Effect of humic acid on technetium sorption on colloids

We studied the effect of humic acid (HA) on the sorption of technetium on hematite and alumina colloidal particles using the short lived isotope 95Tcm as a gamma emitting radiotracer of technetium. 95Tc tracer (t160 d, Eγ = 204 KeV) was produced following the reaction 96Mo(p,2n)95Tc by bombardment of MoO3 target with 22 MeV proton beam from the Pelletron accelerator at Mumbai. 95Tcm was separated as TcO4- from the irradiated target by solvent extraction followed by purification using an alumina column. Indigenously synthesized hematite and commercially available γ-alumina colloids having particle size 700-900 nm and 950 nm, respectively, were used for the sorption study. The zeta potentials of the hematite and alumina colloids were measured by photon correlation spectroscopy (PCS), and their point of zero charge (PZC) were found to be 6.5 and 9.0, respectively. Sorption of technetium on hematite and alumina as a function of pH was studied by adjusting the colloidal dispersion to the required pH and leaving for 48 hr after addition of the tracer, while in the presence of humic acid colloidal dispersion was first equilibrated for an additional 48 hr before addition of the tracer. Solutions were then centrifuged at 7000 rpm for 45 min to separate the colloidal particles from the suspension. The gamma activity of 95Tcg was measured in the liquid phase before and after the phase separation to obtain the percentage sorption. Tc(VII) was found to be sorbed on hematite at lower pH (< 5) values in absence of HA. The sorption, however, decreases slightly in presence of HA. Negligible sorption of technetium was observed at higher pH values both in absence and presence of HA. In the case of alumina, technetium was found to be sorbed uniformly at all pH values up to 9, which was not affected by the presence of HA. These observations can be explained in terms of the electrostatic effects. Negatively charged TcO4- ions are adsorbed on the hematite and alumina colloids in the pH range in which their zeta potential is positive, while there is no adsorption at pH values above the PZC, where the zeta potential of the colloids becomes negative. In presence of HA the PZC of hematite is not affected, while that of alumina is decreased to lower pH value. Though the sorption of TcO4- on hematite colloids is reduced slightly at lower pH, no such effect was observed in the case of alumina. Thus, the sorption behaviour of TcO4- does not appear to be affected significantly by the presence of HA, which itself is strongly adsorbed on these colloids. Technetium was found to be readily reduced to Tc(IV) by solid humic boom clay under the anaerobic conditions. The sorption behaviour of Tc(IV) is expected to be quite different from that of Tc(VII). Our results show that under the conditions of the present study technetium remains as TcO4- in presence of HA. (author)