Recent findings on the phytoremediation of soils contaminated with environmentally toxic heavy metals and metalloids such as zinc, cadmium, lead, and arsenic

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 aboveground 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. 1. Environmental metal pollution and phytoremediation Soil pollution has recently been attracting considerable public attention since the magnitude of the problem in our soils calls for immediate action (Garbisu & Alkorta 2003). As a result of human activities such as mining and smelting of metalliferous, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, etc., metal pollution has become one of the most serious environmental problems today. Due to their immutable nature, metals are a group of pollutants of much concern. In fact, although several metals are essential for biological systems and must be present within a certain concentration range (Garbisu & Alkorta 2003), at high concentrations, metals can act in a deleterious manner by blocking essential functional groups, displacing other metal ions, or modifying the active conformation of biological molecules (Collins & Stotzky 1989). Metal toxicity for living organisms involves oxidative and/or genotoxic mechanisms (Briat & Lebrun 1999). Based on their chemical and physical properties, three different molecular mechanisms of heavy metal toxicity can be distinguished: (i) production of reactive species by autooxidation and Fenton reaction (Fe, Cu), (ii) blocking of essential functional groups in biomolecules (Cd, Hg), and (iii) displacement of essential metal ions from biomolecules (Schutzendubel & Polle 2002). Reviews in Environmental Science and Bio/Technology 3: 71–90, 2004. 2004 Kluwer Academic Publishers. Printed in the Netherlands. 71