Treatment of Soil Contaminated with Arsenic with Phytoremediation

Environmental Science, Environment
Treatment of Soil Contaminated with Arsenic with Phytoremediation
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Arsenic has been widely used in the past decades for the production of chromated copper arsenate (CCA), a wood preservative for garden equipment, playground equipment and several other lumber uses. The levels currently detected in soil near such wooden installations are 20 times higher than normal levels. Although, its use is no longer applicable, the existence of CCA-treated lumber residential equipment will continue to leach the toxic metal into the soil for years and even decades to come. A possible action step for the treatment of soil contaminated with arsenic, would be a phytoremediation method that uses ferns to remove the dangerous quantities of arsenic from the ground, as discussed in more detail below.

Arsenic Exposure Effects/Symptoms

Arsenic is a white or silver metallic element. It is found in nature mostly combined with other elements in both organic and inorganic compounds. The most toxic for human health are considered to be the inorganic compounds. Arsenic exposure can occur through various ways such as drinking contaminated water, digging, breathing or coming into any kind of contact with contaminated soil dust. It may even occur when breathing dust or smoke from the burning wood that has been treated with arsenic and can be proven extremely dangerous for children who eat dirt and play with the soil at playgrounds.

The effects on human health depend on the level of exposure. Low level exposure over a long time may cause skin, liver, bladder, lungs and kidneys cancer. High level exposure when drinking water, may lead to feet and hand sores. Larger doses of arsenic are lethal for the human body.

Phytoremediation

One of the most promising methods to clean the ground is using phytoremediation techniques. These techniques introduce the use of plants that hyperaccumulate or absorb larger than normal amounts of heavy metals in soils, and hence they are called hyperaccumulators. These plants have the ability to concentrate the dangerous metals in their stems, shoots, and leaves, and then these parts are collected and can be recycled or disposed.

How is this possible? The concept is the following: researchers have isolated a gene found in a specific type of fern, Pteris vittata, that increases the tolerance against arsenic by up to 100 to 1,000 times more than other plants. This specific gene encodes a protein that stores the toxic metal away from the cytoplasm, so it can’t hurt the plant. No similar gene has ever been found in other plants, however researchers, at least in theory, could put them in any organism to help cleaning soils and contaminated water.

The technology can be used for both small and large sites. Treatment for gardens and backyards can be addressed using simple home gardening techniques, whereas lumber storage areas can be treated with larger scale agricultural techniques. In particular, Edenspace Systems Corporation has developed such a technology with the help of EPA’s SBIR Program. The company’s aim is mostly focused on individual homeowners. There are indications that there is a strong customer interest in phytoremediation of soil arsenic deposited from CCA-treated wood, with a projected U.S. market of $162-$194 million.

Despite the great potential, the disposal of harvested plants is a major problem. Arsenic cannot be destroyed by recycling the plants, and the toxic metal returns to the soil or water eventually. Therefore, one of the best tactics for heavy metal accumulating plants is to incinerate them and dispose of the ashes instead of disposing of the contaminated soil directly. Homeowners can store low quantities of harvested ferns in watertight containers and dispose of them the same way as regular trash. It is suggested however, that larger quantities handled by businesses and municipal parks should be sent to suitable waste landfills.

Ferns Used to Accumulate Arsenic from the Soil

Other Hyperaccumulators

Apart from ferns, there is a wide variety of other hyperaccumulators - that is plants that hyperaccumulate zinc, copper, chromium, manganese, and other heavy metals from soils. An example of a common plant that is actually a very good hyperaccumulator is the sunflower (sunflower phytoremediation), which has the ability to absorb a variety of metals from the soil such as chromium, cesium, strontium, copper, manganese, zinc ,and lead.

Finally, in order to determine the safety levels of metal concentrations, authorities use soil screening levels. These are concentrations that indicate potential risk and differ from clean up levels. The arsenic cleanup number for residential soils is 50 ppm. Available screening levels for antimony, arsenic, lead and many more metals are available here.

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