The effect of EDTA, NTA and picolinic acid on Th(IV) mobility in a ternary system with natural sand

Organic complexing agents, such as ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and picolinic acid, have been widely used at nuclear sites and are therefore found as common co-contaminants in radioactive contaminated land. This study has explored the mechanisms by which these three complexing agents affect the sorption of Th(IV) to pure silica and a natural sand. EDTA, NTA and, to a lesser extent, picolinic acid decreased the sorption of Th to silica, demonstrating the formation and solubility of Th complexes. However, Th sorption to sand was kinetically controlled and complexation enhanced the rate of Th sorption. EDTA and NTA did not sorb significantly to the sand, and metal desorption indicated that the mechanism involved exchange with sand-associated metals. At equilibrium, however, Th sorption was not affected by the presence of the ligands, and modelling suggested that the interaction between Th and the surface binding sites controlled Th sorption thermodynamically.

► Multi-analytical technique approach taken to define sorption processes.
► Picolinate complexes Th, but to a lesser extent than EDTA or NTA.
► Complexation decreased Th sorption to silica but increased its sorption rate to sand.
► Rapid sorption involved metal exchange rather than long term binding as ternary surface complexes.
► Initial Th complexation did not affect the sorption equilibrium.