pH-dependent synergistic enhancement of U(VI) transport by phosphate and bentonite colloids

Bentonite colloids and phosphate exert both competing and complementary influences on U(VI) mobility in high-level radioactive waste repositories. Phosphate immobilizes uranium through precipitation, yet it also competes with pH-sensitive bentonite colloids for U(VI) adsorption. In this study, the interactions and cotransport of uranium with bentonite colloids and phosphate were examined using quartz-sand and granite-mineral columns. Results show that U(VI) transport is strongly pH-dependent. Specifically, phosphate (P) enhances U(VI) mobility through the formation of intrinsic uranyl-phosphate colloids above pH 3.0. In contrast, under more acidic conditions (pH 2.5), neither phosphate nor U(VI) can form such colloids, and thus phosphate has no influence on U(VI) transport. Although granite adsorption tends to immobilize U(VI), elevated bentonite colloids (BC) concentrations substantially increase uranium mobility, with recovery rates varying by mineral type: biotite (16.45 %) < illite (44.21 %) < K-feldspar (73.75 %) < quartz (90.54 %). Within the P-BC-U(VI) ternary system, bentonite colloids synergistically facilitates U(VI) transport by functioning as both carrier and stabilizer. An independent two-site model (ITSM) successfully reproduced the cotransport behavior by quantifying adsorption-desorption dynamics and colloid retention. These findings underscore the critical role of pH in governing radionuclide mobility mediated by environmental colloids, offering key insights for the safety assessment and transport modeling of geological repositories.