URANIUM UPTAKE AND TRANSLOCATION IN PLANT

AARJMD VOLUME 1 ISSUE 20 (APRIL 2014) ISSN : 2319 - 2801

Asian Academic Research Journal of Multidisciplinary

www.asianacademicresearch.org

262

A Peer Reviewed International Journal of Asian

Academic Research Associates

AARJMD

ASIAN ACADEMIC RESEARCH

JOURNAL OF MULTIDISCIPLINARY

URANIUM UPTAKE AND TRANSLOCATION IN PLANT

M.F. ABDEL-SABOUR*

*Nuclear Research Center, Atomic Energy Authority,

Abstract

Uranium and related radionuclides exist in the environment naturally and, in recent

times, have been added by nuclear power and weapons. The carcinogenic nature and

long half-lives of many radionuclides make them a potential threat to human health.

Plant uptake of radionuclides into the human food chain is one of several vectors used

for calculating exposure rates and perregion forming risk assessment Results reveled

that U uptake by plants seems to be pH-dependent in most studies. Soil acidity and the

saturation condition at the tailings impoundment edge tend to enhance U availability

for plant uptake. Uranium transportation in soil is dependent on the direction of the

surface or soil water flow when it is soluble and mobile (consequently being

bioavailable). This was supported with the finding that migration of U from the

contaminant band was substantial only in the sandy soil. Moreover, investigations

indicated that adsorption of contaminated water was the main source of the U-

accumulation in the different plant organs.

Key words: Food chain, Accumulation, U-translocation, Concentration ratio.

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Introduction

In addition, phytoremediation has been used to extract radionuclides and other pollutants from

contaminated sites. The accuracy and success of these applications depend on an understanding of

the processes involved in plant uptake of radionuclides. One important impediment to such

understanding is that plant tissue concentrations of radionuclides have rarely shown a linear

relationship to radionuclide concentrations in the substrate (soil).

One of the pathways neglected in radionuclides risk assessments, which has subsequently been

shown to be important in many cases (e.g. Nair et al., 1982; kelly et al., 1983) is the food chain

pathway. After Chernobyl accident, all pathways are being investigated, the ingestion pathway to

men must be included therefore, the chemical characteristics of the radioactive material become

important. Initially, following an atmospheric release to the environment, various radionuclides will

be deposited onto plants by wet or dry deposition depending on meteorological conditions, and the

point of deposition will depend on a number of factors, including wind speed and radioactive half

life. At the point of deposition the chemical form of a particular radionuclide could be very different

from that at the time of release, but the processes involved in uptake of the radionuclide into the

crop and hence the subsequent ingestion by man will involve folior absorption, root uptake,

resuspension and translocation, all dependent on the chemical properties of the particular

radionuclide. Plant uptake of radionuclides is one of many vectors for introduction of contaminants

into the human food chain. Thus, it is critical to understand soil-plant relationships that control

nuclide bioavailability.

The contamination of land by naturally occurring radionuclides in the wastes from “non-

nuclear” industries is an area of increasing concern. This has led to the introduction of new safety

standards to control such wastes. Industries affected by this will include uranium mining and

milling, metal or coal mining, radium and thorium factories and the processing of materials

containing technologically enhanced levels of natural radioactivity. However, wastes produced

previously by these industries are often stored in conditions, which do not meet the more stringent

safety standards. Netten and Morley (1983) investigated uranium, molybdenum, copper, and

selenium uptake by the radish plant grown on uranium-rich soils. Radishes were grown in the

naturally occurring uranium-containing soils found in the Okanagan Valley of British Columbia,

Canada. They indicated that bioaccumulation was not observed for U, Cu and, with some

exceptions, Se. Mo bio-accumulates readily in the radish. Their data reveled that U and Mo uptake

seems to be pH-dependent in this plant. As expected radish roots can be a significant source of U

and Mo. Ingestion of radishes from these soils could easily surpass the maximum daily intakes of U

set by the Canadian government.

The activity concentrations of natural uranium isotopes (238

U and 234

U), thorium isotopes

(232

Th, 230

Th and 228

Th) and 226

Ra were studied in soil and vegetation samples (Fabaceae, Poaceae

and Asteraceae) from a disused uranium mine located in the Extremadura region in the south-west

of Spain (Vera-Tome et al., 2002) . Their results characterized radio-logically the area close to the

installation and one affected zone was clearly manifested as being dependent on the direction of the

surface water flow from the mine. The activity concentration mean values (Bq/kg) in this zone

were: 10924, 10900, 10075 and 5289 for 238

U, 234

U, 230

Th and 226

Ra, respectively, in soil samples

and 1050, 1060, 768 and 1141 for the same radionuclides in plant samples. In an unaffected zone,

the activity concentration mean values (Bq/kg) were: 184, 190, 234 and 7251 for 238

U, 234

U, 230

Th

and 226

Ra, respectively, in soil samples and 28, 29, 31 and 80 in plant samples. The activity

concentrations obtained for 232

Th and 228

Th showed that the influence of the mine was only

important for the uranium series radionuclides. The relative radionuclide mobilities were

determined from the activity ratios. Correlations between radionuclide activity concentrations and

stable element concentrations in the soil samples are a good tool to understand the possible

distribution paths for natural radionuclides.




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Activity concentrations and plant/soil concentration ratios (CRs) of 239

,240

Pu, 241

Am, 244

Cm, 232

Th and 238

U were determined for 3 vegetable crops grown on an exposed, contaminated lake bed

of a former reactor cooling reservoir in South Carolina, USA(Whicker et al., 1999) . The crops were

turnip greens and tubers (cv. White Globe), bush beans (Phaseolus vulgaris) and husks and kernels

of sweetcorn cv. Silver Queen. Although all plots were fertilized, some received K2SO4, while

others received no K2SO4. The K2SO4 fertilizer treatment generally lowered activity concentrations

for 241

Am, 244

Cm, 232

Th and 238

U, but differences were statistically significant for 241

Am and 244

Cm

only. Highly significant differences occurred in activity concentrations among actinides and among

crops. In general, turnip greens exhibited the highest uptake for each of the actinides measured,

while corn kernels had the least. For turnip greens, geometric mean CRs ranged from 2.3 X 10-3

for 239,240

Pu to 5.3 X 10-2 for 241

Am (no K2SO4 fertilizer). For corn kernels, geometric mean CRs

ranged from 2.1 X 10-5 for 239,240

Pu and 232

Th to 1.5 X 10-3 for 244

Cm (no K fertilizer). In general,

CRs across all crops for the actinides were in the order: 244

Cm > 241

Am > 238

U > 232

Th > 239,240

Pu.

Lifetime health risks from consuming crops contaminated with anthropogenic actinides were

similar to the risks from naturally occurring actinides in the same crops (total 2 X 10-6

); however,

these risks were only 0.3% of that from consuming the same crops contaminated with 137

Cs.

Plant uptake of uranium and radionuclides:

Morton et al., (2002) investigated the extent of U and Th uptake and cycling by blueberry

(Vaccinium pallidum) in native habitat. Also, they identified the soil properties and processes that

contribute most to U and Th bioavailability in this natural system. Composite samples of plant

leaves and stems, and samples from surface (AE) horizons and from the upper part of the Bs

horizon (fragmental, mixed, frigid, Typic Haplorthod) at two sites in New Hampshire, USA were

collected. They calculated the concentration ratios (CRs) for U and Th for all plant tissues, using

both the AE and Bs horizons as the base. Soil concentrations of U ranged from 16 to 25 µg g-1,

with a mean of 21.1 µg g-1. Soil concentrations of Th ranged from 14 to 97 µg g-1, with a mean of

41.8 µg g-1. Mean U concentrations were 8.65 x 10-3 µg g-1 in leaf tissue, and 7.95 x 10-3 µg g-1

in stem tissue. Mean Th concentrations were 1.59 x 10-1 µg g-1 in leaf tissue, and 9.10 x 10-2 µg g-

1 in stem tissue. They stated that Blueberry plants are cycling both U and Th in this system, with

Th cycling occurring to a greater extent than U. In addition, Th was translocated preferentially to

plant leaves while U concentrations showed little preferential translocation. Uranium uptake,

however, seemed more sensitive than Th uptake to soil properties.

Mortvedt (1994) discussed the soil-plant relationships of U, Th and Po. He stated that plant

concentrations have been related to total contents of these radionuclides in the soil as a plant/soil

concentration ratio (CR), even though the fraction of these radionuclides which may be available to

plants and their soil/plant relationships are not well known. CR values have been used to predict

transport of radionuclides and other elements through the food chain and for biogeochemical

exploration for U. Little information is available on the uptake and transport mechanisms of

radionuclides in plants, though recent advances have been made regarding the effects of soil pH,

soil texture, and organic matter contents on radionuclide uptake, as well as their mobility in soil. He

suggested a mechanisms relating to Ca uptake and translocation in plants and may be similar to

those of some radionuclides, especially 226

Ra.

Lakshmann and Venkateswarlu (1988) investigated U-uptake by potatoes, Raphanus sativus,

Lagenaria leucantha [L. siceraria], Solanum melongena and Abelmoschus [Hibiscus] esculentus in

pots (spiking the soil and irrigation water with U). Increase in U-uptake was observed with

increased U level in water but not in soil. However, the concentration factor for uptake of U by

vegetables decreased with increasing U levels in the water. Rice was similarly grown and the




265

distribution of U within plant parts was measured. They indicated that concentration in the grain

was significantly less than in the husk, which was significantly less than in straw.

Uptake of 226Ra, 210Pb, and 210Po in mature big sagebrush (Artemisia tridentata) was

studied by Simon and Fraley (1986) in the environs of a U mine in Shirley Basin, WY.

Radionuclide solutions were injected into the soil to elevate the soil activity while minimizing root

disturbance and preventing soil surface contamination. The objectives of this study were to measure

the concentration in leaves resulting from root uptake as a function of time and to determine the

equilibrium concentration ratio (CR) for each radionuclide. The vegetation was sampled

approximately every 90 d for a 2-yr period. The maximum internal plant concentration of 226

Ra, 210

Pb, and 210

Po was detected at the first sampling (81 d after soil injection for 226

Ra, 28 d for 210

Pb

and 210

Po). The concentration in leaves decreased over time following the first sampling to an

apparent steady-state value. The steady-state CR values for 226

Ra, 210

Pb, and 210

Po, as determined in

mature sagebrush, were approximately 0.04, 0.009, and 0.08, respectively, and were determined as

the geometric mean of the CR data pooled over the 2nd-yr period. The CR data were fit by

nonlinear least squares to a exponential function that decreased with time to a constant value.

In field lysimeters Sheppard et al., (1984) studied plant growth, plant uptake, and the

redistribution in soil of U and Cr. The objective of the experiment was to characterize the plant

uptake and migration in the soil of U and Cr when placed at different depths in the unsaturated soil

zone within the influence of a water table. Total uptake of Cr by the plants was directly proportional

to Cr concentration in the soil, over eight orders of magnitude. Thus, independent of the amount of

Cr applied, the plant took up a similar fraction, about 0.007. Chromium placement depth in the soil

did not influence Cr uptake by the plants although the Cr was relatively immobile in both soils.

Plant uptake of U was independent of plant species and U placement depth for loam soil, but highly

dependent on placement depth in sandy soil. In the latter, much more U was taken up by the plants

when it was placed near the soil surface, implying that uptake was dependent upon root activity.

Migration of U from the contaminant band was substantial only in the sandy soil.

Dreesen and Cokal (1984) evaluated a technique for assessing the potential uptake of

contaminants growing on chemical waste burial sites using several plant species; i.e. Atriplex

canescens, Kochia scoparia, barley, lucerne and Melilotus officinalis; growing on uranium mill

tailings materials. They noticed that plant availability of Mo, Se, Cl and other trace metals in the

waste was greater than in the surface soil from a uranium mining area. They observed a significant

differences between spp. in the content of nutrients and contaminants in their aerial parts. Barley

contained higher levels of U and much higher levels of Si than the other spp., while lucerne had

higher levels of Al, Ba, Co and V and M. officinalis had higher levels of Ba and V than barley.

The water table interface is the transition between unsaturated, usually aerated, soil and

water-saturated, often anaerobic soil. Plant roots may approach this transition zone and absorb

elements soluble in and below the transition zone. Sheppard and Evenden (1985) examined this

possibility; elements that differ in solubility were studied as a function of oxidation-reduction

potential. Barley (Hordeum vulgare cv. Conquest) was grown in field lysimeters, and treatments

included various elements (iron, technetium and uranium plus phosphorus), element placement

depths, and water table regimes. Measurements included plant uptake of each element, plant root

distribution, and soil profiles of total and extractable amounts of each study element and related

elements. They stated that transition from aerobic to anaerobic conditions occurred 10 cm above the

water table interface. The mobility of the elements decreased in the order Tc > U > P > Fe. They

noticed that the redox gradient markedly modified the mobility of Tc and U. while Fe was more

soluble in the reducing environment than in the oxidizing environment, but no migration of Fe was

detected. The plant roots were almost entirely confined to the oxidized layers of soil. The plants

absorbed no treatment Fe, very little treatment P and U (from the shallow placement only), but

substantial amounts of Tc. A fluctuating water table further restricted plant uptake. The water table




266

interface markedly changed the mobility of some of the studied elements but the effect on rooting

depth was more profound. Thus, the water table interface did not present a unique condition for

plant roots to absorb elements as the elements diffused upward from the reducing environment.

Sheppard nd Evenden (1988) evaluated the major assumption in the use of concentration ratios

to describe the transfer of elements in the environment. The ratios examined in detail were the

'concentration ratio' (CR) of leaf to soil and the 'partition coefficient' (Kd) of solid- to liquid-phase

concentrations in soil. The 'translocation ratio' of berry to leaf was also computed when possible.

Soil was experimentally contaminated to evaluate this linearity over more than a 1000-fold range in

concentration. A secondary objective was to determine CR and Kd values in a long-term (2 y)

outdoor study using a peat soil and a slow-growing native plant species, blueberries (Vaccinium

angustifolium). The elements I, Se, Cs, Pb and U were chosen as environmentally important

elements expected to provide a broad range in CR and Kd values. Their results indicated that

relationships of leaf and leachate concentrations were not consistently linearly related to the total

soil concentrations for each of the elements.

A field study was conducted in an area of enhanced natural radioactivity to assess

concentration ratios (CR = concentration in dry plant /concentration in dry soil) of 232

Th, 230

Th, 226

Ra, 228

Ra, and the light rare earth elements (REE's) La, Ce and Nd (Linsalata et al,. 1989).

Twenty-nine soil and 42 plant samples consisting of relatively equal numbers of seven crop

varieties were obtained from 11 farms on the Pocos de Caldas Plateau, Minas Gerais, Brazil. This

region is the site of a major natural analogue study to assess the mobilization and retardation

processes affecting Th and the REE's at the Morro do Ferro ore body and U series radionuclides at a

nearby open pit U mine. Thorium (IV) serves as a chemical analogue for quadrivalent Pu and the

light REE's (III) as chemical analogues for trivalent Am and Cm. The geometric mean CR's (X10-4)

decreased as follows: 228

Ra (148) > 226

Ra (76) > La (5.4) > Nd (3.0) = Ce (2.6) > 232

Th (0.6). They

indicted that these differences may reflect the relative availability of these metals for plant uptake.

Significant differences were found in the CR's (for any given analyte) among many of the plants

sampled. The CR's for the different analytes were also highly correlated. The reasons for the

correlations in CR's among elements with such diverse chemistries as Ra-REE or Ra-Th are not

clear but are apparently related to the essential mineral requirements or mineral status of the

different plants sampled. This conclusion is based on the significant correlations obtained between

the Ca content of the dried plants and the CR's for all of the elements studied.

Samples of upper and lower foliage of Quercus ilex (and associated soil) were taken in summer

and winter from 41 areas evenly distributed over the urban territory of Rome, and from 5 non-urban

(control) areas; in addition, one tree was sampled over the course of one year (Capannesi et al.,

1988). Elemental concentrations were determined by gamma-spectrometric measurements on

neutron flux-irradiated samples. Results indicated a direct relation between concentration of

pollutant elements (e.g. Pb) and the presence of pollution sources, i.e. vehicular traffic. Urban

structure exerted a major role in that, to a certain extent independent of traffic level, dense building

formed a barrier hindering dispersion of particulates and favouring resuspension of pollutant

elements from the soil (and also leading to relatively high concentration of elements not

characteristic of pollution sources such as Ta, Th, U and rare earths). Some elements, particularly

Mn, showed an inverse relation with pollution levels. No significant seasonal trends were observed.

It is concluded that the results confirmed the validity of use of Q. ilex as a biological monitor of

pollution by stable elements.

Steubing et al., (1993) investigated the potential of using higher plants as indicators of uranium

distribution in soil at a site in Germany where uranium concentrations ranged from 5-1500 mug/g

soil and reached a maximum of 1860 mug/kg in soil water. Results indicated that Sambucus nigra

was the best indicator of uranium contamination. Chemical analysis of its leaves provided more

detailed information regarding uranium distribution than soil analyses. The plants not only indicate




267

the location of mineralization but also the migration pathway of U-containing soil-water. They

indicated that adsorption of contaminated water was the main source of the U-accumulation in the

different plant organs.

Ibrahim and Whicker (1992) determined radionuclide concentrations in soil and tailings and in

mixed grasses of Agropyron, Koeleria, Hordeum and Oryzopsis spp., mixed forbs of Melilotus,

Kochia and Salsola spp., and Artemisia spp., at an uranium mill in Wyoming and were found to be

elevated. Plant:soil concentration ratios were in the order 238

U > 230

Th > 210

Po > 226

Ra > 210

Pb. It

was concluded that for sulfuric acid leached tailings, Ra and Pb are sequestered as sulfates which

were highly insoluble relative to U and Th sulfates, resulting in reduced availability for plant

uptake. Soil acidity and the saturation condition at the tailings impoundment edge tend to enhance

radionuclide availability for plant uptake.

Pinto beans (Phaseolus vulgaris), sweetcorn, and zucchini squash (Cucurbita pepo) were grown

in a pot study using alluvial soils contaminated with various radionuclides (Fresquez et al., 1998).

Soils as well as washed edible (fruit) and non-edible (stems and leaves) crop tissues were analysed

for tritium (3H),

137Cs,

90Sr,

238Pu,

239,240Pu,

241Am, and total uranium (totU). Most radionuclides,

with the exception of 3H and totU, in soil and crop tissues were detected in significantly higher

concentrations (P<0.05) than in soil or crop tissues collected from regional background locations.

Significant differences in radionuclide concentrations among crop species (squash were generally

higher than beans or sweetcorn) and plant parts (non edible tissue were generally higher than edible

tissue) were observed. Most soil:plant concentration ratios for radionuclides in edible and non-

edible crop tissues grown in the studied soils were within default values in the literature commonly

used in dose and risk assessment models. Overall, the maximum net positive committed effective

dose equivalent of beans, sweetcorn, and squash in equal proportions was 74 mrem/year (740

µS/year). This upper bound dose was below the International Commission on Radiological

Protection permissible dose limit of 100 mrem/year (1000 µS/year) from all pathways and

corresponds to a risk of an excess cancer fatality of 3.7 X10-5

(37 in a million), below the US

Environmental Protection Agency's guideline of 10-4

.

Two sampling campaigns were performed in 1993 at the marsh area (Odiel marsh) located in

the city of Huelva, SW Spain. Spartina densiflora and substrate soil (5 cm deep) samples were

collected in several locations across the area (Martinez et al., 1997). Activity concentrations of 210

Po, U and Th isotopes were determined in the plant and substrate samples. The production of

phosphoric acid from phosphate mineral in the vicinity enhanced the concentrations of these

radionuclides in certain areas of the marsh. Moreover, concentrations in plants were affected by the

concentration of the same element in its substrate. Indeed, high concentration levels in plants were

coincident with high concentration in soils. However, concentration ratios (CR), defined as the ratio

between the concentration of an element in the plant and of that in its substrate, were higher when

substrate concentrations were low, whereas low CR values were found in areas where substrate

concentrations were high. Moreover, both variables (CR and soil concentration) were non-linearly

related, at least in the case of radio nuclides from the 238U decay chain.

Levels of 210

Pb were measured in 13 edible vegetables and their associated soils from a

vegetable garden and a farm close to the uranium mining and milling facilities of Pocos de Caldas,

Brazil, (Santos et al., 1993) an area with a high radiation background. The 210

Pb concentration were

compared with those found at a control vegetable garden distant from the mining and milling

operations. Edible vegetables collected at the farm and vegetable garden in the mining area had

higher levels of 210

Pb levels than those from the control site. Soil contamination in the mine vicinity,

occurring by mineral dust deposition or by natural fallout, diminished with increasing distance from

the mineral processing facility. The 210

Pb levels in the soils studied were 8-220 Bq/kg. The

surface/mass ratio and hairiness affected the plants' ability to take up 210

Pb and 210

Po. For lichens

and bryophytes the 210

Pb levels were higher than those in the control region.




268

Effect of fertilizers on uptake of Uranium by plants:

The use of P fertilizers is necessary for a sustainable agriculture, but it involves radioactive

contamination of soils. Phosphorus fertilizers contain different amounts of trace elements. The

problem of cadmium (Cd) loads and other heavy metals is well known. However, only a limited

number of investigations examined the contamination of phosphates with the two heaviest metals,

uranium (U) and thorium (Th), which are radioactive. Also potassium (K) is lightly radioactive.

Measurements were made on the radioactivity content of phosphates, P fertilizers and soils. The

radiation doses to workers and public as well as possible contamination of soils from phosphate

rock or fertilizer caused by these elements or their daughter products is of interest with regard to

radiation protection. Variable amounts of the radionuclides U, Th and Ra can remain in P fertilizers,

originating in the phosphate rock. Experimental work has shown that plant uptake of radionuclides

in P fertilizers or phosphogypsum is minimal (Mortvedt 1992).

In some phosphates uranium and thorium are enriched compared to the average content

in the earth’s crust or soil. The question arises if the appertaining radioactivity in fertilizers applied

over many years becomes harmful compared to the natural background. Heavy metals contents in

raw materials used in phosphate fertilizer industry as well as phosphate products were determined

(Abdel-Sabour and Rizk, 2002). The samples were collected from Abu-Zabal phosphate factory in

El-Qalubia governorate Egypt. The samples include: Rock phosphate, gypsum, limestone, sulfur (as

raw materials), sulfuric and phosphoric acids, mono phosphate (soft and granulate), triple phosphate

(as final products). The aim of the study was to determine the elemental pattern in phosphate

ingredient materials as well as the fertilizers produced. The results revealed that the main source of

heavy metals including U and rare earths in the produced phosphate fertilizers is mainly due to the

impurities in rock phosphate. Rock phosphate exhibited the highs 238

U and 232

Th activities

compared to limestone and sulfur. While the highest 40

K activities was noticed in case of limestone

followed by sulfur raw material. They concluded that a potential metal accumulation in soil is

expected on the long run due to the high application rate of phosphate fertilizers.

Phosphogypsum (PG) is a residue of the phosphate fertilizer industry that has relatively high

concentrations of 226Ra and other radionuclides. Thus, it is interesting to study the effect of PG

applied as a Ca amendment on the levels and behavior of radionuclides in agricultural soils. A study

involving treatments with 13 and 26 Mg ha-1

of PG and 30 Mg ha-1

of manure was performed,

measuring 226Ra and U isotopes in drainage water, soil, and plant samples (El-Mrabet et al., 2003).

The PG used in the treatment had 510 ± 40 Bq kg-1

of 226

Ra. The 226

Ra concentrations in drainage

waters from PG-amended plots were similar (between 2.6 and 7.2 mBq L-1

) to that reported for

noncontaminated waters. Although no significant effect due to PG was observed, the U

concentrations in drainage waters (200 mBq L-1

for 238

U) were one order of magnitude higher than

those described in noncontaminated waters. This high content in U can be ascribed to desorption

processes mainly related to the natural adsorbed pool in soil (25 Bq kg-1

of 238

U). This is supported

by the 234

U to 238

U isotopic ratio of 1.16 in drainage waters versus secular equilibrium in PG and P

fertilizers. The progressive enrichment in 226

Ra concentration in soils due to PG treatment cannot be

concluded from their data. They concluded that PG treatment did not determine any significant

difference in 226

Ra concentration in drainage waters or in plant material [cotton (Gossipium

hirsutum L.) leaves] except 40

K were found in the vegetal tissues.

The effects of large applications of fertilizers on the uptake of uranium and thorium by cotton

and wheat from soils containing small amounts of natural radionuclides were studied by Butnik and

Ishchenko (1990). The uranium content of cotton plant parts decreased by a factor of 2.9 and that of

thorium by a factor of 3.8; the uptake of uranium in wheat grain was less than the control by a

factor of 1.4 and that of thorium by a factor of 2.6. They noticed that complete mineral fertilizer

treatment had a greater effect on radionuclide uptake than individual applications of mineral




269

nutrients. The smallest uptake of uranium and thorium by cotton resulted from combined use of

complete mineral fertilizer and manure treatment. Their results showed that fertilizer application

altered the relative amounts of water-soluble, exchangeable, acid-soluble and fixed forms of the

radionuclides in the soil. Phosphorus fertilizers decreased the exchangeable thorium fraction by

20%, and, in combination with manure, by 40%.

A greenhouse experiment was conducted to evaluate differences in growth and uptake (U),

influx (IN) and transport (TR) of nutrients by 40 exotic genotypes (Baligar et al., 1993). Genotypes

were selected from the world collection (27 entries originated in Africa (Ethiopia, Sudan, Nigeria,

Uganda and Tanzania), 8 were from India and 5 were from the USA or of unknown origin) and

were tested in a dark red latosol at 2, 41 or 64% Al saturation. Growth of shoots and roots and

nutrient uptake parameters were affected by soil Al saturation, genotype and their interactions.

Growth and uptake parameters were used to separate the genotypes into most efficient and least

efficient categories at various levels of Al saturation. Shoot nutrient U, IN and TR were positively

correlated with shoot and root dry weight, and inversely related to soil Al saturation and shoot Al

concentration. The method appeared to be reliable for separating genotypes into Al-tolerant and

sensitive types. Genotypes used in the study showed intra-specific genetic diversity in growth and

U, IN and TR of essential elements. Their results suggested that selection for acid-tolerant

genotypes and further breeding for Al tolerance is feasible; SC283 (IS7173C) from Tanzania

appears to be of most use in breeding programmes.

Laboratory and greenhouse studies were conducted to quantify apatite and phillipsite (zeolite)

sequestration of selected metal contaminants (Knox et al., 2003). The laboratory batch study

measured the sorption of aqueous Co2+

, Ba2+

, Pb2+

, Eu3+

, and UO22+

. They indicated that apatite

sorbed more Co2+

, Pb2+

, Eu3+

, and UO22+

from the spike solution than phillipsite, resulting in

distribution coefficients (Kd values) of > 200 000 L kg-1

. However, phillipsite was more effective

than apatite at sorbing aqueous Ba2+

. Results from the laboratory study were used to design the

greenhouse study that used a soil affected by a Zn-Pb smelter from Pribram, Czech Republic. Two

application rates (25 and 50 g kg-1

) of phillipsite and apatite and two plant species, maize (Zea mays

L.) and oat (Avena sativa L.), were evaluated in this study. Apatite and, to a slightly lesser extent,

phillipsite additions significantly enhanced plant growth and reduced Cd, Pb, and Zn concentrations

in all analysed tissues (grain, leaves, and roots). The sequestering agents also affected some

essential elements (Ca, Fe, and Mg). Phillipsite reduced Fe and apatite reduced P and Fe

concentrations in oat tissues; however, the level of these elements in oat leaves and grains remained

sufficient. Sequential extractions of the soil indicated that the Cd, Pb, and Zn were much more

strongly sorbed onto the amended soil, making the contaminants less phytoavailable.

The contents of rare earth elements (REEs; La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb,

and Lu) and other trace elements (Be, As, Ag, Cd, Sb, Cs, Bi, and U) in waste water treatment

sludges were determined (Kawasaki et al., 1998). In night-soil sludges and sludges of waste water

treatment plants in the food industry, the distribution patterns of REEs normalized vs. average REEs

were almost flat. It was considered that the REE patterns of uncontaminated sludges reflected the

pattern of the continental crust. The crust-normalized REE patterns of sludges of waste water

treatment plants in the chemical industry and municipal sewage sludges did not always show flat

plots. The sludges that did not show a flat REE pattern were considered to be contaminated with

some of the REEs. The coefficient of variation of each element determined among the 10 samples

of night-soil sludges and the 14 samples of sewage sludges ranged from 34 to 77% and from 26 to

84%, respectively. Among the 10 samples of food industry sludges and the 10 samples of chemical

industry sludges, the coefficient ranged from 60 to 143% and from 67 to 172%, respectively. The

variations of the content of each element among the food industry sludges or the chemical industry

sludges were larger than those among the night-soil sludges or the sewage sludges. The contents of

Be, As, Cs, REEs, and U in all the sludges were lower than or the same as those in a field soil.




270

Some of the food and chemical industry sludges contained larger amounts of Ag, Cd, and Sb than

the soil. All the night-soil sludges and sewage sludges contained much larger amounts of Ag and Bi

than the soil.

The effects of iron oxides and organic matter on the partitioning and chemical lability of U and

Ni were examined for contaminated riparian sediments from the U.S. Department of Energy's

Savannah River Site (Andrew et al., 2003) . In sequential extractions of four sediments that

ranged

from 12.7 to 82.2 g kg-1

in organic carbon, U was found almost exclusively in moderately labile

fractions (93% in acid-soluble + organically bound). Nickel was distributed across all operationally

defined fractions, including substantial amounts in the very labile fractions (4–15% in water-soluble

+ exchangeable), noncrystalline and crystalline iron oxides (38–49%), and

in the nonlabile residual

fraction (25–34%). Aqueous U concentrations in 1:1 sediment–water extracts were highly

correlated

to dissolved organic carbon (DOC) (R2 = 0.96; p <

0.0001) and ranged from 29 to 410 µg L

-1.

Aqueous concentrations of Ni exceeded U by two to three orders of magnitude (124–2227

µg L

-1)

but were not correlated with DOC (R2 = 0.04; p

= 0.53). Partitioning and solubility trends suggest

that Ni availability is controlled primarily by iron-oxide phases, whereas

U availability is dominated

by naturally occurring organic carbon. Discrete mineral phases were also identified as nonlabile

reservoirs of anthropogenic metals. In spite of comparably high sediment

concentrations, Ni appears

to be significantly more available than U in riparian sediments and therefore warrants greater

consideration in terms of environmental consequences (i.e., transport, biological uptake, and

toxicity).

Abdel-Haleem et al., (1997) studied the U and Th Enrichment in Corn and Sesame Seeds due

to Organic Waste Application to Sandy Soil. Uptake by plant depends upon several factors

including plant species and above, all the availability of U in soil. Uranium content in most

biological and plant materials are often very low and methods with low detection limits are needed

to obtain results of reasonable accuracy. Neutron activation analysis methods are very often applied

for the determination of U and Th because of the low detection limits for these elements. An

attempt is made to evaluate the uranium and thorium quantities transferred to corn and sesame seeds

due to organic waste disposal to sandy soil. The tested organic wastes were biosolids (Bs) and

municipal solid waste (MSW) applied at increasing rates. Sludge samples showed higher values of

both Np and Pa (derivatives of U and Th) concentration compared to MSW samples. They indicated

that increasing organic waste rate of application, significantly increases Np-239 and Pa-232

contents in seeds of tested crops. However, MSW treatment resulted always a higher accumulation

of Np-239 and Pa-232 in sesame seeds than Bs treatments. A significant linear regression equation

was obtained between the rate of application for any tested organic source and Np-239 or Pa-232

content in sesame seeds or corn grains. This linear relation indicates that these metals are readily

available to the plant. They stated that always, transfer values for corn in general were higher than

those for sesame by one order of magnitude. The data can also be used for evaluation of the U and

Th accumulation parameter due to the organic waste disposal to agriculture land.




271

Conclusion

Results reveled that U uptake by plants seems to be pH-dependent in most studies. Soil acidity

and the saturation condition at the tailings impoundment edge tend to enhance U availability for

plant uptake. Uranium transportation in soil is dependent on the direction of the surface or soil

water flow when it is soluble and mobile (consequently being bioavailable). This was supported

with the finding that migration of U from the contaminant band was substantial only in the sandy

soil. Moreover, investigations indicated that adsorption of contaminated water was the main source

of the U-accumulation in the different plant organs.

Correlations between radionuclide activity concentrations and stable element concentrations in

the soil samples are a good tool to understand the possible distribution paths for natural

radionuclides.

Concentration ratio (CR) values have been used to predict transport of radionuclides and other

elements through the food chain and for biogeochemical exploration for U. Frequently, The CR data

were fitted by nonlinear least squares to a exponential function that decreased with time to a

constant value. Which was confirmed later with the results indicating that relationships of leaf and

leachate concentrations were not consistently linearly related to the total soil concentrations for U.

Uranium uptake, however, seemed more sensitive than Th uptake to soil properties. However

several translocation studies in general indicated that U-concentration in the grain was significantly

less than in the husk, which was significantly less than in straw. Repeatedly, reports shown the

tendency of U like other heavy metals to be accumulated in plant roots. Thus it is expected that

radish and like roots can be a significant source of U. A significant correlations obtained between

the Ca content of the dried plants and the U-CR which may suggested the close relation between Ca

and U uptake.

Fertilizer application (chemical and organic) can alter the relative amounts of water-soluble,

exchangeable, acid-soluble and fixed forms of the U in the soil. Variable amounts of the

radionuclides U, Th and Ra can remain in P fertilizers, originating in the phosphate rock. This lead

to a conclusion that a potential U accumulation in soil is expected on the long run due to the high

application rate of phosphate fertilizers.

Increasing organic waste (biosolids (Bs) or municipal solid waste (MSW) compost) rate of

application, significantly increases U and Th contents in seeds of tested crops (sesame and corn). It

was noticed that always, transfer values for corn in general were higher than those for sesame by

one order of magnitude. However, MSW treatment resulted always a higher accumulation of U and

Th in sesame seeds than Bs treatments. This indicates that these metals are readily available to the

plant due to the organic compounds in the soil.




272

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