International Journal of Environment Science and Technology Center for Environment and Energy Research and Studies (CEERS) ISSN: 1735-1472 EISSN: 1735-2630 Vol. 5, Num. 4, 2008, pp. 501-508

International Journal of Enviornmental Science and Technology, Vol. 5, No. 4, Autumn 2008, pp. 501-508

Biosorption of metal dye from aqueous solution onto Agave americana (L.) fibres

*A. M. Ben Hamissa; M. C. Ncibi; B. Mahjoub; M. Seffen

Applied Chemistry and Environment Research Unit, EPAM Sousse 4000 and Laboratory of Chemistry, Higher Institute of Agronomy, Chott Meriam 4042, Sousse, Tunisia
*Corresponding Author Email: bhmenyar@yahoo.fr Tel.: +21 6732 42632, Fax: +21 6733 27591

Received 13 April 2008; revised 30 May 2008; accepted 16 July 2008

Code Number: st08056

ABSTRACT

In this research, a new low cost and abundant biosorbent; Agave americna (L.) fibres has been investigated in order to remove metal dye (Alpacide yellow) from aqueous solutions. In order to optimize the biosorption process, the effect of pH, temperature, contact time and initial solution concentration was investigated in batch system. The results indicated that acidic pH=2 was favourable for metal dye removal. The increase of temperature increases the velocity of the biosorption reaction. The biosorption kinetics of alpacide yellow were closer to the pseudo-second order than to the first order model for all concentrations and temperature. The calculated thermodynamic parameters such as ΔG^0, ΔH⁰ and ΔS⁰ indicated a spontaneous and endothermic biosorption process of metal dye onto Agave americana fibres. The equilibrium data were analysed using the Langmuir and Freundlich isotherms and showed a good fit with Langmuir model at lower temperatures and with Freundlich model at 50 °C.

Key words: Sorption, dyes, cellulosic biomass, isotherms, kinetics, modeling

Introduction

Dyes usually have a synthetic origin. Metal dyes are used by a wide number of industries and their effluents are discharged into the aquatic environment. These wastewaters, containing dyes and other contaminants, cause many damages to the ecological system (Ohea et al., 2004; Nosheen et al., 2000). Thus, it is necessary to treat these industrial effluents before discharging into the receiving environments. The removal of dyes becomes a major environmental concern because of the difficulty to treat such wastewaters by conventional treatment methods.

Many researches have been investigated in order to remove dyes molecules before disposal of wastewater such as chemical coagulation and ozonation (Kumar et al., 2008, Barredo Damas et al, 2005). Recent research works had proved that adsorption method is becoming the most promising alternative in this domain. The most common adsorbent is the activated carbon (Pala et al., 2003; Kadirvelu et al., 2005; Tan et al., 2008) but its use remains quite expensive. This method do not show significant effectiveness and economic advantage and higher the quality greater the cost (Çiçek et al., 2007; Kiran et al., 2006). Many low cost biosorbents were tested for the removal of dyes from aqueous solutions such as wheat bran (Çiçek et al., 2007), Posidonia oceanica (Ncibi et al., 2007a, b and c); agricultural waste (Aksu and Isoglv, 2006), pine sawdust (Özacar et al., 2005a), orange peel (Sivaraj et al., 2001) and rice husk (Vasanth Kumar and Sivanesan, 2006).

Agave is a succulent plant of a large botanical genus of the same name, belonging to the amaryllis families. Chiefly Mexican, agaves occur also in the central and tropical south America. The plants have a large rosette of fleshy leaves, each ending generally in a sharp point and with a spiny margin. Each rosette grows slowly to flower only once. During flowering a tall stem or "mast" grows from the center of the leaf rosette and bears a large number of flowers. After development of fruit the plant dies, but suckers are frequently produced from the base of the stem which becomes new plants. One of the most familiar species is Agave americana, a native of tropical America. Agave americana, was introduced into Europe and north Africa and is now widely cultivated for its handsome appearance. (Cuénod et al., 1954). Agave americana is a very abundant in Tunisia, but it is not enough valorized.

The exploitation of this bio-resource in order to develop a low-cost and available biosorbent can have economical benefits. In this work, the use of A. americana fibres as biosorbent for the removal of metallic dyes from aqueous solution was investigated. The effect of pH, contact time, initial concentration and temperature were tested in order to optimize the biosorption process. Besides, equilibrium, kinetic and thermodynamic studies were performed in order to investigate the mechanisms probably involved in the present biosorption system.

Materials and Methods

The adsorbent used in this study was the fibres extracted from agave leaves collected from Hergla region in Sousse of Tunisia. They were submitted to a salted hydrolysis at 80 °C for 8 h. beaten with a mallet, thrashings energetically with a scraper. The extracted fibres were washed abundantly with water to remove the parenchyma. Fibres were then cut to about 4 cm in order to obtain uniform length followed by oven drying at 70 °C to constant weight the dried biomass was kept in desiccators for subsequent use in biosorption studies.

Metal dye used in this study is the Alpacide yellow dye (ALY), obtained from textile industry (Chimitex-Tunisia). The metal dye chemical structure is unknown. The synthetic effluent was prepared by dissolving equal amounts of dye in distilled water to produce a stock of 100 mg/L. All working solutions were prepared by diluting the stock solution with distilled water to the needed concentrations.

The biosorption experiments were carried out in batch reactor by adding 0.5 g dried fibres in 100 mL of dye solution (i.e. a liquid/solid ratio of 200) with desired concentration, pH and temperature. The determination of dye concentrations was carried out by spectrophotometry analysis (Camspec M330) at lmax of 440 nm. To investigate the effect of pH on biosorption process, a series of dye solution was prepared by adjusting pH over a range of 1 - 10 using 1 M HCl and NaOH solutions. All experiments were conducted in duplicate and the negative controls (without biomass) were simultaneously carried out to ensure that the biosorption capacity was solely by the Agave fibres and not by the container. The biosorption capacity q and percent removal of ALY dye were determined respectively according to the following Eq.:

q =[(C₀ −C_e )×V W] (mg/ g) (1)

% dye removal = (C0 −Ce ) ×100 C0 (%) (2)

Results and discussion

Effect of initial solution pH

The initial solution pH was considered as a key factor for the removal of dyes on cellulosic biomass (Ncibi et al., 2007a; Aksu and Isoglv, 2006) due to its impact on both the surface binding-sites of the biosorbent and the ionisation process of the dye molecule (Ncibi et al., 2007a). In this study, the pH of dye solution was varied in the pH range of 1 to 10 at 30 °C, with an initial dye concentration of 10mg/g and a solid/liquid ratio of 0.5:100 to determine the optimum pH range for dye biosorption by Agave americna (L.) fibres (AAF). The results represented in Fig.1 show that the biosorption of the ALY dye exhibited a slight variation with pH. The biosorption dye uptake reaches maximum at pH=2 (99.5 % for dye removal with q =1.96 mg/g of fibres) and then declined with further increase in pH. For this reason, the pH=2 was selected for the other experiments. This phenomenon can be explained by the ionisable nature of both ALY dye and the functional groups of the biomass surface (mainly carboxyl and hydroxyl groups) (Ncibi et al., 2007a). Indeed, at lower pH, a fraction of the functional groups of the biomass surface is probably in a protonated form (type: -OH²⁺ and =OH⁺), meaning that it will have a positive net charge therefore solution pH would affect both aqueous chemistry and surface binding sites (Ncibi et al., 2007a). Thus, the dye uptake increases due to electrostatic attraction between negatively charged dye anions and positively charged adsorbent surface (Çiçek et al., 2007). On the other hand, the decrease in the biosorption capacity at alkaline pH can be principally explained by the electrostatic repulsion between the anionic dye molecules present in solution and the ionised functional groups (i.e. alcoolate and carboxylate groups) of the biosorbent surface (Çiçek et al., 2007). In addition, the increase of OH¯ ions in solution probably causes a competition with the dye species for the biosorption sites resulting in a decrease of biosorption amounts. Similar results have been reported for dye biosorption onto Posidonia oceanica (Ncibi et al., 2007a) and Agricultural waste (Aksu and Isogvl, 2006). However, under alkaline solution pH, a significant biosorption fraction still occurs.

Effect of temperature

The effect of temperature on dye removal ability was carried out at 20, 30 and 50 °C. Fig. 2. shows the biosorption kinetics of ALY dye at initial dye concentration of 80 mg/L. It was observed that the biosorption capacity increases as the temperature increased at the range of 20 to 50 °C. Indeed, the biosorption of dyes is a thermo-dependent process. Similar results are observed in other researchers (Ncibi et al., 2007c). A larger amount of ALY was removed by AAF in the first 120 min. of contact time. Indeed, when the temperature of dye solution was raised from 20 to 50 °C, the biosorption capacity of ALY increased from 6 to 12 mg/g. The equilibrium is attainted after 390 min. at 50 °C but at 20 and 30 °C the equilibrium takes more time to be established. From this figure, it's clear that the temperature has an effect on the biosorption speed.

Effect of contact time and initial dye concentration

The effects of contact time and initial dye concentration on biosorption of metal dye by AAF are presented in Fig. 3. It was studied by varying the concentration from 10 to 100 mg/L (10, 20, 40, 60, 80 and 100 mg/L) at pH=2 and 50 °C. From the figure it was observed that the amount of dye uptake, qt (mg/g), increased with contact time at all initial dye concentrations. Similar trend was observed on the industrial effluent by fly ash (Basava Rao et al., 2006). The removal of metal dye increased quite rapidly in the initial stages (first 60 min.) and become slower in the stages until the attainment of equilibrium. Moreover, during the first 60 min., an increase in initial dye concentration from 10 to 100 mg/L increased the biosorption rate from 1.96 to 12 mg/g. Similar results are showed in the adsorption of metal complex dye onto pine sawdust (Özacar and Sengil, 2005b) Such trend could be caused by an increase in the driving force of the concentration gradient of the metal dye molecules and the biomass, which leads to higher amounts of adsorbed dye as the initial dye concentration is increased (Özacar and Sengil, 2005a). Furthermore, the biosorption dynamic profile shows that equilibrium has been reached more rapidly for low concentration (i.e. about 150 min. for 10, 20 mg/g initial dye concentrations and after 270 min. for the higher concentrations). Hence, the initial dye concentration has a significant effect on equilibrium time. In fact, at lower dye concentrations, the available biosorption sites are relatively high and consequently the dye species can find easily the accessible biosorption sites (Vadivelan and Vasanth Kumar, 2005). However, at higher concentrations the available site of biosorption become fewer and consequently the dye ions take more time in order to reach the last available sites (Ben Hamissa et al., 2007).

Equilibrium modeling

In this study, the most frequently used adsorption models, Langmuir (Langmuir, 1918) and Freundlich (Freundlich, 1906) models were studied to describe the equilibrium data in order to discover the biosorption capacity of AAF for ALY dye at different temperatures (Fig. 4). The Langmuir theory is intended for homogeneous type of biosorption meaning that once a dye molecule occupies a site. Whereas Freundlich isotherm is considered suitable for highly heterogeneous surfaces and the biosorption capacity was related to the equilibrium dye concentration (Özacar and Sengil, 2005a).

The Langmuir and Freundlich expression were given by the Eq.:

Langmuir: q_e = (q₀ K_L C_e ) (1+K_L C_e ) (3)

The linearized forms of the Langmuir and Freundlich equations can be written as fellows:

C_e /q_e = 1 / K_L q₀ +C_e /q₀ (5)

log q_e = log K _f + 1 /n log Ce (6)

Table 1 shows the calculated Langmuir and Freundlich constants at 20, 30 and 50 °C. It was seen that Langmuir biosorption isotherm models showed satisfactory fit to the data at different temperatures with a correlation coefficient value (r² = 0.9690.997) and declined with increasing temperature. But, the highest Freundlich correlation coefficient (0.986) was showed at the highest temperature and also the highest Kf and n were found as 5.103 and 1.8, respectively at 50 °C. The calculated n values were between 1 and 10, which represent a beneficial biosorption process (Vadivelan and Vagnth Kumar, 2005). In this case, it can suggest that the heterogeneity of the surface binding site increase with increasing temperature. The average absolute percentage deviation between the experimental and predicted values (Ncibi et al., 2007a), % D, is calculated using Eq. (7).

The isotherm model was considered to describe the satisfactorily fit with the biosorption process if the value of % D was less than 10 %. From Table 1, the Freundlich isotherm fitted the experimental data well with the less average percentage deviation (8.41 %) at 50 °C. The Langmuir isotherm could fit the equilibrium data with an average derivation less than 12.28 % and the best average derivation was performed at 20 °C suggesting a good fit with the biosorption data of metal dye at the low temperature.

The high correlation coefficients (>0.95) show that both the Langmuir and Freundlich models are suitable, for describing the biosorption equilibrium of ALY dye by AAF. From Table 1 it was observed that the maximum biosorption capacity of AAF for ALY was found to be 21.41 mg/g at 50 °C.

The essential characteristics of the Langmuir isotherm can be expressed in terms of dimensionless constant separation factor R_L (Hall et al., 1966), given by Eq. (8)

R_L = 1 /(1 + K_L C₀ ) (8)

The calculated R_L values at different temperatures are shown in Table 2.From this table; it was observed that the values of R_L computed are observed to be in the range of 01, indicating that the biosorption process is favourable for this low-cost adsorbent (El Qada et al., 2008). The R_L values decrease with increasing concentrations confirms the dye uptake process is more favourable with higher concentration.

Kinetic modeling

The kinetics of dye biosorption onto Agave fibres biomass, were analyzed using the pseudo-first and second order kinetic models (Ho, 2004). The pseudo first and second order kinetic equations are

dq_t dt = k₁ (q_e − q_t ) (9)

dq dt = k₂ (q_e − q_t )² (10)

The integrated and the linear form of Eqs. (9 and 10) becomes

log (q_e /q_e − q_t ) = − k₁ t /2.303 (11)

t /qt = 1/ k₂ q² _e + t/ q_e (12)

The pseudo first and second order rate constant values for the biosorption of ALY dyes onto AAF are determined from the slop of log (q_e q_t) against t and of t/q_t against t respectively. The constant K₂ is used to calculated the initial biosorption rate h (mg/g min.), at t → 0 as follows (Ho and Mekay, 1999).

The rate constants k₁, k₂ and h experimental, the calculated q_e values and correlation coefficients are given in Table 3. From Table 3, the data showed a good compliance with the pseudo-second order model with squared correlation coefficient greater than 0.997 and the predicted values of q_e nearly matched the experimental values thus suggesting the goodness of the plot. The value of rate constant, k₂ were found to decrease with increasing concentration and increase with increasing temperature. The values of h increased with increasing concentration presumably due to the enhanced mass transfer of dye molecules to the surface of the biomass and also increased with increasing temperature.

Determination of thermodynamic parameters

To estimate the effect of temperature on the biosorption of metal dyes on AAF at different concentrations, the free energy ΔG°, enthalpy change ΔH° and entropy change ΔS° were determined (Nacèra and Aieha, 2006). The apparent equilibrium constant K_d of the adsorption (Aravindhan et al., 2007) is defined as:

The K_d value is the distribution coefficient for the adsorption used to determine the Gibbs free energy of biosorption in the following equation

The relationship between the K and temperature is given by the Van't Hoff equation (Aravindhan et al., 2007)

The enthalpy and entropy can be obtained from the slope and intercept of van't Hoff plot of ln k versus 1/T.

The change in free energy can be used to determine the nature of the biosorption process. Therefore, the process can be considered as physiosorption when ΔG° is between -20 and 0 kJ/mol and when ΔG° is between -80 and -400 kJ/mol the process is considered as chimiosorption (Aravindhan et al., 2007). In our study, the change in free energy for the biosorption of metal dye onto A.americana fibres is range to -4.4 to -0.52 kJ/mol for all studied concentrations and temperatures (Table 4). Hence, this process can be considered as physiosorption. The negative values of ΔG° at different concentrations and temperatures confirm the spontaneous nature of the biosorption process with a high performance and a high affinity degree of ALY dye molecules for the Agave americana fibres. Table 4 shows that the negative value of ΔG° decreased with an increase in temperature for all concentrations. Thus, the spontaneous nature of biosorption of metallic dyes is inversely proportional to the temperature (Han et al., 2007). And also the increase in initial solution concentration increases the values of ΔG°. The positive values of ΔS° confirm the increased randomness at the solid-solute interface during the biosorption process. The increase in initial dye concentration decreases the ΔS° values. The positive value of ΔH^° confirms the endothermic character of biosorption of dyes on AAF. Similar trend was observed by other authors (Aravindhan et al., 2007).

Conclusion

In the present study, Agave americana (L.) fibres biosorbent was applied successfully for the biosorption of metal dye from aqueous solution namely the Alpacide yellow dye. The amount of dye biosorbed was found to vary with initial solution pH, contact time, initial dye concentration and temperature. The amount of dye uptake (mg/g) was found to increase with increase in solution concentration, temperature and contact time and decrease with increase in initial solution pH. The mechanism follows a pseudo-second order reaction model. Langmuir model present good fit with the experimental data at lowest temperature but Freundlich model fitted well with highest temperature. Thermodynamic parameter shows that the process is endothermic and spontaneous.

Acknowledgment

The authors express their sincere gratitude to Tunisian Ministries of Defence and higher education for the financial support of this study.

Nomenclature

Ca_eq dye concentration on the adsorbent at equilibrium (mg/L)

C_e equilibrium concentration of metal dye in the solution (mg/L)

C₀ initial concentration of metal dye in the solution (mg/L)

D average absolute percentage deviation (%)

h initial biosorption rate

K₁ rate constant of first-order kinetic model (min^-1)

K₂ rate constant of pseudo-second-order kinetic model (g/mg min)

K_d biosorption distribution coefficient

K_F Freundlich isotherm constant [(mg/g) (L/mg)(1-n/n)]

K_L Langmuir isotherm constant (L/mg)

N number of experimental data points

n Freundlich exponent related to adsorption intensity

q experimental amount of metal dye adsorbed per unit of biomass (mg/g)

q_e calculated amount of metal dye adsorbed per unit of biomass (mg/g)

q_pred uptake of dye onto the biosorbent as predicted by the particular isotherm employed (mg/g)

q_t amount of metal dye adsorbed per unit of biomass at time t (mg/g)

q_o Langmuir monolayer adsorption capacity (mg/g)

R universal gas constant: 8.314 j/mol k

R_L dimensionless separator factor

r² squared regression correlation coefficient

T absolute temperature (K)

t time (min)

V solution volume (L)

W weight of A.americana fibres (g)

rG⁰ Gibbs adsorption free energy change (kJ/mol)

rH⁰ adsorption enthalpy change (kJ/mol)

rS⁰ adsorption entropy change [J/(mol.K)]

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