Discrete fragment model for apparent formation constants of actinide ions with humic substances

Abstract

A semi-empirical thermodynamic model was applied to estimate the apparent formation constants of actinide ions, i.e., Th(IV), Pu(IV) and Np(V), with humic substances (HSs), including humic and fulvic acids, over a wide range of solution conditions, i.e., pH, ionic strength, and HS and metal concentrations. The hypothetical HSs consist of humic and fulvic acids with nine types of simple organic ligands, which include aromatic and aliphatic carboxyl groups and phenol groups, as binding sites. The abundance of each binding site in the hypothetical HSs was determined via a fitting analysis using an aciddissociation dataset for several HSs. To determine the apparent formation constant of a given metal ion with HSs, 54 specific binding sites were considered, including nine monodentate sites (1 : 1 metal/ligand complexes) and 45 bidentate sites (1 : 2 metal/ligand complexes). The formation constant of each monodentate binding was determined from the experimental data, while those of the bidentate bindings were determined by considering two monodentate bindings and the chelating effect, for which one of the adjustable parameters was introduced in the model. Introduction of the other parameter, which is related to the fraction of monodentate to bidentate sites (i.e., the heterogeneity), afforded the parameter values with good correlation with the apparent formation constant data. The present model with adjusted parameter values well reproduced the experimental apparent complex formation constants for actinide ion interaction with HSs in a wide range of solution conditions except for those obtained at trace concentrations.

Thermodynamic, Spectroscopic, and Structural Study on a Sodium Uranyl Tri-2-Methoxybenzoate Dodecahydrate Coordination Compound with Considerably Low Solubility in Acidic Conditions

“…In the field of actinide chemistry, experimental and computational investigations of the U­(VI) complexation with aromatic carboxylates in both solution and solid states are subjects of continued interest, from the perspectives of not only comprehending the environmental transport of U­(VI) − but also developing the novel U­(VI)-organic hybrid materials displaying distinct crystal structures and advanced engineering functionalities, such as photochemical and catalytic properties. − Specifically, the aromatic carboxyl groups constitute the principal part of natural organic macromolecules and mainly responsible for binding of U­(VI) in natural aquatic system; examination of reliable thermochemical data (Δ G °, Δ H °, and Δ S °) of U­(VI) complexes with simple aromatic model ligands has been extensively conducted to enhance the accuracy of discrete fragment modeling , and quantitative structure–property relationship modeling, , which are used to describe the chemical interactions between metal cations and complicatedly formed molecular moiety of heterogeneous organic matters. In a different chemical perspective, the aromatic carboxylates are also considered to be superb building components for the synthesis of uranyl-ligand coordination compounds as they make the architectures rigid and involve various packing structures of unit complex through not only the formation of strong bonds but also the relatively weaker noncovalent interactions, such as the π-stacking and ion−π interactions. − Indeed, the benzene skeleton of aromatic ligand is an excellent molecular template in which diverse functional groups can be easily substituted, and the ligand’s chemical properties are controllable accordingly.…”

Spectroscopic Study on Aqueous Uranyl(VI) Complexes with Methoxy- and Methylbenzoates: Electronic and Steric Effects of the Substituents

“…1−3 In particular, the chemical interactions between U(VI) and the heterogeneously composed molecular framework of intricate organic compounds can be quantitatively modeled using the thermodynamic data of simple U(VI) complexes with model ligands. 4,5 In this regard, reliable thermodynamic ΔG°, ΔH°, and ΔS°d ata of the U(VI) complexation reactions with diverse simple aromatic acids enable a more extensive description of the chemical interactions between U(VI) and natural organic macromolecules. It is also expected to ultimately improve the understanding of the fate and transport of U(VI) in the environment.…”

“…Thus, an in-depth understanding of the coordination and thermodynamic properties of the related complexations is essentially required for predicting and controlling the chemical behaviors of aqueous actinide ions. Among a variety of organic functionalities, aromatic carboxyl groups have been of utmost interest in the field of environmental chemistry of U­(VI) because they are ubiquitous in natural organic macromolecules (e.g., natural organic matters such as humic substances) and primarily responsible for binding of U­(VI) ions in the biosphere. − In particular, the chemical interactions between U­(VI) and the heterogeneously composed molecular framework of intricate organic compounds can be quantitatively modeled using the thermodynamic data of simple U­(VI) complexes with model ligands. , In this regard, reliable thermodynamic Δ G °, Δ H °, and Δ S ° data of the U­(VI) complexation reactions with diverse simple aromatic acids enable a more extensive description of the chemical interactions between U­(VI) and natural organic macromolecules. It is also expected to ultimately improve the understanding of the fate and transport of U­(VI) in the environment.…”