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Abstract

The quantitative kinetic and equilibrium adsorption parameters for chlorure de méthylrosaniline (gentian violet, crystal violet) removed by commercial activated carbon were studied by UV–visible spectroscopy.Activated carbon with a high specific surface area 1250 m²/g was characterized by the Brunauer, Emmett et Teller (BET) method and the zero charge point pH (_pzc). The adsorption properties of both activated carbon with gentian violet were conducted at variable stirring speed 100–700 trs/min, adsorbent dose 1–8 g/l, solution pH 1–14, initial gentian violet concentration 5–15 mg/l, contact time 0–50 min, and temperature 299–323 K using batch mode operation to find the optimal conditions for a maximum adsorption. The adsorption mechanism of gentian violet was studied using the pseudo-first-order, pseudo-second-order, and Elovich kinetic models. The adsorption kinetics was found to follow a pseudo-second-order kinetic model with a determination coefficient (R²) of 0.999. The Weber–Morris diffusion model was applied for the adsorption mechanism. The equilibrium adsorption data of gentian violet were analyzed by the Langmuir, Freundlich, Elovich, and Temkin models. The results indicate that the Langmuir model provides the best correlation (q_max = 22.727, 32.258 mg/g at 26 and 40°C, respectively). The adsorption isotherms at different temperatures have been used for the determination of thermodynamic parameters, i.e. free energy (ΔG° = − 2.30 to −5.34 kJ/mol), enthalpy (ΔH° = 36.966 kJ/mol), entropy (ΔS° = 0.131 kJ/mol K), and activation energy (Ea) 40.208 kJ/mol of gentian violet adsorption. The negative ΔG° and positive ΔH° indicate that the overall adsorption is spontaneous and endothermic in nature.

Keywords

Gentian violet adsorption isotherm kinetics thermodynamics equilibrium modeling

Introduction

Water pollution is one of the most undesirable environmental problems in the world and therefore requires urgent solutions. The aquatic environment is the favorable site for receiving human and industrial wastes. The latter has caused increasing pollution, threatening both the environment and human health (Hayzoun et al., 2014). The impurities of water are of various origins and can be classified into differing categories including suspended elements, mineral solutes, organic solutes, dissolved gases, and microorganisms (Komissarchik and Nyanikova, 2014). Although most of them can be eliminated by conventional methods, the concentration of toxic substances remaining in water after treatment may in many cases exceed the permissible limit (Benjelloun et al., 2016). They are mainly organic substances such as dyes released in large quantities by the textile industry and are responsible for the toxicity, odor, unpleasant taste, and water color, thus causing degradation of water quality and subsequent disappearance of aquatic life. Hence, there is an urgent need to treat colored effluents before being discharged into the environment (Lairini et al., 2017). Manufacture and use of synthetic dyes for dyeing fabrics has become an industry solid; about 700,000 tons of dyes are produced annually around the world. About 20% of this quantity is industrial waste that is not loaded without prior treatment. However, their use has become a matter of serious concern to environmentalists. Synthetic dyes are highly toxic causing negative effects on all forms of life because they present sulfur, dyes, nitrates, acetic acid, surfactants, enzymes, chromium compounds, and heavy metals such as copper, arsenic, lead, cadmium, mercury, nickel, cobalt, and certain auxiliary chemicals. GV is a synthetic cationic dye also known as Basic Violet 3, gentian violet, and methyl violet 10B belong to the group of triarylmethane. This dye is used extensively in the textile industry to dye cotton, wool, silk, nylon, in the manufacture of printing inks and also the biological stain, a dermatological agent in veterinary medicine (Ayed et al., 2009). Gentian violet (GV) is toxic and may be absorbed through the skin causing irritation and is harmful by inhalation and ingestion. In extreme cases, it can lead to kidney failure and eye irritation leading to permanent blindness and cancer (Mittal et al., 2010). Therefore, removal of GV from wastewater is of great importance. There are several methods available for color removal from waters such as membrane separation, aerobic and anaerobic degradation using various microorganisms, chemical oxidation, coagulation and flocculation, reverse osmosis, and fungal decolorization (Dabrowski, 2001). Some of these techniques are effective but have some limitations such as excess amount of chemical usage, accumulation of concentrated sludge that has serious disposal problems, and lack of effective color reduction (Orthman and Hy and Lu Gq, 2003). The adsorption remains one of the most effective processes of advanced wastewater treatment, where industries employ to reduce hazardous pollutants present in the effluents. This is a well-known and superior technique to other processes for the dyes elimination due to initial cost, operating conditions, and simplicity of design (Fungaro and Magdalena, 2012). Therefore, it was necessary to understand how the dye GV interacts with the adsorbent during discoloration and to describe the potential processes involved in these interactions. For this purpose, we carried out a parametric study of the adsorption, by studying the effect of several significant parameters on the decolorizing power of the material used particularly the contact time, the dose adsorbent, the pH, the stirring speed, and the temperature highlighted experimentally. Furthermore, the adsorption isotherms were carried out and their modeling was achieved by applying known models. The performances of AC on dye adsorptions were evaluated using equilibrium, kinetic, and thermodynamic studies.

Experimental

Preparation of polluting solution

GV (cationic form, chemical formula: C₂₅H₃OClN₃, molar mass 407.979 g/mol) used in this work was purchased from Sigma-Aldrich and used without any further purification. The molecular structure and characteristic of the dye are illustrated in Table 1. This dye is widely used in microbiology for Gram stain, color indicator in assays reactions, and fingerprint revelation. The stock GV solution was prepared by dissolving the accurate weight of GV in distilled water to a concentration of 1 g/l. The experimental solution of the desired concentrations was obtained by successive dilutions. The concentration of GV dye was measured at λ_max (= 590 nm), using UV–visible spectrophotometer (UV 2300). The dye can also be prepared by the condensation of formaldehyde and dimethylaniline to give a leuco dye

{CH}_{2} O + {3C}_{6} H_{5} N {({CH}_{3})}_{2} \to CH {(C_{6} H_{4} N {({CH}_{3})}_{2})}_{3} + H_{2} O

Table 1.

Characteristics of gentian violet.

Names	Chemical structure	Molecular formula	Mw (g/mol)	Nature
Gentianviolet Crystal violetchlorure de méthylrosaniline,Methyl violet 10B		C₂₅H₃₀ClN₃	407.98	Cationic λ_max = 590 nm

Then, this colorless compound is oxidized to the colored cationic form (a typical oxidizing agent is manganese dioxide)

CH {(C_{6} H_{4} N {({CH}_{3})}_{2})}_{3} + HCl + 1 / {2 O}_{2} \to [C {(C_{6} H_{4} N {({CH}_{3})}_{2})}_{3}] Cl + H_{2} O

The effect of pH on the elimination of GV was examined pH of 2 and 14. The signal has a significant dependence on the pH. In acidic, neutral, and slightly alkaline media, GV remains colored due to the presence of the cationic form of GV. Above pH 10, the solution becomes colorless due to the formation of carbinol base

The adsorbent used in this work is a granulated commercial activated carbon (AC) with a very high specific surface area.

pH of point of zero charge pH_PZC

The point of zero charge (pH_pzc) was carried out to determine the pH value for which the surface net charges of the AC are zero; pH_pzc of AC is equal to 5.23 (Figure 1). This behavior is due to the fact that the AC surface is negatively charged at pH > pH_pzc, thus promoting the adsorption of the cationic materials. Conversely, for pH < pH_pzc values, the AC surface is positively charged and therefore capable of repelling the cations (Harrache et al., 2019).

Figure 1.

Determination of the pH of zero point charge (PZC) of AC.

Batch mode adsorption studies

The effects of the initial GV concentration 5–15 mg/l, solution pH 2–14, adsorbent dose 1–8 g/l, agitation speed 100–700 trs/min, and temperature 298–338 K on the GV adsorption were investigated in batch configuration for variable specific periods from 0 to 90 min. The GV solutions were made up by dissolving the accurate amount of GV (99%) in distilled water, used as stock solution and diluted to the required concentrations; pH was adjusted with HCl or NaOH (0.1 mol/l). For the kinetic studies, desired quantities of AC were contacted with 10 ml of GV solutions in Erlenmeyer flasks and placed on a rotary shaker at 300 r/min; the aliquots were withdrawn at regular times and subjected to centrifugation at 3000 trs/min (10 min).

The remaining GV concentration was titrated on a Perkin Elmer UV–visible spectrophotometer model 550S (λ_max = 590 nm). The amount of GV adsorbed q_t (mg/g) by AC was calculated from the relation

q_{t} = \frac{(C_{0} - C_{t}) \cdot V}{m}

(1)where C₀ is the initial GV concentration and C_t is the GV concentrations (mg/l) at time (t), V is the volume of solution (l), and m is the mass of AC (g).

The surface area of the sample was determined by the BET method using a Quantachrome AsiQuin, Automated Gas Sorption Analyser Instrument Version 2.02. The specific surface area and pore structure of the ACs were characterized by N₂ adsorption–desorption isotherms at −196°C using the ASAP 2010 Micromeritics equipment (Figure 2).

Figure 2.

Nitrogen adsorption isotherm of AC at −196°C.

Error functions

In the recent decades, linear regression is one of the most viable tools defining the best fitting relationship quantifying the distribution of adsorbates. It analyzes mathematically the adsorption systems and verifies the consistency and theoretical assumptions of the isotherm model. Due to the inherent bias resulting from the transformation to a diverse form of parameters estimation errors and fits distortion, several mathematically rigorous error functions were tested. Because of the inherent bias resulting from linearization of isotherm models, the nonlinear regression root mean square error (RMSE) equation (2), the sum of error squares equation (3), and Chi-squares (χ²) equation (4) test are used as criteria for the fitting quality. The smaller RMSE value indicates the better curve fitting (Abbas and Trari, 2015)

RMSE= \sqrt{\frac{1}{N - 2} × \sum_{1}^{N} {(q_{e,exp} - q_{e,cal})}^{2}}

(2)

SSE= \frac{1}{N} \sum_{n =1}^{\infty} {({qe}_{cal} - {qe}_{\exp})}^{2}

(3)

X^{2} = \sum_{1}^{N} \frac{{(q_{e,exp} - q_{e,cal})}^{2}}{q_{e,cal}}

(4)where q_e,exp (mg/g) is the uptake experimental, q_e,cal is the calculated value of uptake using a model (mg/g), and N is the number of observations in the experiment (the number of data points). The smaller RMSE value indicates the better curve fitting (Abbas et al., 2016).

Effect of different parameters of adsorption processes of GV onto AC

The initial concentrations of dye were 5, 10, and 15 mg/l for GV at different pH, temperature, stirring speed, and mass dose. The effect of pH on the rate of color removal was analyzed at pH 2, 6, 8, 10, and 12 at 26°C; 150 trs/min; 1 mg of AC; and 10 ml of GV concentration 5–15 mg/l. The pH was adjusted with 0.1 N NaOH and 0.1 N HCl solutions by using an Orion 920 A pH-meter with a combined pH electrode. The pH-meter was standardized with NBS buffers before every measurement. The effect of adsorbent dose was studied by agitating in different masses from 0.01 to 0.08 g, at 26°C, 150 trs/min, 0.01 mg of AC, and 10 ml of GV dye concentration 5–15 mg/l. The effect of temperature to the adsorption capacity of AC was carried out at 26, 35, and 45°C in a constant temperature bath at pH 11, 400 trs/min, contact time 45 min, 1 mg of AC, and 10 ml of dye concentration 5–15 mg/l.

Results and discussion

BET surface area and pore size

The specific surface area was determined by the BET equation (Figure 3) while the external surface area, micropore area, and micropore volume were calculated by the t-plot method. The total pore volume was evaluated from the liquid volume of N₂ at high relative pressure near unity 0.99. The mesopore volume was deduced by subtracting the micropore volume from the total volume. The pore size distribution was determined from the density functional theory model. All characteristics of AC are reported in Table 2.

Figure 3.

Determination of specific area by the t-plot method.

Table 2.

Physicochemical characteristics of AC.

Specific surface area (m²/g)	Porous volume (cm³/g)
S_BET: 1250.230	V_T: 0.6834
S_ext: 527.372	V_mes: 0.6684
S_mic: 753.332	V_mic: 0.0150
Pore diameter per DFT (nm)	1.581
Particle size (mm)	1.25 < Ø < 2.5
Humidity (%)	0.737
Moisture content (%)	0.743
Apparent density (g/cm³)	0.506
Reel density (g/cm³)	1.249
Total pore volume (cm³/g)	1.176
Total porosity	0.595

DFT: density functional theory.

Optimization study of operating conditions

The pH of the solution plays an important role in the adsorption process, particularly on the uptake capacity. It is observed that the percentage of GV removal is increasing consistently with increasing pH (Figure 4). The effect of pH on GV adsorption by AC can be explained on the basis of pH_pzc = 5.23. The AC surface charge is negative above pH_pzc. So, as the pH of the solution increases, the number of positively charged sites decreases and favors the GV adsorption by electrostatic attraction.

Figure 4.

Evolution of the adsorption of GV dye onto AC as a function of pH.

The adsorption capacity of GV increases over time to reach a maximum after 30 min and thereafter tends toward a constant value indicating that no more GV ions are removed from the solution. The equilibrium time averages 35 min but for practical reasons, the adsorption experiments are run up to 45 min. With raising the initial GV concentration, adsorbed increases from 2.29 to 4.32 mg/g at pH 5.23 and from 4.55 to 14.30 mg/g at pH 11 (Figure 5); from these results, we can deduce that the adsorption of GV onto AC is done in three stages:

Fast adsorption of GV due to the presence of free sites on the adsorbent surface which translates the linear increase of the adsorption capacity over time. This step lasts 10 min under the operating conditions undertaken.

Reduction of the adsorption rate, reflected by a small increase in the adsorption capacity attributed to the decrease in the quantity of GV in solution and the number of available unoccupied sites. This stage lasts 10–20 min.

Stability of the adsorption capacity is observed, probably due to the total occupation of adsorption sites: the establishment of the level therefore reflects this stage.

The GV molecules are initially adsorbed on the external surface area of AC which makes the adsorption rate easy and fast. When the external surface is saturated, the GV molecules entered inside the pores and absorb on the internal surface and such phenomenon takes relatively longer time and high energy. This may be attributed to an increase of the driving force due to the concentrations gradient with increasing C₀ which overcomes the mass transfer resistance of GV between the aqueous and solid phases. Therefore, a higher initial GV concentration C₀ increases the adsorption capacity.

Figure 5.

Evolution of the adsorption of GV dye onto AC as a function of time.

The effect of the stirring speed in the range 100–700 trs/min on the adsorption capacity onto AC is also investigated. The optimal adsorption capacity is obtained for a speed of 400 trs/min (Figure 6) which gives the best homogeneity of the mixture suspension. Therefore, a speed of 400 trs/min is selected for further experiments.

Figure 6.

Evolution of the adsorption of GV dye onto AC as a function of stirring speed.

The first stage of batch experiments on AC and the effect of adsorbent dose on the GV adsorption are examined. Significant variations in the uptake capacity and removal efficiency are observed at different adsorbent dose (1–8 g/l), indicating that the best adsorption is obtained with a dose of 1 g/l (Figure 7). This result was subsequently used in all isotherm adsorption experiments.

Figure 7.

Evolution of the adsorption of GV dye onto AC as a function of adsorbent dose.

Adsorption of GV onto AC

Adsorption kinetic study

The kinetic study is important since it describes the uptake rate of adsorbate and controls the residual time of the whole process. Several models were proposed to study the mechanisms controlling the adsorption. In this study, the experimental data of GV adsorption are examined using a pseudo-first-order and pseudo-second-order kinetic model.

The pseudo-first-order equation (Lagergren, 1998) is given in equation (5)

\log (q_{e} - q_{t}) = \log q_{e} - \frac{K_{1}}{2.303} \cdot t

(5)

The pseudo-second-order model (Ho and McKay, 1998) is expressed by equation (6)

\frac{t}{q_{t}} = \frac{1}{K_{2} \cdot q_{e}^{2}} + \frac{1}{q_{e}} \cdot t

(6)

For the pseudo-second order, the initial adsorption rate h (mg/g min) is expressed by equation (7)

h = K_{2} q_{e}^{2}

(7)where q_t (mg/g) is the amount of GV adsorbed on AC at the time t (min). K₁ (min⁻¹) and K₂ (g/mg min) are the pseudo-first-order and pseudo-second-order kinetics constants, respectively. The slope and intercept of the plots ln(q_e − q_t) versus t and t/q_e versus t were used to determine the first-order rate constants K₁ and q_e and second-order rate constants K₂ and q_e, respectively.

The rate constants predict the uptakes and the corresponding correlation coefficients for AC summarized in Table 3. For the pseudo-first-order kinetic (Figure 8), the experimental data deviate from linearity, as evidenced from the low values of q_e and C ₀ and the model is inapplicable for the present system. By contrast, the correlation coefficient and q_e,_cal determined from the pseudo-second-order kinetic model agree with the experimental data (Figure 9) and its applicability suggests that the adsorption of GV onto AC is based on chemical reaction (chemisorption), involving an exchange of electrons between the adsorbent and adsorbate. The dye is attached to the adsorbent surface by chemical bond and tends to find sites that maximize their coordination number with the surface.

Table 3.

Pseudo-first-order, pseudo-second-order, and Elovich model constants and determination coefficients for GV adsorption onto AC.

	Second	order				Pseudo first	order
C₀ (mg/l)	q_ex (mg/g)	q_cal (mg/g)	R ²	Δq/q (%)	K₂ (g/mg min)	q_cal (mg/g)	R ²	Δq/q (%)	K₁ (min⁻¹)
5	4.3	04.545	0.998	5.39	0.103	3.258	0.990	31.98	0.175
10	9.2	10.000	0.998	8.00	0.031	9.840	0.973	6.50	0.212
15	14.3	15.385	0.999	7.05	0.024	14.421	0.940	0.08	0.193
		Elovich			Diffusion
C₀(mg/l)	R²	β (g/mg)	α (mg/g min)		C₀ (mg/l)	K_in (mg/g min^1/2)	R²	C (min^1/2)	D cm²/s
5	0.940	1.312	7.289		5	1.043	0.986	0.595
10	0.947	0.520	7.649		10	2.492	0.937	0.076	5 × 10⁻⁷
15	0.980	0.375	17.553		15	3.241	0.986	1.838

Figure 8.

First-order kinetic model fit for the adsorption of GV dye onto AC.

Figure 9.

Pseudo-second-order model fit for the adsorption of GV dye onto AC.

The Elovich kinetic equation is related to the chemisorption process (Figure 10) and is often validated for systems where the surface of the adsorbent is heterogeneous (Juang and Chen, 1997); the linear form is given by

q_{t} = (\frac{1}{β}) \ln α \cdot β + (\frac{1}{β}) lnt

(8)where α (mg/g min) is the initial adsorption rate and β (mg/g) is the relationship between the degree of surface coverage and the activation energy involved in the chemisorption.

Figure 10.

Elovich model for the adsorption of GV dye onto AC.

Adsorption mechanism

The adsorption kinetics is usually controlled by different mechanisms. The intraparticle diffusion derives from the Fick’s law and assumes that the diffusion of the liquid film surrounding the adsorbent is negligible, and the intraparticle diffusion is the only rate controlling step of the adsorption process. Equation (9) shows the mathematical expression used to study this phenomenon.

The functional relationship common to most adsorption processes varies proportionally with t^1/2, the Weber–Morris plot (q_t versus t^1/2) rather than with the constant time t (Weber and Morris, 1963). K_in is the intraparticle diffusion rate constant. The intercept C gives information about the thickness of the boundary layer. This is due to the instantaneous utilization of the readily available adsorption sites on the surface. The constants K_in and C are deduced from the slope and intercept of linear plots.

It is well known that a performed batch experiment gives valuable data to evaluate the diffusion coefficients. Under real conditions, the mass transport resistance in the solid is larger than the external fluid film on the solid particles

q_{t} = K_{in} t^{\frac{1}{2}} + C

(9)

The values of C and K_in for the linear segments are shown in Figure 11. It is possible, from the plots of q_t against t½, to distinguish three phases in the adsorption processes and the adsorption processes involve more than a single kinetic stage.

Figure 11.

Intraparticle diffusion model for the adsorption of GV dye onto AC.

The first stage is an instantaneous adsorption due to strong electrostatic attraction between the dye and the outer surface of the adsorbent. The second stage is a gradual adsorption stage attributed to intraparticle diffusion of dye molecules through the pores of the adsorbent.

If intraparticle diffusion is involved in the adsorption, then the plot of q_t against t^½ should be linear and allows calculation of K_in from the slope. The C intercept values give an idea of the boundary layer thickness, i.e. the larger the intercept, the greater the effect of the boundary layer. The final stage corresponds to the equilibrium adsorption, when the dye molecules occupy all active sites on the adsorbent. The values of K_in are determined from the slopes of the respective linear plots (Table 3). The straight lines with higher slope values have larger K_in and consequently a higher adsorption rate per unit time. It can be seen in that as time passes the rate of adsorption decreases, being lower in the equilibrium stage of the system (third step), when all the adsorption sites are already occupied (Figure 11).

The mechanism is discussed chemically, based on the endothermic reaction of the process and on attraction forces according to the isoelectric pH. In our case, based on the fact that the adsorption process that took place is a chemisorption, therefore there is the formation of new chemical bonds between the negatively charged functional groups and the positively charged pollutant (attractive electrostatic forces).

The adsorption mechanism can be summarized in the following four steps:

Transfer of the pollutant from the outer layer to the inner layer (very fast step).

Displacement of the pollutant until it comes into contact with the adsorbent (rapid step).

Diffusion of the pollutant into the adsorbent under a concentrations gradient (slow step).

Adsorption of the pollutant in a micropore (very fast step).

Adsorption isotherms

The aim of this study was to understand the GV adsorbent interaction through the validity of the models and also find parameters that allow for comparison, interpretation, and prediction of the adsorption data of AC. Three isotherms are applied to fit the equilibrium data of this study: Langmuir, Freundlich, and Temkin.

The Langmuir (1918) isotherm assumes monolayer coverage of adsorbate on a homogeneous surface of adsorbent. Graphically, the Langmuir isotherm is characterized by a plateau region. Therefore, at equilibrium the saturation is reached when the adsorption cannot occur. Adsorption occurs at specific sites on the homogeneous surface of the adsorbent. Once a dye molecule occupies a site, there can be no further absorption. A well-known linear expression for the Langmuir isotherm is given by

\frac{C_{e}}{q_{e}} = \frac{1}{q_{m a x}} C_{e} + \frac{1}{K_{L} q_{m a x}}

(10)where C_e is the equilibrium concentration (mg/l), q_max is the monolayer adsorption capacity (mg/g), and K_L the constant related to the free adsorption energy (Langmuir constant, l/mg). Equilibrium parameter for the isotherm called separation factor (R_L) is expressed by equation (10). The parameter indicates the shape of the isotherm: R_L > 1 is unfavorable, R_L = 1 is linear, R_L = 0 is irreversible, and finally 0 < R_L < 1 is favorable

R_{L} = \frac{1}{1 + K_{L} C_{0}}

(11)

The Freundlich (Freundlich, 1906) isotherm is an empirical model applicable to adsorption on heterogeneous surfaces, not restricted to the formation of a monolayer. It is assumed that an increase in the adsorbate concentration increases the amount adsorbed on the surface of the adsorbent

\ln q_{e} = \ln K_{F} + \frac{1}{n} \ln C_{e}

(12)where K_F is the constant indicative of the adsorption capacity of the adsorbent (l/g) and 1/n shows adsorption intensity of the dye onto AC. For a suitable adsorption system, 1/n varies between 0 and 1.

Temkin and Pyzhev (1940) considered the effects of some indirect/adsorbent interactions in the adsorption isotherm, suggesting that for these interactions the adsorption heat of all molecules in the layer decreases linearly with coverage

q_{e} = B_{T} \ln C_{e} + B_{T} \ln A_{T}

(13)

B_{T} = RT / b

The constant B_T (l/mg) is related to the heat of adsorption, A_T (mg/l) is a constant of the Temkin isotherm, b (J/mol) is the energy constant of the Temkin isotherm, R (8.314 J/K mol) is the gas constant, and T (K) is the absolute temperature. The calculated parameters characteristics of each isotherm model are presented in Table 4. It can be seen from the results that the Langmuir isotherm fits better than Freundlich and Temkin ones with higher correlation coefficients R² close to unity, and smaller RMSE, the X² values indicates the better curve fitting. For all studied temperatures, we can note that R_L is the range (0–1), indicating that GV adsorption onto AC is a favorable process.

Table 4.

Parameters of the adsorption isotherms for GV dye onto AC.

40°C	Langmuir	Freundlich	Temkin	Dubinin-R	Elovich
	K_L: 0.08 l/mg	1/n: 0 886	B: 5.924	K: 0.839 mol² K/J²	K_E: 0.086 l/mg
	q_max_: 90.91 mg/g	K_F: 6.514 mg/g	A_T: 4.088 l/mg	q_max: 13.28 mg/g	q_max: 83.33 mg/g
			ΔQ: 39.935 KJ/mol	E: 0.772 kJ/mol
RMSE	1.5833	1.5366	2.1303	1.6982	1.7832
SSE	1.8800	1.7708	3.4037	6.1270	2.3849
X²	1.2585	1.2473	22.4389	4.4040	1.6188
R²	0.993	0.970	0.863	0.900	0.164
25°C
K_L	0.11 l/mg	1/n: 0.838	B: 4.734	K: 1.380 mol² K/J²	K_E: 0.133 l/mg
q_max	35.71 mg/g	K_F: 3.452 mg/g	A_T: 2.447 l/mg	q_max: 10.35 mg/g	q_max: 29.41 mg/g
			ΔQ: 18.691 KJ/mol	E: 0.602 J/mol
RMSE	0.7457	0.7412	1.4636	2.5740	0.9429
SSE	0.4171	0.4120	1.6065	4.9691	0.6668
X²	0.3174	0.3214	11.4628	4.3389	0.4968
R²	0.996	0.993	0.904	0.873	0.742

RMSE: root mean square error; SSE: sum of error squares.

Adsorption thermodynamics

Figure 12 clearly shows that the adsorption capacity of AC increases with increasing temperature from 299 to −323 K, indicating that the adsorption is favored at high temperature. The adsorption reaction of coloring molecules on a surface implies a variation of the free energy between the initial and final state at constant pressure

Δ G ° = Δ H ° - T Δ S °

(14)

Figure 12.

Effect of temperature for the adsorption of GV dye onto AC.

ΔG° is composed of two terms, the enthalpy which expresses the energies of interactions between the molecules and the adsorbent surface, and an entropic term which expresses the modification and arrangement of the molecules in the liquid phase and on the surface. The thermodynamic behavior for adsorption of GV on AC was further investigated. The thermodynamic parameters ΔG°, ΔH°, and ΔS° are calculated using the following equations

Δ G ° = - RT \ln K_{0}

(15)where K₀ is the apparent equilibrium constant. ΔH° and ΔS° of the adsorption are calculated from adsorption data at different temperatures

\ln K_{0} = - \frac{Δ H °}{RT} + \frac{Δ S °}{R}

(16)lnK₀ is obtained from the equilibrium constant (K_s) for the adsorption as follows (Li et al., 2010)

K_{s} = \frac{q_{e}}{C_{e}} \times \frac{γ_{1}}{γ_{2}}

(17)where γ₁ is the activity coefficient of the adsorbed solute and γ₂ is the activity coefficient of the solute in equilibrium suspension. The ratio of activity coefficients is assumed to be uniform in diluted solutions. As the concentrations of the dye in the solution approached zero, the activity coefficient approached unity

\lim_{C_{e \to 0}} \frac{q_{e}}{C_{e}} = K_{0}

(18)

The values of K₀ determined from intercept (figures not shown), by plotting ln(q_e/C_e) versus C_e and extrapolating to C_e = 0.

The ΔH° and ΔS° values are obtained from the slope and intercept of the plots lnK versus 1/T (Figure 13). In the rate law, temperature dependence appears in the rate constant and the dependence of rate constants on temperature over a limited range can usually be represented by the empirical equations (16) and (19).

Figure 13.

(a) Regression of Van’t Hoff for thermodynamic parameters of GV adsorption on AC and (b) determination of activation energy.

The adsorption capacity of AC increases with raising temperature from 299 to 313 K

\ln K_{2} = \ln A - Ea / R

(19)where A is a pre-exponential factor, Ea is the activation energy, and K₂ (g/mg min) is the pseudo-second-order kinetics constant at different temperatures. The energy Ea is obtained by plotting ln K₂ against T⁻¹ (Figure 13). All the calculated thermodynamic parameters are presented in Table 5.

Table 5.

Thermodynamic functions ΔG°, ΔS°, and ΔH° of GV adsorbed on the AC.

T(K)	K_e	R ²	K₂ (g/mg min)	R ²	ΔG° (KJ/mol)	ΔH° (KJ/mol)	ΔS° (KJ/mol k)	Ea (KJ/mol)
299	2.33	0.998	0.03145	0.9976	−2.203
313	4.91	0.999	0.05945	0.9996	−4.037	36.966	0.131	40.208
323	7.31	0.996	0.10479	0.9994	−5.342

Performance of the AC

It is interesting for a comparative purpose to report the values of the adsorption capacity of some adsorbents available in the literature. In Table 6, we report different values of the Langmuir maximum adsorption capacity q_max of different adsorbents cited in previous works.

Table 6.

Comparison of maximum adsorption capacities for GV dye with literature data.

Adsorbents	q_max (mg/g)	References
Fly ash	74.6	(Eren et al., 2010)
Bottom ash	12.1	Gandhimathi et al. (2012)
Sepiolite	77.0
Sulfuric acid	85.8	(Senthilkumaar et al., 2006)
Activated carbon	19.8	(Malarvizhi and Ho, 2010)
ZFA Zeolites From Coal fly Ash	19.6	Tharcila et al. (2013)
ZBA Zeolites from Bottom Ash	17.6	Tharcila et al. (2013)
Poly(acrylicacid-acrylamide-methacrylate)	35	Li (2009)
Raw kaolin	26	Nandi et al. (2008)
Bagasse fly ash	79	Mall et al. (2006)
Carrageenan-Alginate/Montmorillonite nanacomposite hydrogels	89	Mahdavinia et al. (2013)
Activated carbon	65	Mohanty et al. (2006)
Activated carbon	175	Wang and Zhu (2007)
Sewage sludge	263	Otero et al. (2003)
Raw bentonite	130	Eren (2009)
Modified bentonite	457	Eren (2009)
Chitosan-g-(4-hydroxybenzoic acid)	18	Chao et al. (2004)
Poly(vinyl benzyl chloride) beads	160	Kaner et al. (2010)
Ball clay	169	Monash et al. (2011)
Magnetic kappa-carrageenan beads	85	Mahdavinia et al. (2014)
Kappa-carrageenan-poly(acrylic acid)/multi-W. carbon nanotube	118	Hosseinzadeh (2015)
Water hyacinth	32.28	Rajeswari Kulkarni et al. (2017)
Potato peels (Solanum tuberosum)	555	Lairini et al. (2017)
Activated carbon 40°C	90.91	This study
Activated carbon 26°C	35.71

ZBA: Zeolites from Bottom Ash; ZFA: Zeolites From Coal fly Ash.

One can see that the GV adsorption observed in this work is well positioned with respect to other researches. In our study, the maximum adsorption capacity q_max: 32.258 mg/g of GV at 313 K is interesting compared to other adsorbents. The differences of GV uptake are due to the properties of each adsorbent like the structure, the functional groups, and the surface area. AC is an attractive adsorbent for basic dyes owing to the isoelectric point pH_pzc.

Desorption is an unavoidable process in adsorption and an intermediate stage toward regeneration. The latter is an essential tool to estimate the reuse of any adsorbent for industrial applications, owing to the ecological concerns and the needs for sustainable development. For the regeneration of the adsorbent, several methods are distinguished among which we can mention regeneration: electrochemical, microbiological, thermal, and chemical (Aksil et al., 2019). However, we have opted for chemical regeneration because of its low economic cost.

Regeneration and reuse of the adsorbent for other cycles is important from an economic point of view. The desorption of GV from AC was evaluated using two different solutions: NaOH and HCl (0.01 M).

The desorbed coal is again submitted to the next batch to verify the desorption and reusability of the AC. The quantity of the desorbed GV was calculated from the concentration of desorbed purple gentian in the liquid phase using a UV–visible spectrophotometer at 590 nm. The percentage of GV desorbed from the AC was calculated according to the following equation

Desorption (%) = (Desorbed mass / Adsorbed mass) \times 100

(20)

It can be seen that the highest desorption was achieved in the NaOH solution with 28.8% for AC and 13.6% in HCl solution. This may be due to the increase in the number of negatively charged sites with high pH above 5.23, which increases the electrostatic repulsion, which releases the GV from the AC. To check the efficiency of the adsorption, the desorbed AC was dried overnight and subjected to new adsorption/desorption cycles. During the second cycle, the adsorption capacities were 25.23 and 12.50%, respectively, for the basic and acid solutions. A nonsignificant decrease in the adsorption capacity of AC was observed from the third cycle onwards and this can be attributed to the exhaustion of the active sites of the adsorbent occupied by the dye molecules. With increasing the number of cycles, the desorption rate also remains constant after the third cycle (Figure 14).

Figure 14.

Desorption ratio (%) of GV from AC using 0.01 mol/l NaOH and HCl after three cycles.

This study has given encouraging results, and we wish to carry out column adsorption tests under the conditions applicable to the treatment of industrial effluents and to test the homogeneous photodegradation of GV on the semiconductor SnO₂ is our future objective. Preliminary tests are satisfactory; the experiments are currently under way and will be reported very soon in the next paper.

Conclusions

In this work, an experimental study on the utilization of AC for the removal of GV from aqueous solution was investigated. The potential of this adsorbent was studied for decolonization of GV. Influence of different parameters such as initial pH, initial GV concentration, contact time, adsorbent dosage, stirring speed, and temperature on GV adsorption was examined.

The adsorption capacity of GV increased with raising the initial dye concentration, time, and pH; the optimized pH for adsorption was 11. The kinetics of GV removal indicated an optimum contact time of 45 min via two stage process of the adsorption kinetic profile (initial fast and subsequent slow equilibrium). The adsorption of GV ions onto AC follows a pseudo-second-order kinetic model with determination coefficients R² of 0.999 with an activation energy of 40.208 kJ/mol, which relies on the assumption that chemisorption may be the rate-limiting step. In chemisorption, the GV molecules are attached to the adsorbent surface by chemical bond and tend to find sites that maximize their coordination number with the surface. To get an idea on the adsorption mechanism, we applied the Weber–Morris diffusion model. The equilibrium GV adsorption on AC was analyzed by the Langmuir, Freundlich, Elovich, and Temkin models. The results indicate that the first model provides the best correlation.

The negative ΔG° and positive ΔH^o indicate that the GV adsorption is spontaneous and endothermic over the studied temperatures range. The enthalpy clearly indicates that GV is strongly attached to the adsorbent surface by forming chemical bond. The positive ΔS° value states clearly that the randomness increases at the solid–solution interface during the GV adsorption, indicating that some structural exchange may occur among the active sites of the adsorbent and the ions.

The comparison of the adsorption capacity of the prepared adsorbent with other adsorbents shows that it has attractive properties from industrial and economic interests. This study has given encouraging results, and we wish to carry out column adsorption tests under the conditions applicable to the treatment of industrial effluents and to test the homogeneous photodegradation of GV on SnO₂ is the future objective of this work.

Highlights

An activated carbon is used to remove gentian violet from aqueous solution.

The equilibrium experimental data were well fitted by the Langmuir isotherm models.

The results indicate that the Langmuir model provides the best correlation (q_max = 22.727, 32.258 mg/g at 26 and 40°C, respectively)

The adsorption of the dye follows the pseudo-second-order kinetic model.

A thermodynamic study showed the spontaneous and endothermic nature of the adsorption.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Removal of gentian violet in aqueous solution by activated carbon equilibrium,kinetics,and thermodynamic study

Abstract

Keywords

Introduction

Experimental

Preparation of polluting solution

pH of point of zero charge pHPZC

Batch mode adsorption studies

Error functions

Effect of different parameters of adsorption processes of GV onto AC

Results and discussion

BET surface area and pore size

Optimization study of operating conditions

Adsorption of GV onto AC

Adsorption kinetic study

Adsorption mechanism

Adsorption isotherms

Adsorption thermodynamics

Performance of the AC

Conclusions

Highlights

Footnotes

Declaration of Conflicting Interests

Funding

References

pH of point of zero charge pH_PZC