Effect of precursor concentration on the characteristics of nanoscale zerovalent iron and its reactivity of nitrate
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 reductant to remove nitrate in water, and its application has been reported in several publications in recent years (Siantar et al., 1996; Cheng et al., 1997, Cheng et al., 1997; 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 Fe0 is 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 prepared by borohydride reduction of an aqueous iron salt in 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 Fe0 particles (Wang and Zhang, 1997; Zhang et al., 1998; Choe et al., 2000; Ellott and Zhang, 2001). The other method is by reducing FeSO4 with NaBH4 (Ponder et al., 2000; Schrick et al., 2002). Fe0 particles formed by the above methods were found to have a BET surface area in the range 18–33.5 m2 g−1 and 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 Fe0 particles preparation by precipitation is usually the result of three processes: (1) particle nucleation, (2) particle growth and (3) secondary changes in the resulting particle suspension by agglomeration (Söhnel 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 concentration 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 Fe0 particles 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.
Section snippets
Chemicals
Potassium nitrate and sodium nitrite were purchased from Aldrich (99+%, Milwaukee, WI). Nessler's Reagent (Fluka) was used for ammonia measurement. The FeCl3·6H2O 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-Q™ system (18.2 MΩ cm−1).
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,
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 Fe0 prepared by 0.01, 0.05, 0.1, 0.5, and 1.0 M, respectively (Fig. 1). Nanoscale Fe0 particles prepared by 0.01 M had diameters in the range 9–10 nm. As the precursor concentration was increased, the range of iron
Conclusions
The results of this study indicate that the precursor concentration 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 L−1 nitrate solution at initial neutral pH. The results obtained in this study have demonstrated the following:
- (1)
The majority of nanoscale particles of and are in the size range of 20–60 nm and 20–70 nm, and the specific surface are 16.16 and
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