Physico-chemical conditions controlling the radionuclides mobilisation in various granitic environments

Abstract

ABSTRACT Nine samples were chemically characterized and radiometrically analyzed for determination of 234U, 238U, 235U, 230Th, 232Th, 226Ra and 40K by γ-ray spectrometer Hyper Purity Germanium (HPGe detector). 238U activity concentrations reach up to 79.78 Bq Kg-1in the altered granites, 1714.11 and 2223.48 Bq Kg-1in the episyenite and pegmatite. Average 234U activity concentrations in the altered granite is 59.26 Bq Kg-1, between 175.88-543.47 Bq Kg-1and 48.19-1102.88 Bq Kg-1in the episyenite and pegmatite, respectively. 232Th activity concentrations vary between 14.16-1024.8 Bq Kg-1in the episyenite, 11.46-240.27 in the pegmatite and reach up to 201.45 Bq Kg-1in the altered granites. 40K activity concentrations range between 4.42-9.15, 2.76-7.34 and 2.68-9.94 Bq Kg-1in the altered granite, episyenite and pegmatite, respectively. 235U activity concentration ranges between 3.74–2.56, 12.45-78.86 and 0.58-102.34 Bq Kg-1 in the altered granite, episyenite and pegmatite, respectively. The studied altered granite, episyenite and pegmatite samples have relatively higher values than recorded in the international standards with the exception of potassium which has lesser amounts. It is demonstrated that pH changes, oxidation-reduction conditions, mineralogical constituents, in addition to alteration processes in these variable granitic environments were the main factors controlling uranium mobilization according to valid processes. The difference in alteration processes and La/Y ratio clarified the presence of more than one event of uranium accumulation and/or leaching in these rocks as confirmed by the presence of uranium accumulation and uranium vacant occurrences. Also, the isotopic ratios of uranium as 238U/235U,234U/235U, 234U/ 238U, 226Ra/ 238U,226Ra/ 230Th, 210Pb/ 226Ra, 230Th/ 238U, 230Th/ 234U with major, trace and rare earth elements were also affected by physico-chemical conditions changes. The present study shows that the samples of episyenite and pegmatite have been affected by leaching-accumulation events in the period from (1 × 105 to ˃2 × 105years).