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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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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


1.1 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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