Removal of nitrobenzene from aqueous solution by adsorption onto carbonized sugarcane bagasse

Abstract

A sorbent was prepared by charring sugarcane bagasse (SCB) and used to remove nitrobenzene from aqueous solution. The surface area, morphology, and functional groups of the adsorbent were characterized by Brunauer–Emmett–Teller method, scanning electron microscopy, and Fourier transforms infrared spectroscopy. Analysis indicated that oxygen-containing functional groups, such as C = O, –OH, –COOH, and C–O–C, may be involved in the adsorption process. The adsorption of nitrobenzene was investigated under different operating conditions, including adsorbent dosage, initial nitrobenzene concentration, pH, and contact duration. Four kinetic models were applied to describe the adsorption process. Results revealed that the optimal sorbent mass was 0.3 g/50 mL at pH 5.8 and 25°C. The kinetic data obeyed the pseudo-second-order kinetic model (R² > 0.9965). In addition, Langmuir and Freundlich isotherm models were employed to describe the adsorption equilibrium. The Freundlich model presented better fitting for the adsorption equilibrium, suggesting that the carbonized SCB surface had a heterogeneous nature. The maximum adsorption capacities calculated by the Langmuir model were 38.27, 41.72, and 44.70 mg/g at 25°C, 35°C, and 45°C, respectively. The calculated values of ΔG⁰ and ΔH⁰ indicated the spontaneous and exothermic nature of the adsorption process at the considered temperature range. The adsorption mechanism of nitrobenzene onto carbonized SCB cannot be described either as physical adsorption or chemisorption. This study demonstrated that SCB biochar is a potential sorbent for removing nitrobenzene from aqueous solutions.

Keywords

Nitrobenzene adsorption carbonization sugarcane bagasse kinetic model

Introduction

Nitrobenzene (NB), which is a well-known highly toxic organic compound, can present high risks to ecological and human health even at low concentrations (Wang et al., 2009), and it is widely used in the manufacture of dyes, explosives, petrochemicals, and pesticides. After being used, the NB in solutions is generally discharged into wastewater treatment plants, where a substantial proportion of it cannot be removed by conventional treatment processes and is discharged into the surrounding aquatic environment (Qin and Xu, 2016). Given its poor biodegradability, long-term residue, and accumulation in the environment, many countries have listed NB as a priority pollutant. In 2014, the Office of Water, Science, and Technology in the US Environmental Protection Agency (EPA) decreased the tolerance limit of NB in water and organisms from 17 to 10 μg/L to promote public health and environmental protection (Dai et al., 2016). Therefore, strategies for removing NB from aqueous solutions must be developed.

The conventional methods for NB removal from aqueous solutions can be divided into three main categories: physical, chemical, and biological treatments (Wei et al., 2010). Adsorption is the most versatile and effective method for NB removal (Jin et al., 2011). Examples of adsorbents for NB adsorption include nanocrystalline hydroxyapatite (Wei et al., 2010), MCM-41 (Qin and Ma, 2010), marine sediments (Zhao et al., 2003), carbon materials released during woody biomass combustion (Dai et al., 2010), activated sludge modified by cetyltrimethylammonium bromide (Pand and Guan, 2010), and lipoid adsorption materials (Wen et al., 2012). Activated carbon is a commonly used adsorbent that possesses a large pore structure and excellent surface chemical adsorption characteristics (Wei et al., 2017); however, it is not economical. Therefore, low-cost adsorbents, such as industrial waste, natural ores, and agricultural by-products, are preferable (Sharma and Kaur, 2011).

As the main by-product of the sugar industry, sugarcane bagasse (SCB) is the fibrous residue of sugarcane stems after the extraction of sugar juice by crushing and pressing. SCB accounts for approximately 25% of the dry weight of sugarcane. The main components of SCB are cellulose, hemicellulose, and lignin, all of which contain various functional groups, such as hydroxyl, carboxyl, phenolic hydroxyl, and so on. Given that SCB has a high carbon content, is available abundantly, and is a non-toxic material, it can serve as a natural feedstock for manufacturing biocarbon to improve waste management and protect the environment. The biochar is characterized by a high surface area, a porous structure, and functional groups characterizing, suggesting that it might be a good alternative for removing different pollutants from aqueous solution (Abdelhafez and Li, 2016; Bhatnagar and Sillanpää, 2011). In addition, previous biochar adsorption studies have indicated that the magnitude adsorption capacity of NB on biochar varies at different temperatures. The reason is that the adsorbability of biochar depends strongly on its physical and chemical properties, which are affected by temperature. The adsorption isotherm equation can be used to understand the surface characteristics of the adsorbent and its interaction with the adsorbate. Thus, studies on equilibrium adsorption are vital to further elucidate the adsorption mechanism.

This study mainly aimed to examine the adsorption capacity of NB in aqueous solutions by using carbonized SCB. The adsorbent was characterized using Brunauer–Emmett–Teller (BET) method, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) techniques. The influences of several operating parameters, such as pH, contact time, temperature, initial NB concentration, and adsorbent dosage, were determined. The kinetic and equilibrium data of adsorption were analyzed to investigate the adsorption mechanism of NB.

Materials and methods

Materials

SCB was obtained and selected from a sugar factory in Guangxi, China. NB (98%, without further purification) was supplied by Shenyang Chemical Company, China. n-Hexane (C₆H₁₄) was acquired from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used as a chromatographic-grade reagent. All other chemicals used were of analytical grade.

Preparation of carbonized SCB

The carbonized SCB adsorbent used in this study was prepared as follows: Prior to its use, SCB was rinsed with running water, soaked in ultrapure water for 24 h, and then dried at 110°C for 24 h. The dried SCB was placed in a carbonization furnace at 500°C for 4 h. After natural cooling, the sample was grinded to powder (60 mesh). The obtained powder was the carbonized SCB. The adsorbent was stored in a desiccator for adsorption analyses.

Characteristics of adsorbent

The specific surface area of the carbonized bagasse adsorbent was determined using the NOVAe1200 automatic surface analyzer (Quantachrome, USA) for nitrogen adsorption–desorption experiments under nitrogen atmosphere at 77 K. The samples were degassed at 120°C for 3 h prior to the measurements. The specific surface area was calculated using BET equation. Total pore volume (V_total) and average pore diameter (d_av) were derived through Barrett–Joyner–Halenda (BJH) method.

The FTIR spectra of the samples were recorded on a NEXUS470 FTIR spectrometer (ThermoNicolet, USA) to characterize the various functional groups present in the carbonized bagasse. Analysis was conducted using KBr pellets made by mixing KBr with the fine powder of the carbonized bagasse samples (20:1 mass ratio of KBr to SCB). The resolution was set to 4 cm⁻¹, and the range was between 4000 and 500 cm⁻¹. The morphology of the adsorbent was directly observed through S–4800 scanning electron microscopy (Hitachi, Japan) analysis. The elemental compositions of samples were determined with an EA2400 II elemental analyzer (PerkinElmer, USA).

Adsorption experiments

Effects of adsorption conditions

The effects of the adsorption conditions on the NB adsorption onto the carbonized SCB were studied at pH 5.8. Desired amounts of carbonized SCB sorbent powder were placed in 50 mL of NB solution in a set of conical flasks and agitated. Then, the samples were placed in a water bath oscillator at a constant temperature (25°C, 35°C, or 45°C) and an agitation speed of 200 r/min until the adsorption equilibrium of the solid–solution mixture was reached. All samples were collected and filtered by a micropore membrane with a pore size of 0.45 μm, and 1–2 mL of the filtrate was discarded. The residual concentrations of the NB solution were determined by gas chromatography (Agilent6890, USA).

Seven initial concentrations of NB (50, 100, 150, 200, 300, 400, and 500 mg/L) were used to determine the effects of the initial concentration at 25°C, 35°C, or 45°C. The effect of the adsorbent dosage was studied using different adsorbent doses (0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, and 1.0 g) and 50 mL of 200 mg/L NB solution at 25°C. The NB solution was studied at different pH (2–10), and the solutions were adjusted using HCl and NaOH solutions.

Adsorption kinetics

Kinetic studies were performed in a set of conical flasks (100 mL) at a constant temperature (25°C) by shaking 0.3 g of carbonized SCB sorbent powder in 50 mL NB solutions (50, 200, and 400 mg/L; initial pH = 5.8). Then, each capped conical flask was placed in a water bath oscillator at an agitation speed of 200 r/min. Aqueous samples were obtained from different conical flasks at different time intervals (5, 10, 15, 20, 30, 40, 50, 60, 70, and 90 min). All samples were filtered similarly through a 0.45 μm membrane filter to remove the carbon fines. The total NB concentration in aqueous solutions was determined through gas chromatography (Agilent6890, USA). The adsorption capacity of NB at time t, q_t (mg·g⁻¹) was calculated using the following equations

q_{t} = (C_{0} - C_{t}) V / W

(1)

where C₀ (mg·L⁻¹) is the initial concentration of NB, C_t (mg·L⁻¹) is the liquid phase concentration of NB at any time, V (L) is the volume of the solution, and W (g) is the quality of the SCB used.

Adsorption isotherm

The NB adsorption capacity of the carbonized SCB was obtained using adsorption isotherms. In this experiment, 50 mL of NB solutions at different concentrations (50, 100, 150, 200, 300, 400, and 500 mg/L) was mixed with 0.3 g carbonized SCB at 25°C, 35°C, or 45°C in a water bath oscillator at an agitation speed of 200 r/min. After 90 min, the bottles were removed from the oscillator, and the final concentrations of NB in the solution were analyzed. The amount of adsorbed NB per unit of adsorbent mass, q_e (mg·g⁻¹), was calculated using equation (2)

q_{e} = (C_{0} - C_{e}) V / W

(2)

where C₀ and C_e (mg·L⁻¹) are the initial and equilibrium concentrations, respectively, of the adsorbate; V (L) is the experimental volume of the adsorbate in the solution; and W (g) is the mass of the adsorbent used.

Result and discussion

Characterization of the sorbent

Table 1 shows the porosity of the raw and carbonized SCB. SCB possesses not only a larger pore size than the diameter of the NB molecule (0.33 ± 0.02 nm) but also a larger specific surface area. Therefore, SCB can be used as an adsorbent for NB adsorption. The trend of the BET surface area followed the order of carbonized SCB (191.1 m²·g⁻¹) > raw SCB (65.89 m²·g⁻¹), indicating that the surface area of the carbonized SCB sorbents significantly increased after modification. The carbonized SCB exhibited a larger total pore volume and average pore diameter than those of the SCB. After calcination, the components of the SCB were decomposed, and the volatiles were released. The volume of the pores was increased, and new pores were formed.

Table 1.

Porosity parameter of different sorbents.

Sample	Specific surface area (m²·g⁻¹)	Total pore volume (cm³·g⁻¹）	Average pore diameter (nm)
Raw SCB	65.89	0.04	1.91
Carbonized SCB	191.1	0.17	1.95

Figure 1 presents the SEM images of the SCB and carbonized SCB sorbent. Figure 1(a) and (b) shows the SEM diagrams of the SCB and the partially magnified SCB, and Figure 1(c) and (d) depicts the SEM diagrams of the carbonized SCB and the partially magnified carbonized SCB. As illustrated in Figure 1(a), the bagasse was in the form of debris, and the surface of the sample contained few unevenly distributed holes. As shown in Figure 1(b), numerous micropores were observed in the inner channel of the SCB. After carbonization, the carbonized SCB exhibited a sponge-like structure, and the number of small pores were significantly increased compared with that in the SCB.

Figure 1.

SEM images of (a) SCB, (b) partially magnified SCB, (c) carbonized SCB, and (d) partially magnified carbonized SCB.

Figure 2 shows the FTIR spectra of the unmodified and carbonized SCB. The SCB presented six peaks located at 3439.88, 2921.12, 1630.70, 1383.77, 1058.06, and 670.51 cm⁻¹. The band observed at 3439.88 cm⁻¹ represented the stretching vibration of the hydroxyl groups. The band at 2921.21 cm⁻¹ was attributed to the CH₂ units in the biopolymers (Chen et al., 2005). The peak at 1630.70 cm⁻¹ was assigned to C = O vibrations in hemicellulose. The band at 1383.77 cm⁻¹ was attributed to the bending vibration of –COOH and the phenol groups (Xu et al., 2008). An adsorption band appeared at 1058.06 cm⁻¹, which represented the aliphatic C–O–C and alcohol –OH in cellulose (Chen et al., 2008). The peak at 670.51 cm⁻¹ was assigned to the out-of-plane bending of –OH. All these bands changed after pyrolysis. In view of the humid climate in March and April in Guilin, the carbonized SCB was wet, such that the –OH peak (3440.24 cm⁻¹) intensity was too strong. The band intensities at 2923.24 (CH₂) and 1121.21 cm⁻¹ (aliphatic C–O–C and alcohol–OH) were decreased, whereas the bands at 1629.90 (C = O) and 1383.94 cm⁻¹ (–COOH and phenol group) were preserved. The intense band for –OH (670.51 cm⁻¹) disappeared after carbonization, indicating that the hydrogen bonds among cellulose, lignin, and hemicellulose were ruptured during pyrolysis. Analysis suggested that oxygen-containing functional groups (e.g., C = O, –OH, –COOH, and C–O–C) may be involved in the adsorption process and that they can provide sufficient adsorption sites for adsorption.

Figure 2.

FTIR spectra for raw SCB and the carbonized SCB.

Table 2 presents the elemental analysis results for the SCB and the carbonized SCB. Carbon and oxygen were the main components of biochar. The carbon content increased from 44.28 to 56.02% after carbonization, followed by a simultaneous decrease in oxygen and hydrogen contents. The molar ratios of O/C and (N + O)/C also decreased because of the reduction of the surface hydrophilic and polar functional groups. The H/C ratio > 1.0 suggested that the carbonized SCB contained a good amount of original organic residues (Chen et al., 2008).

Table 2.

Elemental analysis results of samples.

Sample	Raw SCB	Carbonized SCB
C (%)	44.28	56.02
H (%)	5.83	5.41
O (%)	48.51	21.98
N (%)	0.18	0.45
C/H	1.58	1.16
O/C	0.82	0.29
(N+O)/C	0.83	0.30

Effects of sorption conditions

The effects of contact time, mass of carbonized SCB, and initial concentration on the adsorption of NB were investigated. All parameters, except the desired one, were set as constant.

Contact time

Equilibrium time is an important parameter used to assess the efficiency and feasibility of an adsorbent (Sharma, 2003). Figure 3 shows the effect of contact time on the adsorption of the carbonized SCB. The NB concentration decreased sharply during the first period of contact (20 min) possibly due to the abundant available vacant sites on the adsorbent surface (Kavitha and Namasivayam, 2007). Figure 3 reveals that the amount of adsorbed NB (mg/g) increased with increasing contact time until it gradually approached the equilibrium state because of the continuous decrease in driving force and NB molecular diffusion rate. After adsorption equilibrium, the amount of adsorbed NB and the concentration of NB in the liquid phase remained nearly constant, and the average removal efficiency of NB reached approximately 99.2% (8.25 mg/g adsorption capacity), 71.2% (23.69 mg/g adsorption capacity), and 53.6% (35.62 mg/g adsorption capacity) at 25°C and pH 5.8 at initial NB concentrations of 50, 200, and 400 mg/L, respectively. The removal rate of NB at the adsorption equilibrium decreased with increasing initial NB concentration. This finding could be due to the limited number of adsorption sites available for the uptake of NB at a fixed adsorbent dosage.

Figure 3.

Effect of the contact time on NB adsorption by the carbonized SCB (initial pH = 5.8, amount of sorbent 0.3 g/50 mL, 25°C).

Adsorbent dose

The adsorption of NB onto the carbonized SCB sorbent was studied by varying the adsorbent quantity in the test solution while maintaining the contact duration, initial NB concentration, and initial pH fixed at 25°C. Figure 4 demonstrates that the removal ratio of NB increased with increasing adsorbent dosage because of the larger surface area and the availability of numerous adsorption sites on the surface (Abdelhafez and Li, 2016). However, the amount of adsorbed NB per unit mass of the adsorbent decreased with increasing adsorbent mass. This finding may be attributed to the overlapped or aggregated adsorbent surface area available to NB and to the increased diffusion path length (Yinian et al., 2011). The percentages of adsorbed NB increased from 40.30% to 88.00% on the carbonized SCB, whereas the amount of adsorbed NB decreased from 80.28 mg/g to 8.8 mg/g as the dose of the carbonized SCB adsorbent was increased from 0.05 g/50 mL to 1.0 g/50 mL. A ratio higher than 10 g/L did not linearly improve the adsorption rate, indicating that the availability of the adsorption surface was not the limiting factor in the process. Thus, the remaining experiments applied an adsorbent dose of 0.3 g/50 mL for the carbonized SCB.

Figure 4.

Effect of adsorbent doses on the adsorption of NB on the carbonized SCB (initial concentration 200 mg/L, at pH = 5.8 and 25°C).

Initial NB concentration

Kinetic dependencies were measured at various initial NB concentrations (50–500 mg/L) at 25°C, 35°C, and 45°C while the other experimental parameters were kept constant. Figure 5 shows the dependencies of the adsorption capacity and the removal percentage versus the initial concentration. The overall trend was similar for different temperatures: as the NB concentrations in the test solution increased, the actual amount of adsorbed NB per unit mass of adsorbent increased from 8.26 mg/g to 36.67 mg/g at 25°C, from 8.29 mg/g to 40.47 mg/g at 35°C, and from 8.32 mg/g to 43.76 mg/g at 45°C. When the initial concentration was less than 400 mg/L, the number of consumed adsorption sites decreased with increasing initial concentration of NB, resulting in decreased number of remaining adsorption sites on NB. At this stage, the adsorption capacity constantly increased. As the initial concentration increased, the total available adsorption sites were completely replaced by NB, and the saturation adsorption capacity was achieved. By contrast, the adsorption rate decreased continuously from 99.20% to 44.06% at 25°C, from 99.67% to 48.66% at 35°C, and from 99.81% to 52.62% at 45°C as the initial NB concentration increased from 50 to 500 mg/L.

Figure 5.

Effect of initial concentration on NB sorption onto the carbonized sorbent prepared from SCB (initial pH = 5.8, amount of sorbent 0.3 g/50 mL).

Solution pH

To determine the effect of pH on the adsorption capacity of the carbonized SCB, solutions were prepared at different pH values ranging from 2 to 10. Figure 6 presents the dependence of pH on the adsorption of NB. The adsorption effect of the carbonized SCB on NB in aqueous solution was hardly affected by changing the pH of the solution. When the pH was increased from 5.8 to 10, the amount of adsorbed NB decreased from 29.54 to 28.72 mg/g. Given that NB is a nonionizable compound, the change in pH exerted an insignificant effect on the solubility of NB. Therefore, in this study, the pH was 5.8 under all experimental conditions (the amount of adsorbed NB was relatively high).

Figure 6.

Effect of pH on NB adsorption (initial concentration 200 mg/L, amount of sorbent 0.3 g/50 mL, 25°C).

Sorption kinetics

Kinetics is a critical parameter in the design of the sorption process and in evaluating the suitability of a sorbent because it controls the size of the sorption units (Rezgui et al., 2017). Four famous kinetic models, namely, pseudo-first-order, pseudo-second-order, modified pseudo-first-order, and intraparticle diffusion, were used to fit the experimental data of adsorption time and capacity by linear fitting.

Pseudo-first-order kinetic model

The pseudo-first-order kinetic model, which is commonly used to determine the adsorption rate based on the adsorption capacity (Somasekhara et al., 2012), can be expressed as follows

In (q_{e} - q_{t}) = In q_{e} - k_{1} t

(3)

where q_t (mg·L⁻¹) is the amount of adsorbed NB at time t (min), q_e (mg·L⁻¹) is the amount of the adsorbed NB on the adsorbent at time under equilibrium conditions, and k₁ is the pseudo-first-order rate constant (min⁻¹).

After the values of ln(q_e−q_t) versus t were plotted, a straight line was obtained with slope −k₁ and intercept lnq_e (Figure 7). Table 3 lists the values of k₁ and q_e at different initial NB concentrations. The correlation coefficient value R² for NB adsorption onto the carbonized SCB sorbent changed in the range of 0.9547–0.9909. Thus, the experimental data did not agree well with the pseudo-first-order kinetic model.

Figure 7.

Four kinetic models for NB sorption onto the carbonized sorbent prepared from SCB (initial pH = 5.8, amount of sorbent 0.3 g/50 mL, 25°C).

Table 3.

Kinetic parameters for NB sorption onto the carbonized sorbent prepared from sugarcane.

Initial NB concentration (mg/L)	k₁/(1/min)	q_e (mg/g)		R ²
Pseudo-first-order constants
50	0.0586	4.58		0.9547
200	0.0612	20.69		0.9909
400	0.0289	19.23		0.9559
Modified pseudo-first-order constants
50	0.0515	8.32		0.9738
200	0.0528	34.14		0.9776
400	0.0224	32.82		0.9747
Initial NB concentration (mg/L)	k₂/(g/(mg min))	q_e (mg/g)	h/(g/(mg min))	R ²
Pseudo-second-order constants
50	0.0180	8.92	1.4304	0.9981
200	0.0038	26.65	2.7013	0.9984
400	0.0030	37.86	4.2985	0.9965
Initial NB concentration (mg/L)	K_d/(g/(mg min^1/2))		R ²
Intraparticle diffusion constants
50	0.5902		0.7262
200	1.9743		0.8924
400	2.5135		0.9139

Modified pseudo-first-order kinetic model

The pseudo-first-order equation was modified by changing the rate constant (Yang and Al-Duri, 2005). By denoting the rate constant in the modified pseudo-first-order rate equation as K₁ (min⁻¹), the following equation was proposed

k_{1} = K_{1} \frac{q_{e}}{q_{t}}

(4)

The modified pseudo-first-order equation can be obtained as follows

\frac{q_{t}}{q_{e}} + In (q_{e} - q_{t}) = In (q_{e}) - K_{1} t

(5)

If the adsorption process followed the modified pseudo-first-order kinetic model represented by equation (5), then the plot of q_t/q_e+ln(q_e−q_t) against t should be a straight line (Figure 7). Table 3 presents the values of K₁ and q_e at different temperatures. The correlation coefficient values for NB adsorption onto the carbonized SCB sorbent changed in the range of 0.9738–0.9776. Thus, the experimental data did not fit well with the modified pseudo-first-order kinetic model.

Pseudo-second-order kinetic model

The pseudo-second-order kinetic model, which assumes that sorption is the interaction between the functional groups on the adsorbent surface and the adsorbate (Somasekhara et al., 2012), can be represented as follows

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

(6)

The initial adsorption rate, h (mg/(g min)), at t = 0 can be defined as follows

h = k_{2} q_{e}^{2}

(7)

where the initial adsorption rate (h), the equilibrium adsorption capacity (q_e), and the pseudo-second-order constants k₂ (g/(mg min)) can be determined experimentally from the slope and intercept of plot t/q_t versus t (Figure 7). The calculated correlations were closer to unity for the pseudo-second-order kinetics model (R² = 0.9965–0.9984); therefore, the adsorption kinetics favorably fitted with the pseudo-second-order kinetic model for the NB adsorption onto the carbonized SCB sorbent. The k₂ and h values were calculated from Figure 7 as 0.0030–0.0180 g/(mg·min) and 1.4304–4.2985 mg/(g·min), respectively (Table 3). The equilibrium adsorption capacities (q_e) were close to the actual values determined by the experiment.

Intraparticle diffusion kinetic model

The reaction rate can be modeled using the resistance to intraparticle diffusion if the sorption is not limited by the reaction between the sorbate and the active sites. A kinetic model was developed by Weber and Morris Rezgui et al. (2017) and defined as follows

q_{t} = K_{d} t^{1 / 2} + C

(8)

where K_d is the adsorption constant of W-M (mg·g⁻¹·min^−1/2), and C is constant. According to equation (8), the plot of q_t versus t^1/2 should be a straight line with slope K_d and intercept C if the adsorption mechanism followed the intraparticle diffusion process. The values of the intercept represented the thickness of the boundary layer; i.e., a larger intercept corresponded to a greater boundary layer effect.

Figure 7 shows the plot of the mass of adsorbed NB per unit mass of adsorbent; q_t versus t^1/2 is presented for NB. The values of K_d obtained from the slope of the straight lines were 0.5902–2.5135 g/(mg·min^1/2). As shown in Table 3, the correlation coefficient value R² for the NB adsorption onto the carbonized SCB sorbent changed in the range of 0.7262–0.9139. Thus, the experimental data did not agree well with the intraparticle diffusion kinetic model.

Kinetic profiles can be used not only to determine the equilibrium time (as a function of experimental conditions) but also to identify the controlling (limiting) mechanisms and steps (Rezgui et al., 2017). Among the four kinetic models, the pseudo-second-order equation generates the best fit to the experimental data of the three investigated adsorption systems at initial NB concentrations of 50, 200, and 400 mg/L for the entire adsorption period. The regression coefficients were higher than 0.9965 for the concentration range used. The calculated q_e from the model was close but not equal to the experimental values. Thus, the adsorption of NB onto the carbonized SCB was associated with a mechanism involving the chemical interaction between the sorbent and the solute (Zabihi et al., 2010), implying that NB adsorption probably occurred via surface exchange reactions until the surface functional sites were fully occupied. After which, the NB molecules diffused into the adsorbent network for further interactions.

The other models generated a poor fit to the results of the experiments conducted in this study. The pseudo-first-order kinetic model and the modified pseudo-first-order kinetic model exhibited lower correlation coefficients than the pseudo-second-order kinetic model, and the calculated q_e differed from the experimental q_e. This finding suggested the insufficiency of the model fitted the kinetic data for the initial concentrations examined. After a short period, the experimental data deviated considerably from linearity.

In the intraparticle diffusion model, if intraparticle diffusion is involved in the sorption process, then the plot of q_t against t^1/2 should be linear; if these lines pass through the origin (if the intercept C is equal to zero), then intraparticle diffusion is the rate-controlling step (Banat et al., 2003). The values of the constant C at different concentrations were not zero, indicating that particle diffusion was involved but was not the limiting step. Furthermore, the intraparticle diffusion model was inapplicable to the investigated adsorption systems (Yang and Al-Duri, 2005).

Sorption isotherm

Adsorption isotherms can describe how adsorbate molecules interact with the adsorbent surface between the liquid phase and the solid phase at equilibrium (Chai et al., 2016). The two isotherm models, Langmuir and Freundlich, are widely employed to describe the experimental data of adsorption isotherms.

Langmuir isotherm

The Langmuir equation is applicable to homogeneous adsorption, where the adsorption of each molecule onto the surface has an equal adsorption activation energy (Crini, 2008). The linear form of the Langmuir isotherm equation was represented as follows (Jianlong, 2002)

C_{e} / q_{e} = 1 / (q_{m} \times K_{L}) + C_{e} / q_{m}

(9)

where C_e is the concentration of the NB solution (mg·L⁻¹) at equilibrium; q_m signifies the maximum adsorption capacity (mg·g⁻¹); and K_L is Langmuir constant related to the free energy of adsorption (L·mg⁻¹). The linear plot of C_e/q_e versus C_e showed that the adsorption followed the Langmuir isotherm (Figure 8). q_m and K_L were calculated from the slope and intercept of the linear plot (Table 4). The R² values were 0.9879, 0.9931, and 0.9955 for the adsorption at 25°C, 35°C, and 45°C, respectively. Increasing the temperature improved the adsorption capacities from 38.27 mg/g at 25°C to 41.72 mg/g at 35°C and to 44.70 mg/g at 45°C. This finding demonstrated that the adsorption process was endothermic; i.e., increasing the adsorption temperature can improve the adsorption capacity of the adsorbent.

Figure 8.

Isotherm plots for NB adsorption onto the carbonized SCB sorbent. (a) Langmuir isotherm; (b) Freundlich isotherm.

Table 4.

Isotherm parameters for NB sorption onto the carbonized SCB sorbent.

Temperature (°C)	q_m (mg/g)		K_L (L/mg)		R _L		R ²
Langmuir constants
25	38.27		0.061		0.0317–0.2466		0.9879
35	41.72		0.084		0.0231–0.1915		0.9931
45	44.70		0.129		0.0152–0.1339		0.9955
Temperature (°C)		K_F (mg^{1 − 1/n}L^1/ng⁻¹)		1/n		R ²
Freundlich constants
25		9.96		0.2351		0.98873
35		12.00		0.2237		0.99366
45		14.18		0.2167		0.99654

The R_L parameter is considered a more reliable indicator of adsorption. R_L values have four possibilities: (1) for favorable adsorption, 0 < R_L < 1; (2) for unfavorable adsorption, R_L > 1; (3) for linear adsorption, R_L = 1; and (4) for irreversible adsorption, R_L = 0 (Senturk et al., 2009).

Table 4 shows that the R_L near 0 confirmed the favorable uptake of the NB process, which indicated a favorable adsorption.

Freundlich isotherm

The Freundlich isotherm is an empirical equation describing the adsorption of an adsorbate onto a heterogeneous surface of an adsorbent as well as multilayer adsorption (Chai et al., 2016). This equation is expressed as follows

In q_{e} = In K_{F} + 1 / n In C_{e}

(10)

where q_e refers to the amount of adsorbed NB (mg·g⁻¹) at equilibrium; C_e is the equilibrium concentration of NB in the solution (mg·L⁻¹); the Freundlich constant K_F (mg^{1 − 1/n}·L^1/n·g⁻¹) is an approximate indicator of the adsorption capacity; and 1/n is the adsorption intensity.

K_F and n were calculated from the intercept and the slope of plots (Figure 8, Table 4). The obtained R² values were 0.9887, 0.9937, and 0.9965 for the adsorption at 25°C, 35°C, and 45°C, respectively. 1/n is a heterogeneity factor that can be used as a measure of the deviation from linearity of the adsorption (Crini, 2008). In this study, the 1/n values were calculated from the slope as 0.2167–0.2351, which ranged from 0 to 1 and implied that the adsorption involved chemical reactions.

The Langmuir and Freundlich equations were well described for NB adsorption onto the carbonized SCB under isothermal conditions. However, according to the correlation coefficient R², the Freundlich isotherm could yield a better fit than the Langmuir isotherm, indicating that the adsorption of the carbonized bagasse on NB was dominated by the multi-molecular layer and heterogeneous nature of the adsorbent surface. At the same time, the correlation coefficient R² of the Langmuir equation was also above 0.98, suggesting that the adsorption on the carbonized SCB was a monolayer adsorption, which depended on the diffusion force.

Thermodynamic parameters

Thermodynamic parameters were calculated to describe the sorption of NB onto the carbonized SCB and to gain insight into the mechanism involved in the sorption process. The thermodynamic parameters for the adsorption of ΔH⁰ (enthalpy), ΔS⁰ (entropy), and ΔG⁰ (Gibbs energy) were calculated using equations (11) and (12)

Δ G^{0} = - R T In K_{L}

(11)

Δ G^{0} = Δ H^{0} - T Δ S^{0}

(12)

where R is the gas constant and has the value of 8.314 J K⁻¹·mol⁻¹, K_L is the Langmuir constant, and T is the experiment temperature expressed in Kelvin (K).

The values of ΔH⁰ and ΔS⁰ were determined from the slope and intercept of the linear plot of ΔG⁰ versus T. The negative values of the calculated ΔG⁰ (−5.002, −5.998, and −7.321 kJ/mol) confirmed the spontaneous nature of the NB adsorption at 25°C, 35°C, and 45°C, and verified the affinity of the sorbent for NB. The positive calculated values of ΔH⁰ (29.63 J/mol) showed the endothermic nature of adsorption. The positive values of ΔS⁰ (0.1160 kJ/(mol·K)) suggested the increased randomness at the solid/solution interface during the adsorption of NB onto the adsorbent.

Regeneration

Regeneration studies can reveal the possibility of recycling the adsorbent. Desorption was performed using EtOH. The adsorption capacity significantly changed after the third reuse cycle, and the regeneration efficiencies for the three times were 29.56%, 10.38%, and 8.27%. This result suggested that the carbonized SCB was unsuitable for many cycles.

Comparisons of NB adsorption

To assess the performance of the carbonized SCB as an adsorbent for NB, it was compared with other adsorbents, and the data are presented in Table 5. As expected, a good adsorbent should have a large surface area and sufficient active sites to generate a great adsorption capacity. The maximum adsorption capacity in Table 5 was calculated by the Langmuir isotherm model. The results suggested that the amount of adsorbed NB onto raw SCB was insignificant compared to that onto the carbonized SCB. This finding is consistent with the results obtained by BET method and SEM images. Moreover, the adsorption capacity of the carbonized SCB for NB was higher than natural or organic synthetic adsorbent but lower than that of activated carbon. The low adsorption capacity was most probably due to the small surface area of carbonized SCB (191.1 m²/g) compared to activated carbons (around 1400 m²/g) (Dai et al., 2010; Villacañas et al., 2006). The carbonized SCB adsorbent utilizes agricultural waste, has a simple preparation process, and is environmentally friendly. Therefore, the carbonized SCB is a potential biochar material adsorbent for NB removal from aqueous solutions.

Table 5.

Comparison of the adsorption capacities of NB for various adsorbents.

Adsorbent	Temperature (°C)	q_m (mg/g)	References
Carbonized SCB	25	38.27	This work
Raw SCB	25	2.69	This work
Nanocrystalline hydroxyapatite	25	5.754	Wei et al. (2010)
Marine sediment	25	4.755	Zhao et al. (2003)
Wood-based activated carbon	25	263	Dai et al. (2010)
Granular activated carbon	25	234	Villacañas et al. (2006)

Mechanism of NB adsorption

Given that SCB has a high carbon content, it can pyrolyze the carbonization of SCB under complete or partial anoxic conditions to prepare bio-carbonaceous materials. In general, biochar will generate a highly aromatic and insoluble solid material after pyrolysis carbonization. This substance has a similar structure and polarity to NB. As its specific surface area increases, it will develop a good stability and an improved pore structure. Therefore, carbonaceous materials have the potential adsorption capacity of NB in environmental governance. In this study, the considerable number of surface oxygen-containing functional groups in the surface of carbonized SCB could provide adequate adsorption sites and improve adsorption capacity. From a kinetic point of view, the adsorption of NB onto carbonized SCB was associated with a mechanism involving chemical interactions. However, from the adsorption isotherm analysis, the adsorption process presented monolayer and multi-molecular layer adsorption processes. To a certain extent, the magnitude of the enthalpy can be classified as physical adsorption and chemisorption. The range of 5–40 kJ/mol of ΔH⁰ indicates to a physisorption mechanism, and the range of 40–800 kJ/mol suggests a chemisorption mechanism (Gerçel et al., 2007). The positive values of ΔH⁰ (29.63 J/mol) indicated the adsorption of NB was an endothermic and physisorption process. Therefore, the mechanism of NB sorption from aqueous solution onto carbonized SCB can be described neither as a physical adsorption nor a chemisorption.

Conclusion

Carbonized SCB can be used as a low-cost sorbent for NB removal from aqueous solutions. BET, SEM, and FTIR analyses were conducted to characterize the adsorbent. Oxygen-containing functional groups (e.g., C = O, –OH, –COOH, and C–O–C) may be involved in the adsorption process. The adsorption capacity was affected by the adsorbent dosage, contact time, pH, and initial concentration. The amount of adsorbed NB gradually increased with increasing contact time and was maintained until equilibrium was reached. At pH 5.8 and 25°C, the adsorption rates were 99.2%, 71.2%, and 53.6%, and the adsorption capacities were 8.25, 23.69, and 35.62 mg/g at initial NB concentrations of 50, 200, and 400 mg/L, respectively. Furthermore, the optimal adsorbent dose for the carbonized SCB was 0.3 g/50 mL. The adsorption kinetics and isotherms followed the pseudo-second-order model and the Freundlich equation. The maximum adsorption capacities q_m were 38.27, 41.72, and 44.70 mg/g. The calculated values of ΔG⁰ and ΔH⁰ indicated the spontaneous and exothermic nature of the adsorption process at the examined temperature range. Analysis implied that the mechanism of NB sorption from aqueous solution onto carbonized SCB can be described neither as a physical adsorption nor a chemisorption.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article:This work was financially supported by the National Natural Science Foundation of China (no. 51368012, 51668014); the Natural Science Foundation of Guangxi Province, China (no. 2016GXNSFDA380007); the Special Funding for “BaGui Scholars” Construction Projects; the Guangxi Scientific Experiment Center of Mining, Metallurgy and Environment, and the Guangxi Talent Highland for Hazardous Waste Disposal Industrialization.

References

Abdelhafez

and Li

(2016) Removal of Pb(II) from aqueous solution by using biochars derived from sugar cane bagasse and orange peel. Journal of the Taiwan Institute of Chemical Engineers 61: 367–375.

Banat

Al-Asheh

and Al-Makhadmeh

(2003) Evaluation of the use of raw and activated date pits as potential adsorbents for dye containing waters. Process Biochemistry 39(2): 193–202.

Bhatnagar

and Sillanpää

(2011) A review of emerging adsorbents for nitrate removal from water. Chemical Engineering Journal 168: 493–504.

Chai

Meng

Zhao

et al . (2016) Removal of color compounds from sugarcane juice by modified sugarcane bagasse: Equilibrium and kinetic study. Sugar Tech 18(3): 317–324.

Chen

Johnson

Chefetz

et al . (2005) Sorption of polar and nonpolar aromatic organic contaminants by plant cuticular materials: The role of polarity and accessibility. Environmental Science & Technology 39: 6138–6146.

Chen

Zhou

and Zhu

(2008) Transitional adsorption and partition of nonpolar aromatic contaminants by biochars of pine needles with different pyrolytic temperatures. Environmental Science & Technology 42: 5137–5143.

Crini

(2008) Kinetic and equilibrium studies on the removal of cationic dyes from aqueous solution by adsorption onto a cyclodextrin polymer. Dyes and Pigments 77: 415–426.

Dai

Mihara

Tanaka

et al . (2010) Nitrobenzene-adsorption capacity of carbon materials released during the combustion of woody biomass. Journal of Hazardous Materials 174: 776–781.

Dai

Zhang

and Zhang

(2016) Nitrobenzene-adsorption capacity of NaOH-modified spent coffee ground from aqueous solution. Journal of the Taiwan Institute of Chemical Engineers 68: 232–238.

10.

Gerçel

Özcan

et al . (2007) Preparation of activated carbon from a renewable bio-plant of Euphorbia rigida by H₂SO₄ activation and its adsorption behavior in aqueous solutions. Applied Surface Science 253: 4843–4852.

11.

Jianlong

(2002) Biosorption of copper (II) by chemically modified biomass of Saccharomyces cerevisiae. Process Biochemistry 37: 847.

12.

Jin

and Wei

(2011) Selective adsorption of phenol and nitrobenzene by β-cyclodextrin-intercalated layered double hydroxide: Equilibrium and kinetic study. Chemical Engineering & Technology 34: 1559–1566.

13.

Kavitha

and Namasivayam

(2007) Experimental and kinetic studies on methylene blue adsorption by coir pith carbon. Bioresource Technology 98(1): 14–21.

14.

Pan

and Guan

(2010) Adsorption of nitrobenzene from aqueous solution on activated sludge modified by cetyltrimethylammonium bromide. Journal of Hazardous Materials 183: 341–346.

15.

Qin

and Ma

(2007) Adsorption of nitrobenzene from aqueous solution by MCM-41. Journal of Colloid and Interface Science 315: 80–86.

16.

Qin

and Xu

(2016) Enhanced nitrobenzene adsorption in aqueous solution by surface silylated MCM-41. Microporous and Mesoporous 232: 143–150.

17.

Rezgui

Guibal

and Boubakera

(2017) sorption of hg(ii) and zn(ii) ions using lignocellulosic sorbent (date pits). The Canadian Journal of Chemical Engineering 95: 775–782.

18.

Senturk

Ozdes

Gundogdu

et al . (2009) Removal of phenol from aqueous solutions by adsorption onto organomodified Tirebolu bentonite: Equilibrium, kinetic and thermodynamic study. Journal of Hazardous Materials 172: 353–362.

19.

Sharma

and Kaur

(2011) Sugarcane bagasse for the removal of erythrosin B and methylene blue from aqueous waste. Applied Water Science 1: 135–145.

20.

Sharma

(2003) Cr(VI) removal from industrial effluents by adsorption on an indigenous low-cost material. Colloids and Surfaces A: Physicochemical and Engineering Aspects 215(1–3): 155–162.

21.

Somasekhara

RCM

Sivaramakrishna

and Varada

(2012) The use of an agricultural waste material, Jujuba seeds for the removal of anionic dye (Congored) from aqueous medium. Journal of Hazardous Materials 203–204: 118–127.

22.

Villacañas

Pereira

MFR

Orfao

JJM

et al . (2006) Adsorption of simple aromatic compounds on activated carbons. Journal of Colloid and Interface Science 293: 128–136.

23.

Wang

et al . (2009) New evidence for the importance of Mn oxides contributed to nitrobenzene adsorption onto the surficial sediments in Songhua River, China. Journal of Hazardous Materials 172: 755–762.

24.

Wei

Lin

et al . (2017) Bagasse activated carbon with TETA/TEPA modification and adsorption properties of CO₂. Water, Air, & Soil Pollution 228: 128.

25.

Wei

Sun

Cui

et al . (2010) Removal of nitrobenzene from aqueous solution by adsorption on nanocrystalline hydroxyapatite. Desalination 263: 89–96.

26.

Wen

Chen

Lian

et al . (2012) Removal of nitrobenzene from aqueous solution by a novel lipoid adsorption material (LAM). Journal of Hazardous Materials 209–210: 226–232.

27.

Xu Di Tan

Chen

et al . (2008) Removal of Pb(II) from aqueous solution by oxidized multiwalled carbon nanotubes. Journal of Hazardous Materials 154: 407–416.

28.

Yinian

Meina

Rongrong

et al . (2011) Phosphorus removal from aqueous solution by Fe(III)-impregnated sorbent prepared from sugarcane bagasse. Fresenius Environmental Bulletin 20: 1288–1296.

29.

Yang

and Al-Duri

(2005) Kinetic modeling of liquid-phase adsorption of reactive dyes on activated carbon. Journal of Colloid and Interface Science 287(1): 25–34.

30.

Zabihi

Asl

and Ahmadpour

(2010) Studies on adsorption of mercury from aqueous solution on activated carbons prepared from walnut shell. Journal of Hazardous Materials 174: 251–256.

31.

Zhao

Yang

and Gao

(2003) Studies on the sorption behaviors of nitrobenzene on marine sediments. Chemosphere 52: 917–925.