Adsorption and desorption of iodine by various Chinese soils: I. Iodate

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Abstract

In order to assess the adsorption of iodate by different soils from China, a series of batch experiments were conducted. It was found that soils rich in iron oxide had high affinity for iodate. Iodate adsorption isotherms could be well fitted with both Langmuir and Freundlich equations. Iodate adsorption by 20 different soils from China revealed that iodate adsorption was significantly correlated with soil organic matter negatively and positively with free iron oxide contents. At initial concentration of 4 mg I L−1, iodate adsorption ranged between 9 and 34 mg kg−1 soil. No correlation between iodate adsorption and cation exchange capacity and soil pH was found. For a single soil, there was a significant linear relationship between the amounts of iodate adsorbed and desorbed, but for a group of different soils, the relationship between the amounts of iodate adsorption and desorption followed a nonlinear relationship, the deviation mainly occurred at high adsorption side. The relationship between Kd and free aluminum oxide and free iron oxide contents showed an exponential relationship for various soils with exception of the soil from Hetian in Xinjiang.

Introduction

Iodine is an essential element for human and animals. It has been well established that environmental iodine deficiency can cause a number of health problems known as iodine deficiency disorders (IDD), such as goiter, spontaneous abortion or sterility (Ahuka and Geelhoed, 1997) and mental retardation (Delange et al., 2001). It has been estimated that around 1570 million people in the world are at the risk of IDD, with 650 million suffering from goiter (14% of the world population) (Delange, 1998). Apart from the visual symptom of iodine deficiency-goiter, it is generally held that iodine deficiency may cause a loss of up to 10 IQ points per person across the community (Stewart et al., 2003).

Supplementation of I in salts has been proven to be a useful approach to eliminating IDD in many parts of the world, but the actual effectiveness may depend on a number of environmental and socioeconomic factors in a particular region. The use of iodized salt has been promoted in China from early 1980s; however, IDD remains a major health problem in remote areas such as Xinjiang Province. In these regions, I intake by the population is inherently low because there is generally less I in soil and drinking water than in coastal regions. Iodine deficiency in Xinjiang, China is further exacerbated by two factors. Firstly, the ethnic minority people in Xinjiang prefer the rock salt that is available free of charge. Secondly, Chinese cooking methods using oil at high temperature may result in substantial loss of I via volatilization (Zhang et al., 2000). A recent survey in Xinjiang showed that about 23% of children aged between 8 and 10 had goiter (XJDCC, 2000). It is therefore suggested that approaches to improving human iodine nutrition other than salt iodination may be necessary for some regions taking into consideration of socioeconomic and environmental factors (Zhu et al., 2003). Iodine supplementation through the food chain has recently been attempted in Xinjiang, for example by the irrigation of paddy soil with iodized water to reduce I deficiency (Cao et al., 1994). Supplementation of I in leafy vegetables has also been studied recently in controlled environments (Zhu et al., 2003). However, efficient management of I in soil–plant system depends on the understanding of the fate and behavior of I in soil.

The other concern with iodine in the environment is that the accumulation of radioactive isotopes of I can be deleterious. Radionuclides of I, primary 131I (half-life 8.04 days) and 129I (half-life 1.6×107 years), are products of nuclear fission. Radioactive I released from nuclear facilities will contribute to the global I budget, and consequently bioaccumulated in organisms posing health risk to humans through food chain transfer Fuhrmann et al., 1998, Hou et al., 2003. The availability of I for plant uptake and its migration depend largely on its interactions with various soil components (Yoshida et al., 1992). Several studies have been reported on the role of soil components (such as organic matter and clay minerals) in I (I and IO3) adsorption Whitehead, 1973, Whitehead, 1974, Whitehead, 1978, Muramatsu et al., 1990, Yoshida et al., 1992, Fuhrmann et al., 1998. In these studies, only limited soil types have been used, and studies involving more diverse soils with wide range of soil properties are needed for better understanding I geochemistry in different environments. Furthermore, with the rapid development of nuclear industry and the prevalence of IDD in China, there is an urgent need to characterize I chemistry, particularly adsorption–desorption processes in different soils. Therefore, the current study aims to conduct a series of experiments to investigate iodate adsorption by various soils from China, and to identify principal factors controlling iodate adsorption on soil.

Section snippets

Soils

Soil samples with different properties were collected at 20 locations in China. All of the samples were air-dried, and were then sieved through a 1-mm sieve for soil pH, iodate adsorption and desorption studies; ground to pass through a 0.25-mm sieve for cation exchange capacity (CEC); free Fe/Al oxides and through a 0.125-mm sieve for organic matter.

Determination of soil properties

Soil pH was determined with Thermo Orion (Model 828) in a 1:2.5 suspension in H2O. Organic matter was determined by oxidation with potassium

Adsorption isotherms for three soil types

To assess the effect of adsorbate concentrations on iodate adsorption by various soils, three soils types, with distinct properties in terms of soil organic matter, pH, CEC and free Fe/Al oxides (Table 1) were selected to conduct adsorption isotherms. Adsorption isotherms were illustrated in Fig. 1; it can be seen from this figure that Perudic Ferrisols soil from Hainan Province had the highest capacity to adsorb iodate, and the Udic Isohumisols soil from Jilin Province had the lowest capacity

Acknowledgements

This project is supported by the Chinese Academy of Sciences (kzcx1-sw-19 Hundred Talent Program).

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