2.6 Translocation of metals within plants
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).
2.7 Uptake
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
21
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
53
2.5 ppm w/ o plant 2.02
5 ppm w/ plant 3.45
0
5 ppm w/ o plant 2.9
10 ppm w/ plant 5.38
0
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
0
Ctr. w/ o plant 0.03
5 ppm w/ plant 5.4
4.8
5 ppm w/ o plant 5.67
10 ppm w/ plant 10.2
4.7
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.
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