Uptake and depuration of 131I by the macroalgae Catenella nipae potential use as an environmental monitor for radiopharmaceutical waste.
Ross Kleinschmidta,b∗ a
Health Physics Unit, Queensland Health Forensic and Scientific Services, PO Box 594
Archerfield, Queensland. Australia. 4108. b
School of Physical and Chemical Sciences, Queensland University of Technology. GPO Box
2434, Brisbane, Queensland. Australia. 4000.
Abstract A study was initiated to establish the suitability of the macroalgae, Catenella nipae as an environmental surveillance monitor for radiopharmaceutical waste discharges to aquatic environments. A series of experiments were conducted to establish the radioactive iodine (131I) concentration factor, and uptake & depuration characteristics of C. nipae. The steady state concentration factor was estimated to be 630 ± 80 mLg-1, with an uptake half-time of 160 ± 20 minutes. Elimination of 131I was found to follow a two phase model, the first having a rapid elimination rate with a half-time of less than ∗
Corresponding author. Address: Health Physics Unit, Forensic & Scientific Services, Queensland Health, PO Box 594, Archerfield, Qld 4108, Australia. Tel.: +61 7 3274 9124; Fax.: +61 7 3274 9123. E-mail:
[email protected]
one minute, followed by the second phase with a half-time of 3.2 days. Application of the Michaelis-Menton model allowed calculation of an estimate for activity concentration of 131I in environmental waters from C. nipae sampling devices in the Brisbane River estuary, Australia. The results suggest that C. nipae may be used as an environmental radioactive waste sentinel.
Keywords Australia, radioactivity, iodine 131, macroalgae, monitoring, recycled water
Introduction Radiopharmaceuticals discharged to domestic sewers, by way of excretion from patients undergoing both diagnostic and therapy procedures, may become concentrated in wastewater treatment plant (WWTP) waste streams. The radiopharmaceuticals are administered on either an inpatient or outpatient basis, therefore providing a diffuse source load to the domestic wastewater system. While local radiation control regulations prescribe allowable disposal concentrations to the sewer (OQPC 2004) for controlling inpatient discharges, outpatient discharge is generally not controlled. The presence, and in some cases the impact, of these radioactive waste discharges has been documented internationally (Ault 1989, Barquero 2008a, Barquero 2008b, EU 1995, Fenner & Martin 1997, Ipek et al 2004, Larsen et al 1995, Larsen et al 2001, Martin &
Fenner 1997, Miller et al 1996, Sundell-Bergman et al 2008 and Titley et al 2000), however, a limited number of studies have been undertaken and published in Australia.
A review of the available literature indicates considerable variation in published radionuclide partitioning values within WWTPs (Ham et al 2003), and the fate of waste streams (i.e. liquid effluent & biosolids) that may contain reconcentrated radioactive wastes. At a local level, the assessment of the impact of reconcentrated radionuclides has become more prevalent with the development of major infrastructure programs to establish wastewater recycling systems for the production of high quality, potable water. The advanced water treatment plant (AWTP) associated with this scheme in Brisbane, Australia, uses secondary and tertiary treated sewage effluent as feed water to a series of microfiltration, reverse osmosis and oxidation processes prior to release for indirect potable reuse. While the impact of any radiopharmaceutical wastes on the indirect potable supply will be minimal due to the designed multibarrier system, the reverse osmosis concentrate (ROC) waste stream is discharged to local rivers and estuaries via outfalls and submarine diffusers. Due to the presence of radiopharmaceuticals in the feed water, 131I concentrations of greater than 100 BqL-1 are not uncommon in the ROC waste stream (Kleinschmidt 2008). The measurement of radioactive wastes in the environment is required to enable assessment and long term monitoring of the radiological impact of the ROC discharge to the receiving estuary. Bioaccumulators such as macroalgae and crustaceans have been documented as being effective in monitoring for the presence of contaminants that would otherwise be difficult to quantify (Costanzo 2001, Costanzo 2005, Evans & Hammand 1995, Runcie
et al 2004, Solimabi and Das 1977, Sombrito et al 1982, Vives i Batlle et al 2005, Wilson et al 2005).
If the 131I uptake and depuration characteristics of the macroalgae Catenella nipae, already recognised for its use in monitoring stable isotope pollutants (Costanzo 2001, Costanzo 2005) and as a estuarine bioindicator (Melville and Pulkownik 2006), allow for reliable modelling, then the macroalgae can be implemented for use as a sentinel for measurement of radiopharmaceutical wastes in the aquatic environment.
Methods Uptake and elimination studies were conducted using native C. nipae collected from mangrove pneumatophores (Figure 1) along the foreshores of Moreton Bay in Queensland, Australia (270S 28.543’, 1530E 11.506’). The harvested C.nipae was transferred to a clean plastic container holding 5 L of seawater and stored at a temperature of approximately 100C during shipping to the laboratory. A further 20 L of seawater was collected from the same location to be used during the uptake and depuration experiments, and was stored at 100C in a dark environment prior to use.
A solution of radiopharmaceutical-purity sodium iodide (131I, half-life 8.04 days) mixed with fresh seawater was used as the tracer for the uptake and depuration experiments. Five 10 g portions of C. nipae were rinsed in seawater, weighed (wet) and transferred to separate glass tanks holding 2000 mL of fresh seawater each. The tanks were exposed
to normal laboratory lighting, and conditioned to the controlled laboratory temperature of 22 ± 20C for the extent of the trials. Varying concentrations of 131I were introduced to three tanks (Tank A - 9.44 BqmL-1, Tank B - 4.85 BqmL-1 and Tank C - 0.76 BqmL-1 respectively), with a fourth tank, Tank D, established as a duplicate to Tank A. A fifth tank, Tank E, was used as a control (9.32 BqmL-1) where no C. nipae was added to the spiked seawater, to monitor iodine losses via pathways other than uptake or depuration. Volatilisation of iodine to the atmosphere has been reported as being less than 0.1% under the test conditions used (Evans et al 1993), and therefore can be considered negligible compared to the counting uncertainty. Activity concentration and counting times were selected to ensure that the counting uncertainty was maintained at less than 2%, and yet maintain an activity working range that could be encountered under environmental surveillance applications. Uptake rate was derived from measurements of a 10 g aliquot of water from each tank at selected time intervals and counting on the described radiation measurement system. Measurement of the change in water activity concentration, as opposed to direct measurement of C. nipae activity, was adopted to allow for the rapid measurement of uptake and depuration in the initial stages of each experiment as the algae did not require removal from the tank. Measurement aliquots were immediately returned to the tank on the completion of counting.
The 131I depuration, or elimination, rate was determined by placing C. nipae previously immersed in Tank A and Tank D into 2 x 1000 mL glass tank respectively with fresh seawater containing no radioactive tracer. The depuration rate was derived from measurements of a 10 g aliquot of water from each tank at selected time intervals and counting on the radiation measurement system. The aliquot was immediately returned
to the tank on the completion of counting. Counting times were selected to ensure that the counting uncertainty was maintained at less than 5%.
Radioactivity measurement for uptake and elimination trials was performed using a 75mm NaI(Tl) scintillation well detector (Bicron Model 3MW3/3, 30 mm dia. x 50 mm deep blind well) in a 100 mm thick lead environmental shield, connected to a multichannel analyser (EG&G µNOMAD) with computer analysis software (EG&G ScintiVision). Water aliquots of 10 mL were collected and counted in standard 20 mL polyethylene liquid scintillation vials, using a region of interest centred on the predominant 364 keV 131I photopeak. Energy calibration of the system was conducted using a set of multi-nuclide reference sources (Amersham Gamma Reference Sources, Model QCR.11). Measurement count times were chosen to meet the desired sensitivity requirements for each experiment.
Field application of the method for surveillance monitoring was tested under two scenarios. A sampling system was developed, based on that used by Costanzo et al, 2001 (Figure 2). In the first experiment three units were deployed in proximity to a WWTP effluent outfall, near the mouth of the Brisbane River. The samplers, each containing approx. 50 g of C. nipae, were submerged for a period of 6 hours. Two litre water samples were collected at the initiation of sampling, after 3 hours, and on extraction of the samplers after 6 hours. The second experiment was conducted by deploying 3 sampling units in the open effluent channel leading from the wastewater treatment plant to the estuary discharge outfall. In this case the samplers were deployed
for varying lengths of time, periods being 100 min, 220 min and 280 min. Effluent samples were collected at times representing half time periods, i.e. at 50 min, 110 min and 140 min.
Measurement of radioactivity in the C. nipae from the sampling devices, and environmental waters & effluent was conducted using high resolution quantitative gamma spectrometry (EG&G GAMMA-X detector, 20% relative efficiency) and computer analysis software (EG&G GammaVision). System calibration for energy and efficiency was conducted using a multinuclide reference source (Eckert & Ziegler Isotope Products 7503-7500 ML + 241Am + 210Pb), traceable to NIST, for waters, and a uranium reference standard (IAEA RGU-1 material) for C. nipae. Environmental and effluent water samples were counted directly in 2000 mL Marinelli beakers without pretreatment, for a live counting time of at least 250000 seconds. C. nipae samples were weighed (wet weight) and compressed into a standard 100 mL jar geometry prior to counting, with a minimum live count time of 10000 seconds. All measurements were corrected for decay from time of exposure to completion of counting.
Results and Discussion Uptake Water activity concentrations measurements were initially taken at short intervals ranging from 1 minute to 10 minutes over the first 60 minutes of the experiment, with
longer periods between measurements as accumulation saturation was observed. Water activity concentration values were converted to 131I specific activity in C. nipae, corrected for mass and radioactive decay. The specific activity results, in Bqg-1, were plotted against exposure time (Figure 3). All sets of uptake results gave a good statistical fit, with correlation coefficients for measured and modelled data ranging from 0.983 to 0.998, to the Michaelis-Menton uptake model (Lopez et al 2000): at = Asat.t/(Km + t)
[1]
where at is the activity concentration in Bqg-1 at a given time, Asat is the saturation activity concentration, Km is the Michaelis-Menton curvature constant, and t is the time of exposure via immersion. A correlation co-efficient of 0.988 was observed for Tank A and Tank D (duplicate) results. Table 1 shows that observed values of 160 ± 20 minutes for the curvature constant, Km indicate that the uptake rate was similar over the range of initial water activity concentration values used.
The concentration factor was calculated for the saturation activity concentrations using the formula: CF = Asat /Aw
[2]
where CF is the concentration factor with units of Lkg-1, Asat is the C. nipae specific activity in Bqg-1, and Aw is the water activity concentration in BqmL-1. Uptake and concentration factor results are given in Table 1. A mean concentration factor of 630 ± 80 mLg-1 was determined from all data sets. This figure is less than the published concentration factor of 10000 mLg-1 as provided by IAEA 2004 for macrophytes, but
within the range of published data for a number of macroalgae species, ranging from 150 mLg-1 for Caulerpa racemosa (Sombrito et al 1982), to greater than 9400 mLg-1 for Chondrus crispus (Wilson et al 2005).
Depuration As for the uptake study, water activity concentrations measurements were taken initially taken at short intervals ranging from 1 minute to 5 minutes over the first 10 minutes of the experiment, with longer periods between measurements as the experiment progressed. Water activity concentration values were converted to 131I specific activity in C. nipae, and corrected for mass and radioactive decay. The specific activity results, in Bqg-1, were plotted against immersion time (Figure 4). Both sets (Tank A and Tank D) of depuration results gave a good statistical fit to a biphasic exponential loss model: at = A1.e-k1.t + A2.e-k2.t
[3]
where at is the activity concentration in Bqg-1 at a given time, A1 and A2 are the activity distribution concentrations for each of the two compartments, k1 and k2 are the respective excretion constants (min-1), and t is the immersion time in minutes. A correlation co-efficient of 0.951 was observed for Tank A and Tank D (duplicate) results (Table 2). Analysis of A1 and A2 values indicates that greater than 96% of the 131
I is retained in C. nipae after a fast initial depuration phase with the half time
estimated to be less than half a minute. The longer, second phase depuration half time is approximately 3.2 days.
These results suggest that C. nipae would be suitable for use as a bioaccumulator based sentinel monitoring system for radioiodine in estuarine waters due to the fast uptake rate and high iodine retention characteristics.
Environmental Monitors For the first experiment, the wet weight 131I activity concentration in the algae retrieved from the deployed sampling devices in the river ranged from 0.28 ± 0.03 Bqg-1 to 0.37 ± 0.04 Bqg-1 after approximately 6 hours immersion in tidal waters (Table 3). The average water activity, aw, over an elapsed time, te, can be estimated using: aw = at.(Km+te)/te /CF.1000
[4]
being derived from formula [1], where at is the wet weight activity concentration of the algae in Bqg-1, Km is the mean curvature constant of 160 (Table 1), te is the average immersion time of 363 minutes, CF is the concentration factor, with units of mLg-1, as determined using formula [2], and the factor of 1000 is used to convert the result to units of BqL-1. Using this data, the mean water activity concentration was calculated to be 0.7 ± 0.2 BqL-1. Direct water 131I water activity concentration, as determined by quantitative gamma spectrometry, varied between 0.5 ± 0.1 BqL-1 and 1.1 ± 0.2 BqL-1, with a mean value of 0.8 ± 0.6 BqL-1. Variations in the activity concentration can be attributed to the tidal nature of the sampling location, with water samples being taken on the flood, peak and ebb tides.
For the second experiment, the wet weight 131I activity concentrations in the algae retrieved from the deployed sampling devices in the effluent channel were 2.47 ± 0.18 Bqg-1, 6.70 ± 0.50 Bqg-1 and 6.30 ± 0.40 Bqg-1 after exposure times of 100 min, 220 min and 280 min respectively (Table 4). The water activity, aw, was determined for each time period as for experiment 1, using formula [4]. The effluent activity concentrations were calculated to be 10.2 ± 1.5 BqL-1, 18.4 ± 2.2 BqL-1 and 15.7 ± 1.9 BqL-1. Direct effluent 131I activity concentrations, as determined by quantitative gamma spectrometry, were 11.1 ± 0.8 BqL-1, 16 ± 1 BqL-1 and 17 ± 1 BqL-1 for elapsed time periods of 50 min, 110 min and 140 mins respectively.
Agreement between the modelled water and effluent 131I concentrations and measured activity concentration confirms that the monitoring method provides representative results under the measurement conditions stated. Depuration was not considered to have a significant impact on the monitor results under the deployment conditions.
Conclusions A series of radioactive iodine (131I) uptake and depuration experiments were conducted using the macroalgae C. nipae. The experiments were designed to establish 131I uptake, concentration factor and depuration characteristics of C. nipae, and establish if the macroalgae could be utilised as a means of monitoring iodine based radioactive waste in an aquatic environment. The results presented indicate that the Michaelis-Menton model adequately describes uptake of 131I by C. nipae, and that the uptake rate, as represented by the curvature constant, applies over the range of 131I water activity
concentrations used in this study. The iodine concentration factor was calculated to be 630 mLg-1 for C. nipae. This value falls within the wide range of published values for iodine concentration in macrophytes and more specifically macroalgae. Depuration results were characterised by a biphasic model with a fast initial elimination component with a half time of less than one minute, followed by a longer phase with a 3.2 day halftime. Greater than 96% of the 131I was retained after the initial phase. Uptake and depuration results were observed to be reproducible under laboratory conditions.
Suitability for environmental monitoring applications was assessed by deploying C. nipae based sampling devices in the Brisbane River estuary, Australia. The sampling devices were submerged and anchored in aquatic environments known to contain iodine based radiopharmaceutical wastes. Results from measurement of the 131I activity concentration in the macroalgae after an immersion, and application of the MichaelisMenton model using parameters determined in this study, compared favourably with direct water and effluent activity concentration measurements with the advantages of shorter radioanalytical counting periods and temporal averaging over the exposure. It is acknowledged that physiological (e.g. reproduction stages) and environmental parameters may affect C. nipae uptake & depuration characteristics (Ngan and Price 1980, Hirano et al 1983), and therefore concentration factors as determined in this study. The model will be applied to historical C. nipae activity datasets to estimate 131I activity in water, and establish baseline environmental data for comparison with future radioactivity concentrations in the estuary.
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Methodology for assessing the radiological consequences of routine
releases of radionuclides to the environment. European Commission Report No. EUR 15760. European Commission. Luxembourg.
Evans, G.J., Mirbod, S.M. and Jervis, R.E. (1993) The Volatilization of iodine species over dilute iodide solutions. The Canadian Journal of Chemical Engineering. 71, 761765.
Fenner, F.D. and Martin, J.E. (1997) Behavior of Na131I and meta(131I) Iodobenzylguanidine (MIBG) in municipal sewerage. Health Physics. 73, 333-339
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Table 1: Uptake experiment results. Total uncertainty values are quoted as 2σ (95%). Tank
Description
Initial solution
C. nipae
Km
Correlation
activity
Saturation
(min)
Co-efficient
(Bq.mL-1)
Activity
Concentration Factor – CF (mL.g-1)
(Bq.g-1) A
~ 10 g C. nipae
9.4 ± 0.3
760 ± 50
163
0.997
568
B
~ 10 g C. nipae
4.9 ± 0.1
380 ± 20
146
0.984
613
C
~ 10 g C. nipae
0.76 ± 0.04
63 ± 8
159
0.983
718
D
Duplicate - Tank A
9.5 ± 0.3
730 ± 40
155
0.998
601
E
Control1
9.3 ± 0.3
-
-
-
-
-
Mean results
-
-
160 ± 20
-
630 ± 80
NOTES: 1
Final control activity (decay corrected) = 9.2 ± 0.2
Table 2: Depuration experiment results. Total uncertainty values are quoted as 2σ (95%). Tank
Description
A1
k1
A2
k2
(Bq.g-1)
(min-1)
Bq.g-1
(min-1)
A
~ 10 g C. nipae
2.2E+01
1.7E+00
6.3E+02
7.4E-05
D
Duplicate - Tank A
3.0E+01
1.3E+00
7.1E+02
6.9E-05
-
Mean results
(2.6 ± 0.8)E+01
(1.5 ± 0.5)E+00
(6.7 ± 0.8)E+02
(7.2 ± 0.4)E-05
Experiment 1 - Environmental monitoring results for estimating 131I
Table 3:
concentration in estuary water using C. nipae sampling devices. Total uncertainty values are 2σ (95%).
Description
Deployment Time
Elapsed Time
C. nipae activity -1
(Bqg )
Modelled
Water activity
water activity
(BqL-1)
(BqL-1)
(min) Monitor 1
08:35 - 14:30
355
0.28 ± 0.03
0.64 ± 0.07
-
Monitor 2
08:40 - 14:45
365
0.30 ± 0.03
0.68 ± 0.07
-
Monitor 3
08:50 – 15:00
370
0.37 ± 0.04
0.84 ± 0.09
-
Water sample 11
08:15
-
-
-
0.7 ± 0.1
2
12:05
-
-
-
1.1 ± 0.2
3
Water sample 3
14:25
-
-
-
0.5 ± 0.1
Mean
-
363
-
0.7 ± 0.2
0.8 ± 0.6
Water sample 2
NOTES: 1 flood tide 2 high tide 3 ebb tide
Table 4:
Experiment 2 - Effluent monitoring results for estimating 131I
concentration using C. nipae sampling devices. Total uncertainty values are 2σ (95%).
Description
Deployment
Elapsed
C. nipae
Modelled
Effluent
Time
Time
activity
effluent activity
activity
-1
-1
(min)
(Bqg )
(BqL )
(BqL-1)
Monitor 1
09:05 - 10:45
100
2.47 ± 0.18
10.2 ± 1.5
-
Monitor 2
09:05 - 12:45
220
6.70 ± 0.50
18.4 ± 2.2
-
Monitor 3
09:05 – 14:25
280
6.30 ± 0.40
15.7 ± 1.9
-
Effluent sample 1
09:55
50
-
-
11.1 ± 0.8
Effluent sample 2
10:55
110
-
-
16 ± 1
Effluent sample 3
11:25
140
-
-
17 ± 1
Figure 1: Catenella nipae: a) attached to a mangrove pneumatophore, and b) as an individual plant.
a)
b)
Figure 2: C. nipae sampling device used for estimating 131I water concentration in an estuary
POLYSTYRENE FLOAT
VENTED HDPE CONTAINER HOLDING ~ 50 g ALGAE
ANCHOR
Figure 3:
131
I uptake by C.nipae showing experimental results (solid symbols) and
modelled data (open symbols) for three different water concentrations. Uncertainty is calculated at 2σ (95%).
UPTAKE TIME (minutes)
1500
1400
300
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
I-131 ACTIVITY IN ALGAE (Bq.g-1 wet weight) 800
700 TANK A & TANK D
600
500
400
TANK B
200
100
0 TANK C
Figure 4:
131
I elimination from C.nipae showing normalised, mean (Tank A and
Tank D) experimental results (solid symbols) and modelled data (open symbols + broken line).
1.00
INSET
0.90
NORMALISED 131I RETENTION
0.80 0.70 0.60 0.50
INSET
1.00
0.40 0.30 0.20 0.95
0.10
0
0.00 0
5
2000
10
15
4000
20
25
6000 8000 TIME (minutes)
10000
12000
