3.3 Sample Analyses
3.3.1 Relative Growth
Relative growth of control and treated plants was calculated
to assess the effects of zinc and chromium concentrations on water hyacinth
plant growth in relationship with time. The formula bellow was used to
calculate the relative growth:
Where FFW denotes final fresh weight
(g); IFW denotes initial fresh weight (g) and
RG denotes the relative growth of water hyacinth
plants which is dimensionless.
3.3.2 Bioconcentration Factor
The BCF (bioconcentration factor) provides an index of the
ability of the plant to accumulate the metal with respect to the metal
concentration in the substrate. The BCF was calculated as follows:
Concentration of metal in plant tissue
BCF =
Initial concentration of metal in external solution (3.2)
(source: Xiaomei et al., 2004)
or
BCF = (P/E)i (3.3)
(Source: Liao and Chang, 2004)
Where I denotes the heavy metals,
BCF the bioconcentration factor and
P represents the trace element concentration in plant
tissues (mg.Kg-1), E represents the trace
element concentration in the water (mgL-1) or in the sediment
(mgkg-1 dry wt). A larger ratio implies better phytoaccumulation
capability.
3.3.3 Metals Accumulation
Metals accumulation in plant and water samples was measured.
Digestion of samples in this study was performed according to the Standard
Methods by APHA.7 (APHA/AWWA/WEF, 2005) Plant biomass samples was decomposed to
dry matter by heating at 105°C for 24 hours in a hot air oven and the fine
particles were digested with nitric acid (HNO3) and hydrogen peroxide (H2O2),
filtered through a wathman paper filter into a volumetric flask before Atomic
Absorption Spectrophotometer analyses. The two following mechanisms were
performed in analyses to differentiate the metal adsorbed and up taken by water
hyacinth during experiment period.
a. Adsorption
The adsorption consists on metal attached to the outer
surface of the roots. To quantify the metal adsorbed by water hyacinth after
the plant exposure to different concentrations of chromium and zinc in
different periods of times (1week, 2weeks and 4 weeks). After test duration of
observation, the adsorption was determined by putting roots of water hyacinth
plant in nine beakers containing 20 ml of EDTA-Na2 respectively for 5, 10, 15,
20, 25, 30, 35, 40 and 45 min for removal of zinc and chromium trace elements
on the outer surface of the roots. Those EDTA-NA2 solutions were filtered,
acidified by 5 drops of Nitric acid (HNO3) and analysed by Atomic Adsorption
Spectrophotometer (AAS) for zinc and chromium adsorbed by the plants.
b. 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. To assess this mechanism during our research;
after period observation, water hyacinth plants were taken out form the small
buckets, roots, petioles and leaves were separated, dried in dry oven at
105°C during 24h. Plant samples were digested and analyzed by AAS to
identify the zinc and chromium concentrations in plant biomass (roots, leaves
and petioles).
c. Translocation ability (TA)
The translocation ability shows the ability of water hyacinth
plants to transport across the metal ions in the shoot tissues. It was
calculated by dividing the concentration of a trace element accumulated in the
root tissues by that accumulated in shoot tissues (Wu and Sun 1998). TA
is given by:
TA = (Ar Ú As)I (3.4)
Where i denote the heavy metal,
TA is the translocation ability and is dimensionless.
Ar represents the amount of trace element accumulated
in the roots (mg.Kg-1 dw), and As
represents the amount of trace element accumulated in the shoots
(mg.Kg-1 dw).
Statistics were used to assess the variations and correlations
between parameters studied. The following were used: standard deviation,
regression analyses, analyse of variance 2 (ANOVA 2) with replications and
other tools in MS-Excel such as average, mean values, etc.
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