Radionuclide uptake by colloidal and particulate humic acids obtained from 14 soils collected worldwide

Uptake of six particle-reactive and/or redox-sensitive radionuclides (²¹⁰Pb, ²³⁴Th, ⁷Be, ⁵⁹Fe, ²³⁷Np and ²³³Pa) by 14 humic acids (HAs) was investigated in artificial groundwater under mildly acidic conditions (pH~5.5). In HA-groundwater slurry, Pb, Be, Fe and Pa bound strongly to particulate HA (>0.45 µm), supporting their application as tracers of soil erosion. Th bound strongly to the colloidal HA (3 kDa-0.45 µm) and as such, would not be a good candidate as a tracer for monitoring soil erosion. HAs likely reduced the oxidized neptunyl form (Np(V)O₄⁺) to Np(IV) based on its enhanced particle-reactivity and Np uptake by particulate HAs, partially retarding the movement of anthropogenic ²³⁷Np in field polluted environments. Particulate/colloidal carbonyl/O-aryl (likely through hydroquinone/quinone) functionalities in the HA correlated to Np and Pa uptake, but only particulate O-aryl functionalities was responsible for Fe uptake. The carboxylate- and carbonyl/O-aryl-containing organic functionalities in the HA correlated strongly with Th uptake. In contrast, no significant correlations between organic parameters and Pb or Be uptake implied their predominance of uniform surface adsorption onto particles. This study provides novel insight into the binding of six radionuclides with different organic functionalities of three size fractions, as well as its possible impact on their application in the soil-tracing research.