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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
  

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4.2 pH effects and metal concentrations remained in controls (blanks) 4.2.1 pH effects in blank samples

Figure 4.3 shows the variations of pH in blanks which are due to elements contained in blank samples such as bacteria, phytoplanktons, zooplanktons, also the variations of temperature can affects pH in blank samples. The correlation between metal removal and the role of experimental containers exists. The increasing or decreasing of pH in blank samples without water hyacinth plants indicates that some elements of metal were fixed on the internal surface of experimental buckets.

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)

pH

pH, 1mg/L pH, 3mg/L pH, 6mg/L

10

8

6

4

2

0

Figure 4.3: variations of pH in blank samples

4.2.2 Zinc concentrations remaining in blank samples

Figure 4.4 shows the trend of zinc concentration in blank water samples at different
initial concentrations (1, 3 and 6 mg/L) and at different periods of time. It was shown

that experimental small buckets may fix some trace elements of zinc on internal surface of buckets or some trace elements were accumulated in sediment because of the variation in metal concentration during the exposure time. For 1 mg/L, the removal of zinc follows a linear trend of decreasing concentration with the increasing of exposure time.

0.2

0.18

 

Zn2+, 1mg/L Zn2+, 3mg/L Zn2+, 6mg/L

0.16

 

Exposure time (h)

1 hr

Oh

6 hr

3 hr

33 hr

21 hr

15 hr

10 hr

57 hr

105 hr

177 hr

537 hr

393 hr

273 hr

705 hr

0.14

0.12

0.1

0.08

0.06

Conc. (mg/L)

0.04

0.02

0

Figure 4.4: Zinc conc. remaining in blank water samples over time

4.2.3 Chromium concentrations remained in blank samples

Figure 4.5 shows that chromium was quickly fixed on the internal surface of experimental buckets and also due to phytoplankton, zooplanktons in water samples. Some trace elements were analyzed in water from 1 to 10 hr only and after 177 hr, the internal surface releases chromium concentration in water and other trace elements were accumulated in sediment.

Exposure time (h)

Conc. (mg/L)

0.16

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

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0

Oh

1 hr

3 hr

6 hr

10 hr

15 hr

21 hr

57 hr

33 hr

105 hr

177 hr

273 hr

705 hr

537 hr

393 hr

Figure 4.5: Chromium conc. remaining in blank water samples over time

4.2.4 Discussions of pH effects on metal concentrations in blank samples

It was reported that pH variations affect metal concentrations in blank samples. According to Barron et al. (1982), if metals are present in wastewaters that contain hexavalant chromium, this chromium must be reduced prior to metal removal. In general, hydroxides usually prove to be the controlling species for adsorbing metal from industrial or domestic wastewater.

The ANOVA analyses show that there is no effect of exposure time to metal concentrations in blank samples for 1 and 3 mg/L (0.7 < 2.1; 1.6 < 2.1 respectively) but for 6 mg/L, the exposure time shows a significant effect on metal concentrations (2.7 > 2.1). According to pH variations, type of metal (zinc and chromium) and concentrations (1, 3 and 6 mg/L), there is a high significant difference (P = 0.05) observed during the experiment.

4.3 pH variations and Zn(II) and Cr(VI) concentrations in water

samples with water hyacinths

4.3.1 Variations of pH on metal removal by the plants

The pH is an important parameter affecting the rate and the extent of biosorption of metal ions onto bioadsorbents such as water hyacinth plants. The variation of pH may affect the surface charge of roots of water hyacinth plants and also the solubility of metal ions. Some metal ions are known to be adsorbed or absorbed in the form of

hydroxides at high pH values such as pH>6. For this reason the effects of initial pH on biosorption of Zn (II) and chromium (VI) ions onto water hyacinth plants was investigated for the initial pH values equals to 6.7.

The variations of pH, zinc and chromium concentrations in water samples with water hyacinth plants after 4 weeks of lab experiments in which was observed an increasing in pH up to pH > 7.5 at 105 hr for 1 mg/L, 3 mg/L and 6 mg/L and then the situation changes after 105 hr of exposure to metal.

It was shown that pH variations affects metal removal during the experiment. Figure 4.6 showed that the pH slightly increased from the starting time (0 hr) (pH= 6.7) to 105 hr (pH= 7.64 to 7.86). However, after 105 hr of experiment, the pH decreased due to the saturation of adsorption sites, so some H+ are released in water samples that caused the decreasing of pH.

Zinc and chromium ions removal from solution was almost completed within 105 hours for pH values above 6 and bellow 8 because of adsorption. The pH of 6 is the critical point for zinc ions because of zinc hydroxide adsorption or absorption. Therefore, it can be said that the optimum pH for adsorption and absorption of zinc (II) and chromium (VI) ions by water hyacinth plants in lab experimental set up is about pH =7.5.

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)

pH, 1mg/L pH, 3mg/L pH, 6mg/L

pH

4

9

8

7

6

5

Figure 4.6: pH variations in plant water samples over time

4.3.2 Zinc concentrations remaining in water samples after 4 weeks of experiment.

Figure 4.7 plotted out the remaining zinc concentration in water samples after four weeks of lab experiments. It was observed that about 60% of zinc (II) was removed within 21 hours. Water hyacinths are effective plants for zinc (II) ions removal form wastewater in the range of concentration between 1 to 6 mg/L. Because the outer surfaces of water hyacinth roots are negatively charged with some acetate groups and metal ions are positively charged, roots attract metal ions, but when adsorption sites of roots are saturated, it is expected that metal ions can be released in water samples. The detention time must be determined. The passage in time influence the metal removal, as the plant can once again release the metal ion back into the water column when the adsorption sites become saturated. The decrease of pH in water samples related to growing time is an important factor in metal absorption and adsorption mechanisms by the plants. It is very clear that after 21 hr, little trace elements of metal are still present in water samples with water hyacinth plants.

0.25

Exposure time (h)

57 hr

Oh

1 hr

537 hr

6 hr 10 hr

177 hr
273 hr

705 hr

393 hr

105 hr

33 hr

21 hr

15 hr

3 hr

Zn2+, 1mg/L
Zn2+, 3mg/L
Zn2+, 6mg/L

0.2

0.15

Conc. (mg/L)

0.1

0.05

0

Figure 4.7: Zinc conc. remaining in water samples with water hyacinth plants over time

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