In this study, durian (Durio zibethinus Murray) skin was examined for its ability to remove methylene blue (MB) dye from simulated textile wastewater. Adsorption equilibrium and kinetics of MB removal from aqueous solutions at different parametric conditions such as different initial concentrations (2–10 mg/L), biosorbent dosages (0.3–0.7 g) and pH solution (4–9) onto durian skin were studied using batch adsorption. The amount of MB adsorbed increased from 3.45 to 17.31 mg/g with the increase in initial concentration of MB dye; whereas biosorbent dosage increased from 1.08 to 2.47 mg/g. Maximum dye adsorption capacity of the durian skin was found to increase from 3.78 to 6.40 mg/g, with increasing solution pH. Equilibrium isotherm data were analyzed according to Langmuir and Freundlich isotherm models. The sorption equilibrium was best described by the Freundlich isotherm model with maximum adsorption capacity of 7.23 mg/g and this was due to the heterogeneous nature of the durian skin surface. Kinetic studies indicated that the sorption of MB dye tended to follow the pseudo second-order kinetic model with promising correlation of 0.9836 < R2 < 0.9918.

INTRODUCTION

The increasing use of dyes in the textile industry has been one of the main contributors to the environmental pollution. Over 7 × 105 metric tons of approximately 10,000 synthetic dyes and pigments are produced annually per year (Cervantes et al. 2001; Wawrzkiewicz & Hubicki 2009). In addition, the presence of small amounts of dyes as coloring agents in water are undesirable because they are highly visible and at the same time might significantly affect the process of photosynthesis in aquatic life where the light penetration is low (Hartono et al. 2005; Jumasiah et al. 2005; Crini 2006; Gurses et al. 2006). Meanwhile, Nam & Renganathan (2000) reported that these dyes have become a great concern in effluent treatment due to their color and potential toxicity to human and animals. Dyes might cause toxicity to aquatic life and severe damage to human beings, for instance dysfunction of the kidneys, liver, brain, central nervous system and reproductive system and may also be mutagenic and carcinogenic (Kadirvelu et al. 2003; Dinçer et al. 2007; Shen et al. 2009).

Methods that are most widely used for the removal of dyes from wastewater systems are physicochemical, chemical and biological methods, such as coagulation (Tan et al. 2000), filtration (Zouboulis et al. 2002), advanced oxidation processes (Esplugas et al. 2002), solvent extraction (Lin & Juang 2002), cation exchange membrance (Wu et al. 2009), adsorption/precipitation (Gupta et al. 1998, 2011a; Jain et al. 2003a, b; Zhu et al. 2007), photocatalytic degradation (Sleiman et al. 2007; Sohrabi & Ghavami 2008; Gupta et al. 2011b, 2012a, b; Karthikeyan et al. 2012; Saleh & Gupta 2012a, b) and electrochemical degradation (Fan et al. 2008; Khani et al. 2010). Among these methods, adsorption has been shown to be the most effective process for the removal of dyes from waste effluents due to its high potential of removing different types of coloring materials from wastewater (Jain et al. 2003a, b; Mittal et al. 2009a, b, 2010a, b; Gupta & Nayak 2012).

Numerous studies were done with the use of agricultural wastes as low-cost adsorbent for removing the dyes, such as coconut husk (Tan et al. 2008), wheat straw (Wu et al. 2009), wheat husk (Bulut & Aydin 2006; Gupta et al. 2007), coir pitch (Namasivayam et al. 2001), garlic peel (Hameed & Ahmad 2009), olive wastes (Nyazi et al. 2005), almonds (Christopher & Wayne 2002) and orange peel (Sivaraj et al. 2001). The agricultural wastes are inexpensive, available in abundance, and mainly consist of lignin, cellulose and hemicelluloses, which together act as an effective adsorbent for a wide range of pollutants because their functional groups such as hydroxyl, carboxyl, phenols, and methoxy participate in binding with the pollutants (Anisuzzaman et al. 2015; Joseph et al. 2014).

Durian (Durio zibethinus Murray) is the king of tropical fruit in Southeast Asia. It is a member of the family Bombacaceae (Polderdijk & Van Der Valk 2002). According to the statistical data reported by the Ministry of Agricultural and Agro-Based Industry Malaysia, the local production of durian fruit in 2011 was projected at 300,470 metric ton, translating to approximately 255,353 metric ton of durian shells as byproducts. Durian waste would be viable as an adsorbent as durian rind contains high fiber content. However, only one-third of the durian is eatable, whereas the seeds and the shell are usually thrown away.

Therefore, in this study, durian skin was used as an alternative sorbent in order to determine its effect on the removal of methylene blue (MB) from simulated wastewater, with the view of replacing activated carbon (AC) for water treatment in industrial fields. Durian skins are being proposed in this study without complex and costly pretreatment steps and an activation process for wastewater treatment. Suitability of durian skin as MB adsorbent in aqueous solutions with parametric variables can be illustrated by the different initial dye concentrations, biosorbent dosages and pH of the solution. The MB dye adsorption process was thoroughly studied from both kinetic and equilibrium conditions.

MATERIALS AND METHODS

Adsorbate

MB (C16H18N3SCI) was obtained from Sigma-Aldrich and used as the adsorbate without further purification. The wavelength of maximum adsorption of this dye was 665 nm. Stock solution was prepared by dissolving 1.0 g of MB in 1 L of distilled water.

Preparation and characterization of the biosorbent

Durian skin was collected as solid waste from the local market in Kota Kinabalu, Sabah, Malaysia. Prior to the process, the durian skins were washed several times and then soaked overnight using distilled water in order to remove unwanted dirt particles or other inorganic impurities that might affect the accuracy of the result. After the cleaning step, the durian skin was cut into small pieces of approximately 1–2 cm with a knife and then was left dry in an oven for 48 hours, with a temperature of 130 °C. Thereafter, the dried durian skin was ground and sieved in order to obtain the approximate size of 250 μm. Subsequently, the resulting product was stored in an air-tight container for further use. No other chemical or physical treatments were applied prior to adsorption experiments.

Surface chemistry of the prepared durian skin powder was analyzed using Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer Spectrum 100, USA) to determine the surface functional groups, where the spectra were recorded in the range of 4,000–650 cm−1. A scanning electron microscope (SEM) (JEOL JSM-5610LV, Japan) at 10 kV with a magnification of 500× was used to observe the porosity and morphological surface of the dried durian skin powder, while the surface properties of durian skin such as specific surface area, pore size distribution and total pore volume were analyzed by using the Brunauer–Emmett–Teller (BET) method. A Jasco V-530 UV-visible (UV–Vis) spectrophotometer was used in this study in order to determine the maximum adsorption value of the solution and the maximum wavelength of MB.

Effect of initial dye concentration

Initial concentrations of 2–10 mg/L were prepared in a series of 250 mL volumetric flasks. The initial concentrations were analyzed by measuring the absorbance values. After that, 0.1 g of durian skin was added into each labeled Erlenmeyer flask and the flasks (covered with parafilm) were then placed in an orbital shaker and shaken for 60 minutes at a constant speed of 150 rpm until equilibrium was achieved. The experiment was conducted at room temperature. Aqueous samples of 2 mL aliquots were taken from every flask using a 0.45 μm filter syringe and then the concentration of each aliquot was analyzed by using a UV–Vis spectrophotometer. The final concentration was measured and the amount of dye adsorbed, (mg/g), was calculated by using Equation (1). 
formula
1
where (mg/L) is the initial concentration and (mg/L) is the final concentration of the MB, V is the volume of the dye solution (L) and W is the mass of the adsorbent used (g).

Effect of biosorbent dosages

The effect of the biosorbent amount on the biosorption of MB was investigated by adding different amounts (0.3, 0.5 and 0.7 g) of dried durian skin into a 250 mL Erlenmeyer flask containing a desired volume of MB solution (10 mL) with a fixed initial concentration (4 mg/L) of dye solution. The Erlenmeyer flasks were then labeled and placed in an orbital shaker at around 150 rpm for 30 minutes until the equilibrium state was achieved. Aqueous samples of 2 mL aliquots were taken from every flask using a 0.45 μm filter syringe and then the concentration of each aliquot was analyzed by using a UV–Vis spectrophotometer. The experiment was conducted at room temperature without changing the pH.

Effect of pH solution

To observe the effect of pH solution on the adsorption uptake, 10 mL of MB dye solution of 4 mg/L initial concentration at different pH values of 4, 7 and 9 were agitated, respectively, with 0.1 g of dried durian skin in an orbital shaker under room temperature. Agitation was carried out for 25 minutes at a constant agitation speed of 150 rpm in order to achieve the equilibrium condition. Aqueous samples with 2 mL aliquots were taken from every flask using a 0.45 μm filter syringe and then the concentration of each aliquot was analyzed by using a UV–Vis spectrophotometer. The pH solution was adjusted by using 0.1 M of HCl and 0.1 M of NaOH solutions.

The final dye concentration was measured and the percentage removal of dye was calculated using Equation (2). 
formula
2

RESULTS AND DISCUSSION

FTIR analysis of durian skin

By comparing the spectra of durian skin before and after adsorption, it was clearly observed that there were IR peaks that had shifted, while some had disappeared after undergoing the adsorption process. This was due to interaction between the functional groups on the biosorbent and the MB. As shown in Figure 1, before adsorption, the broad, intense spectrum band observed at 3,384.92 cm−1 indicated O–H stretching and H-bonding. The peak at 2,623.01 cm−1 before adsorption was due to the O–H stretching vibrations of carboxylic acids, shifted to 2,936.50 cm−1 after the adsorption process. The peaks at 1,637.22 cm−1, 1,407.26 cm−1, 1,345.81 cm−1 and 1,151.54 cm−1 appeared after the adsorption process and may be assigned to the –C = C– stretching vibrations of alkenes, C–C stretching (in-ring) of aromatics, C–H rocking of alkanes and C–H wagging (–CH2Cl) of alkyl halides, respectively. The peaks at 1,246.69, 1,103.96 and 1,026.65 cm−1 after the adsorption corresponded to C–N stretching of aliphatic amines. The peak at 1,054.40 cm−1 before the adsorption process shifted to 1,050.44 cm−1 of the C–O stretching of carboxylic acids after the adsorption process. The spectra at 858.14 cm−1 before adsorption shifted to 862.11 cm−1 after the adsorption process and this was due to the C–H group (alkanes). Another peak at 737.22 cm−1 was observed before the adsorption and this was due to presence of the C–Cl stretching of alkyl halides. Then, it had shifted to 705.50 cm−1 after the adsorption process. The significant shifts of these specific peaks to the higher or lower wave numbers after MB ion biosorption suggested that hydroxyl, carboxyl and amide groups on the durian skin surfaces were involved in the biosorption of the MB.

Figure 1

Combined FTIR spectra of durian skin before and after adsorption at different initial concentrations of 2, 4, 6, 8, and 10 mg/L.

Figure 1

Combined FTIR spectra of durian skin before and after adsorption at different initial concentrations of 2, 4, 6, 8, and 10 mg/L.

SEM analysis of durian skin

Figure 2(a) and 2(b) show the macropore structure of durian skin before and after adsorption under resolution of 500× using an SEM. The samples were mounted on a stub and coated with about 200 Å gold layers in a vacuum chamber and scanned in order to identify the surface texture. The micrograph of the biosorbent showed some cavities in the surface's structure, capable of uptaking MB ions, as well as an irregular and porous microstructure. The surface was observed to contain little amounts of small pores. As a result, it was clearly observed that there was a possibility that these pores may provide a ready access under the small surface area for the sorption process. The surface adsorbent before MB adsorption (Figure 2(a)) is different from the surface of MB-loaded adsorbent (Figure 2(b)), which confirms that the surface of durian skin is covered with dye molecules.

Figure 2

SEM micrograph of durian skin (a) before adsorption and (b) after adsorption of MB ions.

Figure 2

SEM micrograph of durian skin (a) before adsorption and (b) after adsorption of MB ions.

BET analysis of durian skin

The surface properties of durian skin such as specific surface area, pore size distribution and total pore volume were measured by N2 adsorption–desorption isotherms and are presented in Table 1. An adsorption isotherm was obtained by measuring the amount of N2 gas adsorbed on the surface of biosorbent material at 77 K, whereas desorption isotherms were obtained by measuring the amount of gas removed from the surface of biosorbent material as pressure is gradually reduced. From the data, it was inferred that the BET surface area, Langmuir surface area and average pore diameter of durian skin were limited. Durian skin used in this study had a good porous structure, but with a very low BET surface area and average pore diameter, 0.58 m2/g and 57.32 nm, respectively, as compared to previous studies. Earlier studies showed that durian shell AC had mesoporosity and BET surface area of 1,404 m2/g for acid concentration of 30% (Tham et al. 2011).

Table 1

Surface characteristics of the neat durian skin

Surface characteristics Values 
Total surface area 0.58 m2/g 
Total pore volume 0.00837 cc/g 
Average pore diameter 57.32 nm 
Single point surface area 1.56 m2/g 
Langmuir surface area 2.06 m2/g 
Surface characteristics Values 
Total surface area 0.58 m2/g 
Total pore volume 0.00837 cc/g 
Average pore diameter 57.32 nm 
Single point surface area 1.56 m2/g 
Langmuir surface area 2.06 m2/g 

According to the International Union of Pure and Applied Chemistry (IUPAC) classification on pore dimensions, the pores of adsorbents are grouped into micropore (d < 2 nm), mesopore (d = 2–50 nm) and macropore (d > 50 nm). Thus, this shows that the majority of the pores fall into the range of macroporous with an average pore diameter of 57.32 nm. The structural heterogeneity can be identified from the pore size distribution (Foo & Hameed 2011). In general, the size of pores must be larger than the adsorbate molecule volume in order to allow the adsorbate to enter inside the adsorbent particle pores. The N2 adsorption–desorption isotherm (Figure 3) obtained from the neat durian skin corresponded to the Type III isotherm (shows large deviation from Langmuir model and explains the formation of a multilayer), in accordance with the IUPAC classification. This was due to weak interactions between the durian skin and the gas particles. Moreover, the adsorption proceeds as the interaction between adsorbent surfaces is less than the interaction of the adsorbate with an adsorbed layer, which was characterized by heats of adsorption less than the adsorbate heat of liquefaction (Udoji et al. 2010).

Figure 3

N2 adsorption–desorption isotherm of the neat durian skin.

Figure 3

N2 adsorption–desorption isotherm of the neat durian skin.

Effect of initial concentration on dye adsorption

The experimental results of the sorption of MB onto the durian skin at various initial concentrations are shown in Figure 4. For all these runs, it was observed that the MB dye adsorption rate was rapid within the first 20 minutes of contact time and then became slower until it achieved dynamic equilibrium state after approximately an hour, when an almost constant value was observed. A similar result was reported for adsorption of MB on langsat (Lansium domesticum) peel which revealed that amount of MB adsorbed increased with the increase in the initial dye concentrations of MB, thus concluding that a higher initial concentration of dye will enhance the adsorption process (Salleh et al. 2012). At the equilibrium state, only a limited amount of dye could be removed from the dye solution using the adsorbent. During the initial stage, the large surface area of the durian skin was favorable for the adsorption of MB molecules; however, after most of the surface sites became occupied by these molecules, repulsion between the solute molecules of the solid and bulk phases caused difficulties for the remaining surface sites to be occupied. As shown in Figure 4, the adsorption capacity at equilibrium increased from 3.45 to 17.48 mg/g, with an increase in the initial MB concentrations from 2.0 to 10.0 mg/L.

Figure 4

Equilibrium isotherm for the effect of the initial concentrations on the amount of MB adsorbed against the contact time using durian skin (C0 = 2–10 mg/L; W = 0.1 g; t = 60 minutes; agitation speed = 150 rpm).

Figure 4

Equilibrium isotherm for the effect of the initial concentrations on the amount of MB adsorbed against the contact time using durian skin (C0 = 2–10 mg/L; W = 0.1 g; t = 60 minutes; agitation speed = 150 rpm).

Effect of biosorbent dosages on dye adsorption

As shown in Figure 5, there is an increase in adsorption as the biosorbent dosage increased from 0.3 to 0.7 g. This can be attributed to the increased adsorbent surface area and more available adsorption sites resulting from the increase in the dose of the adsorbent (Salleh et al. 2012). The biosorption capacity of biosorbent increased from 1.08 to 2.47 mg/g. A further increase in the biosorbent dosage beyond 0.7 g did not change the biosorption capacity much. This was due to the binding of almost all MB ions to the surface of durian skin and the equilibrium condition achieved between the MB molecules in the durian skin and in the solution (Akar et al. 2009).

Figure 5

Equilibrium isotherm for the effect of the biosorbent dosage on the amount of MB adsorbed against the contact time using durian skin (C0 = 4 mg/L; W = 0.3 g, 0.5 g, 0.7 g; V = 0.20 L; t = 30 minutes; agitation speed = 150 rpm).

Figure 5

Equilibrium isotherm for the effect of the biosorbent dosage on the amount of MB adsorbed against the contact time using durian skin (C0 = 4 mg/L; W = 0.3 g, 0.5 g, 0.7 g; V = 0.20 L; t = 30 minutes; agitation speed = 150 rpm).

Effect of pH solution on dye adsorption

As shown in Figure 6, the amount adsorbed increased gradually when the solution pH increased from 4 to 9, and the equilibrium was reached within 10 minutes. The percentage removal of MB dye increased from 47.19% to 80%, whereas the maximum dye adsorption capacity of the durian skin was found to be increased from 3.78 to 6.40 mg/g. This showed that adsorption of MB dye by durian skin adsorbent was unfavorable at low pH. Thus, the results suggested that the adsorption of MB on durian skin is higher in alkaline solutions than in neutral and acidic conditions. At the low pH condition, the competition between excess hydroxyl ions, H+ and the cationic groups on the dye for adsorption sites will cause the adsorbent surface to be positively charged. Vice versa, when the pH of the solution is high, the adsorbent surface may get negatively charged, which will enhance the positively charged dye cations through electrostatic forces of attraction (Salleh et al. 2012). These results suggested that at pH 9, the durian skin consists of both hydroxyl and carboxylic groups, where these groups are deprotonated and it was demonstrated that these carboxylic sites were also responsible for MB dye binding through electrostatic interactions between the positively charged biosorbent surface and the negatively charged dye anions (Tham et al. 2011).

Figure 6

Equilibrium isotherm for the effect of the pH on the amount of MB adsorbed against the contact time using durian skin (C0 = 4 mg/L; W = 0.1 g; pH = 4, 7, 9; V = 0.20 L; t = 25 minutes; agitation speed = 150 rpm).

Figure 6

Equilibrium isotherm for the effect of the pH on the amount of MB adsorbed against the contact time using durian skin (C0 = 4 mg/L; W = 0.1 g; pH = 4, 7, 9; V = 0.20 L; t = 25 minutes; agitation speed = 150 rpm).

Adsorption isotherms

An adsorption isotherm is the relationship between the amount of adsorbate adsorbed and its concentration in the equilibrium solution (Hamdaoui & Naffrechoux 2007). In the present study the MB adsorption was analyzed by Langmuir and Freundlich isotherm models in order to describe the sorption equilibrium.

To find the regression coefficient , and (Langmuir constant related to rate of adsorption (L/mg)) for the Langmuir isotherm for adsorption of MB on durian skin, the graph of against was plotted and is shown in Figure 7. The value of , maximum adsorption capacity, was determined from the Langmuir plots and it was found to be 7.23 mg/g, whereas the value of (dimensionless equilibrium parameter) in the present investigation was found to be 0.0003. Therefore, it was confirmed that the uptake of MB dye was favorably within the range 0–1 of values. Table 2 summarizes the maximum adsorption capacity of durian skin and other adsorbent materials.

Table 2

Comparison of maximum adsorption capacity of different adsorbents towards MB

Adsorbent Maximum adsorption capacity (mg/g) References 
Pistachio hull powder 602 Moussavi & Khosravi (2011)  
Sugar beet pulp 714.29 Vučurović et al. (2011)  
Natural banana peel 18,647 Amela et al. (2012)  
Activated carbon coconut husk 434.80 Tan et al. (2008)  
Langsat (Lansium domesticum) peel 45.45 Salleh et al. (2012)  
Garlic peel 142.86 Hameed & Ahmad (2009)  
Oil palm empty fruit bunch (EFB) fibers modified using citric acid 103.10 Shaiful et al. (2012)  
EFB fibers modified using polyethylenimine 158.70 Shaiful et al. (2012)  
Durian (Durio zibethinus Murray) skin 7.23 This work 
Adsorbent Maximum adsorption capacity (mg/g) References 
Pistachio hull powder 602 Moussavi & Khosravi (2011)  
Sugar beet pulp 714.29 Vučurović et al. (2011)  
Natural banana peel 18,647 Amela et al. (2012)  
Activated carbon coconut husk 434.80 Tan et al. (2008)  
Langsat (Lansium domesticum) peel 45.45 Salleh et al. (2012)  
Garlic peel 142.86 Hameed & Ahmad (2009)  
Oil palm empty fruit bunch (EFB) fibers modified using citric acid 103.10 Shaiful et al. (2012)  
EFB fibers modified using polyethylenimine 158.70 Shaiful et al. (2012)  
Durian (Durio zibethinus Murray) skin 7.23 This work 
Figure 7

Langmuir isotherm for MB dye adsorption onto durian skin (C0 = 2–10 mg/L; W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

Figure 7

Langmuir isotherm for MB dye adsorption onto durian skin (C0 = 2–10 mg/L; W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

The Freundlich isotherm corresponds to adsorption on a heterogeneous surface and also possibly to multilayer biosorption (Udoji et al. 2010). To find the value of , n and (Freundlich exponent) for the Freundlich isotherm for adsorption of MB on durian skin, the graph of log against log was plotted and is shown in Figure 8. The value of for adsorption of MB on durian skin was 1.05 mg/g, while the heterogeneity factor, n value, was found to be 1.0426. Since the value of n was greater than unity, suggesting that the conditions favored adsorption.

Figure 8

Freundlich isotherm for MB dye adsorption onto durian skin (C0 = 2–10 mg/L; W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

Figure 8

Freundlich isotherm for MB dye adsorption onto durian skin (C0 = 2–10 mg/L; W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

On the basis of the value, the experimental data were more suitable for the Freundlich isotherm than to the Langmuir isotherm. This was due to the value for the Freundlich isotherm (0.9627) being higher than that of the Langmuir (0.0158), and thus explained the heterogeneous nature of the durian skin surface.

Adsorption kinetics

The adsorption kinetics of MB dye onto durian skin was analyzed using the pseudo first-order kinetic model and pseudo second-order adsorption kinetic model at 2–10 mg/L initial concentrations. The values of the adsorption rate constants, and , for MB adsorption on durian skin were determined from Figures 9 and 10, respectively. By comparing the two graphical charts in both figures, it was observed that the correlation coefficients, , for the pseudo second-order model are closer to unity than those of the pseudo first-order model, and hence indicate a better fit with the pseudo second-order model. The correlation coefficients, , for the pseudo second-order were in the range of 0.9836–0.9918, which was higher than the for the pseudo first-order (0.7351–0.8707). Hence, it was proven that the adsorption process of MB dye by using durian skin as biosorbent followed the pseudo second-order kinetic model.

Figure 9

Pseudo first-order kinetic model for MB dye adsorption onto durian skin biosorbent (C0 = 2–10 mg/L, W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

Figure 9

Pseudo first-order kinetic model for MB dye adsorption onto durian skin biosorbent (C0 = 2–10 mg/L, W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

Figure 10

Pseudo second-order kinetic model for MB dye adsorption onto durian skin biosorbent (C0 = 2–10 mg/L, W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

Figure 10

Pseudo second-order kinetic model for MB dye adsorption onto durian skin biosorbent (C0 = 2–10 mg/L, W = 0.1 g; V = 0.20 L; t = 60 minutes; agitation speed = 150 rpm).

Pseudo first-order and pseudo second-order for MB dye adsorption were compared for their adsorption rate constants, values and the differences between the experimental and calculated values at different initial concentrations. According to Table 3, the calculated values were approximately equal to experimental values for the pseudo second-order model. As a result, both results suggested that the adsorption kinetics of MB on durian skin is better represented by the pseudo second-order kinetic model.

Table 3

Kinetics study of MB dye onto durian skin

Initial conc. (mg/L) qe, exp (mg/g) First-order kinetic model
 
Second-order kinetic model
 
k1 (min-1qe, cal (mg/g) R2 k2 (g/(mg·min)) qe, cal (mg/g) R2 Unitless h (mg/(g·min)) 
3.45 0.1019 3.48 0.8611 0.0472 3.73 0.9847 0.6581 
7.14 0.0908 7.77 0.7351 0.0209 7.58 0.9836 1.1966 
10.71 0.1271 14.21 0.7450 0.0185 11.33 0.9903 2.3727 
13.37 0.1030 15.70 0.7872 0.0092 14.60 0.9809 1.9691 
10 17.31 0.0948 16.03 0.8707 0.0103 18.59 0.9918 3.5586 
Initial conc. (mg/L) qe, exp (mg/g) First-order kinetic model
 
Second-order kinetic model
 
k1 (min-1qe, cal (mg/g) R2 k2 (g/(mg·min)) qe, cal (mg/g) R2 Unitless h (mg/(g·min)) 
3.45 0.1019 3.48 0.8611 0.0472 3.73 0.9847 0.6581 
7.14 0.0908 7.77 0.7351 0.0209 7.58 0.9836 1.1966 
10.71 0.1271 14.21 0.7450 0.0185 11.33 0.9903 2.3727 
13.37 0.1030 15.70 0.7872 0.0092 14.60 0.9809 1.9691 
10 17.31 0.0948 16.03 0.8707 0.0103 18.59 0.9918 3.5586 

The pseudo second-order kinetic analysis reveals that the values of the initial adsorption rates, , increases with an increase in the initial MB concentration (Kumar & Kirthika 2009). The lower the concentration of MB ions in the solution, the lower the probability of collisions between these species and hence the faster MB ions could be bonded to the active sites on the surface of the adsorbent (Wong et al. 2003). However, the equilibrium adsorption capacity, , increased with an increase in initial MB concentration and this was because of the large number of MB ions adsorbed at the available adsorption sites. The equilibrium data obtained from the batch test may not be always applicable in real applications, which are generally performed by column test, and a further study is required.

CONCLUSIONS

This study showed that the durian skin is a suitable biosorbent to be used in the removal of MB from simulated textile wastewater. Maximum adsorption capacity increased from 3.45 to 17.31 mg/g with an increase in the initial MB dye concentration of 2–10 mg/L and increased from 1.08 to 2.47 mg/g with increasing biosorbent dosage of 0.3–0.7 g. As the pH of the aqueous solutions increases, the adsorption of MB also increases. Optimum pH was determined to be 9, while maximum adsorption capacity was identified as 6.40 mg/g. Meanwhile, the sorption equilibrium was best described by the Freundlich isotherm model with maximum adsorption capacity of 7.23 mg/g and this was due to the heterogeneous nature of the durian skin surface. Kinetic studies indicated that the sorption of MB dye tended to follow the pseudo second-order kinetic model with promising correlation of 0.9836 < < 0.9918.

ACKNOWLEDGEMENT

This work was fully supported by the Centre of Research & Innovation, Universiti Malaysia Sabah (Grant No. SBK0058-SG-2013), and is gratefully acknowledged.

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