Ind. Eng. Chem. Res. 2008, 47, 9715–9723
9715
Batch and Dynamic Flow Biosorption Potential of Agaricus bisporus/Thuja orientalis Biomass Mixture for Decolorization of RR45 Dye Tamer Akar,*,† Burcu Anilan,‡ Zerrin Kaynak,§ Asli Gorgulu,‡ and Sibel Tunali Akar† Department of Chemistry, Faculty of Arts and Science, Eskis¸ehir Osmangazi UniVersity, 26480, Eskis¸ehir, Turkey, Department of Elementary Education, Faculty of Education, Eskis¸ehir Osmangazi UniVersity, 26480, Eskis¸ehir, Turkey, and The Program of Chemistry, Vocational School of Higher Education, Bilecik UniVersity, 11210 Bilecik, Turkey
This work reports the batch and dynamic flow biosorption conditions for Reactive Red 45 dye using Agaricus bisporus/Thuja orientalis biomass mixture (ABTOC). Experiments were performed to determine optimum pH, biomass amount, contact time, temperature, dye concentration, and flow rate. The applicability of different kinetic and isotherm models for the biosorption process was evaluated. Biosorption showed a highly pH dependent profile. Under optimized batch conditions up to 93.04% dye could be removed from solution in a relatively short time. Kinetic experiments suggest that the biosorption process followed the pseudo-secondorder model in comparison to intraparticle diffusion and the pseudo-first-order models. Thermodynamic data confirm that the biosorption process is spontaneous and endothermic in nature. Besides, the highest regression coefficient (r2 ≈ 1) for the Langmuir model indicates the monolayer coverage of biomass by RR45 dye molecules (qmax ) 108.90 mg g-1). Column studies showed that ABTOC effectively removes RR45 dye with a maximum biosorption yield of ∼100%. ABTOC was able to give nearly 96% dye removal in the presence of Na+, K+, Mg2+, Ca2+, Pb2+, Ni2+, Cu2+, and Cd2+ ions. Our results revealed that ABTOC could be employed as an effective and low-cost alternative biosorbent material for removal of reactive textile dyes from contaminated effluents. 1. Introduction Contamination of water sources by synthetic dyes arises as a result of many industrial activities such as dye manufacturing, craft mills, tannery, textile, pulp, paper mill, and other industries (e.g., food and cosmetic).1 It was reported that approximately 700 000 metric tones and 10 000 different types of dyes and pigments are commercially available worldwide and 5-10% of the dye is lost in the process effluents.2 Discharge of colored effluents into the aquatic environment without adequate treatment is a major problem due to their damages toward the environment and human health.3 Various physical and chemical processes can be used for decolorization of dye-containing wastewater such as oxidation, ozonation, chemical coagulation/flocculation, irradiation, and precipitation. However, their applications are restricted due to some disadvantages such as excess dosage of chemicals, difficulty and high cost of operation, production of sludge or potential toxic byproduct, and ineffective removal of chrominance.4 Also, use of a treatment process alone may not completely remove dye from wastewater. For this reason a combination of these processes is necessary to achieve the desirable goal.1 Use of biomaterials for dye removal from contaminated effluents has gained growing attention in recent years due to their cost effectiveness, ability to produce less sludge, and environmental friendliness.5-8 Currently a number of studies have focused on the biodegradation and biosorption ability of some microbial origin biomass including fungi, bacteria, and algae, which are capable of decolorizing dye wastewater.9 * To whom correspondence should be addressed. Tel.: +90 222 2393750/2871. Fax: +90 222 2393578. E-mail:
[email protected]. † Faculty of Arts and Science, Eskis¸ehir Osmangazi University. ‡ Faculty of Education, Eskis¸ehir Osmangazi University. § Bilecik University.
Among the commonly available sorbent materials, fungal biomass seems to be a good biosorbent because it can be cultivated economically in substantial amounts using simple fermentation techniques and economical growth media. Fungal biomass is also generated as a byproduct from different industrial fermentation processes.10,11 The advantages of macro fungusbased biomasses such as being easily and economically available anywhere, chemical stability in most alkaline and acidic conditions, and good mechanical properties make them attractive in biosorption studies.10,12 Furthermore, the fruiting bodies of the macro fungi have a tough texture when dried and other physical characteristics which are conductive to their development into adsorbents.13 Agaricus bisporus, commonly called mushroom, is a commercially available macro fungus, and it was chosen as a biosorbent material because of the relative lack of information on its dye biosorption abilitiy. The main components of the fungal cell wall are polysaccharides (80-90% of the dry mass), and chitin is a characteristic component of the basidomycetes.14 Previous studies have revealed that chitin and chitosan in the cell wall play important role in the sorption of dyes.9,15 Thuja orientalis cones are economical and broadly available in large quantity in nature, and the mature cones contain cellulose, hemicellulose, lignin, rosin, and tannins.16 On the other hand, previous research demonstrated that T. orientalis cones show metal16-18 and acid dye19 binding capacity. The objective of this study was to explore the biosorption potential of A. bisporus/T. orientalis biomass mixture for removal of RR45 textile dye from aqueous solutions. Batch biosorption conditions were optimized by investigating the different experimental parameters including initial pH, sorbent dosage, and biosorption time. Kinetic measurements were assessed at different temperatures and evaluated by the pseudofirst-order model of Lagergren, pseudo-second-order model, and intraparticle diffusion model. Freundlich, Langmuir, and
10.1021/ie8007874 CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
9716 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 1. Chemical structure of RR45.
Dubinin-Radushkevich isotherm equations were used for modeling of data. Thermodynamic parameters were also deduced. Likewise, continuous flow experiments were then performed by prepared biosorbent system. Finally, the biosorption performance of the biomass system in the solution containing metal ions was investigated to test the co-ions effect. 2. Materials and Methods 2.1. Preparation of the Biosorbent Material. Fresh fungal biomass of A. bisporus and T. orientalis cones were purchased from a commercial company and collected from nature in July 2007, respectively. They were washed repeatedly with deionized water to remove the adhering dirt and soluble impurities, dried at 80 °C for 24 h, and crushed and sieved to select a particle size of less than 150 µm using ASTM Standard sieve. The powdered biosorbents were stored in glass bottles prior to use. The biomass mixture was prepared using a procedure previously recommended by Mahan and co-workers.20 A 1.5 g amount of dry A. bisporus powder was mixed with 2.0 g of T. oriantalis. The powder mixture was wetted with 5 mL of deionized water and thoroughly mixed. The paste was heated in an oven at 80 °C for 24 h to dry the mixture. Then, the biomass was sieved to an original particle size of 150 µm. The prepared biomass was stored in a glass bottle prior to use in biosorption studies and is labeled as “ABTOC” in this manuscript. Elemental analysis of biosorbent was performed on a Carlo-Erba 1106 model instrument. FTIR spectral analysis of unloaded and dye-loaded biosorbent was recorded in a PerkinElmer Spectrum 100IR infrared spectrometer in the region of 400-4000 cm-1. 2.2. Dye Solutions. The Reactive Red 45 (RR45) textile dye, which has wide industrial application, was selected as sorbate, and its chemical structure is given in Figure 1. A stock solution (1.0 g L-1) of RR45 was prepared by dissolving an appropriate amount in deionized water, and the other concentrations were obtained by diluting this stock dye solution. The pH of the dye solutions was adjusted to desired values with 0.1 M HCl and/ or 0.1 M NaOH solutions. 2.3. Biosorption Studies in Batch Mode. Biosorption of RR45 textile dye onto ABTOC was first studied in batch system, and the parameters affecting the biosorption process such as initial pH, biomass and initial dye concentration, contact time, and temperature were optimized. The effect of initial pH of the dye solution on the biosorption capacity of ABTOC was investigated in the pH range of 1-10. A 0.1 g amount of ABTOC biomass was added to dye solutions at different pHs which magnetically stirred at 200 rpm for 1 h. All of the parameters, biosorbent concentration (2.0 g L-1), temperature (20 °C), initial dye concentration (100 mg L-1), and contact time(60min),werekeptconstantateachpHvalue.Biosorbent-sorbate mixture was then centrifuged at 4500 rpm for 3 min, and the remaining RR45 concentration in the supernatant was determined using a UV spectrophotometer (Unicam UV2-100) at 520 nm. The same procedure stated here was carried out for determination of RR45 concentrations in solutions for all batch biosorption studies. The biosorbent concentration was changed
Figure 2. Effect of initial pH on the biosorption of RR45 by ABTOC.
from 0.4 to 4.0 g L-1 in order to elucidate the optimum biosorbent dosage for biosorption of RR45 by ABTOC. The biosorption equilibrium time profile was examined in the time range of 5-60 min as a function of temperature (20-40 °C) in order to understand the biosorption behavior of biosorbent. For this purpose the pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic models were applied to the obtained data. Finally, in order to investigate the isothermal behavior of ABTOC the initial RR45 concentration was changed between 75 and 400 mg L-1 at different temperatures and the equilibrium data were applied to three isotherm models which are Freundlich, Langmuir, and Dubinin-Radushkevich (D-R). 2.4. Biosorption Studies in Continuous Mode. In order to elucidate the biosorption performance of ABTOC in continuous mode, up-flow packed column was prepared and used. The effect of flow rate of sorbate, biosorbent concentration, and internal diameter (i.d.) of the column on the biosorption efficiency of RR45 was investigated. Continuous biosorption experiments were conducted in a glass column (9 mm i.d. and 100 mm height) packed with 0.1 g of ABTOC in order to optimize the flow rate of sorbate. A 50 mL amount of RR45 solution (100 mg L-1 at pH 2.0) was pumped upward through the column, and the flow rates were varied from 0.5 to 6.0 mL min-1 using a peristaltic pump (Ismatec ecoline). All column studies were performed at a room temperature of 25 ( 0.5 °C. The biomass dosage was also optimized in a continuous system in order to compare the performance of batch and column modes for biosorption of RR45 onto ABTOC. The biomass concentration was changed between 0.4 and 4.0 g L-1 as studied in the batch system. The effect of column i.d. on the biosorption performance was investigated using different columns (9-19 mm i.d. and 100 mm height). Effluent samples left the column entirely were collected (50 mL), and the RR45 dye concentration was analyzed as explained before. A synthetic water sample having 100 mg L-1 of Na+, K+, Mg2+, Ca2+, Pb2+, Ni2+, Cu2+, and Cd2+ ions in the same mixture including RR45 dye was prepared, and the proposed continuous biosorption method for dye removal was applied to this mixture. 3. Results and Discussion 3.1. Effect of Initial pH. The effect of pH on the dye-binding capacity of sorbents is an important factor. Figure 2 shows the biosorption capacity of ABTOC for RR45 dye at various pH values ranging from 1.0 to 10.0. RR45 biosorption capacities
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9717 Table 1. Elemental Analysis Results of Raw and Mixed Biosorbents elemental analysis
A. bisporus
T. orientalis
ABTOC
C (%) H (%) N (%) S (%)
40.97 6.53 5.25 0.25
49.97 6.34 1.41 0.12
45.99 6.46 3.64 0.18
were found as 42.27 and 41.35 mg g-1 at pH 2.0 for A. bisporus and T. orientalis, respectively (Figure 2). The difference between the biosorption capacities of these biosorbents may be due to the variations in their chemical compositions. The elemental analysis results in Table 1 showed that A. bisporus differs from T. orientalis especially in terms of carbon and nitrogen content. In the case of a combination of these biosorbents, the biosorption capacities of T. orientalis and A. bisporus slightly increased. The mixed biosorbent has a biosorption capacity of 49.87 mg g-1 at the same experimental conditions (pH 2.0). The most important advantage of the use of mixed biosorbent (ABTOC) is the low cost of the biosorption process. T. orientalis cones are widely available in large quantity. Hence, almost the same amount of RR45 pollution could be removed with a lower amount of A. bisporus in a mixture of raw materials. As seen from Figure 2 the biosorption capacity of ABTOC biomass was strongly pH dependent. Previous research revealed the necessity of lower pH values for maximum reactive dye biosorption.21-24 At strongly acidic conditions the surface of the biosorbent gets positively charged, thus providing attractive forces for negatively charged dye molecules. It was reported that, in particular, protonated amines or imidazoles, nitrogencontaining functional groups present on the biosorbent surfaces, are mainly the responsible sites for dye removal.25 Thus, electrostatic attractions between anionic dye and protonated amino groups on the biosorbent surface could be an effective mechanism for biosorption of RR45 onto ABTOC. As the pH increases, repulsive forces between anionic dye molecules and binding sites on the biosorbent increase due to the enhanced negative charge distribution on the biomass surface. In this study maximum biosorption was observed at pH 1.0 and 2.0. The biosorption capacity of mixed biosorbent lessened notably with an increase in pH up to 5.0, and at a pH above 5.0 there was a little uptake (Figure 2). Therefore, pH 2.0 was selected as the optimum pH in the following experiments. Although similar findings for reactive dye biosorption were reported in the literature,23,24 acidic pH conditions can be a limitation for application of real wastewater treatments. Vijayaraghavan and Yun (2008) stated that, if necessary, an appropriate effluent pretreatment should be performed prior to applying the biosorption process.26 In order to successfully apply the proposed biosorption process to real wastewater, costly pH adjustments may be necessary. 3.2. Effect of Biosorbent Dosage. Figure 3 shows the plot of biosorption capacity (q, mg g-1) and percentage color removal against the ABTOC dosage (g L-1) employed. It is clear from Figure 3 that biosorption yield increased from 26.63% to 93.04% as the biosorbent concentration was increased from 0.4 to 2.0 g L-1. However, the biosorbed RR45 per unit weight of ABTOC was reduced. This can be attributed to the surface area of biosorbents which was increased with increasing amount of biosorbent. A decrease in biosorption capacity with increasing concentration of biomass may be explained by several factors such as unsaturation of biosorption sites and particle interaction. Similar findings have also been obtained by other researchers.22-29 The further increase in biosorbent dosage from 2.0 g L-1 did not significantly improve biosorption due to establishment of an equilibrium between biosorbent and dye molecules.28 Thus,
Figure 3. Effect of biosorbent concentration on the biosorption of RR45 by ABTOC.
Figure 4. Time profiles for biosorption of RR45 by ABTOC at different temperatures.
the optimal biomass concentration was selected as 2.0 g L-1 for biosorption of RR45 onto ABTOC in batch system. 3.3. Equilibrium Time as a Function of Temperature. Biosorption studies were carried out for 60 min in order to evaluate the effect of time on biosorption of RR45 by ABTOC, and the time profiles as a function of temperature are shown in Figure 4. For the given temperatures the biosorption capacity of biomass increased linearly with time beginning, after which, the biosorption rates slowed. It was observed that about 92% of the dye molecules were removed in 50 min of contact at all temperatures examined. After this period biosorption of RR45 remained nearly constant, suggesting that the biosorption equilibrium was established in 50 min. At the beginning of the biosorption process the higher biosorption rate may have been due to an increase in dye binding vacant sites. This resulted in an increased concentration gradient between sorbate in the solution and at the biomass surface. Since biosorption of dye molecules onto vacant sites decreased during the later stages with an increase in contact time, this concentration gradient was reduced.30 The effect of temperature on the biosorption capacity of ABTOC was studied at 20, 30, and 40 °C (Figure 4). The results show that the biosorption equilibrium capacity increased from 46.76 to 51.13 mg g-1 with an increase in the temperature from 20 to 40 °C. The increasing equilibrium capacity with temperature suggests that biosorption of RR45
9718 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 2. Kinetic Parameters for the Biosorption of RR45 by ABTOC T (°C)
KL (min-1)
q1 (mg g-1)
r1 2
k2 (g mg-1 min-1)
q2 (mg g-1)
r2 2
kp (mg g-1 min-1/2)
C (mg g-1)
rp2
20 30 40
5.859 × 10-2 5.340 × 10-2 8.909 × 10-2
24.19 14.07 21.23
0.977 0.884 0.880
3.35 × 10-3 6.25 × 10-3 7.79 × 10-3
51.11 53.43 53.49
0.999 0.999 0.999
4.035 3.397 2.804
20.37 30.56 34.78
0.995 0.955 0.933
0.999 and greater than the other kinetic models studied, and the calculated equilibrium biosorption capacity values showed good agreement with those obtained experimentally. The biosorption capacity of ABTOC obtained from this model increased from 51.11 to 53.49 mg g-1 with an increase in the temperature from 20 to 40 °C. Therefore, biosorption of RR45 onto ABTOC tends to increasing temperature and is endothermic in nature. Biosorption kinetic data were further processed to investigate whether intraparticle diffusion is rate limiting. The intraparticle diffusion equation33 can be written as follows qt ) kpt1⁄2 + C
Figure 5. Pseudo-second-order kinetic plots for the biosorption of RR45 by ABTOC.
dye by ABTOC biomass was an endothermic process and dominant at higher temperatures. 3.4. Biosorption Kinetics. Sorption kinetics explain the factors affecting the biosorption process and predict the rate of biosorption. The nature of the biosorption process depends on the physical and chemical properties of biosorbent-sorbate system. The principle behind the biosorption kinetics includes the search for a best model that well represents the experimental data.30 In order to obtain rate constants, equilibrium biosorption capacity, and mechanism, the biosorption data were analyzed using three simplest kinetic models, pseudo-first-order, pseudosecond-order, and intraparticle diffusion kinetic models in the present investigation. The rate constants, equilibrium biosorption capacities, and r2 values of all kinetic models studied are represented in Table 2. The pseudo-first-order rate expression suggested by Lagergren is given in the following equation31 ln(qe - qt) ) ln qe - KLt
(1)
where qe and qt are the dye biosorbed at equilibrium and time t (mg g-1), respectively. KL is the rate constant for pseudofirst-order biosorption (min-1). A straight line of ln (qe - qt) vs t (figure not shown) suggests the applicability of this kinetic model, and KL and qe were determined from the slope and intercept of the plot. From the kinetic data in Table 2 it can be clearly seen that biosorption of RR45 onto ABTOC does not follow the pseudo-first-order kinetics. The biosorption kinetics can also be described by a pseudosecond-order rate equation (eq 2), and the model plots are given in Figure 5 1 1 t + t ) qt k q2 q2
(2)
2 2
where q2 is the maximum biosorption capacity (mg g-1) for the pseudo-second-order biosorption, qt is the amount of RR45 dye biosorbed at time t (mg g-1), and k2 is the equilibrium rate constant of pseudo-second-order biosorption (g mg-1 min-1). Values of k2 and q2 were calculated from the plot of t/qt against t.32 All correlation coefficients at different temperatures were
(3)
where C is the intercept and kp is the intraparticle diffusion rate constant (mg g-1 min-1/2). According to the Weber-Morris model, the plot of uptake, qt, versus the square root of time, t1/2 (figure not shown), should be linear if intraparticle diffusion is involved in the biosorption system and if these lines pass through the origin, then intraparticle diffusion is the ratecontrolling step.34-37 When the plots do not pass through the origin, this is indicative of some degree of boundary layer control, and this further indicates that intraparticle diffusion is not the only rate-limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously. Therefore, the slope of the linear portion of the figure is defined as rate parameter, kp, for the intraparticle diffusion and biosorption rate characteristic in this region where intraparticle diffusion is the rate-limiting factor.38 The correlation coefficients (rp2) for the intraparticle diffusion model are lower than that of the pseudo-secondorder model (Table 2), but this model indicates that biosorption of RR45 onto ABTOC may be followed by an intraparticle diffusion model up to 30 min. 3.5. Effect of Initial RR45 Concentration on the Biosorption and Biosorption Isotherms. The effect of initial dye concentration on the biosorption capacity of ABTOC for RR45 textile dye was studied between 75 and 400 mg L-1 at an initial pH of 2.0 and three different temperatures (figure not shown). The biosorption capacity of the biosorbent increased with initial concentration of dye and tends to saturation at higher dye concentrations. The biosorption yield was higher for low dye concentrations at all temperatures due to availability of vacant binding sites on the biosorbent surface. Decolorization yield reduced with an increase in the initial RR45 concentration because of nearly complete coverage of the biomass surface including active sites at high dye concentrations.25 The maximum biosorption capacities were found as 79.64, 104.19, and 108.90 mg g-1 at 20, 30, and 40 °C, respectively, at a 350 mg L-1 initial concentration of RR45. The temperature also affected the biosorption capacity, and biosorption was in favor of higher temperatures. The concentration of sorbate in the solid phase as a function of the concentration in the liquid phase of sorbate at equilibrium called “isotherm” and biosorption data are most commonly evaluated by the equilibrium isotherm.10 In this study three isotherm models were used to fit the experimental data obtained at different temperatures: Freundlich, Langmuir, and DubininRadushkevich (D-R) models, and the isotherm constants and r2 values are tabulated in Table 3.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9719 Table 3. Isotherm Constants for the Biosorption of RR45 by ABTOC Langmuir
Freundlich
Dubinin-Radushkevich (D-R)
T (°C)
qmax (mol g-1)
KL (L mol-1)
rL2
RL
n
KF (L g-1)
rF2
qmax (mol g-1)
β (mol2 kJ-2)
rD-R2
E (kJ mol-1)
20 30 40
1.03 × 10-4 1.29 × 10-4 1.39 × 10-4
1.70 × 105 1.89 × 105 2.15 × 105
0.996 0.986 0.995
0.270 0.288 0.309
1.10 × 10-2 9.97 × 10-3 8.77 × 10-3
6.16 × 10-4 7.21 × 10-4 8.11 × 10-4
0.813 0.908 0.885
2.26 × 10-4 2.72 × 10-4 2.99 × 10-4
1.31 × 10-3 1.22 × 10-3 1.19 × 10-3
0.861 0.930 0.914
17.02 18.29 18.34
Figure 6. Freundlich isotherm plots for biosorption of RR45 by ABTOC.
The Freundlich isotherm model is an empirical equation usually adopted for heterogeneous biosorption and frequently used to describe the biosorption.21 The linearized form of the Freundlich equation39 is ln qe ) ln KF + 1 ⁄ n ln Ce
(4)
-1
where KF (L g ) and n (dimensionless) are Freundlich sorption isotherm constants and magnitudes of these constants indicate the binding capacity has reached its highest value; the affinity between the biomass and dye molecules was also higher.30 Figure 6 shows the Freundlich isotherm plots for RR45 biosorption onto ABTOC at different temperatures, and the values of KF and n at the different temperatures were between 6.16 × 10-4-8.11 × 10-4 L g-1and 1.10 × 10-2-8.77 × 10-3, respectively. The Langmuir model describes monolayer biosorption onto a surface with a finite number of identical binding sites28 and was derived from gas-phase adsorption onto a homogeneous surface of glass and metals.10,23 The linear expression of the Langmuir isotherm model40 is
(
)
1 1 1 1 ) + qe qmax qmaxKL Ce
(5)
where qe and qmax are the equilibrium and monolayer biosorption capacities of the sorbent (mol g-1), respectively, Ce is the equilibrium RR45 concentration in solution (mol L-1), and KL is the biosorption equilibrium constant (L mol-1) related to the free energy of biosorption. qmax indicates a practical limiting biosorption capacity when all binding sites are occupied by dye molecules, and it assists in the comparison of biosorption performance.28 The Langmuir isotherm plots as a function of temperature are shown in Figure 7, and isotherm constants and r2 values are also included in Table 3. The Langmuir model provided a better correlation than the other isotherm models studied. It could be concluded that the biosorption process of RR45 onto ABTOC was a monolayer, and the maximum monolayer biosorption capacities were found to be between 1.03 × 10-4 (78.43 mg g-1) and 1.39 × 10-4 mol g-1 (105.85 mg
Figure 7. Langmuir isotherm plots for biosorption of RR45 by ABTOC.
g-1) at different temperatures. Biosorption results of various reactive dyes reported in the literature by different biosorbent materials together with operating conditions are summarized in Table 4. The biosorption capacity of ABTOC obtained for a reactive dye in our study was found to be comparable to and moderately higher than those of many corresponding sorbent materials. The essential feature of the Langmuir isotherm can be expressed by means of RL, a dimensionless constant referred to as a separation factor or equilibrium parameter, and RL is calculated using the following equation RL )
1 1 + KLCo
(6)
where KL is the Langmuir constant (L mol-1) and Co is the highest initial RR45 concentration (mol L-1). By the separation factor value, the value of the isotherm can be assessed by classification: RL > 1, unfavorable isotherm; RL ) 0, linear isotherm; 0 < RL < 1, favorable isotherm; RL < 0, irreversible isotherm.41,42 In this study the values of RL found between 0.270 and 0.309 indicate that the RR45 biosorption system under study can be considered to be a favorable condition. The Freundlich and Langmuir isotherm models do not give any idea about the mechanism of biosorption. In order to predict the biosorption mechanism the equilibrium data were tested with the Dubinin-Raduskevich isotherm model (D-R isotherm). The D-R model describes the biosorption nature of the sorbate on biosorbent, and it is used to calculate the mean free energy of biosorption. The characteristic biosorption curve is related to the porous structure of the biosorbent according to this model, and its linearized equation43 can be represented as ln qe ) ln qm - βε2
(7)
where ε ) RT ln (1 + 1/Ce) (Polanyi potential), qe the amount of RR45 biosorbed per unit weight of biosorbent at equilibrium (mg g-1), qm the biosorption capacity (mg g-1), Ce the equilibrium concentration of RR45 in aqueous solution (mg L-1), β the constant related to the biosorption energy, R the
9720 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 4. Biosorption Results of Different Reactive Dyes from the Literature by Various Biosorbent and Operating Conditions operating conditions biosorbent material
dye
pH
T (°C)
Lentinus sajur-caju (base treated) 15 LSIFB16 Phanerochaete chrysosporium16 CMC immobilized A. fumigatus 17 Aspergillus fetidus (NaOH treated) 18 Aspergillus fetidus 18 Laminaria sp. 23 Escherichia coli57 Escherichia coli57 Pseudomonas luteola57 Trametes Versicolor (autoclaved)58 Rhizopus nigricans59 Rhizopus nigricans59 Rhizopus nigricans59 Rhizopus nigricans59 ABTOC
RR120 RBBR RBBR RBR K-2BP RB5 RB5 RB5 RB5 RY2 RV2 RBMR RB8 RB9 RB3 RB38 RR45
3.0 2.0 2.0 2.0 2.3 2.3 1.0 3.0 3.0 3.0 2.0 6.0 6.0 6.0 6.0 2.0
25 30 30 45 50 50 40 28 28 28 27 29 29 29 29 40
a
( ( ( ( (
2 1 1 1 1
initial concentration range (mg L-1)
biosorbent dosage (g L-1)
biosorption capacity (mg g-1)
25-800 10-500 10-500 33.9-186.6 10-400 10-400 50-1000 200 200 200 20-50 50-350 50-350 50-350 50-350 75-400
1.0
57.20 101.06 85.21 41.10 106.40 76.00 101.5 89.40 52.40 96.40 51.55 122 112 123 161 105.85a 108.90b
2.0 2.0 2.0 2.5
20 10 10 10 10 2.0
qmax value obtained from the Langmuir isotherm model. b Experimental qmax.
gas constant, and T the absolute temperature (K). D-R isotherm constants β and qm can be calculated from the slope and intercept of the plot of ln qe against 2, respectively (Figure 8), which are illustrated in Table 3. The mean free energy of biosorption (E), defined as the free energy change when 1 mol of ion is transferred to the surface of the solid from infinity in solution, can be calculated from the β value obtained from the above equation44 E ) 1 ⁄ (2β)1⁄2 (8) From the magnitude of E, the type of biosorption such as chemisorption or physical sorption can be determined. If E ) 8-16 kJ mol-1, then the reaction is due to the chemical ion exchange. If E < 8 kJ mol-1, then biosorption takes place physically. The mean free energy of biosorption (E) is found between 17.02 and 18.34 kJ mol-1 at different temperatures, which implies that biosorption of RR45 on ABTOC is purely by chemical biosorption. 3.6. Thermodynamic Analysis of Biosorption. The values of thermodynamic parameters are relevant for the practical application of biosorption process.45 Isotherm data for biosorption of RR45 dye by ABTOC were analyzed to obtain thermodynamic parameters such as the changes in free energy (∆Go), enthalpy (∆Ho), and entropy (∆So) associated with the biosorption process. For this purpose KL, the Langmuir constant, was used in the following equations ∆Go ) -RTln KL o
o
(9) o
∆H ∆S ∆G )+ (10) RT RT R The plot of ln KL as a function of 1/T yields a straight line from which ∆Ho and ∆So were calculated from the slope and intercept, respectively. The plot is presented in Figure 9, and the calculated thermodynamic parameters are given in Table 5. ∆Go values of -38.28, -39.59, and -40.89 kJ mol-1 were obtained at 20, 30, and 40 °C, respectively, and thus, RR45 biosorption increased at higher temperatures. Negative values of ∆Go indicate the biosorption process was spontaneous in nature and confirm the affinity of the biosorbent toward the sorbate. Since the ∆Ho value was found to be 8.95 kJ mol-1, the biosorption process was endothermic and thus favored by an increase in temperature as this will activate the sorption sites.30 The positive entropy value (130.62 J K-1mol-1) indicates the increasing randomness at the solid/liquid interface ln KL ) -
Figure 8. D-R isotherm plots for biosorption of RR45 by ABTOC.
during the biosorption process46 and suggests good affinity of the dye toward the adsorbent.47 3.7. Continuous System Studies. Although batch biosorption results give the fundamental information related to the dye biosorption performance of biosorbent,30 a continuous mode of operation was preferred in most industrial wastewater treatment units.48 Hence, up-flow packed column experiments were also carried out. 3.7.1. Effect of Flow Rate. In the first stage of column studies the effect of flow rate on the biosorption capacity of biosorbent was investigated by keeping initial dye concentration (100 mg L-1), pH (2.0), and biosorbent concentration (2.0 g L-1) constant and varying the flow rate from 0.5 to 6 mL min-1 (Figure 10). A 50 mL amount of 100 mg L-1 RR45 solution was completely passed through the column with a 9 mm i.d. and 100 mm height, and effluent was analyzed for unbiosorbed RR45 concentration. The flow rate strongly influenced the RR45 uptake capacity, and as the flow rate increased from 0.5 to 6 mL min-1 the biosorption capacity of biomass decreased from 52.31 to 29.21 mg g-1. This behavior can be explained by Vijayaraghavan et al. (2004) in two ways: (1) the reducing residence time of solute in a column at higher flow rates which causes the dye solution to pass the column before equilibrium establishes; (2) if intraparticle mass transfer controlled the process, a slower flow rate favors biosorption, and in an external mass transfer controlled process a higher flow rate decreases film resistance.49 Therefore, a lower flow rate or longer contact
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9721
Figure 9. Plot of ln KL vs 1/T for estimation of thermodynamic parameters. Figure 11. Effect of biomass dosage on biosorption of RR45 by ABTOC in a continuous system. Table 5. Thermodynamic Parameters for Biosorption of RR45 by ABTOC T (°C)
∆G° (kJ mol-1)
∆H° (kJ mol-1)
∆S° (J K-1 mol-1)
20 30 40
-38.28 -39.59 -40.89
8.95
130.62
time may be required for effective biosorption of RR45, and an optimal flow rate was selected as 0.5 mL min-1 in this study. 3.7.2. Effect of Biosorbent Dosage (Bed Height). The effect of biosorbent dosage or bed height on biosorption of RR45 by ABTOC was investigated in continuous mode, and the results are illustrated in Figure 11. The amount of biosorbent loading into column was changed from 0.4 to 4.0 g L-1, and inlet RR45 concentration, column i.d., and flow rate were kept constant at 100 mg L-1, 9.0 mm, and 0.5 mL min-1. As the biosorbent amount in the column was increased, the biosorption yield was enhanced as a result of an increased bed height. This trend was expected since the biosorption performance of biosorbents usually depends on available sorbent amount for biosorption.50 As shown in Figure 11, biosorption yield of RR45 was enhanced from 9.06% to 100% with an increase in the amount of biomass from 0.4 to 3.0 g L-1. The higher biosorption yields for higher bed heights due to the increase in the surface area of biosorbent providing more binding sites for biosorption.51 After a definite amount of biomass the biosorption yield did not change since
Figure 10. Effect of flow rate on biosorption of RR45 by ABTOC in a continuous system.
Figure 12. Effect of column i.d. on biosorption of RR45 by ABTOC.
the biosorbent surface was saturated with dye molecules. Therefore, 3.0 g L-1 was selected as the optimum dosage of biomass for further column studies. 3.7.3. Effect of Column Size. In order to evaluate the effect of column size on the biosorption performance, column i.d. was varied from 9 to 19 mm and the inlet RR45 concentration, column height, amount of biomass, and flow rate were kept constant at 100 mg L-1, 100 mm, 0.15 g, and 0.5 mL min-1, respectively. The biosorption capacity of ABTOC versus column i.d. was plotted in Figure 12. The RR45 biosorption capacity of ABTOC lessened from 35.05 to 24.53 mg g-1 when the column i.d. was increased from 9 to 19 mm. As the column i.d. increases the bed height decreased because the amount of loading biomass into the column was kept constant. The bed height is the most important parameter for continuous biosorption systems as discussed above. Therefore, the maximum biosorption capacity was obtained in a column with the lowest i.d. 3.8. Effect of Metals. In order to investigate the effect of metal ions on dye removal potential of ABTOC biosorption studies were also carried out in a dye-metal mixture. 95.68% of RR45 was removed from this mixture, and the biosorption capacity was 49.90 mg g-1. It was seen that the presence of metal ions does not significantly inhibit RR45 removal by ABTOC at pH 2.0. This result may be explained by the pH value of 2.0 had a selective effect for removal of dye in the
9722 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 6. Characteristic Absorption Bands of Biosorbent suggested assignment
frequency (cm-1)
-OH and/or -NH2 stretching -CH2-symmetric stretching -CH2-asymmetric stretching -CdN streching -Amid-I band -C-O stretching -CH2 bending vibrations -OH bending vibrations -SO3 group -PdO stretching -P-OH stretching
3429 2923 2852 1697 1632 1452 1383 1262 1236 1156 1030
mixture of RR45 and metal ions. Similar observations were reported by other researchers.52,53 3.9. Biosorbent Characterization. In order to elucidate the nature of the possible dye-biosorbent interaction the FTIR spectra of unloaded and dye-loaded ABTOC were taken, and the characteristic absorption bands of raw biosorbent are presented in Table 6. The FTIR spectrum (figure not shown) of unloaded biomass shows the complex structure of biosorbent and indicates the presence of several functional groups on the biomass surface such as amine, phosphate, sulfonate, etc. The FTIR spectrum of dye-loaded ABTOC indicated no significant changes in the absorption bands of unloaded ABTOC with the exception of a frequency shift at 1697 (indicative of -CdN- streching), 1262 (indicative of -OH bending vibrations), and 1236 (-SO3 groups) cm-1. These functional groups are likely to be responsible for dye binding. Also, a new peak at 1535 cm-1 (indicative of an amidII band),54 which was absent on the surface of unloaded biosorbent, was revealed in the FTIR spectra of dye-loaded ABTOC. This may be attributed to biosorption of RR45 molecules onto the biomass surface. Similar dye-biosorbent interactions were reported by other researchers.55,56 4. Conclusions In this research the biosorption performance of the new and low-cost biomaterial (ABTOC) was investigated for removal of a reactive dye (RR45) from contaminated solutions in a batch and continuous packed bed column. The biosorption reaction was found to be pH, biomass dosage, agitation time, initial dye concentration, and temperature dependent. Under optimized conditions the removal efficiency reached 93.04% in a relatively short operating time for batch process. Biosorption kinetics is well described by the pseudo-second-order model. Thermodynamic parameters indicated that this process is spontaneous and endothermic in nature. The fitness of the biosorption data into the Langmuir model confirmed the monolayer coverage of dye molecules onto biomass. The possible dye-biosorbent interactions were evaluated by FTIR analysis. In addition, ABTOC was successfully used for dye removal from metal ion-rich synthetic solution. Since this investigation was carried out on a laboratory scale, the operative conditions such as pH, contact time, and biomass dosage may be far from the real parameters of wastewaters. However, the mixed biosorbent developed in this study can be regarded as a promising candidate for biosorption of RR45 dye due to the fact that it is a low-cost, abundant, and effective biosorbent. Literature Cited (1) Hao, O. J.; Kim, H.; Chiang, P. C. Decolorization of Wastewater. Crit. ReV. EnViron. Sci. Technol. 2000, 30, 449.
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ReceiVed for reView May 16, 2008 ReVised manuscript receiVed July 24, 2008 Accepted September 22, 2008 IE8007874
