THE UNIFORM RATE OF COPPER UPTAKE BY Lemna Minor L. AND AN ALTERNATIVE TO USING FLAME ATOMIC ABSORPTION SPECTROPHOTOMETRY FOR DETERMINING COPPER CONTENT

Abstract

The floating aquatic plant Lemna minor L., a species of duckweed, has been used as an indicator of heavy metal pollution and also as a method of removing metals and nutrients from wastewater at treatment plants. However, the rate of metal uptake by duckweed has not been thoroughly investigated, and plant operators need to know the optimum time for duckweed removal and replacement so that metal removal from the effluent stream will remain efficient. In addition, the method of determining the amount of metals in plant tissue usually involves the use of a flame atomic absorption spectrophotometer (AAS), which is not commonly available in a wastewater treatment plant. The first goal of this study was to track the rate of copper uptake by L. minor. The second goal was to develop and evaluate a new and different method of copper determination in plant tissues. This new method, which was designed for ease of use, required only nitric acid for frond digestion and sodium hydroxide and ammonium hydroxide solutions for sample processing to allow for visible spectrophotometric analysis on site at a treatment plant. It was found that during days 10-20 of exposure to media containing copper, the amount of copper present in L. minor samples decreased. This indicates that under the environmental conditions of this study, the optimum time for duckweed removal was in the first 10 days. It was also found that this experimental colorimetric method had a detection limit between 200-250 ppm (w/v ) copper. The results of this investigation demonstrate that duckweed fronds did not take up enough copper for the samples to be read colorimetrically, so the method could not be thoroughly evaluated. However, previous experiments have shown that under different environmental conditions, duckweed has the ability to take up far more copper than was observed. Although the colorimetric method was not useful for determining copper content at this time, it is possible that under different conditions it may become a practical method.