Phytoremediation of Copper-Contaminated Soil

Abstract

Concentration of Cu in European topsoils vary in a broad range (0.8–256 mg/kg), which is strongly related to regional and local geology, with additional impacts from anthropogenic pollution. An important anthropogenic source of local soil enrichment in Cu is deposition of airborne dust released during metal processing—copper metallurgy is a large source of emissions containing Cu and other metals, such as Pb, Cd, sometimes Ni, and highly toxic, semimetallic As. Urban and industrial areas are also affected by a wide variety of activities, such as traffic, housing, and metal industry. When the level of soil contamination is too high and dangerous for humans and animals it should be reduced. Soil remediation consists of actions of removal, control, containment, or reduction of contaminants in soil to a level safe for human health and the environment.

The basic principle of soil cleanup involves the removal of pollutants from the soil and recovery of decontaminated soil. Various technical methods have been developed for cleaning up soils contaminated with heavy metals. All of them are technologically much more advanced—in consequence also more expensive—than the methods of metal immobilization.

Better results may be obtained by electrokinetic processes carried out ex situ. In practice, however, the removal of metallic pollutants, including Cu, from soils by electrochemical methods poses a lot of problems. Bioleaching can be used in the case of soils contaminated with copper—both in situ and ex situ, on the heaps or in bioreactors. The technology involves different groups of bacteria, usually of the genera Acidithiobacillus, Acetobacter, Acidophilum, Arthrobacter, and Pseudomonas.

Phytoremediation applies to a group of technologies that use either naturally occurring or genetically engineered plants to improve the properties of polluted soils or to remove contaminants from soils. Phytostabilization may be basically considered as an extension of in situ immobilization technology and focuses on long-term stabilization and containment of pollutants in soil by their sequestration in the plant rhizosphere and chemical fixation with amendments. Phytoextraction is a concept of soil decontamination that involves the use of plants to remove (extract) pollutants, especially heavy metals and metalloids, from soil by root uptake and subsequent transport to aerial parts of plants. The species in which Cu occurs in the reclaimed soil may undergo considerable changes in various geochemical processes, such as mineral weathering. Moreover, the changes in Cu speciation and potential mobility may also result from biochemical processes associated with improved soil biological activity.

Hyperaccumulation of copper is an effect of active uptake of metals from the soil and is an efficacious defense mechanism against toxicity, based mainly on the formation of complexes with citric acid, phytochelatins PC2 and PC3, and metallothioneins.

It is worth underlining that transgenic plants able to take up extraordinarily high amounts of metals from soil have been introduced. There is potential use for hyperaccumulators, estimated at about 450 plant species, especially in phytoremediation of areas highly polluted with trace elements. Generally, the use of hyperaccumulating plant species is connected with phytoextraction time ranging from 2 to 60 years, whereas nonhyperaccumulating plants require 25–2,800 years.