1 Supporting Information 2 3 4 Kaolinite Enhances the Stability of the Dissolvable and Undissolvable Fractions of Biochar via Different Mechanisms 5 6 7 Fan Yang a, b, Zibo Xu a, Lu Yu a, Bin Gao c, a, Xiaoyun Xu a, Ling Zhao a, Xinde Cao a, d, * a School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 8 b School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 9 10 200093, China c Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA 11 d Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China 12 13 14 * Corresponding author tel: +86 21 54743926, fax: +86 21 54740825, e-mail: xdcao@sjtu.edu.cn 15 16 17 Supporting Information consists of 15 pages, including 1 equation, 3 tables and 6 figures. 18 S1
19 20 21 22 23 24 25 26 27 28 29 30 31 Biochar Production and Characterization The walnut shell material was air-dried, ground to less than 2 mm and placed into a lab-scale stainless-steel pyrolysis reactor. The reactor was charged with N 2 for about 10 min at a flow rate of 1.5 L min -1 to create an O 2 -limited condition. Then the reactor was put into a Muffle Furnace and heated to 500 C with a heating rate of 20 C min -1 then held at the highest temperature for 2 h. The ph value of the biochar was measured with the solid to water ratio of 1:20 (w/v) (EUTECH ph 510, USA). The proximate analysis of the biochar, including the contents of the moisture, volatile, fixed carbon and ash, was determined using the standard method (ASTM D3172-13). The concentrations of C, H, and N in the biochar were determined using an elemental analyzer (Vario ELIII, Elementar, Germany). The specific surface area and pore size distribution of the biochar were determined using a BET-N 2 SA analyzer (JW-BK222, Jwgb, China). The inorganic element concentrations in the biochar were measured using inductively coupled plasma (ICP-AES, ICAP 6000 Radial, Thermo, English) following biochar digestion using the USEPA method 3050B. 32 S2
33 34 35 36 37 38 39 40 41 42 Experiment of Ca 2+ Induced Flocculation Multivalent cations such as Ca 2+, may promote intermolecular linkages within organic molecules, leading to flocculation of dissolvable biochar. 1 Therefore, a flocculation experiment of dissolvable biochar was conducted in 0.01 M CaCl 2 solution without kaolinite. The concentration of dissolvable biochar in the flocculation experiment was selected as 20 mg C L -1, where the contribution of Ca 2+ accounted for a large proportion (65.1%, Figure 2), meanwhile, the three mechanisms (Ca 2+ bridging, ligand exchange, and van der Waals attraction ) coexisted (10-100 mg C L -1, Figure 2). The ph was adjusted to 4, coincident with the sorption experiment, and ph 7 was also included as a comparison to explore the effect of ph on the Ca 2+ induced flocculation of dissolvable biochar. All the operating conditions of the flocculation experiment were the same as the sorption experiment, except for the absence of kaolinite. 43 S3
44 45 46 47 48 49 50 51 52 XPS Analysis for the Association of Undissolvable Biochar and Kaolinite Since the electrons carrying chemical data in the X-ray photoelectron spectroscopy (XPS) (AXIS UltraDLD, Japan) were emitted from the outer ~10 nm of the specimen surface, XPS was used to analyze the association of kaolinite onto undissolvable biochar surface. The XPS was equipped with a monochromatic Al Kα X-ray source (1486.6 ev) and was operated at a base pressure of 8 10-8 Pa with pass energy of 23.5 ev. The X-rays were focused onto a spot size of 10 μm 100 μm on each sample. All survey and high resolution scans were taken with the charge neutralizer operating at 30 V to avoid charging caused by any insulating components of the samples. The calibration of the spectra binding energy was performed with the C1s peak of the aliphatic carbons at 284.8 ev. 53 54 55 56 S4
57 58 59 60 61 62 63 64 65 TGA and Activation Energy Calculation The association energy of the undissolvable biochar with kaolinite was determined by a thermogravimetric analysis (TGA). In the TGA analysis, the kaolinite-associated undissolvable biochar particles were heated from 25 C to 425 C under O 2 atmosphere at a rate of 20 C min -1. The TGA curve could be divided into three stages by two temperatures; one was the minimum mass loss temperature (approximately 175 C), and the other was the starting temperature of quick decomposition (approximately 300 C). These three stages were the water loss stage, O 2 chemisorption stage and quick decomposition stage (Figure 4). The activation energy of the undissolvable biochar was obtained from a first-order kinetics analysis of the O 2 chemisorption stage using Arrhenius equation: 2, 3 66 67 68 69 70 71 (S1) where α is the sample conversion ratio, A is the frequency factor, E is the activation energy, β is the heating rate and R is the gas constant. A linear regression was fit to the data points on the graph of ln[-ln(1-α)] versus 1/T based on the TGA curve. The line slope was used to calculate the activation energy E. It should be noted that the content of the attached minerals was subtracted from the TGA results in order to eliminate the disturbance of the minerals on the calculation. 72 73 S5
74 The Stability Measurement Experiments 75 For the K 2 Cr 2 O 7 treatment, 0.1 g C of undissolvable biochar or dissolvable biochar was treated in a glass 76 test tube with 40 ml of a 0.1 M K 2 Cr 2 O 7 /2 M H 2 SO 4 solution at 55 C for 60 h. 4 The results were expressed as 77 the fraction of total C oxidized by the K 2 Cr 2 O 7. 78 For the biodegradation experiments, different amounts of undissolvable biochar and dissolvable biochar 79 were added into a 200-mL glass vial. The dissolvable biochar containing 0.05 g C was selected to ensure 80 adequate CO 2 emission for measurement. The undissolvable biochar containing 0.5 g C was added to get 81 adequate number of solid biochar particles distributing evenly in the aqueous nutrient solution. Then, a 20 ml 82 aqueous nutrient solution [60 g L -1 (NH 4 ) 2 SO 4 + 6 g L -1 KH 2 PO 4 ] was prepared. 4 A total of 0.4 ml of 83 microbial inoculate, the supernatant of paddy soil after 24 h of shaking in deionized water at a ratio of 1:5 84 (w/v) and subsequent centrifugation, was added to the system. The vials were incubated at 25 C. Before the 85 headspace CO 2 was determined, the air was replaced by simulated air without CO 2 (pure O 2 and N 2 ). After 86 incubation for 24 h, the headspace CO 2 content was measured. The incubation lasted for 56 days, and the CO 2 87 was determined intermittently using a gas chromatograph (GC-14B, Shimadzu, Japan). 88 89 90 91 92 93 94 95 96 97 98 Reference 1. Philippe, A.; Schaumann, G. E. Interactions of dissolved organic matter with natural and engineered inorganic colloids: a review. Environ. Sci. Technol. 2014, 48 (16), 8946-62. 2. Xie, Z. Q.; Ma, X. Q. The thermal behaviour of the co-combustion between paper sludge and rice straw. Bioresour. Technol. 2013, 146, 611-618. 3. Yin, K.; Zhou, Y. M.; Yao, Q. Z.; Fang, C.; Zhang, Z. W. Thermogravimetric analysis of the catalytic effect of metallic compounds on the combustion behaviors of coals. React. Kinet., Mech. Catal. 2012, 106 (2), 369-377. 4. Yu, S.; Liu, J.; Yin, Y.; Shen, M. Interactions between engineered nanoparticles and dissolved organic matter: A review on mechanisms and environmental effects. J. Environ. Sci. 2018, 63 (1), 198-217. 99 S6
Table S1. Selected properties of the biochar Item Unit Value Basic properties Yield % 30.5 ph - 8.22 SA m 2 g -1 24.0 PV cm 3 g -1 0.321 Moisture % 2.70 Volatiles % 16.8 Fixed Carbon % 78.2 Ash % 2.30 DBC mg C g -1 10.3 Organic elements C g kg -1 880 N g kg -1 3.40 H g kg -1 31.9 O g kg -1 84.7 Inorganic elements K g kg -1 5.60 Ca g kg -1 6.06 Si g kg -1 4.43 Na g kg -1 0.745 Mg g kg -1 0.665 Al g kg -1 0.328 Fe g kg -1 0.114 SA, BET-N 2 surface area; PV, pore volume; DBC, dissolvable biochar S7
Table S2. Main components of dissolvable biochar determined by GC-MS (the rest of the components with a relative area percentage less than 1% were not listed). Categories Component Names Molecular Formula Area (%) Total - - 66.7 Alcohols 1-Ethoxy-2-propanol C 2 H 5 OCH 2 CH(CH 3 )OH 37.0 Esters Acetic acid, 1-methylpropyl ester C 2 H 5 CH(CH 3 )OC(=O)CH 3 6.1 Acetic acid, 2-methylpropyl ester CH 3 CH(CH 3 )CH 2 OC(=O)CH 3 2.0 Acetic acid, butyl ester C 4 H 9 OC(=O)CH 3 3.2 1, 2-Benzenedicarboxylic acid, bis (2-methylpropyl) C 6 H 4 (COOCH 2 CH(CH 3 ) 2 ) 2 5.3 ester 1,2-Benzenedicarboxylic acid, dibutyl ester C 6 H 4 (COOC 4 H 9 ) 2 2.5 1,2-Benzenedicarboxylic acid, bis (6-methylheptyl) ester C 6 H 4 (COO(CH 2 ) 5 CH(CH 3 ) 2 ) 2 3.5 Ketones 3-Methyl-2-pentanone C 2 H 5 CH(CH 3 )C(=O)CH 3 4.0 Aldehydes 2-Ethylbutanal C 2 H 5 C(C 2 H 5 )C=O 3.1 S8
Table S3. The decreased C loss of different biochar fractions under chemical oxidation and biodegradation conditions (the undissolvable biochar and the dissolvable biochar in kaolinite+cacl 2 treatments were selected for these estimates). Proportion of Fractions (%) C Loss Rate Without kaolinite (%) C Loss Rate With kaolinite (%) C Loss Without kaolinite (%) C Loss With kaolinite (%) Decreased ratio of C Loss (%) Chemical Oxidation Total Biochar 100 / / 3.98 2.28 42.5 Undissolvable 98.8 2.83 1.50 2.80 1.48 33.1 Biochar Dissolvable Biochar 1.2 98.0 66.8 1.18 0.80 9.4 Biodegradation Total Biochar 100 / / 4.25 2.15 49.4 Undissolvable 98.8 4.19 2.12 4.14 2.09 48.2 Biochar Dissolvable Biochar 1.2 9.04 4.71 0.11 0.06 1.2 S9
Figure S1. Extent of flocculation of dissolvable biochar induced by CaCl 2 at ph 4 and 7 (CaCl 2 concentration = 0.01 M, initial dissolvable biochar concentration = 20 mg C L -1 ). S10
Figure S2. SEM elemental mapping of the kaolinite particle cross-section before (a) and after association with dissolvable biochar under the CaCl 2 background electrolyte (b) (scale bar = 1 μm). S11
Figure S3. Accumulated C mineralization of free dissolvable biochar and dissolvable biochar sorbed via three binding mechanisms (a), and undissolvable biochar and kaolinite-associated undissolvable biochar (b) during the 56-day incubation. Error bars represent the standard error of the mean (n = 3). S12
Figure S4. X-ray photoelectron spectroscopy (XPS) of the undissolvable biochar and the kaolinite-associated undissolvable biochar. S13
Figure S5. A first-order kinetics analysis was conducted to fit the data points based on the O 2 chemisorption stage of the TGA curves. Figure S5 shows the first-order kinetics fitting of the kaolinite-associated undissolvable biochar. The linear fits have high degrees of closeness (R 2 =0.9859-0.9945), and the line slopes (red numbers in the graphs) were used to calculate the activation energy. The line slope of the undissolvable biochar+kaolinite+cacl 2 treatment (1414.3) is higher than that of the control (1256.9) but lower than the that of undissolvable biochar+kaolinite treatment (1421.6), which indicates CaCl 2 has a weakening effect on kaolinite by strengthening the undissolvable biochar chemisorption of O 2. The calculations of the activation energy are summarized in Table 1. S14
Figure S6. The decreased ratio of C loss affected by kaolinite under chemical oxidation and biodegradation conditions. (The undissolvable biochar+kaolinite+cacl 2 treatment and the undissolvable biochar associated with kaolinite in CaCl 2 treatment were selected for this estimation). S15