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)(2)CO3(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.