Speciation of Iodide and Iodate Anions in Geochemical Environments: A Theory Driven Experimental Study

Special Article: Thorium Fuel Production

Austin Environ Sci. 2023; 8(3): 1093.

Speciation of Iodide and Iodate Anions in Geochemical Environments: A Theory Driven Experimental Study

Mahesh Sundararajan1,2*; Jayshree Ramkumar2,3; Arulkumar Rasu4; Ponnambalam Venuvanalingam4

1Theoretical Chemistry Section, Chemistry Division, Bhabha Atomic Research Centre, India

2HomiBhabha National Institute, India

3Analytical Chemistry Division, Bhabha Atomic Research Centre, India

4Theoretical and Computational Chemistry Laboratory, School of Chemistry, Bharathidasan University, India

*Corresponding author: Mahesh Sundararajan Theoretical Chemistry Section, Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India Tel: +91-22-25595607 Email: smahesh@barc.gov.in

Received: July 31, 2023 Accepted: September 09, 2023 Published: September 16, 2023

Abstract

Speciation of heavy metal ions in geochemical environment is utmost importance due to its environmental relevance. Iodine exists as iodide and iodate anions and are heavier anions, and they can be more labile in the absence of humic environments. This study particularly focusses on iodine speciation in Humic Acid (HA) environments. First, we have carried out extensive quantum chemical calculations to understand the structure-function relationship of iodide and iodate binding in humic acids. We predict that iodide anion binds through hydrogen bonding and are thus weak in acidic conditions. Iodate anion binds at the macrocyclic pocket of the humic acid, and is somewhat pH independent. Computed structure of the complex and binding energies explain the binding affinities of iodide and iodate with HA and quantum theory of atoms and molecules QTAIM analysis reveal the nature of forces involved in each case. To justify our computational claim, sorption experimental studies are carried out on the uptake of iodine species with humic acid. The studies are quite encouraging and there is a close agreement between the theoretical and experimental results.

Introduction

With diminishing fossil fuel resources, it is vital to increase the energy production to serve the growing population. Significant advances in nuclear technologies make nuclear power a viable solution to the ongoing energy demands. Nuclear energy is advantageous in several means such as well-established technology, harnessing maximum energy through efficient reprocessing and little waste being generated [1]. On the other hand, nuclear accidents are dangerous and can release several toxic, radioactive and hazardous nuclear waste that can be detrimental to the environment. In the last five decades, only two major nuclear accidents with INES level of 7 have occurred namely Chernobyl in 1986 and Fukushima in 2011 [2]. In both accidents, significant amount of radioactive materials such as Sr90, Cs137 and I131 were released in the environment [3,4]. Radionuclide iodine, I131, with the half-life of 8 days is one of the hazardous wastes of fuel reprocessing plant which get released to the environment during normal plant operation [5]. Due to its lower half life, it is less important in biological point of view. On the other hand, I129 (half-life = 1.57×107yrs) is a radionuclide of interest for disposal of high level waste produced by nuclear fuel reprocessing plant and also found in the seawater offshore Fukushima [6,7]. On the other hand, radioiodine is indeed administered in nuclear medicine such as hyperthyroidism [8].

Though heavy ion, iodine species can be dispersed through air and can be found in different environments such as those reported in Antarctica and can contaminate the natural habitat [9]. The oxidation state of iodine varies from –1 to +7 and thus it exists in various forms in nature at different pHs after getting released to the environment. The main chemical forms of iodine are iodide (I-), iodate (IO3-) and molecular iodine (I2) and it may also exist in organic iodine form (CH3I). Besides this, iodine is an essential micronutrient for the production of thyroid hormones in animals and humans. Radioactive iodine is concentrated in the thyroid gland can lead to the direct threat to human populations [10].

Natural Organic Matter (NOM) such as Humic (HA) and Fulvic Acids (FA) are known to bind several ions including radionuclides such as uranyl, Cs+, Sr2+ and iodide ions [11-15]. With less oxygen content, HAs are more hydrophobic than FA, there by binds bulky iodine species better through favorable van der Waals interactions. Due to the availability of deprotonated Lewis basic sites, FA binds cationic species strongly in neutral to alkaline pH medium. It is for these reasons, the mobility of Sr2+ and uranyl ion are controlled by NOM environment, whereas the bulky Cs+ ion is more mobile with varying water concentration even in the presence of NOM environment [12].

Stable iodine I127 and I129 are major bi-product of nuclear fission and undergoes complex geochemical cycling in the environment. Only handful reports are available on the speciation of iodine species with NOM is available [16-18]. In this paper, we have carried out detailed Density Functional Theory (DFT) calculations on the structures and binding modes and affinities of iodide and iodate anions in HA. The computed geometries and energetics could easily explain the interesting results obtained from the sorption experiments of iodide and iodate with humic acid as sorbent.

Experimental Details

Choice of Models

We have taken the Stevenson’s model for HA that satisfies several structural requirements. The model consists of several functional groups such as aromatic hydroxo, quinone, sugar, aromatic carboxylic acid and amino acids such as tyrosine that are key moieties present in HA revealed by several spectroscopic techniques. We have used this model earlier to understand the speciation of uranyl in HA environments [12]. With unknown X-ray structures, we have carried out extensive multi-scale model simulations to derive a reasonable structure that is used to understand the speciation of heavy metal ions.

Computational Calculations

Both geometry optimizations and vibrational frequency computations were carried out using Density Functional Theory (DFT) [19]. DFT calculations were carried out as implemented in the TURBOMOLE 7.2 version of ab initio quantum chemistry program [20]. Geometry optimizations were performed with BP86 functional [21,22,33] including Grimme’s D3 dispersion correction [23] with Becke-Johnson damping factor (D3-BJ) [24]. All atoms except I are represented using the def2- SV (P) basis set [25]. For I, a def2-SV (P) basis set and small core pseudopotential (Z=28) for core electrons is used. The calculations are accelerated using a Resolution of Identity (RI) approximation by incorporating the corresponding auxiliary basis set. Analytical vibrational frequencies within the harmonic approximation were computed with the abovementioned basis sets to confirm proper convergence to well-defined minima. Standard approximation was used to obtain zero-point vibrational energy and entropy corrections. We obtained solvation energies using the optimized gas-phase structures from the COSMO solvation model with dielectric constant e=80 (water) using the default radii. Single point calculations were carried out with M06 functional with def2-TZVP basis set for all atoms with def2-ECP for I. We have earlier used this computational strategy for several systems and their agreement with experiments have been satisfactory [26,27]. Chimera 1.12c software [28] is used to plot the molecular orbitals (iso value of 0.04).

In addition, Quantum Theory of Atoms In Molecules (QTAIM) assessment of topologies of the electron density in weak bonds holding the host and guest is used to better understand the current bonding situation in HA complexes [29,30]. Based on Bader's theory, QTAIM analysis describes the bonding region between atoms in terms of topological characteristics of the electron density (ρ(r)) and Laplacian of the electron density (∇2ρ(r)). For instance, the Bond Critical Bond (BCP), which is represented as a (3,-1) critical point in the QTAIM topography, denotes a chemical bond between two atoms, while a (3,+1) critical point (RCP) denotes a ring structure in a molecular system. Particularly, ρ(r) values at the BCPs can be connected to bond strength. Hybrid B3LYP functional [21,22,31] with the dispersion correction D3(BJ) has been used to perform QTAIM calculations. BP86-D3(BJ)/def2-SV(P) optimized geometries are used to create primary wave function file for AIM calculations. def2-SVP basis set is used for all atoms. B3LYP- D3(BJ)/def2-SVP energy gradients are calculated using the nuclear coordinates generated at BP86-D3(BJ)/def2-SVP level during the single point calculations using “FORCE” keyword. We have used QTAIM calculations to understand the nature of weak interactions in several studies [32-34].

Experimental Setup

The sorption studies were carried out in batch mode [35]. A known concentration of pure solution of iodide / iodate (5ppm), maintained at different pH values between 1.0-4.0, was contacted with approximately 0.05g of humic acid for a fixed time of 30 mins. At the end of equilibration time, the solid was completely removed by centrifugation and the amount of iodide / iodate left in solution was analyzed spectrophotometrically using methylene blue [36]. From these data, the amount taken up (%) was calculated.

Results and Discussion

Geometric Structures of Iodine Species and HA

We have taken solvated iodide and iodate as possible species of iodine studied here. The optimized species are shown in Figure 1. Unlike chloride and bromide anions, the solvated structure of iodide is somewhat different [32]. The solvated structure of iodide is partially exposed to vacuum due to its bulky nature of anions. This is not unexpected as for bulky ions such as iodide a similar trend is also noted for alkali metal ions such as the solvated structure of Cs+ ions [37,38]. For iodate anion, a planar structure is noted. The major difference between the iodide and iodates are the charges carried on the iodine moiety due to the differing oxidation state of the species investigated here. In iodide, the oxidiation state of iodine is –I that carries partial negative charge, whereas for iodate anion, the oxidation of iodine is +VII and thus carries a partial positive charge. These variations can lead to the different binding modes with HA.