Fitomineração induzida e fracionamento de níquel, cromo e cobalto em solo ultramáfico tratado com ácido cítrico

Abstract

Ultramafic soils originate from the weathering of ultrabasic rocks, rich in mafic minerals. Due to the difficulty of developing agriculture in these areas, there is an opportunity to commercially phytomine ultramafic soils to produce a biomass that can generate a bio-ore, Hyperaccumulator plants have been used to this aim. However, non-accumulating plant species with high biomass production can be chemically induced by chelating agents to mobilize metals from the soil to the shoot, a common practice in phytoextraction from contaminated soils, but not tested for phytomining yet. In this sense, the present study aimed to evaluate the effects of citric acid on the induced extraction of Ni, Cr and Co by maize plants (Zea mays) cultivated in an ultramafic soil and to determine the redistribution of these metals between soil fractions as a result of the application of the acid. Emphasis was given to the element Ni, due to the high commercial value of this metal and the possibility of its recycling from plant biomass. The experiment was carried out in a greenhouse with application of citric acid to the soil at concentrations of 0, 5, 10, 20 and 40 mmol kg-1 of soil, in three successive croppings. The results showed that the available concentrations of Ni, Cr and Co increased by the rates of citric acid after each of the croppings. The sequential extraction showed that the acid increased the percentages of Ni and Co in the exchangeable fraction of the soil. The chelator had no effect on the available concentrations of Cr, which was mainly retained in the residual fraction. The application of citric acid at doses of 20 and 40 mmol kg-1 of soil resulted in the highest concentrations of Ni in the aerial part of the plants, with no significant difference between these two rates. Therefore, 20 mmol kg-1 recommended to induce Ni phytoaccumulation. Despite the considerably high Ni concentrations in the shoots (500-600 mg kg-1), assisted phytomining using maize plants, for the soil and concentrations tested here, is not an alternative to the use of hyperaccumulators since the recovered Ni would be far from what is considered adequate to make the technique economically viable.