G. Shaw

Soil contamination with radionuclides and potential remediation.

Soils contaminated with radionuclides, particularly 137Cs and 90Sr, pose a long-term radiation hazard to human health through exposure via the foodchain and other pathways. Remediation of radionuclide-contaminated soils has become increasingly important. Removal of the contaminated surface soil (often up to 40 cm) or immobilization of radionuclides in soils by applying mineral and chemical amendments are physically difficult and not likely cost-effective in practicality. Reducing plant uptake of radionuclides, especially 137CS and 90Sr by competitive cations contained in chemical fertilizers has the general advantage in large scale, low-level contamination incidents on arable land, and has been widely practiced in central and Western Europe after the Chernobyl accident. Phytoextraction of radionuclides by specific plant species from contaminated sites has rapidly stimulated interest among industrialists as well as academics, and is considered to be a promising bio-remediation method. This paper examines the existing remediation approaches and discusses phytoextraction of radionuclides from contaminated soils in detail.

Competitive effects of potassium and ammonium on caesium uptake kinetics in wheat

Caesium (Cs) uptake by roots of winter wheat is known to obey Michaelis-Menten kinetics. As a result, the solution-to-root transfer factor for radiocaesium is depressed in a non-linear fashion by increasing the concentration of stable caesium (133Cs) in the external solution. It is likely that 133Cs concentrations in the environment are insufficient to result in this effect being observed. However, any environmentally abundant ion which demonstrates a general physiological equivalence with the Cs+ ion may exert a similar effect on the soil-to-plant transfer factor for radiocaesium. The K+ and NH4+ ions are members of the same homologous series to which the Cs+ ion belongs. A mechanistic scheme of root uptake competition between these ions is proposed which envisages Cs uptake as a reaction which may be competitively inhibited by any ‘analogous’ ion. A series of equations is presented describing this inhibition of Cs uptake in terms of its effect on the solution-to-root transfer factor for radiocaesium. Experimental data gathered in the cases of the inhibition of radiocaesium uptake by the K+ and NH4+ ions show a generally good agreement with the proposed theoretical relationships over the concentrations ranges of K+ and NH4+ (0–10 μM) examined.

Radiocaesium uptake and translocation in wheat with reference to the transfer factor concept and ion competition effects

A theoretical scheme is presented which considers the uptake of radiocaesium by roots of wheat and its subsequent translocation to above ground tissues. This is tested experimentally against data derived for actively growing plants in the presence of increasing concentrations of stable caesium (133Cs), potassium (K) and ammonium (NH4). Transfer functions for 137Cs are defined in the presence of each of these ions. Mean transfer factors derived for roots over the ion concentration ranges examined were approximately one order of magnitude greater than those for shoots and transfer functions in all cases were distinctly non-linear. Experimental data adhered closely to the model in all instances except that of K+ at an external concentration greater than 20 μM. This anomaly is identified as an ion-specific effect with important radioecological implications. The uses and limitations of the soil-to-plant transfer concept and simple mechanistic models such as that presented here are discussed with reference to the problem of modelling the movement of radioactive substances within the environment.

Predicted spatio-temporal dynamics of radiocesium deposited onto forests following the Fukushima nuclear accident

The majority of the area contaminated by the Fukushima Dai-ichi nuclear power plant accident is covered by forest. To facilitate effective countermeasure strategies to mitigate forest contamination, we simulated the spatio-temporal dynamics of radiocesium deposited into Japanese forest ecosystems in 2011 using a model that was developed after the Chernobyl accident in 1986. The simulation revealed that the radiocesium inventories in tree and soil surface organic layer components drop rapidly during the first two years after the fallout. Over a period of one to two years, the radiocesium is predicted to move from the tree and surface organic soil to the mineral soil, which eventually becomes the largest radiocesium reservoir within forest ecosystems. Although the uncertainty of our simulations should be considered, the results provide a basis for understanding and anticipating the future dynamics of radiocesium in Japanese forests following the Fukushima accident.

The Kinetics of Caesium absorption by roots of winter wheat and the possible consequences for the derivation of soil-to-plant transfer factors for radiocaesium

Abstract Caesium (Cs) uptake in roots of winter wheat was found to follow a dual pattern similar to that established for potassium uptake in barley roots. This suggests the operation of two discrete uptake systems for Cs, as for potassium. The ‘System 1’ (low concentration) uptake mechanism for caesium, however, can be resolved into two hyperbolic components which both obey Michaelis-Menten kinetics. The Michaelis-Menten equation was used to derive a function which describes the variation in solution-to-root transfer factor for any element for which the appropriate root uptake constants (K m and V max ) can be determined. This function successfully described available data for root uptake of caesium and potassium, predicting that the solution-to-root transfer factor decreases in relation to an increase in the substrate concentration of each respective element. At substrate concentrations equivalent to carrier-free radiocaesium concentrations, however, the solution-to-root transfer factor predicted by the function and by empirical data suggests that the relationship between root uptake and solution concentration of caesium is linear. These findings are discussed in relation to the comparative physiology of caesium and potassium uptake by plant roots and with respect to the application of the soil-to-plant transfer factor concept to radioecological studies.

Blockade by fertilisers of caesium and stronium uptake into crops: effects on the root uptake process

The use of fertilizer application to soils as a countermeasures against the absorption of radiocaesium and radiostrontium by crop roots relies on ionic components of the added materials being able to reduce the trans-membrane flux of radionuclides into the root tissues. This paper reviews evidence which suggests that the potassium (K+) and calcium (Ca2+) ions are the most effective ‘blockers’ of caesium and strontium uptake, respectively, due to their ability to compete directly with these radioions for membrane associated uptake sites. The nature of this competition is highly concentration-dependent giving rise to non-linear soil-to-plant radionuclide transfer functions; in the case of competitive antagonism between Cs and K new evidence suggests that membrane selectivity in favour of the K+ ion may be ‘switched on’ at a threshold concentration resulting in a particularly effective potential exclusion of radiocaesium by this nutrient element. However, it is suggested that at ambient soil solution concentrations of both K and Ca, radiocaesium and radiostrontium uptake may already be so reduced in relation to its maximum potential efficiency that any fertiliser additions will have a relatively small effect in further reducing radionuclide absorption. This supports the observation that fertiliser-based countermeasures are most often effective in nutrient poor soils with a low degree of base saturation.

Interception of caesium-contaminated rain by vegetation

Abstract A series of experiments were carried out to quantify the interception of rain-borne caesium by crops of grass ( Lolium perenne ), broad beans ( Vicia faba ) and wheat ( Triticum aestivum ) under rainfall intensities typical of those encountered in the U.K. (1–18 mm h −1 ). The fraction of contamination deposited on the vegetation decreased with time, as the plant surface approached a saturation level of contamination. Surface contamination was dependent upon the total amount of rainfall rather than the rainfall intensity. The total surface water storage capacity of wheat and beans was determined, and found to approach a saturation level as total rainfall increased in a manner similar to the measured contamination levels. Total accumulation of caesium from aqueous solution onto bean leaves was found to increase linearly with time over the range of concentrations tested (0.0001–10 mM), the rate of accumulation increasing with concentration according to a power law. The combination of results obtained suggests that deposition of rain-borne contaminants could be modelled appropriately by a water storage capacity term and a “chemical term” derived from the contaminant concentration and its affinity for a particular plant surface.

Application of fertilisers and ameliorants to reduce soil to plant transfer of radiocaesium and radiostrontium in the medium to long term - a summary

Abstract Factors influencing the effectiveness of fertilisers and ameliorants as soil-based countermeasures for reducing the transfer of radiocaesium and radiostrontium to plants are described. The applicability of potential treatments and treatment rates for different soil types is discussed. Data from small scale experiments conducted under controlled conditions, as well as field investigations carried out in Belarus and Ukraine are summarised in an overall evaluation of the most common and most effective soil-based agrochemical treatments available.

Soil transport and plant uptake of radio-iodine from near-surface groundwater.

This paper describes a 12-month experiment designed to study the extent of upward migration of (125)I (as a surrogate for (129)I) from near-surface groundwater, through a 50-cm column of soil and into perennial ryegrass. The water table was established at a depth of 45 cm below the soil surface. By 3 months, (125)I had migrated about half way up the soil column. After this, it tended to accumulate just above this mid-point, with only very small amounts being transported to the upper 20 cm of soil. This behaviour seemed to be explained well by soil moisture and redox conditions. The experiment indicated that (125)I was mobile only within the saturated/low redox zone at the base of the soil column and accumulated in the zone of transition between anoxic and oxic soil conditions. Uptake of (125)I by the ryegrass was found to be low.

Effect of potassium starvation on the uptake of radiocaesium by spring wheat (Triticum aestivum cv. Tonic)

Short term experiments were carried out to investigate the effect of internal tissue potassium concentration on the uptake of radiocaesium by spring wheat (Triticum aestivum cv. Tonic). The results showed that potassium starvation increased Cs influx rates by a factor of 10 compared with non-starved plants. Solution to plant tissue transfer factor (TF) values also increased by around an order of magnitude after potassium starvation treatment. The enhancement of Cs influx rates by potassium starvation could be offset by an increase in external potassium concentration: this effect is minimal at external potassium concentrations greater than approximately 200 μM (8 mg L-1). This reveals that Cs influx into plant roots is subject to control by both internal and external potassium status. The kinetics of Cs uptake by potassium in starved and non-starved plants can be described adequately by the Michaelis-Menten equation. It was shown that potassium starvation substantially reduces the Km value from approximately 28 to 6 μM, which suggests that starvation treatment increases significantly the affinity of plant roots for Cs+. Mechanisms involved in K-Cs interactions during plant uptake are discussed in this paper. Finally, the relevance of such mechanisms as determinants of radiocaesium uptake by plants growing under different ecological conditions is emphasised.