Effect of natural organic matter on the adsorption and fractionation of thorium and protactinium on nanoparticles in seawater Peng Lin a,b , Min Chen a, ⁎, Laodong Guo b a College of Ocean and Earth Sciences and State Key Laboratory of Marine Environmental Science, Xiamen University, Xiangan, Xiamen 361102, China b School of Freshwater Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53204, USA abstract article info Article history: Received 27 April 2014 Received in revised form 2 August 2014 Accepted 11 August 2014 Available online 29 August 2014 Keywords: Thorium scavenging Protactinium adsorption Fractionation Nanoparticles Partition coefficient Colloidal organic matter Laboratory experiments were carried out to examine the role of natural organic matter (NOM) in regulating the adsorption and fractionation of 234 Th and 233 Pa in seawater on inorganic nanoparticles (20 nm), including SiO 2 , CaCO 3 , Fe 2 O 3 , Al 2 O 3 and TiO 2 . 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 (K d ) for 234 Th, with logK d values close to 7, while pro- teins showed weaker binding strength with 234 Th and had a logK d value of 5.75. Compared to 234 Th, 233 Pa showed lower affinity for different organic matters, with logK d values ranging from 4.41 on APS to 5.46 on 6-kDa-dextran. In general, macromolecules and COM preferentially complexed with 234 Th over 233 Pa, which resulted in a frac- tionation between 234 Th and 233 Pa on different NOMs with a fractionation factor (F Th/Pa ) following the order of APS N HA ≥ Dextran N BSA. Results from binary-sorbent experiments (nanoparticles plus NOM) indicated that NOMs evidently affected the adsorption of 234 Th and 233 Pa on different inorganic nanoparticles likely through the formation of organic coatings. While SiO 2 preferentially adsorbed 233 Pa over 234 Th, the presence of APS could compete the effective sorption sites and thus reduced the adsorption of 233 Pa on SiO 2 nanoparticles. Based on a two end-member mixing model (Li, 2005), coating extent seemed to depend on the relative abun- dance and chemical composition of organic matter and inorganic nanoparticles. In addition to organic composi- tion, NOM concentrations also affected the partitioning of 234 Th and 233 Pa between seawater and particles, showing a strong ‘colloidal concentration effect’. Nevertheless, there was little change in logK d values of 234 Th and 233 Pa 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Owing to the strong particle-reactive nature in marine environ- ments and constant production rate from the soluble and conservative parent radionuclides ( 234 U, 238 U and 235 U) in the water column (Ku et al., 1977), thorium (Th) and protactinium (Pa) isotopes, such as 230 Th, 234 Th and 231 Pa, have been widely used as oceanographic tracers or proxies for various processes in the ocean, such as export fluxes, boundary scavenging, oceanic circulation, and paleoproductivity (e.g., Anderson et al., 1983a,b; Kumar et al., 1995; Walter et al., 1997; Moran et al., 2005; Gherardi et al., 2009; Lippold et al., 2011). However, most of these applications are based on isotopic ratios of bulk particles. Little is known about the effect of particle composition and organic mat- ter on the scavenging of Th and Pa and their fractionation on colloidal and particulate matter in the ocean. Over the past decades, several studies through field and laboratory experiments have examined the role of particle compositions in the scavenging and fractionation of Th and Pa in the ocean (e.g., Anderson et al., 1992; Luo and Ku, 1999; Guo et al., 2002a; Geibert and Usbeck, 2004; Roberts et al., 2009; Kretschmer et al., 2011; Chuang et al., 2013, 2014). Field studies have elucidated the interrelationship between 230 Th/ 231 Pa ratio and the abundance of major particulate components in the ocean such as biogenic opal, carbonate, and metal oxides (e.g., Walter et al., 1997; Chase et al., 2002; Roy-Barman et al., 2005, 2009). Using 234 Th and 233 Pa as tracers, controlled laboratory experi- ments also revealed different roles of individual inorganic or organic components in the adsorption of 234 Th and 233 Pa in different experi- mental systems including bulk sinking particles collected by sediment traps, model micro-particles and nanoparticles, and marine colloidal or- ganic matter (e.g., Quigley et al., 2001; Guo et al., 2002a; Geibert and Usbeck, 2004; Alvarado-Quiroz et al., 2006; Roberts et al., 2009; Chuang et al., 2013, 2014; Lin et al., 2014). Nevertheless, the effect of natural or- ganic matter (NOM) on the interactions between radionuclides and Marine Chemistry 173 (2015) 291–301 ⁎ Corresponding author. E-mail address: mchen@xmu.edu.cn (M. Chen). http://dx.doi.org/10.1016/j.marchem.2014.08.006 0304-4203/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem