EVALUATION OF LEMNA GIBBA G3 FOR THE BIOREMEDIATION OF SELENIUM CONTAMINATED WASTEWATER

Abstract

Lemna gibba L. G3 (duckweed) was evaluated as a plant for potential use in the bioremediation of selenium contaminated waters. Frond production, growth constant (K), dry weight (DVV) and uptake of selenium were measured after eight days of growth in a growth chamber having a temperature of 28° C and in continuous light. Concentrations of selenium in the growth medium and the plant biomass were determined using the Thermo Jarrell Ash 61E simultaneous emission spectrometer. Selenium was detected in the biomass of all samples grown in the medium containing selenium and was not detected in the corresponding untreated control. Rinsing with 10% Tween-Twentyᵀᴹ a detergent, did not alter the measured concentration of selenium in the plant biomass. L. gibba was able to grow at (K) > 50% of the control (K) in E medium containing concentrations of selenium from 330 to 6500 ppb. However, the rate of growth was strongly influenced by the form of the selenium containing ion, the concentration of sulfur, and the concentration of the E medium. The uptake of selenium by L. gibba appeared to be influenced by both the form of the selenium containing ion and the amount of sulfur present in the growth medium. When selenium was introduced as selenous acid at 800 ppb, uptake was measured at 119 μg Se/gram L. gibba. When selenium was introduced as sodium selenate at 800 ppb, uptake was measured at 564 μg Se/gram L. gibba. When sulfur, in a ratio of S:Se of 32:1, was introduced with 800 ppb selenium (using sodium selenate) uptake was measured at 108 μg Se/gram L. gibba. Increased S:Se ratios in the growth medium appeared to reduce the toxic effects of selenium. Plants grown in medium containing 1600 ppb selenium, as sodium selenate, and a S:Se ratio of 4:1 had a growth rate (K) of 107.6. Plants grown in the same selenium concentration but with a S:Se ratio of 32:1 had a growth rate (K) of 191.4. L. gibba appears to grow in the presence of selenium contaminated growth medium and to absorb selenium from the medium. Thus it would seem to have some potential in the bioremediation of selenium contaminated wastewater. However, the effectiveness of the rate of removal of selenium and the plant's tolerance to various concentrations of selenium appear to be strongly influenced by the form of the available selenium containing ion and the concentration of other chemical constituents, especially sulfur, in the growth medium. Further studies to clarify the mechanisms governing uptake of selenium across the plant membrane and the availability of the selenate ion versus the selenite ion in aqueous solution could prove useful.