In this study, a novel magnetic sludge biochar (MSBC) from sewage sludge was created by the assembly of strontium hexaferrite (SrFe12O19) onto the surface of sewage sludge biochar (SBC) under high-temperature and oxygen-free conditions. The characterization of MSBC was achieved by Fourier transform infrared spectroscopy, X-ray diffraction and vibrating sample magnetometry, and the adsorption properties of the MSBC towards malachite green (MG) from aqueous solution were systematically investigated. The influence of variables (different mass ratio of SBC and SrFe12O19, initial MG concentration, absorbent dosage, pH and contact time) was also studied in detail. The optimal adsorption amount of MG (388.65 mg MG/g) was obtained with 500 mg MG/L, 2.0 g MSBC/L for 40 min under pH of 7.0, with different mass ratios of SBC and SrFe12O19 (1:4, 1:2, 3:4 and 1:1), when the mass ratio of SBC and SrFe12O19 was 3:4 at room temperature, and the Langmuir model was more suitable than the Freundlich model for equilibrium data. Meanwhile, the kinetic models showed that the overall adsorption process was better described by a pseudo-second-order kinetic model. The results indicated that the MSBC was a novel, efficient, magnetically separable adsorbent for the removal of the dye from wastewater.

INTRODUCTION

Sewage sludge is the byproduct of water discharged from wastewater plants. The growing global urbanization of society coupled with the rapid rise of sludge volume, which causes serious pollution of the environment, is forcing both public and private sludge generators to re-evaluate their sludge management strategies (Liu & Tay 2001; Tyagi & Lo 2013). Sewage sludge is a mixture of inorganic and organic materials, and has a good carbonaceous structure. So it could be used as a kind of biochar after being pyrolyzed (Agrafioti et al. 2013; Kong et al. 2013). Due to its renewability and low cost, biochar from sewage sludge is extensively used in industrial purification and chemical recovery operations.

Another environmental problem that pollutes water and soils is effluent that contains dyes (Nethaji et al. 2010). The presence of dyes, even in trace amounts, produces perceptible coloration, inhibits sunlight penetration, and reduces photosynthetic activity (Lee & Pavlostathis 2004). Malachite green (MG), a dark-green cationic triarylmethane dye, is widely used as a biocide, medical disinfectant, and coloring agent in the silk, leather, and paper industries, etc. (Zhang et al. 2008; Nethaji et al. 2010). However, MG can cause injuries to humans and animals due to its mutagenic and carcinogenic characteristics, when it is above the permissible level (Berberidou et al. 2007). Therefore, the removal of MG from wastewater before discharge into the environment is necessary and very important.

Conventional methods to remove dyes from wastewater include oxidation, flocculation, coagulation, membrane filtration, photocatalysis, electrochemical techniques, adsorption, etc. (Sharma et al. 2011). Adsorption is one of the most effective methods for dye removal from aqueous solutions due to its high efficiency and the easy handling and availability of different sorbents (Salleh et al. 2011; Yagub et al. 2014). Among all these sorbents, biochar is the most popular adsorbent for the removal of dyes from wastewater (Wu et al. 2005; El-Hendawy et al. 2008; Ghaedi et al. 2013; Yagub et al. 2014; Tao et al. 2015). Although these biochar have high removal efficiency, they have a disadvantage that the removal of sorbent materials after the dye adsorption from the aquatic system is a tedious and time-consuming task. To overcome this problem, the sorbent material can be given a magnetic functionality (Yang et al. 2008; Ambashta & Sillanp 2010). Hence, introducing magnetic properties to the adsorbent has become a research hotspot, and many researchers have synthesized magnetic biochar to enhance the separation of adsorbents from aqueous solution after adsorption (Shan et al. 2015; Jiang et al. 2015).

In the present study, the magnetic substrate SrFe12O19 was firstly synthesized by a coprecipitation method, and the sludge biochar (SBC) was prepared from sewage sludge under 800 °C and oxygen-free conditions (Werle & Wilk 2010). Then, the SBC was modified on the surface using SrFe12O19 as magnetic substrate under high-temperature and oxygen-free conditions (Xia et al. 2015). The advantage of synthesizing magnetic sludge biochar (MSBC) was basically to acquire the benefit of both materials, such as ease of separation under magnetic influence and high surface area as well as porosity, and these characteristics were considered to be highly desirable for a material used as an adsorbent for wastewater treatment. The adsorption capacity of MG on MSBC was investigated. Batch adsorption experiments were conducted using synthetic aqueous solutions of MG. The effects of the mass ratio of SBC, initial concentration of MG, absorbent dosage and contact time were investigated. The isotherms and kinetics data of the adsorption process were also evaluated to find out the adsorption mechanism of MSBC for MG.

MATERIALS AND METHODS

Preparation of sludge biochar

Sewage sludge taken from the outlet of the anaerobic tank of the Sand Lakes sewage treatment plant in Wuhan, China, was used as a precursor material for the synthesis of biochar. SBC was prepared by a calcination method. The detailed process could be described as follows. Firstly, some amount of sludge was dried at 60°C for 12 h. Then the dried sludge was calcined at 800 °C for 2 h protected by nitrogen gas. Thus the SBC was obtained after being ground into powder.

Preparation of magnetic biochar

The MSBC was synthesized by a calcination method at 800 °C for 2 h protected by nitrogen gas. The detailed process could be described as follows. First, 1.364 g SrCO3 was added in 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. At the addition of NaOH, 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 was added in the mixture to have different mass ratios of SBC and SrFe12O19 (1:4, 1:2, 3:4 and 1:1). Subsequently, the solution was stirred evenly and dried at 60°C for 12 h. At last, the ferrite precursor was calcined at 800 °C for 2 h protected by nitrogen gas. Under this high temperature, a series of reactions, as in Equations (4)–(6), occurred on the surface of the biochar. Thus the SrFe12O19 was produced on the surface of the SBC. At last, MSBC were 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

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

Batch sorption experiment

A series of batch adsorption experiments were carried out to achieve the optimum operating conditions for adsorption of MG on MSBC. The effect of important operating parameters such as the mass ratio of SBC, initial concentration of MG, absorbent dosage and contact time were studied using 50 mL of MG solution in a 250 mL flask at room temperature. The mixture was shaken in a constant temperature shaker at 150 rpm. The samples were withdrawn from the mixture, and thereafter samples were centrifuged at 6,000 rpm for 5 min. The final concentration of MG in supernatant solution was estimated at wavelength λmax = 618 nm by UV-VIS spectrophotometry (SP-1920, China). The percentage removal of MG and adsorption capacity was calculated using the following equation: 
formula
7
where is the amount of MG adsorbed on the magnetic composite at the time of equilibrium (mg MG/L), and are initial and final concentrations of malachite green (mg MG/L), v (ml) is the volume of the sample, w (g) is the mass of adsorbents.

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 MG, the biosorption isotherm models (Langmuir and Freundlich) were used to characterize the interaction of concentrations of dye in solution (; mg MG/L) with the adsorption quantities on the adsorbent (; mg MG/g) 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 MG/g) is the adsorbed MG amount at equilibrium, (mg MG/L) is the supernatant concentration at equilibrium, and (mg MG/g) and b (L/mg MG) are constants representing the maximum adsorption capacity and the Langmuir constant related to the heat of adsorption, respectively.
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 MG. The magnitude of determines the feasibility of the adsorption process. If , adsorption is unfavorable; if , adsorption is linear; if , adsorption is favorable; and if , 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.

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 pseudo-first order and second order, respectively.

Each batch adsorption experiment above was performed in triplicate to obtain results with error <5%. If the 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 the batch experiments listed below in ‘Results and discussion’ were the average values of three tests.

RESULTS AND DISCUSSION

Characterization of magnetic composites

The X-ray diffraction patterns of SBC and MSBC are shown in Figures 1(a) and 1(b), respectively. As shown in Figure 1(a), the SBC had high crystallinity and characteristic peaks were observed at 26.65°, 31.69° and 56.62°, which correspond to (hkl) planes of 111, 220 and 440, respectively; 2θ at around 26.65° is mainly attributed to the diffraction peak of C, while the peaks at around 31.69°and 56.62° shows the existence of Fe3C. In Figure 1(b), the typical peaks from SrFe12O19 (JCPDS No. 33-1340) can be found at 44.69° and 45.45° indicating that the SrFe12O19 structure has been successfully obtained. Comparing Figures 1(a) and 1(b), the SrFe12O19 structure is successfully obtained on the surface of MSBC.
Figure 1

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

Figure 1

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

As shown in Figure 1(b), MSBC not only had the characteristic peak of SBC, but also had the diffraction peak of strontium ferrite, which further illustrated the magnetic component of magnetic biochar was strontium ferrite. In addition, the diffraction peaks that appeared at around 45 ° were from Fe3C, which were relatively obvious (Battezzati et al. 2005). It might be from iron hydroxide or ferric oxide reacting with carbon under the conditions of a nitrogen atmosphere and high temperature. In addition, the diffraction peak 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.

FT-IR investigation revealed the presence of various functional groups. FT-IR spectra of MSBC and MSBC-MG are shown in Figures 2(a) and 2(b), respectively. The strong adsorption peak at 1,039 cm−1, shown in the FT-IR 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 the FT-IR spectra of MSBC (Deligeer et al. 2011). The variation in the band positions resulted from differences in the distances between the 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 (a) MSBC and (b) MSBC-MG.

Figure 2

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

Finally, magnetic measurement of MSBC with different mass ratios of SBC was investigated through a vibrating sample magnetometer (VSM) at room temperature with the applied magnetic field sweeping from −20 to +20 kOe and the VSM curves shown in Figure 3. The magnetization of MSBC was attributed to the presence of SrFe12O19, the higher mass ratio of SBC meaning a lower mass ratio of SrFe12O19. Hence, as shown in Figure 3, the magnetization decreased with increasing mass ratio of SBC. In addition, the magnetic characteristics (zero coercivity and magnetization increasing linearly at high fields) indicated superparamagnetic properties. The minimum magnetization value of MSBC was about 11.11 emu⋅g−1 at 7.5 kOe, which shows the 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.

Effects of different mass ratios of sludge biochar and SrFe12O19

The MSBC was synthesized from SrFe12O19 and biochar, which acquired the characterization of both materials. The different mass ratios of SBC and SrFe12O19 may have some effect on the adsorption. Therefore, in this study, MSBC with different mass ratios of SBC and SrFe12O19 (1:4, 1:2, 3:4 and 1:1) were obtained to study its effect on adsorption. The results are given in Figure 4(a).
Figure 4

The effects of (a) the mass ratio of SBC and initial concentration of MG, (b) absorbent dosage, (c) contact time and (d) pH on the adsorption of MG by MSBC.

Figure 4

The effects of (a) the mass ratio of SBC and initial concentration of MG, (b) absorbent dosage, (c) contact time and (d) pH on the adsorption of MG by MSBC.

As shown in Figure 4(a), the adsorption amount of MG increased with increasing mass ratio of SBC and SrFe12O19, and reached the maximum adsorption amount (388.65 mg MG/g) at 3/7; and then it began to decrease with increasing mass ratio of SBC and SrFe12O19. During the process of the generation of MSBC, some SrFe12O19 might deposit onto the surface of SBC, and some SrFe12O19 was adsorbed into the gaps of the biochar, which reduced the surface-to-volume ratio of MSBC. At last, its adsorbability became strong with the decreasing mass percentage of SrFe12O19 and the increasing mass percentage of SBC. However, with the increasing mass percentage of SBC, the adsorbability of MSBC became stronger and stronger, and it might adsorb more and more SrFe12O19, which might reduce the adsorption amount of MG. Hence, the optimal mass ratio of SBC and SrFe12O19 is 3:4.

Effect of initial concentration

The initial MG concentration was one of the important factors during the adsorption process. In this study, the initial MG concentration, which ranged from 50 to 700 mg MG/L, was investigated, as shown in Figure 4(a). It was obvious that the removals of MG by this adsorbent were dependent on its concentration. The initial MG concentration provided an important driving force to overcome the resistance to the mass transfer of dye from the liquid phase to the solid adsorbent surface. Therefore, a higher initial MG concentration would have a benefit effect on MSBC sorption capacity. Such an effect is clearly shown in Figure 4(a), as the adsorption amount rapidly increased with the initial MG concentration increasing from 50 to 500 mg MG/L, and reached a maximum (388.65 mg MG/L) roughly at 500 mg MG/L. However, when the initial dye concentration continued to increase, the adsorption amount reached equilibrium and all sites were almost saturated with dye. Hence, the rate of increment of adsorption capacity became constant.

Effect of the amount of adsorbent

The adsorbent dose is an important parameter in adsorption studies because it determines the capacity of the adsorbent for a given initial concentration of MG solution. In this study, the effect of the adsorbent dose on the removal of MG was investigated by varying the adsorbent amount from 0.5 to 12 g MSBC/L for this adsorbent with 500 mg MG/L at a pH of 7.0 and room temperature for 4 h.

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

Effect of shaking time

Figure 4(c) shows the removal of MG by magnetic adsorbent as a function of contact time. It is clear that the sorption amount of MG increased with increasing contact time. The sorption of MG increased fast in the first 20 min, and then slowed down until the sorption process achieved equilibrium after 1 h. However, it is worth noting that the instantaneous time for separating the MSBC composite 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 MG from the solution to the external surfaces of the MSBC composite. As the sites were being gradually occupied, the adsorbed MG tended to be transported from the bulk phase to the actual sorption sites (i.e., inner-sphere pores of MSBC). Such a slow diffusion process would decrease the sorption rate of MG at later stages. Overall, the removal process was quite fast and 40 min was enough to reach equilibrium.

Effect of pH

In adsorption studies the pH value is a very crucial parameter that affects the adsorption capacity of MG (Nekouei et al. 2016). To study the effect of pH on the adsorption of MG on MSBC, experiments were carried out in the pH range from 3.0 to 9.0 with 500 mg MG/L, 1 g MSBC/L at room temperature for 40 min, and the result is presented in Figure 4(d). It was found that by increasing pH from 3.0 to 7.0, the percentage removal of MG and MG adsorption capacity were increased from 51.22% to 77.73% and from 256.12 to 388.65 mg MG/g, respectively. At lower pH levels than 7.0, the surface would carry more positive charges and would more significantly repulse the positively charged species in solution. Thus, the lower adsorption of MG at lower pH values was mainly attributed to the increasing repulsion between the more positively charged MG species and positively charged surface sites (adsorbent protonation). Moreover, at lower pH, H+ ions competed with MG ions for the surface binding-sites of the adsorbent. When the pH was beyond 7.0, the percentage removal of MG and MG adsorption capacity did not change significantly. Therefore, the pH for MG adsorption was optimized at 7.0.

Adsorption isotherms

The results and correlation coefficients of the Langmuir and Freundlich models are presented in Figure 5 and listed in Table 1. By comparing the constants and R2 correlation coefficients (Table 1), it can 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 fitted the experimental data very well might be due to the homogenous distribution of active sites on the adsorbent surface. In this situation, the value was calculated as 0.0264 for MG adsorption onto MSBC. So the adsorption of MG onto MSBC was favorable.
Table 1

Adsorption isotherm and kinetic parameters for MB adsorption onto MSBC

Langmuir isotherm Freundlich isotherm 
b (L mg−1qm (mg g−1RL R2 Kɛ (mg1+n/g Lnn R2 
0.0736 404.858 0.0264 0.9930 51.1998 2.2738 0.8574 
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 
207.3910 394.8600 0.0457 0.9523 401.6064 0.0011 0.9947 
Langmuir isotherm Freundlich isotherm 
b (L mg−1qm (mg g−1RL R2 Kɛ (mg1+n/g Lnn R2 
0.0736 404.858 0.0264 0.9930 51.1998 2.2738 0.8574 
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 
207.3910 394.8600 0.0457 0.9523 401.6064 0.0011 0.9947 
Figure 5

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

Figure 5

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

Adsorption kinetics

Figure 6 shows the plots of the linearized forms of the pseudo-first-order and pseudo-second-order kinetic models for the adsorption of MG onto MSBC. The kinetic parameters and correlation coefficients (R2) are shown in Table 1. The R2 values of the pseudo-second-order model were much higher than those of the pseudo-first-order model, and the calculated MG adsorption capacities at equilibrium were all close to the experimental ones. Hence, the pseudo-second-order model was more appropriate to describe the adsorption behavior of MG onto MSBC.
Figure 6

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

Figure 6

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

CONCLUSIONS

In this study, a novel MSBC was created by the assembly of SrFe12O19 onto the surface of SBC under high-temperature and oxygen-free conditions, which combined both features of SrFe12O19 and SBC. The results for the adsorption behavior of MG by MSBC indicate that the maximum adsorption amount of MG was 388.65 mg MG/g with 500 mg MG/L, 2.0 g MSBC/L for 40 min under a pH of 7.0, when the mass ratio of SBC and SrFe12O19 was 3:4 at room temperature. The adsorption isotherm of the MSBC towards MG was a Langmuir isotherm (R2 = 0.993), while the adsorption mechanism followed the pseudo-second-order model (R2 = 0.952). The experimental results showed that the MSBC could be utilized as a magnetically separable and efficient adsorbent for environmental cleanup.

ACKNOWLEDGEMENTS

This research was financially supported by the open fund of the National ‘Twelfth Five-Year’ Plan for Science & Technology Pillar Program (2015BAL01B02), Natural Science Foundation of Hubei Province, China (Nos. 2013CFB289; 2013CFB308) and Major Science and Technology Program for Water Pollution Control and Treatment (No. 2009ZX07317-008-003).

REFERENCES

REFERENCES
Agrafioti
E.
Bouras
G.
Kalderis
D.
Diamadopoulos
E.
2013
Biochar production by sewage sludge pyrolysis
.
Journal of Analytical and Applied Pyrolysis
101
,
72
78
.
Ambashta
R. D.
Sillanpää
M.
2010
Water purification using magnetic assistance: a review
.
Journal of Hazardous Materials
180
(
1–3
),
38
49
.
Battezzati
L.
Baricco
M.
Curiotto
S.
2005
Non-stoichiometric cementite by rapid solidification of cast iron
.
Acta Materialia
53
(
6
),
1849
1856
.
Berberidou
C.
Poulios
I.
Xekoukoulotakis
N. P.
Mantzavinos
D.
2007
Sonolytic, photocatalytic and sonophotocatalytic degradation of malachite green in aqueous solutions
.
Applied Catalysis B: Environmental
74
(
1–2
),
63
72
.
Deligeer
W.
Gao
Y. W.
Asuha
S.
2011
Adsorption of methyl orange on mesoporous γ-Fe2O3/SiO2 nanocomposites
.
Applied Surface Science
257
(
8
),
3524
3528
.
El-Hendawy
A. A.
Alexander
A. J.
Andrews
R. J.
Forrest
G.
2008
Effects of activation schemes on porous, surface and thermal properties of activated carbons prepared from cotton stalks
.
Journal of Analytical and Applied Pyrolysis
82
(
2
),
272
278
.
Ghaedi
M.
Karimi
F.
Barazesh
B.
Sahraei
R.
Daneshfar
A.
2013
Removal of Reactive Orange 12 from aqueous solutions by adsorption on tin sulfide nanoparticle loaded on activated carbon
.
Journal of Industrial and Engineering Chemistry
19
(
3
),
756
763
.
Jiang
T.
Liang
Y.
He
Y.
Wang
Q.
2015
Activated carbon/NiFe2O4 magnetic composite: a magnetic adsorbent for the adsorption of methyl orange
.
Journal of Environmental Chemical Engineering
3
(
3
),
1740
1751
.
Nethaji
S.
Sivasamy
A.
Thennarasu
G.
Saravanan
S.
2010
Adsorption of Malachite Green dye onto activated carbon derived from Borassus aethiopum flower biomass
.
Journal of Hazardous Materials
181
(
1–3
),
271
280
.
Salleh
M. A. M.
Mahmoud
D. K.
Karim
W. A. W. A.
Idris
A.
2011
Cationic and anionic dye adsorption by agricultural solid wastes: a comprehensive review
.
Desalination
280
(
1–3
),
1
13
.
Sharma
P.
Kaur
H.
Sharma
M.
Sahore
V.
2011
A review on applicability of naturally available adsorbents for the removal of hazardous dyes from aqueous waste
.
Environmental Monitoring and Assessment
183
(
1–4
),
151
195
.
Silva
W. M. S.
Ferreira
N. S.
Soares
J. M.
Da Silva
R. B.
Macêdo
M. A.
2015
Investigation of structural and magnetic properties of nanocrystalline Mn-doped SrFe12O19 prepared by proteic sol–gel process
.
Journal of Magnetism and Magnetic Materials
395
,
263
270
.
Tyagi
V. K.
Lo
S.
2013
Sludge: a waste or renewable source for energy and resources recovery?
Renewable and Sustainable Energy Reviews
25
,
708
728
.
Wu
F.
Tseng
R.
Juang
R.
2005
Comparisons of porous and adsorption properties of carbons activated by steam and KOH
.
Journal of Colloid and Interface Science
283
(
1
),
49
56
.
Xia
A.
Ren
S.
Lin
J.
Ma
Y.
Xu
C.
Li
J.
Jin
C.
Liu
X.
2015
Magnetic properties of sintered SrFe12O19–CoFe2O4 nanocomposites with exchange coupling
.
Journal of Alloys and Compounds
653
,
108
116
.
Yagub
M. T.
Sen
T. K.
Afroze
S.
Ang
H. M.
2014
Dye and its removal from aqueous solution by adsorption: a review
.
Advances in Colloid and Interface Science
209
,
172
184
.
Zhang
J.
Li
Y.
Zhang
C.
Jing
Y.
2008
Adsorption of malachite green from aqueous solution onto carbon prepared from Arundo donax root
.
Journal of Hazardous Materials
150
(
3
),
774
782
.