Abstract
Zinc and chromium are some of environmental pollutants and are
toxic even at very low concentrations. Domestic and industrial discharges are
probably the two most important sources for chromium and zinc in the water
environment. Rwanda is still facing problems of heavy metal discharges into
natural ecosystems by factories and household without any prior treatment. The
toxic heavy metals are entering the food chain through drinking water,
agriculture and fisheries activities and therefore endangering human life.
The general objective of this study is to investigate on the
major mechanisms responsible for Cr (VI) and Zn (II) removal form the water
phase by macrophyte plants. Water hyacinth have been used in the remediation
process in the present work because this plant has elaborate much roots system
providing more binding sites for Cr (VI) and Zn (II). Three mechanisms
(Adsorption, uptake and translocation) for fixation of Cr (VI) and Zn (II) by
macrophytes plants had been reported. The investigation had been conducted on
two heavy metals commonly found in polluted industrial wastewater in Rwanda (Cr
(VI) and Zn (II)).
Different parameters were studied in this research such as pH
effects, plant relative growth, trace metal remaining in water samples,
translocation ability, bioconcentration factor, adsorption, bioaccumulation and
uptake mechanisms. The pH slightly increase from starting time 0 hr (pH= 6.7)
to 48 hr (pH= 7.64 to 7.86); but after 48 hr of experiment, the pH decrease due
to the saturation of bound sites so some H+ are released in water
samples which cause the decreasing of pH. The relative growth significantly
decreased (P = 0.05) from 1, 3 and 6 mg/L in 1 week but for 2 and 4 weeks, the
relative growth slightly decreased linearity with the increasing (P = 0.05) of
metal concentrations due to relatively increasing toxicity in contrast to Cr
(VI) and Zn (II) concentration. This study shows that 56.7% of Zn (II) was
accumulated in petioles, 27.0 % in leaves and 16.3% in roots whereas for Cr
(VI) 73.7% was taken up in roots, 14.1% in petioles and 12.2% in leaves. It was
seen that 17.6%, 6.1% and 1.1% were respectively adsorbed for 1 mg/L, 3 mg/L
and 6 mg/L of Zn (II) concentrations by water hyacinth plants; but for Cr (VI),
9.0%, 36.4% and 54.6% were adsorbed respectively for 1 mg/L, 3 mg/L and of 6
mg/L. The order of translocation ability for Cr (VI) was
leaves<petioles<roots in water hyacinth whereas for Zn (II) was
leaves<roots<petioles.
Key words: Chromium, removal mechanisms,
wastewater, water hyacinth, Zinc
1 INTRODUCTION
1.1 Background
Heavy metals are environmental pollutants and some of them are
toxic even at very low concentrations. Pollution of the biosphere with toxic
metals has accelerated dramatically since the beginning of the industrial
revolution (Nriogo, 1979). The primary sources of this pollution are the
burning of fossil fuels, the mining and smelting of metalliferous, municipal
wastes, fertilizers, pesticides and sewage.
Heavy metals are of great concern primarily due to their known
toxicity to aquatic life and human health at trace levels (EPA, 2001; EPA,
2002). It was reported that domestic and industrial discharges are probably the
two most important anthropogenic sources for metals in the water environment
(Stephenson, 1987). However, the lack of a reliable method to predict metals
distribution in treatment units is a key weakness in determining metals fate
and transport in wastewater treatment processes, and therefore, the development
of effective pre-treatment guidelines (Patterson and Kodukula, 1984).
The rapid industrialization in some developing countries with
an enormous and increasing demand for heavy metals, such as zinc (Zn) and
chromium (Cr), causes high emissions of these pollutants into water bodies.
Unlike organic pollutants, metals in wastewater are not degraded through
biological processes, threatening not only the aquatic ecosystems but also
human health through contamination of drinking water. The reuse option of the
treated wastewater is an important strategy for conserving water resources,
particularly in areas suffering from shortage of water.
Several studies have shown that constructed wetlands are very
effective in removing heavy metals from polluted wastewaters (Qian et
al., 1999). Different wetland plant species differ, however, in their
abilities to take up and accumulate various trace elements in their tissues
(Rai et al., 1995). Recently, wetland plant species with high
capacities of trace element (Cu, Ni, Zn, etc.) removal from water were
identified (Zayed et al., 1998a; Zhu et al., 1999) duckweed
(Lemna minor L.) and water hyacinth [Eichhornia crassipes
(Mart.) Solms-Laubach
Heavy metals may come from natural sources, leached from rocks
and soils according to their geochemical mobility or come from anthropogenic
sources, as the result of human land occupation and industrial pollution.
Depending on their solubility, these metals may eventually become associated to
suspended particulates matter and/or accumulate in the bottom sediments
(Espinoza-Quinones et al., 2005).
In Rwanda, the problem regarding waste treatment in general is
still crucial at one side. At the other side the way to deal with such problem
is not easy at all because there is no appropriate technology for waste
treatment. The selection of that technology is not also an easy issue to deal
with because it must take into account many other important aspects like the
financial and social ones. And finally is that selected technology appropriate
in order to meet the effluent standards and is it also cost effective for a
developing country like Rwanda?
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