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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 RESULTS AND DISCUSSIONS

The laboratory experiments for this research were started in March 2007 and ended at the end of October 2007. Water hyacinth plants were collected several times but unfortunately some of them died due to rigorous conditions, more than 1,000 samples (water and plant mixed) were analyzed and the results for this research are average values.

4.1 Variations on plant relative growth

4.1.1 Relative growth of water hyacinth plants

The relative growth was calculated to estimate the effects of zinc and chromium concentrations on plant growth according to exposure time. The relative growth indicates the tolerance of the plants to different concentrations of metal and exposure time to these metals. It was observed that the final fresh weight increased compared to the initial fresh weight in the first week but it decreased when the metal concentration increased, mostly for 6 mg/L. There is a slight stagnation for the second week and fourth week in plant growth in terms of metal concentration. The growth decreased with increasing time and metal concentrations due to a decreasing of essential macro and micro nutrients for the plants in the experimental water in the small buckets.

The growth of water hyacinth at different concentrations of chromium and zinc is shown on Figure 4.1. It was noted that the relative growth of the plants decreased considerably in the first week only and slightly decreased with an increasing metal concentration and exposure time. There was constant trend in the plant growth after 1 week of experiment. However, at higher concentrations of these metals, plant growth was always inhibited. The growth of water hyacinth plants significantly increased (P = 0.05) with the passage of time according to the Figure 4.2.

Relative growth values

5 4 3 2 1 0

 
 

1 wk, 1

1 wk, 3

1 wk, 6

2 wks, 1

2 wks, 3

2 wks, 6

4 wks, 1

4 wks, 3

4 wks, 6

Exposure time (week) and initial conc.

Figure 4.1: Relative growth of water hyacinth plants vs exposure time for different Zn and Cr concentrations

4.1.2 Discussions on relative growth of water hyacinths

For water hyacinth plants treated with Zn and Cr, the relative growth significantly decreased (P = 0.05) from 1, 3 and 6 mg/L in 1 week but for 2 and 4 weeks, the relative growth slightly decreased linearly with the increasing (P = 0.05) of metal concentrations. In the case of zinc and chromium, however, the relative growth exhibited an exponential decrease caused by relatively increasing toxicity in contrast to chromium and zinc concentrations.

The ANOVA 2 shows that for 1 week exposure time, there is a high effect (difference is significant) of initial concentrations (1, 3 and 6 mg/L) to the growth of the plants (P = 0.05), but for 2 and 4 weeks according to initial concentrations, the difference is not significant (P = 0.05).

4.1.3. Correlation between final fresh weight and relative growth

Figure 4.2 shows that there is no correlation between the final fresh weight and the relative growth of water hyacinth plants. The non existence of correlation is reported by the stagnation in plant growth due to the decreasing nutrients and increasing metal concentrations. It was considered that a decrease in the growth was induced by metal toxicity.

Final Fresh weight vs Relative growth

y = 0.009x + 1.4994
R2 = 0.2977

30 50 70 90

Final Fresh Weight (g)

4

3

2

1

0

Relative growth

Figure 4.2: Correlation between Relative Growth of plants and Final Fresh Weight

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