Studies indicated that duckweed absorbed mercury in water (e.g. lakes) strongly and that after three days it contained up to 2000 ppm of mercury by weight of duckweed. The uptake of mercury was little affected by acidity at pH 4.0 and pH 5.0. The presence of copper ion suppressed the mercury uptake significantly. The presence of 1.0 mM EDTA in solution also suppressed the mercury uptake significantly. Further 10 mg of humic acid in 400 mL solution suppressed the mercury uptake to some degree, probably by complexing the mercury.