL-Mammar. Equilibrium and kinetic modeling studies for the adsorption-desorption of methyl violet 10B onto leather waste. Eurasian Chemical Communications, 2022, 4(2), 175-189. Link: http://www.echemcom.com/article_143917.html
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