Phytoremediation of heavy metals and study of the metal coordination by X-ray absorption spectroscopy

Although traditional technologies for cleaning contaminated soils and waters have proven to be efficient, they are usually expensive, labor intensive, and in the case of soil, they produce severe disturbance. More recently, the use of plants in metal extraction (phytoremediation) has appeared as a promising alternative in the removal of heavy metal excess from soil and water. Phytoremediation of polluted waters, is based on the cultivation of aquatic plants such as Eichhornia crassipes and Azolla filiculoides Lam., which have demonstrated a high capability to absorb cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn) from aqueous solutions. Studies have demonstrated that terrestrial plants such as Brassica juncea, Salsola kali, and Prosopis spp. cultivated in hydroponics and agar are able to uptake significant amounts of heavy metals, which indicates their possible utilization in phytoremediation processes. On the other hand, a wide variety of plants have demonstrated the ability to grow and uptake heavy metals from severely polluted sites. Several species of Thlaspi, B. juncea, Salix spp., and Populus spp., among others, have been already tested in pilot projects or are currently in commercial application in phytoremediation projects. Recently, researchers have realized that phytoextraction can also be used for the recovery of precious metals such as gold, silver, platinum, and palladium, which indicates the wide possibilities of the phytoremediation technology with regards to mining. X-ray absorption spectroscopy (XAS) consists of two complimentary techniques X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) which provide invaluable chemical information. XAS has been used to investigate a number of different elements within inorganic chemical systems. However, more recently, it has been applied to investigate metal interactions within biosystems. XAS has provided important information on the coordination chemistry of metals and toxic element interactions with phytoremediation systems. XAS has provided information in terms of the coordination environment of metals absorbed by plants, their atomic geometry, and the bioreduction of metals within phytoremediation systems. In addition, XAS has provided information about the production of gold and silver nanoparticles by the metal interaction with the plants on phytomining systems.