Preparation of magnetic porous carbon from waste hydrochar by simultaneous activation and magnetization for tetracycline removal

Bioresource Technology 154 (2014) 209–214

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Preparation of magnetic porous carbon from waste hydrochar by simultaneous activation and magnetization for tetracycline removal Xiangdong Zhu, Yuchen Liu, Feng Qian, Chao Zhou, Shicheng Zhang ⇑, Jianmin Chen Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China

h i g h l i g h t s  A novel magnetic porous carbon with

c-Fe2O3 particles was prepared from hydrochar.

 Activation and magnetization of hydrochar can be simultaneously obtained.  The as-prepared magnetic porous carbon could remove tetracycline efficiently.

a r t i c l e

i n f o

Article history: Received 9 September 2013 Received in revised form 30 November 2013 Accepted 5 December 2013 Available online 14 December 2013 Keywords: Hydrochar Maghemite Magnetic Tetracycline Removal

a b s t r a c t In the present work, a novel magnetic porous carbon (MPC) with maghemite (c-Fe2O3) particles is facilely prepared from hydrochar (a solid residue of hydrothermal carbonization of biomass) in one step through simultaneous activation and magnetization. The resultant MPC is characterized and utilized as an adsorbent for tetracycline (TC) removal from aqueous solutions. The BET surface area and micropore volume of the MPC are found to be 349 m2 g1 and 0.16 cm3 g1, respectively. The adsorption kinetics data could be well described by the pseudo-second-order model, and the TC adsorption onto MPC is an endothermic and spontaneous process. The enhanced surface area of the MPC, as well as its graphite-like structure, may contribute to the adsorption capacity of TC. After adsorption, MPC could be effectively separated by applying a magnetic field. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Tetracycline (TC) is one of the most widely used antibiotics in the livestock industry, most of which is discharged into aquatic environments in original and metabolized forms (Zhou et al., 2012). The most dangerous effect of antibiotics in the environment is the development of multi-resistant bacterial strains that can no longer be treated with the presently known drugs (Zhu et al., 2013). Due to its potential risk and the ineffective removal by conventional water treatment, it is of great importance to explore efficient and cost-effective treatment technologies for TC removal. Adsorption, due to its high efficiency and easy operation, is one of the most important methods for TC removal (Liu et al., 2012). Carbon-based adsorbents possess large surface areas and abundant pore structures and have shown great potential in the removal of undesirable organic pollutants from aqueous solutions (Zhou et al., 2012). These adsorbents have been verified to be very effective in TC removal, due to the capability of p–p electron coupling

⇑ Corresponding author. Tel./fax: +86 21 65642297. E-mail address: [email protected] (S. Zhang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.12.019

with TC molecules, as well as their large surface areas (Ji et al., 2009; Liu et al., 2012; Zhou et al., 2012). Recently, magnetic carbon composites have been developed and are not only high-efficiency adsorbents for removal of pollutants from aqueous solutions, but are also easily collected magnetically after adsorption (Zhang et al., 2012). The introduction of a magnetic medium (such as maghemite, c-Fe2O3) to carbon-based adsorbents by a pyrolysis activation (simultaneous activation and magnetization) or chemical co-precipitation reaction is an efficient method to enable the adsorbent to be efficiently separated with an external magnetic field. However, for a low surface area of carbonaceous material, co-precipitation reaction is not an efficient and facile method due to its negative effect on porosity of products (Oliveira et al., 2002). But, both an enhanced surface area and excellent magnetization property can be simultaneously obtained via a pyrolysis activation (Zhang et al., 2012). However, few studies were presented to develop this promising technology for the preparation of MPC. Hydrochar is a by-product of hydrothermal carbonization (HTC) of waste biomass for bio-oil production (Hu et al., 2010a; Sevilla et al., 2011). As a carbon-rich, functional group abundant and pollution-free solid residue, hydrochar can be selected as an ideal

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carbon-based precursor for assembling magnetic porous carbon (MPC) (Liu et al., 2012). However, one of the main limiting factors hindering the effective exploitation of hydrochar for environmental applications is their low surface area and poor porosity (Falco et al., 2013). Hence, to enhance pollutant removal efficiency, a facile and efficient approach for the activation of hydrochar is needed to explore. To the best of the authors’ knowledge, little information has been made available regarding the simultaneous activation and magnetization of hydrochar. In the present study, a novel MPC with c-Fe2O3 particles was prepared in one step with the thermal pyrolysis of ferric chloride (FeCl3) pretreated hydrochar. Nitrogen (N2) BET (Brunauer– Emmett–Teller) surface area, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared (FTIR) and Raman spectroscopy were used to characterize the structural differences between the hydrochar and MPC. Adsorption behaviors of TC onto MPC were then investigated. The effects of the solution pH and temperature on TC adsorption were also evaluated. The main objective of the present work was the investigation of a facile and efficient modification route of hydrochar for improvement in its performance for environmental and agricultural applications.

2. Methods 2.1. Materials Analytical grade ferric chloride hexahydrate (FeCl36H2O) and tetracycline (TC) were purchased from China National Medicines Corporation Ltd. and Aladdin Reagent Co., respectively. All chemical solutions were prepared using deionized water (18.2 MX). The hydrochar material was obtained from our pilot-scale HTC unit. An aqueous solution/dispersion of salix psammophila, at a concentration of 80 g L1, was placed in a stainless steel autoclave and heated up to 573 K and then cooled with circulating water to room temperature (Sevilla et al., 2011). The resulting solid product was recovered by filtration and washed with distilled water and ethyl acetate, and finally dried at 373 K for 2 h. 2.2. Synthesis of magnetic porous carbon The preparation of the MPC from the hydrochar material was carried out as follows: the hydrochar material (20 g) was immersed into the prepared FeCl3 solution (26.7 g FeCl3 in 120 mL of water) for 12 h, and then the hydrochar material was separated by filtration and dried at 353 K for 2 h under air. The obtained material was pyrolyzed at the temperature of 973 K for 1 h under nitrogen (N2) flow of 1 L min1 at a heating rate of 4 K min1. The carbonized sample was washed with 0.1 M of hydrochloric acid (HCl) and water. The prepared material, denoted as MPC, was milled and sieved to obtain particles in powder form (<0.15 mm). 2.3. Characterization of the samples Nitrogen gas sorption isotherms and textural properties of the hydrochar and MPC were determined by the Quantasorb SI instrument (Quantachrone, USA). The surface area was calculated using the BET method with relative pressures of 0.06–0.2, and the total pore volume was determined from the amount of N2 absorbed at a relative pressure of 0.99. The micropore volume was obtained through a t-plot analysis. Magnetic measurement was carried out

at 300 K by a vibrating sample magnetometer (VSM) with a maximum magnetic field of ±4000 Oe. The microscopic features of the hydrochar and MPC were characterized by SEM (XL300, Philips) equipped with an energy-dispersive X-ray (EDX, Link 300) analyzer and TEM (H-600, Hitachi). The power XRD patterns were recorded on the X’Pert PRO system equipped with a Cu Ka radiation (40 kV, 40 mA) over the 2h range of 10–80°. The surface functional groups of hydrochar and MPC were determined by FTIR spectroscopy using a Nicolet (Nexus 470) spectrometer. The spectra were performed at 4 cm1 resolution with a 400–4000 cm1 scan range. Raman spectra were recorded by the LabRam-1B spectrometer with He–Ne laser operating at a wavelength of 514 nm, and the curve fitting were performed with the combination of Gaussian line shapes that gave the minimum fitting error. XPS experiment was carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Ka radiation (hv = 1253.6 eV). In general, the X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle at 54°. Binding energies was calibrated by setting C to 1s at 284.6 eV. The measurement of pH of zero point charges (pHPZC) of the MPC sample was determined with the following procedure: (1) A 25 mL sodium chloride (NaCl, 0.01 M) solution was placed into 60 mL glass bottle. The initial solution pH was adjusted to successive initial values between 2.0 and 12.0, and 0.05 g of the MPC sample was added to the glass bottle. (2) The glass bottle was filled with N2 to eliminate the effect of carbon dioxide (CO2) on the pH change, and then shaken at 313 K. (3) The final solution pH was measured after a desired contact time of 48 h. (4) The difference between the final pH and the initial pH, denoted as DpH, was plotted against the initial pH. The solution pH at which the curve crosses the line of DpH = 0 was taken as the pHPZC of sample (Liu et al., 2011, 2012). 2.4. Batch adsorption experiments Adsorption kinetics of TC on MPC were performed by placing 0.05 g of MPC in a glass bottle containing a 50 mL TC solution on a shaker at 298 K, with initial TC concentrations of 10, 30, 50 mg L1. Samples were taken and then subjected to filtration through a 0.22 lm nylon membrane filter at appropriate time intervals. The TC concentrations were determined using an ultraviolet–visible (UV–vis) spectrophotometer (UV–vis 4802, Unico Instruments Co. Ltd., Shanghai) at a wavelength of 360 nm. Equilibrium isotherm experiments were performed at 303 K over the initial TC concentration range from 5 to 80 mg L1. The glass bottles were shaken for 5 days, this period having been previously determined by kinetics experiments. The amount of TC adsorbed on the adsorbent at equilibrium (qe, mg g1) was calculated using the following equation (Eq. (1)):

qe ¼

ðC 0  C e ÞV m

ð1Þ

where C0 and Ce are the initial and equilibrium concentrations of TC (mg L1), m is the mass of adsorbent (g), and V is the volume of reaction solution (L). To examine the effect of pH on TC adsorption onto the MPC, TC adsorption was carried out with different aqueous solutions (pH = 3.1, 4.9, 5.9, 7.0, 8.1, 9.1 and 10.1), and, the solution pH was adjusted with NaOH or HCl (0.1 M) solutions. The TC concentration and temperature of adsorption were fixed at 30 mg L1 and 298 K, respectively.

X. Zhu et al. / Bioresource Technology 154 (2014) 209–214

3. Results and discussion 3.1. Characterization of hydrochar and magnetic porous carbon The MPC was prepared by the physical activation of FeCl3 pretreated hydrochar under N2 atmosphere. After activation, the textural properties of hydrochar and MPC were evaluated by N2 adsorption at 77 K. The N2 adsorption isotherm of the hydrochar showed a very low uptake at low relative pressures (shown in Fig. S1), with a slight enhancement at values of P/P0 near 1.0, revealing a scant porosity with a contribution of mesoporosity (Román et al., 2012). In addition, it can be seen that activation resulted in significant microporosity development, due to the release of small organic molecules and unconverted compositions of waste biomass during the HTC process (Titirici et al., 2012). Table 1 shows the textural properties of hydrochar and MPC. It is evident that the MPC sample had a large BET surface area and high pore volume compared with the starting material. The activation of hydrochar caused near fiftyfold and fortyfold increases of the BET surface area and micropore volume, respectively. The XRD patterns of samples are presented in Fig. S2. The major crystalline phases in the hydrochar sample were quartz and calcium oxalate. After activation, the calcium oxalate was disappeared, and, maghemite (c-Fe2O3) was characterized as the major crystalline phase in the MPC with diffraction peaks at 29.9°, 35.2°, 44.5°, 53.3°, 56.8°, 62.5° and 72.6° (Li et al., 2013; Zhang et al., 2012), and other peaks related to the presence of quartz (peaks at 2h of 26.5° and 67.8°) (Dai et al., 2013). The diffraction peaks of c-Fe2O3 and Fe3O4 are very similar, hence, the formation of c-Fe2O3 particles in MPC was further confirmed by XPS measurement (Fig. S3). Distinct peak at 711.3 eV and a satellite peak at 718.2 eV are observed from Fe 2p3/2 spectrum, suggesting that the electron structure of Fe3+ and absence of Fe2+ (Xiao et al., 2013; Li et al., 2013). Results from the XRD and XPS analyses confirmed the iron phase was c-Fe2O3. The process of heating also gave rise to broad peaks around 23.2° and 42.9°, which was indicative of an amorphous material with some degree of short-range order and can be attributed to the formation of turbostratic crystallite (Keiluweit et al., 2010). In addition, the broad peak shape reflected both the low degree of crystallinity and the small crystal size of the MPC sample (Bourke et al., 2007). The morphology of the hydrochar and MPC were investigated by SEM and TEM studies (shown in Figs. S4–S7). The surface of the hydrochar appeared rough (Fig. S4), resulting from the lignin component having only partial degradation (Fuertes et al., 2010). Sphere-like microparticles (diameter of approximately 15.0 lm) were observed, which were caused by the degradation of the cellulose component during the HTC process and its subsequent precipitation and growth as spheres (Sevilla and Fuertes, 2009b). In addition, there were some rudimentary pores within the structure result from the preliminary constitute decomposition (Fig. S4b) (Liu and Zhang, 2011). After activation of the FeCl3 pretreated hydrochar, the c-Fe2O3 particles had been dispersed on the surface of the porous carbon and developed individually without obvious

Table 1 Textural characteristic of hydrochar and magnetic porous carbon (MPC). Sample

Hydrochar MPC

BET-surface area (m2 g1) 7.16 349

Total pore volume (cm3 g1)

Micropore volume (cm3 g1)

Pore size (nm)

0.054 0.24

0.0044 0.16

30.5 2.75

211

aggregation, and a smooth surface was observed due to the further decomposition of the carbonaceous material (Fig. S5) (Zhang et al., 2012). The element composition of the MPC was also analyzed by the EDX instrument. It was revealed that the MPC contained mainly carbon, oxygen and iron and that the mass percentage of iron was 0.70% (Fig. S6). TEM images of the MPC (Fig. S7) clearly showed that the c-Fe2O3 particles had been well dispersed within the pores of the bulk of the carbonaceous material matrix (Zhang et al., 2012). Irregular shaped particles with sizes ranging from 20 to 300 nm were observed. The magnetic separation properties of the MPC were investigated at 300 K by measuring magnetization curves (Fig. S8) (Hu et al., 2010b). The MPC sample exhibited a relatively low saturation magnetization (0.7 emu g1). However, the particles of the MPC could also be easily attracted by an external magnetic field (Fig. S8 insert), and, the clear solution could be easily removed by a pipet. Based on the EDX measurement, about 1.0% of the weight of the MPC came from c-Fe2O3, hence, the saturation magnetization of MPC was calculated as 0.76 emu g1 (the saturation magnetization of pure c-Fe2O3 is 76.0 emu g1) (Zhu et al., 2007), which is consistent with the VSM measurement. The functional groups of the surfaces of the hydrochar and MPC were investigated by FTIR spectra, as shown in Fig. S9, and, both samples exhibited similar bands. In comparison with the hydrochar, the intensities of the bands of the MPC were weaker, due to oxygen and aromatic carbon removal, such as the peaks located at 3424 cm1 (OAH), 1700 cm1 (C@O), 1607 and 1450 cm1 (C@C). This information also indicated the higher degree of condensation of MPC sample. The peak at 3424 cm1 can be attributed to the hydrogen bonded OAH (hydroxyl or carboxyl) stretching vibrations (Unur, 2012). The band at 2915 cm1 can be assigned to the stretching vibration of aliphatic CAH (Sevilla and Fuertes, 2009a), and, the peak that appeared at 2850 cm1 may be due to the –CH2 stretching vibration (Uchimiya et al., 2011; Zhang et al., 2013). The bands at 1700 and 1607 cm1 (together with the band at 1450 cm1) can be ascribed to C@O (carbonyl, quinone, ester, or carboxyl) and C@C vibrations, respectively, which reveals the aromatization of the hydrochar. The bands in the 1400– 1000 cm1 region can be attributed to CAO (hydroxyl, ester, or ether) and OAH bending vibrations (Sevilla and Fuertes, 2009a). The bands at 870 cm1 and 777 cm1 can be assigned to aromatic CAH out of plane bending vibrations. The sharp peak at 460 cm1 can be ascribed to the adsorption of Si–O, indicating the presence of quartz, which is in accordance with the XRD results (Liou, 2004). Fig. S10 shows the Raman spectra for the hydrochar and MPC samples. Both samples exhibited the typical spectra of carbonaceous materials with two main peaks in the D (around 1335 cm1) and G bands (around 1591 cm1) (Fuertes et al., 2010; Sevilla and Fuertes, 2009a). A wide band appeared at 100– 500 cm1 in the MPC sample, which can be attributed to the presence of c-Fe2O3 (Han et al., 2007). The deconvolution of the overlapping band of the hydrochar sample gave rise to one additional peak, which can be ascribed to aryl–alkyl ether due to the aromatization of the sample (Sevilla and Fuertes, 2009a). The G band corresponds to stretching vibrations with the basal graphene layers in all carbonaceous materials (Pyrzyn´ska and Bystrzejewski, 2010), and a stronger intensity of the G band was observed in the MPC sample. The G/D ratios (indicator of the sample’s crystallinity) of the hydrochar and MPC samples were found to be 1.42 and 0.38, respectively; hence, the degree of crystallinity of the hydrochar was weaker than that of the MPC sample, which agrees with the XRD results. In addition, some defects and semi-crystalline graphitic layers existed in the MPC sample, which was concluded from the G/D ratio of the MPC sample (Bystrzejewski et al., 2009). This

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may be attributed to the constituent decomposition during the thermal treatment process. 3.2. Adsorption of tetracycline by magnetic porous carbon

not cross the origin, suggesting that pore diffusion was not the rate-limiting step for the TC adsorption onto MPC. Hence, for the beginning stage, a film diffusion or chemical reaction may control the rate of adsorption.

3.2.1. Adsorption kinetics The effect of contact time on the adsorption of TC was studied for initial concentrations of 10, 30 and 50 mg L1, as shown in Fig. 1a, the adsorption process of TC by MPC was very slow (Liu et al., 2012), and the equilibrium time increased with the increasing initial TC concentrations. To evaluate the mechanism of the kinetics of TC onto the MPC, four kinetics models were used to interpret the experimental data: the pseudo-first-order model (Eq. (2)), pseudo-second-order model (Eq. (3)), Elovich equation (Eq. (4)) and intra-particle diffusion model (Eq. (5)).

3.2.2. Adsorption isotherm The adsorption isotherm of TC over MPC at 303 K was performed. To illustrate the adsorption characterization of the MPC for TC, the adsorption data were analyzed on the basis of the nonlinear Freundlich (Eq. (6)) and the Langmuir equation (Eq. (7)):

lnðqe  qt Þ ¼ ln qe  k1 t

ð2Þ

t 1 t ¼ þ qt k2 q2e qe

ð3Þ

where KF is the Freundlich constant closely bound up with the adsorption capacity (mg11/n L1/n g1), KL is the Langmuir bonding term related to interaction energies (L mg1), n is related to the adsorption intensity, and Qm is the maximum adsorption capacity. The Freundlich and Langmuir parameters, together with regression coefficients, are also listed in Fig. 2. As shown in this figure, the adsorption of TC onto MPC was well fitted with Freundlich model. Compared with the TC adsorption capacities of other adsorbents reported in literature, the MPC exhibited an excellent adsorption performance (Liu et al., 2012). More graphitic layers, as confirmed by the Raman results, existed in the MPC, acting as a p-acceptor during the p–p electron donor–acceptor interaction. There were also more pores for trapping TC molecules in the MPC. These graphitic layers and great numbers of pores may contribute to the adsorption capacity of TC (Ji et al., 2009, 2011). To determine if the adsorption was favorable or unfavorable with the Langmuir type adsorption process, the isotherm was classified by the separation factor RL, which is defined in Eq. (8):

qt ¼

1 1 lnðabÞ þ lnðtÞ b b

ð4Þ

qt ¼ ki t 0:5 þ X

ð5Þ

1

1

1

1

0.5

where k1 (h ), k2 (g mg h ) and ki (mg g h ) are the rate constants of the pseudo-first-order model, pseudo-second-order model and the intra-particle diffusion equation, respectively; qt (mg g1) is the amount of TC adsorbed onto MPC at time t; a (mg g1 h1) is the initial adsorption rate; b is the desorption constant (g mg1); and, X is the intercept reflecting the boundary layer thickness (Zhou et al., 2012). The pseudo-first-order and pseudo-second-order models describe the mononuclear and binuclear adsorption concerning the adsorbent capacity, respectively; while the Elovich equation is an empirical equation that considers the contribution of desorption (Zhang et al., 2012). The intra-particle diffusion model is used to determine the rate-limiting step of the adsorption process (Zhou et al., 2012). The estimated parameters values are presented in Tables S1 and S2. As the results showed, the coefficient of determination (R2) values of the pseudo-second-order model were higher than those of the other three models; and, the values of qexp were closer to the theoretical values calculated by the pseudo-secondorder model, indicating that the pseudo-second-order model was the most suitable in describing the adsorption kinetics of TC onto MPC. As shown in Fig. 1b, the plots were multilinear in two distinct regions; and, the first linear segments of those three curves did

20

a

C0: 50 mg L-1

qe ¼ K F C 1=n e qe ¼

RL ¼

Q mK L Ce 1 þ K LCe

ð8Þ

The values of KL are listed in Fig. 2. When the RL values are between 0 and 1, the adsorption process is favorable. In the present work, the RL values were found to be within the range of 0.011–0.16 for TC adsorption by the MPC, respectively, suggesting that the adsorption process was favorable for the as-prepared sample. 3.2.3. Adsorption thermodynamics To determine the effect of temperature on TC adsorption, adsorption experiments were also conducted at 298 to 318 K. The change of the thermodynamic parameters of adsorption process is obtained by the following equations:

20

C0: 30 mg L-1

b

-1

10 mg L -1 30 mg L -1 50 mg L

16

-1

qt (mg g )

-1

ð7Þ

1 1 þ K LC0

16

q (mg g ) t

ð6Þ

12

C0: 10 mg L-1

8

12 8

4 4

0 0

20

40

60

80

Contact time (h)

100

120

2

4

6 1/2

t

8

10

1/2

(min )

Fig. 1. (a) Kinetics of TC adsorption on MPC with different initial concentrations at 298 K, (b) intra-particle diffusion plot for the TC adsorption with different initial concentrations at 298 K.

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was more favorable at higher temperatures. The positive values of DS° indicated that the adsorption process was irreversible and favored sorption stability. The numerical value of DG° decreased with increasing temperature at the same TC concentration, indicating that the reaction was more favorable at higher temperatures. However, the negative values of DG° suggested that the adsorption of TC onto MPC was spontaneous. In addition, the values of DG° in the present study were within the ranges of 20 and 0 kJ mol1, which indicated that the mechanism of TC adsorption onto MPC was mainly a physical adsorption (Feng et al., 2011).

Freundlich

30 25 -1

qe (mg g )

Langmuir

20 15 KF

10

Freundlich 2 1/n R

11.78 0.23

0.99

Langmuir 2 Qm R

KL 1.08

25.44 0.85

5 0

10

20

30

40

50

-1

Ce (mg L ) Fig. 2. Sorption isotherms of TC by MPC as the adsorbent at 303 K (markers are experimental data, and lines are the data predicted by Langmuir and Freundlich model, respectively).



ln K d ¼ Kd ¼ 

DH DS þ RT R



ð9Þ

qe Ce

ð10Þ 

DG ¼ DH  T DS



ð11Þ

where Kd is the distribution coefficient, DH° is the change of enthalpy (kJ mol1), DS° is the change of entropy (J mol1 K1), DG° is the change of Gibbs free energy (kJ mol1), T is the absolute temperature in Kelvin (K), and R is the gas constant (8.314 J mol1 K1). The results are listed in Table 2. The positive values of DH° indicated that the adsorption was an endothermic process. This endothermic characteristic revealed that the adsorption process

3.2.4. The effect of solution pH on TC adsorption The adsorption of TC onto MPC for different solution pH values was investigated, and the results are shown in Fig. 3a. TC is an amphoteric molecule with multiple ionizable functional groups at different pH values, and its predominant species are H4TC+ at pH <3.4, H3TC at 3.4 < pH < 7.6, H2TC at 7.6 < pH < 9.0, and HTC2 at pH >9.0, respectively (Kang et al., 2011; Zhu et al., 2013). It is evident that the adsorption amounts of TC decreased with increased solution pH values. The di-anionic species (HTC2) inhibited the adsorption of TC onto MPC, indicating that the affinity of TC for the MPC surface was higher at low solution pH. A strong p–p electron donor–acceptor interaction between the TC molecule and the surface of carbonaceous adsorbents has been verified (Ji et al., 2009; Yang et al., 2011). Hence, the lower adsorption amounts under an alkali solution may result from the weak interaction of cation-p bonding and p–p stacking with MPC during the TC adsorption process (Gao et al., 2012). The pHPZC value was measured as 7.5 (Fig. 3b); hence, few differences in the TC adsorption amounts were observed under near natural solution pH conditions, due to the buffering effect of the MPC (Liu et al., 2012).

Table 2 Thermodynamic parameters for the adsorption of TC onto MPC. C0 (mg L1)

Temperature (K)

Kd

DG° (kJ mol1)

DH° (kJ mol1)

DS° (J mol1 K1)

R2

20

298 303 308 313 318 298 303 308 313 318

3.37 4.95 9.66 11.3 14.9 1.53 2.14 2.22 4.40 5.92

3.40 4.54 5.68 6.82 7.96 0.95 1.94 2.93 3.92 4.91

70.2

228

0.96

63.0

198

0.93

30

a

b

3

16

pHPZC=7.5

Δ pH

-1

qe (mg g )

2 12 1

8 0 4 -1 0 3

5

6

7

Initial pH

8

9

10

2

4

6

8

10

Initial pH

Fig. 3. (a) Effect of initial pH on the adsorption of TC onto MPC, and (b) pHPZC of the MPC sample.

12

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4. Conclusions A novel MPC with c-Fe2O3 particles can be synthesized in one step with the thermal pyrolysis of FeCl3 pretreated hydrochar. The analysis results obtained by SEM indicate that c-Fe2O3 particles had been individually dispersed onto the surface of the porous carbon. The spent MPC can be separated and recovered from aqueous solutions by magnetic separation. The prepared MPC is an excellent adsorbent for the removal of tetracycline. Our results demonstrate that the hydrochar is a potential material for the preparation of MPC, which is a promising adsorbent for environmental and agricultural applications. Acknowledgements The authors are thankful for the financial support from the Jinlvyuan Green Engineering Co. Ltd. and the Shanghai Science and Technology Committee. The authors also thank the anonymous reviewers for fruitful suggestions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013. 12.019. References Bourke, J., Manley-Harris, M., Fushimi, C., Dowaki, K., Nunoura, T., Antal, M.J., 2007. Do all carbonized charcoals have the same chemical structure? 2. A model of the chemical structure of carbonized charcoal. Ind. Eng. Chem. Res. 46, 5954– 5967. Bystrzejewski, M., Pyrzyn´ska, K., Huczko, A., Lange, H., 2009. Carbon-encapsulated magnetic nanoparticles as separable and mobile sorbents of heavy metal ions from aqueous solutions. Carbon 47, 1201–1204. Dai, Z., Meng, J., Muhammad, N., Liu, X., Wang, H., He, Y., Brookes, P.C., Xu, J., 2013. The potential feasibility for soil improvement, based on the properties of biochars pyrolyzed from different feedstocks. J. Soils Sediments 13, 989–1000. Falco, C., Marco-Lozar, J., Salinas-Torres, D., Morallón, E., Cazorla-Amorós, D., Titirici, M., Lozano Castello, D., 2013. Tailoring the porosity of chemically activated hydrothermal carbons: influence of the precursor and hydrothermal carbonization temperature. Carbon 62, 346–355. Feng, Y., Yang, F., Wang, Y., Ma, L., Wu, Y., Kerr, P.G., Yang, L., 2011. Basic dye adsorption onto an agro-based waste material-Sesame hull (Sesamum indicum L.). Bioresour. Technol. 102, 10280–10285. Fuertes, A., Arbestain, M.C., Sevilla, M., Maciá-Agulló, J., Fiol, S., López, R., Smernik, R.J., Aitkenhead, W., Arce, F., Macias, F., 2010. Chemical and structural properties of carbonaceous products obtained by pyrolysis and hydrothermal carbonisation of corn stover. Soil Res. 48, 618–626. Gao, Y., Li, Y., Zhang, L., Huang, H., Hu, J., Shah, S.M., Su, X., 2012. Adsorption and removal of tetracycline antibiotics from aqueous solution by graphene oxide. J. Colloid Interface Sci. 368, 540–546. Han, Q., Liu, Z., Xu, Y., Chen, Z., Wang, T., Zhang, H., 2007. Growth and properties of single-crystalline c-Fe2O3 nanowires. J. Phys. Chem. C 111, 5034–5038. Hu, B., Wang, K., Wu, L., Yu, S.H., Antonietti, M., Titirici, M.M., 2010a. Engineering carbon materials from the hydrothermal carbonization process of biomass. Adv. Mater. 22, 813–828. Hu, J., Shao, D., Chen, C., Sheng, G., Li, J., Wang, X., Nagatsu, M., 2010b. Plasmainduced grafting of cyclodextrin onto multiwall carbon nanotube/iron oxides for adsorbent application. J. Phys. Chem. B 114, 6779–6785. Ji, L., Chen, W., Duan, L., Zhu, D., 2009. Mechanisms for strong adsorption of tetracycline to carbon nanotubes: a comparative study using activated carbon and graphite as adsorbents. Environ. Sci. Technol. 43, 2322–2327.

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