The biochar was prepared by pyrolyzing the roots of cauliflowers, at a temperature of 500 °C under oxygen-limited conditions. The structure and characteristics of the biochar were examined using scanning electron microscopy, an energy dispersive spectrometer, a zeta potential analyzer, and Fourier transform infrared spectroscopy. The effects of the temperature, the initial pH, antibiotic concentration, and contact time on the adsorption of norfloxacin (NOR) and chlortetracycline (CTC) onto the biochar were investigated. The adsorption kinetics of NOR and CTC onto the biochar followed the pseudo-second-order kinetic and intra-particle diffusion models. The adsorption isotherm experimental data were well fitted to the Langmuir and Freundlich isotherm models. The maximum adsorption capacities of NOR and CTC were 31.15 and 81.30 mg/g, respectively. There was little difference between the effects of initial solution pH (4.0–10.0) on the adsorption of NOR or CTC onto the biochar because of the buffering effect. The biochar could remove NOR and CTC efficiently in aqueous solutions because of its large specific surface area, abundant surface functional groups, and particular porous structure. Therefore, it could be used as an excellent adsorbent material because of its low cost and high efficiency and the extensive availability of the raw materials.

Cauliflower (Brassica oleracea var. botrytis) is cultivated and consumed worldwide. According to the statistics of the Food and Agriculture Organization of the United Nations (UNFAO), the cultivation area of cauliflower has been growing continuously worldwide. The global cauliflower production had reached 20,884,672 tonnes in 2011. China has been ranked as the largest exporter of cauliflower in the world, accounting for 43.21% of the global total (Li et al. 2014). As a result, the production of cauliflowers yields a significant amount of cauliflower roots. A small portion of the roots can be used as firewood, while the majority are discarded without any processing. Step by step, a huge amount of vegetable waste has piled up around the country, a heavy burden on the local agro-environment. In addition, the decay of this waste causes the contamination of surface and ground water through leachate, soil contamination through direct waste contact or leachate, air pollution by malodorous gases, all of which give rise to complex environmental pollution problems (Fitzgibbon et al. 1995). Therefore, it is imperative to explore the utilization of cauliflower waste resources.

Antibiotics are widely used in human therapy and the livestock industry to prevent and control infectious diseases. They enter the environment through various pathways which lead to unexpected environmental contamination because they are poorly metabolised and continually applied (Ankley et al. 2007). The fate of these compounds in the water environment has raised great concerns. Considerable concentrations (μg/L) of fluoroquinolones (FQs) and tetracyclines (TCs) have been detected frequently in surface water and wastewater effluent (Batt et al. 2007; Senta et al. 2013), and even drinking water (Carmona et al. 2014). Norfloxacin (NOR) and chlortetracycline (CTC) have also been found in the same media (Kim et al. 2010; Senta et al. 2013). While the concentrations of FQs and TCs are high their input into and output from these water systems occur continuously, which leads to their accumulation in the water. Additionally, the reported higher concentrations for antibiotic residue in hospital and pharmaceutical manufacturing wastewater could up to 100–500 mg/L (Larsson et al. 2007; Jing et al. 2014). Therefore, it is essential to remove the antibiotics from hospital and pharmaceutical manufacturing wastewater as early as possible so as to prevent them from entering the environment.

As a more practical and environmentally friendly treatment method, the adsorption of NOR or CTC is often carried out by adsorbents such as activated carbon (Ocampo-Péreza et al. 2015), novel carbon nanotubes and graphene (Devaraj et al. 2013; Yu et al. 2016). These adsorbents suffer from complicated operation and high cost in manufacturing, which tend to be more expensive than other adsorbents in practical engineering applications. Thus, we are in sore need of developing novel low-cost adsorbents and exploring their adsorption mechanism for adsorbing NOR and CTC.

Biochar, as a type of organic material, is produced from the pyrolysis of large amounts of lignocellulosic biomass such as agricultural and forest residues (Yao et al. 2011). Previous experimental studies have assessed the capabilities for the removal of pollutants and the environmental benefits of biochar including carbon sequestration and biowaste valorization. Adsorption with biochar is an effective treatment for wastewater that has shown lower environmental impacts than other options (e.g., ozone/ultraviolet-light oxidation), because there would be no risk of highly toxic byproducts (Thompson et al. 2016). Therefore, biochar has been widely used to reduce toxicity and content of organic compounds, including pesticides, antibiotics, and other chemicals, partly because of its low cost, low risk of causing secondary pollution and its applicability in large-scale regions. To our knowledge, however, none of the previous studies have exploited the novel type of biochar prepared from cauliflower roots to carry out studies of the adsorption of NOR and CTC.

Biochar derived from cauliflower roots was developed in this study, which could be applied to remove NOR and CTC from aqueous solutions. The structure and characteristics, adsorption performance and properties of the biochar, as well as the factors which may influence the adsorption of NOR or CTC, were all examined. The thermodynamic parameters (Gibbs free energy, enthalpy change, and entropy change), and the interaction mechanism between the biochar and NOR or CTC were also evaluated. It may help to consider the potential of the novel adsorbent material for pollutants removal from polluted wastewater.

Chemicals

Chlortetracycline hydrochloride (CTC, C22H23ClN2O8, molar mass 478.88 g/mol, purity >97%) (Figure 1(a)) and norfloxacin (NOR, C16H18FN3O3, molar mass 319.34 g/mol, purity >98%) (Figure 1(b)) were purchased from the Sigma Chemical Co., Ltd, USA. All other chemicals used in these experiments were analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd, China. No further purification was carried out for these chemicals.

Figure 1

The molecular structure of chlortetracycline (a) and norfloxacin (b).

Figure 1

The molecular structure of chlortetracycline (a) and norfloxacin (b).

Close modal

The preparation of biochar

The biochar was prepared from the roots of cauliflowers from agricultural green waste. The feedstock was collected from Lintao County, Gansu Province, Northwest China. The details of the process were as follows: the cauliflower roots were pretreated by being washed with deionized water several times and oven-dried for 12 hours at 80 °C, then smashed and put through a 0.90 mm sieve. The biochar was produced by slow pyrolysis of the pretreated cauliflower roots under O2-limited conditions at 500 °C with the heating rate of 20 °C/min for 6 hours in a muffle furnace. After cooling to room temperature, the sample was rinsed and dried, crushed and passed through a 0.15 mm sieve before analysis, and then stored in desiccators before use.

Characterization

The physical properties of the biochar samples such as total pore volume, porosity, and specific surface area were measured by using the Brunauer–Emmett–Teller method (BET, Tristar II 3020, Micromeritics Instrument, USA). The particle size and surface morphology of the biochar were characterized by scanning electron microscopy (SEM, JSM-5600LV, JEOL, Japan). The biochar was mixed with KBr, pressed into pellets and scanned by Fourier transform infrared spectroscopy (FTIR, Nexus 670, Thermo-Nicolet, USA) to analyze the change in functional groups (FGs). FTIR spectra were recorded from 4,000 to 400 cm−1. The content of the element and type of material were analyzed by Energy Dispersive Spectrometer (EDS, IE250, Oxford Instruments, UK). The zeta potential of the biochar was measured by a Zeta Potential Analyzer (Nano ZEN3600, Malvern Zetasizer Nano, Malvern Instruments Ltd, UK).

Adsorption experiments

The adsorption kinetics of NOR or CTC by biochar were measured in batch-mode experiments at 298.15 K. The NOR or CTC solution contained 0.01 M CaCl2 as the background electrolyte. The CaCl2 solution was prepared using ultra-pure water from a Millipore system with a resistivity of 18.2 MΩ·cm. The initial pH values of NOR or CTC solutions in the adsorption experiments were adjusted to 6.5 with 0.01 M HCl or 0.01 M NaOH solutions. Every polyethylene tube containing 0.05 g of the biochar and 25 mL of NOR (10 mg/L) or CTC (10 mg/L) solution was sealed and shaken at 200 rpm. At time intervals of 5, 10, 20, 30, 60, 120, 240, 360, 480, 720, 1,080, 1,440, 2,160 and 2,880 minutes, the contents were centrifuged (4,500 rpm, 5 minutes) and filtered immediately. Afterwards, NOR and CTC were analyzed directly by a UV-vis spectrometer (Unicam UV300, ThermoSpectronic, USA) at a wavelength of 273 nm and 274 nm, respectively (Jing et al. 2014). Control and blank experiments were performed in duplicate. Adsorption isotherms under different temperatures (288.15 K, 298.15 K, 308.15 K) were carried out by shaking a fixed amount of the biochar (0.05 g) in polyethylene tube in which 10–300 mg/L NOR or CTC solution were added respectively by using the same procedures. The removal efficiency (R) and adsorption capacity (Qe, mg/g) on the biochar were calculated as shown in Equations (1) and (2):
formula
(1)
formula
(2)
where C0 (mg/L), Ct (mg/L), and Ce (mg/L) are the initial, final and equilibrium concentrations of NOR or CTC, V (L) is the volume of solution, and m (g) is the mass of the biochar.

As powerful tools in measuring the adsorption capacities and adsorption process, kinetic models can provide useful information about the reaction mechanism and pathways. To gain insight into the adsorption dynamics and processes of NOR and CTC onto the biochar, the kinetic models of pseudo-first-order, pseudo-second-order, and intra-particle diffusion were applied in the present study.

Adsorption isotherms play a major role in adsorption systems. They can provide information about the distribution of adsorbate between the liquid and solid phases at various equilibrium concentrations. Here, the Langmuir and Freundlich isotherm models were applied to assess the adsorption equilibrium characteristics of NOR or CTC onto the biochar. Kinetic and isotherm models were used to fit the experimental data using the equations listed in Table 1.

Table 1

Equations of kinetic and isotherm models

ModelsEquations
Pseudo-first-order  
Pseudo-second-order  
Intra-particle diffusion  
Langmuir  
Freundlich  
ModelsEquations
Pseudo-first-order  
Pseudo-second-order  
Intra-particle diffusion  
Langmuir  
Freundlich  

In these formulas, k1 and k2 are rate constants of pseudo-first- and pseudo-second-order adsorption; kbi (mg/(h0.5·g)) is the rate parameter of stage i; Ci is the intercept corresponding to the boundary layer of stage i; kf ((mg/g)·(L/mg)1/n) and n (unitless) are the Freundlich adsorption coefficient and the linear index; b (L/mg) and Qm (mg/g) are the adsorption constants for characterization of the surface strength and maximum adsorption capacity; Qt (mg/g) is the adsorption capacity at time t (min); RL is the dimensionless constant separation factor.

Adsorption thermodynamics

The effect of different temperature (288.15 K, 298.15 K, and 308.15 K) on adsorption of NOR or CTC on the biochar were examined to further understand the adsorption mechanism. The thermodynamic parameters, Gibbs free energy (ΔG, kJ/mol), enthalpy change (ΔH, kJ/mol), entropy change (ΔS, kJ/(mol·K)), and Kd values were calculated using the following Equations (3) and (4):
formula
(3)
formula
(4)
where Kd is calculated based on b (Langmuir isotherm constants) and involved in the unit conversion here, R is the universal gas constant (8.314 J/mol K), T (K) is temperature, and Kd is the distribution coefficient for the adsorption.

Characteristics of the biochar

The pH, zeta potential, yield, specific surface area, pore volume parameters, and elemental compositions of the biochar were estimated and summarized in Table 2. As shown, the biochar produced by pyrolyzing the roots of cauliflowers in the present study had a larger specific surface area (232.15 m2/g) than that produced from rice-husk (34.40 m2/g) (Liu et al. 2012), alkali biochar (117.80 m2/g) (Liu et al. 2012), and MeOH-char (65.97 m2/g) using KOH solution and methanol modification (Jing et al. 2014). This was likely due to the high lignocellulosic content of the roots of cauliflowers compared to the others. The large specific surface area of the biochar may provide a large number of active adsorption sites for NOR and CTC molecules. The SEM image of the biochar is shown in Figure 2. It can be seen that the biochar has a lamellar structure and a large porous surface with many pore channels which may be favorable to the adsorption of NOR and CTC molecules. The EDS measurement provided the elemental compositions of the biochar samples, which was dominated by C (84.40 atom%), O (12.89 atom%), Si (0.15 atom%), and N (0 atom%). The zeta potential of the biochar was −57.5 ± 0.8 mV, the surface charge of the biochar was negative, which could generate the electrostatic attraction force when NOR or CTC exist in cationic form.

Table 2

Characteristics of biochar prepared from cauliflower roots

CharacteristicsValues
pH 9.62 
Zeta potential (mV) −57.5 ± 0.8 
Yield (%) 32.33 
Specific surface area (m2/g) 232.15 
Pore characteristics 
 Total pore volume (cm3/g) 0.15 
 Micropore volume (m3/g) 0.07 
 Mesopore volume (cm3/g) 0.08 
Elemental composition (atom %) 
 Carbon 84.40 
 Oxygen 12.89 
 Nitrogen 0.00 
 Silicon 0.15 
CharacteristicsValues
pH 9.62 
Zeta potential (mV) −57.5 ± 0.8 
Yield (%) 32.33 
Specific surface area (m2/g) 232.15 
Pore characteristics 
 Total pore volume (cm3/g) 0.15 
 Micropore volume (m3/g) 0.07 
 Mesopore volume (cm3/g) 0.08 
Elemental composition (atom %) 
 Carbon 84.40 
 Oxygen 12.89 
 Nitrogen 0.00 
 Silicon 0.15 
Figure 2

SEM images of biochar samples: (a) ×2,000, (b) ×500, (c), (d); (c), (d) are the enlarged parts of (b) shown in the black squares.

Figure 2

SEM images of biochar samples: (a) ×2,000, (b) ×500, (c), (d); (c), (d) are the enlarged parts of (b) shown in the black squares.

Close modal

The estimated N2 adsorption–desorption isotherms are shown in Figure 3. As can be seen, the N2 isotherm of the biochar evolved to the type I and IV isotherm defined in the International Union of Pure and Applied Chemistry (IUPAC) classification. This suggests that the biochar developed in our study was of microporous and mesoporous structures. It could be also observed from Figure 3 that the isotherm displays a hysteresis curve of the biochar, indicating the presence of narrow cracks and small mesopore volume in the adsorbent materials (Jing et al. 2014), as can be also seen in Table 2.

Figure 3

Nitrogen adsorption–desorption isotherms of the biochar.

Figure 3

Nitrogen adsorption–desorption isotherms of the biochar.

Close modal

The FGs of absorbents contribute significantly to the adsorption process. Further insight into the differences between the adsorption of NOR and CTC on the biochar can be gained by illustrating the FTIR spectra of the biochar before and after the adsorption of NOR and CTC. As shown in Figure 4, the most significant peaks of the biochar were 572.1, 1,044.3, 1,420.1, 1,572.6 and 3,400.5 cm−1. The highest intensity peak at 3,400.5 cm−1 of the biochar sample was attributed to stretching vibrations of O−H groups (from carboxyl and phenol) (Pezoti et al. 2016), followed by 1,572.6 cm−1 which resulted from C═C (aromatic) stretching vibrations (Kyzas & Deliyanni 2015), 1,420.1 cm−1 was identified by highly conjugated C═O stretching in carboxyl groups, 1,044.3 cm−1 which was attributed to C−OH bending vibrations (from carboxylate and ether structures) and O−H stretching vibrations (phenol structures) (Torrellas et al. 2015), and 572.1 cm−1 because of the benzene derivatives or aromatic and polysaccharide contents in biochar (Özdemir et al. 2011). Other peaks of the biochar were 1,384.7 cm−1, 872.8 cm−1, and 472.9 cm−1 which were attributed to C−H deformation of aliphatic groups, Si−O−Si symmetrical stretching, and Si−O−Si flexural vibration, respectively. After NOR and CTC adsorption onto the biochar, the bands were enhanced from 1,572.6 cm−1 to 1,766.3 cm−1 and 1,772.0 cm−1, respectively. These changes may result from the NOR and CTC molecule adsorption onto the biochar surface (Koutroumanis et al. 2010). In addition, the band at 1,572.6 cm−1 shifted to 1,577.9 and 1,599.3 cm−1, which was mainly attributable to the interaction of π-π electron coupling between NOR and/or CTC molecules and aromatic rings in the biochar (Ji et al. 2009). Likewise, the band at 3,400.5 and 1,420.1 cm−1 also shifted, which was mainly attributed to the reaction of the O−H and C=O groups of the biochar with NOR and CTC molecules. On the surface of the prepared biochar, there were much more O-containing FGs which could facilitate the formation of the hydrogen bonding with NOR or CTC molecules. This may further increase the adsorption capacity of the biochar.

Figure 4

FTIR spectra of the biochar before and after adsorption of NOR and CTC.

Figure 4

FTIR spectra of the biochar before and after adsorption of NOR and CTC.

Close modal

Adsorption kinetics

Adsorption kinetics data of NOR and CTC on the biochar were fitted with the pseudo-first-order kinetic, pseudo-second-order kinetic, and intra-particle diffusion models, which were then applied to analyze the adsorption processes of two types of antibiotics. Figure 5 showed the adsorption processes of NOR and CTC by the biochar. The adsorption of NOR or CTC could be divided into two steps describing a rapid and a slow adsorption process (Ho & McKay 1999). During the first step, about 83.7% of NOR was adsorbed within 4 hours, then a slow adsorption process followed with a slow increase of the adsorbed amount in 24 hours. For CTC, the first step adsorption process was faster than NOR, at about 90.1% of CTC within 1 hour. When equilibrium was reached, the removal rate of NOR and CTC were about 92.3% and 93.2%, respectively.

Figure 5

Adsorption kinetics of NOR and CTC by the biochar.

Figure 5

Adsorption kinetics of NOR and CTC by the biochar.

Close modal

The correlation coefficient (R2), k1, k2, kbi and predicted Qe (theory), and measured Qe (exp) are shown in Table 3. As can be seen, the R2 values of the pseudo-first-order and intra-particle diffusion equations are low for both NOR and CTC. Also, there are large differences between the Qe (exp) and Qe (theory) for the two types of antibiotics, suggesting it was the poor fitting to the experimental data by using pseudo-first-order kinetic and intra-particle diffusion model. Nevertheless, the R2 of the pseudo-second-order model were high, and the values of Qe (exp) agreed better with the predicted Qe (theory). This suggested that the pseudo-second-order kinetic model could better describe the adsorption processes of NOR and CTC. The results indicated that the rate-limiting step of the adsorption of NOR and CTC onto the biochar may be a chemical reaction or a chemisorption process involving the hydrogen bonding, π-π electron coupling (between NOR or CTC molecules and aromatic rings of the biochar), ion exchange and electrostatic interactions (with biochar) to control the rate of adsorption (Ho & McKay 1999; Liu et al. 2012).

Table 3

Adsorption kinetic parameters

Models Pseudo-first-order
Pseudo-second-order
Intra-particle diffusion
ParametersQe (exp) (mg/g)Qe (theory) (mg/g)k1 (min)R2Qe (theory) (mg/g)k2 (g/(mg·min))R2kbi (mg/(h0.5·g))CiR2
NOR 4.13 1.02 0.0021 0.6337 4.630 0.0108 0.9998 0.0393 3.0056 0.7902 
CTC 4.66 0.17 0.0012 0.4507 4.651 0.0442 0.9999 0.0088 4.2942 0.4191 
Models Pseudo-first-order
Pseudo-second-order
Intra-particle diffusion
ParametersQe (exp) (mg/g)Qe (theory) (mg/g)k1 (min)R2Qe (theory) (mg/g)k2 (g/(mg·min))R2kbi (mg/(h0.5·g))CiR2
NOR 4.13 1.02 0.0021 0.6337 4.630 0.0108 0.9998 0.0393 3.0056 0.7902 
CTC 4.66 0.17 0.0012 0.4507 4.651 0.0442 0.9999 0.0088 4.2942 0.4191 

Based on the data of Table 3, the fit was poor for NOR and CTC using the intra-particle diffusion model. However, it could be observed that the adsorption process tended to be followed by multi-linearity, indicating that three or more steps occurred in the adsorption processes of NOR and CTC. Generally, the first stage represented for macropore diffusion; the second and third stages represented micropore diffusion (Liu et al. 2016). The parameters of the three stages of the intra-particle diffusion are shown in Table 4. However, these lines do not pass through the origin, suggesting that the intra-particle diffusion model was involved in the adsorption processes of NOR and CTC but it was not the only rate-controlling step (Liu et al. 2016). The kbi value of the three stages decreased, indicating that the rate of adsorption was initially fast and gradually slowed. The kb1 value of the first stage of the CTC was much greater than the NOR, which confirmed the data and results above (Table 2 and Figure 3). Ci is the intercept corresponding to the boundary layer of stage i, which is proportional to extent of the thickness of boundary layer, i.e. the larger the intercept (Ci), the greater the boundary layer effect. The Ci value increased gradually from the first stage to the third stage, which implied that the boundary layers became thicker during the adsorption process (McKay et al. 1980; Singh et al. 2012).

Table 4

Intra-particle diffusion parameters

AdsorbateIntra-particle diffusion
kb1 (mg/(h0.5·g))C1kb2 (mg/(h0.5·g))C2kb3 (mg/(h0.5·g))C3R32
NOR 0.1169 2.4943 0.9942 0.0669 2.7960 0.9649 0.0202 3.8620 0.9236 
CTC 0.1918 3.4723 0.9443 0.0056 4.4536 0.9349 0.0019 4.5703 0.9775 
AdsorbateIntra-particle diffusion
kb1 (mg/(h0.5·g))C1kb2 (mg/(h0.5·g))C2kb3 (mg/(h0.5·g))C3R32
NOR 0.1169 2.4943 0.9942 0.0669 2.7960 0.9649 0.0202 3.8620 0.9236 
CTC 0.1918 3.4723 0.9443 0.0056 4.4536 0.9349 0.0019 4.5703 0.9775 

Since the adsorption of NOR and CTC on the biochar tended to fit the pseudo-second-order kinetic and intra-particle diffusion models, physi- and chemi-adsorption might be the major adsorption mechanism involved in macropore and micropore diffusion, electrostatic interaction and ion exchange (Ho 2006). In this study, given that the structure of the biochar is dominated by micropores and mesopores (Figure 3), the NOR or CTC molecules were firstly adsorbed on the pore surface of the biochar via diffusion, forming a monolayer of NOR or CTC molecules. Secondly, as the monolayer approached saturation, the adsorption for NOR or CTC gradually increased with a rearrangement process of NOR or CTC molecules. The difference in the adsorption rates of NOR and CTC depended on the interaction between the abundant FGs of biochar and the different molecular structures of NOR or CTC, which was consistent with the results of the FTIR spectra.

Adsorption isotherm

The Langmuir and Freundlich adsorption isotherms were used to fit the experimental data for the adsorption of NOR and CTC onto the biochar at different temperatures (288.15 K, 298.15 K, and 308.15 K). The isothermal parameters are summarized in Table 5. The Langmuir and Freundlich isotherm, illustrated in Figure 6, showed the highest R2 values for both NOR and CTC.

Table 5

Adsorption isotherm parameters

AdsorbateTemperature (K)Langmuir isotherm
Qm (mg/g)b (L/mg)R2
NOR 288.15 26.18 0.0967 0.9961 
298.15 29.50 0.1020 0.9937 
308.15 31.15 0.1166 0.9942 
CTC 288.15 81.30 0.0455 0.9828 
298.15 76.92 0.0375 0.9861 
308.15 76.34 0.0211 0.9599 
Freundlich isotherm
kf ((mg/g)·
AdsorbateTemperature (K)(1/mg)1/n)nR2
NOR 288.15 5.0107 3.1008 0.9851 
298.15 6.1376 3.3278 0.9915 
308.15 6.8517 3.4247 0.9923 
CTC 288.15 6.3709 1.9414 0.9911 
298.15 5.1547 1.8587 0.9887 
308.15 3.3574 1.7176 0.9906 
AdsorbateTemperature (K)Langmuir isotherm
Qm (mg/g)b (L/mg)R2
NOR 288.15 26.18 0.0967 0.9961 
298.15 29.50 0.1020 0.9937 
308.15 31.15 0.1166 0.9942 
CTC 288.15 81.30 0.0455 0.9828 
298.15 76.92 0.0375 0.9861 
308.15 76.34 0.0211 0.9599 
Freundlich isotherm
kf ((mg/g)·
AdsorbateTemperature (K)(1/mg)1/n)nR2
NOR 288.15 5.0107 3.1008 0.9851 
298.15 6.1376 3.3278 0.9915 
308.15 6.8517 3.4247 0.9923 
CTC 288.15 6.3709 1.9414 0.9911 
298.15 5.1547 1.8587 0.9887 
308.15 3.3574 1.7176 0.9906 
Figure 6

(a), (b) Langmuir isotherm (temperatures 288.15, 298.15, 308.15 K); (c), (d) Freundlich isotherm (temperatures 288.15, 298.15, 308.15 K); (a), (c) NOR; (b), (d) CTC.

Figure 6

(a), (b) Langmuir isotherm (temperatures 288.15, 298.15, 308.15 K); (c), (d) Freundlich isotherm (temperatures 288.15, 298.15, 308.15 K); (a), (c) NOR; (b), (d) CTC.

Close modal

From Table 5, the maximum adsorption capacities of NOR and CTC were 31.15 and 81.30 mg/g, respectively. Clearly, the adsorption capacities for NOR increased with increasing temperature. By contrast, the adsorption capacities for CTC decreased with increasing temperature, suggesting that the adsorption is endothermic and exothermic for NOR and CTC, respectively. The adsorption behavior for NOR and CTC onto the biochar were further evaluated by RL. As described by Hall et al. (1966), RL values indicate the shape of isotherm process, i.e. whether it was favorable (0 < RL < 1), linear (RL = 1), irreversible (RL = 0) and unfavorable (RL > 1). The RL values calculated from the Langmuir isotherm model lie between 0 and 1, suggesting that adsorption for NOR and CTC onto the biochar were favorable.

Here, n > 1 indicated that the adsorption for both antibiotics onto the biochar were favorable. The NOR capacities increased and the CTC capacities decreased with increasing temperature. kf could be also considered as an indicator for the adsorption capacity. That is, greater kf suggested higher adsorption capacity, and vice versa (Xu & Li 2010). When temperature increased, kf for NOR increased from 5.0107 to 6.8517 (mg/g)·(L/mg)1/n in the adsorption system but decreased from 6.3709 to 3.3574 (mg/g)·(L/mg)1/n for CTC. These results indicated that the adsorptions are endothermic and exothermic for NOR and CTC, respectively. The effects of temperature on the adsorption of NOR and CTC onto the biochar were further revealed.

Overall, the Langmuir and Freundlich isotherms were more applicable in the adsorption of NOR and CTC. These results suggest that the dominant NOR and CTC adsorptions are characterized by physi- and chemi-adsorption processes. In the Langmuir isotherm, the maximum adsorption capacities were 31.15 and 81.30 mg/g, which were higher than other adsorbents (Taheran et al. 2016; Li et al. 2017) for the removal of NOR and CTC from aqueous solution. According to Table 6, the cauliflower biochar showed a remarkable adsorption capacity for NOR or CTC in this study that was considerably higher than those obtained in previous investigations with different biochar adsorbent materials and some of the modified biochars, such as potato stems biochar, rice husk biochar, pinewood biochar, acid/alkali-treated biochar (feedstock, rice husk) and clay-biochar composite (feedstock, potato stems and attapulgite), etc. Additionally, these adsorbents suffered from complicated operation and high cost in manufacturing compared with cauliflower biochar (Cernansky 2015; Ahmed et al. 2015), which would limit their practical application. These results revealed that the low-cost biochar has greater potential for removal of NOR and CTC from water contaminated by antibiotics, suggesting that the biochar could become an efficient adsorbent for wastewater treatment.

Table 6

Comparison of removal of NOR and CTC family with different adsorbent materials

AdsorbentAdsorbateAdsorption conditionsMaximum adsorption (mg/g)References
Potato stem biochar NOR 308 K, 24 hours, neutral pH 3.12 Li et al. (2017)  
Clay–biochar composite NOR 308 K, 24 hours, neutral pH 5.24 
Iron-doped activated alumina NOR 304 ± 1 K, 16 hours, pH 6.5 6.89 Liu et al. (2011)  
Cauliflower roots biochar NOR 308 K, 24 hours, pH 6.5 31.15 This study 
Pinewood biochar CTC 298 K, 24 hours, pH 5 2.01 Taheran et al. (2016)  
Rice husk biochar TC 303 K, 24 hours, neutral pH 16.95 Liu et al. (2012)  
Acid-treated biochar TC 303 K, 24 hours, neutral pH 23.26 
Alkali-treated biochar TC 303 K, 24 hours, neutral pH 58.8 
Clays, humic substances, and clay–humic complexes CTC 297 ± 2 K, 24 hours, pH 7 12.0a Pils & Laird (2007)  
Cauliflower root biochar CTC 308 K, 24 hours, pH 6.5 81.30 This study 
AdsorbentAdsorbateAdsorption conditionsMaximum adsorption (mg/g)References
Potato stem biochar NOR 308 K, 24 hours, neutral pH 3.12 Li et al. (2017)  
Clay–biochar composite NOR 308 K, 24 hours, neutral pH 5.24 
Iron-doped activated alumina NOR 304 ± 1 K, 16 hours, pH 6.5 6.89 Liu et al. (2011)  
Cauliflower roots biochar NOR 308 K, 24 hours, pH 6.5 31.15 This study 
Pinewood biochar CTC 298 K, 24 hours, pH 5 2.01 Taheran et al. (2016)  
Rice husk biochar TC 303 K, 24 hours, neutral pH 16.95 Liu et al. (2012)  
Acid-treated biochar TC 303 K, 24 hours, neutral pH 23.26 
Alkali-treated biochar TC 303 K, 24 hours, neutral pH 58.8 
Clays, humic substances, and clay–humic complexes CTC 297 ± 2 K, 24 hours, pH 7 12.0a Pils & Laird (2007)  
Cauliflower root biochar CTC 308 K, 24 hours, pH 6.5 81.30 This study 

aApproximate maximum adsorption capacities value from the literature.

Effect of pH

The influence of pH values ranging from 2.1 to 11.0 on the adsorption of NOR and CTC onto the biochar were investigated, using NOR or CTC initial concentrations of 10 and 30 mg/L, with a shaking time of 1,440 minutes at 298.15 K. These initial pH values of the NOR or CTC solution were adjusted to the required value by adding 0.10 M HCl or NaOH. The point of zero charge (pHpzc) of the biochar was determined using the solid addition method (Hameed 2009). Each tube containing 0.05 g of the biochar and 25 mL of NaCl solution (pHi, 2.0–11.0) was shaken and measured using a pH meter to obtain the final pH values (pHf). The ΔpH versus pHi curve was plotted, and the point of intersection with the axis of the abscissas was the pHpzc.

As shown in Figure 7(d), the pH of zero point charges (pHpzc) of the biochar sample is 8.87. In this sense, the biochar at different pH values presents the various states: the surface charge of the sorbent was positive at pH < pHpzc, while it was negative at pH > pHpzc (Güzel et al. 2017). NOR and CTC are amphoteric molecules, which contain multiple ionizable FGs. NOR has two values of pKa (6.22, 8.51), and CTC has three values of pKa (3.30, 7.44, and 9.27). Thus they can coexist in several states under different conditions of pH: cationic form NOR+ (pH < 6.22) or CTC+ (pH < 3.30), zwitterionic and neutral form NOR±/NOR0 (6.22 < pH < 8.51) or CTC±/CTC0 (3.30 < pH < 7.44), and anionic form NOR (pH > 8.51) or CTC (pH > 7.44) under the different pH conditions. Previous studies have shown that the final pH values (representing the equivalent pH values) played a more important role on the adsorption of biochar (Ahmed & Theydan 2014). The effects of initial solution pH on the adsorption of the biochar were studied in the pH range of 2.1–11.0 for different concentrations of 10 and 30 mg/L (Figure 7). In the initial pH range of 4.0–10.0, the equivalent pH values were almost identical (about 8.4–9.2). The results showed that there was little difference between the effects of initial solution pH on the adsorption of NOR or CTC onto the biochar because of the buffering effect. NOR and CTC were mainly negatively charged under the experimental conditions (initial pH range of 4.0–10.0), in which the differences of electrostatic attraction between the biochar and NOR or CTC could be less significant. Accordingly, if the biochar would be applied to the natural environmental conditions, the effect of pH could be ignored.

Figure 7

(a), (b) The final pH vs. the initial pH; (c) effect of initial pH on the adsorption of NOR and CTC; (d) pHpzc of the biochar.

Figure 7

(a), (b) The final pH vs. the initial pH; (c) effect of initial pH on the adsorption of NOR and CTC; (d) pHpzc of the biochar.

Close modal

Effect of temperature and adsorption thermodynamics

According to Equations (3) and (4), the ΔH and ΔS parameters for NOR and CTC were calculated from the slope and intercepts of the linear plots of lnKd versus 1/T, as shown in Figure 8. The thermodynamic parameters, Gibbs free energy (ΔG), enthalpy change (ΔH), and entropy change (ΔS), were presented in Table 7. It can be shown that the ΔG values range from −20.9103 to −24.8671 kJ/mol for NOR adsorption and from −21.7902 to −20.9636 kJ/mol for CTC adsorption on the biochar in temperatures ranging from 288.15 K to 308.15 K. The ΔG values were negative, consistent with the spontaneous adsorption process of NOR and CTC onto the biochar (Ahmad & Kumar 2010). For NOR, the enhancement of ΔG negative values with increasing temperature showed a rapid and more feasible adsorption in higher temperatures. By contrast, the smaller ΔG negative values of CTC with increasing temperature indicated a slow and more difficult adsorption in higher temperatures, further confirming the results of the Langmuir and Freundlich isotherm models.

Table 7

Thermodynamic adsorption parameters

AdsorbateT (K)ΔG (kJ/mol)ΔH (kJ/mol)ΔS (kJ/(mol·K))
NOR 288.15 −20.9103 36.1235 0.1980 
298.15 −22.9692 
308.15 −24.8671 
CTC 288.15 −21.7902 −33.6551 −0.0410 
298.15 −21.5083 
308.15 −20.9636 
AdsorbateT (K)ΔG (kJ/mol)ΔH (kJ/mol)ΔS (kJ/(mol·K))
NOR 288.15 −20.9103 36.1235 0.1980 
298.15 −22.9692 
308.15 −24.8671 
CTC 288.15 −21.7902 −33.6551 −0.0410 
298.15 −21.5083 
308.15 −20.9636 
Figure 8

Plots of In Kd versus 1/T.

Figure 8

Plots of In Kd versus 1/T.

Close modal

The ΔH values of NOR and CTC adsorption were 36.1235 and −33.6551 kJ/mol, respectively, showing the endothermic and exothermic natures for NOR and CTC adsorption, respectively. Similar results were reported by Pezoti et al. (2016) using NaOH-activated carbon adsorption of amoxicillin, and by Ersan et al. (2013) using tannin-based cryogels to remove TC. In general, an adsorption process could be identified as physi-adsorption if 8 < ΔH < 25 kJ/mol, as chemi-adsorption if 83 < ΔH < 830 kJ/mol, or as both physi- and chemi-adsorption if 25 < ΔH < 83 kJ/mol (Zhang et al. 2013). As a result, we could conclude that the dominant adsorption by the biochar were the physi- and chemi-adsorption processes for NOR and CTC. The result was consistent with the one analyzed by the adsorption kinetic model.

The ΔS values were 0.1980 for NOR and −0.0410 kJ/mol for CTC. The positive ΔS value showed for an increase in the randomness at the solid/liquid interface during the adsorption of NOR on the biochar. The negative ΔS value indicated a decrease in the degree of freedom of irregularities at adsorbate–solution interface during the adsorption process.

The biochar in the present study derived from the roots of cauliflowers presented a large specific surface area (232.15 m2/g), abundant surface FGs, and a particular porous structure. It was able to remove NOR and CTC efficiently due to its good adsorption properties, as demonstrated by the remarkable adsorption capacities compared with other biochar materials (31.15 and 81.30 mg/g) and removal rate (92.3% and 93.2%). The equilibrium experiments data of NOR and CTC onto the biochar were well described by Langmuir and Freundlich isotherm models. The working process of NOR adsorption was endothermic, while the process of CTC adsorption was exothermic. The adsorption kinetic data fitted nicely with the pseudo-second-order kinetic and intra-particle diffusion models. There was little effect of the initial pH of the solution on the adsorptions due to the buffering effect of the biochar. The thermodynamic parameters showed the physi- and chemi-adsorption favored, and spontaneous adsorption for NOR and CTC onto the biochar. Overall, the biochar developed from the roots of cauliflower exhibited good removal efficiency of NOR and CTC from aqueous solution, suggesting that the biochar may have great potential as an efficient adsorbent for contaminated water remediation.

This work was supported by the National Natural Science Foundation of China (21307050), the Fundamental Research Funds for the Central Universities at Lanzhou University (lzujbky-2016-162).

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