Soils polluted with heavy metals have become common across the globe due to increase in geologic and anthropogenic activities. Plants growing on these soils show a reduction in growth, performance, and yield. Bioremediation is an effective method of treating heavy metal polluted soils. It is a widely accepted method that is mostly carried out in situ; hence it is suitable for the establishment/reestablishment of crops on treated soils. Microorganisms and plants employ different mechanisms for the bioremediation of polluted soils. Using plants for the treatment of polluted soils is a more common approach in the bioremediation of heavy metal polluted soils. Combining both microorganisms and plants is an approach to bioremediation that ensures a more efficient clean-up of heavy metal polluted soils. However, success of this approach largely depends on the species of organisms involved in the process.

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Heavy Metal Polluted Soils: Effect on Plants and Bioremediation Methods

The use of higher plants to remediate contaminated land is known as phytoremediation, a term coined 15 years ago Among green technologies addressed to metal pollution, phytoextraction has received increasing attention starting from the discovery of hyperaccumulator plants, which are able to concentrate high levels of specific metals in the above-ground harvestable biomass The small shoot and root growth of these plants and the absence of their commercially available seeds have stimulated study on biomass species, including herbaceous field crops We review here the results of a bibliographical survey from 1995 to 2009 in CAB abstracts on phytoremediation and heavy metals for crop species, citations of which have greatly increased, especially after 2001 Apart from the most frequently cited Brassica juncea (L) Czern, which is often referred to as an hyperaccumulator of various metals, studies mainly focus on Helianthus annuus L, Zea mays L and Brassica napus L, the last also having the greatest annual increase in number of citations Field crops may compensate their low metal concentration by a greater biomass yield, but available data from in situ experiments are currently very few The use of amendments or chelators is often tested in the field to improve metal recovery, allowing above-normal concentrations to be reached Values for Zn exceeding 1,000 mg kg−1 are found in Brassica spp, Phaseolus vulgaris L and Zea mays, and Cu higher than 500 mg kg−1 in Zea mays, Phaseolus vulgaris and Sorghum bicolor (L) Moench Lead greater than 1,000 mg kg−1 is measured in Festuca spp and various Fabaceae Arsenic has values higher than 200 mg kg−1 in sorghum and soybean, whereas Cd concentrations are generally lower than 50 mg kg−1 Assisted phytoextraction is currently facilitated by the availability of low-toxic and highly degradable chelators, such as EDDS and nitrilotriacetate Currently, several experimental attempts are being made to improve plant growth and metal uptake, and results are being achieved from the application of organic acids, auxins, humic acids and mycorrhization The phytoremediation efficiency of field crops is rarely high, but their greater growth potential compared with hyperaccumulators should be considered positively, in that they can establish a dense green canopy in polluted soil, improving the landscape and reducing the mobility of pollutants through water, wind erosion and water percolation

Field crops for phytoremediation of metal-contaminated land. A review

Due to their immutable nature, metals are a group of pollutants of much concern. As a result of human activities such as mining and smelting of metalliferous ores, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, etc., metal pollution has become one of the most serious environmental problems today. Phytoremediation, an emerging cost-effective, non-intrusive, and aesthetically pleasing technology, that uses the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues, appears very promising for the removal of pollutants from the environment. Within this field of phytoremediation, the utilization of plants to transport and concentrate metals from the soil into the harvestable parts of roots and above-ground shoots, i.e., phytoextraction, may be, at present, approaching commercialization. Improvement of the capacity of plants to tolerate and accumulate metals by genetic engineering should open up new possibilities for phytoremediation. The lack of understanding pertaining to metal uptake and translocation mechanisms, enhancement amendments, and external effects of phytoremediation is hindering its full scale application. Due to its great potential as a viable alternative to traditional contaminated land remediation methods, phytoremediation is currently an exciting area of active research.

Recent Findings on the Phytoremediation of Soils Contaminated with Environmentally Toxic Heavy Metals and Metalloids Such as Zinc, Cadmium, Lead, and Arsenic

A multi-process phytoremediation system for removal of polycyclic aromatic hydrocarbons from contaminated soils.

Metabolic adaptations to arsenic-induced oxidative stress in Pteris vittata L and Pteris ensiformis L

A process for removal of metal ions from soil and methods for effecting such removal are described. The process is based on manipulating the growth of crop and crop-related members of the plant family Brassicaceae in metal-containing soils so that the metal in the soils is made more available to the plants. These particular plants will absorb metals into their roots making the metals non-leachable from the soils or will absorb the metal into their roots and transfer them to the shoots and/or roots which can be easily harvested.

Phytoremediation of metals

The present invention provides a method for remediating soil contaminated with Cr(VI) by reducing the Cr(VI) to Cr(III) in the soil. The method involves contacting the contaminated soil with a plant that accomplishes the reduction. Removal of the plant from the environment is not required; in fact, preferred embodiments of the invention involve plowing the plant back into the soil environment and replanting the soil.

Slavik Douchenkov

Papers

Phytoremediation of metals

Conversion of metal oxidation states by phytoreduction