Effects of rice straw biochar on sorption and desorption of di-n-butyl phthalate in different soil particle-size fractions

Journal Pre-proofs Effects of rice straw biochar on sorption and desorption of di-n-butyl phthalate in different soil particle-size fractions Lei Xiang, Li-Juan Zeng, Pei-Pei Du, Xiao-Dan Wang, Xiao-Lian Wu, Binoy Sarkar, Huixiong Lü, Yan-Wen Li, Hui Li, Ce-Hui Mo, Hailong Wang, QuanYing Cai PII: DOI: Reference:

S0048-9697(19)34870-3 https://doi.org/10.1016/j.scitotenv.2019.134878 STOTEN 134878

To appear in:

Science of the Total Environment

Received Date: Revised Date: Accepted Date:

12 August 2019 5 October 2019 6 October 2019

Please cite this article as: L. Xiang, L-J. Zeng, P-P. Du, X-D. Wang, X-L. Wu, B. Sarkar, H. Lü, Y-W. Li, H. Li, C-H. Mo, H. Wang, Q-Y. Cai, Effects of rice straw biochar on sorption and desorption of di-n-butyl phthalate in different soil particle-size fractions, Science of the Total Environment (2019), doi: https://doi.org/10.1016/ j.scitotenv.2019.134878

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Effects of rice straw biochar on sorption and desorption of di-n-butyl phthalate in different soil particle-size fractions

Lei Xiang a,1, Li-Juan Zeng a,1, Pei-Pei Du a, Xiao-Dan Wang a, Xiao-Lian Wu b, Binoy Sarkar c Huixiong Lü d, Yan-Wen Li a, Hui Li a, Ce-Hui Mo a, Hailong Wang b,e, Quan-Ying Cai a *

a

Guangdong Provincial Research Center for Environment Pollution Control and Remediation Materials, College of Life Science and Technology, Jinan University, Guangzhou 510632, China

b

Biochar Engineering Technology Research Center of Guangdong Province, School of Environmental and Chemical Engineering, Foshan University, Foshan, Guangdong 528000, China

c

Department of Animal and Plant Sciences, The University of Sheffield, Sheffield, S10 2TN, United Kingdom

d

College of Natural Resources and Environment, South China Agricultural University, Guangzhou, 510642, China

e

Key Laboratory of Soil Contamination Bioremediation of Zhejiang Province, Zhejiang A&F University, Hangzhou, Zhejiang 311300, China

1These

authors contribute equally to this paper.

*Corresponding

Author

E-mail: [email protected] (Q-Y Cai).

1

Abstract: Sorption of organic contaminants by biochar greatly affects their bioavailability and fate in soils. Nevertheless, very little information is available regarding the effects of biochar on sorption and desorption of organic contaminants in different soil particle-size fractions. In this study, di-n-butyl phthalate (DBP), a prevalent organic contaminant in agricultural soils, was taken as a model contaminant. The effects of biochar on DBP sorption and desorption in six particle-size fractions (i.e., coarse sand, fine sand, coarse silt, fine silt, clay, and humic acid fractions) of paddy soil were investigated using batch sorption-desorption experiments. A straw-derived biochar with high specific surface area (116 m2/g) and high content of organic matter (OM) rich in aromatic carbon (67%) was prepared. Addition of this biochar (1% and 5%) significantly promoted the sorption and retention of DBP in all the paddy soil particle-size fractions at environmentally relevant DBP concentrations (2~12 mg/L) with 1.2~132-fold increase of the Kd values. With increasing addition rates of biochar, DBP retention by the biochar enhanced. The biochar’s effectiveness was remarkably influenced by the physicochemical properties of the soil particle-size fractions, especially, the OM contents and pore size showed the most striking effects. A parameter (rkd) reflecting the biochar’s effectiveness showed negative and positive correlations with OM contents and pore size of the soil particle-size fractions, respectively. Accordingly, strong effect of the biochar was found in the soil fractions with low OM contents and high pore size. The findings of this study gave insight into the effects and influencing factors of biochar on sorption and desorption of organic contaminants in soils at scale of various particle-size factions.

Keywords: Phthalate; Sorption, Biochar, Agricultural soil; Particle-size fraction

2

1. Introduction Phthalic acid esters (PAEs) or phthalates are widely used as plasticizers in plastic products and non-polymeric products. PAEs have been frequently detected in soils with applications of fertilizers and plastic films for mulching (Lü et al., 2018). Especially in China, higher concentrations of PAEs (up to dozens and even hundreds mg/kg) in soils (Lü et al., 2018) were observed compared to those (generally < 3 mg/kg) of other countries such as Scotland (Rhind et al., 2012), Netherlands (Peijnenburg et al., 2006), and Serbia (Škrbić et al., 2016). Among the PAE compounds detected in soils of China, di-n-butyl phthalate (DBP) and di-(2-ethylhexyl) phthalate (DEHP) were the predominant compounds. For example, the average concentrations of DBP and DEHP in soils were detected up to 57.5 and 149 mg/kg in Xinjiang province (Guo and Wu, 2011), 17.5 and 2.48 mg/kg in Guangdong province (Yang et al., 2007), 15.7 and 5.32 mg/kg in Shandong province (Chai et al, 2014), and 14.6 and 4.2 mg/kg in Heilongjiang province (Xu et al., 2008), respectively. PAEs in soils can be taken up and accumulated by crops, thus threatening human health through the food chain (Lü et al., 2018). Human exposure to PAEs via dietary pathway (especially consuming cereals and vegetables) in China, especially the Pearl River Delta, was higher compared to other countries (Wang et al., 2018). PAEs are typical endocrine disruptors and have serious adverse effects (e.g., reproductive developmental and neurological toxicity) on human and other organism (Katsikantami et al., 2016). Both DBP and DEHP were identified as priority pollutants by the U.S. Environment Protection Agency (USEPA). Considering the frequent detection and high concentrations of DBP and DEHP in agricultural soils and their toxicities (Lü et al., 2018), it is urgent to develop effective methods to lower the uptake, translocation, and accumulation of PAEs from soils to crops. Biochar, a promising material with high porosity and surface area, functional surface groups, and specific mineralogical composition (Liu et al., 2018), is widely used for conditioning soil physicochemical properties, remediating contaminants, and sequestrating carbon in soils (Sopeña et al., 2013; Mohan et al., 2014; Purakayastha et al., 2019). Many studies have reported sorption and 3

desorption for organic and inorganic contaminants in soils added with various biochars under different experimental conditions (Rajapaksha et al., 2016; Peiris et al., 2017; Li et al., 2019). Biochar exhibited high sorption or retention capacities for different organic contaminants, including PAEs (Jin et al., 2014; Wang et al., 2016a), polycyclic aromatic hydrocarbons (PAHs) (Lamichhane et al., 2016; Tang et al., 2016), polychlorinated biphenyls (Huang et al., 2018; Wang et al., 2016b), pesticides (Sun et al., 2012a; Martin et al., 2012; Liu et al., 2018), antibiotics (Peiris et al., 2017). By inactivation of organic contaminants (e.g., PAEs, PAHs, pesticides) owing to surface sorption, pore

filling,

and

functional

groups-derived

interactions,

biochar

lowered

their bioavailability in contaminated soils (He et al., 2016; Lamichhane et al., 2016; Liu et al., 2018). For example, Chen et al. (2019a) found that biochar significantly reduced the uptake of DEHP by Brassica chinensis L. It was noted that the effectiveness of biochar in the inactivation of organic contaminants in soils generally increased with increasing application rates but decreased with aging (Martin et al., 2012; Zhang et al., 2016). In addition, the soil physicochemical properties, especially the content of organic matter (OM) significantly enhanced the biochar effectiveness owing to increasing hydrophobic interaction, π-π coordination, etc. (Zhang et al., 2016; Wu et al., 2019; Chen et al., 2019a). Although effects of application rates, aging, and soil properties on biochar effectiveness have been extensively reported, little information is available regarding the effectiveness and influencing factors of biochar on sorption-desorption of organic contaminants in different soil particle-size fractions up to date. Soil is made up of various particle-size fractions (Qi and Zhang, 2016; Xiang et al., 2019). Sorption of organic contaminants to soil particle-size fractions not only differs greatly from that of the whole soil sample but also varies remarkably among individual particle size fractions (Thiele-Bruhn et al., 2004; Sun et al., 2012b; Xiang et al., 2018a, 2019). Our previous study found that DBP was strongly adsorbed to humic acid fraction of paddy soil but its sorption was reversely associated with the particle sizes of the other fractions, including coarse sand, fine sand, coarse silt, fine silt, and clay (Xiang et al., 2019). The similar sorption pattern relevant to the soil particle-size 4

fractions were also observed for perfluorooctanoic acid and phenoxyalkanoic herbicides (Haberhauer et al., 2000; Xiang et al., 2018a) but not for phenanthrene (Gao et al., 2015). Gao et al. (2015) found that silt fraction had the highest carbon-normalised distribution constant (Koc) for phenanthrene which was higher by five times than the other size fractions, despite distribution coefficient (Kd) of the fraction for phenanthrene being the lowest. Therefore, the sorption of different soil particle-size fractions could give new insights into the sorption behaviours of organic contaminants, which could not be obtained from the experiments using the whole soil sample (Sun et al., 2012b; Xiang et al., 2018a, 2019). Furthermore, it could be expected that biochars could show various effects on organic contaminants in different soil particle-size fractions when they were used as additives to remediate soils polluted by organic contaminants. Accordingly, it is necessary to investigate the effects and key factors of biochars on the sorption and desorption of organic contaminants in different soil particle-size fractions. In this study, DBP as a typical PAE compound was chosen as the model sorbate due to its high toxicity and frequent occurrence in soils of China. Paddy soil was selected because it was usually more heavily polluted by DBP in Guangdong Province, China, compared to other land-use types of soils (Lü et al., 2018). Paddy soil was separated into six particle-size fractions (i.e., coarse sand, fine sand, coarse silt, fine silt, clay, and humic acid) for the sorption/desorption experiments. The objectives of this study are to: (1) investigate the variations in DBP sorption/desorption in the six soil particle-size fractions with and without biochar application, and (2) analyse the influencing factors on the biochar’s effectiveness in DBP remediation using Pearson correlation analysis and canonical correspondence analyses (CCA). The findings of this study would advance our understanding in the effects and influencing factors of biochar on sorption and desorption of organic contaminants in the soil particle-size factions.

2. Materials and Methods 2.1. Materials and regents. 5

Standard DBP with purity of 98% was obtained from Aladdin Industrial Corporation (Shanghai, China). Its water solubility and octanol–water partition coefficient (log Kow) were 11.2 mg/L (20 ℃) and 4.45, respectively. Chromatographically pure methanol was obtained from Anpel Scientific Instrument (Shanghai) Co., Ltd. All other regents were of analytical grade, which were purchased from Guangzhou Chemical Reagent Co. Ltd., China. Uncontaminated surface soil (0–20 cm depth) was collected from a paddy field in Guangdong province, China, and its DBP concentration was less than the limit of detection (12 μg/kg). The soil was air-dried and then ground to pass through a 2-mm sieve. The physicochemical properties of the soil were determined in our previous study and the results presented as follows: 27.1 g/kg of OM, 119 cmol/kg of cation exchange capacity (CEC), 6.5 of pH, 1.26 g/kg of total nitrogen, 1.76 g/kg of total phosphorus, and total 18.0 g/kg of total potassium (Xiang et al., 2019). 2.2. Instrumentation Main instruments used in this study were listed as follows: KSL-1100X-S muffle oven (Hefei Ke Jing Material Technology, Co. Ltd., China), EA2400Ⅱ Elementar Vario El elemental analyser (Perkin Elmer, Waltham, MA), Micromeritics ASAP 2020 automated system (Micromeritics Instrument Corp., Norcross, GA, U.S.A.), field emission scanning electron microscopy (FESEM, JEOL, JSM-6330F), Nicolet Nexus 6700 FTIR spectrometer (Thermo Nicolet Corporation, U.S.A.), HNY-2102C shaker (Honour, Tianjin, China), ultrasonic bath (KH-250E, Kunshan, China), and Agilent 1100 HPLC coupled with a UV detector (Agilent Technologies, San Diego, CA). 2.3. Preparation of biochar and characterization of its physicochemical properties. Rice straw was used to prepare the biochar, according to a reported procedure with minor modification (Li et al., 2019). Briefly, rice straw was cut into pieces and then cleaned with deionized water to remove the surface adhesives, followed by drying at 80 °C. The dried rice straw was subjected to pyrolysis at 600 ℃ for 4 h in a ceramic pot covered with a tight-fitting lid and placed under N2 atmosphere in a muffle oven. After it cooled to room temperature, the biochar was collected and washed with 0.1 M HCl for demineralization, followed by washing with deionized 6

water to remove the residual acids thoroughly. The obtained biochar was oven-dried at 80 °C, gently ground, and homogenized to pass through a 0.15 mm sieve. The obtained biochar was used for characterization of its physicochemical characteristics. Elemental analyses (C, H, N, S) were conducted by the elemental analyser. Specific surface areas, pore volume, and pore size were analysed with the Micromeritics system using the Brunauer–Emmett–Teller (BET) method via N2 sorption/desorption measurements. Morphologies of the biochar sample were investigated using the field emission scanning electron microscopy after spray-coating the sample with a thin layer of gold (Xiang et al., 2018b). Surface functional groups were analysed by fourier transform infrared (FTIR) spectroscopy using KBr pellet method (at the ratio of 1.5/300 sample/KBr, m/m), in the scanning region of 500-4000 cm-1 with 4 cm-1 of resolution (Xiang et al., 2019). The ash content in the biochar was obtained by measuring the residual weight after heating the sample at 750 °C for 6 h (Fu et al., 2019). The organic matter content was calculated by subtracting the ash content from the total mass of the samples. The oxygen content was obtained by subtracting the contents of carbon, hydrogen, and nitrogen, and sulphur from the organic matter content (Jung et al., 2013). 2.4. Fractionation of soil particle-size fractions and characterization of the physicochemical properties. The soil samples (2 mm) was fractionated into six particle-size fractions, namely coarse sand, fine sand, coarse silt, fine silt, clay, and humic acid fractions, following a wet-sieving method assisted by sonication and sedimentation (Sun et al., 2012b; Xiang et al., 2018a, 2019). The detailed procedures are presented in the Supplementary Data (Supplementary T1). During the fractionation process, 92% of the mass of the original soil was recovered (Xiang et al., 2019). The obtained soil particle-size fractions were used for characterization of their physicochemical properties with the results listed as follows: mean fraction size = 15~209 μm, OM = 5.1~39.5 g/kg, CEC = 26.2~420 cmol/kg, specific surface area = 1.6~33.4 m2/g, and pH = 3.8~7.9. More details of the results on the physicochemical characteristics were shown in Supplementary Data (Table S1) (Xiang et al., 7

2019). 2.5. Sorption and desorption experiments Sorption and desorption of DBP to the six soil particle-size fractions with and without biochar amendment were conducted by batch experiments following the OECD guideline 106 (OECD, 2006). Two addition rates (1% and 5%, w/w) of rice straw biochar in the soil particle-size fractions were used according to the previous study (He et al., 2019). The treatment without addition of biochar served as the control. Before sorption experiments, biochar was mixed well with each soil particle-size fractions. DBP dissolved in methanol was diluted with the background solution which contained 0.01 mol/L CaCl2 and 200 mg/L NaN3. To avoid the solvent interference, the concentration of methanol in the final solution was kept below 0.5% (v/v) (Xiang et al., 2019). In the sorption isotherm experiments, five initial DBP concentrations in the solutions (2, 4, 6, 8, and 12 mg/L) were used, following its water solubility (11.2 mg/L) (Staples et al., 1997) and the reported data for environmentally relevant concentrations of DBP (Xiang et al., 2019). The biochar-amended soil particle-size fraction (0.50 g) and 25 mL DBP solution were placed into a Teflon-lined centrifuge tube (50 mL). After vortexed for 1 min, the solution in the centrifuge tube was shaken at 200 rpm and 25 °C on a shaker. The shaking time was set as 192 h, considering equilibrium sorption of DBP in some particle-size fractions of the paddy soil required 144-192 h including fine sand, fine silt, clay, and coarse silt (Xiang et al., 2019). After shaking, the supernatant obtained via centrifugation at 3782 rpm for 10 min was passed through a 0.22-μm Teflon syringe filter for DBP analysis using high performance liquid chromatography (HPLC, see section 2.5). DBP loss resulting from biodegradation, volatilization, and sorption to the centrifuge tube wall was evaluated by conducting the same DBP concentration series (2-12 mg/L) without the soil sample fractions or the biochar-amended soil sample fractions under the abovementioned conditions in triplicate. The DBP concentration in the biochar-amended soil sample fractions after sorption was analyzed using our reported methods (Xiang et al., 2019). The results showed the DBP loss was below 10% (Supplementary Data, Table S2) and the mass balance rate was in the range of 8

85% to 94%. After sorption and removing the supernatant, one step desorption experiments were conducted (Martin et al., 2012). Fresh background solution (25 mL) containing 0.01 M CaCl2 and 200 mg/L NaN3 was added into each tube. The solution in tube was shaken for 192 h, and then centrifuged at 3782 rpm for 10 min. DBP in the supernatants was analyzed as described above. The remaining volume of the supernatant from the previous step (i.e., sorption experiment) was obtained by the variations of weights of the centrifuge tubes (Qi et al., 2014). According to the remaining volume of the supernatants and their known DBP concentrations before the desorption step, the actual desorbed amount of DBP was corrected by subtracting the carry-over amount. All of the sorption and desorption experiments were performed in triplicates. 2.6. Analysis of DBP DBP concentrations were measured using an Agilent 1100 HPLC coupled with a UV detector (Wu et al., 2018; Xiang et al., 2019). DBP was separated by a reverse-phase C18 column (250 mm  4 mm, 5 μm; Agilent) set at 25 °C. Mobile phase is made up of 80% acetonitrile and 20% deionised water (v/v) at a flow rate of 1 mL/min. The injection volume was 10 μL. DBP was detected using UV detector at 242 nm, with the retention time of 4.8 min. DBP quantitation was conducted using a calibration curve with seven concentrations (0, 0.5, 1, 2, 4, 8, and 10 mg/L). The limits of detection for DBP in supernatant and soil were 10 μg/L and 12 μg/kg, respectively. Samples including supernatant and soil fractions spiked at different DBP concentrations (0.1, 0.5, 2, 4, 8 mg/L or mg/kg) were determined to evaluate the recoveries and precision. Results showed that the DBP recoveries in soil fractions and supernatant ranged from 87%-95% and 92%-98%, respectively, with relative standard deviation (RSD) lower than 10% (Supplementary Data, Table S3). 2.7. Data analysis The adsorbed amounts qe (mg/g) of DBP in different soil particle-size fractions with and without biochar amendment were estimated by the mass difference between the initial and final concentrations in the solutions according to Eq. (1): 9

𝑞𝑒 = (𝑐0 ― 𝑐𝑒)·𝑣/𝑚 (1) where ce (mg/L) is the DBP concentrations in equilibrium solution (i.e., final concentration); c0 (mg/L) is the initial DBP concentration in solution; v is the solution volume (0.025 L), and m indicates the mass (0.50 g) of the soil particle-size fractions with and without biochar amendment. The Freundlich and Langmuir equations were used to assess the sorption isotherm data (Langmuir, 1918; Freundlich, 1922). Their linearized forms are presented as follows: Ln 𝑞𝑒 = ln 𝐾𝑓 ＋ 𝑛 ·ln 𝑐𝑒

(2)

1/ 𝑞𝑒 = 1/ 𝑄𝑚 + 1/(𝐾𝐿·𝑄𝑚·𝑐𝑒)

(3)

where qe presents the amount adsorbed (mg/g) at equilibrium time; ce (mg/L) has been defined in Eq. (1); Kf is the Freundlich coefficient [(mg/g)/(mg/L)n], indicating the sorption affinity of the focused compound; n, a dimensionless empirical parameter, indicates the isotherm linearity, if convex (n < 1), linear (n = 1), or concave (n > 1); Qm presents the maximum sorption capacity (mg/g); and KL presents the Langmuir constant associated with the sorption binding energy (Xiang et al., 2016 and 2019). The single-point sorption partition coefficient (Kd, mL/g) and the organic carbon content-normalised sorption coefficient (logKoc) at given equilibrium concentrations of DBP were calculated to assess the sorption capacity using Eq. (4) and (5). 𝐾𝑑 = 𝑞𝑒 /𝑐𝑒

(4)

log𝐾𝑜𝑐 = log[ (100𝐾𝑑/OM%)·1.724]

(5)

where ce is the equilibrium concentration of DBP in solution; qe is the equilibrium sorption capacity computed by the given ce value and the equation well fitted to the DBP sorption isotherm; OM% is the percentage of OM in the biochar-amended soil particle-size fractions; 1.724 is the conversion coefficient between OM content and organic carbon content (Xiang et al., 2018a). In this study, Freundlich equation was used to compute the qe and corresponding Kd and logKoc values. Standard Gibbs free energy (ΔG0, kJ/mol) was calculated to evaluate the thermodynamic property of the DBP sorption using Eq. (5) and (6) (Sukul et al., 2008; Gao et al., 2015). 10

Δ𝐺0 = ― 𝑅𝑇 In𝐾OM 𝐾OM =

100𝐾f OM%

(6)

(7)

where R is the universal gas constant with the value of 8.314 J/kmol, T is the absolute temperature in Kelvin (K) with the value of 298 K; KOM is the organic matter (OM)-normalised Freundlich coefficient (Gao et al., 2015); Kf and OM% have been defined in the Eq. (2) and (5), respectively. Desorption rate (rd, %) was calculated to evaluate the effects of the biochar on DBP desorption from the soil particle-size fractions Eq. (8): 𝑟𝑑 = (𝑐𝑑𝑒·𝑣 ― 𝑐𝑠𝑒·𝑣𝑟) /(𝑞𝑠𝑒·𝑚) (8) where ces and qes are the equilibrium concentrations of DBP in the solution and the soil particle-size fractions, respectively, after sorption experiments; ced is the equilibrium concentrations of DBP in the solution after desorption experiments; vr is the remaining volume after removing the supernatants before desorption experiments, which is obtained according to the weight variations between before and after removing the supernatants (Qi et al., 2014); v and m have been defined in the Eq. (1). 2.8. Statistical analysis The basic statistical analysis of the sorption–desorption data was conducted in Microsoft© Excel 2013, including data tabulation, mean calculation, and standard deviation (SD) calculation. All the data were presented as mean ± SD. The Canonical Correspondence Analysis (CCA) was carried out using Canoco 5.0 (Huang et al., 2019). One-way analysis of variance (ANOVA) with a Duncan’s (D) test and Pearson correlation analysis were carried out by SPSS 21.0 with the statistical significance defined at P < 0.05.

3. Results and discussion 3.1. Characteristics of the prepared biochar As shown in Fig. 1, the prepared biochar had high specific surface area (116 m2/g), which is 11

comparable with that of rice hull-derived biochar (95.7 m2/g) but far higher than those of maize straw-, peanut-, bamboo-, and chestnut-derived biochars (< 2 m2/g, Wu et al., 2019). The high specific surface area of the biochar could be associated with its porous structure (Chen et al., 2019a). According to the International Union of Pure and Applied Chemistry (IUPAC), there are three types of pores depending on the pore size, i.e., micropore (< 2 nm), mesoporous (2~50 nm), and macropore (> 50 nm) (Xiang et al., 2018a). Accordingly, the biochar had the mesoporous structure, since its average pore size was 7.1 nm (Fig. 1). This was further confirmed by the SEM image that clearly showed a vascular structure and porous structure of the biochar (Fig. 1). It could be expected that after addition of the biochar, its high specific surface area relevant to the mesoporous structure could effectively increase DBP sorption via surface sorption and pore filling (Chen et al., 2019b) in the soil particle-size fractions with low specific surface area (1.6~33.4 m2/g; Supplementary Data, Table S1). On the other hand, the biochar possessed high content of organic matter (67%, Fig. 1), which was comparable with those of chestnut- and poultry litter-derived biochars (53.4%~58.1%, Wu et al., 2019; Sun et al., 2011) but higher than those of rice hull- and maize straw -derived biochars (33.6%~43.4%, Wu et al., 2019) and the soil particle-size fractions (0.51%~3.95%; Supplementary Data, Table S1). Furthermore, the organic matter of the biochar was rich in aromatic moieties, as indicated by its extremely low H/C (0.36) but high C/N (38.2) ratios (Fig. 1) (Schmidt and Noack, 2000; Jin et al., 2014 and 2018). The high content of organic matter rich in aromatic carbon in the biochar could enhance the DBP sorption in the partition domain of the soil particle-size fractions amended with biochars (Ren et al., 2018a,b). In addition, the biochar contained many specific functional groups, which could strongly adsorb DBP. As shown in Fig. 1, there were several characteristic absorption bands found in the FTIR spectrum of the biochar, which were assigned to hydroxyl (–OH) groups at 3200~3500 cm-1, C-H bonds of CH3 and CH2 at 2850~2900 cm-1, aromatic C=C and carbonyl/carboxyl C=O bonds at 1400~1700 cm-1, and C-O-C bonds at ~1100 cm-1 (Ren et al., 2018ab; Chen et al., 2019a, Xiang et al., 2019). Among these functional groups in the biochar, the aliphatic carbon (CH3 and CH2) had 12

the potential to adsorb DBP via hydrophobic force, while the aromatic C=C and hydroxyl (–OH) groups could interact with DBP via π-π coordination and H-bonding, respectively (Chen et al., 2019a; Xiang et al., 2019). 3.2. Sorption isotherms of different soil particle-size fractions with the biochar Langmuir and Freundlich isotherm equations widely used to describe the sorption equilibrium data of biochar-amended soils were applied in this study (Martin et al., 2012; Chen et al., 2019b). The former model is usually used to evaluate ideal sorption regarding a homogeneous sorbent (Langmuir et al., 1918), while the latter model is often applied to non-ideal sorption process governed by complex sorption mechanism (Freundlich, 1922). The isotherm parameters of Freundlich and Langmuir equation are summarized in Table 1. In comparison to the Langmuir equations, the Freundlich equation generally described better the isotherm data for the soil particle-size fractions with or without biochar amendment (Fig. 2 and Supplementary Data, Fig. S1), since the relatively higher regression coefficients of Freundlich model (R2: 0.72~0.93) than those of Langmuir model (R2: 0.19~0.93) (Table 1). Furthermore, it was obviously unreasonable that Qm and KL values in the Langmuir equations that described isotherm data of the coarse sand and 5% biochar-amended fine silt were negative. Accordingly, the Freundlich equation was used to describe the DBP sorption isotherm data in the present study. Previous studies also showed better fittings of the Freundlich equations than Langmuir equations to isotherm data of DBP sorption to soil particle-size fractions or minerals and organic matter fractions derived from soils (Jin et al., 2015; Gao et al., 2016; Wu et al., 2018a; Xiang et al., 2019). The better fitting of the Freundlich equations suggested DBP sorption to the soil particle-size fractions with and without biochar was non-ideal sorption relevant to heterogeneous sorption instead of monolayer sorption. The parameters of Freundlich model were compared among the soil particle-size fractions with and without biochar hereafter. The n values (0.23 to 0.84) of the Freundlich equations for all the soil particle-size fractions with biochar were below 1 (Table 1), indicating their convex DBP sorption isotherms, i.e., DBP 13

sorption capacity decreased with the increase of DBP concentrations (Xiang et al., 2019). Compared to the soil particle-size fractions without biochar, the n values for soil fractions with biochar were 1.4~6.0 times lower. This suggested that the addition of biochar enhanced the nonlinearity of DBP isothermal sorption to different soil particle-size fractions, which could be related to the increase of extraneous organic matter from biochar addition. As reported by Wu et al. (2018b), the addition of extraneous organic matter decreased the n values of the Freundlich equations for DBP sorption. It was noted that the n values increased in coarse sand, fine sand, coarse silt, and fine silt fractions but decreased in clay and humic acid fractions, with increasing addition rates of biochar (Table 1). This was likely due to the variations in physicochemical properties among various soil particle-size fractions. As for clay and humic acid fractions, their activated sites relevant to high OM content (22.6~39.5 g/kg) and CEC (388~420 cmol/kg, Supplementary Data, Table S1) could interact with biochar components (Martin et al., 2012), and thus likely resulted in the increasing non-linear DBP sorption and decreasing n values, with an increase of biochar addition rate. On the contrary, the other soil particle-size fractions with low OM contents (5.1~29.4 g/kg) and CECs (26.2~251 cmol/kg, Supplementary Data, Table S1) had weak interaction with biochar components. Accordingly, with the increasing addition rates of biochar, the biochar components had increased effective sties in the soil particle-size fractions for DBP sorption, and thus likely led to the reducing non-linear DBP sorption and increased n values. The Freundlich coefficients Kf associated with sorption capacity ranged from 0.01 to 0.61 [(mg/g)/(mg/L)n] for DBP sorption to the soil particle-size fractions with and without biochar (Table1). It was obvious that the Kf values increased with increasing addition rates of biochar no matter which soil particle-size fractions were used (Table1). Specifically, the Kf values for DBP sorption with 1% and 5% of biochar were higher by 1.5~14.0 and 6.9~29 times respectively than those of the control without biochar. Similar results were also found in the case of diethyl phthalate (DEP) sorption to bamboo biochar-amended soils and oxyfluorfen sorption to rice hull biochar-amended soils when addition rates of biochar increased (Zhang et al., 2016; Wu et al., 14

2019). These results confirmed that addition of biochar was highly effective in enhancing the sorption affinity of organic contaminants to soils and the effectiveness increased with the increasing addition rates of biochar. This is because higher addition rates of biochar could provide increased active sorption sites in soil particle-size fractions for sorption of organic contaminants via surface sorption, pore filling, hydrophobic interaction, π-π coordination, and H-bonding (Martin et al., 2012; Chen et al., 2019ab). It was noted that the increase in Kf values after addition of biochar was more distinct in soil coarse fractions (e.g., coarse and fine sand fractions) by factor of 4.9~29, compared to the soil fine fractions (e.g., fine silt and clay fractions) and humic acid fractions by factor of 1.5~10.8 (Supplementary Data, Table S4). This was likely attributed to higher OM contents (22.6~39.5 g/kg) of the soil fine fractions and humic acid fractions (Supplementary Data, Table S1) that could easily interact with biochar components and cover the active sorption sites (Martin et al., 2012; Chen et al., 2019a), and thus led to decreased effectiveness in increasing the sorption affinity of DBP after biochar addition. Previous studies also found that addition of biochar was more effective for increasing sorption affinity of DEP and oxyfluorfen in low organic carbon soils than high organic carbon soils by a factor up to 10 (Zhang et al., 2016; Wu et al., 2019). 3.3. Effects of biochar addition on DBP sorption capacity to different soil particle-size fractions. To evaluate the effects of biochar addition on DBP sorption capacities of various soil particle-size fractions, the single-point partition coefficient (Kd) were calculated following the Eq. 4, based on the series of given DBP Ce and fitting results of the Freundlich model. In the present study, five ce values, i.e., 2, 4, 6, 8, 12 mg/L corresponding to the sorption/desorption experiments were applied. As shown in Fig. 3, Kd values in the biochar-amended soil particle-size fractions ranged from 32~1902 mL/g, which were higher by 1.2~132 than those of the soil particle-size fractions without biochar (Supplementary Data, Table S5). This indicated that the addition of biochar significantly increased the capacity of DBP sorption to all the soil particle-size fractions. Similarly, Zhang et al. (2016) found that addition of 0.5% biochar enhanced DEP sorption to soils containing different OM contents by 3-98 times. The significant increase in DBP sorption to the soil 15

particle-size fractions after biochar addition could be attributed to two main aspects. On the one hand, the prepared biochar had much higher sorption capacity of DBP than all the soil particle-size fractions, and thus directly improved DBP sorption after its addition. As shown in Supplementary Data, Fig. S2, the biochar also showed a fitting to the Freundlich isotherm model for DBP sorption, and the Kf values (2.2 (mg/g)/(mg/L)n) were higher by 44~220 times than those of the soil particle-size fractions [0.01~0.05 (mg/g)/(mg/L)n, Table 1]. On the other hand, addition of the biochar could reduce the bulk density of the soil particle-size fractions (Verheijen et al., 2019) and improve their specific surface area and active sorption sites relevant to OM content and functional groups. For example, addition of the biochar improved the OM contents of the soil particle-size fractions by a factor of 1.2~7.5 (Supplementary Data, Fig. S3). As for each soil particle-size fraction, addition of biochar proved more effective in increasing the Kd values in coarse and fine sand fractions, compared with the other soil particle-size fractions (Supplementary Data, Table S5). This was generally consistent with the pattern of Kf values for DBP sorption to the soil particle-size fractions with and without biochar addition (Supplementary Data, Table S4). The variations in the biochar effectiveness of biochar addition among different soil particle-size fractions were attributed to their various physicochemical properties (Singh and Kookana, 2009; Martin et al., 2012). Zhang et al. (2016) found that biochar was more effective to promote the sorption of DEP to soils with low organic carbon content than soils with high one. Anasonye et al. (2018) observed more increase in PAH sorption to sandy loam soil (OM content 1.4%) than a sandy soil after addition of biochar. In the present study, compared to coarse and fine sand fractions, the other soil particle-size fractions had much higher OM contents and CEC by a factor of 1.2~7.7 and 3.8~16.0, respectively. Their organo-mineral components relevant to the high OM content and CEC could interact with or block the active sorption sites of the biochar, and thus reduce the effectiveness of DBP sorption after addition of biochar (Singh and Kookana, 2009; Martin et al., 2012). A previous study reported that removal of minerals and organic matter in soils using 2% HF and UV treatment caused a remarkable enhancement in sorption of carbaryl and 16

ethion in the biochar-amended soils (Singh and Kookana, 2009). In addition, more increase in Kd values were generally observed due to the addition of 5% biochar (by a factor of 1.2~85.3) than 1% biochar (by a factor of 5.6~132) for all the soil particle-size fractions (Supplementary Data, Table S5). This was because an increased biochar addition could provide more active sites for DBP sorption. As reported by Zhang et al. (2016), promotion of DEP sorption to soils by biochar increased with an increasing biochar addition rate. It was noted that the Kd values generally increased first and then decreased with increasing ce values of DBP for all the soil particle-size fractions amended with biochar (Fig. 3). This was likely associated with the convex DBP sorption isotherms for the biochar-amended soil particle-size fractions due to the n values below 1, causing a decrease of DBP sorption capacity with increasing ce values (Table 1). The organic carbon content-normalised sorption coefficients (logKoc) corresponding to the Kd values were calculated based on the Eq. 5. As displayed in Table S6 of Supplementary Data,, the logKoc values of the biochar-amended soil particle-size fractions ranged from 3.30 to 4.99, which were obviously higher than those of the ones without biochar addition (2.84 to 3.89). Furthermore, the promotion of logKoc values generally enhanced with increasing addition rates of biochar for all the soil particle-size fractions, which corresponded to the variations in OM contents after the biochar addition (Supplementary Data, Table S6 and Fig. S3). These results suggested that the addition of biochar not only increased the OM contents but also improved their types and characteristics. The extremely low H/C (0.36) but high C/N (38.2) indicated that the prepared biochar was rich in carbon with aromatic structure (Fig. 1) (Schmidt and Noack, 2000; Jin et al., 2014 and 2018). This was further confirmed by the characteristic absorption bands at 1500~1700 cm-1 observed in the FTIR spectrum of the biochar (Fig. 1). The high contents of aromatic carbon were favorable for DBP sorption via π-π coordination (Xiang et al., 2019; Chen et al., 2019b; Ghaffar et al., 2015), thus promoting the logKoc values for the soil particle-size fractions after the biochar addition. Following the Test Guidelines of the Environmental Safety Assessment for Chemical Pesticides-Part 4 (Adsorption/Desorption in Soils) in China (Standardization 17

Administration of China, 2014), there are five types of organic pollutants depending on their logKoc values, namely, easy sorption of compounds (logKoc > 4.30), sub-easy sorption of compounds (3.70 < logKoc ≤ 4.30), moderate sorption of compounds (3.00 < logKoc ≤ 3.70), sub-difficult sorption of compounds (2.30 < logKoc ≤ 3.00), and difficult sorption of compounds (logKoc < 2.30) (Xiang et al., 2018c). Accordingly, addition of the biochar generally made the DBP sorption from moderate or sub-difficult sorption of compounds to sub-easy or easy sorption of compounds in all the soil particle-size fractions (Supplementary Data, Table S6). This confirmed that the biochar was effective in immobilizing DBP in all the soil particle-size fractions. The standard Gibbs free energy (△G0) was calculated to evaluate the effects of biochar on thermodynamic properties of DBP sorption to the soil particle-size fractions based on Eq. 5 and 6. All the △G0 values were negative under the presence or absence of the biochar, indicating DBP sorption was a spontaneous process (Table 1). In general, △G0 values above -20 kJ indicate a physical sorption (Xiang et al., 2016 and 2018c). Accordingly, DBP sorption to all the soil particle-size fractions without biochar was a physical process, since the △G0 values ranged from -18.4 to -15.2 kJ (Table 1). After addition of the biochar, DBP sorption to the soil particle-size fractions was dominated by a physical process associated with chemical sorption, because the △G0 values were in the range of -20.3 to -23.0 kJ, lower than -20 kJ (Xiang et al., 2019). This suggested that the addition of the biochar could enhance the sorption energy of DBP in the soil particle-size fractions, which could address why biochar improved the sorption affinity of DBP to the soil particle-size fractions and enabled increasing Kf values (Table 1). 3.4. Effects of the biochar on DBP desorption from different soil particle-size fractions Desorption or release behaviour of organic contaminants adsorbed by soil and biochar could influence the mobility and bioavailability of the contaminants (Liu et al., 2018). To evaluate the effects of the biochar on DBP desorption from the soil particle-size fractions, desorption experiments were conducted. As illustrated in Table 2, the desorption rates of DBP ranged from 44.2% to 86.9 % for different soil particle-size fractions without biochar. Lower desorption rates 18

were observed in the humic acid fraction (44.2~75.2%) compared to the other soil particle-size fractions (> 68%), which was attributed to higher OM content and CEC of humic acid fraction, thus leading to its stronger retention of DBP (Xiang et al., 2018a). Addition of the biochar significantly decreased the DBP desorption, with the desorption rates generally below 55% (Table 2). To be specific, after addition of 1% biochar, the desorbed amounts of DBP for all the soil particle-size fractions (except humic acid) at 2 mg/L of DBP initial concentration were below the detection limit (10 μg/L). Similar results were observed for coarse silt and fine silt at 4 mg/L of DBP initial concentration, indicating extremely low DBP desorption in such conditions. In the other situations, addition of 1% biochar decreased the DBP desorption rates by 10~50%, compared to the control without biochar (Table 2). When addition rate of the biochar was up to 5%, hardly any DBP was desorbed from all the soil particle-size fractions, except low DBP desorption (13.7~19.4%) was observed for the coarse sand and fine sand fractions at 12 mg/L of DBP initial concentration and for humic acid fractions at 8 and 12 mg/L of DBP initial concentrations. These results demonstrated that the biochar was highly effective in the retention of DBP in the soil particle-size fractions, especially with high addition rate (5%). Similar results were reported in other studies. For example, addition of 3% biochar reduced desorption rates of soil atrazine to 13.7~19.4% despite of three successive desorption cycles conducted (Ren et al., 2018a). Addition of biochar decreased desorption of 17α-ethinylestradiol in both sandy loam and clay soils by about 30~45% (Wei et al., 2019). Biochar addition also increased the retention of DEHP in soils by about 25~50% (He et al., 2016). Addition of 1% wheat straw biochar in soil decreased the desorption rate of herbicide 4-chloro-2-methylphenoxy acetic acid from 64.2% to 55.1% (Tatarková et al., 2013). In addition, DBP desorption from biochar-amended soil was influenced by the physicochemical properties of the soil particle-size fractions. Fewer decrease in DBP desorption rates (1.5~20.9%) were observed for humic acid fraction than for the other soil particle-size fractions (19.2~47.1%) after the addition of 1% of biochar. This was likely because the humic acid fraction had higher OM 19

contents and CEC relevant to soil minerals (Supplementary Data, Table S1), which could interact with the components of biochar and thus lower the effectiveness of biochar on DBP sorption (Singh and Kookana, 2009; He et al., 2016). Qin et al (2018) found that the addition of pig-derived biochar significantly decreased the leaching loss for DBP up to 88% from the soil with low organic carbon, whereas no significant impact was observed in the soil with high organic carbon. He et al. (2016) found that physicochemical properties of soils, especially the organic carbon/matter contents, were the key factors influencing the desorption and bioavailability of DEHP in soils. 3.5. Effects of physicochemical properties of soil particle-size fractions on biochar effectiveness As mentioned above, the physicochemical properties of the soil particle-size fractions had profound influences on the biochar’s effectiveness in regulating sorption and desorption of DBP. To better understand the influences, a parameter (rkd) reflecting the biochar effectiveness was defined, which was the ratio of Kd values between the soil particle-size fractions with and without biochar addition at given ce values of DBP (2~12 mg/L). Furthermore, Pearson correlation analysis and canonical correspondence analysis (CCA) between the rkd values and the physicochemical characteristics of the soil particle-size fractions were conducted, respectively. Generally, an arrow in a CCA figure indicates a variable; the length of the arrow represents effect intensity between this variable and other variables; while the angle between different variables presents their correlation type; either positive correlation (acute angle), negative correlation (obtuse angle), or no correlation (right angle) (Huang et al., 2019). Fig. 4 showed that all the physicochemical properties of the soil particle-size fractions except pH value had obvious correlation with the rkd values. Among these properties, OM contents and pore size could be the most effective on the rkd values, since they had longer arrows and the angle relations between their arrows and the arrows of the rkd values were consistent no matter which ce value was given or biochar addition rate was used (Fig. 4). Generally, the rkd values showed negative and positive correlations with OM content and pore size, respectively, because there were acute angle relations and negative Pearson coefficients between the rkd values and OM contents, but 20

the obtuse angle relations and positive Pearson coefficients between the rkd values and pore sizes (Fig. 4, Supplementary Data, Table S7). These indicated lower OM content and higher pore size were beneficial to the biochar effectiveness for DBP sorption in the soil particle-size fractions. This was likely attributed to that lower OM content interact less with the activated sites of the biochar and ensure the biochar function for DBP sorption and retention (Singh and Kookana, 2009; Martin et al., 2012). In term of pore size, great pore size of the soil particle-size fractions was likely conducive to the distribution of the biochar, giving full play to their talent for DBP sorption and retention. Accordingly, more effectiveness of the biochar for sorption and retention of DBP was observed in the coarser soil particle-size fractions with lower OM contents and higher pore size (e.g. coarse sand and fine sand), compared with the other soil particle-size fractions. In addition, there were generally positive correlations between pore volumes and the rkd values (Fig. 4, Supplementary Data, Table S7), likely owing to the positive relations between pore volumes and pore sizes (Xiang et al., 2019). As for the other physicochemical properties, their correlations with the rkd values depended on the given ce values and addition rates of the biochar (Fig. 4, Supplementary Data, Table S7). To be specific, under addition of 1% biochar, the rkd values showed positive correlations with specific surface areas at given ce values of 4 and 8 mg/L, and with mean fraction size at given ce values of 2 and 12 mg/L. Similarly, under addition of 5% biochar, the rkd values positively correlated with both specific surface areas and pore volume at given ce values of 6 and 12 mg/L, and with mean fraction size at given ce values of 2 and 4 mg/L. In the other situations, rkd values generally showed negative correlations with specific surface areas, CECs, and mean fraction size. These results suggested the complexity of interactions between components of the biochar and the soil particle-size fractions. More systematic studies should be carried out in the future to reveal the mechanisms controlling the interaction between biochar and soil properties. 4. Conclusion In the present study, a rice straw-derived biochar was prepared that had high specific surface 21

area and organic matter (OM) content with aromatic carbon. The prepared biochar was highly effective in improving the sorption and retention of DBP in all the paddy soil size-particle fractions at environmentally relevant concentrations of DBP. Furthermore, effectiveness of the biochar in retaining DBP increased with increasing biochar addition rates but varied among the soil particle-size fractions, which was remarkably influenced by the physicochemical properties of the soil particle-size fractions. Stronger retention of DBP by the biochar was found in the soil particle-size fractions with lower OM contents and higher pore size than the other soil fractions with higher OM contents and lower pore size. Results of this study gave an insight into the effects of biochar on sorption and desorption of DBP in the soil particle-size factions with different physicochemical properties. Meanwhile, this study also provided an effective approach to immobilize DBP and reduce its bioavailability in the soils, especially in those soils with coarse particle-size fractions and low OM contents. Obviously, more systematic studies need to be conducted to reveal the mechanisms underlying the interaction between components of the biochar and the soil particle-size fractions in the presence of emerging organic contaminants such as DBP.

Acknowledgments This work was funded by the National Natural Science Foundation of China (Nos. 41573087, 41773108, 41807339), the NSFC-Guangdong Joint Fund (U1501233), the Research Team Project of the Natural Science Foundation of Guangdong Province (2016A030312009, 2017A030311019), the Program of the Guangdong Science and Technology Department (2016B020242005, 2015B020235008), and Special Fund of Guangdong Province for Science, Technology, and Innovation (2018A030310629).

Appendix A. Supplementary data Supplementary data to this article can be found online at https: .

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Figure caption: Fig. 1. Physicochemical characteristics of the prepared rice straw biochar: (a) SEM image, (b) FTIR spectrum, (c) pore diameter distribution, (d) elemental content (%), and (e) contents (%) of ash and organic matter. SSA, PV, PS indicate specific surface area (m2/g), pore volume (cm3/g), and average pore size (nm), respectively. Fig. 2. Sorption isotherms of DBP to paddy soil size-particle fractions with and without rice straw biochar addition: (a) Relationships between equilibrium concentrations of DBP in supernatants and in the soil particle-size fractions, and (b) linear Freundlich sorption isotherms. “1%” and “5%” indicate the soil particle-size fractions with 1% and 5% of biochar addition rates (m/m), respectively, while CK indicates the soil particle-size fractions without biochar. Fig. 3. Effects of biochar addition on Kd values of DBP sorption to soil particle-size fractions at given Ce values of DBP (2~12 mg/L) based on Freundlich sorption isotherms. “1%” and “5%” indicate the soil particle-size fractions with 1% and 5% of biochar (m/m), respectively, while CK indicates the soil particle-size fractions without biochar. Fig. 4. Canonical correspondence analysis (CCA) between rkd values and the physicochemical characteristics of the soil particle-size fractions with the addition of 1% biochar (a) and 5% biochar (b). The rkd2, rkd4, rkd6, rkd8, and rkd12 indicate the ratios of Kd values in the soil particle-size fractions with and without biochar addition at given DBP ce values of 2, 4, 6, 8, and 12 mg/L, respectively.

28

Fig. 1. Physicochemical properties of the prepared rice straw biochar: (a) SEM image, (b) FTIR spectrum, (c) pore diameter distribution, (d) elemental content (%), and (e) contents (%) of ash and organic matter. The specific surface area, pore volume, average pore size, and pH were detected as 116. 1 m2/g, 0.01 cm3/g, 7.09 nm, and 11.25, respectively.

29

(a)

(b)

Fig. 2. Sorption isotherms of DBP to paddy soil size-particle fractions with and without addition of rice straw biochar: (a) Relationships between equilibrium concentrations of DBP in supernatants and in the soil particle-size fractions, and (b) Linear Freundlich sorption isotherms. “1%” and “5%” indicate the soil particle-size fractions with 1% and 5% of biochar addition rates (m/m), respectively, while CK indicates the soil particle-size fractions without biochar. 30

c

b b

Kd (mL/g)

c

400

c c b

b

100 50

a

a

0

2

a

4

1600

6

ce (mg/L)

c b b a

0

12

a

2

4

6

a

8

12

Fine silt

c

c

1200 c

b b c

400 200 100

c

b b

50

a

0

2

b

a

4

a

a

6

8

Kd (mL/g)

900

a

c c

600 300

b

100

b

a

0

12

b

b

200

a

a

2

4

2500

a

6

ce (mg/L)

a

a

8

b

12

ce (mg/L)

c

Clay

c

b

800

Humic acid

b

2000 600

1500

c

1000

c c

500

c

b

200

Kd (mL/g)

Kd (mL/g)

b

a

1500

Coarse silt

c

0

a

a

ce (mg/L)

c

800

100

c

b

100

1200

Kd (mL/g)

c

400 200

a

8

Fine sand

c

800

b

a a

c

1200

CK 1% 5%

1200 800

1600

Coarse sand

Kd (mL/g)

1600

400

100 b

b a

a

2

a

a

4

6

b a

8

b

a

b a a

a

a

50

b

b

200

0

12

2

4

a

6

a

8

a

a

12

ce (mg/L)

ce (mg/L)

Fig. 3. Effects of biochar addition on Kd values of DBP sorption to soil particle-size fractions at given

Ce

values of DBP (2~12 mg/L) based on Freundlich sorption isotherm. “1%” and “5%”

indicate the soil particle-size fractions with 1% and 5% of biochar (m/m), respectively, while CK indicates the soil particle-size fractions without biochar.

31

Fig. 4. Canonical correspondence analysis (CCA) between rkd values and the physicochemical properties (Supplementary Data, Table S1) of the soil particle-size fractions with the addition of 1% and 5% biochar. The rkd2, rkd4, rkd6, rkd8, and rkd12 indicate the ratios of Kd values in the soil particle-size fractions with and without biochar addition at given DBP ce values of 2, 4, 6, 8, and 12 mg/L, respectively. BET, PV, PS, and FS indicate specific surface area, pore volume, pore size, and mean fraction size, respectively.

32

Table 1. Isotherm parameters for DBP sorption to rice straw biochar-amended soil particle-size fractions. Soil fractions

Biochar (%)

Coarse sand

0

Langmuir equation KL Qm R2 a -0.13(0.03) -0.07(0.05) 0.83(0.05)

1

7.44(0.87)

0.18(0.03)

0.70(0.01)

0.23(0.01)

0.14(0.02)

0.73(0.01)

-23.0(0.4)

5

2.30(0.68)

0.49(0.01)

0.69(0.14)

0.39(0.06)

0.29(0.03)

0.76(0.09)

-22.1(0.3)

0

0.13(0.08)

0.42(0.43)

0.89(0.05)

1.66(1.19)

0.02(0.02)

0.86(0.04)

-15.2(5.9)

1

0.91(0.06)

0.23(0.05)

0.91(0.04)

0.47(0.01)

0.10(0.02)

0.93(0.05)

-21.2(0.5)

5

1.91(1.10)

0.65(0.17)

0.65(0.12)

0.42(0.09)

0.31(0.05)

0.72(0.07)

-21.9(0.4)

0

0.39(0.10)

0.16(0.05)

0.77(0.06)

0.69(0.13)

0.04(0.02)

0.82(0.08)

-17.6(1.4)

1

10.1(3.24)

0.18(0.04)

0.60(0.25)

0.23(0.01)

0.14(0.02)

0.74(0.17)

-20.4(0.5)

5

1.26(0.02)

0.56(0.07)

0.68(0.01)

0.49(0.02)

0.34(0.04)

0.81(0.00)

-21.4(0.3)

0

0.19(0.34)

0.18(0.13)

0.84(0.11)

1.16(0.76)

0.05(0.03)

0.89(0.06)

-18.4(3.5)

1

0.56(0.30)

0.43(0.07)

0.87(0.02)

0.45(0.10)

0.13(0.04)

0.76(0.05)

-21.6(0.8)

5

-0.19(1.09)

-1.54(3.03)

0.71(0.15)

0.84(0.22)

0.61(0.02)

0.78(0.13)

-23.5(0.1)

0

0.20(0.13)

0.14(0.018)

0.74(0.06)

0.66(0.21)

0.04(0.02)

0.78(0.09)

-17.8(1.7)

1

0.70(0.26)

0.30(0.01)

0.93(0.07)

0.49(0.05)

0.11(0.03)

0.91(0.03)

-20.3(0.6)

5

6.40(0.16)

0.46(0.04)

0.67(0.10)

0.28(0.02)

0.39(0.03)

0.77(0.08)

-21.8(0.2)

0

0.04(0.17)

0.60(0.64)

0.85(0.03)

0.99(0.37)

0.05(0.03)

0.87(0.05)

-17.2(1.7)

1

0.12(0.15)

0.21(0.50)

0.82(0.01)

0.76(0.16)

0.08(0.03)

0.83(0.01)

-18.3(0.9)

5

0.74(0.20)

1.02(0.11)

0.19(0.02)

0.57(0.03)

0.35(0.03)

0.86(0.01)

　 -21.1(0.2)

Fine sand

Coarse silt

Fine silt

Clay

Humic acid

a Values

in parentheses are the standard deviation (n = 3).

33

　

Freundlich equation n Kf R2 1.39(0.19) 0.01(0.01) 0.88(0.07)

　 　

△G -18.4(1.7)

Table 2. Parameters of DBP desorption from biochar-amended soil particle-size fractions Soil fractions Coarse sand

cd (mg/g) b 0.043(0.005)

rd (%) c

Biochar addition 　 1% 　 cd (mg/g) rd (%)

　 5% 　 cd (mg/g)

rd (%)

-d

-

-

-

4

0.056(0.006)

76.8(11.6)

0.053(0.007)

41.1(5.1)

-

-

6

0.072(0.010)

78.6(11.4)

0.071(0.011)

36.8(5.7)

-

-

8

0.091(0.014)

71.9(10.9)

0.087(0.011)

51.3(6.4)

-

-

12

0.143(0.021)

74.4(11.0)

0.109(0.016)

28.8(4.3)

0.071(0.011)

16.7(2.7)

2

0.043(0.007)

68.1(10.5)

-

-

-

-

4

0.057(0.008)

68.4(10.1)

0.050(0.007)

46.5(6.8)

-

-

6

0.063(0.010)

77.9(11.8)

0.055(0.008)

49.8(7.5)

-

-

8

0.117(0.018)

72.3(11.1)

0.087(0.012)

53.1(7.5)

-

-

12

0.142(0.021)

72.9(11.0)

0.110(0.017)

38.9(5.9)

0.065(0.007)

15.4(2.4)

2

0.049(0.007)

79.6(11.3)

-

-

-

-

4

0.060(0.009)

71.0(10.7)

-

-

-

-

6

0.079(0.012)

74.0(11.0)

0.067(0.010)

44.8(6.5)

-

-

8

0.102(0.015)

79.5(12.0)

0.088(0.013)

45.1(6.8)

-

-

12

0.144(0.021)

72.1(10.5)

0.110(0.016)

40.6(6.0)

-

-

2

0.046(0.007)

84.1(12.7)

-

-

-

-

4

0.061(0.009)

85.1(12.6)

-

-

-

-

6

0.090(0.014)

68.1(10.6)

0.068(0.010)

32.3(4.9)

-

-

8

0.108(0.017)

71.7(11.6)

0.069(0.011)

35.6(5.4)

-

-

12

0.155(0.024)

70.0(10.7)

0.114(0.017)

38.4(5.8)

-

-

2

0.042(0.006)

78.1(11.3)

-

-

-

-

4

0.058(0.009)

78.1(11.8)

0.052(0.008)

41.0(6.2)

-

-

6

0.065(0.010)

77.5(11.5)

0.057(0.008)

33.9(5.0)

-

-

8

0.106(0.016)

86.9(13.4)

0.078(0.012)

39.8(6.0)

-

-

12

0.138(0.020)

77.1(11.3)

0.114(0.017)

42.6(6.2)

-

-

2

0.046(0.007)

67.1(10.1)

0.042(0.006)

46.2(7.0)

-

-

4

0.071(0.011)

66.8(10.2)

0.058(0.009)

49.4(7.3)

-

-

6

0.088(0.013)

61.4(9.3)

0.082(0.013)

43.6(6.8)

-

-

8

0.112(0.019)

75.2(12.6)

0.102(0.017)

66.8(10.8)

0.065(0.010)

19.4(2.9)

12 0.129(0.018) 44.2(6.1) 　 0.118(0.018) 42.7(6.5) 　 0.069(0.011) ci (mg/L) indicates the initial DBP concentration in the solution used for sorption experiment.

13.7(2.1)

Coarse silt

Fine silt

Clay

Humic acid

b

0 69.7(10.3)

Fine sand

a

ci (mg/L) a 2

cd (mg/g) indicates the DBP amount desorbed from the soil particle-size fractions after a sorption and a successive desorption

experiment. c

rd (%) indicates desorption rates of DBP from the soil particle-size fractions after a sorption and a successive desorption experiment.

d

“-” indicates that the DBP concentration in the supernatant was below the detection limit (10 μg/L).

e Values

in parentheses are the standard deviation (n = 3).

34

TOC graphic

35

Highlights (1) A straw-derived biochar with high OM content and great SSA was prepared. (2) Biochar markedly promoted sorption and retention of DBP in six soil particle-size fractions. (3) Biochar effectiveness was affected by the physicochemical characteristics of soil fractions. (4) OM contents and pore size had the most effective on the biochar effectiveness.

36

Note: All the authors declare no conflict of interest.

37