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1 Journal of Nanoparticle Research 2 Supplementary Material 3 4 Facilitated transport of titanium dioxide nanoparticles by humic 5 substances in saturated porous media under acidic conditions 6 7 Ruichang Zhang a, c, Haibo Zhang b, Chen Tu b, Xuefeng Hu b, Lianzhen Li b, 8 Yongming Luo a, b, * 9 10 a 11 Science, Chinese Academy of Sciences, Nanjing 210008, China 12 b 13 Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 14 264003, China 15 c Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Key Laboratory of Coastal Environmental Processes and Ecological Remediation, University of Chinese Academy of Sciences 16 17 18 19 *Corresponding author. Tel./fax: +86-535-2109 007. E-mail: [email protected] (Luo 20 YM). 1 1500 Intensity (Counts) Anatase 1000 500 0 10 20 30 40 60 70 80 2 21 22 50 Fig. S1 XRD pattern of TiO2 NPS in the present study 23 2 60 TiO2 NPs Zeta Potential (mV) 40 HS Quartz 20 0 -20 -40 -60 2 3 4 5 7 8 pH 24 25 6 Fig. S2 Zeta potentials of bare TiO2 NPs, HS and quartz in deionized water 3 2937 937 1106 1036 1387 1631 2874 3455 4000 3500 3000 2500 2000 1500 1000 500 -1 26 27 Wave number (cm ) Fig. S3 FTIR spectrum of HS in this study 4 28 Table S1 The assignmen of absorption bands in FTIR spectrum of HS Wavenumber (cm-1) Assignment 3455 2937 2874 1631 1387 1106 1036 937 OH stretching vibration Aliphatic CH2 stretching vibration CH2 symmetric stretching vibration Aromatic C=C stretching vibration COO symmetric stretching vibration Aliphatic C-OH stretching vibration C-O-C stretching Aliphatic C-C vibration 29 5 1.0 y=0.039x+0.006 2 R =0.999 0.8 Abs 0.6 y=0.011x+0.003 2 R =0.999 0.4 TiO2 NPs at 343 nm 0.2 HS at 228 nm 0.0 0 20 40 60 80 -1 30 31 Concentration (mg L ) Fig. S4 Calibration curves for UV light absorbance of TiO2 NPs and HS 6 32 Table S2 Physiochemical parameters of transport experiments of TiO2 NPs. Background condition pH Na+ (mmol L-1) 0 mg L-1 HS 0.5 mg L-1 HS 1 mg L-1 HS 5 mg L-1 HS 10 mg L-1 HS pH 4.0 pH 5.0 pH 6.0 1 mmol L-1 NaCl 10 mmol L-1 NaCl 100 mmol L-1 NaCl 250 mmol L-1 NaCl 0.5 mmol L-1 CaCl2 1 mmol L-1 CaCl2 2 mmol L-1 CaCl2 5 mmol L-1 CaCl2 4.0 4.0 4.0 4.0 4.0 4.0 5.0 6.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 1 10 100 250 Ca2+ (mmol L-1) HS (mg L-1) Porosity Flow velocity (cm min-1) 0.5 1 2 5 0 0.5 1 5 10 0.5 0.5 0.5 5 5 5 5 5 5 5 5 0.45 0.42 0.47 0.46 0.45 0.42 0.45 0.43 0.43 0.45 0.43 0.42 0.44 0.42 0.42 0.46 0.36 0.36 0.36 0.38 0.37 0.36 0.36 0.36 0.36 0.37 0.35 0.37 0.37 0.37 0.36 0.36 7 33 Adsorption of HS onto TiO2 NPs 34 To quantitatively evaluate the effects of HS on the stability and transport behavior of 35 TiO2 NPs observed in column experiments, adsorption studies were conducted to 36 determine the amount of HS adsorbed onto TiO2 NPs in conditions identical to those 37 used in transport experiments. The suspended particles in TiO2 NPs suspension 38 prepared above were pelleted by sequential centrifugation (Chen et al. 2012). Briefly, 39 10 mL suspensions were added to Teflon centrifuge tubes and centrifuged for 20 40 minutes at 9,400 g (3K 15, Sigma Laborzentrifugen). Then 8 mL of supernatant 41 were carefully withdrawn from each tube and transferred into another clean centrifuge 42 tube for centrifugation. This procedure was repeated until the TiO2 NPs was 43 completely removed from the solution. The HS concentration in the supernatant was 44 determined using a spectrophotometer (GENESYS 10S UV-Vis, Thermo Scientific) 45 at 228 nm. The adsorbed HS were then determined by the difference between the 46 initial and final HS concentrations in the aqueous phase. Control experiments with 47 TiO2 NPs-free solutions showed no variations in HS concentrations before and after 48 the centrifugation processes in the range of HS concentrations tested. 8 pH 4.0 pH 5.0 pH 6.0 a 2 Adsorbed amount (mg/m ) 0.8 0.6 0.4 0.2 0.0 1 10 -1 HS (mg L ) b 2 Adsorbed amount (mg/m ) 0.35 0.30 0.25 0.20 0.15 0.10 1 10 100 -1 NaCl (mmol L ) 49 50 Fig. S5 Amounts of HS a dsorbedonto the TiO2 NPs surface as a function of pH and 51 HS concentration (0.1 mmol L-1 NaCl) (a), and NaCl concentration (5 mg L-1 HS and 52 pH 5.0) (b) in the TiO2 suspension 9 3413 2928 1625 1042 1389 1118 514 HS coated TiO2 NPs TiO2 NPs 4000 3500 3000 2500 2000 1500 1000 500 -1 53 Wave number (cm ) 54 Fig. S6 FTIR spectra of TiO2 NPs and HS coated TiO2 NPs. Peaks at 2928 cm-1 for 55 aliphatic CH2, 1118 cm-1 for aliphatic C-OH, and 1042 cm-1 for C-O-C in FTIR 56 spectra of HS coated TiO2 NPs could reasonably inferred the adsorption of HS onto 57 the surface of TiO2 NPs. 10 58 Particles attachment efficiency 59 Using the single collector efficiency model, a dimensionless contact efficiency η0 60 could be calculated as follows (Tufenkji and Elimelech 2004), 0 D I G 61 (S1) 62 Where ηD, ηI, and ηG are collector efficiency components for particles transported to 63 the collector due to diffusion, interception, and gravity, respectively. Diffusion is 64 associated with smaller particles when they undergo Brownian motion due to random 65 bombardment by molecules of the suspending medium and as a result come in contact 66 with the collector surface. Interception occurs when the moving particles contact with 67 the collector grain while traveling along a streamline. Gravitational sedimentation is 68 related to the settling of particles on collector grains by the combined effects of the 69 buoyant weight of the particle and the fluid drag on the particles (Rahman et al. 2013; 70 Yao et al. 1971). 71 Single collector contact efficiency can also be calculated using the following equation 72 in detail (Tufenkji and Elimelech 2004): 73 74 0 2.4 As1/3 N R 0.081 N Pe 0.715 NvdW 0.052 0.55N R1.675 N A0.125 0.22 N R 0.24 NG1.11 NvdW 0.053 (S2) Where As is the porosity-dependent parameter of Happel’s model: 2(1 p 5 ) As 2 3 p 3 p5 2 p6 75 (S3) 76 Here p is determined according to p (1 )1/3 , where ε is the porosity of a porous 77 medium. 78 NR is the aspect radio, which is defined as: 11 NR 79 dp (S4) dc 80 Where dp and dc are the diameter of TiO2 NPs aggregate and quartz packed in column 81 respectively. 82 NPe is Peclet number, which is described as: N Pe 83 Ud c D (S5) 84 Where U is the approach (superficial) velocity of the TiO2 NPs suspension, and D∞ is 85 the diffusion coefficient in an infinite medium, which is calculated using: D 86 kT 6 a p (S6) 87 Here, k is the Boltzmann constant, 1.3805×10-23, T is the temperature in degree of 88 Kelvin, 293 K, μ is the absolute viscosity of fluid, and ap is the radius of TiO2 NPs. 89 NvdW is the van der Waals number, which is defined as: A kT 90 N vdW 91 Where A is the Hamaker constant for the interacting system of TiO2 NPs -water-quartz, 92 and was assumed to be 1.0×10-20. 93 NG is the gravity number obtained by the following equation: 94 NG 2a p 2 ( p f ) g 9U (S7) (S8) 95 Where ρp and ρf is the density of TiO2 NPs and fluid respectively, and g is the 96 gravitational acceleration, 9.81 m s-2. 97 NA is the attraction number, which is calculated using: 98 NA A 12 a p 2U 12 (S9) 99 Essentially, η0 is the probability that a particle will collide with the collector grain 100 through one of the three modes of transport (diffusion, interception, and transport), 101 depending on system hydrodynamics, particle size, density, and van der Waal’s forces 102 (Saleh et al. 2008). 103 Under conditions relevant to most aquatic systems, the single collector removal 104 efficiency η is lower than the single collector contact efficiency η0 due to repulsive 105 colloidal interactions between particles and collector grains (Tufenkji and Elimelech 106 2004). The actual single collector removal efficiency is often expressed using eq. S10, 107 2d c ln(Ci / C0 ) 3(1 ) L (S10) 108 Here, dc is the average diameter of the collector particles, ε is packed bed porosity, L 109 is the length of the column, and C0 and Ci are influent and effluent particle 110 concentration, respectively. 111 The attachment efficiency or “sticking coefficient” α in eq. S11 represents the fraction 112 of collisions between particles and collectors that result in attachment, i.e., describes 113 the ratio between the experimental single collector removal efficiency η and the 114 predicted single collector contact efficiency η0. 115 116 13 0 (S11) 117 Derjaguin–Landau–Verwey–Overbeek (DLVO) theory 118 DLVO theory was applied to evaluate the role of electrostatic and van der Waals 119 interactions on the interaction between the nanoparticles and the nanoparticle-quartz 120 surfaces. Total (h) vdW (h) dl (h) 121 (S12) 122 DLVO interaction energies between TiO2 NPs were calculated assuming 123 sphere-sphere geometry by utilizing the following equations (Gregory 1981): vdW (h) 124 A101a p 12h(1 14h / ) (S13) 125 dl (h) 2 0 r a p p 2 ln 1 exp( h) 126 Interaction profiles for nanoparticles and quartz sand particles were developed 127 assuming sphere-plate geometry and the following equations were used for 128 calculation (Gregory 1981): vdW (h) 129 A102 a p 6h(1 14h / ) (S14) (S15) 130 1 exp( h) dl (h) 0 r a p 2 p c ln p 2 c 2 ln 1 exp(2 h) (S16) 1 exp( h) 131 In DLVO interaction energy profiles, positive interaction energy values represent 132 repulsive condition whereas negative interaction energy values correspond to 133 attraction. 134 When DLVO interaction energy between TiO2 NPs is calculated, ap is the radius of 135 the initial TiO2 NPs, 30 nm, and in the case of the energy between TiO2 NPs and 136 quartz surface, the radius of an equivalent sphere for the nanoparticle aggregates 137 which were measured by DLS has been used as the nanoparticle radius (ap). h denotes 14 138 the (minimum) surface-to-surface separation distance between the spheres (for 139 sphere–sphere geometry) or between a sphere and a plate (for sphere–plate geometry). 140 A characteristic wavelength (λ) of 100 nm was assumed in the calculations. 141 Permittivity of free space (ε0) and dielectric constant (εr) of water are 8.854×10-12 C 142 V-1 m-1 and 81.5 respectively, κ is the inverse Debye length (m-1) which was estimated 143 for each electrolyte solution using eq. S17, and ψp and ψc are the surface potentials of 144 TiO2 NPs and quartz collector (V), respectively. For the calculation of interaction 145 profiles, zeta potentials of TiO2 NPs and quartz were measured under different 146 chemical conditions and these values were used instead of surface potentials. The 147 Hamaker constant for TiO2 NPs–water–TiO2 NPs interaction system (A101) used was 148 3.7×10-20 J (Shih et al. 2012) and for TiO2 NPs–water–quartz system (A102) 1.0×10-20 J 149 was used (Chowdhury et al. 2011). 103 e 2 N A (2 I ) 0 r kT 150 1/ 2 ？ 151 (S17) 152 Where e is the electron charge, 1.60×10-19 C, NA is Avogadro’s constant, 6.02×1023 153 mol-1, and I is the ionic strength of the solution. 15 Interaction Energy (kT) 20 -1 0 mg L -1 0.5 mg L -1 1 mg L -1 5 mg L -1 10 mg L a 15 10 5 0 -5 -10 0 10 20 30 40 Separation Distance (nm) Interaction Energy (kT) 10 5 0 -5 -10 154 pH 4.0 pH 5.0 pH 6.0 b 0 10 20 30 40 Separation Distance (nm) 155 Fig. S7 Calculated DLVO interaction energy between TiO2 NPs (based on primary 156 size) under varying HS concentrations (pH 4.0 and 0.1 mmol L-1 NaCl) (a), and under 157 varying pH (5 mg L-1 HS and 0.1 mmol L-1 NaCl) (b) 16 1.0 0.8 C/C0 0.6 0.4 0.2 0.0 0 1 2 3 158 159 160 4 5 PV Fig. S8 Breakthrough curve of conservative Br-tracer (0.1 mmol L-1, pH 4.0 and 0.1 mmol L-1 NaCl) in quartz sands. 17 1.0 -1 a 0 mg L -1 1 mg L -1 5 mg L -1 10 mg L 0.8 C/C0 0.6 0.4 0.2 0.0 0 1 2 3 5 PV 1.0 -1 b 0 mg L -1 1 mg L -1 5 mg L -1 10 mg L 0.8 0.6 C/C0 4 0.4 0.2 0.0 0 161 162 163 1 2 3 4 5 PV Fig. S9 Breakthrough curves for TiO2 NPs under different HS concentrations at pH 5.0 (a) and pH 6.0(b) 18 Interaction Energy (kT) 600 -1 a 0 mg L -1 0.5 mg L -1 1 mg L -1 5 mg L -1 10 mg L 400 200 0 -200 -400 0 10 20 30 40 Interaction Energy (kT) Separation Distance (nm) 400 b pH 4.0 pH 5.0 pH 6.0 200 0 -200 0 164 165 166 167 10 20 30 40 Separation Distance (nm) Fig. S10 Calculated DLVO interaction energy between TiO2 NPs (based on aggregated size) and quartz under varying HS concentrations (pH 4.0 and 0.1 mmol L-1 NaCl) (a), and under varying pH (0.5 mg L-1 HS and 0.1 mmol L-1 NaCl) (b) 19 30 pH 5.0 -1 1 mg L HS -1 2 mmol L CaCl2 Number (%) 25 20 15 10 5 0 100 168 169 170 171 1000 10000 Hydrodynamic Diameter (nm) Fig. S11 Representative number-weighted hydrodynamic diameter distribution of the TiO2 NPs in the influent samples at 0.5 mg L-1 HS, 1 mg L-1 HS and 2 mmol L-1 CaCl2. 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