Supporting Information

Transport of Surface-Modified Nano Zero-Valent Iron (SM-NZVI) in

Saturated Porous Media: Effects of Surface Stabilizer Type,

Subsurface Geochemistry and Contaminant Loading

Haoran DONG1,2 and Irene M. C. LO1*

1Department of Civil and Environmental Engineering, The Hong Kong University of

Science and Technology, Hong Kong, China

2College of Environmental Science & Engineering, Hunan University, Changsha,

Hunan, China

*Corresponding author: Email: cemclo@ust.hk; Fax: 852-23581534; Tel: 852-

23587157.

− Pages: 1-10

− Figures: Fig. S1-S8

− Table: Table S1

Preparation and characterization of SM-NZVIs

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Two commercial NZVIs in aqueous dispersion form were supplied by the

NANOIRON® Company (Czech Republic, EU): pristine NZVI (Nanofer 25,

produced from nanosized ferrihydrite) and surface coated by PAA (Nanofer 25S).

Nanofer 25 (referred to as CNZVI in this study) was used for further modification by

using Tween-20 or starch. Information on the three surface stabilizers is summarized

in Table S1. Deoxygenated ultrapure water was used for the preparation of SM-

NZVIs to avoid the oxidation of Fe0 during the modification process. PAA-modified

CNZVI (i.e., Nanofer 25S), Tween-20-modified CNZVI and starch-modified CNZVI

are referred to as P-CNZVI, T-CNZVI and S-CNZVI, respectively, in the following.

T-CNZVI was prepared by dispersing CNZVI particles in aqueous Tween-20 to

result in suspensions comprising iron nanoparticles (1.0 g L-1) and Tween-20 (35 wt

%), followed by sonication for 30 min. The method of preparation of S-CNZVI is as

described below. Briefly, a 0.4 wt% starch solution was prepared by mixing 0.8 g of

potato starch with 200 mL of ultrapure water and heating the mixture to 100 oC. Once

the starch solution started boiling, the heating was removed and the solution was

allowed to cool at room temperature. The cooled starch stock solution was then

introduced into CNZVI stock suspensions to result in suspensions comprising iron

nanoparticles (1.0 g L-1) and starch (0.4 wt%), followed by sonication for 30 min. The

SM-NZVI suspensions were freshly prepared before each experiment. Mössbauer

spectroscopy confirmed that ~85% were in zero-valent state in CNZVI, and no

obvious loss of Fe0 was observed during the surface modification.

Morphological analysis of NZVI particles was performed by TEM (JEOL 2010

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TEM). The individual particles of P-CNZVI, T-CNZVI and S-CNZVI appear

spherical and have an average diameter of about 7 nm, 9 nm and 4 nm, respectively.

The hydrodynamic particle sizes of P-CNZVI, T-CNZVI and S-CNZVI (Measured at

100 mg L-1) were 183 nm, 136 nm and 361 nm, respectively, and were determined by

using dynamic light scattering (DLS) (Zetaplus, LaborScience S.A.). Aggregation of

the nanoparticles with time was monitored by measuring the time-dependent

hydrodynamic diameter via DLS.

Zeta potential measurement of porous media

The streaming potential of the porous medium (sand and soil) was measured

using an Electro Kinetic Analyzer (Anton Paar GmbH, Graz, Austria) equipped with a

cylindrical cell to house the granular porous medium. Sand or soil after saturation

with various flushing solutions was wet-packed into the cylindrical cell in a solution

as same as the flushing solution used during the corresponding column experiments.

Before the start of each measurement, the cell was equilibrated by circulating the

solution for 20 min. Streaming potentials were converted to zeta potentials using the

Helmholtz–Smoluchowski equation.

Determination of As(V) sorption on T-CNZVI

Sorption kinetics experiments were conducted using a 100 mg L-1 T-CNZVI

suspension and 1 mg L-1 As(V) in synthetic groundwater at pH 7. The final solutions

(40 mL) in 41-mL glass vials sealed with Teflon caps were shaken in an end-over-end

rotator at 26 rpm, at room temperature (25 ºC). At pre-determined time intervals,

suspensions were filtered using 0.2-μm pore size cellulose nitrate filters. It should be

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noted that all three types of nanoparticles had particle sizes smaller than 200 nm

before the reaction and could possibly pass through the filters; thus the iron

concentration in the filtered solution was analyzed. However, the results showed that

the concentrations of iron were non-detectable, indicating that all the SM-NZVI could

be retained by the filter. This should be ascribed to the formation of larger aggregates

(d > 200 nm) after the reaction with As(V). The filtrates were acidified, diluted (if

necessary) and analyzed using a Graphite Furnace-Atomic Absorption

Spectrophotometer (GF-AAS) (Hitachi, Z-8200) for total arsenic concentration. The

results are shown in Fig. S2 and the equilibrium time for As(V) adsorption by T-

CNZVI was determined to be 120 min. As(V) sorption capacity of T-CNZVI was then

evaluated. A range of As(V) (0-20 mg L-1) was reacted with T-CNZVI (100 mg L-1) in

the synthetic groundwater. After reaching the adsorption equilibrium, the T-CNZVI

suspensions were filtered and the arsenic concentration in the filtrate was measured.

As the predominant mechanism of As(V) removal by NZVI is adsorption onto

the NZVI corrosion products (Kanel et al., 2006), the adsorption isotherm for the

As(V) adsorption on T-CNZVI was examined and the plots are shown in Fig. S3. A

Langmuir adsorption isotherm (Eq. 1), was able to describe As(V) adsorption by T-

CNZVI.

qe = qmax αCe / (1+αCe) Eq. 1

where qe is the amount of As sorbed (mg g-1), α (L mg-1) is a parameter related to the

affinity of the sorbent for the sorbate, qmax (mg g-1) is the maximum As sorption

capacity, and Ce is the equilibrium As concentration in the solution (mg L-1). For T-

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NZVI, the equation (R2 > 0.992) gives values of qmax (13.9 mg g-1 at pH 7).

Fig. S1 Schematic of column experiments for the transport of SM-NZVI

Fig. S2 Kinetics of arsenic removal by T-CNZVI at pH 7 (Fe0=100 mg L-1, As(V)=1 mg L-1).

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Fig. S3 Adsorption isotherm plots (a) and Langmuir adsorption plots (b) for the adsorption of As(V) on T-CNZVI at pH 7 (Fe0=100 mg L-1).

Fig. S4 Distribution of (a) P-CNZVI; (b) S-CNZVI and (c) T-CNZVI along the length of column at the end of injection: (1) DI water system with 100 mg L -1 of Fe0; (2) Synthetic groundwater system with 100 mg L-1 Fe0; (3) DI water system with 1 g L-1

of Fe0 and (4) Synthetic groundwater system with 1 mg L -1 Fe0. Note: Column sectioning is from the inlet to the outlet. In X-axis, “3” represents the column section of “0-3 cm” and “6” represents the column section of “3-6 cm”, etc.

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Fig. S5 Aggregation of P-CNZVI in DI water and synthetic groundwater (GW) in the absence and presence of HA. (a) Fe0=100 mg L-1; (b) Fe0=1 g L-1

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Fig. S6 Zeta potential at pH 7 of (a) P-CNZVI, (b) T-CNZVI and (c) S-CNZVI in DI water and synthetic groundwater (GW) in the absence and presence of HA. Error bars represent the standard deviations of duplicate experiments

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Fig. S7 TEM images of P-CNZVI in the presence of HA (10 mg L -1): (a) P-CNZVI=100 mg L-1; (b) P-CNZVI=1 g L-1

Fig. S8 Aggregation of (a, b) T-CNZVI and (c, d) S-CNZVI in DI water and synthetic groundwater (GW) in the absence and presence of HA.

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Table S1 Three types of surface stabilizers for modification to NZVI

Surface-Modified NZVI

Category of Surface Stabilizer

Molecular Structure of Stabilizer

Stabilization Mechanism

P-CNZVI PAA (Polyelectrolytes)1

Electrosteric

T-CNZVI Tween-20(Non-ionicSurfactant)2

Steric

S-CNZVI Starch(Hydrophilic biopolymers)3

Steric

1 Hydutsky et al., 20072 Kanel et al., 20073 He and Zhao, 2005

References(1) Hydutsky, B. W., Mack, E. J., Beckerman, B. B., Skluzacek, J. M., Mallouk, T. E.,

2007. Optimization of nano- and microiron transport through sand columns using

polyelectrolyte mixtures. Environmental Science & Technology 41, 6418–6424.

(2) Kanel, S. R., Nepal, D., Manning, B., Choi, H., 2007. Transport of surface-

modified iron nanoparticle in porous media and application to arsenic(III)

remediation. Journal of Nanoparticle Research 9, 725–735.

(3) He, F., Zhao, D., 2005. Preparation and characterization of a new class of starch-

stabilized bimetallic nanoparticles for degradation of chlorinated hydrocarbons in

water. Environmental Science & Technology 39, 3314–3320.

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