Adsorption of Thorium-234 onto particles differing in surface area and surface quality

Thorium-234 (²³⁴Th) has been widely used as tracer for particle related processes such as horizontal and vertical particle transport, export fluxes and sediment dynamics. To gain insights into the factors controlling ²³⁴Th scavenging onto particle, we investigated the adsorption of ²³⁴Th onto natural particles differing with respect to surface area and quality. In controlled laboratory experiments the adsorption of ²³⁴Th onto fine sediment fraction <100µm (Baltic Sea sediment, boulder clay) and different types of microorganisms such as Rhodomonas spp., Synechococcus spp. and Surirella spp. were examined. Based on preliminary calculations the distribution coefficient (logK_d), which describes the ratio of particulate to dissolved ²³⁴Th phase in relation to particle concentration, is highest for the fine sediment fraction followed by Syncheococcus spp. > Rhodomonas spp. > Surirella spp.. Considering the surface area of the particles this pattern, however reverses with highest activity to surface ratios (logASR; in analogy to logK_d) of 2.5 ± 0.17 to 2.3 ± 0.15 for the organic coated particles of Rhodomonas spp. and Suriralla spp. declining to 1.2 ± 0.08 for the fine sediment fraction. In addition the logK_d decreases with increasing particulate organic carbon content. Our experimental results indicate that different natural particle types have varying ²³⁴Th uptake potential in relation to their surface area and composition. We concluded therefore that these findings could have significant influence for the application of ²³⁴Th as particle tracer and interpretation of ²³⁴Th based data.