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The effect of humic acids (HA) on neptunium redox behavior and sorption onto hematite, -Fe2O3, and goethite, -FeOOH, colloids was established in batch sorption experiments that were carried out in broad pH interval. The sorption isotherms were provided for two samples of HA: commercial sample of leonardite humic acid and its hydroquinone-enriched derivative obtained using formaldehyde copolycondensation. The distribution of Np fitted the distribution of hydroquinone-enriched HA at low pH values in case of both solids while the influence of parent HA on Np sorption was negligible. This is due to Np(V) reduction upon interaction with hydroquinone-enriched derivative having higher reducing capacity compared to the parent HA. The order of components addition was found to be significant for Np retention. © 2007 Elsevier B.V. All rights reserved.

Humic substances play an important role in the speciation of actinides in the environment due to complexing, redox and sorptive interactions. This work is devoted to plutonium(V) reduction studies in solutions containing different humic substances, eluted from Mollisol and Sod-podzolic soil samples, as compared to real Pu distribution is contaminated soils. On the basis of kinetic curve slopes, the reducing ability is increased in the order: HA < FA ∼ low molecular weight fraction. The data obtained show that the different humic compounds play different roles in Pu(V) migration ability due to differing reducing and complexing properties.

Alkoxysilylated humic derivatives were used to create covalently bound humic coatings on silica gel. The corresponding reaction was conducted in an aqueous milieu that was feasible due to the water solubility of these derivatives. The alkoxysilylated humic derivatives were obtained by alkoxysilylation of leonardite humic acid (HA) and hydroquinone-modified HA (HQ) with enhanced redox properties. The silica gel coated with covalently bound HA and HQ was tested for sequestration of aqueous Np(V) and Pu(V) under anaerobic conditions. Greater sequestration with respect to Pu(V) (up to 97%) as compared to Np(V) (up to 60%) was demonstrated for samples of silica gel coated with HA and HQ, whereas sorption of both actinides on pure silica gel did not exceed 20%. Results provide bench-scale proof that alkoxysilylated humic derivatives can be used as reagents for the in situ installation of permeable reactive barriers designed to remediate actinide-contaminated groundwater.

Natural humic materials are known to cause a reduction of penta- and hexavalent plutonium; however, contradicting results have been previously reported for neptunium. The goal of this study was to determine if incorporation of additional reducing centers into humic structure would yield material with enhanced reducing performance with respect to higher valent actinides, nominally, Pu(V) and Np(V). Hydroquinone-moieties were used as the redox centers. They were incorporated into humic structures using formaldehyde condensation at different monomer-to-humic ratios to obtain materials of different reducing properties. Reduction of model actinides by the hydroquinone-enriched humic materials was studied under both oxic and anoxic conditions at acidic and neutral pH values. Redox speciation of plutonium and neptunium was monitored using solvent extraction techniques and liquid scintillation counting. It was shown that both nonmodified leonardite humic acid and all the obtained derivatives were able to reduce Pu(V) quantitatively both at neutral and acidic pH and under oxic and anoxic conditions. The reduction rate increased with an increase in the degree of modification of the humic material and reached its maximum for HQ500 derivative. For neptunium, reduction was observed only under anoxic conditions at acidic pH. The hydroquinone-modified derivatives had much higher reducing performance when compared to nonmodified leonardite humic acid. The results obtained show good prospects for a use of hydroquinone-modified humic materials as reactive agents for remediation of actinide contaminated aquifers.