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

Marine Chemistry 173 (2015) 291–301

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Marine Chemistry journal homepage: www.elsevier.com/locate/marchem

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 b

College of Ocean and Earth Sciences and State Key Laboratory of Marine Environmental Science, Xiamen University, Xiangan, Xiamen 361102, China School of Freshwater Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53204, USA

a r t i c l e

i n f o

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

a b s t r a c t 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 N HA ≥ Dextran N 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. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Owing to the strong particle-reactive nature in marine environments and constant production rate from the soluble and conservative parent radionuclides (234U, 238U and 235U) in the water column (Ku et al., 1977), thorium (Th) and protactinium (Pa) isotopes, such as 230 Th, 234Th and 231Pa, 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 matter on the scavenging of Th and Pa and their fractionation on colloidal and particulate matter in the ocean. ⁎ Corresponding author. E-mail address: [email protected] (M. Chen).

http://dx.doi.org/10.1016/j.marchem.2014.08.006 0304-4203/© 2014 Elsevier B.V. All rights reserved.

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/231Pa 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 234Th and 233Pa as tracers, controlled laboratory experiments also revealed different roles of individual inorganic or organic components in the adsorption of 234Th and 233Pa in different experimental systems including bulk sinking particles collected by sediment traps, model micro-particles and nanoparticles, and marine colloidal organic 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 organic matter (NOM) on the interactions between radionuclides and

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conducted to examine the effect of NOM concentration and size on the partitioning of 234Th and 233Pa in seawater.

particle surfaces and the role of organic ligands or functional groups in influencing the scavenging of radionuclides in the ocean remain poorly understood. Accumulating evidence has shown the strong binding ability of organic ligands with trace elements (Bruland et al., 1991; Hirose, 2007; Vraspir and Butler, 2009), as well as the important role of organic matter in regulating the transport and biogeochemical cycling of particle-reactive radionuclides in marine environments, especially colloidal organic matter (e.g., Baskaran et al., 1996, 2003; Guo et al., 1997; Xu et al., 2011; Chuang et al., 2013, 2014). In fact, marine colloidal and suspended particles are mostly organic in nature and levels of dissolved organic matter (DOM) are several orders of magnitude higher than those of particle-reactive radionuclides in seawater (Santschi et al., 2006). Therefore, systematic studies are needed to better understand the influence of NOM, including molecular compositions and functional groups on the scavenging and fractionation of Th and Pa isotopes in marine environments, and to better use these radionuclides as oceanographic tracers or proxies. So far, the scavenging and the fractionation of Th and Pa were mostly simulated with individual inorganic or organic matter in previous laboratory adsorption experiments (e.g., Hirose and Tanoue, 2001; Guo et al., 2002a; Quigley et al., 2002; Alvarado-Quiroz et al., 2006). Binary-sorbent adsorption experiments containing both organic and inorganic sorbents are still few (e.g., Roberts et al., 2009; Chuang et al., 2014) although such adsorption systems are more close to natural seawater conditions. Thus, interactions between different types of organic matter and inorganic particles, as well as their influence on the scavenging and fractionation of Th and Pa need to be better quantified. Recently, we have examined the role of particle composition and size in the adsorption of 234Th and 233Pa in seawater and their fractionation on pure inorganic nanoparticles with known composition and size (Lin et al., 2014). In the present study, controlled laboratory experiments were carried out to examine the adsorption of 234Th and 233Pa on inorganic nanoparticles in the presence or absence of organic matter in seawater in order to further determine the role of organic ligands or functional groups in regulating the scavenging and fractionation of 234 Th and 233Pa in the ocean. Natural and model macromolecular organic matters were used including humic substances (humic acid, HA), acid polysaccharides (xanthan gum, APS), protein (bovine serum albumin, BSA), dextran with different molecular weights, and natural colloidal organic matter (COM) isolated from river water and seawater. Inorganic nanoparticles used for the adsorption experiments included SiO2, CaCO3, Al2O3, TiO2, and Fe2O3. Additional experiments were also

2. Methods 2.1. Nanoparticles and organic matter Natural seawater was used in all adsorption experiments, including ultraviolet irradiated natural seawater (UVSW) and natural seawater (NSW) containing DOM. Seawater was collected from the northern Gulf of Mexico (salinity 35) and filtered with a 0.4 μm Nuclepore filter for laboratory experiments with a dissolved organic carbon (DOC) concentration b1.2 mg-C/L. Aliquots of filtered seawater were further UVirradiated (500 W) for over 48 h to remove active DOM (Chen et al., 2004). The background DOC concentration in this UVSW was about 0.2 mg-C/L. Inorganic nanoparticles for the adsorption experiments included SiO2, CaCO3, Fe2O3, Al2O3 and TiO2 (Table 1), representing opal, carbonate, metal oxides/hydroxides and lithogenic particles, respectively, in the ocean. All nanoparticles used had the same particle size of 20 nm and should have the same specific surface areas, allowing the evaluation of the effect of particle composition and organic matter on the adsorption and fractionation of Th and Pa in seawater. Types of organic matter used in our laboratory adsorption experiments are listed in Table 1. Similar to previous studies (e.g., Guo et al., 2002a; Quigley et al., 2002; Yang et al., 2013), acid polysaccharides (APS, xanthan gum), dextran with different molecular weights (i.e., 6, 40, and 500 kDa), proteins (bovine serum albumin, BSA), and humic acid (HA) were used. In addition to model macromolecular organic matter, freeze-dried natural COM collected from the Yukon River (YR-COM) and Gulf of Mexico (GoM-COM) were also used in the adsorption experiments. Hereafter, natural organic matter (NOM) is used to represent both natural COM and model macromolecular organic matter. 2.2. Adsorption experiments A schematic of the adsorption experiment of 234Th and 233Pa is shown in Fig. 1. Different inorganic and organic sorbents were used in adsorption experiments including 1) single-sorbent experiments containing either inorganic nanoparticles or NOM, 2) binary-sorbent experiments containing nanoparticles–NOM mixtures, and 3) particle-

Table 1 Partition coefficient (Kd) values of 234Th and 233Pa (in logKd) and fractionation factor between 234Th and 233Pa (FTh/Pa) in different adsorption systems. All inorganic nanoparticles have a nominal size of 20 nm and a specific surface area between 89 and 103 m2/g (Beijing Nachen Science & Technology Co. Ltd). Model colloidal organic matter (COM) included dextran with different molecular weights, acid polysaccharides (APS), humic acids (HAs), and proteins (BSA). Natural COM (1 kDa-0.4 μm) included COM collected from the Yukon River (YR-COM) and the Gulf of Mexico (GoM-COM). Macromolecules or COM

Without nanoparticle 234

Th

233

Pa

FTh/Pa

SiO2 234

Th

CaCO3 233

Pa

FTh/Pa

234

Th

Fe2O3 233

Pa

FTh/Pa

234

Th

Al2O3 233

a

FTh/Pa

234

Th

TiO2 233

Pa

UV-irradiated natural seawater experiments (UVSW) UVSWa – – – Dextran 6 kDa 6.80 5.46 22 Dextran 40 kDa 6.82 5.41 26 Dextran 500 kDa 6.85 5.38 30 APS 6.76 4.41 224 HA 6.90 5.38 33 BSA 5.75 4.84 8.1

4.73 5.54 5.67 5.24 5.60 5.13 5.42

5.39 4.32 4.32 4.32 3.91 4.15 4.12

0.22 17 22 8.3 49 9.5 20

4.86 – – – 5.08 4.65 5.82

4.08 – – – 3.85 3.79 4.05

6.0 – – – 17 7.2 59

5.59 – – – 5.57 5.03 5.31

4.01 – – – 3.67 4.05 4.53

38 – – – 79 9.5 6.0

5.01

5.16 5.42 5.08

3.99 – – – 3.95 3.65 4.14

Natural seawater experiments (NSW) NSW 7.41 – APS 6.38 – HA 6.42 – Dextran 6 kDa 6.14 5.22 YR-COM 6.32 5.04 GoM-COM 5.86 4.72

7.72 6.91 6.45 6.52 – 6.21

– – – 6.21 – 5.80

– – – 2.0 – 2.6

7.48 6.30 6.27 3.69 5.87 6.01

– – – 3.19 4.40 4.83

– – – 3.2 30 15

7.77 7.23 6.76 6.69 – 6.32

– – – 6.32 – 5.69

– – – 2.3 – 4.3

6.74 6.15 6.14 – – –

– – – – – –

“–” denotes no data. a Data from Lin et al. (2014).

– – – 8.3 19 14

FTh/Pa

234

11 – – – 16 59 8.7

4.39

– – – – – –

Th

233

Pa

FTh/Pa

6.37 5.29 4.62

4.48 – – – 4.29 4.18 4.20

0.81 – – – 120 13 2.6

6.64 7.15 6.64 – – –

– – – – – –

– – – – – –

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Fig. 1. Schematic diagram of 234Th and 233Pa adsorption experiments in single-sorbent and binary-sorbent systems. UVSW denotes ultraviolet irradiated natural seawater, NSW denotes natural seawater without ultraviolet irradiation, and P.C.E. denotes particle concentration effect.

concentration-effect experiments containing the same sorbent but different concentrations (Fig. 1). Briefly, all adsorption experiments were carried out in a stirred cell ultrafiltration unit with a 1 kDa membrane (Amicon, YM1) under constant stirring. Before the experiments, all Teflon beakers and stirred cell units were cleaned with 10% HCl solution and deionized water. The stirred cell unit was then pre-conditioned with UVSW or NSW (Fig. 1). Predetermined amounts of NOMs and/or nanoparticles were added into UVSW or NSW solution buffered with Tris–HCl for a pH of 8.0, with a final concentration of 50 mg/L for nanoparticles and 1 mg/L for NOMs, respectively. Then, 200 to 250 Bq of 234 Th milked from the parent nuclide 238U or 233Pa equilibrated with 237 Np, both in 200 μL of 1 M HCl solution, was added into the above experimental system under constant stirring. An equilibrium time of 2 h was consistently used for all adsorption experiments, which allowed data comparisons with other previous studies (Quigley et al., 2001; Guo et al., 2002a; Yang et al., 2013; Lin et al., 2014). Measurements of pH before and after the experiments showed no significant shift in pH (±0.3 pH) during the adsorption experiments. After adsorption equilibrium, the experimental solution was ultrafiltered to separate nanoparticles and/or NOMs from the b1 kDa dissolved phase to examine the partitioning of 234Th and 233Pa between colloidal and dissolved phases. Aliquots of initial adsorption solution, retentate (i.e., colloids or nanoparticles) and the b 1 kDa solution were sampled for the measurement of 234Th and 233Pa, to evaluate the mass balance of 234Th and 233Pa, which was used to check possible loss of 234 Th and 233Pa on the container wall during the adsorption experiments. Total recoveries in all adsorption experiments were 92 ± 7% for 234Th and 95 ± 9% for 233Pa, respectively. Mass balance higher than 90% indicated that sorption losses of 234Th and 233Pa were indeed minimal during adsorption experiments and filtration processes. Additional adsorption experiments with different concentrations of dextran (6 kDa) were designed to investigate whether NOM or colloids had similar particle concentration effect on the partitioning of Th and Pa in marine environments as those observed for micro-particles in field and laboratory studies (e.g., Honeyman and Santschi, 1989; Guo et al., 1995; Lin et al., 2014). Dextrans with different molecular weights,

including 6 kDa, 40 kDa, and 500 kDa were also used as sorbents to examine the molecular weight or size effect of NOMs on the adsorption of 234 Th and 233Pa in seawater. 2.3. Measurements of 234Th and 233Pa For samples containing 234Th and 233Pa, the activities of 234Th and Pa were determined on a Canberra gamma spectrometer with a ultra low background high purity Germanium well detector at 63.5 keV for 234Th and 312 keV for 233Pa, respectively (Guo et al., 2002a; Lin et al., 2014). Some samples containing only 234Th were counted on a liquid scintillation counter (LSC, Tris-Carb 2900TR, PerkinElmer) along with blank scintillation solution and initial spike solution. Samples were counted to obtain a counting error of b5% or better depending on specific samples. The ingrowth of 233Pa from 237Np after adsorption experiments was minimized by counting the samples within 1 d and thus the ingrowth was generally negligible. 233

2.4. Partition coefficient and fractionation factor Similar to previous studies (e.g., Quigley et al., 2001; Guo et al., 2002a; Yang et al., 2013), distribution coefficient (Kd) of particlereactive elements was used to quantify the partitioning and adsorption behavior of 234Th and 233Pa and their affinity for inorganic nanoparticles and NOMs in seawater. While the traditionally defined Kd is the partition coefficient of radionuclides between the traditionally defined dissolved (b0.4 μm) and particulate phases, the same Kd is defined here as the partition coefficient of 234Th or 233Pa between the b1 kDa dissolved and nanoparticle or colloidal (N1 kDa) phases (Guo et al., 1997; Lin et al., 2014): Kd ¼

Ac Ad  Cp

where Ac is the activity of 234Th or 233Pa in the colloidal or nanoparticle phase (in Bq/L); Ad is the activity of 234Th or 233Pa in the b 1 kDa

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dissolved phase (in Bq/L); and Cp is the concentration of nanoparticles or NOMs (in g/mL or kg/L). Therefore, the dimension of Kd values is mL/g or L/kg. To quantify the fractionation between 234Th and 233Pa in a specific adsorption system, a fractionation factor between 234Th and 233Pa (FTh/Pa) or between 233Pa and 234Th (FPa/Th) has been used (Guo et al., 2002a; Roberts et al., 2009; Yang et al., 2013; Lin et al., 2014). Similarly, FTh/Pa was defined as the ratio of Kd values: FTh=Pa ¼ Kd ðThÞ=Kd ðPaÞ FTh/Pa values equal to unity mean no fractionation between 234Th and Pa during their adsorption on particle surfaces. FTh/Pa values N 1 indicate the specific particles preferentially scavenge 234Th over 233Pa, while 233 Pa is selectively removed by particles when FTh/Pa values are b 1. 233

3. Results 3.1. Partitioning of 234Th and 233Pa 3.1.1. UV-radiated natural seawater experiments (UVSW) The logKd values of 234Th and 233Pa derived from different adsorption experiments with different NOMs (single sorbent) and nanoparticles– NOM (binary sorbents) in UVSW are summarized in Table 1 and depicted in Figs. 2 and 3. In single-NOM adsorption experiments, APS, HAs and dextrans with different molecular weights generally had higher and comparable logKd values for 234Th, ranging from 6.76 to 6.90. However, protein (BSA), with a logKd value of 5.75, showed a significantly lower affinity for 234 Th compared to other NOMs. Compared with 234Th, 233Pa generally had lower logKd values, varying from 4.41 on APS to 5.46 on dextran 6 kDa with the sequence of dextran N HA N BSA N APS (Table 1,

Fig. 2. Variations in logKd values of 234Th and 233Pa as well as the fractionation factor between Th and Pa (FTh/Pa, defined as the ratio of Kd-Th to Kd-Pa) in different experimental treatments: ultraviolet irradiated natural seawater (UVSW, left panels) and natural seawater (NSW, right panels). 6 k, 40 k and 500 k denote dextran with molecular weight of 6, 40 and 500 kDa, respectively, while YR and GoM denote natural COM collected from the Yukon River and the Gulf of Mexico, respectively.

P. Lin et al. / Marine Chemistry 173 (2015) 291–301

295

the logKd values of 234Th on TiO2 ranged from 4.62 in the presence of BSA to 6.37 in the presence of APS (Table 1). In the presence of APS, TiO2 had the highest logKd value for 234Th among different nanoparticles, followed by SiO2, Fe2O3, Al2O3 and CaCO3 (Fig. 3). In the presence of HA or BSA, variations in logKd values of 234Th followed a different order among different nanoparticles compared to nanoparticles–APS adsorption systems (Fig. 3), showing the order of CaCO3 N SiO2 N Fe2O3 N Al2O3 N TiO2 for nanoparticles–BSA and Al2O3 N TiO2 N SiO2 N Fe2O3 N CaCO3 for nanoparticles–HA adsorption systems. In contrast, 233 Pa generally had lower and less variable logKd values between different nanoparticle–NOM adsorption systems (Fig. 3), ranging from 3.67 on Fe2O3 to 4.29 on TiO2 in nanoparticles–APS treatments (following the sequence of TiO2 N Al2O3 N SiO2 N CaCO3 N Fe2O3), from 4.05 on CaCO3 to 4.53 on Fe2O3 in nanoparticles–BSA treatments (Fe2O3 N TiO2 N Al2O3 N SiO2 N CaCO3), and from 3.65 on Al2O3 to 4.18 on TiO2 in nanoparticles–HA treatments (TiO2 N SiO2 N Fe2O3 N CaCO3 N Al2O3), respectively (Table 1). 3.1.2. Natural seawater experiments (NSW) As shown in Table 1 and Fig. 4, in single-nanoparticle adsorption experiments with NSW, the logKd values of 234Th were generally higher than those in single-nanoparticle UVSW treatments. The logKd values also varied significantly among different nanoparticles, ranging from 6.64 on TiO2 to 7.77 on Fe2O3 with the order of Fe2O3 N SiO2 N CaCO3 N Al2O3 N TiO2, showing the effect of NOMs on the partitioning of 234Th between nanoparticles and seawater. Similar to the results in UVSW treatments, different NOMs also had different affinities for 234Th and 233Pa in NSW treatments, with distinct effect on the adsorption of 234Th and 233Pa on nanoparticles (Fig. 2). For example, the logKd values of 234Th varied from 5.86 on GoM-COM to 6.42 on HA without inorganic nanoparticles (Fig. 2). For binarysorbent experiments with both nanoparticles and NOM in NSW, Fe2O3 generally had the highest logKd values for 234Th among the five nanoparticles (Figs. 4 and 5). Furthermore, the addition of NOMs seemed to somewhat decrease logKd values of 234Th or 233Pa compared with those from single-nanoparticle adsorption systems with NSW (e.g., 6.52 for 234Th on SiO2–dextran vs. 7.72 for 234Th on SiO2 nanoparticles). In contrast, the addition of most nanoparticles into single-NOM

Fig. 3. Variations in logKd values of 234Th and 233Pa as well as the fractionation factor between Th and Pa (FTh/Pa, defined as the ratio of Kd-Th to Kd-Pa) in different adsorption experiments, including nanoparticles-COM and single-inorganic nanoparticles (INP, data from Lin et al., 2014) experiments in UVSW treatments.

Fig. 2). Significant differences in the logKd values between NOMs for 234 Th or 233Pa or between 234Th and 233Pa on the same NOM indicated that different NOMs could have different affinities for different nuclides in seawater. In binary-sorbent adsorption experiments with both nanoparticles and NOM, organic composition of NOMs had a distinct influence on the partitioning of 234Th and 233Pa, especially on 234Th. For example,

Fig. 4. Comparisons in logKd values of 234Th between UV-irradiated seawater (UVSW) and natural seawater (with dissolved organic matter, DOM) experiments using different inorganic nanoparticles. Data of single-nanoparticle (INP) in UVSW treatments are from Lin et al., 2014.

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3.2. Fractionation between 234Th and 233Pa during adsorption The logKd values between 234Th and 233Pa were evidently different regardless of single-sorbent or binary-sorbent (i.e., nanoparticles– NOM) adsorption experiments in UVSW and NSW treatments (Table 1), indicating different adsorption intensities between theses two radionuclides and significant fractionation between 234Th and 233 Pa during adsorption. For UVSW, as shown in Figs. 2 and 3, values of fractionation factor between 234Th and 233Pa (FTh/Pa) varied among different adsorption experiments, but they were all higher than unity, indicating that these adsorption systems all preferentially adsorbed 234 Th over 233Pa. For example, APS had the highest FTh/Pa value of 224, followed by HA, dextran, and BSA in UVSW treatments (Table 1, Figs. 2 and 6). Similarly, APS–nanoparticle treatments also had higher FTh/Pa values in NOM–nanoparticle experiments, ranging from 17 to 120 (average 56 ± 44, Table 1), while BSA– and HA–nanoparticle treatments had comparable moderate FTh/Pa values, with a range from 2.6 to 59 (average 19 ± 23) and from 7.2 to 59 (average 19 ± 22), respectively (Fig. 3). For the adsorption experiments in NSW, FTh/Pa values higher than unity (Figs. 2 and 5) also indicated the preferential scavenging of 234 Th over 233Pa in the presence of different NOMs, although the FTh/Pa values were somewhat lower than those from UVSW treatments (Table 1). 3.3. Partitioning of 234Th and 233Pa under different NOM concentrations The logKd values of 234Th and 233Pa under different dextran concentrations are summarized in Table 2 and depicted in Fig. 6 to examine possible colloidal particle concentration effect on the partitioning of 234 Th and 233Pa between seawater and NOM. As shown in Fig. 6, both 234 Th and 233Pa had a general decrease in logKd values with increasing dextran (6 kDa) concentrations from 1 mg/L to 25 mg/L, showing a strong colloidal particle concentration effect on the adsorption of 234 Th and 233Pa. While logKd values of both 234Th and 233Pa decreased with increasing dextran concentration, the FTh/Pa values increased in general with increasing dextran concentrations, especially at higher dextran concentrations (Fig. 6). 3.4. Partitioning of 234Th and 233Pa on different size of NOM As shown in Fig. 7, there was little difference in the logKd values for both 234Th and 233Pa on different sizes of dextran between 6 and 500 kDa or 2 and 10 nm (converted from their relationship shown in Guo and Santschi, 2007). Similarly, in adsorption experiments with dextran plus SiO2 nanoparticles, logKd values of both 234Th and 233Pa did not seem to vary with increasing dextran molecular weight between 6–500 kDa although logKd values of 234Th decreased a bit when the dextran molecular weight or size reached 500 kDa or ~10 nm (Fig. 7). Consistent with their differences in particle reactivity, 234Th had consistently higher logKd values compared to 233Pa in both dextran and dextran–SiO2 adsorption experiments. 4. Discussion 4.1. Colloidal concentration and size effect

Fig. 5. Variations in logKd values of 234Th and 233Pa on nanoparticles and their fractionation factor (FTh/Pa, defined as Kd-Th/Kd-Pa) in natural seawater treatments containing both natural DOM and additional macromolecular organic matter (including 6 kDa-dextran, YR-COM and GoM-COM).

adsorption systems in NSW treatments appeared to increase logKd values, except for Al2O3–COM and CaCO3–dextran adsorption experiments (Fig. 2 vs. Figs. 4 and 5).

Particle concentration effect on the partitioning of radionuclides has been widely reported in both field and laboratory studies (Honeyman and Santschi, 1989; Guo et al., 1997; Lin et al., 2014). Our hypothesis was that, similar to micro-particles, colloids and nanoparticles should also have a particle concentration effect or colloidal concentration effect on the partitioning of trace elements in seawater. The inverse correlation between logKd values and dextran concentrations (in logCdextran, Fig. 6) clearly showed an influence on the adsorption of 234Th and 233 Pa and suggested an evident colloidal concentration effect, which was similar to other observations for micro-particles or nanoparticles

P. Lin et al. / Marine Chemistry 173 (2015) 291–301

8

40 2

Th Pa

y = 7.04 - 1.06x R = 0.95 2 y = 5.88 - 1.25x R = 0.98

2

y = 12.7 + 14.1x R = 0.59

7

30

FTh/Pa

LogKd

297

6

5

20

10

4

0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

Log(Cdextran)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Log(Cdextran)

Fig. 6. Relationship between concentrations of 6 kDa-dextran (in logCdextran) and logKd values of 234Th and 233Pa (left panel) or the fractionation factor (FTh/Pa, right panel).

(e.g. Honeyman and Santschi, 1989; Benoit, 1995; Guo et al., 1995; Luo et al., 1995; Lin et al., 2014). The presence of colloidal particles has been used to explain the observed particle concentration effect in marine environments (Honeyman and Santschi, 1989). In the present study, only the 6 kDa (or 2 nm) dextran was added into natural seawater in which natural colloids had been removed through pre-filtration, followed by ultrafiltration and UV-irradiation (i.e., UVSW). The colloidal concentration effect shown in Fig. 6 could thus be explained by the colloid–colloid and particle–particle interactions especially when the NOM concentration increased. Exopolymeric substances and acid polysaccharides have been shown to form gels and could enhance particle aggregation in the ocean, especially at a critical particle concentration (Jackson, 1990; Logan et al., 1995; Santschi et al., 2006; DeLa Rocha et al., 2008). Under high NOM concentrations, colloidal aggregation and coagulation should become the predominant process causing decreased Kd values and the colloidal concentration effect observed here (Fig. 6). Even though both 234Th and 233Pa showed similar decrease in logKd with increasing NOM concentration (Fig. 6), their fractionation factor increased somewhat with increasing dextran concentrations, especially when logCp N1 or NOM concentration N 10 mg/L (Fig. 6), suggesting that the colloidal concentration effect was slightly different on the adsorption of 234Th and 233Pa under low and high NOM or colloidal concentrations in seawater. On the other hand, little variation in the logKd values of both 234Th and 233Pa with increasing NOM size or molecular weight (Table 1, Fig. 7) suggested negligible NOM size effect on the partitioning of Th and Pa when the NOM composition was the same or the size was b10 nm. Previous studies have examined the colloidal size distribution of 234Th in the b0.4 μm dissolved phase (e.g., Guo and Santschi, 1997; Zhang et al., 2005; Kretschmer et al., 2010), but specific colloidal size Table 2 LogKd values of 234Th and 233Pa, as well as the fractionation factor between 234Th and 233Pa (FTh/Pa) under different dextran (6 kDa) concentrations in UV-irradiated natural seawater. Dextran concentration (mg/L)

234

1 2 3 5 7 10 15 20 25

6.97 6.81 6.67 6.29 6.03 5.75 5.89 5.71 5.60

FTh/Pa

logKd Th

233

Pa

5.80 5.53 5.42 4.94 4.76 4.46 4.28 4.27 4.06

15 19 18 22 19 20 41 28 35

effect was not identified because the chemical composition of marine colloids could change with size (Guo and Santschi, 2007; Stolpe et al., 2010). Nevertheless, particle size effect on the adsorption of 234Th and 233 Pa has been observed on nanoparticles and micro-particles (e.g., Lin et al., 2014). Thus, change in chemical composition with particle size might be partly responsible for the particle size effect on the scavenging and fractionation of Th and Pa observed in marine environments. 4.2. Role of organic functionalities In order to test our hypothesis that the effect of organic matter on the adsorption of 234Th and 233Pa is organic ligand specific and to evaluate the role of different organic functionalities, selected model macromolecular organic matter with known functionalities was used in our adsorption experiments (Fig. 2). For discussion purposes, NOMs were categorized into three different compound classes: carbohydrates (acid polysaccharides and dextran with different molecular weights), protein (BSA) and humic substances (HAs) in addition to natural COM. Acid polysaccharide and dextran (6 kDa, 40 kDa and 500 kDa) had comparable and highest logKd values for 234Th (Fig. 2). High logKd values of 234Th observed on carbohydrates were consistent with those previously reported in laboratory experiments (Guo et al., 2002a; Quigley et al., 2001, 2002) and field studies (e.g., Guo et al., 2002b; Santschi et al., 2003; Passow et al., 2006; Xu et al., 2011), showing the essential role of carbohydrates in the scavenging of Th isotopes in marine environments, especially for the acid polysaccharides. In contrast to 234Th, 233Pa had the lowest logKd values on acid polysaccharides compared to dextran and other organic compounds. Hence, effect of organic matter on the adsorption of nuclides in seawater is also nuclide- or element-specific. Low affinity of acid polysaccharides for 233Pa but high for 234Th led to preferential scavenging of 234Th over 233Pa, and thus a high fractionation factor between 234Th and 233Pa (Fig. 2). Proteins had the lowest FTh/Pa values during the adsorption of 234Th and 233Pa due to the lowest logKd values for 234Th on proteins (Fig. 2). Relatively lower affinity of proteins for 234Th might be related to its functional groups with more N- and S-containing soft ligands compared with carbohydrates. Several acidic functional groups in acid polysaccharide compounds, like glucuronic acid, sulfate and phosphate, had strong binding and complexation ability with particle-reactive metals and radionuclides (Alvarado-Quiroz et al., 2006; Zhang et al., 2008; Hassler et al., 2011). Similar to carbohydrates, humic acids also had the highest logKd values for both 234Th and 233Pa, although the affinity of humic acid for 233 Pa was lower than that for 234Th (Fig. 2). High binding ability of humic acids with 234Th was consistent with the fact that humic substances also contain a certain number of acidic functional groups, such

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Fig. 7. The effect of NOM size on the adsorption of 234Th and 233Pa (in terms of logKd values) in the absence (left panel) or presence (right panel) of SiO2 nanoparticles. Numbers of kDa were converted into nm based on their relationship shown in Guo and Santschi (2007).

as carboxyl and phenolic hydroxyl functional groups (Santschi et al., 2006; Yang et al., 2013). The difference between the binding of 234Th (or 233Pa) with protein and the binding with acid polysaccharide or humic substances indicated a strong preference of Th binding to acidic functional sites of natural colloidal and particulate organic matter in the ocean (Quigley et al., 2002; Santschi et al., 2003). In terms of fractionation between 234Th and 233Pa, humic acids also preferentially bound 234Th over 233Pa, which was consistent with previous observations (e.g., Guo et al., 2002a), likely due to their difference in oxidation states (e.g., Th(IV) vs. Pa(V) in oxic environments). 4.3. Interactions between NOM and inorganic nanoparticles Our single-sorbent adsorption experiments were designed to examine the interactions of 234Th or 233Pa with individual sorbents including NOMs and inorganic nanoparticles (e.g., Fig. 2). Nevertheless, natural colloids, particles, or phytoplankton cells should contain both organic and inorganic components in marine environments (Chuang et al., 2013, 2014). Our binary-sorbent adsorption experiments were then used to elucidate the interactions between NOM and inorganic nanoparticles and how NOMs might have altered the partitioning of 234Th and 233Pa between nanoparticles and seawater as well as the mechanisms through which 234Th and 233Pa bound with organic ligands and

particle surfaces. It is likely that the effect of NOMs on the adsorption of nuclides is a synergistic one, resulting in positive or negative impacts through change in surface properties of particles and chemical speciation of nuclides in the presence of different organic ligands. Based on the data in Fig. 3, Kd ratios in UVSW treatments were calculated as the ratio of Kd values in nanoparticle–NOM adsorption systems over those in single-nanoparticle adsorption systems to further examine the role of NOMs in the adsorption of 234Th and 233Pa on nanoparticles. As shown in Fig. 8, it was clear that sorption of 234Th on inorganic nanoparticles was enhanced to a different extent (Kd ratios N 1) in the presence of different NOMs except for NOM–Fe2O3 and the treatment with HA–CaCO3. Similar cases were also found in NSW treatments showing increased logKd values in the presence of seawater DOM as compared with those in UVSW treatments (Figs. 4 and 5). However, this enhancement was not observed for the sorption of 233Pa on inorganic nanoparticles in UVSW except for treatments with BSA–Fe2O3 and BSA–Al2O3 (Fig. 8). The most noticeable sorption depression for 233 Pa was observed in the treatment with SiO2 nanoparticles (Fig. 8) that had been found to have the highest affinity for 233Pa (Walter et al., 1997; Guo et al., 2002a; Chase et al., 2002; Lin et al., 2014). Due to the high affinity of 233Pa on SiO2 nanoparticle surfaces, the presence of NOM in turn reduced considerably the active surface sites for 233Pa and thus depressed the sorption of 233Pa on SiO2 surfaces (Fig. 3).

Fig. 8. Variation in Kd ratios of 234Th and 233Pa with different adsorption systems in UVSW treatments. The Kd ratio is defined as the ratio of Kd values derived from nanoparticle–NOM adsorption experiments over Kd values in single-nanoparticle adsorption experiments (data from Lin et al., 2014) with UVSW.

P. Lin et al. / Marine Chemistry 173 (2015) 291–301

Consequently, the addition of NOMs significantly changed the fractionation between 234Th and 233Pa during their adsorption on nanoparticles in seawater (Figs. 3 and 5). For example, the preferential scavenging of 233 Pa over 234Th in single-SiO2 adsorption experiments was evidently altered by NOMs (Table 1, Fig. 3). Similarly, NOMs also changed the fractionation between 234Th and 233Pa in NSW treatments (Fig. 5). Such change in the scavenging and fractionation of 234Th and 233Pa in the presence of NOMs was clearly synergistic effect and consistent with those observed in previous studies (e.g., Roberts et al., 2009; Yang et al., 2013; Chuang et al., 2014). This also pointed to potential change in chemical properties of nanoparticle surfaces, likely resulting from the formation of organic coatings and other interactions between NOM and nanoparticles. In addition, the difference in the affinity of NOMs and inorganic nanoparticles for 234Th and 233Pa might also play a role (Fig. 2). If organic matter is coated on the surface of nanoparticles, 234Th and 233 Pa would have to interact with organic ligands before being adsorbed onto nanoparticles, and the partitioning of 234Th and 233Pa in seawater might predominantly depend on the chemical composition of NOMs and the interactions between organic and inorganic sorbents. Although surface coating by organic matter may vary with specific particles, inorganic nanoparticles could serve as a carrier and influence the partitioning or Kd values of 234Th and 233Pa in adsorption experiments with different treatments. Interestingly, the results from NSW treatments seemed to support the existence of organic coatings during the adsorption of 234Th and 233 Pa on nanoparticle surfaces in seawater. The logKd values of 234Th or 233Pa in binary-sorbent experiments with both nanoparticle and NOM (Figs. 4 and 5) were generally on the same order of magnitude as those in single-NOM adsorption experiments (Fig. 2) except for CaCO3–dextran treatment (Table 1). Furthermore, the addition of inorganic nanoparticles into NOM adsorption systems all enhanced the adsorption of 234Th and 233Pa relative to single-NOM adsorption experiments in NSW, likely through the formation of organic coating and increase in the effectiveness of organic ligands (e.g., Reiler et al., 2002; Chen and Wang, 2007). However, the logKd values of 234Th and 233Pa in nanoparticle–NOM adsorption experiments with UVSW containing no natural COM (Fig. 3) were generally lower than those in singleNOM adsorption systems (Fig. 2), suggesting less organic coating formation without natural COM. It was likely that the freshness of DOM and its diverse functionalities in seawater (e.g., Chuang et al., 2014) also play a role in organic coating formation and enhanced the adsorption of 234Th and 233Pa in seawater. 4.4. Factors affecting organic coating extent To further evaluate organic coating extent and the relative importance of individual inorganic nanoparticles and NOMs in the scavenging

299

and fractionation of 234Th and 233Pa in seawater, predicted Kd values (Kd-P) based on a two end-member mixing model (Li, 2005) were compared with measured Kd values in the present study. Kd-P values were calculated from the following equation: Kd −P ¼ f N Kd‐N þ f O Kd‐O f N þ f O ¼ 100% where Kd-N and Kd-O represent the partition coefficient values of a given radionuclide on pure nanoparticles and pure NOMs, respectively; fN and fO are the weight percentages of nanoparticles and NOMs in adsorption systems, respectively. Similarly, Kd-O was the partition coefficient in single-NOM adsorption experiments and Kd-N was that of singlenanoparticle experiments in the UVSW (Table 1). The partitioning of 234 Th was used here as an example for discussion since Th isotopes have been shown to strongly bind with organic ligands (Guo et al., 2002b; Alvarado-Quiroz et al., 2006; Zhang et al., 2008; Roberts et al., 2009; Chuang et al., 2014). The predicted Kd values of 234Th as well as the ratios (R) of measured Kd over Kd-P values are summarized in Table 3 and depicted in Fig. 9. Measured logKd values in NSW treatments generally fell above the one-to-one correlation line between predicted and measured Kd values, while they somewhat followed the 1:1 correlation line for logKd values in UVSW treatments. The difference here further supported higher organic coating extent in NSW treatments compared to UVSW treatments. Based on the definition of partition coefficient, Kd values are highly inversely dependent on mass concentrations of colloids or particles. Considering the difference in concentrations of NOM between UVSW and NSW (with a relatively higher overall NOM concentration) treatments, higher R values and thus stronger organic coating extent in NSW treatments (Table 3) suggested that the NOM abundance controlled the organic coating extents and thus influenced the scavenging and fractionation of 234Th and 233Pa. Additionally, among each data cluster, there existed different R values between different adsorption systems (Table 3, Fig. 9), suggesting different surface coating extents with different organic ligands and inorganic compositions. In general, carbohydrates including acid polysaccharides and dextran seemed to have lower R values and thus lower surface coating extent on most inorganic nanoparticles except for SiO2 and TiO2 (Table 3). Such observation implied that carbohydrates could strongly interact with SiO2 particles, affecting the adsorption and fractionation of Th and Pa in the ocean. For example, although SiO2 had a strong affinity for Pa over Th (e.g., Chase et al., 2002; Guo et al., 2002a; Scholten et al., 2005; Lin et al., 2014), bulk biogenic SiO2 particles might still preferentially interact with Th if they were coated with carbohydrates and coating extents were high.

Table 3 The predicted logKd values (logKd-P) of 234Th derived from a two end-member mixing model (Li, 2005), as well as the ratios (R) of measured-logKd to predicted logKd (logKd-P) values in nanoparticles–NOM adsorption experiments. “–” denotes no data. COM

CaCO3

SiO2 logKd-P

R

logKd-P

Fe2O3 R

Al2O3

TiO2

logKd-P

R

logKd-P

R

logKd-P

R

UV-irradiated natural seawater experiments (UVSW) APS 5.22 2.4 BSA 4.80 4.1 HA 5.32 0.65 Dextran (6 kDa) 5.25 2.0 Dextran (40 kDa) 5.26 2.6 Dextran (500 kDa) 5.28 0.91

5.26 4.91 5.36 – – –

0.65 8.1 0.20 – – –

5.69 5.59 5.73 – – –

0.75 0.52 0.20 – – –

5.33 5.05 5.41 – – –

0.68 1.1 1.0 – – –

5.14 4.55 5.25 – – –

17 1.2 1.1 – – –

Natural seawater experiments (NSW) NSW 6.18 HA 5.39 APS 5.36 Dextran (6 kDa) 5.18 GoM-COM 5.01

6.18 5.42 5.39 5.23 5.08

20 7.1 8.2 0.03 8.5

6.26 5.74 5.73 5.67 5.62

32 10 32 11 5.0

6.19 5.46 5.44 – –

3.5 4.8 5.2 – –

6.17 5.34 5.30 – –

3.0 20 71 – –

35 12 36 22 16

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8

7

LogKd-measured

composition. The specific chemical mechanisms in the interactions between different NOMs and inorganic nanoparticles in marine environments are still poorly understood. Further studies are needed to better understand these processes.

NSW NSW-APS NSW-HA NSW-Dextran NSW-GoM COM UVSW-APS UVSW-BSA UVSW-HA UVSW-Dextran

5. Conclusions

6

5

4

3

3

4

5

6

7

8

LogKd-predicted Fig. 9. Relationship between predicted logKd values (logKd-predicted) and measured logKd values (logKd-measured) for 234Th, including data points from adsorption experiments with different colloidal organic matters in both natural seawater (NSW, containing natural DOM) and UV-irradiated seawater (UVSW, DOM-free). Predicted logKd values are based on a two end-member mixing model of Li (2005). Solid line represents one-to-one correlation or logKd-predicted equals to logKd-measured.

Compared to carbohydrates, protein generally had a R value higher than unity for most nanoparticles, except for Fe2O3 (Table 3), suggesting stronger interaction and higher coating extent of proteins on nanoparticle surfaces in seawater. Similar to carbohydrates, in the presence of proteins, both SiO2 and CaCO3 could increase the fractionation extent between Th and Pa in the ocean (Fig. 3) due to the weaker binding strength between Pa isotopes and proteins. In contrast, since the FTh/Pa value on protein was less than that on single-Fe2O3 (Table 1) and the low coating extent of protein on Fe2O3 nanoparticle surfaces (R = 0.52, Table 3), protein seemed to depress the fractionation between Th and Pa on Fe2O3 in seawater (Fig. 3). On the other hand, protein did not seem to significantly alter the partitioning of 234Th and 233Pa on two lithogenic nanoparticles (i.e., Al2O3 or TiO2, Figs. 3 and 8). Humic substances generally had R values lower than or close to unity (Table 3), suggesting low coating extent on the selected nanoparticles in seawater, especially in UVSW treatments. This was somewhat different from the observation that some inorganic micro-particles, like Al2O3, kaolinite and hematite, could be effectively coated by humic substances (e.g., Murphy et al., 1999; Hizal and Apak, 2006; Yang et al., 2013), possibly due to the difference in particle sizes (micro-particles vs. nanoparticles) and other factors. Although the coating extent of HA was relatively low in UVSW, the addition of HA into NSW also enhanced surface coating on nanoparticles, especially on Al2O3 and TiO2. Thus, humic substances might still play an important role in the scavenging of Th and Pa in the ocean. Obviously, the surface properties of inorganic colloids and particles could be altered by organic matter or functional groups through surface coating formation and/or organic gel layer on the particle surface (Decho, 1990; Leppard, 1995), with organic ligands playing a predominant role in the scavenging and fractionation of Th and Pa in seawater, especially for Th. The interactions of NOMs with inorganic nanoparticles or the coating extent were highly variable with chemical compositions of NOMs and inorganic particles (Table 3). Therefore, previous observations about the relationship between Th and particle composition in the ocean, like lithogenics (e.g., Luo and Ku, 1999, 2004; Roy-Barman et al., 2005, 2009) and biogenic carbonate (e.g., Chase et al., 2002), might be the net results of organic matter interactions with inorganic particles (e.g., Roberts et al., 2009). However, it is still not clear how organic matter alters surface properties on particles with different chemical

Laboratory adsorption experiments were conducted to examine the role of different organic ligands in the adsorption of 234Th and 233 Pa in seawater. Carbohydrates and humic substances had comparable logKd values for 234Th, ranging from 6.76 to 6.90 and showing higher affinity than protein. 233Pa had a relatively lower affinity for organic matter with logKd values varying from 4.41 to 5.46. Selected organic matter seemed to preferentially complex with 234Th over 233 Pa, resulting in a fractionation between 234Th and 233Pa with a fractionation factor (FTh/Pa) following the order of carbohydrates N humic substances N proteins. The adsorption experiments of 234Th and 233Pa with different nanoparticles in the presence or absence of various macromolecular organic matters further attested the important role of organic matter and demonstrated different roles of different organic functional groups in governing the adsorption of 234Th and 233Pa on inorganic nanoparticles in seawater. The effect of organic matter on the partitioning of 234Th and 233 Pa in seawater was likely through the formation of organic coating on inorganic nanoparticle surfaces and changes in surface properties or decease in sorption sites of carrier phases. Comparisons of the measured Kd values with predicted ones derived from a two end-member mixing model (Li, 2005) revealed different organic coating extents for different NOMs during the adsorption of Th and Pa in seawater. They also indicated the effect of organic matter might be associated with the relative abundance and chemical compositions of organic matter and inorganic nanoparticles. Carbohydrates seemed to have the strongest fractionation effect on 234Th and 233Pa, with high FTh/Pa values and strong coating extent on SiO2 and TiO2 nanoparticles. In addition to chemical composition and organic coating, concentration of NOMs or colloidal particles also had a significant effect on the adsorption and fractionation of 234Th and 233Pa in seawater, showing a clear colloidal concentration effect, with a decrease in logKd values with increasing NOM concentration, and role of colloid-colloid interactions. The partitioning of 234Th and 233Pa and adsorption intensities changed little with the molecular weight or size of NOMs between 2 and 10 nm or 6–500 kDa, suggesting negligible NOM size effect on the scavenging of Th and Pa in seawater. However, particle size effect became evident for larger nanoparticles and micro-particles, as shown in previous studies, implying that particle size effect may also vary with size and chemical composition. Acknowledgments We thank Yusheng Qiu and Zhengzhen Zhou for their technical assistance during seawater sampling and laboratory experiments, and two anonymous reviewers for their constructive comments that improved the presentation of this manuscript. This work was supported in part by grants from the Natural Science Foundation of China (#41125020 to M.C.), NSF (OCE#0850957 to L.G.), and China Ocean Mineral Resources R & D Association (COMRA) Program (#DY125-13E-01 to M.C.). References Alvarado-Quiroz, N., Hung, C.-C., Santschi, P.H., 2006. Binding of thorium(IV) to carboxylate, phosphate and sulfate functional groups from marine exopolymeric substances (EPS). Mar. Chem. 100, 337–353. http://dx.doi.org/10.1016/j.marchem.2005.10.023. Anderson, R.F., Bacon, M.P., Brewer, P.G., 1983a. Removal of 230Th and 231Pa at ocean margins. Earth Planet. Sci. Lett. 66, 73–90. Anderson, R.F., Bacon, M.P., Brewer, P.G., 1983b. Removal or 230Th and 231Pa from the open ocean. Earth Planet. Sci. Lett. 62, 7–23.

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