Adsorption of Eosin Dye from Aqueous Solution Using Groundnut Hull–Based Activated Carbon: Kinetic,Equilibrium,and Thermodynamic Studies

Abstract

This article reports the use of activated carbon prepared from groundnut hulls (GHAC) in the adsorption of eosin dye from aqueous solution in batch process. The prepared GHAC was characterized using Brunauer, Emmett and Teller (BET), Fourier transform infrared, and scanning electron microscopy. Operational parameters such as agitation time, initial dye concentration, pH, and effect of temperature were studied. Equilibrium time for the adsorption process was attained in 6 h. Adsorption isotherms used to test the adsorption data were Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms. Adsorption equilibrium data were best described by these isotherms in the following order: Temkin > Langmuir > Freundlich > D-R. Kinetic data were fit using pseudo-first- and second-order kinetic models. Adsorption of eosin dye onto GHAC was best described by the pseudo-second-order kinetic model. Thermodynamic parameters such as ΔG⁰, ΔH⁰, and ΔS⁰ of the adsorption process were determined; the adsorption process was feasible, endothermic, and spontaneous with increased randomness at the solid–solution interface. The mean energy of adsorption (<8 kJ/mol) obtained from D-R isotherm showed that the reaction followed a physisorption mechanism. Regeneration and reusability of GHAC were assessed for four successive adsorption–desorption cycles and was found to retain its adsorptive capacity as high as 98%. This study has shown that GHAC is a good adsorbent in the treatment of eosin dye from aqueous solution.

Introduction

In recent years, increasing awareness of water pollution and its far-reaching effects have prompted concerted effort toward pollution abatement (Ahmet et al., 2008). Much of the treatments ongoing within industries are focused toward treating water contaminated with organic pollutants, because organics are common waste constituents. The fact that synthetic dyes are largely used in many industries presents lot of hazards and environmental problems. Today, there are more than 10,000 dyes with different chemical structures available commercially (Arivoli and Thenkuzhali, 2008). Dyes are classified in three broad classes: (a) anionic: direct, acid, and reactive dyes; (b) cationic: all basic dyes; and (c) nonionic: dispersed dyes. Eosin dyes are used in wool and silk industries to give red color with a yellow fluorescence (Finar, 1973).

Many investigators have studied different techniques for removal of colored dye from wastewater, for example, chemical coagulation/flocculation, different advanced oxidation processes (Malik and Saha, 2003; Bali, 2004), ozonations (Bes-Pia´ et al., 2003; Wang et al., 2003), cloud point extraction (Purkait et al., 2004a, 2004b, 2004c), nanofiltration (Chakraborty et al., 2003), micellar enhanced ultrafiltration (Purkait et al., 2004a, 2004b, 2004c), and adsorption onto agricultural solid waste (Rengaraj et al., 2002), calcined alunite (Ozacar and Sengil, 2003), various types of activated carbon (Al-Degs et al., 2001; Jana et al., 2003), surfactant-impregnated montmorillonite (Bae et al., 2000), etc. Ultrafiltration and nanofiltration can be used for complete removal of all classes of dyes, but care is needed to avoid membrane clogging, which decreases the flux. Recently, efforts have been concentrated on the preparation of activated carbon from agricultural byproducts such as almond shell (Demirbas et al., 2008), bean pod (Cabal et al., 2009), rice husk (Mohan et al., 2008), cherry stone (Jaramillo et al., 2009), date palm seed (El Nemr et al., 2008), sunflower seed hull (Thinakaran et al., 2008), waste apricot (Onal et al., 2007), oil palm fiber (Tan et al., 2007), bamboo (Hameed et al., 2007), plum kernel (Tseng, 2007), periwinkle shells (Bello et al., 2008), coconut husk (Tan et al., 2008a, Tan et al., 2008b), and sawdust (Bello et al., 2010). In practice, the feasibility of activated carbon adsorption process depends on many factors including the feasibility of regeneration and disposal of spent activated carbon. Therefore, the spent activated carbon should have high regeneration efficiency for wider application in adsorption process.

Groundnut hull (GH) is an agricultural waste material that is readily available. Activated carbon prepared from GH (GHAC) has been utilized for the sorption of dyes such as methylene blue (Kannan and Sundaram, 2001) and malachite green (Malik et al., 2007). To the best of our knowledge, no study on the utilization of GH for adsorption of eosin dye has been reported in literature. In the present study, GHAC was prepared; its potential to remove eosin dye from aqueous solutions was investigated. Effects of operational parameters such as agitation time, initial eosin dye concentration, pH, and temperature were studied. The feasibility of regenerating the spent GHAC carbon was also determined.

Materials and Methods

Preparation of eosin dye solution

The dye used in this study is eosin, it is anionic in nature (purity: 98.7%; FW: 691.86; color: red; λ_max: 517 nm). The structure of the dye is shown in Fig. 1. Eosin dye solutions were prepared by dissolving accurately weighed amounts of dye in distilled water at concentrations of (10–60 mg/L). The pH of the solution was adjusted to observe the effect of pH on eosin dye adsorption. Solutions of 0.1 M hydrochloric acid and 0.1 M sodium hydroxide were used to adjust the pH of the dye solution. The pH was measured using a pH meter.

FIG. 1.

Structure of eosin dye.

Preparation of GHAC

GH used for preparation of adsorbent was obtained from a local market in Ogbomoso, Oyo State, Nigeria. It was first washed with water to remove dirt from its surface and subsequently dried at 105°C for 24 h in an oven to remove the moisture content. The dried GH was ground and sieved to the desired particle size of 100–200 μm. The sieved sample was then loaded into a stainless-steel vertical tubular reactor placed in a tube furnace. Carbonization of the dried GH was carried out at 700°C, with heating rate of 10°C min⁻¹ under purified nitrogen flown through at a flow rate of 150 mL min⁻¹ for 2 h. A certain amount of produced char was then soaked with potassium hydroxide (KOH) at an impregnation ratio of 1:1 (KOH pellets:char). The mixture was dehydrated in an oven overnight at 105°C, then pyrolyzed in a stainless-steel vertical tubular reactor placed in a tube furnace under high-purity nitrogen (99.995%) flow of 150 cm³/min (second pyrolysis) to a final temperature of 778°C, and activated for 2 h. Once the final temperature was reached, the gas flow was switched over from nitrogen to CO₂ while activation was held for 2 h. The activated product referred to as GHAC was then cooled to room temperature under nitrogen flow and then washed with hot deionized water and hydrochloric acid (0.1 M) until the pH of the solution used for washing reached 6.5–7. GHAC prepared was stored in an airtight container.

Characterization of GHAC

Textural characterization of the GHAC was carried out by N₂ adsorption at 77 K using Autosorb I, supplied by Quantachrome Corporation. Ten milligrams of each of GHAC before and after eosin dye adsorption was ground with 200 mg KBr (spectroscopic grade) in a mortar and pressed into 10-mm-diameter disks less than 10 tonnes of pressure and high vacuum for 10 min separately. Fourier transform infrared (FTIR) spectra of these samples were obtained on a JASCO FTIR-3500 spectrometer. Analysis conditions used were 16 scans at a resolution of 4 cm⁻¹ measured between 400 and 4000 cm⁻¹ (Fig. 2). The surface morphology of the GHAC samples before and after eosin dye adsorption was examined using a scanning electron microscope (SEM; model VPFESEM Supra 35VP). Elemental analysis of GHAC was carried out (Table 1). The scanning electron micrographs of GHAC before and after adsorption are shown in Fig. 3.

FIG. 2.

Fourier transform infrared (FTIR) spectrum of (a) raw groundnut hull (GH) and (b) activated carbon prepared from GH (GHAC).

FIG. 3.

Scanning electron micrograph of (a) raw GH and (b) GHAC.

Table 1.

Elemental Analysis of Activated Carbon Prepared from Groundnut Hulls

Element	C	H	N	O	Ca	Fe	Mg	Mn	Na	K
Percentage	45.5	6.7	0.9	33.9	0.28	0.001	0.009	0.002	0.024	0.019

Batch equilibrium method

Batch adsorption process of eosin dye from its aqueous solution was carried out by agitating 0.2 g of 200-μm-particle-sized GHAC with 100 mL of various concentrations of eosin dye solutions in 250-mL conical flasks. This was done by setting the samples into a bath shaker and the samples were shaken for 48 h at 30°C–60°C until equilibrium was reached. Ten milliliters of the supernatants were extracted and filtered using filter paper. Five milliliters of the filtrates were withdrawn and analyzed using UV–visible spectrophotometer (model A Analyst 800; Perkin-Elmer) at a wavelength of 517 nm to determine the amount of adsorbate adsorbed on GHAC and the percentage of eosin dye removed, using Equations (1) and (2). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\begin{align*} q_e = \frac {(C_0 - C_e)V} {W} \tag{1} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \% \ {\rm Removal} = \frac {(C_o - C_e)} {C_o} \times 100 \tag {2} \end{align*} \end{document} where q_e is the amount of eosin dye adsorbed (mg/g) at equilibrium, V the volume of solution (dm³), C_o and C_e are the initial and equilibrium concentration of eosin dye remaining in the solution (mg/L), and W is the dry weight of the GHAC used (g).

Batch kinetic studies

The procedures for kinetic experiments were identical to those of equilibrium tests. The aqueous samples were taken at preset time intervals and the concentrations of eosin dye were determined similarly. The amount of eosin dye adsorbed at time t, that is, q_t (mg/g), was calculated using Equation (2). Each experiment was carried out in triplicate under identical conditions.

Results and Discussions

Characterization of GHAC

BET analysis

It was found that the BET surface area, pore volume, average pore diameter, and bulk density of the GHAC were 385 m²/g, 0.732 cm³/g, 4.89 nm, and 0.415 g/cm³, respectively. The porosity of GHAC was well developed with higher surface area, total pore volume, and larger average pore diameter, which are advantageous for eosin dye adsorption. The average pore diameter of the prepared sample was found to be 4.89 nm. This indicated that the prepared GHAC was mesoporous. Stavropoulos and Zabaniotou (2005) stated that KOH is dehydrated to K₂O, which reacts with CO₂ produced by the water-shift reaction, to give K₂CO₃. Intercalation of metallic potassium appeared to be responsible for the drastic expansion of the carbon material, and hence, a high specific surface area and high pore volume are created. Similar observation was reported by Tseng et al. (2006), wherein CO₂ gasification was found to promote the formation of mesopores and enhance the surface area of activated carbon.

FTIR analysis

The FTIR spectrum of GHAC before adsorption is shown in Fig. 2a. A broad peak at 3400 cm⁻¹ is an indicator of -OH and -NH groups. The stretching of the -OH groups bound to methyl radicals presented a signal between 2950 and 2887 cm⁻¹. The peaks located at 1737 and 1633 cm⁻¹ are characteristics of carbonyl group stretching from aldehydes and ketones. The presence of -OH group, along with carbonyl group, confirms the presence of carboxylic acid groups in GHAC. The peaks at 1508 cm⁻¹ are associated with the stretching in aromatic rings. The peaks observed at 1071 and 1024 cm⁻¹ are due to C-H and C-O bonds. The -OH, NH, carbonyl, and carboxylic groups are important sorption sites (Volesky, 2003). The FTIR spectrum of the GHAC after adsorption of eosin dye is shown in Fig. 2b. Compared with GHAC spectrum before adsorption, the broadening of -OH peak at 3400 cm⁻¹ and carbonyl group peak at 1633 cm⁻¹ was observed. This indicates the involvement of hydroxyl and carbonyl groups in the adsorption of eosin dye. A similar result was obtained by Suleman et al. (2009).

SEM analysis

Figure 3a and b shows the SEM images of raw GH and GHAC, respectively. SEM images are very useful to obtain accurate adsorption details of activated carbons before and after adsorption experiments. Physical properties and the surface morphology of activated carbons influence their adsorption capacities. Raw GH (Fig. 3a) does not have well-developed pores; however, GHAC has a lot of pores and higher surface area (Fig. 3b). This observation is supported by the BET surface area, total pore volume, and average pore diameter of the GHAC (385 m²/g, 0.732 cm³/g, and 4.89 nm) mentioned earlier. This shows that KOH and CO₂ are effective in creating well-developed pores on the surface of the precursor, leading to GHAC with high surface area and porous structure (Suzuki et al., 2007; Hameed and Daud, 2008). These pores allowed a good surface for eosin dye to be trapped and adsorbed into. Similar observations have been reported by other investigators (Amin, 2008; Hameed and Ahmad, 2009)

Effect of agitation time and initial dye concentration

The effect of agitation time and initial concentration on the uptake of eosin dye by GHAC is shown in Fig. 4 for various concentrations at 30°C. The process showed rapid adsorption of eosin dye in the first 5 h for all the concentrations studied, and thereafter, the increase became gradual until a point where there is no change in the amount adsorbed, that is, a constant value when the amount of desorbed dye is proportional to the amount adsorbed on GHAC, inferring a dynamic equilibrium (at 6 h). The adsorption capacity at equilibrium increases from 4.78 to 28.47 mg/g with an increase in the initial dye concentration from 10 to 60 mg/L. As there is no significant difference in the adsorption values at 6 h and 8 h, after 6-h contact, a steady-state approximation was assumed and a quasi-equilibrium situation was reached. Further experiments were conducted for 6 h contact time only. The adsorption curves were single, smooth, and continuous, leading to saturation, and indicated the possible monolayer coverage on the surface of GHAC by the eosin; dye molecules (Langmuir, 1916; Malik and Saha, 2003).

FIG. 4.

Effect of agitation time and initial dye concentrations on the adsorption of eosin dye on GHs at 30°C.

Effect of pH

pH plays an important role in the adsorption capacity by influencing the chemistry of both the eosin dye molecule and the GHAC in aqueous solution. The percentage of eosin dye adsorbed by GHAC at different pH levels is shown in Fig. 5 for an initial dye concentration of 30 mg/L. Eosin is a dipolar molecule at low pH, as shown in Fig. 1. GHAC contains oxygen complexes on its surface, for example, carboxylic groups and carbonyl groups. These groups are nucleophilic in nature. With a decrease in the pH of the dye solution, more dye molecules are protonated and get adsorbed on the surface of the GHAC. It was observed from Fig. 5 that at pH 2, eosin dye adsorption was about 98% for an initial dye concentration of 30 mg/L. Figure 5 shows that the percentage of adsorption decreased with an increase in pH; the removal was 70% at pH 12. This is due to the fact that the dye molecules become nucleophilic at higher pH (basic pH), which results in less adsorption on the nucleophilic sites of the GHAC. Similar works have been reported in literature (Hameed and Rahman, 2008, Hameed et al., 2009).

FIG. 5.

Effect of pH on the percentage adsorption of the dye. Initial concentration=30 mg/L; time=60 min; temperature=30°C.

Effect of temperature

The adsorption of eosin dye on GHAC was investigated as a function of temperature and maximum dye removal was obtained at 60°C. Experiments were performed at different temperatures (30°C–60°C) for initial eosin dye concentrations of 10–60 mg/L. The percentage of adsorption increased from 51.96 to 55.95%, 55.42 to 62.67%, 52.85 to 54.36%, 53.14 to 55.21%, 49.44 to 51.96%, and 47.45 to 49.38% for the initial eosin dye concentrations of 10–60 mg/L, respectively, with the rise in temperature from 30°C to 60°C (Fig. 6). This is mainly due to the increased surface activity suggesting that adsorption between eosin dye and GHAC was an endothermic process.

FIG. 6.

Effect of temperature on the adsorption of eosin dye unto GHAC at different initial concentration and temperatures.

Adsorption kinetics

The pseudo-first-order kinetic model

The pseudo-first-order equation is generally expressed as follows (Lagergren, 1898): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \ln (q_e - q_t) = \ln q_e - k_1 t \tag{3} \end{align*} \end{document} where q_e and q_t are the adsorption capacities at equilibrium and at time t, respectively (mg/g). k₁ is the rate constant for pseudo-first-order adsorption (h⁻¹). A plot of ln(q_e − q_t) against t at various concentrations and temperatures resulted in graphs with negative slopes. k₁ and q_e are calculated from the slopes and intercepts, respectively (figure not shown). Although the correlation coefficients were high, on comparing the q_e,calc to the q_e,exp, the values do not agree. The results are presented in Table 2. Therefore, the adsorption of eosin dye using GHAC does not follow the pseudo-first-order kinetics.

Table 2.

Pseudo-First- and Second-Order Adsorption Kinetics Model Rate Constants for the Adsorption of Eosin Dye onto Activated Carbon Prepared from Groundnut Hulls at Various Temperatures

			Pseudo-first-order model				Pseudo-second-order model
Temp.	C_o (mg/L)	q_e,exp (mg/g)	k₁ (h⁻¹)	q_e,_calc (mg/g)	R²	SSE (%)	k₂ (g/mg/hr)	q_e,_calc (mg/g)	R²	h_o	SSE (%)
30°C	10	4.78	0.5589	5.76	0.9637	0.4	0.1007	5.84	0.9169	3.44	0.43
	20	11.08	0.8168	10.43	0.9767	0.27	0.2401	11.59	0.9961	32.26	0.21
	30	16.55	0.912	10.55	0.9613	2.45	0.3769	16.89	0.9991	107.53	0.14
	40	21.14	0.8532	13.03	0.9471	3.31	0.2991	21.55	0.9993	138.89	0.17
	50	24.72	0.8651	16.52	0.9653	3.35	0.2341	25.25	0.9991	149.25	0.22
	60	28.47	1.0245	15.64	0.9536	5.24	0.3762	28.82	0.9997	312.5	0.14

SSE, sum of square errors.

The pseudo-second-order kinetic model

The pseudo-second-order equation is expressed as follows (Ho and McKay, 1999): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \frac {t} {q_t} = \frac {1} {k_2 q^2 e} + \frac {1} {q_e} t \tag {4} \end{align*} \end{document} where q_e and q_t are the adsorption capacities at equilibrium and at time t, respectively (mg/g). k₂ is the rate constant of pseudo-second-order adsorption (g/mg/min). The initial adsorption rate, h (mg/g/min), is \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} h = k_2 q^2 e \tag{5} \end{align*} \end{document}

Plots of t/q_t versus t gave linear graphs, from which q_e and k₂ were estimated from the slopes and intercepts of the plot (figure not shown) at 30°C–60°C. The correlation coefficients were as high as 0.99 and there was good agreement between q_e,calc and q_e,exp data obtained. The results are presented in Table 2. The good agreement shows that the pseudo-second-order kinetic equation fits the adsorption data well.

Test of kinetic models

To complement R², the applicability and validity of each kinetic model was verified through the sum of square errors (SSE %) given by \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} SSE \ \% = \sqrt \frac {{\sum (q_{\rm e,exp} - q_ {\rm e,calc}) ^2}} {N} \tag {6} \end{align*} \end{document} where N is the number of data points. Higher value of R² and lower value of SSE suggests a good data fit. From Table 2, it was found that the adsorption of eosin dye on GHAC can be best described by the second-order kinetic model.

Adsorption isotherms

Langmuir isotherm model

Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of adsorption sites that are uniform for adsorption with no trans-migration of adsorbate in the plane of surface (Langmuir, 1916). The linearized form of Langmuir adsorption model is expressed as

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \frac {C_{\rm e}} {q_{\rm e}} = \frac {C_{\rm e}} {q_{\rm o}} + \frac {1} {q_{\rm o} b} \tag {7} \end{align*} \end{document}

where C_e is the dye concentration in the solution at equilibrium (mg/L), q_e is the dye concentration at equilibrium (mg/g), q_o is the monolayer adsorption capacity of GHAC (mg/g), and b is the Langmuir sorption constant (L/mg). A plot of C_e/q_e versus C_e gave a straight line with a slope 1/q_o and an intercept of 1/q_ob (figure not shown). The higher R² values of Langmuir isotherm when compared with Freundlich isotherm indicate that the adsorption of the eosin dye onto GHAC fits the Langmuir isotherm better. Values of q_o and b are calculated and reported in Table 3. The value of q_o obtained was compared with those from other adsorbents (Table 4). To confirm the favorability of the process, the dimensionless equilibrium parameter (R_L) defined by Equation (8) was used.

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} R_{\rm L} = \frac {1} {(1 + bC_{\rm o})} \tag {8} \end{align*} \end{document}

Table 3.

Adsorption Isotherm Model Parameters for Adsorption of Eosin Dye onto Activated Carbon Prepared from Groundnut Hulls at Different Temperatures

	Langmuir parameters				Freundlich parameters			Temkin parameters			Dubinin-Radushkevich parameters
Temp. (°C)	R²	q_o	b	R_L	R²	k_f	n	R²	A_T	b_T (×10³)	R²	β (×10⁻⁶)	q_o	E (kJ/mol)
30	0.9643	68.97	0.0225	0.8163	0.9542	1.179	1.043	0.9982	0.2679	0.1924	0.9579	0.2679	0.1924	0.240
40	0.9813	63.29	0.0272	0.7862	0.9611	1.5011	1.1203	0.9975	0.3018	0.2063	0.9433	0.3018	0.2063	0.270
50	0.9887	58.82	0.0323	0.7559	0.9828	2.2024	1.2847	0.9902	0.3801	0.2330	0.8960	0.3801	0.2330	0.346
60	0.9805	44.84	0.0604	0.6234	0.9518	2.4077	1.3063	0.9921	0.4095	0.2394	0.9295	0.4095	0.2394	0.353

Table 4.

Comparison of Maximum Monolayer Adsorption of Selected Dyes unto Various Adsorbents

Dyes	Adsorbents	q_o (mg/g)	Reference
Eosin dye	GHAC	68.97	This work
Congo red	Activated red mud	7.08	Tor and Cengeloglu (2006)
Congo red	Coir pith AC	6.70	Namasivayam and Kavitha (2002)
Methylene blue	Apricot stones–AC	4.11	Aygun et al. (2003)
Methylene blue	Walnut shell–AC	3.53	Mall et al. (2005)
Methylene blue	Almond shell–AC	1.33	Aygun et al. (2003)

GHAC, activated carbon prepared from groundnut hulls.

where C_o is the highest initial dye concentration, R_L is used to confirm the favorability of the adsorption process, that is, (0 < R_L < 1) is favorable, R_L=1 is linear, R_L=0 is irreversible, or R_L >1 is unfavorable (Langmuir, 1916). The values of R_L reported in Table 3 obtained at various temperatures were less than 1, indicating that the adsorption of eosin dye unto GHAC is favorable.

Freundlich isotherm model

This assumes heterogeneous surface energies, in which the energy term in Langmuir varies as a function of the surface coverage. The linearized form of Freundlich model is represented as follows (Freundlich, 1906): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \log q_{\rm e} = \frac {1} {n} \log C_{\rm e} + \log k_{\rm f} \tag {9} \end{align*} \end{document} where q_e is the amount of eosin dye adsorbed at equilibrium (mg/g), C_e is the equilibrium concentration of the adsorbate (mg/L), and k_f and n are constants incorporating the factors affecting the adsorption capacity and the degree of nonlinearity between the solute concentration in the solution and the amount adsorbed at equilibrium, respectively. Plots of log q_e versus log C_e gave linear graphs (figure not shown). Comparing the value of R² obtained with that of Langmuir model shows that the adsorption data fit the Langmuir model better (Table 3). The values of k_f and n are reported in Table 3; it shows the heterogeneity of the material as well the possibility of multilayer adsorption of eosin dye through the percolation process. The values of n greater than 1 indicate that the adsorption is favorable.

Temkin isotherm model

To consider the effect of the adsorbate interaction on GHAC, Temkin isotherm model was used to test the experimental data (Temkin and Pyzhev, 1940). It is expressed as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} q_{\rm e} = \frac {RT} {b_{\rm T}} \ln (A_{\rm T} C_{\rm e}) \tag {10} \end{align*} \end{document} where A_T and b_T are constants, R is the gas constant, and T is the temperature. A plot of q_e versus ln C_e gave a straight-line graph (figure not shown), from which b_T and A_T values were calculated. These values are reported in Table 3. This shows the applicability of Temkin isotherm as having a best fit to the experimental data, indicated by the highest values of the correlation coefficients than all the models tested. b_T and A_T increases as the temperature increases, thereby inferring that the adsorbate interaction with GHAC increased with increasing temperature, and hence, higher rate of sorption was observed as energy increases.

Dubinin-Radushkevich isotherm model

The Dubinin-Radushkevich isotherm model (D-R model) (Dubinin and Radushkevich, 1947), which does not assume a homogeneous surface or a constant sorption potential as the Langmuir model, was used to estimate the characteristic porosity of GHAC and the apparent energy of adsorption. It was also used to test the experimental data. It is expressed as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} Q_{\rm e} = q_{\rm o} e^{- \beta \varepsilon^2} \tag{11} \end{align*} \end{document} where β is the free energy of sorption per mole of the sorbate as it migrates to the surface of GHAC from an infinite distance in the solution (mol² J⁻²), q_o is the maximum adsorption capacity, and ɛ is the Polanyi potential (J mol⁻¹) that can be written as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \varepsilon = RT \ln (1 + 1 / C_e) \tag{12} \end{align*} \end{document}

A plot of ln Q_e versus ɛ² gave a linear plot (figure not shown), from which β and q_o were obtained from the slopes and intercepts, respectively. The values are presented in Table 3. Similarly, the β value obtained was then used to estimate the mean free energy of adsorption (E). The results are presented in Table 3. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} E = 1 / \sqrt{2} \beta \tag{13} \end{align*} \end{document}

The values of E were found to be between the range of 0.240 and 0.353 kJ/mol over the range of temperatures used in this study. As E<8 kJ/mol, it suggests that the adsorption mechanism is physical in nature (Helfferich, 1962). Similar results were obtained by other researchers (Arivoli and Thenkuzhali, 2008, Dang et al., 2009).

Thermodynamic studies

The thermodynamic parameters ΔG^o, ΔH^o, and ΔS^o were also determined to investigate the feasibility, spontaneity, and the nature of the reaction. This was achieved by using the following equations: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} k_{\rm o} = q_{\rm e} / C_{\rm e} \tag{14} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \Delta G^{\rm o} = - RT \ln k_{\rm o} \tag{15} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \ln k_o = \frac {\Delta S^{\rm o}} {R} - \frac {\Delta H^{\rm o}} {RT} \tag{16} \end{align*} \end{document} where k_o is the equilibrium constant, q_e is the amount removed at equilibrium (L/mg), C_e is the concentration at equilibrium (mg/L), T is the temperature in Kelvin, and R is the gas constant. A plot of ln k_o against 1/T gave linear plots (figure not shown), from which ΔH^o and ΔS^o values were obtained from the slopes and intercepts. The results are presented in Table 5. The low value of ΔH^o, which falls between the range of 1 and 93 kJ/mol, signifies that the mechanism of adsorption follows physisorption. The positive value of ΔH^o affirms the endothermic nature of adsorption. These are in line with the findings by Arivoli and Thenkuzhali (2008). The low ΔH^o values depict that the dye is physisorbed onto GHAC. The positive values of ΔS^o show the increased disorder and randomness at the solid/solution interface of eosin dye on GHAC. The negative values of ΔG^o show that the adsorption is favorable and spontaneous.

Table 5.

Thermodynamic Parameters for the Adsorption of Eosin Dye onto Activated Carbon Prepared from Groundnut Hulls at Different Initial Concentrations and Temperatures

	ΔG^o (×10³ kJ/mol)
C₀	30°C	40°C	50°C	60°C	ΔH^o (kJ/mol)	ΔS^o (J/mol/K)
10	−0.22	−0.20	−1.06	−1.11	14.84	48.38
20	−0.54	−0.78	−1.02	−1.30	6.99	24.87
30	−0.52	−0.69	−0.87	−1.43	8.22	28.64
40	−0.29	−0.40	−0.54	−0.66	5.50	12.48
50	−0.27	−0.32	−0.12	−0.22	4.79	9.02
60	−0.26	−0.19	−0.13	−0.07	3.39	10.27

Repeated adsorption–desorption studies

GHAC was subjected to adsorption–desorption cycles through regeneration step in between. It is clearly observed from Table 6 that the efficiency of adsorption was retained only when GHAC after desorption was given a treatment with 0.2 M HCl solution. When the GHAC was reused without such treatment, there was a decrease in its adsorptive capacity.

Table 6.

Percentage of Adsorption of Eosin Dye on Regenerated Activated Carbon Prepared from Groundnut Hulls

	Percentage of eosin dye adsorbed by GHAC
Regeneration without HCl	47.05
Regeneration with 0.2 M HCl
I Cycle	94.93
II Cycle	95.91
III Cycle	96.87
IV Cycle	98.45

Conclusions

The optimum pH for the highest easin dye removal was 2.0. Adsorption data followed the pseudo-second-order kinetic model. Temkin isotherm exhibited the best fit for the adsorption data than the Langmuir, Freundlich, and D-R isotherms. Low value of mean energy (<8 kJ/mol) obtained from D-R isotherm confirmed that the adsorption process followed a physisorption mechanism. Negative value of ΔG^o shows that the process is spontaneous at all temperatures. The positive and low values of ΔH^o (1–93 kJ/mol) show that the process is endothermic in nature. The positive values of ΔS^o show that there was increased disorder and randomness at the solid/solution interface of eosin dye and GHAC. This work has shown that GHAC is a good adsorbent in the treatment of eosin dye from aqueous solution.

Footnotes

Acknowledgments

The authors gratefully acknowledge the 1-year postdoctoral fellowship jointly awarded by USM-TWAS to O.S.B. (FR number: 3240223483 in year 2009). The 12-month study leave granted to O.S.B. by his home institution to honor this fellowship is also acknowledged.

Author Disclosure Statement

No competing financial interests exist.

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