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Characteristics of tetracycline adsorption by cow manure biochar prepared at different pyrolysis temperatures
Bioresource Technology 285 (2019) 121348
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Characteristics of tetracycline adsorption by cow manure biochar prepared at different pyrolysis temperatures Peizhen Zhang, Yanfei Li, Yaoyao Cao, Lujia Han
T
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Laboratory of Biomass and Bioprocessing Engineering, College of Engineering, China Agricultural University, Box 191, Beijing 100083, China
ARTICLE INFO
ABSTRACT
Keywords: Biochar Cow manure Pyrolysis temperature Tetracycline Adsorption
This study aimed to explore the feasibility of using cow manure biochar (CMBC) for adsorption of tetracycline for realizing farm waste treatment and recycling. Three kinds of pyrolysis-temperature CMBCs were prepared and characterized. There were significant differences in the specific surface area, pores structure, surface charge, and oxygen-containing functional groups. The effect of adsorption was not only related to the physicochemical properties of CMBC but also the dosage, solution pH, and ambient temperature. CMBC showed surface heterogeneity, and the adsorption of tetracycline was mainly chemical. Controlling the rate of adsorption was achieved by combining internal particle diffusion and liquid film diffusion. Furthermore, the adsorption was a spontaneous and endothermic process. The use of CMBC as an adsorbent for tetracycline represents a new method for treating and recycling waste in farms. These results could aid in further studies on the adsorption mechanism and optimizing the adsorption process.
1. Introduction With the rapid development of large-scale aquaculture, antibiotic pollution has become a problem requiring urgent solution (Lupo et al., 2012; Norvill et al., 2017). Antibiotics are widely used in modern aquaculture because they not only inhibit the growth of bacteria in animals (Zhao et al., 2010), but also promote feed utilization and animal growth rate (Phillips et al., 2004; Sarmah et al., 2006). In practice, a large number of antibiotics are not fully absorbed and utilized by animals, thus entering the soil and water as the excrement is discharged (Li et al., 2012), and ultimately entering the human body through the food chain (Pouretedal & Sadegh, 2014). There are many methods of treating antibiotics in wastewater, such as membrane separation (Aydin et al., 2016), oxidation (Souza et al., 2018), photochemical degradation (Le-Minh et al., 2010), electrochemical treatment (Liang et al., 2018), biological treatment (Chen et al., 2017), and adsorption (Zhou et al., 2017). Adsorption has become the primary method in practical applications because it is highly efficient, easy to perform, economical, and practical (Acosta et al., 2016). The adsorption of more than 30 types of antibiotics involving a number of adsorbents (Ahmed et al., 2015), including activated carbon (Acosta et al., 2016), bentonite (Ravi and Pandey, 2019), titanium dioxide nanoparticles (Qi et al., 2018), steel chips (Tran et al., 2017), sawdust biochar (Zhou et al., 2017), wheat straw biochar (Li et al.,
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2018), and sludge biochar (Rivera-Utrilla et al., 2013) has been studied. Preparation of biochar as an adsorbent with low-cost biomass has recently become a research hotpot because of biochar’s strong ability to remove organic pollutants in aqueous solutions (Güzel & Sayğılı, 2016; Zhu et al., 2014). Biochar is a carbon-based material obtained by pyrolyzing biomass feedstock under high temperature and anaerobic conditions (Zhang et al., 2015). The physicochemical properties of biochars prepared at different pyrolysis temperatures, such as oxygen-containing functional groups (carboxyl, hydroxyl, etc.), aromatization structure, specific surface area and porosity, and degree of graphitization, often significantly affect the adsorption and fixation of contaminants (Ahmad et al., 2014). In addition, the adsorption process is not only related to the characteristics of the biochar, but also to the types of adsorbed pollutants, the solution concentration, pH and ambient temperature (Zeng et al., 2018). Presently, research on adsorption of antibiotics by biochars mainly focuses on the adsorption effect of lignocellulosic biochar (Cao et al., 2019; Pouretedal & Sadegh, 2014; Zhang et al., 2015), while the preparation of biochar from livestock manure as a raw material can effectively utilize a large amount of livestock waste and solve the problems associated with the treatment of cultured solid waste. On the other hand, the pyrolysis process can also decompose antibiotics carried in livestock manure (Cervini et al., 2016). The use of livestock manure
Corresponding author. E-mail address: [email protected] (L. Han).
https://doi.org/10.1016/j.biortech.2019.121348 Received 6 March 2019; Received in revised form 10 April 2019; Accepted 13 April 2019 Available online 15 April 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
Bioresource Technology 285 (2019) 121348
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Table 1 Physicochemical properties of CMBCs. Cow manure biochar Elemental compositions
Atom ratio Ash% BET surface area (m2/g) Total volume of pores (cm3/g) Micropores volume (cm3/g) Average of Pore width (nm) pH
CMBC300 C% H% N% S% O% H/C O/C (O + N)/C
CMBC500 a
47.25 ± 0.40 4.23 ± 0.04a 3.26 ± 0.04a 0.94 ± 0.16a 11.23 ± 0.40a 1.07 0.18 0.24 33.08 ± 0.19c 1.55 0.002848 / 7.34 8.62 ± 0.03b
CMBC700 b
43.08 ± 1.59 1.60 ± 0.05b 2.15 ± 0.06b 0.56 ± 0.02b 7.17 ± 1.39b 0.45 0.13 0.17 45.44 ± 0.55b 1.77 0.002612 0.000567 5.89 10.75 ± 0.03a
42.56 ± 0.26b 0.72 ± 0.06c 1.79 ± 0.03c 0.57 ± 0.12b 2.73 ± 0.03c 0.20 0.05 0.08 51.63 ± 0.29a 31.23 0.023430 0.004945 3.00 10.83 ± 0.08a
Data are expressed as mean values ± standard deviations. a–c Indicates that the average of the different superscripts in the same row is significantly different at p < 0.05.
biochars were cooled to room temperature, further pulverized to pass through a 0.12 mm sieve, and stored in closed plastic bottles. Biochars obtained using the different pyrolysis temperatures were labeled as CMBC300, CMBC500, and CMBC700, respectively. 2.2. Batch adsorption experiment Tetracycline hydrochloride (TC; C22H25CIN2O8; purity, 90%) is an amphoteric compound with multiple ionizable functional groups. Based on the solubility of TC in water (e.g., 201 mg/L (at 15.15 °C), 311 mg/L (at 20.15 °C), and 430 mg/L (at 30.15 °C)), and the range of the concentration of antibiotic residues in actual aquaculture wastewater (ng – μg/L), 50 mg/L of TC was used for all adsorption experiments except for the adsorption isotherms (10–80 mg/L with the gap of 10 mg/L) (Caço et al., 2008; Jang et al., 2018; Zhu et al., 2014). A blank control sample (without biochar) was prepared for each batch of experiments to subtract the loss of TC during the adsorption process. All experiments were performed in duplicate.
Fig. 1. The effect of CMBCs and dosages on the adsorption of tetracycline. The adsorption conditions were 25 °C, pH 6, tetracycline concentration of 50 mg/L, and reaction time of 24 h.
2.2.1. Different CMBCs dosages for adsorption experiments Different dosages (0.25 g/L, 0.5 g/L, 0.75 g/L, 1 g/L, 1.25 g/L, 1.5 g/L, 1.75 g/L, 2 g/L) of biochar were added to 50 mL plastic centrifuge tubes, to which 40 mL TC solution (pH 6, 50 mg/L) was added. The mixtures were shaken at 150 rpm to an adsorption equilibrium at 25 ± 0.5 °C in the dark. The supernatants were filtered through a 0.22 μm membrane to obtain samples to be tested. The concentration of TC remaining in the solutions was determined using HPLC.
biochar as an adsorbent for the removal of antibiotics from aquaculture wastewater presents an effective way to achieve recycling of aquaculture waste. As tetracycline antibiotics are one of the most abundant antibiotics in livestock farming wastewater (Selvam et al., 2017), in this study, tetracycline hydrochloride was selected as the target adsorbate, and the cow manure biochars obtained at different pyrolysis temperatures were used as the adsorbent. Based on the analysis of the physicochemical properties of cow manure biochars, the characteristics of tetracycline hydrochloride adsorption and its effects on cow manure biochars were studied, providing a foundation for further exploration of adsorption mechanisms and optimization of the adsorption process.
2.2.2. Different pH solutions for adsorption experiments The Britton-Robinson buffer solution can be used to prepare a buffer with a wide pH range (pH 1.8–11.9). The Britton-Robinson buffer was prepared by mixing phosphoric acid (H3PO4; purity, ≥85%), boric acid (H3BO3; purity, ≥99.5%), and acetic acid (CH3COOH; purity, ≥99.5%) at a concentration of 0.04 mol/L, and its pH was adjusted by adding a solution of 0.2 mol/L sodium hydroxide (NaOH; purity, ≥96%). The initial pH of the solution was adjusted to 3–10 by adding BR buffer solution. The experiments were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (50 mg/L) and 50 mg biochar (1.25 g/L, w: v). Other adsorption conditions are similar to those in subsection 2.2.1.
2. Materials and methods 2.1. Preparation of biochars from cow manure Raw cow manure samples were collected from a dairy farm in Beijing, China. The collected samples were dried in a 70 °C oven for 48 h and pulverized for subsequent carbonization. A quartz boat containing a sample of smashed cow manure was placed in a tube furnace and passed through N2 to create an oxygenlimited or oxygen-free environment for pyrolysis carbonization. The initial temperature was set to 25 °C, the heating rate was 20 °C/min, and the pyrolysis carbonization was completed after heating at the target temperatures (300 °C, 500 °C, and 700 °C) for 1 h. After the obtained
2.2.3. Adsorption kinetics The adsorption kinetics were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (50 mg/L, pH 6) and 50 mg biochar (1.25 g/L, w: v) and shaken at 150 rpm for 48 h in the dark at 25 ± 0.5 °C. The supernatant was extracted at 1 min, 2 min, 3 min, 2
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Fig. 2. (a) The relationship between zeta potential and pH of CMBCs. (b) Species distribution of tetracycline at different pH values. (c) Effect of initial pH of solution on adsorption of tetracycline by CMBCs. The adsorption conditions were 25 °C, tetracycline concentration of 50 mg/L, biochar dosage of 1.25 g/L, and reaction time of 24 h.
and micropore volume (MPV) of biochar were analyzed by N2 adsorption at 77 K by a multifunctional gas analyzer (TRISTAR II 3020 M, USA). The adsorption data were used to calculate the SSA according to Brunauer-Emmett-Teller theory. The desorption isotherm data were used to calculate the PV based on the Barrett-Joyner-Halenda theory. The surface functional group information of the biochar before and after adsorption was obtained by Fourier-transform infrared spectroscopy (FTIR, Spectrum 400, PerkinElmer, Waltham, MA, USA) and scanned using a KBr tablet method at a resolution of 4 cm−1 in the wavelength range of 4000–400 cm−1. The amount of surface charge of the biochar was measured by a zeta potential analyzer (SZ-100Z, Horiba, Kyoto, Japan) at 25 °C. The crystal composition and degree of graphitization of the biochar were analyzed by X-ray diffraction (XRD; Bruker D8 advance, Bruker AXS, Billerica, MA, USA) using a Cu target (40 kV, 30 mA) of 5°–60° at 2θ with scanning in 0.02° increments. The pH was measured by mixing 1 g biochar with 20 mL deionized water and shaking in 25 ℃ water bath for 24 h using an FE20 pH meter (Mettler Toledo, USA).
4 min, 5 min, 10 min, 0.5 h, 1 h, 2 h, 5 h, 8 h, 12 h, 24 h, 36 h and 48 h. Other adsorption conditions are similar to those in subsection 2.2.1. 2.2.4. Adsorption isotherms The adsorption isotherms were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (10–80 mg/L, pH 6) and 50 mg biochar (1.25 g/L, w: v). Other adsorption conditions are similar to those in subsection 2.2.1. 2.2.5. Adsorption thermodynamic The adsorption thermodynamic were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (50 mg/L, pH 6) and 50 mg biochar (1.25 g/L, w: v) and shaken at a constant temperature (15 °C, 25 °C, and 35 °C) at 150 rpm to an adsorption equilibrium. Other adsorption conditions are similar to those in subsection 2.2.1. 2.3. Characterization of CMBCs The content percentages of carbon, hydrogen, nitrogen, and sulfur in biochar samples were determined using an elemental analyzer (Vario Macro Elementar, Elementar, Langenselbold, Germany). According to the ASTM standard E870-82, the percentage of oxygen by mass difference subtraction was calculated as 100% - C% - H% - N% - S% - Ash%. The ash content was analyzed using a fully automated analyzer (YXGYFX7705, U-Therm, Hunan, China). The biochar was placed in a crucible and heated at 525 °C for 6 h. The mass ratio of the obtained mass to the original biochar was expressed as the ash content. The surface morphology of the biochar was analyzed by scanning electron microscopy (SEM; SU3500, Hitachi, Tokyo, Japan). The specific surface area (SSA), pore volume (PV), pore width (PW),
2.4. Tetracycline analysis by HPLC and UV/Vis detector The TC concentration was measured and calculated by HPLC (Waters, Milford, MA, USA) using a C18 reverse phase column (5 μm, 4.6 mm × 250 mm) and a UV/Vis detector (2489, Waters, USA). The TC concentration in the solution was measured at a wavelength of 355 nm. For the liquid-phase, the column temperature was 30 °C, the mobile phase consisted of acetonitrile (C2H3N, HPLC grade, 20%) and 0.01 M oxalic acid (C2H2O4, AR, 80%), the flow rate was 1 mL/min, the injection volume was 20 μL, and the retention time was 3.7 min. The detected solute was quantified using an external standard method, and 3
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Fig. 3. Adsorption kinetics. (a) Pseudo-first order. (b) Pseudo-second order. (c) Elovich. (d) Intra-particle diffusion. (e) Liquid film diffusion. Reaction time 48 h. (f) Adsorption isotherms. The adsorption conditions were 25 °C, tetracycline solution concentration of 50 mg/L, pH 6, biochar dosage of 1.25 g/L, and reaction time of 24 h.
the curve-fitting coefficient was greater than 0.99. The relationship between TC concentration and peak area was as follows: 5 mg/L – 1711491; 10 mg/L – 351471; 25 mg/L – 867310; 50 mg/L – 1742354.
initial and adsorption equilibrium, respectively, and V (L) and M (g) represent the volume of the TC solution and the mass biochar, respectively. In this study, five adsorption kinetics models, namely pseudo-first order, pseudo-second order, elovich, intra-particle diffusion and liquid film diffusion, and three commonly used adsorption isotherms, namely Langmuir, Freundlich, and Temkin, were used to analyze the experimental data. The characteristics of the adsorption process were also explored from an energy perspective, providing useful data regarding the energy changes during the adsorption process. Adsorption thermodynamics were analyzed using adsorption equilibrium data at different temperatures. The standard Gibbs free energy change (ΔG), standard
2.5. Statistical analysis of the adsorption experiment data The adsorption capacity Q (mg/g) and removal rate R (%) of biochar were calculated according to the following equations:
Q = [(C0
Ce )/M ] × V
(1)
R = [(C0
Ce )/C0] × 100%
(2)
where C0 (mg/L) and Ce (mg/L) represent the TC concentration at the 4
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Table 2 Parameters and correlation coefficients of tetracycline adsorption kinetic model of CMBCs. Models and Parameters
CMBC300
CMBC500
CMBC700
Pseudo-first order Qe K1 R2
5.258 18.895 0.929
7.173 19.036 0.795
11.735 10.972 0.746
Pseudo-second order Qe K2 R2
5.485 6.646 0.932
7.438 3.544 0.881
11.793 1.281 0.818
Elovich a b R2
2.863 4.000 0.398
3847.732 1.400 0.992
5475.563 1.002 0.917
Intra-particle diffusion Ki Ki1 Ki2 Ki3 Ci R2
0.043 1.068 0.009 0.026 3.961 0.644
0.099 0.954 0.106 0.007 4.537 0.892
0.133 0.863 0.229 −0.057 5.785 0.931
Liquid film diffusion Kfd R2
0.283 × 10−2 0.290
0.131 × 10−2 0.841
0.877 × 10−5 0.816
Table 4 Thermodynamic parameters of tetracycline adsorption on CMBCs.
lnK = S / R
(3) (4)
( H / RT )
where R (8.314 J·mol−1·K) is the ideal gas constant, T (K) is the absolute temperature, and K is the adsorption partition coefficient; and (5)
K c = as / ae = vs Qe / ve Ce
T (K)
lnK
ΔG (KJ/mol)
ΔH (KJ/mol)
ΔS (J/mol·K)
CMBC300
288 298 308
5.096 5.039 4.904
−12.201 −12.484 −12.558
−7.020
179.383
CMBC500
288 298 308
5.329 5.566 6.238
−12.760 −13.790 −15.975
33.342
159.454
CMBC700
288 298 308
5.461 5.649 6.523
−13.077 −13.995 −16.704
38.865
18.102
the pyrolysis carbonization temperature increased (p < 0.05), while the content of C decreased with the increase of pyrolysis temperature. When the pyrolysis temperature increased from 300 °C to 500 °C, the C content decreased significantly (p < 0.05), but there was no significant change when the temperature was increased to 700 °C, which was consistent with previously obtained results (Wang et al., 2018). H/C, O/ C and (O + N)/C ratio decreased as the pyrolysis temperature increased, indicating that the aromaticity of the biochar gradually increases, the hydrophilicity decreases (i.e., hydrophobicity is enhanced), and the polarity decreases. The pH of CMBC700 and CMBC500 was significantly higher than that of CMBC300, while the first two did not differ significantly. CMBC700 had a larger specific surface area and pores structure distribution than did CMBC300 and CMBC500. The specific surface area of CMBC700 was 31.23 m2/g, which was about 20.14 and 17.64 times that of CMBC300 and CMBC500, respectively, while there was no significant difference between CMBC300 and CMBC500. The total pore volume of CMBC700 was 0.023430 cm3/g, which was about 8.23 and 8.97 times that of CMBC300 and CMBC500, respectively, and there was no significant difference between the latter two. The micropore volume of CMBC700 (0.004945 m3/g) was 8.72 times that of CMBC500 (0.000567 m3/g), while no micropores were detected in CMBC300. The proportion of micropores in CMBC700 and CMBC500 was 21.10% and 21.71%, respectively. It was found that the proportion of micropores in the total pores distribution was low. The average pore width of CMBC700 was 3.00 nm, which was about 1/3 and 1/2 times that of CMBC300 and CMBC500, respectively. It can be seen that as the pyrolysis temperature increased, the specific surface area of the CMBC increased and the pores distribution became better. When the pore size of the adsorbent is 1.7–3 times higher than the molecular size of the adsorbate, the adsorbent may exhibit the best adsorption performance (if the adsorbent needs to be recovered, the pore size of the adsorbent may even be 3–6 times higher than the molecular size of the adsorbate). The molecule TC has a length of 1.41 nm, width of 0.46 nm, and height of 0.81 nm. Therefore, CMBC has an effective pore size distribution for adsorbing TC molecules. According to FTIR analysis, the characteristic peaks and peak widths of CMBC at different pyrolysis temperatures slightly differed. Namely, the peak at 3385 cm−1 corresponded to the eOH in phenol, and the peaks at 2926 cm−1 and 2857 cm−1 corresponded to the eCH2 and eCH3 in aliphatic hydrocarbons. The intensities of the two peaks of CMBC700 were lower those of the peaks of CMBC300 and CMBC500
enthalpy change (ΔH), and standard entropy change (ΔS) were calculated separately using the following formulae:
G = - RTlnK
Adsorbent
where Kc (L/g) is the ratio of the adsorption amount of the adsorbate on the adsorbent to the residual amount in the solution during adsorption equilibrium, and Qe/Ce represents the adsorption coefficient. When lnK is plotted against 1/T, a straight line with a slope of −ΔH/R and an intercept of ΔS/R is obtained. The data in the tables and figures were expressed as the replicated mean values ± standard deviation. The analysis of variance (ANOVA) based on the Duncan’s multiple comparison test was obtained using SPSS 20 software. 3. Results and discussion 3.1. Physical and chemical properties of CMBCs There was a significant difference in the physical and chemical properties of CMBCs obtained at different pyrolysis temperatures (p < 0.05; Table 1). CMBC700 had the highest ash content (51.63%), which was significantly higher than that of CMBC500 (45.44%) and CMBC300 (33.08%) as well as that of straw biochars (Zhang et al., 2012). The content of O and H in the CMBC significantly decreased as Table 3 Adsorption isotherms parameters for tetracycline on CMBCs. Adsorbent
CMBC300 CMBC500 CMBC700
T (℃)
25℃
Langmuir model
Freundlich model
Temkin model
Qm
KL
R2
KF
1/n
R2
KT
B
R2
26.727 15.061 22.553
0.008 0.052 0.035
0.970 0.865 0.885
0.416 2.123 2.065
0.751 0.420 0.496
0.979 0.947 0.944
0.194 1.193 0.778
3.396 2.560 3.711
0.928 0.885 0.876
5
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(Wang et al., 2018), indicating that the content of oxygen-containing functional groups and aliphatic functional groups decreased with the increase of pyrolysis temperature (Keiluweit et al., 2010). The peak at 1594 cm−1, representing the stretching vibration peak of aromatic C]C, indicated that a few relatively stable aromatic compounds were gradually formed. This demonstrated that aliphatic, amide, and aromatic amines in cow manure will decrease with increasing pyrolysis temperature, resulting in a relatively stable aromatic structure. The characteristic peak at 1429 cm−1 represented saturated eCH3 in-plane bending vibration (Artz et al., 2008). The characteristic peak at 1068 cm−1 was attributed to the C-O bond stretching of ethers, esters and carbonates (Dai et al., 2016). The peaks at 975 cm−1 and 795 cm−1 represented the aromatic eCH out-of-plane bending, indicating the presence of adjacent aromatic hydrogens (Ahmed et al., 2016). The above results were consistent with the results of the elemental analysis. With the increase of pyrolysis temperature, the aromaticity increased and the proportion of polar and surface oxygen-containing functional groups (eCOOH, eOH, etc.) decreased. The XRD patterns of different CMBCs were obtained. A 2θ of around 25°, indicates the presence of an ordered graphite structure. However, according to the peak intensity, there was no significant difference in the degree of graphitization among the three biochars. The surface of CMBC300 was relatively smooth and the sheet structures were more obvious, at the same time, a small number of narrow and long pores appeared. The surface of CMBC500 was rough and the degree of cracking increased markedly. CMBC700 had the largest surface rupture and a significant increase in micropores and pore structures. In summary, as the pyrolysis carbonization temperature increased, the surface of the resulting CMBC roughened, and the number of internal pores structure increased. This is because volatile components are gradually released from cow manure with the increase of temperature, resulting in improved pores structure. A larger specific surface area, larger pore volume, and smaller average pore diameter formed, contributing to improved adsorption performance (Ahmad et al., 2012).
might be pores filling, π-π EDA interaction, and H-bonding. 3.3. Effect of solution pH value on the adsorption of tetracycline As the solution pH value increased from 3 to 10, the removal rate of TC by CMBC700 and CMBC500 decreased, while that of CMBC300 increased and then decreased (Fig. 2c), indicating that adsorption of TC is dependent on pH value. Further, the surface of the biochar was negatively charged, and the charge increased with increasing pH (Fig. 2a). TC exhibited different species distributions at different pH values and the dissociation constants (pKa) of TC are 3.3, 7.7, and 9.7 (Fig. 2b). When the pH of the solution was lower than 3.3 (pKa1), the main species of TC was protonated TC (TC+), while the surface of the biochar was negatively charged; thus, the electrostatic interaction between TC and biochar was mainly electrostatic attraction and the adsorption effect was optimal. When the pH of the solution was higher than 3.3 (pKa1), the main species of TC was TC0, and the negative charge of biochar further increased. Therefore, when the pH was between 3.3 and 7.7, the electrostatic attraction became weak and the adsorption efficiency decreased. When the pH was in the range of 7.7 (pKa2) – 9.7 (pKa3), TC became TC−, and when the pH was higher than 9.7 (pKa3), TC became TC2−. The amount of negative charge on the surface of biochar gradually increased, and the electrostatic repulsion between TC and biochar became stronger, further reducing the adsorption efficiency. In summary, CMBC700 and CMBC500 have the highest removal rate of TC at low pH, and the adsorption of both was greatly affected by pH. CMBC300 did not show a consistent trend of pH and charge, indicating that other adsorption mechanisms, such as surface complexation and cation bridging, were dominant. Some studies have shown that the ash removal treatment affected the specific surface area and porosity of biochar, as well as changed the ash composition and content, and accordingly influenced the adsorption effect (Li et al., 2017). The ash content of the CMBCs differed and was the key factor that affected their pH and zeta potential. Therefore, the difference in adsorption effect at different pH values may have been caused by ash. Subsequent studies should specifically analyze the effect of ash contents on the adsorption mechanism. In summary, the change in pH affected the surface charge of biochar and the species distribution of TC, influencing the adsorption of TC through electrostatic interaction.
3.2. Effects of CMBCs and dosage on the adsorption of tetracycline With the increase of CMBC dosage, the removal efficiency of TC gradually increased, and the removal rate decreased with the increase of dosage (Fig. 1). At lower CMBC dosage, the adsorption site on the saturated biochar limited the increase in the removal rate, resulting in a larger adsorption capacity. In the case of a higher CMBC dosage, the biochar accumulation limited the exposure of the surface sites, leading to slower increase in the removal rate and lower adsorption capacity (Zeng et al., 2018). With the increase of pyrolysis temperature, the removal rate of TC by CMBC increased. The removal rate of TC by CMBC700 was significantly higher than that by CMBC300 and CMBC500 (p < 0.05), possibly due to the richer pores structure of CMBC (Zhu et al., 2018). The characteristic peak of aromatic C]C shifted from 1594 cm−1 to 1603 cm−1 (CMBC300), and the peaks at 1598 cm−1 (CMBC500) and 1600 cm−1 (CMBC700) were significantly enhanced. Moreover, due to the graphitized surface of CMBC and the benzene ring in TC (XRD results confirmed that CMBC has a graphitized structure, acting as a πdonor during π-π electron donor-receptor interaction), TC and CMBC combined more easily, indicating the existence of π-π EDA interaction between them, which is considered to be one of the major mechanism of TC adsorption to biochar. The eOH peaks in CMBCs significantly shifted, and their intensity changed (3383 cm−1 to 3396 cm−1, 3400 cm−1 to 3398 cm−1, 3432 cm−1 to 3399 cm−1). Only CMBC300 showed a change in the content of the eCOOH functional group, indicating that the oxygen-containing functional groups on the surface of the biochar can be used as a receptor for binding phenolic hydroxyl groups on the surface of TC, while the carbonyl group binds to it via Hbonds. In summary, the main mechanism of TC adsorption by CMBC
3.4. Adsorption kinetics of CMBCs Dynamics trends are characterized by three stages: rapid adsorption, slow adsorption, and balance (Tang et al., 2018). In the rapid adsorption process, about 90% of the adsorption capacity occurred within the first 8 h (Fig. 3), possibly due to the high concentration difference at the interface between the biochar and the solution, which promoted a large mass transfer driving force, leading to the rapid occupation of the adsorption sites on the biochar surface by TC (Zhou et al., 2017). No significant change was observed in Qt from 24 h to 48 h, indicating that the adsorption equilibrium was reached by 24 h. The interaction between the hydroxyl group in the TC and the oxygen-containing functional groups on the surface of CMBCs is thought to occur via hydrogen bonds and π-π electron donor-acceptor interactions, accounting for the rapid adsorption of TC (Zhou et al., 2017). The pseudo-first order model is based on the assumption that the adsorption is controlled by the diffusion step. The pseudo-second order model is based on the assumption that the adsorption rate is determined by the square of the number of adsorption sites that are not occupied by the adsorbent surface and that the adsorption process is controlled by the chemisorption mechanism. The elovich model is suitable for processes with large changes in activation energy during the reaction. The adsorption kinetic parameters are shown in Table 2. The experimental data fitted well with the pseudo-second order kinetic model and the elovich model (CMBC500 and CMBC700). The former indicated that the adsorption rate of TC is mainly determined by chemisorption, including electron 6
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sharing or electron transfer between the adsorbent and the adsorbate, which is consistent with the results of other studies (Zhu et al., 2014). The latter indicated that the surface of the biochars is heterogeneous in energy and that chemical adsorption occurs on the surface. To further understand the mechanism of adsorption kinetics, the particle internal diffusion and liquid film diffusion models were used to study the adsorption process. The curve of Qt on t1/2 is multi-linear (three linear parts), indicating that the adsorption process has multiple stages, and the order of the rate constants of the three adsorbents was Ki1 > Ki2 > Ki3, indicating that the internal diffusion of the particles is related to adsorption (Fig. 3d and Table 2). That is, the adsorption process of TC to CMBCs includes three stages: external diffusion, adsorption, and balance (Tang et al., 2018). In general, the first stage is related to the external conduction resistance, while the second and third stages are related to the internal diffusion model of the particle. In the present study, the first stage had a higher slope (Ki1) due to external diffusion, indicating that the TC molecule migrates from the solution to the outer surface of the biochar. The second stage indicated that TC molecules enter the adsorption process and exhibit a relatively high adsorption capacity. At this stage, the TC molecules diffuse from the outer surface of the biochar to the adsorption sites and are adsorbed to the active sites of the biochar. The third stage was the adsorption equilibrium, which may have occurred due to the reduction of the free adsorption sites of the biochar and an increase in electrostatic repulsion between the TC molecules adsorbed on the surface of the biochar and the TC molecules in the solution. Reportedly, if the curve of the intraparticle diffusion model is linear and passes through the origin, the rate control of the adsorption process is caused by pores diffusion; however, if the linear curve does not pass through the origin, this indicates that intra-particle diffusion is not the only process determining the adsorption rate of TC, but that there are also other processes involved, such as initial external mass transfer or chemical reactions (Zhu et al., 2014). Fig. 3e and Table 2 show the linear fitting results and parameters of liquid film diffusion. Since the R2 value (0.841 and 0.816) of CMBC500 and CMBC700 was much larger than that of CMBC300 (0.290), it could be concluded that the former biochars had a step of diffusing the liquid film on the surface of the adsorbent, while the latter biochar may have had little or no such diffusion. In summary, the process of combining TC molecules with CMBC500 and CMBC700 is as follows: the liquid film is first diffused; it then enters the inner surface through the pores of the biochar; and, finally, the whole process of adsorption is controlled by intra-particle diffusion.
on heterogeneous surfaces and is not limited to a single layer; 0.1 < 1/ n < 0.5 indicates that TC is easily adsorbed to CMBCs. With the increase of pyrolysis temperature, KF (0.416–2.065) gradually increased, resulting in stronger adsorption capacity. The Temkin isotherm model is used to reflect chemisorption based on indirect interactions between adsorbed molecules. The experimental data can also be well-fitted to the Temkin model, indicating that biochar has strong intermolecular forces in the process of adsorbing TC. Therefore, it was shown that biochar adsorption was controlled by a variety of mechanisms. Compared with the adsorbents reported in the literatures, the CMBCs exhibited better adsorption capacity. The maximum adsorption capacity of various adsorbents for TC under different conditions indicated that CMBC700 can be used as an effective adsorbent for removing TC from aquaculture wastewater. 3.6. Thermodynamic study of CMBCs The ΔG value was calculated by lnK, and the ΔG values of CMBCs were all negative, which is consistent with the previous research results, indicating that the adsorption of TC by biochar was thermodynamically spontaneous (Table 4) (Tang et al., 2018; Zhu et al., 2014). The absolute value of ΔG increased as the temperature of the adsorption conditions increased, indicating that the adsorption process was thermodynamically favorable at higher temperatures. The ΔH values were 38.865 kJ·mol−1 (CMBC700) and 33.342 kJ·mol−1 (CMBC500), indicating that the adsorption process was an endothermic process and that adsorption is more favorable at higher temperatures, while the ΔH of CMBC300 was −7.020 kJ·mol−1, indicating that the adsorption process was an exothermic reaction. Since the ΔH value increased with the pyrolysis temperature, the more energy the TC molecule obtained, the more it interacted with the biochar surface active sites, which may have resulted in the relatively high adsorption capacity of CMBC700. The positive value of ΔS indicates that the reversibility of adsorption is poor. The ΔS value at 18.102–179.383 J·mol−1·K−1 indicated that the randomness of the biochar/TC interface increased with temperature during the adsorption process, rather than the ordered process. 4. Conclusion This study showed that CMBCs represent a cost-effective adsorbent for TC in aquaculture wastewater and that environmental factors and physicochemical properties affect TC adsorption. The pseudo-second order, elovich, and the Freundlich models best explained the kinetics and isotherms, suggesting that CMBC surface has a heterogeneous structure and that adsorption of TC is mainly chemical. The main control of adsorption rate and the limiting step are determined by a combination of internal particle diffusion and liquid film diffusion. Pores filling, π-π EDA interactions, hydrogen bonding, and electrostatic interactions between CMBC and TC represent the most possible mechanisms of adsorption.
3.5. Adsorption isotherms of CMBCs The adsorption isotherm refers to the relationship between the equilibrium concentration (Ce) of the adsorbate in the aqueous solution and the equilibrium adsorption capacity (Qe) when the adsorption reaches equilibrium under a certain temperature condition. The law of variation of the adsorption isotherm curve is helpful for analyzing the interaction between adsorbate and adsorbent and the structural characteristics of the adsorption layer (Jang et al., 2018). With the increase of the initial concentration, the adsorption capacity of biochar gradually increased (Fig. 3f). This phenomenon may be due to the increase in the driving force of the concentration difference, and the increase in TC molecules provides more capture opportunities for biochar. The order of fitting results of the CMBC isothermal adsorption experimental data is Freundlich > Langmuir > Temkin (Table 3), indicating that TC adsorption to biochar is a heterogeneous process. For the Langmuir model, the assumption is based on the uniform distribution of adsorbed molecules on the surface of the adsorbent (monolayer adsorption). Qm and KL increased with the increase of pyrolysis temperature of biochar, indicating that CMBC500 and CMBC700 have better adsorption effects. The linear correlation coefficient of the Freundlich model was 0.944 ≤ R2 ≤ 0.979, based on which it is speculated that the adsorption of TC to the surface of CMBC occurs
Acknowledgments This study was funded by the National Natural Science Foundation of China (No. 31271611) and Innovation Team Project of Education Ministry (No. IRT_17R105). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.121348. References Acosta, R., Fierro, V., Martinez de Yuso, A., Nabarlatz, D., Celzard, A., 2016. Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char.
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Characteristics of tetracycline adsorption by cow manure biochar prepared at different pyrolysis temperatures Peizhen Zhang, Yanfei Li, Yaoyao Cao, Lujia Han
T
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Laboratory of Biomass and Bioprocessing Engineering, College of Engineering, China Agricultural University, Box 191, Beijing 100083, China
ARTICLE INFO
ABSTRACT
Keywords: Biochar Cow manure Pyrolysis temperature Tetracycline Adsorption
This study aimed to explore the feasibility of using cow manure biochar (CMBC) for adsorption of tetracycline for realizing farm waste treatment and recycling. Three kinds of pyrolysis-temperature CMBCs were prepared and characterized. There were significant differences in the specific surface area, pores structure, surface charge, and oxygen-containing functional groups. The effect of adsorption was not only related to the physicochemical properties of CMBC but also the dosage, solution pH, and ambient temperature. CMBC showed surface heterogeneity, and the adsorption of tetracycline was mainly chemical. Controlling the rate of adsorption was achieved by combining internal particle diffusion and liquid film diffusion. Furthermore, the adsorption was a spontaneous and endothermic process. The use of CMBC as an adsorbent for tetracycline represents a new method for treating and recycling waste in farms. These results could aid in further studies on the adsorption mechanism and optimizing the adsorption process.
1. Introduction With the rapid development of large-scale aquaculture, antibiotic pollution has become a problem requiring urgent solution (Lupo et al., 2012; Norvill et al., 2017). Antibiotics are widely used in modern aquaculture because they not only inhibit the growth of bacteria in animals (Zhao et al., 2010), but also promote feed utilization and animal growth rate (Phillips et al., 2004; Sarmah et al., 2006). In practice, a large number of antibiotics are not fully absorbed and utilized by animals, thus entering the soil and water as the excrement is discharged (Li et al., 2012), and ultimately entering the human body through the food chain (Pouretedal & Sadegh, 2014). There are many methods of treating antibiotics in wastewater, such as membrane separation (Aydin et al., 2016), oxidation (Souza et al., 2018), photochemical degradation (Le-Minh et al., 2010), electrochemical treatment (Liang et al., 2018), biological treatment (Chen et al., 2017), and adsorption (Zhou et al., 2017). Adsorption has become the primary method in practical applications because it is highly efficient, easy to perform, economical, and practical (Acosta et al., 2016). The adsorption of more than 30 types of antibiotics involving a number of adsorbents (Ahmed et al., 2015), including activated carbon (Acosta et al., 2016), bentonite (Ravi and Pandey, 2019), titanium dioxide nanoparticles (Qi et al., 2018), steel chips (Tran et al., 2017), sawdust biochar (Zhou et al., 2017), wheat straw biochar (Li et al.,
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2018), and sludge biochar (Rivera-Utrilla et al., 2013) has been studied. Preparation of biochar as an adsorbent with low-cost biomass has recently become a research hotpot because of biochar’s strong ability to remove organic pollutants in aqueous solutions (Güzel & Sayğılı, 2016; Zhu et al., 2014). Biochar is a carbon-based material obtained by pyrolyzing biomass feedstock under high temperature and anaerobic conditions (Zhang et al., 2015). The physicochemical properties of biochars prepared at different pyrolysis temperatures, such as oxygen-containing functional groups (carboxyl, hydroxyl, etc.), aromatization structure, specific surface area and porosity, and degree of graphitization, often significantly affect the adsorption and fixation of contaminants (Ahmad et al., 2014). In addition, the adsorption process is not only related to the characteristics of the biochar, but also to the types of adsorbed pollutants, the solution concentration, pH and ambient temperature (Zeng et al., 2018). Presently, research on adsorption of antibiotics by biochars mainly focuses on the adsorption effect of lignocellulosic biochar (Cao et al., 2019; Pouretedal & Sadegh, 2014; Zhang et al., 2015), while the preparation of biochar from livestock manure as a raw material can effectively utilize a large amount of livestock waste and solve the problems associated with the treatment of cultured solid waste. On the other hand, the pyrolysis process can also decompose antibiotics carried in livestock manure (Cervini et al., 2016). The use of livestock manure
Corresponding author. E-mail address: [email protected] (L. Han).
https://doi.org/10.1016/j.biortech.2019.121348 Received 6 March 2019; Received in revised form 10 April 2019; Accepted 13 April 2019 Available online 15 April 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Physicochemical properties of CMBCs. Cow manure biochar Elemental compositions
Atom ratio Ash% BET surface area (m2/g) Total volume of pores (cm3/g) Micropores volume (cm3/g) Average of Pore width (nm) pH
CMBC300 C% H% N% S% O% H/C O/C (O + N)/C
CMBC500 a
47.25 ± 0.40 4.23 ± 0.04a 3.26 ± 0.04a 0.94 ± 0.16a 11.23 ± 0.40a 1.07 0.18 0.24 33.08 ± 0.19c 1.55 0.002848 / 7.34 8.62 ± 0.03b
CMBC700 b
43.08 ± 1.59 1.60 ± 0.05b 2.15 ± 0.06b 0.56 ± 0.02b 7.17 ± 1.39b 0.45 0.13 0.17 45.44 ± 0.55b 1.77 0.002612 0.000567 5.89 10.75 ± 0.03a
42.56 ± 0.26b 0.72 ± 0.06c 1.79 ± 0.03c 0.57 ± 0.12b 2.73 ± 0.03c 0.20 0.05 0.08 51.63 ± 0.29a 31.23 0.023430 0.004945 3.00 10.83 ± 0.08a
Data are expressed as mean values ± standard deviations. a–c Indicates that the average of the different superscripts in the same row is significantly different at p < 0.05.
biochars were cooled to room temperature, further pulverized to pass through a 0.12 mm sieve, and stored in closed plastic bottles. Biochars obtained using the different pyrolysis temperatures were labeled as CMBC300, CMBC500, and CMBC700, respectively. 2.2. Batch adsorption experiment Tetracycline hydrochloride (TC; C22H25CIN2O8; purity, 90%) is an amphoteric compound with multiple ionizable functional groups. Based on the solubility of TC in water (e.g., 201 mg/L (at 15.15 °C), 311 mg/L (at 20.15 °C), and 430 mg/L (at 30.15 °C)), and the range of the concentration of antibiotic residues in actual aquaculture wastewater (ng – μg/L), 50 mg/L of TC was used for all adsorption experiments except for the adsorption isotherms (10–80 mg/L with the gap of 10 mg/L) (Caço et al., 2008; Jang et al., 2018; Zhu et al., 2014). A blank control sample (without biochar) was prepared for each batch of experiments to subtract the loss of TC during the adsorption process. All experiments were performed in duplicate.
Fig. 1. The effect of CMBCs and dosages on the adsorption of tetracycline. The adsorption conditions were 25 °C, pH 6, tetracycline concentration of 50 mg/L, and reaction time of 24 h.
2.2.1. Different CMBCs dosages for adsorption experiments Different dosages (0.25 g/L, 0.5 g/L, 0.75 g/L, 1 g/L, 1.25 g/L, 1.5 g/L, 1.75 g/L, 2 g/L) of biochar were added to 50 mL plastic centrifuge tubes, to which 40 mL TC solution (pH 6, 50 mg/L) was added. The mixtures were shaken at 150 rpm to an adsorption equilibrium at 25 ± 0.5 °C in the dark. The supernatants were filtered through a 0.22 μm membrane to obtain samples to be tested. The concentration of TC remaining in the solutions was determined using HPLC.
biochar as an adsorbent for the removal of antibiotics from aquaculture wastewater presents an effective way to achieve recycling of aquaculture waste. As tetracycline antibiotics are one of the most abundant antibiotics in livestock farming wastewater (Selvam et al., 2017), in this study, tetracycline hydrochloride was selected as the target adsorbate, and the cow manure biochars obtained at different pyrolysis temperatures were used as the adsorbent. Based on the analysis of the physicochemical properties of cow manure biochars, the characteristics of tetracycline hydrochloride adsorption and its effects on cow manure biochars were studied, providing a foundation for further exploration of adsorption mechanisms and optimization of the adsorption process.
2.2.2. Different pH solutions for adsorption experiments The Britton-Robinson buffer solution can be used to prepare a buffer with a wide pH range (pH 1.8–11.9). The Britton-Robinson buffer was prepared by mixing phosphoric acid (H3PO4; purity, ≥85%), boric acid (H3BO3; purity, ≥99.5%), and acetic acid (CH3COOH; purity, ≥99.5%) at a concentration of 0.04 mol/L, and its pH was adjusted by adding a solution of 0.2 mol/L sodium hydroxide (NaOH; purity, ≥96%). The initial pH of the solution was adjusted to 3–10 by adding BR buffer solution. The experiments were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (50 mg/L) and 50 mg biochar (1.25 g/L, w: v). Other adsorption conditions are similar to those in subsection 2.2.1.
2. Materials and methods 2.1. Preparation of biochars from cow manure Raw cow manure samples were collected from a dairy farm in Beijing, China. The collected samples were dried in a 70 °C oven for 48 h and pulverized for subsequent carbonization. A quartz boat containing a sample of smashed cow manure was placed in a tube furnace and passed through N2 to create an oxygenlimited or oxygen-free environment for pyrolysis carbonization. The initial temperature was set to 25 °C, the heating rate was 20 °C/min, and the pyrolysis carbonization was completed after heating at the target temperatures (300 °C, 500 °C, and 700 °C) for 1 h. After the obtained
2.2.3. Adsorption kinetics The adsorption kinetics were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (50 mg/L, pH 6) and 50 mg biochar (1.25 g/L, w: v) and shaken at 150 rpm for 48 h in the dark at 25 ± 0.5 °C. The supernatant was extracted at 1 min, 2 min, 3 min, 2
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Fig. 2. (a) The relationship between zeta potential and pH of CMBCs. (b) Species distribution of tetracycline at different pH values. (c) Effect of initial pH of solution on adsorption of tetracycline by CMBCs. The adsorption conditions were 25 °C, tetracycline concentration of 50 mg/L, biochar dosage of 1.25 g/L, and reaction time of 24 h.
and micropore volume (MPV) of biochar were analyzed by N2 adsorption at 77 K by a multifunctional gas analyzer (TRISTAR II 3020 M, USA). The adsorption data were used to calculate the SSA according to Brunauer-Emmett-Teller theory. The desorption isotherm data were used to calculate the PV based on the Barrett-Joyner-Halenda theory. The surface functional group information of the biochar before and after adsorption was obtained by Fourier-transform infrared spectroscopy (FTIR, Spectrum 400, PerkinElmer, Waltham, MA, USA) and scanned using a KBr tablet method at a resolution of 4 cm−1 in the wavelength range of 4000–400 cm−1. The amount of surface charge of the biochar was measured by a zeta potential analyzer (SZ-100Z, Horiba, Kyoto, Japan) at 25 °C. The crystal composition and degree of graphitization of the biochar were analyzed by X-ray diffraction (XRD; Bruker D8 advance, Bruker AXS, Billerica, MA, USA) using a Cu target (40 kV, 30 mA) of 5°–60° at 2θ with scanning in 0.02° increments. The pH was measured by mixing 1 g biochar with 20 mL deionized water and shaking in 25 ℃ water bath for 24 h using an FE20 pH meter (Mettler Toledo, USA).
4 min, 5 min, 10 min, 0.5 h, 1 h, 2 h, 5 h, 8 h, 12 h, 24 h, 36 h and 48 h. Other adsorption conditions are similar to those in subsection 2.2.1. 2.2.4. Adsorption isotherms The adsorption isotherms were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (10–80 mg/L, pH 6) and 50 mg biochar (1.25 g/L, w: v). Other adsorption conditions are similar to those in subsection 2.2.1. 2.2.5. Adsorption thermodynamic The adsorption thermodynamic were performed in 50 mL plastic centrifuge tube by mixing 40 mL tetracycline solution (50 mg/L, pH 6) and 50 mg biochar (1.25 g/L, w: v) and shaken at a constant temperature (15 °C, 25 °C, and 35 °C) at 150 rpm to an adsorption equilibrium. Other adsorption conditions are similar to those in subsection 2.2.1. 2.3. Characterization of CMBCs The content percentages of carbon, hydrogen, nitrogen, and sulfur in biochar samples were determined using an elemental analyzer (Vario Macro Elementar, Elementar, Langenselbold, Germany). According to the ASTM standard E870-82, the percentage of oxygen by mass difference subtraction was calculated as 100% - C% - H% - N% - S% - Ash%. The ash content was analyzed using a fully automated analyzer (YXGYFX7705, U-Therm, Hunan, China). The biochar was placed in a crucible and heated at 525 °C for 6 h. The mass ratio of the obtained mass to the original biochar was expressed as the ash content. The surface morphology of the biochar was analyzed by scanning electron microscopy (SEM; SU3500, Hitachi, Tokyo, Japan). The specific surface area (SSA), pore volume (PV), pore width (PW),
2.4. Tetracycline analysis by HPLC and UV/Vis detector The TC concentration was measured and calculated by HPLC (Waters, Milford, MA, USA) using a C18 reverse phase column (5 μm, 4.6 mm × 250 mm) and a UV/Vis detector (2489, Waters, USA). The TC concentration in the solution was measured at a wavelength of 355 nm. For the liquid-phase, the column temperature was 30 °C, the mobile phase consisted of acetonitrile (C2H3N, HPLC grade, 20%) and 0.01 M oxalic acid (C2H2O4, AR, 80%), the flow rate was 1 mL/min, the injection volume was 20 μL, and the retention time was 3.7 min. The detected solute was quantified using an external standard method, and 3
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Fig. 3. Adsorption kinetics. (a) Pseudo-first order. (b) Pseudo-second order. (c) Elovich. (d) Intra-particle diffusion. (e) Liquid film diffusion. Reaction time 48 h. (f) Adsorption isotherms. The adsorption conditions were 25 °C, tetracycline solution concentration of 50 mg/L, pH 6, biochar dosage of 1.25 g/L, and reaction time of 24 h.
the curve-fitting coefficient was greater than 0.99. The relationship between TC concentration and peak area was as follows: 5 mg/L – 1711491; 10 mg/L – 351471; 25 mg/L – 867310; 50 mg/L – 1742354.
initial and adsorption equilibrium, respectively, and V (L) and M (g) represent the volume of the TC solution and the mass biochar, respectively. In this study, five adsorption kinetics models, namely pseudo-first order, pseudo-second order, elovich, intra-particle diffusion and liquid film diffusion, and three commonly used adsorption isotherms, namely Langmuir, Freundlich, and Temkin, were used to analyze the experimental data. The characteristics of the adsorption process were also explored from an energy perspective, providing useful data regarding the energy changes during the adsorption process. Adsorption thermodynamics were analyzed using adsorption equilibrium data at different temperatures. The standard Gibbs free energy change (ΔG), standard
2.5. Statistical analysis of the adsorption experiment data The adsorption capacity Q (mg/g) and removal rate R (%) of biochar were calculated according to the following equations:
Q = [(C0
Ce )/M ] × V
(1)
R = [(C0
Ce )/C0] × 100%
(2)
where C0 (mg/L) and Ce (mg/L) represent the TC concentration at the 4
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Table 2 Parameters and correlation coefficients of tetracycline adsorption kinetic model of CMBCs. Models and Parameters
CMBC300
CMBC500
CMBC700
Pseudo-first order Qe K1 R2
5.258 18.895 0.929
7.173 19.036 0.795
11.735 10.972 0.746
Pseudo-second order Qe K2 R2
5.485 6.646 0.932
7.438 3.544 0.881
11.793 1.281 0.818
Elovich a b R2
2.863 4.000 0.398
3847.732 1.400 0.992
5475.563 1.002 0.917
Intra-particle diffusion Ki Ki1 Ki2 Ki3 Ci R2
0.043 1.068 0.009 0.026 3.961 0.644
0.099 0.954 0.106 0.007 4.537 0.892
0.133 0.863 0.229 −0.057 5.785 0.931
Liquid film diffusion Kfd R2
0.283 × 10−2 0.290
0.131 × 10−2 0.841
0.877 × 10−5 0.816
Table 4 Thermodynamic parameters of tetracycline adsorption on CMBCs.
lnK = S / R
(3) (4)
( H / RT )
where R (8.314 J·mol−1·K) is the ideal gas constant, T (K) is the absolute temperature, and K is the adsorption partition coefficient; and (5)
K c = as / ae = vs Qe / ve Ce
T (K)
lnK
ΔG (KJ/mol)
ΔH (KJ/mol)
ΔS (J/mol·K)
CMBC300
288 298 308
5.096 5.039 4.904
−12.201 −12.484 −12.558
−7.020
179.383
CMBC500
288 298 308
5.329 5.566 6.238
−12.760 −13.790 −15.975
33.342
159.454
CMBC700
288 298 308
5.461 5.649 6.523
−13.077 −13.995 −16.704
38.865
18.102
the pyrolysis carbonization temperature increased (p < 0.05), while the content of C decreased with the increase of pyrolysis temperature. When the pyrolysis temperature increased from 300 °C to 500 °C, the C content decreased significantly (p < 0.05), but there was no significant change when the temperature was increased to 700 °C, which was consistent with previously obtained results (Wang et al., 2018). H/C, O/ C and (O + N)/C ratio decreased as the pyrolysis temperature increased, indicating that the aromaticity of the biochar gradually increases, the hydrophilicity decreases (i.e., hydrophobicity is enhanced), and the polarity decreases. The pH of CMBC700 and CMBC500 was significantly higher than that of CMBC300, while the first two did not differ significantly. CMBC700 had a larger specific surface area and pores structure distribution than did CMBC300 and CMBC500. The specific surface area of CMBC700 was 31.23 m2/g, which was about 20.14 and 17.64 times that of CMBC300 and CMBC500, respectively, while there was no significant difference between CMBC300 and CMBC500. The total pore volume of CMBC700 was 0.023430 cm3/g, which was about 8.23 and 8.97 times that of CMBC300 and CMBC500, respectively, and there was no significant difference between the latter two. The micropore volume of CMBC700 (0.004945 m3/g) was 8.72 times that of CMBC500 (0.000567 m3/g), while no micropores were detected in CMBC300. The proportion of micropores in CMBC700 and CMBC500 was 21.10% and 21.71%, respectively. It was found that the proportion of micropores in the total pores distribution was low. The average pore width of CMBC700 was 3.00 nm, which was about 1/3 and 1/2 times that of CMBC300 and CMBC500, respectively. It can be seen that as the pyrolysis temperature increased, the specific surface area of the CMBC increased and the pores distribution became better. When the pore size of the adsorbent is 1.7–3 times higher than the molecular size of the adsorbate, the adsorbent may exhibit the best adsorption performance (if the adsorbent needs to be recovered, the pore size of the adsorbent may even be 3–6 times higher than the molecular size of the adsorbate). The molecule TC has a length of 1.41 nm, width of 0.46 nm, and height of 0.81 nm. Therefore, CMBC has an effective pore size distribution for adsorbing TC molecules. According to FTIR analysis, the characteristic peaks and peak widths of CMBC at different pyrolysis temperatures slightly differed. Namely, the peak at 3385 cm−1 corresponded to the eOH in phenol, and the peaks at 2926 cm−1 and 2857 cm−1 corresponded to the eCH2 and eCH3 in aliphatic hydrocarbons. The intensities of the two peaks of CMBC700 were lower those of the peaks of CMBC300 and CMBC500
enthalpy change (ΔH), and standard entropy change (ΔS) were calculated separately using the following formulae:
G = - RTlnK
Adsorbent
where Kc (L/g) is the ratio of the adsorption amount of the adsorbate on the adsorbent to the residual amount in the solution during adsorption equilibrium, and Qe/Ce represents the adsorption coefficient. When lnK is plotted against 1/T, a straight line with a slope of −ΔH/R and an intercept of ΔS/R is obtained. The data in the tables and figures were expressed as the replicated mean values ± standard deviation. The analysis of variance (ANOVA) based on the Duncan’s multiple comparison test was obtained using SPSS 20 software. 3. Results and discussion 3.1. Physical and chemical properties of CMBCs There was a significant difference in the physical and chemical properties of CMBCs obtained at different pyrolysis temperatures (p < 0.05; Table 1). CMBC700 had the highest ash content (51.63%), which was significantly higher than that of CMBC500 (45.44%) and CMBC300 (33.08%) as well as that of straw biochars (Zhang et al., 2012). The content of O and H in the CMBC significantly decreased as Table 3 Adsorption isotherms parameters for tetracycline on CMBCs. Adsorbent
CMBC300 CMBC500 CMBC700
T (℃)
25℃
Langmuir model
Freundlich model
Temkin model
Qm
KL
R2
KF
1/n
R2
KT
B
R2
26.727 15.061 22.553
0.008 0.052 0.035
0.970 0.865 0.885
0.416 2.123 2.065
0.751 0.420 0.496
0.979 0.947 0.944
0.194 1.193 0.778
3.396 2.560 3.711
0.928 0.885 0.876
5
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(Wang et al., 2018), indicating that the content of oxygen-containing functional groups and aliphatic functional groups decreased with the increase of pyrolysis temperature (Keiluweit et al., 2010). The peak at 1594 cm−1, representing the stretching vibration peak of aromatic C]C, indicated that a few relatively stable aromatic compounds were gradually formed. This demonstrated that aliphatic, amide, and aromatic amines in cow manure will decrease with increasing pyrolysis temperature, resulting in a relatively stable aromatic structure. The characteristic peak at 1429 cm−1 represented saturated eCH3 in-plane bending vibration (Artz et al., 2008). The characteristic peak at 1068 cm−1 was attributed to the C-O bond stretching of ethers, esters and carbonates (Dai et al., 2016). The peaks at 975 cm−1 and 795 cm−1 represented the aromatic eCH out-of-plane bending, indicating the presence of adjacent aromatic hydrogens (Ahmed et al., 2016). The above results were consistent with the results of the elemental analysis. With the increase of pyrolysis temperature, the aromaticity increased and the proportion of polar and surface oxygen-containing functional groups (eCOOH, eOH, etc.) decreased. The XRD patterns of different CMBCs were obtained. A 2θ of around 25°, indicates the presence of an ordered graphite structure. However, according to the peak intensity, there was no significant difference in the degree of graphitization among the three biochars. The surface of CMBC300 was relatively smooth and the sheet structures were more obvious, at the same time, a small number of narrow and long pores appeared. The surface of CMBC500 was rough and the degree of cracking increased markedly. CMBC700 had the largest surface rupture and a significant increase in micropores and pore structures. In summary, as the pyrolysis carbonization temperature increased, the surface of the resulting CMBC roughened, and the number of internal pores structure increased. This is because volatile components are gradually released from cow manure with the increase of temperature, resulting in improved pores structure. A larger specific surface area, larger pore volume, and smaller average pore diameter formed, contributing to improved adsorption performance (Ahmad et al., 2012).
might be pores filling, π-π EDA interaction, and H-bonding. 3.3. Effect of solution pH value on the adsorption of tetracycline As the solution pH value increased from 3 to 10, the removal rate of TC by CMBC700 and CMBC500 decreased, while that of CMBC300 increased and then decreased (Fig. 2c), indicating that adsorption of TC is dependent on pH value. Further, the surface of the biochar was negatively charged, and the charge increased with increasing pH (Fig. 2a). TC exhibited different species distributions at different pH values and the dissociation constants (pKa) of TC are 3.3, 7.7, and 9.7 (Fig. 2b). When the pH of the solution was lower than 3.3 (pKa1), the main species of TC was protonated TC (TC+), while the surface of the biochar was negatively charged; thus, the electrostatic interaction between TC and biochar was mainly electrostatic attraction and the adsorption effect was optimal. When the pH of the solution was higher than 3.3 (pKa1), the main species of TC was TC0, and the negative charge of biochar further increased. Therefore, when the pH was between 3.3 and 7.7, the electrostatic attraction became weak and the adsorption efficiency decreased. When the pH was in the range of 7.7 (pKa2) – 9.7 (pKa3), TC became TC−, and when the pH was higher than 9.7 (pKa3), TC became TC2−. The amount of negative charge on the surface of biochar gradually increased, and the electrostatic repulsion between TC and biochar became stronger, further reducing the adsorption efficiency. In summary, CMBC700 and CMBC500 have the highest removal rate of TC at low pH, and the adsorption of both was greatly affected by pH. CMBC300 did not show a consistent trend of pH and charge, indicating that other adsorption mechanisms, such as surface complexation and cation bridging, were dominant. Some studies have shown that the ash removal treatment affected the specific surface area and porosity of biochar, as well as changed the ash composition and content, and accordingly influenced the adsorption effect (Li et al., 2017). The ash content of the CMBCs differed and was the key factor that affected their pH and zeta potential. Therefore, the difference in adsorption effect at different pH values may have been caused by ash. Subsequent studies should specifically analyze the effect of ash contents on the adsorption mechanism. In summary, the change in pH affected the surface charge of biochar and the species distribution of TC, influencing the adsorption of TC through electrostatic interaction.
3.2. Effects of CMBCs and dosage on the adsorption of tetracycline With the increase of CMBC dosage, the removal efficiency of TC gradually increased, and the removal rate decreased with the increase of dosage (Fig. 1). At lower CMBC dosage, the adsorption site on the saturated biochar limited the increase in the removal rate, resulting in a larger adsorption capacity. In the case of a higher CMBC dosage, the biochar accumulation limited the exposure of the surface sites, leading to slower increase in the removal rate and lower adsorption capacity (Zeng et al., 2018). With the increase of pyrolysis temperature, the removal rate of TC by CMBC increased. The removal rate of TC by CMBC700 was significantly higher than that by CMBC300 and CMBC500 (p < 0.05), possibly due to the richer pores structure of CMBC (Zhu et al., 2018). The characteristic peak of aromatic C]C shifted from 1594 cm−1 to 1603 cm−1 (CMBC300), and the peaks at 1598 cm−1 (CMBC500) and 1600 cm−1 (CMBC700) were significantly enhanced. Moreover, due to the graphitized surface of CMBC and the benzene ring in TC (XRD results confirmed that CMBC has a graphitized structure, acting as a πdonor during π-π electron donor-receptor interaction), TC and CMBC combined more easily, indicating the existence of π-π EDA interaction between them, which is considered to be one of the major mechanism of TC adsorption to biochar. The eOH peaks in CMBCs significantly shifted, and their intensity changed (3383 cm−1 to 3396 cm−1, 3400 cm−1 to 3398 cm−1, 3432 cm−1 to 3399 cm−1). Only CMBC300 showed a change in the content of the eCOOH functional group, indicating that the oxygen-containing functional groups on the surface of the biochar can be used as a receptor for binding phenolic hydroxyl groups on the surface of TC, while the carbonyl group binds to it via Hbonds. In summary, the main mechanism of TC adsorption by CMBC
3.4. Adsorption kinetics of CMBCs Dynamics trends are characterized by three stages: rapid adsorption, slow adsorption, and balance (Tang et al., 2018). In the rapid adsorption process, about 90% of the adsorption capacity occurred within the first 8 h (Fig. 3), possibly due to the high concentration difference at the interface between the biochar and the solution, which promoted a large mass transfer driving force, leading to the rapid occupation of the adsorption sites on the biochar surface by TC (Zhou et al., 2017). No significant change was observed in Qt from 24 h to 48 h, indicating that the adsorption equilibrium was reached by 24 h. The interaction between the hydroxyl group in the TC and the oxygen-containing functional groups on the surface of CMBCs is thought to occur via hydrogen bonds and π-π electron donor-acceptor interactions, accounting for the rapid adsorption of TC (Zhou et al., 2017). The pseudo-first order model is based on the assumption that the adsorption is controlled by the diffusion step. The pseudo-second order model is based on the assumption that the adsorption rate is determined by the square of the number of adsorption sites that are not occupied by the adsorbent surface and that the adsorption process is controlled by the chemisorption mechanism. The elovich model is suitable for processes with large changes in activation energy during the reaction. The adsorption kinetic parameters are shown in Table 2. The experimental data fitted well with the pseudo-second order kinetic model and the elovich model (CMBC500 and CMBC700). The former indicated that the adsorption rate of TC is mainly determined by chemisorption, including electron 6
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sharing or electron transfer between the adsorbent and the adsorbate, which is consistent with the results of other studies (Zhu et al., 2014). The latter indicated that the surface of the biochars is heterogeneous in energy and that chemical adsorption occurs on the surface. To further understand the mechanism of adsorption kinetics, the particle internal diffusion and liquid film diffusion models were used to study the adsorption process. The curve of Qt on t1/2 is multi-linear (three linear parts), indicating that the adsorption process has multiple stages, and the order of the rate constants of the three adsorbents was Ki1 > Ki2 > Ki3, indicating that the internal diffusion of the particles is related to adsorption (Fig. 3d and Table 2). That is, the adsorption process of TC to CMBCs includes three stages: external diffusion, adsorption, and balance (Tang et al., 2018). In general, the first stage is related to the external conduction resistance, while the second and third stages are related to the internal diffusion model of the particle. In the present study, the first stage had a higher slope (Ki1) due to external diffusion, indicating that the TC molecule migrates from the solution to the outer surface of the biochar. The second stage indicated that TC molecules enter the adsorption process and exhibit a relatively high adsorption capacity. At this stage, the TC molecules diffuse from the outer surface of the biochar to the adsorption sites and are adsorbed to the active sites of the biochar. The third stage was the adsorption equilibrium, which may have occurred due to the reduction of the free adsorption sites of the biochar and an increase in electrostatic repulsion between the TC molecules adsorbed on the surface of the biochar and the TC molecules in the solution. Reportedly, if the curve of the intraparticle diffusion model is linear and passes through the origin, the rate control of the adsorption process is caused by pores diffusion; however, if the linear curve does not pass through the origin, this indicates that intra-particle diffusion is not the only process determining the adsorption rate of TC, but that there are also other processes involved, such as initial external mass transfer or chemical reactions (Zhu et al., 2014). Fig. 3e and Table 2 show the linear fitting results and parameters of liquid film diffusion. Since the R2 value (0.841 and 0.816) of CMBC500 and CMBC700 was much larger than that of CMBC300 (0.290), it could be concluded that the former biochars had a step of diffusing the liquid film on the surface of the adsorbent, while the latter biochar may have had little or no such diffusion. In summary, the process of combining TC molecules with CMBC500 and CMBC700 is as follows: the liquid film is first diffused; it then enters the inner surface through the pores of the biochar; and, finally, the whole process of adsorption is controlled by intra-particle diffusion.
on heterogeneous surfaces and is not limited to a single layer; 0.1 < 1/ n < 0.5 indicates that TC is easily adsorbed to CMBCs. With the increase of pyrolysis temperature, KF (0.416–2.065) gradually increased, resulting in stronger adsorption capacity. The Temkin isotherm model is used to reflect chemisorption based on indirect interactions between adsorbed molecules. The experimental data can also be well-fitted to the Temkin model, indicating that biochar has strong intermolecular forces in the process of adsorbing TC. Therefore, it was shown that biochar adsorption was controlled by a variety of mechanisms. Compared with the adsorbents reported in the literatures, the CMBCs exhibited better adsorption capacity. The maximum adsorption capacity of various adsorbents for TC under different conditions indicated that CMBC700 can be used as an effective adsorbent for removing TC from aquaculture wastewater. 3.6. Thermodynamic study of CMBCs The ΔG value was calculated by lnK, and the ΔG values of CMBCs were all negative, which is consistent with the previous research results, indicating that the adsorption of TC by biochar was thermodynamically spontaneous (Table 4) (Tang et al., 2018; Zhu et al., 2014). The absolute value of ΔG increased as the temperature of the adsorption conditions increased, indicating that the adsorption process was thermodynamically favorable at higher temperatures. The ΔH values were 38.865 kJ·mol−1 (CMBC700) and 33.342 kJ·mol−1 (CMBC500), indicating that the adsorption process was an endothermic process and that adsorption is more favorable at higher temperatures, while the ΔH of CMBC300 was −7.020 kJ·mol−1, indicating that the adsorption process was an exothermic reaction. Since the ΔH value increased with the pyrolysis temperature, the more energy the TC molecule obtained, the more it interacted with the biochar surface active sites, which may have resulted in the relatively high adsorption capacity of CMBC700. The positive value of ΔS indicates that the reversibility of adsorption is poor. The ΔS value at 18.102–179.383 J·mol−1·K−1 indicated that the randomness of the biochar/TC interface increased with temperature during the adsorption process, rather than the ordered process. 4. Conclusion This study showed that CMBCs represent a cost-effective adsorbent for TC in aquaculture wastewater and that environmental factors and physicochemical properties affect TC adsorption. The pseudo-second order, elovich, and the Freundlich models best explained the kinetics and isotherms, suggesting that CMBC surface has a heterogeneous structure and that adsorption of TC is mainly chemical. The main control of adsorption rate and the limiting step are determined by a combination of internal particle diffusion and liquid film diffusion. Pores filling, π-π EDA interactions, hydrogen bonding, and electrostatic interactions between CMBC and TC represent the most possible mechanisms of adsorption.
3.5. Adsorption isotherms of CMBCs The adsorption isotherm refers to the relationship between the equilibrium concentration (Ce) of the adsorbate in the aqueous solution and the equilibrium adsorption capacity (Qe) when the adsorption reaches equilibrium under a certain temperature condition. The law of variation of the adsorption isotherm curve is helpful for analyzing the interaction between adsorbate and adsorbent and the structural characteristics of the adsorption layer (Jang et al., 2018). With the increase of the initial concentration, the adsorption capacity of biochar gradually increased (Fig. 3f). This phenomenon may be due to the increase in the driving force of the concentration difference, and the increase in TC molecules provides more capture opportunities for biochar. The order of fitting results of the CMBC isothermal adsorption experimental data is Freundlich > Langmuir > Temkin (Table 3), indicating that TC adsorption to biochar is a heterogeneous process. For the Langmuir model, the assumption is based on the uniform distribution of adsorbed molecules on the surface of the adsorbent (monolayer adsorption). Qm and KL increased with the increase of pyrolysis temperature of biochar, indicating that CMBC500 and CMBC700 have better adsorption effects. The linear correlation coefficient of the Freundlich model was 0.944 ≤ R2 ≤ 0.979, based on which it is speculated that the adsorption of TC to the surface of CMBC occurs
Acknowledgments This study was funded by the National Natural Science Foundation of China (No. 31271611) and Innovation Team Project of Education Ministry (No. IRT_17R105). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.121348. References Acosta, R., Fierro, V., Martinez de Yuso, A., Nabarlatz, D., Celzard, A., 2016. Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char.
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