The Role of Chloride Salts in Chemically Enhanced Phytoextraction of Heavy Metals From a Contaminated Agricultural Soil

Abstract

Chemically enhanced phytoextraction of heavy metals has proved in several cases to be a perspective and costeffective method of soil remediation. The main drawback of this method continues to be the low mobility (and thus the bioavailability) of some heavy metals (e.g., lead [Pb]). The use of synthetic chelating agents, such as ethylenediaminetetraacetic acid (EDTA), significantly increased the mobility of heavy metals in soils and their translocation from the roots to the shoots of several plants (Blaylock et al., 1997; Huang et al., 1997). However, because of their high solubility and persistence in soils, chelates can possibly leach down the soil profile. Thus, they pose an important environmental risk for groundwater quality (Romkens et al., 2002). In addition, free EDTA and EDTAheavy metal complexes are potentially toxic to plants (Vassil et al., 1998). For these reasons, several alternatives to EDTA have been proposed. Among many, (S, S)-N, N’-ethylenediaminedisuccinic acid (EDDS), a biodegradable chelating agent, was introduced to minimize unwanted negative effects associated with the use of EDTA during phytoextraction experiments (Kos and Lestan, 2003). Xiong and Feng (2001) introduced chloride salts as another alternative for enhancing Pb accumulation in Brassica pekinensis. Highly soluble chloride salts dissociate in soil solution and thus provide (1) cations capable of exchanging the adsorbed heavy metals and (2) ligands needed to form watersoluble mobile complexes such as CdCl, CdCl3 , or CdCl4 . Other studies (Li et al., 1994; Smolders et al., 1998) also have proved the positive influence of chloride salts on heavy metal (especially Cd) accumulation in plants. The authors add that the use of chloride salts is preferred over the use of EDTA because of the lower costs and shorter dwell time of the formed soluble complexes in soils. Furthermore, the application of sodium chloride (NaCl) did not negatively influence plant biomass production to the extent that EDTA did (Xiong and Feng, 2001). However, many authors have focused on soils artificially spiked with a single metal in a form that does not correspond to natural conditions (e.g., Pb[NO3]2 or Cd[NO3]2) (e.g., Begonia et al., 2003, 2004; Hovsepyan and Greipsson 2005; Xiong and Feng, 2001). In addition, the application of mobilizing agents to soils contaminated with multiple metals can lead to a reduction in plant biomass yields and the total amount of heavy metals phytoextracted due to the toxicity of other metals present at high concentrations in the soil solution (Chen and Cutright, 2001). The main objectives of this study were (1) to determine the effect of different chloride salt (NH4Cl) concentrations on the mobility of cadmium (Cd), zinc (Zn), and Pb in a contaminated agricultural soil originating from the contaminated mining and smelting area of Přibram, Czech Republic, (2) to evaluate the phytoextraction efficiency of a hybrid poplar (Populus nigra · Populus maximoviczii) and maize (Zea mays) after the application of different NH4Cl concentrations, (3) and to compare the effectiveness of NH4Cl and EDTA in the phytoextraction process. M. Komarek (&) P. Tlustos J. Szakova Department of Agrochemistry and Plant Nutrition, Czech University of Agriculture in Prague, Kamýcka 129, 165 21 Prague 6, Czech Republic e-mail: komarek@af.czu.cz