Review Chelate-enhanced phytoremediation of soils polluted with heavy metals I. Alkorta 1 , J. Herna´ndez-Allica 2 , J.M. Becerril 3 , I. Amezaga 3 , I. Albizu 2 , M. Onaindia 3 & C. Garbisu 2, * 1 Unidad de Biofı´sica, Centro Mixto UPV/EHU, Apdo. 644, E-48080 Bilbao, Spain; 2 NEIKER, Basque Institute of Agricultural Research and Development, c/ Berreaga 1, E-48160 Derio, Spain; 3 Department of Plant Biology and Ecology, University of the Basque Country, Apdo. 644, E-48080 Bilbao, Spain (*author for correspondence, phone: +34 94 403 43 00; fax: +34 94 403 43 10; e-mail: cgarbisu@neiker.net) Key words: assisted phytoremediation, bioavailability, chelating agents, enhanced phytoremediation, ethylenediaminetetraacetic acid (EDTA), induced phytoremediation, phytoextraction Abstract In general, hyperaccumulators are low biomass, slow-growing plants. High biomass non-hyperaccumulator plants by themselves are not a valid alternative for phytoextraction as they also have many limitations, such as small root uptake and little root-to-shoot translocation. In this context, chemically-induced phytoex- traction (based on the fact that the application of certain chemicals, mostly chelating agents, to the soil significantly enhances metal accumulation by plants) has been proposed as an alternative for the cleaning up of metal polluted soils. But chelate-induced phytoextraction increases the risk of adverse environmental effects due to metal mobilization during extended periods of time. In order to minimize the phytotoxicity and environmental problems associated with the use of chelating agents, nowadays, research is being carried out on the gradual application of small doses of the chelating agent during the growth period. However, EDTA utilization in the future will most likely be limited to ex situ conditions where control of the leachates can be achieved. There are other mobilizing agents which are much less harmful to the environment such as citric acid, NTA, and particularly EDDS. Research should also be aimed towards more innovative agronomic practices. Environmentally safe methods of chelate-induced phytoextraction must be developed before steps towards further development and commercialization of this remediation technology are taken. Most importantly, more applied projects in this field are needed to clarify the real potential and risks of this technology. 1. Introduction 1.1. Metal pollution Heavy metals are present in soils and aqueous streams as both natural components or as a result of human activity (i.e., metal-rich mine tailings, metal smelting, electroplating, gas exhaust, energy and fuel production, downwash from power lines, intensive agriculture, sludge dumping, etc.) (Ra- skin et al. 1994). Widespread low to medium pol- lution of agricultural land represents a specific problem and, in Europe, the polluted agricultural lands likely encompass several million of ha (Flathman & Lanza 1998). According to a report (ETCS 1998), there are 1,400,000 sites contami- nated with heavy metals and/or organic pollutants in Western Europe alone. A European Union Council Directive (86/278/EEC, 1986) limited values for concentrations of heavy metals in arable soils to 3 mg kg )1 for Cd, 140 mg kg )1 for Cu, 75 mg kg )1 forNi,300 mg kg )1 forPb,300 mg kg )1 for Zn, and 1.5 mg kg )1 for Hg (Grcˇman et al. 2001). Besides, heavy metals are highly persistent in soils, with residence times in the order of thousands of years (McGrath 1987). Unless remediation action is undertaken, the availability of arable land for cultivation will decrease because of stricter Reviews in Environmental Science and Bio/Technology 3: 55–70, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 55