The present paper deals with a complete batch adsorption study of 4-nitrophenol (4NP) from aqueous solution onto activated carbon prepared from Acacia glauca sawdust (AGAC). The surface area of the adsorbent determined by methylene blue method is found to be 311.20 m2/g. The optimum dose of adsorbent was found to be 2 g/l with 4NP uptake of 25.93 mg/g. The equilibrium time was found to be 30 minutes with the percentage removal of 96.40 at the initial concentration of 50 ppm. The maximum removal of 98.94% was found to be at pH of 6. The equilibrium and kinetic study revealed that the Radke–Prausnitz isotherm and pseudo second order kinetics model fitted the respective data well. In the thermodynamic study, the negative value of Gibbs free energy change (−26.38 kJ/mol at 30°C) and enthalpy change (−6.12 kJ/mol) showed the spontaneous and exothermic nature of the adsorption process.

INTRODUCTION

Phenol and its derivatives are highly toxic in nature and they have capability to persist in aqueous media. Amongst the phenol derivatives, 4-nitrophenol (4NP) is one of the highly toxic compounds. It has a nitro group at the opposite side of a hydroxyl group on the benzene ring. It is commonly found in wastewater from many pharmaceuticals, agricultural, pesticides, petrochemicals, petroleum refineries, coke oven, steel foundry, insecticides and herbicides industries (Kumar et al. 2007). It is a precursor to the rice herbicide fluorodifen and the pesticide parathion. It is also an intermediate in the synthesis of paracetamol. The United States Environmental Protection Agency (USEPA) listed 4NP as one of the toxic pollutants. Its acute ingestion in humans causes headaches, drowsiness, nausea, and cyanosis (blue colour in lips, ears, and fingernails). Contact with eyes causes irritation in humans; however, its long term effects on human and animal health and its carcinogenicity was not reported by USEPA. It is soluble in water and very toxic in nature. Due to its solubility, stability and toxicity in water, it is essential to remove 4NP from wastewater before discharging it into rivers or any disposal site (Marais & Nyokong 2008).

The conventional methods like chemical oxidation and biodegradation are the methods used for the treatment of 4NP but they are not very efficient and are time consuming. The modern methods such as advanced oxidation, membrane filtration, electrochemical oxidation and photocatalytic degradation are highly efficient but are more costly. Among all available methods, adsorption is an efficient low-cost method for treatment of wastewater. Many adsorbent materials such as charred sawdust (Dutta et al. 2001), fly ash (Singh & Nayak 2004), activated carbon fibres (Tang et al. 2007; Liu et al. 2010), acid-activated jute stick char (Ahmaruzzaman & Gayatri 2010) and activated carbon prepared from babool sawdust (Ingole & Lataye 2015) have been used for the adsorption of phenol and its compounds.

In the present study, an activated carbon prepared from Acacia glauca sawdust, a waste generated from saw mills and carpentry workshops, has been utilized for the removal of 4NP. The effect of adsorbent dose (m), initial pH of the solution (pH0), contact time (t), initial 4NP concentration (C0) and temperature (T) on the removal of 4NP has been studied. The adsorption equilibrium, kinetics and thermodynamics of the adsorption process have been reported in the present paper. Comparisons of the performance of various adsorbents and a batch process design have also been reported in the present study.

MATERIALS AND METHODS

Adsorbent

The raw material, i.e. Acacia glauca sawdust, was collected from local sawmills in Nagpur (India). The vernacular name of this tree is ‘Subabool’. The material was sieved to obtain desirable size (300 to 600 μ). The sieved sawdust was washed thoroughly with double distilled water (DDW) to remove any impurities such as dust and clay. The washed material was kept to sun dry in intense sunlight for a day and then it was kept in a hot air oven at the temperature of 105 °C for about 6 hrs. The dried sawdust was mixed with orthophosphoric acid in the ratio of 1:0.5 (g:ml) and was kept for 24 hrs. The material was kept in a muffle furnace at the temperature of 450 ± 10 °C for 1 hr, to convert it into a char. The charred material was kept in 2 molar liquid ammonia solution in the ratio of 1:3 (g:ml) for 2 hrs for the complete removal of acid, and then washed with DDW until neutral pH. The prepared activated carbon (AGAC) was dried in a hot air oven at 120 °C for 6 hrs. Proximate analysis was performed as per IS 1350 (I) 1984, scanning electron micrographs were taken by a 6380A (JEOL, Japan) scanning electron microscope (SEM), and Fourier transform infrared (FTIR) analysis, to understand the presence of various functional groups on adsorbent, was done by an IR-Affinity FTIR spectrophotometer (Shimadzu, Japan).

Adsorbate

All the chemicals used in present study were of analytical reagent (AR) grade. 4NP was obtained from Loba Chemie Pvt. Ltd, Mumbai (India). The stock solution of 1,000 mg/l concentration of 4NP was prepared by dissolving 1 g of 4NP in 1 litre of DDW. The various required concentrations; ranges from 1 to 500 mg/l of adsorbate test solutions were prepared from the stock solutions by dilution with DDW. The other chemicals used in the present study, viz. HCl, NaOH, KNO3, NaCl etc., were supplied by Merck Specialities Pvt. Ltd, Mumbai, India.

Batch experimental programme

From the stock solution the standard solutions of 2, 4, 6, 8 and 10 mg/l were prepared for the preparation of a standard graph for the measurement of concentration of unknown solutions. The absorbance was taken at 318 nm wavelength by using a UV–visible (UV-VIS) spectrophotometer (Shimadzu, Tokyo, Japan, model: UV-2450).

A temperature-controlled orbital shaker (REMI Instruments Mumbai, India, model: CIS-24BL) was used for the batch adsorption study. For each experimental run, 50 ml aqueous solution of known concentration was placed in a 250 ml conical flask containing a known mass of the adsorbent and agitated at a constant speed of 150 rpm at a required temperature. The initial pH (pH0) of the adsorbate solution was adjusted using 0.1 N HCl or 0.1 N NaOH aqueous solutions without any further adjustment during the sorption process. The time study was performed by taking out samples at various time intervals. The amount of 4NP adsorbed per unit weight of the adsorbent was calculated by: 
formula
1
The percentage removal of 4NP was determined as follows: 
formula
2
where Co and Ce, initial and equilibrium concentrations of 4NP, mg/l; V, volume of 4NP solution, l; Ma, mass of the adsorbent, g; and qe, amount of 4NP adsorbed per unit weight of the adsorbent, mg/g.

Surface area determination by methylene blue dye

Activated carbon with a mesopore (20–50 Å) structure adsorbs medium size molecules like methylene blue (MB) dye in a single layer. Based on this, for better understanding of the monolayer surface area due to mesopores, the MB surface area method can be used (Kaewprasit et al. 1998), which is explained below.

MB dye solution of various concentrations from 50 to 1,000 mg/l was prepared. 50 ml solution of different concentrations of MB was placed in 250 ml conical flasks. The AGAC dose of 0.5 g was added to each of the conical flasks and agitated in an orbital shaker at the speed of 150 rpm at the temperature of 25 °C for 2 hrs. After 2 hrs agitation the flasks were kept for 24 hrs and filtered. The final concentration of each solution was determined at 664 nm wavelength by using a UV-VIS spectrophotometer. The following Langmuir equation was used to determine the constants and surface area. 
formula
3
where C, equilibrium concentration of MB (mol/L); N, number of moles of MB adsorbed per gram of adsorbent; Nm, number of moles of MB per gram of adsorbent required to form monolayer; KL, Langmuir constant. By using Equation (3) (Langmuir's modified equation for MB surface area (Kaewprasit et al. 1998)) plots of C on x-axis and C/N on y-axis give a straight line. The values of constants Nm and KL were determined from the slope and intercept of the straight line. For the calculation of surface area, the following equation has been used. 
formula
4
 
formula
5
where SMB, specific surface area (10−3 km2/kg); Ng, number of MB molecules adsorbed at the monolayer of adsorbent in kg/kg; ɑMB, (197.2 Å2) occupied surface area of one molecule of MB; NAVO, Avogadro's number (6.02 × 1023) per mole; M, molecular weight of MB (319.84 g/mol).

Adsorption kinetics and equilibrium

Kinetic models describe the rate and order of an adsorption system based on the concentration of solution. The pseudo first order kinetic equation is best suited for molecular sorption as the rate of reaction as a function of sorption capacity and the validity of the equation are only up to initial time of sorption. The pseudo first order (Lagergren equation) equation is as given below (Uzun & Guzel 2004): 
formula
6
where kf (min−1) is the Lagergren rate constant.
A pseudo second order equation is used to describe the chemo-sorption (dissociative sorption) involving valence forces through the sharing or exchange of electrons between adsorbate and adsorbent as covalent bond or ion exchange. The pseudo second order equation is as given below (Tang et al. 2007): 
formula
7
where ks (g/(mg min)) is the second order rate constant and qe and qt are the amounts of 4NP adsorbed (mg/g) at equilibrium and at time t, respectively.

The slope and intercept of the first order plot of ln(qeqt) versus t for different concentrations were used to determine the kf and qe. Values of ks and equilibrium sorption capacity qe were calculated from the intercept and slope of the plot of t/qt versus t for the pseudo second order kinetic model.

Isotherm models and error analysis

To adequately correlate the experimental data, six isotherm equations were tested to find the best-fit isotherm using linear regression coefficient and two different error analysis functions of non-linear regression. The different equilibrium adsorption isotherm equations used to investigate the equilibrium study are given in Table 1.

Table 1

Various isotherm equations used for adsorption of 4NP onto AGAC surface (Lataye et al. 2008a, 2011)

Isotherms Equation 
Langmuir  
Freundlich  
Temkin  
Redlich–Peterson  
Toth  
Radke–Prausnitz  
Isotherms Equation 
Langmuir  
Freundlich  
Temkin  
Redlich–Peterson  
Toth  
Radke–Prausnitz  

qe is amount of 4-NP adsorbed at equilibrium (mg/g); qm and qTh are monolayer adsorption capacity (mg/g); KL, KF, KT, KR and KRP are Langmuir, Freundlich, Temkin, Redlich–Peterson and Radke–Prausnitz constants (l/mg); KTh is Toth constant (mg/l); B, Th and P are dimensionless constant of Temkin, Toth and Radke–Prausnitz isotherm; 1/n is heterogeneity factor; aR is isotherm constant and β is exponent in Redlich–Peterson isotherm; krp is Radke–Prausnitz isotherm constant.

A detailed error analysis was carried out to investigate the best-fit adsorption isotherm which describes the overall adsorption process. The two different error analysis functions studied are as follows:

  1. Hybrid fractional error function (HYBRID) – Ho et al. (2002) developed this function in order to improve the fit of sum of square method at lower concentration: 
    formula
    8
  2. Chi-squared (χ2) – the non-linear χ2 test is the sum of the squares of the differences between the experimental data and the calculated data of isotherms divided by the corresponding calculated data (Lataye et al. 2011): 
    formula
    9

Thermodynamics of adsorption

The Gibbs free energy change of the adsorption process is related to the adsorption equilibrium constant by the classical Van 't Hoff equation: (Lakshmi et al. 2009; Lataye et al. 2009): 
formula
10
The Gibbs free energy is also the change in entropy and the heat of adsorption at a constant temperature as given below: 
formula
11
Therefore, Kad can be obtained from Equations (10) and (11) as follows: 
formula
12
where ΔG0, free energy change (kJ/mol); ΔH0, change in enthalpy (kJ/mol); ΔS0, entropy change (kJ/(mol K)); Kad, equilibrium constant of interaction between the adsorbate and the AGAC surface; T, absolute temperature (K), and R, universal gas constant (8.314 J/(mol K)). ΔH0 and ΔS0 were obtained from the slope and intercept of the plot of lnKad versus 1/T.

RESULTS AND DISCUSSION

Characterization of adsorbent

The adsorbent AGAC used in the present study was characterized for proximate analysis, SEM micrographs, point of zero charge (pHPZC), FTIR spectroscopy and MB surface area. Characterization shows the physical and chemical activeness of AGAC to remove the impurities and also the purity of carbon prepared.

Proximate analysis

The results of proximate analysis are shown in Table 2. The ash content was found to be 20.23% and shows the non-carbonate contents of material. The amount of fixed carbon was 55.38%, which directly depends on the lignocellulose contents of raw material. The higher amount of fixed carbon shows the better sorption ability. Moisture and volatile contents were 10.07 and 14.32%, which inversely affect the sorption capacity.

Table 2

Proximate analysis of AGAC

Sr. No. Characteristics AGAC 
Moisture content 10.07% 
Ash content 20.23% 
Volatile matter 14.32% 
Fixed carbon 55.38% 
Sr. No. Characteristics AGAC 
Moisture content 10.07% 
Ash content 20.23% 
Volatile matter 14.32% 
Fixed carbon 55.38% 

SEM analysis

Figure 1(a) and (b) show the SEM images of virgin and 4NP-loaded AGAC. Figure 1(a) shows distinctly the porous surface (micro- and mesopores) of the adsorbent before adsorption, which are blocked after the adsorption of 4NP. Figure 1(b) reveals the adsorption of 4NP onto the AGAC surface and pores. The surface morphological features are comparable before and after adsorption of 4NP.
Figure 1

SEM picture of AGAC (a) before 4NP adsorption and (b) after 4NP adsorption onto surface.

Figure 1

SEM picture of AGAC (a) before 4NP adsorption and (b) after 4NP adsorption onto surface.

Point of zero charge

The pHPZC of the adsorbent was determined by using the solid addition method (Lataye et al. 2006). The pHPZC is the pH at which the adsorbent has neutral charge on the surface. The pHPZC for the AGAC was found to be 6.15 (Figure 2). During the adsorption study it was observed that the pH of the solution was increased after addition of the adsorbent. If pH of the solution is less than pHPZC, the adsorbent surface becomes positive and attracts anions from the solution, and if pH of the solution is greater than pHPZC, the surface becomes negative and attracts cations from the solution.
Figure 2

Point of zero charge for AGAC tested in 0.1 N NaCl solution.

Figure 2

Point of zero charge for AGAC tested in 0.1 N NaCl solution.

FTIR spectra

The FTIR study was performed for the adsorbent before and after the adsorption of 4NP (shown in Figure 3). Peaks found at 3,500.8 cm−1 and 3,300.2 cm−1 indicate OH stretching. Peaks at 3,030.17, 2,831.5, 2,362.8 and 1,707 cm−1 indicate C—H stretching of alkanes, aldehyde, C ≡ C alkynes and C = O carboxylic acid groups. The peak at 1,577.77 cm−1 indicates the nitro group, which confirms the adsorption of 4NP onto AGAC. Similarly, the slight increase in the intensities of peaks at 1,700 and 893.04 cm−1 shows the adsorption of 4NP (Figure 3).
Figure 3

FTIR spectrum of AGAC before and after 4NP adsorption.

Figure 3

FTIR spectrum of AGAC before and after 4NP adsorption.

MB surface area

By the MB surface area method, surface area of AGAC (SMB) was calculated. The SMB was found to be 311.20 m2/g, which was the monolayer surface area for adsorbent, mainly due to the mesopores of the material, which enhanced the adsorptive capacity of AGAC.

Batch adsorption study

The effect of various adsorption parameters, viz. adsorbent dose (m), initial pH (pH0), contact time (t), initial concentration (C0) of adsorbate and temperature (T) has been studied. The detailed discussion on the results of the batch adsorption study are given in the following sections.

Effect of adsorbent dose

The effect of adsorbent dose on removal of 4NP was studied by varying adsorbent dosage from 0.02 to 0.25 g at constant volume of 50 ml at initial concentration of 50 mg/l. The results are shown in Figure 4. It is observed that the percentage removal of 4NP is increased with the increase in adsorbent dose. The percentage removal of 4NP is increased rapidly from 74.17% at the dose of 0.4 g/l to 97.5% at the dose of 2 g/l, after which it becomes almost constant. The initial increase in percentage removal of the adsorption may be due to the availability of more adsorption sites as well as more surface area with increasing adsorbent dosage. However, the adsorption capacity (qe) decreases with increasing adsorbent dosage. The decrease in adsorption capacity with respect to adsorption dose (m) is due to the saturation of adsorption sites and the particle–particle interaction, which may lead to solid aggregation (Lataye et al. 2011). Since there is no increase in the adsorption beyond 2 g/l, the quasi-equilibrium condition has been considered at this dose and it is considered as the optimum adsorbent dose. The further study was conducted by using 2 g/l as an optimum adsorbent dose.
Figure 4

Effect of AGAC dose on adsorption of 4NP (C0 = 50 mg/l, volume of sample = 50 ml, temperature = 30°C, time of contact = 60 min, rpm = 150).

Figure 4

Effect of AGAC dose on adsorption of 4NP (C0 = 50 mg/l, volume of sample = 50 ml, temperature = 30°C, time of contact = 60 min, rpm = 150).

Effect of pH0

To study the effect of pH0 on adsorption of 4NP, the pH0 of solution with initial concentration 50 mg/l was varied from 2 to 12 by adding 0.1 N HCl or 0.1 N NaOH solution. The temperature of the solution was kept at 30 °C. The effect of pH0 on adsorption is shown in Figure 5. It is observed that the percentage removal of 4NP is more in the acidic pH range than the basic range. It does not change much with increase in initial pH of the solution from 2 to 6 where percentage removal was observed in the range of 98.38–98.94%, and after that it was decreased to 85.38% when pH0 was increased to 12. The maximum percentage removal of 98.94% was found at pH0 6.
Figure 5

Effect of pH on adsorption of 4NP onto AGAC (C0 = 50 mg/l Ma = 0.1 g, volume of sample = 50 ml, temperature = 30°C, rpm = 150).

Figure 5

Effect of pH on adsorption of 4NP onto AGAC (C0 = 50 mg/l Ma = 0.1 g, volume of sample = 50 ml, temperature = 30°C, rpm = 150).

4NP is a weak acid with pKa value of 7.15 (Carrera et al. 2011). When initial pH of the solution is in acidic range, especially less than 6, the majority of 4NP molecules remain in molecular form (shown in Figure 6); at the same time the AGAC surface possesses positive charge as the pH of solution is less than pHPZC (i.e. 6.15); due to this the removal was more in the acidic range. When initial pH of the solution is in alkaline range, the dissociation of 4NP into 4NP anions takes place (shown in Figure 6) and at the same time the AGAC surface possesses negative charge (because pH > pHPZC). In this state electrostatic repulsion dominates between the adsorbate and adsorbent surface, resulting in considerable decrease in percentage removal of 4NP in alkaline range. At pH of 6.15 the net charge on the AGAC surface was zero so there was no electrostatic repulsion and hence the removal of 4NP is more than in alkaline range. A similar pattern was observed by Tang et al. (2007).
Figure 6

Log C–pH diagram for 4NP.

Figure 6

Log C–pH diagram for 4NP.

Effect of contact time

Figure 7 shows the effect of contact time on adsorption of 4NP. It is noticed that from 2 minutes to 30 minutes there was significant increase in adsorption capacity and percentage removal of 4NP. The significant increase in percentage removal of 4NP is because of availability of ample adsorption sites at the initial stage of adsorption. Thereafter, it became constant when equilibrium was reached. The same trend was observed for both 50 and 100 mg/l concentrations. The 4NP removal was instantaneous: about 96.5% of 4NP was removed in 30 minutes at C0 = 50 mg/l. The residual 4NP concentration was found to be 1% after 1 hr. The percentage removal and adsorption capacity remained almost constant after 1 hr. Since the difference in 4NP removal at 30 minutes and 2 hrs was 2% only, 30 minutes has been considered as equilibrium time and further experiments were conducted at 30 minutes.
Figure 7

Effect of time of contact on adsorption of 4NP onto AGAC (C0 = 50 mg/l and 100 mg/l, Ma = 0.1 g, volume of sample = 50 ml, temperature = 30°C, rpm = 150).

Figure 7

Effect of time of contact on adsorption of 4NP onto AGAC (C0 = 50 mg/l and 100 mg/l, Ma = 0.1 g, volume of sample = 50 ml, temperature = 30°C, rpm = 150).

Effect of initial concentration and temperature

The effect of initial concentration C0 (50 mg/l ≤ C0 ≤ 500 mg/l) and temperature (10 °C ≤ T ≤ 50 °C) on the uptake of 4NP by AGAC at m = 2 g/l and t = 30 minutes was studied and results are shown in Figure 8.
Figure 8

Effect of initial concentration and temperature on adsorption of 4NP onto AGAC (Ma = 0.1 g, volume of sample = 50 ml, time of contact = 30 min, rpm = 150).

Figure 8

Effect of initial concentration and temperature on adsorption of 4NP onto AGAC (Ma = 0.1 g, volume of sample = 50 ml, time of contact = 30 min, rpm = 150).

It is observed that percentage removal of 4NP decreased, whereas adsorption capacity (qe) increased with increase in initial concentration. The decrease in percentage removal of 4NP with increasing concentration is because the fixed adsorbent mass adsorbs a fixed amount of 4NP at constant volume of adsorbent. The adsorption capacity (qe) increased because the resistance to the uptake of 4NP from the solution decreases (i.e. mass transfer driving force increases) with the increase in initial concentration (Lataye et. al. 2008b).

The temperature has a prominent effect on the adsorption capacity of AGAC. From Figure 8, it is observed that adsorption of 4NP decreased with increase in temperature, which indicates the exothermic nature of the process.

Adsorption kinetics and equilibrium

All the adsorption kinetic constants are shown in Table 3. The qe,cal and qe,exp values from the pseudo second order kinetic model are very close to each other and the correlation coefficients R2 are also close to unity as compared to the pseudo first order model (as shown in Table 3). Therefore the sorption can be more appropriately represented by the pseudo second order kinetic model, which indicates the sorption of 4NP over AGAC as dissociative sorption. The pseudo second order model was also suitable for the adsorption of lower molecular weight adsorbate on smaller adsorbent particles (Liu et al. 2010), which shows the applicability of this study.

Table 3

Comparison of the pseudo first and second order sorption rate constant, and calculated and experimental qe values for sorption of different concentration 4NP onto AGAC

C0 (mg/l) Pseudo first order kinetic model
 
Pseudo second order kinetic model
 
kf (min−1qe,cal (mg/g) qe,exp (mg/g) R2 ks (g/(mg min)) qe,cal (mg/g) qe,exp (mg/g) R2 
50 0.018 1.629 26.07 0.6448 0.065 26.110 26.07 
100 0.035 7.506 51.25 0.753 0.021 51.546 51.25 
C0 (mg/l) Pseudo first order kinetic model
 
Pseudo second order kinetic model
 
kf (min−1qe,cal (mg/g) qe,exp (mg/g) R2 ks (g/(mg min)) qe,cal (mg/g) qe,exp (mg/g) R2 
50 0.018 1.629 26.07 0.6448 0.065 26.110 26.07 
100 0.035 7.506 51.25 0.753 0.021 51.546 51.25 

On the basis of the error functions and from correlation coefficient R2, the order of fitting of isotherm data was found as Radke–Prausnitz > Toth > Redlich–Peterson > Langmuir > Freundlich > Temkin (Table 4 and Figure 9). A comparison of the experimental adsorption data with the linearized plot of Langmuir, Freundlich, Temkin, Redlich–Peterson, Toth and Radke–Prausnitz models is shown in Figure 9, which indicates that the Radke–Prausnitz isotherm is almost superimposed by the experimental data sequentially followed by Toth and Redlich–Peterson. Freundlich, Temkin and Langmuir showed moderate fitting. Thus the analysis of the linear isotherm plots suggested that the Radke–Prausnitz model yields much better fit than the other models. The theoretical maximum adsorption capacity (qmax) values calculated were 210.67, 210.80, 204.51, 198.41 and 189.63 mg/g at 10 °C, 20 °C, 30 °C , 40 °C and 50 °C, respectively. There is a decrease in qmax value from 10 °C to 50 °C, showing that higher temperature is not favourable for 4NP adsorption onto AGAC.
Table 4

Isotherms constants and their respective error functions for adsorption of 4NP onto AGAC

Isotherms Constants Temperatures (°C)
 
10 20 30 40 50 
Langmuir qm, mg/g 227.27 227.27 222.22 212.77 204.08 
KL, l/mg 0.09 0.09 0.09 0.07 0.07 
R2 (Linear) 0.99 0.99 0.99 0.98 0.99 
R2 (Non-Linear) 0.99 0.99 0.99 0.99 0.99 
Hybrid 11.01 9.94 9.51 12.07 10.47 
χ2 11.74 9.51 10.27 14.63 12.07 
Freundlich KF, l/mg 31.06 30.46 29.90 27.55 25.96 
2.23 2.20 2.26 2.31 2.35 
1/n 0.45 0.45 0.44 0.43 0.43 
R2 (Linear) 0.98 0.98 0.98 0.99 0.99 
R2 (Non-Linear) 0.99 0.99 0.99 0.99 0.99 
Hybrid −0.95 −1.36 −1.38 −0.52 −0.64 
χ2 5.61 6.40 6.47 2.63 3.67 
Temkin 39.89 40.37 38.60 36.54 34.94 
KT, l/mg 1.64 1.56 1.55 1.40 1.28 
R2 (Linear) 0.98 0.98 0.99 0.97 0.98 
R2 (Non-Linear) 0.99 0.99 0.99 0.98 0.99 
Hybrid 7.12 6.99 6.47 8.30 7.14 
χ2 8.01 7.10 6.18 16.63 11.33 
Redlich–Peterson KR, l/mg 38.18 35.70 35.04 49.47 40.75 
β 0.82 0.83 0.82 0.70 0.72 
aR, l/mg 0.40 0.36 0.38 0.99 0.82 
R2 (Linear) 1.00 1.00 1.00 1.00 1.00 
R2 (Non-Linear) 1.00 1.00 1.00 1.00 1.00 
Hybrid −0.58 −0.53 −0.35 −0.02 0.01 
χ2 1.43 0.98 0.74 0.16 0.06 
Toth Th 0.38 0.42 0.41 0.25 0.31 
qTh, mg/g 450.76 403.32 389.19 801.63 499.47 
KTh, (mg/l)Th 0.54 0.48 0.50 0.73 0.62 
R2 (Linear) 1.00 1.00 1.00 1.00 1.00 
R2 (Non-Linear) 1.00 1.00 1.00 1.00 1.00 
Hybrid −0.12 −0.02 −0.12 −0.14 0.04 
χ2 0.93 0.57 0.55 0.31 0.03 
Radke–Prausnitz 0.72 0.75 0.75 0.67 0.74 
krp, [(mg/g)/(mg/l)1/P63.69 70.92 67.57 42.92 55.56 
KRP, l/g 0.83 0.63 0.65 1.59 0.64 
R2 (Linear) 1.00 1.00 1.00 1.00 1.00 
R2 (Non-Linear) 1.00 1.00 1.00 1.00 1.00 
Hybrid −0.14 −0.02 −0.12 −0.14 0.23 
χ2 0.62 0.31 0.29 0.28 0.06 
Isotherms Constants Temperatures (°C)
 
10 20 30 40 50 
Langmuir qm, mg/g 227.27 227.27 222.22 212.77 204.08 
KL, l/mg 0.09 0.09 0.09 0.07 0.07 
R2 (Linear) 0.99 0.99 0.99 0.98 0.99 
R2 (Non-Linear) 0.99 0.99 0.99 0.99 0.99 
Hybrid 11.01 9.94 9.51 12.07 10.47 
χ2 11.74 9.51 10.27 14.63 12.07 
Freundlich KF, l/mg 31.06 30.46 29.90 27.55 25.96 
2.23 2.20 2.26 2.31 2.35 
1/n 0.45 0.45 0.44 0.43 0.43 
R2 (Linear) 0.98 0.98 0.98 0.99 0.99 
R2 (Non-Linear) 0.99 0.99 0.99 0.99 0.99 
Hybrid −0.95 −1.36 −1.38 −0.52 −0.64 
χ2 5.61 6.40 6.47 2.63 3.67 
Temkin 39.89 40.37 38.60 36.54 34.94 
KT, l/mg 1.64 1.56 1.55 1.40 1.28 
R2 (Linear) 0.98 0.98 0.99 0.97 0.98 
R2 (Non-Linear) 0.99 0.99 0.99 0.98 0.99 
Hybrid 7.12 6.99 6.47 8.30 7.14 
χ2 8.01 7.10 6.18 16.63 11.33 
Redlich–Peterson KR, l/mg 38.18 35.70 35.04 49.47 40.75 
β 0.82 0.83 0.82 0.70 0.72 
aR, l/mg 0.40 0.36 0.38 0.99 0.82 
R2 (Linear) 1.00 1.00 1.00 1.00 1.00 
R2 (Non-Linear) 1.00 1.00 1.00 1.00 1.00 
Hybrid −0.58 −0.53 −0.35 −0.02 0.01 
χ2 1.43 0.98 0.74 0.16 0.06 
Toth Th 0.38 0.42 0.41 0.25 0.31 
qTh, mg/g 450.76 403.32 389.19 801.63 499.47 
KTh, (mg/l)Th 0.54 0.48 0.50 0.73 0.62 
R2 (Linear) 1.00 1.00 1.00 1.00 1.00 
R2 (Non-Linear) 1.00 1.00 1.00 1.00 1.00 
Hybrid −0.12 −0.02 −0.12 −0.14 0.04 
χ2 0.93 0.57 0.55 0.31 0.03 
Radke–Prausnitz 0.72 0.75 0.75 0.67 0.74 
krp, [(mg/g)/(mg/l)1/P63.69 70.92 67.57 42.92 55.56 
KRP, l/g 0.83 0.63 0.65 1.59 0.64 
R2 (Linear) 1.00 1.00 1.00 1.00 1.00 
R2 (Non-Linear) 1.00 1.00 1.00 1.00 1.00 
Hybrid −0.14 −0.02 −0.12 −0.14 0.23 
χ2 0.62 0.31 0.29 0.28 0.06 
Figure 9

Comparison of the fit of the various equilibrium isotherm equations with the experimental sorption data for 4NP onto AGAC.

Figure 9

Comparison of the fit of the various equilibrium isotherm equations with the experimental sorption data for 4NP onto AGAC.

Thermodynamics of adsorption

The thermodynamic parameters ΔG0, ΔH0 and ΔS0 were calculated by using Equations (11) and (12) and the plot of lnKad versus 1/T (plot not shown here), and the values are presented in Table 5. The values of Gibbs free energy change are negative, showing that the adsorption of 4NP on AGAC is spontaneous. If the ΔG0 value is in the range of 0 to −20 kJ/mol it shows physical adsorption and if its range is −80 to −400 kJ/mol it refers to chemo-sorption (Liu et al. 2010). In this study all ΔG0 are in the range of −25.04 to −27.72 kJ/mol, indicating that the process is mainly physical adsorption which was enhanced by chemo-sorption. The positive value of ΔS0 (66.86 J/mol) shows a greater stability of the adsorption process with no structural changes at the solid–liquid interface. The negative value of ΔH0 (−6.12 kJ/mol) shows the exothermic nature of the adsorption process.

Table 5

Thermodynamic parameters for the adsorption of 4NP onto AGAC

ΔG0 (kJ/mol)
 
ΔH0 (kJ/mol) ΔS0 (J/(mol K)) 
283 K 293 K 303 K 313 K 323 K 
−25.04 −25.72 −26.38 −27.05 −27.72 6.12 66.86 
ΔG0 (kJ/mol)
 
ΔH0 (kJ/mol) ΔS0 (J/(mol K)) 
283 K 293 K 303 K 313 K 323 K 
−25.04 −25.72 −26.38 −27.05 −27.72 6.12 66.86 

Adsorption mechanism

4NP contains the aromatic ring of benzene with hydroxyl and nitro groups on opposite sides. This aromatic ring of phenol contains π electrons which have strong interaction toward the π electron of the activated carbon layer, resulting in π–π interaction. The electron withdrawing groups (like nitro group) enhanced the π–π interaction and reduces the repulsion between π electrons of consecutive aromatic rings, resulting in more sorption of 4NP on the AGAC surface.

Comparative study of AGAC with other adsorbents

Table 6 shows the comparison of AGAC with other adsorbents used for the removal of 4NP. From the table it can be interpreted that 4NP adsorption efficiency of AGAC is much better than the other adsorbents in several parameters.

Table 6

Comparison of AGAC with other adsorbent materials for removal of 4NP

Sr no. Adsorbent material Adsorbent dose, m (g/l) Time, t % Removal Monolayer qe (mg/g) Reference 
AGAC 30 minutes 98.94 204.79 Present study 
Charred sawdust 10 80 minutes 82.00 – Dutta et al. (2001)  
Activated carbon fibre 24 hrs 98.75 247.85 Tang et al. (2007)  
Amberlite IRA-900 – 30 minutes 86.00 – Marais & Nyokong (2008)  
Activated jute stick char 10 4 hrs – 39.38 Ahmaruzzaman & Gayatri (2010)  
Gemini surfactant modified montmorillonite – 40 minutes – 81.30 Xue et al. (2013)  
Cocoa shell based activated carbon 25 minutes – 167.17 Ahmad et al. (2011)  
Sr no. Adsorbent material Adsorbent dose, m (g/l) Time, t % Removal Monolayer qe (mg/g) Reference 
AGAC 30 minutes 98.94 204.79 Present study 
Charred sawdust 10 80 minutes 82.00 – Dutta et al. (2001)  
Activated carbon fibre 24 hrs 98.75 247.85 Tang et al. (2007)  
Amberlite IRA-900 – 30 minutes 86.00 – Marais & Nyokong (2008)  
Activated jute stick char 10 4 hrs – 39.38 Ahmaruzzaman & Gayatri (2010)  
Gemini surfactant modified montmorillonite – 40 minutes – 81.30 Xue et al. (2013)  
Cocoa shell based activated carbon 25 minutes – 167.17 Ahmad et al. (2011)  

Design of batch sorption system from isotherm data

An isotherm study can be used for design of a batch sorption system (Ozacar & Sengil 2004; Hameed et al. 2008; Li et al. 2010). It gives better understanding in industrial applications of the quantity of adsorbent required for achieving desired efficiency when treating 4NP wastewater. Figure 10 shows the schematic diagram for the single-stage batch sorption model for 4NP onto AGAC, where Ma is the mass of AGAC adsorbent (g), V is the volume of 4NP solution (l), C0 and C1 are the initial and final concentration of 4NP in effluent (mg/l), q0 and q1 are mg of 4NP adsorbed per g of AGAC for fresh and loaded adsorbent.
Figure 10

A single-stage batch sorption mass balance diagram.

Figure 10

A single-stage batch sorption mass balance diagram.

The mass balance for the single-stage batch sorption process is given by: 
formula
13
For equilibrium condition C1=Ce and q1=qe and q0 = 0 for fresh AGAC, Equation (13) can be modified and rearranged as: 
formula
14
The Radke–Prausnitz is the best fitted isotherm for adsorption of 4NP onto AGAC; so in place of qe we put the given equation: 
formula
15
By rearranging Equation (14): 
formula
16
On the basis of Equation (14) the value of Ma can be calculated for different volume of 4NP solution. The required mass of AGAC in (g) for varying volume of 4NP solution for the initial concentration of 100 mg/l, temperature of 30 °C and for different percentage removal efficiency (50, 60, 70, 80, 90 and 95%) for the batch sorption process is shown in Figure 11. For example, for volume of 50 l the required AGAC masses are 15.01, 19.31, 24.72, 32.47, 47.60 and 68.52 g for removal efficiency of 50, 60, 70, 80, 90 and 95% of 4NP, respectively.
Figure 11

AGAC adsorbent mass (Ma) against volume of 4NP (V) treated for different percentage removal efficiency.

Figure 11

AGAC adsorbent mass (Ma) against volume of 4NP (V) treated for different percentage removal efficiency.

CONCLUSIONS

The particle size of AGAC used in the study is 300–600 μ. The SEM micrograph clearly shows the pores on the surface, which make it effective for adsorption. The monolayer surface area for AGAC was found to be 311.20 m2/g; this large surface area makes the material more efficient for the sorption of 4NP. It is observed that adsorption of 4NP onto AGAC is maximum at pH0 of 6, with the removal of 98.94%. The optimum adsorbent dose was found to be 2 g/l at an initial concentration of 50 mg/l at normal temperature. The equilibrium time was found to be 30 minutes. The maximum and minimum removal was found to be 98.06 and 80.38% at initial concentration of 50 and 500 mg/l, respectively, at 30 °C temperature. The Radke–Prausnitz isotherm is found to be the best-fit isotherm in all the cases. With increase in temperature, adsorption decreases, which shows the exothermic nature of the process. The kinetic study indicates that the pseudo second order kinetic equation is applicable for the entire process. The overall results on the adsorption of 4NP onto AGAC shows the applicability of the adsorption study. The comparative study of the adsorbent revealed that AGAC is a better adsorbent as compared to the other adsorbents, with maximum adsorption efficiency (202.06 mg/g) and less equilibrium time (30 minutes). The adsorbent can be used as a low-cost adsorbent for the removal of phenolic compounds like 4NP. Because of the high sorptive capacity of prepared AGAC, it can be used as an alternative to commercial activated carbon in an economical way in the treatment of different industrial wastewaters and air for removal of various toxic pollutants. The AGAC remaining after 4NP sorption can be disposed of by incineration, which also produces usable thermal heat energy, and the ash formed after incineration can be either disposed of in landfill sites or used as partial replacement in cement manufacturing.

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