Biosorption Isotherms and Kinetics Studies for the Removal of 2,6-Dichlorophenolindophenol Using Palm Tree Trunk (Elaeis guineensis)

In this work, the potential of natural and pretreated palm tree trunk (PTT) as agents for adsorption of an organic dye, 2,6-dichlorophenolindophenol (2,6-DCPIP) from aqueous solutions was probed. Natural and acetic acid treated PTT were characterized by Fourier transform infrared (FT-IR) spectroscopy and by the point of zero charge (pzc). The biosorption of 2,6-DCPIP was investigated in batch mode using natural and treated PTT. This study was achieved by highlighting several parameters such as the contact time, biosorbents dosage, the initial concentration of 2,6-DCPIP, the pH of the solution, the ionic strength and the interfering ions. The results showed that 2,6-DCPIP was successfully adsorbed from aqueous solutions by natural and treated PTT. The equilibrium was attained after 40 minutes for treated PTT and 20 minutes for natural PTT. The maximum capacity of adsorption was obtained at pH = 2. The adsorption isotherms were investigated and it was found that the experimental data were best described by the Dubinin-Radushkevich isotherm for the natural PTT (R = 0.979) and by the Temkin isotherm for the treated PTT (R = 0.976). The maximum adsorption capacities determined by Langmuir isotherm were found as 108.932 and 157.233 μmol∙g for natural and treated PTT, respectively. The adsorption kinetics was analyzed and was best described by the pseudo-second order model (R ≥ 0.998). The diffusion mechanism was studied and the result showed that external mass transfer is the main rate controlling step. The desorption of 2,6-DCPIP is favorable in alkaline medium.


Introduction
Dyes are synthetic aromatic water-soluble dispersible organic colorants, having potential applications in various industries. They are widely used in textile, paper, plastic, food and cosmetic industries in order to give the certain coloration to the desired product and also consume substantial volumes of water. It is reported that, over 100,000 dyes are commercially available and more than 700,000 ton/year are produced in the world [1] [2]. Most of the dyes are highly toxic, mutagenic, carcinogenic and recognized pollutants [3] [4] [5]. It is for this reason that, some dyes discharged from the effluents of these industries into receiving streams are sources of water pollution. Water effluents containing dyes have a very hazardous impact on environment, thus causing chronic and acute diseases [6]. Dyes can significantly affect visibility, photosynthesis and also aquatic life due to the presence of aromatics, metals and chlorides [7].
Hence the elimination of dyes from waste water is essential to prevent continuous environmental pollution. The commonly used procedures for removing organic dyes from aqueous solution include flocculation, electroflotation, chemical precipitation, electrokinetic coagulation, ion exchange, membrane filtration, electrochemical destruction, irradiation, ozonation and microbial biodegradation [1] [8] [9]. However, these methods are ineffective, non-economical and have many disadvantages such as high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal or treatment [5]. There is thus a need to search for new processes that could remove organic dyes. The adsorption technique is one of the preferred methods of advanced wastewater treatment which industries employ to reduce hazardous organic dyes present in the effluent because it is efficient, simple to design and does not produce any sludge [1] [2] [10] [11]. At first, the adsorbent used in adsorption was commercial activated carbon. However, the high cost of activated carbon used in adsorption and problems of regeneration of adsorbent after adsorption [12] led to the search for new adsorbents that are lower in cost, locally available and efficient [11]. Several researchers were led to the use of agricultural solid wastes. Ever since, many agricultural solid wastes were used to adsorb organic dyes in aqueous solutions. Examples are: rice husk, tea leaves, orange peel, corncobs, coconut husk, barley husks, peanut hull, coir pith, wheat bran, eggshell, cocoa shell [2], hazelnut shells [13], olive stone [14], sugarcane bagasse [15] and wood sawdust [16]. All of these are excellent for organic dyes removal. The aim of this study was to investigate about a natural and abundant lignocellulosic material, palm tree trunk, for the adsorption of organic dyes which are mostly toxic nowadays. 2,6-DCPIP was chosen as an example of an organic dye. The effects of some important parameters such as treatment of biosorbent, initial dye concentration, pH of solution, adsorbent dosage, ionic strength, contact time and interfering ions on the adsorption of 2,6-DCPIP onto both natural and treated PTT were compared. The possibility to reuse the materials was also examined.

Materials, Preparation and Characterization of Biomass
All chemical reagents used in this experiment were of analytical grade, purchased and used without further purification. NaCl, CaCl 2 and NaOH were purchased from Fisher, CH 3 COOH and BaCl 2 were purchased from BDH, HNO 3 and 2,6-DCPIP were purchased from Riedel-de-Häen and HCl was purchased from Phillip Harris. The structure of 2,6-DCPIP is illustrated in Figure 1.
The palm tree trunk used in this work was collected from a local agricultural field of the Littoral region in Cameroon. The biomass was cut into small pieces, washed several times with tap water to remove dust and soil particles, and then dried in sun for 8 days. The dried biomass was ground into fine powder and sieved to obtain sizes ranging from 0 -100 µm. The powder was washed several times with distilled water, air-dried for 2 days and then in an oven at 110˚C for 24 h before being kept in a bottle for further use.
In view of studying the effect of chemical pretreatment of biomass on 2,6-DCPIP uptake capacity, the biomass was submitted to pretreatment with acetic acid, according to the following procedure: 5 g of the natural biomass was put into contact with 100 mL of 2 mol•L −1 acetic acid solution. The mixture was stirred in a mechanical platform shaker (EDMUND BÜHLER GmbH) for 2 h at a speed of 200 rpm. The resultant biomass was washed several times with distilled water in order to remove the excess of acetic acid until the pH (6.3) of washed water was stable. After washing thoroughly, the biosorbent was air-dried for 2 days and then in an oven at 110°C for 24 h before being kept in a bottle for further use.
The determination of the pzc of natural PTT and treated PTT was performed according to the method previously described by [17]. The pzc was measured by adding 0.1 g of biomass to 50 mL of 0.1 mol•L −1 NaCl solution whose initial pH ( i pH ) was measured and adjusted between 1 and 12 with NaOH or HCl solutions. The containers were sealed and placed on a mechanical platform shaker for 48 h at a speed of 150 rpm after which the final pH (pH f ) was measured. The pzc occurs when there is no change in pH after contact with the biosorbent. The pzc corresponds to the point where the curve of ( ) The natural and treated PTT were also characterized by using Fourier transform infrared spectroscopy (FT-IR) which allowed to identify different chemical Figure 1. Chemical formula of 2,6-dichlorophenolindophenol [4].

Batch Biosorption and Desorption Studies
The stock solution of 2,6-DCPIP was prepared at 10 −3 mol•L −1 by dissolving 0.327 g of the hydrated sodium salt of 2,6-dichlorophenolindophenol in 1 L of distilled water. Solutions of different concentrations (2 × 10 −5 -10 −4 mol•L −1 ) were prepared by dilution of the stock solution with distilled water. The pH of each solution of 2,6-DCPIP was adjusted to the required value using HCl or NaOH solutions.
The biosorption studies were achieved in aqueous solution, in a shake flask, at room temperature. In this study, the batch biosorption experiments were carried out by mixing pre-weighted amounts (5 -60 mg) of biosorbent with 10 mL of 2,6-DCPIP of various initial concentrations (2 × 10 −5 -10 −4 mol•L −1 ) into a flask.
The mixture was stirred at constant agitation speed of 150 rpm for an interval time between 5 -70 min on a mechanical platform shaker. After agitation, the suspensions were filtered using whatman filter paper. The filtrates were analyzed by measuring the absorbance using UV-Vis Spectrophotometer (JENWAY) at a maximum adsorption wavelength of 600 nm [4]. The obtained absorbance was converted to the corresponding concentration C e by using the equation of calibration curve. The amount of 2,6-DCPIP adsorbed at equilibrium q e (mol•g −1 ) (Equation (1)), at time t q t (mol•g −1 ) (Equation (2)) and the percentage of adsorption ( ads % ) (Equation (3)) were calculated as follows [14] [18] [19]: where i C , e C and t C (mol•L −1 ) are the initial concentration, the final con- form shaker. After agitation, the suspensions were filtered. The filtrates were also analyzed by measuring the absorbance using UV-Vis Spectrophotometer. The desorption percentages ( des % ) were calculated as follows (Equation (4)) [20]: where; f C and r C (mol•L −1 ) are the initial concentration and the final concentration of 2,6-DCPIP loaded biosorbents, respectively.

Fourier Transform Infrared (FT-IR) Spectroscopy
The FT-IR spectra of PTT were recorded in order to explore the surface functional groups as shown in Figure 2 [22]. The region below 1000 cm −1 is the fingerprint zone and the absorption cannot clearly be assigned to any particular vibration because they correspond to complex interacting vibration systems [22].
The FT-IR spectrum of natural and treated PTT had similar bands with distinctive peak intensities ( Figure 2).

Point of Zero Charge (pzc)
The pzc of a material in a solution is the pH value at which the net surface charge of the material is equal to zero [17]. The PZC value of PTT was 4.8

Effect of Pretreatment of Biomass
The biosorbent was treated with acetic acid. The treatment affected the functional groups contained at the surface of the material. Figure

Effect of Biosorbents Dosage
The effect of biosorbents dosage on the removal of 2,6-DCPIP was studied and the results of this study are shown in Figure 5. The removal percentage of 2,6-DCPIP increased from 8.251% to 81.169% and from 22.789% to 89.755% for natural and treated PTT, respectively. As can be seen, the removal percentage of 2,6-DCPIP increased considerably with increasing adsorbent dosage. This is because of the greater surface area, the availability of more adsorption sites (carboxyl groups) for 2,6-DCPIP adsorption during the adsorption reaction [19]. A further increase in adsorbent dosage for treated PTT (>4.0 g/L) did not cause significant improvement in 2,6-DCPIP adsorption. This may be due to the adsorption of almost all the 2,6-DCPIP onto the biosorbent and the establishment of equilibrium. This situation can also be attributed to overlapping of adsorption sites as a result of overcrowding of biomass [24]. A quite similar tendency was reported by [19] for the adsorption of malachite green by sea shell powder.

Effect of Contact Time
The effect of contact time on biosorption of 2,6-DCPIP is presented in Figure 6.
It can be observed from this figure that, rapid adsorption of 2,6-DCPIP takes place in the first 5 minutes for the two materials, thereafter, the rate of adsorption decreases gradually with the progress of adsorption and reaches equilibrium in about 20 minutes for natural PTT and 40 minutes for treated PTT. No significant change in 2,6-DCPIP removal is obtained after the equilibrium time. The initial rapid phase may be due to rapid attachment of 2,6-DCPIP to the biosorbent surface or to the availability of more adsorption vacant sites at the initial stage [15]. But after some of the easily available active sites are used up, the dye needs some time to find out more active sites for binding until the equilibrium time is reached [10] [25].

Effect of Initial Concentration
The effect of initial concentration on the biosorption of 2,6-DCPIP was investigated and the results are shown in Figure 7. It can be observed from this figure that, the amount of 2,6-DCPIP adsorbed at equilibrium increases from 3.

Effect of Solution pH
The pH is an important factor that affects biosorption processes. It is used in

Effect of Ionic Strength
The ionic strength of the solution is one of the factors that control both electrostatic and non-electrostatic interactions between the adsorbate and the adsorbent surface [7]. The effect of ionic strength on biosorption of 2,6-DCPIP was carried out using the initial concentration of NaCl. The results illustrated in Figure 9 reveal that an increase of ionic strength causes an increase in the adsorption ca-

Effect of Interfering Ions
The effect of interfering ions on biosorption of 2,6-DCPIP was carried out using different salts; BaCl 2 , CaCl 2 and NaCl. The results show that the capacity of adsorption increases with the presence of salts in the order NaCl < CaCl 2 < BaCl 2 ( Figure 10). The capacity of adsorption is higher with divalent ions. This is because the doubly charged ions increase the surface charges of biosorbents, which increases the electrostatic interaction and/or anionic exchange between the

Adsorption Isotherms
In order to understand the mechanism of biosorption, it is important to perform the adsorption isotherms. In this study, four adsorption isotherms were used to describe the obtained equilibrium data: Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherms.

Langmuir Isotherm
The general equation of Langmuir isotherm is described as follows (Equation (5) The value of L R indicates whether the type of adsorption isotherm will be favorable ( )

Freundlich Isotherm
The general equation of Freundlich isotherm is described as follows (Equation (7)) [12] [29]: is the Freundlich constant related to the biosorption capacity and 1 n is an empirical parameter related to the biosorption intensity of the adsorbent. The Freundlich isotherm constants 1 n and f K were calculated from the slopes and intercepts of linear plots of log e q versus log e C .

Dubinin-Radushkevich Isotherm
The general formula of Dubinin-Radushkevich isotherm is given by the following Equation (8) (8) where; β (mol 2 •kJ −2 ) is the activity coefficient related to the mean free energy (E (kJ•mol −1 )) obtained from Equation (10) and ε is polanyi potential which is determined from Equation (9): where, R is the universal gas constant (8.314 × 10 −3 kJ•mol −1 •K −1 ) and T is the absolute temperature in kelvin in our case (298 K). The constants β and max q were calculated from the slopes and the intercepts of linear plot of ln e q versus 2 ε .

Temkin Isotherm
The general formula of Temkin isotherm is given by the following Equation (11 The isotherms obtained for 2,6-DCPIP adsorption onto natural and treated PTT are shown in Figure 11 and the corresponding adsorption constants are   The adsorption energies obtained from Dubinin-Radushkevich isotherm (Table 1) are between 8 and 16 kJ•mol −1 , implying that chemisorption (anionic exchange) is the mechanism which controls the biosorption process [31]. The positive values of the heat of adsorption from Temkin isotherm (Table 1) indicate that the adsorption process is exothermic [28] [32]. Moreover, the heat of adsorption of treated PTT is greater than the heat of adsorption of natural PTT.
The values of the separation factor determined by the Langmuir isotherm ( Figure 12) are found between 0 -1 (0.612 -0.888 for treated PTT and 0.691 -0.918 for natural PTT), meaning that biosorption is favorable for both biosorbents. This result is in agreement with what was already noted with the Freundlich isotherm. The lower L R values at higher initial 2,6-DCPIP concentrations showed that the adsorption was more favorable at higher concentrations [33]. The values of L R of treated PTT are lower than the values of natural PTT, implying that the adsorption of 2,6-DCPIP is more favorable with treated PTT rather than natural PTT [30].

Adsorption Kinetics
The controlling mechanisms of adsorption process such as chemical reaction, diffusion control or mass transfer coefficient are used to determine kinetic models. Thus, the kinetics of dye onto various adsorbent materials was analyzed using different kinetic models which are presented below.

Pseudo-First Order Model
The pseudo-first order equation of Lagergren is generally expressed as follows (Equation (12) (12) where; e q and t q (µmol•g −1 ) are the amounts of 2,6-DCPIP adsorbed at

Pseudo-Second Order Model
The pseudo-second order equation is generally expressed as follows (Equation (13) (14)) and the half time of the reaction t 1⁄2 (min) (Equation (15)).
where; α (mmol•g −1 •min −1 ) is the initial adsorption rate and β (g•µmol −1 ) is related to the extent of surface coverage and activation energy for chemisorption.
The values of β and α were calculated from the slopes and intercepts of the linear plots of t q versus ln t .

Intraparticle Diffusion Model
Weber and Morris demonstrated that in intraparticle diffusion studies, rate processes are usually expressed in terms of square root of time [12] [19] [34].
The intraparticle diffusion model is defined by Equation (17).
where; ip K (µmol•g −1 .min −1/2 ) is the intraparticle diffusion rate constant and C (µmol•g −1 ) is the thickness of boundary layer. The values of ip K and C were calculated from the slopes and intercepts of the linear plots of t q versus 1 2 t .

External Mass Transfer Resistance Model
This model assumes that the surface solute concentration s C , on the sorbent is negligible at t = 0, and that intraparticle diffusion is also negligible; it is used to calculate the initial rate of solute sorption. The initial rate of sorption can be determined using the classical mass transfer equation, which describes the evolu- where; L β is the external mass transfer coefficient, t C is the liquid phase solute concentration at time t, s C is the liquid phase solute concentration at the particle surface and S is the specific surface area for mass transfer. This equation can be simplified by substituting the following boundary conditions: [12] [35]: So the external mass transfer rate L S β , was approximated by the initial slope of the 0 t C C versus time graph.

Boyd Model
In order to interpret the rate-controlling step during the adsorption process, the experimental data were further analyzed by the model given by Boyd (Equation (20)) [12] [29] [35]: Since t e q F q = , Bt could be represented as follows (Equation (21)): where; F is the fraction of solute sorbed at different times t and Bt is a mathematical function of F.
where, r represents the mean radius of the particle calculated by sieve analysis and by assuming them as spherical particles.
Kinetics of The 2,6-DCPIP adsorption onto natural and treated PTT are shown in Figure 13 and the corresponding constants are given in Table 2.  From Figure 13(c), the curves of intraparticle diffusion present two linear portions. The first portion located at the beginning of the process is attributed to the film diffusion, where the 2,6-DCPIP diffuses through the solution to the external surface of adsorbent or boundary layer diffusion. The second portion located at the end of the process describes the pore diffusion [37]. It is also clear from Figure 13(c) and Table 2 that the first stage is faster than the second one, which may be ascribed to the very slow diffusion of the 2,6-DCPIP from the surface film into the micropores which represent the least accessible sites for M. C. D. Ngaha et al. adsorption. Also, the intraparticle diffusion model curve does not pass through the origin, which is an indication that 2,6-DCPIP diffusion in the bulk of natural and treated PTT is not the only process that governs the biosorption [9] [15]. Moreover, the values of the thickness of boundary layer C for each linear portion are not zero, indicating that intraparticle diffusion is present as a part of diffusion process, but it is not the sole rate-controlling step in all the stages [29]. Figure 13(e) shows that the straight line obtained by the Boyd model does not pass through the origin of the graph, implying that external mass transfer is the main rate controlling step at the initial stages [12] [36]. The values of i D presented in Table 2 lie in the range 10 −13 to 10 −5 cm 2 ⁄s, indicating that chemisorption occurs during the biosorption process [17]. This result is in agreement with the pseudo-second order model and Dubinin-Radushkevich isotherm. Thus, the biosorption of 2,6-DCPIP on the natural and treated PTT is best described by external mass transfer diffusion rather than internal diffusion.

Desorption
The repeated availability of the adsorbents after adsorption-desorption cycles is crucial to illustrate the stability and potential recovery of the adsorbents. In this study, NaOH, HNO 3 and H 2 O were used as desorbing agents to regenerate the biosorbents. The results show that the maximum desorption percentage, 67.371% for natural PTT and 54.260% for treated PTT is obtained in NaOH medium ( Figure 14). This can be explained by the phenomenon of anionic exchange between the hydroxyl ions (OH − ) of NaOH solution and 2,6-DCPIP loaded biosorbent. However, the low percentage of desorption obtained with treated PTT compared to natural PTT is due to the strong bond formed between the 2,6-DCPIP and the treated PTT [20].

Conclusions
From this study, the capacity of using natural and treated PTT for the removal of Journal of Encapsulation and Adsorption Sciences 2,6-DCPIP from aqueous solution has been proven. Both materials are efficient biosorbents, but the treated PTT showed better performance than natural PTT.
The adsorption was highly dependent on various operating parameters such as; treatment of biosorbent, contact time, pH of solution, biosorbents dosage, initial concentration of 2,6-DCPIP, ionic strength and interfering ions. The adsorption isotherms indicate that the equilibrium data are the best described by the Dubinin-Radushkevich and Temkin isotherms for natural and treated PTT, respectively. Results of adsorption kinetics demonstrated that the adsorption processes were controlled by pseudo-second order kinetics. The mechanism of diffusion was studied and the results showed that external mass transfer was the main rate controlling step. Desorption using NaOH as desorbing agent recovers a maximum quantity of 2,6-DCPIP. From the results obtained, the utilization of PTT for the removal of 2,6-DCPIP from aqueous solution is promising.