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A theoretical and experimental study of the biogeochemical behavior of Americium-241 in simplified rhizospheric conditions Application to a calcareous agricultural soil

Thomas PERRIER, doctorate thesis of the University of Henri Poincarré Nancy I Speciality Sciences of soil and Geomicrobiology, 337p. defended on the 15th June 2004.

Summary

The Americium 241 is an artificial element produced during the nuclear fuel cycle, and one of the most (radio)toxic contaminants. It is found in every compartment of the environment, especially soils. The biogeochemical behaviour of americium in subsurface soils plays a dominant role on the potential migration of the element, but is currently poorly known. The main aims of the study were to identify, quantify and model the impact of the different processes that govern 241Am mobility in the rhizosphere of agricultural soils. The physico-chemical of both soil and soil solution as well as the potential role of microorganisms on sorption/desorption processes and on americium solution speciation were specifically investigated.
241Am remobilization was studied in laboratory under controlled conditions by lixiviation tests on columns filled with a natural calcareous 2 mm-sieved soil, artificially contaminated with 241Am. The columns were percolated under contrasted hydrodynamic conditions (water saturated and unsaturated) with solution of varied compositions, simulating simplified conditions that could be found in a rhizospheric soil horizon (modelled exudates chosen: glucose and citrate). The physico-chemical parameters of the outlet solution were monitored (pH, conductivity, major ions, Fetot, organic acids, 241Am), as well as microbial biomass and the contamination profile obtained at the end of the experiments.
The results show that 241Am remobilization is contrasted and highly dependant on the studied condition (pH, ionic force, glucose and/or citrate concentration). Thus, a solution in chemical equilibrium with the soil or containing low concentrations of exudates (10-4 M), releases only very small amounts of the americium fixed on the solid phase. 241Am desorption corresponds in that case to a solid/liquid partition coefficient (Kd) of about 105 L.kg-1. Adding high concentrations of glucose, though inducing advanced dissolution of soil carbonates by indirect action of microorganisms, did not enhance Am remobilization either. It is the addition of high concentrations of citrate (> 10-2 M), with or without glucose, that induced releases 300 to 10000 times greater, by complexing the 241Am released in solution after the dissolution of the carrier phases. At last, colloidal transport of 241Am was systematically observed, with a limited but significant amplitude that could account for more than 90% of the total transported contamination.
Finally, the whole set of acquired results demonstrate that the evaluation of americium mobility in the rhizosphere is impossible without taking into account the complexity and the inter-connectivity of the processes involved: Am solid speciation (mainly associated with carbonates and iron oxides), Am speciation in solution directly governed by microbial activity (complexation by citrate ions, pH, etc.) and also the dynamic of solute transfers. The relative contribution of each processes remains difficult to estimate with the macroscopic tools used in the study. The main directions proposed to model the system are discussed on the basis of the different sets of experimental data obtained (retention profiles, physico-chemical variations, Kd), with the help of a consistent and completed thermodynamical data base.


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