A novel magnetic biochar from sewage sludge (MSBC) using SrFe12O19 as magnetic substrate was successfully synthesized under high-temperature and oxygen-free conditions. Several techniques and methodologies (X-ray diffraction, Fourier transform infrared spectroscopy and vibrating sample magnetometer) were used to determine the surface functional groups and physicochemical properties of MSBC, which showed that the MSBC combined the features of both SrFe12O19 and sludge biochar (SBC). And then the adsorption behavior of methyl orange (MO) from aqueous solution onto the MSBC was investigated. And the influence of variables including pH, initial concentration of MO, adsorbent dosage and contact time was studied in detail. The optimal adsorption amount of MO (149.18 mg·g−1) was obtained with 600 MO mg·L−1, 2 MSBC g·L−1, at pH of 5 for 40 min. The equilibrium data were evaluated using Langmuir and Freundlich isotherms. The Langmuir model better described the absorption of MO. Besides, the kinetic data were analyzed using pseudo-first-order and pseudo-second-order equations, and the pseudo-second order exhibited the better fit for the kinetic studies (R2 = 0.9982). This study showed that MSBC could be utilized as an efficient, magnetically separable adsorbent for the environmental cleanup.

INTRODUCTION

Due to its potentially carcinogenic and highly toxic nature, the pollution caused by dyes in water bodies becomes more and more serious around the world (Silveira et al. 2009; Chen et al. 2010a; Zhu et al. 2010; Bhattacharyya & Ray 2015). Besides, even with a small amount of dyes in water, the dyes are also easily noticeable and may be perceived as contaminators and consequently unacceptable (Zhu et al. 2010a, 2010b; Mekatel et al. 2015). Therefore, in respect of environmental safety, the removal of dyes from wastewater is very essential before it is discarded into water bodies. Several conventional methods like chemical precipitation, flocculation, membrane separation, ion exchange, oxidation, degradation by biological means and adsorption have been used to remove dyes from colored wastewater (Zhang et al. 2014; Manivel et al. 2015). Among these methods, adsorption is considered quite attractive in terms of its high adsorption properties, low cost, low energy requirements and simple operation (Jeon et al. 2008; Zhang et al. 2014). Today, due to high specific surface area and appreciable amounts of active sites available for adsorption, biochar is the most popular adsorbent for the removal of dyes from wastewater (Kalderis et al. 2008). Numerous researches have been performed with the aim of obtaining low-cost and high-adsorption capacity biochar from renewable sources, such as coconut shells, olive husks, cotton stalks, groundnut shell, pistachio nut shells and sludge (Wu et al. 2005; El-Hendawy et al. 2008; Michailof et al. 2008; Tao et al. 2015). Although these adsorbent materials have high removal efficiency, they suffer from a serious problem, that the separation of adsorbent materials after the dye adsorption from the aquatic system is a tedious and time-consuming task.

Recently, a magnetic separation technique, in which magnetic properties were introduced into the adsorbent to degrade and treat the toxic pollutants and hazardous chemicals in water, developed to solve environmental problems, and has received considerable attention in recent years (Yang et al. 2008; Ambashta & Sillanp 2010). And many researchers have synthesized magnetic biochar to enhance the separation of adsorbents from aqueous solution after adsorption (Zhu et al. 2010a, 2010b; Jiang et al. 2015; Shan et al. 2015).

In the present study, sewage sludge, which is enriched with organic matter and microorganisms, has been used for the preparation of biochar under high-temperature and oxygen-free conditions. Then, the biochar was modified on the surface using SrFe12O19 as magnetic substrate under high-temperature and oxygen-free conditions. The advantage of the magnetic biochar from sewage sludge (MSBC) is basically to acquire the benefit of both materials, such as ease of separation under magnetic influence; high surface area as well as porosity, and these characteristics were considered to be highly desirable for the material used as an adsorbent for water treatment. Batch adsorption experiments were conducted using synthetic aqueous solutions of methyl orange (MO). The effects of initial dye concentration, contact time, pH, and adsorbent dosage were investigated. The isotherms and kinetics data of adsorption process were also evaluated to find out the adsorption mechanism of the adsorption of MO onto MSBC.

MATERIALS AND METHODS

The preparation of sludge biochars

Sewage sludge taken from the outlet of anaerobic tank of the Sand lakes sewage treatment plant in Wuhan, China, was used as a precursor material for the synthesis of biochar. Firstly, the sludge was dried at 60 °C for 12 h, and then it was calcined at 800 °C for 2 h under the protection of nitrogen gas. After being cooled to room temperature, the sludge biochar (SBC) was obtained.

Preparation of magnetic biochar from sewage sludge

To obtain the MSBC, the molar ratio of Fe3+/Sr2+ was kept at 10.85. The detailed process could be described as follows. Firstly, 1.364 g SrCO3 was added to 20 mL deionized water, and 1 N HCl was added to completely dissolve SrCO3 as in Equation (1). Secondly, 27 g FeCl3·6H2O was dissolved in 50 mL deionized water. Then the two mixtures were evenly mixed together. Thirdly, the pH value of this mixture was adjusted to 10 by 1 N NaOH. When NaOH solution was added gradually, Fe3+ firstly became Fe(OH)3 as in Equation (2). At the same time, Sr2+ became Sr(OH)2 as in Equation (3). Then some amount of SBC, whose quality accounted for 75% of the total quality of SrCO3 and FeCl3·6H2O, was added to the mixture. Subsequently, the solution was stirred evenly and dried at 60 °C for 12 h. Finally, the ferrite precursor was calcined at 800 °C for 2 h under the protection of nitrogen gas. Under this high temperature, a series of reactions, as Equations (4)–(6), occurred on the surface of SBC. Thus the SrFe12O19 was produced on the surface of SBC. Finally, MSBC was obtained after being washed with plenty of deionized water to neutral and air-dried. 
formula
1
 
formula
2
 
formula
3
 
formula
4
 
formula
5
 
formula
6

Characterization of magnetic biochar from sewage sludge

Fourier transform infrared (FTIR) spectra of raw MSBC and dye loaded MSBC were recorded using KBr pellets with a FTIR (Continuum IR Microscope, USA) in the wave number range of 4,000–400 cm−1. The crystalline structure of the samples was identified from X-ray diffraction (XRD) patterns recorded in the 2θ range 20–60° using an X'PERT-PRO diffractometer (PANalytical, The Netherlands). The magnetic properties of magnetic nanocomposites were studied using a vibrating sampling magnetometer (Mo del 4HF VS, USA).

Determination of MO adsorption capacity

A series of batch adsorption experiments were carried out to achieve the optimum operating conditions for adsorption of MO onto MSBC. The effect of important operating parameters such as initial dye concentration, adsorbent dose, contact time and pH were studied using 50 mL of MO solution in a 250 mL flask at room temperature. The pH of the solution was adjusted with 1 N HCl or 1 N NaOH solution. The mixture was shaken in a constant temperature shaker for the desired time at 150 rpm. The samples were withdrawn from the mixture at optimum contact time, and thereafter samples were centrifuged at 6,000 rpm for 10 min. The final concentration of MO in supernatant solution was estimated at wavelength (λmax = 646 nm) by UV-VIS spectrophotometry (SP-1920, China). The percentage removal of MO and adsorption amount of MO () were calculated using the following equation (Equation (7)): 
formula
7
where is the adsorption amount of MO onto MSBC at time of equilibrium (mg·g−1), and are initial and final concentration of MO (mg·L−1), v (mL) is the volume of sample, (g) is the mass of adsorbent.

Each batch adsorption experiment above was performed in triplicate to obtain results with error <5%. If error was found to be more than 5%, more experiments were performed. The experimental data could be reproduced with accuracy greater than 95%. All the data of batch experiments listed in ‘Results and discussion’ were the average values of three tests.

RESULTS AND DISCUSSION

Characterization of magnetic biochar from sewage sludge

The X-ray diffraction patterns of SBC and MSBC are shown in Figure 1(a) and 1(b), respectively. Figure 1(a) shows that the SBC had several peaks. It indicates that the disappearance of the amorphous organic matter and the increase of ash content in sludge resulted in more crystal phase in sludge after carbonization. Due to the presence of a small amount of inorganic salts and heavy metals, there were some weaker peaks in the SBC.
Figure 1

XRD patterns of SBC (a) and MSBC (b).

Figure 1

XRD patterns of SBC (a) and MSBC (b).

As shown in Figure 1(a) and 1(b), the MSBC not only had the characteristic peak of SBC, but also had the diffraction peak of strontium ferrite, which further illustrated that the magnetic component of magnetic biochar was strontium ferrite. Besides, the diffraction peaks appearing at around 45 ° were SrFe12O19 diffraction peak, which were relatively obvious (Battezzati et al. 2005). It might be that iron hydroxide or ferric oxide reacted with carbon under the conditions of nitrogen atmosphere and high temperature. In addition, the diffraction peak's intensity of MSBC was weaker than that of SBC, which could be attributed to the reduction of the unit of relative content of SBC in the quality of MSBC. Figure 1(a) and 1(b) indicate that the disappearance of the amorphous organic matter and the increase of ash content in sludge resulted in more crystal phase in sludge after carbonization.

FTIR investigation revealed the presence of various functional groups. FTIR spectra of MSBC and MSBC-MO are shown in Figure 2(a) and 2(b), respectively. The strong adsorption peak at 1,039 cm−1, shown in the FTIR spectrum of MSBC, belonged to the symmetric stretching vibrations of C-O-C (Jiang et al. 2015). The peaks at 3,463 and 1,630 cm−1 attributed to the stretching and bending vibrations of O-H appeared in FTIR spectra of MSBC (Deligeer et al. 2011). The variation in the band positions resulted from differences in the distances between tetrahedral and octahedral sites at which they occurred in the intervals of 400–700 cm−1 (Silva et al. 2015).
Figure 2

Infrared spectra of MSBC (a) and MSBC-MO (b).

Figure 2

Infrared spectra of MSBC (a) and MSBC-MO (b).

Finally, magnetic measurement of MSBC had been investigated through a vibrating sample magnetometer (VSM) at room temperature in the applied magnetic field sweeping from −20 to +20kOe and the VSM curve shown in Figure 3. In Figure 3, the magnetization value of MSBC was 16.123 emu.g−1 at 7.5kOe, which showed that MSBC had reasonable magnetic responsivity and could be separated easily from the aqueous solution with the help of an external magnetic field (Raj & Joy 2015).
Figure 3

Room temperature M versus H curves of MSBC.

Figure 3

Room temperature M versus H curves of MSBC.

Effect of initial concentration

The effect of initial concentration on the absorption of MO by MSBC was investigated in the initial concentration range from 10 to 1,000 mg·L−1 with 2 MSBC g·L−1 at pH of 6 for 4 h. As shown in Figure 4(a), the adsorption amount increased with the increase of the initial concentration of MO, and after certain equilibrium concentration the adsorption amount became constant. However, the removal percentage of MO by MSBC decreased with the increase of the initial concentration of MO. The initial concentration provided an important driving force to overcome the resistance to the mass transfer of MO from liquid phase to the solid adsorbent surface (Deng et al. 2015). Hence, a higher initial MO concentration would have a beneficial effect on the sorption capacity of MSBC. Such an effect is clearly shown in Figure 4(a): the adsorption amount rapidly increased with the initial concentration of MO increasing from 10 to 600 mg·L−1, and roughly reached a maximum (149.31 mg·g−1) at amount 600 mg·L−1. However, when the initial concentration of MO continued to increase, the adsorption amount reached equilibrium and all sites were almost saturated with MO. Hence, the rate of increment of adsorption capacity became constant.
Figure 4

The effects of initial concentration (a), pH (b), biosorbent dosage (c) and contact time (d) on the adsorption of MO by MSBC.

Figure 4

The effects of initial concentration (a), pH (b), biosorbent dosage (c) and contact time (d) on the adsorption of MO by MSBC.

Effect of initial pH

The pH value is a very crucial parameter that affects the adsorption capacity of MO, because it could influence both the properties of the absorbent and the structural stability of MO. Hence, the effect of pH on the structural stability of MO was firstly considered. MO solutions were kept for 4 h after pH adjustment. It was found that MO was stable at pH from 5 to 10. To study the effect of pH on absorption of MO on MSBC, experiments were carried out in the pH range from 5 to 10 with 600 MO mg·L−1, 2 MSBC g·L−1 at room temperature for 4 h.

As shown in Figure 4(b), the adsorption amount dropped from 145.7 mg·g−1 to 99.52 mg·g−1 as the pH value increased from 5 to 10, while the removal percentage of MO dropped generally. Therefore, pH value of 5 was selected for the subsequent experimental study. The main mechanism for the MO adsorption onto adsorbents was related to electrostatic interaction. At lower pH (under acidic conditions), MO occurred as the quinoid form, and it rearranged into its azo structure at higher pH (Zhu et al. 2010a, 2010b). The basic and acidic structures of MO are given below:

The dissociation constant pKa for MO was 3.46; therefore MO was predominantly present as azo structure while pH ranged from 5 to 10. Azo structure of MO possessed one main active position –S=O group that could form normal hydrogen bonding. As a result, a lower amount of normal hydrogen bonding between MO and MSBC was formed when MO was azo type structure, which resulted in the decrease of adsorption quantity (Liu et al. 2015). In addition, π type of hydrogen bonding interaction between the other electronegative residues such as −N=N− group in azo type structure of MO and single −OH group of MSBC surface, as well as Yoshida hydrogen bonding interaction between benzene ring in both azo type and quinone structures of MO and single −OH groups of MSBC, competed with MO anions and hindered its adsorption.

Effect of the amount of adsorbent

The adsorbent dose was an important parameter in adsorption studies because it determined the capacity of adsorbent for a given initial concentration of MO solution. In this study, the effect of the adsorbent dose on the removal of MO was investigated by varying the adsorbent amount from 0.25 to 8 g·L−1 with 600 MO mg·L−1 at pH of 5 and room temperature for 4 h.

In Figure 4(c), it was observed initially that the adsorption amount increased with the increase of adsorbent dose until it reached 2 g·L−1, and after the critical dose, the adsorption amount began to decrease with the increase of adsorbent dose, while the removal percentage of MO dropped rapidly. This could be attributed to increase in adsorbent surface area and the availability of more adsorption sites to increase the dosage of the adsorbent (Wang & Li 2007).

Effect of contact time

The contact time was also an important parameter in adsorption studies because it determined the adsorption rate of adsorbent. Figure 4(d) shows the effects of contact time on the absorption of MO by MSBC. In this study, the effect of contact time on the removal of MO was investigated by varying the contact time from 0 to 120 min with 600 MO mg·L−1 , 2 MSBC g·L−1 at pH of 5 and room temperature for 120 min.

As shown in Figure 4(d), the absorption amount of MO increased with the increased contact time. The sorption of MO increased fast in the first 20 min, and then slowed down until the sorption process achieved equilibrium after 1 h. However, it was worth noting that the instantaneous time for separating MSBC from solution by external magnet could be neglected when testing the total contact time. The fast removal rate during the initial stage might be attributed to the rapid diffusion of MO from the solution to the external surfaces of MSBC. As the sites were gradually occupied, the adsorbed MO tended to be transported from the bulk phase to the actual sorption sites (i.e., inner-sphere pores of MSBC). Such slow diffusion process would decrease the sorption rate of MO at later stages. Overall, the removal process was quite fast and 1 h was enough to reach equilibrium.

Adsorption isotherms

For a solid–liquid system, the equilibrium of adsorption is one of the important physico-chemical aspects in the description of adsorption behavior. In this study, in order to further investigate the biosorption mechanism of MO, the absorption isotherm models (Langmuir and Freundlich) were used to characterize the interaction of concentrations of MO in solution (; mg·L−1) with the adsorption amount on adsorbent (; mg·g−1) at equilibrium (Chen et al. 2010). The parameters obtained from the different models provided important information on the sorption mechanisms and the surface properties and affinities of the absorbent.

The Langmuir isotherm theory assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface. The linearized Langmuir isotherm equation is represented by Equation (8). 
formula
8
where (mg·g−1) is the adsorbed MO amount at equilibrium, (mg·L−1) is the supernatant concentration at equilibrium, and (mg·g−1) and b (L·mg−1) are constants representing the maximum adsorption capacity and the Langmuir constant related to the heat of adsorption, respectively. The values of and b were calculated from the slopes (1/) and intercepts (1/) of the linear plots of versus in Figure 5(a).
Figure 5

The adsorption isotherms of MO onto MSBC Langmuir isotherm (a) and Freundlich isotherm (b).

Figure 5

The adsorption isotherms of MO onto MSBC Langmuir isotherm (a) and Freundlich isotherm (b).

The Langmuir isotherm can be expressed in terms of a dimensionless separation factor, , which describes the type of isotherm: 
formula
9
where is the initial concentration of MO. The magnitude of determines the feasibility of the adsorption process. If > 1, adsorption is unfavorable; if = 1, adsorption is linear; if < 1, adsorption is favorable; and if = 0, adsorption is irreversible.
The Freundlich isotherm is an empirical equation based on sorption on a heterogeneous surface or surface supporting sites of varied affinities (Güzel et al. 2015). The linear form of the Freundlich isotherm is given by the following equation: 
formula
10
where and n are the Freundlich constants characteristic of the system. The values of and 1/ were calculated from intercept and slope of the straight line plot of ln versus ln in Figure 5(b).

The results of Langmuir and Freundlich isotherms are presented in Figure 5(a) and 5(b), respectively. And the correlation coefficients of Langmuir and Freundlich isotherms are listed in Table 1. By comparing the constants and correlation coefficients R2 (Table 1), it could be seen that the Langmuir model was more suitable for the experimental equilibrium sorption data than the Freundlich model. The fact that the Langmuir isotherm fits the experimental data very well might be due to homogeneous distribution of active sites on the adsorbent's surface. In this situation, the value was calculated as 0.1266 for the adsorption of MO onto MSBC. So the adsorption of MO onto MSBC was favorable.

Table 1

Adsorption isotherm parameters for MO adsorption on the MSBC

Langmuir isotherm
 
Freundlich isotherm
 
b (L mg−1qm (mg g−1RL R2 Kε (mg1+n/g LnR2 
0.0115 173.6111 0.1266 0.9886 7.7547 2.039 0.9548 
Langmuir isotherm
 
Freundlich isotherm
 
b (L mg−1qm (mg g−1RL R2 Kε (mg1+n/g LnR2 
0.0115 173.6111 0.1266 0.9886 7.7547 2.039 0.9548 

Adsorption kinetics

The adsorption kinetic studies can provide important information on the adsorption rate and mechanism. The linear forms of pseudo-first-order rate and pseudo-second-order rate equations are expressed by Equations (11) and (12) 
formula
11
 
formula
12
where and (mg·g−1) are the amounts of the metal ions adsorbed at t (min) and equilibrium, respectively. And (1/min) and (g/mg min) are the rate constants of the pseudo-first-order and pseudo-second-order, respectively.
Figure 6(a) and 6(b) show the plots of the linearized form of the pseudo-first-order and pseudo-second-order kinetic models for the adsorption of MO onto MSBC. The kinetic parameters and correlation coefficients (R2) are shown in Table 2. The R2 values of the pseudo-second-order model were much higher than that of the pseudo-first-order model, and the calculated MO adsorption capacities at equilibrium were all close to the experimental ones. Therefore, the pseudo-second-order model was more appropriate to describe the adsorption behavior of MO onto MSBC.
Figure 6

Plot of pseudo-first-order kinetic equation (a) and second-order kinetic equation (b).

Figure 6

Plot of pseudo-first-order kinetic equation (a) and second-order kinetic equation (b).

Table 2

Kinetic parameters for MO adsorption on the MSBC

Pseudo-first-order
 
Pseudo-second-order
 
qe,cal (mg g−1qe,exp (mg g−1k1 (min−1R2 qe,cal (mg g−1k2 (g mg−1 min−1R2 
121.0124 149.18 0.0457 0.9400 172.4138 0.0005 0.9982 
Pseudo-first-order
 
Pseudo-second-order
 
qe,cal (mg g−1qe,exp (mg g−1k1 (min−1R2 qe,cal (mg g−1k2 (g mg−1 min−1R2 
121.0124 149.18 0.0457 0.9400 172.4138 0.0005 0.9982 

CONCLUSIONS

In this study, the MSBC from sewage sludge was synthesized by the SrFe12O19 as magnetic substrate. The characterizations of MSBC, which was determined by XRD, FTIR and VSM, showed that the MSBC combined the features of both SrFe12O19 and SBC. And the study of the adsorption behavior of MO from aqueous solution onto the MSBC showed that the optimal adsorption amount of MO (149.18 mg·g−1) was obtained with 600 MO mg·L−1, 2 MSBC g·L−1, at pH of 5 for 40 min. The absorption parameters for the Langmuir and Freundlich isotherms were determined and the equilibrium data were better described by the Freundlich isotherm. Besides, the adsorption kinetics could be successfully fitted to a pseudo-second-order kinetic model. The experimental results showed that the magnetic sludge-derived biochar could be utilized as a magnetically separable and efficient adsorbent for the environmental cleanup.

ACKNOWLEDGEMENTS

This research was financially supported by the open fund of the National Science and Technology Pillar Program (2014BAL04B04) and Major Science and Technology Program for Water Pollution Control and Treatment (No. 2009ZX07317-008-003).

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