Phytoextraction and fractionation of heavy metals in soil after multiple applications of natural chelants

Évio Eduardo Chaves De Melo,Clístenes Williams Araújo Do Nascimento,Ana Cristiane Queiroz Santos,Adriana Maria De Aguiar Accioly

doi:10.1590/s0103-90162008000100009

Évio Eduardo Chaves De Melo, Clístenes Williams Araújo Do Nascimento + Show 2 more

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https://doi.org/10.1590/s0103-90162008000100009

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Abstract

Chelate-assisted phytoextraction of heavy metals is a promising approach to clean up polluted soils. However, the most successful chelants tested so far are synthetic agents that barely degrade in soil, increasing the metal leaching risks. Natural organic acids have been proposed to enhance phytoextraction due to their higher biodegradability, but they can also be a drawback for efficient phytoextraction. This work was carried out to compare the effectiveness of multiple applications of citric and gallic acids on the availability in soil and accumulation of Cd, Pb, Cu, and Zn by velvet bean (Stizolobium aterrimum) plants. The organic acids were added as follows: 5 mmol kg-1 in the 28th cultivation day; two doses of 5 mmol kg-1 at the 28th and 31st day; and three applications at the same rate at the 28th, 31st, and 34th day of velvet bean cultivation. Soil samples were sequentially extracted and soil solution metal concentrations assessed. Neither citric acid nor gallic acid was efficient for metals phytoextraction. In general, low molecular weight organic acids (LMWOA) application increased the metal concentrations in the water soluble and exchangeable fractions. Zinc and Cu were retained mostly in the organic matter fraction. Zn was remobilized from the organic matter fraction through LMWOA application into the water soluble and exchangeable fractions. LMWOA mobilized Pb and Cu from iron oxides, but such an increase in solubility was not high enough to affect phytoextraction.

Highlights

Environmental restoration of polluted soils by conventional technologies demands large economic resources (Cunningham & Berti, 2000; Ensley, 2000; Nascimento & Xing, 2006)
Taking into account the low efficiency of a single application of low molecular weight organic acids (LMWOA) on phytoextraction enhancement, we studied the effect of multiple applications of citric and gallic acids on the phytoextraction of Cd, Pb, Cu, and Zn by velvet bean plants (Stizolobium aterrimum)
Plants produced less biomass on the heavy metal contaminated soil when no acids were added as compared to the ones grown on the non-contaminated soil (Table 1)

Summary

Introduction

Environmental restoration of polluted soils by conventional technologies demands large economic resources (Cunningham & Berti, 2000; Ensley, 2000; Nascimento & Xing, 2006). Phytoextraction - the use of plants to remove. Metals from soils by storing them in above-ground biomass - has been developed as an economically and environmentally attractive technology to decontaminate heavy metal polluted soils (Raskin et al, 1997; Terry & Bañuelos, 2000). The first one is the use of hyperaccumulating plants that naturally concentrate metals in their aboveground tissues. The other strategy makes the use of high biomass plants that are induced to accumulate metals through chelants applications in soils (Blaylock et al, 1997; Huang et al, 1997; Cooper et al, 1999; Wu et al, 1999; Shen et al, 2002; Melo et al, 2006). The main drawback for the public acceptance of this technique is the risk that synthetic chelants pose to environment due to a possible excessive metal solubilization and consequent leaching (Shen et al, 2002; Chen et al, 2003)

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