USE OF AQUEOUS PLANTS IN AQUEOUS MEDIUM
DECONTAMINATION FROM RADIONUCLIDES
Olga K. Karlina, Helen V. Kropotova, Igor A. Sobolev
Scientific and Industrial Association "Radon"
The 7-th Rostovsky Lane
2/14, Moscow, 119121, Russia
Tel. (095)248 1680
Fax (095)248 1941
E-mail oj@nporadon.msk.ru
ABSTRACT
In the present work we consider the possibility of the decontamination of aqueous medium ( SIA "Radon" surface flow ) from radionuclides by the use of aqueous plant species. Higher aqueous plants are used for the experiment, which include: Elodea canadensis, Lemna minor; moss Leptodictium riparium, filamentous alga Cladophora glomerata. The capacities of the mentioned plants to accumulate radionuclides are studied under laboratory and natural conditions.
INTRODUCTION
At present practically all natural waters contain a quantity of radionuclides of anthropogeneous origin. For enterprises which use radiation sources the problem of radionuclide presence in the surface flows is very urgent. Although activity level of such waters is sufficiently low, many of the hydrobionts have the capability to selectively accumulate radionuclides which may result in their redistribution and more significant contamination of the local environment.
For development of a technique which uses aqueous plants for surface water decontamination, it is essential to select aqueous plants which have adapted to regional climatic conditions. In this case an optimum aqueous plant is one having sufficiently high accumulating property in relation to radionuclides as well as satisfactory mass growth during plant development.
For the use of aquatic vegetation in removal of radionuclides from water the following stages of the process have been proposed:
For the experiment to be successful study is carried out with the following objectives:
EXPERIMENT
Laboratory - scale experiments on 137Cs accumulation by various plants were conducted in aquariums with volumes of 30 L each where water temperature of 20 ° C was maintained. Water aeration and water periodical replacement was provided once a week. Water specific activity was held at level of 2,0*102 to 4,5*103 Bq/1 137Cs.
In an effort to determine dynamics of 137Cs accumulation the following species of aqueous flora were used in the laboratory experiments: the higher aqueous plants -Elodea canadensis and Lemna minor, moss - Leptodictyum riparium.
The plants selected for natural setting tests were based on the results of laboratory-experiments. These natural setting tests were conducted at the "proving ground" ( in pond - settlers of enterprise's strict regime zone ) . During plant transplanting at the proving ground the effects of water hardness, organic compounds content, temperature on the adaptation, plant mass growth and capability to accumulate radionuclides were studied. In the course of the experiment radiochemical and chemical analyses of water and plant samples were performed.
RESULTS AND DISCUSSION
Experiments in the aquariums showed that maximum 137Cs accumulation by plants was found at 7 - 14th days of experiment. Specific activity of plants increased as the water specific activity increased while accumulation coefficient was held constant at approximately 300 for Elodea canadensis, 270 for Lemna minor. The accumulation coefficient was determined as the ratio of specific activity of dry plant-mass to specific activity of water. Results obtained as a whole correlate well with analogous literature data [1, 2]. No literature data are available for radionuclide accumulation by Leptodictyum riparium. In our experiments the accumulation coefficient for Leptodictyum riparium was 4000. For comparison, coefficient of 137Cs accumulation by aqueous moss of Fontinalis kind is 1100 [2].
Natural experiments have revealed different adaptabilities of the plants selected during their cultivation at the proving ground. Thus, adaptation rate of plants during their transplanting into contaminated basin depends on transplanting season and water temperature ( Elodea canadensis, Lemna minor ). Mass growth of the mentioned plants, especially at their adaptation and initial development, correlates with increased water temperature in the basin (Fig. l a). No similar dependence is found for Leptodictyum riparium (Fig. lb). Moreover, it has been noted that Lemna minor is adapted easily in the experimental basin, if it grew in the basin with harder water before transplanting.
Figure 1. Variations in Lemna minor (a) and Leptodictyum riparium (b) Mass and Water
Temperature During Cultivation of the Plants Under Natural Conditions:
1 - Variations in Mass, 2 - Variations in Temperature.
Mass of plants under study increased under natural conditions by the factor 8-10 for Lemna minor; 2-2,5 for Elodea canadensis; 2-3 for Leptodictyum riparium; mass growth of plants together with settled suspended solids was on the average 1,5-2 times higher.
Coefficients of radionuclide accumulation by plants under study exceeded appropriate characteristics obtained under laboratory conditions. Maximum radionuclide accumulation by plants was found at 18-20th days of the experiment for Elodea canadensis and Lemna minor and at 7th day - for Leptodictyum riparium, at 14th day - for Cladophora glomerata (Table I). At a later time plant activity increased mostly due to fine suspensions settling on their surface. Activity of plant samples not being washed was on the average 1,3 times higher than activity of samples washed in pure water before analysis.
Table I. Results of Investigation of the Accumulation Capacity of
Aqueous Plants Under Natural Conditions
CONCLUSIONS
The objectives of further investigations will be to develop equipment for implementation of the proposed decontamination technique and to test this technique under natural conditions. Test results and evaluation of decontamination efficiency will be presented for the proposed technique.
REFERENCES