Association behaviour of 241Am(III) on SiO2(amorphous) and SiO2(quartz) colloids

SiO2 colloids have been identified as a potential vector for enhancing radionuclide transport in granitic groundwater and in concrete pore water. The sorption behaviour of 241Am(III) on SiO2 colloids was studied as a function of americium concentration pH (5-12), colloid concentration, ionic strength, temperature and SiO2 allotropic species. The Am(III) sorption mechanism on amorphous silica is different from that on quartz. For SiO2(amorphous) solution, the variation of log Kp (ml g-1) with pH is linear (pH=5-9) with a slope of +1 indicating a one proton exchange mechanism. The colloid concentration (ppm) affects the sorption and log Kp 3.7-0.67 log [SiO2] (pH = 6). Kp increases insignificantly when the ionic strength decreases. It shows no significant variation, however, with the Am concentration. On amorphous silica, the Am(III) sorption is driven by proton exchange from the silanol groups. For SiO2(quartz), log Kp is constant over a large range of quartz concentration in suspension and the variation of log Kp with pH is about linear (pH = 5-12), with a slope of 0.28, indicating a more complex exchange mechanism. Reactions taking into account the interaction of positive Am(OH)w(3-w)+ species on to the negatively charged quartz surface are suggested. (author)