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The Status of Iodine and Selenium in Waikato Soils

Abstract
This thesis investigates the status of the essential trace elements iodine and selenium in Waikato soils. Soil samples (368) representing different Soil Orders, land uses and soil depths were examined. A tetramethyl ammonium hydroxide extraction method followed by ICP-MS was validated for iodine determination. This method was suitable for total iodine determination and also gave a reliable estimate of the total selenium content of soils, based on analysis of Certified Reference Materials. Acid extraction of selenium was unsuitable because of difficulties with recoveries and interferences in the ICP-MS, although the use of methane and DRC-ICP-MS reduced interferences. Time and equipment constraints limited the complete validation using acid extraction for total selenium determination. Waikato soils had a mean iodine content of 20.9 mg kg-¹, geometric mean of 13.7 mg kg-¹ and range of 1.5 – 122.8 mg kg-¹. Allophanic and Granular soils contained the highest mean iodine contents with Pumice soils displaying the lowest. The iodine content was shown to increase with soil depth, with the Waikato soils showing no evidence of coastal enrichment, though this could be explained by the losses of iodine being equal to, or exceeding the additions to soil. Land use appeared to have an effect on the iodine content of soils with background soils displaying more iodine on average than both farmed and forestry soils. Forestry soils displayed the lowest mean iodine content. Farming and forestry both appear to reduce the amount of iodine in soils. Iodine was correlated strongly to aluminium and iron, indicating that clay minerals and iron oxides are the most important in the retention of iodine, with organic matter appearing to be less important in iodine retention. There was also a strong correlation of iodine with selenium and mercury, suggesting an association between these elements. The selenium status of Waikato soils showed a mean concentration of 1.77 mg kg-¹, geometric mean 1.33 mg kg-¹ and a range in concentration of 0.18 – 12.1 mg kg-¹. Like iodine, selenium also displayed the highest mean concentrations in Allophanic and Granular soils, with the lowest concentrations in Pumice soils. The concentration of selenium also increased with soil depth, with parent material appearing to affect the selenium content of soils. Selenium appeared to be more concentrated in the soils closest to the coast than those more inland; with the relative enrichment suggesting that the losses of selenium are likely to equal or exceed the inputs from the surface. Selenium concentration in relation to land use indicated that farming and forestry may be depleting selenium from soils, with background soils displaying more selenium on average than both farmed and forestry soils. Again selenium showed similar behaviour to iodine in that it was strongly correlated to aluminium, iron and manganese, indicating that clay minerals and iron and manganese oxides are the most important factors in selenium retention in Waikato soils. Organic matter was less important in retention shown by the less significant correlation with selenium. Mercury and iodine were both strongly correlated to selenium suggesting that chalcophilic elements (mercury) are strongly associated to selenium. The strong correlation between iodine and selenium also explained the similarities in the relationships of both elements with other soil properties. The results presented for both iodine and selenium indicate that the status of both elements in the Waikato Region may be better than previously thought, with the soils showing mean concentrations that suggest they may not be as deficient as thought.
Type
Thesis
Type of thesis
Series
Citation
McNally, S. R. (2011). The Status of Iodine and Selenium in Waikato Soils (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/5375
Date
2011
Publisher
University of Waikato
Rights
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