The term “trace elements” and/or “heavy elements” is generally used for cationic metals and oxyanions, which naturally occur in soils and plants in concentrations less than 1 g/kg; therefore, they usually pose no environmental risk. Some trace elements, such as zinc (Zn), iron (Fe), cobalt (Co), copper (Cu), molybdenum (Mo), manganese (Mn), and nickel (Ni), are present in small quantities in plants and participate in metabolism. However, there are other trace elements that have no biological role, including lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), and other metalloids, which are toxic to the environment. Mining and ore processing significantly contribute to environmental trace element contamination. However, despite the high concentration of toxic elements in soils near mining activities, some plants adapt and develop mechanisms to accumulate toxic trace elements and even neutralise their toxicity. Thorough research of such plant species triggered the development of new methods of remediation, where plants are used to treat contaminated soil, air, and water. This method is called phytoremediation. Depending on the contaminant, different techniques of phytoremediation are used and are described as follows:
- Phytoextraction is the absorption of pollutants; this technique is utilised in mining and refers to phytomining.
- Phytodegradation is the transformation of organic contaminants through metabolism or mineralisation in the plant by enzymes into other, usually less toxic or mobile, compounds.
- Phytofiltration is the absorption of such contaminants by roots in hydroponic systems with a continuous effluent flow. Plants with high root surface area and tolerance to contaminants are optimal for this method.
- Phytostabilisation is the accumulation and immobilisation of elements in the roots to reduce their transport further.
- Phytovolatilisation uses a plant’s ability to absorb an element through its roots, convert it into a less toxic and volatile form, and release it to the atmosphere. This technique can be applied to some metals and metalloids as well as organic compounds.