Zinc and chromium are environmental pollutants that are toxic even at very low concentrations. Domestic and industrial wastewater discharges are probably the two most important sources for chromium and zinc in water. In Rwanda, the discharge into natural ecosystems of untreated wastewater containing heavy metals by factories and households is a growing problem. A bench-scale study was therefore conducted from May to October 2007 to investigate the major mechanisms responsible for Cr (VI) and Zn

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... (II) removal from industrial wastewater using water hyacinth. The pH effects, plant relative growth, trace metal remaining in water samples, translocation ability, bioconcentration factor, adsorption, bioaccumulation and uptake mechanisms were investigated. The pH slightly increased from the start time pH = 6.7 (0 hr) to pH = 7.64 to 7.86 (48 hr); but after 48 hours of experiment, the pH decreased due to the saturation of bond sites, resulting in some H+ being released back into the water. The relative growth significantly decreased (P ≤ 0.05) from 1, 3 and 6 mg/L in 1 week but it slightly decreased linearly after 1 week with increasing metal concentrations (P ≤ 0.05). About 56.7 % of Zn (II) was accumulated in petioles, 27.0 % in leaves and 16.3 % in roots. For Cr (VI) 73.7 % was taken up in roots, 14.1 % in petioles and 12.2 % in leaves. It was observed that 17.6 %, 6.1 % and 1.1 % were adsorbed for 1, 3 and 6 mg/L of Zn (II) concentrations, respectively, by water hyacinth plants. For Cr (VI), 9.0 %, 36.4 % and 54.6 % were adsorbed for 1, 3 and 6 mg/L, respectively. The order of translocation ability for Cr (VI) was leaves<petioles<roots in water hyacinth whereas for Zn (II) it was leaves<roots<petioles.