Environ. Sci. Technol. 2008, 42, 3780–3785
Temperature Programmed Reduction for Measurement of Oxygen Content in Nanoscale Zero-Valent Iron JIASHENG CAO,† XIAOQIN LI,† JAVAD TAVAKOLI,‡ AND W E I - X I A N Z H A N G * ,† Department of Civil & Environmental Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, and Department of Chemical Engineering, Lafayette College, Easton, Pennsylvania 18042
Received November 11, 2007. Revised manuscript received February 10, 2008. Accepted March 5, 2008.
Nanoscale zerovalent iron (nZVI) has increasingly been used for environmental remediation and in toxic waste treatment. Most applications exploit its large surface area and high reactivity, the latter being a function of zerovalent iron content. In this work, temperature programmed reduction was applied to measure oxygen in nZVI. Iron oxides in nZVI were reduced by hydrogen to form metallic iron and water, which was then measured with an online mass spectrometer to determine oxygen content of the sample. For fresh nZVI prepared by sodium borohydride reduction of iron salts, average oxygen content was 8.21%. Total iron content was approximately 90.35% by the method of acid digestion; Fe(III) content was estimated at 14.37%, and that of zerovalent iron [Fe(0)] at 75.98%. The oxygen content quickly increased to 26.14% after purging with oxygen for four hours. Several other techniques were also used to characterize the iron nanoparticles. High resolution TEM provided direct evidence of the oxide shell structure and indicated that the shell thickness was predominantly in the range of 2–4 nm. The surface elemental composition was determined from highresolution X-ray photoelectron spectroscopy. The nZVI oxygen content results fill a knowledge gap on nZVI composition.
ide as the outer layers (7, 8, 12). The shell grows with the progress of ZVI oxidation. The reactivity and total reduction capacity of nZVI are functions of the zerovalent iron content in nZVI particles. A number of recent publications have examined nZVI reactivity with chlorinated aliphatic compounds. However, very little information is available on the nZVI composition and surface structure (12). It is often difficult to determine the exact content of zerovalent or metallic iron in a sample due to the presence of oxidized iron on its outer shell. Traditionally, the ZVI content is estimated based upon production of hydrogen gas under acidic conditions or reactions with targeted contaminants such as chlorinated organic compounds (13). Those experiments are often tedious and use hazardous chemicals. In this work, the oxygen content of nZVI produced chemically by the reduction of ferric or ferrous iron salts was measured. During synthesis, a core–shell nanoparticle structure forms. Previous works using X-ray photoelectron spectroscopy (XPS) have shown the basic core–shell structure of nZVI (7, 8, 11). The particle size distribution has also been measured by transmission electron microscopy (TEM) and light scattering methods. These studies have shown that nZVI prepared by the solution method is polydispersed, with an average diameter of 60 nm and a standard deviation of approximately 15 nm. TEM images have revealed that the shell thickness varies significantly from 1 to 20 nm. Nonetheless, the chemical or elemental compositions, ZVI content, or the degree of iron oxidation within the core–shell have not yet been reported. The purpose of this work was to investigate the feasibility of a temperature-programmed reduction (TPR) technique for measurement of oxygen content in nZVI. Information on the oxygen content was then applied to determine the iron content and the degree of iron oxidation. TPR is a wellestablished technique used in the characterization of solid materials such as catalysts (15–21), but its use in environmental research is still very limited. The key feature of TPR is the reduction of a solid material in a reducing atmosphere with a precisely controlled temperature program. Hydrogen and carbon monoxide are the common reducing gases. The consumption of the reducing gas and the generation of specific product(s) can be precisely measured and used for measurements of particle compositions (14–16).
Introduction As an environment-friendly reductant, nanoscale zerovalent iron (nZVI) has been increasingly used in the treatment of toxic and hazardous chemicals and remediation of contaminated soil and groundwater, as nZVI can degrade a wide range of organic (e.g., chlorinated hydrocarbons) (1–5) and inorganic (nitrate, chromate, perchlorate, metal ions) (6–10) pollutants. Chemically, zerovalent iron serves as a costeffective electron donor. Structurally, the small particle size of nZVI provides a high surface area-to-volume ratio, promoting mass transfer to and from the iron surface, increasing the adsorption and reaction capacity for contaminant removal and/or degradation. In engineering practices, the small nZVI particle size may offer additional advantages in easy mixing and mobility when used in groundwater applications (5). It is generally accepted that nZVI has a core–shell structure with zerovalent iron in the nucleus and iron oxide/hydrox* To whom correspondence should be addressed. Email: wez3@ lehigh.edu. † Lehigh University. ‡ Lafayette College. 3780
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 10, 2008
Materials and Methods Materials. Ultra high purity argon (Ar) and hydrogen (H2, 10% in Ar) were used in all experiments. Ferric chloride anhydrous (FeCl3) was obtained from Alfa Aesar. Sodium borohydride (NaBH4, 98%) was purchased from Finnish Chemicals, Finland. High purity iron(III) oxide (Fe2O3, 99.99%, 99%, ∼10 mesh, Aldrich) with known oxygen contents were used as reference materials for the oxygen measurements. Several other iron particles were also tested in this work, including iron materials from BASF (4.1 µm, BASF, Germany), Hoeganaes (H-200, >200 mesh, D50 ) 85 µm, Hoeganaes, New Brunswick, NJ), Fisher (40 mesh, 420 µm), and Fisher (325 mesh, 44 µm). The purchased iron particles were pretreated with high purity argon flowing at 20 mL/min for 1 h before use. Additional information on the iron materials used in this work is summarized in Table 1. Preparation of Iron Nanoparticles. Two iron nanoparticles were analyzed with the TPR techniques, fresh and aged nZVI. Fresh nZVI (10∼100 nm) was prepared by the reduction of ferric chloride with sodium borohydride (1, 11). The 10.1021/es7027845 CCC: $40.75
2008 American Chemical Society
Published on Web 04/16/2008
TABLE 1. Summary of nZVI and Iron Oxide Investigated in This Work sample
source
size
surface iron
oxygen (%)
total iron (%)
Fe2O3 Fe2O3 FeO Fe Fe Fe Fe nZVI
Aldrich Aldrich Aldrich BASF Hoeganaes (H-200) Fisher Fisher Lehigh
5∼25 nm 99.9%) were measured using the same procedure. The theoretical oxygen contents of Fe2O3 and FeO were 30.08 and 22.2%, respectively. Figure 4 depicts the reduction profiles of 5.9 mg Fe2O3 (
