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In the present study, modified brown seaweeds, Sargassum binderi, was used as biosorbent for the removal of acid orange 8 (AO8) and methyl orange (MO) from aqueous solutions. Different type of chemically modified seaweeds and initial dye concentration (75-500 mg/L) was studied in the batch biosorption process. The results revealed that the protonation of S. binderi with 1.0 M hydrochloric acid (HCl) gave relatively high and comparable percentage of dye removal as compared to other treatments. The ability of HCl treated S. binderi in dye removal was dependent on the initial dye concentration. The dye uptake was found to decrease with an increase in initial dye concentration of AO8. Percentage of MO removal at equilibrium increased slightly from 75 to 200 mg/L, but decreased drastically from 200 to 500 mg/L. Biosorption of AO8 and MO by modified S. binderi were best described by the Langmuir adsorption isotherm model, in which the process can be concluded as monolayer sorption process of dye ions onto seaweeds. Biosorption kinetics of AO8 and MO onto modified S. binderi were best described by a pseudo-second-order model which indicated the occurrence of chemisorptions occur in the system. FTIR analysis showed that the dyes were grafted on the biomass surface via interactions with the hydroxyl and other groups on the seaweeds.
Keywords: Dye, Biosorption, Sargassum sp., Kinetic, Isotherm
Dye wastewater generated from textile and associated industry is the largest portion of the total wastewater. Acid dyes such as acid orange 8 and methyl orange are the largest class of dyes used in the world. Acid dyes are anionic compounds that are mainly used for dyeing nitrogen containing fabrics like wool, polyamine and silk (Somasiri, Ruan, Li & Jian, 2006). Removing colour from the wastewater is important because very low concentration of dyes is clearly visible and can affect the water environment considerably (Habibi, Hassanzadeh & Mahdavi, 2005). Dye wastewater discharge in effluent without treatment will affects the aquatic flora and fauna and causes many water borne diseases. Dyes can also cause allergic dermatitis and skin irritation (Somasiri et al., 2006). Some of dyes are proven as carcinogen and mutagen. Therefore, the proper treatment technologies have appeared of importance for ensuring healthy environment.
Sargassum seaweeds used as low cost biosorbent in this study consist of high alginic acid content which contributes to the binding capability towards dye compound in solution (Tan, Wong, Ong & Hii, 2009). The aim of the modification of seaweed by various chemical is to enhance the sorption capacities of seaweeds toward the anionic dyes and thus improve the treatment processes. There are various pre-treatments used such as heat inactivated, acid, base and otherwise chemically treated have been used to remove hazardous pollutant from aqueous medium (Ozer, Akkaya & Turabik, 2005). In this study, the adsorption capacity of natural and chemical modified of seaweed toward acid dye AO8 and MO present in aqueous solution was studied.
MATERIALS AND METHODS
Collection of Seaweed: S. binderi were collected from Cape Rachado at Port Dickson, Negeri Sembilan, Malaysia. Seaweeds were cleaned thoroughly under the running tapped water for several times to remove dirt, epiphytes and sand particles. Then, it was rinsed with distilled water and dried in the oven at 50°C for 24 hours. Seaweeds were modified by using different types of chemical in order to enhance the sorption capacities of seaweeds toward the dyes. The type of modified seaweed which possessed higher percentage of uptake was chosen for subsequent study.
Modification of Seaweed: Modification was carried out by treating the natural seaweeds with 0.1 M sodium hydroxide (NaOH), 1.2 M citric acid (C6H8O7), 1.2 M nitrilotriacetic acids (C6H9NO6), 0.1 M and 1 M hydrochloric acid (HCl) solution, respectively. The mixture was then filtered and washed with distilled water until neutral and dried in the oven overnight at 50°C. The modified seaweed was labeled as base treated seaweed (BS), citric acid treated seaweed (CAS), nitriloacetic acid treated seaweed (NTAS) and hydrochloric acid treated seaweed (1.0 &0.1 M AS).
Preparation of Dye Solution: Acid orange 8 (C17H14N2NaO4S, abbreviated as AO8) and methyl orange (C14H14N3NaO3S, abbreviated as MO) were selected as the key sorbates in this study. Stock solution of the two dyes with concentration of 1000 mg/L was prepared by dissolving 1 g of dye powders in 1000 mL of distilled water. Dye solutions for AO8 and MO of different concentrations were prepared by adequate dilution on the stock solution with distilled water when necessary.
Figure 1: Structure of AO8 (Sigma Aldrich) Figure 2: Structure of MO (Sigma Aldrich)
Batch Biosorption Experiment: In this study, the biosorption experiments were carried out in an orbital shaker (LabTech) at a constant speed of 130 rpm at 30°C using 250 mL conical flask. The flasks contain 1.0 g of dried seaweed in 100 mL dye solution with 100 mg/L of concentration was placed in the shaker unless otherwise stated. A flask with only seaweed and distilled water was used as control. In the sorption experiment, samples were withdrawn from the flasks at pre-determined time intervals to measure the intensity of dye. The remaining dye concentrations in the solution were estimated by measuring the absorbance at maximum wavelength of dye using Ratio Beam UV-VIS spectrophotometer (Hitachi U-1800). All the experiments were carried out in triplicates and the results are presented in the percentage uptake and sorption capacity. The percentage removal of dye in this biosorption experiment was calculated using Eq. 1:
where Ci and Ce (mg/L) are initial and equilibrium concentrations. The amount of dye adsorbed on the seaweed at equilibrium was calculated from the mass balance of the equation as given below:
where qe (mg/g) is equilibrium dye concentration at any time, M (g) is the mass of the seaweed used and V (L) is volume of the dye solution.
Fourier Transform Infrared (FT-IR) Spectroscopy Technique: In this study, Fourier Transform Infra Red (FT-IR) Spectroscopy (Perkin-Elmer Inc., United States of America) was used to observe the changes in functional groups of the natural seaweeds, and chemical modified seaweeds before and after absorption. The dried seaweeds were first ground in an agate mortar and pestle into powder form, and mixed with KBr powder by the ratio of 1:10 in the sample disk. The mixture was pressed at 20 tonnes for 3 to 4 minutes to form the pellets. The background obtained from scan of pure KBr was automatically subtracted from the sample spectra. All the spectra obtained were plotted using the same scale on the absorbance axis in the range of 400-4000 cm-1.
RESULTS AND DISCUSSION
Dyes Removal (AO8 and MO) by Using Natural S. binderi: Figure 3 showed the natural S. binderi exhibited low affinity for AO8 and MO. Negative carboxyl and phosphonate groups which are redundant in the seaweed would repeal the AO8 and MO dye which contain negatively charged sulphonic groups (Yun & Volesky, 2003). Thus, S. binderi did not have the ability in removing the anionic dye (Yun & Volesky, 2003) despite of its high adsorption ability toward cationic dye (Tan et al., 2009).
Figure 3: Comparison of AO8 and MO removal using S. binderi
Effect of Modified S. binderi on uptake of AO8 & MO: Figure 4 and 5 showed that 1.0 M AS was the most efficient in removal of AO8 from solution while BS had the lowest biosorption capacity compare with the other type of modified seaweeds. Figure 4 showed approximately 80% of AO8 uptake by 1.0 M AS recorded at equilibrium state. Besides, Figure 5 indicated that the modified seaweed with 1.0 M AS showed maximum increase on biosorption of AO8 by approximately four times higher in comparison with BS. Figure 6 and 7 showed the high in uptaking of MO (60%) as compared to other types of modified seaweed. The three acid treated seaweeds showed approximately 30-50% of AO8 and MO uptake. The removal of MO by using BS was considered as zero. The result showed that the modified biomass has greater dye affinity compared to its natural form except for the BS.
The acidic groups of the seaweed's polysaccharides are converted to the hydrogen form after acid treatment (Hartley et al., 2007) and the amine groups (-NH2) mainly found in protein molecules in the biomass (Fu & Viraraghavan, 2001) can be protonated as a form of -NH3+. Such positively charged groups are likely the binding sites for negatively charged dye solutions. The concentration of H+ ions was reduced during the base modification in the seaweeds and negatively charged biomass surface cannot interact with the negatively charged dye ions. This was likely to occur because NaOH would contribute hydroxide (OH-) ions during the base treatment which repulse the negatively charged dye molecules. The adsorbability of modified seaweeds on the removal of AO8 followed the sequence: 1.0 M AS > 0.1 M AS > NTAS > CAS > BS. On the other hand, the effect of modified seaweeds on the removal of MO followed the sequence: 1.0 M AS > 0.1 M AS > CAS > NTAS > BS. Hydrochloric acid is known as strong mineral acid while citric acid and nitrilotriacetic acid are weak acid in solution. The more acidic of chemical to modify the seaweed, the more positively charged present onto the seaweed surface (Hartley et al., 2007). Thus, 1.0 M AS was the most efficient in removing the AO8 and MO from the solution and chosen for the subsequent studies.
As shown in Figure 5 and 7, 1.0 M AS treated seaweed was more efficient on removal of AO8 (7.921 mg/g) from the solution as compared to MO (5.92 mg/g). Thus, the adsorbability of acid dyes by modified seaweed depends on the dye property. Although AO8 has larger molecular mass than MO (Figure 1 and 2), the rate of sorption was higher for the former. This may be due to the difference in the solubility in water of the two dyes since both of the dyes contain same functional group which has a sulphonic group and an azo group. AO8 was highly soluble in water while MO was soluble in hot water only (ChemicalLAND21.com, 2008). The high solubility of AO8 in water hence increased the degree of fixation between the dye and the modified biomass even though AO8 was less advantageous in its molecular size.
Figure 4: Different Type of Modified seaweed on uptake of AO8.
[Symbols: (â™¦) - BS; (â-²) - CAS; (â- ) - NTAS; (Ã-) - 0.1 M AS; (â-) - 1.0 M AS]
Figure 5: Effect of modified seaweeds on the removal of AO8.
Figure 6: Different type of modified seaweed on uptake of MO.
[Symbols: (â™¦) - BS; (Ã-) - CAS; (â-²) - NTAS; (Ð-) - 0.1 M AS; (â- ) - 1.0 M AS]
Figure 7: Effect of modified seaweeds on the removal of MO.
Effect of Initial Dye Concentration of AO8: Figure 8 showed the initial concentration of AO8 increased from 75 to 200 mg/L, the equilibrium sorption capacity increased from 6.154 to 13.641 mg of AO8 per gram of modified seaweed due to increased in the number of ions competing for the available binding sites in the biomass. The uptake of AO8 by the seaweed slightly decreased at 200-500 mg/L showing the saturation of binding sites at higher concentration levels (Basha et al., 2008). Figure 9 showed the percentage of AO8 removal at equilibrium decreased from 82.18% to 26.31% with increasing of initial dye concentration from 75 mg/L to 500 mg/L. It was clear that the removal of dye was dependent on the concentration of dye. This was due to limitation of the adsorbent mass where the binding site of adsorbent or solute ratio was smaller (Azira, Wong, Robiah & Chuah, 2004). Moreover, the solution with higher initial concentration required a longer the time to remove the dye (Marungrueng & Pavasant, 2006).
Effect of Initial Dye Concentration of MO: Figure 10 showed the initial concentration of MO increased from 75 to 500 mg/L, the equilibrium uptake capacity of 1.0 M AS increased from 4.171 to 15.994 mg of MO per gram of modified seaweed. High initial concentration provided increase driving force to overcome all mass transfer resistance of dye ions between the aqueous and solid phases resulting in higher probability of collision between MO and modified seaweed (Basha et al., 2008). Hence, a higher initial concentration of dye may enhance the adsorption process. Figure 11 showed the percentage of dye removal at equilibrium increased slightly from 75 to 200 mg/L, and decreased drastically from 200 to 500 mg/L. At low initial dye concentrations, the removal process occurred very fast owing to the large difference in concentration between the biosorbent surface and solution (Ozer et al., 2005). The solution with higher initial concentration required longer time to remove the dye (Marungrueng & Pavasant, 2006). Increased in percentage of removal with low initial concentration might be due to higher availability of dye ions in the solution. This also might indicated exist of reductions in immediate solute adsorption from 200 to 500 mg/L, owing to lack of available active sites or saturation of the sorption sites on the adsorbent required for high initial dye concentration of MO (Eren & Acar, 2006).
Figure 8: Sorption capacities of S. binderi at equilibrium in different dye concentrations of AO8.
Figure 9: Percentage uptake of AO8 at equilibrium in various initial dye concentrations.
Figure 10: Sorption capacities of modified S. binderi at equilibrium in different dye concentration of MO.
Figure 11: Percentage uptake of MO at equilibrium in various initial dye concentrations.
Adsorption Isotherm Analysis: In this study, both Langmuir's and Freundlich's absorption isotherm equilibrium models were used for the analysis of the algal-dye sorption system. The linearised form of Langmuir isotherm (Langmuir, 1917) was used to characterise the adsorption process of AO8 and MO onto S. binderi.
Where Ce is equilibrium concentration, qe is amount of dye absorbed (mg/g) at equilibrium, qmax is the maximum of Langmuir monolayer adsorption capacity and b is Langmuir constant.
The Freundlich isotherm (Freundlich, 1906) is the earliest known relationship describing the sorption equation. The linearised form of Freundlich isotherm is shown in Eq. 4:
Where Ce is equilibrium concentration, qe is amount of dye absorbed (mg/g) at equilibrium, Kf is Freundlich constant and 1/n is exponential constant.
The Langmuir biosorption isotherm models showed better fit to the data at different initial dye concentration of AO8 and MO with a correlation coefficient value (R² = 0.937 and 0.942) as compared to Freundlich correlation coefficient (R² = 0.649 and 0.895) (Table 1). Langmuir model predicts that maximum adsorption occurs when a saturated monolayer of dye molecules is present on the modified seaweed surface, and the energy of adsorption is constant and there is no migration of absorbate molecules in the surface plane of the seaweed (Ozacar & Sengil, 2005). The maximum Langmuir monolayer adsorption capacities of AO8 and MO were estimated at 16.129 and 17.241 mg/g, respectively. Adsorption capacities for the dyes determined by qmax parameter of Langmuir model or Kf parameter of Freundlich model followed the same order which was MO > AO8 (17.241 > 16.129 for qmax; 5.260 > 4.875 for Kf). This order could be correlated with the sizes of dye molecules as well as the pore size distribution of modified seaweed used. Visually, it was obvious from the molecular structures or molecular mass given in Figure 1 and Figure 2 that the size of AO8 is significantly larger than MO. In the present adsorption systems, Freundlich constants (nf) values are 5.155 for AO8 and 5.208 for MO which are between 1 and 10, represent a beneficial biosorption process (Vadivelan & Kumar, 2005). In this case, it indicated that adsorption intensity is favourable over the entire range of concentrations studied and the heterogeneity of the surface binding site increased with increasing initial concentration. The magnitude of Freundlich adsorption capacity (Kf) was found to be 4.875 and 5.260 for AO8 and MO, respectively. The Kf value showed the dye ions were easy separation from aqueous medium (Ahalya, Kanamadi & Ramachandra, 2005).
Table 1: The parameters of Langmuir and Freundlich isotherm equations for the adsorption of dyes
Adsorption Kinetics and Modelling: Kinetic adsorption data of AO8 and MO by S. binderi at various initial dye concentrations were treated with pseudo-first-order (Eq. 5) and pseudo-second-order kinetic models (Eq. 6).
A linear form of pseudo-first-order model is:
A linear form of pseudo-second-order model (Ho & McKay, 2000) is:
Where qe is amount of dye adsorbed (mg/g) at the equilibrium, q is amount of dye uptake (mg/g) at time t (min) and k1 is rate constant of pseudo-first-order biosorption, k2 is equilibrium rate constant pseudo-second-order (g/ mg min) and h (k2qe2) = the initial sorption rate.
Table 2 and 3 showed the correlation coefficients (R2) values obtained from both kinetic models of AO8 and MO adsorption were close to 1, which reflecting a compatible relation with experimental data. However, the experimental qe, values agreed well with the calculated qe for pseudo-second-order kinetics model for both of the dyes tested. This showed that the pseudo-second-order kinetic model was found to be more suitable than the pseudo-first-order kinetic model for AO8 and MO. The experimental qe values increased when the initial concentration of dye increased (Table 2 and 3), indicating the dye removal is dependent on initial concentration. Adsorption of AO8 and MO onto modified S. binderi was a chemisorption process which required exchange or sharing of electrons between dye cations and functional groups of adsorbent (Chandra, Mirna, Sudaryanto & Ismadji, 2007). Karadag (2007) mentioned that pseudo-second-order model provided the best correlation of the experimental data on the adsorption of acid orange 8 onto surfactant modified clinoptilolite. The results obtained by Ni et al. (2007) showed the same phenomenon that the MO adsorption by calcined layered double hydroxides (LDO) with Zn/Al was fitted best by the pseudo-second-order model.
Table 2: Pseudo-first-order and Pseudo-second-order rate constants at different initial dye concentrations of AO8
Pseudo-first-order rate constants
Pseudo-second-order rate constants
7.221 x 10-3
3.830 x 10-2
4.597 x 10-3
3.379 x 10-3
3.255 x 10-3
2.493 x 10-3
3.014 x 10-4
2.144 x 10-3
Notes: qe, exp - experimental qe values
qe, cal - calculated qe values based on model
Table 3: Pseudo-first-order and Pseudo-second-order rate constants at different initial dye concentrations of MO
Pseudo-first-order rate constants
Pseudo-second-order rate constants
7.357 x 10-3
3.502 x 10-3
6.202 x 10-3
2.686 x 10-3
3.987 x 10-3
Notes: qe, exp - experimental qe values
qe, cal - calculated qe values based on model
Fourier Transform Infrared (FT-IR) Spectroscopy Analysis: The changes in functional groups of S. binderi before and after adsorption were interpreted using FT-IR technique in the range of 400 - 4000 cm-1 to obtain information on the nature of cell wall and dye interaction. Table 4 showed the FT-IR spectrum of pure seaweed exhibited the broad band at 3500 - 3400 cmâˆ’1 indicated the presence of hydroxyl group (-OH) stretching vibrations, 2925 - 2927 cm-1 for carboxyl group (-COOH), 1637 - 1640 cm-1 for C-O stretching, 601-619 cm-1 is recognised as S-O stretching (Skoog & Leary, 1992). A new peak at around 1163 and 1162 cmâˆ’1 was indicated from the AO8 and MO attached modified seaweeds, respectively, was characteristic of sulfite (S=O stretching) (Rathinam, Jonnalagadda & Balachandran, 2007).
The seaweed showed a peak at around 1739-1738 cm-1 after acid modification, which could be attributed to biosorption of HCl on the modified seaweed (Rathinam et al., 2009). According to Huang and Huang (1996), acid treated biomass contained a higher percentage of surface nitrogen. This indicated that acid treatment might dissolve polysaccharide compounds in the outer layer of the cell wall and therefore produce additional binding sites. Therefore, the adsorptive capacities of the pretreated biosorbents were greater than the natural form as shown in the present study. After the AO8 and MO had been adsorpted onto the modified seaweed, the spectrum showed some changes, with new peaks presented at 1162 and 1163 cmâˆ’1, respectively, characteristic of sulfite (S=O stretching). The structural formulae (Figure 1 and 2) of both of the acid dyes in this study showed the presence of the sulfonic acid groups which has S=O functional group. This provided evidence that the dyes molecules were indeed adsorbed onto the modified biomass surface (Low et al., 2007). FT-IR results confirmed the chemisorption process between dye molecules and functional groups of S. binderi.
Table 4: Changes in the functional groups in the dried pure and modified S. binderi before and after AO8 and MO adsorption.
Standard Wavenumber (Skoog &Leary, 1992)
Wavenumber from the pure alga (cm-1) (max. values)
Wavenumber from the modified alga (cm-1) (max. values)
Wavenumber from the AO8 and MO attached onto modified alga, recpectively (cm-1) (max. values)
3250 - 3700
2400 - 3300
1670 - 1780
1040 - 1200
550 - 650
Figure 12: FT-IR spectrum of (1) 1.0 M HCl Treated Seaweed, (2) Natural
Seaweed, (3) 1.0 M HCl Treated Seaweed with MO, (4) 1.0 M HCl Treated Seaweed with AO8.
The present study showed that S. binderi modified by 1.0 M HCl can improve removal of acid dye AO8 and MO from aqueous solutions. 1.0 M HCl modified S. binderi was able to remove AO8 (80%) better than MO (60%). The adsorption of lead ions was highly dependent on initial dye concentration. Adsorption isotherms of AO8 and MO were well described by the Langmuir model and the process could be concluded as monolayer sorption process of dye ions onto seaweed. The adsorption kinetics of AO8 and MO fitted well to pseudo-second-order kinetic model which indicated the chemisorptions occur in the system. FTIR analysis showed that the dyes were grafted on the biomass surface via interactions with the hydroxyl and other groups on the seaweeds.