Adsorption and fractionation of thorium and protactinium on nanoparticles in seawater

Marine Chemistry 162 (2014) 50–59

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Adsorption and fractionation of thorium and protactinium on nanoparticles in seawater Peng Lin a,b, Laodong Guo b,⁎, Min Chen a a b

College of Ocean and Earth Sciences and State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China School of Freshwater Sciences, University of Wisconsin–Milwaukee, 600 East Greenfield Avenue, Milwaukee, WI 53204, USA

a r t i c l e

i n f o

Article history: Received 21 November 2013 Received in revised form 8 March 2014 Accepted 14 March 2014 Available online 21 March 2014 Keywords: Thorium Protactinium Marine scavenging Nanoparticles Partition coefficient Adsorption Fractionation

a b s t r a c t Laboratory adsorption experiments were conducted to examine the role of particle composition in the scavenging and fractionation of 234Th and 233Pa 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 234Th and 233Pa, giving rise to different partition coefficient (Kd) values. Compared to 234Th, log Kd values of 233Pa had a higher variability on different nanoparticles, ranging from 3.10 to 5.39. In general, the log Kd values for 234Th on CaCO3, Fe2O3, and Al2O3 were higher than those for 233Pa, while the opposite was true on SiO2 and TiO2 nanoparticles, resulting in a significant fractionation between 234Th and 233Pa during their adsorption on nanoparticle surfaces. Among the selected nanoparticles, Fe2O3 had the highest fractionation factor between 234Th and 233Pa (FTh/Pa) while SiO2 had the lowest, following the order of Fe2O3 N CaCO3 N Al2O3 N TiO2 N SiO2. In addition to the effect of chemical composition, the concentration of nanoparticles or micro-particles also significantly affected the partitioning of 234Th or 233Pa between dissolved and particulate phases, showing a strong particle concentration effect with a general decrease in log Kd values with increasing particle concentration. Interestingly, results from adsorption experiments with binary nanoparticles, containing both SiO2 and CaCO3, clearly demonstrated an increase in log Kd values of 233Pa with increasing SiO2/CaCO3 ratios of the adsorbent, but a decrease in the fractionation factor between 234Th and 233 Pa. However, the enhanced adsorption of 233Pa on SiO2 nanoparticles or the increased log Kd value of 233Pa 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. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Thorium (Th) and protactinium (Pa) isotopes, such as 230Th, 234Th and 231Pa, are naturally occurring radionuclides in the ocean. While their parent nuclides (234U, 238U and 235U, respectively) are highly soluble and demonstrate conservative behavior and homogeneous distributions in marine environments (Ku et al., 1977), Th(IV) and Pa(IV, V) are particle-reactive elements and can be quickly scavenged by suspended and sinking particles in the water column resulting in a disequilibrium between Th (or Pa) isotopes and their corresponding U isotopes (Bacon and Anderson, 1982; Baskaran et al., 1996; Clegg and Whitfield, 1991; Dezileau et al., 2003; Lippold et al., 2012). The disequilibrium between daughter and parent nuclides such as 234Th/238U and 230 Th/234U, as well as isotope activity ratios such as 230Th/231Pa, have ⁎ Corresponding author. Tel.: +1 414 382 1742. E-mail address: [email protected] (L. Guo).

http://dx.doi.org/10.1016/j.marchem.2014.03.004 0304-4203/© 2014 Elsevier B.V. All rights reserved.

long been used as oceanographic tracers or proxies for particle fluxes, oceanic circulation, boundary scavenging, and paleoproductivity (Kumar et al., 1993; Scholten et al., 1995; Walter et al., 2001; Edmonds et al., 2004; Scholten et al., 2008; Gherardi et al., 2009; Lippold et al., 2011; Venchiarutti et al., 2011). However, the molecular mechanisms governing the interactions between these particlereactive nuclides and oceanic particulate matter remain elusive, even though understanding them is important for better use of Th and Pa isotopes as oceanographic proxies, especially in the context of the GEOTRACES program. Recent field studies have recognized the important role of particle composition in the scavenging of Th and Pa as well as their fractionation in the ocean (e.g., Luo and Ku, 1999, 2004a,b; Chase et al., 2002; Chase and Anderson, 2004; Roy-Barman et al., 2005, 2009; Xu et al., 2011). In addition, increased controlled laboratory experiments have shown the effect of particle composition, mostly on micro-particles or sinking particles, on the adsorption of Th and Pa in seawater (Guo et al.,

P. Lin et al. / Marine Chemistry 162 (2014) 50–59

2002a; Geibert and Usbeck, 2004; Roberts et al., 2009; Chuang et al., 2013). Both field investigations and laboratory studies have elucidated preferential scavenging of Pa by opal and Th by carbonates. However, the key particle components responsible for the fractionation between Th and Pa in seawater are still under debate. Furthermore, many previous laboratory studies used particles with unknown or uncharacterized sizes (e.g., Anderson et al., 1992; Guo et al., 2002a) or pre-neutralized Th and Pa tracers (e.g., Geibert and Usbeck, 2004), which may result in different experimental results. Thus, more studies with normative and specific laboratory experimental conditions using consistent particle size and well-controlled pH are needed to better evaluate the influence of particle composition on the scavenging and fractionation of Th and Pa and to better understand the molecular mechanism of the interactions of Th/Pa with particle surfaces in the ocean. So far, most laboratory studies focus on the adsorption of particlereactive elements on micro-particles or sinking particles collected by sediment traps (Guo et al., 2002a; Geibert and Usbeck, 2004; Roberts et al., 2009; Chuang et al., 2013; Yang et al., 2013). Few studies concentrate on the interactions between radionuclides (Th or Pa) and colloids or nanoparticles in seawater (Quigley et al., 2001, 2002; Zhang et al., 2008). The mass concentration of colloids has been shown to outweigh that of suspended particles in oceanic environments (e.g., Guo et al., 1994; Guo and Santschi, 2007). However, the role of colloids or nanoparticles in the adsorption and scavenging of Th or Pa in the ocean remains elusive. Furthermore, the effects of colloidal/nanoparticle composition, particle size or specific areas, and particle abundance (i.e., particle concentration effect) on the adsorption of Th and Pa have rarely been quantified, but knowledge of these effects is important to allow better interpretation of field data and the construction of radionuclide-related models describing particle dynamics and fluxes in the ocean. In the present study, controlled laboratory experiments were carried out to examine the interactions of Th and Pa with pure inorganic nanosized particles with known chemical compositions and size, including SiO2 and CaCO3 (representing biogenic components), Al2O3 and TiO2 (representing lithogenic) and Fe2O3 (representing metal oxides) in natural seawater systems. Ultraviolet irradiated natural seawater and artificial seawater were used as the adsorption medium to exclude the effect of pre-existing natural organic matter and/or colloids/nanoparticles. In addition, adsorption experiments using binary nanoparticle pairs were also carried out to determine the relationship between the particulate SiO2/CaCO3 ratio and Pa/Th fractionation during their scavenging in the ocean. Finally, the effects of particle concentrations and specific surface area on the adsorption and partitioning of 234Th and 233 Pa were also examined through experiments using nanoparticles with the same composition and size but different concentrations and using different particles with the same composition but different sizes (including nanoparticles and micro-particles), respectively. 2. Methods and materials 2.1. Seawater, nanoparticles and experimental conditions Controlled laboratory adsorption experiments were conducted in batch experiments using two different seawater solutions, including ultraviolet irradiated natural seawater (UVSW) and artificial seawater (ASW), to exclude the effect of pre-existing particles and natural dissolved organic matter on the adsorption of 234Th and 233Pa on nanoparticles. For the UVSW, natural seawater collected from the Gulf of Mexico (salinity 35) or off Xiamen Bay was pre-filtered with a 0.4 μm Nuclepore filter followed by ultrafiltration with a 1 kDa membrane to remove suspended and nanoparticles (Guo and Santschi, 2007) and then exposed to UV irradiation (500 W or 500 × 107 erg/s) for 48 h to remove active dissolved organic matter (Chen et al., 2004) to produce particlefree and dissolved organic carbon (DOC)-free seawater for adsorption experiments. Artificial seawater (salinity 35) was prepared based on the synthetic seawater recipe of Horne (1969).

51

Types of selected inorganic nanoparticles and their corresponding particle sizes (20 nm) are listed in Table 1. Similar to a previous study (Yang et al., 2013), SiO2 and CaCO3 nanoparticles were used in the adsorption experiments to represent biogenic SiO2 and CaCO3 particles produced by diatoms and calcareous plankton in the ocean, respectively. In addition, Al2O3 and TiO2 nanoparticles were used to represent lithogenic components. Fe2O3 nanoparticles were used in adsorption experiments to represent metal oxides/hydroxides in seawater. Adsorption equilibrium time was determined by the results of kinetic experiments (Lin, 2012). Removal percentages of 234Th and 233Pa generally reached the highest values before 1.5 h and remained constant by 2 h; therefore, an equilibrium time of 2h was used for all 234Th and 233Pa adsorption experiments. The same adsorption equilibrium time had also been used in adsorption experiments for Th, Pa and other particle-reactive radionuclides (e.g., Quigley et al., 2001; Guo et al., 2002a; Roberts et al., 2009; Yang et al., 2013), making comparisons between different studies feasible. 2.2. Adsorption experiments Experimental procedures for the adsorption of 234Th and 233Pa on single nanoparticles and binary nanoparticles are schematized in Fig. 1. All Teflon beakers and apparatus were cleaned with a 10% HCl solution and triple-rinsed with deionized water to minimize the effect of colloidal impurities. The stirred cell ultrafiltration unit with a 1 kDa membrane (Amicon YM1) was first preconditioned with UVSW or ASW seawater. Then, 49 mL of seawater was pipetted into the stirred cell unit along with 1 mL of non-complexing Tris–buffer solution (25 mM) to maintain a constant pH value of around 8.0 during the adsorption experiments (Roberts et al., 2009; Yang et al., 2013). For single-nanoparticle experiments, 2.5 mg of nanoparticles was added, resulting in a final particle concentration of 50 mg/L. Then, 200 to 250 Bq of 234Th milked from the parent nuclide 238U or 233Pa equilibrated with 237Np were added into the adsorption system under constant stirring. No pre-neutralization was conducted for the 234Th or 233 Pa spike solution to avoid possible formation of “pseudo-colloids” during neutralization (Roberts, 2008). Finally, the pH value of each adsorption solution was measured and then stirred for 2 h on a magnetic stirrer before ultrafiltration. After adsorption equilibrium, the pH of each adsorption solution was measured again. No significant pH shift (±0.3 pH) was found during the adsorption experiments. The adsorption solution was ultrafiltered through a 1 kDa membrane (Amicon YM1) to isolate nanoparticles from the b1 kDa dissolved phase using a concentration factor (CF) of ~ 10. To examine the mass balance or possible loss of 234Th and 233Pa onto the container wall during adsorption experiments, aliquots of initial adsorption solution, retentate (containing nanoparticles), and the b 1 kDa permeate solution were sampled and measured for 234Th and 233Pa. Control experiments without particles for different treatments were also carried out to examine the sorption loss of 234Th and 233Pa. To examine the particle concentration effect on the partitioning of Th and Pa in seawater, different amounts of nanoparticles (SiO2, 20 nm) or micro-particles (SiO2, 4 μm) were added into the adsorption solution to produce different concentrations of nanoparticles or microparticles. Other experimental procedures were the same as those described above for single-nanoparticle adsorption experiments. For sorption experiments using SiO2 micro-particles, 0.4 μm Nuclepore filters were used to separate SiO2 micro-particles from dissolved phases (Fig. 1). Similar to other adsorption experiments, UV-irradiated natural seawater was used in the particle concentration effect experiments. SiO2 and CaCO3 have been proposed to be the main components governing the removal and fractionation of Th and Pa in the ocean (Chase et al., 2002; Chase and Anderson, 2004; Guo et al., 2002a; Kretschmer et al., 2011). Therefore, SiO2 and CaCO3 nanoparticles were selected for the binary-nanoparticle experiments to examine the relative importance between SiO2 and CaCO3 in the scavenging of Th

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P. Lin et al. / Marine Chemistry 162 (2014) 50–59

Table 1 Mass balance, log Kd values, and fractionation factors between 234Th and 233Pa (FTh/Pa) on different nanoparticles in laboratory experiments. All selected nanoparticles have a nominal size of 20 nm and a specific surface area of 96 ± 10 m2/g (Beijing Nachen Science & Technology Co. Ltd). Nanoparticles

Artificial seawater experiments SiO2 CaCO3 Fe2O3 Al2O3 TiO2 Average

234

233

Th

Pa

FTh/Pa

Mass balance (%)

Log Kd

Mass balance (%)

Log Kd

90 91 92 94 86 91 ± 3

4.68 4.80 5.59 5.01 4.39 4.89 ± 0.45

97 95 101 109 107 102 ± 6

5.14 3.65 3.10 4.45 4.57 4.08 ± 0.98

0.35 14.1 309 3.6 0.66

4.73 4.86 5.59 5.00 4.39 4.91 ± 0.44

100 97 101 104 103 101 ± 3

5.39 4.08 4.01 3.99 4.48 4.39 ± 0.59

0.23 6.0 38 10.2 0.81

UV-irradiated natural seawater experiments 94 SiO2 CaCO3 96 Fe2O3 101 Al2O3 102 90 TiO2 Average 97 ± 5

and Pa in the ocean. Briefly, SiO2 and CaCO3 were mixed in various proportions (in wt.%) to give rise to different relative concentrations of these two nanoparticles. The experimental conditions, including equilibrium time, particle concentration, and filtration procedures were the same as for the single-nanoparticle adsorption experiments. For all samples of adsorption experiments, the activities of 234Th and 233 Pa were determined by either liquid scintillation counting (LSC) or gamma spectrometry (Guo et al., 2002a). Samples were counted to obtain a counting error of b 5–10% or better depending on specific samples. The ingrowth of 233Pa from 237Np after adsorption experiments was corrected although this was generally negligible since the samples were counted within one day. 2.3. Partition coefficient values and fractionation factor The partitioning of radionuclides between seawater and particulate or colloidal phases could be quantified using the distribution coefficient (Kd), which has been applied to investigations on the partitioning and

adsorption behavior of various particle-reactive elements in seawater (e.g., Quigley et al., 2001, 2002; Roberts et al., 2009; Lin et al., 2012; Yang et al., 2013). In the present study, Kd was used to evaluate the partitioning of 234Th and 233Pa between the b1 kDa dissolved and nanoparticle or colloidal phases (Honeyman and Santschi, 1989; Guo et al., 1997), following the equation:

Kd ¼

Ap Ad  Cp

where Ap was the activity of 234Th or 233Pa in the particulate phase (in Bq/L). For nanoparticle experiments, Ap was calculated from the activity concentration of 234Th or 233Pa in the retentate solution divided by the concentration factor (CF = volume of initial solution/volume of retentate solution) of ultrafiltration experiments; Ad was the activity of 234Th or 233Pa in the dissolved phase, either in the b 1 kDa permeate solution or in the b 0.4 μm fraction (in Bq/L); Cp was the concentration

Fig. 1. Schematic diagram of adsorption experiments, including single-component, binary-component, and particle concentration effect (P.C.E.) experiments for 234Th and 233Pa in artificial seawater (ASW) and ultraviolet-irradiated natural seawater (UVSW).

P. Lin et al. / Marine Chemistry 162 (2014) 50–59

of nanoparticles or micro-particles (in kg/L). Hereafter, Kd values (in mL/g or L/kg) of 234Th or 233Pa are reported in terms of log Kd. A fractionation factor (F) was defined as the ratio of Kd values between two different radionuclides and used to evaluate the fractionation between 234Th and 233Pa (FTh/Pa) by a specific nanoparticle (Scholten et al., 2005; Roberts et al., 2009; Yang et al., 2013): FTh=Pa ¼ Kd ðThÞ=Kd ðPaÞ When the FTh/Pa value is equal to unity, no fractionation exists between 234Th and 233Pa during adsorption. If FTh/Pa is N1, the specific nanoparticles adsorb 234Th preferentially over 233Pa, while 233Pa is preferentially scavenged over 234Th by the specific nanoparticle when FTh/Pa is b1. 3. Results 3.1. Mass balance of 234Th or 233 Pa The mass balance of 234Th or 233Pa in control experiments and adsorption experiments was used to evaluate possible adsorption losses to filters and the apparatus wall during adsorption experiments. The recovery percentage or mass balance was calculated as the sum of the measured activities in both the retentate and permeate solutions divided by the initial total activity of 234Th or 233Pa. As shown in Table 1, the mass balance for 234Th in the ASW treatment varied from 86% in the TiO2 adsorption experiment to 94% in the Al2O3 adsorption experiment, with an average of 91 ± 3%. For the UVSW treatment, the mass balance of 234Th was higher (average of 97 ± 5%) than those in the ASW treatment, ranging from 90% in the TiO2 adsorption system to 102% in the Al2O3 adsorption system. Compared to 234Th, the mass balance of 233Pa was generally higher, varying from 95% to 109% (average of 102 ± 6%) for the ASW treatments and from 97% to 104% (average of 101 ± 3%) for the UVSW treatments (Table 1). A slightly lower recovery for 234Th than 233Pa was consistent with the higher particle reactivity and log Kd values of 234Th (see also Table 1) for most nanoparticles. Overall, mass balance results of 234Th and 233Pa were all close to 100%, indicating that adsorption losses of 234Th and 233Pa were minimal during adsorption experiments and filtration process. 3.2. Partitioning of 234Th and 233Pa between nanoparticles and seawater The partition coefficient values (in log Kd) of 234Th and 233Pa on selected nanoparticles with different chemical compositions are given in Table 1 and depicted in Fig. 2. Log Kd values of 234Th varied significantly between different nanoparticles, ranging from 4.39 to 5.59 in 234Th adsorption experiments with either ASW or UVSW (Table 1). Similarly, 233 Pa also had a large variability in its log Kd values between different nanoparticles, varying from 3.10 to 5.14 in adsorption experiments with ASW and from 4.01 to 5.39 in adsorption experiments with UVSW (Fig. 2). Distinct differences in the partition coefficient values between 234Th and 233Pa indicated that the affinity of different nanoparticles for 234Th or 233Pa varied considerably (Table 1). Among different nanoparticles in the ASW treatment, Fe2O3 had the highest log Kd value for 234Th, followed by Al2O3, CaCO3, SiO2 and TiO2 (Fig. 2a). The log Kd values for 233Pa had a somewhat opposite order compared to 234Th, with the highest log Kd value for SiO2 but the lowest one for Fe2O3 (Fig. 2c). Additionally, the results from UVSW treatments (Fig. 2b and d) showed comparable log Kd values for 234Th and 233Pa on different nanoparticles to those from ASW experiments (Fig. 2a and c) based on t-test analysis. Similar log Kd values, especially for 234 Th, between different experimental media indicated that residual dissolved organic matter (DOM) in UV-irradiated seawater was minimal and the effect of residual DOM was negligible on the adsorption of 234 Th or 233Pa on nanoparticle surfaces.

53

3.3. Fractionation between 234Th and 233Pa during adsorption As shown in Fig. 2, 234Th and 233Pa had different Kd values for most nanoparticles regardless of the ASW or UVSW adsorption system, indicating that there was fractionation between 234Th and 233Pa during their adsorption. The fractionation factor defined as the Kd ratio between 234 Th and 233Pa (FTh/Pa) generally deviated from unity (Table 1, Fig. 2e and f). FTh/Pa values higher or lower than unity indicated either positive or negative fractionation between 234Th and 233Pa during their adsorption on nanoparticle surfaces, which was due to different particle compositions. For example, the measured log Kd value for 234Th on Fe2O3 was 5.59 in the ASW and UVSW treatments but was 3.10 and 4.07 for 233 Pa in the ASW and UVSW treatments, respectively, resulting in a high FTh/Pa value of 309 and 38 on Fe2O3. Similar fractionation could be found on both Al2O3 and CaCO3, with a moderate FTh/Pa value varying from 3.6 to 14.1. However, in contrast to Fe2O3, Al2O3 and CaCO3 nanoparticles which had a FTh/Pa N 1, SiO2 nanoparticles had a FTh/Pa value b1, ranging from 0.35 to 0.23 in the ASW and UVSW experimental treatments, respectively. TiO2 nanoparticles, on the other hand, had FTh/Pa values close to unity, showing little fractionation between 234Th and 233 Pa (Fig. 2e and f). 3.4. Partitioning of 234Th and 233Pa under different particle concentrations Log Kd values of 234Th and 233Pa under different particle concentrations of nanoparticles or micro-particles are shown in Table 2 and Fig. 3 to demonstrate the effect of particle concentration on the partitioning of 234Th and 233Pa between seawater and particle surfaces. For SiO2 nanoparticles with particle concentrations ranging from 25 to 75 mg/L, the log Kd values for both 234Th and 233Pa showed a general decrease with increasing particle concentrations (Fig. 3), although the log Kd values of 233Pa at particle concentrations of 50 mg/L and 75 mg/L were somewhat similar. Similar to the results in single-nanoparticle adsorption experiments (Fig. 2), higher log Kd values were also measured for 233Pa on SiO2 nanoparticles than for 234Th under different nanoparticle concentrations (Table 2). For adsorption experiments using SiO2 micro-particles with particle concentrations ranging from 7.9 to 60.9 mg/L, the log Kd values for 233Pa also decreased with increasing micro-particle concentrations, showing a strong particle concentration effect during the adsorption of 233Pa on SiO2 micro-particles (Fig. 3). Data of particle concentration effect for 234 Th on SiO2 micro-particles were not available for comparison with 233 Pa. We expected that similar particle concentration effect should also exist for the adsorption of 234Th on micro-particles, as observed in field and laboratory studies (e.g., Honeyman and Santschi, 1989; Guo et al., 1995, 1997). 3.5. Partitioning of 234Th and 233Pa with binary nanoparticles Variations in log Kd values of 234Th and 233Pa, as well as in the fractionation factor between 233Pa and 234Th (FPa/Th) in the binarynanoparticle adsorption experiments and varying proportions of CaCO3 and SiO2 (in wt.%) are shown in Table 3 and depicted in Fig. 4. For 234Th, log Kd values remained somewhat similar with increasing %CaCO3 or decreasing %SiO2, while the log Kd values of 233Pa generally increased with increasing %SiO2. Additionally, as shown in Fig. 4, the FPa/Th values generally increased with increasing SiO2 content (in wt %), consistent with the higher affinity of SiO2 but lower removal ability of CaCO3 for 233Pa, as shown in Table 1 and Fig. 2. 4. Discussion 4.1. Effect of nanoparticle composition on the adsorption of 234Th and 233Pa As summarized in Table 1 and Fig. 2, the log Kd values of 234Th and Pa and their fractionation factor (FPa/Th) for each pure nanoparticle

233

54

P. Lin et al. / Marine Chemistry 162 (2014) 50–59

ASW

UVSW

7 6

7

a

Th

6

4 3

1

1

6

0

SiO2 CaCO3 Fe2O3 Al2O3 TiO2

6

LogKd

3

d

4 3

2

2

1

1 0

SiO2 CaCO3 Fe2O3 Al2O3 TiO2

1000

1000

e

100

SiO2 CaCO3 Fe2O3 Al2O3 TiO2

f

FTh/Pa

100

10

1

0.1

Pa

5

4

0

SiO2 CaCO3 Fe2O3 Al2O3 TiO2

7

c

Pa

5

LogKd

3 2

7

FTh/Pa

4

2

0

b

5

LogKd

LogKd

5

Th

10

1

SiO2 CaCO3 Fe2O3 Al2O3 TiO2

0.1

SiO2 CaCO3 Fe2O3 Al2O3 TiO2

Fig. 2. Variations in partition coefficient, log Kd, values of 234Th and 233Pa between nanoparticles and dissolved (b1 kDa) phases, and the fractionation factor (FTh/Pa, defined as the ratio of Kd-Th/Kd-Pa) between 234Th and 233Pa during their adsorption on nanoparticles in artificial seawater (ASW, left panels) and ultraviolet-irradiated natural seawater (UVSW, right panels).

Table 2 Log Kd values of 234Th and 233Pa under different concentrations of nanoparticles and micro-particles in UV-irradiated natural seawater. SiO2 nanoparticle and micro-particle had a nominal size of 20 nm and 4 μm, as well as a specific surface area of 96 m2/g and 0.6 m2/g, respectively. Particle concentration (mg/L)

Log Kd 234

233

SiO2 nanoparticle 25 50 75

5.13 4.73 4.59

5.98 5.39 5.47

SiO2 micro-particle 7.9 17.4 25.8 32.1 40.0 49.9 60.9

– – – – – – –

4.47 4.14 4.16 4.12 4.11 3.86 3.83

Th

“–” denotes no data available.

Pa

with the same particle size (20 nm) clearly demonstrated that different types of nanoparticles had distinctly different affinities for 234Th and 233 Pa, affecting their adsorption and scavenging and thus causing fractionation between Th and Pa in the ocean with a changing particle regime. Based on the chemical composition of nanoparticles used in the present study, the selected nanoparticles could be categorized into three different particle types for further discussion: lithogenic particles (Al2O3 and TiO2), biogenic particles (SiO2 and CaCO3), and metal oxides/ hydroxides (e.g., Fe2O3). For lithogenic nanoparticles, Al2O3 has been shown to be a major lithogenic component and was used as a proxy for lithogenic particles in the ocean (Dean et al., 1999; Chase et al., 2002; Vijver et al., 2008). It had the second highest log Kd value for 234Th and showed preferential scavenging for 234Th over 233Pa. TiO2, on the other hand, showed little fractionation between 234Th and 233Pa even though it seemed to preferentially scavenge 233Pa over 234Th to a small extent (Fig. 2e and f). If Al2O3 was the major component of lithogenic particles in the ocean, lithogenic particles should have an overall fractionation factor N 1 between 234Th and 233Pa, with a preferential removal characteristics for

P. Lin et al. / Marine Chemistry 162 (2014) 50–59

SiO2 nanoparticle

SiO2 micro-particle

7 6.5

55

5

Pa

Th Pa

2

y = 1.19 - 0.64x R = 0.86

4.5

LogKd

LogKd

6 5.5

4

5 3.5 4.5 4 -4.7

-4.6

-4.5

-4.4

-4.3

-4.2

-4.1

LogCp

3 -5.2

-5

-4.8

-4.6

-4.4

-4.2

-4

LogCp

Fig. 3. Variations in log Kd values of 234Th and 233Pa with different particle concentrations (Cp, in log Cp) of SiO2 nanoparticle (left panel) and SiO2 micro-particle (right panel). An evident “particle concentration effect” was observed with a consistent decrease in log Kd value with increasing particle concentration (in log Cp) for both 234Th and 233Pa. 234

Th (Table 1). This was consistent with the fact that lithogenic particles were considered important scavengers for Th isotopes (Luo and Ku, 1999, 2004a), especially in low latitude oceans with a predominance of lithogenic particles in the particle flux, such as the Middle Atlantic Bight (Chase et al., 2002) and the northwestern North Pacific Ocean (Narita et al., 2003). SiO2 and CaCO3 are the two major components of biogenic particles, which represented diatoms or biogenic opal and other plankton with carbonates in the ocean, respectively. As shown in Fig. 2, SiO2 and CaCO3 had different adsorption selectivities between 234Th and 233Pa. While the log Kd values of 234Th were virtually the same on SiO2 and CaCO3 nanoparticles, the adsorption capacity of 233Pa on SiO2 and CaCO3 was distinctly different with a much higher log Kd value for SiO2, resulting in a fractionation factor b1 between 234Th and 233Pa on SiO2 (Fig. 2). The preferential scavenging of 233Pa by SiO2 observed here and in several previous studies (Guo et al., 2002a; Roberts et al., 2009) had important implications for understanding the scavenging of radionuclides and thus the particulate Th/Pa ratio in the ocean, as well as the control of major phytoplankton species or the relative abundance of biogenic SiO2 and CaCO3 in the water column. For example, low FTh/Pa values (around 1 or b1) were usually found in oceanic regions where diatoms dominated or seasonal diatom blooms occurred in the water column, such as the Southern Ocean and the Labrador Sea (Siddall et al., 2005; Guihou et al., 2011; Kretschmer et al., 2011). On the other hand, high FTh/Pa values were observed in oceanic regions where biogenic carbonate components dominated the particle flux, like the North Atlantic and the Equatorial Pacific Ocean (Chase et al., 2002; Scholten et al., 2005). However, in addition to the influence of calcareous biogenic particles, high FTh/Pa values might also be related to the influence of

Table 3 Log Kd values of 234Th and 233 Pa, as well as the fractionation factor between 233Pa and 234 Th (FPa/Th) in the binary-nanoparticle adsorption experiment. Log Kd and FPa/Th values on pure SiO2 or CaCO3 were taken from single-nanoparticle adsorption experiments (Table 1). SiO2 (wt.%)

CaCO3 (wt.%)

Log Kd 233

4.80 4.37 4.70 4.74 4.77 4.83 4.62 4.75 4.48 4.68

3.65 3.75 4.07 4.08 4.13 4.22 4.16 4.34 4.27 5.14

Th

0 10 20 40 50 60 70 80 90 100

100 90 80 60 50 40 30 20 10 0

FPa/Th

234

Pa 0.07 0.24 0.23 0.22 0.23 0.25 0.35 0.39 0.62 2.86

lithogenic particles in the ocean since the affinity of Al2O3 for 234Th was generally higher than CaCO3 and SiO2 (Table 1 and Fig. 2a and b). Among selected nanoparticles, the only log Kd value N 5.0 for 233Pa was on SiO2 with a FTh/Pa value b1 (Table 1). Therefore, biogenic silica should have the highest affinity for Pa in the ocean. Among selected nanoparticles, ferric oxide showed the strongest affinity for 234Th (Fig. 2a and b), consistent with the wide application of Fe2O3 as the co-precipitation carrier of 234Th in seawater (Waples et al., 2004; Rutgers van der Loeff et al., 2006). Together with the lowest log Kd value for 233Pa on Fe2O3 and the highest FTh/Pa value (Figs. 2e and f), metal oxides/hydroxides may play an important role in the scavenging and fractionation of Th and Pa in the ocean, even though the abundance of metal oxides/hydroxides in the ocean could be low. 4.2. Comparisons of partition coefficient and fractionation factor with literature data Previous studies, including both field studies and laboratory experiments, had shown evident fractionation between Th and Pa during their adsorption on different particle surfaces in the ocean (e.g., Geibert and Usbeck, 2004; Li, 2005; Dutay et al., 2009). As shown in Table 4, there was a broad agreement in FTh/Pa values between our results and other studies. For example, compared to most inorganic particles, FTh/Pa values on SiO2 were generally lower, ranging from b 1 to 5, while carbonate (CaCO3) had higher FTh/Pa values N10 (Table 4). There are few data available for FTh/Pa values on Fe2O3, Al2O3 and TiO2 for comparisons. Our FTh/Pa values for Fe2O3 nanoparticles were higher than those reported for Fe2O3 micro-particles by Guo et al. (2002a). The difference in FTh/Pa values between nanoparticles and micro-particles with the same chemical compositions (Table 4) likely reflected the effect of particle size or specific surface area. By extrapolation, Chase et al. (2002) reported evidently higher partition coefficient values for Th with respect to pure carbonate (CaCO3) than to pure opal (SiO2) in oceanic particles. In the present study, log Kd values for Th were similar between SiO2 and CaCO3 nanoparticles (Table 4). The difference was probably due to the more complex characteristics of natural particles compared to those of pure nanoparticles used here, such as the effect of organic matter and particle size. In addition, our laboratory results seemed to favor the conclusions from Chase et al. (2002), Chase and Anderson (2004) over those from Luo and Ku (1999, 2004a,b), showing stronger interactions of Pa with opal than with lithogenic particles (Table 4). Overall, both laboratory results and field data on the partitioning and fractionation of Th and Pa (Table 4) consistently support the conclusion that the scavenging of Th, Pa and other particle-reactive trace elements should be regulated not only by particle fluxes (Yu et al., 2001; Moran

56

P. Lin et al. / Marine Chemistry 162 (2014) 50–59

5.2

4.6

Th

2

4.4

LogKd

LogKd

4.8

4.4

4

3.6

Pa

y = 3.83 + 0.0056x R = 0.77

4.2 4 3.8 3.6

0

20

40

60

80

0

100

20

%CaCO3

40

60

80

100

8

10

%SiO2

3

0.7 2

y = 0.21 + 0.046x R = 0.97

2.8

0.6

FPa/Th

FPa/Th

2.6 2.4 0.8 0.6 0.4

0.5 0.4 0.3

0.2 0

0

20

40

60

80

100

%SiO2

0.2

0

2

4

6

SiO2/CaCO3 ratio

Fig. 4. Variations in log Kd values of 234Th and 233Pa, as well as FPa/Th with the abundance of CaCO3 or SiO2 (wt.%) and the SiO2/CaCO3 ratio in binary-nanoparticle adsorption experiments.

Table 4 Compilation of Log Kd values of Th and Pa and their fractionation factor (FTh/Pa) based on both laboratory experiments and field studies. Particle type

Log Kd-Th

Log Kd-Pa

FTh/Pa

Reference

Hematite SiO2 MnO2 Al2O3 Coastal sediments MnO2 (5 μm) Fe2O3 (5 μm) CaCO3 Humic acid Chitin SiO2 (1–5 μm) Carrageenan Opal Carbonate Opal Carbonate Lithogenics Smectite MnO2 CaCO3 Opal Opal Carbonate Lithogenics Organic carbon SiO2 + EPS SiO2 CaCO3 CaCO3 + EPS SiO2 (20 nm) CaCO3 (20 nm) Fe2O3 (20 nm) Al2O3 (20 nm) TiO2 (20 nm)

7.28 5.41 5.32 7.06 6.70 6.14 5.83 5.60 5.58 3.50 3.98 7.78 5.59 6.95 5.40 6.00 8.36 6.85 7.66 6.86 6.08 5.54 6.58 6.99 7.11 5.74 5.54 5.70 6.58 4.71 4.83 5.59 5.01 4.39

7.04 5.36 5.43 5.96 5.57 6.20 5.15 3.68 4.31 3.69 5.09 7.49 6.15 5.34 6.08 5.00 7.08 6.20 7.79 5.23 5.70 6.00 5.32 5.79 5.93 4.34 4.39 5.11 5.26 5.27 3.87 3.56 4.22 4.53

1.7 1.1 0.80 12 8.9 0.87 4.8 83 19 0.65 0.08 1.9 0.28 41 0.21 10 20 0.87 0.75 42 2.4 0.35 18 16 15 25 14 3.8 20 0.28 9.2 107 6.1 0.73

Anderson et al. (1992)

Guo et al. (2002a)

et al., 2002, 2005) but also by particle types and compositions (Chase et al., 2002; Luo and Ku, 2004a; Chuang et al., 2013; Yang et al., 2013). The relationship between Th or Pa and specific particle composition derived from controlled laboratory experiments was also consistent with field observations in oceanic environments. For example, the enhanced scavenging of Pa resulted from an elevated opal percentage in higher latitude regions or during a diatom bloom (e.g., Rutgers van der Loeff and Berger, 1993; Walter et al., 1999; Kretschmer et al., 2011); increased contribution to the removal of Th from lithogenic particles in the Middle Atlantic Bight and biogenic carbonate particles in the Equatorial Pacific Ocean (Chase et al., 2002; Dutay et al., 2009).

4.3. Effects of particle concentration and size on the scavenging of Th and Pa Chase et al. (2002) Luo and Ku (2004a)

Geibert and Usbeck (2004)

Li (2005)

Roberts et al. (2009)

This study

The adsorption experiments clearly elucidated the effect of particle concentration on the scavenging of Th and Pa, showing a decrease in log Kd values of both 234Th and 233Pa with increasing nanoparticle concentrations (Fig. 3). Additionally, a significantly inverse correlation between log Kd values and particle concentrations (in log Cp) was consistently observed for micro-particle experiments (Fig. 3). Such an inverse relationship between particle concentrations and log Kd values or particle concentration effect for both 234Th and 233Pa is consistent with previous studies for particle-reactive trace metals and radionuclides in laboratory experiments and field measurements (e.g., Honeyman and Santschi, 1989; Guo et al., 1995; Guo et al., 1997; Benoit and Rozan, 1999). If the partitioning of particle-reactive elements between dissolved and particle surfaces was controlled by thermodynamic equilibrium, the distribution coefficient should be independent of the particle concentration (Stumm and Morgan, 1981). The particle concentration effect observed here suggested the possible role of other processes in the adsorption of Th and Pa on the nanoparticles or micro-particles.

P. Lin et al. / Marine Chemistry 162 (2014) 50–59

Indeed, the particle concentration effect had been interpreted as the intermediary role of colloids and/or physical particle–particle interactions during the partitioning of radionuclides between particles and seawater (Honeyman and Santschi, 1989). In our adsorption experiment, natural DOM and/or colloids were pre-removed by ultraviolet irradiation and filtration/ultrafiltration. Therefore, the observed particle concentration effect may be largely due to interactions between nanoparticles and other physicochemical processes such as coagulation and aggregation (Honeyman and Santschi, 1989). Similarly, the particle concentration effect observed in micro-particle adsorption experiments (Fig. 3) was consistent with that in nanoparticle experiments and demonstrated the role of particle–particle interactions. Interestingly, when the particle composition was the same (i.e., SiO2) for both nanoparticles and micro-particles the log Kd values of 233Pa decreased with increasing particle size (Table 2), demonstrating the role of particle size and thus specific surface areas in controlling the adsorption of radionuclides in aquatic environments. In addition, the Kd ratios (34–66) of 233Pa between nanoparticles and micro-particles were generally lower than the ratio of their specific surface area (~ 160, Table 2) in our experiments, with a higher Kd ratio at lower particle concentrations (e.g., Kd ratio of 34 at 50 mg/L vs. 66 at 25 mg/L). The difference between Kd ratios and specific surface area ratio indicated again the effect of particle concentrations and the importance of particle– particle interactions. More comprehensive and systemic studies are needed to better understand the effect of particle size and particle concentration as well as physicochemical and compositional controls on the adsorption of radionuclides on particle surfaces.

4.4. Relative role of opal and carbonate Both field studies and controlled laboratory experiments supported the consensus that, in addition to specific surface area, particle composition should control the adsorption or scavenging of particle-reactive radionuclides in the ocean (e.g., Luo and Ku, 1999; Chase et al., 2002; Guo et al., 2002a; Roy-Barman et al., 2005, 2009; Chuang et al., 2013; Yang et al., 2013). In general, Th seemed to have higher particle reactivity than Pa (Table 1) and could be preferentially removed by carbonate particles (e.g., Chase et al., 2002) while Pa could be preferentially adsorbed on biogenic opal (e.g., Walter et al., 1997, 2001; Guo et al., 2002a). Nevertheless, the relative roles between different carrier phases, e.g., opal vs. carbonate, in the scavenging of particle-reactive radionuclides had rarely been examined by laboratory studies. Binarynanoparticle experiments with SiO2 and CaCO3 should allow one to examine the variation in the partitioning of Th and Pa with the abundance of SiO2 or CaCO3 and thus the relative control of biogenic opal and carbonate components in the scavenging and fractionation of Th and Pa in the ocean. As shown in Fig. 4, the log Kd values of 233Pa increased with increasing SiO2 abundance, showing a significant positive correlation with percentage of SiO2, while the log Kd values of 234Th changed little with the content of CaCO3 in the adsorption experiment. This suggested that the partitioning of Pa on particle surfaces was rather sensitive to the abundance of biogenic silica but the removal of Th was less sensitive to the variation in CaCO3 contents in oceanic environments. Therefore, the positive correlation between the partition coefficient of Th and biogenic carbonate observed in the oceanic environment (e.g., Chase et al., 2002) might be the net result of different particle types and/or the effect of organic matter as a particle coating, since Th was shown to have a strong affinity for extracellular polymeric substances (Guo et al., 2002b; Quiroz et al., 2006; Zhang et al., 2008; Roberts et al., 2009). For Pa, the relatively lower affinity of Pa to other particles and organic matter (Fig. 2, and Roberts et al., 2009) did not seem to change the strong selectivity of Pa to SiO2, resulting in similar results on the increasing log Kd of Pa with biogenic silica abundance in both laboratory experiments (Fig. 4) and field studies (e.g., Chase et al., 2002).

57

Moreover, values of FPa/Th generally increased with increasing SiO2 content (in wt %), which is consistent with those of previous studies demonstrating that SiO2 had the highest affinity for Pa (e.g., Table 1, Chase et al., 2002; Guo et al., 2002a). As shown in the following relationships between the fractionation factor (FPa/Th) and %SiO2, our result here (Eq. (1), based on data in Fig. 4) was surprisingly similar to the field results (Eq. (2)) as compiled by Scholten et al. (2005): FPa=Th ¼ ð0:098  0:310Þ exp ½ð2:294  0:435Þ  %SiO2 Þ

ð1Þ

2

ðR ¼ 0:59; n ¼ 10Þ

FPa=Th ¼ 0:096 exp ð2:757  %SiO2 Þ

ð2Þ

2

ðR ¼ 0:68; n ¼ 97Þ As shown in Table 3 and Fig. 4, when %SiO2 was comparable to %CaCO3, or between 20 and 60%, values of FPa/Th were somewhat similar, showing a consistently lower fractionation factor between 233Pa and 234 Th (FPa/Th) or higher fractionation factor between 234Th and 233Pa (FTh/Pa) during their adsorption. However, values of FPa/Th increased considerably after the %SiO2 reached or exceeded 60% (Fig. 4). Such a trend indicated that increasing fractionation between Pa and Th during their scavenging by marine particles occurred only when biogenic SiO2 predominated the total particle pool in the water column. These results on the variation in FPa/Th with the abundance of SiO2 should provide new insights into the fractionation between Th and Pa in the ocean, and they could help explain why the Southern Ocean was distinctly different from most other oceanic regions (Dutay et al., 2009; Kretschmer et al., 2011; Venchiarutti et al., 2011). The Southern Ocean had been found to have over 60% of biogenic SiO2 in the total particle pool, leading to FPa/Th values near or somewhat higher than unity in most cases (Chase et al., 2002; Scholten et al., 2005). Again, when the FPa/Th was plotted against the SiO2/CaCO3 ratio from our binary-nanoparticle adsorption experiments, there existed a significantly positive correlation between FPa/Th and the SiO2/CaCO3 ratio (Fig. 4, right lower panel), similar to the observation from Chase et al. (2002). Clearly, the relative contents of SiO2 and CaCO3 particles were a key factor responsible for the fractionation between Th and Pa during their scavenging although in some oceanic regions during certain seasons, lithogenic and metal oxide particles in addition to biogenic opal and carbonate particles, may become significant in the water column. This may occur in regions such as the Northeast Atlantic Ocean, the Mediterranean Sea and the northwestern North Pacific Ocean (Narita et al., 2003; Roy-Barman et al., 2005, 2009). 5. Conclusions Laboratory adsorption experiments of 234Th and 233Pa on different inorganic nanoparticles showed that the chemical composition of nanoparticles played an important role in regulating the adsorption and scavenging of 234Th and 233Pa in seawater. Among selected nanoparticles studied, Fe2O3 had the highest affinity for 234Th, followed by lithogenic Al2O3, biogenic particles (SiO2 and CaCO3), and TiO2. For 233Pa, SiO2 had the highest affinity, followed by TiO2 and other nanoparticles. While the difference in log Kd values of 234Th seemed generally small between different nanoparticles with the exception of TiO2, the log Kd value of 233 Pa was distinctly high for SiO2 compared to other nanoparticles. In general, 234Th demonstrated higher affinity with most inorganic nanoparticles than 233Pa, except for SiO2 and TiO2 resulting in distinct fractionation between 234Th and 233Pa during their adsorption on different nanoparticle surfaces thus indicating control of particle composition in the scavenging and fractionation of Th and Pa in the ocean. In addition to the effect of chemical composition, the concentration of nanoparticles or micro-particles also significantly affected the partitioning of 234Th and 233Pa between seawater and particles, showing an evident particle

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concentration effect with a decrease in log Kd values with increasing particle concentration. This particle concentration effect suggested the importance of particle–particle interactions during the partitioning of Th and Pa on particle surfaces in the ocean. In addition, under the same particle composition (SiO2), the log Kd value of 233Pa increased with decreasing particle size or specific surface areas, supporting the physicochemical control of the partitioning of radionuclides in the ocean. The log Kd value of 233Pa increased consistently with increasing SiO2 content in the adsorbent. Consequently, the fractionation factor between Pa and Th (FPa/Th) also increased with increasing SiO2/CaCO3 ratios, especially when SiO2 became predominant or the %SiO2 reached 60% or higher. Our results indicated that Th/Pa ratios of oceanic particles could be largely controlled by the abundance of biogenic opal in colloidal or particulate pools, although biological uptake and other particle components are also important factors. Overall, our results on the adsorption behavior of 234Th and 233Pa and their fractionation on well-defined and pure inorganic nanoparticles consistently elucidated the important role of particle composition in the adsorption and fractionation of 234Th and 233Pa in seawater. In addition, differences between laboratory and field results on the interaction between Th and biogenic carbonate also pointed to the role of natural organic matter in affecting particle surface properties and thus the scavenging and fractionation of Th and Pa in the ocean. Future studies are needed to evaluate other controlling factors, especially the role of natural organic matter in the adsorption and fractionation of particlereactive radionuclides in the ocean in order to better use them as oceanographic tracers or proxies.

Acknowledgment We thank Yusheng Qiu, Zhengzhen Zhou, and Run Zhang for their technical assistance during seawater sampling and laboratory experiments and Weifeng Yang for discussion. Constructive comments from Peter Santschi and two anonymous reviewers improved earlier versions of this manuscript. This work was supported in part by grants from the NSF (OCE#0850957 to L.G.), Chinese Natural Science Foundation (#41125020 to M.C.), and State Oceanic Administration of China (#2010050012-3 to M.C.).

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