Adsorptive removal of copper(II) from aqueous solutions on activated carbon prepared from Tunisian date stones: Equilibrium, kinetics and thermodynamics

Adsorptive removal of copper(II) from aqueous solutions on activated carbon prepared from Tunisian date stones: Equilibrium, kinetics and thermodynamics

Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 741–749 Contents lists available at SciVerse ScienceDirect Journal of the Taiwan Ins...

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Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 741–749

Contents lists available at SciVerse ScienceDirect

Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice

Adsorptive removal of copper(II) from aqueous solutions on activated carbon prepared from Tunisian date stones: Equilibrium, kinetics and thermodynamics F. Bouhamed 1, Z. Elouear *, J. Bouzid 2 Laboratoire Eau Energie et Environnement, Ecole Nationale d’Inge´nieurs de Sfax, B.P.W 3038 Sfax, Tunisia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 November 2011 Received in revised form 17 February 2012 Accepted 23 February 2012 Available online 29 March 2012

In this study, activated carbon produced from Tunisian date stones, a low-cost agricultural by-product, by chemical activation using H3PO4 as an activator was used as adsorbent for the removal of copper(II) ions from aqueous solutions. To optimize the preparation method, the effect of the main process parameters (such as acid concentration, impregnation ratio, and temperature of pyrolysis step) on the performances of the obtained activated carbons was studied. The optimal activated carbon was fully characterized considering its adsorption properties as well as its chemical structure and morphology. Optimum adsorption conditions were determined as a function of pH, initial copper concentration, contact time and temperature of solution for copper(II) removal. The results showed that the adsorption of copper(II) onto activated carbon produced by the optimum conditions was maximal at about pH 5.0. The rates of adsorption were found to conform to the pseudo-second-order kinetic model. The application of the intra-particle diffusion model revealed that the adsorption mechanism of copper(II) is rather a complex process and the intra-particle diffusion is involved in the overall rate of the adsorption process but it is not the only rate-controlling step. The isotherm equilibrium data were well fitted by the Langmuir and Dubinin–Radushkevich isotherm models with a monolayer maximum adsorption capacity of 31.25 mg/g. According to the experimental results, the adsorbent derived from this material is expected to be an economical product for metal ion remediation from water and wastewater. ß 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Activated carbon Date stones Adsorption Copper Isotherm

1. Introduction The presence of metal ions in municipal or industrial wastewater and their potential impact have been a subject of scientific environmental research for a long time because of their extreme toxicity even at low concentrations, and their tendency to accumulate in the food chain [1]. Copper pollution arises from copper mining and smelting, brass manufacture, electroplating industries and excessive use of Cu-based agri-chemicals [2]. Ingestion of high quantities of copper may cause gastrointestinal bleeding, hypotension, convulsions, and significant DNA damage [3]. Therefore, it is urgent to remove those toxic heavy metals from wastewater. Although heavy metal removal from aqueous solutions can be achieved by conventional methods, including chemical precipitation, oxidation/reduction, electrochemical treatment, evaporative recovery, filtration, ion exchange and

* Corresponding author. Tel.: +216 23 543 734; fax: +216 74 665 190. E-mail addresses: [email protected] (F. Bouhamed), [email protected] (Z. Elouear), [email protected] (J. Bouzid). 1 Tel. + 216 23 160 511. 2 Tel. + 216 20 413 488.

membrane technologies, they may be ineffective or cost-expensive, especially when the metal ion concentrations in solution are in the range of 1–100 mg/L [4,5]. Recently, adsorption technology has become one of the alternative treatments in either laboratory or industrial scale [6,7]. There are many adsorbents in use. Activated carbons are known as very effective adsorbents due to their highly developed porosity, large surface area, variable characteristics of surface chemistry, and high degree of surface reactivity [8]. However, due to their high production costs, these materials tend to be more expensive than other adsorbents. This has led a growing research interest in the production of activated carbons from renewable and cheaper precursors. The choice of precursor largely depends on its availability, cost, and purity, but the manufacturing process and intended applications of the product are also important considerations [9]. Several suitable agricultural by-products (lignocellulosics) including fruit stones [10], olive waste cake [11,12], pine bark [13], rice husks [14], pistachio-nut shells [15] and wheat bran [16] have been investigated in the last years as activated carbon precursors and are still receiving renewed attention. Tunisia produces about 90,000 tons of dates annually, which can yield a minimum of 9000 metric tons of date stones [17]. A reasonable fraction of this quantity can be reclaimed easily from dates processing plants and can be used as a raw material for

1876-1070/$ – see front matter ß 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2012.02.011

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production of activated carbon. The use of this material as precursor for the preparation of activated carbon produces not only a useful adsorbent for the purification of contaminated environments, but also contributes to minimizing the undesirable solid wastes. Previous studies [18–24] on the production of activated carbon from date stones were concentrated mostly on the development of the high quality activated carbon. They did not consider the detailed studies of the adsorption process parameters, the kinetics or the thermodynamics of heavy metals adsorption. Therefore, the purpose of this work was to evaluate the adsorption potential of date stones-based activated carbon for copper ions as a model for metallic species. The adsorbent used in this work was prepared in the laboratory scale, from date stones, via chemical activation using phosphoric acid as a dehydrating agent. Optimization of some process parameters and characterization of the optimal activated carbon were first investigated. Thereafter, the adsorption of the copper ions was undertaken. The adsorption isotherms, kinetic and thermodynamic aspects of the retention process were explored. 2. Materials and methods 2.1. Preparation of activated carbons The palm dates were initially scraped with a knife to remove all fibers present at surface. The collected date stones were washed and dried in an air oven at 70 8C for 48 h and then crushed and sieved to the desired particle size (<1.5 mm). The resultant sieve cut was used for the production of activated carbons via chemical activation. For this purpose, phosphoric acid (analytical grade) was retained as a dehydrating agent. Each preparation test was conducted as follows: 40 g of the crushed (Ø < 1.5 mm) and dried precursor was mixed with H3PO4 solutions having different concentrations (25–85% H3PO4 in weight). The impregnation ratio, defined as: weight of impregnant (H3PO4)/weight of precursor (date stones), was 1, 1.25, 1.5, 1.75 and 2. The impregnation was carried out in a stirred pyrex reactor equipped with a reflux condenser. Stirring was used to ensure the access of the acid to the interior of the date stones cake particles. The temperature and the duration of the reaction were 104 8C and 2 h, respectively. Agitation and heating were ensured by a heating magnetic stirrer with connected temperature regulator probe made of Teflon. The pyrolysis of the impregnated material was conducted in a cylindrical stainless steel reactor, inserted into a tubular regulated furnace under continuous nitrogen flow (0.5 L/ min). Pyrolysis temperature ranged from 350 to 650 8C, while pyrolysis time was maintained at 2 h. After cooling down to room temperature, under the same flow of nitrogen, the obtained activated carbon was repeatedly washed with hot distilled water until neutral pH. The sample was then dried at 105 8C overnight, ground (until a granulometry ranging between 100 and 160 mm) and finally kept in hermetic bottle for subsequent uses. 2.2. Characterization 2.2.1. Characterization of the prepared adsorbents Some parameters which had effect on the activated carbon namely acid concentration, impregnation ratio, temperature of pyrolysis step was studied to determine the optimum conditions for producing the prepared activated carbons. In this work, efficiency and quality of the activated carbon were preliminarily characterized by measuring both iodine and methylene blue number. The iodine number can be used for estimation of the relative surface area and measurement of porosity, the pores size less than 1.0 nm in diameter. It was obtained on the basis of the Standard

Test Method, ASTM Designation D4607-86 by titration with sodium thiosulphate [25]. The concentration of iodine solution was thus calculated from total volume of sodium thiosulphate used and volume dilution factor. In case of methylene blue number, it is also one of the most widely recognized probe molecules for assessing the removal capacity of the specific carbon for moderate-size pollutant molecules (1.5 nm) [26]. Methylene blue adsorption tests were conducted by mixing 0.3 g of the prepared activated carbons with 100 ml of 1000 mg/L methylene blue solution [15]. After agitation during 24 h, the suspension was filtered and the MB residual concentration was measured at 660 nm, using an UV/vis spectrophotometer (OPTIMA, SP-3000 plus). A previously established linear Beer–Lambert relationship was used for the concentration determination. 2.2.2. Characterization of the activated carbon prepared under optimal conditions The physico-chemical characteristics of activated carbon produced by the optimum conditions in this experiment were determined. Nitrogen adsorption–desorption isotherms at 196 8C were measure with an automatic adsorption instrument (ASAP 2010, Micromeritics). The specific surface area was determined by the BET isotherm equation, and the total pore volume (Vtot) was calculated by the adsorption data at P/P0 = 0.995. Prior to the measurements, the samples were out gassed at 300 8C under nitrogen for at least 3 h. The point of zero charge (pHZPC) of the adsorbent was determined by the method described by Bouzid et al. [27]. Bulk density was determined as follows: activated carbon sample was placed in a graduated cylinder, tapped several times until constant volume and then weighted. The bulk density was calculated as the ratio of the weight sample to its volume and expressed in g/cm3 [28]. Crystalline phases eventually present in the adsorbent material were analyzed by powder X-ray diffraction (XRD analyzer Philips X Pert). Microstructure of the raw material and the activated carbon prepared in the optimal conditions were examined using a scanning electron microscopy (SEM, Philips XL30). 2.3. Batch mode adsorption studies All chemicals used in this work, were of analytical reagent grade and were used without further purification. Solutions were prepared by dissolving the corresponding reagents in bidistilled water. The activated carbon, prepared under the optimal conditions, was retained for all the following adsorption experiments. In order to determine the sorption capacity of activated carbon for Cu(II), as well as the influence of the contact time, pH, and temperature on Cu(II), sorption experiments were performed by batch equilibration technique. Solutions were prepared from CuSO45H2O salt and distilled water. After adsorption process the adsorbent separated from the samples by filtering and the filtrate was analyzed for copper(II) using a flame atomic absorption spectrophotometer (HITACHI Z6100). Analytical errors were estimated to be of the order of 3%. All the experiments were duplicated to assure the veracity of the experimental results. 2.3.1. Effect of equilibration time Sorption experiments for the kinetics study were conducted as follows: 0.5 g of activated carbon was suspended in 200 mL solution containing 100 mg/L of Cu(II) ions. The suspensions were stirred for different time intervals, filtrated, and analyzed to determine the Cu(II) ions concentration.

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According to literature, several models can be used to express the kinetics of the sorption processes, e.g. pseudo-first order, pseudo-second order and intraparticle diffusion models [29]. A pseudo-first-order reaction model was applied to our experimental data in order to determine the sorption rate constant, at room temperature. The linear form of pseudo-first-order rate expression is [30,31]

solutions was adjusted from 2 to 7 by adding NaOH or HNO3 solutions. The suspensions were stirred for 2 h. Measurements of the initial pH of the various metal tested solutions are carried out using a pH meter of laboratory models pH 540 GLP equipped with an electrode of glass combined SENTIX41. A preliminary calibration is systematically carried out using suitable buffer solutions.

dqt ¼ K 1 ðqe  qt Þ dt

2.3.3. Effect of initial Cu2+ concentration Activated carbon was equilibrated with Cu(II) solutions of different initial concentrations 10–100 mg/L for 2 h at three different temperatures, i.e. 10, 20 and 40 8C. After filtration, the final concentrations of copper in the solutions were measured. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) adsorption isotherms, often used to describe the sorption of solutes from a liquid phase, were applied to our experimental results. Linear form of the Langmuir equation can be expressed as follows [34]:

(1)

where qe and qt refer to the amount of Cu2+ (mg/g) adsorbed at equilibrium and at any time, t (min), respectively, and K1 is the equilibrium rate constant of pseudo-first-order sorption (1/min). Integration of Eq. (1) for the boundary conditions t = 0 to t and qt = 0 to qt, gives logðqe  qt Þ ¼ log qe 

K1 t 2:303

(2)

Kinetic data were further treated with the pseudo-second-order kinetic model [29]. The differential equation is the following: dqt ¼ K 2 ðqe  qt Þ dt

(3)

where K2 is the equilibrium rate constant of pseudo-second-order adsorption (g/mg min). Integrating Eq. (3) for the boundary condition t = 0 to t and qt = 0 to qt, gives: t 1 1 ¼ þ t qt K 2 q2e qe

(4)

The following expression denotes the initial sorption rate h (mg/g min): h ¼ K 2 qe2

(5)

In addition, the intraparticle diffusion model [32] was considered in order to determine the participation of this process in the sorption of copper(II) by activated carbon. In this model, the rate of intra-particle diffusion is a function of t0.5 and can be defined by Eq. (6) as follows: qt ¼ K d t 0:5 þ C

(6)

where qt is the amount of sorbate on the surface of the sorbent at time t (mg/g), Kd is the intraparticle rate constant (mg/g min0.5), t is the time (min) and C is the intercept. According to this model, the plot of uptake (qt), versus the square root of time (t0.5) should be linear with a slope Kd and intercept C if intraparticle diffusion is involved in the overall adsorption mechanism. Further, if this line passes through the origin then the intraparticle diffusion is the rate controlling step of the process. If the plots do not pass through the origin, this is indicative of some degree of boundary layer control and this further shows that the intraparticle diffusion is not the only ratelimiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously. In most cases these plots give general features of three stages; initial curved portion, followed by an intermediate linear portion and a plateau. The initial portion due to external mass transfer, the intermediate linear part is due to intra-particle diffusion and the plateau to the equilibrium stage where intraparticle diffusion starts to slow down due to extremely low solute concentrations in the solution [33]. 2.3.2. Effect of pH Sorption experiments for the influence of pH were conducted as follows: 0.5 g of activated carbon were suspended in 200 ml solution containing 100 mg/L of copper ions. The pH of the various

Ce 1 Ce þ ¼ qe q0 b q0

(7)

where Ce is the equilibrium concentration (mg/L), qe is the amount of copper sorbed at equilibrium, b is the sorption constant (L/mg) (at a given temperature) related to energy of sorption, q0 is the maximum sorption capacity (mg/g). Weber and Chakravorti [35] expressed the essential characteristics and the feasibility of the Langmuir isotherm in terms of a dimensionless constant separation factor or equilibrium parameter RL, which is defined as, RL ¼

1 1 þ bC 0

(8)

The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). The Freundlich isotherm is an empirical model that is based on adsorption on heterogeneous surface and active sites with different energy. The linearized Freundlich isotherm equation is represented by the following equation [36]: 1 ln C e (9) n where Kf and n are Freundlich constants, indicating the adsorption capacity and the adsorption intensity, respectively. Kf and n are, respectively, determined from the intercept and slope of plotting ln qe versus ln Ce. The D–R equation, based on the heterogeneous surface of the adsorbate, has the form [37]: ln qe ¼ ln K f þ

ln C ads ¼ ln q0  be2

(10)

where Cads (mol/g) is the amount of the metal ions adsorbed per unit weight of the adsorbent, q0 (mol/g) is the maximum sorption capacity, b (mol2/J2) is the activity coefficient related to the mean sorption energy, and e is the polanyi potential. e can be calculated in the following way:   1 e ¼ RT ln 1 þ (11) Ce where R (8.314 J/mol K) is the gas constant and T (K) is the temperature. The saturation limit q0 may represent the total specific micropore volume of the sorbent. The slope of the plot of ln Cads versus e2 gives b and the intercept yields the sorption capacity, q0. The mean sorption energy E (kJ/mol) can also be calculated using the following relationship: 1 E ¼ pffiffiffiffiffiffiffi 2b

(12)

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2.3.4. Estimation of thermodynamic parameters Thermodynamic parameters such as free energy change (DG8), enthalpy change (DH8), and entropy change (DS8) for the sorption of copper(II) on activated carbon was determined using the following equations [38,39]:

DG ¼ RT ln b

(13)

D G ¼ D H   T D S 

(14)

where DG8 is the change in free energy (kJ/mol), DH8 the change in enthalpy (kJ/mol), DS8 the change in entropy (J/mol K), T the absolute temperature K, R the gas constant (8.314  103), and b is the equilibrium constant of sorption. From Eqs. (13) and (14), it can be rewritten as: ln b ¼

DS R



DH RT

(15)

The enthalpy and entropy changes can be respectively determined from the slope and intercept of the plot ln b against 1/T. 3. Results and discussion 3.1. Preparation of activated carbons: effect of processing parameters

500 400 300 200 100 0 0

20

40

60

80

100

[H3PO4] concentration (Wt%) Fig. 2. Effect of H3PO4 concentration on the iodine number (impregnation ratio: 1.75; pyrolysis temperature: 450 8C; pyrolysis duration: 2 h).

the iodine number occurring in the acid concentration range 60– 85% H3PO4 and the handling difficulties of the most concentrated commercial phosphoric acid (85% H3PO4 in weight) owing especially to its high viscosity, a concentration of 60% H3PO4 seems to be the most suitable for the development of the best iodine number and, consequently, to the best development of microporous structure. Contrary to iodine number, methylene blue is one of the most widely recognized probe molecules for assessing the removal capacity of the specific carbon for moderate-size pollutant molecules via its macroporosity (1.5 nm) [26]. The effect of temperature on methylene blue number is shown in Fig. 3. It was noticed that, as the pyrolysis temperature increases from 350 to 450 8C the quantity of methylene blue adsorbed begins to increase. Beyond 450 8C a decrease on this quantity was noticed. Thus, keeping the pyrolysis temperature at around 450 8C leads to a better development of the sorbent porosity. Note that this result is in agreement with those reported in several papers dealing with activation of other agricultural materials (woods, coconut shell, grain sorghum) with H3PO4 [26,40–42]. The above results show that the most efficient activated carbon is that obtained under the following optimal conditions: an acid concentration equal to 60% in weight, an impregnation ratio of 1.75 and a pyrolysis temperature of 450 8C. It is to note that results obtained when studying the effect of each parameter (acid concentration, impregnation ratio and pyrolysis temperature) on the whole considered properties, namely iodine and methylene blue numbers and specific area (results not shown), lead to the same optimal conditions.

Methylene blue(mg/g)

In this work, the efficiency and the quality of the produced activated carbon from date stones were characterized by measuring the iodine and the methylene blue numbers. These characteristics were plotted and analyzed for the variables of activation temperature, impregnation ratio and acid concentration of the date stones to find the optimum conditions at which the iodine number and the methylene blue of the activated carbon is obtained. Figs. 1–3 illustrate the effect of the impregnation ratio, acid concentration and temperature on the iodine and the methylene blue numbers of the prepared activated carbon. Fig. 1 shows the effect of changing the impregnation ratio on the iodine number. The highest iodine number was obtained at an impregnation ratio of 1.75. At a value more or less than 1.75, the iodine number decreased. Therefore, 1.75 is the suitable impregnation ratio value leading to the best iodine number and, consequently, to the best development of microporous structure. The effect of acid concentration on the iodine number is shown in Fig. 2. As it can be seen from this figure, the iodine number increases rapidly with an increase of acid concentration from 25% to 60% H3PO4 while it increases slightly with acid concentration higher than 60% H3PO4. Taking into account this slight increase of

600 Iodine number(mg/g)

744

300 250 200 150 100 50 0 200

300

400

500

600

700

Temperature (°C) Fig. 1. Effect of impregnation ratio on the iodine number (H3PO4 concentration: 60%; pyrolysis temperature: 450 8C; pyrolysis duration: 2 h).

Fig. 3. Effect of temperature on methylene blue number (H3PO4 concentration: 60%; impregnation ratio: 1.75; pyrolysis duration: 2 h).

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Fig. 6, panels a and b, shows SEM micrographs of the raw date stones residue and the resulting activated carbon prepared under the optimal conditions. For the date stones residue, the surface was quite smooth with very little pores as shown in Fig. 6(a). After activation process, the sorbent surface demonstrated a well developed and uniform surface, forming an orderly porous structure and a predominately microporous character which is responsible of the high developed surface area of this material (Fig. 6(b)). 3.3. Metal ion adsorption study

Fig. 4. Nitrogen adsorption–desorption isotherms of the optimal activated carbon.

Table 1 Characteristics of the activated carbon prepared under optimal condition. Parameter

Value

Specific surface area (m2/g) Total pore volume (cm3/g) pHzpc Iodine number (mg/g) Methylene blue number (mg/g) Bulk density (g/cm3)

826 0.46 3.34 472 259 0.56

3.2. Characteristics of the activated carbon prepared under the optimal conditions Nitrogen adsorption is a standard procedure for determination of porosity of carbonaceous adsorbents. From nitrogen adsorption isotherm analysis (Fig. 4), it was inferred that the isotherm of the activated carbon prepared under the optimal conditions is Langmuirian in shape being typical type I in the BDDT classification [43]. This type of isotherm is usually exhibited by microporous solids. Meanwhile, the desorption branch presents a hysteresis loop at high relative pressures, characteristic of mesoporosity [44]. The results of the physical properties of the adsorbent are shown in Table 1. The XRD pattern of the sorbent shown in Fig. 5 exhibit broad peaks and absence of a sharp peak that revealed predominantly amorphous structure.

Fig. 5. The X-ray diffraction pattern of the optimal activated carbon.

3.3.1. Adsorption kinetics The effect of equilibration time on the sorption of copper(II) by activated carbon was investigated for time periods from 15 min up to 8 h. From Fig. 7, it is concluded that rate of metal ion adsorption increases sharply at short contact time and slowed gradually as equilibrium was approached. The necessary time to reach the equilibrium is about 2 h. This behavior may be due to the, availability of initial large number of vacant surface active sites for adsorption and sorption rate is very fast thus rapidly increases the amount of adsorbate accumulated on the carbon surface mainly within the first 2 h of adsorption. Afterward the filling of vacant sites becomes difficult due to repulsive forces between copper ion adsorbed on solid surface and copper ion from solution. In order to investigate the mechanism of copper(II) adsorption on activated carbon prepared under optimal conditions pseudo-first order, pseudo second order and intraparticle diffusion model were used. All kinetic data for the adsorption of copper(II) ions onto activated carbon, calculated from the related plots, are summarized in Table 2. The validity of the exploited models is verified by the correlation coefficient, R2. Comparison of the R2 values for different models suggests that the pseudo-second-order kinetic model fits best since its highest value (R2 = 0.997). Pseudo-second-order kinetic model implies that the predominant process here is chemisorption, which involves a sharing of electrons between the adsorbate and the surface of the adsorbent [45,46]. The intraparticle diffusion model was tested to identify the diffusion mechanism. Plot of the quantity of copper(II) adsorbed against square root of time is given in Fig. 8. It is shown in Fig. 8 that the plots were not linear over the whole time range and the graphs of this figure reflect a dual nature, with initial linear portion followed by plateau. This implies that the external surface adsorption (stage 1) is relatively very fast and the stage of intra-particle diffusion (stage 2) is rapidly attained and continued to 120 min. Finally, equilibrium adsorption (stage 3) starts after 120 min. The cations are slowly transported via intra-particle diffusion into the particles and are finally retained in the pores. The linear portion of the curve does not pass the origin and the latter stage of copper(II) adsorption does not follow Webber–Morris equation. It may be concluded that the adsorption mechanism is rather a complex process and the intra-particle diffusion was not the only ratecontrolling step. The Values of intercept (Table 2) gave an idea about the thickness of boundary layer, i.e. the larger the intercept the greater will be the boundary layer effect [47].The results obtained agreed with those found by Nadeem et al. [48] for the adsorption of lead onto chemically modified activated carbon. 3.3.2. Effect of pH The pH of the aqueous solution has been identified as the most important variable governing metal adsorption onto adsorbents. This is partly due to the fact that hydrogen ions themselves are a strongly competing adsorbate and because the solution pH influences the ionization of surface functional groups. In order to establish the effect of pH on the adsorption of copper(II), the

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Fig. 6. SEM micrographs of (a) the raw date stones residue and (b) the optimal activated carbon.

batch equilibrium studies at different pH values were carried out in the range of 2–7 (Fig. 9). We note that as the pH of the solution increased from 2.0 to 7.0, the adsorption capacity of copper(II) increased up to pH 5.0 and then decreased at pH > 5.0. The amount adsorbed increased as pH increased from 2.0 to 5.0 may be due to the presence of negative charge on the surface of the adsorbent that may be responsible for metal binding. However, as the pH is lowered, the hydrogen ions compete with the metal ions for the sorption sites in the sorbent; the overall surface charge on the adsorbent becomes positive and hinds the binding of positively charged metal ions [49]. At pH higher than 5.0, the precipitation of insoluble metal hydroxides takes place restricting the true adsorption studies [50]. So, an optimized pH of 5.0 is taken for all the adsorption experiments. The effect of pH may be explained in relation to the interaction of Cu(II), Cu(OH)+ and Cu(OH)2 with surface functional groups present in activated carbon as follows: ROH2 þ $ ROH þ Hþ

(16)

ROH $ RO þ Hþ

(17)

RO þ Cu2þ $ ROCuþ

(18)

RO þ CuðOHÞþ $ ROCuðOHÞ

(19)

where R represents the surface sites of activated carbon; R–OH2+, R–OH, R–O represent protonated, neutral, and ionized surface hydroxyl functional groups; R–OCu+ and R–OCu(OH) are formation of the bonding complexes. It can be seen that at low pH values, H+ competes with the Cu ions for the active surface sites and,

14

3.3.3. Adsorption isotherms Graphic presentations of the adsorption isotherms of copper(II) onto activated carbon prepared under the optimal conditions along with its validation with the Langmuir and Freundlich isotherms at 20 8C are illustrated in Fig. 10. The values of isotherm constants are given in Table 3. Comparison of the R2 values shows that the Langmuir isotherm fitted quite well with the experimental data with a high correlation coefficient (R2 = 0.995). The Langmuir model is basically developed to describe the sorption processes where no interaction between sorbate species occur on sites having the same sorption energies independent of surface coverage [52]. Maximum monolayer adsorption capacity q0 of copper(II) onto activated carbon was 31.25 mg/g. RL value was calculated as 0.08, which is greater than zero and less than unity, showing favorable adsorption of copper(II) onto activated carbon obtained from date stones residue by chemical activation with H3PO4 under optimal conditions. The Dubinin–Radushkevich (D–R) isotherm fitted quite well with the experimental data with a high correlation coefficient (R2 = 0.999). From the intercept of the plots, the q0 value was found to be 14.9 mg/g. The mean adsorption energy was calculated and the result indicated that the adsorption of copper(II) onto activated carbon may be carried out via chemically. Table 2 Kinetic model parameters for the adsorption of copper(II) onto optimal activated carbon.

12

qt(mg/g)

moreover, the less functional groups, i.e. R–O are ionized (deprotonated) in this region, and it is difficult that they form Cu complexes. Noted here Cu2+ and Cu(OH)+ are the dominant species involved in the adsorption below pH 5.0, thus other species Cu(OH)2 and Cu(OH)3 were not accounted in the formation of surface complexes [51].

10 8 6

Kinetic model

Parameter

Value

Pseudo-first order

K1 (1/min) R2 K2 (gm/g min) R2 h (mg/g min) Kd1 (mg/g min0.5) R21 C1 Kd2 (mg/g min0.5) R22 C2

0.029 0.959 0.011 0.997 2.221 0.977 0.957 1.407 0.029 0.954 11.405

Pseudo-second order

4 2 0 0

100

200

300

400

500

Time (min) Fig. 7. Kinetic study for copper(II) adsorption on optimal activated carbon.

Intraparticle diffusion model

The first phase

The second phase

F. Bouhamed et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 741–749

14

Table 3 Equilibrium model parameters for adsorption of copper(II) onto optimal activated carbon.

12

qt(mg/g)

747

10

Isotherms

Parameter

Value

Langmuir

q0 (mg/g) b (L/mg) RL R2 Kf n R2 q0 (mg/g) b (mol2/J2) E (kJ/mol) R2

31.25 0.11 0.08 0.99 0.37 1.16 0.94 14.9 8.71  109 7.75 0.99

8 6 Freundlich

4 Dubinin–Radushkevich

2 0 5

0

10 1/2

t

15

(min

1/2

20

25

)

Fig. 8. Plots of copper(II) adsorption against square root of contact time.

14 12 qe(mg/g)

10 8 6 4 2 0 0

2

4

6

8

pH

Table 4 Results for adsorption of copper(II) by activated carbons obtained from various plants, agricultural and wood based materials. Activated carbon source

pH

Adsorbant capacity (mg/g)

References

Date stones Apricot stone Soybean hulls Olive waste cakes Kraft lignin Rice husk Ceiba pentandra hulls Phoenix dactylifera stone carbon Palm shell carbon Coconut coir activated carbon Palm oil empty fruit bunch Grapeseed carbon Cassava peel carbon Rubber wood sawdust carbon

6 6.5 5 – – 6 6 6 5

31.25 22.85 39.37 12 136 29 20.78 37.3 22 84.74 0.84 48.78 52 5.72

This work [53] [54] [11] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64]

– 5 6 6

Fig. 9. Effect of pH on the adsorption of copper(II) on optimal activated carbon.

Values of the adsorption capacity of other low-cost activated carbons derived by thermal method from the literature are given in Table 4 for comparison. The value for copper(II) sorption observed in this work is in good agreement with values found by other researchers. Differences of metal uptake are due to the properties of each adsorbent such as structure, functional groups and surface area.

qe(mg/g)

3.3.4. Effect of temperature and thermodynamic parameters Temperature has a pronounced effect on the adsorption capacity of activated carbon. It was found that the copper(II) adsorption uptake increases with increasing solution temperature from 10 to 40 8C, indicating that the adsorption is an endothermic

16 14 12 10 8 6 4 2 0

Table 5 Thermodynamic parameters of copper(II) adsorption onto activated carbon by date stones. Temperature (8C)

DG8 (kcal/mol)

DH8 (kcal/mol)

DS8 (cal/mol K)

10 20 40

3.39 3.48 3.72

39.06 39.06 39.06

25.22 25.22 25.22

process. Similar findings have been reported by other researchers working on removal of heavy metal ions from aqueous solution by activated carbon [65,66]. The thermodynamic parameters for the adsorption process such as free energy change (DG8), enthalpy change (DH8), and

Langmuir isotherm Freundlich isotherm

0

20

40

60

80

Ce(mg/L) Fig. 10. Validation of copper(II) adsorption isotherms on optimal activated carbon at 20 8C.

Fig. 11. Van’t Hoff plot of adsorption equilibrium constant b.

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entropy change (DS8) were calculated to evaluate thermodynamic feasibility of the sorption process and to confirm its nature. The Gibbs free energy indicates the degree of spontaneity of sorption process, and the higher negative value reflects a more energetically favorable sorption. DH8 and DS8 were obtained from the slop and intercept of a plot of ln b against 1/T (Fig. 11). The calculated parameters are given in Table 5. The negative value of DG8 indicates the spontaneous nature of sorption of copper(II) by activated carbon. The positive value of DH8 showed that the sorption process was endothermic in nature and positive value of DS8 shows the increasing randomness at solid/liquid interface during the reaction. 4. Conclusion Batch adsorption studies for the removal of copper(II) from aqueous solutions have been carried out using activated carbon prepared from Tunisian date stones by chemical activation using phosphoric acid as activation agents. The most efficient activated carbon is that obtained under the following optimal conditions: an acid concentration equal to 60% H3PO4, an impregnation ratio of 1.75, and a pyrolysis temperature of 450 8C. The adsorption isotherm studies showed that Langmuir adsorption isotherm model and Dubinin–Kaganer–Radushkevich model adequately described the adsorption of copper(II) onto activated carbon and the maximum adsorption capacity (q0) was found to be 31.25 mg/g. It was shown that the pseudo-second order kinetic model better described the sorption data; this suggests that the rate-limiting step may be chemical sorption rather than diffusion. The thermodynamic parameters DG8, DH8 and DS8 showed a chemically favored, spontaneous and exothermic adsorption. The present study concludes that date stones derived activated carbon prepared under these conditions may be used effectively for removal of copper(II) ions from wastewater since it is a low-cost and locally available adsorbent. Acknowledgments The authors gratefully acknowledge financial support from the Tunisian Chemical Group. They also wish to express their gratitude to Mr. A. Charfi, Mr. L. Fourati and Mme. N. Ammar for their help and support. Thanks are due to Mr. Z. Fakhfakh from Faculty of Science of Sfax for the assistance in MEB. References [1] Mohan D, Singh KP. Single and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse-an agricultural waste. Water Res 2002;2304:36. [2] Dean JC, Bosqui FL, Lanovette KH. Removing heavy metals from waste water. Environ Sci Technol 1972;6:509. [3] Yu B, Zhang Y, Shukla A, Shukla SS, Dorris KL. The removal of heavy metal from aqueous solutions by sawdust adsorption-removal of copper. J Hazard Mater 2000;80:33. [4] Liang S, Guo XY, Feng NC, Tian QH. Application of orange peel xanthate for the adsorption of Pb2+ from aqueous solutions. J Hazard Mater 2009;170:425. [5] Dhakal RP, Ghimiere KN, Inoue K. Adsorptive separation of heavy metals from an aquatic environment using orange waste. Hydrometallurgy 2005;79:182. [6] Kumar U, Bandyopadhyay M. Sorption of cadmium from aqueous solution using pretreated rice husk. Bioresour Technol 2006;97:104. [7] Singh KK, Talat M, Hasan SH. Removal of lead from aqueous solutions by agriculture waste maize bran. Bioresour Technol 2006;97:2124. [8] Zhang W, Chang QG, Liu WD, Li BJ, Jiang WX, Fu LJ. Selecting activated carbon for water and wastewater treatability studies. Environ Prog 2007;26:289. [9] Prahas D, Kartika Y, Indraswati N, Ismadji S. Activated carbon from jackfruit peel waste by H3PO4 chemical activation: pore structure and surface characterization. Chem Eng J 2007;140:32. [10] Malik DJ, Strelko Jr, Streat M, Puziy AM. Characterisation of novel modified active carbons and marine algal biomass for the selective adsorption of lead. Water Res 2002;36:1527.

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