Metal Oxides In Soils: The Effects Of Hematite And Magnetite
Whilst it is true that hematite and magnetite are mostly known as sources of iron, there are an array of use cases used to improve and remediate soils. Superior quality magnetite and hematite, milled to any specification, are available from African Pegmatite
Introduction To Iron In Soils
Soil can, naturally, be high in iron content. Soils bearing a string 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).
Other pathways that use magnetite in the assistance of immobilising heavy metals in soils include where the magnetite has been treated with a starchy coating. This coated magnetite was applied to controlled sandy soil systems which has arsenic concentrations of 31.45 mg kg-1. The combined magnetite and starch system was able to remove some 93% of arsenic containing molecules and reduce leachibility by in excess of 83%(8). Similar studies - albeit in the absence of the starchy resin - have shown that magnetite in soil can effectively remove more than 90% of arsenic residues and polycyclic aromatic hydrocarbons (PAH) when as little as 1% by weight magnetite was used(9).
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(10). 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(11).
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(12). Phosphates are known contaminants to soils that commonly occur from leaching from plants treated with NPK-type fertilisers. Biochar is an organic material produced by the combustion of small organic debris such as pelletised agricultural and forestry waste - it is often mixed into solis to enhance their properties in terms of stability and organic availability. Biochar that has been coated with powdered magnetite has been shown experimentally - in the laboratory and in the field - to remove excess phosphates from the soil at a rate of 3.38 mg phosphate per gram of coated biochar(13). This represents a reduction in phosphate leaching of around two thirds compared to untreated soil.
Magnetite And Biological Pathways In The Soil
Methanogenesis is a naturally occurring phenomenon in anoxic soils which is the final stage in the decomposition of organic matter - therefore to form soil itself. The process can be broadly thought of as the biological ‘fixing’ of carbon dioxide and methanol into the soil - they are both transformed via the help of bacterial organisms to methane. The produced methane is a significant proportion of accrued natural gas accumulations. Recent research into this area has probed additives to the already existing soil which may accelerate the methanogenesis pathway - experimental studies in the laboratory have added magnetite and granular activated carbon to microbial communities and have shown that magnetite increases methanogenesis (and therefore produced methane) by almost 20%(14). Researchers reported that the presence of magnetite activated certain gene activation functions.
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(15).
Furthermore, magnetite can react with nitrites in soils(16). 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(17), 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(18).
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-1. 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(19). 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(20).
A broader study looked into the effects in general of hematite present in arsenic containing soil on the efficacy rates of bacteria present and it was found that when gram negative bacteria Pseudomonas jinjuensis was used in testing, the presence of iron containing compounds such as hematite boosted the activities of the bacteria by a modest - but noticeable - 8%(21)
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(22). The γ-Fe2O3 on the hematite was the arsenic ‘sponge’.
Other potentially harmful contaminants to soils include chromium - including toxic hexavalent chromium (chromium-vi). Methods exist to ‘fix’ the chromium in the soil to prevent washout and/or leaching, but approaches to immobilise and then reduce the chromium(vi) have been developed. Using hematite treated biochar (see earlier for a brief introduction to biochar), reduction of the chromium(vi) increased from 28% to 39%(23). Hematite coated biochar was particularly effective at immobilisation of the chromium when Pseudomonas putida was present - the chromium release rate from the biochar soil was reduced by more than 50%. An unrelated study looked into the effect of hematite presence on the rate of bacteria catalysed methanogenesis in red mud(24). The study revealed that methane production increased by around 35% in the presence of hematite relative to the control, with multivalent cations provided by the hematite promoting the formation of compact aggregates (thereby increasing soil strength) and also allowing more redox active transfers.
Phosphate and glyphosate, components of fertiliser and industrial herbicides respectively, have been shown to be adsorbed in soil by hematite(25). 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. Copper, cadmium and phosphorus residues have been shown to be treated in paddy fields where the soil had been treated with hematite - authors noted that the presence of the iron oxide limited the overall redox potential of the soil, enhanced Cu and Cd immobilisation and decreased phosphorus availability overall(26).
Weathered petroleum/oil containing soil is regarded as difficult to remediate - this poses a problem as natural decomposition of such materials in the soil takes significant periods of time, all the while allowing potential exposure to harmful materials. Outside treatment of the most severe cases is possible by combining pyrolysis with a mixed carbon and hematite top layer soil additive. Research has shown that under laboratory conditions, soil pyrolysed with as little as 5% hematite by weight was able to reduce the amount of asphaltenes, resins and polyaromatic hydrocarbons by 68%, 52% and 67% respectively(27).
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(28). These effects are observed in a different way with thorium in laboratory testing(29), 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 polyaromatic hydrocarbons, oily residues, arsenic and other heavy metals including cadmium
- In some cases, magnetite and hematite in soils are beneficial for the organic and/or biological breakdown of materials in the soil, including via methanogenesis
- 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
Both hematite and magnetite have a wealth of uses in the soil treatment and improvement world. Crucial to their efficacy is ensuring that the hematite and magnetite are the best quality as supplied - such as that which is available from African Pegmatite, the mineral partner.
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