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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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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:

RG =

FFW (3.1)


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


Initial concentration of metal in external solution (3.2)

(source: Xiaomei et al., 2004)


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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