Np(V) sorption by goethite and hematite in the presence of humic acid

Full text of publication follows: Understanding effects of organic acids on actinide sorption to mineral surfaces is important for risk assessment of long-term storage of nuclear wastes and for modeling of radionuclide behavior in the environment. The sorption of Np(V) by well-characterized synthesized goethite (α-FeOOH) and hematite (α- Fe2O3) samples is studied in the absence and presence of non-modified and modified humic acids in the broad pH range and different ionic strengths. Leonardite humic acids isolated from the commercially available potassium humate Powhumus (Humintech Ltd, Duesseldorf) were used as the parent non-modified humic material (CHP). For the synthesis of modified humic acids the reaction of formaldehyde co-poly-condensation between parent sample (CHP) and hydroquinone was used. The condensation was conducted at the monomer to humic ratio of 100 mg per 1 g of CHP. Batch experiments were carried out in two component systems (Np(V)-ligand, Np(V)-mineral, mineral-ligand) and three component system (Np(V)-mineral-ligand) under N2-atmosphere. Micro- and ultra-filtrations were used for solid/solution separation. The redox speciation of Np(V), formation of actinide-humate complexes and determination of humic acid concentration in the solution during the sorption process were studied by solvent extraction techniques (TTA and HDEHP), spectrophotometry and X-ray photoelectron spectroscopy. The kinetics investigations showed that the equilibrium at the Np(V)-goethite and Np(V)-hematite systems is achieved in a week while in the ternary systems it takes more than 20 days to equilibrate. Slow reduction of Np(V) took place upon interaction with modified humic acid with no effect for non-modified sample. The presence of modified humic acid enhanced Np(V) sorption by goethite and hematite at low pH values and slightly decreased it at high pH values. The decrease of free humic acid concentration in the solution was established in the ternary system experiments with decreasing pH-value that indicates formation of ternary surface complexes. The FITEQL software was used to provide information on chemical equilibrium (reactions and corresponding equilibrium constants) for simulation of obtained experimental data in ternary system through the combination of the binary systems data. The work was supported by US DOE under special RFP in Russia (grant RUC2-20008 MO-04, administrated through CRDF). (authors)