Sorption and speciation of iodine in groundwater system: The roles of organic matter and organic-mineral complexes

Iodine enrichment in the groundwater in South China and its hydrogeochemical control

In North China, iodine-rich groundwater has been extensively studied, but few in South China. This study aimed to investigate the characteristics of iodine-rich groundwater in South China and identify potential contamination sources. The results revealed that the average concentration of iodine in groundwater was 890 µg/L, with a maximum concentration of 6350 µg/L, exceeding the permitted levels recommended by the World Health Organization (5-300 µg/L). Notably, the enrichment of iodide occurred in acidic conditions (pH = 6.6) and a relatively low Eh environment (Eh = 198.4 mV). Pearson correlation and cluster analyses suggested that the enrichment of iodide could be attributed to the intensified redox process involving Mn(II), iodine (I2), or iodate (IO3-) in the soil. The strong affinity between Mn(II) and I2/IO3- facilitated their interaction, resulting in the formation and mobilization of I- from the soil to the groundwater. Leaching experiments further confirmed that reducing substances (such as sodium sulfides, ascorbic acids, and fulvic acids) in the soil with low dissolved oxygen (DO) levels (< 1.0 mg/L) enhanced the dissolution of iodine species. Conversely, higher DO content (> 3.8 mg/L) promoted the oxidation of I- into I2 or IO3-, leading to its stabilization. This research provides new insights into the characteristics and mechanisms of I- enrichment in groundwater in South China, and emphasizes the significance of the redox reactions involving Mn(II) and I2/IO3-, as well as the influence of soil properties in regulating the occurrence and transportation of iodine species within groundwater systems.

Record High Iodate Anion Capture by a Redox-Active Cationic Polymer Network

As a critical radioactive anionic contaminant, traditional adsorbents primarily remove iodate (IO3 -) through ion exchange or hard acid-hard base interactions, but suffer from limited affinity and capacity. Herein, employing the synergistic effect of ion exchange and redox, we successfully synthesized a redox-active cationic polymer network (SCU-CPN-6, [C9H10O2N5 ⋅ Cl]n) by merging guanidino groups with ion-exchange capability and phenolic groups with redox ability via a Schiff base reaction. SCU-CPN-6 exhibits a groundbreaking adsorption capacity of 896 mg/g for IO3 -. The inferior adsorption capacities of polymeric networks containing only redox (~0 mg/g) or ion exchange (232 mg/g) fragments underscore the synergistic "1+1>2" effect of the two mechanisms. Besides, SCU-CPN-6 shows excellent uptake selectivity for IO3 - in the presence of high concentrations of SO4 2-, Cl-, and NO3 -. Meanwhile, a high distribution coefficient indicates its exemplary deep-removal performance for low IO3 - concentration. The synergic strategy not only presents a breakthrough solution for the efficient removal of IO3 - but also establishes a promising avenue for the design of advanced adsorbents for diverse applications.

Hidden Role of Organic Matter in the Immobilization and Transformation of Iodine on Fe-OM Associations

The biogeochemical processes of iodine are typically coupled with organic matter (OM) and the dynamic transformation of iron (Fe) minerals in aquifer systems, which are further regulated by the association of OM with Fe minerals. However, the roles of OM in the mobility of iodine on Fe-OM associations remain poorly understood. Based on batch adsorption experiments and subsequent solid-phase characterization, we delved into the immobilization and transformation of iodate and iodide on Fe-OM associations with different C/Fe ratios under anaerobic conditions. The results indicated that the Fe-OM associations with a higher C/Fe ratio (=1) exhibited greater capacity for immobilizing iodine (∼60-80% for iodate), which was attributed to the higher affinity of iodine to OM and the significantly decreased extent of Fe(II)-catalyzed transformation caused by associated OM. The organic compounds abundant in oxygen with high unsaturation were more preferentially associated with ferrihydrite than those with poor oxygen and low unsaturation; thus, the associated OM was capable of binding with 28.1-45.4% of reactive iodine. At comparable C/Fe ratios, the mobilization of iodine and aromatic organic compounds was more susceptible in the adsorption complexes compared to the coprecipitates. These new findings contribute to a deeper understanding of iodine cycling that is controlled by Fe-OM associations in anaerobic environments.

Environmental iodine as a natural iodine intake in humans and environmental pollution index: a scientometric and updated mini review

Although almost a third of the world's population is exposed to iodine deficiency (ID), and supplementation programs such as enriching table salt have been carried out or are being carried out at the global and national level, in many regions of the world, people are facing an increase in iodine intake, which is mainly due to the presence of large amounts of iodine in water, soil, agricultural products, or high consumption of seafood. Published articles were indexed in the Scopus database (from 2000 to 1 April 2023) were reviewed and analyzed by VOSviewer software. The results showed the growing interest of researchers over the last 20 years in environmental iodine intake. The results of this study can have a significant impact on the planning and policy-making of relevant officials and communities to supply the needed iodine.

Co-Occurrence of Bromine and Iodine Species in US Drinking Water Sources That Can Impact Disinfection Byproduct Formation

Bromine and iodine species are precursors for forming disinfection byproducts in finished drinking waters. Our study incorporates spatial and temporal data to quantify concentrations of inorganic (bromide (Br-), iodide (I-), and iodate (IO3-)), organic, and total bromine (BrT) and iodine (IT) species from 286 drinking water sources and 7 wastewater effluents across the United States. Br- ranged from <5-7800 μg/L (median of 62 μg/L in surface water (SW) and 95 μg/L in groundwater (GW)). I- was detected in 41% of SW (1-72 μg/L, median = <1 μg/L) and 62% of GW (<1-250 μg/L, median = 3 μg/L) samples. The median Br-/I- ratio in SW and GW was 22 μg/μg and 16 μg/μg, respectively, in paired samples with detect Br- and I-. BrT existed primarily as Br-, while IT was present as I-, IO3-, and/or total organic iodine (TOI). Inorganic iodine species (I- and IO3-) were predominant in GW samples, accounting for 60-100% of IT; however, they contributed to only 20-50% of IT in SW samples. The unknown fraction of IT was attributed to TOI. In lakes, seasonal cycling of I-species was observed and was presumably due to algal productivity. Finally, Spearman Rank Correlation tests revealed a strong correlation between Br- and IT in SW (RBr-,IT = 0.83) following the log10 (Br-, μg/L) = 0.65 × log10 (IT, μg/L) - 0.17 relationship. Br- and I- in treated wastewater effluents (median Br- = 234 μg/L, median I- = 5 μg/L) were higher than drinking water sources.

Spatiotemporal Variability of Groundwater Iodine in the Northern Arid Basins: Significance for Safe Water Supply

The genesis of geogenic iodine (I)-contaminated groundwater poses a significant threat to long-term water exploitation. Safe and sustainable water supply, particularly in the northern arid basins, demands a quantitative prediction of the high variability of I distribution over hydrogeological timescales. Here, bioenergetics-informed reactive transport modeling was combined with high-resolution molecular characterization of fueling organic matter to decipher the time-controlled interactions between vertical flow and (bio)geochemical processes in I transport within the Datong aquifers. The declining reactivities of I-bearing organic matter and Fe oxides in the 15-40 m depth decreased the rate of I release, while a growing number of pore volumes flushed through the aquifers to leach out I^- and organic I. This removal effect is compensated by the desorption of I^- from Fe oxides and secondary FeS generated from the concurrent reduction of Fe oxides and SO₄^2-. Consequently, peak concentrations of groundwater I^- may have appeared, depending upon the vertical recharge rate, at the first several pore volumes flushed through the aquifers. The current vertical distributions of the various I species likely represent a quasi-steady state between I mobilization and leaching. These new mechanistic insights into the dynamic hydrogeological-(bio)geochemical processes support secure groundwater use in the I-affected northern arid basins.

Deficiency and excess of groundwater iodine and their health associations

More than two billion people worldwide have suffered thyroid disorders from either iodine deficiency or excess. By creating the national map of groundwater iodine throughout China, we reveal the spatial responses of diverse health risks to iodine in continental groundwater. Greater non-carcinogenic risks relevant to lower iodine more likely occur in the areas of higher altitude, while those associated with high groundwater iodine are concentrated in the areas suffered from transgressions enhanced by land over-use and intensive anthropogenic overexploitation. The potential roles of groundwater iodine species are also explored: iodide might be associated with subclinical hypothyroidism particularly in higher iodine regions, whereas iodate impacts on thyroid risks in presence of universal salt iodization exhibit high uncertainties in lower iodine regions. This implies that accurate iodine supply depending on spatial heterogeneity and dietary iodine structure optimization are highly needed to mitigate thyroid risks in iodine-deficient and -excess areas globally.

1H-13C heteronuclear single quantum coherence NMR evidence for iodination of natural organic matter influencing organo-iodine mobility in the environment

The complex biogeochemical behavior of iodine (I) isotopes and their interaction with natural organic matter (NOM) pose a challenge for transport models. Here, we present results from iodination experiments with humic acid (HA) and fulvic acid (FA) using ¹H-¹³C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy. Even though not a quantitative approach, ¹H-¹³C HSQC NMR corroborated that iodination of NOM occurs primarily through aromatic electrophilic substitution of proton by I, and also revealed how iodination chemically alters HA and FA in a manner that potentially affects the mobility of iodinated NOM in the environment. Three types of iodination experiments were conducted with HA and FA: a) non-enzymatic iodination by IO₃^- (pH 3) and I^- (pH 4 and 7), b) addition of lactoperoxidase to promote I^--iodination in the presence of the co-substrate, H₂O₂ (pH 7), and c) addition of laccase for facilitating I^--iodination in the presence of O₂, with or without a mediator (pH 4). When mediators or H₂O₂ were present, extracellular oxidases and peroxidases enhanced I^- incorporation into NOM by between 54% and 3400%. Iodination of HA, which was less than that of FA, enhanced HA's stability (inferred from increases in aliphatic compounds, decreases in carbohydrate moieties, and thus increased molecular hydrophobicity) yet reduced HA's tendency to incorporate more iodine. As such, HA is expected to act more as a sink for iodine in the environment. In contrast, iodination of FA appeared to generate additional iodine binding sites, which resulted in greater iodine uptake capability and enhanced mobility (inferred from decreases in aliphatic compounds, increases in carbohydrates, and thus decreases in molecular hydrophobicity). These results indicate that certain NOM moieties may enhance while others may inhibit radioiodine mobility in the aqueous environment.

Natural organic matter-enhanced transportation of iodine in groundwater in the Datong Basin: Impact of irrigation activities

Local residents in the Datong Basin of northern China are exposed to groundwater with elevated iodine concentrations. Natural organic matter (NOM) has been linked to the heterogeneous distribution of elevated iodine in groundwater used for irrigation purposes, but little is known about the effects of hydrologic fluctuations and NOM characteristics on the transport and enrichment of iodine in the groundwater. Cl/Br molar ratios in Datong Basin groundwater range widely from 133 to 2099. A rapid increase in Cl/Br molar ratio with increasing Cl content indicates hydrologic fluctuations from the upper groundwater to the deeper aquifer due to large-scale irrigation activities in the Basin. A two end-member model of groundwater δ²H and δ¹⁸O values suggests the contribution of upper water recharging groundwater ranges from 20.7 to 49.5%. This vertical recharge process predominantly controls iodine enrichment and distribution in the groundwater. Additionally, the correlation between DOC concentration and δ¹⁸O signatures indicates considerable fresh organic matter is imported into the aquifer during the vertical recharge process. Iodine mobilization is likely promoted by young carbon transported to the deeper aquifer in the organo‑iodine form. Excitation-emission matrix (EEM) results indicate humic-like substances dominate NOM in the groundwater. Evidence from a PARAFAC model suggests organic matter in groundwater samples is associated with microbially-mediated degradation processes in an anaerobic environment. The drawdown migration of organic matter from the upper soil/sediments or surface could provide an extra energy source that promotes microbial activity. Buried sedimentary iodine coupled with anaerobic microbial respiration of subsurface organic carbon within the aquifer could lead to the release of iodine into the groundwater. These findings pave the way for a more comprehensive assessment of the susceptibility of drinking water aquifers, thereby supporting the management of groundwater resources.

Iodine immobilization by materials through sorption and redox-driven processes: A literature review

Radioiodine-129 (¹²⁹I) in the subsurface is mobile and limited information is available on treatment technologies. Scientific literature was reviewed to compile information on materials that could potentially be used to immobilize ¹²⁹I through sorption and redox-driven processes, with an emphasis on ex-situ processes. Candidate materials to immobilize ¹²⁹I include iron minerals, sulfur-based materials, silver-based materials, bismuth-based materials, ion exchange resins, activated carbon, modified clays, and tailored materials (metal organic frameworks (MOFS), layered double hydroxides (LDHs) and aerogels). Where available, compiled information includes material performance in terms of (i) capacity for ¹²⁹I uptake; (ii) long-term performance (i.e., solubility of a precipitated phase); (iii) technology maturity; (iv) cost; (v) available quantity; (vi) environmental impact; (vii) ability to emplace the technology for in situ use at the field-scale; and (viii) ex situ treatment (for media extracted from the subsurface or secondary waste streams). Because it can be difficult to compare materials due to differences in experimental conditions applied in the literature, materials will be selected for subsequent standardized batch loading tests.

A review of the behavior of radioiodine in the subsurface at two DOE sites

Multiple processes affect the fate of the radioactive isotope ¹²⁹I in the environment. Primary categories of these processes include electron transfer reactions mediated by minerals and microbes, adsorption to sediments, interactions with organic matter, co-precipitation, and volatilization. A description of dominant biogeochemical processes is provided to describe the interrelationship of these processes and the associated iodine chemical species. The majority of the subsurface iodine fate and transport studies in the United States have been conducted at U.S. Department of Energy (DOE) sites where radioisotopes of iodine are present in the environment and stored waste. The DOE Hanford Site and Savannah River Site (SRS) are used to illustrate how the iodine species and dominant processes at a site are controlled by the prevailing site biogeochemical conditions. These sites differ in terms of climate (arid vs. sub-tropical), major geochemical parameters (e.g., pH ~7.5 vs. 4), and mineralogy (carbonate vs. Fe/Al oxide dominated). The iodine speciation and dominant processes at a site also have implications for selection and implementation of suitable remedy approaches for ¹²⁹I.

Microbial Methylation of Iodide in Unconfined Aquifer Sediments at the Hanford Site, USA

Incomplete knowledge of environmental transformation reactions limits our ability to accurately inventory and predictably model the fate of radioiodine. The most prevalent chemical species of iodine include iodate (IO₃⁻), iodide (I⁻), and organo-iodine. The emission of gaseous species could be a loss or flux term but these processes have not previously been investigated at radioiodine-impacted sites. We examined iodide methylation and volatilization for Hanford Site sediments from three different locations under native and organic substrate amended conditions at three iodide concentrations. Aqueous and gaseous sampling revealed methyl-iodide to be the only iodinated compound produced under biotic conditions. No abiotic transformations of iodide were measured. Methyl-iodide was produced by 52 out of 54 microcosms, regardless of prior exposure to iodine contamination or the experimental concentration. Interestingly, iodide volatilization activity was consistently higher under native (oligotrophic) Hanford sediment conditions. Carbon and nutrients were not only unnecessary for microbial activation, but supplementation resulted in >three-fold reduction in methyl-iodide formation. This investigation not only demonstrates the potential for iodine volatilization in deep, oligotrophic subsurface sediments at a nuclear waste site, but also emphasizes an important role for biotic methylation pathways to the long-term management and monitoring of radioiodine in the environment.

Effects of depositional environment and organic matter degradation on the enrichment and mobilization of iodine in the groundwater of the North China Plain

Groundwater iodine has direct importance for human dietary iodine intake in areas where drinking water is of groundwater origin. However, little is known about enrichment and mobilization mechanisms of groundwater iodine in the North China Plain (NCP). Geochemistry, inorganic/organic carbon isotope and biomarker of groundwater and sediment samples were studied to reveal the effects of depositional environment and organic matter (OM) degradation on the generation of high iodine groundwater (>100 μg/L) in NCP. Results showed that groundwater iodine had a range of 7.2-800 μg/L and was increasing with increase in HCO₃ concentration and decrease in groundwater δ¹³C_DIC value, indicating the potential effects of microbial activity on the elevation of groundwater iodine. Sediments iodine ranged from 0.03 to 2.54 μg/g and higher contents occurred under the oxidizing depositional environment (higher Pr/Ph ratios). Biomarker analysis indicated that the marine iodine-rich OM is considered as the main source of groundwater iodine, which is prone to be released into groundwater by the microbial degradation under the reducing conditions. The hypothesis was evidenced by the ¹³C_org, ¹³C_DIC and 3-D excitation emission matrices of groundwater. These results suggest that carbon-related biogeochemical cycling and redox condition are important in the enrichment and mobilization of iodine in groundwater system.