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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 Bioaccumulation

Metal 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 plants

The 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 zinc

Figure 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.

 

1 0.8 0.6 0.4 0.2

0

Total desorption of Zinc

 

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

conc. (mg/L)

 
 
 
 

1 week 2 weeks 4 weeks

 
 

Exposure time (week)

Figure 4.14: Total desorption of Zinc

4.5.3 Adsorption of chromium by water hyacinth plants

The 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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