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pH value in water 7, period of the experiment 21 days pH value in water 7, period of the experiment 21 days

The removal percentage is calculated then on the basis of the concentration in plant tissue. The total dry biomass in all the cases and the total amount of each element absorbed and accumulated by the plants were determined. This value was related to the initial amount added in the treatments, and then the percentage of removal was calculated. The values are shown in the Table 24.2. It is observed that the majority of the elements has percentage of removal below 10% after 21 days of treatment, with the exception of the removal of Hg and Pb (30%). The elements were not detected in the water or plant tissue of the control groups as it is expected in the cases of toxic elements that are not present in the culture water. The Lemna minor is well-suited for the absorption of the elements from water, but this fact is negatively influenced by the lesser amount of biomass production when compared to the water hyacinth (Alvarado et al. 2008). The experiments demonstrated that the removal of each element is affected by the biomass production capacity of the plant and the kinetic of each element itself in water that changes the availability for the plant at pH 7. Regarding to the chromium species, significant differences were found between the Cr+3 and Cr+6 , and are consequence of the higher toxicity of the Cr+6 (Aportela and González 2001; Rodríguez et al. 2002) that affects the absorption of the element from water. Under the conditions of the experiment and due to the chromium compound used (potassium dichro-mate), the element was mainly in the form of chromate and precipitated (Netzahuatl-Muñoz et al. 2008; Rodriguez et al. 2002). The Cr in the Cr+3 treatment also precipitated, as shown in Fig. 24.2, but in a minor level, since it was added as the very soluble nitrate. It is important to point that the assays were carried out using low concentrations of the elements in water to avoid the effect of the toxicity (Zayed et al. 1998b). All concentrations were below 4 mg L- 1. For mercury and Lemna 0.113 mg L-1, arsenic 0.150 mg L-1, chromium 0.5 mg L-1, and 1.5 and 0.7 mg L)1 for Pb. Higher concentrations could be deleterious for the species viability and the process efficiency (Posada and Arroyave 2006).

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