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Abstract
The effect of the matrix composition (main constituents) on the concentration and
chemical behavior of uranium in phosphogypsum stack solutions and leachates has been
investigated. Solid and aqueous samples were taken from three different sub-areas
of a phosphogypsum stack at a coastal area in Vasilikos (Cyprus). The sub-areas are
characterized whether by their acidity (e.g. "aged" and "non-aged" phosphogypsum)
or by their salt content, originating from pulping water during wet stacking or (after
deposition) from the adjacent sea. Measurements in stack solutions and leachates showed
that phosphogypsum characteristics affect both, the concentration and the chemical
behavior of uranium in solution. Uranium concentration in solutions of increased salinity
is up to three orders of magnitude higher than in solutions of low salinity and this
is attributed to the effect of ionic strength on the solubility of phosphogypsum.
Modelling showed that uranium in stack solutions is predominantly present in the form
of uranium(VI) phosphate complexes (e.g. UO(2)(H(2)PO(4))(2), UO(2)HPO(4)), whereas
in leachates uranium(VI) fluoro complexes (e.g. UO(2)F(2), UO(2)F(3)(-)) are predominant
in solution. The latter indicates that elution of uranium from phosphogypsum takes
places most probably in the form of fluoro complexes. Both, effective elution by saline
water and direct migration of uranium to the sea, where it forms very stable uranium(VI)
carbonato complexes, indicate that the adjacent sea will be the final receptor of
uranium released from Vasilikos phosphogypsum.