The agronomical behavior of the plant was tested with the variables fresh and percentage of dry weight. There are significant differences between the percent of dry biomass at the beginning with respect to the end of the experiment (21 days). P=0.05 for both groups (control and experimental). No significant differences were found between the groups regarding to the percentage of dry matter at the end of the experiment.
In the control group (Lemna in water without mercury), an increment in the fresh biomass of 64.46 g was found at the end of the experiment (Table 24.3).
Table 24.3 Agronomical variables of Lemna minor in contaminated water and controls
Fresh initial Initial dry Initial dry Fresh Final dry Final % of
Group biomass (g) weight (g) weight % biomass (g) weight (g) dry weight Mercury
Control 100.00 5.3 ± 0.4 5.3 ± 0.4 164 ± 36 10 ± 3 6.7 ± 0.3
Experimental 100.00 5.3 ± 0.4 5.3 ± 0.4 162 ± 24 11 ± 2 6.9 ± 0.8 Arsenic
Control 100.00 9 ± 1 9 ± 1 75 ± 10 5.9 ± 0.8 8 ± 1
Experimental 100.00 9 ± 1 9 ± 1 60 ± 5 7 ± 1 12 ± 2 Chromium (III)
Control 100.00 5.3 ± 0.4 5.3 ± 0,4 109 ± 5 7.9 ± 0,6 6.5 ± 0.3
Experimental 100.00 5.3 ± 0.4 5.3 ± 0,4 121 ± 4 9 ± 2 9 ± 1 Chromium (VI)
Control 100.00 5.3 ± 0.4 5.3 ± 0.4 109 ± 5 9 ± 2 9 ± 1
Experimental 100.00 5.3 ± 0.4 5.3 ± 0.4 116 ± 5 9 ± 2 8 ± 1
In the experimental group (Lemna with mercury), the biomass increment was 62.18 and had no significant differences with respect to the control at the end of the experiment; P=0.05. In general, an increment in fresh and dry biomass was observed with respect to the beginning of the experiment. No significant differences were found also in the dry biomass at the end of the experiment between the control and experimental groups; P=0.05.
These facts demonstrated that the concentration of the element in water did not had a hard toxic effect on the plants, evidencing the tolerance for the biological development at the level of contamination used in the experiment (0.133 mg L-1) in concordance to Posada and Arroyave (2006). In that work, in the range of 0.1-1 mg L-1, the plant adapted and observed a fast recovery in the growth.
An important diminution of the mercury concentration was observed in the control of mercury without plants after 6 days of the experiment (from 0.22 at the day 0 till 0.031 mg L-1 at the 6th day). After this period, the concentration did not change significantly. This fact implies that the kinetic of the element in water must be taken into account. Part of the added amount of Hg possibly precipitated or was adsorbed in the vessel. It must be taken into account also that the element is volatile and the climatic conditions as temperature and evapotranspiration could also activate the loss of mercury. In the experimental group of mercury with plants, the initial concentration was 0.135 mg L-1 and fell to 0.027 mg L-1 at the 6th day. At the end of the experiment, the mercury foliar concentrations were 40 mg g-1 dry weight, demonstrating that a significant part of the mercury was absorbed by the plant. The removal capacity is then 30% as mentioned before and it is shown in Table 24.2.
The bioremediation is a two-step process according to Wang et al. (1996), and the metal accumulation by aquatic individuals consists in a first step of the fast absorption in the biological surface. The second step is the slow and irreversible transport into the cells (bioaccumulation) by diffusion of the metallic ion through the cell membrane or by the active transport using pro teins (Metcalf & Eddy, Inc. 1995; Miretzky et al. 2006). As a surface phenomenon, there is saturation according to the monocape model (Oporto et al. 2001). After the 6th day, part of the retained mercury is released to the water, making it available. Then, the concentration in water raised. At the day 13, the results are according to Burke and Weis (2000). They established that aquatic plants accumulate heavy metals in tissue, which are further released to the water. Posada and Arroyave (2006) observed that Lemna minor, at the concentration range of 0.10-1 mg L-1, experimented a decrease in growth till the 4th day. After that period, Lemna minor recovered and grew up constantly. According to this experience of Posada and Arroyave (2006), a percentage of the plants died, releasing the absorbed element till the day 13. After this day, the reabsorption occurs again within a new recovery period. As a consequence, the concentration in water decreases till day 20. The adaptation to the contaminated medium was observed. A process of absorption, release, and reabsorption occurs in a cyclic fashion, as happened with the arsenic (Alvarado et al. 2008). The mass balance shows that the decrease of the element concentration in water could be the result not only of the bioabsorption by plants, but also a consequence of different processes as precipitation, adsorption in vessels, and even phytovolatilization. 300 mg of Hg were found in the whole Lemna minor tissue and the final mass in water was 520 mg (60); the remaining 180 mg of the added 1,000 mg could be not only precipitated, but also volatized as showed by Wollwnberg and Peters, 2006. The transpiration of Hg(0) was demonstrated by the plants. Carvalho (2001) also reported this behavior of the macrophytes with the selenium.
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