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Zinc and Chromium removal mechanisms from industrial wastewater by using water hyacinth, eicchonia crassipes( Télécharger le fichier original )par John Gakwavu Rugigana National University of Rwanda - Master's in WREM (water resources and environmental management) 2007 |
4.5 BioaccumulationMetal accumulation in plant and water samples was measured. Digestion of samples in this study was performed according to the Standard Methods (APHA/AWWA/WEF, 2005). Plant biomass samples were decomposed to dry matter by heating at 105°C for 24 hours in a hot air oven. According to Lin and Zhang (1990), metal accumulations by macrophytes can be affected by metal concentrations in water and sediments. The accumulation of metal in plant material is expressed as mg of metal per kg of dry matter. The data for accumulation of chromium and Zinc are expressed in different forms such as uptake, translocation ability and adsorption, both for the top (shoots) and the roots of plants exposed to metal-containing water. The zinc concentration in both the roots and the shoots tended to increase with increasing concentration of zinc and also with the passage of time. 4.5.1 Adsorption of Zinc by water hyacinth plantsThe adsorption mechanism was performed by using EDTA-Na2 to remove metals fixed on outer surface of the roots performed the adsorption mechanism. This mechanism help to understand the ability of water hyacinth plants to fix metals on the roots. The adsorption behavior of zinc was assessed by immersing roots in different volumetric beakers with 100 ml EDTA-Na2 at different periods of time for desorption. The Figures 4.11, 4.12 and 4.13 shows that the metal concentration decreases when the passage in time of desorption increases (Figure 4.13). Except some differences observed in Figures 4.11, and 4.12, the situation looks to be the same in general. The high concentration adsorbed in 1 week was around 0.036 mg/L (1 mg/L initial concentration), 0.16 mg/L for 2 weeks (3 mg/L) and 0.2 mg/L for 4 weeks (1 mg/L).
conc. in mg/L 0.04 0.03 0.02 0.01 0 5 min Desorption of Zinc for 1week 10 min Period of time for removal 15 min 1 mg/L 3mg/L 6 mg/L 20 min 25 min 30 min 35 min 40 min 45 min Figure 4.11: Desorption of Zinc after 1 week Desorption of Zinc for 2 weeks 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min 45 min Period of time for removal
conc. in mg/L 0.2 0.15 0.1 0.05 0 1 mg/L 3 mg/L 6mg/L Figure 4.12: Desorption of Zinc after 2 weeks Desorption of Zinc for 4 weeks
1 mg/L 3 mg/L 6 mg/L 0.25 0.2 0.15 0.1 0.05 0 conc. in mg/L 45 min 5 min 10 min 15 min 20 min 35 min 30 min 25 min 40 min Period of time for removal Figure 4.13: Desorption of Zinc after 4 weeks 4.5.2 Total adsorption of zincFigure 4.14 show that the adsorption for the 1 mg/L zinc initial concentration increased with the exposure time, but higher concentrations (3 and 6 mg/L) reduce the ability of water hyacinth plants to adsorb metal. The majority of molecules are adsorbed onto the roots.
Exposure time (week) Figure 4.14: Total desorption of Zinc 4.5.3 Adsorption of chromium by water hyacinth plantsThe adsorption of chromium presented on Figure 4.15 shows a decrease in concentration desorbed on external surface of roots according to time. The capacity of water hyacinth plants to adsorb trace elements of chromium depend on several factors which can affect this mechanism This means that more trace elements of chromium was removed in 5 to 15 minutes and the high concentration observed for 6 mg/L was around 2 mg/L, for 3 mg/L was around 1.6 mg/L and for 1 mg/L was around 0.3 mg/L. It shows the same situation as for zinc that 5 to 15 minutes are sufficient to remove the maximum quantity of chromium on roots and the concentration of trace elements decreases with the passage of time of desorption. Period of time for removal (min.)
conc.in (mg/L) 1 mg/L 3 mg/L 6 mg/L 3 2 2 1 1 0 5 min 15 min 10 min 20 min 35 min 30 min 25 min 40 min 45 min Figure 4.15: Desorption of Chromium |
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