| Summary: | NSF (OCE) [0850957]; Chinese Natural Science Foundation [41125020]; State Oceanic Administration of China [2010050012-3] Laboratory adsorption experiments were conducted to examine the role of particle composition in the scavenging and fractionation of Th-234 and Pa-233 on nanoparticles in seawater. Nanoparticles with known chemical composition and size were used, including SiO2, CaCO3, Fe2O3, Al2O3 and TiO2, representing biogenic opal, carbonate, metal oxides, and lithogenic particles, respectively. The results indicated that nanoparticles with the same size but different chemical compositions had different affinities for Th-234 and Pa-233, giving rise to different partition coefficient (K-d) values. Compared to Th-234, log IQ values of Pa-233 had a higher variability on different nanopartides, ranging from 3.10 to 5.39. In general, the log K-d values for Th-234 on CaCO3, Fe2O3, and Al2O3 were higher than those for Pa-233, while the opposite was true on SiO2 and TiO2 nanoparticles, resulting in a significant fractionation between Th-234 and Pa-233 during their adsorption on nanoparticle surfaces. Among the selected nanoparticles, Fe2O3 had the highest fractionation factor between Th-234 and Pa-233 (F-Th/Pa) while SiO2 had the lowest, following the order of Fe2O3 > CaCO3 > Al2O3 > TiO2 > SiO2. In addition to the effect of chemical composition, the concentration of nanoparticles or micro-particles also significantly affected the partitioning of Th-234 or Pa-233 between dissolved and particulate phases, showing a strong particle concentration effect with a general decrease in log K-d values with increasing particle concentration. Interestingly, results from adsorption experiments with binary nanoparticles, containing both SiO2 and CaCO3, clearly demonstrated an increase in log K-d values of Pa-233 with increasing SiO2/CaCO3 ratios of the adsorbent, but a decrease in the fractionation factor between Th-234 and Pa-233. However, the enhanced adsorption of Pa-233 on SiO2 nanoparticles or the increased log K-d value of Pa-233 occurred only when the abundance of SiO2 reached 60% (in wt.) or higher. Thorium, on the other hand, was less sensitive to particle composition during its adsorption on nanoparticles in seawater. Thus, it seems that the Th/Pa ratios of oceanic particles could be largely controlled by the relative abundance of biogenic silica. Further studies are needed to examine the role of natural organic matter, which affects surface properties and chemical speciation of trace elements, in regulating the adsorption of radionuclides on nanoparticles. (C) 2014 Elsevier B.V. All rights reserved.
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