Feasibility of using living alfalfa plants in the phytoextraction of cadmium(II), chromium(VI), copper(II), nickel(II), and zinc(II): Agar and soil studies.

摘要

Trace level concentrations of heavy metals are found everywhere on earth. However, activities such as mining and localized intensive agriculture, have contributed to an undesirable accumulation of toxic metals in many areas worldwide. Current technologies used in the cleaning process of contaminated sites are expensive and frequently not environmentally friendly. Phytoremediation, the use of plants to remove, stabilize, or degrade soil contaminants, is a promising remediation technology, which has several advantages over traditional clean up methodologies. This investigation demonstrated the capabilities of alfalfa (Medicago sativa) to clean up soils contaminated with Cd(II), Cr(VI), Cu(II), Ni(II), and Zn(II).; Experiments conducted using agar-based media showed that the concentration of 5 mg/L of these heavy metals, individually, increased the growth of the alfalfa plants. The dose of 10 mg/L of Cr(VI), 20 mg/L of Cd(II), Cu(II) or Ni(II) significantly reduced the germination and development of the alfalfa seedlings. However the plants were able to tolerate 40 mg/L of Zn(II). Plants gown in the agar-based media contaminated with 10 mg/L of Cd(II), Cr(VI), Cu(II), and Ni(II) accumulated in the shoot dry tissues 2,427 mg/kg of Cd, 909 mg/kg of Cr, 757 mg/kg of Cu, and 713 mg/kg of Ni, respectively. However, the plants treated with 40 mg/L of Zn(II) accumulated 4,036 mg/kg of Zn in the shoot dry tissues. In another experiment alfalfa plants were cultivated in a montmorillonite-based medium individually contaminated with the heavy metals indicated above, whose solutions were adjusted at pH 4.5, 5.8, and 7.1. Our results showed that alfalfa plants were able to tolerate 80 mg/L of Cd(II), Cu(II), and Ni(II), and 160 mg/L of Zn(II). Alfalfa plants did not show capabilities to tolerate more than 10 mg/L of Cr(VI) in the cultivation medium. Furthermore, alfalfa plants were able to remove up to 86 mg/kg of Cd(II) and 185 mg/kg of Cu(II) at pH 7.1, as well as 174 mg/kg of Ni(II) at pH 5.8 and 398 mg/kg of Zn(II) at pH 4.5. Experiments conducted in the same substratum using a mixture of 50 mg/kg of Cd(II), Cu(II), Ni(II) and Zn(II) showed that alfalfa plants absorbed more Ni than any of the other metals. The concentrations of Ni in the alfalfa shoot dry tissues were 437 mg/kg, 333 mg/kg, and 308 mg/kg at pH 7.1, 5.8, and 4.5, respectively. The second metal with the highest uptake potential was cadmium since it was found in concentrations of 202, 124, and 132 mg/kg at pH 7.1, 5.8, and 4.5, respectively; while zinc was third, followed by copper. These experiments demonstrated that alfalfa plants successfully compete with the soil matrix for the adsorbed metal cations. Further experiments performed in a silt soil showed that at the growth stage of 20 days, alfalfa plants were able to tolerate up 500 mg/L of Cd(II), Cu(II), and Zn(II). In these conditions, the alfalfa shoot dry tissues accumulated up to 1079 mg/kg of Cd, which represented 26% of the Cd concentrated in the root tissues. This result indicated that alfalfa could be included as a Cd hyperaccumulator species. Also, other experiments performed in the same soil, demonstrated that Zn(II) reduced the toxic effects of Ni(II) to alfalfa plants, which could represent an important information for the use of living alfalfa plants in the phytoremediation of nickel contaminated soils.