Assessing the Aging Effect on the Biovailability of Selenium-79 and Technetium-99 in soils using the Diffusive Gradients in Thin-Films Technique (DGT)

Alex Chapman, Jackie Pates & Hao Zhang

(1) Introduction & Rationale

(2) Approach

(3) Results

(4) Conclusions

Selenium-79 (79Se) and technetium-99 (99Tc) have been identified as significant radionuclides in long-term safety assessments of nuclear waste disposal owing to their long half-life ( ≥105 years) and high mobility.

Understanding the bioavailability of these radionuclides and how they age within different soil types is crucial in order to evaluate their potential transfer from the soil and their incorporation into the biosphere. Entry

into the biosphere would occur as a result of plant uptake in the event that these radionuclides become mobilised within the soil following disposal of nuclear waste in deep geological repositories, or due to aerial

deposition. Once taken up by plants there is then significant potential for redistribution throughout the food chain which can ultimately lead to human consumption of contaminated agricultural produce and livestock.

Figure 1. (Above) Schematic diagram showing

cross-section through DGT device during soil

deployment along with the established

concentration gradient.

Organic carbon is the dominant soil property influencing the availability of both Tc and Se.

The aging of both Tc and Se can be described well by a pseudo-second-order equation. Tc ages faster in soils of higher organic carbon content. Se ages faster in soils of higher organic carbon and in particular

soils with a higher content of Al + Fe. Tc ages faster in soils of higher organic carbon.

Twenty soil samples encompassing a range of properties and land use types were artificially spiked in the laboratory

with selenium (Se) Tc and U. The soils were incubated in the dark at 10-14°C, with DGT deployments made at

progressively-increasing time intervals throughout the incubation period (Figure 2). For the DGT deployments, the soils

are wetted to saturation and subsequently returned to an air-dry state.

3.1. Technetium

How does DGT work?

Based upon the principles of Fick’s first law of diffusion, it acts as sink to promote the continued diffusion of a target element across a well-defined diffusion

pathway (see Figure 1).

Rapid immobilisation of kinetically labile or available species occurs through a selective binding agent within the binding layer (A; Figure 1). A polyacrylamide

diffusive gel layer (B) is interposed between the binding layer and filter membrane (C) to constrain the flux.

Elements are eluted from the binding layer using a suitable acid prior to quantitative analysis.

Figure 2. (Left) DGT deployment in incubated soil sample.

Why DGT?

Conventional extraction techniques for assessing availability have a number of inherent limitations and uncertainties. E.g. their reproducibility and comparability have

been questioned and there is the possibility of induced changes in speciation during the extraction procedure. They are based on an equilibrium approach to

determining exchangeable soil fractions, yet the soil system is a complex dynamic system where a variety of processes operate as sources and sinks such that it is never

in a state of true equilibrium.

DGT offers an in-situ kinetic approach to determining availability by perturbing the soil system in the same manner as a plant would by mimicking the rate of removal of

an element by a plant. A number of studies have demonstrated significant linear relationships between DGT uptake and plant uptake for a range of trace elements and

plant types.

A B C

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3.2. Selenium

• A significant (p < 0.05) relationship was observed between soil organic carbon

content and the DGT-measured availability of Tc (TcDGT) at short (3-100 days)

and longer (549 days) periods of aging.

• Resupply of Tc from the soil solid phase was very slow at both short and long

periods of aging and did not appear to be governed by any particular soil

property.

• Aging in all soils could be described particularly well by a pseudo-second-order

equation as given by:

𝑄𝑡 =𝑡

1𝑘

𝑄𝑒2 +𝑡𝑄𝑒

where Qt is availability at time t (days), Qe is availability at equilibrium (where

aging is negligible) and k is the pseudo-second-order rate constant (synonymous

to the aging rate constant).

• The pseudo-second-order rate constant (k) was found to be positively

correlated (R2 = 0.5) with soil organic carbon.

• DGT was found to be unsuitable for the measurement of Se in soils of higher

alkalinity (> 10 mM HCO3-). DGT uptake of selenate (SeO4

2-) and selenite

(SeO32-) anions was suppressed due to competition with bicarbonate (HCO3

-)

anions.

• A highly significant relationship (p < 0.01) was observed between soil organic

carbon and the DGT-measured availability of Se (SeDGT) at shorter periods of

aging (3 – 100 days). A significant relationship ( p < 0.05) was observed with

the total Al + Fe content of the soil over the same time period.

• Resupply of Se from the soil solid phase was generally very slow and appeared

to be controlled by the soil organic carbon.

• The pseudo-second-order aging rate constant was found to be positively

correlated with both organic carbon (R2 = 0.49) and total Al + Fe (R2 = 0.76).

Figure 3. DGT-measured availability of Tc in all 20 incubated soils over 549 days aging after normalising each time point to day three. Soils are highlighted according to organic carbon content.

Figure 4. DGT-measured availability of Se in nine incubated soils over 549 days aging after normalising each time point to day three. Soils with an alkalinity > 10 mM HCO3

- are omitted. All soils are highlighted according to organic carbon content and texture.

> 4%% organic C: < 4% 4 - 8%

> 4%% organic C: < 4% 4 - 8%