Adsorption of methylene blue and malachite green on micro-mesoporous carbon materials

Abstract

The aim of this work was to investigate the determination of adsorption kinetics and adsorption isotherms for methylene blue (MB) and malachite green (MG) from aqueous solutions, on micro-mesoporous carbon materials, C–HA, C–HA–CO₂, C–CA and C–CA–CO₂, obtained by soft-templating method. In most cases, the adsorption kinetics proceeded in compliance with pseudo II-order reaction model, only in one case with pseudo I-order reaction model: C–CA (MG). Adsorption, in equilibrium conditions, was described by Langmuir and Freundlich equations. In five cases, the adsorption proceeded in compliance with the Langmuir model: C–HA (MB, MG), C–HA–CO₂ (MB, MG), C–CA–CO₂ (MB); in three cases, the adsorption was described using Freundlich equation: C–CA (MG, MB), C–CA–CO₂ (MG).

Introduction

Colorants, from a long time, are applied in dyes that are used for textiles, paper, skin, cosmetics, plastics, foodstuff, etc. (McKay, 1979). The disposal of these colorants during the mentioned processes can be dangerous and can cause serious environmental problems (Algidsawi, 2011; Charri and Jamoussi, 2011).

Adsorption on carbon materials is one of the most popular techniques for numerous organic pollutants, also colored organic compounds from aqueous solutions and sewage (Choma et al., 2015).

Many research works have been carried out on the adsorption of colorants, namely methylene blue (MB) and malachite green (MG), on various adsorbents. For example, Yuan et al. (2007) compared MB adsorption on two adsorbents: mesoporous carbon with varied pore volumes and specific surface area (OMC-70, OMC-40, OMC-100) and microporous carbon (CFY). Derylo-Marczewska et al. (2010) applied the obtained mesoporous carbons for MB adsorption and Anbia and Ghaffari (2011) for MG adsorption. Other authors, e.g. Böke et al. (2013) used ordered mesoporous carbons for MB adsorption and Tian et al. (2011) for MG adsorption. Other researchers used active carbons obtained from various kinds of floral or biodegradable wastes for MG adsorption (Akar et al., 2013; Hajati et al., 2015) and for MB adsorption (Álvarez-Torrellas et al., 2016; Nunes et al., 2011).

Experimental

Micro-mesoporous carbons, obtained by the soft-templating method in the presence of hydrochloric acid (C–HA) and citric acid (C–CA) in compliance with Choma et al. (2013), were used as the adsorbents. The obtained carbon materials were additionally activated by carbon dioxide. Activation was carried out in 1123 K temperature for 8 hours (C–HA–CO₂, C–CA–CO₂) in accordance with the procedure proposed by Wickramaratne and Jaroniec (2013).

MB (C₁₆H₁₈ClN₃S, M_mol = 319.85 g mol⁻¹) and MG (C₂₃H₂₅ClN₂, M_mol =382.93 g mol⁻¹) from Sigma-Aldrich (Germany) were used as the adsorbates.

The porous structure of the applied adsorbents was determined on the basis of low-temperature nitrogen adsorption isotherms (77 K) by the volumetric adsorption analyzer ASAP 2020 from Micromeritics (Norcross, GA, USA), in Structural Research Laboratory, Jan Kochanowski University in Kielce. Before the adsorptive measurements, all the samples were degassed at 473 K for 2 hours. On the basis of experimental, low-temperature nitrogen adsorption isotherms, standard parameters characterizing the porous structure of the adsorbents were determined (the methods for the calculation of the structure parameters used in this work and the appropriate references are described in Choma et al., 2013).

The images of the studied materials were taken by a scanning electron microscope (SEM) from Zeiss mod. Ultra Plus, EDS Bruker Quantax 400. During the measurements, a 5 kV voltage was applied.

The concentration of the colorants before and after adsorption was determined by spectrophotometric method on SP-830 PLUS from Metertech. During the investigation, the following wavelengths were applied: 665 nm (MB) and 615 nm (MG). Adsorption studies were performed in Erlenmeyer flaks, vol. 100 mL, containing 0.1 g of studied carbon, 50 mL of colorant solution.

The adsorption kinetics of MB and MG on all the studied adsorbents was determined for the initial concentration of 3 mg/dm³. Adsorption equilibrium settled after 6 h: C–HA–CO₂ (MB and MG); C–HA (MG), after 8 h: C–CA (MG), after 10 h: C–CA–CO₂ (MB and MG), after 12 h: C–HA (MB) and after 22 h: C–CA (MB). Adsorption isotherms were performed for the following initial concentration of colorants: MB: 25 mg/dm³–315 mg/dm³, MG: 8 mg/dm³–200 mg/dm³. Kinetics and adsorption measurements were carried out at 298 K temperature with a mixing speed of 120 r/min.

Results and discussion

The obtained nitrogen adsorption isotherms (Figure 1) are type IV according to the IUPAC classification. By analyzing the porous structure parameters (Table 1), it can be noticed that the specific surface area S_BET increases for adsorbents after CO₂ activation, e.g., from 742 m²/g for C–HA to 1960 m²/g for C–HA–CO₂. In materials obtained after activation, the total pore volume also increased, e.g., from 0.79 cm³/g (C–HA) to 1.67 cm³/g (C–HA–CO₂), mesopores volume, e.g., from 0.66 cm³/g (C–HA) to 1.20 cm³/g (C–HA–CO₂) and micropores volume, e.g., from 0.13 cm³/g (C–HA) to 0.47 cm³/g (C–HA–CO₂). The investigated adsorbents have a large advantage of mesopores volume (72–85%). The pore volume distribution functions (Figure 2) contain two peaks: first is from micropores (1.9–2.06 nm) and the second is from mesopores (5.36–7.41 nm).

Figure 1.

Nitrogen adsorption isotherms for micro-mesoporous carbons.

Table 1.

Porous structure parameters for studied micro-mesoporous carbon materials.

Carbon	SBET (m2/g)	Vt (cm3/g)	Vmi (cm3/g)	Vme (cm3/g)	Sext (m2/g)	wmi (nm)	wme (nm)	Mesoporosity (%)
C–HA	742	0.79	0.13	0.66	5.4	1.90	7.41	84
C–HA–CO2	1960	1.67	0.47	1.20	19.0	2.05	7.37	72
C–CA	724	0.53	0.08	0.45	0.3	2.03	5.85	85
C–CA–CO2	1080	0.72	0.15	0.57	0.5	2.06	5.36	79

Figure 2.

Pore size distributions for micro-mesoporous carbons.

Figure 3 shows the SEM image of the studied adsorbents’ surface. In Figure 3(a), for carbon C–HA, the ordered structure of the material can be clearly seen. Parallel lines are mesopores divided by carbon walls. Figure 3(b) confirms the ordered structure of this material “similar to honeycomb.” In Figure 3(c), homogenous mesopores can be seen (black points) building ordered structure of the carbon C–CA. Figure 3(d) and (e) presents the adsorbents’ structure after CO₂ activation (C–HA–CO₂ and C–CA–CO₂). Less homogenous mesopores can be seen with the dimension of 6.16 nm (Figure 3(d)) and less ordered structures.

Figure 3.

SEM images of studied adsorbents surface: (a) and (b) (C–HA), (c) (C–CA), (d) (C–HA–CO₂) and (e) (C–CA–CO₂).

Adsorption kinetics (Figure 4(a) and (b)) for most of the studied carbon materials and colorants can be described as pseudo II-order reaction (equation (1)), where the velocity constant k value is in the range from 0.211 to 2.438 g mg⁻¹h⁻¹ (Table 2). The exception is the MG adsorption process on C–CA carbon, where the adsorption process proceeds in compliance with the mechanism of pseudo I-order reaction (equation (2)), (k = 0.635 h⁻¹) (Table 2)

$$\frac{t}{q_1}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$

(1)

$$\ln (q_e-q_t)=\ln q_e-k_{1}t$$

(2) where k₁, k₂—reaction rate constants (min⁻¹), (g mg⁻¹ min⁻¹); t—time (min); q_e—adsorption value after the equilibrium stabilization (mg/g); q_t—adsorption value in given time t (mg/g).

Figure 4.

Adsorption kinetics of MB: (a) and MG: (b) on studied adsorbents.

Table 2.

Adsorption kinetics—determined constants.

Adsorbent	Adsorbate	qe (mg/g)	Pseudo I-order		Pseudo II-order
			k1 (h−1)	R 2	k2 (g mg−1 h−1)	R 2
C–HA	MB	1.41	0.127	0.9007	0.372	0.9864
	MG	1.52	0.542	0.9870	2.085	0.9975
C–HA–CO2	MB	4.64	0.522	0.9368	0.385	0.9983
	MG	1.52	0.648	0.9902	2.438	0.9997
C–CA	MB	1.31	0.153	0.9430	0.211	0.9849
	MG	2.73	0.625	0.9813	0.182	0.9726
C–CA–CO2	MB	1.56	0.455	0.9700	0.532	0.9983
	MG	1.39	0.356	0.9163	0.562	0.9977

The adsorption isotherms of MB and MG received on the studied adsorbents are shown in Figure 5(a) and (b). In most cases, the adsorption process of the studied systems is described by a Langmuir adsorption isotherm (equation (3)). The adsorption data for materials C–CA (MB, MG) and C–CA–CO₂ (MG) are in compliance with Freundlich equation (equation (4)). The maximum adsorption capacity (q_m) is in the range from 13.24 to 80 mg/g (Table 3)

$$\frac{c_e}{q}=\frac{1}{q_m}{C}_e+\frac{1}{q_{m}K}$$

(3)

$$\log q=\log K_F+n\log C_e$$

(4) where C_e—equilibrium concentration of colorant aqueous solution (mg/dm³); q—amount of substance adsorbed by elementary mass of adsorbent (mg/g); q_m—maximum adsorption capacity (mg/g); K—adsorption equilibrium constant (dm³/g), K_F, n—Freundlich isotherm constants, describing relative intensity and adsorption ability, respectively.

Figure 5.

Adsorption isotherms of MB: (a) and MG: (b) on studied adsorbents.

Table 3.

Langmuir and Freundlich isotherms—determined constants and ΔG.

Adsorbent	Adsorbate	Langmuir isotherm			Freundlich isotherm			ΔG (kJ/mol)
		qm (mg/g)	K (dm3/g)	R 2	1/n	KF	R 2
C–HA	MB	39.84	50	0.9500	1.74	3.72	0.9550	−9.68
	MG	21.65	94	0.9505	2.38	2.95	0.9158	−11.24
C–HA–CO2	MB	73.3	178	0.9980	3.95	9.33	0.5874	−27.11
	MG	25.57	500	0.9968	3.68	7.94	0.8971	−15.39
C–CA	MB	18.52	45	0.9575	2.21	1.66	0.9864	−9.53
	MG	13.24	36	0.9130	1.98	1.02	0.9612	−8.89
C–CA–CO2	MB	80	277	0.9952	3.35	22.91	0.4242	−28.23
	MG	16.78	169	0.9602	2.97	4.07	0.9655	−27.45

The adsorbents after CO₂ activation are much better than the adsorbents without CO₂ activation for the both studied dyes (Table 3). This effect is greater for MB adsorption (Table 3). The results suggested that the value of q_m depends on the porous structure of the studied adsorbents (Table 1).

The calculated value of free enthalpy ΔG (equation (5)), (Table 3) changes from −28.23 to −8.89 kJ/mol. The adsorption occurs as a spontaneous process (ΔG < 0)

$$\Delta G=-\mathrm{RT}\ln K$$

(5) where ΔG—free enthalpy; R—gas constant (J/mol × K); T—temperature (K); K—Langmuir constant (dm³/mol).

Conclusion

In this work, adsorption of MB and MG on micro-mesoporous carbon materials, obtained by soft-templating method, was investigated. The adsorption kinetics for most of the studied adsorbents and colorants can be described as pseudo II-order reaction (except C–CA–MG). Adsorption data for most of the studied materials are in compliance with Langmuir equation, and in three cases, they are in compliance with the Freundlich equation.