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Sorption of tetracycline on H3PO4 modified biochar derived from rice straw and swine manure
Bioresource Technology 267 (2018) 431–437
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Sorption of tetracycline on H3PO4 modified biochar derived from rice straw and swine manure ⁎
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T
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Tingwei Chena,1, Ling Luoa,1, , Shihuai Denga, , Guozhong Shib, , Shirong Zhanga, Yanzong Zhanga, Ouping Dengc, Lilin Wanga, Jing Zhanga, Luoyu Weib a
College of Environmental Sciences, Sichuan Agricultural University, Chengdu 611130, People’s Republic of China Biogas Institute of Ministry of Agriculture, Chengdu 610041, People’s Republic of China c College of Resources, Sichuan Agricultural University, Chengdu 611130, People’s Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Swine manure biochar Tetracycline Sorption Chemisorptions Electrostatic attraction
Currently, the information about the sorption of tetracycline (TC) on animal manure derived biochar was rare although plant residue derived biochar showed high sorption of TC). Therefore, this study explored the sorption of TC on swine manure derived biochar, and compared with rice straw derived biochar simultaneously. Also, H3PO4 was adopted to modify both types of biochar. The sorption kinetic and isotherm data showed H3PO4 modification enhanced the sorption of TC on both types of biochar (especially swine-manure-biochar), and indicated the chemisorptions including H-bonding and π-π electron donor acceptor interaction might be the primary mechanism. Moreover, the strengthened electrostatic attraction between TC and biochars might largely explain the enhanced sorption capacity of TC along with pH increasing from 5.0 to 9.0. At the same conditions, swine manure derived biochar demonstrated lower sorption capacity of TC than rice straw biochar, but still could be good material for the sorption of TC.
1. Introduction Recent years, antibiotics have gained significant attention due to the extensive usage of antibiotics and the resulting environmental pollution (Chen et al., 2017). Usually, antibiotics are extensively used in human and animal healthcare and are also used as an animal growth promoter in the farming industry (Bao et al., 2010). Antibiotics are not easily biodegraded, therefore, they are frequently detected in soils, sediments, aquatic environment, and so on (Jing et al., 2014). When used widely, antibiotics can cause the proliferation of antibiotic resistance genes (ARGs) in the environment, which might pose potential hazards to human health and ecology (Chen et al., 2017). Tetracyclines (TCs) are one of the most frequently used antibiotics for human healthcare and feed additives of animals. According to statistics, the consumption of TCs was ranked second in the world, but first in China (Bao et al., 2010). Approximate 70–90% TCs can’t be metabolized by body, and then released in the environment as unchanged form (Xu and Li, 2010). The concentrations of TCs in the environment reported in literatures were ranging from ng kg−1 to mg kg−1 (e.g., 500 mg kg−1 in hospital and pharmaceutical manufacturing wastewater) (Jing et al., 2014). TCs may be carcinogenic on chronic
exposure, hence, the removal of TCs should be paid significant attention (Peiris et al., 2017). Until now, several technologies have been tired to remove TCs from wastewater, such as ozone oxidation, anaerobic biodegradation, photocatalytic degradation, and so on (Chen and Zhang, 2013; Lucas et al., 2016; Sousa et al., 2017). These technologies can remove TCs effectively, but the cost is relatively expensive and the operation is often complex or unstable. Compared with these technologies, sorptive removal of TCs by biochar has received numerous research focus because biochar showed the great benefit of adsorption properties, cost effectiveness and easy operation (Peiris et al., 2017; Zhu et al., 2014). Biochar, biomass-derived carbonaceous material, gained enormous attention due to its role in sequestering carbon (Luo and Gu, 2016), improving soil fertility (Ding et al., 2016) and removing environmental pollutants such as heavy metals (Li et al., 2017a), hydrophobic organic contaminants (HOCs) (Wang et al., 2016), ionic and nonionic organic pollutants (Kim and Hyun, 2018), and so on. In general, biochar is produced from oxygen-limited pyrolysis of biomass, including plant residues, sewage sludge, and animal manures (Jang et al., 2018; Xu et al., 2014; Wang et al., 2016). Plant residues were the most widely used feedstock than animal manures and sewage sludge to produce
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Corresponding authors. E-mail addresses: [email protected] (L. Luo), [email protected] (S. Deng), [email protected] (G. Shi). 1 These authors contributed equally. https://doi.org/10.1016/j.biortech.2018.07.074 Received 28 May 2018; Received in revised form 10 July 2018; Accepted 14 July 2018 0960-8524/ © 2018 Elsevier Ltd. All rights reserved.
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at 105 °C (Luo et al., 2011; Peng et al., 2017). The H3PO4 modified biochar derived from rice straw and swine manure were recorded as RCA and SCA, respectively. For understanding the role of H3PO4 modification in enhancing sorption capacity of TC, rice straw derived biochar and swine manure biochar were only washed by distilled water with the similar procedures, and labeled as RC and SC.
biochar, especially rice straw (Luo et al., 2011; Wang et al., 2017). Also, there is an increasing interest in biochar derived from animal manures. For example, several studies reported swine manure derived biochar showed good potential to adsorb HOCs (Wang et al., 2016), heavy metals (Meng et al., 2014) and pesticides (Zhang et al., 2013). Relative to rice straw, swine manures were less used to produce biochar and the related research literatures were also quite limited. Currently, rice straw and swine manure are two of the most abundant agricultural wastes in China (Meng et al., 2018). Considering the annual production of rice straw (approximate 0.12 billion tonnes) and swine manure (approximate 3.8 billion tonnes), swine manure may be a promising potential feedstock for biochar production. It is reported that the animal manure derived biochar usually contains high ash content that might interact with pollutants (Zhang et al., 2013; Wang et al., 2016). Besides, animal manure derived bicohar may have higher pH and O/C ratio as well as lower surface area and carbon content than plant residue derived biochar (Zhao et al., 2016). Therefore, it is assumed that the sorption of TCs on animal manure derived biochar may be different with plant residue derived one. To date, there are several studies about the sorption of TCs on biochar, and most of them are related to plant residue derived biochar (Meng et al., 2013; Tsai et al., 2012; Wang et al., 2016). The studies about animal residue derived biochar are less, and the mechanism of TC on animal derived biochar is not clear. Moreover, the comparison of sorption properties between animal manure and plant residue derived biochar is less explored. For better utilization of animal manure derived biochar in adsorbing TCs, it is necessary to understand the mechanism of TCs on animal manure derived biochar, and compare the sorption properties of animal manure derived biochar to plant residue derived biochar. In this study, rice straw and swine manure were chosen as feedstocks of biochar due to their abundance in agricultural wastes and the difference of their physicochemical properties. In order to enhance sorption capacity of TC, H3PO4 was applied to modify biochars. The aim of this study was 1) to compare the characteristics of rice straw-derived biochar with swine manure-derived biochar; 2) to investigate the performance and mechanism of rice straw-derived and swine manure-derived biochars as sorbents for TC removal. Furthermore, the effect of pH on the sorption capacity of TC was explored to further understand the mechanism of TC on both biochars.
2.3. Characteristics of biochars The structure and morphology of biochars were characterized using a scanning electron microscopy equipped with an energy dispersive spectrometer (SEM -EDS, Zeiss Sigma300, Germany). Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer Frontier, America) of biochars was recorded with 4 cm−1 resolution between wavenumbers of 4000 cm−1 and 400 cm−1. The results of SEM-EDS and FTIR were presented in Supporting Information. The elemental compositions of biochars were determined by an element analyzer (YX-CHN5000, China). Moreover, Brunauer-Emmett-Teller (BET, ASAP2460, Micromeritics, America) was applied to determine the surface area, porosity and pore volume of biochars. The pH of zero point charges (pHPZC) of SC, SCA, RC and RCA were measured according to the methods reported in the previous research (Jang et al., 2018). The measurement of pHPZC was presented in Supporting Information. 2.4. Sorption experiments 2.4.1. Sorption kinetics Sorption kinetics experiments were performed to evaluate the equilibrium time for the subsequent sorption isotherm experiments.10 mg of RCA and SCA and 20 mg of RC and SC were added to 50 mL centrifuge glass tubes containing 30 mL TC solution with concentration of 120 mg L−1, respectively. All tubes were put in a table concentrator shaker and shaken at 200 rpm at 25 °C from 0.5 h to 216 h. Each treatment was conducted in triplicates, and also the tubes containing only 30 mL TC solution of the same concentration was used for observing the loss of TC during this procedure. At predetermined times, the tubes were taken out and centrifuged at 3000 rpm for 20 min and then filtered by millipore membranes (0.45 µm). Thereafter, the filtrate was determined by a V-5000 spectrophotometer at wavelength of 360 nm (Zhu et al., 2014).
2. Materials and methods
2.4.2. Batch sorption experiments Sorption isotherms of TC to modified biochars were studied. Sorption isotherms for each biochar were determined at different initial solution concentrations of 30, 40, 50, 60, 80, 100, 150, 200 mg L−1. Each concentration for each biochar had triplicate treatments. To each 50 mL centrifuge glass tube, 5–10 mg of biochar (the weighed amounts depending on the concentration of TC and also the type of biochars) and 30 mL TC solution with varying concentrations were combined and shaken at 200 rpm at 25 °C to reach apparent equilibrium based on the sorption kinetics of Section 2.4.1. Moreover, tubes adding only TC solution were used as blanks to test the loss of TC during the periods of experiments. Afterward, the following procedures such as centrifugation, filtration and measurement, were the same as Section 2.4.1.
2.1. Materials Tetracycline hydrochloride (TC) (purity ≥ 95%) was purchased from Sigma. Chemicals used in this study were all analytical grade. Rice straw was collected from Chongzhou experimental base of Sichuan Agricultural University, Chengdu, China, while swine manure was obtained from the experimental farm of Sichuan Agricultural University, Ya’an, China. Both rice straw and swine manure were air-dried and ground to pass through a 2 mm sieve before producing biochars. 2.2. Biochars As previously described (Zhang et al., 2013), the biomass was packed into 30 mL ceramic pot covered with lid to limit oxygen supply and then heated at 700 °C in a preheated muffle furnace for 2 h. The produced biochars were ground and passed through a 100 mesh sieve (0.15 mm). H3PO4 solution was applied to modify the biochars for enhancing the sorption capacity for adsorption of TC since H3PO4 is nonpolluting and easily washed away by water (Peng et al., 2017). Briefly, 20 g biochars were immersed in 40 mL 14% H3PO4 solution for 24 h at 25 °C. After that, the H3PO4 modified biochars were washed by distilled water until the pH of supernatants was stable. Subsequently, the supernatants were discarded and the biochars were oven-dried overnight
2.4.3. Effect of pH on sorption isotherms The pH values of TC solution with varying concentrations were adjusted to pH 5, pH 7 and pH 9 by using 0.1 mol L−1 HCl and NaOH solution. After that, sorption isotherms were conducted at 25 °C in the same way as described in Section 2.4.2. Equilibrium pH of solution was measured after sorption experiments. 2.5. Sorption data analysis The experimental data of sorption kinetics were examined by the pseudo-first-order (PFO) and pseudo-second-order (PSO) (Zhou et al., 432
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content and ash content in this study (data not shown). On the other hand, it was found that the ratios of O/C, (O + N)/C as well as H/C of SCA and RCA were decreased relative to SC and RC, respectively. This decrease indicated the biochars modified by H3PO4 became less hydrophilic with weaker polar groups (Zama et al., 2017). The FTIR spectra of four biochars presented in Supporting Information were similar, and the major bands occurred at the wavenumber of 3436 cm−1 (-OH), 1591 cm−1 (aromatic C=O, C=C), 1400 cm−1 (C-O), and 1050 cm−1 (SiO32−) (Kizito et al., 2015). It can be seen that there were no obvious changes between RC and RCA, as well as SC and SCA, indicating H3PO4 modification might have no significant improvement on functional groups of raw biochars on a qualitative level. However, according to the previously XPS results reported by Peng et al. (2017), H3PO4 modification could increase the number of -COOH and -OH groups in the surface of biochar. Therefore, quantitative analysis of functional groups should be paid more attention to clarify the difference between rice straw and swine manure derived biochar, as well as the variations between H3PO4 modified biochar and raw biochar in future studies. After H3PO4 modification, the porous structure of both SC and RC was more apparent revealed by SEM images presented in Supporting Information. The EDS results presented in Supporting Information demonstrated that Ca, F, Na, and Mg of raw biochars were lost after H3PO4 modification. Moreover, it can be known that Si was difficult to be removed by H3PO4 modification. Furthermore, the removal of Ca, F, Na, etc. might be partially attributed to the increase of surface area and pore volume in Table 1, especially SCA.
2017). The equations were presented as follows: Pseudo-first-order:
qt = qe (1−exp (−k1 t ))
(1)
Pseudo-second-order:
qt =
qe2 k2 t 1 + qe k2 t
(2)
To investigate the reaction behavior between biochars and TC, the data of sorption isotherms were fitted by three models as follows (Xu and Li, 2010; Zhou et al., 2017): Freundlich:
qe = Kf Cen
(3)
Langmuir:
qe =
qmax KL Ce (4)
1 + KL Ce 1−n n
−1
where Kf (mg L g ) is the Freundlich affinity coefficient indicating adsorption strength, and considered as a relative indicator of sorption capacity, n (unitless) is the Freundlich linearity index (Ji et al., 2011), qmax is the maximal sorption capacity, and KL (L mg−1) is a solutesurface interaction energy-related parameter (Luo et al., 2011). 3. Results and discussion 3.1. Characterization of biochars
3.2. Sorption kinetics
Table 1 demonstrates the surface area, pore volume, elemental compositions and aromatic ratio, and ash content of four biochars. In general, the surface area decreased as the order of SC (227.56 m2 g−1) < SCA (319.04 m2 g−1) < RC (369.26 m2 g−1) < RCA (372.21 m2 g−1). This result indicated rice straw-derived biochars (RC and RCA) had higher surface area than swine manure-derived biochars (SC and SCA) no matter if H3PO4 treatment was applied. Moreover, it can be seen that the H3PO4 modification could enhance the surface area of biochars produced from these two different origins, although the change of surface area between RC and RCA was slight. After H3PO4 modification, SCA presented higher total pore volume (Vt: 0.25 cm3 g−1), micropore volume (Vmic: 0.09 cm3 g−1) and mesopore volume (Vmes: 0.17 cm3 g−1) than SC (Vt: 0.14 cm3 g−1, Vmic: 0.07 cm3 g−1, and Vmes: 0.07 cm3 g−1), but there was no change found between RC and RCA (Vt: 0.23 cm3 g−1, Vmic: 0.09 cm3 g−1, and Vmes: 0.14 cm3 g−1). The elemental composition of four biochars in Table 1 suggested that H3PO4 modification generally enhanced the content of C, N and S, but reduced the content of O of raw biochars. The content of H showed no obvious change between SC and SCA, while H content was decreased in RCA relative to RC. Furthermore, the lower ash content of SCA (43.98%) and RCA (55.27%) than SC (60.73%) and RC (58.97%) implied that H3PO4 modification could partially remove ash of raw biochars (i.e., SC and RC). Actually, the reason for the increase of C content might be caused by the decrease of ash content by H3PO4 modification because a significantly negative relationship was found between C
Fig. 1 represents the sorption of TC on four biochars at different time at 25 °C. It can be seen that the adsorption process of TC by four biochars was slow, which was consistent with Liu et al (2012). After 168 h, there was no obvious difference between 192 h and 216 h, therefore, 192 h was considered to be a sufficient equilibrium time for TC adsorbed onto four biochars (Bao et al., 2010). This equilibrium time was longer than the equilibrium time of TC adsorbed onto sediments (less than 20 h) (Xu and Li, 2010) and soils (approximately 8 h) (Bao et al., 2010). Furthermore, it was found that the sorption performance of SCA and RCA were enhanced relative to SC and RC, respectively. In this study, PFO and PSO kinetic models were used to fit the adsorption data for understanding possible mechanism associated with the adsorption of TC on these biochars. Usually, the more optimal the model is, the higher the R2 value is (Bao et al., 2010). From Table 2, the R2 values of PSO model (0.901–0.911) were higher than those of PFO model (0.806–0.862), similar with the majority of TC sorption studies (Jing et al., 2014; Liu et al., 2012; Zhu et al., 2014). Moreover, the experimental qe values were closer to the theoretical qe values calculated by PSO model (Table 2), further suggesting that the sorption kinetics of TC on the biochars could be preferably described by PSO model. The PSO model demonstrates chemisorptions occurred between TC and these four biochars involving valency forces via sharing or
Table 1 Elemental compositions, surface area, pore volume of SCA, SC, RCA, and RC. Sample
SC SCA RC RCA
Surface Areaa (m2 g−1)
227.56 319.04 369.26 372.21
Vtb (cm3 g−1)
0.14 0.25 0.23 0.23
Vmicc (cm3 g−1)
0.07 0.09 0.09 0.09
Vmesd (cm3 g−1)
0.07 0.17 0.14 0.14
Elemental compositions (%)
Atomic ratio
Ash (%)
C
H
O
N
St
O/C
N/C
H/C
(O + N)/C
31.96 48.35 31.77 37.77
0.66 0.66 0.98 0.43
4.77 4.41 7.23 5.31
1.60 2.23 0.96 1.05
0.28 0.37 0.09 0.17
0.15 0.09 0.23 0.14
0.05 0.05 0.03 0.03
0.02 0.01 0.03 0.01
0.15 0.09 0.23 0.14
60.73 43.98 58.97 55.27
a Calculated by the BET method. bThe total pore volume, calculated at P/P0 = 0.99. cThe micropore volume, Calculate by Vt-Vmic. dThe Mesopore volume, Calculated by the DFT method.
433
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Fig. 1. Adsorption kinetics of TC with the concentration of 120 mg L−1 on SC, SCA, RC and RCA at 25 °C.
Fig. 2. Adsorption isotherms of TC on SCA and RCA at 25 °C. Table 3 Sorption isotherm parameters of TC on SCA and RCA.
Table 2 Adsorption kinetics parameters of TC onto SC, SCA, RC, and RCA. Sample
SC SCA RC RCA
Pseudo-first-order
Sample
Pseudo-second-order
qe (mg g−1)
k1 (h−1)
R2
qe (mg g−1)
k2 (g mg−1 h−1)
R2
109.5 141.5 132.7 153.7
0.03 0.03 0.06 0.07
0.862 0.806 0.812 0.833
127.8 159.7 150.2 166.3
0.0003 0.0003 0.0004 0.0005
0.903 0.901 0.908 0.911
Langmuir qmax (mg g
SCA RCA
160.3 167.5
Freundlich −1
)
KL (L mg 0.13 0.29
−1
)
R
Kf (mg1−nLn g−1)
n
R2
0.973 0.981
70.3 86.8
0.15 0.13
0.981 0.986
2
the qmax of TC by SCA (160.3 mg g−1) was slightly lower than RCA (167.5 mg g−1). From Freundlich model, Kf of SCA (70.3 mg1−nLn g−1) was also lower than that of RCA (86.8 mg1−nLn g−1), and the n value of SCA (0.15) was slightly higher than RCA (0.13). These results suggested that RCA had stronger sorption ability and higher sorption capacity than SCA, which might be mainly explained by the higher surface area and O/C ratio of RCA than SCA. In this study, both Freundlich and Langmuir model could fit the sorption data of TC on RCA and SCA well, indicating the sorption of TC on biochars might be affected by multiple mechanisms. Peiris et al. (2017) summarized the primary sorption mechanisms of TC on biochar surfaces, including cation exchange, π-π electron donor acceptor (EDA) interaction, H bonding, electrostatic interaction and surface complexation. H3PO4 modification removed the cations such as Ca2+, Mg2+ and Na+, indicating the cation exchange as well as the surface compexation (not common in biomass derived biochars) should not be the primary mechanisms for the sorption of TC on both types of biochar (Peiris et al., 2017). Meanwhile, the data of Fig. 1 and Table 2 revealed that the TC was adsorbed mainly by chemisorptions including H bonding and π-π EDA interaction. Compared the O/C content of RCA with SCA (Table 1), the higher O/C of RCA suggested that RCA might have more O-containing functional groups which could serve as H-bond acceptors, and thus the sorption of TC on RCA was higher than SCA (Wang et al., 2018). On the other hand, physical sorption might also partially contribute to the higher sorption capacity of RCA because surface area of RCA was higher than SCA (Table 1), although chemical sorption was dominated in this study. However, it is very difficult to quality the contribution of H bonding, π-π EDA interaction as well as physical sorption for adsorption of TC on both types of biochar (Yang and Xing, 2010).
exchange of electrons between TC and biochars (Peiris et al., 2017; Zhou et al., 2017). By comparison, the qe values of H3PO4 modified biochars (SCA and RCA) were higher than raw biochars (SC and RC). For example, the theoretical qe of TC on SCA was 159.7 mg g−1, 25% higher than that of SC (Table 2). It is obvious that the surface area of SCA increased 40% and the total pore volume increased 79% after SC modified by H3PO4 (Table 1), indicating surface adsorption and pore-filling might explain the higher qe of SCA than SC (Fig. 1 and Table 2). Besides, the H3PO4 modification could increase the amount of functional groups of biochar (Peng et al., 2017), therefore, the chemical interactions between TC molecules and functional groups of SCA might also partially contribute to the higher qe value of SCA relative to SC (Jing et al., 2014). However, the reason for enhanced qe of RCA relative to RC might be different with SCA to SC because there was no obvious change of surface area and pore volume between RCA and RC (Table 1). Thus, the possible reason for the higher qe of RCA than RC might be explained by the enhanced interactions between RCA and TC due to the increased amount of functional groups, such as -COOH, -OH, and so on (Peng et al., 2017). 3.3. Sorption isotherm Since the H3PO4 modified biochars showed higher sorption capacity than raw biochars (Table 2), therefore, SCA and RCA were employed to explore the difference of sorption ability between plant-derived and animal manure-derived bicohar. Sorption data of TC onto SCA and RCA at 25 °C are plotted in Fig. 2. The sorption data of SCA was quite close to these of RCA, and RCA showed slightly higher sorption capacity than SCA. The parameters of TC adsorbed to SCA and RCA fitted by Langmuir and Freundlich model are listed in Table 3. Both the Langmuir and Freundlich model described the sorption isotherms well because the R2 values of Freundlich (0.981–0.986) were similar with R2 of Langmuir model (0.973–0.981). According to the qmax fitted by Langmuir model,
3.4. Effect of pH Multiple factors can determine the sorption mechanisms of TC on biochars, including the properties of TC, physicalchemical properties of biochars as well as the conditions where the sorption is carried out 434
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Table 4 Sorption isotherm parameters of TC on SCA and RCA at different initial pH of solution. Sample
Langmuir qmax (mg g−1)
Freundlich KL (L mg−1)
R2
Kf (mg1−nLn g−1)
n
R2
Initial pH = 5 SCA 143.9 RCA 131.9
0.28 0.51
0.931 0.861
76.1 88.0
0.1 0.1
0.784 0.977
Initial pH = 7 SCA 174.0 RCA 266.8
0.23 0.06
0.902 0.980
69.7 49.3
0.2 0.3
0.968 0.998
Initial pH = 9 SCA 365.4 RCA 552.0
0.03 0.02
0.963 0.991
33.3 24.0
0.5 0.6
0.988 0.997
is not in line with the findings of other relevant studies (Jang et al., 2018; Peiris et al., 2017; Zhou et al., 2017). In these studies, a decline in sorption at elevated pH was widely reported and mainly explained by the π-π EDA interaction as well as electrostatic interaction (Jang et al., 2018; Peiris et al., 2017; Zhou et al., 2017). Generally, the properties of TC and biochar were both altered by pH of solution, and thus influencing the sorption capacity of TC on biochar. The pKa of TC and pHPZC of biochar were proposed as the main reasons for the changes of sorption capacity of TC on biochar under different pH. It is well known that TC is an amphoteric molecule with three values of pKa (3.3, 7.7 and 9.7) (Jang et al., 2018). At environmentally relevant pH values, TC can exist as a cation, zwitterion, as well as anion (Sassman and Lee, 2005). For example, at pH 2, TC is about 95% TCH3+; at pH 5, TC is more than 95% TCH20; at pH 7, TC is about 75% TCH20 and 25% TCH−; at pH 9, TC is about 70% TCH− and 30% TC2− (Gu and Karthikeyan, 2005; Sassman and Lee, 2005; Xu and Li, 2010). Besides, the surface charge of biochar was positive when the pH of solution was lower than their pHPZC, and vice versa. At low pH, TC was cationic and the surface of biochar is commonly positive, therefore, the π-π electron EDA interaction was weak. Along with pH increasing, TC became zwitterionic, and the EDA interaction between TC and biochar was strengthened. Thus, the enhanced sorption of TC was observed. In summary, the altered charges of both TC and biochar should reasonably explain the changes of sorption capacity of TC on biochars under different pH. In this study, although the initial pH of solution was set as 5.0, 7.0 and 9.0, the equilibrium pH values of TC adsorbed on SCA and RCA were 5.07, 5.94 and 7.12, and 5.67, 6.18, and 7.09, respectively. Taking the pHPZC of SCA (7.02) and RCA (7.08) (Fig. 3b) into consideration, the surface charges of SCA and RCA were positive when pH was lower than 7.0. Along with pH increasing from 5.0 to 7.0 (equal to the equilibrium pH of TC adsorbed on SCA and RCA), TCH20 decreased from 95% to 75%, and TCH− approximately increased from 0% to 25% (Sassman and Lee, 2005). Therefore, the enhanced sorption of TC on RCA and SCA at elevated pH could be largely explained by the strengthened electrostatic attraction between positive surface charge of both biochars and negative charge of TC.
Fig. 3. (a) Effect of pH onto the sorption isotherms of TC onto SCA and RCA (pHi means the initial pH of solution) 25 °C; (b) the pHPZC of SC, SCA, RC and RCA 25 °C.
(Peiris et al., 2017). In this study, the impact of pH on sorption of TC on both H3PO4 modified biochars was explored because TC forms and sorption mechanisms of TC on biochars were largely affected by pH (Jang et al., 2018; Zhou et al., 2017). Fig. 3 shows the effect of pH in the sorption of TC on on H3PO4 modified biochars (i.e., SCA and RCA) at 25 °C. Generally, the sorption data of TC on SCA were quite close to these of RCA at all tested pH values. A increase in sorption at elevated pH was observed on both RCA and SCA, and maximum sorption of TC on both biochars was found at pH 9 in this study. The sorption data of TC on SCA and RCA under different solution pH were fitted by Langmuir and Freundlich model (Table 4). Taking R2 values into consideration, both Freundlich model and Langmuir could fit all the sorption data (except sorption data of TC on SCA at pH 5) well, especially Freundlich model. In general, Kf of TC sorption on SCA and RCA decreased but n increased with increasing pH. Along with the initial pH of solution increasing from 5 to 9, the fitted Kf of SCA decreased from 76.08 to 33.29 mg1−nLn g−1 and RCA declined from 88.03 to 23.99 mg1−nLn g−1. Meanwhile, the n values increased from 0.13 to 0.48 for TC adsorbed on SCA and from 0.09 to 0.60 for TC adsorbed on RCA, respectively. On the other hand, the qmax of TC on RCA increased from 131.9 mg g−1 to 552 mg g−1 with pH increasing from 5 to 9. As for SCA, the qmax changed from 143.9 mg g−1 to 365.4 mg g−1. These results obviously suggested that the sorption capacities of TC on both biochars were enhanced by elevated pH of solution (from 5 to 9), which
3.5. Sorption properties of biochars from different origins In order to further understand the sorption ability of TC on SCA and RCA relative to other biochars, the qmax and Kf values were compared with biochars produced from different biomass including sawdust, rice husk, chicken feather, chicken bone, apricot nut shell, and so on (Table 5) (Li et al., 2017b; Liu et al., 2012; Marzbali et al., 2016; Oladipo et al., 2017; Wang et al., 2017; Zhou et al., 2017). It is evident that the sorption of TC was not only affected by the precursors and treatment, but also by the tested pH and temperature. At the same temperature, the qmax and Kf values of SCA and RCA 435
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Table 5 Comparison of TC Langmuir and Freundlich model parameters of biochar with different feedstocks and modification methods in the literature. Different biochar
Adsorption temperature and pH
Surface
Langmuir 2
Area (m g
Acid-treated rice straw biochar Acid-treated swine manure biochar Rice straw biochar Iron and zinc doped sawdust biochar Alkali-treated rice husk biochar Alkali-treated chicken feather biochar Acid-treated apricot nut shell biochar Magnetic chicken bone-based biochar Magnetic chicken bone-based biochar
25 °C, 25 °C, 25 °C, 25 °C, 30 °C, 30 °C, 30 °C,
pH = 9.0 pH = 9.0 Ambient pH pH = 6.0 Ambient pH Ambient pH pH = 6.5
50 °C, pH = 10.0
−1
)
KL (L mg
−1
Freundlich )
qmax (mg g−1)
Kf (mg
1−n n
L g
References −1
)
n
372.2 319.0 – – 117.8 1838.9 307.6
0.02 0.03 0.71 0.02 0.02 0.03 1.35
552.0 365.4 14.2 102.0 58.8 388.3 200.0
24.0 33.3 4.8 3.4 9.4 79.0 148.8
0.6 0.5 0.6 0.6 0.3 4.0 0.1
This work This work (Wang et al., 2017) (Zhou et al., 2017) (Liu et al., 2012) (Li et al., 2017a,b) (Marzbali et al., 2016)
–
11.30
63.3
5.3
1.6
(Oladipo et al., 2017)
were higher than that of rice straw biochar (Wang et al., 2017) and Fe/ Zn doped sawdust biochar (Zhou et al., 2017), especially at pH 9.0. However, compared with sorption conducted at 30 °C (ambient pH), the sorption of SCA and RCA at 25 °C (ambient pH) was lower than alkalitreated chicken feather biochar (Li et al., 2017b) but higher than alkalitreated rice husk biochar (Liu et al., 2012). The higher surface area of alkali-treated chicken feather biochar than SCA and RCA as well as the sorption temperature should be largely contributed to its higher sorption capacity. Although the temperature is an important factor for the sorption of TC, the origins of biochars as well as the preparation methods of biochars are also critical gradients for sorption behaviors. For example, sorption capacity and ability of TC on magnetic chicken bone-based biochar at 50 °C was significantly lower than sorption of TC on SCA and RCA at 25 °C (Oladipo et al., 2017). From Table 5, it can be concluded that H3PO4 modified biochars derived from rice straw and swine manure showed good sorption abilities and capacities for TC relative to other biochars. Therefore, SCA and RCA are assumed to be good sorption materials for removing TC from water.
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4. Conclusions H3PO4 modification enhanced the sorption of TC on both types of biochar, especially SCA (approximate 25% enhancement). The sorption of TC on H3PO4 modified biochars was significantly enhanced along with pH increasing from 5.0 to 9.0, especially RCA. The H-bonding and π-π EDA interaction might be the primary sorption mechanisms for the sorption of TC on RCA and SCA. Relative to other biochars, RCA and SCA demonstrated high sorption abilities and capacities of TC and might be good sorption materials for TC removal from water, although the sorption of TC on SCA was lower than RCA. Acknowledgement This work was supported by Education Department of Sichuan Province, China [grant number 18ZA0373]; and the Agricultural Science and Technology Innovation Program (ASTIP), Chinese Academy of Agricultural Sciences. Conflict of interests The authors declare that they have no conflict of interests. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.biortech.2018.07.074. References Bao, Y.Y., Zhou, Q.X., Ying, W., Qiang, Y., Xie, X.J., 2010. Effects of soil/solution ratios
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Sousa, J.M., Macedo, G., Pedrosa, M., Becerracastro, C., Castrosilva, S., Pereira, M.F., Silva, A.M., Nunes, O.C., Manaia, C.M., 2017. Ozonation and UV254nm radiation for the removal of microorganisms and antibiotic resistance genes from urban wastewater. J. Hazard. Mater. 323, 434–441. Tsai, W.T., Liu, S.C., Chen, H.R., Chang, Y.M., Tsai, Y.L., 2012. Textural and chemical properties of swine-manure-derived biochar pertinent to its potential use as a soil amendment. Chemosphere 89 (2), 198–203. Wang, Z., Han, L., Ke, S., Jie, J., Ro, K.S., Libra, J.A., Liu, X., Xing, B., 2016. Sorption of four hydrophobic organic contaminants by biochars derived from maize straw, wood dust and swine manure at different pyrolytic temperatures. Chemosphere 144, 285–291. Wang, H., Chu, Y., Fang, C., Huang, F., Song, Y., Xue, X., 2017. Sorption of tetracycline on biochar derived from rice straw under different temperatures. PLoS ONE 12 (8), e0182776. Wang, H., Fang, C., Wang, Q., Chu, Y., Song, Y., Chen, Y., Xue, X., 2018. Sorption of tetracycline on biochar derived from rice straw ans swine manure. RSC Adv. 8, 16260–16268. Xu, X., Cao, X., Zhao, L., Sun, T., 2014. Comparison of sewage sludge- and pig manurederived biochars for hydrogen sulfide removal. Chemosphere 111, 296–303. Xu, X.R., Li, X.Y., 2010. Sorption and desorption of antibiotic tetracycline on marine
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Sorption of tetracycline on H3PO4 modified biochar derived from rice straw and swine manure ⁎
⁎
T
⁎
Tingwei Chena,1, Ling Luoa,1, , Shihuai Denga, , Guozhong Shib, , Shirong Zhanga, Yanzong Zhanga, Ouping Dengc, Lilin Wanga, Jing Zhanga, Luoyu Weib a
College of Environmental Sciences, Sichuan Agricultural University, Chengdu 611130, People’s Republic of China Biogas Institute of Ministry of Agriculture, Chengdu 610041, People’s Republic of China c College of Resources, Sichuan Agricultural University, Chengdu 611130, People’s Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Swine manure biochar Tetracycline Sorption Chemisorptions Electrostatic attraction
Currently, the information about the sorption of tetracycline (TC) on animal manure derived biochar was rare although plant residue derived biochar showed high sorption of TC). Therefore, this study explored the sorption of TC on swine manure derived biochar, and compared with rice straw derived biochar simultaneously. Also, H3PO4 was adopted to modify both types of biochar. The sorption kinetic and isotherm data showed H3PO4 modification enhanced the sorption of TC on both types of biochar (especially swine-manure-biochar), and indicated the chemisorptions including H-bonding and π-π electron donor acceptor interaction might be the primary mechanism. Moreover, the strengthened electrostatic attraction between TC and biochars might largely explain the enhanced sorption capacity of TC along with pH increasing from 5.0 to 9.0. At the same conditions, swine manure derived biochar demonstrated lower sorption capacity of TC than rice straw biochar, but still could be good material for the sorption of TC.
1. Introduction Recent years, antibiotics have gained significant attention due to the extensive usage of antibiotics and the resulting environmental pollution (Chen et al., 2017). Usually, antibiotics are extensively used in human and animal healthcare and are also used as an animal growth promoter in the farming industry (Bao et al., 2010). Antibiotics are not easily biodegraded, therefore, they are frequently detected in soils, sediments, aquatic environment, and so on (Jing et al., 2014). When used widely, antibiotics can cause the proliferation of antibiotic resistance genes (ARGs) in the environment, which might pose potential hazards to human health and ecology (Chen et al., 2017). Tetracyclines (TCs) are one of the most frequently used antibiotics for human healthcare and feed additives of animals. According to statistics, the consumption of TCs was ranked second in the world, but first in China (Bao et al., 2010). Approximate 70–90% TCs can’t be metabolized by body, and then released in the environment as unchanged form (Xu and Li, 2010). The concentrations of TCs in the environment reported in literatures were ranging from ng kg−1 to mg kg−1 (e.g., 500 mg kg−1 in hospital and pharmaceutical manufacturing wastewater) (Jing et al., 2014). TCs may be carcinogenic on chronic
exposure, hence, the removal of TCs should be paid significant attention (Peiris et al., 2017). Until now, several technologies have been tired to remove TCs from wastewater, such as ozone oxidation, anaerobic biodegradation, photocatalytic degradation, and so on (Chen and Zhang, 2013; Lucas et al., 2016; Sousa et al., 2017). These technologies can remove TCs effectively, but the cost is relatively expensive and the operation is often complex or unstable. Compared with these technologies, sorptive removal of TCs by biochar has received numerous research focus because biochar showed the great benefit of adsorption properties, cost effectiveness and easy operation (Peiris et al., 2017; Zhu et al., 2014). Biochar, biomass-derived carbonaceous material, gained enormous attention due to its role in sequestering carbon (Luo and Gu, 2016), improving soil fertility (Ding et al., 2016) and removing environmental pollutants such as heavy metals (Li et al., 2017a), hydrophobic organic contaminants (HOCs) (Wang et al., 2016), ionic and nonionic organic pollutants (Kim and Hyun, 2018), and so on. In general, biochar is produced from oxygen-limited pyrolysis of biomass, including plant residues, sewage sludge, and animal manures (Jang et al., 2018; Xu et al., 2014; Wang et al., 2016). Plant residues were the most widely used feedstock than animal manures and sewage sludge to produce
⁎
Corresponding authors. E-mail addresses: [email protected] (L. Luo), [email protected] (S. Deng), [email protected] (G. Shi). 1 These authors contributed equally. https://doi.org/10.1016/j.biortech.2018.07.074 Received 28 May 2018; Received in revised form 10 July 2018; Accepted 14 July 2018 0960-8524/ © 2018 Elsevier Ltd. All rights reserved.
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at 105 °C (Luo et al., 2011; Peng et al., 2017). The H3PO4 modified biochar derived from rice straw and swine manure were recorded as RCA and SCA, respectively. For understanding the role of H3PO4 modification in enhancing sorption capacity of TC, rice straw derived biochar and swine manure biochar were only washed by distilled water with the similar procedures, and labeled as RC and SC.
biochar, especially rice straw (Luo et al., 2011; Wang et al., 2017). Also, there is an increasing interest in biochar derived from animal manures. For example, several studies reported swine manure derived biochar showed good potential to adsorb HOCs (Wang et al., 2016), heavy metals (Meng et al., 2014) and pesticides (Zhang et al., 2013). Relative to rice straw, swine manures were less used to produce biochar and the related research literatures were also quite limited. Currently, rice straw and swine manure are two of the most abundant agricultural wastes in China (Meng et al., 2018). Considering the annual production of rice straw (approximate 0.12 billion tonnes) and swine manure (approximate 3.8 billion tonnes), swine manure may be a promising potential feedstock for biochar production. It is reported that the animal manure derived biochar usually contains high ash content that might interact with pollutants (Zhang et al., 2013; Wang et al., 2016). Besides, animal manure derived bicohar may have higher pH and O/C ratio as well as lower surface area and carbon content than plant residue derived biochar (Zhao et al., 2016). Therefore, it is assumed that the sorption of TCs on animal manure derived biochar may be different with plant residue derived one. To date, there are several studies about the sorption of TCs on biochar, and most of them are related to plant residue derived biochar (Meng et al., 2013; Tsai et al., 2012; Wang et al., 2016). The studies about animal residue derived biochar are less, and the mechanism of TC on animal derived biochar is not clear. Moreover, the comparison of sorption properties between animal manure and plant residue derived biochar is less explored. For better utilization of animal manure derived biochar in adsorbing TCs, it is necessary to understand the mechanism of TCs on animal manure derived biochar, and compare the sorption properties of animal manure derived biochar to plant residue derived biochar. In this study, rice straw and swine manure were chosen as feedstocks of biochar due to their abundance in agricultural wastes and the difference of their physicochemical properties. In order to enhance sorption capacity of TC, H3PO4 was applied to modify biochars. The aim of this study was 1) to compare the characteristics of rice straw-derived biochar with swine manure-derived biochar; 2) to investigate the performance and mechanism of rice straw-derived and swine manure-derived biochars as sorbents for TC removal. Furthermore, the effect of pH on the sorption capacity of TC was explored to further understand the mechanism of TC on both biochars.
2.3. Characteristics of biochars The structure and morphology of biochars were characterized using a scanning electron microscopy equipped with an energy dispersive spectrometer (SEM -EDS, Zeiss Sigma300, Germany). Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer Frontier, America) of biochars was recorded with 4 cm−1 resolution between wavenumbers of 4000 cm−1 and 400 cm−1. The results of SEM-EDS and FTIR were presented in Supporting Information. The elemental compositions of biochars were determined by an element analyzer (YX-CHN5000, China). Moreover, Brunauer-Emmett-Teller (BET, ASAP2460, Micromeritics, America) was applied to determine the surface area, porosity and pore volume of biochars. The pH of zero point charges (pHPZC) of SC, SCA, RC and RCA were measured according to the methods reported in the previous research (Jang et al., 2018). The measurement of pHPZC was presented in Supporting Information. 2.4. Sorption experiments 2.4.1. Sorption kinetics Sorption kinetics experiments were performed to evaluate the equilibrium time for the subsequent sorption isotherm experiments.10 mg of RCA and SCA and 20 mg of RC and SC were added to 50 mL centrifuge glass tubes containing 30 mL TC solution with concentration of 120 mg L−1, respectively. All tubes were put in a table concentrator shaker and shaken at 200 rpm at 25 °C from 0.5 h to 216 h. Each treatment was conducted in triplicates, and also the tubes containing only 30 mL TC solution of the same concentration was used for observing the loss of TC during this procedure. At predetermined times, the tubes were taken out and centrifuged at 3000 rpm for 20 min and then filtered by millipore membranes (0.45 µm). Thereafter, the filtrate was determined by a V-5000 spectrophotometer at wavelength of 360 nm (Zhu et al., 2014).
2. Materials and methods
2.4.2. Batch sorption experiments Sorption isotherms of TC to modified biochars were studied. Sorption isotherms for each biochar were determined at different initial solution concentrations of 30, 40, 50, 60, 80, 100, 150, 200 mg L−1. Each concentration for each biochar had triplicate treatments. To each 50 mL centrifuge glass tube, 5–10 mg of biochar (the weighed amounts depending on the concentration of TC and also the type of biochars) and 30 mL TC solution with varying concentrations were combined and shaken at 200 rpm at 25 °C to reach apparent equilibrium based on the sorption kinetics of Section 2.4.1. Moreover, tubes adding only TC solution were used as blanks to test the loss of TC during the periods of experiments. Afterward, the following procedures such as centrifugation, filtration and measurement, were the same as Section 2.4.1.
2.1. Materials Tetracycline hydrochloride (TC) (purity ≥ 95%) was purchased from Sigma. Chemicals used in this study were all analytical grade. Rice straw was collected from Chongzhou experimental base of Sichuan Agricultural University, Chengdu, China, while swine manure was obtained from the experimental farm of Sichuan Agricultural University, Ya’an, China. Both rice straw and swine manure were air-dried and ground to pass through a 2 mm sieve before producing biochars. 2.2. Biochars As previously described (Zhang et al., 2013), the biomass was packed into 30 mL ceramic pot covered with lid to limit oxygen supply and then heated at 700 °C in a preheated muffle furnace for 2 h. The produced biochars were ground and passed through a 100 mesh sieve (0.15 mm). H3PO4 solution was applied to modify the biochars for enhancing the sorption capacity for adsorption of TC since H3PO4 is nonpolluting and easily washed away by water (Peng et al., 2017). Briefly, 20 g biochars were immersed in 40 mL 14% H3PO4 solution for 24 h at 25 °C. After that, the H3PO4 modified biochars were washed by distilled water until the pH of supernatants was stable. Subsequently, the supernatants were discarded and the biochars were oven-dried overnight
2.4.3. Effect of pH on sorption isotherms The pH values of TC solution with varying concentrations were adjusted to pH 5, pH 7 and pH 9 by using 0.1 mol L−1 HCl and NaOH solution. After that, sorption isotherms were conducted at 25 °C in the same way as described in Section 2.4.2. Equilibrium pH of solution was measured after sorption experiments. 2.5. Sorption data analysis The experimental data of sorption kinetics were examined by the pseudo-first-order (PFO) and pseudo-second-order (PSO) (Zhou et al., 432
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content and ash content in this study (data not shown). On the other hand, it was found that the ratios of O/C, (O + N)/C as well as H/C of SCA and RCA were decreased relative to SC and RC, respectively. This decrease indicated the biochars modified by H3PO4 became less hydrophilic with weaker polar groups (Zama et al., 2017). The FTIR spectra of four biochars presented in Supporting Information were similar, and the major bands occurred at the wavenumber of 3436 cm−1 (-OH), 1591 cm−1 (aromatic C=O, C=C), 1400 cm−1 (C-O), and 1050 cm−1 (SiO32−) (Kizito et al., 2015). It can be seen that there were no obvious changes between RC and RCA, as well as SC and SCA, indicating H3PO4 modification might have no significant improvement on functional groups of raw biochars on a qualitative level. However, according to the previously XPS results reported by Peng et al. (2017), H3PO4 modification could increase the number of -COOH and -OH groups in the surface of biochar. Therefore, quantitative analysis of functional groups should be paid more attention to clarify the difference between rice straw and swine manure derived biochar, as well as the variations between H3PO4 modified biochar and raw biochar in future studies. After H3PO4 modification, the porous structure of both SC and RC was more apparent revealed by SEM images presented in Supporting Information. The EDS results presented in Supporting Information demonstrated that Ca, F, Na, and Mg of raw biochars were lost after H3PO4 modification. Moreover, it can be known that Si was difficult to be removed by H3PO4 modification. Furthermore, the removal of Ca, F, Na, etc. might be partially attributed to the increase of surface area and pore volume in Table 1, especially SCA.
2017). The equations were presented as follows: Pseudo-first-order:
qt = qe (1−exp (−k1 t ))
(1)
Pseudo-second-order:
qt =
qe2 k2 t 1 + qe k2 t
(2)
To investigate the reaction behavior between biochars and TC, the data of sorption isotherms were fitted by three models as follows (Xu and Li, 2010; Zhou et al., 2017): Freundlich:
qe = Kf Cen
(3)
Langmuir:
qe =
qmax KL Ce (4)
1 + KL Ce 1−n n
−1
where Kf (mg L g ) is the Freundlich affinity coefficient indicating adsorption strength, and considered as a relative indicator of sorption capacity, n (unitless) is the Freundlich linearity index (Ji et al., 2011), qmax is the maximal sorption capacity, and KL (L mg−1) is a solutesurface interaction energy-related parameter (Luo et al., 2011). 3. Results and discussion 3.1. Characterization of biochars
3.2. Sorption kinetics
Table 1 demonstrates the surface area, pore volume, elemental compositions and aromatic ratio, and ash content of four biochars. In general, the surface area decreased as the order of SC (227.56 m2 g−1) < SCA (319.04 m2 g−1) < RC (369.26 m2 g−1) < RCA (372.21 m2 g−1). This result indicated rice straw-derived biochars (RC and RCA) had higher surface area than swine manure-derived biochars (SC and SCA) no matter if H3PO4 treatment was applied. Moreover, it can be seen that the H3PO4 modification could enhance the surface area of biochars produced from these two different origins, although the change of surface area between RC and RCA was slight. After H3PO4 modification, SCA presented higher total pore volume (Vt: 0.25 cm3 g−1), micropore volume (Vmic: 0.09 cm3 g−1) and mesopore volume (Vmes: 0.17 cm3 g−1) than SC (Vt: 0.14 cm3 g−1, Vmic: 0.07 cm3 g−1, and Vmes: 0.07 cm3 g−1), but there was no change found between RC and RCA (Vt: 0.23 cm3 g−1, Vmic: 0.09 cm3 g−1, and Vmes: 0.14 cm3 g−1). The elemental composition of four biochars in Table 1 suggested that H3PO4 modification generally enhanced the content of C, N and S, but reduced the content of O of raw biochars. The content of H showed no obvious change between SC and SCA, while H content was decreased in RCA relative to RC. Furthermore, the lower ash content of SCA (43.98%) and RCA (55.27%) than SC (60.73%) and RC (58.97%) implied that H3PO4 modification could partially remove ash of raw biochars (i.e., SC and RC). Actually, the reason for the increase of C content might be caused by the decrease of ash content by H3PO4 modification because a significantly negative relationship was found between C
Fig. 1 represents the sorption of TC on four biochars at different time at 25 °C. It can be seen that the adsorption process of TC by four biochars was slow, which was consistent with Liu et al (2012). After 168 h, there was no obvious difference between 192 h and 216 h, therefore, 192 h was considered to be a sufficient equilibrium time for TC adsorbed onto four biochars (Bao et al., 2010). This equilibrium time was longer than the equilibrium time of TC adsorbed onto sediments (less than 20 h) (Xu and Li, 2010) and soils (approximately 8 h) (Bao et al., 2010). Furthermore, it was found that the sorption performance of SCA and RCA were enhanced relative to SC and RC, respectively. In this study, PFO and PSO kinetic models were used to fit the adsorption data for understanding possible mechanism associated with the adsorption of TC on these biochars. Usually, the more optimal the model is, the higher the R2 value is (Bao et al., 2010). From Table 2, the R2 values of PSO model (0.901–0.911) were higher than those of PFO model (0.806–0.862), similar with the majority of TC sorption studies (Jing et al., 2014; Liu et al., 2012; Zhu et al., 2014). Moreover, the experimental qe values were closer to the theoretical qe values calculated by PSO model (Table 2), further suggesting that the sorption kinetics of TC on the biochars could be preferably described by PSO model. The PSO model demonstrates chemisorptions occurred between TC and these four biochars involving valency forces via sharing or
Table 1 Elemental compositions, surface area, pore volume of SCA, SC, RCA, and RC. Sample
SC SCA RC RCA
Surface Areaa (m2 g−1)
227.56 319.04 369.26 372.21
Vtb (cm3 g−1)
0.14 0.25 0.23 0.23
Vmicc (cm3 g−1)
0.07 0.09 0.09 0.09
Vmesd (cm3 g−1)
0.07 0.17 0.14 0.14
Elemental compositions (%)
Atomic ratio
Ash (%)
C
H
O
N
St
O/C
N/C
H/C
(O + N)/C
31.96 48.35 31.77 37.77
0.66 0.66 0.98 0.43
4.77 4.41 7.23 5.31
1.60 2.23 0.96 1.05
0.28 0.37 0.09 0.17
0.15 0.09 0.23 0.14
0.05 0.05 0.03 0.03
0.02 0.01 0.03 0.01
0.15 0.09 0.23 0.14
60.73 43.98 58.97 55.27
a Calculated by the BET method. bThe total pore volume, calculated at P/P0 = 0.99. cThe micropore volume, Calculate by Vt-Vmic. dThe Mesopore volume, Calculated by the DFT method.
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Fig. 1. Adsorption kinetics of TC with the concentration of 120 mg L−1 on SC, SCA, RC and RCA at 25 °C.
Fig. 2. Adsorption isotherms of TC on SCA and RCA at 25 °C. Table 3 Sorption isotherm parameters of TC on SCA and RCA.
Table 2 Adsorption kinetics parameters of TC onto SC, SCA, RC, and RCA. Sample
SC SCA RC RCA
Pseudo-first-order
Sample
Pseudo-second-order
qe (mg g−1)
k1 (h−1)
R2
qe (mg g−1)
k2 (g mg−1 h−1)
R2
109.5 141.5 132.7 153.7
0.03 0.03 0.06 0.07
0.862 0.806 0.812 0.833
127.8 159.7 150.2 166.3
0.0003 0.0003 0.0004 0.0005
0.903 0.901 0.908 0.911
Langmuir qmax (mg g
SCA RCA
160.3 167.5
Freundlich −1
)
KL (L mg 0.13 0.29
−1
)
R
Kf (mg1−nLn g−1)
n
R2
0.973 0.981
70.3 86.8
0.15 0.13
0.981 0.986
2
the qmax of TC by SCA (160.3 mg g−1) was slightly lower than RCA (167.5 mg g−1). From Freundlich model, Kf of SCA (70.3 mg1−nLn g−1) was also lower than that of RCA (86.8 mg1−nLn g−1), and the n value of SCA (0.15) was slightly higher than RCA (0.13). These results suggested that RCA had stronger sorption ability and higher sorption capacity than SCA, which might be mainly explained by the higher surface area and O/C ratio of RCA than SCA. In this study, both Freundlich and Langmuir model could fit the sorption data of TC on RCA and SCA well, indicating the sorption of TC on biochars might be affected by multiple mechanisms. Peiris et al. (2017) summarized the primary sorption mechanisms of TC on biochar surfaces, including cation exchange, π-π electron donor acceptor (EDA) interaction, H bonding, electrostatic interaction and surface complexation. H3PO4 modification removed the cations such as Ca2+, Mg2+ and Na+, indicating the cation exchange as well as the surface compexation (not common in biomass derived biochars) should not be the primary mechanisms for the sorption of TC on both types of biochar (Peiris et al., 2017). Meanwhile, the data of Fig. 1 and Table 2 revealed that the TC was adsorbed mainly by chemisorptions including H bonding and π-π EDA interaction. Compared the O/C content of RCA with SCA (Table 1), the higher O/C of RCA suggested that RCA might have more O-containing functional groups which could serve as H-bond acceptors, and thus the sorption of TC on RCA was higher than SCA (Wang et al., 2018). On the other hand, physical sorption might also partially contribute to the higher sorption capacity of RCA because surface area of RCA was higher than SCA (Table 1), although chemical sorption was dominated in this study. However, it is very difficult to quality the contribution of H bonding, π-π EDA interaction as well as physical sorption for adsorption of TC on both types of biochar (Yang and Xing, 2010).
exchange of electrons between TC and biochars (Peiris et al., 2017; Zhou et al., 2017). By comparison, the qe values of H3PO4 modified biochars (SCA and RCA) were higher than raw biochars (SC and RC). For example, the theoretical qe of TC on SCA was 159.7 mg g−1, 25% higher than that of SC (Table 2). It is obvious that the surface area of SCA increased 40% and the total pore volume increased 79% after SC modified by H3PO4 (Table 1), indicating surface adsorption and pore-filling might explain the higher qe of SCA than SC (Fig. 1 and Table 2). Besides, the H3PO4 modification could increase the amount of functional groups of biochar (Peng et al., 2017), therefore, the chemical interactions between TC molecules and functional groups of SCA might also partially contribute to the higher qe value of SCA relative to SC (Jing et al., 2014). However, the reason for enhanced qe of RCA relative to RC might be different with SCA to SC because there was no obvious change of surface area and pore volume between RCA and RC (Table 1). Thus, the possible reason for the higher qe of RCA than RC might be explained by the enhanced interactions between RCA and TC due to the increased amount of functional groups, such as -COOH, -OH, and so on (Peng et al., 2017). 3.3. Sorption isotherm Since the H3PO4 modified biochars showed higher sorption capacity than raw biochars (Table 2), therefore, SCA and RCA were employed to explore the difference of sorption ability between plant-derived and animal manure-derived bicohar. Sorption data of TC onto SCA and RCA at 25 °C are plotted in Fig. 2. The sorption data of SCA was quite close to these of RCA, and RCA showed slightly higher sorption capacity than SCA. The parameters of TC adsorbed to SCA and RCA fitted by Langmuir and Freundlich model are listed in Table 3. Both the Langmuir and Freundlich model described the sorption isotherms well because the R2 values of Freundlich (0.981–0.986) were similar with R2 of Langmuir model (0.973–0.981). According to the qmax fitted by Langmuir model,
3.4. Effect of pH Multiple factors can determine the sorption mechanisms of TC on biochars, including the properties of TC, physicalchemical properties of biochars as well as the conditions where the sorption is carried out 434
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Table 4 Sorption isotherm parameters of TC on SCA and RCA at different initial pH of solution. Sample
Langmuir qmax (mg g−1)
Freundlich KL (L mg−1)
R2
Kf (mg1−nLn g−1)
n
R2
Initial pH = 5 SCA 143.9 RCA 131.9
0.28 0.51
0.931 0.861
76.1 88.0
0.1 0.1
0.784 0.977
Initial pH = 7 SCA 174.0 RCA 266.8
0.23 0.06
0.902 0.980
69.7 49.3
0.2 0.3
0.968 0.998
Initial pH = 9 SCA 365.4 RCA 552.0
0.03 0.02
0.963 0.991
33.3 24.0
0.5 0.6
0.988 0.997
is not in line with the findings of other relevant studies (Jang et al., 2018; Peiris et al., 2017; Zhou et al., 2017). In these studies, a decline in sorption at elevated pH was widely reported and mainly explained by the π-π EDA interaction as well as electrostatic interaction (Jang et al., 2018; Peiris et al., 2017; Zhou et al., 2017). Generally, the properties of TC and biochar were both altered by pH of solution, and thus influencing the sorption capacity of TC on biochar. The pKa of TC and pHPZC of biochar were proposed as the main reasons for the changes of sorption capacity of TC on biochar under different pH. It is well known that TC is an amphoteric molecule with three values of pKa (3.3, 7.7 and 9.7) (Jang et al., 2018). At environmentally relevant pH values, TC can exist as a cation, zwitterion, as well as anion (Sassman and Lee, 2005). For example, at pH 2, TC is about 95% TCH3+; at pH 5, TC is more than 95% TCH20; at pH 7, TC is about 75% TCH20 and 25% TCH−; at pH 9, TC is about 70% TCH− and 30% TC2− (Gu and Karthikeyan, 2005; Sassman and Lee, 2005; Xu and Li, 2010). Besides, the surface charge of biochar was positive when the pH of solution was lower than their pHPZC, and vice versa. At low pH, TC was cationic and the surface of biochar is commonly positive, therefore, the π-π electron EDA interaction was weak. Along with pH increasing, TC became zwitterionic, and the EDA interaction between TC and biochar was strengthened. Thus, the enhanced sorption of TC was observed. In summary, the altered charges of both TC and biochar should reasonably explain the changes of sorption capacity of TC on biochars under different pH. In this study, although the initial pH of solution was set as 5.0, 7.0 and 9.0, the equilibrium pH values of TC adsorbed on SCA and RCA were 5.07, 5.94 and 7.12, and 5.67, 6.18, and 7.09, respectively. Taking the pHPZC of SCA (7.02) and RCA (7.08) (Fig. 3b) into consideration, the surface charges of SCA and RCA were positive when pH was lower than 7.0. Along with pH increasing from 5.0 to 7.0 (equal to the equilibrium pH of TC adsorbed on SCA and RCA), TCH20 decreased from 95% to 75%, and TCH− approximately increased from 0% to 25% (Sassman and Lee, 2005). Therefore, the enhanced sorption of TC on RCA and SCA at elevated pH could be largely explained by the strengthened electrostatic attraction between positive surface charge of both biochars and negative charge of TC.
Fig. 3. (a) Effect of pH onto the sorption isotherms of TC onto SCA and RCA (pHi means the initial pH of solution) 25 °C; (b) the pHPZC of SC, SCA, RC and RCA 25 °C.
(Peiris et al., 2017). In this study, the impact of pH on sorption of TC on both H3PO4 modified biochars was explored because TC forms and sorption mechanisms of TC on biochars were largely affected by pH (Jang et al., 2018; Zhou et al., 2017). Fig. 3 shows the effect of pH in the sorption of TC on on H3PO4 modified biochars (i.e., SCA and RCA) at 25 °C. Generally, the sorption data of TC on SCA were quite close to these of RCA at all tested pH values. A increase in sorption at elevated pH was observed on both RCA and SCA, and maximum sorption of TC on both biochars was found at pH 9 in this study. The sorption data of TC on SCA and RCA under different solution pH were fitted by Langmuir and Freundlich model (Table 4). Taking R2 values into consideration, both Freundlich model and Langmuir could fit all the sorption data (except sorption data of TC on SCA at pH 5) well, especially Freundlich model. In general, Kf of TC sorption on SCA and RCA decreased but n increased with increasing pH. Along with the initial pH of solution increasing from 5 to 9, the fitted Kf of SCA decreased from 76.08 to 33.29 mg1−nLn g−1 and RCA declined from 88.03 to 23.99 mg1−nLn g−1. Meanwhile, the n values increased from 0.13 to 0.48 for TC adsorbed on SCA and from 0.09 to 0.60 for TC adsorbed on RCA, respectively. On the other hand, the qmax of TC on RCA increased from 131.9 mg g−1 to 552 mg g−1 with pH increasing from 5 to 9. As for SCA, the qmax changed from 143.9 mg g−1 to 365.4 mg g−1. These results obviously suggested that the sorption capacities of TC on both biochars were enhanced by elevated pH of solution (from 5 to 9), which
3.5. Sorption properties of biochars from different origins In order to further understand the sorption ability of TC on SCA and RCA relative to other biochars, the qmax and Kf values were compared with biochars produced from different biomass including sawdust, rice husk, chicken feather, chicken bone, apricot nut shell, and so on (Table 5) (Li et al., 2017b; Liu et al., 2012; Marzbali et al., 2016; Oladipo et al., 2017; Wang et al., 2017; Zhou et al., 2017). It is evident that the sorption of TC was not only affected by the precursors and treatment, but also by the tested pH and temperature. At the same temperature, the qmax and Kf values of SCA and RCA 435
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Table 5 Comparison of TC Langmuir and Freundlich model parameters of biochar with different feedstocks and modification methods in the literature. Different biochar
Adsorption temperature and pH
Surface
Langmuir 2
Area (m g
Acid-treated rice straw biochar Acid-treated swine manure biochar Rice straw biochar Iron and zinc doped sawdust biochar Alkali-treated rice husk biochar Alkali-treated chicken feather biochar Acid-treated apricot nut shell biochar Magnetic chicken bone-based biochar Magnetic chicken bone-based biochar
25 °C, 25 °C, 25 °C, 25 °C, 30 °C, 30 °C, 30 °C,
pH = 9.0 pH = 9.0 Ambient pH pH = 6.0 Ambient pH Ambient pH pH = 6.5
50 °C, pH = 10.0
−1
)
KL (L mg
−1
Freundlich )
qmax (mg g−1)
Kf (mg
1−n n
L g
References −1
)
n
372.2 319.0 – – 117.8 1838.9 307.6
0.02 0.03 0.71 0.02 0.02 0.03 1.35
552.0 365.4 14.2 102.0 58.8 388.3 200.0
24.0 33.3 4.8 3.4 9.4 79.0 148.8
0.6 0.5 0.6 0.6 0.3 4.0 0.1
This work This work (Wang et al., 2017) (Zhou et al., 2017) (Liu et al., 2012) (Li et al., 2017a,b) (Marzbali et al., 2016)
–
11.30
63.3
5.3
1.6
(Oladipo et al., 2017)
were higher than that of rice straw biochar (Wang et al., 2017) and Fe/ Zn doped sawdust biochar (Zhou et al., 2017), especially at pH 9.0. However, compared with sorption conducted at 30 °C (ambient pH), the sorption of SCA and RCA at 25 °C (ambient pH) was lower than alkalitreated chicken feather biochar (Li et al., 2017b) but higher than alkalitreated rice husk biochar (Liu et al., 2012). The higher surface area of alkali-treated chicken feather biochar than SCA and RCA as well as the sorption temperature should be largely contributed to its higher sorption capacity. Although the temperature is an important factor for the sorption of TC, the origins of biochars as well as the preparation methods of biochars are also critical gradients for sorption behaviors. For example, sorption capacity and ability of TC on magnetic chicken bone-based biochar at 50 °C was significantly lower than sorption of TC on SCA and RCA at 25 °C (Oladipo et al., 2017). From Table 5, it can be concluded that H3PO4 modified biochars derived from rice straw and swine manure showed good sorption abilities and capacities for TC relative to other biochars. Therefore, SCA and RCA are assumed to be good sorption materials for removing TC from water.
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