Phytoremediation Technology: Hyper-accumulation Metals in Plants Prabha K. Padmavathiamma & Loretta Y. Li Received: 13 October 2006 /Accepted: 1 April 2007 / Published online: 22 May 2007 # Springer Science + Business Media B.V. 2007 Abstract This paper reviews key aspects of phyto- remediation technology and the biological mechanisms underlying phytoremediation. Current knowledge re- garding the application of phytoremediation in alleviat- ing heavy metal toxicity is summarized highlighting the relative merits of different options. The results reveal a cutting edge application of emerging strategies and technologies to problems of heavy metals in soil. Progress in phytoremediation is hindered by a lack of understanding of complex interactions in the rhizo- sphere and plant based interactions which allow metal translocation and accumulation in plants. The evolution of physiological and molecular mechanisms of phyto- remediation, together with recently-developed biologi- cal and engineering strategies, has helped to improve the performance of both heavy metal phytoextraction and phytostabilization. The results reveal that phytoreme- diation includes a variety of remediation techniques which include many treatment strategies leading to contaminant degradation, removal (through accumula- tion or dissipation), or immobilization. For each of these processes, we review what is known for metal pollutants, gaps in knowledge, and the practical impli- cations for phytoremediation strategies. Keywords Metals . Phytoremediation . Pollution . Hyper accumulation . De-contamination . Excluders . Chelation 1 Introduction Heavy metals are ubiquitous environmental contami- nants in industrialized societies. Soil pollution by metals differs from air or water pollution, because heavy metals persist in soil much longer than in other compartments of the biosphere (Lasat 2002). Over recent decades, the annual worldwide release of heavy metals reached 22,000 t (metric ton) for cadmium, 939,000 t for copper, 783,000 t for lead and 1,350,000 t for zinc (Singh et al. 2003). Sources of heavy metal contaminants in soils include metalliferous mining and smelting, metallurgi- cal industries, sewage sludge treatment, warfare and military training, waste disposal sites, agricultural fertilizers and electronic industries (Alloway 1995). For example, mine tailings rich in sulphide minerals may form acid mine drainage (AMD) through reaction with atmospheric oxygen and water, and AMD contains elevated levels of metals that could be harmful to animals and plants (Stoltz 2004). Ground-transportation also causes metal contami- nation. Highway traffic, maintenance, and de-icing operations generate continuous surface and ground- Water Air Soil Pollut (2007) 184:105���126 DOI 10.1007/s11270-007-9401-5 P. K. Padmavathiamma Department of Soil Science, University of British Columbia, 2357 Main Mall, Vancouver, BC, Canada V6T 1Z4 L. Y. Li (*) Department of Civil Engineering, University of British Columbia, 6250 Applied Science Lane, Vancouver, BC, Canada V6T 1Z4 e-mail: lli@civil.ubc.ca

water contaminant sources. Tread ware, brake abra- sion, and corrosion are well documented heavy metal sources associated with highway traffic (Ho and Tai 1988 Fatoki 1996 Garc��a and Mill��n 1998 S��nchez Mart��n et al. 2000). Heavy metal contaminants in roadside soils originate from engine and brake pad wear (e.g. Cd, Cu, and Ni) (Viklander 1998) lubricants (e.g. Cd, Cu and Zn) (Birch and Scollen Figure Metal a Mn 64.487x 0.1919 0.2453 Zn 25.616x-0.427 0.2013 Pb 31.996x-0.0989 0.0359 b Pb 347.5x-0.8549 0.966 Cu 43.347x-0.3368 0.9625 Zn 110.66x-0.3295 0.9971 c Pb 319.69x-1.1831 0.8269 Cu 197.25x-1.0689 0.8548 Zn 271.6x-0.6321 0.804 d Pb 206.93x -0.6 0.9731 Zn 227.69x-0.1842 0.7593 0 20 40 60 80 100 120 140 160 0 5 10 15 Distance from the Highway (m) Concentration (mg/kg) Mn Zn Pb Pow er (Mn) Pow er (Pb) Pow er (Zn) a 0 25 50 75 100 0 10 20 30 40 50 60 Distance from the Highway (m) Concentration (mg/kg) Pb Cu Zn Pow er (Pb) Pow er (Zn) Pow er (Cu) b 0 50 100 150 200 250 300 350 0 5 10 15 20 25 30 35 Distance from the Highway (m) Concentration (mg/kg) Pb Cu Zn Pow er (Pb) Pow er (Cu) Pow er (Zn) c 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Distance from the highway (m) Concentration (mg/kg) Pb Zn Pow er (Pb) Pow er (Zn) d Regression Equation, y Correlation, R2 Fig. 1 Heavy metal content of road-side soils from a Brussels-Ortend, Belgium (Albasel and Cottenie 1985) b Osogobo, Nigeria (Fakayode and Olu-Owolabi 2003) c West bank, Palestine (Swaileh et al. 2004) d A31 between Nancy and France (Viard et al. 2004) 106 Water Air Soil Pollut (2007) 184:105���126

2003 Turer et al. 2001) exhaust emissions, (e.g. Pb) (Gulson et al. 1981 Al-Chalabi and Hawker 2000 Sutherland et al. 2003) and tire abrasion (e.g. Zn) (Smolders and Degryse 2002). The concentration ranges of metals of greatest importance in roadside soils are given in Fig. 1. Toxic heavy metals cause DNA damage, and their carcinogenic effects in animals and humans are prob- ably caused by their mutagenic ability (Knasmuller et al. 1998 Baudouin et al. 2002). Exposure to high levels of these metals has been linked to adverse effects on human health and wildlife. Lead poisoning in children causes neurological damage leading to reduced intel- ligence, loss of short term memory, learning disabilities and coordination problems. The effects of arsenic include cardiovascular problems, skin cancer and other skin effects, peripheral neuropathy (WHO 1997) and kidney damage. Cadmium accumulates in the kidneys and is implicated in a range of kidney diseases (WHO 1997). The principal health risks associated with mercury are damage to the nervous system, with such symptoms as uncontrollable shaking, muscle wasting, partial blindness, and deformities in children exposed in the womb (WHO 1997). Metal-contaminated soil can be remediated by chem- ical, physical or biological techniques (McEldowney et al. 1993). Chemical and physical treatments irrevers- ibly affect soil properties, destroy biodiversity and may render the soil useless as a medium for plant growth. These remediation methods can be costly. Table 1 summarizes the cost of different remediation technol- ogies. Among the listed remediation technologies, phytoextraction is one of the lowest cost techniques for contaminated soil remediation. There is a need to develop suitable cost-effective biological soil remedi- ation techniques to remove contaminants without affecting soil fertility. Phytoremediation could provide sustainable techniques for metal remediation. This paper summarizes the development of phytoremedia- tion for metals in the past two decades. Phytoremediation involves the use of plants to remove, transfer, stabilize and/or degrade contami- nants in soil, sediment and water (Hughes et al. 1997). The idea that plants can be used for environ- mental remediation is very old and cannot be traced to any particular source. The concentration of metal uptake in plants is shown in Fig. 2. A series of fascinating scientific discoveries, combined with inter- disciplinary research, has allowed phytoremediation to develop into a promising, cost-effective, and environmentally friendly technology. The term phytoremediation (���phyto��� meaning plant, and the Latin suffix ���remedium��� meaning to clean or restore) refers to a diverse collection of plant- based technologies that use either naturally occurring, or genetically engineered, plants to clean contaminat- ed environments (Cunningham et al. 1997 Flathman and Lanza 1998). Some plants which grow on metalliferous soils have developed the ability to accumulate massive amounts of indigenous metals in their tissues without symptoms of toxicity (Reeves and Brooks 1983 Baker and Brooks 1989 Baker et al. 1991 Entry et al. 1999). The idea of using plants to extract metals from contaminated soil was re- introduced and developed by Utsunamyia (1980) and Chaney (1983). The first field trial on Zn and Cd phytoextraction was conducted by Baker et al. (1991). Several comprehensive reviews have been written, summarizing many important aspects of this novel plant- based technology (Salt et al. 1995, 1998 Chaney et al. 1997 Raskin et al. 1997 Chaudhry et al. 1998 Wenzel et al. 1999 Meagher 2000 Navari-Izzo and Quartacci 2001 Lasat 2002 McGrath et al. 2002 McGrath and Zhao 2003 McIntyre 2003 Singh et al. 2003 Garbisu and Alkorta 2001 Prasad and Freitas 2003 Alkorta et al. 2004 Ghosh and Singh 2005 Pilon- Smits 2005). These reviews give general guidance and recommendations for applying phytoremediation, highlighting the processes associated with applica- tions and underlying biological mechanisms. The present review is intended to give an updated, more concise version of information so far available with respect to different subsets of phyoremediation. It provides a critical overview of the present state of the art, with particular emphasis on phytoextraction and phytostabilization of soil heavy metal contaminants. Table 1 Cost of different remediation technologies (Glass 1999) Process Cost (US$/ton) Other factors Vitrification 75���425 Long-term monitoring Land filling 100���500 Transport/excavation/ monitoring Chemical treatment 100���500 Recycling of contaminants Electrokinetics 20���200 Monitoring Phytoextraction 5���40 Disposal of phytomass Water Air Soil Pollut (2007) 184:105���126 107