Sorption of humic acid and europium onto montmorillonite as a function of pH, ionic strength and metal ion concentration

In this work, the effect of pH and ionic strength was investigated in the ternary system of montmorillonite, humic acid and europium. The binary interactions of the metal ion and the humic acid with the montmorillonite have also been studied. The sorption of humic acid on montmorillonite is at a maximum (~95%) between pH 2 to 5. Above pH 5, the sorption starts decreasing. Increasing the pH increases the sorption of Eu3+ onto montmorillonite in the absence of humic acid. The behaviour in the binary Eu/montmorillonite is consistent with a surface complexation mechanism across the pH range. In the presence of humic acid, the Eu3+ behaviour is different. It shows a similar trend to that in the absence of humic acid in the range of pH from 2 to 5, but above pH 5 the amount sorbed decreases rapidly. Increasing HA concentration reduces Eu sorption. The sorption of HA and Eu3+ (in presence of HA) was also studied as a function of ionic strength. In both cases, the Kd value increases with increasing concentration of NaClO4 from 0.01 to 0.2 mol/l at pH 6. The presence of humic reduces Eu sorption in the ternary system, as would be expected for a competing ligand, but there is evidence from the data that Eu binds to sorbed humic in the ternary system, and that the sorbed humic has a higher affinity for the Eu than that left in the solution phase.