ZnCl2 modified biochar derived from aerobic granular sludge for developed microporosity and enhanced adsorption to tetracycline

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ZnCl2 modified biochar derived from aerobic granular sludge for developed microporosity and enhanced adsorption to tetracycline Lilong Yana, Yue Liua, Yudan Zhanga, Shuang Liua, Caixu Wanga, Wanting Chena, Cong Liua, ⁎ Zhonglin Chenb, , Ying Zhanga a b

School of Resource and Environment, Northeast Agricultural University, Harbin 150030 China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Tetracycline Aerobic granular sludge Biochar Modification

In this study, biochar derived from aerobic granular sludge was modified by ZnCl2 (Zn-BC) to improve the adsorption performance of tetracycline (TC). The surface area, pores, and functional groups of Zn-BC were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) and the effects of initial pH, TC concentration, and temperature on TC adsorption performance were analyzed. At the same time, the adsorption kinetics, isotherms, thermodynamics and diffusion models were studied. The results showed that the BET surface area and micropore volume of Zn-BC were 852.41 m2·g−1 and 0.086 cm3·g−1, respectively. The maximum adsorption performance of TC was 93.44 mg·g−1, and it was less influenced by pH. The adsorption of TC on Zn-BC agreed well with the pseudo-second-order model and the Langmuir isotherm. The thermodynamic parameters indicated that the adsorption process was a spontaneously endothermic reaction.

1. Introduction Antibiotics such as tetracyclines, sulfonamides, macrolides and quinolones are widely used in animal and human medicine to prevent and treat infectious diseases (Chen and Zhang, 2013; Yu et al., 2016). The annual use of antibiotics in China is estimated to exceed 25 000

⁎

tons, and among them, more than 8 000 tons of antibiotics are used as feed additives (Ben et al., 2008). Because of its low cost and good effects, tetracycline (TC) has become the most widely used antibiotic in China, and it has been widely used in agriculture and animal husbandry as a veterinary medicine and growth promoter (Bao et al., 2010; Ji et al., 2009; Yu et al., 2016). TC is a typical sterilizing medicine (Dai

Corresponding author. E-mail address: [email protected] (Z. Chen).

https://doi.org/10.1016/j.biortech.2019.122381 Received 31 August 2019; Received in revised form 24 October 2019; Accepted 4 November 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Lilong Yan, et al., Bioresource Technology, https://doi.org/10.1016/j.biortech.2019.122381

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good sedimentation, rich microbial composition (Yan et al., 2015), and a large number of small pores during carbonization, which greatly improve its adsorption performance. It has broad application prospects and is expected to become a novel adsorbent. Although there are some studies on BC derived from AGS used to adsorb Pb2+, Cu2+ (Wei et al., 2018) and dyes (Zhang et al., 2015), little research has been done on the adsorption of TC. Ordinary BC has a limited surface area and a less-developed pore structure (Jang and Kan, 2019), so its adsorption performance on macromolecules such as TC is limited. Therefore, to improve its pore structure and increase the specific surface area, it is essential to activate and modify BC (Wang et al., 2017). At present, BC has been activated by ZnCl2 (Liou, 2010; Wei et al., 2018), FeCl3 (Zhu et al., 2014a), O3 (Huber et al., 2005), heat treatment (Zhu et al., 2018), H3PO4 (Chen et al., 2018; Liou, 2010; Sun et al., 2016), NaOH (Jang et al., 2018; Jang and Kan, 2019), and KOH (Aghababaei et al., 2017; Muniandy et al., 2014), to change its surface structure. BC has also been modified by introducing nanoscale iron (Dong et al., 2017a; Zhang et al., 2014), Fe3O4 (Cui et al., 2014; Pi et al., 2017), iron salt (Peng et al., 2014; Yang et al., 2016), bimetallic double hydroxide (Fe/Zn (Zhou et al., 2017), La3+/La(OH)3 (Dong et al., 2017b), Mg/Al (Tan et al., 2016; Wan et al., 2017)), amino (Chen et al., 2018), and cosolvents (Jing et al., 2014; Kim and Hyun, 2018) to improve its adsorption performance. Among these techniques, the pore-forming effect of ZnCl2 decomposition during heat treatment (Wei et al., 2018) can accelerate the reaction rate, increase the surface area, and provide more sites on BC for the adsorption of TC (Liou, 2010; Wei et al., 2018). The purpose of this paper is to (1) prepare Zn-BC by using AGS as a substrate and ZnCl2 as an activator; and (2) study the factors and mechanisms of TC adsorption on Zn-BC. AGS was activated by ZnCl2 to prepare Zn-BC. The operating conditions were optimized, and the specific surface area, porosity, structural characteristics and functional groups were characterized. At the same time, the adsorption kinetics, thermodynamics, isotherms and diffusion models of TC on Zn-BC derived from AGS were analyzed to provide a new approach for the efficient removal of TC.

et al., 2019), and its antibacterial broad spectrum and stable naphthalene structure resist degrading (Chen et al., 2016). Approximately 70–90% of TC cannot be metabolized (Bao et al., 2010; Chen et al., 2018) and is subsequently released into the environment in a constant manner. Therefore, antibiotics are often detected in soil, sediment and aquatic environments (Gao et al., 2012; Jing et al., 2014; Xu and Li, 2010), and these trace amounts of antibiotics can cause adverse consequences (Chen et al., 2016): for example, they can cause bacterial resistance and alter microbial ecological functions (Pi et al., 2017). Antibiotics can reduce human immunity and induce the production of antibiotic resistance genes (ARGs) in microorganisms (Chen and Zhang, 2013; Xu and Li, 2010; Zhou et al., 2017). Some antibiotics are carcinogenic, teratogenic, mutagenic or play a hormonal role, seriously interfering with human physiological functions (Yu et al., 2016). Therefore, controlling and handling antibiotic contaminants are critical to environmental safety. Membrane technology, bioremediation, oxidation, anaerobic biodegradation and photocatalytic degradation can effectively remove TC (Huber et al., 2005; Tang et al., 2018), but the cost is relatively expensive and the procedure is usually complicated (Chen et al., 2018). In comparison, adsorption technique has the advantages of simplicity, low cost, high removal efficiency and nontoxicity (Rivera-Utrilla et al., 2013; Yu et al., 2016; Zhu et al., 2014a), and it has been widely used in environmental restoration. A variety of adsorbents have been used to remove contaminants, including carbon nanotubes (Ji et al., 2009; Yu et al., 2016), palygorskite (Chang et al., 2009), kaolinite (Li et al., 2010), graphene (Gao et al., 2012; Yu et al., 2016; Zhao et al., 2013) and apatite (Cui et al., 2014), but the above materials probably have present difficulties such as cost, limited adsorption performance or potential risks to the environment (Chen et al., 2018). In contrast, biochars (BCs) from plant residues (Kim and Hyun, 2018; Sun et al., 2016; Wang et al., 2017), animal waste (Chen et al., 2018; Wang et al., 2016), sewage sludge (Xu et al., 2014; Yang et al., 2016) and algae (Peng et al., 2014) are less toxic to the environment and have been widely used to remove TC from wastewater. As a biomass-derived carbonaceous material, BC, produced by the pyrolysis of biomass under oxygen-limited conditions (Kim and Hyun, 2018; Tang et al., 2018), is a carbon-rich and fine-grained porous material (Aghababaei et al., 2017; Li et al., 2017). Carbon materials are inexpensive, nontoxic and have a high specific surface area, a large pore volume and abundant binding sites; in addition, their surface properties can be simply changed by modification and introducing more oxygencontaining functional groups (Li et al., 2017; Wan et al., 2017), so that BC and contaminants can be specifically combined via H bonding, π-π interaction and covalent bonding (Jing et al., 2014), which have been widely used in research. BC plays an important role in carbon sequestration, improving soil fertility, and removing heavy metals and hydrophobic organic pollutants (Mohan et al., 2014; Wang et al., 2016). Generally, BC derived from straw has a low carbon content and high consumption, and its ability to adsorb TC is limited. BCs derived from salix (Zhu et al., 2014a), rice husk (Chen et al., 2016), microalgae (Peng et al., 2014), peanuts hulls (Torres-Pérez et al., 2012), bagasse (Tan et al., 2016), sawdust (Zhou et al., 2017), spent coffee grounds (Dai et al., 2019), bacterial extracellular polymer substance (EPS) (Pi et al., 2017) and bamboo (Wan et al., 2017), exhibit a general adsorption performance on TC. BCs derived from swine manure (Chen et al., 2018; Wang et al., 2016), rice straw (Chen et al., 2018), pinus taeda (Jang et al., 2018), alfalfa (Jang and Kan, 2019), and sugar beet pulp (TorresPérez et al., 2012), exhibit excellent adsorption performance, but the adsorption time is too long, which is not suitable for practical application. BC derived from sludge has been widely used to remove a variety of pollutants due to its rich functional groups and wide sources (RiveraUtrilla et al., 2013; Xu et al., 2014; Yang et al., 2016). Compared with activated sludge, aerobic granular sludge (AGS) has a high carbon content, large surface area (Zhang et al., 2015), compact structure,

2. Materials and methods 2.1. Reagents and materials TC (C22H24N2O8; CAS Number: 60-54-8) was purchased from the Macklin Biochemical Co. (Shanghai, China). All other chemicals used in this study were of analytical grade. AGS was taken from a sequencing batch reactor (SBR) in the laboratory (Yan et al., 2015). 2.2. Adsorbents preparation AGS was air-dried and pulverized to pass through a 100 mesh sieve, then immersed in 5 mol·L-1 ZnCl2 solution for 24 h, dried at 105 °C, and placed in a tube furnace (SK-3-9 K, China). N2 was introduced into the tube furnace for 10 min to ensure an inert atmosphere. Finally, the pretreated sludge was calcined at 700 °C for 2 h with a heating rate of 5 °C·min−1 with continuous nitrogen flow, and then cooled to room temperature. Subsequently, the sample was washed with 2 mol·L−1 HCl, dried at 105 °C, sealed and stored. Zn-BC was finally obtained. For comparison, BC was treated in the same way as Zn-BC, except for immersion in the ZnCl2 solution. 2.3. Characteristics of adsorbents The structures and morphologies of adsorbents were characterized using scanning electron microscopy (SEM, SU8010, Japan). A Brunauer-Emmett-Teller analysis (BET, ASAP2020, USA) was applied to determine the surface area, porosity and pore volume of the adsorbent. Moreover, Fourier Transform Infrared Spectroscopy (FTIR, 2

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PerkinElmer, USA) of Zn-BC was recorded with 4 cm−1 resolution between wavenumbers of 4000 cm−1 and 400 cm−1. X-ray diffraction (XRD, D8 ADVANCE, Germany) was performed using CuKá as the radiation source. The crystal structure property was clarified at 2θ = 10°–90°, and the scanning rate was 3°·min−1. The point of zero charge (pHpzc) of Zn-BC was estimated by the pH drift method (Jang et al., 2018).

where qe and qt are the adsorption capacity (mg·g−1) of the adsorbent at equilibrium and at a certain time, respectively, and K1 is the pseudo-first-order adsorption rate constant (min−1). 2.5.1.2. Pseudo-second-order model (Zhu et al., 2014a). The pseudosecond-order model assumes that chemisorption controls the rate of adsorption, involving electron transfer and electron sharing between the adsorbate and the adsorbent during the reaction. The equation and linear relationship are as follows:

2.4. Sorption experiments

t 1 t = + qt K2 qe 2 qe

2.4.1. Effect of initial pH, concentration and temperature To investigate the effect of the initial solution pH on TC adsorption, batch experiments were performed at different pH values ranging from 4 to 9. The pH of the initial solution was adjusted to 4–9 by using 0.1 mol·L-1 NaOH and HCl solution. Next, 0.015 g Zn-BC was added to 20 mL TC solution (80 mg·L-1) and agitated without light (to minimize photodegradation) at 160 rpm and 25 °C for 24 h. Afterwards, the supernatant was filtrated using a 0.22 μm syringe filter and then measured by spectrophotometry at a wavelength of approximately 365 nm (Zhu et al., 2014a). Control groups were assessed using a TC solution without Zn-BC and performed simultaneously with experimental groups to minimize the error. All experiments were performed in three parallel samples at the same time. To investigate the effect of initial TC concentration and temperature on adsorption, batch experiments were performed at different initial TC concentrations of 10–100 mg·L−1 and at different temperatures of 15, 25 and 35 °C; other steps were the same as above. The adsorption capacity is calculated by using equation (1) (Jang and Kan, 2019):

qe =

(C0 − Ce ) V W

where K2 is the pseudo-second-order adsorption rate constant (g·mg−1·min−1). 2.5.1.3. Elovich equation (Zhou et al., 2017).

1 1 qt = ⎛ ⎞ ln ab + ⎛ ⎞ ln t ⎝b⎠ ⎝b⎠

t0 =

−1

qt t

where t (minute) represents time and qt (mg·g amount of adsorption at t.

qt = Kt t + Ct

(7) −1

1/2

where Kt (mg·g ·min ) is the intraparticle diffusion rate constant, and C (mg·g−1) is the constant related to the thickness and the boundary layer; the larger the C value, the greater the influence of the boundary layer on the adsorption rate.

(2) −1

) represents the 2.5.1.5. Liquid film diffusion (Jang et al., 2018). To determine whether the actual rate-controlling step results from film diffusion or pore diffusion, the kinetic data were analyzed using liquid film diffusion, which is given as follows:

2.4.2. Adsorption kinetics and isotherms experiments Adsorption kinetics were investigated by adding 0.015 g of Zn-BC into 20 mL TC solution (80 mg·L−1, pH = 5). The solution was then shaken without light at 160 rpm and 25 °C for 72 h while the samples were collected at a regular intervals and measured. In addition, adsorption isotherm experiments were carried out by stirring 20 mL TC solution (10–100 mg·L−1, pH = 5) mixed with 0.015 g Zn-BC without light for 48 h and then measured.

q ln ⎛⎜1 − t ⎟⎞ = −Kfd t q e⎠ ⎝ where Kfd (min

−1

(8)

) is the liquid film diffusion coefficient.

2.5.2. Isotherms models 2.5.2.1. Langmuir (Gao et al., 2012).

2.5. Experimental data modeling

1 1 1 = + qe qm KL Ce qm

Five kinetics models (pseudo-first-order, pseudo-second-order, Elovich, intraparticle diffusion and liquid film diffusion), three isotherms models (Langmuir, Freundlich and Temkin), and thermodynamic models were used to fit experimental data to investigate the reaction behavior between TC and Zn-BC.

(9)

−1

where qm (mg·g ) is the maximum amount of adsorption corresponding to the monolayer coverage, and KL (L·mg−1) is the Langmuir constant related to the adsorption energy. The separation factor (RL) is used to evaluate the adsorption process, and it is defined as follows:

2.5.1. Kinetics models 2.5.1.1. Pseudo-first-order model (Bao et al., 2010). The pseudo-firstorder model assumes that the adsorption is controlled by the diffusion step; the equation and linear relationship are as follows:

lg(qe − qt ) = lg qe − K1 t

(6)

2.5.1.4. Intraparticle diffusion (Jang et al., 2018). For the solid-liquid adsorption process, the solute is transferred by external liquid film diffusion or intraparticle diffusion or a combination of the two. The intraparticle diffusion model is based on Fick's 2nd law of diffusion; assuming that the external resistance acts only in the initial stage of diffusion, the direction of particle diffusion is random, the concentration of the adsorbate does not change with the position of the particle, and the intraparticle diffusion coefficient is constant and does not change with time. The equation is as follows:

where C0 (mg·L ) is the initial TC concentration, and Ce (mg·L ) is the TC concentration at equilibrium. V (L) is the volume of the TC solution. W (g) is the amount of Zn-BC. The adsorption rate is calculated by using equation (2) (Bao et al., 2010):

Vt =

1 ab

(5)

where constant a is related to the rate of chemisorption and constant b is related to the surface coverage.

(1)

−1

(4)

RL =

1 1 + KL C1

(10) −1

where C1 (mg·L ) is the highest initial TC concentration. RL indicates that the isotherm is unfavorable (RL > 1), favorable (RL < 1), linear (RL = 1), or irreversible (RL = 0).

(3) 3

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2.5.2.2. Freundlich (Bao et al., 2010).

ln qe = ln KF +

1 Ce n

(11)

where KF is the adsorption capacity in the unit concentration, and n−1 is the intensity of adsorption. n−1 represents that the isotherm is irreversible (n−1 < 0), desirable (0 < n−1 < 1), or undesirable (n−1 > 1). 2.5.2.3. Temkin (Zhou et al., 2017).

q e = a ln KT + a ln Ce

(12)

where KT is the equilibrium bond constant related to the maximum energy of the bond. 2.5.3. Thermodynamic models (a) Gibbs (Li et al., 2010):

ΔG = −RT ln K d

(13)

(b) Van't Hoff (Zhu et al., 2014a):

ln K d =

ΔS ΔH − R RT

Fig. 1. Nitrogen adsorption-desorption curve and pore size distributions of ZnBC.

(14)

where Kd is the dispersion coefficient of the adsorption process, ΔS (J·mol−1·K−1), ΔH (kJ·mol−1) and ΔG (kJ·mol−1) are the entropy, enthalpy, and the standard Gibbs free energy change, respectively, and R (8.314 J·mol−1·K−1) is the universal gas constant.

matter, and a large number of pores are necessary for the rapid transfer of adsorbate to adsorbent (Liou, 2010). A previous study showed that carbon materials should have a sufficiently large pore diameter to allow contaminant molecules to easily enter the adsorption sites in the pores (Rivera-Utrilla et al., 2013). When the average pore diameter is less than 1.7 times the second broad size of the molecule, the target compound cannot be effectively adsorbed due to the size exclusion effect (Zhang et al., 2014; Zhu et al., 2014b; Zhu et al., 2018). The size of the TC molecule is less than 1.27 nm, and the size of fully protonated TC is 12.9 Å, 6.2 Å, and 7.5 Å (Chang et al., 2009; Li et al., 2010), indicating that Zn-BC can sufficiently adsorb TC and pore filling participates in the adsorption process (Ji et al., 2009; Wang et al., 2016; Wang et al., 2017). Analysis by FTIR spectra in the Supplementary data showed that ZnBC had abundant oxygen-containing polar functional groups. The peak at 3444 cm−1 represented the stretching vibration of –OH associated with intermolecular hydrogen in alcohol and phenol (Tan et al., 2016; Zhu et al., 2018) and the peak at 1127 cm−1 represented the characteristic peak of the benzene ring or the aromatic (C]C) (Muniandy et al., 2014), indicating that Zn-BC contains benzene ring substances (Zhu et al., 2014a). At the same time, it was observed that the stretching vibration of C-O-C in saturated six-membered dioxane ether at the peak of 573 cm−1 and C]O at the absorption peak of 1559 cm−1 (Muniandy et al., 2014), these oxygen-containing functional groups acted as π-electron acceptors during the reaction (Jing et al., 2014; Kim and Hyun, 2018). The dehydration of ZnCl2 in the carbonization process led to aromatization of the carbon skeleton (Liou, 2010), resulting in π-π conjugation between the aromatic structure in Zn-BC and the aromatic ring structure in TC (Wang et al., 2017). In addition, the formation of hydrogen bonds between the phenolic group of TC and the oxygen-containing carbon group could also facilitate the adsorption process (Dai et al., 2019; Rivera-Utrilla et al., 2013). After the adsorption of TC, the composition of the functional group did not change significantly, but some peaks became sharp. This was probably because the main functional groups of Zn-BC were similar to TC, resulting in an increase in the content of some functional groups after adsorption. No –COOH was found throughout the process, probably because –COOH was ignited at the higher temperature (700 °C), resulting in the loss of –COOH (Li et al., 2017). Analysis by XRD in the Supplementary data showed that Zn-BC had three diffraction peaks at diffraction angles 2θ of 15°, 26° and 43°, mainly the 101 peak and 002 peak of crystalline carbon fiber and its

3. Results and discussion 3.1. Characterization of Zn-BC The SEM images of Zn-BC in the Supplementary data showed that remarkable amounts of pores and voids were observed on the surface of Zn-BC compared with BC and exhibited a neatly arranged honeycomb porous structure, and the pore distribution was concentrated and uniform. The pore-forming effect of ZnCl2 decomposition during heat treatment (Wei et al., 2018) could improve the breakage of organic matter in the sludge and gradually recombine the solid matrix to form a developed porous structure (Yang et al., 2016), which could accelerate the reaction rate, increase the surface area, and provide more sites for the adsorption of TC. After the adsorption of TC, the pore structure on the surface of Zn-BC disappeared and showed a wrinkled structure at the magnification of 4.50 k and 30.0 k, inferred that TC molecules were likely adsorbed on the surface of Zn-BC through pore filling (Wang et al., 2016; Wang et al., 2017; Zhou et al., 2017), which requires further verification. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the nitrogen adsorption–desorption isotherm of Zn-BC (Fig. 1) complied with the class I adsorption isotherm and was a typical monolayer adsorption, which is the feature of microporous material for micropore filling (Jing et al., 2014). At the same time, the pore size distribution (Fig. 1) was determined using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm. According to the pore diameter, IUPAC divides the pores into micropores (d < 2 nm), mesopores (2 nm < d < 50 nm) and macropores (d > 50 nm) (Li et al., 2017). After ZnCl2 modification, the average pore diameter, pore volume and BET surface area of BC changed from 169.322 Å, 0.008216 cm3·g−1, 6.3482 m2·g−1 to 24.61 Å, 0.086 cm3·g−1, 852.41 m2·g−1, respectively. The microporous structure and high specific surface area facilitated ion exchange and diffusion (Zhou et al., 2017; Zhu et al., 2014b) and ensured that TC molecules were evenly distributed on the surface of Zn-BC to provide more contact area between TC and Zn-BC (Dong et al., 2017a), providing a premise and basis for the superior adsorption performance of Zn-BC. The pore structure of carbon materials affects the adsorption process of organic 4

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Fig. 2. (continued) Table 1 Thermodynamic parameters of TC adsorption on Zn-BC. T (K)

lnKd

ΔG (kJ·mol−1)

ΔH (kJ·mol−1)

ΔS (J·mol−1·K−1)

288 298 308

0.3555 0.9534 1.4297

−0.85 −2.36 −3.66

39.65

0.14

Tan et al., 2016). The initial pH determines the charge and the electron density of the Zn-BC surface, the ionic form of the solution and the form of TC substance (Chen et al., 2016; Kim and Hyun, 2018). As shown in Fig. 2a, the pHpzc of Zn-BC was approximately 3.6. When the pH of the solution is less than pHpzc, the surface of the adsorbent is positively charged; when the pH of the solution is more than pHpzc, the surface of the adsorbent is negatively charged (Jang et al., 2018). The effect of the initial solution pH on the removal of TC was shown in Fig. 2b. The results showed that when the initial solution pH was increased from 4 to 5, the adsorption performance gradually increased, and the adsorption performance of TC on Zn-BC was maximized at pH = 5 (74.79 mg·g−1), which is similar to other studies (Jang et al., 2018; Premarathna et al., 2019). However, rice straw and swine manure biochars modified by H3PO4 showed maximum adsorption performance at pH = 9 (Chen et al., 2018). The optimum pH for TC adsorption by different carbon materials is also different (Premarathna et al., 2019). At pH = 5.0, the main substance was TC H20. At the same time the pH of the TC solution was greater than pHpzc, and the surface of Zn-BC was negatively charged. The electrostatic repulsion force was the lowest at this time (Aghababaei et al., 2017), resulting in the highest adsorption capacity (Zhou et al., 2017). When the pH was further increased from 7.7, the dominant species became TC H- and TC2− , so electrostatic repulsion increased between the anionic TC and the negatively charged Zn-BC (Dai et al., 2019; Rivera-Utrilla et al., 2013). This led to a reduction in the adsorption affinity of Zn-BC for TC, and the adsorption capacity weakened (Xu and Li, 2010). However, a large amount of TC molecules were still adsorbed at this time,

Fig. 2. (a) The pHpzc of Zn-BC; (b) effect of initial solution pH on adsorption; (c) effect of initial solution concentration on adsorption; (d) effect of temperature on adsorption; (e) thermodynamic model.

secondary diffraction peak 100 (Muniandy et al., 2014; Wang et al., 2017), which are characteristics of amorphous carbon. The carbon rings were randomly stacked (Muniandy et al., 2014), indicating the formation of graphite structures in disordered carbon (Zhu et al., 2018). The structure of Zn-BC was gradually converted to microcrystalline carbon fiber and the crystallinity was increased (Zhu et al., 2014a). In addition, Zn-BC was converted into a more stable carbon compound, which was also the reason why the properties of Zn-BC were more stable. Since ZnBC was soaked in ZnCl2 solution and contained salt, the diffraction peak was sharp. 3.2. Effect of initial solution pH, concentration and temperature TC is a hydrophilic amphiphilic molecule with various functional groups, such as phenol and alcohol (Zhou et al., 2017; Zhu et al., 2014b), and can exist in different forms under different acid dissociation constants such as TC H3+ (pH < 3.3), TC H20 (3.3 < pH < 7.7), TC H− (7.7 < pH < 9.7), and TC 2− (pH > 9.7) (Gao et al., 2012; 5

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Fig. 3. (continued)

obvious change. More TC molecules were transferred from the aqueous solution to the surface of Zn-BC because of the large surface area, abundant adsorption sites and high concentration of TC solution in the initial stage of adsorption (Zhou et al., 2017), and Zn-BC had a higher TC adsorption capacity at this time. The limited adsorption sites were mostly combined, the remaining adsorption sites would decrease, and the adsorption rate became slower when the TC concentration increased (Dai et al., 2020; Zhou et al., 2017). With increasing temperature, the adsorption of TC on Zn-BC gradually increased (Fig. 2d). Higher temperature led to more intense intermolecular motion and a higher rate of TC diffusion to the surface of

Fig. 3. Adsorption kinetics: (a) adsorption of TC with time and adsorption rate at different times; (b) pseudo-first-order; (c) pseudo-second-order; (d) Elovich; (e) liquid Film diffusion; (f) intraparticle diffusion.

indicating that in addition to the electrostatic action, other effects such as pore filling, π-π action and H bonding also played important roles in the adsorption process (Jang and Kan, 2019; Wang et al., 2017). The effect of initial solution concentration on the removal of TC on Zn-BC was shown in Fig. 2c. The adsorption capacity of TC gradually increased with increasing initial concentration, when the concentration was higher than 80 mg·L-1, the adsorption performance almost had no 6

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Table 2 Comparison of the adsorption of TC on various adsorbents. Raw material

Surface area (m2·g−1)

Qm (mg·g−1)

References

Biochar (CH3OH-modified) Hydrochar (FeCl3 modified) Pinus taeda (NaOH activated) Palygorskite Graphene oxide Carbon nanotubes Kaolinite Municipal sewage sludge Microalgae ((NH4)2SO4·FeSO4·6H2O modified) Graphene oxide–chitosan aerogel Sludge (FeSO4 modified) Spent coffee grounds Spent coffee grounds waste AA dregs Rice straw (H3PO4 modified) Swine manure (H3PO4 modified) Alfalfa hays (NaOH modified) Magnetic hypercrosslinked resins Sugar beet pulp Peanuts hulls Klebsiella sp. J1 (Fe3O4 modified) Bagasse (Mg/Al modified) Forest (H3PO4 modified) Aerobic granular sludge (ZnCl2 modified)

65.97 349 959.90 126 – 370 13.10 67.387 128.30 345 126.86 451 419 46.56 372.21 319.04 796.50 1322 821 829 49.20 – 354 852.41

< 100 20 274.81 210 (mmol kg−1) 386.85 900 (mmol kg−1) 9 (mmol kg−1) 120 95.86 1470 40.80 64.89 123.46 11.9 552 365.40 302.37 90–140 288.3 28 56.04 17.70 263.80 93.44

Jing et al., 2014 Zhu et al., 2014a Jang et al., 2018 Chang et al., 2009 Gao et al., 2012 Ji et al., 2009 Li et al., 2010 Tang et al., 2018 Peng et al., 2014 Zhao et al., 2013 Yang et al., 2016 Dai et al., 2019 Dai et al., 2019 Dai et al., 2020 Chen et al., 2018 Chen et al., 2018 Jang and Kan, 2019 Zhang et al., 2014 Torres-Pérez et al., 2012 Torres-Pérez et al., 2012 Pi et al., 2017 Tan et al., 2016 Aghababaei et al., 2017 this study

swine manure (Chen et al., 2018), rice straw (Chen et al., 2018), municipal sewage sludge (Tang et al., 2018), and sugar beet pulp (TorresPérez et al., 2012) (Table 2). However, compared with spent coffee grounds (Dai et al., 2019) and sugar beet pulp, the source of sludge used is more extensive in this study; compared with swine manure and rice straw, the adsorbent used in this study has a fast adsorption rate for TC, and the adsorption equilibrium time (48 h) is much shorter than that of pig manure and rice straw (72 h). In summary, the adsorbent in this study can sufficently adsorb TC and can reach equilibrium in a short time, which has broad application prospects in removing TC. To clarify the adsorption mechanism, pseudo-first-order, pseudosecond-order and Elovich model were used to analyze the dynamic properties of TC, and the results as well as the kinetic parameters were shown in Fig. 3b, c, d and Table 3, respectively. The experimental results showed that the pseudo-second-order model (R2 = 0.992) better fitted the dynamics of TC compared with the pseudo-first-order (R2 = 0.970) and Elovich model (R2 = 0.945), which is consistent with previous results (Peng et al., 2014; Yang et al., 2016). The process of the pseudo-second-order could be divided into two dynamic steps. TC molecules were first adsorbed on the surface of micropores of Zn-BC to form a single layer of TC molecules (Jing et al., 2014). The rate of the adsorption reaction was mainly controlled by the surface reaction process and not the mass transfer process; as monolayer physical adsorption approached saturation, Zn-BC began to adsorb TC by chemisorption (Zhou et al., 2017). Zn-BC combined with TC had stable chemical and physical properties and did not cause secondary pollution to the environment due to this chemical interaction. According to the calculation results of the pseudo-second-order model, the maximum adsorption performance of TC on Zn-BC was 87.72 mg·g−1, which was close to the experimental maximum adsorption capacity (93.44 mg·g−1). In addition, the adsorption rate constant K was less than 1, indicating that the reaction process was rapid. The adsorption process generally consists of liquid film diffusion, intraparticle diffusion and an interaction stage (Sun et al., 2016), but the third phase cannot be considered a speed limit step. Therefore, the adsorption rate is controlled by liquid film diffusion, intraparticle diffusion or both. To understand the mechanism and rate control steps affecting the adsorption kinetics, the adsorption process was studied by intraparticle diffusion and liquid film diffusion (Fig. 3d, e and Table 3). The results indicated that intraparticle diffusion participated in the

Table 3 Kinetic parameters of TC adsorption on Zn-BC. Kinetic models

Kinetic parameters

Pseudo-first order Pseudo-second order Elovich Liquid Film diffusion Intra-particle diffusion

R2 = 0.970 R2 = 0.992 R2 = 0.945 R2 = 0.908 R2 = 0.958

q = 58.03 K1 = 3.55 × 104 q = 87.72 K2 = 8.90 × 10−5 a = 5.44 b = 0.09 t0 = 2.11 Kt = 1.07 × 10−3 Ct = 27.89 Kfd = 1.21

Zn-BC (Gao et al., 2012), which facilitated the adsorption of TC on ZnBC. Further thermodynamic analysis showed (Fig. 2e and Table 1) that ΔG < 0 indicates that the whole adsorption reaction was a spontaneous process (Dai et al., 2019); ΔH > 0 indicates that the adsorption of TC on Zn-BC was an endothermic process (Tan et al., 2016; Wang et al., 2017); ΔS > 0 indicates that the adsorption process was irreversible, which was conducive to the stability of adsorption (Zhu et al., 2014a). Higher temperature promoted the entire adsorption process and the adsorption process worked better at higher temperature (Zhao et al., 2013), but there was still a higher adsorption capacity at room temperature (25 °C), which was convenient for practical operation. Therefore, 25 °C was selected as the optimum temperature for the study. 3.3. Adsorption kinetics Fig. 3a showed that the adsorption consisted of a fast adsorption phase and a slow adsorption phase. The adsorption performance increased rapidly in the first 40 min, due to the large surface area and abundant active sites of Zn-BC (Wang et al., 2017) and the high initial concentration of TC. The adsorption rate decreased with time as active sites gradually reached saturation, and the performance of adsorption had no obvious difference between 48 h and 72 h; therefore, 48 h was considered to be a sufficient equilibrium time (Chen et al., 2018; Jang et al., 2018). The maximum adsorption capacity of Zn-BC (93.44 mg·g−1) was much larger than that of BC (15.01 mg·g−1) in this study, which is higher than that of salix (Zhu et al., 2014a), rice husk ash (Chen et al., 2016), waste AA dregs (Dai et al., 2020), sludge (Yang et al., 2016), peanut hulls (Torres-Pérez et al., 2012), EPS of Klebsiella sp. J1 (Pi et al., 2017), and bagasse (Tan et al., 2016), but lower than that of pinus taeda (Jang et al., 2018), graphene oxide (Gao et al., 2012), 7

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Table 4 Isotherm parameters of TC adsorption on Zn-BC. Isotherm models

Isotherm parameters

Langmuir Freundlich Temkin

R2 = 0.998 qm = 1394.57 KL = 8.32 × 10−4 R2 = 0.842 n = 47.85 KF = 15.72 R2 = 0.942 a = 38.43 KT = 0.10

et al., 2014). Further analysis of the liquid film diffusion model found that the intercept was almost zero (−0.001), indicating that liquid film diffusion (R2 = 0.908) was the rate-limiting step for the adsorption of TC (Tan et al., 2016; Zhou et al., 2017). 3.4. Adsorption isotherms The Langmuir adsorption isotherm model is based on the assumption that adsorption occurs on uniform adsorbent sites (Jang et al., 2018). The Freundlich adsorption model holds that adsorption occurs on heterogeneous surface and unequal binding sites (Xu and Li, 2010). The Temkin isotherm contains interacting factors between the adsorbent and the adsorbate. The results and the relevant parameters calculated by the above mentioned isotherm models were shown in Fig. 4 and Table 4. Obviously, compared with the Freundlich model, the Langmuir model fitted the experimental data better (R2 = 0.998), which is similar to other studies (Gao et al., 2012; Peng et al., 2014; Rivera-Utrilla et al., 2013). The maximum adsorption performance calculated according to the Langmuir adsorption model was 1394.572 mg·g−1, indicating that Zn-BC in this study had excellent adsorption performance compared with a previous study (Jang and Kan, 2019). Furthermore, the calculated separation factor RL and n-1 were 0.923 and 0.021, respectively, which indicated that the adsorption process of TC on Zn-BC was favorable under such operating conditions (RL < 1, 0 < n-1 < 1) (Jang et al., 2018; Zhou et al., 2017; Zhu et al., 2014a). The Temkin adsorption isotherm model is general and has a wide application range for adsorption, and the correlation coefficient was 0.942. This result indicated that the energy change of the adsorption of TC on Zn-BC was affected by temperature and electrostatic interaction occurred throughout the process (Li et al., 2010). 3.5. Adsorption mechanism In general, the adsorption mechanism of TC on Zn-BC mainly includes pore filling, electrostatic interaction, hydrophobic interaction, H bonding and π-π interaction (Dai et al., 2019; Zhou et al., 2017). The initial pH affects the existence of TC, and it is generally believed that pH has a great influence on the adsorption of TC. The initial pH had an effect on the adsorption of TC on Zn-BC in this study, indicating that electrostatic action participated in the adsorption process (Tan et al., 2016). Zn-BC has a developed microporous structure and a large surface area. The surface area and pore diameter of Zn-BC were 852.41 m2·g−1 and 24.61 Å, respectively, which were conducive to ion exchange and diffusion, and the pore volume of Zn-BC was larger than the size of the TC molecule. Analysis of the pore size and surface area structure of ZnBC after adsorption in the Supplementary data showed that pores on the surface of Zn-BC disappeared and the surface was covered with more ridges, indicating that the surface of Zn-BC adsorbed a large amount of TC, which inferred that pore filling participated in the adsorption process (Ji et al., 2009; Wang et al., 2017). Analysis by FTIR in the Supplementary data revealed that after the adsorption of TC, the position of the stretching peak of each functional group had shifted, and the obvious shifts were C–O–C in the aromatic ring structure and C]O and O–H in alcohol and phenol, from which it was concluded that the aromatic group participated in the entire adsorption process. π-π conjugation formed between aromatic structures,

Fig. 4. Adsorption isotherms: (a) Langmuir model; (b) Freundlich model; (c) Temkin model.

diffusion process (R2 = 0.958), and adsorption mainly occurred on the surface. However, the intercept of the fitting curve was large (27.887), indicating that intraparticle diffusion was not the main factor controlling the overall adsorption reaction rate (Tan et al., 2016), and the reaction rate might be affected by other factors (Jang et al., 2018; Jing 8

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including C–O–C and C]O in Zn-BC and the aromatic ring structure in TC (Wang et al., 2017). In addition, the H bonding between –OH and the oxygen-containing carbon group also facilitated the adsorption process (Rivera-Utrilla et al., 2013). 3.6. Reusability For carbon materials, reusability is critical in practical applications. To investigate the reusability, Zn-BC was desorbed by NaOH solution and alcohol, respectively. After three cycles of reuse, the adsorption performance of Zn-BC desorbed by NaOH solution was 81.41, 80.03, 77.09 mg·g−1, respectively, and that of Zn-BC desorbed by alcohol was 81.41, 78.07, 73.17 mg·g−1, respectively, showing Zn-BC is a promising adsorbent with excellent reusability and stability. 4. Conclusions Zn-BC derived from AGS and modified by ZnCl2 had well-developed microporous structures and a high surface area. The pseudo-secondorder kinetic model and Langmiur isotherm model better fitted the experimental data. Both the intraparticle diffusion model and liquid film diffusion participated in the adsorption process, but liquid film diffusion was the main rate-limiting step. TC adsorbed onto the surface of Zn-BC mainly by electrostatic interaction, pore filling, π-π conjugation and H bonding. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by National Key Research and Development Program of China (2018YFD0800105); Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. QA201819); and Heilongjiang Provincial Key Laboratory of Soil Protection and Remediation. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122381. References Aghababaei, A., Ncibi, M.C., Sillanpää, M., 2017. Optimized removal of oxytetracycline and cadmium from contaminated waters using chemically-activated and pyrolyzed biochars from forest and wood-processing residues. Bioresour. Technol. 239, 28–36. Bao, Y.Y., Zhou, Q.X., Wan, Y., Yu, Q., Xie, X.J., 2010. Effects of soil/solution ratios and cation types on adsorption and desorption of tetracycline in soils. Soil Sci. Soc. Am. J. 74 (5), 1553–1561. Ben, W.W., Qiang, Z.M., Adams, C., Zhang, H., Chen, L.P., 2008. Simultaneous determination of sulfonamides, tetracyclines and tiamulin in swine wastewater by solidphase extraction and liquid chromatography-mass spectrometry. J. Chromatogr. A 1202, 173–180. Chang, P.H., Li, Z.H., Yu, T.L., Munkhbayer, S., Kuo, T.H., Hung, Y.C., Jean, J.S., Lin, K.H., 2009. Sorptive removal of tetracycline from water by palygorskite. J. Hazard. Mater. 165, 148–155. Chen, H., Zhang, M.M., 2013. Occurrence and removal of antibiotic resistance genes in municipal wastewater and rural domestic sewage treatment systems in eastern China. Environ. Int. 55 (4), 9–14. Chen, T.Y., Luo, L., Deng, S.H., Shi, G.Z., Zhang, S.R., Zhang, Y.Z., Deng, O.P., Wang, L.L., Zhang, J., Wei, L.Y., 2018. Sorption of tetracycline on H3PO4 modified biochar derived from rice straw and swine manure. Bioresour. Technol. 267, 431–437. Chen, Y.J., Wang, F.H., Duan, L.C., Yang, H., Gao, J., 2016. Tetracycline adsorption onto rice husk ash, an agricultural waste: its kinetic and thermodynamic studies. J. Mol. Liq. 222, 487–494. Cui, L.M., Xu, W.Y., Guo, X.Y., Zhang, Y.K., Wei, Q., Du, B., 2014. Synthesis of strontium hydroxyapatite embedding ferroferric oxide nano-composite and its application in

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