Study on speciation and salinity-induced mobility of uranium from soil

A study was carried out to investigate the impact of salinity on mobility of uranium (U) in soil and the mechanism involved. In order to explore the involved process, speciation study was carried out and it confirmed that U was present in the moderately reduced soil as a mixture of U(IV) and U(VI) with U(IV) as dominating (71 %) state. Sequential extraction of soil samples showed that major fraction is bound to clay mineral (49.31 %) followed by oxides and hydroxide of Fe/Mn (19.58 %), organic phase (10.75 %), exchangeable (10.41 %) and remaining to carbonate phase (9.96 %). The speciation study of U(VI) in soil solution revealed that UO₂CO₃, (UO₂)₂CO₃(OH) ₃ ⁻ , UO₂(CO₃) ₂ ²⁻ , UO₂OH⁺ and UO₂(OH)₂ are predominate species in the ambient condition. The effect of salinity induced by CaCl₂, MgCl₂, NaCl, NaNO₃ and Na₂SO₄ on the mobility of uranium elucidates that increases in ionic strength mobilize U(VI) from soil exchange sites forcing it into solution. Although the desorption capacity of cations is directly proportional to ionic radius (Ca²⁺ > Mg²⁺ > Na⁺) and strength of solution it fails in case of Na₂SO₄ and NaNO₃. It was observed that at high salinity, U is removed from solution as Na₂U₂O₇ (s) at near neutral pH. At high salinity NaNO₃ induces highest desorption of U from soil subjected to its oxidizing properties. The study confirms that ion exchange mechanism is involved in the desorption of U from soil CaCl₂, MgCl₂, NaCl, Na₂SO₄ whereas NaNO₃-induced uranium desorption involves both ion exchange mechanism as well as oxidative dissolution of U(IV) bound to Fe/Mn⁻ oxides and organic matter.