The effect organic matter on plutonium sorption to the iron oxyhydroxide mineral goethite was studied at a bulk and molecular level. Batch sorption experiments quantified sorption of plutonium (10-11 M) to goethite (0.1 g�L-1) in the presence of representative natural organic matter (NOM) with respect to NOM concentration, pH, and time. Leonardite humic acid, Suwannee River fulvic acid, citric acid, and deferoxamine-B (DFOB) were considered at concentrations of 5 mgC�L-1 and 50 mgC�L-1. Plutonium and NOM sorption was measured after 1, 7, 14, and 30 day sorption periods by measuring the loss of plutonium and NOM from the aqueous phase. Radiolabeled organic ligands provided a complimentary and simultaneous study of ligand sorption to goethite to inform possible ternary surface-ligand-plutonium complex formation. Generally, sorption of plutonium was decreased at circumneutral pH values relative to a ligand free system. The magnitude of the decrease in plutonium sorption followed the trend DFOB > fulvic acid > humic acid > citric acid. Thus the influence of NOM on plutonium sorption follows the strength of the aqueous Pu-NOM complexes. Modeling exercises considering plutonium speciation using FITEQL were performed using aqueous, including complexation with NOM, and surface complexation constants from open literature. The model could not accurately predict the increased sorption of plutonium observed at low pH values where the ligands exhibit significant sorption. Thus, it appears ternary surface-ligand-Pu complexes may be responsible for increased sorption of plutonium at low pH values. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to probe the sorption mechanisms of organic matter to the goethite surface at the molecular level. Carboxylic acid and carboxylate ion stretches were found to be the molecular vibrations of greatest sorption significance. A change in the relationship between the asymmetric and symmetric carbonyl vibrations upon sorption to goethite of an NOM species was used to propose mondentate, bidentate-mononuclear, and bridging-bidentate surface complexes.

Effect of natural organic matter on the sorption of plutonium to goethite

248th National Meeting (2014)

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