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Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes



A.O. Alade, O.S. Amuda, A.O. Afolabi and F.E. Adelowo
 
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ABSTRACT

The suitability and the performance of activated carbon produced from flamboyant pod back, milk bush kernel shell and rice husk for the effective removal of acenaphthene from simulated wastewater under the influence of carbonization temperature and initial concentration were investigated. The adsorption capacities of all the activated carbons obtained from the selected raw materials are influenced by increasing carbonization temperature. Activated carbons obtained from rice husk at carbonisation temperature of 600°C had the maximum adsorption capacity (5.554 mg g-1) while carbons produced from milk bush at carbonisation temperature of 300°C had the minimum adsorption capacity (1.386 mg g-1), for the adsorption of acenaphthene from the simulated wastewater. The removal efficiencies of the investigated adsorbents generally rank high and the highest value (80.56%) was obtained for the adsorption of acenaphthene by rice husk carbonized at 600°C. Furthermore, the removal efficiencies obtained in the study decreased as the initial concentrations of the adsorbate increased. The four selected isotherm models; Freundlich, Langmuir, Temkin and Dubinin-Radushkevich described well the equilibrium adsorption of acenaphthene unto activated carbon derived from Flamboyant pod bark, milk bush kernel shell and rice husk. Sequence of suitability of the selected isotherms in the study was Temkin ≈ Freundlich >Dubinin-Radushkevich>Langmuir for adsorption of acenaphthene. It therefore shows that Temkin isotherm is the most suitable model for fitting experimental data obtained from adsorption of acenaphthene from simulated wastewater unto activated carbon produced from Flamboyant pod bark, milk bush kernel shell and rice husk.

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A.O. Alade, O.S. Amuda, A.O. Afolabi and F.E. Adelowo, 2012. Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes. Journal of Environmental Science and Technology, 5: 192-209.

DOI: 10.3923/jest.2012.192.209

URL: https://scialert.net/abstract/?doi=jest.2012.192.209
 
Received: November 03, 2011; Accepted: February 23, 2012; Published: March 28, 2012



INTRODUCTION

The search for the removal of organic pollutants using alternative low cost adsorbents is now on the rise by many researchers (Mckay, 1995; Mackay and Gschwend, 2000; Gupta et al., 2002; Boving and Zhang, 2004; Zheng et al., 2004; Gokturk and Kaluc, 2008). Few of these reasons are:

Commercial activated carbons are expensive above $2.00/lb but agricultural by-products at low cost of $0.45/lb (Ahmedna et al., 2000)
Agricultural wastes have low ash contents and high density (Johns et al., 1999; Wartelle and Marshall, 2001)
Activated carbon with a developed microporosity and wide micropore size distribution which are characteristics of activated carbon produced from agriculture origin, can improve the adsorption of PAHs (Murillo et al., 2004)
Commercial activated carbons have recorded difficulty in regeneration and high cost of disposal (Darwish et al., 1996; Gupta et al., 1998; Flock et al., 1999; Burleigh et al., 2002; Singh et al., 2003)
Some agricultural base activated carbon compare favourably well in physical and chemical properties with some commercial activated carbon (Ahmedna et al., 1997; Johns et al., 1999; Pendyal et al., 1999; Al-Barrak and Elsaid, 2005)

Wide varieties of high carbon content materials such as wood, coal, peat; nutshells, sawdust, bones, husk, petroleum coke and others have been utilized to produce activated carbon of varying efficiencies (Savova et al., 2001; Ekinci et al., 2002; Aygun et al., 2003; Dutta and Basu, 2011). These materials, usually in irregular and bulky shapes, are always adjusted to exhibit the desired final shapes, roughness and hardness. Generally, the production of activated carbon involves pyrolysis or carbonization and activation as the two main production processes (Vitisidant et al., 1999).

Numerous carbonaceous materials, particularly, those of agricultural base, are being investigated to posses potential as activated carbon. The suitable ones have minimum amount of organic material and a long storage life. Similarly they consist of hard structure to maintain their properties under usage conditions. They can be obtained at a low cost. Some of the materials that meet the above conditions have been used, in past works, to produce activated carbons which were subsequently used for the treatment of wastewater and adsorption of hazardous gases. Agricultural by-products like rice straw, soybean hull, sugarcane bagasse, peanut shell, pecan shell and walnut shells were used by Johns et al. (1998) to produce Granulated Activated Carbons (GACs). Similarly, Atmani et al. (2009) used acid, base and salt treatment process to activation of skin almond waste in order to improve its adsorption capacity for acid and basic dye from waste water.

The choice of a particular material for the production of effective adsorbent (activated carbon) is based on low cost, high carbon and low inorganic content (Tsai et al., 1997). Agricultural materials have attracted the interest of researchers for the production of adsorbents because of their availability in large amount and at a low cost (Ioannidou and Zabaniotou, 2007). The selected materials employed in this study were rice husk (Oryza sativa), flamboyant (Delonix regia) pod back and milk bush (Thevetia peruviana) kernel shell. Rice husk has been used in the production of activated carbon (Daifullah et al., 2003; Subramanian et al., 2004) but flamboyant pod bark and milk bush kernel shell are untapped agricultural produce that attracted the interest of this research. Use of agricultural by-product for the production of activated carbon is primarily for economic and ecological advantages (Itodo et al., 2011). Acenaphthene is also one of the common PAHs in the environment. It is otherwise called 1, 2-hydrocenbaphtylene and occurs in coal tar (Finar, 2005).

MATERIALS AND METHODS

Agricultural raw materials: The natural precursor used in the production of the adsorbent include rice husk which were obtained from Arowomole Rice Mill, Ogbomosho. Others include flamboyant pod bark which was sourced from the campus yard of Ladoke Akintola University of Technology, Ogbomosho, while milk bush kernel shell was obtained from its trees scattered all over various secondary schools in Ogbomosho.

Reagents: The reagents used during the course of the experimental activities include sodium bicarbonate (NaHCO3), phosphoric acid (H2PO4 58%), acetone (BDH Chemicals Ltd., MW 58.08 g, spg 0.789-0.791 g°C), distilled water, polycyclic aromatic hydrocarbon (PAHs) of 2-rings, Naphthalene (100 g , Merck, MW 128.18 g) and 3-ring, Acenaphthene (100 g, Merck, MW 178.23). All reagents are analytical grade.

Materials processing: The materials were sorted to remove stones, shaft and debris, thereafter the backs of the flamboyant pod were removed mechanically and kept separately. Similarly, the shells of the milk bush kernel were mechanically cracked with nut cracker and the shells were carefully collected by hand for further treatment. The back and shell obtained were then washed with distilled water, to remove surface impurities (Bulut and Tez, 2007; Jayarajan et al., 2011) and later dried in the oven at a temperature of 105°C overnight (Amuda and Ibrahim, 2006) to constant moisture level before crushing with milling machine so as to reduce their sizes to pellet-form and increase their surface area (Bulut and Tez, 2007). They were then stored in dry containers prior to carbonization. Furthermore, since rice husk were in chaff form, they were washed with distilled water, before being dried in the oven at a temperature of 105°C to a constant weight (Amuda and Ibrahim, 2006).

Carbonization: 1 kg of each agricultural material was charged into the furnace (Vecstra, Model 184A, Italy) which was then heated to the desired temperature (300°C). The resulting charred material was collected and cooled at room temperature. The procedure was repeated for all the materials separately at carbonization temperatures of 300-600°C. The domain of variation of these factors is defined according to Bornemann et al. (2007).

Activation: Samples of the carbonized material were weighed, soaked in excess phosphoric acid (H3PO4) for 3 h (Kadirvelu et al., 2001) and charged inside an oven at temperature of 200°C for 24 h (Kadirvelu et al., 2001) to ensure proper adsorbate drying. ZnCl2 always produce wide pore surface area than base (Ahmadpour et al., 1998), H3PO4 produces a better pore surface area and are relatively safer than ZnCl2 (Johns et al., 1999) and hence the choice for this study. The materials were then removed from the oven, cooled for 2 h and then washed with distilled water until leachable impurities due to free acid and adherent powder were removed (Amuda and Ibrahim, 2006). The samples were later soaked in 2% (w/v) NaHCO3 to remove any residual acid left. The resulting mixture were further washed with distilled water to bring the pH to 7.0 and finally drained and dried overnight in an oven at 110°C (Amuda and Ibrahim, 2006).

Preparation of simulated water: Calculated amount (50-150 mg) of the desired PAH was weighed and added to 300 mL of acetone in 1 dm3 standard flask. The mixture was carefully swirled together for 10 min to allow proper dissolution. Then distilled water was added to the mixture to make up to the mark (Crisafully et al., 2008) thus a solution of 50 mg L-1 of PAH was produced. The above procedure was repeated for the preparation of 75, 100, 125 and 150 mg L-1 of acenaphthene under this study.

Determination of adsorption capacity: Weighed amount (1 g) of each activated carbon was added to 50 mL of the 50 mg L-1 stock solution in 250 mL conical flask. The mixture in the flask was covered and placed on magnetic stirrer at 150 rpm for 2 h (Lemic et al., 2007) at ambient temperature (28±2°C) and pH 7.5 (Crisafully et al., 2008). After which the content was allowed to stand for 1 h and the supernatant solution was filtered with Whatman filter paper (15 mm) into sample bottles (Crisafully et al., 2008). Starting with 50 mg L-1 of the simulated wastewater prepared, the process was repeated for 75, 100, 125 and 150 mg L-1 of acenaphthene. Furthermore this procedure was repeated for same activated carbon produced at various temperatures. The resulting filtrates were subjected to analysis.

Analytical measurement: The unadsorbed concentration of either naphthalene or acenaphthene in the filtrate was quantified using gas chromatography coupled with flame ionization detection (GC-FID) (Crisafully et al., 2008; Bornemann et al. (2007). A HP-5 capillary of 30 m with internal diameter of 0.25 mm and film thickness of 0.25 μm was used. The column temperature was set to 60°C for 2 min and then ramped to 320°C programmed at 10°C min-1. Nitrogen was used as carrier gas at a constant pressure of 35 psi while hydrogen and air flow rate pressure were 22 and 28 psi, respectively. Injector port and detection temperature were 250 and 320°C, respectively while 1.0 μL of sample was injected, before analysis, calibration standard was run to check column performance peak height and resolution and the limits of detection of the compound was identified mainly by its retention time. The abundance of quantification of analyte with respect to authentic PAH standard detection limits was derived from replicate procedure.

Quantification of adsorption capacity: The adsorption capacities of the materials carbonized at different temperature were determined using Eq. 1 (Crisafully et al., 2008):

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(1)

where qe is the adsorption capacity of adsorbent (mg/g), Co is the initial concentration of the adsorbate in the solution (mg L-1); Ce is the final concentration of the adsorbate in the solution quantified with GC-FID (mg L-1); V is the volume of the solution (mL) and w is the weight of the adsorbent (g). The removal efficiency RE (%) of each activated carbon at different concentration of selected adsorbate was calculated according to equation 2 (Maryam et al., 2008; Amuda et al., 2008):

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(2)

where, Co and Ce are initial of final concentration of absorate, respectively.

Adsorption isotherm models: Adsorption Isotherm equations applicable to single-solute were used to describe the experimental sorption data obtained in this study and the parameters of the isotherm models were obtained from graphical plotting of the experimental data. Four models used in this study include Freundlich, Langmuir, Temkin and Dubinin-Radushkevich isotherm models.

Langmuir isotherm model: The linearized form of the Langmuir Isotherm equation is expressed as:

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(3)

where, Ce (mg L-1) is the equilibrium concentration of the adsorbates, qe (mg/g) is the amount of adsorbate adsorbed per unit mass of adsorbent. QL and KL are related adsorption capacity and rate of adsorption respectively and were determined by plotting Ce/qe against Ce. Importance of Langmuir isotherm is often investigated with dimensionless separation factor (RL) (Hameed and Rahman, 2008; Amuda et al., 2008) which is expressed in the form (equation 4). Where KL is the Langmuir constant and Co is the highest concentration of adsorbate (mg L-1) in the solution. The value of RL obtained indicates Langmuir isotherm to be unfavourable (RL>1), linear (RL=1) and favourable (0< RL <1) or irreversible (RL = 0) (Hameed and Rahman, 2008):

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(4)

Freundlich isotherm model: The Freundlich Isotherm equation is an empirical equation expressed in linear logarithmic form (Cheung et al., 2009; Amuda et al., 2008) as (5). A plot of In qe against In Ce is used to determine the Freundlich constants, Kf and 1/n, respectively:

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(5)

Temkin isotherm model: This equation focuses on the effects of indirect adsorbate-adsorbate interaction on adsorption (Olgun and Atar, 2009). It is expressed as:

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(6)

where, A and B are the Temkin Constants which are evaluated by plotting qe against In Ce. B is further determined from the expression:

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(7)

where, b is related to the heat of adsorption, T is the adsorbate room temperature and R is the universal gas constant (8.314 mol-1 K-1).

Dubinin-Radushkevich isotherm (D-R) model: D-R isotherm equation is usually use to estimate the porosity, apparent free energy and the characteristics of adsorption (Nemr et al., 2009). It is commonly applied in its linear logarithm form as shown in Eq. 8:

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(8)

where, k is a constant related to adsorption energy and QD-R (mg/g) is the theoretical saturation capacity. The Polanyi potential is calculated from Eq. 9:

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(9)

where, R is the universal gas constant (8.314 mol-1 K-1), T is the adsorbate room temperature and Ce (mg L-1) is the equilibrium concentration of the adsorbates. D-R constants; k and QD-R (mg g-1) were evaluated by plotting In qe against ε2.

Mean free energy of adsorption: The mean free energy of adsorption is defined as the free energy change when one mole of adsorbate is transferred to the surface of the adsorbent from infinity in the solution (Nemr et al., 2009). It is calculated from the values of K obtained from the plot of Dubinin-Radushkevich using the relation (Kundu and Gupta, 2006):

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(10)

where, K is a constant related to the adsorption energy (J mol-1)

Adsorption isotherms error analysis: In order to determine the ability of the selected adsorption isotherm models, employed in linear method, the experimental data and parameter of the isotherms were subjected to statistical error analysis The selected error analysis are the Average Relative Error (ARE) and Average Relative Standard Error (ARS) Their values are expected to be very low for a given isotherm model to be favourable particularly under the linear method (Han et al., 2009). The error analysis was employed in this study in order to find the best correlation between the non-linear isotherm equations and experimental data points (Cheung et al., 2009). The ARE and ARS are determined using the Eq. 11 and 12, respectively (Han et al., 2009).

The Average Relative Error (ARE):

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(11)

The average relative standard error (ARS):

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
(12)

where, qc is the calculated quantity of adsorbate adsorbed onto the adsorbent and qe is the experimental data and ‘n’ is the number of experimental data points.

RESULTS AND DISCUSSION

In this study, each of the agricultural materials employed were carbonized at 300-600°C and thus flamboyant pod back are coded FB300-FB600; rice husk, RH300-RH600 while milk bush kernel shell carbons are coded MB300-MB600.

Influence of carbonization temperature and initial concentration on adsorption capacities of the activated carbons: The adsorption capacity of FB300 for the adsorption of acenaphthene from the simulated wastewater increased from 1.466-4.207 mg g-1 as the initial concentration of the polyaromatic hydrocarbon increased from 50-150 mg L-1. Similarly, the adsorption capacities of FB500 increased from 1.538-4.504 mg g-1 as the initial concentration increased from 50-150 mg L-1. FB 600 showed increased adsorption capacities from 1.634-4.754 mg g-1 as the initial concentration of acenaphthene increased from 50-150 mg L-1. Furthermore, the result shows that maximum adsorption capacity of rice husk for acenaphthene increased from 5.353 to 5.554 mg g-1 as the carbonization temperature increased from 300-600°C at a concentration o f 150 mg L-1. Similar trend was observed at other initial concentrations (50-125 mg L-1) of acenaphthene in the aqueous media as the carbonization increased from 300-600°C. The adsorption capacity of MB300, MB500 and MB600 increased from 1.388-3.419 mg g-1 as the initial concentration of acenaphthene increased from 50-150 mg L-1. MB500 showed adsorption capacities which increased from 1.449-4.086 mg g-1 as the initial concentration of acenaphthene increased from 50-150 mg L-1. Similarly, as the initial concentration of acenaphthene increased from 50-150 mg L-1, the adsorption capacity of MB600 also increased from 1.469 to 4.331 mg g-1. The results further showed that the maximum adsorption capacities of activated carbon obtained from milk bush kernel shell increased from 3.419-4.331 mg g-1 as the carbonization temperature increased from 300-600°C. Maximum adsorption capacities recorded for Flamboyant pod back are 4.207, 4.504 and 4.754 mg g-1. The results show that there is increase in adsorption capacity as carbonization temperatures increased. Based on the maximum adsorption capacities obtained for the activated carbon produced from the selected agricultural raw material, the order of capacity is given as rice husk> flamboyant pod back > milk bush kernel shell. Bornemann et al. (2007) equally observed that carbonization temperature influenced the sorption behaviour of aromatic hydrocarbon on charcoals prepared from grass and wood. Similarly, carbonization temperature influenced the adsorption of organic compound like phenol and methylene blue unto activated carbon produced from hazelnut (Ioannidou and Zabaniotou, 2007). Furthermore, high adsorption capacities exhibited by these adsorbent may be attributed to the dispersion interactions between the Ð (pi)-electrons of the aromatic ring (Moreno-Castilla, 2004).

Influence of carbonization temperature and initial concentration on removal efficiencies of the activated carbons: Fig. 1 shows the Removal Efficiencies, RE (%) of the three activated carbons produced for the adsorption of naphthalene and acenaphthene under the influence of increasing carbonization temperature (300-600°C) and initial concentration (50-150 mg L-1) of the two adsorbates. Removal efficiencies of FB300, FB500 and FB600 decreased from 58.65-56.09, 61.53-60.01 and 65.39-63.38%, respectively as the initial concentration of acenaphthene increased from 50-150 mg L-1. However, the maximum removal efficiencies of the activated carbon flamboyant pod back; 58.65, 61.53 and 65.39% increased as the carbonization temperature increased from 300, 500 and 600°C, respectively. It can be deduced that increasing carbonization temperature favour the efficiencies of activated carbon obtained from flamboyant pod back for the removal of acenaphthene from water medium, however this can be improved at higher carbonization temperature and initial concentration lower than 50 mg L-1. Similar trend was recorded for the adsorption of acenaphthene from the simulated wastewater unto the activated carbon obtained from milk bush kernel shell at various carbonization temperatures. The removal efficiencies of MB300, MB500 and MB600 decreased from 54.48-45.59, 57.96-54.4 and 58.75-57.74%, respectively as the initial concentration of acenaphthene increased from 50-150 mg L-1 The maximum removal efficiencies of carbon produced from milk bush kernel at carbonization temperature of 300, 500 and 600°C for the removal of acenaphthene (54.48, 57.96 and 58.75%) are higher than the maximum removal efficiencies obtained for the removal of naphthalene (48.18, 53.55 and 56.41), respectively.

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
Fig. 1: Removal efficiencies (%) of activated carbons at different initial concentrations of acenaphthene in simulated waste water

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
Fig. 2: Freundlich adsorption isotherm of acenaphthene unto activated carbons produced

It shows that efficient activated carbon from milk bush kernel shell may be produced at carbonization temperature of 600°C and used to remove polyaromatic hydrocarbon at low concentration. The study showed that the removal efficiencies of all the agricultural raw materials investigated for the adsorption of acenaphthene increased with increasing carbonization temperature but decreased with increased initial concentrations. this showed increased removal efficiencies as initial concentration increased from 50-150 mg L-1. Furthermore, the removal efficiencies of all the materials studied for the removal of acenaphthene are higher than those observed for naphthalene at a given initial concentration and carbonization. This may be attributed to the high partition coefficients (Log Kow) of acenaphthene (3.989) and this has been reported to be a factor that influences the adsorption of polyaromatic hydrocarbons in aqueous medium onto activated carbon (Lemic et al., 2007).

Effect of carbonization temperature on Freundlich isotherm model: Figure 2 shows the plots of Inqe (adsorption capacity) versus InCe (equilibrium concentration of adsorbate) used for determining the Freundlich isotherm parameters for naphthalene and acenaphthene adsorption, respectively, onto activated carbons produced. Freundlich isotherm parameters obtained for the adsorption of naphthalene and acenaphthene onto the adsorbents produced from agricultural raw materials are shown in Table 1. The Freundlich exponent ‘n’ for the adsorption of acenaphthene unto FB300, FB500 and FB600 are 1.10, 1.06 and 1.08, respectively. Furthermore, the Freundlich exponent ‘n’ for the adsorption of acenaphthene unto MB300, MB500 and MB600 are 1.45, 1.14 and 1.04, respectively. For the activated carbons derived from rice husk, RH300, RH500 and RH600, the values of ‘n’ obtained for adsorption of acenaphthene are 1.08, 1.29 and 1.38, respectively. The Freundlich exponent ‘n’ obtained for this study are above those obtained from a related study of adsorption of naphthalene and acenaphthene unto activated carbons derived from agricultural materials (Table 2) except for FB300 and RH600 which are less than value obtained for sugarcane bagasse (1.02). Furthermore, the Freundlich exponent ‘n’ obtained in this study are less than values obtained for adsorbent derived from resin (Long et al., 2008). However, these values satisfy the condition, 1<n<10 which in dicate that the adsorption of acenaphthene unto the activated carbon produced from the selected agricultural materials is favourable (Rao and Viraraghavan, 2002).

Table 1: Freundlich isotherms model parameters for acenaphthene adsorption onto activated carbons produced
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
KF and No. are Freundlich parameters while R2 is the correlation coefficient

Table 2: Freundlich exponent n of adsorption of acenaphthene unto some selected activated carbons
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
Fig. 3: Langmuir adsorption isotherm of acenaphthene unto activated carbons produced

Table 3: Langmuir isotherms model parameters for naphthalene and acenaphthene adsorption onto Selected Adsorbents
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
QL, KL and RL are Langmuir parameters while R2 is the correlation coefficient

The corresponding large values of correlation coefficient (R2) (Table 4-7) of R2 (0.997-1.000) showed that the Freundlich model is good for describing the adsorption process of acenaphthene unto carbon derived from the agricultural raw materials investigated in this study (Crisafully et al., 2008). The appropriateness of this model to the selection of acenaphthene is an indication of heterogeneity of the adsorbents (Olgun and Atar, 2009).

Effect of carbonization temperature on Langmuir isotherm model: Fig. 3 shows the plots of Ce/qe (ratio of equilibrium concentration of adsorbate to adsorption capacity) versus qe (adsorption capacity) using linear method and this was used to determine the Langmuir isotherm parameters for the adsorption of acenaphthene onto the three types of adsorbents produced at carbonization temperatures of 300, 500 and 600°C. In the adsorption study of acenaphthene unto the activated carbon derived from flamboyant pod back, (Table 3) the monolayer capacity, QL, obtained for FB300, FB500 and FB600 are 29.07, 51.28 and 38.72 mg g-1, respectively. With separation factor (RL) of 0.73, 0.81 and 0.73 which satisfy the condition 0<RL<1, shows that Langmuir isotherm is favourable (Hameed and Rahman, 2008). The monolayer capacity, QL, of MB300, MB500 and MB600 are 7.59, 21.41 and 71.94 mg g-1, respectively which increased with increasing carbonization temperature of milk bush kernel shell. However, the QL; 45.87, 15.82 and 13.37 mg g-1, obtained for RH300, RH500 and RH600, respectively, decreased as the carbonization temperature increased from 300-600°C. The corresponding values of RL which ranged between 0.28-0.68, satisfied the condition 0<RL<1 and this shows that Langmuir isotherm is favourable (Hameed and Rahman, 2008).

Table 4: Langmuir isotherms model parameters for organic compounds adsorption onto some adsorbents
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes

Furthermore, MB600 posses the largest monolayer capacity, 71.94 mg g-1 for the adsorption of acenaphthene unto the activated carbon produced while FB500 posses the largest monolayer, 294.118 mg g-1, for the adsorption of naphthalene from the simulated water. The positive values of QL indicate high adsorption capacity of the activated carbon obtained from these raw materials (Rao and Viraraghavan, 2002). These values are however, below those obtained for the adsorption of organic compounds unto commercial activated carbons but compare well with activated carbon derived from agricultural raw materials (Table 4). Generally, the correlation coefficients (R2) of these activated carbons are high (0.9172-0.9437) and such high degree of R2, particularly for the Langmuir isotherm model, suggests that these activated carbons exhibit monolayer coverage with constant activation energy (Cheung et al., 2009; Han et al., 2009).

Effect of carbonization temperature on Temkin isotherm model: Table 5 shows the Temkin isotherm parameters (A, B and b) and their corresponding correlation coefficients (R2). These parameters were obtained from the plot of qe (adsorption capacity) versus InCe (equilibrium concentration of adsorbate) (Fig. 4) for acenaphthene adsorption onto the activated carbons produced at various carbonization temperatures. Temkin parameter constant (b) is related to the heat of adsorption in the adsorption process of acenaphthene unto the activated carbons produced from the selected agricultural raw materials. Heat of adsorption of acenaphthene unto carbon produced from flamboyant pod back (Table 5) ranges from 861.64-984.44 J mol¯1 which decreased as the carbonization increased from 300-600°C. Heat of adsorption of acenaphthene unto the MB and the values of ‘b’ obtained ranged from 911.02-1495.70 J mol-1. The heat of adsorption of naphthalene unto MB500 and MB600 are higher than heat of adsorption of acenaphthene unto the carbon derived from milk bush kernel shell (MB) at the same carbonization temperatures.

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
Fig. 4: Temkin adsorption isotherm of acenaphthene unto activated carbons produced

Table 5: Temkin isotherm model parameters for acenaphthene adsorption onto selected adsorbents
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
A, B and b are Temkin parameters while R2 is the correlation coefficient

Heat of adsorption of acenaphthene unto carbon obtained from rice husk ranged from 764.65 to 920.56 J mol-1, this increased as the carbonization temperature increased from 300 to 600°C. Generally, with the highest value of heat of adsorption, the three activated carbon showed the same decreasing order (MB>FB>RH) of heat of adsorption for acenaphthene. Temkin isotherm is generally applied to the study of dye (Nemr et al., 2009; Olgun and Atar, 2009; Han et al., 2009) and the values of heat of adsorption of Db-86 (dye) unto carbon produced from orange peels which ranged from 355.9 to 680.8 J mol-1 (Nemr et al., 2009) are fairly less than those obtained in this study. Similarly, the heat of adsorption of phenol (79.78 J mol-1) unto activated carbon derived from Rattan Sawdust (Hameed and Rahman, 2008) is less than the heat of adsorption of acenaphthene in this study. The large values of correlation coefficients (R2) obtained for the adsorption of acenaphthene unto selected adsorbents range from 0.9667 to 0.9766. Since these values compare well above the correlation coefficients (R2) obtained for Langmuir isotherm model it suggests that Temkin isotherm model can be applied to the study of adsorption capacity of activated carbon produced from milk bush kernel shell and rice husk for the removal of acenaphthene in simulated wastewater.

Effect of carbonization temperature on Dubinin-Radushkevich isotherm model: Values of Dubinin-Radushkevich (D-R) isotherm model parameters (KT and QD-R) obtained from the plot of Inqe (adsorption capacity) versus ε2 (Polanyi potential) in Fig. 5 for the adsorption of acenaphthene unto the selected raw materials carbonized at various temperature of 300, 500 and 600°C, are shown in Table 6.

Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
Fig. 5: Dubinin-Radushkevich adsorption isotherm of acenaphthene unto activated carbons produced

The essential characteristic of the Dubinin-Radushkevich isotherm is the estimation of the mean free energy of adsorption (E) often defined as the free energy change involved in the transfer of one mole of adsorbate unto the surface of adsorbent (Nemr et al., 2009). The mean free energy of adsorption (E) obtained for the adsorption of acenaphthene unto carbon derived from flamboyant pod back range from 7.86-10.10 kJ mol-1. Constant value of mean free energy of adsorption (7.07 kJ mol-1) was obtained for the adsorption of acenaphthene unto MB300 and MB500 with mean free energy of adsorption of 7.86 kJ mol-1. High mean free energy of adsorption is obtained for the adsorption of acenaphthene unto rice husk ranged from 8.84 to 14.14 kJ mol-1, this and increased as the carbonization temperature increased from 300 to 600°C though less than the mean free energy of adsorption of acenaphthene, 17.68, 35.35 and 35.35 kJ mol-1 unto RH300, RH500 and RH600, respectively. Nemr et al. (2009) suggested that the closeness of the values of mean free energy of adsorption as observed for the adsorption of acenaphthene unto MB300, MB500 and MB60 indicate that physico-sorption play the significant roles in the adsorption of these adsorbates from the simulated wastewater. Moreover, the values of mean free energy of adsorption observed in this study fall between 5 and 40 kJ mol-1 it shows the type of adsorption involved in the study is physisorption (physical sorption) which usually takes place at low temperature (Cooney, 1999; Inglezakis and Poulopoulo, 2006). Furthermore, the applicability of this isotherm to the study of adsorption of naphthalene and acenaphthene was based on the large values of their correlation coefficients (R2) (Table 6).

Error analysis: The suitability of selected adsorption isotherms employed in predicting the fits of adsorption capacity is based on the low values of their Average Relative Errors (ARE) and Average Relative Standard error (ARS) (Han et al., 2009; Cheung et al., 2009). The average relative errors (ARE) and average relative standard error (ARS) of Freundlich, Langmuir, Temkin and Dubinin-Radushkevich isotherm employed in this study are recorded in Table 7. The average relative errors of Freundlich, Langmuir, Temkin and Dubinin-Radushkevich isotherm for fitness in describing the adsorption of acenaphthene unto FB300, FB500 and FB600 ranged between 0.955-0.962, 11.33-13.67, 0.961-0.963 and 0.775-0.811, respectively. The large values of Langmuir confirm that it is less suitable for studying the adsorption of acenaphthene unto carbon derived from flamboyant pod back.

Table 6: Dubinin-Radushkevich isotherm Model parameters for acenaphthene adsorption onto selected adsorbents
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
Q o-r and KT are Dubinin-Radushkevich parameters, R2 is the correlation coefficient while E is the adsorption energy (kJ mol-1)

Table 7: Values of error analysis about isotherm equations for acenaphthene adsorption unto the activated carbons
Image for - Adsorption of Acenaphthene unto Activated Carbon Produced from Agricultural Wastes
ARE: Average relative error, ARS: Average relative standard error, D-R: Dubinin-Radushkevich

With large values of ARE, Langmuir isotherm is less suitable for describing the adsorption of properties of acenaphthene unto MB300, MB500 and MB600. In the study of adsorption of acenaphthene, the ranges of ARE are 0.923-0.967, 2.62-29.13, 0.949-0.965 and 0.580-0.906 for the above stated isotherms, respectively. Error analysis of Freundlich, Langmuir, Temkin and Dubinin-Radushkevich range for the adsorption of acenaphthene is 0.88-0.944, 3.19-14.43, 0.670-0.955 and 0.669-0.802 for the isotherm models, respectively. In this case, only Langmuir isotherm is not applicable to the study of adsorption of acenaphthene unto rice husk. Based on the ranges of ARE obtained in the study, the sequence of applicability of the selected adsorption isotherm is Dubinin-Radushkevich>Freundlich ≈ Temkin>Langmuir for acenaphthene as applied in the work of Baral et al. (2009). The fitness of the isotherms to the experimental data can be optimized by error analysis particularly Average Relative Standard error (ARS), (Cheung et al., 2009). Isotherm models with very low values of ARS are the most suitable to the study (Han et al., 2009). The ARS for the adsorption of acenaphthene unto the same adsorbent ranged between 1.17-1.18, 15.41-26.54, constant at 1.18 and 1.35-1.41, respectively. With relatively low values of ARS it shows that Temkin and Freundlich isotherms are the suitable models for fitting adsorption of acenaphthene unto carbon developed from Flamboyant pod back. Ranges of ARS of Freundlich, Langmuir, Temkin and Dubinin-Radushkevich isotherm models used to fit the adsorption capacities of acenaphthene unto carbon produced from milk bush kernel shell were in the ranges 1.13-1.18, 3.61-39.45, 1.16-1.18 and 0.98-1.62, respectively. From this, adsorption capacities fitted best with Freundlich and Temkin isotherm models. Furthermore, the ranges of ARS calculated for Freundlich, Langmuir, Temkin and Dubinin-Radushkevich isotherm models for the adsorption of acenaphthene unto rice husk were 1.07-1.16, 4.48-19.72, 1.00-1.17 and 1.14-1.40 for the above stated isotherm models respectively. Thus adsorption data of acenaphthene can be fitted with Temkin and Freundlich isotherms. Sequence of suitability of the selected isotherms in the study was Temkin ≈ Freundlich>Dubinin-Radushkevich>Langmuir for adsorption of acenaphthene (Baral et al., 2009). It therefore shows that Temkin isotherm is the most suitable model for fitting experimental data obtained from adsorption of acenaphthene from simulated water unto activated carbon produced from rice husk, flamboyant pod back and milk kernel shell.

CONCLUSION

This study was conducted to determine the suitability and the performance of activated carbon produced from flamboyant pod back, milk bush kernel shell and rice husk for the effective removal of naphthalene and acenaphthene from simulated wastewater under the influence of carbonization temperature and initial concentration. The following conclusions could be made based on the analysis of the results obtained in this work. The adsorption capacities of all the activated carbons obtained from the selected raw materials are influenced by increasing carbonization temperature. Activated carbons obtained from rice husk at carbonisation temperature of 600°C had the maximum adsorption capacity (5.554 mg g-1) for the adsorption of acenaphthene while carbons produced from milk bush at carbonisation temperature of 300°C had the minimum adsorption capacity (1.386 mg g-1), from the simulated wastewater. The removal efficiencies of the investigated adsorbents generally rank high and the highest value (80.56%) was obtained for the adsorption of acenaphthene by rice husk carbonized at 600°C. Furthermore, the removal efficiencies obtained in the study decreased as the initial concentrations of the adsorbate increased. The four selected isotherm models; Freundlich, Langmuir, Temkin and Dubinin-Radushkevich described well the equilibrium adsorption of naphthalene and acenaphthene unto activated carbon derived from Flamboyant pod bark, milk bush kernel shell and rice husk. Sequence of suitability of the selected isotherms in the study was Temkin≈Freundlich>Dubinin-Radushkevich>Langmuir for adsorption of acenaphthene. It therefore shows that Temkin isotherm is the most suitable model for fitting experimental data obtained from adsorption of acenaphthene from simulated wastewater unto activated carbon produced from Flamboyant pod bark, milk bush kernel shell and rice husk.

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