Zinc and Chromium removal mechanisms from industrial wastewater by using water hyacinth, eicchonia crassipes
par John Gakwavu Rugigana
National University of Rwanda - Master's in WREM (water resources and environmental management) 2007
Once the ions have been absorbed through the roots or leaves and have been transported to the xylem and phloem vessels. There is a possibility of movement throughout the whole plant (Alloway, 1990; Streit and Stumm, 1993). The mobility of different metal ions varies and the rate and extent of movement within plants depends on the metal, the plant organ and the age of the plant. Zn, Cu and Pb fall in the category of metals which are readily, immediately and least translocated respectively. In the xylem heavy metals will usually only be transported if special chelates are formed. For example, zinc may be transported by chelation to organic acids, copper transported in complex with amino acids, nickel can be transported as nickel peptide complex and lead may be transported as a Pb-EDTA complex (Greger, 1999; Saxena et al., 1999).
The uptake process is a mechanism by which metal ions are transported across the cell membrane and can be used in a building of new biomass or stored in vacuoles. Streit and Stumm (1993) reported that little is known about the mechanism involved in the absorption and translocation of heavy metal to the host plant root cells. The presence of carboxyl groups at the roots system induces a significant cation exchange capacity and this may be the mechanism of moving heavy metal in the roots system where active absorption takes place.
Table 2.2 shows that water hyacinth plants are able to remove the maximum of chromium concentration from wastewater at low concentration (72.3 %). When metal concentration increases in wastewater, the removal capacity of water hyacinth plants decrease linearity.
Table 2.2: Chromium uptake by water hyacinths during a period of 17 days from Keith et al., 2006
Sample ID Chromium % removal vs. control
Ctr. w/ plant 0.067
Ctr. w/ o plant 0.014 0
7 ppm w/ plant 1.98
7 ppm w/ o plant 7.16 72.3
14 ppm w/ plant 10.4
14 ppm w/ o plant 13.1
28 ppm w/ plant 20.7
28 ppm w/ o plant 25.6 19.1
Comparing to the above results in Table 2.2, the Table 2.3 depicts the phenomenon for copper removal which follow the same trend as for chromium, but the removal capacity by the plants is less than for chromium (53 %).
The most important parameter to consider is the pH (Kelly, 1988). Generally when the pH decreases, the toxicity of metal ions increases because the proportion of the adsorbed ion on the root system decreases (Harding and Whitton, 1977).
Table 2.3: copper uptake by water hyacinth during a period of 17 days.from Keith et al., 2006
Sample ID Copper % removal vs. control
Ctr. w/ plant 0.567
Ctr. w/ o plant 0.266 0
2.5 ppm w/ plant 0.949
2.5 ppm w/ o plant 2.02
5 ppm w/ plant 3.45
5 ppm w/ o plant 2.9
10 ppm w/ plant 5.38
10 ppm w/ o plant 2.39
Table 2.4 presents the results of Arsenic removal by water hyacinth plants and it is shown that water hyacinth is not able to remove Arsenic from wastewater even if at low concentration.
Table 2.4: Arsenic uptake by water hyacinth during a period of 17 days from Keith et al., 2006
Sample ID Arsenic % removal vs. control
Ctr. w/ plant 0.056
Ctr. w/ o plant 0.03
5 ppm w/ plant 5.4
5 ppm w/ o plant 5.67
10 ppm w/ plant 10.2
10 ppm w/ o plant 10.7
20 ppm w/ plant 20.4
20 ppm w/ o plant 19.6 0
The results show how a floater plant like water hyacinth affects arsenic, chromium, and copper. Water hyacinths do not seem to remove large amounts of arsenic or copper from contaminated water.
The water hyacinth appeared to be a good choice for removing chromium from polluted water. At low concentrations, the plant removed about 70% of the chromium in the water (Table 2.2). As the concentrations increase, the plant appeared to be unable to take up as much as possible percent chromium (21%). In conclusion, the effectiveness removal order of these metals was arsenic<copper<chromium.