Metal Oxides In Soils: The Effects Of Hematite And Magnetite
Introduction To Iron In Soils
Soil can, naturally, be high in iron content. Soils bearing a strong red or orange colour are often iron rich, due to the presence of natural oxides of iron such as pyrite, magnetite or hematite. These oxides, whilst all oxides of the same base metal, can impart wildly different properties to the soil beyond just colour. Such phenomena will be discussed below. Important to note is the potential of iron, in any oxidation state, to be reduced or oxidised under relatively mild conditions by the presence of certain types of bacteria(1). Iron compounds have long been added to soils. In soils, because the iron oxide is so dispersed relative to other compounds and the soil itself, these iron oxides when sourced from soil are not regarded as useful for iron/steel production(2).
Magnetite In Soils
Magnetite is a form of iron oxide, Fe3O4, which possesses uniquely iron in both the Fe2+ and Fe3+ oxidation states. Bearing the form of an inverse spinel, it’s magnetic properties are highly valued. Addition of magnetite to soil is advantageous as it can later be removed easily by using magnets. It can occur naturally in soil, or be added as a soil treatment agent.
Magnetite’s major function as an additive in soils is to catalyse the degradation of contaminants in the soil. Such contaminants include, but are not limited to, industrial pollutants, aromatic organic compounds and others. Soil composition is an important factor in choosing what to plant in the soil, therefore it is important to eliminate contaminants where possible.
Dealing With Contaminated Soils
Magnetite is useful in the removal of other heavy metals from contaminated soil, with one study reporting the effective immobilisation/removal of metals including cadmium, lead and uranium via the application of ca. 1.5 weight% magnetite nanoparticles(3). This study claims that magnetite bears strong adsorption properties to other metals, and may be useful in the in situ removal of pollutants from contaminated soils. Reductive chlorination of chlorinated ethylenes, such as trichloroethylene and vinyl chloride, has been demonstrated from soils on a laboratory scale(4). The dechlorination followed a hydrolysis-type pathway, which was accelerated by a factor of ten by the addition of Fe2+ from magnetite. Magnetite has also been used for the reduction of nitrobenzene from soils, associated with industrial runoff, and it has been suggested that increasing stoichiometries of magnetite can reduce the nitrobenzene in the absence of fully soluble Fe2+(5). It can be used to stabilise excess arsenic in mining tailings(6), though in most cases of soil/tailings arsenic contamination removal, the addition of zinc aids the process in terms of efficiency(7).
In addition to chlorinated ethylenes, the removal of other industrial wastes that can catalysed by magnetite include processes that eliminate polycyclic aromatic hydrocarbons, n-alkanes and refractory oil residues as soil contaminants. Fenton-like peroxide and persulfate oxidation degradation processes catalysed by ground magnetite in soil have been demonstrated(8). Studies have shown that more complex iron(ii) catalysts are outperformed by simply using powdered magnetite in eliminating up to 90% of crude oil contaminants from soil in as little as seven days, favourably compared to just 15% of contaminant removal for a commercially obtained iron catalyst(9).
Magnetite’s use to relieve contamination of soils from industrial and agricultural runoff is particularly valued due to its lack of human or animal toxicity. Other industrial byproducts often found in soils include phenols and related aromatic hydrocarbons, the removal of which is catalysed by magnetite powder under ultraviolet light. Reduction from Fe3+ to Fe2+ is the leading explanation of the catalytic process. Notably, this process is not enhanced by the magnetite being in nanoparticle form(10).
Conversion Of Magnetite To Maghemite And Other Oxides Of Iron
Maghemite, γ-Fe2O3, is formed when magnetite is exposed to temperatures in the 350-400 °C region, or when under oxidising conditions. Some naturally magnetite-rich soils can contain moderate-to-high amounts of maghemite, such as those found in tropical climates. Magnetite is not stable in wet soils over long time frames, converting to magnemite(11).
Furthermore, magnetite can react with nitrites in soils(12). Such nitrites are often components of fertilisers, and if magnetite is present bearing Fe3+ ions in the presence of nitrite, it can remove nitrate from the soil, eventually reducing it to nitric oxide and then to nitrous oxide gas, which leaves the soil. Soil denitrification is not ideal as it causes a requirement for more nitrogen to be fixed into the soil, as per the nitrogen cycle. In steatite-derived soils, magnetite can be converted to hematite(13), during soil formation.
Hematite In Soils
Like magnetite, hematite is an oxide of iron and it has the formula Fe2O3. Hematite is not magnetic, and so is not readily removed from soils to which it has been added. It also can occur naturally in soil.
Hematite is the most prevalent oxide of iron present in the ground, as such, it is also the largest source of iron for iron/steel production. Lateritic soils rich in hematite are often used as components in bricks and other building materials in the developing world, displaying a strong red colouration. Hematite is also present in bauxitic soils alongside alumina, which are also the major components in ‘red mud’ - the waste stream from the Bayer process(14).
Treatment Of Contaminated Soils
Like its cousin magnetite, hematite is also a good soil additive for the removal of harmful or potentially harmful contaminants. Such an example for hematite is the reduction of arsenic concentration in soils used for the growth of corn. Corn is a major agricultural product, reaching billions of humans every day. Arsenic is toxic to human life, and also slows plant growth. It is therefore crucial to remove arsenic from soil. One study applied between 0 and 0.2 weight% hematite nanoparticles to contaminated soil with arsenic contents of between 0 and 96 mg/kg. It was found that the amount of arsenic uptake into the roots and leaves of the corn plants was significantly reduced when the soil had been treated with hematite(15). Hematite was found to be ‘immobilising’ the arsenic, preventing uptake. When utilised in soils that are also high in alumina, it has been found that hematite is more effective at arsenic immobilisation(16). Hematite has also been used as part of a magnetic biochar formed from it and pinewood, which was used to remove arsenic from soils, and is particularly useful as the arsenic-loaded hematite biochar can be removed using magnets(17). The γ-Fe2O3 on the hematite was the arsenic ‘sponge’.
Phosphate and glyphosate, components of fertiliser and industrial herbicides respectively, have been shown to be adsorbed in soil by hematite(18). Interestingly, when the hematite is hydrated to either goethite or ferrihydrite, adsorption favours phosphate, whilst unhydrated hematite favours the herbicide. It should be noted that glyphosate is a suspected carcinogen and is toxic to aquatic life - therefore runoff should be minimised.
Hematite And Humic Acids
Humic acids are a broad class of organic compounds found in humus, the major component of soil. Heavy metal retention is related to humic acid concentration, especially when considering hematite as a method of removal of said heavy metals. Humic acid adsorption onto hematite is said to decrease with increasing pH, with other materials coordinating preferentially. But, in these systems, the humic acid leads to improved adsorption of heavy (and toxic) metals such as cadmium(19). These effects are observed in a different way with thorium in laboratory testing(20), where excess humic acid did not improve thorium adsorption to hematite.
Soil Additives/Iron Deficient Soil
Iron deficiency in soil can be a problem that profoundly affects plants. Typically, this occurs when soil pH is in excess of 6.5. Addition of iron to the soil is not a quick fix unless the iron is biologically available, that is, in a chelated form. Magnetite and hematite are not biologically available and thus treatment of soil containing plants suspected of being iron deficient is futile.
- Hematite and magnetite are oxides of iron that can be naturally found in, and/or added to, soils
- These oxides of iron afford the soils properties including durability (hence their use in building materials) and oftentimes intense colouration
- They are viable soil additives with the potential to remove - or catalyse the removal of - undesirable soil contaminants such as arsenic and other heavy metals
- Iron content in soils needs to be modulated to ensure no problems will arise from runoff into water courses, and to ensure that nitrogen fixation in soils is not unduly diminished
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