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Is the Extraction of Thorium onto MnO2-coated 

Filter Cartridges Uniform?

by 

P. W. Swarzenski 1 and M. Baskaran2

1: U.S. Geological Survey, Center for Coastal and Watershed 

Studies, St. Petersburg, FL 33701

2: Department of Geology, Wayne State University, Detroit, Ml 

48202

USGS OPEN FILE REPORT 

OFR 2004-1421
Abstract:

Large-volume (i.e., 102 - 103 L) seawater samples are traditionally required to study the 

partitioning of particle-reactive radionuclides between solution and size-fractionated particulate 

matter. One of the most frequently used methods to preconcentrate the short-lived isotopes of Th 

(234Th and 228Th) from such large volumes of water involves the effective extraction of Th onto 

two MnO2-coated polypropylene cartridges. Determination of dissolved Th activities assumes 

that the two MnO2-coated filter cartridges extract Th uniformly (same extraction efficiency), but 

this assumption has not been rigorously validated. Any variability in the extraction efficiency of 

the two cartridges connected in series will directly introduce an error in the determination of final 

dissolved Th activity. In this article, we evaluated the variability in the extraction efficiency of 

MnO2 -coated filter cartridges that were prepared under varying conditions.

Thorium-234-spiked seawater was filtered in series through a manifold consisting of six 

MnO2-coated cartridges. From the activities of 234Th retained in each cartridge, the relative 

(calculated from the activities in two successive cartridges) and absolute (ratio of 234Th activity 

retained to the activity entered) extraction efficiency for each of the cartridges was calculated. At 

a constant flow rate and constant KMnO4 saturation, the absolute extraction efficiency varied by 

-40% (from 54.1 to 93.8%) within the first two filter cartridges, and over 50% (from 32.3 to 

89.3%) on all six MnO2 cartridges. Our results confirm that a uniform extraction efficiency using 

two filters connected in series is rarely achieved. Using the average extraction efficiency of all 

cartridges, we propose a new approach that assumes a constant extraction efficiency. This 

method will reduce the error introduced by the assumption of uniform extraction efficiency in the 

determination of dissolved Th activities.
1. Introduction:

In seawater, the distribution of particle-reactive radionuclides such as Th and 2l °Pb in the 

U/Th-series has provided a wealth of information on marine particle dynamics, while the more 

soluble radioisotopes, such as radium, have been effectively utilized to trace water masses and to 

estimate horizontal and vertical mixing coefficients. The activities of many of the short-lived 

U/Th-series radionuclides in aqueous systems are so dilute that most of these nuclides require 

considerable preconcentration prior to their radiochemical separation and analyses. With the 

advent of new mass spectrometric methods (e.g., thermal ionization mass spectrometry and 

inductively coupled plasma mass spectrometry), many U/Th-series radionuclides that have higher 

atomic abundances (half-lives greater than 226Ra, 1,620 yrs), such as 232Th, 230Th, 231 Pa, 234U, 235U 

and 238U, have successfully been measured in small-volume samples (Chen et al., 1986; Toole et 

al., 1991; Cohen et al., 1992; Luo et al., 1997; Fietzke et al., 1999; Hinrichs and Schnetger, 1999; 

Stirling et al., 2000). However, radiometric counting methods are still the only way of measuring 

environmental levels of nuclides with half-lives less than 100 yrs, due to their low atomic 

abundances. For example, the longest-lived radionuclide in the U/Th decay series below 222Rn is 

2l °Pb, with a half-life of 22.3 yrs. If 2l °Pb were to remain in secular equilibrium with 238U in 

ocean water, its atomic abundance would be ~ 4.1 x 107 atoms L" 1 , as opposed to 8.5 x 10 15 

atoms L" 1 for 238U (neglecting atmospheric input to the surface waters). With a relatively short 

residence time of 210Pb in the water column, the concentration of 210Pb is ~ two orders of 

magnitude lower than this value, and hence the atomic abundance would be ~105 atoms L" 1 . 

Similarly, the atomic abundance of 234Th would be ~ 1.2 x 105 atom L' 1 , if 234Th and 238U are 

assumed to be in secular equilibrium.

Therefore, several methods have been developed to preconcentrate radionuclides from 

natural waters. These include co-precipitation with Fe- and Al-hydroxides, and selective sorption 

onto Fe-and/or Mn-coated surfaces. The most widely used preconcentration method for
particulate and dissolved Th is to pass a large volume of water through a set of prefilters that 

ideally retain particulate Th, while dissolved Th will be adsorbed onto the two MnO2 -coated 

cartridges (e.g., Buesseler et al., 1992; Baskaran et al., 1993, 1996, 2003; Colley and Thomson, 

1994; Cochran et al., 1987, 1996; Moran et al., 1997; Charette and Moran, 1999; Benitez-Nelson 

et al., 2000; Guo et al., 2002). The fundamental assumption inherent to this technique is that the 

two Mn filters connected in series extract Th in a uniform and consistent manner (i.e., exactly the 

same extraction efficiency). However, this assumption has not been rigorously tested, and any 

significant deviation from uniform extraction of Th will significantly affect the determination of 

Th activities. In this article, we present results from a series of experiments that address the 

variability of the extraction efficiency of traditional MnO2 filters as a function of variable degree 

of KMnO4 saturation and provide a new method to standardize the extraction efficiency 

calculations.

2. Review of Earlier Methods of Preconcentration For Radionuclides:

Many methods have been developed to preconcentrate radionuclides from large volumes 

of seawater. Merrill et al. (1960) were among the first to propose the use of dispersed insoluble 

hydroxides on cation-exchange columns to remove Be from large volumes of seawater. Lal et al. 

(1964) developed the first method to extract a subset of trace elements such as Si, Be, Al, and Ti 

from coastal seawater by filtering seawater through a low-resistance matrix of ferric hydroxide. 

Krishnaswami et al. (1972) used synthetic acrilon fiber coated with ferric hydroxide to extract Th, 

Pb, Si, and Ra from seawater. Moore (1976) used fluffy acrylic fiber coated with MnO2 to extract 

228Ra from seawater quantitatively. Fluffy acrylic fiber was also shown to extract Ra, Th and Ac, 

isotopes quantitatively (Reid et al., 1979). A potential problem with the use of acrilon fiber is 

that the ash content is relatively high (10-15% of the raw fiber weight) and thus reduces the 

effective volume of water samples that will be counted in a Ge-well detector (for a typical 10-ml 

geometry). Furthermore, acrilon fiber has been shown to contain high blank levels of select
radionuclides (226Ra: 0.1-1.0 dpm/100 g of acrylic fiber, Moore and Reid, 1973; Reid et al., 1979; 

228Ra: 0.1 to 0.2 dpm/100 g of acrylic fiber, Knauss et al., 1978; Reid et al., 1979; 232Th, 230Th, 

228Th: 0.05 to 0.16 dpm/100 g of acrylic fiber, Moore, 1976; Knauss et al., 1978; Moore et al., 

1985). Mann et al. (1984) observed higher blank levels for Pu in cotton fibers as compared to 

polypropylene fiber material. Moore and Reid (1973) used MnO2-impregnated acrylic fiber 

cartridges to extract dissolved Ra from natural waters. Wong et al. (1979) was the first one to 

determine the concentration of Pu from seawater using two filter cartridges connected in series 

and assumed uniform extraction efficiency in the two filters. The distinct advantage of using 

fiber cartridges is that the filtration process can occur in the field (either in situ or in a field-based 

laboratory), instead of having to bring large volumes of seawater back to the lab for subsequent 

processing. Many researchers have started using in-situ filtration pumps to preconcentrate 

radionuclides at various depths in the deep ocean using such a cartridge methodology (e.g., 

Nozaki et al., 1981; Bacon and Anderson, 1982; Baskaran et al., 1993, 1996, 2003; Buesseler et 

al., 1995; Cochran et al. 1996; Guo et al., 1995; Moran et al., 1997). Mann and Casso (1984) 

extended the concept of Wong et al. (1979) to determine the concentrations of 137Cs by passing 

water through a tandem cartridge consisting of twin beds of ion-exchange resin impregnated with 

cupric ferrocyanide. Mann et al. (1984) first used a three-cartridge series to extract transuranics 

from seawater, the first being employed to retain the particulate matter and the remaining two to 

extract dissolved radionuclides. Since then, many other researchers have utilized this method to 

obtain particulate and dissolved phases of Th and 210Pb (e.g., Buesseler et al., 1992; Baskaran et 

al., 1993; Baskaran and Santschi, 2003). The advantages and disadvantages of this method 

include:

a) Simultaneous measurement of gamma-emitting radionuclides, such as 234Th (although 

the most useful gamma line at 63 keV has a branching ratio of only 4%), 210Pb and Ra 

isotopes, although relatively higher amounts of extraction of Th and Pb can be 

accomplished while Ra extraction is highly variable (e.g., Baskaran et al., 1993; Colley
and Thomson, 1994; Baskaran and Santschi, 2003). However, activity ratios of Ra 

(228Ra/226Ra, 223Ra/226Ra, and 224Ra/228Ra) can be readily obtained from large samples.

b) Large volume sampling is still required for 228Th measurements, although atom 

abundances of 230Th and 232Th in seawater are sufficiently high for detection via modern 

mass spectrometric techniques.

c) Selective removal of Th but not U, which eliminates extensive chemical separation of 

U from Th isotopes in the field, as in the case of the Fe(OH)3 -precipitation method;

d) Simultaneous determination of both dissolved and particulate phases is possible; and

e) In studies that involve size-fractionated particulate-Th determination, this method 

allows the filtration of large volumes of water (103 -104 L) to assess the distribution of Th 

in various class-sized particles (e.g., Guo et al., 2002), which is not possible with other 

methods.

In addition, to estimate particulate organic carbon (POC) export from the euphotic zone using 

234Th as a tracer, POC/234Th ratios on sinking particulate matter are needed and obtained by sizefractionated filtration of large seawater samples. The distinct disadvantage lies with the 

assumption that the first and second MnO2 -coated cartridges extract Th uniformly. In fact, some 

earlier studies have shown that this efficiency to be highly variable and can range from < 50% to 

100% (e.g., Bacon et al., 1989; Cochran et al., 1996) for Th isotopes. This range reflects the 

variability in the extraction efficiency from sample to sample, under the assumption that both 

cartridges extract Th uniformly. However, it remains unknown whether these variabilities are 

due to differences in the cartridges or in the samples. The discrete variability in the extraction 

efficiency of the two cartridges has not been systematically determined and is the subject of this 

paper.

3. Materials and Methods:
The most commonly reported parameters that cause variations in the extraction efficiency 

of the MnO2 -coated cartridge filters include: variations in the volume of water filtered, 

fluctuations in the flow rates, and variations in the cartridge-preparation techniques, including the 

temperature during MnO2 -impregnation (e.g., Buesseler et al., 2001). For the following 

extraction efficiency experiments, we kept the volume of water filtered, and the cartridge 

preparation and temperature during MnO2 impregnation constant. The flow rates varied between 

2.8 and 7.5 L min" 1 . We believed this variation in flow rates would not cause any major 

difference in the extraction efficiency, and thus, absolute variations in the extraction efficiencies 

of the MnO2-coated cartridges due to changes in the preparation of MnO2 cartridges can be 

quantitatively assessed.

3.1 Preparation of filter cartridges:

We used 7- x 25-cm filter cartridges, consisting of a spun, woven special polypropylene 

fiber wound over a cylindrical core of perforated polypropylene (Johnson Filtration, Amarillo, 

TX, Polypropylene Media Filter Cartridge, Model: JP0.5R10P). Because the manufacturing 

process of these commercially-available cartridges involves the addition of large amounts of antiwetting surfactant that can interfere with the impregnation of MnO2 , a rigorous cleaning of this 

surfactant was required. The raw filters were first cleaned by soaking a batch of 12 cartridges in 

20-L plastic buckets at ~60°C for ~ 12 hrs in a bath of: 1) 2% Micro cleaning solution; 2) 2 M 

HC1; 3) 0.5 NaOH; and 4) 2 M HNO3 . The filters were rinsed in tap water before and after 

soaking them in cleaning solutions until the pH of the wash solution stabilized at around ~7. An 

average weight of an uncleaned prefilter is -150 g, and the ash weight of the cleaned prefilter is 

less than 0.1% of its original weight. Each precleaned filter was then soaked in a saturated 

KMnO4 solution (250 g KMnO4 per liter of water) in a plastic container for a specific period of 

time at ~70°C. The soaking time in the KMnO4 bath varied from 36 hrs (DARK, D) to 6 hrs 

(LIGHT, L) with a normal soaking time of 12 hrs (NORMAL, N) for this set of experiments.
This procedure for coating the precleaned polypropylene cartridges produced extraction 

cartridges with MnC>2 weights ranging from 0.1 to 10% of the total weight of the prefilter 

cartridges, depending on how long the filters were soaked in the KMnO4 bath. After the KMnO4 

bath, one batch of six cartridges was thoroughly washed with tap water. This water flow was 

connected directly to two DARK MnO2-coated cartridges to remove any radionuclides of interest 

that may be present. We periodically ashed the MnO2 -cartridges to determine potential blank Th 

activities, and we did not find any. The filter cartridges were bagged wet.

3.2 Milking of 234Th:

Thorium-234 spike was obtained by passing a uranyl nitrate solution through two large 

concentrated HCl-conditioned columns (Dowex-1 anion-exchange column: AG 1-X4, 100-200 

mesh, chloride form) successively in order to separate U from Th quantitatively. The amount of 

U loaded onto the column and the saturation capacity of the anion-exchange resin were taken into 

consideration in deciding the amount of resin needed to separate 238U and 234Th. This 234Th 

fraction was subsequently isolated from impurities by passing the spike through an 8 M HN(V 

conditioned Dowex-1 anion-exchange column. Thorium retained on the resin beads was eluted 

with MILLI-Q water. The purified 234Th was evaporated to dryness and oxidized with 

concentrated HNO3 twice, to destroy any organic matter that might have been introduced from 

the elution of the last resin column. One ml of the 234Th spike was taken periodically over a 

period of one half-life of 234Th and assayed by gamma counting using the 63-keV photopeak.

3.3 Extraction efficiency experiment:

For this experiment, a large seawater sample (150 L) was collected from Tampa Bay, 

Florida, (salinity of 26) and initially prefiltered through a precleaned polypropylene filter 

cartridge (0.5-jnm median pore size). A known amount of pH-neutralized spike (31074 dpm/ml 

234Th) was added to the sample, stirred vigorously for -30 min, and then allowed to equilibrate

8
for ~2 hrs. After 2 hrs, the water sample was filtered through a manifold consisting of six preassigned cartridge filters. We ensured that the cartridge housings were operating correctly in that 

flow of water passed directly through the cartridge filters without any leaks or malfunctions. 

After filtration, the cartridges were rinsed with distilled water to remove any salts. From previous 

laboratory experiments, it has been demonstrated that the loss of 2MTh during rinsing process was 

negligible (Santschi et al., 1999). The fibers from the extractor cartridges were cut away from the 

inert cartridge cores, packed into crucibles, and ashed at 500°C for ~6 hrs. The ash was 

homogenized and a known weight of this ash was packed into a gamma-counting vial and gamma 

counted for 234Th (63 keV peak) in a Ge-well detector coupled to Inspector (CANBERRA Inc.). 

The amount of ash utilized for gamma counting varied from 14.3% to 100% (53% of the samples) 

of the total ash weight. The gamma-counting system was calibrated with liquid and sediment 

standards obtained from NIST and IAEA (U-standard from NIST for 234Th and RGU-1 from 

IAEA for 234Th). The densities of the sample and standards were comparable; thus, no selfabsorption correction was applied (Baskaran et al., 1993).

4. Results and Discussion:

4.1 Variations in the ash weight:

In these experiments, we assumed that a cartridge ash weight is a reasonable proxy for 

the amount of MnO2 coated on the fiber (Table 1). Nonetheless, the ash weight varied 

considerably within one batch; for example, for the batch made on 27th June (all DARK cartridges 

- 36-hr soak in KMnO4 bath), the ash content varied between 12.21 and 15.09 g, with a mean 

value of 13.99 g (n=7). In contrast, during the LIGHT coating (6-hr soak in KMnO4 bath), the 

ash content varied between 1.73 and 10.76 g with a mean value of 3.72 g (n-12). However, there 

were two cartridges that had significantly higher ash content (10.53 and 10.76 g), which could be 

due to direct contact with the precipitated MnO2 at the bottom of the KMnO4 bath. Although all 

cartridges in a batch (twelve) were treated the same way and they were constantly moved around
in the KMnO4 bath, the possibility of non-uniform coating still exists. Excluding the outliers, the 

mean value of 2.34 g (range: 1.73 to 2.83 g) was considerably lower (with a higher percentage 

range) than those for the DARK-coated cartridges. In two other batches of DARK cartridges, the 

variations were fairly large. The range in ash content for a normal coating (12-hr soaking in 

KMnO4 bath) was also wide (1.83 to 4.21 g). Thus, there are variations in the ash content that 

could be, at least in part, attributed to cartridge-preparation methods [such as location of a 

cartridge in a bath, contact with the precipitated KMnO4 crystals, distance from the heating 

element(s), rate of mixing of the heated solution in the bath, etc.)]. For example, Mann et al. 

(1984) reported that polypropylene filters do not impregnate uniformly with MnO2 . It is possible 

that the surfactants utilized in making these cartridges were not cleaned properly prior to soaking 

in the KMnO4 bath and caused poor impregnation. The question as to whether the ash content 

(measure of the amount of sorption sites) is related to the absolute or relative extraction efficiency 

of Th is discussed below.

4.2 Derivation of the absolute and relative extraction efficiency:

A schematic of the six-cartridge filtration setup and the corresponding activity in each 

cartridge is shown in Figure 1. If we assume A] to be the initial activity of dissolved 234Th 

entering the first MnO2 filter cartridge, Aj and Aj are the activities in the 'i'th and 'j'th cartridges 

(i = 1,2,3,4,..... and j = i-1). Thus, the absolute extraction efficiency (TJJ) in the 'i'th cartridge 

can be derived as follows:

ili = A1 *100/(AI -IAj) (1)

TU=A,*100/A, (2) 

Tl2 = A2 *100/(A,-A1 ) (3)

For uniform extraction efficiency in the first two cartridge filters (i.e., %=

Therefore, Aj/Aj = A2 / (Ar - AI)

10
or A,A2 = A, A,-A, 2 (4)

or A, =A, /(A,-A2) = A,/(1-A2/A,) (5)

Equation (5) is the standard equation used to determine the Th activity if one assumes that the 

two filter cartridges extract Th uniformly.

4.3 Variations in relative extraction efficiency:

The relative extraction efficiency is calculated using Eq. (5) for the first five cartridges 

and the results are presented in Table 1. The calculated relative efficiency, using 234Th activities 

from the first three cartridges (two values for each set), in all five sets varied from 64.3 to 96.4% 

similar to highly varying extraction efficiencies reported from sample to sample in literature. For 

example, Mann et al. (1984) reported an extraction efficiency using MnO2 -coated cotton filter 

cartridges that varied between 16 and 100% for Pu and 76 to 100% for Am. Livingston and 

Cochran (1987) found that the extraction efficiency using MnO2 -polypropylnene filter cartridges 

varied between 69 to 93% (mean: 82.7%, n=12) and 71 to 94% (mean: 87.1%, n=12), for Th and 

Am, respectively. Bacon et al. (1989) reported extraction efficiencies of 97 to 100% for Th, and 

55 to 80% for Pa using MnO2 -coated acrylic fiber cartridges. Using MnO2-treated polypropylene 

cartridges, Cochran et al. (1987) observed very low extraction for Pu (~10%), and this was 

attributed to the lower scavenging of the oxidized Pu present in the water column. Those authors 

also observed that the extraction efficiency for 230Th and 241 Am varied between 76.4 and 91.2% 

(83.8±7.4%, ISD), and 71.2 and 96.0% (83.6±12.4%, ISD), respectively. Buesseler et al. (1992) 

reported an extraction efficiency that ranged between 70 and 95% (87±8%, ISD, Buesseler et al., 

1992; 79±9%, 1 SD, Buesseler et al., 1995) for 234Th. Cochran et al. (1996) observed a large 

range in the extraction efficiency, from 65 to 100% (74.0±17.0%, ISD), with a few samples less 

than 50%, in the Arctic waters in the Nansen Basin. For the Amerasian Basin, Moran et al. 

(1997) reported an extraction efficiency that varied between 82.3 to 96.9% (89.6 ±7.3%, ISD) for

11
234Th, whereas Baskaran et al. (2003) reported the extraction efficiency to vary between 72 to 

98% for 234Th in the Canada Basin. Baskaran et al. (2003) also reported that a couple of samples 

had less than 65% efficiency, and discarded those samples from further interpretation. Guo et al. 

(2002) found the extraction efficiency to vary between 70 and 92%. From such studies, it is 

evident that ~20% (+/- 1SD) variations in the efficiency from sample to sample are quite 

common.

The relative efficiency in all filters varied from negative values (indicating that the 

second of two cartridges had a higher activity than the first one; such observations have been 

made with Ra isotopes; unpublished data) to values greater than 95% (Table 1). More discussion 

on observed low-extraction efficiencies in 3 rd to 5 th cartridges is given below. In all six cartridges 

connected in series, the relative efficiency is much more variable than the absolute efficiency. 

Thus, it is critical that we evaluate the variations on the individual extraction efficiency (absolute) 

of these cartridges.

4.4 Variations in the Absolute Extraction Efficiency:

In this experiment, we had six cartridges connected in series, and hence 'i' varied from 1 

and 6 and 'j' from 2 to 6. Equation (1) does not assume uniform efficiency, and the ratio of the 

activity retained in the filter to that entering activity yields an absolute efficiency. The massbalance of Th activity is shown in Table 1. In five of the six sets, the mass-balance was within 

5%, while in one it was off by ~9%. The summation of the activities in all six cartridges is used 

as the total activity instead of the actual activity added to the experiment. The difference in 

absolute efficiency ([measured activity in Fi/amount of spike added] x 100 - [measured activity 

in Fi/summation of activity in all the filter cartridges in a set] x 100) for the first cartridge 

calculated using the amount of spike added and the summation of 234Th in each cartridge is 

relatively small (less than 5%, except Set HI, where it is -10%, Table 1). The absolute 

efficiencies calculated from Equation (1) are shown in Table 1. Prior work assumed that the

12
extraction efficiency in the first and second MnO2 -extractor cartridges is identical (r|i = r|2 ). 

Experimental results show that the absolute extraction efficiency in the first two filter cartridges 

varied between 54.1 to 93.8%, with a much large variation in the absolute extraction efficiency, 

as compared to the relative extraction efficiency (Table 1). The absolute extraction efficiency in 

a given set of six cartridges varied by almost over a factor of two, from 32.3 to 89.3% in the 

DARK cartridges, and 39.3 to 93.8% in the LIGHT cartridges. It is noteworthy that the first 

cartridge filter always had the highest absolute extraction efficiency (Table 1). In addition, the 

absolute and relative extraction efficiencies in the first cartridge (relative efficiency was 

calculated using the activity of 234Th retained on the first two cartridges in each set) varied 

between 88.9 and 93.8%, and 89.4 and 94.2%, respectively. There is a general trend that the 3 rd , 

4th , and 5th filter cartridges have the lowest extraction efficiency (Table 1). The plot of the 

relative efficiency against the absolute efficiency (Figure 4) indicates scatter in the plot, 

suggesting that the relative efficiency and absolute efficiency vary, although the correlation is 

significant (P>0.001). Coastal water samples are often enriched with organic colloidal material, 

and it is likely that some of the spike added in the water readily forms a complex with the 

colloidal organic matter and is not retained uniformly by the filter cartridges. If we assume ~ 1- 

10% of colloid-bound 234Th passes through the first cartridge, we can expect absolute efficiencies 

as calculated in this study. However, if varying amounts of this colloidal 234Th are retained in 

subsequent cartridges, then the absolute and relatively efficiencies will be highly variable. Our 

data appear to suggest this. The question then becomes, is this what is causing the variations in 

the extraction efficiency from region to region? It is likely that this is a major factor that causes 

variability in the relative extraction efficiency in seawater. An experiment similar to the one 

conducted in this study with ultrafiltered and UV-treated water could perhaps shed more light on 

this issue.

Using the relative efficiency of 'i'th filter cartridge calculated from the activity of 234Th 

retained on 'i'th and 'i+1' cartridges, the activity of 234Th entering the 'i'th cartridge was

13
calculated and compared to the actual value entering the 'i'th cartridge (Figure 5). The actual 

activity going to the 'i+l'th cartridge was calculated by subtracting the value of 234Th retained by 

the 'i'th cartridge from the value that enters the 'i'th cartridge. The deviation from the calculated 

activity to the actual activity for each of the cartridges (Table 2) indicated the following,

a) In the first two cartridges, the deviation between the actual activity and the activity 

calculated under the assumption that the first three cartridges (to calculate the relative 

extraction efficiency in the first two) extract Th uniformly, ranged between 0.6 -18.8%. 

Thus, there could be an error as high as 18.8% on the determination of 234Th from the 

assumption that the cartridges extract uniformly.

b) The uniform extraction efficiency varied between 32.3 and 89.3% (Set I) in the DARK 

MnO2-coated cartridge filters and was comparable to the range of the LIGHT-coated 

filters (Set II). The lower efficiencies (less than 60% in 11 out of 15) are commonly 

found in the 3rd to 5th filter cartridges, although values >60% (four out of 15) are also 

observed. As discussed before, this could be due to a small fraction of total Th forming a 

Th-ligand complexation that escapes MnO2 surfaces without sorption.

c) The relative efficiency calculated using equation (5) becomes negative when 234Th 

activity in Fj is < FJ+I. In four of the 25 samples, the relative extraction efficiency was 

negative (Table 1).

As discussed, factors that may affect the extraction efficiency are the flow rate (i.e., direct 

measure of the contact time of the nuclide with the MnO2), the amount of MnO2 coated on the 

cartridges (i.e., the rate of reaction between the tracer and the extractor MnO2), and the 

temperature and duration of MnO2 impregnation. There is no relation between the absolute or 

relative extraction efficiency and the amount of ash weight (Figure 2), suggesting that the amount 

of MnO2 coated on the filter cartridge does not control the amount of Th absorbed. This is 

similar to the observation reported for the relative extraction efficiency for Th and the ash weight

14
by Baskaran et al. (1993). The extraction (both absolute and relative) efficiency is found to be 

independent of the flow rate that ranged from 2.8 to 7.5 L min" 1 (Figure 3). When the surface 

area of the fiber remains imsaturated with respect to any element of interest, we expect the 

volume of water filtered to be independent of the extraction efficiency. Mann et al. (1984) did 

not find any clear relation between flow rate and/or volume and extraction efficiency of Pu. 

Several researchers also did not find any correlation between the flow rates (up to 6 L/minute, 

Buesseler et al., 1992; up to 35 L/minute, Baskaran et al., 1993) and extraction efficiency for 

234Th in MnO2-coated, polypropylene filter cartridges, while others have observed only a weak 

dependence of the extraction efficiency on flow rate (-5-16 L/minute, Cochran et al., 1995b; 4.8 

to 13 L/minute, Charette and Moran, 1999; Guo et al., 2002). Charette and Moran (1999) also 

reported the extraction efficiency is independent of the amount of volume, up to 2,000 L on an 

8.5-cm cartridge (= 6,000 L for the cartridge used for this experiment). Thus, it seems that 

contact time of the nuclide with a MnO2-coated surface in some cases plays a role, while in other 

cases, the amount of contact time seems to be independent of the amount of nuclide extracted.

4.5 Use of constant extraction efficiency in calculating dissolved 234Th:

As discussed earlier, both the absolute and relative extraction efficiency of the cartridge 

filters varied considerably from filter to filter, and this variation will introduce an error in the 

final dissolved 234Th activities. In order to circumvent the error on dissolved 234Th activity from 

the implied assumption of uniform extraction efficiency, we evaluated the possibility of 

combining the two filters and assumed constant extraction efficiency for the set of two cartridges. 

The constant extraction efficiency value used for a pair is taken to be the average value of all the 

relative extraction efficiencies in the first two cartridge filters (in our case, calculated from the 

cartridges 1 and 2, and cartridges 2 and 3). The activity of 234Th entering the first cartridge filter 

was then calculated as follows: 

Activity entering the first cartridge (which is the activity in the field sample)

15
= (A,+A2)/[ii +n(1.00-iO] (6) 

where r\ is the average of the relative extraction efficiency (0.852 in our study, average of 12 

values) of all the pairs of filter cartridges (one for each sample). The activity thus obtained is 

compared to the spike added to the experiment (i.e., activity entering the first filter cartridge) in 

Table 2. In ~ 30% of the samples (eight out of 25), the activity calculated using equation (5) that 

assumes uniform extraction efficiency yielded meaningless values, either negative or with errors 

>80%, while the activity calculated using a constant extraction efficiency after combining the two 

successive cartridges yielded a much more realistic value. Thus, it appears that use of constant 

extraction efficiency on the combined activity in two filters provides a more meaningful activity 

than the value obtained using the assumption of uniform extraction efficiency.

4.7 How does the variation in extraction efficiency affect the rate of removal of dissolved 

and participate 234Th as well as residence time of particulate and dissolved 234Th in the 

euphotic zone?

We have taken three data points for 238U, and dissolved and particulate 234Th activities 

from the Canada Basin of the Arctic Ocean: case I (234Thd = 1.97 dpm L" 1 ; 234Thp = 0.20 dpm L' 1 ; 

238U = 2.27 dpm I/ 1 ); case II (234Thd = 1.23 dpm I/ 1 ; 234Thp = 0.09 dpm L' 1 ; 238U = 2.22 dpm L1L-1); and case III (234Thd = 1.79 dpm I/1 ; 234Thp = 0.19 dpm L* 1 ; 238U = 2.35 dpm I/1 ). The 

removal fluxes of dissolved and particulate 234Th and the residence times were calculated using 

±5, ±10, and ±15% errors on the dissolved 234Th activities and the corresponding residence times 

and rate constants and plotted in Figures 6a and 6b. Up to a ±15% variation on the 234Thd 

activities could result in up to orders of magnitude higher or lower rate constants and residence 

times, depending on how close 234Thtotai is to the 238U activity. When the 234Thtotai/238U AR is close 

to 1, the errors on the residence times and rate constants could become very high (Figures 6a, 6b). 

For example, in case I, with ±15% error on the 234Thd values, the removal fluxes of dissolved

16
234Th (JThd) varied between 0.00013 to 0.0171 dpm I/ 1 d' 1 (0.00863 dpm L' 1 d" 1 for 0% error) 

while the removal fluxes of particulate 234Th (JThP) values varied between negative and 0.0114 

dpm L' d' 1 (0.00288 dpm L" 1 d' 1 for 0% error; Figure 6a). The residence times of dissolved 234Th 

varied between 98 and 732 d (228 d for 0% error, neglecting one value in which 234Thtotai/238U 

activity ratio is 1.0), while the corresponding values for the particulate 234Th varied between 

negative values to 4636 d (70 d for 0% error, neglecting two negative values and 4,636 days in 

Figure 6b). Thus, the residence times and removal fluxes of particulate and dissolved 234Th could 

vary over orders of magnitude due to non-uniform extraction efficiency of MnO2 cartridge filters, 

especially in waters where 234Thtotai/238U is close to equilibrium. In case II, the 234Th,otai/238U ratio 

is 0.59. In this case, the residence times and removal rate constants varied within a factor of 2 

(JThd : 0.023 to 0.034 dpm L" 1 d" 1 ; JThp : 0.0205 to 0.0312 dpm L" 1 d" 1 ; residence time of dissolved 

234Th: 31 and 61 days; residence time of particulate 234Th: 2.9 and 4.4 days; Figure 6a). In case 

III, the 234Thtotai/238U ratio is 0.84. In this case, the residence times and removal rate constants 

varied within a factor of ~6 (JThd : 0.00838 to 0.0238 dpm L' 1 d" 1 ; JThp : 0.00292 to 0.01836 dpm L' 1 

d" 1 ; residence time of dissolved 234Th: 64 and 246 days; residence time of particulate 234Th: 10 and 

465 days; Figure 6a).

5. Conclusions:

1) There is a relatively large variation in the absolute efficiency of the first and second 

filter cartridges, and thus, the assumption of uniform extraction efficiency could 

introduce a large error on the dissolved 234Th activity when uniform extraction 

efficiency is assumed using the two Mn-cartridge system.

2) We suggest using constant extraction efficiency on the combined activity of two filter 

cartridges connected in series to obtain a dissolved 234Th activity. The average value 

of the relative efficiency from all the cartridges appears to provide a reasonable 

value.

17
3) In cases where 234Thtotai/238U activity ratios are close to 1 or >1.0, even a relatively 

small error in the extraction efficiency could introduce very high errors in the 

removal-rate constants and residence times of particulate and dissolved 234Th.

Acknowledgments:

We thank Sarah Trimble and Brian Blake-Collins for help with the analytical work in the 

laboratory. We thank Bill Burnett, Kirk Cochran, and an anonymous reviewer for their reviews 

of this manuscript. The use of trade, firm and brand names is for identification purposes only and 

does not constitute endorsement by the U.S. Government. This work was supported, in part, by a 

cooperative grant between Wayne State University and U.S. Geological Survey-St. Petersburg, 

Florida.

18
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21
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22
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23
Table 1: 234Th activities, absolute and relative efficiency for the extraction efficiency experiment+ .

Filter 

Number

SET);

F1

F2

F3

F4

F5

F6

SET II

F1

F2

F3

F4

F5

F6

SET III

F1

F2

F3

F4

F5

F6

SET IV

F1

F2

F3

F4

F5

F6

SETV

F1

F2

F3

F4

F5

F6

Color of Flow Rate 

filter++ (L/min)

D

D

D

D

D

D

L

L

L

L

L

L

D

L

D

L

D

L

D

D

D

L

L

L

N

N

N

N

N

N

2.8

2.8

2.8

2.8

2.8

2.8

6.0

6.0

6.0

6.0

6.0

6.0

6.0

6.0

6.0

6.0

6.0

6.0

7.5

7.5

7.5

7.5

7.5

7.5

6.5 - 7.0

6.5 - 7.0

6.5 -- 7.0

6.5 - 7.0

6.5 - 7.0

6.5 - 7.0

Date made

6/27/2003

6/27/2003

6/27/2003

6/26/2003

6/27/2003

6/26/2003

8/19/2003

8/19/2003

8/19/2003

8/19/2003

8/19/2003

8/19/2003

6/27/2003

8/19/2003

6/27/2003

8/19/2003

6/27/2003

8/19/2003

2/5/2003

2/5/2003

6/26/2003

8/19/2003

8/19/2003

8/19/2003

8/8/2003

8/8/2003

8/12/2003

8/12/2003

8/12/2003

8/18/2003

Total ash 

weight (g)

13.4647

13.6647

15.747

2.3155

13.8596

2.3686

2.3981

1.7252

2.2773

10.7581

2.8317

2.2672

15.0897

2.2985

12.2105

2.5089

13.9214

10.5353

5.3276

2.7194

10.7278

2.2298

2.4789

2.356

10.8926

10.1489

2.1353

4.2065

1.8255

3.223

234Th total activity 

(dpm)

21713213946

16578±622

5912±225

1122±41

18031131

544118

22660512107

80831130

2690199

26481127

892142

614122

18231713101

118521226

1416145

1420158

665168

927163

22704613346

239711363

1681+94

1332153

657117

599144

21901113377

127711406

3351136

993147

595114

646157

'Absolute "Relative 

efficiency (%) efficiency (%)

89.3 (87.3)

63.9

63.0

32.3

76.8

-

93.8(91.2)

54.1

39.3

63.7

59.2

-

91.8(83.8)

72.8

32.0

47.2

41.8

-

88.9(91.3)

84.9

39.4

51.5

52.3

-

92.3(88.1)

69.6

60.0

44.4

47.9

-

92.4

64.3

81.0

-60.7

69.8

-

96.4

66.7

1.5

66.3

31.1

-

93.5

88.1

-0.3

53.2

-39.4

-

89.4

93.0

20.8

50.7

8.8

-

94.2

73.8

70.4

40.0

-8.6

7.9

+: Spike added: set I, II, IV and V: 2.49 x 105 dpm; set III: 2.18 x 1 (rdpm; Mass balances in five sets are:

97.8%, 97.2%, 91.3%, 102.7% and 95.6% in sets I through V, respectively.

++: D: Deep (36 hrs); L: Light (6 hrs) and N: Normal (12 hrs) soaking in KMnO4 bath

*: The absolute extraction efficiency (= amount retained on the filter / summation of the activities in 

all the filters in a set) calculated using Equation (1); number in parenthesis denotes the value 

calculated based on the amount of spike added.

**: The relative extraction efficiency was calculated using Equation (5).

24
Table 2: Activities of measured 234Th, and calculated 234Th based on constant and relative extraction 

efficiency

Sample 

Code

SETI

F1

F2

F3

F4

F5

F6

SET II

F1

F2

F3

F4

F5

F6

SET III

F1

F2

F3

F4

F5

F6

SET IV

F1

F2

F3

F4

F5

F6

SETV

F1

F2

F3

F4

F5

F6

Measured 234Th

(dpm)

217132±3946

16578±622

59121225

1122141

18031131

544118

22660512107

80831130

2690199

26481127

892142

614122

18231713101

118521226

1416145

1420158

665168

927163

22704613346

239711363

1681194

1332153

657117

599144

21901113377

127711406

3351136

993147

595114

646157

234Th entry

(dpm)

243091

25959

9381

3469

2347

544

241533

14928

6845

4155

1507

614

198596

16279

4427

3011

1591

926

255286

28240

4269

2588

1256

599

237367

18356

5585

2234

1241

646

234Th dpm

(constant effi.)*

245941

23667

7402

3077

2470

246970

11336

5618

3726

1586

204330

13962

2985

2194

1674

264153

26994

3171

2093

1322

243912

16965.67

4570.612

1670.986

1306.571

234Th dpm

(relative effi.)"

235080

25768

7297

-1847

2582

234987

12114

174195

3994

2865

194993

13460

-505715

2669

-1686

253847

25779

8100

2628

7495

232573

17314

4761

2479

-6927

*: Assuming a constant efficiency of 85.2% (see text for details)

**: Assuming Equation (5) in the text

25
Figure Caption

Figure 1: Filtration assembly with absolute extraction-efficiency equations.

Figure 2: Extraction efficiency versus ash weight.

Figure 3: Extraction efficiency versus flow rate (L min" 1 ).

Figure 4: Relative efficiency versus absolute efficiency.

Figure 5 The measured and relative efficiency-based calculated 234Th were normalized by dividing 

the F, by 1000, F2 by 100, F3 , F4 and F5 by 10.

Figure 6a: Removal flux of dissolved and particulate 234Th versus percentage error introduced in the 

determination of dissolved of 234Th due to variable extraction efficiency in filters 1 and 2.

Figure 6b: Residence times of dissolved and particulate 234Th versus percentage error introduced in the 

determination of dissolved of 234Th due to variable extraction efficiency in filters 1 and 2. 

The data used are: Case-1: 234Thd = 1.97 dpm L '; 234Thp = 0.20 dpm L' 1 ; 23*U = 2.27 dpm L' 

'; Case II: 234Thd = 1.23 dpm L' 1 ; 234Thp = 0.09 dpm L' 1 ; 238U = 2.22 dpm L' 1 ; and Case III: 

234Thd = 1.79 dpm L' 1 ; 234Thp = 0.19 dpm L' 1 ; 238U = 2.35 dpm L' 1 ; Data taken from 

Baskaran et al. (2003).

26
(AI-A 1 -A2-A3)

A,-A, Ai - A, - Aj | - A2 -

Figure 1: Filtration assembly with absolute extraction efficiency equations.

27
tomxIao

o 

* <

a I

NJ

oo

Extraction Efficien

CD 00 <5"

ho

O

m 

23!

<D

o

 oo

o

o

o

o>
TJ f Extraction Efficien

m

K)

[traction efficiency 1Cfl 0^

§"35(' rc

10

31

O ^

Ss

7)

0)

CD O1

"P3D ^^^ O) 

-Sl

Oft

o to jt o

D O O O O 

------ 1

0*

cr <D ol

mg

5'§

3 o ^5»<

^^^^ 1^

 i i i i i i i i i i 

m 

" * GBO«0

- «QD O 

 

... 1 ... 1 ... 1 ... 
Relative Efficiency 

O)ao

U) o

oo'

13 O

10

o, im

CD o> 

3 o o

00
1 WWW o

Q> 

Cfl

 a 800

c

Q>

o 600

E ill

> 400

fl«

2 200

V

Q> 

M

S n

 i i i i i i i i i i i i i i i i i i

o

 O o

: o : 

o

. <&agg> .

 i i i i i , i i i

200 400 600 800 1000

234. Z Normalized Measured Th

Figure 5 The measured and relative efficiency-based calculated 234Th activities (dpm) 

were normalized by dividing the FI by 1000, ?2 by 100, p3, ?4 and Fs by 10.

31
T-

<7

"i 0.04

0. "O

^c <

f 0.03

TO

^BI > /

S 0.02

 J Thd (I) x '

O J7hd OO

-h Jjhd (III)

i i i r j i i i i

>

N ^

i

(0 1

5 W ^ u

o

§ 0.01

u.

5

o

E 0

_

i i i i i i i i

i i i i i i i i i i i

«

^

zis

4

4

0 D

 Q

i i i .I i i i i i i

D JT*W

X JTh (II)

A J P (lll)

Thp

.,,.,,..,

> yyV ^

t

i

T ^

i

i

0.04

0.03

^0.02

0.01

\

0

-I

 ^

i i i i i i i i i ^1-0.01

» -15 -10 -50 5 10 15

Error in the 234Th activity (%)

d

^

' D

^

E

Q.

^

£

CO

CM

0)

IS

3

O"-E

(0

0. s

X

E

j,,,^

(0

o

0) tt

234r Figure 6a: Removal flux of dissolved and particulate Th versus percentage error 

introduced in the determination of dissolved of 234Th due to variable 

extraction efficiency in filters 1 and 2.

32
g

 E

0)

0

c

5"5

(£

**>

M

0)

> 

o

(0

0

1000

800

600

400

200

0*

^ ^d(I) 

~h T (III)

 i i i i | i i i i | i i i i | i i i i | i r~

_

-

-

-

D

-

D 

D A 

3 A ,

> ± + +

^.i,,*,,,,^,,,,^,,,,^,,

-15 -10 -505

D T p(D 

X Tp(II)

A Tp(III)

 1 1 I 1 1 1 1 

-

-

-

0 -

i

-

A

+ -

..*....;

^^

100^

E

«/> >l=

80 o>

o

c

<D

"60 2

(0

o

tt

40^

CO

cs

20 §

> 3 

'o r

10 15 £

Error in the 234Th Activity (%) d

Figure 6b: Residence times of dissolved and paniculate 234Th versus percentage error

introduced in the determination of dissolved of 234Th due to variable extraction 

efficiency in filters 1 and 2. The data used are: Case 1: 234Thd = 1.97 dpm 1L1 ; 

234Thp = 0.20 dpm L' 1 ; 238U = 2.27 dpm L' 1 ; Case II: 234Thd = 1.23 dpm L/ 1 ; 234Thr

234.

= 0.09 dpm I/ 1 ; 238U = 2.22 dpm L' 1 ; and Case III: 234Thd = 1.79 dpm I/ 1 ; 

0.19 dpm L' 1 ; 238U = 2.35 dpm L' 1 ; Data taken from Baskaran et al. (2003).

TU Thp -

33