In the present study, modified sawdust was used for the removal of an azo dye (Orange G (OG)) from aqueous solutions. The study was carried out in batch mode. Effects of various important parameters such as pH, concentration, temperature, dose and agitation speed on the removal of the dye were investigated for optimization of the process. It was observed that the maximum removal of 78.3% was achieved at the lowest dye concentration of 50 mg/L. The process of removal was found to be exothermic in nature. Adsorption of OG on modified sawdust was rapid and in accordance with pseudo-first-order kinetics. Mass transfer coefficient (βL) was also estimated and found to be 0.33 cm/sec at 30 °C, which indicates that sorption of dye was driven by a film diffusion mechanism. The Langmuir isotherm model agrees well with the sorption isotherm data and also confirms that adsorption took place on the homogenous surface of modified sawdust. The Langmuir capacity was determined and found to be 5.48 mg/g. Therefore, the study recommends that modified sawdust is a promising candidate for the efficient removal of dye-contaminated wastewater.

INTRODUCTION

Effluent from the dye industry causes serious environmental problems. Major sources of dyes in effluent are textiles, hair colorings, printing, paper production dyeing, leather tanning, dyestuff manufacturing and food plants (Banerjee et al. 2013; Mane & Babu 2013). Pollution by dye is of great concern because dyes are toxic, non-degradable and stable in the environment. The presence of dyes inhibits the penetration of sunlight to aqueous systems, which disturbs aquatic species. Some dyes exhibit mutagenic, teratogenic and carcinogenic effects (Garg et al. 2004; Warang et al. 2013). A variety of dyes are used in textile industries (Mishra & Tripathy1993; Srinivasan & Viraraghavan 2010). Among these various dyes, Orange G (OG) is a common dye that is used in textiles, cosmetics, wood stain, paper, leather industries, etc. (Dulman et al. 2012; Banerjee et al. 2014a). OG, an azo dye, is the disodium salt of benzene-azo-6:8-disulphuric acid. In azo dyes, one or more azo groups (–N = N–) are bound to aromatic rings. The widespread applications of azo dyes are accounted for by their chemical stability and versatility, which are due to their molecular structure (Sun et al. 2009; Abdelkader et al. 2011). The molecular formula of OG is C16H10N2Na2O7S2 (wavelength of maximum absorption (λmax) 478 nm).

Although OG is used in various industries, it is also toxic to flora and fauna. OG is reported to cause irritation of the respiratory and gastrointestinal tracts (Dulman et al. 2012). Owing to these environmental problems, treatment of dye effluent is mandatory before it can be discharged into water bodies. Various methods such as coagulation and flocculation, photo-degradation, ozonation, electro-coagulation, chemical oxidation, filtration, ion-exchange, membrane separation and adsorption have been used for the removal of dyes from wastewater (Mohammadi et al. 2011; Banerjee et al. 2013). Most of the suggested methods are costly and generate toxic sludge. Among various techniques, adsorption is most effective and is a low-cost method. Adsorption of dyes on activated carbon is quite popular, but its high cost restricts its widespread application and researchers are working to search for low-cost adsorbents for this purpose (Salvador & Jimenez 1996). In recent years, more attention has been focused on the utilization of a number of non-conventional adsorbents for the removal of dyes from aqueous solutions. Materials such as sugar beet pulp, hen feathers, modified rice straw, date stones, peat, coal, zeolites, silica gel, orange peel, coir pith, rice husks, coffee husks, barley husks, bagasse fly ash and bamboo (Mall et al. 2006; Arulkumar et al. 2011; Saja et al. 2013) have been reported for the removal of dyes from aqueous solutions.

In the present study, modified sawdust was used for the removal of OG from aqueous solutions. The effects of various important parameters such as pH, temperature, concentration and dose were investigated. Kinetic, isotherm and thermodynamic studies were also carried out.

MATERIALS AND METHODS

Adsorbate and adsorbent preparation

OG was obtained from Merck India (P) Mumbai. OG (C.I. No. 16230) of 99.0% purity was used for the preparation dye solution without further purification for adsorption experiments. Stock and standard dye solutions were prepared in distilled water. The sawdust was collected from a local saw mill situated in Allahabad, India. The sawdust was washed with distilled water several times until the solution was free from color. The sawdust was kept in a hot air oven at 105 °C overnight. The dried sawdust was then modified with perchloric acid and dried again. This material was then stored in a desiccator for further use as an adsorbent. Details of adsorbent preparation are reported in our previous article (Banerjee et al. 2013). Modified sawdust was characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy (Varian 1000 FT-IR, Scimitar Series). The Brunauer–Emmett–Teller surface area of the adsorbent was also determined (Micromeritics ASAP 2020, V302G single port).

Batch adsorption experiments

The batch adsorption experiments for the removal of OG were conducted in a batch process by taking aqueous solutions of OG of known concentrations. A known amount, 20 g/L, of the adsorbent was added to 250-mL reagent bottles containing 50 mL solution of different initial concentrations (50–125 mg/L) of dye solution at room temperature. The solution pH was adjusted with NaOH or HCl solutions using a pH meter. Adsorption experiments were carried out in a thermostatic shaker at 100 rpm. After equilibrium was reached, the adsorbent was separated from the dye solution by centrifugation at 5,000 rpm for 10 min. The residual dye concentration in each sample was measured by a UV-Vis spectrophotometer at 478 nm. Preliminary experiments showed that equilibrium was achieved in 90 min. The batch adsorption experiments were conducted in triplicate for accuracy of the results. The amount of dye adsorbed and the percent removal were determined by using the following equations:
formula
1
formula
2
where Ci and Ce (mg/L) are the liquid-phase concentrations of OG at initial and at equilibrium, respectively. V is the volume of the solution (L), W is the mass of dry adsorbent used (g) and qe is the amount adsorbed on per unit mass of the adsorbent (mg/g). The optimum-operating conditions for the experiments can be summarized as follows: contact time 90 min, initial dye concentration 50 mg/L, temperature 30 °C, 2.0 pH, agitation speed 100 rpm and adsorbent dose 10 g/L.

RESULTS AND DISCUSSION

Adsorbent characterization

Modification of sawdust was carried out for enhancement of its adsorption efficiency. A SEM image of the modified adsorbent (Figure 1) shows that the surface was rough with many ‘troughs’ on it, making it suitable for the adsorption process. It was observed that due to modification, active surface sites and pores on the surface were increased. FT-IR suggested the presence of C-OH, N-H, C = O, C-O-C, C-H and C-X groups, which may be responsible for adsorption of OG on modified sawdust. Detailed discussion on this aspect has been reported elsewhere (Banerjee et al. 2013). The average particle bulk density porosity and surface area were found to be 700 μm, 0.25 ± 0.07 g/cm3, 74.4 ± 7.4% and 74.23 m2/g, respectively (Banerjee et al. 2013).

Figure 1

SEM of modified sawdust at 20 μm magnification.

Figure 1

SEM of modified sawdust at 20 μm magnification.

Effect of concentration and contact time

Studying the effect of concentration and contact time revealed that equilibrium was achieved within 90 min, which was similar for all concentration ranges studied (50–125 mg/L). At the initial stages, the rate of removal was faster and after 90 min there was no significant change in the removal. At the initial stages, large numbers of active sites were available but after a certain time, due to saturation of most of the adsorbent sites, the rates of removal decreased. Removal increased from 36.3 to 78.3% by decreasing initial concentration OG from 125 to 50 mg/L. This trend is due to the availability of fixed adsorbent sites for a fixed amount of adsorbent (Roy et al. 2013). For higher concentrations, the ratio of adsorbent sites to dye species decreases, which leads to the decrease in removal.

Effect of agitation speed (rpm)

Agitation speed is an important parameter that affects the removal of any adsorbate because it decides the contact between adsorbate and adsorbent. For this study, batch experiments were carried out at four different shaking speeds, namely, 50, 75, 100 and 125 rpm, while keeping all other parameters constant (temperature 30 °C, dye concentration 50 mg/L, dose 20 g/L). At higher rpm, removal decreases due to improper contact of dye molecules and adsorbent; similar reasons also apply for low rpm. On the basis of this study, a shaking speed of 100 rpm was selected for further studies.

Effect of temperature on removal of dye

Removal of any OG from aqueous solutions is strongly affected by solution temperature. To investigate the effect of temperature on the removal of dye, temperature was varied from 30 to 50 °C. Other parameters, such as pH, concentration dose and rpm were constant during the study of this effect. The effect of solution temperature on the dye removal percent was investigated at various initial concentrations and the results are shown in Figure 2. The adsorption of dye on modified sawdust is exothermic in nature. A similar trend was observed at all concentration ranges. Removal decreased from 78.3 to 37.5%, 58.4 to 31.7%, 46.3 to 30.8% and 36.3 to 22% for 50, 75, 100 and 125 mg/L concentration. The exothermic nature suggests that adsorption is physical in nature.

Figure 2

Effect of temperature on removal of dye for the removal of OG by adsorption on modified sawdust (a) 50 mg/L, (b) 75 mg/L, (c) 100 mg/L and (d) 125 mg/L.

Figure 2

Effect of temperature on removal of dye for the removal of OG by adsorption on modified sawdust (a) 50 mg/L, (b) 75 mg/L, (c) 100 mg/L and (d) 125 mg/L.

Effect of adsorbent dose

The dependence of OG adsorption on the modified sawdust was studied at different amounts of adsorbent ranging from 10 to 25 g/L. The removal of dye increased from 54.8 to 82.6% by increasing the dose from 10 to 25 g/L and keeping other parameters constant, but beyond this value no significant removal was noticed and the percent dye adsorption reached a constant value. With the increase of adsorbent dose, the number of active sites also increases, which results in maximum binding of dye molecules. The maximum adsorption efficiency was observed at an adsorbent dose of 20 g/L therefore, the same amount was selected for further experiments.

Effect of pH on removal of dye

The pH of solution plays an important role in any adsorption process because pH decides the nature of the adsorbent surface when it comes in contact with the adsorbate solution. For investigation of the effect of pH on the removal of OG, the pH of the dye solution was varied from 2.0 to 7.0. According to Figure 3, it is clear that removal was higher at lower pH. Removal increased from 22.1 to 79.3% by decreasing the pH from 7.0 to 2.0.

Figure 3

Effect of pH on removal of dye for the removal of OG by adsorption on modified sawdust.

Figure 3

Effect of pH on removal of dye for the removal of OG by adsorption on modified sawdust.

At lower pH, the adsorbent surface becomes positively charged, which favors the anionic dye species. The tendency for a decreasing trend of removal of dye on increasing pH of dye solution may be attributed to the deprotonation of the adsorbent surface (Atia et al. 2009; Mane & Babu 2013).

KINETIC STUDY

To describe the adsorption kinetics of OG on modified sawdust, two kinetic models, namely pseudo-first-order and pseudo-second-order kinetic models, were applied to the kinetic data. The linearized forms of pseudo-first-order (Lagergren 1898) and pseudo-second-order (Ho & McKay 1999) models can be expressed as follows:

  • (i) pseudo-first-order kinetic equation
    formula
    3
  • (ii) pseudo-second-order kinetic equation
    formula
    4
    where k1(min−1) and k2 (g/mg·min) are the rate constants of the pseudo-first-order and pseudo-second-order processes, qt and qe are the amounts of OG adsorbed at different times t and at equilibrium, respectively. The values of different kinetic parameters for first-order and second-order were determined by plotting graphs of log (qeqt) versus t and t/qt versus t (figure not shown), respectively. The values of different kinetic parameters at different temperatures are given in Tables 1 and 2. It is clear from Tables 1 and 2, that the values of experimental qe and calculated qe agreed well for the pseudo-first-order model, better in comparison with the pseudo-second-order model.

Further, the value of the correlation coefficient obtained for the pseudo-first-order showed a consistent trend. From Tables 1 and 2, all the R2 values obtained from the pseudo-first-order model were close to unity. On the basis of the above discussion, it can be stated that adsorption of OG on modified sawdust is governed by first-order kinetics.

Table 1

Values of different kinetic parameters

Pseudo-first-order kinetic modelPseudo-second-order kinetic model
Temperature (±0.5 °C)Experimental qe (mg/g)qe, cal (mg/g)k1, (min−1)R2qe, cal (mg/g)k2, (g/m g·min)R2
30 3.96 5.177 0.030 0.98 7.90 0.001 0.94 
40 2.56 2.219 0.029 0.99 3.651 0.006 0.97 
50 1.87 2.383 0.027 0.98 4.706 0.001 0.88 
Pseudo-first-order kinetic modelPseudo-second-order kinetic model
Temperature (±0.5 °C)Experimental qe (mg/g)qe, cal (mg/g)k1, (min−1)R2qe, cal (mg/g)k2, (g/m g·min)R2
30 3.96 5.177 0.030 0.98 7.90 0.001 0.94 
40 2.56 2.219 0.029 0.99 3.651 0.006 0.97 
50 1.87 2.383 0.027 0.98 4.706 0.001 0.88 
Table 2

Values of intra particle diffusion constants and mass transfer coefficient

Temperature (±0.5 °C)kid(mg/g·min1/2)R2Coefficient of mass transfer βL (cm/s)R2
30 0.50 0.99 0.33 0.97 
40 0.27 0.95 0.26 0.98 
50 0.24 0.98 0.40 0.97 
Temperature (±0.5 °C)kid(mg/g·min1/2)R2Coefficient of mass transfer βL (cm/s)R2
30 0.50 0.99 0.33 0.97 
40 0.27 0.95 0.26 0.98 
50 0.24 0.98 0.40 0.97 

INTRAPARTICLE DIFFUSION STUDY

Kinetic data for the removal of OG by modified sawdust were analyzed using the intraparticle diffusion model to check the possibility of intraparticle diffusion. The itraparticle diffusion model can be expressed as follows (Salvador & Jimenez 1996):
formula
5
where kid is the intraparticle diffusion rate constant (mg/g·min1/2). The value of kid can be determined from the slope of graph qt versus t½ (figure not shown). Values of kid for the present system are given in Tables 1 and 2. In case of involvement of intraparticle diffusion as a rate-controlling step, the lines of plot pass through the origin. However, in the present case, the lines do not pass through the origin, which suggests that intraparticle diffusion is not the only rate-controlling step (Srivastava et al. 2011).

MASS TRANSFER STUDY

During adsorption of OG on modified sawdust, a number of steps may be involved in the transfer of a dye species from the bulk to the surface of the modified sawdust (Srivastava et al. 2011). In the present study, a mass transfer study was carried out by applying the mass transfer model. The following mass transfer model expression was used for determining the coefficient of mass transfer (Srivastava et al. 2011):
formula
6
where Ss is the specific surface area, ‘k’ is the product of Langmuir's parameters, βL is the coefficient of mass transfer, m is the mass of the adsorbent per unit volume. The values of βL at different temperatures were calculated by the slopes of the plots of ‘ versus t’ (Figure 4).
Figure 4

Mass transfer plots for the removal of OG by adsorption on modified sawdust at different temperatures.

Figure 4

Mass transfer plots for the removal of OG by adsorption on modified sawdust at different temperatures.

The values of ‘m’ and ‘Ss’ were determined as follows (Sharma et al. 2010):
formula
7
formula
8
where δp, εp and dp are the density, porosity and diameter of the adsorbent, respectively. The values of the coefficient of mass transfer at different temperatures were calculated and are given in Tables 1 and 2. The values of the coefficient of mass transfer, βL, were determined from the slopes and intercepts of Figure 5 and were found to be 0.33 cm/sec, 0.26 cm/sec and 0.40 cm/sec at 30 °C, 40 °C and 50 °C, respectively.
Figure 5

(a) Langmuir isotherm plot, (b) Freundlich isotherm plot.

Figure 5

(a) Langmuir isotherm plot, (b) Freundlich isotherm plot.

ADSORPTION EQUILIBRIUM

Adsorption equilibrium is deciphered through various adsorption isotherm models. Isotherm describes the behavior of any adsorbate–adsorbent system and is helpful in designing any system. For the present system, the most widely used adsorption models for single-solute systems, the Langmuir and Freundlich models, were applied to the experimental data. Different isotherm parameters give important illustrations regarding the adsorption mechanism.

Langmuir isotherm

The Langmuir isotherm suggests monolayer coverage of homogeneous adsorbent sites. According to this isotherm, adsorption energy is constant and is independent of surface coverage. The linearized form of the Langmuir isotherm can be expressed as follows (Sharma et al. 2010; Yadav et al. 2012):
formula
9
Ce and qe are the equilibrium concentration and amount of dye adsorbed at equilibrium. Q0(mg/g) and b (L/g) are the Langmuir adsorption capacity and the adsorption intensity, respectively. The values of the Langmuir constant at different temperatures were calculated by the slope and intercept of linear plots of Ce/qe versus Ce (Figure 5(a)). The values of the Q0 were found to be 5.48, 4.62 and 3.70 mg/g at 30, 40 and 50 °C. Langmuir adsorption capacity was found to be higher at the lowest temperature and it decreased with an increase of temperature showing the exothermic nature of the removal process. Adsorption intensity (0.592 L/mg, 0.038 L/mg and 0.036 L/mg at 30 °C, 40 °C and 50 °C, respectively) also decreased with the increase of temperature from 30 to 50 °C. Further, for all temperatures, good correlation coefficients (0.99, 0.99 and 0.95 at 30, 40 and 50 °C) were shown, which indicates the monolayer coverage of dye species on the modified sawdust and that Langmuir's model fits the data well.

Freundlich isotherm

The Freundlich isotherm is based on the assumption of heterogeneous surface of the adsorbent. The heterogeneity of the adsorbent surface arises from the presence of different functional groups over it, and the various adsorbent–adsorbate interactions. The linearized form of the Freundlich isotherm can be expressed as follows (Sharma et al. 2010; Yadav et al. 2012):
formula
10
where Ce and qe are the equilibrium concentration and the amount of dye adsorbed at equilibrium, respectively, and Kf (mg/g) and 1/n (L/mg) are Freundlich constants for the system. 1/n is indicative of adsorption intensity for the present system. The values of different Freundlich isotherm parameters were calculated from the slopes and intercepts of the linear plot log qe versus log Ce (Figure 5(b)).

The Freundlich adsorption capacity was found to be 3.35, 0.73 and 0.51 (mg/g (L/mg)1/n) at 30, 40 and 50 °C. The value of n was greater than 1 at all temperatures (13.38, 2.55 and 2.53 L/mg at 30, 40 and 50 °C), which suggests that the adsorbate was favorably adsorbed on the adsorbent. The higher n value indicates the stronger adsorption intensity. The correlation coefficient was >0.98 for 30 and 50 °C, while it was 0.87 at 50 °C.

Among the Langmuir and Freundlich isotherm models, the Langmuir model was found to be more suitable for the present system. The Langmuir model showed good linearity and high-correlation coefficients at all temperatures in comparison to the Freundlich isotherm model.

THERMODYNAMIC STUDY

A thermodynamic study of the removal of OG by adsorption on modified sawdust was carried out to determine the value of different thermodynamic parameters. Different thermodynamic parameters such as enthalpy change (ΔH0), entropy change (ΔS0) and change in standard free energy (ΔG0), were determined by using the following equations (Banerjee et al. 2014b):
formula
11
formula
12
where Kc (L/g) is the standard thermodynamic equilibrium constant defined by qe/Ce, R (8.314 J/mol·K) is the gas constant and T (K) the absolute temperature. The values ΔH0 and ΔS0 were calculated from the slopes and intercepts of the plot log Kc versus 1/T (figure not shown). The values of ΔG0 at different temperatures were determined by Equation (12). They were calculated to be −3.38 kJ/mol, −0.11 kJ/mol and −1.28 kJ/mol at 298 K, 308 K and 318 K, respectively. The negative value of ΔH° (−75.78 kJ/mol) confirms the exothermic nature of the process of adsorption. The negative value of ΔS0 (−0.259 J/mol·K) suggests that there was a decrease in randomness at the solid–solution interface. The values of ΔG° were found to be negative at all temperatures, which showed the spontaneity of adsorption of OG on modified sawdust.

CONCLUSIONS

The removal of an azo dye, OG, from aqueous solutions was carried out by adsorption on modified sawdust. Sawdust was modified by a simple low-cost method by using perchloric acid. Equilibrium was achieved in 90 min. Solution temperature was found to be an effective parameter, and it was observed that on increasing the temperature from 30 to 50 °C the dye removal percentage decreased from 78.3 to 36.3%, which suggests the exothermic nature of the removal process. Dye removal was at a maximum (78.3%) at pH 2.0. The pseudo-first-order kinetic model was found to be more suitable for removal of dye in the present case. The mass transfer coefficient (βL) was determined and it suggested that the rate of transfer of the mass of dye from the bulk to the adsorbent surface was rapid enough, and the value of the coefficient of mass transfer was found to be 0.33 cm/sec at 30 °C. The equilibrium data fitted Langmuir's model well and the Langmuir adsorption capacities were found to be 5.48 mg/g, 4.62 mg/g and 3.70 mg/g at 30 °C, 40 °C and 50 °C, respectively. Negative values of enthalpy (ΔH0) and standard free energy (ΔG0) suggest an exothermic nature and a spontaneous nature of the process of removal. Finally, sawdust modified with perchloric acid is a comparatively efficient and inexpensive alternative absorbent material for the removal of hazardous azo dye (OG) from aqueous solutions.

ACKNOWLEDGMENT

The author (SB) thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for financial support.

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