Nanoparticles of Magnetite
Many of the high profile uses of magnetite are when magnetite is in the form of a nanoparticle - i.e. in the particle size of below the micrometre scale (3). Such examples of nanoparticle magnetite include as in ferrofluids, which have been demonstrated as useful in drug delivery (4), and water purification. Some of these uses employ microscale-sized magnetite in addition to nanoscale.
Industrial Processes
Perhaps the best-known application of magnetite black sand is in the industrial scale synthesis of ammonia through the Haber-Bosch (H-B) process (5). The H-B process produces ammonia by converting atmospheric nitrogen with hydrogen under elevated temperatures and pressures, employing a heterogeneous iron catalyst. Magnetite is the primary source material for this. Ground magnetite is partially reduced, relieving it of some of its oxygen, leaving a catalyst bearing a magnetite core with an outer shell of ferrous oxide (FeO, würstite). The advantage of this catalyst comes in its porosity, and thus it is a highly active, high surface area material. Ammonia is a major chemical feedstock and is a key component in the manufacture of fertiliser, and the use of magnetite in H-B provides an inexpensive and reliable catalyst for this globally important process (6).
A further high importance application of magnetite black sand lies within the Fischer-Tropsch (F-T) process, where carbon monoxide and hydrogen are converted into small, straight chain hydrocarbons, which can then undergo a cracking/reforming/isomerisation process into synthetic fuels. F-T is an essential part of the global petroleum sector, ensuring supplies of hydrocarbons when traditional production is inhibited, and by consistently producing low-sulfur diesel products. The F-T reaction between carbon monoxide and hydrogen (synthesis gas) is typically catalysed by a transition metal catalyst, such as nickel, cobalt or ruthenium. Iron-based catalysts are also a popular choice due to their ubiquity and relatively inexpensive nature – magnetite oxide being such an example. Powdered magnetite is reduced partially with hydrogen, producing a low-porosity, low-pore size catalyst with diameters in the region of 100 microns. The magnetite catalyst is active with a low loading of a promoter such as silica, and at typical reactor temperatures of 340°C (7). Iron catalysts, like magnetite, have also been shown to be effective in lower temperature F-T processes producing liquid hydrocarbons and waxes. Iron catalysts are less sensitive to poisoning by hydrogen sulfide (a common contaminant in synthesis gas) than cobalt catalysts, and are inherently cheaper than their ruthenium counterparts (8). In addition, magnetite is active in the water-gas shift reaction that accompanies the main F-T reactions (9).
Naturally occurring magnetite has been used as a catalyst for the high-efficiency degradation of hydrogen peroxide into hydroxyl radicals, which then were used to degrade para-nitrophenol. Magnetite black sand powder with a particle size of 75 microns has been used at a loading of 1 g/L at neutral pH to rapidly catalyse the degradation of peroxide to hydroxyl radicals, which rapidly caused the decomposition of the para-nitrophenol present(10). Para-nitrophenol is a known contaminant from a variety of industrial processes, such as petrochemical, pesticide and paper manufacture; and a known pollutant. Magnetite black oxide has been demonstrated working in a similar way in the degradation of 2-chlorobiphenyl, via the superoxide mediated, magnetite assisted production of hydroxyl radicals(11). It was noted in both cases that a small amount of the contaminant was removed by surface adhesion to the magnetite catalyst.
Further degradation processes catalysed by magnetite fertilizer are those which eliminate polycyclic aromatic hydrocarbons, n-alkanes and refractory oil residues as contaminants in soils. In two studies, powdered magnetite fertilizer was demonstrated as a highly effective catalyst for the Fenton-like (peroxide to hydroxyl, as above) and persufate oxidation degradation pathways(12). Magnetite outperformed a soluble Fe2+ catalyst at removing between 80 and 90% of crude oil residues from soil in one week, compared to just 10-15% for the regular iron catalyst(13). The use of magnetite fertilizer as a soil contamination reliever is particularly welcomed owing to magnetite’s relatively low toxic nature. It has also been shown as an effective catalyst in the degradation of phenol - another industrial by-product - under ultraviolet irradiation where the reduction of Fe3+ to Fe2+ was deemed to play a key role in catalyst activity, with the size of magnetite black oxide particles being unimportant(14).
Medicinal Uses
Magnetite has found wide use in the medicinal field. DNA has been shown to be extracted from kernels of maize vie the use of magnete and magnetite-silica composites, both performing better than commercially available DNA extraction kits. The extraction using magnetite black oxide was high yielding and resulted in extracts that were suitable for use in enzyme digestion and the polymerase chain reaction process(15). 5 micron scale magnetite powder has been used as a dye in stained gelatine for the assay of proteolytic activity - the breakdown of proteins into smaller polypeptides and/or amino acids(16).
Magnetic Resonance Imaging (MRI) contrast agents are often reported as high efficacy applications for magnetite due to their superparamagnetic properties - they become magnetic inside the strong magnetic field of the MRI instrument, but loose this magnetism when the field is no longer applied, and are highly detectable(17). Vivo studies with rats have shown that when combined with dextran (a long chain polysaccharide), it will cross the blood-brain barrier and provide effective contrast properties(18). One report showed that the intentional ingestion of powdered magnetite proved an unexpected source of contrast agent, though it should be noted that the intentional consumption of magnetite as a dietary supplement for iron is not recommended(19).
As a ferrofluid, Fe3O4 has found potential use in the treatment of hypothermia(20), whereby a solution of metallic materials (in this case magnetite) was suspended in a commercial gel to mimic mammalian tissue. By passing a current across the magnetite-bearing gel, localised heat was induced. A ferrofluid is a dispersion of an iron-type substance in a liquid medium. Similarly, magnetite has been included in the manufacture of ferrimagnetic glass-ceramics and used as ‘thermoseeds’ for the hyperthermic treatment of cancer cells, with magnetite content of up to 60%. Such ‘thermoseeds’ are implanted around tumours in granular form and hyper-localised heating is induced by the application of a magnetic magnetite field(21), which causes cell death.
Other Uses
As a component in recording media, magnetite has been used - although it is often reduced to gamma-Fe2O3 for high quality recording applications(22), doped with small amounts of cobalt for optimal readability. Magnetite has been found to be a high-performing black pigment in thermal coatings, with higher light absorption than other common inorganic black pigments(23). It is used in toner cartridges in printing applications.
Magnetite has been employed extensively in water purification and has been formed into polymeric microspheres alongside styrene and divinylbenzene to produce magnetic ion-exchange resins(24), showing good efficiency at removing toxic cobalt and nitrate contaminants from water. In a plant in Australia, micron-scale magnetite has been used as a reagent in purification and clarification of water, producing a potable supply from low quality ground and surface water. The issues relating to a ‘loaded’ reagent being hard to remove were resolved by the magnetic magnetite nature (25). Chlorinated hydrocarbons can be removed from water via bacteria that have been adsorbed onto magnetite, which can then be removed using a magnetic field(26).
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