bench-scaled nano-fe0 permeable reactive barrier...

NGWA.org Ground Water Monitoring & Remediation 1

© 2011, The Author(s)Ground Water Monitoring & Remediation © 2011, National Ground Water Association.doi: 10.1111/j1745–6592.2011.01352.x

Bench-Scaled Nano-Fe0 Permeable Reactive Barrier for Nitrate Removalby S. Mossa Hosseini, B. Ataie-Ashtiani, and M. Kholghi

IntroductionThe intensive use of nitrogen fertilizers, waste-water dis-

posal, and manure causes extensive nitrate contamination in subsurface water. Consequences of drinking water contami-nated by nitrate over maximum concentration level (MCL) are related to methemo-globinemia and blue baby disease in infants after it transformed into nitrite (Winneberger 1982; WHO 2005; Joekar-Niasar and Ataie-Ashtiani 2009). Threats of nitrate contamination lead to a strict limit of 10 mg/L of N in many countries. Existing in situ technolo-gies, such as chemical reduction, biological denitrification, and physical adsorption can be used to treat water contami-nated by nitrate. Rocca et al. (2007) overviewed the latest development strategies used in nitrate removal for in situ application. The in situ chemical reductions of nitrate are potentially very low due to post-treatment requirements and needs complicated underground structures which are gen-erally too expensive (Koch and Siegrist 1997; Prüsse and Vorlop 2001). Biological denitrification approach naturally

occurs when certain bacteria consume nitrate in their breath-ing process under anaerobic conditions and transform NO

3−

to N2 gas (Soares et al. 2000).

One of the well-known approaches of in situ nitrate treatment from groundwater is the application of perme-able reactive barrier (PRB) which executes in the forms of continuous trenches, funnel-and-gate and reactive vessel (Day et al. 1999). Continuous trenches and funnel-and-gate systems are the most common types of PRBs (Gillham and Burris 1992). Reductant agents are injected into the exca-vated trench or wells in the aquifer and the contamination plume crosses it, under the natural gradient of groundwater (Powell et al. 1998). In addition, PRBs require no exter-nal energy source. The method is more cost effective than pump-and-treat systems; however, it requires a higher initial capital investment (Agrawal and Tratnyek 1996; Naftz et al. 2002; NFESC 2004).

Nano-zero-valent iron (NZVI) particles are widely used in PRB as reductant agents to remove the nitrate from groundwater. A rapid developing volume of research at various scales of laboratory and field application of NZVI is taking place (Ludwig and Jekel 2007). NZVI particles have large surface areas and high surface reactivity (Zhang 2003). Application of NZVI as reductant agents in PRBs for removing the wide range of pollutants has made the PRBs

AbstractThere are many fundamental problems with the injection of nano-zero-valent iron (NZVI) particles to create permeable

reactive barrier (PRB) treatment zone. Among them the loss of medium porosity or pore blocking over time can be considered which leads to reduction of permeability and bypass of the flow and contaminant plume up-gradient of the PRB. Present study provides a solution for such problems by confining the target zone for injection to the gate in a funnel-and-gate configuration. A laboratory-scale experimental setup is used in this work. In the designed PRB gate, no additional material from porous media exists. NZVI (d

50 = 52 ± 5 nm) particles are synthesized in water mixed with ethanol solvent system. A steady-state

condition is considered for the design of PRB size based on the concept of required contact time to obtain optimum width of PRB gate. Batch experiment is carried out and the results are used in the design of PRB gate width (~50 mm). Effect of high initial NO

3−-N concentration, NZVI concentration, and pore velocity of water in the range of laminar groundwater flow

through porous media are evaluated on nitrate-N reduction in PRB system. Results of PRB indicate that increasing the initial NO

3−-N concentration and pore velocity has inhibitor effect—against the effect of NZVI concentration—on the process of

NO3

−-N removal. Settlement velocity (S.V.) of injected NZVI with different concentrations in the PRB is also investigated. Results indicate that the proposed PRB can solve the low permeability of medium in down-gradient but increasing of the S.V. especially at higher concentration is one of the problems with this system that needs further investigations.

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2 S. M. Hosseini et al./ Ground Water Monitoring & Remediation NGWA.org

as one of the attractive progresses in the record of ground-water remediation technologies since the 1990s (Tratnyek et al. 2003). More than 60% of the 200 PRBs installed world wide by 2004 were iron-based (ITRC 2005).

Two types of reduction process were applied in PRBs to treat groundwater: (1) abiotic reduction and (2) biotic reduction. In the fist type a range of materials are used. For example, limestone to remediate groundwater with acid mine drainage (Pearson and McDonnell 1975), geochemi-cal materials to remediate uranium (U) (Longmire et al. 1991); zero-valent iron (ZVI) to remediate halogenated organic solvents (Gillham and O’Hanneisin 1994), explo-sive 3-nitro-toluene (TNT), 1,3,5-trinitro-perhydro-1,3,5-triazine (RDX) (Nurmi and Tratnyek 2008), and arsenic (Lien and Wilkin 2005). For the second type, Bionolle (Schipper and Vojvodic-Vukovic 2000), sawdust (Boley et al. 2002), leaf compost (Delwiche 1981), and alfalfa (Benner et al. 1997).

Most of the fundamental information about contaminant removal from groundwater by ZVIs is gained from batch reactors in small bottles; however, study on the bench-scale physical model is limited (Tratnyek et al. 2003).

Despite the wide application of PRB-based NZVI, there are many potential problems with these reductant agents to create in situ treatment zones due to buildup precipitates within the PRB treatment zone (Yu 2005). These problems include: significant loss of medium porosity or pore spacing blocking over time which leads to reduction of permeabil-ity, creates an impermeable barrier and bypass of the flow, and contaminant plume up-gradient of the PRB (Kim and Corapcioglu 2002).

In 1996, a PRB continuous wall (35 m length, 1.5 m depth, 1.5 m width in groundwater direction) was installed in Bardowie, north of Newzland to remediate nitrate from groundwater. Reductant agents in this PRB included saw-dust. The installed PRB indicated the high efficiency to nitrate removal from groundwater in the first 5 years. After 5 years of PRB installation, a tracer study with bromide indicated the bypass of flow reaching PRB due to accumu-lation of biomass (Schipper et al. 2004).

Very limited information is available about the effect of mineral precipitations (e.g., greenish iron oxide, calcium carbonate and bicarbonate), cementation and iron corrosion on long-term operation of a PRB system in field applica-tions. Geochemical conditions within PRB cause these problems (Mackenzie et al. 1997).

Corrosive materials of iron such as HCO3− and SO

42− that

exist frequently in groundwater reduce the lifetime of PRB. Philips et al. (2000) investigated the performance of PRB-based ZVI in Oak Ridge (United States) with dimensions of 2 feet (width), 225 feet (length), and 31 feet (depth) which is used to reduce the Uranium from groundwater. Analysis of samples from ZVI in this PRB core after 15 months of installation indicated that a film with thickness of 20 to 50 µm covered the ZVI surface. In addition, they reported that mineral precipitations on the ZVI surface prevent further corrosion and significantly reduces the reactivity of ZVI.

Johnson et al. (2008) studied a field-scale PRB-based ZVI in Nebraska to reduce the TNT and RDX of ground-water. They reported that installed PRB is effective in

removing target contaminants; moreover, mineral precipitates affect hydraulic properties of PRB.

Wilkin et al. (2003) investigated the performance of two PRBs based on ZVI with continuous wall (in Elizabeth city) and funnel-and-gate configuration (in Denver). Investigation of these PRBs during 1996 through 2000 indicated that groundwater chemistry, rate of groundwater passed through PRB, and population of microbes are three influenced fac-tors on the precipitate of the Siderite and Calcite on the ZVI surface. In addition, they reported that precipitates reduce the porosity of reductant agents as 0.35% each year. It was indicated that the performance of funnel-and-gate PRB in reduction of contaminants was better than continuous wall, but some problems accompanied by funnel-and-gate PRB (e.g., hydrological effect on the aquifer) require more studies.

Painter (2004) reported to minimize the installation costs of funnel-and-gate PRB a trade-off between capture zone of PRB, retention time of flow in reactive zone, and PRB thickness are necessary.

According to previous studies, in this study, a novel idea in design of PRB with funnel-and-gate configuration is used to solve the problems of pore blocking and loss of porous media permeability through PRB. Designed PRB in this study, contains no materials of porous medium and is only filled by contaminated water and NZVI injected in the gate of PRB. In addition, the effects of high initial nitrate con-centration, pore water velocity, and NZVI concentration on nitrate-N removal through PRB and settling velocity (S.V.) of injected NZVI with different concentrations in the PRB are investigated.

Materials and Methods

Synthesizing and Characterizing of NZVI ParticlesThe NZVI particles used in this study were synthe-

sized on-site to prevent more oxidation of NZVI surface. To synthesize the NZVI particles, 1.5 M NaBH

4 (99%,

Merk, Germany) solution (solution prepared as 4:1 [v:v] de-ionized water [DI] water/ethanol) was added slowly in the rate of 1 to 2 mL/min into 1.0 M FeCl

3·6H

2O (99%,

Merk) aqueous solution at ambient temperature and vigor-ous stirrer approximately 400 rpm according to Wang and Zhang (1997), Wang et al. (2006), and Lee et al. (2008). All aqueous solutions were deoxidized using N

2 purged DI

water. During this reaction, Ferric ion (FeIII) is reduced into black particles by sodium borohydride as the reductant, as shown in the following reaction (Glavee et al. 1995; Yang and Lee 2005):

4Fe3+(aq)

+ 3BH4 + 9H

2O → 4Fe0

(s) ↓ + 3H

2BO

3 + 12H+

(aq) + 6H

2(g)↑

(1)

The black precipitates were filtered by vacuum filtration through 0.2 µm filter papers and then, washed with DI water and ethanol at least three times. The freshly prepared par-ticles were stored in N

2 purged solution of 10−4 M HCl (pH

= 4). A major advantage of this synthesis method is its rela-tive simplicity and no need for any special instruments (Li


NGWA.org S. M. Hosseini et al./ Ground Water Monitoring & Remediation 3

et al. 2006). In addition, using of ethanol during the synthesis step will result in low concentration of boron in final products (Lee et al. 2008).

To characterize the synthesized NZVI particles, X-ray diffraction (XRD) pattern, transmission electron microscopy (TEM) images, and Brunauer, Emmett and Teller Method (BET) surface area were recorded (Figure 1). According to Figure 1a, the synthesized NZVI is generally spherical in form and exists as chain-like agglomeration according to several references (Liu et al. 2005). On the basis of investiga-tion of over 200 nanoparticles from TEM images, the NZVI particles’ diameters were relatively 52 ± 5 nm (Figure 1b). NZVI particles possess a core-shell formation, in which the shell represents the oxidized part that surrounds the Fe0 core. Synthesized NZVI indicated two phases, α-Fe0 and Fe

3O

4 in agreement to Nurmi et al. (2005). The weak peaks

in XRD spectrum of nanoparticles (2q = 25, 35, 57, and 67) inhibit oxidized shell on the nanoparticles, similar to the previous researches (Coe et al. 2005; Sun 2006). BET sur-face area of synthesized NZVI was obtained as 32.4 m2/g.

According to the core-shell model, the mixed valence iron oxide shell is largely insoluble under neutral pH condi-tions and may protect the NZVI core from rapid oxidation, but it reduces the reactivity rate of NZVI. In acidic condi-tions (pH < 4), the acid dissolves the oxide film on Fe0 to keep a fresh surface and to continue the reduction process (Cheng et al. 1997; Huang and Zhang 2004). It should be mentioned that the NZVI cannot be stored in strong acidic solution for long time because in such condition, NZVI can easily reduce H

2O according to the following reaction:

Fe0 + 2H2O → Fe2+ + H

2 + 2OH− (2)

To avoid similar problems, NZVI was synthesized just before its usage in the PRB and stored as slurry suspen-sion. Mechanism of contaminant reduction by core/shell NZVI particles can be described by a general conceptual model (Scherer et al. 1998). Some details and key elements of this model for NO

3− reduction as model contaminant are

shown in Figure 2. Aqueous nitrate (NO3aqs

−) is transferred

(a) (b)

(c)

Figure 1. Characteristics of synthesized nano-Fe0: (a) TEM image and (b) histogram of synthesized nano-Fe0 particle size, (c) XRD pattern of fresh nano-Fe0.

Figure 2. Conceptual model of NO3− reduction by core/shell

NZVI particles.



to the boundary layer at the interface of oxidized film and water and then adsorbed to it as NO

3ads−. NO

3ads− is diffused

along the boundary and produce complex forms as NH3ads

, NO

2ads−, and NH

4ads by giving electrons (that come from the

core, Fe0) and H+ (that is produced from reduction of H2O

in the boundary). The products in the boundary layer can be desorbed and diffused away from the surface into the solution.

Bench-Scale PRB Setup Reduction of NO

3−-N by NZVI particles was carried out

in a bench-scaled laboratory setup as shown in Figure 3. Uncontaminated sand with non-uniform size (mean diam-eter d

50 = 0.82 ± 2 mm) was used as porous medium. The

length of porous medium before and after the PRB position was considered as 35 and 70 cm, respectively.

The sand properties were estimated as bulk density (r

b = 1.68 g/cm3), particle density (r

s = 2.67 g/cm3), average

porosity of sand (n = 0.37), and average saturated hydraulic conductivity (K = 0.55 cm/s).

The laboratory model was made of Plexi-glass as shown in Figure 3, schematically. A main reservoir was provided to supply influent water with certain NO

3−-N concentration

into porous medium. A reservoir in up-stream and one in down-stream of porous medium were considered to create a steady-state condition of flow through sand. A certain pore velocity through sand was made with regulation of water level in these reservoirs (Ataie-Ashtiani et al. 1999).

The funnel-and-gate configuration of PRB was used with a length of 10 cm perpendicular to the water direc-tion. Contaminated water was passed through with 45° fun-nels convergence angle according to Rocca et al. (2007).

Figure 3. Scheme of laboratory setup.



Efficiency of the PRB for contaminant reduction depends on the gate width (W) in water direction. The value of W must be obtained according to batch experiment results. Volume of 5 mL of solution was withdrawn from each of four points as up-gradient (U.G. point), core (in the cen-ter of gate width), and two points in down-gradient (D.G.1 and D.G.2) of PRB with syringe during each set of experi-ments (Figure 3). Then samples were filtered by a 0.2-µm filter paper before analysis by UV-Vis spectrophotometer to measure the absorbance value of samples according to Wang et al. (2006). For better simulation of groundwater system in the experiments, water which was used in both batch and PRB experiments contained other ions as reported in Table 1. The effect of other ions with different valence in NO

3−-N reduction by NZVI has been investigated by previ-

ous researchers (Ruangchainikom et al. 2006). As shown in Figure 3, reservoir containing NZVI par-

ticles suspended in DI water with certain concentration was placed on top of PRB, such that injection was carried out in the center of gate. A small electromotor with low stirrer was considered in the reservoir of NZVI particles to prevent sedimentation of particles and create a homogeneous injec-tion of particles into gate. In addition, to avoid the NZVI oxidation in the reservoir before the injection to PRB, Ar gas was sparged to the solution. It is important to keep fresh the NZVI surface during synthesis and also before the injec-tion in any reactive zone (Chien et al. 2006). Different levels of NO

3−-N concentration (100, 200, and 300 mg/L of N)

were provided by adding KNO3 to water of main reservoir,

manually. In the bench-scaled PRB, the effect of NZVI concentration, high initial NO

3−-N concentration, and pore

velocity of water through sand in NO3−-N reduction process

were investigated and obtained results are presented in the next sections.

Results and Discussion

Steady-State Design of PRBFrom an engineering point of view, essential concern in

the PRB design is how to ensure enough contact between

contaminant and reductant agent to meet desirable treatment objective (Naftz et al. 2002). On the basis of the study by Hocking et al. (2001), the required contact time of many hours is commonly required for volatile organic compounds (VOC), in which other contaminants (e.g., As, Mo, NO

3−, Se,

and V) reduced to acceptable levels through PRB in less than 6 min. Regardless of convective and dispersive processes in PRB due to low velocity of groundwater and low molecu-lar diffusion and dominating the chemical degradation of N by NZVI, the width of PRB in the direction of groundwater flow must be at least W, calculated as (ITRC 2005):

W = Va·Δt (3)

where W is the required width of PRB gate, Va is the actual linear velocity of water in PRB, calculated from Va = VD · n (VD, Darcy’s velocity in porous media and n, porosity of porous media), and Δt is the necessary contact time between reductant agents (NZVI) and goal contaminant.

Results of Choe et al. (2004) and Yang and Lee (2005) on batch experiments indicated that using of a much greater BET surface area for NZVI (larger than 31.4 m2/g) the first-order or pseudo-first-order reaction could not model the reaction between NZVI and N. However, choosing the best-fitted reaction model is needed for more investigations. In this study, a mixed-order reaction model reported by Huang and Zhang (2004) is used to obtain the Δt value:

⎟⎟⎠

⎞⎜⎜⎝

⎛=−

KN + CC

mdt

d[C] ρmαs (4)

where αs is the specifi c surface area of NZVI (m2/g), rm the mass concentration of Fe0 (g/L), μ the specifi c nitrate reduc-tion rate (mM/m2/min), KN the half saturation concentra-tion of N (mM), and C the NO3

−-N concentration in bulk solution (mM). Integrating Equation 4 with respect to (C0 to C) and (t0 to t) results:

ms

infeffinf

effN

0

)(ln

ρma

CCC

CK

ttt

−+−=−=Δ

(5)

where Ceff and Cinf are the infl uent and effl uent N concen-tration in PRB, respectively. Th e value of rm can be calcu-lated as follows (Tratnyek et al. 2003):

sol

Fem

0

V

M=ρ

(6)

where MFe0 is the mass of NZVI, Vsol is the volume of

solution, and Δt is denoted to the required contact time between NO3

−-N and NZVI in the gate to reduce Cinf to Ceff . In the batch experiment, the effect of initial nitrate con-

centration and also mass of NZVI are investigated similar to values that used in bench-scaled tests. The bottles were mixed

Table 1Average Chemistry Parameters of Used Water in

Experiments

Param eter (Unit) Value

Total dissolved solids (mg/L) 563.00

Calcium (mg/L) 208.42

Magnesium (mg/L) 68.04

Potassium (mg/L) 74.30

Chloride (mg/L) 78.00

Carbonate (mg/L) 0.0

Bicarbonate (mg/L) 163.72

Sodium (mg/L) 25.87

pH (−) 6.90



by rotary shaker at low stirrer during reaction time (100 to 200 rpm). Recorded experimental data from batch experiments (points in Figure 4) were fitted to the Equation 5 and results are shown in Figure 4 (as lines). Estimated parameters of reac-tion equation (µ and K

N) were found to converge to a unique

set of value for a specific mass of NZVI as summarized in Table 2. Correlation coefficient (R2) values of fitted models to observed data are more than 0.99 for all cases which indicates the goodness of fit of Equation 5 to recorded values.

Results of this study indicated that kinetic reaction of NO

3−-N reduced by NZVI particles were well matched with

the form of Equation 5. Similar reaction modeling was done by Huang and Zhang (2004) in which the kinetic param-eters K

N = 2 mM and µ = 0.7 mM/m2/min were obtained

for micro-Fe0 with rm = 10 g/L and a

s = 0.04 m2/g. Plot of

µ vs. rm estimated in this study (Figure 5), shows a linear

relationship with high correlation (R2 = 0.98) which is con-sistent with the results of Bandstra et al. (2005).

A sensitivity analysis of reaction parameters µ and KN in

Equation 5 was carried out to investigate the effects of these parameters on the nitrate removal concentration. In this regard, values of µ and K

N were changed as ±5 and ±10% of their opti-

mum values (Figure 5). Results indicated that the µ parameter value has more effect on the removal of nitrate in Equation 5 compared to the K

N parameter. In addition, two parameters of

KN and µ control the shape of N reduction curve. According to

Figure 6, curves of N reduction can be divided into two parts: liner and nonlinear. Parameter of µ largely control the linear part of the curve, whereas K

N the nonlinear part.

Table 2Estimated Parameters of Reaction Equation for

Different Nano-Fe0 Concentration

rm (g/L) KN (mM) µ (mM/m2/min) R2

2 7.17 0.0027 0.994

5 5.20 0.0046 0.996

8 4.13 0.0079 0.999

Figure 4. Kinetic modeling results of NO3−-N removal during

batch experimental in different initial NO3−-N concentration.

Points and lines show the recorded and fitted values, respec-tively. Parameter used were as = 32.4 m2/g and rm equal (a) 2, (b) 5, and (c) 8 g/L of solution.

Initial N-Conc. (mg/L)

100

(a)

(b)

(c)

200 300

Figure 5. Linear variation of µ vs. rm in batch experiments.

Figure 6. Sensitivity of kinetic parameters µ and KN in Equation 5. Initial NO3

−-N and NZVI concentration were 21.4 mM (300 mg/L) and 2 g/L of solution, respectively.



The required width of PRB (W) to remove certain per-cent of NO

3−-N in different conditions of initial NO

3−-N

concentration (100, 200, and 300 mg/L), NZVI concentra-tion (2, 5, and 8 g/L), and pore velocity of water through PRB (0.125, 0.250, and 0.375 mm/s) was estimated using Equations (3) through (6) and shown in Figure 7. On the basis of obtained results, variation of W vs. different per-centage of nitrate removal (10 to 100%) is linear for NZVI concentrations 2 and 5 g/L. But in the case of NZVI con-centration 2 g/L, these variations show nonlinear shape for percent of nitrate removal higher than 90%. Obviously, when the NZVI concentration is constant, increasing the initial NO

3−-N concentration leads to the increase of Δt

and therefore, the value of W. In addition, increasing the pore velocity of water through sand causes the required width of PRB to increase. Maximum and minimum values for W are obtained, 100 mm and 3 mm, in conditions that

V = 0.375 mm/s, C0 = 100 mg/L, r

m = 2 g/L and V = 0.125

mm/s, C0 = 100 mg/L, r

m = 8 g/L, respectively. In this study,

the PRB width is considered equal to 50 mm. This value of W is similar to the designed width of PRB by Tratnyek et al. (1997) for 1000-fold reduction of methanes (PCM).

Factors Influencing the Nitrate Removal by PRBIn process of NO

3−-N reduction through PRB, the effects

of initial NO3

−-N concentration (100, 200, and 300 mg/L) and NZVI concentration (2, 5, and 8 g/L) are investigated similar to batch experiments. In addition, in PRB experi-ments, the effects of pore water velocity (0.125, 0.250, and 0.375 mm/s) that fall in the range of laminar flow (Reynolds number less than unit) are considered. In PRB experiments, these factors considered in six as shown in Figure 8. Concentrations 2, 5, and 8 g/L of NZVI lead to stoicheiometric ratio of Fe/N 20, 50, and 80, respectively.

Figure 7. Variation of PRB width (W) against percentage of N removal (Ceff/Cin) based on batch experiments (V [mm/s], C0 [mg/L], rm [g/L], µ [mM/m2/min], and KN [mM]).

0

20

40

60

80

100

120

0 20 40 60 80 100

W (

mm

)

a

b

c

d

e

f

Nitrate Removal (%)

a: V=0.375, C0=100, ρm=2, μ=0.0027, KN=7.17 b: V=0.125, C0=300, ρm=2, μ=0.0027, KN=7.17 c: V=0.250, C0=100, ρm=2, μ=0.0027, KN=7.17 d: V=0.125, C0=100, ρm=2, μ=0.0027, KN=7.17 e: V=0.125, C0=200, ρm=5, μ=0.0046, KN=5.20 f: V=0.125, C0=100, ρm=8, μ=0.0079, KN=4.13

Figure 8. Factors investigated in NO3−-N removal by PRB.

Initial NO3--N

Concentration (mg/L)

Pore Water Velocity (mm/s)

0.375

NZVI Concentration (mg/L)

0.250

0.125 2000 300

2000

2000 100

100

200 5000

8000 100 Exp. 1

Exp. 2 Exp. 3

Exp. 4

Exp. 5

Exp. 6



The selected values of NO3−-N concentration and pore water

velocities were based on previous studies (Yang and Lee, 2005) and also some typical experiments.

In the control runs with no addition of nano-Fe0, no removal of nitrate was found over the time period of typical experiments. The variability in performance and the time required to NO

3−-N removal were influenced by operational

conditions. In all cases, a total time of 100 min was consid-ered. Time of NZVI injection was assigned according to set-tling process of these nanoparticles in the PRB as discussed in the next sections.

Effect of Pore Water VelocityThe effects of three pore water velocities through sand

0.125, 0.250, and 0.375 mm/s were investigated in which initial NO

3−-N and NZVI concentrations were constant at

100 mg/L and 2 g/L, respectively. The pore water veloci-ties of 0.125, 0.250, and 0.375 mm/s through sand lead to water velocities of 0.046, 0.092, and 0.138 mm/s through PRB gate, respectively. Since the volume of PRB gate was 3000 mL, to have a concentration of 2 g/L NZVI, 6 g of NZVI was suspended in 100 mL DI water and injected with a constant rate. Results for four sampling points (U.G., core, D.G.1, and D.G.2) are shown in Figure 9. NO

3−-N removal

through PRB indicates steep chemical gradients at the inter-faces between the NZVI zone and the sand, and less reduc-tion in down-gradient where the plume of treated water interacts with the native material. These results are consis-tent with the results reported by Longmire et al. (1991) and Gu et al. (2002).

Increasing the pore velocity of water resulted in less NO

3−-N reduction in core and D.G.1 and D.G.2 in PRB,

possibly due to less contact time of NO3−-N by NZVI par-

ticles during gate width. Significant NO3−-N attenuation in

the PRB exhibited 40% of C0 when pore velocity of water

was 0.125 mm/s after 30 min. NO3−-N attenuation was

observed 35 and 10% of C0 when the pore velocity of water

was 0.250 and 0.375 mm/s. Therefore, the pore velocity of water plays the main role in the NO

3−–N reduction through

such PRB. In addition, forming oxidized layer on the NZVI surface and also, sedimentation of NZVI in PRB are other inhibitor factors in NO

3−-N reduction through PRB that are

described in the next sections.

Effect of NZVI and Higher Initial Nitrate-N ConcentrationAs described before, additional experiments in PRB

were also carried out to compare the efficiencies of NO3−-N

degradation by NZVI in conditions of higher initial NO3−-N

concentration (200 and 300 mg/L) and higher values of NZVI concentration (5 and 8 g/L). Reductions of the NO

3−-N

with NZVI in four sampling points of U.G., core, D.G.1, and D.G.2 are shown in Figure 10. Results in Figure 10 express that the enhancement of influent NO

3−-N concen-

tration, decreases its reduction rate during time (Figures 8 and 9). Significant NO

3−-N attenuation in the PRB exhibited

60% of C0 when pore velocity of water and NZVI concentra-

tion were 0.125 mm/s and 5 g/L, respectively. These results are not consistent with the results reported by Gandhi et al. (2002) and Wang et al. (2006) that showed increasing initial NO

3−-N concentration would result in more NO

3−-N

removal rate in batch experiment. Possibly due to interfer-ence of the other ions with different valences in the water passed the PRB. Ruangchainikom et al. (2006) investi-gated the effects of other ions with different valence (e.g., Na+, Ca2+, Cl−, and Humic acid) on the NO

3−-N removal

by nano-Fe particles coated by CO2 in batch experiments.

They reported that presence of Ca2+ and Humic acid in the solution, reduced the NO

3−-N removal rate. But the Cl− has

inverse effect. They also concluded that the water volume and dissolved oxygen of water (DO) have preventive effect on the NO

3−-N removal rate.

Effect of NZVI concentration (5 and 8 g/L) in NO3−-N

reduction through the PRB in four points of U.G., core, D.G.1, and D.G.2 are shown in Figure 6. Results showed

0.55

0.7

0.85

1

0 20 40 60 80 100

Time (min )

C/C

0

V= 0.375 mm/s

V= 0.250 mm/s

V= 0.125 mm/s

U.G.CoreD.G.2D.G.1

Figure 9. Effect of pore water velocity through sand on the reduction of N through PRB. NZVI concentration and initial nitrate concentration were equal to 2 g/L and 100 mg/L, respectively.



that concentration of injected nano-Fe0 has a direct propor-tion on the rate of NO

3−-N reduction.

On the basis of Yang and Lee (2005) study, a com-plete removal of 150 mg/L as NO

3 was obtained by add-

ing 14.72 mol Fe0/mol N after 45 min of reaction. They observed when the dose of nano-sized ZVI was decreased to a stoicheiometric ratio of 7.36 mole Fe0/mole N, the NO

3−-N removal was only about 83% of C

0 after 60 min of

reaction. In this study, the addition of nano-Fe0 also caused significant effect in NO

3−-N attenuation. On the basis of

obtained results, the best condition for NO3

−-N reduction through designed PRB (between six sets of considered fac-tors in Figure 7) was observed as 80% of C

0 when NZVI

concentration = 8 g/L, V = 0.125 mm/s, and initial concen-tration of N = 100 mg/L.

Sedimentation Mechanism of NZVI in PRBIn developed PRB system, rapid settlement of injected

NZVI in PRB gate was one of challenge to create a long-period reaction zone. In this regards, S.V. of NZVI with concentrations 2, 5, and 8 g/L in the PRB core has been investigated. If settling process of nano-Fe0 particles is considered similar to natural sediments (i.e., no aggrega-tion, gelation), then the S.V. of a Fe0 particle (with average diameter of 50 ± 5 nm) would be estimated about 2.9 × 10−8 mm/s according to method developed by Zhiyao et al. (2008). This value of S.V. leads to a total time of 500 to 600 h (20 to 25 d) required for nanoparticles to move from top of PRB gate to the bottom of it. Results of Phenrat et al. (2007) indicated that the S.V. of NZVI particles are very faster than what considered as natural sediments due to three sequen-tial controlling processes: random motion of individual nanoparticles, rapid aggregation of NZVI to micron-size aggregates, and chain-like aggregation of the micron-size aggregate. These processes are shown schematically in the Figure 11.

In order to evaluate the effect of NZVI concentration on the S.V. in the PRB gate, the variation of total Fe0 concentra-tion in samples withdrawn from gate core have been mea-sured using UV-Vis spectrophotometer. Obtained results are presented as colored points in Figure 12. In this experiment,

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

Time (min )

C/C 0

U. G.

Core

D. G. 1

D. G. 2

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70 80 90 100

Time (min )

C/C 0

U. G.

Core

D. G. 1

D. G. 2

0.6

0.8

1.0

0 10 20 30 40 50 60 70 80 90 100

Time (min )

C/C 0

U. G.

Core

D. G. 1

D. G. 2

(a)

(b)

(c)

Figure 10. Effect of NZVI and initial NO3−-N concentration on

NO3−-N reduction through PRB: (a) NZVI conc. = 2 g/L, initial

N conc. = 300 mg/L; (b) NZVI conc. = 5 g/L, initial N conc. = 200 mg/L; and (c) NZVI conc. = 8 g/L, initial N conc. = 100 mg/L. In all cases V = 0.125 mm/s.

Figure 11. Conceptual model of NZVI settling in the PRB gate: (a) Brownian motion of each particles, (b) rapid aggregation of NZVI, and (c) chain-like aggregation or gelation.

Core

NZVI

Slow Settling

Flow Direction

Gate of PRB

Fast Settling

(c) Gelation

Slow Settling

(a) Brownian Motion (b) Aggregation

Sand

Screen



the velocity of water through PRB gate was 0.046 or 0.125 mm/s in sand.

Three distinct regions could be found in the process of NZVI settlement in PRB gate as follows:

• Region 1 (rapid motion and aggregation): Nanoparticles have random motions in this region during time of aggre-gation (t

agg). The S.V. value of NZVI is low and the

reduction rate of NZVI would be fast (Figure 9). Increase of injected NZVI concentration (from 2 to 8 g/L) lead to reduction of the t

agg value (from 4 to 2 min).

• Region 2 (rapid settlement region): The S.V. of nanopar-ticles in this region is rapid. During time of t

gel, the chain

shaped aggregates grow to form micron-size aggregates and therefore, gelation form of particles (Allain and Cloitre 1993). As NZVI concentration increased (from 2 to 8 g/L), aggregation in this region will be more rapid and resulted less t

gel (from 11 to 4 min). In this region the

rate of reduction is low because of lower contact time between contaminant and reductant agents.

• Region 3: In spite of aggregation of nanoparticles to form the micron-size clusters (as described in Region 2), some aggregates do not have appropriate size to settle rapidly. These aggregates have lower rate of settlement comparing to cluster forms and as a result, the reduction of nitrate continues. Evidently, after settlement of all injected particles, no contaminant reduction is observed (also see Figure 10c).

According to Figure 12, in three cases of NZVI con-centrations (2, 5, and 8 g/L), the required time for complete settling of injected particles were as 13, 19, and 22 min, respectively. These values were considered as the injected time of NZVI particles to the PRB core in the main experiments.

In the higher water velocity through PRB gate (0.092 and 0.138 mm/s), pattern of NZVI settlement was not the same as pattern of velocity 0.046 mm/s that shown in Figure 12. The S. V. of NZVI in presence of water veloc-ity needs more investigation. Similar work is conducted by Phenrat et al. (2007) to investigate the S.V. of three types of nanoparticles Fe0, Fe0/Fe

3O

4, and Fe0/αFe

2O

3 with con-

centrations range of 4 to 1130 mg/L in batch experiments.

They reported that for low concentration of NZVI (~ 4 g/L), the rate of particle settlement is very slow (only 95% of C

0

in 1000 s) and settling process did not follow three regions pattern as described previously. For higher NZVI concentra-tion (>190 mg/L) the settlement rate is faster and follows the three divided patterns. In addition, they concluded that the rank of S.V. of these nanoparticles were Fe0/αFe

2O

3 >

Fe0/Fe3O

4 > Fe0.

To evaluate how much of injected NZVI could trans-port by water flow toward the down-gradient side of core, additional experiment was conducted. In this experiment, maximum pore water velocity through PRB (0.138 mm/s according to Experiment 6 in the Figure 8) was considered to have the highest transport of NZVI. The mass of total Fe that was adsorbed on sand surface in 3 cm layer of sand down-gradient of PRB was measured as 5% of initial injected value (2 g/L). While, mass of total Fe that settled in the bottom of PRB was obtained as 95% of initial injected value. Analysis of the XRD pattern and SEM image of settled particles in the bottom of PRB after 1 month are shown in Figure 13 indi-cated the presence of Fe0 and magnetite (Fe

3O

4). Presence

of Fe0 in the settled NZVI indicates the possible potential of these particles for more reduction and application.

The actual potential of NZVI in contaminants reduction is a very challenging issue accompanied by its field applica-tion and needs more investigations (Henderson and Demond 2007). As shown in Figure 13a, in settled particles at the bottom of PRB, we still have some Fe0 that could be used over. In this regard, Sohn et al. (2006) conducted interesting experiments about the actual potential of NZVI reduction in batch experiments. They used 0.5 g of nano-Fe0 to reduce 50 ppm of N in 350 mL of DI water. After each experiment, the remained particles were used to reduce the nitrate in solution with same concentration, sequentially. Their results indicated that the NZVI is capable of reducing nitrate (with mentioned concentration) in six sequence steps without sig-nificant decreasing in the reaction rate.

Additional experiment was conducted to investigate the effect of installed PRB on the hydraulic properties of porous media. Transport of NZVI particles by water flow toward down-gradient of PRB may reduce the sand permeability. Obtained results based on Darcy’s law indicated insignifi-cant reduction of this parameter (less than 3%) during the PRB experiments. Thus, the proposed system of PRB in this study could solve the problems of medium porosity and permeability reduction during the operation. Settling nanoparticles in short time which associated with main-taining reactive zone of PRB in long period is one of chal-lengeable problems with this PRB. Probably application of a dispersed type of NZVI particles (surface modified) that have very low S.V. and also the management of injection frequency could solve this problem. More investigations are required to apply this PRB system in the filed conditions (in situ application).

To check the conformity between simulated results of maximum N removal based on batch experiments (simulated from Equation 5) and recorded values from PRB experi-ments; scatter plot of Figure 14 is illustrated. The correla-tion coefficient between two set of simulated and recorded data was R2 = 0.60. The discrepancy between results of

Figure 12. Sedimentation pattern of NZVI in PRB core at vari-ous concentrations. C/Cmax lead to measured concentration in each time to maximum measured of related concentration.



batch and PRB was possibly due to the difference between operational conditions in two scales. For example, in the batch experiments all bottle were set on the rotary shaker and therefore this prevented the sedimentation of NZVI, while this phenomenon in the PRB is one of challenges that needs more investigation.

ConclusionIn this study a bench-scale PRB with funnel-and-gate

configuration was designed based on results of batch experi-ments. Only the injected NZVI in center of PRB made a reactive treatment zone for contaminant plume and no addi-tional material from porous medium existed in the gate of PRB. This configuration is capable to solve the problems of pore blocking and loss of porous media permeability down-gradient of the PRB. But the settling of injected particles in the gate, especially in high concentration confronts the such PRBs with challenge to create a long-period reaction zone. In addition, results of experiments indicated that increasing the water velocity through PRB and nitrate concentration of plume interred PRB have negative impact on the reduc-tion rate of N. Increasing the injected NZVI concentration enhance N reduction rate and increases the S.V. Obtaining a trade-off between these two issues needs further studies for in situ application of the proposed PRB system.

AcknowledgmentThe authors appreciate Professor Paul G. Tratnyek,

Oregon Health and Science University, Beaverton, Oregon, for helpful comments and guiding during nanoparticles syn-thesis and application.

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(a) (b)

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Biographical SketchesS. Mossa Hosseini, Ph.D, graduated from the Irrigation and

Reclamation Department, University of Tehran, Karaj, Iran. His study area focuses on the groundwater modeling and remedia-tion using nanoparticles. He is also interested in the field of nano particles transport in saturated porous media; [email protected].

B. Ataie-Ashtiani, corresponding author, Professor of Civil Engineering and Head of the Water and Environmental Engineering Division of Department of Civil Engineering at the Sharif University of Technology. His research interests are in groundwater modeling including contaminant transport. He is also interested in issues including multiphase flow in the sub-surface zone and processes at the ocean-land interface. He has taken a particular interest in sea-water intrusion in coastal aquifers and is currently working on several projects based on actual aquifers as well as more fundamental analyses of specific aspects; [email protected].

M. Kholghi is Associated Professor in the Department of Irrigation and Reclamation Engineering, University of Tehran. He has more than 20 years experience in hydrogeology, groundwater flow and solute transport modeling, groundwater management using simulation-optimization and decision mak-ing, groundwater remediation, groundwater monitoring net-work design, and groundwater-surface water conjunctive use management.

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