ABSORPTION OF THORIUM BY LEMNA GIBBA: A POTENTIAL REMEDIATION APPROACH FOR CONTAMINATED WATERS

Weijin Dong¹*, John T. Barber¹, Allen W. Apblett², Edwin H. Walker, Jr.³, and Adrian T. Korz¹

¹Department of Ecology and Evolutionary Biology, Tulane University, New Orleans, LA 70118, USA;

²Department of Chemistry, Oklahoma State University, Stillwater, OK 74078, USA;

³Department of Chemistry, Southern University, Baton Rouge, LA 70813, USA

Lemna gibba, one member of the duckweed family, was found to be able to efficiently remove thorium from its growth medium free of phosphate and sulphate. Over 7 days of growth period, the EC50 was determined to be 570 mM. Approximately 20% of thorium was removed after 6h of incubation and 40% after 12 h and the concentration was reduced from initial 250 mM to around 12 mM in the medium after 24 hours. The washing-after-incubation experiment showed that after 24-h incubation, 46% of added thorium was associated with plant materials and only less than 1% was washed out from the plants. When homogenized, the lyophilized fronds of L. gibba removed more than 92.5% of the available thorium within half an hour. With the aid of diffuse reflectance infrared Fourier transform spectroscopic analysis, carboxylate groups from the plant materials provided the binding sites for the absorption of thorium as evidenced by the appearance bands at 1581 cm^-1 and 1432 cm^-1 and the disappearance bands at 1756 cm^-1 and 1220 cm^-1. Thorium formed complexes with the carboxylate group of the non-living cell walls. On the other hand, the thorium associated with the living materials existed most likely in the same form because thorium did not have time to enter the live fronds in a large amount. Further data analysis and literature research revealed that the carrying capacity, which was computed around 10%, of lyophilized, homogenized Lemna fronds was comparable to other nonliving biological tissues as well as most inorganic ion exchangers. This study also showed a promising technique for thorium-contaminated sites.

*The author is currently associated with Oak Ridge Associated Universities and the address is Environmental Sciences Division, Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831. ORNL is managed by Battelle Institute and the University of Tennessee for the U.S. Department of Energy.