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IJCRR - 7(9), May, 2015

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KINETICS AND EQUILIBRIUM STUDIES OF THE ADSORPTION OF PHENOL AND METHYLENE BLUE ONTO COLA NUT SHELL BASED ACTIVATED CARBON

Author: Lekene Ngouateu R. B., Kouoh Sone P M. A., Ndi Nsami J., Kouotou D., Belibi Belibi P. D., Ketcha Mbadcam J.

Category: General Sciences

Abstract:The aim of this work was to study the removal of methylene blue (MB) and phenol from aqueous solution using powder activated carbon prepared from cola nut shells chemically activated with KOH solution. The effect of contact time, adsorbent dose, pH and initial concentration has been studied using batch adsorption method. Given the base line for the results, the percentage removal of MB and phenol were 98.19 % and 61.81 % respectively. The maximum adsorption of MB and phenol occur at pH 8 and 6. Also the quantity adsorbed decreases with the adsorbent dose for both adsorbates. The quantity adsorbed of MB
increases with increased in initial concentration whereas reverse result was observed with phenol. The adsorption equilibrium data were confronted to Langmuir, Freundlich, Tempkin and Dubinin-Kaganer-Radushkevich isotherms models. The Langmuir model describes better the adsorption with maximum monolayer adsorption capacities of 65.79 mg.g-1 and 6.22 mg.g-1 for MB and phenol respectively. The kinetics of methylene blue and phenol adsorption were also discussed using pseudo-first-order, pseudo-second-order, Elovich, mass transfer and intra-particle diffusion models. The adsorption process follows the pseudosecond- order and mass transfer kinetic models respectively for methylene blue and phenol owing to their high correlation coefficient. This study shows that the prepared activated carbon can be used as low cost alternative adsorbent for removal of both phenol and MB.

Keywords: Adsorption, Activated carbon, Equilibrium, Kinetics, Methylene blue and Phenol

Full Text:

INTRODUCTION

Many industries discharge wastewaters containing many hazardous substances such as phenols, heavy metals and dyes. The contamination of water by industrial activities is the source of problems to human health and environment (Hameed et al, 2007a). MB in industries has wide variety of application such as colouring of papers, dyeing textiles and cosmetics. Consequently, their effluents contain notable amounts of this dye stuff (Shen et al, 2009; Reddy et al, 2012). On the other hand, phenol found mostly in pesticides, pharmaceuticals, petro-chemicals, steel manufacturing, etc. (Alam et al, 2006; Varghese et al, 2004; Isichei and Okieimen, 2012) causes several problems. In addition, phenol and the MB are very harmful to humans and other aquatic organisms (Hameed et al, 2007b; Ekpete et al, 2012). The presence of MB in water inhibits sun light diffusion into the water, which reduces the photosynthetic process of aquatic plants (Idris et al, 2012a; Ndi et al, 2013). The ingestion of methylene blue and phenol causes many problems like vomiting, mental confusion, diarrhea, dark urine, cyanosis, tissue necrosis and increased heart bit rate (Hameed et al, 2007a; Karabacako?lu et al, 2008). Thus, it is important to treat wastewaters before their rejection into the nature. And for this purpose, many methods of removing these pollutants have been used such as chemical precipitation, filtration, ion exchange, reverse osmosis, ultra-filtration, biological degradation, membrane separation, solvent extraction and adsorption. These methods differ in their efficiency to remove dyes and phenol and are very expensive (Moraitopoules et al, 2009). However, adsorption remains the outmost process because of its simplicity, high efficiency and easy recovery (Ndi Nsami et al, 2013). In the scientific literature, many adsorbents have been used such as zeolite, clay, silica gel, resins, activated alumina and activated carbon (Ketcha et al., 2007; Ketcha Mbadcam et al, 2012a). Activated carbon is the most widely used adsorbents owing to its well-developed pore structure, large active surface area, good mechanical properties and multiple functional groups at their surface (Wu et al, 2005, Kouotou et al, 2013). There are many precursors from which it can be prepared and some of these precursors include corn cobs (Tsai et al, 2001; Ketcha Mbadcam et al, 2012a), apricot stone (Kobya et al, 2005), sugar bagasse, coconut shells (Jaguaribe et al, 2005), peanut hull (Guler et al, 2007), rattan sawdust (Hameed et al, 2007a), bamboo (Hameed et al 2007b), hazelnut bagasse (Karabacako?lu et al, 2008), cotton stalk fibre (Kunquan et al, 2009), rice husk (Yahaya et al, 2011), oil palm shell (Kouotou et al., 2012), cola nut shell (Ndi et al, 2013); olive stone (Hanen and Abdelmottaleb, 2013) The aim of the present study was to examine the adsorption capacities of MB and phenol from aqueous solution onto activated carbon prepared from cola nut shells activated with KOH using batch system. The effects of factors such as contact time, adsorbent dose, pH and initial concentration were investigated. The kinetics and equilibrium data were confronted to several models.

MATERIALS AND METHODS

Preparation of adsorbent

The cola nut shells were initially washed with tap water followed by distilled water, sun dried and mechanically ground using a grinding machine (Retch) then sieved through a sieve (Retch) to get geometrical size less than 1.5 mm. The prior sieved cola nut shells was kept in an oven at 110 °C for a period of 24 hours, then allowed to cooled to room temperature, in a desiccator containing CaCl2 (drying agent) for 30 min. It was removed from the desiccator and a mass reagent ratio 1:1 was applied, mixed, and dried for 24 hours at 110 °C in an oven (Ndi et al, 2013). The carbonization and activation were accomplished in a one-single step by carrying out thermal transformation of cola nut shells impregnated with KOH as activating agent in the absence of air in a Carbolite furnace (OSI) at 500 °C for 1 hour as resident time. After activation, the furnace was allowed to cool to room temperature. The pyrolysed carbons were leached with 1 % HCl (v/v) for 2-3 hours and washed several times with distilled water until a neutral pH was achieved. The activated carbon obtained was later dried in an oven at 110 °C for at least 24 hours and kept for further application (Ndi Nsami et al, 2013).

Characterization of adsorbent

The surface functional groups of the prepared activated carbon were estimated using Fourier Transform Infrared (FTIR) spectroscopy (Alpha - Bruker). The FTIR spectra of our sample were recorded within 400-4000 cm-1. The BET specific surface area, the total pore volume and the pore size of activated carbon were determined by standard multipoint techniques of nitrogen adsorption using Micromeritrics ASAP 2020 equipment. The iodine number which is a measure of micropore content of the activated carbon is determined by the American Standard for Testing of Materials (ASTM D2866-94).

Preparation of solutions

All the reagents used in this study were of analytical grade. Two stocks solutions of MB and phenol of concentration 50 mg.L-1 and 1000 mg.L-1 respectively were prepared by dissolving 0.050 g of MB and 1.000 g of phenol separately in 1000 mL volumetric flask. These solutions were stirred until total dispersion and the volume completed with distilled water up to mark. These solutions were stirred on a magnetic agitator for 6 hours to homogeneity. All the experimental solutions were prepared by diluting the stock solution to the required concentration.

Batch adsorption study

The batch adsorption was carried out at room temperature. The effects of contact time, adsorbent dose, pH of solution and the initial concentration of these adsorbates were studied. In each experiment, 20 mL of MB or phenol solution of known concentration were mixed with a known mass of activated carbon. The pH of the mixture was adjusted either with 0.1 N HCl or 0.1 N NaOH solutions. The suspension was stirred for a given interval of time, using a magnetic agitator and stirrer at a controllable speed. After agitation, the suspensions were filtered using Whatman No°1 filter paper. The concentration of phenol after adsorption was determined using a back titration method as described by Jeffery et al and that of MB was determined using a UV-visible spectrophotometer (CORNING, 256) at the maximum absorption wavelength of 668 nm.

RESULTS AND DISCUSSION

Results

Characterisation of prepared activated carbon The FTIR spectrum of activated carbon is shown in (Fig. 1) with the following significant absorption bands: 3321 cm-1; 873, 802, 752 cm-1; 1574 cm-1; 1163 cm-1; 1089.9 cm-1

The effect of contact time of both adsorbates was determined by agitating 20 mL solution of phenol of initial concentration of 200 mg.L-1 with 0.05 g of activated carbon and 20 mL of MB solution with initial concentration of 18 mg.L-1 and 0.005 g of AC. The experimental The effect of contact time of both adsorbates was determined by agitating 20 mL solution of phenol of initial concentration of 200 mg.L-1 with 0.05 g of activated carbon and 20 mL of MB solution with initial concentration of 18 mg.L-1 and 0.005 g of AC. The experimental results obtained are shown in Fig. 2. The adsorption rate of phenol was slow within the first 10 minutes, and then increased rapidly and reaches equilibrium at 25 minutes.

Adsorbent dose

To study the effect of adsorbent dose on the adsorption of phenol and MB, a series of adsorption experiments were carried out with different adsorbent mass varying from 0.005 to 0.025 g at initial concentration of 18 mg.L- 1 for MB and from 0.05 to 0.2 g at initial concentration of 200 mg.L-1 for phenol. Results (Fig. 3) showed that with an increase in activated carbon dose, the adsorption capacity decreases.

Effect of pH

The pH is an important parameter that influences adsorption of pollutants from aqueous solutions, because it affects both the surface of adsorbent and adsorbate (Jadhav and Vaujara, 2004; Tagne et al, 2013). In this work, the pH was varied between 2 and 10 using 200 mg.L-1 of initial phenol concentration and 0.05 g of activated carbon, 18 mg.L-1 initial MB concentration and 0.005 g of activated carbon

The effect of initial concentration of adsorbents on adsorption of phenol and MB was carried out on different concentrations ranging from 100, to 500 mg.L-1 for phenol and 9 to 18 mg.L-1 for MB.

Adsorption isotherm models

An adsorption isotherm is the relationship between the adsorbate in the liquid phase and the adsorbate adsorbed on the surface of the adsorbent at equilibrium at constant temperature. In order to successfully represent the dynamic adsorptive behaviour of any substance from the fluid to the solid phase, it is important to have a satisfactory description of the equilibrium state between the two phases composing the adsorption system (Ndi Nsami et al, 2013). Classical adsorption models are used to describe the equilibrium established between adsorbed component on the adsorbent and unadsorbed component in solution (represented by adsorption isotherms). Langmuir, Freundlich, Dubini-Radushkevich (D-R) and Tempkin adsorption models were used to analyse the equilibrium data of adsorption of both phenol and methylene blue onto the activated carbon.

Langmuir adsorption isotherm: The Langmuir adsorption equation is one of the most common isotherm equations for modelling equilibrium data in solid-liquid systems. This equation is based on three assumptions: (i) - Adsorption cannot proceed beyond monolayer coverage. (ii) - All the surface sites are equivalent and can accommodate at most one adsorbed atom. (iii) - The ability of a molecule to be adsorbed at a given site is independent of the occupation of neighbouring sites (Ketcha Mbadcam et al, 2011). The general form of the Langmuir equation is:

Freundlich adsorption isotherm: The Freundlich equation is an equation based on adsorption on a heterogeneous surface. Its general form equation is given by equation (6) (Anagho Gabche et al, 2013):

Dubinin-Radushkevich (D-R): This model envisages the heterogeneity of the surface energies and can be written in the following linear form (8) (Ketcha Mbadcam et al, 2012b).

Tempkin adsorption isotherm: Tempkin isotherm assumes that the heat of adsorption decreases linearly with the coverage due to adsorbent - adsorbate interaction (Tagne et al, 2013). The Tempkin isotherm has generally been applied in the following linear form:

The data obtained from these studies have been tested with the Langmuir, Freundlich, Dubinin-Radushkevich and Tempkin linearized equations. The results are summarized in Table 2.

Kinetics adsorption models In the present work, the kinetics of adsorption of both phenol and MB onto activated carbon was studied using five kinetic models:

Mass transfer kinetic model: the general equation of mass transfer kinetic model is as follows (Ketcha Mbadcam et al, 2011):

Intraparticle diffusion kinetic model: the transportation of adsorbate from solution phase to the surface of the adsorbent particles may be controlled either by one or more steps such as: (i) film or external diffusion, (ii) pore diffusion, (iii) surface diffusion and (iv) adsorption on the pore surface, or a combination of more than one step (Kunquan et al, 2009). The general equation of intraparticle diffusion model is given by (Weber and Morris, 1963. Srivastava et al., 1989; Hameed and Daud, 2008; Idris et al, 2012b):

where, is the amount of pollutant adsorbed at time t, (mg.g-1.min-1) is the intraparticle rate constant and C the intercept. In order to obtain parameters related to each kinetic model, experimental data have been tested with linear forms of these models. The straight-line plots are presented on Fig. 6 to 10, while the parametric constants for these models are given in Table 2. It can be noted that the adsorption of MB and phenol onto prepared activated carbon adequately follows respectively pseudo-second-order and mass transfer kinetics models from their correlation coefficient (R2 = 1 and 0.9418)

DISCUSSION

Characterisation of prepared activated carbon The broad adsorption band at 3321 cm-1 is the characteristic stretching vibration of O-H for phenols, alcohols or carbonyls. The bands at 873, 802, and 752 cm-1 are due to plane deformation mode of C-H for different substituted benzene ring (Benadjemia et al, 2011). The band at 1574 cm-1 can be attributed to the stretching of carbonyl group (C=O), and the broad band at 1163 cm-1 represents the haloalkyl group (C-H) (Kunquan et al, 2009). The band at 1089.9 cm-1 is due to internal vibration of Si-O-Si (Maghri et al, 2012; Ravichandra et al, 2012). The prepared activated carbon exhibit a low specific surface area of 2.0448 m2 .g-1. This might be due to the activating reagent (KOH) because same precursor was used with same operating conditions, but at this time with ZnCl2 as activation reagent which present a significant surface area of about 648 m2 /g (Ndi Nsami et al, 2013). The values of pore size and iodine number indicate that the prepared activated carbon consist of micropores and mesopores which are favourable for respective adsorption of MB and phenol.

Contact time

According to Fig.2, the first step might be due to the intraparticle transport of phenol from bulk solution to the external surface of the porous adsorbent and the second step the diffusion into the interior pore (Ranjan et al, 2009; Ndi Nsami et al, 2013). For MB, the amount adsorbed increases rapidly and reaches equilibrium after 5 minutes. This result showed that within this time, the adsorption sites were exhausted and the remaining vacant sites were difficult to be occupied by other molecules due to the repulsive forces between adsorbate present on solid and bulk phase (Tagne et al, 2013).

Adsorbent dose

The decrease in the quantity adsorbed can be due to the agglomeration/aggregation of adsorbent particles at higher masses, which limit the surface area available for the adsorbates and also the increase in the diffusional path length (Kunquan et al, 2009; Essomba et al, 2014)

pH

 For the phenol adsorption (Fig. 4), at low pH value, the surface of activated carbon as well as phenol molecules gets protonated and with formation of positive charges on both adsorbate and adsorbent. This result leads to the reduction of phenol adsorption due to repulsive forces. With the increase of pH to 6, molecular form of phenol persist in the medium and surface protonation is minimum, leading to the enhancement of phenol adsorption and finally reaches its maximum (Jadhav and Vaujara, 2004). Significant decline in removal efficiency was for further increase in pH, which may be attributed to formation of phenolate anions and at the same time the presence of hydroxyl ions on the activated carbon prevents the uptake of phenolate ions (Ekpete et al, 2010). For the MB (Fig. 4), the amount adsorbed increases with an increase in pH value. This is probably due to the presence of delocalized -electrons within the graphene layers which are described as Lewis basic site (electron donor) and which reacts with MB molecule which are positively charged (cationic dye) (Benadjemia et al, 2011).

Concentration

The adsorption capacity (Qe ) increased as initial MB concentration increased (Fig.5); showing that the initial concentration provides a powerful driving force to overcome the mass transfer resistance between the aqueous and solid phases (Tsai et al, 2006). For adsorption of phenol (Fig. 5), within 100 and 200 mg.g-1 the amount adsorbed increases and beyond this interval, the amount adsorbed decreases with increase in concentration. The high sorption at the initial concentration may be due to an increased number of vacant sites on the activated carbon available at the initial stage. As the initial concentration increases there is a decrease in amount of phenol adsorbed due to the accumulation of phenol particles on the surface (Ekpete et al, 2010).

Isotherm

The results in table 2 show clearly that adsorption isotherms are in agreement with the Langmuir adsorption isotherm for both MB and phenol adsorption with higher values of correlation coefficient R2 of 0.9875 and 0.8956 respectively. In addition, the mean sorption energy (E) which is given by the D-R adsorption isotherm is less than 8 kJ.mol-1 (0.05 kJ.mol-1) for phenol adsorption, indicating a physical adsorption. On the other hand, E = 12.9 kJ.mol-1 for MB adsorption indicates the chemical process of adsorption (Ranjan et al, 2009). The values of are 0.23 and 0.0003 for phenol and MB respectively, which indicates that the adsorption is favourable onto prepared activated carbon.

Kinetics

The high value of correlation coefficient implies that MB adsorption on activated carbon may occur through a chemical process involving the valence forces of shared or exchanged electrons (Ho, 2006). The kinetic of adsorption of phenol better fit the mass transfer kinetic model with high correlation coefficient, R2 = 0.9418. In addition, the high value of the intraparticle rate constant (kid =10.899 mg.g-1.min-1) indicates that the particle diffusion mechanism predominates in adsorption of phenol. Similar results have been reported by Idris et al 2012b, Idris et al, 2012.

CONCLUSION

The activated carbon prepared from cola nut shells was successfully employed as an adsorbent for the removal of MB and phenol in aqueous solution. The equilibrium data fitted well the Langmuir adsorption isotherms for both adsorbates. The values of RL were found to be 0.23 and 0.0003 respectively for phenol and MB and confirmed that the prepared activated carbon is favorable for adsorption of both phenol and MB. The kinetics data of adsorption of phenol and MB follow respectively mass transfer and pseudo-second order model. These results indicated that the activated carbon prepared from cola nut shells could be used as low cost alternative adsorbents to commercial activated carbon in the removal of organic compounds such as phenol and MB from wastewater.

 ACKNOWLEDGEMENT

The authors are grateful to Dr. Isaac Nongwe Beas, for recording the IR analysis; the Research Unit- Adsorption and Surface of the Applied Physical and Analytical Chemistry Laboratory of the University of Yaoundé I. The authors are also grateful to the scholars whose articles are cited and included in references of this manuscript, and grateful to authors, editors, and publishers of all the articles, journals and books from where the literature for this article has been reviewed and discussed.

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