Effect of natural organic matter on the adsorption and fractionation of thorium and protactinium on nanoparticles in seawater

Laboratory experiments were carried out to examine the role of natural organic matter (NOM) in regulating the adsorption and fractionation of 234Th and 233Pa in seawater on inorganic nanoparticles (20 nm), including SiO2, CaCO3, Fe2O3, Al2O3 and TiO2. Model macromolecules and natural colloidal organic matter (COM) tested in the present study included dextran with different molecular weights, acid polysaccharides (APS), humic acids (HAs), protein (bovine serum albumin, BSA), and COM isolated from river water and seawater. APS and HAs had comparable affinity and higher partition coefficients (Kd) for 234Th, with logKd values close to 7, while proteins showed weaker binding strength with 234Th and had a logKd value of 5.75. Compared to 234Th, 233Pa showed lower affinity for different organic matters, with logKd values ranging from 4.41 on APS to 5.46 on 6-kDa-dextran. In general, macromolecules and COM preferentially complexed with 234Th over 233Pa, which resulted in a fractionation between 234Th and 233Pa on different NOMs with a fractionation factor (FTh/Pa) following the order of APS > HA ≥ Dextran > BSA. Results from binary-sorbent experiments (nanoparticles plus NOM) indicated that NOMs evidently affected the adsorption of 234Th and 233Pa on different inorganic nanoparticles likely through the formation of organic coatings. While SiO2 preferentially adsorbed 233Pa over 234Th, the presence of APS could compete the effective sorption sites and thus reduced the adsorption of 233Pa on SiO2 nanoparticles. Based on a two end-member mixing model (Li, 2005), coating extent seemed to depend on the relative abundance and chemical composition of organic matter and inorganic nanoparticles. In addition to organic composition, NOM concentrations also affected the partitioning of 234Th and 233Pa between seawater and particles, showing a strong 'colloidal concentration effect'. Nevertheless, there was little change in logKd values of 234Th and 233Pa on dextran with a molecular weight between 6-500 kDa or 2-10 nm in size, indicating negligible NOM size effect from small colloids on the adsorption of Th and Pa in seawater.