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Why is There Excess Manganese in the Soil
The need for heavy metal toxin cleanup is a direct result of the human activities of mining and well drilling. The vast majority of contamination has been caused by strip mining with the residual airborne manganese particles dusting the soil around the mine. This type of topsoil contamination is where phytoremediation makes the most sense.
Waterborne manganese pollution is usually due to high manganese levels in the bedrock surrounding a well. The mineral leaks into the water supply and cannot be easily filtered out without chemical processors. There is a particular aquatic plant that is an excellent phytoextraction device, the Pacific Mosiquitofern.
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What is Phytoremediation?
Removing contaminants with plants is not a new idea, but it has only recently been rediscovered by modern science and looks to be both the most economically and environmentally friendly option available. Phytoremediation is a potential alternative to current engineering procedures that are destructive to the soil. Byproducts of this type of soil detoxification can be used in the manufacturing industry and help eliminate the need for excessive mining operations.
When the hazard level for a given metal is reached, then the land is no longer safe to use for any type of agricultural project. The point at which the metal becomes a hazard is defined by the Pollution Index. Finding the pollution index involves a bit of calculating with the following formula where PI is the pollution index.
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Economic and Environmental Benefits
Mine tailings have been stabilized using commercially available varieties of metal tolerant grasses such as Highland Bent, Goninan, Parys, and Merlinand grass. This revegetation of mine tailings is a common practice to prevent further dispersal of contaminants. It is now beginning to be used as secondary source of income from recently defunct mining operations. Although these grasses are not used specifically for manganese removal, they are use in the phytoremediation of other mining operations with trailings that include iron, copper, and arsenic.
Because phytoremediation is the cheapest and most environmentally friendly soil rehabilitation method and the costs that are incurred often are offset by sale of the heavy metal byproducts, more and more mining companies are actively cleaning up the environment that they once laid waste to.
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Plants Used for Manganese
The plants used specifically for manganese detoxification are usually those that evolved in an area with naturally occurring high levels of the metal. Some of these plants are highly adaptable, while others need a very controlled environment. Thus the choice of plant will be determined by the local environment in which the manganese levels are toxic.
Sunflowers: Although great at removing toxins from the soil and very fast growing, they do not fare well in many soil conditions. They require fertile, moist, well-drained soil with a lot of mulch. Despite this limitation, they were used to remove cesium-137 and strontium-90 from a nearby pond after the Chernobyl disaster and are considered an excellent plant for removing almost all heavy metals. (This puts sunflowers in a category all its own as most other plants are only good for one type of metal extraction.) With enough fertilization and water supply, sunflowers, with their deeper root systems, are able to reach deeper into the soil than other plant choices.
Macadamia neurophylla (Proteaceae) is the absolute best choice for phytoremediation of manganese, if it can be acquired. It can accumulate up to 1000 (?), twice the amount of the nearest other choice. The problem with this plant is that it is found rarely and confined to the southern massifs of Grand Terre in Caledonia. Obtaining a supply and seeding it over a large enough range to produce the intended effect is almost never possible.
Pacific Mosquito Fern (also known as Fairy Moss): This plant has been used as a companion plant for detoxification and nitrogen stabilizing in China for over 1000 years. It is well suited for water born phytoremediation. The downfall of this plant is that is not salt tolerant, dying off at levels above 1%.
Gossia bidwillii (Myrtaceae): This 15 -25 meter tree is a slow growing hyperaccumulator of manganese. Known as the Python Tree and native from New South Wales to Coen in Australia, it hasn’t been picked up as commercial phytoremediation plant because of its need for wet, semi-moist substrate and slow growth rate.
Phytolacca acinosa Roxb. (Phytolaccaceae) is a manganese hyperaccumulator species that grows rapidly and is easily incorporated into several different growing environments. It is a compact (1-3 meter) plant that is a good choice for use with phytomining.
Stanleya Pinnata is a potentially useful species due to its broad adaptation to semi-arid environments and ability to withstand toxic levels of both manganese and selenium.
Other plants that have been identified as potential manganese accumulators are Alyxia rubricaulis (Apocynaceae), Maytemus bureauvianus (Celastraceae), Indian Mustard, and Vetiver grass (Vetiveria zizanioides L.).
When using any of these plants to detoxify a large area of land the natural chelates released by the roots of certain plants can form complexes with metals in the rhizosphere. This can compromise the possible chemical extraction from the harvested plants. These complexes can include siderophores, organic acids and phenolics each of which would require a secondary chemical process to extract the pure metal from the plant.
Another thing that these plants may do (as well as their associated soil microbes) is release chemicals that act as biosurfactants in the soil so as to increase the absorption of metals, including manganese. These metal contaminants can be stabilized in natural and constructed wetlands through a process called phytofiltration. The use of hyperaccumulating plants, especially fast growing ones, in tandem with an induced process that includes using chemicals like synthetic chelates ethylenediaminetetracetic acid (EDTA) or acidifying chemicals (e.g., NH4SO4) increases the bioavailability of metals in the soil can significantly reduce the toxins in a relatively low period of time. A successful phytoremediation plan should take no more than ten years to completely rehabilitate a parcel of land.
It can be extremely hard to foster a long-term sustainable vegetation patch on land contaminated by manganese mine tailings. Using a bean like G.max to solve the problem of nitrogen deficiency in the soil can help replace the missing nutrient and keep the environment healthy enough for phytoremediation plants to grow.
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Heavy Metal Recovery
Once phytoremediation has been chosen as the method to detoxify an area, what should be done with the resulting heavy metal laced plants? There are a few different options with the best being phytomining - a process where the plants are harvested and then processed removing the pure metal from their biomass.
Retrieving the metal from plants is far easier than mining it from the ground. It is cost effective and does far less damage to the physical environment. Of course, phytomining is only possible in areas where there is substantial metal content near the surface of the soil. Any metals beneath the root line won’t be leeched by the plants and remain locked in the soil.
Manganese phytoremediation is a perfect complimentary process to mining. It not only cleans up the areas surrounding the mine and mine tailings, but also it allows a secondary manganese extraction method. The cost effective nature coupled with the environmental stewardship makes it a win-win situation for mining companies. It cleans up their image as they clean up the environment.
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