Equilibrium and thermodynamics of azo dyes biosorption onto Spirulina platensis

Abstract

The equilibrium and thermodynamics of azo dye (tartrazine and allura red) biosorption onto Spirulina platensis biomass were investigated. The equilibrium curves were obtained at 298, 308, 318 and 328 K, and four isotherm models were fitted the experimental data. Biosorption thermodynamic parameters (ΔG, ΔH and ΔS) were estimated. The results showed that the biosorption was favored by a temperature decrease. For both dyes, the Sips model was the best to represent the equilibrium experimental data (R²>0.99 and ARE<5.0%) and the maximum biosorption capacities were 363.2 and 468.7 mg g-1 for tartrazine and allura red, respectively, obtained at 298 K. The negative values of ΔG and ΔH showed that the biosorption of both dyes was spontaneous, favorable and exothermic. The positive values of ΔS suggested that the system disorder increases during the biosorption process.

Biosorption; Spirulina platensis; Azo dyes

THERMODYNAMICS

Equilibrium and thermodynamics of azo dyes biosorption onto Spirulina platensis

G. L. Dotto; M. L. G. Vieira; V. M. Esquerdo; L. A. A. Pinto^*

Unit Operation Laboratory, School of Chemistry and Food, Phone: + (55) (53) 3233 8648, Fax: + (55) (53) 3233 8745, Federal University of Rio Grande, FURG, 475 Engenheiro Alfredo Huch Street, 96203-900, Rio Grande - RS, Brazil

ABSTRACT

The equilibrium and thermodynamics of azo dye (tartrazine and allura red) biosorption onto Spirulina platensis biomass were investigated. The equilibrium curves were obtained at 298, 308, 318 and 328 K, and four isotherm models were fitted the experimental data. Biosorption thermodynamic parameters (ΔG, ΔH and ΔS) were estimated. The results showed that the biosorption was favored by a temperature decrease. For both dyes, the Sips model was the best to represent the equilibrium experimental data (R²>0.99 and ARE<5.0%) and the maximum biosorption capacities were 363.2 and 468.7 mg g^-1 for tartrazine and allura red, respectively, obtained at 298 K. The negative values of ΔG and ΔH showed that the biosorption of both dyes was spontaneous, favorable and exothermic. The positive values of ΔS suggested that the system disorder increases during the biosorption process.

Keywords: Biosorption; Spirulina platensis; Azo dyes.

INTRODUCTION

More than 0.7 million tons of synthetic organic dyes are produced annually worldwide (Koprivanac and Kusic, 2009). Azo dyes make up approximately 70% of all dyes used worldwide by weight. They are extensively used in the textile, paper, food, leather, cosmetics and pharmaceutical industries (Saratale et al., 2011). Discharge of these dyes in effluents affects aquatic plants because they reduce sunlight transmission through water. Also azo dyes may impart toxicity to aquatic life and may be mutagenic, carcinogenic and may cause severe damage to human beings, such as dysfunction of the kidneys, reproductive system, liver, brain and central nervous system (Salleh et al., 2011). Thus, several governments have established environmental restrictions with regard to the quality of colored wastewater and have forced dye-using industries to remove dyes from their effluents before discharging (Mahmoodi et al., 2010). The conventional methods are generally ineffective in color removal, and they are expensive and less adaptable to a wide range of dye wastewaters (Srinivasan and Viraraghavan, 2010). In this way, biosorption is an alternative eco-friendly technology for removal of azo dyes from aqueous solutions, due its simplicity of design, ease of operation, insensitivity to toxic substances and complete removal of pollutants, even from dilute solutions (Aksu, 2005).

Biosorption is the passive uptake of pollutants from aqueous solutions by the use of non-growing or dead microbial biomass, thus allowing the recovery and/or environmentally acceptable disposal of the pollutants (Aksu, 2005). Many biosorbents have been used to remove dyes from aqueous solutions, such as, Chlorella vulgaris (Aksu and Tezer, 2005), Azolla rongpong (Padmesh et al., 2006), Aspergillus foetidus (Patel and Suresh, 2008), Aspergillus parasiticus (Akar et al., 2009), Ulothrix sp. (Dogar et al., 2010) and Nostoc linckia (Mona et al., 2011), but studies about the use of S. platensis are very limited. S. platensis, a member of blue-green algae, contains a variety of functional groups such as carboxyl, hydroxyl, sulfate, amine, phosphate and other charged groups on the surface, which can sequester pollutants (Seker et al., 2008). This microalgae was successfully employed for removal of chromium, cadmium, cooper, lead and nickel from aqueous solutions (Chojnacka et al., 2005; Seker et al., 2008). However, its use to remove dyes is rarely investigated.

The equilibrium and thermodynamic analysis of the biosorption process is fundamental in order to obtain information about the interactions adsorbate-biosorbent, and so increase the process efficiency (Aksu, 2005). The equilibrium curve (isotherms) leads to information about the biosorption mechanism and also about the maximum biosorption capacity (Blázquez et al., 2010). The shape of an isotherm not only provides information on the biosorption affinity of dye molecules, but also reflects thermodynamic data, leading to relevant information about biosorption spontaneity and the stability of the biosorbent-adsorbate interactions (Aksu, 2005; Piccin et al., 2011; Dotto et al., 2011).

In this work, the equilibrium and thermodynamics of azo dye (tartrazine and allura red) biosorption onto S. platensis biomass in batch systems were studied. Temperature effect on the experimental equilibrium curves was verified, and the Langmuir, Freundlich, Dubinin-Radushkevich and Sips models were fitted to the experimental data. The values of the Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) for the biosorption process were estimated.

MATERIALS AND METHODS

Preparation and Characterization of Spirulina platensis

S. platensis strain LEB-52 was cultivated in a 450 L open outdoor photo-bioreactor, under uncontrolled conditions, in the south of Brazil. During these cultivations, water was supplemented with 20% Zarrouk synthetic medium (Costa et al., 2004). At the end of cultivation, the biomass was recovered by filtration, washed with distilled water and pressed to recover the biomass with a moisture content of 76% (wet basis).

The wet biomass (cylindrical pellet form with a diameter of 3 mm) was dried in perforated trays using perpendicular air flow. The drying conditions were: air temperature 60 °C, air velocity 1.5 m s^-1, relative humidity between 7% and 10%, load in tray 4 kg m^-2 (Oliveira et al., 2009). The dried biomass was ground in a mill (Wiley Mill Standard, No. 03, USA) and sieved until the discrete particle size ranged from 68 to 75 µm.

The particles were characterized according to the centesimal chemical composition (AOAC, 1995) and point of zero charge (pH_zpc) using the eleven points experiment (Hao et al., 2004). The functional groups of the S. platensis biomass were identified using infra-red analysis (FT-IR) (Prestige 21, the 210045, Japan) (Mona et al., 2011). The specific surface area, pore volume and average pore radius were determined by standard BET N₂-adsorption methods (Quantachrome, Nova station A, USA) (Schimmel et al., 2010). Scanning electron microscopy (SEM) was used to verify the surface characteristics of the S. platensis particles (Jeol, JSM-6060, Japan) (Bangash and Alan, 2009). The samples were sputter-coated with a thin conductive layer of gold for 400 s, and images were then obtained using an acceleration voltage of 10 kV with magnification ranging from 300 to 2000 fold. The thermal profile of the S. platensis particles was obtained in a N₂ atmosphere using TG and DTG curves (PG instruments, SDT Q600, U.K.) (Gottipati and Mishra, 2010).

Adsorbate

In this work, the azo dyes tartrazine (molecular weight 534.4 g mol^-1, purity 85%, color index 19140, maximum wavelength 425 nm, pKa = 9.4) and allura red (molecular weight 496.4 g mol^-1, purity 85%, color index 16045, maximum wavelength 500 nm, pKa = 11.4) (Pluryquímica Ltda., Brazil) were employed for the batch experiments. Distilled water was used to prepare all solutions. The chemical structures of the dyes are illustrated in Figure 1.

Batch Equilibrium Experiments

The equilibrium biosorption isotherms were determined using batch studies under different temperature conditions (298, 308, 318 and 328 K). S. platensis particles (50 mg dry basis) were added to 90 mL of distilled water. The pH of the suspension was corrected (pH = 4) through addition of 10 mL of disodium phosphate/ citric acid buffer solution (0.1 mol L^-1), which did not present interaction with the dyes. The suspension was agitated at 10000 rpm (Dremel, 1100-01, Brazil) for 5 min. Different volumes of the dye stock solution containing 10 g L^-1 were added to the S. platensis suspensions and adjusted to 200 mL with distilled water. The suspensions were placed in flasks and agitated at 100 rpm using a thermostated Wagner type agitator (FANEM, 315 SE, Brazil). Samples were analyzed every 8 h. The equilibrium was attained when the dye concentration in the liquid did not change between three consecutive measurements. The biomass and biosorbed dyes were removed from the liquid by filtration through Whatmann No. 40 Filter Paper, which did not present interaction with the dyes, and the dye concentration was determined by spectrophotometry (Quimis, Q108, Brazil) (Aksu and Tezer, 2005; Piccin et al., 2009; Dotto and Pinto, 2011a; Dotto and Pinto 2011b). The experiments were carried out in replicate (three times for each experiment) and blanks were performed.

The biosorption capacity at equilibrium (q_e), valid when the solute remaining in the liquid filling the pores is negligible, was determined according to Equation (1):

where C₀ is the initial dye concentration in the liquid phase (mg L^-1), C_e is the dye concentration in the liquid phase at equilibrium (mg L^-1), m is the biosorbent amount (g) and V is the volume of suspension (L).

Equilibrium Isotherm Models

The biosorption equilibrium data, commonly known as biosorption isotherms, describe how pollutants interact with biosorbent materials and so are critical for optimizing the use of biosorbents (Aksu, 2005). This manner is very important to obtain a model that represents the experimental equilibrium data. In this work, Langmuir, Freundlich, Dubinin-Radushkevich and Sips models were fitted to the experimental data.

The Langmuir isotherm is derived by assuming a uniform surface with finite identical sites and mono-layer adsorption of the adsorbate (Langmuir, 1916). The Langmuir isotherm is shown in Equation (2):

where q_m is the maximum monolayer biosorption (mg g^-1) and k_L is the Langmuir constant (L mg^-1).

Another essential characteristic of the Langmuir isotherm can be expressed by the separation factor or equilibrium factor (R_L) (Langmuir, 1916), as demonstrated in Equation (3):

The Freundlich adsorption isotherm gives the empirical relation between q_e and C_e (Freundlich, 1906). The Freundlich isotherm is given by Equation (4):

where k_F is the Freundlich constant ((mg g^-1) (mg L^-1)^-1/n ) and 'n' is the biosorption intensity.

Another equation used in the analysis of isotherms was proposed by Dubinin and Radushkevich (1947), and can be represented by Equation (5):

where q_s is the D-R constant (mg g^-1) and ε can be correlated according to Equation (6):

The constant B (mol² kJ^-2) gives the mean free energy E (kJ mol^-1) of adsorption per molecule of adsorbate when it is transferred to the surface of the solid from infinity in the solution (Dubinin and Radushkevich, 1947), and can be computed using Equation (7):

The Sips isotherm is a combination of the Langmuir and Freundlich isotherms (Sips, 1948), as

demonstrated in Equation (8):

where q_ms is Sips maximum biosorption capacity (mg g^-1), k_S is the Sips constant (L mg^-1) and ms the exponent of the Sips model.

The isotherm parameters were determined by nonlinear regression, using the software Statistica 6.0 (Statsoft, USA.). The fit quality was measured according to the coefficient of determination (R²) and average relative error (ARE) (Piccin et al., 2009).

Biosorption Thermodynamics

The values of ΔG, ΔH and ΔS are important information in order to elucidate the biosorption thermodynamic behavior (Aksu, 2005). In this work, the values of Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were estimated using Equations (9) and (10) as demonstrated by Milonjic, (2007):

where, R is the universal gas constant (8.314 J mol^-1 K^-1), T is the temperature (K), K_D is the thermodynamic equilibrium constant (L g^-1) and ρ is the water density (g L^-1). The K_D values were estimated from the parameters of the best fit isotherm model.

RESULTS AND DISCUSSION

Spirulina platensis Characterization

The percentage chemical composition of S. platensis used in the biosorption tests is shown in Table 1.

The values presented in Table 1 are similar to the percentage chemical composition obtained by Oliveira et al. (2009) and show that S. platensis is composed of several biomolecules. According to Chojnacka et al. (2005), these biomolecules contain functional groups which can sequester pollutants. The functional groups of S. platensis were identified by FT-IR as shown in Figure 2.

In Figure 2 the major intense bands were around 3269, 2918, 1660, 1627, 1548, 1409, 1028, 850, 700 and 543 cm^-1. The O-H bond stretching mixed with NH₂ group can be observed at 3269 cm^-1. The peak 2918 cm^-1 is relative to CH₂ stretching. The scissor bending of NH₂ groups can be observed at 1660 cm^-1. At 1627 and 1548 cm^-1 C=C stretching can be observed. The bands at 1409 and 1028 cm^-1 could be attributed to -S-O and -P-O, respectively. The band of 850 cm^-1 could be attributed to an N-H stretch of amide or amine. The adsorption peaks in the region 700 cm^-1 could be attributed to aromatic -CH bending vibrations. At 1543 cm^-1 R-S-O can be observed. These functional groups may be responsible for dye binding (Aksu, 2005; Chojnacka et al., 2005; Seker et al., 2008).

The values of zero point charge (pH_zpc), specific surface area, pore volume and average pore radius of the biosorbent are very important to understand the biosorption mechanisms (Schimmel et al., 2010). The zero point charge (pHzpc) of the S. platensis biomass (obtained by the eleven point's experiment) was 7, showing that when the pH of the suspension is lower than 7, the surface of the S. platensis biomass is positively charged (Hao et al., 2004). S. platensis particles presented specific surface area of 3.5 m² g^-1, pore volume of 3.9 mm³ g^-1and average pore radius of 22.5Å.

Scanning electron microscopy (SEM) was carried out in order to verify the surface characteristics of S. platensis particles. Figure 3 shows SEM images of S. platensis: (a) distributed particles and (b) particle surface.

It was observed in Figure 3 (a) that the S. platensis particles presented different forms and mean diameter in the range from 68 to 75 μm. In Figure 3 (b) a rough and homogeneous surface was observed. In addition, in Figures 3 (a) and (b), the rigid and non-porous structure of the S. platensis particles can be observed.

To verify the thermal profile of the S. platensis particles used in the biosorption experiments, TG and DTG curves were used. TG and DTG curves of S. platensis particles are shown in Figure 4.

TG and DTG curves demonstrate that S. platensis particle mass loss occurred in three steps. The first mass loss step, from about 25 °C to 140 °C concerns the loss of water, which is adsorbed on the S. platensis particles. The decomposition of the S. platensis particles is observed from about 150 °C to 500 °C. A carbonization of material was observed up to 500 °C. This shows that, under experimental conditions of biosorption, S. platensis particles maintained their physical characteristics.

Biosorption Isotherms

The equilibrium experimental curves of azo dye biosorption onto S. platensis particles at different temperatures (298, 308, 318, 328 K) are presented in Figure 5 (a) tartrazine and (b) allura red.

The biosorption isotherms of tartrazine (Figure 5 (a)), were characterized by an initial step of weak attraction between S. platensis and the dye, followed by a strong increase in the biosorption capacity, and finally a plateau was observed. According to Blázquez et al. (2010), this behavior is common for a type "V" isotherm. On the other hand, in the allura red biosorption isotherms (Figure 5 (b)), the equilibrium curves presented an initial step of increase in biosorption capacity followed by a plateau. In this case, type "I" isotherms were observed. These results suggested that allura red biosorption onto S. platensis particles occurred by formation of a monolayer. However, for tartrazine, biosorption occurred by formation of a multilayer.

It can be observed in Figure 5, for both tartrazine and allura red, that a temperature decrease caused an increase in biosorption capacity, the best results being obtained at 298 K. This behavior can be explained by two main aspects: Firstly, the temperature increase causes an increase in the solubility of the dyes (Koprivanac and Kusic, 2009) so, the interaction forces between the dyes and the solvent become stronger than those between dyes and S. platensis particles. Coupled to this, according to Aksu (2005), at temperatures above 318 K damage of sites on the surface of biomass can occurs and, consequently, a decrease in the surface activity. Piccin et al. (2011) showed that the temperature increase caused a decrease in the adsorption capacity of FD&C red n° 40 onto chitosan. Similar behavior was obtained by Aksu and Tezer (2005) in the biosorption of remazol red and remazol golden yellow using Chlorella vulgaris as biosorbent. They obtained best results at 298 K.

In order to obtain an equation to represent the equilibrium experimental data, Langmuir, Freundlich, Dubinin-Radushkevich and Sips models were fitted to the experimental data. The isotherm parameters, their respective coefficients of determination (R²) and average relative error (ARE) values are shown in Table 2.

It was observed in Table 2 for biosorption of tartrazine onto S. platensis particles, that Dubinin-Radushkevich and Sips isotherms presented high values of the coefficient of determination (R²>0.95), indicating a good fit to the experimental data. However, the D-R model presented high values of the average relative error (ARE>14%), consequently, overestimating the maximum biosorption capacity values. Thus, the Sips model was the best to represent the equilibrium experimental data (R²>0.98 and ARE<5%). In the case of allura red biosorption, the Langmuir and Sips models presented high values of the coefficient of determination and low values of the average relative error (R²>0.99 and ARE<3%), but the Sips model was more appropriate because it estimated more accurately the maximum biosorption capacity values.

The high values of "ms" from the Sips model (Table 2) confirmed that tartrazine biosorption onto S. platensis occurred by formation of a multilayer. On the contrary, the "ms" values for allura red (Table 2) were near to 1, confirming the formation of a monolayer on the S. platensis surface. In summary, for both azo dyes, the Sips model was the best to represent the equilibrium experimental data. The maximum biosorption capacities were 363.2 mg g^-1 and 468.7 mg g^-1 for tartrazine and allura red, respectively, obtained at 298 K. These values are in accordance with the specific literature (Aksu, 2005; Aksu and Tezer, 2005; Srinivasan and Viraraghavan, 2010).

Biosorption Thermodynamics

The biosorption thermodynamic parameters (ΔG, ΔH and ΔS) were estimated using Equations (9) and (10). The K_D values were estimated from the Sips parameters (k_S and q_ms), because this model presented the best fit with the experimental data. The values of ΔG, ΔH and ΔS are shown in Table 3.

The negative values of ΔG presented in Table 3 showed that the biosorption of the azo dyes tartrazine and allura red onto S. platensis particles was a spontaneous and favorable process. The negative values of ΔH (Table 3) suggested that the biosorption was exothermic. The positive values of ΔS indicated that the disorder at the solid-liquid interface increased during the biosorption process. However, Table 3 shows that the enthalpy change (ΔH) contributed more than entropy change (ΔS) to obtain negative values of ΔG. This shows that the azo dyes biosorption onto S. platensis was an enthalpy-controlled process. Similar thermodynamic parameters were obtained by Aksu and Tezer (2005) in the biosorption of reactive dyes onto Chlorella vulgaris, and also by Dotto et al. (2011) in the adsorption of acid blue 9 and food yellow 3 onto chitosan.

CONCLUSION

In this study, the azo dye biosorption onto S. platensis was investigated at different temperatures, using the experimental equilibrium curves and the thermodynamic parameters. The experimental equilibrium curves showed that tartrazine bio-sorption occurred by formation of a multilayer, but for allura red biosorption occurred by formation of a monolayer on the S. platensis surface. For both azo dyes a temperature decrease caused an increase in the biosorption capacity. In all conditions, the Sips model was the best to represent the equilibrium experimental data, the maximum biosorption capacities being 363.2 mg g^-1 and 468.7 mg g^-1 for tartrazine and allura red, respectively, obtained at 298 K.The values of Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) showed that the azo dye biosorption onto S. platensis was spontaneous, favorable, and exothermic, and the system disorder increased during the process.

ACKNOWLEDGEMENTS

The authors would like to thank CAPES (Brazilian Agency for Improvement of Graduate Personnel) and CNPq (National Council of Science and Technological Development) for the financial support.

(Submitted: December 15, 2011 ; Revised: May 23, 2012 ; Accepted: May 23, 2012)

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Publication Dates

• Publication in this collection
  01 Mar 2013
• Date of issue
  Mar 2013

History

• Received
  15 Dec 2011
• Accepted
  23 May 2012
• Reviewed
  23 May 2012