Environ. Sci. Technol. 2009, 43, 5455–5460
Natural Organic Matter Enhanced Mobility of Nano Zerovalent Iron RICHARD L. JOHNSON,* GRAHAM O’BRIEN JOHNSON, JAMES T. NURMI, AND PAUL G. TRATNYEK Division of Environmental and Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Road, Beaverton, Oregon 97006
Received February 13, 2009. Revised manuscript received May 7, 2009. Accepted May 18, 2009.
Column studies showed that the mobility of nanometer-sized zerovalent iron (nZVI) through granular media is greatly increased in the presence of natural organic matter (NOM). At NOM concentrations of 20 mg/L or greater, the nZVI was highly mobile during transport experiments in 0.15-m long columns packed with medium sand. Below 20 mg/L NOM, mobility of the nZVI was less; however, even at 2 mg/L the nZVI showed significantly increased mobility compared to the no-NOM case. Spectrophotometric and aggregation studies of nZVI suspensions in the presence of NOM suggest that sorption of the NOM onto the nZVI, resulting in a reduced sticking coefficient, may be theprimarymechanismofenhancedmobility.Modelingthemobility of nZVI in porous media with filtration theory is challenging, but calibration of a simple model with experimental results from the column experiments reported here allows simulation of transport distances during injection. The simulation results show that the increased mobility due to NOM combined with the decrease in mobility due to decreased velocity with distance from an injection well could produce an injection zone that is wide enough to be useful for remediation but small enough to avoid reaching unwanted receptors.
Introduction Permeable reactive barriers (PRBs) composed of granular zerovalent iron (ZVI) can be used to remove a wide range of contaminants from groundwater (1-3). However, the mechanics of installation limit the use of conventional PRBs to depths of about 70 feet (∼21 m) below ground surface (4, 5). To overcome this limitation, a variety of methods for direct injection of ZVI into the subsurface have been proposed. Some of methods are based on direct injection in conjunction with hydraulic or pneumatic fracturing, which allows the injected iron to both prop open the fractures and serve as a reactive medium. Other methods emphasize ultrafine (nominally “nano”) zerovalent iron (nZVI) injected via a well under conditions where the nZVI is supposed to disperse through the natural pore spaces in the aquifer. If the nZVI is sufficiently mobile in situ, this approach could be used to create deep PRBs (e.g., by overlapping multiple injection zones) or to target source zones of nonaqueous phase liquid (6-9). While a few early studies claimed to have achieved significant mobility of nZVI in the subsurface by the direct * Corresponding author phone: (503)748-1193; e-mail: rjohnson@ ebs.ogi.edu. 10.1021/es900474f CCC: $40.75
Published on Web 06/08/2009
2009 American Chemical Society
injection approach (e.g. ref 10,), further consideration has shown that “bare” nZVI (i.e., without the amendments described below) has very little mobility in porous media under most conditions (7, 8). Laboratory experimental data have shown that it is difficult to transport significant mass loadings of nZVI (e.g., g1 g/L) through porous media without (i) mechanical enhancements to transport in granular media (i.e., induced fractures, pressure pulsing), (ii) very large velocities compared to typical regional groundwater flow velocities (i.e., pressurized injection conditions), and (iii) amendments to the injected material to minimize aggregation, settling, and or filtration of the nZVI (11, 12). Amendments to nZVI materials can include both solution additives and surface modification of the nZVI itself. Building on some early work that used shear thinning fluids to mobilize colloidal ZVI during injection (13, 14), recent work has considered the effectiveness of a wide range of amendments. These amendments can serve several functions, including (i) reducing aggregation, (ii) modifying surface charge of the aggregates to decrease electrostatic interactions between the aggregates and the soil grains (15), and (iii) potentially increasing the role that hydrodynamics can play in removing aggregates from soil surfaces (16). These amendments can be sorbed or covalently bonded to the nZVI and may also help to target contaminant “source zones” containing nonaqueous-phase liquids (17). It is essential that any amendment during nZVI injection be cost-effective and environmentally benign, which has led to interest in natural, “green” polymers. One such green polymer is guar gum which sorbs to nZVI, resulting in more negatively charged surfaces and reduced aggregate size and therefore more stable suspensions (18). Changes in surface charge may also significantly affect the “sticking” of nZVI to subsurface media. Another type of green polymer that could influence aggregation and sticking, and therefore in situ mobility, of nZVI is natural organic matter (NOM). Recent studies have shown that NOM can stabilize suspensions of carbon nanotubes (19-22), fullerenes (23-25), gold (26), and iron oxide (27) nanoparticles. NOM has also been shown to enhance the mobility of hematite through sand columns (28), but the effect of NOM on suspensions of nZVI or its mobility in porous media has not been described. The results presented in this study show that NOM dramatically increases nZVI mobility in porous media compared to nZVI without NOM. The effect occurs over a wide range of NOM concentrations and therefore has the potential both to be significant under natural conditions and to provide a strategy for facilitating nZVI transport during injection. Implications of these results for potential transport of nZVI beyond injection are also discussed.
Materials and Methods Materials. The nZVI used in this study was RNIP-10DS (Toda America Inc., Schaumburg, IL) that was shipped and stored as a powder under dry, anoxic conditions [to minimize aging, which is significant when the material is stored in an aqueous slurry (29)]. The specific surface area (as) of the fresh materialsmeasured by gas adsorptionswas 29 ( 2 m2 g-1 (29). Immediately prior to initiation of each set of experiments, 1 g/L solutions of nZVI were prepared in deoxygenated, deionized water containing 100 mg/L NaHCO3 (1.19 mM) in an anoxic chamber and dispensed into reservoirs used as influent for the column experiments. Stock solutions of NOM were prepared with deionized water containing 100 mg/L NaHCO3. These stock solutions gave circum-neutral pH (7.7 for 20 mg/L NOM and 7.1 for VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Column Experiments expt no. 1 2 3 4 5 6 7 8 9 10
nZVI solvent [NOM] (mg/L)
column pre-exposure [NOM] (mg/L)
column eluant [NOM] (mg/L)
v (cm/s)
% of total mass in effluent
single collector efficiencya
sticking coefficient
0 200 200 200 20 6 2 20 20 20
0 200 200 200 20 6 2 0 0 20
0 200 200 200 20 6 2 0 20 0
0.1 0.1 0.03 0.01 0.1 0.1 0.1 0.1 0.1 0.1
0.9 71.9 57.2 26.2 66.8 31.8 11.6 7.5 13.5 33.0
0.010 0.010 0.023 0.049 0.010 0.010 0.010 0.010 0.010 0.010
1.00 0.070 0.053 0.061 0.084 0.24 0.46 0.55 0.43 0.24
a The single collector efficiency (SCE) for the 0.1 cm/s case was determined by assuming a sticking coefficient of 1 in experiment 1. For the slower velocities, SCE values were calculated using the 0.1 cm/s value and the equations presented in ref 46.
200 mg/L), which decreased only slightly in the presence of 1.0 mg/L nZVI for 24 h (to pH 7.4 and 6.9, respectively). The NOM was provided by Baohua Gu (Oak Ridge National Laboratory), who obtained the raw material (NOM-GT) by reverse osmosis of brown water from a wetland pond in Georgetown, SC (30). NOM-GT has been characterized extensively by a variety of methods (31-34). The carbon content of NOM-GT is 48.3% (35). Column Experiments. Transport experiments were performed in plexiglass columns that were 0.15 m long with an inside diameter (I.D.) of 0.0125 m. Each column was packed with washed silica sand (#17, U.S. Silica, particle diameter ∼300 µm, and negligible iron content (
