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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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4.3.3 Chromium conc. remaining in water after 4 weeks of experiment.

Figure 4.8 highlights the capacity of water hyacinth plants in chromium removal from wastewater. From this figure, it was observed that chromium was fixed on the outer surface of plant roots, but this fixation was not effective because of some trace elements of chromium in water samples. After a certain time, the plant begins to release this metal again in the water due to the saturation of adsorption sites on root system.

Oh

1 hr
3 hr
6 hr

10 hr 15 hr 21 hr 33 hr 57 hr 105 hr 177 hr

273 hr 393 hr 537 hr 705 hr

Exposure time (h)

Conc. (mg/L)

0.16

0.14

0.12

0.08

0.06

0.04

0.02

0.1

0

Cr6+,1mg/L Cr6+, 3mg/L Cr6+,6mg/L

Figure 4.8: Chromium conc. remaining in water samples with water hyacinth plants

4.3.4 Discussions on pH variations and metal removal by the plants

Metal can precipitate as hydroxides simply by raising the pH of the wastewater to the range of pH 8 to 11 (Barron et al., 1982). As a result, the extent of adsorption or absorption was rather low at low pH values. However, in the equilibrium solid phase, Zn (II) and Cr (VI) ion concentrations increased with increasing pH because of increasingly negative charges on the surfaces of the roots at high pH values that attracted positively charged Zn (II) and Cr (VI) ions more strongly.

The ANOVA showed that for 1 mg/L, there is no effect of exposure time (P = 0.05) but a
high effect of pH on metal remained in water samples (P = 0.05). For 3 mg/L, the trend is
the same; no effect of exposure time (P = 0.05) and high effect of pH effects on metal

remaining (P = 0.05). For 6 mg/L there was no effect of exposure time (P = 0.05) but high difference between pH effects and metal remaining (P = 0.05).

4.4 Bioconcentration Factor (BCF) for zinc and chromium

The bioconcentration factor (BCF) is a parameter showing the ability of plant materials to accumulate metals in tissues. It was seen that when the concentration of metal increases, water hyacinth plants are not able to accumulate metal ions. The plants have the limit for metal accumulation in their tissues.

4.4.1 Bioconcentration Factor for zinc

Bioconcentration factor (BCF) is a useful parameter to evaluate the potential of the plants in accumulating metals and this value was calculated according to dry weight basis. Figure 4.9 shows that the bioconcentration factor of zinc decreases while the metal concentration increases. This demonstrates that water hyacinth is able to accumulate zinc at low concentrations, which contributes particularly to plant cells building.

Bioconcentration Factor of Zinc.

initial conc.(mg/L) conc.in plant BCF

tissues (mg/L)

Variation of Zinc conc.

conc. in mg/L

4

8

6

2

0

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

Figure 4.9: Bioconcentration factor of Zinc

I.C: Initial conc. in mg/L conc./PT: conc. in plant tissues in mg/Kg

BCF: bioconcentration factor

4.4.2 Bioconcentration Factor for chromium

Figure 4.10 plotted the bioconcentration factor of chromium and shows the trend as for zinc. The increase in concentration reduces the ability of the plant to accumulate more trace elements of metals. The Bioconcentration factor of chromium appears to be constant, independent to the initial concentration.

Zinc and chromium removal mechanisms from industrial wastewater by water hyacinth (Eichhornia crassipes) (Mart.) Solms-
Laubach

BCF of Cr

10

6

4

2

8

0

I.C Conc./P.T BCF

IC vs Conc. in plant tissues

BCF of Chromium

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

Figure 4.10: Bioconcentration factor of Chromium

I.C: Initial conc. in mg/L conc./PT: conc. in plant tissues in mg/kg

BCF: bioconcentration factor

4.4.3 Discussions on bioconcentration factor

In comparing the two metals studied (Zn (II) and Cr (VI)); the BCF of zinc seems to be higher than the chromium's BCF for 1 and 3 mg/L, but very low for 6 mg/L for zinc. It was seen that the plant accumulated more low concentrations than the high ones.

Tables 4.1 and 4.2 show the variations on bioconcentration factors of zinc and chromium and it is reported that there is no significant difference both for zinc and chromium when comparing initial concentrations to the concentrations in plant tissues and bioconcentration factors (P = 0.05) for zinc and chromium.

Table 4.1: variations on bioconcentration factor of zinc

BCF of Zinc ANOVA

 
 
 
 

Source of Variation

SS

df

MS

F

P-value

F crit

Initial concentrations Conc./PT & BCF

Error

Total

1.3
0.8
0.4

2.5

2

1

2

5

0.7
0.8
0.2

3.3
3.7

0.2
0.2

19.0
18.5

Table 4.2: variations on bioconcentration factor of chromium

BCF of Chromium ANOVA

 
 
 
 
 
 

Source of Variation

SS

df

MS

F

P-value

F crit

initial concentrations Conc./PT & BCF

Error

Total

3.5
4.5
3.6

11.6

2

1

2

5

1.7
4.5
1.8

1.0
2.5

0.5
0.3

19.0
18.5

Zinc and chromium removal mechanisms from industrial wastewater by water hyacinth (Eichhornia crassipes) (Mart.) Solms-
Laubach

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