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Zinc and Chromium removal mechanisms from industrial wastewater by using water hyacinth, eicchonia crassipes

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par John Gakwavu Rugigana
National University of Rwanda - Master's in WREM (water resources and environmental management) 2007

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4.5.4 Discussions on adsorption mechanism

The adsorption ability of water hyacinth plants seems to be different when zinc and chromium are compared. It was seen that for zinc 17.6% of 1 mg/L was adsorbed by the water hyacinth plants, 6.1% of 3 mg/L was adsorbed and the plants adsorbed 1.1% of 6 mg/L. Whereas for chromium, 9.0% of 1 mg/L, 36.4% of 3 mg/L and 54.6% of 6 mg/L were adsorbed on the roots of water hyacinth plants.

4.6 Uptake mechanism

4.6 1 Uptake mechanism for zinc

The uptake mechanism was observed to identify which part of water hyacinth plant contributes much in metal ions accumulation. The variation of uptake versus metal dosage for zinc are shown in Figure 4.16 and exhibited linearity at the low level of exposure time (1 week for petioles and leaves); however, the linearity trend could not be established with confidence for leaves and roots for 1 and 4 weeks. The regression coefficients for zinc (II) were found to be 0.6379 for 1 and 3 mg/L, 0.3195 for 3 and 6 mg/L and 0.3660 for 1 and 6 mg/L during all the experimental period. Thus, the uptake process apparently followed an increasing trend with a linear increase of metal concentrations in petioles for 1 week and 4 weeks but in 2 weeks, the pattern of uptake changes. It was observed that petioles are important parts for metal ions accumulation in water hyacinth plants

exposure time (wk) vs initial conc. (mg/L)

roots petioles leaves

Uptake mechanisms








Conc. (mg/Kg)

1 mg/L

1 mg/L

1 mg/L







4 weeks

2 weeks

1 week

Figure 4.16: Variations of uptake for zinc by the plants

Figure 4.16 depicted the uptake of zinc (II), which shows to be in normal distribution according to metal concentration, but it exhibits the changes when exposure time increases. Thus, the present observations showed that the extent of metals (Zn) uptake by plant was dependent on the concentration of the metal in the solution as well as the length of exposure to the plants.

4.6.2 Uptake mechanism for chromium

The figure 4.17 describes the uptake mechanism which demonstrates the important part of water hyacinth plant in metal accumulation. As seen from this figure, roots are important parts for chromium accumulation in the plants. This show a difference with zinc, which was more accumulated in petioles. This Figure 4.17 continues to show the behavior of chromium in plant tissues and it is clear that roots are the important parts for the accumulation of chromium in the water hyacinth plants. When chromium is mixed with zinc in the same water samples, zinc is more mobile than chromium, so zinc will be absorbed very quickly than chromium. Petioles come in second position in metal uptake for 3 mg/L. The uptake is linear according to concentration for roots and leaves but less for petioles.

dry weight






Uptake mechanism for chromium

roots petioles leaves

1 mg/L 3 mg/L 6 mg/L

Initial conc. (mg/L)

Figure 4.17: Uptake of chromium in plant tissues for different initial concentrations

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