The fractionation of U-Th into the colloidal fraction in acid mine drainage conditions in the Río Tinto areas

Río Tinto provides ideal conditions for studying the role of colloidal particles in concentrating elements and their influence on the transport of them to Ría de Huelva estuary(Spain).Ultrafiltration experiments were developed on the Río Tinto waters,in order to separate colloids by different sizes(50,10,5 and 3kDa).Samples from 50kDa and 3kDa ultrafiltration were analysed to find concentration using ICP-MS,and radionuclides activities have been determined for each of them.An ultrafiltration permeation model describes the relation between log of concentration of the element in the permeate solution and log of CF.If the permeation is constant,the linear pattern resulting can be used to predict the ultrafiltration behaviour of U and Th in this peculiar waters.Experiments were carried out using CF from 1,07 to 1,73.The behaviour of U can be predicted by a permeation model that provides a Pc of 0,813 for U(50kDa) and Pc=0,626 for U(3kDa).The permeation model can just show little retention for U,and that the permeation behaviour of U can be considered the same even at low CF.The application of the permeation model on Th(50kDa) samples failed,the membrane was not retaining quantitatively.A Pc=0,360 for Th(3kDa) was derived.Anyway,this could be due to artefactual retentions.Using low CF,colloidal concentration cannot be estimated by the permeation model for both U and Th.Concentration of U amounts to 4,73-6,54 μg/L,the range for Th is 13,0-28,0 μg/L.Both the elements are in higher concentrations respect to common natural waters,according to their predict character in acidic conditions,in which both can stay in dissolved form,and even colloids will be disintegrated. 234U/238U AR range is 1,761-2,060.The disequilibrium among 234U and 238U isotopes is attributable to the excess of 234U related both to the recoil effect,and the preferential leaching of 234U.230Th activity range is 0,056-0,169 mBq/L;its presence in solution is related to the decay of 234U.