Iodine

Additional information on environmental fate and behaviour

Administrative data

Endpoint:

additional information on environmental fate and behaviour

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: no guideline study, but the test is performed according to good scientific practice and an adequate and reliable documentation is presented.

Data source

Reference

Reference Type:

publication

Title:

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

Author:

Ashworth, D.J., Shaw, G., Butler, A.P., Ciciani, L.

Year:

2003

Bibliographic source:

J. Environ. Radioactivity, 70, 99-114

Materials and methods

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

Several soil samples with simulated near-groundwater are studied for 3, 6, 9, and 12 months. On the soil surface perennial ryegrass was sown to examine the uptake by plants. The grass was cutted and examined on a monthly basis. After the 3, 6, 9, and 12 months the soil samples were destructively sampled by removing the water, the grass and seperating the soil column in different discrete horizontal layers.

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

Sodium iodide

EC Number:

231-679-3

EC Name:

Sodium iodide

Cas Number:

7681-82-5

Molecular formula:

INa

IUPAC Name:

sodium iodide

Details on test material:

Instead of I-127 the radioactive isotope I-125 (Amersham, UK) has been used. It has a physical half-life of around 60 days. As iodide is one of the predominant species of iodine in the soil environment, the substitution of sodium iodide is acceptable.

Results and discussion

Any other information on results incl. tables

The I-125 content of the soil samples was analysed with reference to a sample of the initial I-125 stock solution by using a solid-state scintillation gamma detector (EG&G Wallac 1282 Compugamma CS; Milton Keyes, UK). The detection efficiency for I-125 added to soil was determined to be about 80%.

At 3 -monthly intervalls the columns were destructively analysed. Therefore the water was removed, the grass was cut and the soil was sepereated in discrete horizontal layers. The soil of each layer was homogenised and stored in a plastic bag to prevent moisture loss. Then from each layer triplicate sub-samples were taken and analysed.

The analysis showed that atfer 3 month the iodine had migrated about half way up the soil column. Afterwards, it tended to accumulate at this layer with only very small amounts being transported in the upper 20 cm of the column. By comparing the results of the I-125 measurement and the determination of the redox conditions within the column it can be assumed that soil moisture and redox potential have a strong influence on the mobility of iodine in soil. The experiment gave indication that the iodine was mobile only in the saturated/ anoxic zone at the base of the column. In the transition zone between the oxic and anoxic conditions iodine was accumulated. The analysis of the grass showed that the uptake of iodine was low.

Futhermore the soil extractability of I-125 for the 3 -month samples was investigated. With deionised water about 6.1% of the I-125 activity was extracted. By using 1M sodium hydroxide solution the extractability could be increased to a mean of 60% of the total I-125 activity. This result suggests that a major part of the iodine is bound to the humic material in the soil which is in line with the results of former studies by Whitehead (Whitehead, 1973; Whitehead, 1974), Tikhomirov et al. (Tikhomirov, 1980) and Bors et al. (Bors, 1988).

References:

Bors J, Martens R, Kuhn W (1988). Studies on the role of natural and anthropogenic organic substances in the mobility of radio-iodine in soils, Radiochimica Acta 52/53, 317 -325.

Tikhomirov FA,Kasparov SV, Prister BS, Sal'nikov VG (1980). Role of organic matter in iodine fixation in soils, Soviet Soil Science, 12(1), 64 -72.

Whitehead DC (1973). The sorption of iodine by soils as influenced by equilibrium conditions and soil properties, J Sci Food Agri, 24, 547 -556.

Whitehead DC (1974). The sorption of iodide by soil components, J Sci Food Agri, 25, 73 -79.

Applicant's summary and conclusion

Conclusions:

In contrast to OECD guideline 312 which analyses the leaching of chemicals by rain in soil columns the presented study examines the upward migration of iodine from near-surface groundwater in a soil column and potential uptake by plants. Since both approches investigate the behavior of a chemical in soil the presented study can be considered as relevant for the examination of soil transport.

Executive summary:

Ashworth et al. examined the upward soil migration behaviour of iodine over one year by placing soil sample columns in sealed containers filled with water simulating an artifical groundwater level. After the establishment of a soil moisture equilibrium perennial ryegrass was sown on the soil surface and the pure water was replaced with water containing I-125 which was constantly replaced to maintain the water level. The soil redox conditions were measured by platinum redox electrode probes at different depths of the soil columns. On a 3 -monthly intervall soil columns were destructively analysed and the I-125 content in several discrete horizontal soil layers determined.

After 3 month the iodine had migrated half way up the column, but then it accumulated at this level, with only very small amounts of iodine being transported further in the upper zone of the soil column. This behaviour might be explained by the soil moisture and redox conditions. The experiments indicated that iodine is mobile only in the saturated/ anoxic zone at the base of the column and accumulates in the transition zone between the anoxic and oxic part of the soil.

Additionally, the soil extractabily of iodine was examined for the 3 month samples indicating that a major part of the iodine is probably bound to the humic substances in the soil as the extractability with 1M sodium hydroxide solution was much higher then the extractability with deionised water. This result is in line with former studies of Whitehead (Whitehead, 1973; Whitehead, 1974), Tikhomirov et al. (Tikhomirov, 1980) and Bors et al. (Bors, 1988).

The analysis of the ryegrass showed that only small amounts of iodine were taken up by the plants.

References:

Bors J, Martens R, Kuhn W (1988). Studies on the role of natural and anthropogenic organic substances in the mobility of radio-iodine in soils, Radiochimica Acta 52/53, 317 -325.

Tikhomirov FA,Kasparov SV, Prister BS, Sal'nikov VG (1980). Role of organic matter in iodine fixation in soils, Soviet Soil Science, 12(1), 64 -72.

Whitehead DC (1973). The sorption of iodine by soils as influenced by equilibrium conditions and soil properties, J Sci Food Agri, 24, 547 -556.

Whitehead DC (1974). The sorption of iodide by soil components, J Sci Food Agri, 25, 73 -79.