*Article* **Transfer of Natural Radionuclides from Soil to Abu Dhabi Date Palms**

**Prasoon Raj 1,\*, Nemeer Padiyath 1, Natalia Semioshkina 2, Francois Foulon 1, Ahmed K. Alkaabi 1, Gabriele Voigt <sup>2</sup> and Yacine Addad <sup>1</sup>**


**\*** Correspondence: prasoon.raj@ku.ac.ae or prasoon.raj@outlook.com

**Abstract:** Nuclear power and modern agriculture are two crucial sectors for sustainable development in the United Arab Emirates (UAE). As these industries mature rapidly in the country, their long-term inter-compatibility needs monitoring with local data on transfer of radionuclides from arid sandy soils to farm products. Date palms, main crop from the Arabian Peninsula, remain largely unstudied for radioecological impact assessments. This paper reports the first measurement of soil to UAE date palms concentration ratios for natural radionuclides. Representative samples of soils, fruits, and leaves from seven palms in Abu Dhabi have been studied using gamma-spectrometry. Average activity concentrations in the soils are around 278.9 Bq kg−<sup>1</sup> for 40K, 15.5 Bq kg−<sup>1</sup> for 238U, and 8.3 Bq kg−<sup>1</sup> for 232Th. The latter two decay chains, in the plant samples, are close to detection limits, signifying their lower levels in the UAE flora and the need for upgrading analytical techniques. The geometric means of soil to fruit concentration ratios are 1.12 for 40K, but negligibly low for the others—approximately 0.08 for 238U and 0.17 for 232Th chains. The respective ratios for the leaves are approximately 0.13, 0.36, and 0.77. Personal radiation doses due to soils and dates are very low, posing no danger to the public.

**Keywords:** radioecology; nuclear power; soil radioactivity; plant sciences; agricultural sustainability; energy studies; arid land; impact assessment; NORM; potassium; middle east

#### **1. Introduction**

*1.1. Radioecology vis-à-vis Nuclear Power in the Arabian Peninsula*

In wake of the increasing energy demands in the Arab countries, nuclear power programs are being accelerated in the region, towards a more sustainable future energy mix [1]. Legacy and future nuclear activities are known to put risks of trace radioactivity releases in the environments [2]. The so-released radionuclides may undergo a complex, long-term transport to human food, via wind, rain, waterbodies, soil, plants, animals, etc., as studied under a collective science called radioecology. It is critical to conduct thorough radiological impact assessments (RIA) [3] and to develop nuclear emergencypreparedness systems, for combatting adverse bioecological impacts of such a transfer chain. To that end, at Emirates Nuclear Technology Center (ENTC), we are undertaking baseline radioecological studies in the United Arab Emirates (UAE). A particularly novel challenge is present in the UAE's modern agriculture sector, which is simultaneously progressing to address the food self-sufficiency concerns of the Arab nations [4].

In agricultural farms, radionuclides deposited on the soil are absorbed by plants through their roots [2]. This transfer process is termed as root uptake and is characterized by the radionuclide activity concentrations (AC, units: Bq kg<sup>−</sup>1) in plant parts and rhizosphere soils, as well as their relative value, i.e., the *plant:soil* concentration ratio (CR). The AC and CR parameters are important inputs for computational prediction of the radioisotope

**Citation:** Raj, P.; Padiyath, N.; Semioshkina, N.; Foulon, F.; Alkaabi, A.K.; Voigt, G.; Addad, Y. Transfer of Natural Radionuclides from Soil to Abu Dhabi Date Palms. *Sustainability* **2022**, *14*, 11327. https://doi.org/ 10.3390/su141811327

Academic Editors: Hosam M. Saleh and Mohammad Mahmoud Dawoud

Received: 18 July 2022 Accepted: 3 September 2022 Published: 9 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

phytoextraction efficiencies following a radioactivity release scenario. Such a calculation assists in predicting cumulative doses to the humans and biota, and furthermore, in optimizing the immediate and long-term mitigation solutions. It also supports the future land decontamination and waste management efforts using phytoremediation. In this regard, the soil–plant bio-transfer kinetics of native plants is crucial to investigate further, but remains severely understudied for arid lands [5,6].

Arid region radioecology (ARRE) is a developing subject to gather more radionuclide transfer factor (TF) data, such as CR, specific to crops, climates, and practices prevalent in arid nations, which are substantially different from temperate regions [7]. It attends to the less explored features [6], such as the physiochemistry of sandy soils, bio-saline growth media, perennial crops such as date palms, heavy irrigation, large usage of chemicals, etc. ARRE not only supports the Arab nations, but also the drylands globally, covering around half of the earth's territory. Moreover, it equips us for better decision-making under changing climatic conditions, which may push many more regions towards aridity in the years to come. The ARRE knowledge will help establish the long-term inter-compatibility of nuclear power and modern agriculture, advocate safety of the nuclear operations and waste management plans to the experts and the public, and finally, support the wider adoption of nuclear electricity for sustainable development.

#### *1.2. Soil and Date Palm Studies in the UAE*

Having made substantial progress in nuclear energy through the Barakah Nuclear Power Plant (BNPP) [8], as well as arid agriculture, the UAE serves as a great platform for radioecological activities. Presently, the focus sources of environmental radionuclides in the UAE are naturally occurring radioactive materials (NORM) from fossil fuel industries, and artificial radionuclides (ARN) from the global fallout (mainly 137Cs) [9]. For emergencypreparedness, academic consideration of reactor emissions is also warranted in future activities, especially radionuclides such as 134,137Cs, 89,90Sr, 58,60Co, 131I, etc.

In the UAE's agriculture, date palms (*Phoenix dactylifera* L.) make the main crop, topmost exported entity, and most traditional food. Native date palms are systematically propagated for sustainable land and biodiversity managements. Palm cultivars occupy more than 390 km<sup>2</sup> of land, amounting to an annual produce of approximately 340 kton, valued at USD 500–800 million [10]. With about 40 million trees of over 100 gene-varieties, the UAE's dates constitute 12% of the world's total supply [11]. Besides the fruits, its leaves, pits, wood, and other parts are also used for consumer products [12]. So, we have identified the root uptake CR and bioaccumulation of natural radionuclides from soil to the date palms as an important baseline data to measure in the UAE [6].

Past studies on the UAE soils report approximately mean activity concentrations of the key NORMs; of 238U as 17.8 Bq kg−1, of 232Th as 5.2 Bq kg−1, and of 40K as 182 Bq kg−<sup>1</sup> [9]. Limited measurements of the date fruits from the UAE's commercial producers have concluded low activities of NORMs, with mean values ~0.66 Bq kg−<sup>1</sup> for 226Ra (238U series), ~0.15 Bq kg−<sup>1</sup> for 228Ra (232Th series), and 277 Bq kg−<sup>1</sup> for 40K [9,13,14]. However, *plant:soil* CR estimates of any kind are practically absent from the UAE's literature. For the date palms, CR values are scarce even from the worldwide literature, except for a few cases [15,16].

#### *1.3. Objectives of the Research*

This paper reports a pilot field experiment to estimate baseline *plant:soil* CR of the UAE date palms. The research aims to develop and assess an internationally traceable procedure for CR experiments in arid areas, to obtain the first comprehensive estimates of CRs of natural gamma-emitting radionuclides, to document the farm practices unique to the arid geographies such as the UAE, and, to improve understanding of the NORM migration kinetics in date palms. Seven different palm samples with their surrounding soils have been collected and gamma-assayed in Abu Dhabi (the capital city of the UAE). As the first of its kind, this paper details the consistent and reproducible protocols for sample processing, specific logistical challenges, as well as a detailed analysis of the measured data on soil physiochemistry and CRs. It discusses some essential correlations between soil characteristics and radionuclide speciation and provides conservative risk factors due to background radioactivity in soils and date fruits. Finally, a future program to create a database of radioecological parameters for the UAE agricultural crops is outlined, to complete gaps of the existing global ARRE databases [7].

#### **2. Materials and Methods**

The pilot study on date palm CR measurements was conducted in the *Sas-al-Nakhl* locality (SAN in short, the term literally translated from Arabic stands for 'village of the date palms') of Abu Dhabi (UAE) (Figure 1A). Date palms are perennial, woody trees, living up to 60 years, fruiting from as early as 4 years of age, and are particularly tolerant towards very harsh summer temperatures as well as high soil salinities [17]. Date fruits are rich in nutrients, and often, particularly in urban settings, are allowed to ripen on the tree. Trees can grow to 10–20 m in height. They have compound leaves, 2–4 m long, comprised of a thick midrib, and green-coloured pinnae, also known as leaflets. Depending on the location, the roots extend in soil to a depth of 50–60 cm. Date palms, prime among the few native trees, hold an indispensable position in the Arabic food and traditions, and are important for soil, land, and biodiversity conservation. For their safe cultivation and propagation alongside the radioisotope remnants in environments, detailed research is needed in the UAE on date palm phytoremediation capabilities.

**Figure 1.** (**A**) Sampling location with tree spots (numbered I–VII) in KU SAN Campus, and the nearby Abu Dhabi landmarks in inset; (**B**) a sampled date palm tree with soil sampling spots (yellow circles); (**C**) canopy of a sampled tree with fruit bunches and compound leaves; (**D**) method of soil sampling using a manual Auger; (**E**) photographs of the representative portions of date fruit samples. Several species varieties could be seen in the photographs in (**E**).

#### *2.1. Sampling*

The sampling location is in the SAN campus of Khalifa University (KU), south-east of the Abu Dhabi city (Figure 1A), which has around 100 date palms. Soils are primarily Torripsaments [18] transported from agricultural areas of the UAE, treated monthly with bio compost and macronutrient fertilizers, and watered daily with drip irrigation. These are common practices in the commercial farms as well. Due to saline infertile soil cover in the city, artificial assembly of desert soil layers is done in most of the date palm farms. This makes the KU locations representative for the baseline data measurements.

Date palms typically follow a long growth period, from pollination in February–March to final harvest in July–August. Because of the COVID-19 related restrictions in 2020, the harvests were delayed. In September 2020, 14 palm trees were sampled. Fruits were plucked in their tamr stage (fully ripe), but some kimri and khalal stages (mature but unripe) could also be collected. From 2 kg to 4 kg fresh bunches were carefully detached, from two directions of the tree. Two large leaves with midribs were sampled from two directions. For the barks, around 20 cm long pieces were sliced from surface, but deeper layers could not be sampled to avoid damage to the palms. For each tree, soil was collected at 4–6 randomized spots under the canopy (Figure 1B), to an average depth of 22 cm, using a 10 cm diameter soil Auger (Figure 1D), and mixed to make one composite sample per tree.

#### *2.2. Sample Processing*

Date fruits were detached from the pedicels (see Figure 1C,E). Likewise, green leaflets were separated from the thick midribs of each compound leaf. A sample processing methodology was developed following the internationally acclaimed technique for radionuclide measurements in biological samples [3], and a careful adaptation of similar studies conducted elsewhere on fruits and vegetables [19,20]. In the end, for each of the 14 palms under study, 1 sample each of fruit (pulp with skin), pit/seed, pedicel, leaflet, leaf midrib, upper bark, and soil were obtained. In this phase of the experiment, fruit, leaflet, and soil samples from only seven of the palm trees were completed and reported. Numbered I to VII, in Figure 1, samples with fully ripe fruits were selected. In this paper, only ripe edible pulp (with skin) of the dates are referred to as 'fruit', and the green leaflets as 'leaf'. Quantities of fruit pits, midribs and barks are relatively more difficult to analyse in the current setting of detector-sample geometry and are postponed for the next phase of this study.

Fruits were gently washed 2–3 times under tap water to remove dust and dirt. Then they were washed with distilled water and left on paper towels to air-dry for one night. The following day, they were cut lengthwise into multiple 1–2 cm wide pieces. Date-pits were separated and stored, while the fruit pieces were spread on steel plates and allowed to dry to a constant weight, in a forced-air convection oven at 100–105 ◦C. For the seven fruit samples, the net weight reduction due to heating, which amounts to the water content of the fruits, varied between 24% and 34%, and averaged around 26%. Dried fruit pieces were ground using a heavy-duty centrifugal mill. For some samples, the fruits were too sticky because of their high sugar-content. Those samples were briefly embrittled by dipping in liquid nitrogen bath, then immediately ground, and re-dried for the removal of any additional moisture absorbed during air-condensation [21]. Leaves were cleaned multiple times, cut, oven-dried, ground, and packed in the same manner as the fruits. The average moisture content in the leaves was found to be around 38%. Finally, the dried ground powders of fruits and leaves were sieved to <2 mm grain size and packed in 1 L polyethylene bottles. Homogenous and tight pack geometry was ensured. The average net weight of fruits packed in a bottle was about 704 g, while that of leaves was about 382 g. The bottles were properly capped. A two-layer air-tight sealing was done for the caps using aluminium-backed tapes topped with two rounds of polyvinyl tapes.

Soil samples were at first sieved to remove larger chunks of rocks, debris, roots, etc. Then they were thoroughly homogenized by mixing in large plastic drums. Around 1 kg units of wet soil were packed in air-tight zip lock bags for physiochemical analyses. The rest were oven dried like the plant samples, sieved to <2 mm, and packed in 1 L tin cans, each typically packing 1.2 kg to 1.5 kg of dried soils. After securing the caps, a thin ~5 mm layer of molten wax was applied to seal the containers. All the sealed sample bottles and

cans were stored for around one month (time > ten half-lives of 222Rn/224Ra) to ensure radioactivity secular equilibrium between 226Ra and its progenies [22].

#### *2.3. Analytical Techniques*

Wet (homogenized, unprocessed) soil samples were studied for their physiochemical properties (Table 1) such as pH, organic matter content (OM), cation exchange capacity (CEC), electrical conductivity (EC), moisture content, and USDA texture class [23], using standardized soil analysis techniques [24,25].

**Table 1.** Minimum (Min.), maximum (Max.) and AM (Avg.) of the physicochemical characteristics of the soil samples. Parameters include pH, electrical conductivity (EC, unit: dS m<sup>−</sup>1), organic matter content (OM, unit: weight %), percent moisture content (Mois.), cation exchange capacity (CEC, unit: meq (100 g)<sup>−</sup>1), and soil texture, i.e., the percentage contents of clay, silt, and sand.


The processed samples of soils, fruits and leaves were assayed for their gamma spectra using two cylindrical high-purity germanium (HPGe) detectors, located at the Emirates Nuclear Energy Corporation (ENEC) [8] Environmental Radiochemistry Laboratory (ERL). The detectors' relative efficiencies are 40% and 60%, and the energy resolution is 1.9 keV at 1332 keV. They are calibrated for energy and efficiency using a calibration standard containing 105 kBq mixed gamma radionuclides (EZAG 8503 mixture), in the same geometries as the sample containers. Counting times on HPGe detectors were limited to 4 h for soil samples in tin cans and 16 h for plant samples in plastic bottles. The measurement characteristics are optimized for higher processing speeds as per the program requirements of the ERL. Quality assurance for natural radionuclide quantification in environmental matrices is achieved by frequent comparison with certified reference materials, successful participation in standardized proficiency tests (PT) conducted by International Atomic Energy Agency (IAEA) and other top bodies.

NORM measurement protocols as per Ref. [22] were followed. A wide range of radionuclides with progenies from each series were set to be identified, including but not limited to 235/238U, 228/232/234Th, 224/226/228Ra, 210/212/214Pb, 212/214Bi, 228Ac, 208Tl, and 40K. A priori minimum detectable activities (MDA), in units of Bq kg−1, for the five key radionuclides are: [40K: 2.41, 214Bi: 1.53, 214Pb: 1.65, 228Ac: 2.72, and 208Tl: 2.23] in 1 L plastic bottles used for plant samples, while [40K: 11.7, 214Bi: 2.61, 214Pb: 2.85, 228Ac: 4.65, and 208Tl: 3.70] in 1 L tin cans used for soil samples. The spectra were further analysed using GENIE 2000 [26] to obtain the nuclide's activity concentrations, along with the necessary corrections as needed, such as interference and coincidence summing, background and Compton corrections, etc.

#### *2.4. Data Treatment*

To establish a benchmark for multivariate analyses necessary for the TF experiments in the UAE, the data on soil characteristics were employed in non-parametric Kendall's τ<sup>B</sup> Rank Correlation tests [27,28]. The results were studied in terms of coefficients of correlation (*r*) and statistical significance (*p*), where *r* > 0.7 and *p* < 0.05 have been taken as a measure of strong correlation. This partially helps in understanding radionuclide speciation and fate in the UAE soils, as further nuclear activities and waste remediation efforts are implemented. Based on theoretical considerations, biogeological distributions such as the CRs often follow lognormal statistics. Therefore, central tendencies are reported alternatively as arithmetic means (AM) with 1σ standard deviations (SD), as well as geometric means (GM) with geometric standard deviations (GSD) [2,7].

#### **3. Results and Discussion**

#### *3.1. Physiochemistry and Radioactivity of Soils*

Table 1 reports the range (minimum and maximum) and average values (excluding sample number VI) of the main characteristics for the six soil samples (I–V and VII). Preliminary studies have concluded that sample number VI has higher salinity due to disposal of wastewater and construction debris in the sampling position. So, in Table 1, characteristics of soil sample VI are reported separately, along with the respective averages for all seven samples. A major difference can be noted in the average values of electrical conductivities. Overall, the sampled soils are slightly alkaline, but near neutral, with an average pH of about 7.5. Their organic matter content is between 1.8% and 2.8%, and moisture is between 2% and 16% of the weight. They have been classified under the USDA texture class: sandy (with an average sand content of 93.4%).

Gamma-spectrometry of the soil samples (each one assayed for about 4 h) yielded clear measurements of all the NORM chains. Box-whisker plots in Figure 2 depict variations of the respective radionuclides over the seven soil samples. For 238U chain, 214Pb and 214Bi have been consistently recorded and their activity concentrations agree with each other very well. Likewise, for the 232Th chain, progeny radionuclides measured accurately are 228Ac and 208Th, again with excellent inter-agreement. The weighted AM ± SD values for activity concentrations of the soils in units Bq kg−<sup>1</sup> are: 15.5 <sup>±</sup> 2.8 for 238U, 8.3 <sup>±</sup> 0.8 for 232Th, and 278.9 <sup>±</sup> 57.7 for 40K.

**Figure 2.** Box-whisker plots showing variabilities of activity concentrations (in unit: Bq kg−1) in the seven soil samples for the main measured radionuclides under NORM decay chains of (**A**) 238U, (**B**) 232Th, and (**C**) 40K. X shows the AM of the data.

Radionuclide speciation in soil solids vs. in aqueous solutions in a plant growing soil–water system, are known to be associated with the soil's physiochemical properties [29]. However, such relationships have rarely been established for soils primarily composed of desert sands, as we encountered in this study. A cross-correlation test between various reported parameters have provided some interesting observations, albeit with a very limited number of samples in a small population. There is a clear relationship between

concentrations of 40K and 214Bi. The former is also impacted by moisture content in the soil. Moisture, furthermore, has a strong correlation with the organic matter/carbon retention in the soils. Finally, electrical conductivity is found to be positively associated with silt content of the soils, also observable for sample VI in Table 1.

Figure 3 compares the ranges and mean values NORM activity concentrations between various soil studies in the UAE, nearby countries, and world average. Soil NORM activities as measured in this study agrees well with previous measurements conducted in the UAE, with samples specifically from agricultural lands [30], from coastlines [31], as well as those averaging over all territories within the UAE [9].

**Figure 3.** Range (shown with error bars, black dash showing maximum and green the minimum) and AMs (square markers) of soil activity concentrations for (**A**) 238U-series, (**B**) 232Th-series, and (**C**) 40K, as measured in this study, and compared with other measurements in the region. **Sources**: UAE overall [9], UAE agricultural [30], UAE coastline [31], Saudi Arabia [16], Qatar [32], and World [33].

The eastern and western coastlines on the UAE feature some differences, due to differences in geologies, soil makeup [10,18], more rains on the east, and gradient of salinity between the two seas (Arabian Gulf on west, and Arabian Sea on east). The averaging across all coastlines in Ref. [31] has led to relatively lower potassium and thorium contents than urban soils. Abu Dhabi soil radioactivity is very similar to those in Saudi Arabia and Qatar, within the uncertainty margins.

Compared to the world average [33], the activity concentrations of the NORMs in the UAE soils are consistently lower, and well within the recommended limits. Low retention behaviour of sandy soils can be a reason behind the low U and Th values. A comparatively higher content of potassium can be attributed to large-scale application of potassium-containing fertilizers [2,34] in farms, gardens, etc. However, the mean 40K in soils in the UAE and nearby countries such as Qatar, are lower than the global average. Because caesium and potassium uptakes are known to compete [2], in scenarios of large 137/134Cs depositions in the local soils, it is probable that low K-levels in soils can promote the uptake of radiocaesium by plants [35].

#### *3.2. NORM Concentrations in Date Palms*

Unlike the soils, the fruit and the leaf samples from date palms have considerably lower concentrations of NORM radionuclides, except for 40K. Gamma-spectrometry has quantified the activity concentrations for 238U and 232Th in only 3–5 samples each of fruits and leaves. This is based on the weighted means of the same progenies as in Figure 2 and as shown in Table 2. AM of 238U and 232Th activity concentrations in both leaves and fruits vary between 1.1 and 3.2 Bq kg<sup>−</sup>1, with the average concentrations in leaves greater than those in fruits. In general, 232Th measures slightly higher than 238U for the plant samples. One of the progenies of 238U chain, 210Pb, has been consistently found in 6/7 of the leaf samples, with AM ± SD as 30.3 ± 5.7 Bq kg<sup>−</sup>1, an order of magnitude higher than the weighted mean of the decay chain's radioactivity. This can be due to the superficial contamination of leaves with lead contained in the aerial pollutants, but it requires further experimental verification.

**Table 2.** Measured ranges and averages (in parentheses) of activity concentrations of NORMs radionuclides in leaf and fruit samples from this study. BDL stands for below detection limit. A comparison (bottom part) is shown with similar studies on date fruits in the UAE and Saudi Arabia.


<sup>a</sup> weighted averages of respective progenies are reported. <sup>b</sup> values reported for fresh dates; <sup>c</sup> values reported for date-pits.

The measured concentrations of 40K are much higher (average of leaves and fruits around 271 Bq kg<sup>−</sup>1) than the other two NORMs. This is due to the large appetite of desert plantations towards macronutrients such as potassium. It is also enhanced by the frequent fertigation using NPK supplements in the soils and waters to sustain the growth and improve yields [10,34]. It is worthwhile to reiterate the competition between the Cs and K uptakes in plants [2]. Larger extraction of potassium, even at 40K soil concentrations lower than the world average, could mean much higher risks of 137/134Cs transfers to date palms and other trees in the UAE and nearby countries [35]. Furthermore, phosphates from the fertilizers, depending on their original production process, are known to add considerable amounts of uranium to the soil. This must impact uptake concentrations, which requires a detailed study in future projects.

A pragmatic approach has been taken in estimating the low levels of U and Th concentrations in plant samples. These radionuclides have often suffered with large relative uncertainties in the primary gamma-ray peak area (going to 200%). As they are extremely important data to evaluate, all detected cases providing relative uncertainty up to 50–60%, and within gamma line MDA limits have been accepted, to generate at least their rough estimates. In the future, either much higher efficiency gamma-detection setups should be employed, or alternative techniques with lower limits of detection should be developed. However, it is important to point out that the natural contents of NORMs in the UAE's dates have been verified to be extremely low in other works as well (see Table 2). The literature has merely a few instances of the UAE's date fruits gamma-assays, and the U- and Th-series are often below detection limit (BDL) and difficult to quantify. No measurements on leaves are available for comparison. However, for the fruits, averages of previously measured values are consistently lower than the ones reported in this study, although within the statistical variance margins.

#### *3.3. Concentration Ratio Estimations*

Soil to plant NORM CRs for date palm fruits and leaves have been estimated by taking the ratios of the activity concentrations (Bq kg−1) in the plant part to that in the corresponding soil. After calculating the ratios for each sample, GM and GSD values are calculated to provide central tendencies for each NORM. A complete demonstration of the process is shown for 40K in Figure 4, with sample-wise variation of Bq kg−<sup>1</sup> in soils, fruits, and leaves, along with CR values for *fruit:soil* and *leaf:soil*, as well as the *fruit:leaf* translocation factor.

**Figure 4.** Sample wise distribution of 40K measurements: (**top**) activity concentrations in soil, fruit, and leaf, (**middle**) CR values *fruit:soil* and *leaf:soil*; and (**bottom**) CR *fruit:leaf*. The connecting lines are for guidance only.

For the fruits, 40K CR has a GM of 1.12 ± 1.2, and for leaves it is 0.77 ± 1.6 (see Table 3). This indicates a preferential bioconcentration of 40K in the fruits. Dates and nuts are typically potassium-rich fruits [19], which is a possible explanation for CR > 1. A comparison has been done for the CR values from this study with the much lower ones available in the literature in Table 3. Ref. [16] cites a recorded CR~0.22, while Ref. [15] gives CR~0.55 for 40K. A main driver for potassium uptake in the urban Abu Dhabi date palms is the larger application of NPK fertilizers (with potassium in concentrations of 22 kton [11]—much higher than the global average). This observation highlights the need for a detailed study of the impact of fertilizers on the CR values.

Estimation of CRs for the U- and Th-chains has been difficult due to the low activity concentrations of the corresponding radionuclides in the plant samples. For all the available cases, the measured CRs with 1σ uncertainties are plotted in Figure 5. Regarding both of these decay chains, five samples could provide estimates for *leaf:soil* CR, three for *fruit:soil* CR, and two for *fruit:leaf* CR. The mean values of all CRs are reported in Table 3, and are compared with the two literature cases of date palm CR measurements [15,16]. The *fruit:soil* CR for 238U (GM = 0.08 ± 1.6) is only 20% deviant from the literature value on Saudi dates. The same comparison is more pronounced for CR of 232Th, which deviates by more than 40%. Uncertainties in the case of Th chain are much higher than the U chain (see Figure 5). This is mainly because the inherent thorium content in UAE soils (see Figure 2) is lower by an order of magnitude than uranium, adversely impacting the transfer factor measurements. For some samples, the large uncertainty margins render the 232Th CR < 0, which is a physically impossible extreme. This brings us to the caveat that the estimated CRs for Th- and U-decay chains in date palms are indicative only.

**Table 3.** Overall values expressed using frequency of observations (*n*), GM and GSD, for the soil-toplant CRs for leaf and fruit, with fruit:leaf CR. Literature CR value are also shown for comparison.


a,b arithmetic mean and arithmetic standard deviation are reported, as *<sup>n</sup>* ≤ 2.

**Figure 5.** Sample wise variation of *fruit:soil* CR and *leaf:soil* CR values for (**top**) 238U and (**bottom**) 232Th. Unlike 40K in Figure 4, these NORMs were observed in a few samples only. For some cases, very high 1σ uncertainties yield CR < 0 as (meaningless) limits.

#### *3.4. Aggregated Risk Factors*

Obtained average activity concentrations of the soils and plant samples have been used for estimating the conservative ranges of radiation exposure 1 m above the earth surface using the standard methods proposed in Refs. [33,36,37]. For soil and date palm (leaf and fruit), the radium equivalent (Raeq), internal hazard index (Hin), absorbed dose (Dabs), and effective dose equivalent (Deff) are reported in Table 4.

For the fruits, additional calculations are done for per capita annual radioactivity intake based on the measured indicative activity concentrations. This is further used to evaluate the associated committed effective doses. For the risk factors due to fruits, FAOSAT data on the total annual production of dates in the UAE and total population have been employed [11], adding up to give the maximum committed effective dose from consumption of the studied dates, providing conservative risk estimates. All the aggregated dose factors and hazard indices are well within the UAE national [9] and global

limits [33] and pose no risks to the public, as specified in the table. This demonstrates that the phytoextraction of natural radionuclides in the Abu Dhabi date palms are below the safety limits, promising its large-scale propagation in agriculture and landscaping.

**Table 4.** Summary of aggregated doses and risk factors from exposure to NORM baseline radioactivity in soil, leaf, and fruit of date palms, with (bottom part) ingested doses from date fruits, presented with respective global control limits for comparison.


*3.5. Perspectives and Possible Expansions*

This experiment has strengthened the avenues for future CR measurements in the UAE, but it has also highlighted the necessary developments in our laboratory capabilities. A major shortcoming for measurements of low levels of NORMs in UAE plants is the high detector counting uncertainties. Improved gamma-spectrometry setups, and alternative radioanalytical techniques such as liquid scintillation counting, gross alpha/beta spectrometry, and sequential extraction based isotopic analyses, need to be employed to fill the gaps and improve the confidence in measurements. Using counting times much longer than the one employed in this study (16 h), will also improve the uncertainties and detection limits greatly. Our sample processing steps require longer instrument times and can work with larger quantities (few kg) of samples only. This reduces the speed of our studies, increases manual work, and leads to higher instrumental expenses. Experimental setups adaptable to various quantities and geometries of samples are under development, to reduce the time and expenditure for all steps, from sampling to analysis.

In this pilot study, a limited geographical distribution and statistically smaller number of samples could provide only indicative values of activity concentrations and CRs. In the following stages, many more locations will be sampled, with better focus on the representative, large-scale, commercial farms. An important aspect is to include local agricultural practices in hyper-arid countries, such as the use of modern greenhouses/net houses. Dedicated sampling efforts are necessary to account for the factors capable of systematically impacting the average CR estimates such as the species of fruits sampled, age of trees, seasonal dependency, irrigation, and fertigation practices in the fields, etc. The impact of fertilizers is a highly significant research topic for root uptake studies in UAE farms. More fieldwork is demanded to ascertain the detailed biochemical kinetics of transfer/translocation of radionuclides in date palms, with full-body studies on multiple tree samples. Further sophistication of experiments is expected for the CR estimation for other major crops of the UAE, viz.: vegetables (cucumber, tomato, capsicum, etc.), animal fodder (maize, Rhodes grass, Alfalfa mixture, etc.) and desert plants (Rimth, Ghaf, etc.). Studies addressing the above issues have been planned under the ENTC's coordinated project with IAEA [38].

#### **4. Conclusions**

This paper describes the development and results of a field experiment towards the first estimation of natural radionuclides' soil to plant concentration ratios (CR) in the date palms in the UAE. Date palms are one of the topmost crops, exported entities, and traditional foods of Arab nations. Native date palms have been adopted for enhanced cultivation and propagation with modern agriculture and are utilized in sustainable land and biodiversity managements in arid lands. Upcoming nuclear power development in the region raises the risks of radioactivity release to the environments. In this regard, date palms have not been studied thoroughly, making it a critical subject for obtaining the associated radioecological transfer factors and phytoextraction safety limits.

As part of a pilot study in Abu Dhabi (UAE), seven date palm trees were sampled. Samples of their rootzone soils, fruits and leaves were processed and assayed for gammaemitting NORM radionuclide activities. The soil samples have high concentrations of 40K, but values for 238U and 232Th are much lower. The measured soil radioactivity is below the global averages. The uptake of 40K is considerably high in the dates, leading to GM of CR ~1.12. Only rough estimates of 238U (~0.08) and 232Th (~0.17) could be obtained due to their very low inherent contents in the local dates. The experiment also recorded the uptake to leaf, also a consumer product, as 0.77 for 40K, 0.13 for 238U and 0.36 for 232Th. The values obtained here compare very well with the limited literature measurements but makes a novel and complementary input to the global arid region radioecology database. The developed protocol will be replicated for more accurate estimations of date palm CRs, which can be further utilized in national exercises of risk prediction, radioactive waste management, and nuclear emergency-preparedness. Preliminary calculations suggest negligible personal dose factors due to the baseline radioactivity in Abu Dhabi soils and date palms. More detailed experiments and computational impact assessments are planned based on this experience, expanding to new geographies in the country, as well as new measurement techniques, and other important crops.

**Author Contributions:** Conceptualization, formal analysis, investigation, software, P.R.; methodology, P.R., N.P., N.S. and G.V.; resources, P.R., N.P., Y.A. and A.K.A.; writing—original draft preparation, P.R.; writing—review and editing, N.P., N.S., G.V. and Y.A.; visualization, P.R.; supervision, F.F. and Y.A.; project administration, F.F.; funding acquisition, F.F. and Y.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was sponsored by Khalifa University and received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** All relevant data are included in the article.

**Acknowledgments:** The authors would like to thank the Research and Development team of Emirates Nuclear Energy Corporation (ENEC) for providing access to the radioanalytical instruments in the Environmental Radiochemistry Laboratory, located outside Al Ruwais. Their scientists Michael Millinor and Andrew Maddison are dearly acknowledged for their involvement in the gamma spectrometry and for their several discussions and comments. Likewise, the authors extend their gratitude to the Soil Analysis Laboratory of the International Centre for Biosaline Agriculture (ICBA) Dubai for their support in physiochemical analysis of the soil samples. Sincere thanks are due to Greendreams Landscaping for participation in the field surveys and sampling campaigns.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

