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Abstract

Conducting polymeric composites have attracted great attention over the last years because of their potential uses in chemical, electronic and optical devices, and as catalysts as well as in adsorption processes. Chemical synthesis of polyaniline (PANI) and polyaniline-SiO₂ composite and their adsorptive performance were reported in the present work. These materials were prepared and evaluated for their methylene blue (MB) dye adsorption characteristics from aqueous solution. Adsorption equilibrium kinetic and thermodynamic experiments of MB onto PANI and PANI/SiO₂ were studied. The effects of initial dye concentration, contact time and temperature on the adsorption capacity of PANI/SiO₂ for MB have been investigated. The pseudo-first order and pseudo-second order kinetic models were used to describe the kinetic data. It was found that adsorption kinetics followed the pseudo-second order at all of the studied temperatures. The Langmuir, Freundlich and Dubinin Raduschkevich adsorption models were used for the mathematical description and the HyperChem v8 software was exploited to propose a possible mechanism of the adsorption process.

Keywords

Polyaniline polyaniline composite adsorption methylene blue dye Langmuir isotherm

Introduction

With the rising awareness of the occurrences of industrial activities and pollution instances of aqueous media, textiles industry is considered as the major source of dye contamination.¹ The effective removal of dyes from aqueous wastes is an important issue for many countries.² This is due to their harmful impact on the environment such as toxic and even carcinogenic as well as mutagenic action towards living organisms. Most dyes have a dangerous effect (directly or indirectly) on fish. Direct activity is due to colouring of water and changing its composition, which significantly deteriorates the living conditions, but indirect activity is due to poisonous properties of many dyes. In this respect, adsorption has gained an important credibility during recent years because of its good performance and low cost,³ much selective, easy to operate and a proven efficient process for the removal of dyes from contaminated aqueous media.⁴

Actually, conjugated polymers, particularly those based on polyaniline (PANI), polythiophene (PT) and their derivatives, have gained interesting popularity because of multiple properties such as the low cost synthesis and their numerous applications in rechargeable batteries, display devices and sensors, porous structure, tunable morphology, good electrorheological property, unique redox chemistry, non-toxicity, insolubility in water, environmental stability. In addition, their simple doping and dedoping by acid/base treatment have made them very interesting agents in conducting polymers family. The application of PANI as adsorbent for water purification is due to the large amounts of amine and imine functional groups, which are expected to have interactions with inorganic and organic molecules, such as Hg(II), Cr(VI) and methylene blue.^5–10

Recently, a great work has been made to combine conjugated polyaniline with conventional organic and inorganic adsorbents to form composites or hybrid adsorbents such as PANI-graphene oxide,¹¹ doped polyaniline-potash alum,¹² polyaniline-polystyrene (PANI-PS),¹³ PANI-cellulose,¹⁴ PANI-cellulose fiber,¹⁵ polyaniline-polyethylene glycol (PANI-PEG),¹⁶ polyaniline-magnetite,¹⁷ PANI-chitosan,¹⁸ PANI-sawdust,¹⁹ polyaniline-multiwalled carbon nanotubes (PANI-MWCNT),²⁰ PANI-chitin,²¹ PANI-nickel ferrite.²² These functional materials are combined with PANI to generate PANI-related composites to improve its surface area and change its surface morphology and have been studied like adsorbents for removing dyes, heavy metals and ions from aqueous solutions^23–32 and demonstrate an important attention because of their excellent adsorption performance.^33–36

The aim of this work is the study of the mechanistic nature of methylene blue removal in aqueous solutions at acidic and basic mediums by adsorption using synthesized polyaniline (PANI) and its silica composite (PANI-SiO₂). For this purpose, a combination of adsorption isotherms, kinetic models are used.

Experimental section

Synthesis of polyaniline emeraldine salt

Polyaniline Emeraldine Salt (PANI-ES) can be synthesized from monomeric aniline (99.5%, Biochem) by oxidative chemical polymerization.^37,38 In summary, PANI was prepared as follows: the first solution was prepared with 0.5 M of aniline dissolved in 100 mL of 1 M HCl (32%, VWR) and the second solution with 0.25 M of (NH₄)₂S₂O₈ (98%, VWR) in 100 mL of distilled water. The latter was added slowly to the first solution with continuous stirring. The mixture was placed under stirring for one hour at 0–5°C. The greenish black precipitate resulting from this solution was passed to filtration and washing repeatedly with water first, diluted HCl solution and methanol until the wash liquid was colorless to remove oligomers and other non-polymeric impurities. Then, the collected polymer was dried for 48 h in a 40°C oven, ground and stored for processing.^39–47

Synthesis of PANI/Silica (SiO₂) composite

Determined amount of silica suspension (including 0.23 g of silica/10 mL), aniline (9.313 g, 0.10 mol) and 10 mL of conc. HCl in 200 mL of distilled water was added by mixing in the ice water bath. The mixtures were stirred for a further 30 min. Then, a volume of 100 mL of aqueous APS solution (containing 22.820 g, 0.10 mol of APS) was dropped into the emulsions in 60 min. The mixture was placed under stirring for another 4 h in a cooled water bath. The PANI/silica composite was filtered and washed with water three times. Finally, it was dried in the oven at 40°C for 48 h.^48–52

Sorbate dye (Methylene blue)

To prepare the MB solution 1 g of MB was dissolved in 1 L of distilled water. It has a molecular formula C₁₆H₁₈N₃ClS with molecular weight 319.85 g.mol⁻¹. It is a non-toxic water soluble dye, blue in color (λ_max 661 nm).^50,53 Initial and final concentrations of MB solutions were determined by measuring absorbance at 661 nm using UV visible absorption spectroscopy.

Adsorption experiments

The adsorption study was performed using an aqueous MB dye solution for the determination of adsorption capacity of the synthetic PANI and PANI/SiO₂ composite. Sample concentrations were determined in the UV-visible apparatus at λ = 661 nm. The effect of different parameters was carried out from this study such as the adsorption time (0–120 min), the initial concentration in MB (4–21 mg/L) and the mass of the adsorbents (0.05–1.0 g).

The q_e (equilibrium adsorption amount) was calculated as follows:

q_{e} = \frac{(C_{i} - C_{e}) . V}{m}

(1)where C_i and C_e are the initial concentration (before adsorption) and concentration at equilibrium (after adsorption) in mg.L⁻¹ of MB, respectively; V and m are the volume (L) of the MB solution and the weight (g) of the used material (adsorbent).^54–58

Results and discussion

Study of adsorption of the dye (MB)

The influence of the contact time

The contact time effect on the quantity of adsorbed methylene blue dye is shown in Figure 1(a) and (b). First, it was seen that the MB concentration after adsorption decreases continuously when the contact time increases with the two adsorbents PANI and PANI/SiO₂ composite.

Figure 1.

Effect of (a) and (b) contact time on the removal of MB by PANI and PANI/SiO₂ composite in acidic and alkaline mediums (T = 25°C, C_o = 3 × 10⁻⁵ mol/l, m = 0.133 g) (c) Effect of the mass of PANI and PANI-SiO₂ composite (d) Effect of the initial concentration of MB.

Also, the dye removal rate is faster in the first 25 min, which is due to the large number of free sites for adsorption of MB; therefore, it reaches equilibrium in the 60^th minute and subsequently remains constant. The quantity of MB adsorbed q_t increases from 3.53 mg.g⁻¹ in the acidic medium to 5.2 mg.g⁻¹ in the alkaline medium when the PANI homopolymer is used. With the PANI/SiO₂ composite, the amount of adsorbed dye q_t increases from 2.23 mg.g⁻¹ in acidic medium to 6.97 mg.g⁻¹ in alkaline medium.

By comparing the effect of the medium (acidic or alkaline), the PANI homopolymer and the PANI/SiO₂ composite exhibit high adsorption efficiencies in the alkaline medium compared to those in the acidic medium, this can be explained for PANI/SiO₂ by the fact that the particles are more and more negatively charged as the pH becomes more and more basic. This is due to the deprotonation of surface silanols (Si-OH) by OH^- hydroxyls in solution to form silanolates (SiO^-). For the PANI, the ES form transforms into the EB form at higher pH. The negative charged surface revealed and the negative charge density increased with the increasing pH value. Therefore, the adsorption capacity increased with the increasing pH value.

Effect of adsorbent dosing

With increasing the adsorbent mass, the number of adsorption sites increases, which consequently increases the adsorption of more dye molecules. By checking the Figure 1(c), it was seen a decrease in the values of q_e (mg.g⁻¹) with increasing the mass of PANI and PANI/SiO₂ composite from 0.05 to 1 g. The marginal improvement in the adsorption of the dye can be explained by the reason that a fixed volume of dye solution with a known C_o of dye has a defined number of total dye molecules and in this condition the dose of adsorbent becomes very important. Therefore, many adsorbent adsorption sites remain empty, resulting in a decrease in q_e.⁵⁹ Consequently, selecting the appropriate dose becomes a very important factor for an effective, efficient treatment process. In this case, the 0.133 g adsorbent mass was selected for all adsorption studies.

Of the two adsorbents used in the study, the PANI/SiO₂ composite exhibits greater elimination at all levels of the adsorbent dosing than pure PANI because the specific surface area of PANI/SiO₂ (43.7 m².g⁻¹) is higher than that of pure PANI (30.3 m².g⁻¹).

Effect of the dye initial concentration

It was seen from the figure 1(d) that the dye adsorption increases almost linearly with the increase in C_o of the dye (from 4 to 21 mg.L⁻¹) in both cases of PANI and PANI/SiO₂ composite. At higher concentrations, a large number of dye molecules completely occupy the binding sites of adsorbent materials that were not possible at low dye concentration. Therefore, the C_o equal to 3 ×10⁻⁵ mol.L⁻¹ was chosen as the ideal adsorbate concentration for further studies.

Adsorption kinetics

Two kinetic models have been studied in this part, those of pseudo first and second order. The kinetic curves are shown in Figure 2.

Figure 2.

(a) and (b) Pseudo first-order model of PANI and PANI/SiO₂, (c) and (d) pseudo-second-order model of PANI and PANI/SiO₂ kinetic plot for adsorption of MB in acidic and basic mediums.

The Lagergren’s first order pseudo kinetic model (linear form) is expressed by

\ln (q_{e} - q_{t}) = \ln q_{e} - K_{1} t

(2)where K₁ is the adsorption rate constant (min⁻¹), q_e and q_t are the adsorbed amount of dye in equilibrium and at time t (in mg.g⁻¹), respectively.^54,60 K₁, q_e and R² (correlation coefficient) were obtained graphically and are illustrated in Table 1.

Table 1.

Kinetic parameters of adsorption of MB on PANI and PANI/SiO₂ in acidic and alkaline mediums.

Model	Parameters	PANI		PANI/SiO₂
Model	Parameters	pH = 2	pH = 11	pH = 2	pH = 11
Pseudo-first order	K₁ (min⁻¹)	0.03	0.04	0.04	−0.03
	q_e (mg/g)	0.22	2.43	2.13	0.009
	R ²	0.921	0.988	0.80	0.85
Pseudo-second order	K₂ (min⁻¹)	0.48	0.027	0.01	−0.32
	q_e (mg/g)	3.83	4.34	3.61	6.66
	R ²	1.000	0.990	0.965	0.999
Intraparticle diffusion	K_id1	0.15	0.41	0.63	0.006
	C₁	3.03	0.97	−0.98	6.88
	R ²	0.922	0.99	0.98	1.00
	K_id2	0.007	0.02	0.068	−0.029
	C₂	3.73	3.80	2.11	6.97
	R ²	0.999	1.00	0.85	0.76

As evident in Table 1, R² values varies from 0.92 to 0.98 for PANI and from 0.80 to 0.85 for PANI/SiO₂ in acid and basic medium respectively. As you can see, the (cal. q_e) estimated by the velocity equation is significantly different from the experimental value of q_e (exp. q_e). Also, R² value was not very nearby to 1. Therefore, the pseudo first-order model does not fit our experimental data.

In addition, the second order pseudo kinetic model (linear form) is expressed by

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

(3)where K₂ is the adsorption rate constant for the pseudo-second order (in g mg⁻¹ min⁻¹), q_t and q_e were described before. K₂, q_e and R² were obtained graphically³⁴ and are illustrated also in Table 1. The correlation coefficients (R²) of the graphs for various adsorbents vary from 0.99 to 1.00 for PANI in acidic and basic medium and from 0.965 to 0.999 for PANI/SiO₂ in acidic and basic medium, which suggests that adsorption of MB on PANI homopolymer and PANI/SiO₂ composite follows the pseudo-second order model. In addition, (q_e, exp) are very close to the (q_e, cal).

The intraparticle diffusion model was used to analyze kinetic data of the MB adsorption process, this kinetic model is given by the following equation:

q_{t} = K_{id} {(t)}^{0.5} + C

(5)where, K_id and C are the intraparticle diffusion rate constant and a constant related to the thickness of the boundary layer, these constants are obtained graphically.^57,61,62 If the plot is a line, the adsorption of MB is controlled by diffusion resistance. Intraparticle diffusion plots for the adsorption of MB on pure PANI and PANI/SiO₂ composite are illustarted in Figure 3.

Figure 3.

Intraparticle diffusion plots for MB adsorption onto (a) (b) PANI and (c) (d) PANi/SiO₂ in acidic and basic mediums.

The above figure show two stages with different slopes for the polyaniline/SiO₂ composite and for pure polyaniline. The intraparticle diffusion constants (K_id and C) for all steps are given in Table 1. As evident from the PANI plot, the MB molecules are scattered in adsorbent particles and then spread in their macropores at the beginning of the adsorption process. Subsequently, MB molecules are propagated in polymeric micropores then the equilibrium take place. However, the PANI/SiO₂ plot also involves two steps. This can be explained by the presence of silica nanoparticles and, consequently, to the raise in surface area of the composite. As well illustrated in the first three figures, the intraparticle diffusion stage is a gradual process. High C values show that the transfer of additional mass of MB molecules on PANI is significant and take place in the beginning of the adsorption process.

Adsorption isotherms

Adsorption isothermal studies are required to apply the adsorption technique for practical purposes. The adsorption mechanism could be determined by evaluating the equilibrium data also known as adsorption data obtained from the experiments. An equilibrium relationship could be established between the amounts of dye adsorbed on the surface of an adsorbent through the adsorption isotherms. In this study, several isothermal models (Langmuir, Freundlich and Dubinin-Radushkevich) were used to examine the adsorption data.

Langmuir isotherm

The basic assumption of this isotherm is that monolayer formation occurs so that only one dye molecule could be absorbed at an adsorption site and that intermolecular forces decrease with distance. It is also assumed that the surface of the adsorbent has a homogeneous character and has identical and energy equivalent adsorption sites. The Langmuir model is given by the equation below:

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

(7)where q_m is the maximum adsorption amount with full monolayer coverage on the surface of the adsorbent (mg.g⁻¹) and K_L is the Langmuir adsorption constant for adsorption energy (L.g⁻¹).⁶³

The values of q_m and K_L can be determined from Figure 4 from the slopes and the interception of the linear curve of C_e/q_e in function of C_e and are represented in Table 2 together with R² (correlation coefficient).

Figure 4.

Langmuir isotherm plot for MB adsorption by (a) (b) PANI and (c) (d) PANI/SiO₂ composite in acidic and basic mediums.

Table 2.

Adsorption isotherms parameters of MB adsorption on PANI and PANI/SiO₂ composite at pH2 and 11.

Isotherm		PANI (pH2)	PANI (pH11)	PANI/SiO₂ (pH2)	PANI/SiO₂ (pH11)
Langmuir	K_L (L g⁻¹)	0.57597	0.09081	1.77328	0.20906
	q_m (mg g⁻¹)	7.89889	13.86385	1.30303	31.42677
	R _L	0 < R_L < 1	0 < R_L < 1	0 < R_L < 1	0 < R_L < 1
	R ²	0.88845	0.17234	0.91575	0.3616
Freundlich	k_f (L g⁻¹)	3.56744	3.92062	0.67684	6.14691
	n _f	3.55252	6.6212	3.92572	1.60462
	R ²	0.80783	0.08493	0.58776	0.69909
Dubinin-K-R	q_m	20.09799	0.0205	1.424 0 × 10⁻⁵	0.00106
	B	−0.01766	−0.09925	−0.02045	−0.04175
	R ²	0.74661	0.85615	0.60007	0.69486
	E_ads (kJ/mol)	5.32	2.24	4.94	3.46

The isotherm is linear for all concentrations range and shows a reasonable adaptation to the adsorption data.

The preference and viability of the adsorption process can be determined by the separation factor R_L in the data analysis using the Langmuir isotherm. It is given by the equation below:

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

(8)

If: 0 < R_L <1 adsorption is favorable, but if 1 < R_L adsorption is unfavorable. If R_L = 0: adsorption is irreversible and if R_L = 1: adsorption is reversible.⁶⁴ In this case study, in all the cases: 0 < R_L < 1 indicating that MB adsorption on PANI and PANI/SiO₂ composite is favorable.

Freundlich isotherm

Freundlich’s model is applicable to heterogeneous systems and involves the formation of multilayers. This adsorption isotherm is given by:

\log q_{e} = \log k_{f} + \frac{1}{n} \log C_{e}

(9)where k_f and n are the Freundlich constants and represent, respectively, the adsorption capacity and the measure of heterogeneity.⁶⁵ The values of k_f and n were determined from the linear diagram of ln q_e in function of ln C_e (Figure 5) and the values are represented in Table 2.

Figure 5.

Freundlich isotherm plot for MB adsorption by (a) (b) PANI and (c) (d) PANI/SiO₂ composite in acidic and basic mediums.

The linear plot of ln q_e vs. ln C_e shows Freundlich adsorption. In adsorption, if k_f value increases, the quantity of dye adsorbed onto the surface of PANI and PANI/SiO₂ will increase (Figure 5 and Table 2). The results in Table 2 demonstrate that Freundlich isotherm have low agreement with the experimental data (low R² values). However, k_f values increase and the quantity of dye adsorbed onto the surface of PANI and PANI/SiO₂ increase (best values at pH 11).

In this study, n > 1 in all cases, which indicate the favorability of the adsorption process.² The data evaluation of the linearly calculated model above sometimes does not provide satisfactory results since linear equations can provide multilinear graphs.

Dubinin - Kaganer - Radushkevich isotherm

In adsorption studies, the different isotherms studied before are generally used to describe single layer adsorption and cannot determine the mechanisms and energy of adsorption. It is the Dubinin - Kaganer - Radushkevich (DKR) isotherm that can provide the adsorption mechanism and the energy of the adsorption process, which is expressed linearly:

\ln q_{e} = \ln q_{m} - B ε^{2}

(12)where q_m is the theoretical monolayer saturation capacity (mg.g⁻¹), B is the constant, called D-R model constant, and ε² is the Polanyi potential,⁶⁶ which is determined by the equation

ε = R T \log (1 + \frac{1}{C_{e}})

(13)where R is the general gas constant and T is the absolute temperature.⁶⁷

Low R² values demonstrate that DKR isotherm have low agreement with the experimental data. The calculated adsorption energy values ranging from 2.24 to 5.32 < 8 kJ.mol⁻¹, indicates that the adsorption of MB onto PANI and PANI/SiO₂ surfaces are physical in nature (Figure 6).^66,67

Figure 6.

Dubinin-Kaganer-Radushkevich isotherm plot for MB adsorption by (a) (b) PANI and (c) (d) PANI/SiO₂ composite in acidic and basic mediums.

A comparison of the efficacy of the synthesized materials on the adsorption process is provided in Table 2.

The validity of the models was verified by the good correlation with the exprimental data assessed by correlation coefficients R² values. The equilibrium adsorption isotherms data of the two adsorbents (Table 2) demonstrate that Freundlich and Dubinin–Radushkevich isotherms have low agreement with the experimental data, whereas Langmuir model has produced the best agreement but not in all cases (In case of PANI and PANI/SiO₂ at pH2).

Thermodynamic study

Thermodynamic parameters, such as E_a (activation energy), ∆G (Gibb free energy change), ∆H (enthalpy change) and ∆S (entropy change) are useful to explain the nature of adsorption. The E_a is calculated from the Arrhenius equation, shown below

k = A . \exp (- Ea / RT)

(14)where T is the absolute temperature, A is the Arrhenius constant and k is the frequency constant.⁶¹ The ∆G is calculated by the following equation⁵⁵

Δ G = - R T \ln \frac{q_{e}}{C_{e}}

(15)

The ∆H and ∆S are calculated from the Van’t Hoff equation by plotting ln q_e/C_e vs. 1/T, as indicated below.

\ln \frac{q_{e}}{C_{e}} = \ln k = \frac{{∆ S}^{0}}{R} - \frac{{∆ H}^{0}}{R T}

(16)where T is the temperature. Values of ΔH° and ΔS° (Table 3) were calculated from the slope and intercept of Van’t Hoff plots of ln Kd versus 1/T.⁶⁸ (Figure 7).

Table 3.

Thermodynamic parameters of MB adsorption onto PANI and PANI/SiO₂ composite.

	T (°C)	∆H° (kJ mol⁻¹)	∆S° (kJ mol⁻¹ K⁻¹)	∆G° (kJ mol⁻¹)
PANI pH2	30	−1.21	0.106	−31.22 < 0
	35			−34.95 < 0
	40			−39.22 < 0
	45			−44.03 < 0
PANI pH11	30	1.37	0.105	−33.43 < 0
	35			−37.13 < 0
	40			−41.36 < 0
	45			−46.10 < 0
PANI-SiO₂ pH2	30	−14.1	0.12	−22.54 < 0
	35			−26.95 < 0
	40			−30.40 < 0
	45			−37.58 < 0
PANI-SiO₂ pH11	30	14.56	0.076	−37.80 < 0
	35			−40.47 < 0
	40			−43.50 < 0
	45			−47.00 < 0

Figure 7.

Determination of standard enthalpy change for the adsorption of MB by (a) (b) PANI and (c) (d) PANI/SiO₂ composite in acidic and basic mediums.

Figure 7 shows the Van’t Hoff diagram, obtained by plotting ln k vs. 1/T after adsorption of MB. The enthalpy change value is useful to explain the adsorption phenomenon. Rehman et al.⁶⁹ described the criteria for physisorption and chemisorption between adsorbate and adsorbent in relation with enthalpy change (∆H°): 4–10 kJ mol⁻¹ (physical adsorption), 2–40 kJ mol⁻¹ (Hydrogen bonding forces) and >40 kJ mol⁻¹ (chemical adsorption).⁷⁰ The enthalpy variation values in the present work, as illustrated in Table 3, are −1.21 and +1.37 kJ mol⁻¹ for the adsorption of MB on PANI (at pH 2 and 11), −14.1 and +14.56 kJ mol⁻¹ for the PANI/SiO₂ composite, these values with their negative sign confirm the physical and exothermic process at acidic pH, and endothermic process at alkaline pH, respectively. The ∆G° values are also useful to explain the adsorption phenomenon. When ∆G° is between −20–0 kJ mol⁻¹ show physisorption and −400 to −80 kJ mol⁻¹ show chemisorption.^12,71 In this part, ∆G° values decreases with increasing temperatures in the ranges 30–45°C. For Methylene Blue adsorption on the pure PANI and PANI/SiO₂ composite, the values of ∆G° varies from −31.22 to −46.10 kJ mol⁻¹ and −22.54 to −47 kJ mol⁻¹, respectively. This result confirms that MB adsorption onto PANI and its derivatives is spontaneous and physical in nature.

The ΔG° value are more negative indicating that adsorption is more spontaneous. The positive value of ΔS° reveals the increased randomness at the dye and doped PANI and PANI/SiO₂ interfaces during the adsorption process.

Theoretical calculation (modelling approach)

Mulliken atomic charges and electrostatic potential distribution

The possible interactions between MB dye and PANI, PANI-SiO₂ sorbent sites were considered through theoretical calculation of electronic charges and electrostatic potential distribution in methylene blue using HyperChem v8 software.⁷² The geometry optimization of the MB, Mulliken atomic charges and electrostatic potential were assessed by the semi empirical PM3 method derived from the Hartree-Fock theory.⁷³

Figure 8 shows the calculated Mulliken atomic charges of MB at pH2 and 11. For MB molecules in acidic medium pH = 2, the totality of the positive charges were centered onto alkyl amine groups in both ends of the benzenic cycles (left and right sides –N⁺H(Me)₂; +0.976e^-), in amine group –NH(+0.38e^-) and sulfur atom S (+0.35e^-) in the central ring, in the benzenic cycles with sulfur and nitrogen atoms (+0.108e^-). It is interesting to note that, alkyl amine groups –N⁺H(Me)₂ in both sides are the most positively charged. Also, the charge in –NH amine group is higher than in sulfur atom (Figure 8 above). However, in alkaline medium at pH = 11, the charges of sulfur atom in the centred ring (+0.097e^-) and alkyl amine groups –N(Me)₂; +0.02e^-) at the ends of MB molecule are highly reduced. The benzenic cycles adjacent to the central ring having sulfur and nitrogen heteroatoms are completely charged negatively (−1.043e^-) (Figure 8 below).

Figure 8.

Semi-empirical PM3 calculated Mulliken atomic charges and (3D) mapped isosurface surrouding the MB molecule ((↑) MB at pH = 2 and (↓) MB at pH = 11).

The chemical reactivity of materials is better perceptive by the electric potential surface (EPS). It is useful to detect favorable interaction sites in molecules. Figure 8 illustrates the PM3 calculated 3D mapped isosurface of the electrostatic potential surrounding the MB dye in acidic and basic solution (pH = 2 and pH = 11). Methylene blue molecule in acidic pH close to 2, shows an electrostatic potential characterized by the red colors indicating negative potential regions (0.370–0.0 esu), followed by green colors, which denote strong positive potential regions (0.0–1.304). In alkaline medium, MB dye shows negative potential regions (red colors; −0.334–0.0 esu), followed by, strong positive potential regions (green colors; 0.0–0.500). Positively and/or negatively charged molecules tend to interact with the sorbent sites where the electrostatic potential is strongly negative and positive, respectively.

Adsorption mechanism

Before talking about the MB adsorption mechanism onto PANI and PANI-SiO₂, the following points needs to be taken in consideration to explain the nature of adsorption process:

1-Species to be considered in a system with MB in aqueous solution as function of pH are schematized below. As can be seen in Figure 9, MB possess three basic sites represented by the nitrogen atoms in the middle ring and in alkyl amine groups, the pKa values reported respectively are close to pKa₁ = 2.6, pKa₂ = 11.2 and pKa₃ = 11.2,⁷⁴ this indicates that the nitrogen atom in the middle ring is the least basic. The calculated isoelectric point (IEP) of MB molecule was 8.33 (calculated as the average pKa values). From the speciation forms in Figure 9, MB dye can adopt three chemical forms, the tri-protonated form as MBH₃²⁺ in very acidic medium (pKa < 2.6) were the ends alkylamine groups (-NH(CH₃)⁺₂) are both charged positively. For 2.6 < pKa <8.33, methylene blue is in the mono protonated (MBH) and di-protonated MBH₂⁺ forms. However, for pKa values greater than 8.33 the nitrogen atom in the central cycle of MB molecule is completely deprotonated, the MB dye become negatively charged (MB^-).

Figure 9.

Speciation of MB forms in water.

2-The proposed mechanisms of adsorption of methylene blue onto PANI and PANI-SiO₂ in the different cases (pH2 and 11) are shown below (Figures 10 and 11). All theoretical calculations were done according to semi-empirical PM3 calculated Mulliken atomic charges using HyperChem v8 software.

Figure 10.

The proposed mechanisms of adsorption of methylene blue onto PANI at pH2 and 11.

Figure 11.

Proposed mechanisms of adsorption of methylene blue onto PANI-SiO₂ at pH2 and 11.

The last proposed mechanisms are also argued by recording the FTIR spectrum of both dye loaded PANI and PANI-SiO₂.

The PANI FTIR spectra shown in Figure 12(a) where the different absorption bands represent the characteristic bands of the PANI. Peak at 798 cm⁻¹ attributed to the out-of-plane bending of C-H in benzenoid ring, 1234 cm⁻¹ and 1116,6 cm⁻¹ correspond to C-H and C-N stretching vibration of the quinoid rings, 1295,1 cm⁻¹ correspond to C-N stretching vibration of the benzenoid unit, 1460,3 cm⁻¹ and 1598 cm-1 correspond to C = C stretching vibration of benzenoid and quinoid rings, 2933,3 cm⁻¹: protonated imine –NH⁺–3442,8 cm⁻¹ secondary amine –NH–. Else, for PANI MB loaded (after adsorption), it can be seen that the intensity of the peaks 1460,3 cm⁻¹ and 1598 cm⁻¹ corresponding to C = C stretching vibration of benzenoid and quinoid rings decrease due to the weak interactions between the PANI and the dye molecules. However, in PANI-SiO₂ (Figure 12(b)), the peak at 1650 cm⁻¹ belong to bending motions of absorbed H₂O on SiO₂, and the peaks show slight displacement, which could be due to weak interactions such as van der Waals forces and the hydrogen bond between PANI and SiO₂.⁵⁵

Figure 12.

FTIR spectra of (a) pure PANI (b) PANI-SiO₂ composite before and after adsorption.

Also, for both adsorbents and after adsorption, appearance of the peak around 1045 cm⁻¹ corresponding to C-S stretching vibration which indicates the occurrence of adsorption.

Conclusions

In the present study, PANI and PANI/SiO₂ composite were successfully synthesized and used as adsorbents for removal of MB (cationic dye) from aqueous solutions. The adsorption of MB was studied as a function of contact time, initial MB concentration and sorbent dosing. A high amount of dye (6.97 mg.g⁻¹) was adsorbed on PANI/SiO₂ composite in comparison to that adsorbed with pure PANI (5.2 mg.g⁻¹). By comparing the effect of the medium (acidic or alkaline), the PANI homopolymer and the PANI/SiO₂ composite exhibit high adsorption efficiencies in the alkaline medium compared to those in the acidic medium. Therefore, the adsorption capacity increased with the increasing pH value. The pseudo second order is more adequate for the adsorption kinetics of MB by PANI and PANI/SiO₂ composite in acidic and basic mediums. From the intraparticle diffusion results, the MB molecules are scattered in adsorbent particles and then propagated in polymeric micropores. The Langmuir isothermal model fitted more closely to the data of MB adsorption in this study. The negative sign of ∆H values confirm the physical and exothermic process at acidic pH, and endothermic one at alkaline pH. The ∆G variation confirms that the process of adsorption between MB and both adsorbents used in this study is physical and spontaneous. Theoretical calculation of electronic charges and electrostatic potential distribution in methylene blue using HyperChem v8 software were used to propose a mechanism of adsorption between MB and the two adsorbents (PANI and PANI/SiO₂).

Supplemental Material

Supplemental Material - Study of methylene blue dye elimination from water using polyaniline (PANI) and PANI/SiO₂ composite

Supplemental Material for Study of methylene blue dye elimination from water using polyaniline (PANI) and PANI/SiO2 composite by Abderrahim Bensedira, Nacerddine Haddaoui, Rachida Doufnoune, Ouahiba Meziane and Nouar Sofiane Labidi in Polymers and Polymer Composites

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. All authors declare that they have no conflict 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.

ORCID iDs

Abderrahim Bensedira

Nacerddine Haddaoui

Ouahiba Meziane

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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Study of methylene blue dye elimination from water using polyaniline (PANI) and PANI/SiO 2 composite

Abstract

Keywords

Introduction

Experimental section

Synthesis of polyaniline emeraldine salt

Synthesis of PANI/Silica (SiO2) composite

Sorbate dye (Methylene blue)

Adsorption experiments

Results and discussion

Study of adsorption of the dye (MB)

The influence of the contact time

Effect of adsorbent dosing

Effect of the dye initial concentration

Adsorption kinetics

Adsorption isotherms

Langmuir isotherm

Freundlich isotherm

Dubinin - Kaganer - Radushkevich isotherm

Thermodynamic study

Theoretical calculation (modelling approach)

Mulliken atomic charges and electrostatic potential distribution

Adsorption mechanism

Conclusions

Supplemental Material

Supplemental Material - Study of methylene blue dye elimination from water using polyaniline (PANI) and PANI/SiO2 composite

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Supplemental Material

References

Supplementary Material

Synthesis of PANI/Silica (SiO₂) composite

Supplemental Material - Study of methylene blue dye elimination from water using polyaniline (PANI) and PANI/SiO₂ composite