J. Phys. Chem. C 2007, 111, 10247-10253
10247
Influence of Aluminum Substitution on the Reactivity of Magnetite Nanoparticles Teresa L. Jentzsch, Chan Lan Chun, Rachel S. Gabor, and R. Lee Penn* Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: March 22, 2007; In Final Form: May 4, 2007
Magnetite is an iron oxide that commonly forms in soils under reducing conditions. It is an important material in natural and engineered remediation systems because it can affect the reductive degradation of environmental toxins such as halogenated hydrocarbons. To examine the influence of redox-inactive dopants and particle size on the reactivity of magnetite nanoparticles, samples with varying size and amounts of aluminum cation substitution (0-8.07 mol %) were synthesized. These materials were characterized, and their reactivity was examined using the organic molecules benzoquinone and carbon tetrachloride. Activation energies and frequency factors for the reaction of magnetite with benzoquinone were determined from a series of variable temperature kinetic studies. The ratio of products formed during the degradation of carbon tetrachloride was also studied to assess the influence of aluminum doping on the mechanism of reaction. The activation energy and frequency factor both initially decreased with increasing aluminum substitution, then increased as the aluminum content increased from 2.14% to 8.07%. Overall, the reactivity of magnetite nanoparticles decreased as a function of aluminum substitution in both systems, while the ratio of products resulting from the reaction with carbon tetrachloride seemed to be unaffected.
Introduction The study of iron oxides is interesting and important because these compounds play significant roles in the environment. They are found in waterways, soils, and rocks, and they often occur as nanoparticles in the 3-100 nm size range. The formation of iron oxides occurs through a variety of biotic and abiotic mechanisms, including precipitation, weathering, and redox processes.1 Iron oxides are influential in the fate and transport of environmental toxins because they can affect both oxidative and reductive transformations and have a high affinity for the sorption of heavy metals and other chemical species. For example, Fe(II) in magnetite has been shown to reduce aqueous chromate ions, which are mutagenic toxins, to precipitate relatively harmless chromium hydroxides.2,3 Magnetite (Fe3O4) is an iron oxide that is frequently found as a component of soils and rocks, and it is one of the most common mixed-valence iron oxides in the earth’s crust. It can be produced through a variety of redox and transformation processes, and most of the other iron oxides can be converted to magnetite under the appropriate pH and redox conditions.1 Magnetite can be formed through the biotic reduction of Fe(III), in which iron-reducing bacteria couple the reduction with the metabolism of organic material.4 Magnetite also plays a significant role within biological organisms. Magnetotactic bacteria5 have been shown to produce small chains of magnetite particles,6 called magnetosomes,7 that are believed to aid them in navigation via alignment with the earth’s magnetic field.8 The death of these magnetotactic bacteria results in the incorporation of small magnetite particles into the surrounding soil.9 Magnetite crystals have also been discovered in the abdomens of bees,10 the skulls of homing pigeons,11 and even in the brains of humans.12 Because of its prevalence in the environment and natural occurrence with iron, aluminum is often incorporated into the * Author to whom correspondence should be addresssed. Phone: 612626-4680. Fax: 612-626-7541. E-mail:
[email protected].
structures of iron oxides; natural samples of goethite, for example, have been shown to contain up to 32% aluminum cation substitution.1 The incorporation of aluminum into the structure of iron oxide particles is expected to change the chemical and physical properties of the materials. Previously, the effects of aluminum substitution on the properties of other iron oxides such as goethite13 and ferrihydrite14 have been studied, as has the kinetic behavior of magnetite samples substituted with aluminum and chromium.15 To our knowledge, however, neither aluminum-substituted magnetites containing less than 10% substitution nor their reactions with organic molecules in solution have been studied. To examine the redox behavior of aluminum-substituted magnetites, kinetic studies of benzoquinone reduction by synthetic magnetites were performed. Quinone species have been shown to serve as electron acceptors during the reduction of humic acids by bacteria16 and as electron shuttles throughout the bacterial reduction of iron oxides.17 Its environmental relevance was a significant reason for the selection of benzoquinone as the probe for observing the differences in reactivity of the doped magnetite samples. Other reasons included its ease of use and characterization, and the fact that its reaction with iron oxide species has been studied previously.18,19 Carbon tetrachloride (CT) was also used to probe the reactivity of Al-doped magnetite because it has been found as a common groundwater contaminant in the United States20 and has been used in many detailed studies of the reactivity of iron and iron oxides.21-26 In the presence of reduced iron species, CT is known to form a trichloromethyl radical (CCl3‚) and a chlorine ion (Cl-) via one-electron transfer as the first step. The intermediate trichloromethyl radical reacts by competing electron and atom transfer steps, resulting in two pathways: one producing persistent and toxic chloroform (CF) and the other forming relatively benign products like carbon monoxide, methane, and/or formate. The branching ratio between these two pathways reflects competition among elementary reaction steps
10.1021/jp072295+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007
10248 J. Phys. Chem. C, Vol. 111, No. 28, 2007 and has practical implications for environmental engineering because CF is an undesirable product.24,27 Previous studies showed that the rate of remediation of carbon tetrachloride by magnetite and the ratio of products formed may be affected by the presence of other ions.18,28 Thus, to better understand any effect of aluminum doping on reactivity, we examined reduction kinetics of CT and the yield of CF, which is one measure of the branching between the one- and two-electron-transfer pathways.26,29,30 The objective of this study was to examine the effects of the doping of magnetite nanoparticles with aluminum, a redoxinactive metal cation, on their reductive reactivity with two organic molecules: benzoquinone and carbon tetrachloride. Temperature studies were performed to determine the activation energy and frequency factor associated with the reactions between benzoquinone and the aluminum-substituted magnetites. In the case of carbon tetrachloride, the branching ratio was also monitored. Experimental Methods Synthesis of Aluminum-Substituted Magnetites. The synthesis is an adaptation of that of Vayssieres et al.31 For a typical synthesis, a solution of 1.0 M NaNO3 (J.T. Baker, ACS grade) was prepared and purged with nitrogen for a period of 30 min to remove oxygen and then adjusted to pH 12.0 with 1 M NaOH (Mellinckrodt, ACS grade); this solution was used to maintain a constant ionic strength during the syntheses. For one of the undoped syntheses, the concentration of NaNO3 was increased to 3.0 M, as increased ionic strength has been shown to decrease the resulting particle size,31 and this sample was used to probe the effects of particle size. A solution of 2.0 M Fe(NO3)3‚9H2O (Fisher, ACS grade) and 1.0 M FeCl2‚4H2O(Fisher) was prepared, and an appropriate amount of NaNO3 was added to match the concentration of the NaNO3 solution. For aluminumsubstituted syntheses, a fraction of the Fe(NO3)3 (2-10 mol %) was replaced by Al(NO3)3‚9H2O (Fisher, ACS grade), so that the total concentration of M3+ ions remained constant. To the pH 12 NaNO3 solution, 8.0 mL of the metal salt solution was added in 100 µL aliquots, with approximately 500 µL of 1 M NaOH solution added between each addition to maintain the pH at 12.0 ( 0.1. The solution was continually purged with nitrogen during the synthesis. The resulting suspension was initially a greenish-brown color that darkened to a brown-black as the synthesis proceeded. The suspension was then sealed in a Nalgene bottle and stirred overnight. The following day, the bottles were placed on a magnet to separate the magnetic material over a period of 1-4 h, with particles containing higher aluminum contents requiring longer settling times. The supernatant was discarded, and the particles were washed with deaerated, purified water. The samples were then centrifuged in 50 mL tubes at 6000 rpm (4588g) for 10 min. The washing and centrifuging steps were repeated 3 times to remove any remaining salts. The wet solids were then placed in an anaerobic chamber (
