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Effect of precursor concentration on the characteristics of
nanoscale zerovalent iron and its reactivity of nitrate
Ya Hsuan Liou
a, Shang-Lien Lo
a,, Wen Hui Kuan
b, Chin-Jung Lin
a, Shih Chi Weng
aaResearch Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan
b
Department of Environmental and Safety Engineering, Ming-Chi Institute of Technology, Taishan, Taipei hsien 243, Taiwan
a r t i c l e
i n f o
Article history: Received 30 July 2005 Received in revised form 11 April 2006 Accepted 26 April 2006 Keywords: Precursor concentrations Nanoscale Fe0 Nitrate A BST RA CT
Differing precursor concentrations, 1.0, 0.1, and 0.01 M FeCl36H2O, were performed to produce nanoscale Fe0and the results were discussed in terms of the specific surface area, particle size and electrochemical properties. The results indicated that the nanoscale Fe0 prepared by 0.01 M FeCl3 had absolutely reduced in size (9–10 nm) and possessed the greatest specific surface area (56.67 m2g1). These synthesized nanoscale Fe0particles were attempted to enhance the removal of 40 mg-N L1unbuffered nitrate solution. The first-order degradation rate constants (kobs) increased significantly (5.5–8.6 times) with nanoscale Fe0 prepared by 0.01 M precursor solution ðFe0
0:01 MÞ. When normalized to iron surface area concentration, the specific rate constant (kSA) was increased by a factor of approximately 1.7–2.4 using Fe0
0:01 M (6.84 104L min1m2 for Fe00:01 M, 4.04 104L min1m2for Fe00:1 Mand 2.80 104L min1m2for Fe01 M). The rise of reactivity of the reactive site on the Fe0
0:01 M surface was indicated by the specific rate constant (kSA) calculation and the i0value of the electrochemical test.
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1.
Introduction
Global contamination of groundwater with nitrate has spurred an intense effort to find efficient and cost effective treatment methods. Chemical reduction processes have become a new focus in recent studies. Zerovalent iron (Fe0), the most commonly used material, is a conventional reduc-tant to remove nitrate in water, and its application has been reported in several publications in recent years (Siantar et al., 1996;Cheng et al., 1997a, b;Huang et al., 1998;Zhang et al., 1998; Kielemoes et al., 2000; Alowitz and Scherer, 2001;
Schlicker et al., 2003; Westerhoff and James, 2003; Choe et al., 2004;Su and Puls, 2004). Unlike halogenated hydrocarbon reduction, nitrate reduction reaction by Fe0 is relatively sensitive to the solution pH; and nitrate is well known as an oxidizing inhibitor to iron corrosion due to the formation of
an overlying oxide layer. Therefore, the nitrate reduction from unbuffered water at initial neutral pH by Fe0 has relatively rarely been reported. The use of nanoscale Fe0is currently getting the most attention.Choe et al. (2000)indicated that reducing the size of reductants to nanoscale-dimension would obtain some advantages in Fe0/nitrate unbuffered system, including: (1) an increase in reductive degradation reaction rate, (2) a decrease of the reductant dosage, (3) control over the risk of toxic intermediates release and (4) a nontoxic end product, nitrogen gas, is found (Choe et al., 2000).
Previously, it was usually believed that the reduction of particle size would be one of the important parameters to the reduction of nitrate (Wang and Zhang, 1997; Alowitz and Scherer, 2001). Recently, nanoscale Fe0 particles were pre-pared by borohydride reduction of an aqueous iron salt in
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Corresponding author. Tel.: +886 2 23625373; fax: +886 2 23928821. E-mail address:sllo@ntu.edu.tw (S.-L. Lo).
several studies (Ichinose et al., 1992;Wang and Zhang, 1997;
Zhang et al., 1998;Choe et al., 2000;Ponder et al., 2000;Ellott and Zhang, 2001;Schrick et al., 2002;Liao et al., 2003); also, two synthesizing methods were found for nanoscale Fe0 particle manufacturing. One is by mixing NaBH4 and FeCl3 solutions to form Fe0particles (Wang and Zhang, 1997;Zhang
et al., 1998; Choe et al., 2000; Ellott and Zhang, 2001). The other method is by reducing FeSO4with NaBH4(Ponder et al.,
2000;Schrick et al., 2002). Fe0particles formed by the above methods were found to have a BET surface area in the range 18–33.5 m2g1and the particles are in the size range 1–100 nm (Wang and Zhang, 1997;Zhang et al., 1998;Ponder et al., 2000;
Ellott and Zhang, 2001;Schrick et al., 2002;Liao et al., 2003;
Choe et al., 2004). The Fe0particles preparation by precipita-tion is usually the result of three processes: (1) particle nucleation, (2) particle growth and (3) secondary changes in the resulting particle suspension by agglomeration (So¨hnel and Garside, 1992). However, the process of particle growth by precipitation is complex and no simple way to control the relative rate of nuclear formation and growth has yet been found. Despite the growing number of publications on nitrate reduction by nanoscale Fe0 particles, there is still limited knowledge to evaluate the influence of parameters during the preparation of such nano-particles. The precursor concentra-tion of Fe0 may play an important role on the particle formation and surface characteristics.
In this study, three different concentrations of iron chloride, 1.0, 0.1 and 0.01 M, were employed to prepare the nanoscale Fe0particles for the reduction of nitrate reaction. Attention was also given to the surface characterization through both the investigation of kinetic control and the identification of the electrochemical properties.
2.
Material and methods
2.1. Chemicals
Potassium nitrate and sodium nitrite were purchased from Aldrich (99+%, Milwaukee, WI). Nessler’s Reagent (Fluka) was used for ammonia measurement. The FeCl36H2O and NaBH4 were obtained from Adlich. All other chemicals used in this work were analytical reagent grade, and solutions were prepared in water purified with a Milli-QTM system (18.2 MO cm1).
2.2. Methods for synthesis
Nanoscale Fe0 particles were prepared by modifying the method of previous literature (Wang and Zhang, 1997;Zhang et al., 1998;Ellott and Zhang, 2001;Choe et al., 2004). Three different concentrations of FeCl36H2O, 1.0, 0.1, and 0.01 M, were prepared with 30% technical grade ethanol, 70% deionized water (v/v). Due to the spillage of hydrogen foaming, NaBH4solution was carefully added to FeCl36H2O aqueous solution for the production of synthesized particles. In order to compare the capability of reducing nitrate, the same mass of Fe0has to be produced from various precursor concentration systems. Therefore, the reaction solution
volumes were 0.5, 5, and 50 mL for the 1.0, 0.1, and 0.01 M FeCl3systems, respectively.
The reduction of ferric iron (Fe3+) leading to Fe0was the result of redox reaction in which electrons from a reducing agent (NaBH4) were transferred to iron according to the following schematic chemical equation:
FeðH2OÞ3þ6 þ3BH4þ3H2O ! Fe0# þ3BðOHÞ3þ10:5H2. (1) The reaction above was carried out at ambient temperature with magnetic stirring. Here, the nanoscale Fe0 particles prepared by 1.0, 0.1 and 0.01 M FeCl36H2O are expressed as Fe0
1 M, Fe00:1 Mand Fe00:01 M, respectively.
2.3. Characterization of synthesized particles
Surface area of the nanoscale Fe0 particles was measured using isothermal nitrogen adsorption method with an ASAP 2010 surface analyzer. Three different concentrations of FeCl36H2O (0.01, 0.1 and 0.01 M) produced the synthesized particles which were observed with a Hitachi H-7100 trans-mission electron microscopy (TEM) to characterize the size and size distribution of the metal particles.
The electrochemical analysis experiments were performed using a standard three-electrode cell and the micro-proces-sor-controlled electronic potentiostat (EG&G, Princeton Ap-plied Research; model 273A) with software Zplot2 and Zview2. The working electrode was a cavity microelectrode (CME), similar to that described byVivier et al. (1999). The cavity was then filled up with nanoscale Fe0particles using the electrode as a pestle. A Pt wire was used as the counter electrode and an Ag/AgCl electrode was used as the reference. The polarization curves were recorded in the potential range from 0.5 to 0.8 V at a scan rate of 0.5 mV s1. An Ar-purged 40 mg-N L1 nitrate solution was employed to examine the electro-chemical corrosion properties of the nanoscale Fe0particles. All experiments were performed stirring at 200 rpm at 2570.1 1C.
2.4. Batch experiment with NO3
Kinetic batch experiments were conducted to investigate reactivity of Fe0
1 M, Fe00:1 M and Fe00:01 M particles for nitrate
reduction reaction. Oxygen has been removed by purging the nitrate solution with argon. Plastic bottles with a 75 mL capacity were filled with 75-mL samples of 40 mg NO3-N/L nitrate aqueous solution and 0.0265 g of the Fe0particles. The bottles were capped with Teflon silicone septa and aluminum seals. They were then mixed at 200 rpm using a reciprocal shaker water bath (Yihder, BT-350R) at ambient temperature (25 1C) without pH control. Then, the samples were filtered using a Milipore filter (25 mm diameter, 0.2 mm pore size) at certain time intervals. Filtrates were collected and analyzed immediately.
2.5. Sample analysis
Target pollutant, nitrate, and intermediate, nitrite, were measured using an ion chromatograph (Model: Dionex DX-100TM) with a column of IonPac AS4A-SC (4.0 mm 4.0 mm I.D.). A mixed solution of 1.7 mM Na2CO3and 1.8 mM NaHCO3
was used as the mobile phase at a flow rate of 1.0 mL min1. Ammonium was analyzed by indophenol method using a spectrometer (UV SPECTRONIC 20 GENESYS). The pH was measured with a Beckman Model 71 pH meter.
Nitrogen gas was identified by HP5830 GC with a 180 0.63 cm packed molecular sieve No. 5A column and a thermal conductivity detector. Helium was used as the carrier gas at a flow rate of 30 mL min1. A 1 mL gaseous sample was withdrawn from the headspace of the reactor. The column temperature was kept at 40 1C. The injection and detector temperature were 100 1C. Peaks were quantified by comparing retention time and peak areas with standard gas (Supelco).
3.
Results and discussions
3.1. Characterization of nanoscale zerovalent iron Five precursor concentrations, 0.01, 0.05, 0.1, 0.5, and 1.0 M were used to prepare the nanoscale Fe0. Iron grain sizes were determined by measurement of TEM micrographs. The techniques agreed reasonably to give an average particle size of 9.5, 42.5, 40.0, 45.0, and 45.0 nm for nanoscale Fe0prepared by 0.01, 0.05, 0.1, 0.5, and 1.0 M, respectively (Fig. 1). Nanoscale Fe0particles prepared by 0.01 M had diameters in the range 9–10 nm. As the precursor concentration was increased, the range of iron particles expanded widely (Fig. 1). This result was immediately apparent in the synthesis that the precursor concentration had a strong effect on the nanoscale Fe0 particle size in the reaction production.
Three precursor concentrations, 0.01, 0.1, and 1.0 M, were chosen for the TEM observation.Figs. 2(a) and (b) present a well-aggregated structure for Fe01 M and Fe00:1 M samples. TheFe00:01 Msample shows isolated particles that are distinctly different from the aggregation. It is small in particle size and, moreover, it is separately formed. The nanoscale Fe0particles produced by chemical precipitation process are usually the result of three processes: (1) particle nucleation, (2) nuclei
growth to primary particles, and (3) secondary changes in the resulting particle suspension by agglomeration (So¨hnel and Garside, 1992). The number of nuclei depends on the concentration of precursor solute (Siantar et al., 1996). The same number of nuclei was produced in this experiment because the amounts of ferric ion were equal in different precursor concentrations. Since the nucleation is seldom in the final stage of the Fe0particle formation, primary particles form thereafter. The precursor concentration decreases as particle growth proceeds, and therefore, large primary particles are produced under dilute precursor solute. As a rule, these primary particles aggregate to form final Fe0 particles due to their large free energy. However, primary particles and their agglomerates are clearly observed in TEM image (Figs. 2a–c). Von Smoulchowski established that the van der Waals forces become significant when two particles approach within a certain distance. He also defined a sphere action with radius R where R is proximally double the radius of particles (r). Up to separation distance R particles approach without any interaction. On the other hand, when the distance between particles becomes smaller than R, the particles adhere irreversibly. In this study, equal amounts of Fe0 per unit were obtained from different volumes and different concentrations of precursor solute. Therefore, a dense solute presents a reduced volume, and compact aggregates occur with strong attraction between particles. HenceFigs. 2(a) and (b)show a compact aggregate structure built up by numerous small primary particles. Due to the weaker bonded aggregate structure, theFe00:01 M particles are observed as show inFig. 2(c).
3.2. Kinetics of nitrate reduction by nanoscale Fe0 The reaction rate was evaluated with nitrate unbuffered solution (40 mg-N L1) containing 0.0265 g of Fe0
1 M, Fe00:1 M, and
Fe0
0:01 M particles (Fig. 3). The reduction of nitrate followed
pseudo-first-order kinetics with respect to the concentration
0 10 20 30 40 50 60 70 80 90 100 0.001 0.01 0.1 1 10 Precursor concentration (M) A v
erage particle size (nm)
of nitrate r ¼d NO 3 dt ¼kobs NO 3 , (2)
where kobs is the observed pseudo-first-order reaction rate constant (min1). The reduction of nitrate using Fe0
1 M and
Fe0
0:1 M exhibited a stagnation phenomenon at a rate of 0.16
and 0.25 102min1, respectively. Compared with Fe0
1 Mand
Fe0
0:1 M, the kobsvalue of Fe00:01 Mwas promoted by a factor of
about 5.5–8.5 to 1.37 102min1. Generally, the nitrate reduction rate is proportional to the amounts of exposed iron surface. Therefore, regarding the iron activity per unit surface area, the kobsnecessarily normalize according to the surface area and the mass concentration of iron particles. The surface area normalized rate constant (kSA) can be calculated by
kSA¼kobs=ra, (3)
where rais the surface area concentration of iron in m2L1, and here, the kSAis a parameter of assessment of the overall surface reactivity. The BET surface areas are 16.16 m2g1for
Fe01 M, 17.50 m2g1 for Fe00:1 M, and 56.67 m2g1 for Fe00:01 M (Table 1). The value of ra is 5.71 m2L1for Fe01 M, 6.18 m
2 L1 for Fe0
0:1 M, and 20.02 m2L1 for Fe00:01 M in the batch
experi-ments. Thus the kSAfor Fe01 M, Fe00:1 M, and Fe00:01 Mwere 2.80, 4.04, and 6.84 104min1m2L, respectively, as shown in
Table 1. The reactivity of Fe0
0:01 Mwas higher relative to both
Fe01 Mand Fe00:1 Mas indicated by a larger KSAfor Fe00:01 M. This fact is due to the decrease of the precursor concentration to 0.01 M with the rise in chemical reactivity in each reactive site on the iron surface (Fig. 3).
Besides being a degradation test of nitrate by Fe0
0:01 M, this
research also investigates the reaction stoichiometry and the total nitrogen mass balance (Fig. 4). During experiments, the concentration of nitrate, nitrite, ammonium, nitrogen gas, and pH value of the solution were monitored. Until now, a pathway for nitrate reduction by Fe0has not been proposed well.Cheng et al. (1997a, b)andHuang et al. (1998)observed complete reduction of nitrate to ammonia with a pH buffer using a commercially microscale iron powder. In contrast, in nanoscale iron particle condition,Choe et al. (2000)reported 100% mass recovery as nitrogen gas; however, in some Fig. 2 – Transmission electron microscopy image of nanoscale Fe0particles produced by: (a) 1.0 M FeCl
3, (b) 0.1 M FeCl3, and (c) 0.01 M FeCl3. A: aggregates of small particles with stronger bond. B: aggregates of small particles with weaker bond.
Table 1 – The specific surface area, pseudo-first-order rate constant and surface area normalized rates for Fe0
0:01 M, Fe00:1 M
and Fe01 M
Fe0
1 M Fe00:1 M Fe00:01 M
BET specific surface area (m2g1) 16.16 17.50 56.67
kobs(102min1) 0.16 0.25 1.37 kSA(104L min1m2) 2.80 4.04 6.84 0 0.8 0.7 0.9 0.6 0.5 0.4 0.3 0.2 0.1 1 0 20 40 60 80 100 Time (min) Nitrate conc.(mg-N L -1) Fe0 0.01 M Fe0 0.1 M Fe0 1.0 M
Fig. 3 – A plot of nitrate concentration vs. reaction time. Initial nitrate concentration was 40 mg NO3–N/L. Fe0 content 0.3533 g L1. 0 5 10 15 20 25 30 35 40 45 0 50 100 150 200 250 Time (min) Conentration (mg-N L -1) 7 8 9 10 11 p H f inal
ammonium nitrogen gas pH
nitrate N-balance
Fig. 4 – Disappearance of nitrate and formation of ammonia with pH increase. Initial nitrate concentration was 40 mg NO3–N/L. Fe00.0265 g/75 mL.
studies, nitrite was found to accumulate as an intermediate product (Westerhoff and James, 2003).
In this study, as nitrate disappeared, ammonium formed accordingly (Fig. 4). Nitrite was not detected while using an ion chromatograph (the detection limit: 0.1 mg-N L1). Two pathways by which Fe0may reduce nitrate to nitrogen gas or ammonium are as follows:
5Fe0þ2NO 3þ12H þ 25Fe2þþN 2þ6H2O; (4) 4Fe0þNO 3þ10Hþ24Fe2þþNHþ4þ3H2O: (5)
The total nitrogen mass balance was about 95.7% in which nitrogen as ammonium accounted for 57.4% and as nitrate for 30.8% (Fig. 4). Moreover, a small amount of nitrogen gas (7.5%) was detected.
Five organic buffers, 2-(N-morpholino)ethanesulfonic acid (MES), 3-(N-morpholino)propanesulfonic acid (MOPS), piper-azine-N, N-bis(2-ethanesulfonic acid) (PIPES), (2-hydroxye-hyl)piperazine-(2-ethanesulfonic acid) (HEPES) and N-(tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS), were chosen by Alowitz and Scherer (2001) to adequately maintain the initial pH at a range of 5.5–9.0. Some nitrogen loss unaccounted for in this batch experiment may be attributed to: (1) Nitrite sorbed on the iron surface:Liou et al. (2005)have proposed that nitrite remained sorbed to the high reactivity surface of nano-Fe0particles until the forma-tion of ammonium or nitrogen was achieved, similar to the sequential dehalogenation of carbon tetrachloride to methane as reported by Matheson and Tratnydk (1994). (2) Ammonium adsorption on iron surface was observed in a batch experiment of N-Nitrodimethylamine reduction in previous study (Gui et al., 2000). At initial stage, the total nitrogen mass balance was 99+% because the ammonium formation was not significant. As the amount of ammonium increased, the nitrate removal decreased and remained steady afterward.
3.3. Electrochemical properties
Perng and Wu (2003) indicated that the overall reaction sequence of contaminant-free water/Fe0could be summar-ized as Hþ þe þFe0!Fe H ðadsÞ #
þFe HðadsÞ!2Fe HðadsÞ!H2þFe2þþ2e. ð6Þ Here, the adsorbed hydrogen atoms (Hads), serving as the reducing agent, are formed via the reduction of protons from water at the iron surface (Cheng et al., 1997a, b; Lin et al., 2004). When nitrate was added to the solution, nitrate may have been adsorbed onto the iron surface, and rapidly reduced to ammonia or nitrogen by neighboring Hads, as shown in Eqs. (4) and (5). The hydrogen concentration on the iron surface, [Fe–H(ads)], plays an important role in the nitrate reduction reaction by Fe0. The calculation of [Fe–H
(ads)] can be expressed by (Perng and Wu, 2003):
½Fe HðadsÞ ¼Ci1=20 , (7)
where i0 is exchange current density (A cm2) and C is constant. The relative magnitude of i0 gives a measure of reaction kinetics. The Tafel profiles of log|i0| vs. the applied potentials for Fe0
1 M, Fe00:1 M, and Fe00:01 Mare shown inFig. 5. The
i0may be found at the intersection of the extrapolated linear regions of the Tafel curve (Bard and Faulkner, 1980). The i0 values are shown inTable 2(1.40, 3.30 and 15.8 107A cm2 for Fe0
1 M, Fe00:1 M and Fe00:01 M, respectively). The larger i0
demonstrates that Fe0
0:01 M is more reactive in the nitrate
solution than both Fe0
0:1 Mand Fe01 M, and is consistent with the
increase of the denitrification rate exhibited by Fe00:01 M. The ratio of [Fe–H(ads)]0.01 M:[Fe–H(ads)]0.1 M:[Fe–H(ads)]1.0 Mis approxi-mately 3.35:1.53:1. Besides, the [kSA]0.01 M:[kSA]0.1 M: [kSA]1.0 Mis 2.44:1.44:1. These trends indicate that the increase of i0with the rise in Hads production led to the rise in chemical
-9 -8 -7 -6 -5 -4 0.5 0.55 0.6 0.65 0.7 0.75 0.8 Potential (V) vs. Ag/AgCl log l i l (A cm -2) Fe0 0.01 M Fe0 0.1 M Fe0 1.0 M
reactivity in each reactive site on the iron surface. This fact confirms that decreasing the precursor concentration of nanoscale Fe0 increased the chemical reactivity of the particle.
4.
Conclusions
The results of this study indicate that the precursor concen-tration is a critical parameter, which controls the nanoscale Fe0 particle production. The synthesized nanoscale Fe0 particles were employed for the denitrification of unbufferd 40 mg-N L1nitrate solution at initial neutral pH. The results obtained in this study have demonstrated the following: (1) The majority of nanoscale particles of Fe01 Mand Fe00:1 Mare
in the size range of 20–60 nm and 20–70 nm, and the specific surface are 16.16 and 17.50 m2g1, respectively. However, an absolutely reduced size (9–10 nm) and great-est specific surface area (56.67 m2g1) results with produc-tion under 0.01 M condiproduc-tion.
(2) The nanoscale Fe0 particles could effectively remove nitrate without acidification. The first-order degradation rate constants (kobs) follow the trend Fe00:01 M4Fe00:1 M4 Fe0
1 M.
(3) The reactivity of Fe0
0:01 Mparticles surface was higher than
that of Fe01 M and Fe00:1 M as indicated by a larger kSA for
Fe00:01 M. A rising reactivity 1.69–2.44 times that of Fe00:01 M
particles on a mass basis.
(4) The largest i0 demonstrated the rise in Hads production, and led to highest chemical reactivity in each reactive site on the iron surface.
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![Table 2 – Exchange current density (i 0 ) and [Fe–H (ads) ] j M ratio of Fe 0 0:01 M , Fe 0 0:1 M and Fe 0 1 M](https://thumb-ap.123doks.com/thumbv2/9libinfo/8781235.215909/7.892.79.823.129.272/table-exchange-current-density-fe-ratio-fe-fe.webp)
