ISSN 1 746-7233, England, UKWorld Journal of Modelling and Simulation

Vol. 9 (2013) No. 4, pp. 243-254

A new adsorption isotherm model of aqueous solutions on granular activatedcarbon

Hossein Shahbeig1, Nafiseh Bagheri2, Sohrab Ali Ghorbanian1, Ahmad Hallajisani1, Sara Poorkarimi2

1 School of Chemical Engineering, University College of Engineering, University of Tehran, P.O. Box 11155/4563Tehran, Iran

2 Department of Chemical Engineering, North Tehran Branch, Islamic Azad University, Tehran, Iran

(Received January 01 2013, Revised May 18 2013, Accepted September 22 2013)

Abstract. Equilibrium adsorption of aniline, benzaldehyde, and benzoic acid on granular activated carbon(GAC) has been investigated. Experiments were carried out on two sizes of activated carbon. The equilibriumdata were analyzed using the ordinary least square approach. The results show that the adsorption processwas more effectively described by Hill and Koble-Corrigan isotherm models based on the values of the leastsquare parameter, Durbin-Watson Test, and mean relative percent error. Furthermore, the results show thatbenzoic acid has been adsorbed more in comparison with the other adsorbates and benzaldehyde has beenadsorbed more than aniline. Moreover, a new isotherm model has been presented which is in good agreementwith experimental data. The suggested model has a 7.41% smaller error than the Hill and Koble-Corriganmodels.

Keywords: adsorption, isotherm, aniline, benzaldehyde , benzoic acid, modelling, GAC

1 Introduction

Over the past decades, the population and social civilization have grown. Due to changes in daily lifeand resource use, and continuing improvement of the industrial and technologies has been accompanied bya quick modernization and metropolitan development[11, 38]. The increasing of industrial activities intensifiedanthropogenic attacks on ecosystems, seriously threatening human health and the environment[12, 24]. Increas-ing concern for public health and environmental quality has led to the establishment of rigid limits on theacceptable environmental levels of specific pollutants[10, 41, 48]. Thus, the removal or destruction of aniline,benzaldehyde, and benzoic acid from process or waste streams becomes a major environmental problem. Thepresence of these compounds even at low concentrations can be an obstacle for the reuse of water. Thesecompounds are present in the wastewaters from coking plants, fertilizer, pharmaceutical, plastics, organicchemical, steel, and petroleum industries, dye manufacturing, and paint-stripping operations. They are con-sidered as pollutants because of their toxic and carcinogenic characteristics[33, 40]. Thus, the removal of thesepollutants from wastewater stream is one of the major environmental challenges. Several methods are avail-able for removal of this material such as photocatalytic degradation[4, 28], combined photo-Fenton and bio-logical oxidation[31], advanced oxidation processes[42], aerobic degradation[34], nanofiltration membranes[2],ozonation[29] and adsorption. In this respect, adsorption on activated carbon is an effective and useful tech-nique in the treatment of organic components containing wastewater[33, 37]. Activated carbon adsorption hasbeen cited by the US Environmental Protection Agency as one of the best available environmental controltechnologies[5, 46]. Despite its industrial importance, adsorption from liquid phase has been studied much lessextensively than adsorption from the vapor phase[6, 47]. Adsorption equilibrium occurred when an adsorbate Corresponding author. Tel.: +982166957781.

E-mail address: shahbeig@ut.ac.ir

Published by World Academic Press, World Academic Union

244 H. Shahbeig et al.: A new adsorption isotherm model

containing phase has been contacted with the adsorbent for adequate time and adsorbate concentration as thebulk solution is in a dynamic balance with the interface concentration[9, 12, 30, 33]. Physical modelling for anyengineering application is usually based on proposing empirical relations with a large amount of experimentaldata and the relevant non-dimensional parameters using regression techniques[7]. The empirical correlation,which represents a significant role towards the modelling analysis, operational design and applied practice ofthe adsorption systems, is typically displayed by graphically expressing the solid-phase against its residualconcentration[12, 25]. In the present study, the adsorption capacity and equilibrium coefficients for adsorptionof aniline, benzaldehyde, and benzoic acid by two sizes of activated carbon have been obtained. In order tocorrelate our experimental data, several equations (Langmuir, Freundlich, Jovanovic, Redlich-Peterson, Hill,and Koble-Corrigan) that have been proposed in the literature were investigated. Finally, a new and modifiedmodel was proposed for correlating of adsorption isotherm data.

2 Experimental

2.1 Materials

The adsorbents used in the experiments were GAC with two different sizes (1.5 and 2.5mm). Further-more, Aniline (A), Benzaldehyde (B), and Benzoic Acid (BA) were used as adsorbates. All of used sub-stances were purchased from Merck Company, Lindenplatz, Germany. The properties of solutes and GAC arepresented in Tab. 1 and Tab. 2.

Table 1. Solutes properties

A B BAMolecular weight (kg/kmole) 93.12 106.12 122.12Density (kg/m3) 1024 1048 1086Solubility in water (kg/m3) 3.6 3.3 2.9Maximum adsorption capacity of adsorbent (1.5 mm), qm (mg/g) 161.57 233.44 250.00Maximum adsorption capacity of adsorbent (2.5 mm), qm (mg/g) 112.54 165.87 187.38

Table 2. Granular activated carbon properties

Charcoal activated granular, extra pure 1.5mm 2.5mmIdentity Conforms ConformsSubstances soluble in nitric acid 0.7% 7%Chloride (Cl) 500ppm 200ppmCyano compound (CN) Passes test Passes testAs (Arsenic) 5ppmPb (Lead) 20ppm 20ppmFe (Iron) 500ppmZn (zinc) 100ppm 100ppmPolycyclic aromates Passes test Passes testTar products Passes test Passes testn-Hexane adsorption 33% 30%Residue on ignition (600 0C) 5% 8%Loss on drying 5% 10%

2.2 Method

The study of adsorption was carried out at a constant temperature (20 1 oC) under continuous stirring.Two series of experiments have been done. The first and second series of experiments have been performed

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World Journal of Modelling and Simulation, Vol. 9 (2013) No. 4, pp. 243-254 245

with adsorbent size 1.5 and 2.5 mm, respectively. For both series, various weights of activated carbon (0.01to 0.05 g) were agitated in 100 mL of a solution containing an adsorbate with a concentration of 300 mg/L.Furthermore, each adsorbate was treated with nine different weights of activated carbon for both series ofexperiments. Therefore, 27 experiments were performed for each series of experiments, 54 experiments totally.The contact time to reach equilibrium was set for two days. In addition, the equilibrium concentration of thesolutions was obtained by a Unico UV/visible spectrophotometer. The adsorbed amount was determined fromthe following formula:

qe =V (C0 Ce)

m, (1)

where qe is the amount of adsorbed solute per weight of adsorbent at equilibrium, V is the solution vol-ume, m is the mass of activated carbon, C0 and Ce are the initial and equilibrium concentration of adsorbaterespectively.

3 Results and discussion

3.1 Adsorption isotherms

The adsorbed amount is expressed in mg of adsorbate weight per gramme of adsorbent. The adsorp-tion isotherms were measured at 293.15 K. To correlate our experimental adsorption data. The Langmuir,Freundlich, Jovanovic, Redlich-Peterson, Hill, and Koble-Corrigan equations were used.

3.1.1 Langmuir isotherm model

Langmuir adsorption isotherm was originally developed to describe gas-solid phase adsorption on acti-vated carbon[12, 15]. This model is based upon two assumptions that the forces of interaction between adsorbedmolecules are negligible and once a molecule occupies a site no further sorption takes place. In its derivation,Langmuir isotherm refers to homogeneous adsorption, with no transmigration of the adsorbate in the plane tothe surface[1, 12, 25, 33, 36]. The model has the following hypotheses:

(1) Monolayer adsorption (the adsorbed layer is one molecule thick);(2) Adsorption takes place at specific homogeneous sites within the adsorbent;(3) Once a dye occupies a site, no further adsorption can take place at that site;(4) Adsorptional energy is constant and does not depend on the degree of occupation of an adsorbents

active center;(5) The strength of the intermolecular attractive forces is believed to fall off rapidly with distance;(6) The adsorbent has a finite capacity for the dye (at equilibrium, a saturation point is reached where no

further adsorption can occur);(7) All sites are identical and energetically equivalent;(8) The adsorbent is structurally homogeneous;(9) There is no interaction between molecules adsorbed on neighboring sites.

qe =Q0bCe1 + bCe

, (2)

where qe is the amount of solute adsorbed per unit weight of the adsorbent at equilibrium (mg g1), Ce theequilibrium concentration of the solute in the bulk solution (mg L1), Q0 the maximum adsorption capacity(mg g1), and b is the constant related to the free energy of adsorption (L mg1). Eq. (2) can be linearized tofour different linear forms as shown in Tab. 3.

The constants of all models were obtained by ordinary least squares using the Application software[35].The calculation results from the Langmuir model were shown in Tab. 4.

In Application software, several examinations were performed for the analyzing and fitting of data. Themodels are developed based on the statistical function such as the least square parameter (R2) and Durbin-Watson Test