Complexation of Metal Ions with Humic Acid: Metal Ion Charge Neutralization Model

Abstract

The humic acid complexation reaction presented in this work is based on the concept of metal ion charge neutralization upon complexation to humic acid functional groups. Experimental results from the complexation of UOi + , Am 1+ , and Cm'' with humic acids of different origin are presented to demonstrate and validate the proposed model. By introducing the operational humic acid concentration and the loading capacity, the complexation constants are derived, which are independent of the metal ion concentration, pH, and origin of humic acid. The loading capacity is an operational term needed for the intercomparison of stability constants and is the mole fraction of the maximum available complexing sites of humic acid under a given set of experimental conditions. The metal ion charge neutralization model adequately describes the thermodynamic equilibrium reaction of metal ions with humic acid and allows a direct application of the resulting stability constants for geochemical modeling of actinide migration at an environmentally relevant pH.

XAFS Study of Actinide Coordination Structure in Np(IV)-Fulvates

“…The increase of the loading capacity (LC) of humic substances for actinide cations as a function of pH is greater the larger the effective charge on the actinide. 3 ) For example, the slope of LC versus pH 4) follows the order: Np4+»Am3+>U022+>Np02+. The LC offulvates for Np(IV) exhibits an remarkably large increase going from pH 1.0 to 1.5.…”

“…'anomaly' for tetravalent neptunium lies in the presence of Np(OH)"FA(IV-x); the LC is calculated assuming a charge neutralization of the Np4+ cation through fonnation of a Np-FA(IV) complex, where the FA(IV) denotes four functional groups from fulvate neutralizing the Np4+ cation charge. 3 ) For each coordinating hydroxyl group, one less functional group . from the fulvate is needed to neutralize the Np(OH)x(4-X)+ cation charge.…”

Simplified Modelling of the Complexation of Humic Substance for Equilibrium Calculations

“…Therefore, numerous attempts have been carried out to model and describe the interactions of HS with metal ions. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] A few of the generally acknowledged models are the NICA-Donnan model [7][8][9] and Model VI. 2,6) Although they can describe cation-binding equilibria of humic substances fairly well, the forms of the description model are somewhat complicated and are not well suited to be incorporated into a chemical equilibrium calculation code with the use of stability constants.…”

Complexation of Radionuclides with Humic Substances

“…Among them are the "conservative roof" model [11], the charge neutralization model [12], the polyelectrolyte model [13], and other models presented in [10][11]14] with references therein.…”

“…The charge neutralization model used by Kim and Czerwinski [12] In this model, the ligand concentration is equal to the number of ionised sites at the pH of the study. Hence, it is equal to the proton exchange capacity (eq/g) multiplied by the ionisation degree HA and the ligand concentration (g/L).…”