Supporting Information

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1 1 Supporting Information Transport of Biochar Particles in Saturated Granular Media: Effects of Pyrolysis Temperature and Particle Size Dengjun Wang,, Wei Zhang, # Xiuzhen Hao, and Dongmei Zhou,, Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing , China # Department of Plant, Soil and Microbial Sciences; Environmental Science and Policy Program, Michigan State University, Michigan 48824, United States University of Chinese Academy of Sciences, Beijing , China 12 Table of Content Title Page S1. Biochar Characterization S2-S4 S2. Dynamic Light Scattering Measurements S4 S3. Interaction Energies between Biochar Particles and Sand Surfaces S4-S7 S4. Transport Model S7-S9 Table S1. Selected Characteristics of the Biochars S10 Table S2. Contents of Oxygen Surface Functional Groups of the Biochars S11 Table S3. Contact Angle Measurements, Surface Tension Components, S12 Hamaker Constant, and G AB h0 for the Biochar Particles Figure S1. UV/Vis Scanning Spectra of Biochar MP Suspensions S13 Figure S2. Calibration Curves of Biochar MP Suspensions S14 Figure S3. Acid-Base Titration Curves of the Biochars S15 Figure S4. Comparison of the AB, EDL, and LW Interaction Energies S16 Figure S5. DLS Sizes of Biochar MP and NP Suspensions S17 Reference S18-S20 Corresponding author: Professor Dongmei Zhou (Ph.D.), dmzhou@issas.ac.cn. S1

2 S1. Biochar Characterization The bulk density of the biochar samples was measured using a filling and tapping procedure. 1 Briefly, the biochar sample is loaded in discrete portions into a glass column with known volume. After filling of each portion, the cylinder was tapped onto a bench until no volume changed was observed. The final volume and sample weight were recorded. Triplicate measurements were done and the standard error was less than 1.0%. The ph of the biochars was measured in deionized (DI) water at a ratio of 1: 10 wt/wt. The specific surface area of the biochars was determined using the Brunauer-Emmett-Teller (BET) nitrogen adsorption technique (Micrometrics Accusorb 2100E, USA) at 77 K. Elemental C, N, and H contents of the biochars were measured by a Vario EL II Elemental Analyzer (Elementar, Germany). The extractable constituents of biochars were measured using the acid digestion method (HF: HClO 4 : HNO 3 = 1: 1: 1). 2 The content of K was determined by a FP640 Flame Photometry (Shanghai Yuefeng Instruments & Meters Co., Ltd., China), Ca, Mg, Fe, Zn, and Mn were measured by Atomic Absorption Spectrometry (Hitachi Z-2000, Japan), and the P content was determined using ascorbic acid-nh 4 -molybdate blue colorimetry at 700 nm. 2 The sizes of the biochar particles were determined using a Particle Size Analyzer (Malvern Mastersizer 2000, UK). In the acid-base titrations of wheat and pine-derived biochars, g of the biochar samples (150 µm sieve) was placed in 50 ml plastic bottles. Twenty milliliters of DI water was then added to each bottle, and each of the bottles was S2

3 stirred on a magnetic stirrer for 2 h at 25 C. Then, the samples were titrated with 0.10 M HCl at 25 C to the end point of ph 2.0 using an automatic titration instrument (TIM 854, Radiometer) with magnetic stirring. The titration rate was kept at 0.5 ml min L 1, with data collected every 6 s. The oxygen surface functional groups (carboxylic, lactonic, and phenolic groups) on biochars were measured by acidimetric titration method. 3 Briefly, g of the biochar samples was placed in 100 ml plastic bottles. Then, 50 ml of 0.05 M NaHCO 3, Na 2 CO 3, and NaOH was added to each bottle, and each of the bottles was shaken at the speed of 30 rpm for 24 h at 25 C. Afterwards, the samples were filtered through 0.45 µm filters and the filtrates were titrated with 0.20 M HCl at 25 C to the end point of ph 7.0 using the automatic titration instrument (TIM 854, Radiometer). The titration rate was kept at 1.0 ml min L 1. The contents of oxygen surface functional groups on the biochars were calculated on the basis of the assumption that NaHCO 3 neutralizes carboxylic groups (-COOH) only, Na 2 CO 3 neutralizes carboxylic and lactonic groups (-COH), and NaOH neutralizes all the acidic groups including phenolic groups (-OH). Static contact angles of the probe liquids (i.e., water, glycerol, and n-decane) on the surfaces of the wheat and pine-derived biochars were measured by the sessile drop method, using a 3 µl probe liquid droplet in a telescopic goniometer at room temperature (Rame-Hart, Model ). The telescope with a magnification power of 23x was equipped with a protractor of 1 graduation. For each contact angle S3

4 55 reported, at least ten measurements on different surface locations were averaged S2. Dynamic Light Scattering (DLS) Measurements The average hydrodynamic sizes and the intrinsic size distributions of the biochar MP and NP suspensions (Table 1) were determined using DLS (BI-200SM, Brookhaven Instruments Corporation, Holtsville, NY). The DLS measurements were performed with a multi-detector light unit which employs a laser with a wavelength of 640 nm. For each experiment, 3.0 ml of biochar MP or NP suspension (Table 1) was introduced into a glass vial. The scattered light intensity was detected by a photodetector at a scattering angle of 90 o, and each auto-correction function was accumulated for 15 s. The intensity-weighted hydrodynamic particle size distributions were determined from the intensity-autocorrelation functions using the NNLS algorithm assuming the Stokes-Einstein equation for spherical particles. 4,5 The intensity-weighted particle size distributions were further converted to number-weighted size distributions. The latter number-weighted sizes were used in the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) calculations. All characterization measurements were conducted at least in duplicate using freshly prepared suspensions for each condition S3. Interaction Energies between Biochar Particles and Sand Surfaces To qualitatively understand the transport and retention behaviors of the biochar S4

5 particles in saturated granular media (quartz sand), the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was applied to calculate the total biochar particle-particle and particle-sand interaction energies as the sum of the Lifshitz-van der Waals (LW), the electrostatic double layer (EDL), and the Lewis acid-base (AB) interactions. The particle-particle interaction energy was determined by assuming a sphere-sphere configuration; whereas the particle-sand interaction was calculated by assuming a sphere-plate configuration The LW interaction energies ( ) for the particle-particle ( ) and particle-sand ( ) systems were calculated using equations [2] and [3], respectively: where A is the Hamaker constant; a p is the radius of the biochar particle; h is the separation distance between the particle-particle (or particle-sand) surface; h 0 represents the minimum equilibrium distance between the biochar particle and sand surface and equals to nm; 6 and,,and are the LW interfacial tension parameters for biochar particle, water, and sand, respectively. The electrostatic double layer interaction energies ( ) for the particle-particle ( ) and particle-sand ( ) systems were determined using equations [5] and [6], respectively: S5

6 ln ln 1 exp ln ln 1 exp where ε 0 is the dielectric permittivity of vacuum; ε w is the dielectric constant of water; ψ p and ψ s are the zeta potentials of the biochar particle and sand, respectively; κ is the inverse of Debye length; e is the electron charge; n i0 and z i are the number concentration and valence of ion i in the bulk suspension; k is the Boltzmann constant; and T is the absolute temperature. The AB interaction energies ( ) for the particle-particle ( ) and particle-sand ( ) systems were calculated using equations [8] and [9], respectively: Δ exp 2 Δ exp where λ w is the characteristic decay length of AB interactions in water (1.0 nm at 20 C) and Δ largely represents the AB interaction free energies per unit area corresponding to h 0. Δ where Υ and Υ are the electron-acceptor and electron-donor parameters of the surface, respectively, and the subscripts of p, w, and s represent biochar particle, water, S6

7 and sand, respectively. For equations [4] and [10], the values of,,and for water are 21.8, 25.5, and 25.5 mj m 2, respectively. 7 Values of,,and for quartz sand are reported to be 39.2, 1.4, and 47.8 mj m 2, respectively. 8 For biochar particles, the values of,,and were determined through measuring the air-liquid-biochar contact angles (θ) using three different probe liquids (water, glycerol, and n-decane) with known surface tension parameters and by solving Young-Dupré equation in the following form: cos where the subscript i represents water ( 72.8, 21.8,and 25.5 mj m ), glycerol ( 64.0, 34.0, 3.92 and 25.5 mj m ) or n-decane ( 23.8, 23.8,and 0 mj m ) S4. Transport Model The transport and retention of biochar particles through the packed columns were described using a one-dimensional form of the convection-dispersion equation with two kinetic retention sites as: 10, where θ is the volumetric water content [ ], C is the biochar particle concentration in the aqueous phase [N L 3, where N and L denote number and length, respectively], t is S7

8 the time [T], ρ b is the bulk density of the porous matrix [M L 3, where M denotes mass], x is the vertical spatial coordinate [L], D is the hydrodynamic dispersion coefficient [L 2 T 1 ], q is the Darcy velocity [L T 1 ], and S 1 and S 2 are the solid phase concentrations associated with retention sites 1 and 2, respectively. The two kinetic retention sites described mass transfer of biochar particle between the aqueous and solid phase. The site 1 (equation [13]) assumes reversible retention, whereas site 2 (equation [14]) assumes irreversible, depth-dependent retention as: where k 1 [T 1 ] and k 2 [T 1 ] are first-order retention coefficients on sites 1 and 2, respectively, k 1d [T 1 ] is the first-order detachment coefficient, and ψ x [ ] is a dimensionless function to account for depth-dependent retention. The value of ψ x is given as: where d c [L] is the median diameter of the sand grains, x 0 [L] is the coordinate of the location where the depth-dependent retention starts and β is an empirical factor controlling the shape of the spatial distribution. Bradford et al. 11 found that β = provides an optimal value for experiments in which significant depth-dependent retention occurred. The above approach assumes that retention near the column inlet is dominated by site 2, and away from the inlet by site 1. S8

9 It should be mentioned that some researchers have attributed attachment/detachment and straining mechanisms of colloid retention to sites 1 and 2, respectively However, additional microscopic information is frequently needed to substantiate this assumption. In this study, we do not attempt to attribute specific biochar particle retention mechanisms to a given site without other experimental evidence. Rather, equations [12]-[15] are viewed as a simple and flexible approach to describe biochar breakthrough curves (BTCs) and retention profiles (RPs) that are not exponential with depth. BTCs and RPs were analyzed using the HYDRUS-1D code, 14 which allow us to fit the biochar particle transport parameters (k 1, k 1d, and k 2 ) using a nonlinear least squared optimization routine based on the Levenberg-Marquardt algorithm. 15 The approach velocity and dispersivity that were used in the HYDRUS-1D code were obtained by fitting the solution of the convection-dispersion equation for the tracer (bromide) BTCs. S9

10 Table S1. Selected Characteristics of Biochar Samples Used in This Study Bulk density (g cm 3 ) ph BET (m 2 g 1 ) Elemental composition (w%) C N H Extractable constitutes (g kg 1 ) P K Ca Mg Fe Zn Mn Particle size (µm) b MP mass fraction (w%) c NP mass fraction (w%) d Wheat straw Pine needle 350 C 550 C 350 C 550 C W350 W550 P350 P a a 7.4 a 65 b 55 a 1.1 a 5.9 ab 1.1 a 11 a 32 a 2.7 a 1.4 a a 0.13 b 148 c 4.3 a 1.6 a 0.41 b 9.7 b 205 d 69 bc 1.1 a 8.5 c 2.5 b 13 a 44 b 4.2 b 1.7 b 0.12 b 0.20 d 136 bc 5.5 b 2.2 b 0.30 a 7.4 a 30 a 67 b 2.8 c 6.7 b 1.2 a 15 a 28 a 6.2 c 1.4 a a a 124 ab 4.9 ab 1.9 ab 0.37 b 9.9 b 83 c 76 c 2.5 b 4.6 a 2.7 b 27 b 50 b 4.1 b 2.0 c a 0.17 c 112 a 6.5 c 2.6 c a Mean values in each row followed by the same lowercase letters are not significantly different using Tukey s HSD test at p < b The size of the biochar powder was measured by the Particle Size Analyzer (Malvern Mastersizer 2000). c, d Mass fraction of the micron-particle (MP) or nanoparticle (NP) was evaluated by drying the isolated biochar MP or NP suspension. S10

11 Table S2. Contents of Oxygen Surface Functional Groups of the Biochars Biochar Carboxylic Lactonic Phenolic Total cmol kg 1 W c a 17 c 95 b 135 c W a 20 d 42 a 68 b P b 12 a 169 c 201 d P a 14 b 35 a 54 a a Mean values in each column followed by the same lowercase letters are not significantly different using Tukey s HSD test at p < S11

12 Table S3. Contact Angle Measurements, Surface Tension Components, Hamaker Constant, and G h0 AB for Biochar Particles Properties W350 W550 P350 P550 contact angle ( C) water 93.9 b a 110 c 86.2 a 94.8 b glycerol 93.6 c 95.3 d 84.3 b 69.6 a n-decane surface tension components γ LW (mj m 2 ) γ c a b 3.56 d γ b a 9.53 c a Hamaker constant (10 20 J) G h0 AB (mj m 2 ) c b 3.34 d a a Mean values in each row followed by the same lowercase letters are not significantly different using Tukey s HSD test at p < S12

13 Absorbance λ = 221 nm W350 W550 P350 P Wavelength (nm) 175 Figure S1. Representative UV/Vis scanning spectra of 200 mg L 1 biochar MP 176 suspensions. The peak/optimal wavelength for W350, W550, P350, and P550 was 177 identical (λ = 221 nm). S13

14 Absorbance W350 W550 P350 P550 R 2 = R 2 = R 2 = R 2 = Concentration (mg L -1 ) Figure S2. Representative calibration curves of W350, W550, P350, and P550 MP 180 within the concentration range of mg L 1 at the wavelength of 221 nm. High 181 linear correlation coefficients were achieved for all the biochars in this study, and the 182 calibration curves for the biochar MP and NP suspensions were identical. S14

15 ph W Volume of HCl (ml) W550 9 P Figure S3. The acid-base titration curves of the biochars used in this study. A constant ph automatic potentiometric titration was performed to obtain the acid-base titration curves for the biochars to probe the origin of the biochar alkalinity. The titration data indicated that all the biochar samples were alkaline, and their alkalinity increased with increasing pyrolysis temperature. For example, a total of 1.20 ml of HCl was consumed when the final ph of W350 was decreased to 2.0, however, the amount of the consumed HCl was increased to 1.65 ml for W550. Similar trends were observed for the titration data of pine needle biochars (consumed HCl volumes were 0.32 and 1.32 ml, respectively, for P350 and P550 biochars) Volume of HCl (ml) P350 S15

16 Interaction energy (kt) P350_NP AB Interaction EDL Interaction LW Interaction W350_NP AB Interaction EDL Interaction LW Interaction Interaction energy (kt) P550_NP AB Interaction EDL Interaction LW Interaction W550_NP AB Interaction EDL Interaction LW Interaction h (nm) h (nm) Figure S4. Comparison of the AB, EDL, and LW interaction energies for P350/P550/W350/W550 NP-sand systems. The AB interaction energy for P350-sand was repulsive and primarily responsible for the magnitude of the energy barrier. S16

17 DLS Size (nm) (a) W350_MP W550_MP P350_MP P550_MP (b) W350_NP W550_NP P350_NP P550_NP Time (h) Time (h) Figure S5. Representative DLS sizes of biochar MP (a) and NP (b) suspensions (200 mg L 1 ) as a function of time over 2 days. Error bars represented the standard deviations. Both the biochar MPs and NPs were stable in this study. S17

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