Iodine release associated with the transformation of iron minerals in
natural sediments
Junxia Lia, X.J. Xiea, M. Siebeckerb and D.L. Sparksc
a School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China
bDepartment of Plant and Soil Science, Texas Tech University, Lubbock, Texas, 79409, United States
c Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19716, United States
jxli@cug.edu.cn
Iodine is an essential element for thyroid hormone synthesis. According
to a recent worldwide survey of median urinary iodine, 11 countries
around the world have an excess of iodine intake, which can trigger
iodine toxicity disorders (IED) and cause immune responses such as
autoimmune thyroiditis. In China, excessive iodine intake is widespread,
with approximately 31 million people in 11 provinces affected. Our
previous studies found the occurrence of high iodine groundwater in the
Datong basin and North China Plain, which increases hypothyroidism risk
for residents using the groundwater as the main source of drinking
water. However, the current understanding of the hydrochemical cycling
of iodine in the groundwater systems is very limited.
Our previous studies found that iodine concentration in groundwater can
be up to 1187 µg/L at the Datong basin. In the basin, the iodine-loaded
metal (oxy)hydroxides are considered as an primary hosts of
soil/sediment iodine, and dissimilatory reduction of metal
(oxy)hydroxides would lead to iodine mobilization from sediment into
groundwater. Therefore, in order to better understand the iodine
mobilization in this groundwater system, a series of batch incubation
experiments investigating the transformation of iron minerals were
performed under both aerobic and anaerobic conditions.
Two sediment samples of DXZ04 and DXZ147 at depths of 4.35 m and 281 m,
respectively, were collected from the iodine-affected area of the Datong
basin to perform the batch incubation experiments which involved the
iron reducer Shewanella oneidensis MR-1. Iron K-edge X-ray absorption
near edge spectroscopy (XANES) and PHREEQC (Version 3, 3.4.0-12927) were
used to identify the transformation of Fe phases and iodine species,
respectively. The results showed that for the shallow sediment DXZ04,
after treatment with the iron reducer MR-1 and Na-lactate (used as an
electron donor for MR-1), iodine release from sediment into solution was
observed under anaerobic conditions compared with no evident release of
iodine under aerobic conditions. The redox-derived transformation of
sediment Fe phases from the crystalline Fe to HCl-extractable Fe phases
was considered as the main factor causing the iodine release. For the
untreated deep sediment DXZ147, a higher ratio of HCl-extractable Fe
fraction to natural sediment Fe phases was observed. Consistently, the
groundwater from the deep aquifer had the higher iodine concentrations
(up to 732 µg/L) than the shallow aquifer, suggesting that the long-term
transformation of Fe minerals under the reducing conditions of the deep
aquifer led to iodine mobilization from the sediment into the
groundwater. In addition, during the iodine release, the transformation
of iodine species from organic iodine or iodate to iodide further
promotes its mobilization. The evidence gained from this study provides
new insights on the genesis of natural high iodine groundwater.