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Ageing impact on the transfer factor of 137Cs and 90Sr to lettuce and winter wheat
Journal of Environmental Radioactivity 164 (2016) 19e25
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Ageing impact on the transfer factor of winter wheat
137
Cs and
90
Sr to lettuce and
Lina Al Attar*, Mohammad Al-Oudat, Bassam Safia, Basem Abdul Ghani Department of Protection and Safety, Atomic Energy Commission of Syria, Damascus, P.O. Box 6091, Syria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 20 June 2016 Accepted 23 June 2016
The study focuses on long-term (extending from 1 to 10 years) lysimeter experiments of the transfer factor of 137Cs and 90Sr to lettuce and winter wheat crops. Transfer factors (Fvs) were the ratio of the activity concentrations of the radionuclides in crops to those in soil, both as dry weight (Bq kg1). Fvs of 137 Cs to lettuce decreased significantly with ageing; geometric means for the 1st, 2nd and 10th year contaminated soil were 0.114, 0.030 and 0.013, respectively. However, a significant decline of Fvs for 137Cs was only seen between the 1st and 2nd year for both wheat compartments (straw and grains) which disappeared thereafter. The dynamic of 137Cs Fvs may be explained according to the distribution coefficient experiment (Kd) which had a value of 3600 L kg1 showing a high affinity of the clay minerals for caesium. Desorption data revealed that Cs fixation enhanced with ageing. The mechanism involved may be an initial sorption of caesium species to the surface soil particles followed by progressive irreversible fixation to the interlayer of the porous clay minerals. Fvs of 90Sr were high and showed trivial variation for both crops for the time course studied. Sorption of Sr2þ species to the clay mineral may be the governing process, which was supported by high desorption percentage (ranged 77%) with low Kd, i.e. 10 L kg1. In general, higher Fvs of 137Cs and 90Sr for lettuce was observed in comparison to winter wheat. The diversity of plant species and root systems would play essential roles for such behaviours. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Transfer factor Aging Caesium Strontium Winter wheat Lettuce
1. Introduction Following nuclear weapons testing and nuclear power plant accidents, for instant Chernobyl in 1986 and Fukushima in 2011 (Steinhauser et al., 2014; Thielen, 2012; IAEA, 2010), radionuclides are released to the environment, especially the fission products 137 Cs and 90Sr. These long-lived contaminants form potential pathway of radiation to human health through food chain and contribute substantially to overall radiological dose. Transfer factor is a vital parameter to evaluate the radiological impact assessment and environmental safety studies. The soil-to-plant transfer factor accounts for the uptake of radionuclides via plant roots and represents the activity concentration ratio of the radionuclide per unit dry mass in the plant (Bq kg1) to that in the soil, designated as Fv (IAEA, 2009, 2010). The dynamic of Fv differ by provenance, hence, there has been an interest to determine Fv for various foodstuff worldwide. Empirical Fvs for most important agricultural products in much of Europe and the USA are already documented; however,
* Corresponding author. E-mail address: prscientifi[email protected] (L. Al Attar). http://dx.doi.org/10.1016/j.jenvrad.2016.06.019 0265-931X/© 2016 Elsevier Ltd. All rights reserved.
data for certain climate and soil types are not fully covered (Frissel et al., 2002; IAEA, 2006). The Atomic Energy Agency (IAEA) made extensive reviews on the radionuclide aggregated-transfer factors for a variety of foodstuff (IAEA, 2009, 2010). The data included were useful for routine release of radionuclides to the environment from nuclear facilities, in which equilibrium has been established between the movements of radionuclides into and out of the environment compartments. For cases dealing with acute contamination, such as nuclear accidents, equilibrium cannot be assumed, and the rate of transfer between compartments must be assumed to vary with time (IAEA, 2006, 2010). The migration and mobility characteristics of radionuclides in soil differ depending on the soil properties (soil pH, texture, concentrations of exchangeable calcium and potassium, organic matter content, etc.), climate conditions, plant species, land use and management practice (Fernandez et al., 2006; IAEA, 2006; Krouglov et al., 1997; Yamaguchi et al., 2007). On a long-term, there will be a continuous change of radionuclide bioavailability in the soil due to the radionuclide mobility (related to its soluble forms) and redistribution of the radionuclide in the root zone. Several investigations have been carried out to evaluate the impact of
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L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
ageing on the transfer factor of 137Cs and 90Sr (analogues of Kþ and Ca2þ, respectively) from contaminated soil to various plants (Choi et al., 2011; Djingova et al., 2005; Frissel et al., 2002; Gerstman and Schimmack, 2006; Gomez and Brown, 2015; Nisbet and Shaw, 1994a; Noordijk et al., 1992; Schimmack et al., 2004, 2007; Twining et al., 2004). There is little information on the transfer of anthropogenic radionuclides to plants in semiearid and arid regions in comparison to humid and temperate regions, with the fact that the climate in Syria is semi-arid. The work performed by Al-Oudat and Al-Asfary (2006) depicted the transfer factor and ageing impact of 137Cs and 90 Sr in local conditions to several groups of crops including cereals and leafy vegetables as well as orchard trees, i.e. olive and apricot trees and grape vines. Yassine et al. (2003) investigated the effect of ageing and soil characteristics on the transfer factor of 137Cs and 90 Sr to the edible part of cereals, leafy, non-leafy and leguminous vegetables. Recently, transfer factor of 90Sr and 137Cs to lettuce and winter wheat at different growth stage applications using lysimeters in an open field experiment has been released (Al Attar et al., 2015). The present study focuses on the effect of ageing, i.e. 1e10 years, on the transfer factor of 137Cs and 90Sr to lettuce (Lactuca sativa L.) and winter wheat (Triticum aestivum L.). The chosen crops have different physiological functioning and are representative of leafy vegetables and cereals, respectively, according to the IAEA classifications of plants (2009). In addition, wheat is one of the main contributors to the Syrian economy with an annual production of 3.1 million tonnes in 2010 (FAO STAT, 2015) and it forms a foremost portion of the daily-diet of many countries. Lettuce is one of the major leafy vegetables in the Mediterranean region and basically consumed throughout the year. The data acquired form a baseline for future radiological assessment studies for the influence of longterm contamination by these fission radionuclides. 2. Materials and methods 2.1. Experimental design and climate conditions The experiment was conducted using lysimeters in an open field at the agricultural station of the Atomic Energy Commission of Syria, viz. Soujeh, 28 km northwest of Damascus (33 470 latitude, 36 070 longitude, ca. 1100 m above the sea-level). The climate of the site is humid with annual rainfall of 311 mm and mean temperature of 14 C (January 6.2 C, July 25.3 C) with wind speed ranging 0.0e3.28 m s1 and air humidity 26.8e68.3%. To comply with radiological protection legislations, the site was lined with high-density polyethylene sheet. To avoid site disturbance by birds or animal incursion during the growing seasons, it was necessary to build an enclosed barbed wire chain-link fence, which was reinforced at the base. Each cylindrical lysimeter (40 cm diameter 50 cm height) was filled-up with ca. 40 kg of dry Incepisol soil. They were sown in December with seeds of cultivars of lettuce (Lactuca sativa L. var. local) and winter wheat (Triticum aestivum L. var. ACSAD), with a particular concern that this cultivation was the solo practice for the oldest soil during the 10 years. Sowing density was ca. 20 g m2. The experiment was performed without the addition of fertilizer in order to meet the local conditions at the area of study. Five replicates per crop and per year were carried out, giving a total of 10 lysimeters per year/treatment. They were distributed in randomised block design. Sowing and harvest were implemented according to the standard local agricultural scheme for one cycle of cultivation with manual remove of weeds. The total growth-period of the crops at the year of the experiment was 153 and 160 days for lettuce and wheat, respectively. Watering was performed on a
regular basis throughout the growth-period of the crops with an average rate of 1.5 L per week for each lysimeter that met the field capacity so that drought stress and water-leakage were avoided. 2.2. Soil characterisation The soil used was Inceptisol, which is Calcisols according to FAO WRB (2014). The soil is brown reddish and is the main agricultural soil in the country. The relative abundance of the clay minerals in the soil was determined using powder X-ray diffraction, XRD, (Stoe Transmission diffractometer, Model STADI-P). Soil pH was measured in soil/CaCl2 (0.01 M) solution 1:5. Soil texture was determined in accordance to particle size distribution (Black et al., 1984). The organic matter and cation exchange capacity (CEC) were analysed as described by Page et al. (1982). Ca2þ, Mg2þ, Naþ and Kþ were determined by Atomic Absorption Spectroscopy using Analytik Jena-AGVario 6. Using XRD, the most abundant fraction of clays size fraction is montmorillonite. The physical and chemical properties of soil are summarised in Table 1. According to IAEA classification of soil-type (IAEA, 2010), when clay content exceeds 35% the soil is clay-group. Since silt content was over 50%, it is a silty-clay soil. The neutral pH and low percentage of organic matter were the main characteristic features of the soil studied. 2.3. Deposition of radionuclides Each lysimeter of the 1st year was contaminated, before sowing, with 1 L of a mixed carrier free radioactive solution with activity of 148 ± 7 of 137Cs and 127 ± 5 kBq of 90Sr. The chosen activities of the radionuclides were similar to those used for the contamination of 2nd and 10th year lysimeters. Uncertainty of the activity concentrations of the stock solutions was estimated (according to Uncertainty Estimation No. PM-09/1, Atomic Energy Commission of Syria, AECS) at 95% confidence level and found to be ±6% for 137Cs and ±1.9% for 90Sr. Contamination was performed by pipetting the radioactive solution carefully and uniformly on the surface of each lysimeter in about 40 points. To prevent the radionuclides falling into, and concentrating within, soil cracks, 1 L water was applied to each lysimeter two days before the addition of the radioactive solution in order to stack the soil. This in turn ensured that the radioactive solution was spread evenly over the soil surface and less likely to resuspend by wind action. Homogenisation of the top 20 cm was performed manually.
Table 1 Mechanical and chemical properties of soil prior sowing (soils of the years studied). Analyst
Result
pH (in H2O) pH (in CaCl2) Soil size distribution Clay% Silt% Sand% Exchangeable cations (cmolc kg1) Kþ Naþ Ca2þ Mg2þ Cation exchange capacity, CEC (cmolc kg1) Soluble cations (cmolc kg1) Kþ Naþ Ca2þ Mg2þ Organic matter %
8.1 ± 0.3 7.5 ± 0.1 38.7 ± 8.3 51.0 ± 6.4 10.3 ± 1.9 1.3 ± 0.1 0.9 ± 0.1 20.0 ± 0.5 33.6 ± 0.7 55.8 0.30 ± 0.02 11.2 ± 0.9 18.1 ± 1.4 9.0 ± 0.7 0.8 ± 0.2
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
2.4. Sampling and preparation According to the common agricultural practices in the region, lettuce was harvested in May and winter wheat in June (code 51 and 92, respectively, Meier, 2001). Plants were manually cut at about 2 cm above the soil level. Caution was taken during harvest to prevent contaminating the crop with soil particles. Wheat crop was divided into two plant compartments, straw and grains. Gains were peeled prior to preparation and the husks were excluded from the study. Plant samples of both crops, including peeled grains, were gently washed with distilled water to rinse adhering particles of soil and husk dust. Thereafter, the plant materials were weighed fresh and then oven-dried at 60 C for about 2 weeks until reaching constant weights. Next, they were ground, mixed thoroughly using a Turbula mixer (Basel, Schweiz) to produce a homogenous sample and placed into cylindrical geometry of 18 mL for activity measurement. At harvest time, three soil cores were collected (with a 5 cm diameter corer) down to 20 cm depth in each lysimeter; so that a composite sample was prepared. Care was taken to avoid sampling close to the edges of the lysimeter or previously coring spots. Roots, small stones and other untypical materials were hand removed. Soil samples were dried (at 105 C for 72 h till constant weighing), ground, sieved through 500 mm (Retsch, Germany), homogenised and placed into cylindrical geometry for gamma spectrometry measurement.
2.5. Distribution coefficient and extraction experiments The Kd is commonly used as a means of assessing the mobility of radionuclides in the environment and for comparing sorption data obtained from different sources. Thus, sorption experiments were performed by placing an adequate weight (i.e. 0.5 g) of the dried soil in contact with 50 mL of 137Cs/85Sr-tracer solution into a polypropylene tube (solution: solid ratio 100). Equilibrium was achieved by rotating the tubes on a mineralogical roller for 24 h. This was accomplished after carrying out primary kinetic studies for 1e48 h (Al Attar et al., 2010; Twining et al., 2004). A time of 24 h was adequate to attain equilibrium between the two phases. The suspensions were then centrifuged for 15 min at 4000 rpm using Hettich Universal-3100 (Germany), followed by filtration through 0.45 mm Sartorius Minisart RC/SRP membranes. The pH of the filtrate was measured using Sartorius Basic meter PB-11, fitted with an Ag/AgCl electrode. The Activity concentrations of 137Cs and 85Sr in the supernatants were determined via gamma spectrometry. Sorption of radionuclides on the tube-wall was negligible since the recovery of blank experiments (without soil material) were found to range 98e101% for 137Cs and 94e101% for 85Sr. Sorption of the radionuclide on soil matrix was then estimated in terms of distribution coefficient (Kd) given in the following equation,
A A V t Kd mL g 1 ¼ i M At where Ai and At are, respectively, the initial and equilibrium solution activities of the radionuclide of interest, V/M is the volume-tomass ratio, representing the volume of the radionuclide solution (L) that can be treated by a given mass of the soil (kg). The distribution coefficients obtained in this study were the mean of two independent experiments. In order to study the radiocaesium and strontium sorption reversibility, desorption experiments were carried out in a similar manner to sorption but using NH4Cl reagent. Table 2 summarises Kd and desorption data.
21
Table 2 Soil-solution distribution coefficient (Kd, L kg1) for 137Cs and 85Sr and % desorption. Kd (L kg1)
137 85
Cs Sr
3635 ± 370 10.1 ± 1.8
% Desorption 1st year soil
2nd year soil
10th year soil
36 ± 7 72 ± 5
31 ± 6 75 ± 6
16 ± 2 83 ± 3
Values are the arithmetic mean of triplicates and ±2s.
2.6. Radiochemical measurements Gamma measurement was performed using HPGe detector (PeType, Eurisys, France), high resolution (1.85 keV at 1.33 MeV), high relative efficiency (80%) and fitted with InterWinner software (Version 4). Efficiency calibration was made using point source of 137 Cs with radioactivity of 40.33 kBq (reference date 1st Sep 1987, Amersham, UK). Counting was carried out to attain 1e5% statistical accuracy at the appropriate chosen geometry. However, because of the low activity concentration in grain samples, counting uncertainties were 5e10%. Minimum detectable activities (MDAs) of 137 Cs were derived from the background measurement at 100,000 s and found to be 0.1 and 6 Bq kg1 for soil and plant, respectively. Quality control was carried out using the reference materials Soil375 and Grass-373 provided by IAEA. In the case of 85Sr, CRM QCY48 supplied by AEA Technology, QSA GmbH (reference date 1st December 2008) was used for efficiency calibration. The activity of 90Sr in soil and plant samples was estimated ^ through its progeny 90Y via. Cerenkov after the radiochemical separation process, which is described in an earlier work (Al Attar et al., 2015). The method involves the separation of 90Sr by acid extraction, multiple precipitation and eliminating the radiochemical impurities. When secular equilibrium between 90Sr and 90Y was established, the latter was extracted and dissolved in HCl. The radiation of the beemitter of 90Y was determined using a Wallac 1414 (Perkin Elmer, Finland) liquid scintillation counting, efficiency of 56%. The lower limit of detection was 0.07 Bq kg1. Certified reference samples, i.e. Soil-375 and Grass-373 supplied by IAEA, were used to assure the quality control of the measurements. 2.7. Calculation of transfer factor The soil-to-plant transfer factors for 137Cs and 90Sr were calculated, for each lysimeter individually, according to the IAEA (2010) definition,
Fv ¼
Activity concentration of radionuclide in cropðdwÞ Activity concentration of radionuclide in soilðdwÞ
Radionuclides activities were decay-corrected to harvest date. Transfer factors were defined on dry weight bases for both of the crop and soil in order to reduce associated uncertainties (IAEA, 2009). The number of observations per treatment (year) of each crop is n ¼ 5. Thus, the annual Fv data of each year studied was expressed as geometric mean (GM), with the fact that the geometric mean indicates the central tendency of a number of data (IAEA, 2009). The uncertainties assigned to the geometric mean were estimated by the geometrical standard deviation (GSD) which is the exponent of the standard deviation of the natural logarithms of the individual values at a confidence interval of 95%. In addition, higher and lower limits and the uncertainty were calculated. The results are shown in Table 3. It is worth pointing out that the number of observations is critical for worldwide database analysis, as great caution should be taken when transfer factor is reported without an estimation of the extent of uncertainty or a range (IAEA, 2010).
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L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Table 3 Geometric mean, lower and upper limits and uncertainty of Fvs for Crop
Lettuce
Wheat-straw
Wheat-grains
137
Year
st
1 2nd 10th 1st 2nd 10th 1st 2nd 10th
137
Cs and
90
Sr in lettuce and wheat compartments. 90
Cs
GM
GSD
LCI
UCI
0.114 0.030 0.013 0.030 0.003 0.003 0.005 0.003 0.004
1.201 1.231 1.317 1.258 1.194 1.385 1.243 1.483 1.318
0.095 0.025 0.011 0.026 0.002 0.003 0.005 0.002 0.003
0.136 0.036 0.014 0.033 0.003 0.004 0.006 0.003 0.004
Un % 36 37 25 23 31 29 22 35 25
Sr
GM
GSD
LCI
UCI
0.977 1.033 1.033 0.053 0.043 0.047 0.017 0.016 0.016
1.227 1.172 1.092 1.389 1.229 1.157 1.140 1.170 1.115
0.882 0.898 0.956 0.045 0.036 0.041 0.015 0.014 0.014
1.082 1.187 1.116 0.063 0.052 0.053 0.019 0.018 0.017
Un % 20 28 15 33 36 26 26 28 19
GM is the geometric mean, GSD is the geometrical standard deviation, LCI and UCI are lower and upper limits at 95% confidence interval, Un % is the uncertainty percentage.
2.8. Statistical analysis Empirical Evidence based on the central locations of data showed that the arithmetic and geometric means and median were comparable. Applying the normality test as Shapiro-Wilk on log-Fv values, the data indicated normal distribution with pn values (i.e. p of the normality test) were greater than alpha (0.05). One-way analysis of variance (ANOVA) was performed to evaluate the significant difference for log-Fv values using Fisher’s test at a confidence interval of 95% via. XLSTAT (2015, Version 2015.3.01). The least significant differences (LSD) were calculated with a significant level of 5%, indicating whether the difference among the values is significant or not. The data are shown in Table 4. 3. Results and discussion 3.1. Distribution coefficient and desorption experiments Investigating the sorption behaviour of the radionuclides from simulated solution on the soil made it possible to evaluate Kd values and predict Fvs of 137Cs and 90Sr to the crops studied. Table 2 summarises batch distribution coefficients (Kd) and desorption proportion of the radionuclides of interest. The high Kd for 137Cs, i.e. 3.6 103 L kg1, may indicate a strong affinity of caesium to the clay content of soil, i.e. montmorillonite. However, 90Sr showed weak sorption to the clay mineralogy with Kd of around 10 L kg1. Kd values obtained were in the same order to those reported by GilGarcía et al. (2009) where the Kds are grouped according to soil texture and organic matter criterion, i.e. geometric mean of 95 and 5.5 103 L kg1 for Sr and Cs, respectively, in clay soil. Similarly, Twining et al. (2004) reported Kds in the order of 103 L kg1 for 134 Cs and in the range 30e60 L kg1 for 85Sr. It is well known (Al Attar et al., 2010) that montmorillonite sorbs cations at surface sites as well as interlayer positions by three possible mechanisms that are surface sorption, and/or ion exchange of the metal ions with ions on the surface and sorption/penetration through the crystal lattice. The first two sites would be available for both Table 4 Significant variation of log-Fv for
137
Cs and
Crop/plant compartment
nuclides. However, the interlayer sites would require the hydrated cations to denude their hydration shell in order to facilitate their bonding to the sorption interface. Monovalent cations, in respect to divalent ones, have larger ionic radii but smaller hydrated radii that make them easily strip their weak-hydration sphere and penetrate into the crystal lattice (Horvath, 1985). Therefore, Csþ would firmly bond to the clay mineral than Sr2þ. The radionuclide speciation in the solid phase might change with time, so an estimation of the change in the reversibility of the sorption in the short and medium term is also required in any experimental approach designed to derive information on sorption dynamics (Vidal et al., 2009). Data of desorption experiments (Table 2) were strongly supportive to the conclusion drawn earlier by batch distribution coefficients. Desorption proportion of 137Cs were 36, 31 and 16% for the 1st, 2nd and 10th year contaminated soils, respectively. The irreversible sorption of caesium (poorly hydrated monovalent cation) as a consequence of its penetration and fixation to the crystal lattice sites of the clay mineral might be responsible for the decrease of the extractable percentage of caesium. In the case of 90Sr, constant desorption with a mean of 77% inferred that sorption of the divalent cation to the surface sites may be the governing mechanism. 3.2. Transfer factor of
*
Significantly different at p 0.05,
**
Sr in lettuce
Sr to lettuce and winter wheat at various age of soil-contamination. pn values (Normality test)
Cs Sr Cs Sr Cs Sr
Winter wheat (grains)
90
Transfer factors values for 137Cs and 90Sr to lettuce according to ageing of soil contamination are illustrated in Fig. 1. Clearly, Fv of 137 Cs decreased continuously over time, while variation of Fv 90Sr among years were insignificantly different. Ageing seemed to have a great influence on the Fvs of 137Cs, which decreased with increasing time following contamination. The geometric mean of Fv for the 1st year was 0.114, became 0.03 at the 2nd year and reached 0.013 for the 10th year with uncertainty did not exceed 37% (see Table 3). The difference of Fvs along the time course studied was highly significant (p < 0.0001, Table 4). The results of the 2nd and 10th year were in agreement to previous findings (Al-Oudat and Al-Asfary, 2006; Yassine et al., 2003) where
p-values of the significance variation of log Fv for 90Sr and 137Cs with ageing 1st vs 2nd
Winter wheat (straw)
Cs and
90
Radionuclide
Lettuce (whole plant)
137
p 0.005,
0.180 0.534 0.200 0.371 0.164 0.891 ***
p 0.0005.
***
<0.0001 S 0.582 <0.0001 S*** 0.108 0.005 S** 0.608
1st vs 10th ***
<0.0001 S 0.581 <0.0001 S*** 0.249 0.080 0.054
2nd vs 10th <0.0001 S*** 0.998 0.305 0.609 0.147 0.133
LSD 0.140 0.088 0.145 0.136 0.179 0.098
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Fig. 1. Geometric mean of Fvs for (number of observation is n ¼ 5).
137
Cs and
90
Sr to lettuce as a function of ageing
Fv for cabbage, as a leafy vegetable, ranged between 0.02 and 0.01 after 2 and 3 years of contamination, respectively. The IAEA (2010) reference value for the Fv of 137Cs from clayey soil to leafy crops is 0.018 (ranging from 0.0005 to 0.72), so the data obtained in this study fell within the reference range. The decline seen of Fvs was by a factor of 4 after 2 years and by a factor of 9 after 10 years. Choi and co-authors (2011) who studied ageing effect on the transfer factor of 137Cs to soybean found it decreased by a factor of 3 after 4 years; however the authors used a sandy soil with 4% of clay content and acidic pH (i.e. 5.2). This confirms that soil characteristics (soil texture, organic matter content, exchangeable K and pH) play substantial roles in Fv of 137Cs in addition to the dissolved chemical forms of the radionuclide and their interaction with the soil matrix (IAEA, 2006). Herein, the behaviour of 137Cs at the 1st year might reflect a physiochemical process as desorption reached 36% for 137 Cs. In other words, the radionuclide species may be sorbed on the planer surface of soil particles by initial reversible sorption. Later, the continuous decrease in Fv at the 2nd and 10th year contaminated soils was associated with the fixation of 70% and over 84% of 137Cs respectively (as shown by desorption experiments, see Table 2). The findings were consistent with those reported by Djingova et al., 2005 who stated that fixation of 80% of radiocaesium to the resistant phase of soil occurred after 2-year of soil contamination. The drop of Fvs observed at the 2nd and 10th years soils would mirror the diffusion of Csþ, as monovalent cation, to the interlayer sites of the porous montmorillonite mineral, i.e. retention of 137Cs by the crystal lattice of the clay with Kd measurement was as high as 3600 L kg1. Consequently, there would be a decrease in the bioavailability of 137Cs to plant uptake. The dynamic of 137Cs Fvs with time would be a reflection of increasing the irreversible fixation of the radionuclide, which is estimated to be about 15% per year, taking into account that the percentage may be much higher or lower according to the soil characteristics and systems (IAEA, 2006). In contrast to 137Cs, no dynamic of Fv for 90Sr as a function of ageing was observed. The average geometric mean value of Fv over the years was ca. 1.01 with uncertainty ranging 15e28% (Table 3). An earlier study carried out by Yassine and co-authors (2003) also showed insignificant difference in the transfer factor of 90Sr for cabbage, as a leafy vegetable, after three years. Noordijk and workers (1992) investigated the impact of ageing for several crops in lysimeter experiments, including spinach and cabbage on clay, loam and sandy soils; they found that the transfer of 90Sr did not change over a 7 year period. Usually, strontium is mainly fixed to the organic matter in the soil by complexation and/or ion exchange (IAEA, 2009; Yamaguchi et al., 2007); however, the soil used in this study was poor in organic matter (1%). The low Kd value, i.e. 10 L kg1 and high desorption proportion of 77% for the three contaminated soils (1st, 2nd and 10th year, Table 2) confirmed the
23
constant and high Fv values obtained. This might mirror instant reversible sorption of Sr2þ species that are dominant in natural waters over a broad pH range, i.e. 2e9 (IAEA, 2009) with the fact that the pH of soil studied was ca. 8 (see Table 1). It is documented (Vidal et al., 2009) that in most soil types Sr does not undergo complexation in soil-solution and hence remains in the exchangeable form. Meaning, radiostrontium was available for root uptake throughout the time course studied. It was notable that Fvs of 90Sr were much enhanced in comparison to those of 137Cs, by a factor of 9 in the 1st year and a factor of 80 for the 10 year contaminated-soil. As found with Twining et al. (2004) the higher Fv values for 85Sr compared with 134Cs are, generally, consistent with the overall lower Kds for Sr. In this study, sorption experiments indicated high Kd for 137Cs ca. 3600 L kg1 and low extractable proportion (desorption) in comparison with 90 Sr. Although Kd values in laboratory batch experiments should not be assumed to be identical to field sorption values, this type of partitioning information can be used to compare the likely mobility of radionuclides in the soil-plant environment and initial retardation of radionuclide by the soil type (Sheppard and Thibault, 1990).
3.3. Transfer factor of
137
Cs and
90
Sr in winter wheat
Fvs values of 137Cs and 90Sr to wheat-straw and grains according to ageing of soil contamination are demonstrated in Figs. 2 and 3, respectively. As for lettuce, Fvs 90Sr were constant along the timescale studied. Fvs of 137Cs showed a significant decrease only between the 1st and 2nd year for both straw and grains. In the case of wheat-straw, a distinct drop of one order of magnitude in the Fvs of 137Cs was seen between the 1st and 2nd years of soil contamination, accompanied with geometric means of 0.030 and 0.003 respectively (p value < 0.0001). No change was observed in the Fv of 137Cs after 10 years (geometric mean 0.003 with uncertainty of 29%, Table 3). A similar trend was reported by Djingova et al. (2005) when they studied the transfer factors of 137 Cs to wheat-straw for three successive years, with mean Fvs of 0.055, 0.032 and 0.035, respectively. In our study, Fv of the 1st year for wheat-straw was in the range stated in the literature for cereals i.e. 0.015e0.1 (Frissel et al., 2002) which is characteristic for medium nutrient soil with pH > 4.8. In addition, the obtained geometric means for 137Cs for the time-lag studied was in the range of IAEA database for cereals in clayey soil, i.e. 0.0043e0.53 (IAEA, 2010). Squire and Middleton (1966) estimated the decrease of the transfer factor of 137Cs with ageing for various crops to be a factor of 4 after 20 years on clayey soil. The dynamic of Fv in this study may be explained by the same scenario previously stated for lettuce, where initial rapid sorption of Cs may occur at the surface soil particles. Afterwards, irreversible fixation of Cs on clay minerals
Fig. 2. Geometric mean of Fvs for 137Cs and 90Sr to winter wheat-straw as a function of ageing (number of observation is n ¼ 5).
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L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Fig. 3. Geometric mean of Fvs for 137Cs and 90Sr to winter wheat-grains as a function of ageing (number of observation is n ¼ 5).
may take place by ion exchange to a number of sites that are accessible only to poorly hydrated cations and show high selectivity for Csþ over Kþ. This was reflected by high Kd derived and low desorption proportion. Fig. 2 shows that Fvs of 90Sr did not varied with ageing. Fvs after 1, 2 and 10 years of soil contamination were 0.053, 0.043 and 0.047 (uncertainty of 33, 36 and 26%), respectively, associated with insignificant difference (see Table 4). The findings were in agreement to earlier work performed by Yassine et al. (2003) and Noordijk et al. (1992) who stated a trivial change of the transfer factor of 90Sr to several crops when time elapsed increased up to three and seven years, respectively, between contamination and harvest. Nisbet and Shaw (1994a) also found that there were insignificant differences in the concentration ratios of 90Sr for barely, cabbage and carrot crops grown in peat soil over 5-year study. Based on Kd and desorption tests performed (i.e. 10 L kg1 and 77%, respectively) Sr would be available for root uptake along the time of the study. For grains (Fig. 3), both radionuclides gave lower Fvs when compared to wheat straw, which was similar to the findings of others (Nisbet and Shaw, 1994b; Yassine et al., 2003). However, Fvs of 137Cs and 90Sr show comparable trends to those for wheat-straw, with decrease in Fvs for 137Cs between the 1st and 2nd year (Fvs were 0.005 and 0.003, respectively, with p value of 0.005) and no significantly variation for 90Sr over the timescale studied (ranged 0.016e0.017). The comparable trends for wheat straw and grains would be based on the fact that the radionuclides existed in grains will mainly translocate from the foliar part (Nisbet and Shaw, 1994b; IAEA, 2009). The Fvs obtained for 137Cs in wheat-grains were consistent with the literature values (Al-Oudat and AlAsfary, 2006) i.e. 0.001 for edible parts of wheat on the same silty-clay soil and to the recorded value of 0.002 in wheat-grains after 2 or 3 years (Djingova et al., 2005; Yassine et al., 2003). The IAEA (2010) Fv in cereals grains for clayey soil ranges 2 104e9 102 and 0.0053e0.71 for 137Cs and 90Sr, respectively. Although Cs has higher biological mobility than Sr within the plant, Fv of 90Sr in grains was almost one order of magnitude higher than those of 137Cs, which may reflect enhanced concentration of 90Sr in lettuce leaves or wheat-straw (IAEA, 2009).
3.4. Comparison of transfer factor of winter wheat-straw
137
Cs and
90
Sr to lettuce and
Analysis of data indicated that Fvs of 90Sr always exceeded those of 137Cs and both radionuclides revealed higher Fvs to lettuce compared to winter-wheat straw. The higher uptake of 90Sr with regard to the crop groups may depend on the different chemistry of
the respective radionuclides and their interactions with soil components. Since Csþ being more firmly bound to the crystal lattice of the clay minerals than Sr2þ, the latter remained available in the soil along the time. The higher Fvs of the radionuclides in lettuce compared to winter-wheat may be related to the diversity of plant species and mainly the root system. Lettuce has massive, dense and superficial taproots whereas winter-wheat is characterised by fibrous roots that is less intense at the top 20 cm of soil surface (Strebl et al., 2009; Thorup-Kristensen et al., 2009). This would mirror the higher uptake of the radionuclides by the roots of lettuce and consequently higher Fvs, with the fact that the activity concentrations of the radionuclides usually remain in the top 20 cm of the soil surface for all crops, with an exception to grass, i.e. 10 cm (IAEA, 2010). In a similar manner, Twining et al. (2004) reported higher Fvs of 134Cs and 85Sr to mung leaf (leguminous vegetable) in comparison to sorghum (cereals). They inferred that the lower radioactivity uptake to sorghum is due to the deep root penetration to a depth of 1.5 m or more whilst mung bean roots are generally located within the top 20 cm. The trend of Fv for 137Cs in lettuce was somewhat different to that noted in wheat-straw (see Figs. 1 and 2); it decreased according to age of the contamination in the soil for the former plant while it remained constant after the 2nd year for the latter. The type of the root system would again influence the transfer factors. Lettuce roots might detect the lessen concentration of Cs in the soil after the 2nd year while it would be hardly sensed by wheat roots. In parallel, Djingova et al. (2005) stated that the behaviour of 137Cs would differ according to the crop and found that the dependency of the transfer factor on 137Cs concentration in soil was clear for cabbage but not for wheat. Despite that no explanation was given, the authors concluded that cabbage, as leafy vegetable, might be considered a bioindicator for soil contamination. Concerning 90Sr, no change of Fvs with time was observed in this study, giving comparable trends for both crops but higher on lettuce than wheatstraw. These findings supported that Sr2þ were available for root uptake through the timescale studied and that the characters of root distribution in soil influence the radionuclide transfer factors (IAEA, 2010).
4. Conclusion The data obtained indicated that Fvs of 137Cs decreased significantly for lettuce, as leafy vegetable, with ageing of soil contamination. Significant decline was only observed between the 1st and 2nd years and vanished thereafter for winter wheat (straw and grains). The high affinity of clay minerals for 137Cs was associated with high Kd (ca. 3600 L kg1). Sorption reversibility experiments revealed low desorption proportion of Cs from the 1st, 2nd and 10th year contaminated soils (ranging 36e16%). Rapid sorption of 137Cs to soil surface particles followed by retention of 137Cs by the crystal lattice of the clay mineral after the 1st year may be responsible for the behaviour noted of caesium. In contrast, Fvs of 90Sr showed little variation for both crops over the timeframe studied, with Fvs higher in lettuce than wheat straw. The dominance of Sr2þ species in the soil-solution system might maintain the bioavailability of radiostrontium for root uptake with Kd around 10 L kg1 and average desorption as high as 77%, indicating that reversible sorption was the governing mechanism. The low Fvs observed for both radionuclides in wheat could be attributed to its fibrous and deep penetrating root system which differ to lettuce massive, dense and superficial roots. The Fvs of 137Cs and 90Sr to the group crops studied were in the range reported in the literature and fitted to IAEA database characteristic to clayey soil and semi-arid environment.
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Acknowledgment Great appreciation is due to Prof. I. Othman (Director General of AECS) for his invaluable encouragement and funding the project. The ex-head of the Chemistry Department Dr. T. Yassine is acknowledged for providing 90Sr. The authors would like to thank W. Doubal and Mr. M. Hasan for gamma measurements, Mr. A. Nashawati for analysis by liquid scintillation counter and Ms. Rolana Bouzou for the analysis by Atomic Absorption Spectroscopy. Prof. A. Dyer is sincerely thanked for reviewing the English language of the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvrad.2016.06.019. References Al Attar, L., Al-Oudat, M., Safia, B., Abdul Ghani, B., 2015. Transfer factor of 90Sr and 137 Cs to lettuce and winter wheat at different growth stage applications. J. Environ. Radioact. 150, 104e110. Al Attar, L., Al Masri, M.S., Budeir, Y., Al Chayah, O., 2010. Sorption of 226Ra from waste effluents using Syrian bentonite. Environ. Technol. 31, 1587e1599. Al-Oudat, M., Al-Asfary, F., 2006. Transfer factor of caesium-137 in arid and semiarid regions. In: IAEA, Classification of Soil Systems on the Basis of Transfer Factors of Radionuclides from Soil to Reference Plants. Technical Document No. 1497. IAEA, Vienna, pp. 139e144. Black, C.A., Evans, D.D., Ensminger, L.E., White, J.L., Clarck, F.E., 1984. Methods of Soil Analysis, I. Physical and Mineralogical Properties Including Statistics of Measurements and Sampling. American Society of Agronomy, Madison, WI. Choi, Y.H., Lim, K.M., Jun, I., Keum, D.K., Han, M.H., 2011. Time-Dependent transfer of 54 Mn, 60Co, 85Sr and 137Cs from a sandy soil to soybean plants. Prog. Nucl. Sci. Technol. 1, 392e395. Djingova, R., Kovacheva, P., Todorov, B., Zlateva, B., Kuleff, I., 2005. On the influence of soil properties on the transfer of 137Cs from two soils (Chromic Luvisol and Eutric Fluvisol) to wheat and cabbage. J. Environ. Radioact. 82, 63e79. FAO STAT (International wheat production statistics). Accessed on line: 15th July 2015: https://en.wikipedia.org/wiki/International_wheat_production_statistics. FAO WRB, 2014. World reference base for soil resources updated 2015. Accessed on line: 18th January 2016. http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/ nrcs142p2_050915.pdf. Fernandez, J.M., Piault, E., Macouillard, D., Juncos, C., 2006. Forty years of 90Sr in situ migration: importance of soil characterisation in modelling transport phenomena. J. Environ. Radioact. 87, 209e226. Frissel, M.J., Deb, D.L., Fathony, M., Lin, Y.M., Mollah, A.S., Ngo, N.T., Othman, I., Robison, W.L., Skarlou-Alexiou, V., Topcuoglu, S., Twining, J.R., Uchida, S., Wasserman, M.A., 2002. Generic values for soil-to-plant transfer factors of radiocesium. J. Environ. Radioact. 58, 113e128. Gerstman, U.C., Schimmack, W., 2006. Soil-to-grain transfer of fallout 90Sr for 28 winter wheat. Radiat. Environ. Biophys. 45, 187e194. Gil-García, C., Rigol, A., Vidal, M., 2009. New best estimates for radionuclide solidliquid distribution coefficients in soils. Part 1: radiostrontium and radiocaesium. J. Environ. Radioact. 100, 690e696. Gomez, A.E., Brown, J., 2015. Determination of Root Uptake to Vegetables Grown in Soil Contaminated for Twenty-five Years. Accessed on line: 18th April 2015. https://www.gov.uk/government/uploads/system/uploads/attachment_data/ file/337136/HPA-CRCE-047_for_website.pdf. Horvath, A.L., 1985. Handbook of Aqueous Electrolyte Solutions. Ellis Horwood Ltd.,
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New York, p. 344. IAEA, 2006. Classification of Soil Systems on the Basis of Transfer Factors of Radionuclides from Soil to Reference Plants. IAEA, Vienna. Technical Document No. 1497. IAEA, 2009. Quantification of Radionuclide Transfer in Terrestrial and Freshwater Environments for Radiological Assessments. IAEA, Vienna. TecDoc-1616. IAEA, 2010. Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Terrestrial and Freshwater Environments. IAEA, Vienna. Technical Report Series No. 472. Krouglov, S.V., Filipas, A.S., Alexakhin, R.M., Arkhipov, N.P., 1997. Long-term study on the transfer of 137Cs and 90Sr from Chernobyl-contaminated soils to grain crops. J. Environ. Radioact. 34, 267e286. Meier, M. BBCH Monograph, 2001. Growth Stages of Mono and Dicotyledonous Plants, second ed. Federal Biological Research Centre for Agriculture and Forestry. Accessed on line: 29th July 2015. http://www3.syngenta.com/country/ se/SiteCollectionDocuments/Crop%20Protection/BBCH-Skala-English.pdf. Nisbet, A.F., Shaw, S., 1994a. Summary of a 5-year lysimeter study on the time dependent transfer of 137Cs, 90Sr, 239,240Pu and 241Am to crops from three contrasting soil types. 1 Transfer to edible portion. J. Environ. Radioact. 23, 1e17. Nisbet, A.F., Shaw, S., 1994b. Summary of a five-year lysimeter study on the time dependent transfer of 137Cs, 90Sr, 239,240Pu and 241Am to crops from three contrasting soil types. 2 Distribution between different plant parts. J. Environ. Radioact. 23, 171e187. Noordijk, H., van Bergeijk, K.E., Lembrechts, J., Frissel, M.J., 1992. Impact of ageing and weather conditions on soil-to-plant transfer of radiocesium and radiostrontium. J. Environ. Radioact. 15, 277e286. Page, A.L., Miller, R.H., Keency, D.R., 1982. Methods of Soil Analysis. American Society of Agronomy Inc. Publisher, Madison, Wisconsin, pp. 771e1159. €pp, V., Schimmack, W., Gerstman, U.C., Schultz, W., Sommer, M., Tscho Zimmermann, G., 2007. Intra-cultivar variability of the soil-to-grain transfer of 137 90 fallout Cs and Sr for winter wheat. J. Environ. Radioact. 94, 16e30. Schimmack, W., Zimmermann, G., Sommer, M., Dietl, F., Schultz, W., Paretzke, H.G., 2004. Soil-to-grain transfer of fallout 137Cs for 28 winter wheat cultivars as observed in field experiments. Radiat. Environ. Biophys. 42, 275e284. Sheppard, M.I., Thibault, D.H., 1990. Default soil solid/liquid partition coefficients, Kds, for four major soil types: a compendium. Health Phys. 59, 471e482. Strebl, F., Gerzabek, M., Kirchner, G., Ehlken, S., Bossew, P., 2009. Vertical migration of radionuclides in undisturbed soils. In: Quantification of Radionuclide Transfer in Terrestrial and Freshwater Environments for Radiological Assessments. IAEA, Vienna, pp. 103e120. TecDoc-1616. Squire, H.M., Middleton, L.J., 1966. Behaviour of 137Cs in soils and pastures, a longterm experiment. Radiat. Bot. 6, 413e423. Steinhauser, G., Brandl, A., Johnson, T.E., 2014. Comparison of the Chernobyl and Fukushima nuclear accidents: a review of the environmental impacts. Sci. Total Environ. 470471, 800e817. Thielen, H., 2012. The Fukushima Daiichi nuclear accidentean overview. Health Phys. 103, 169e174. n Cortasa, M., Loges, R., 2009. Winter wheat roots Thorup-Kristensen, K., Salmero grow twice as deep as spring wheat roots, is this important for N uptake and N leaching losses? Plant Soil 322, 101e114. Twining, J.R., Payne, T.E., Itakura, T., 2004. Soilewater distribution coefficients and plant transfer factors for 134Cs, 85Sr and 65Zn under field conditions in tropical Australia. J. Environ. Radioact. 71, 71e87. Vidal, M., Rigol, A., Gil-García, C.J., 2009. Soil-radionuclides interactions. In: Quantification of Radionuclide Transfer in Terrestrial and Freshwater Environments for Radiological Assessments. IAEA, Vienna, pp. 71e97. TecDoc-1616. XLSTAT, (Addinsoft™) version 2015.3.01.19349. Available from http://www.xlstat. com/en/download.html. Yamaguchi, N., Seki, K., Komamura, M., Kurishima, K., 2007. Long-term mobility of fallout 90Sr in ploughed soil, and 90Sr uptake by wheat grain. Sci. Total Environ. 372, 595e604. Yassine, T., Othman, I., Al-Oudat, M., 2003. Transfer of 137Cs and 90Sr from typical Syrian soil to crops. J. Food Phys. XVI, 73e79.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Ageing impact on the transfer factor of winter wheat
137
Cs and
90
Sr to lettuce and
Lina Al Attar*, Mohammad Al-Oudat, Bassam Safia, Basem Abdul Ghani Department of Protection and Safety, Atomic Energy Commission of Syria, Damascus, P.O. Box 6091, Syria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 20 June 2016 Accepted 23 June 2016
The study focuses on long-term (extending from 1 to 10 years) lysimeter experiments of the transfer factor of 137Cs and 90Sr to lettuce and winter wheat crops. Transfer factors (Fvs) were the ratio of the activity concentrations of the radionuclides in crops to those in soil, both as dry weight (Bq kg1). Fvs of 137 Cs to lettuce decreased significantly with ageing; geometric means for the 1st, 2nd and 10th year contaminated soil were 0.114, 0.030 and 0.013, respectively. However, a significant decline of Fvs for 137Cs was only seen between the 1st and 2nd year for both wheat compartments (straw and grains) which disappeared thereafter. The dynamic of 137Cs Fvs may be explained according to the distribution coefficient experiment (Kd) which had a value of 3600 L kg1 showing a high affinity of the clay minerals for caesium. Desorption data revealed that Cs fixation enhanced with ageing. The mechanism involved may be an initial sorption of caesium species to the surface soil particles followed by progressive irreversible fixation to the interlayer of the porous clay minerals. Fvs of 90Sr were high and showed trivial variation for both crops for the time course studied. Sorption of Sr2þ species to the clay mineral may be the governing process, which was supported by high desorption percentage (ranged 77%) with low Kd, i.e. 10 L kg1. In general, higher Fvs of 137Cs and 90Sr for lettuce was observed in comparison to winter wheat. The diversity of plant species and root systems would play essential roles for such behaviours. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Transfer factor Aging Caesium Strontium Winter wheat Lettuce
1. Introduction Following nuclear weapons testing and nuclear power plant accidents, for instant Chernobyl in 1986 and Fukushima in 2011 (Steinhauser et al., 2014; Thielen, 2012; IAEA, 2010), radionuclides are released to the environment, especially the fission products 137 Cs and 90Sr. These long-lived contaminants form potential pathway of radiation to human health through food chain and contribute substantially to overall radiological dose. Transfer factor is a vital parameter to evaluate the radiological impact assessment and environmental safety studies. The soil-to-plant transfer factor accounts for the uptake of radionuclides via plant roots and represents the activity concentration ratio of the radionuclide per unit dry mass in the plant (Bq kg1) to that in the soil, designated as Fv (IAEA, 2009, 2010). The dynamic of Fv differ by provenance, hence, there has been an interest to determine Fv for various foodstuff worldwide. Empirical Fvs for most important agricultural products in much of Europe and the USA are already documented; however,
* Corresponding author. E-mail address: prscientifi[email protected] (L. Al Attar). http://dx.doi.org/10.1016/j.jenvrad.2016.06.019 0265-931X/© 2016 Elsevier Ltd. All rights reserved.
data for certain climate and soil types are not fully covered (Frissel et al., 2002; IAEA, 2006). The Atomic Energy Agency (IAEA) made extensive reviews on the radionuclide aggregated-transfer factors for a variety of foodstuff (IAEA, 2009, 2010). The data included were useful for routine release of radionuclides to the environment from nuclear facilities, in which equilibrium has been established between the movements of radionuclides into and out of the environment compartments. For cases dealing with acute contamination, such as nuclear accidents, equilibrium cannot be assumed, and the rate of transfer between compartments must be assumed to vary with time (IAEA, 2006, 2010). The migration and mobility characteristics of radionuclides in soil differ depending on the soil properties (soil pH, texture, concentrations of exchangeable calcium and potassium, organic matter content, etc.), climate conditions, plant species, land use and management practice (Fernandez et al., 2006; IAEA, 2006; Krouglov et al., 1997; Yamaguchi et al., 2007). On a long-term, there will be a continuous change of radionuclide bioavailability in the soil due to the radionuclide mobility (related to its soluble forms) and redistribution of the radionuclide in the root zone. Several investigations have been carried out to evaluate the impact of
20
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
ageing on the transfer factor of 137Cs and 90Sr (analogues of Kþ and Ca2þ, respectively) from contaminated soil to various plants (Choi et al., 2011; Djingova et al., 2005; Frissel et al., 2002; Gerstman and Schimmack, 2006; Gomez and Brown, 2015; Nisbet and Shaw, 1994a; Noordijk et al., 1992; Schimmack et al., 2004, 2007; Twining et al., 2004). There is little information on the transfer of anthropogenic radionuclides to plants in semiearid and arid regions in comparison to humid and temperate regions, with the fact that the climate in Syria is semi-arid. The work performed by Al-Oudat and Al-Asfary (2006) depicted the transfer factor and ageing impact of 137Cs and 90 Sr in local conditions to several groups of crops including cereals and leafy vegetables as well as orchard trees, i.e. olive and apricot trees and grape vines. Yassine et al. (2003) investigated the effect of ageing and soil characteristics on the transfer factor of 137Cs and 90 Sr to the edible part of cereals, leafy, non-leafy and leguminous vegetables. Recently, transfer factor of 90Sr and 137Cs to lettuce and winter wheat at different growth stage applications using lysimeters in an open field experiment has been released (Al Attar et al., 2015). The present study focuses on the effect of ageing, i.e. 1e10 years, on the transfer factor of 137Cs and 90Sr to lettuce (Lactuca sativa L.) and winter wheat (Triticum aestivum L.). The chosen crops have different physiological functioning and are representative of leafy vegetables and cereals, respectively, according to the IAEA classifications of plants (2009). In addition, wheat is one of the main contributors to the Syrian economy with an annual production of 3.1 million tonnes in 2010 (FAO STAT, 2015) and it forms a foremost portion of the daily-diet of many countries. Lettuce is one of the major leafy vegetables in the Mediterranean region and basically consumed throughout the year. The data acquired form a baseline for future radiological assessment studies for the influence of longterm contamination by these fission radionuclides. 2. Materials and methods 2.1. Experimental design and climate conditions The experiment was conducted using lysimeters in an open field at the agricultural station of the Atomic Energy Commission of Syria, viz. Soujeh, 28 km northwest of Damascus (33 470 latitude, 36 070 longitude, ca. 1100 m above the sea-level). The climate of the site is humid with annual rainfall of 311 mm and mean temperature of 14 C (January 6.2 C, July 25.3 C) with wind speed ranging 0.0e3.28 m s1 and air humidity 26.8e68.3%. To comply with radiological protection legislations, the site was lined with high-density polyethylene sheet. To avoid site disturbance by birds or animal incursion during the growing seasons, it was necessary to build an enclosed barbed wire chain-link fence, which was reinforced at the base. Each cylindrical lysimeter (40 cm diameter 50 cm height) was filled-up with ca. 40 kg of dry Incepisol soil. They were sown in December with seeds of cultivars of lettuce (Lactuca sativa L. var. local) and winter wheat (Triticum aestivum L. var. ACSAD), with a particular concern that this cultivation was the solo practice for the oldest soil during the 10 years. Sowing density was ca. 20 g m2. The experiment was performed without the addition of fertilizer in order to meet the local conditions at the area of study. Five replicates per crop and per year were carried out, giving a total of 10 lysimeters per year/treatment. They were distributed in randomised block design. Sowing and harvest were implemented according to the standard local agricultural scheme for one cycle of cultivation with manual remove of weeds. The total growth-period of the crops at the year of the experiment was 153 and 160 days for lettuce and wheat, respectively. Watering was performed on a
regular basis throughout the growth-period of the crops with an average rate of 1.5 L per week for each lysimeter that met the field capacity so that drought stress and water-leakage were avoided. 2.2. Soil characterisation The soil used was Inceptisol, which is Calcisols according to FAO WRB (2014). The soil is brown reddish and is the main agricultural soil in the country. The relative abundance of the clay minerals in the soil was determined using powder X-ray diffraction, XRD, (Stoe Transmission diffractometer, Model STADI-P). Soil pH was measured in soil/CaCl2 (0.01 M) solution 1:5. Soil texture was determined in accordance to particle size distribution (Black et al., 1984). The organic matter and cation exchange capacity (CEC) were analysed as described by Page et al. (1982). Ca2þ, Mg2þ, Naþ and Kþ were determined by Atomic Absorption Spectroscopy using Analytik Jena-AGVario 6. Using XRD, the most abundant fraction of clays size fraction is montmorillonite. The physical and chemical properties of soil are summarised in Table 1. According to IAEA classification of soil-type (IAEA, 2010), when clay content exceeds 35% the soil is clay-group. Since silt content was over 50%, it is a silty-clay soil. The neutral pH and low percentage of organic matter were the main characteristic features of the soil studied. 2.3. Deposition of radionuclides Each lysimeter of the 1st year was contaminated, before sowing, with 1 L of a mixed carrier free radioactive solution with activity of 148 ± 7 of 137Cs and 127 ± 5 kBq of 90Sr. The chosen activities of the radionuclides were similar to those used for the contamination of 2nd and 10th year lysimeters. Uncertainty of the activity concentrations of the stock solutions was estimated (according to Uncertainty Estimation No. PM-09/1, Atomic Energy Commission of Syria, AECS) at 95% confidence level and found to be ±6% for 137Cs and ±1.9% for 90Sr. Contamination was performed by pipetting the radioactive solution carefully and uniformly on the surface of each lysimeter in about 40 points. To prevent the radionuclides falling into, and concentrating within, soil cracks, 1 L water was applied to each lysimeter two days before the addition of the radioactive solution in order to stack the soil. This in turn ensured that the radioactive solution was spread evenly over the soil surface and less likely to resuspend by wind action. Homogenisation of the top 20 cm was performed manually.
Table 1 Mechanical and chemical properties of soil prior sowing (soils of the years studied). Analyst
Result
pH (in H2O) pH (in CaCl2) Soil size distribution Clay% Silt% Sand% Exchangeable cations (cmolc kg1) Kþ Naþ Ca2þ Mg2þ Cation exchange capacity, CEC (cmolc kg1) Soluble cations (cmolc kg1) Kþ Naþ Ca2þ Mg2þ Organic matter %
8.1 ± 0.3 7.5 ± 0.1 38.7 ± 8.3 51.0 ± 6.4 10.3 ± 1.9 1.3 ± 0.1 0.9 ± 0.1 20.0 ± 0.5 33.6 ± 0.7 55.8 0.30 ± 0.02 11.2 ± 0.9 18.1 ± 1.4 9.0 ± 0.7 0.8 ± 0.2
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
2.4. Sampling and preparation According to the common agricultural practices in the region, lettuce was harvested in May and winter wheat in June (code 51 and 92, respectively, Meier, 2001). Plants were manually cut at about 2 cm above the soil level. Caution was taken during harvest to prevent contaminating the crop with soil particles. Wheat crop was divided into two plant compartments, straw and grains. Gains were peeled prior to preparation and the husks were excluded from the study. Plant samples of both crops, including peeled grains, were gently washed with distilled water to rinse adhering particles of soil and husk dust. Thereafter, the plant materials were weighed fresh and then oven-dried at 60 C for about 2 weeks until reaching constant weights. Next, they were ground, mixed thoroughly using a Turbula mixer (Basel, Schweiz) to produce a homogenous sample and placed into cylindrical geometry of 18 mL for activity measurement. At harvest time, three soil cores were collected (with a 5 cm diameter corer) down to 20 cm depth in each lysimeter; so that a composite sample was prepared. Care was taken to avoid sampling close to the edges of the lysimeter or previously coring spots. Roots, small stones and other untypical materials were hand removed. Soil samples were dried (at 105 C for 72 h till constant weighing), ground, sieved through 500 mm (Retsch, Germany), homogenised and placed into cylindrical geometry for gamma spectrometry measurement.
2.5. Distribution coefficient and extraction experiments The Kd is commonly used as a means of assessing the mobility of radionuclides in the environment and for comparing sorption data obtained from different sources. Thus, sorption experiments were performed by placing an adequate weight (i.e. 0.5 g) of the dried soil in contact with 50 mL of 137Cs/85Sr-tracer solution into a polypropylene tube (solution: solid ratio 100). Equilibrium was achieved by rotating the tubes on a mineralogical roller for 24 h. This was accomplished after carrying out primary kinetic studies for 1e48 h (Al Attar et al., 2010; Twining et al., 2004). A time of 24 h was adequate to attain equilibrium between the two phases. The suspensions were then centrifuged for 15 min at 4000 rpm using Hettich Universal-3100 (Germany), followed by filtration through 0.45 mm Sartorius Minisart RC/SRP membranes. The pH of the filtrate was measured using Sartorius Basic meter PB-11, fitted with an Ag/AgCl electrode. The Activity concentrations of 137Cs and 85Sr in the supernatants were determined via gamma spectrometry. Sorption of radionuclides on the tube-wall was negligible since the recovery of blank experiments (without soil material) were found to range 98e101% for 137Cs and 94e101% for 85Sr. Sorption of the radionuclide on soil matrix was then estimated in terms of distribution coefficient (Kd) given in the following equation,
A A V t Kd mL g 1 ¼ i M At where Ai and At are, respectively, the initial and equilibrium solution activities of the radionuclide of interest, V/M is the volume-tomass ratio, representing the volume of the radionuclide solution (L) that can be treated by a given mass of the soil (kg). The distribution coefficients obtained in this study were the mean of two independent experiments. In order to study the radiocaesium and strontium sorption reversibility, desorption experiments were carried out in a similar manner to sorption but using NH4Cl reagent. Table 2 summarises Kd and desorption data.
21
Table 2 Soil-solution distribution coefficient (Kd, L kg1) for 137Cs and 85Sr and % desorption. Kd (L kg1)
137 85
Cs Sr
3635 ± 370 10.1 ± 1.8
% Desorption 1st year soil
2nd year soil
10th year soil
36 ± 7 72 ± 5
31 ± 6 75 ± 6
16 ± 2 83 ± 3
Values are the arithmetic mean of triplicates and ±2s.
2.6. Radiochemical measurements Gamma measurement was performed using HPGe detector (PeType, Eurisys, France), high resolution (1.85 keV at 1.33 MeV), high relative efficiency (80%) and fitted with InterWinner software (Version 4). Efficiency calibration was made using point source of 137 Cs with radioactivity of 40.33 kBq (reference date 1st Sep 1987, Amersham, UK). Counting was carried out to attain 1e5% statistical accuracy at the appropriate chosen geometry. However, because of the low activity concentration in grain samples, counting uncertainties were 5e10%. Minimum detectable activities (MDAs) of 137 Cs were derived from the background measurement at 100,000 s and found to be 0.1 and 6 Bq kg1 for soil and plant, respectively. Quality control was carried out using the reference materials Soil375 and Grass-373 provided by IAEA. In the case of 85Sr, CRM QCY48 supplied by AEA Technology, QSA GmbH (reference date 1st December 2008) was used for efficiency calibration. The activity of 90Sr in soil and plant samples was estimated ^ through its progeny 90Y via. Cerenkov after the radiochemical separation process, which is described in an earlier work (Al Attar et al., 2015). The method involves the separation of 90Sr by acid extraction, multiple precipitation and eliminating the radiochemical impurities. When secular equilibrium between 90Sr and 90Y was established, the latter was extracted and dissolved in HCl. The radiation of the beemitter of 90Y was determined using a Wallac 1414 (Perkin Elmer, Finland) liquid scintillation counting, efficiency of 56%. The lower limit of detection was 0.07 Bq kg1. Certified reference samples, i.e. Soil-375 and Grass-373 supplied by IAEA, were used to assure the quality control of the measurements. 2.7. Calculation of transfer factor The soil-to-plant transfer factors for 137Cs and 90Sr were calculated, for each lysimeter individually, according to the IAEA (2010) definition,
Fv ¼
Activity concentration of radionuclide in cropðdwÞ Activity concentration of radionuclide in soilðdwÞ
Radionuclides activities were decay-corrected to harvest date. Transfer factors were defined on dry weight bases for both of the crop and soil in order to reduce associated uncertainties (IAEA, 2009). The number of observations per treatment (year) of each crop is n ¼ 5. Thus, the annual Fv data of each year studied was expressed as geometric mean (GM), with the fact that the geometric mean indicates the central tendency of a number of data (IAEA, 2009). The uncertainties assigned to the geometric mean were estimated by the geometrical standard deviation (GSD) which is the exponent of the standard deviation of the natural logarithms of the individual values at a confidence interval of 95%. In addition, higher and lower limits and the uncertainty were calculated. The results are shown in Table 3. It is worth pointing out that the number of observations is critical for worldwide database analysis, as great caution should be taken when transfer factor is reported without an estimation of the extent of uncertainty or a range (IAEA, 2010).
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L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Table 3 Geometric mean, lower and upper limits and uncertainty of Fvs for Crop
Lettuce
Wheat-straw
Wheat-grains
137
Year
st
1 2nd 10th 1st 2nd 10th 1st 2nd 10th
137
Cs and
90
Sr in lettuce and wheat compartments. 90
Cs
GM
GSD
LCI
UCI
0.114 0.030 0.013 0.030 0.003 0.003 0.005 0.003 0.004
1.201 1.231 1.317 1.258 1.194 1.385 1.243 1.483 1.318
0.095 0.025 0.011 0.026 0.002 0.003 0.005 0.002 0.003
0.136 0.036 0.014 0.033 0.003 0.004 0.006 0.003 0.004
Un % 36 37 25 23 31 29 22 35 25
Sr
GM
GSD
LCI
UCI
0.977 1.033 1.033 0.053 0.043 0.047 0.017 0.016 0.016
1.227 1.172 1.092 1.389 1.229 1.157 1.140 1.170 1.115
0.882 0.898 0.956 0.045 0.036 0.041 0.015 0.014 0.014
1.082 1.187 1.116 0.063 0.052 0.053 0.019 0.018 0.017
Un % 20 28 15 33 36 26 26 28 19
GM is the geometric mean, GSD is the geometrical standard deviation, LCI and UCI are lower and upper limits at 95% confidence interval, Un % is the uncertainty percentage.
2.8. Statistical analysis Empirical Evidence based on the central locations of data showed that the arithmetic and geometric means and median were comparable. Applying the normality test as Shapiro-Wilk on log-Fv values, the data indicated normal distribution with pn values (i.e. p of the normality test) were greater than alpha (0.05). One-way analysis of variance (ANOVA) was performed to evaluate the significant difference for log-Fv values using Fisher’s test at a confidence interval of 95% via. XLSTAT (2015, Version 2015.3.01). The least significant differences (LSD) were calculated with a significant level of 5%, indicating whether the difference among the values is significant or not. The data are shown in Table 4. 3. Results and discussion 3.1. Distribution coefficient and desorption experiments Investigating the sorption behaviour of the radionuclides from simulated solution on the soil made it possible to evaluate Kd values and predict Fvs of 137Cs and 90Sr to the crops studied. Table 2 summarises batch distribution coefficients (Kd) and desorption proportion of the radionuclides of interest. The high Kd for 137Cs, i.e. 3.6 103 L kg1, may indicate a strong affinity of caesium to the clay content of soil, i.e. montmorillonite. However, 90Sr showed weak sorption to the clay mineralogy with Kd of around 10 L kg1. Kd values obtained were in the same order to those reported by GilGarcía et al. (2009) where the Kds are grouped according to soil texture and organic matter criterion, i.e. geometric mean of 95 and 5.5 103 L kg1 for Sr and Cs, respectively, in clay soil. Similarly, Twining et al. (2004) reported Kds in the order of 103 L kg1 for 134 Cs and in the range 30e60 L kg1 for 85Sr. It is well known (Al Attar et al., 2010) that montmorillonite sorbs cations at surface sites as well as interlayer positions by three possible mechanisms that are surface sorption, and/or ion exchange of the metal ions with ions on the surface and sorption/penetration through the crystal lattice. The first two sites would be available for both Table 4 Significant variation of log-Fv for
137
Cs and
Crop/plant compartment
nuclides. However, the interlayer sites would require the hydrated cations to denude their hydration shell in order to facilitate their bonding to the sorption interface. Monovalent cations, in respect to divalent ones, have larger ionic radii but smaller hydrated radii that make them easily strip their weak-hydration sphere and penetrate into the crystal lattice (Horvath, 1985). Therefore, Csþ would firmly bond to the clay mineral than Sr2þ. The radionuclide speciation in the solid phase might change with time, so an estimation of the change in the reversibility of the sorption in the short and medium term is also required in any experimental approach designed to derive information on sorption dynamics (Vidal et al., 2009). Data of desorption experiments (Table 2) were strongly supportive to the conclusion drawn earlier by batch distribution coefficients. Desorption proportion of 137Cs were 36, 31 and 16% for the 1st, 2nd and 10th year contaminated soils, respectively. The irreversible sorption of caesium (poorly hydrated monovalent cation) as a consequence of its penetration and fixation to the crystal lattice sites of the clay mineral might be responsible for the decrease of the extractable percentage of caesium. In the case of 90Sr, constant desorption with a mean of 77% inferred that sorption of the divalent cation to the surface sites may be the governing mechanism. 3.2. Transfer factor of
*
Significantly different at p 0.05,
**
Sr in lettuce
Sr to lettuce and winter wheat at various age of soil-contamination. pn values (Normality test)
Cs Sr Cs Sr Cs Sr
Winter wheat (grains)
90
Transfer factors values for 137Cs and 90Sr to lettuce according to ageing of soil contamination are illustrated in Fig. 1. Clearly, Fv of 137 Cs decreased continuously over time, while variation of Fv 90Sr among years were insignificantly different. Ageing seemed to have a great influence on the Fvs of 137Cs, which decreased with increasing time following contamination. The geometric mean of Fv for the 1st year was 0.114, became 0.03 at the 2nd year and reached 0.013 for the 10th year with uncertainty did not exceed 37% (see Table 3). The difference of Fvs along the time course studied was highly significant (p < 0.0001, Table 4). The results of the 2nd and 10th year were in agreement to previous findings (Al-Oudat and Al-Asfary, 2006; Yassine et al., 2003) where
p-values of the significance variation of log Fv for 90Sr and 137Cs with ageing 1st vs 2nd
Winter wheat (straw)
Cs and
90
Radionuclide
Lettuce (whole plant)
137
p 0.005,
0.180 0.534 0.200 0.371 0.164 0.891 ***
p 0.0005.
***
<0.0001 S 0.582 <0.0001 S*** 0.108 0.005 S** 0.608
1st vs 10th ***
<0.0001 S 0.581 <0.0001 S*** 0.249 0.080 0.054
2nd vs 10th <0.0001 S*** 0.998 0.305 0.609 0.147 0.133
LSD 0.140 0.088 0.145 0.136 0.179 0.098
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Fig. 1. Geometric mean of Fvs for (number of observation is n ¼ 5).
137
Cs and
90
Sr to lettuce as a function of ageing
Fv for cabbage, as a leafy vegetable, ranged between 0.02 and 0.01 after 2 and 3 years of contamination, respectively. The IAEA (2010) reference value for the Fv of 137Cs from clayey soil to leafy crops is 0.018 (ranging from 0.0005 to 0.72), so the data obtained in this study fell within the reference range. The decline seen of Fvs was by a factor of 4 after 2 years and by a factor of 9 after 10 years. Choi and co-authors (2011) who studied ageing effect on the transfer factor of 137Cs to soybean found it decreased by a factor of 3 after 4 years; however the authors used a sandy soil with 4% of clay content and acidic pH (i.e. 5.2). This confirms that soil characteristics (soil texture, organic matter content, exchangeable K and pH) play substantial roles in Fv of 137Cs in addition to the dissolved chemical forms of the radionuclide and their interaction with the soil matrix (IAEA, 2006). Herein, the behaviour of 137Cs at the 1st year might reflect a physiochemical process as desorption reached 36% for 137 Cs. In other words, the radionuclide species may be sorbed on the planer surface of soil particles by initial reversible sorption. Later, the continuous decrease in Fv at the 2nd and 10th year contaminated soils was associated with the fixation of 70% and over 84% of 137Cs respectively (as shown by desorption experiments, see Table 2). The findings were consistent with those reported by Djingova et al., 2005 who stated that fixation of 80% of radiocaesium to the resistant phase of soil occurred after 2-year of soil contamination. The drop of Fvs observed at the 2nd and 10th years soils would mirror the diffusion of Csþ, as monovalent cation, to the interlayer sites of the porous montmorillonite mineral, i.e. retention of 137Cs by the crystal lattice of the clay with Kd measurement was as high as 3600 L kg1. Consequently, there would be a decrease in the bioavailability of 137Cs to plant uptake. The dynamic of 137Cs Fvs with time would be a reflection of increasing the irreversible fixation of the radionuclide, which is estimated to be about 15% per year, taking into account that the percentage may be much higher or lower according to the soil characteristics and systems (IAEA, 2006). In contrast to 137Cs, no dynamic of Fv for 90Sr as a function of ageing was observed. The average geometric mean value of Fv over the years was ca. 1.01 with uncertainty ranging 15e28% (Table 3). An earlier study carried out by Yassine and co-authors (2003) also showed insignificant difference in the transfer factor of 90Sr for cabbage, as a leafy vegetable, after three years. Noordijk and workers (1992) investigated the impact of ageing for several crops in lysimeter experiments, including spinach and cabbage on clay, loam and sandy soils; they found that the transfer of 90Sr did not change over a 7 year period. Usually, strontium is mainly fixed to the organic matter in the soil by complexation and/or ion exchange (IAEA, 2009; Yamaguchi et al., 2007); however, the soil used in this study was poor in organic matter (1%). The low Kd value, i.e. 10 L kg1 and high desorption proportion of 77% for the three contaminated soils (1st, 2nd and 10th year, Table 2) confirmed the
23
constant and high Fv values obtained. This might mirror instant reversible sorption of Sr2þ species that are dominant in natural waters over a broad pH range, i.e. 2e9 (IAEA, 2009) with the fact that the pH of soil studied was ca. 8 (see Table 1). It is documented (Vidal et al., 2009) that in most soil types Sr does not undergo complexation in soil-solution and hence remains in the exchangeable form. Meaning, radiostrontium was available for root uptake throughout the time course studied. It was notable that Fvs of 90Sr were much enhanced in comparison to those of 137Cs, by a factor of 9 in the 1st year and a factor of 80 for the 10 year contaminated-soil. As found with Twining et al. (2004) the higher Fv values for 85Sr compared with 134Cs are, generally, consistent with the overall lower Kds for Sr. In this study, sorption experiments indicated high Kd for 137Cs ca. 3600 L kg1 and low extractable proportion (desorption) in comparison with 90 Sr. Although Kd values in laboratory batch experiments should not be assumed to be identical to field sorption values, this type of partitioning information can be used to compare the likely mobility of radionuclides in the soil-plant environment and initial retardation of radionuclide by the soil type (Sheppard and Thibault, 1990).
3.3. Transfer factor of
137
Cs and
90
Sr in winter wheat
Fvs values of 137Cs and 90Sr to wheat-straw and grains according to ageing of soil contamination are demonstrated in Figs. 2 and 3, respectively. As for lettuce, Fvs 90Sr were constant along the timescale studied. Fvs of 137Cs showed a significant decrease only between the 1st and 2nd year for both straw and grains. In the case of wheat-straw, a distinct drop of one order of magnitude in the Fvs of 137Cs was seen between the 1st and 2nd years of soil contamination, accompanied with geometric means of 0.030 and 0.003 respectively (p value < 0.0001). No change was observed in the Fv of 137Cs after 10 years (geometric mean 0.003 with uncertainty of 29%, Table 3). A similar trend was reported by Djingova et al. (2005) when they studied the transfer factors of 137 Cs to wheat-straw for three successive years, with mean Fvs of 0.055, 0.032 and 0.035, respectively. In our study, Fv of the 1st year for wheat-straw was in the range stated in the literature for cereals i.e. 0.015e0.1 (Frissel et al., 2002) which is characteristic for medium nutrient soil with pH > 4.8. In addition, the obtained geometric means for 137Cs for the time-lag studied was in the range of IAEA database for cereals in clayey soil, i.e. 0.0043e0.53 (IAEA, 2010). Squire and Middleton (1966) estimated the decrease of the transfer factor of 137Cs with ageing for various crops to be a factor of 4 after 20 years on clayey soil. The dynamic of Fv in this study may be explained by the same scenario previously stated for lettuce, where initial rapid sorption of Cs may occur at the surface soil particles. Afterwards, irreversible fixation of Cs on clay minerals
Fig. 2. Geometric mean of Fvs for 137Cs and 90Sr to winter wheat-straw as a function of ageing (number of observation is n ¼ 5).
24
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Fig. 3. Geometric mean of Fvs for 137Cs and 90Sr to winter wheat-grains as a function of ageing (number of observation is n ¼ 5).
may take place by ion exchange to a number of sites that are accessible only to poorly hydrated cations and show high selectivity for Csþ over Kþ. This was reflected by high Kd derived and low desorption proportion. Fig. 2 shows that Fvs of 90Sr did not varied with ageing. Fvs after 1, 2 and 10 years of soil contamination were 0.053, 0.043 and 0.047 (uncertainty of 33, 36 and 26%), respectively, associated with insignificant difference (see Table 4). The findings were in agreement to earlier work performed by Yassine et al. (2003) and Noordijk et al. (1992) who stated a trivial change of the transfer factor of 90Sr to several crops when time elapsed increased up to three and seven years, respectively, between contamination and harvest. Nisbet and Shaw (1994a) also found that there were insignificant differences in the concentration ratios of 90Sr for barely, cabbage and carrot crops grown in peat soil over 5-year study. Based on Kd and desorption tests performed (i.e. 10 L kg1 and 77%, respectively) Sr would be available for root uptake along the time of the study. For grains (Fig. 3), both radionuclides gave lower Fvs when compared to wheat straw, which was similar to the findings of others (Nisbet and Shaw, 1994b; Yassine et al., 2003). However, Fvs of 137Cs and 90Sr show comparable trends to those for wheat-straw, with decrease in Fvs for 137Cs between the 1st and 2nd year (Fvs were 0.005 and 0.003, respectively, with p value of 0.005) and no significantly variation for 90Sr over the timescale studied (ranged 0.016e0.017). The comparable trends for wheat straw and grains would be based on the fact that the radionuclides existed in grains will mainly translocate from the foliar part (Nisbet and Shaw, 1994b; IAEA, 2009). The Fvs obtained for 137Cs in wheat-grains were consistent with the literature values (Al-Oudat and AlAsfary, 2006) i.e. 0.001 for edible parts of wheat on the same silty-clay soil and to the recorded value of 0.002 in wheat-grains after 2 or 3 years (Djingova et al., 2005; Yassine et al., 2003). The IAEA (2010) Fv in cereals grains for clayey soil ranges 2 104e9 102 and 0.0053e0.71 for 137Cs and 90Sr, respectively. Although Cs has higher biological mobility than Sr within the plant, Fv of 90Sr in grains was almost one order of magnitude higher than those of 137Cs, which may reflect enhanced concentration of 90Sr in lettuce leaves or wheat-straw (IAEA, 2009).
3.4. Comparison of transfer factor of winter wheat-straw
137
Cs and
90
Sr to lettuce and
Analysis of data indicated that Fvs of 90Sr always exceeded those of 137Cs and both radionuclides revealed higher Fvs to lettuce compared to winter-wheat straw. The higher uptake of 90Sr with regard to the crop groups may depend on the different chemistry of
the respective radionuclides and their interactions with soil components. Since Csþ being more firmly bound to the crystal lattice of the clay minerals than Sr2þ, the latter remained available in the soil along the time. The higher Fvs of the radionuclides in lettuce compared to winter-wheat may be related to the diversity of plant species and mainly the root system. Lettuce has massive, dense and superficial taproots whereas winter-wheat is characterised by fibrous roots that is less intense at the top 20 cm of soil surface (Strebl et al., 2009; Thorup-Kristensen et al., 2009). This would mirror the higher uptake of the radionuclides by the roots of lettuce and consequently higher Fvs, with the fact that the activity concentrations of the radionuclides usually remain in the top 20 cm of the soil surface for all crops, with an exception to grass, i.e. 10 cm (IAEA, 2010). In a similar manner, Twining et al. (2004) reported higher Fvs of 134Cs and 85Sr to mung leaf (leguminous vegetable) in comparison to sorghum (cereals). They inferred that the lower radioactivity uptake to sorghum is due to the deep root penetration to a depth of 1.5 m or more whilst mung bean roots are generally located within the top 20 cm. The trend of Fv for 137Cs in lettuce was somewhat different to that noted in wheat-straw (see Figs. 1 and 2); it decreased according to age of the contamination in the soil for the former plant while it remained constant after the 2nd year for the latter. The type of the root system would again influence the transfer factors. Lettuce roots might detect the lessen concentration of Cs in the soil after the 2nd year while it would be hardly sensed by wheat roots. In parallel, Djingova et al. (2005) stated that the behaviour of 137Cs would differ according to the crop and found that the dependency of the transfer factor on 137Cs concentration in soil was clear for cabbage but not for wheat. Despite that no explanation was given, the authors concluded that cabbage, as leafy vegetable, might be considered a bioindicator for soil contamination. Concerning 90Sr, no change of Fvs with time was observed in this study, giving comparable trends for both crops but higher on lettuce than wheatstraw. These findings supported that Sr2þ were available for root uptake through the timescale studied and that the characters of root distribution in soil influence the radionuclide transfer factors (IAEA, 2010).
4. Conclusion The data obtained indicated that Fvs of 137Cs decreased significantly for lettuce, as leafy vegetable, with ageing of soil contamination. Significant decline was only observed between the 1st and 2nd years and vanished thereafter for winter wheat (straw and grains). The high affinity of clay minerals for 137Cs was associated with high Kd (ca. 3600 L kg1). Sorption reversibility experiments revealed low desorption proportion of Cs from the 1st, 2nd and 10th year contaminated soils (ranging 36e16%). Rapid sorption of 137Cs to soil surface particles followed by retention of 137Cs by the crystal lattice of the clay mineral after the 1st year may be responsible for the behaviour noted of caesium. In contrast, Fvs of 90Sr showed little variation for both crops over the timeframe studied, with Fvs higher in lettuce than wheat straw. The dominance of Sr2þ species in the soil-solution system might maintain the bioavailability of radiostrontium for root uptake with Kd around 10 L kg1 and average desorption as high as 77%, indicating that reversible sorption was the governing mechanism. The low Fvs observed for both radionuclides in wheat could be attributed to its fibrous and deep penetrating root system which differ to lettuce massive, dense and superficial roots. The Fvs of 137Cs and 90Sr to the group crops studied were in the range reported in the literature and fitted to IAEA database characteristic to clayey soil and semi-arid environment.
L. Al Attar et al. / Journal of Environmental Radioactivity 164 (2016) 19e25
Acknowledgment Great appreciation is due to Prof. I. Othman (Director General of AECS) for his invaluable encouragement and funding the project. The ex-head of the Chemistry Department Dr. T. Yassine is acknowledged for providing 90Sr. The authors would like to thank W. Doubal and Mr. M. Hasan for gamma measurements, Mr. A. Nashawati for analysis by liquid scintillation counter and Ms. Rolana Bouzou for the analysis by Atomic Absorption Spectroscopy. Prof. A. Dyer is sincerely thanked for reviewing the English language of the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvrad.2016.06.019. References Al Attar, L., Al-Oudat, M., Safia, B., Abdul Ghani, B., 2015. Transfer factor of 90Sr and 137 Cs to lettuce and winter wheat at different growth stage applications. J. Environ. Radioact. 150, 104e110. Al Attar, L., Al Masri, M.S., Budeir, Y., Al Chayah, O., 2010. Sorption of 226Ra from waste effluents using Syrian bentonite. Environ. Technol. 31, 1587e1599. Al-Oudat, M., Al-Asfary, F., 2006. Transfer factor of caesium-137 in arid and semiarid regions. In: IAEA, Classification of Soil Systems on the Basis of Transfer Factors of Radionuclides from Soil to Reference Plants. Technical Document No. 1497. IAEA, Vienna, pp. 139e144. Black, C.A., Evans, D.D., Ensminger, L.E., White, J.L., Clarck, F.E., 1984. Methods of Soil Analysis, I. Physical and Mineralogical Properties Including Statistics of Measurements and Sampling. American Society of Agronomy, Madison, WI. Choi, Y.H., Lim, K.M., Jun, I., Keum, D.K., Han, M.H., 2011. Time-Dependent transfer of 54 Mn, 60Co, 85Sr and 137Cs from a sandy soil to soybean plants. Prog. Nucl. Sci. Technol. 1, 392e395. Djingova, R., Kovacheva, P., Todorov, B., Zlateva, B., Kuleff, I., 2005. On the influence of soil properties on the transfer of 137Cs from two soils (Chromic Luvisol and Eutric Fluvisol) to wheat and cabbage. J. Environ. Radioact. 82, 63e79. FAO STAT (International wheat production statistics). Accessed on line: 15th July 2015: https://en.wikipedia.org/wiki/International_wheat_production_statistics. FAO WRB, 2014. World reference base for soil resources updated 2015. Accessed on line: 18th January 2016. http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/ nrcs142p2_050915.pdf. Fernandez, J.M., Piault, E., Macouillard, D., Juncos, C., 2006. Forty years of 90Sr in situ migration: importance of soil characterisation in modelling transport phenomena. J. Environ. Radioact. 87, 209e226. Frissel, M.J., Deb, D.L., Fathony, M., Lin, Y.M., Mollah, A.S., Ngo, N.T., Othman, I., Robison, W.L., Skarlou-Alexiou, V., Topcuoglu, S., Twining, J.R., Uchida, S., Wasserman, M.A., 2002. Generic values for soil-to-plant transfer factors of radiocesium. J. Environ. Radioact. 58, 113e128. Gerstman, U.C., Schimmack, W., 2006. Soil-to-grain transfer of fallout 90Sr for 28 winter wheat. Radiat. Environ. Biophys. 45, 187e194. Gil-García, C., Rigol, A., Vidal, M., 2009. New best estimates for radionuclide solidliquid distribution coefficients in soils. Part 1: radiostrontium and radiocaesium. J. Environ. Radioact. 100, 690e696. Gomez, A.E., Brown, J., 2015. Determination of Root Uptake to Vegetables Grown in Soil Contaminated for Twenty-five Years. Accessed on line: 18th April 2015. https://www.gov.uk/government/uploads/system/uploads/attachment_data/ file/337136/HPA-CRCE-047_for_website.pdf. Horvath, A.L., 1985. Handbook of Aqueous Electrolyte Solutions. Ellis Horwood Ltd.,
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