Environmental context. Organic ligands, especially those derived from humic acid (HA), play a major role in the fate and transport of metal ions in the environment. For the modelling of subsurface pollutant transport, it is important to understand which components of a heterogeneous humic material interact most strongly with multivalent cations.

Abstract. The luminescence sensitisation and anisotropy characteristics of a series of Tb^III complexes with a leonardite humic acid (LHA) were investigated in order to evaluate the interactions between the metal and different components of the humate. Ultrafiltration was used to separate LHA into six size fractions, which ranged from 500 Da to 0.2 μm, and were then used to form the Tb^III complexes. Each fraction was first characterised by ¹³C NMR and UV-Vis spectroscopy, which showed that the smaller ones (<3 kDa) had a significantly lower aliphatic content than the larger ones. These smaller components were good energy donors, which could effectively sensitise Tb^III luminescence. At the same time, the luminescence anisotropy of Tb^III increased significantly when these LHA fractions were added, which indicated the formation of tightly bound complexes. In this sense, the smaller LHA fractions were comparable to ethylenediaminetetraacetate, although their effect was not as strong. In contrast, the larger LHA sizes had little or no influence on Tb^III sensitisation or anisotropy. The results obtained suggest that the sizes and aliphatic content of humic polymers play a major role in their aqueous interactions with trivalent metal ions. Divalent metals are expected to behave in a similar way.