Journal of Radioanalytical and Nuclear Chemistry, VoL 241, No. 3 (1999) 627-636 Cesium, strontium, europium(Ill) and plutonium(IV) complexes with humic acid in solution and on montmorillonite surface F. Maefigek,* Ibrahim S. Shaban, L. Mfitel Department of Nuclear Chemistry, Faculty of Natural Sciences, Comenius University, SK-842 15 Bratislava, Slovakia (Received March 24, 1999) The effect of Aldrich humic acid (HA) on the mobility of ~37Cs, 85Sr, ~SZEu and 239pu radionuclides was studied in Ca-montmorillonite suspensions. Verified 2-sites-2-species (2s2s) models correspond to an intensive interaction of all elements with humificated surface, what is in a remarkable contrast with the weak complexation of cesium and even strontium in solutions - the neutral ligand interaction constants 13(1/mol) are log 13<-9.9 and 7.56+0.21 for Cs and Sr, respectively. The result for europium complexation in solution, log 13= 12.49-2-_0.18 is in a good agreement with literature data. For plutonium(IV) not only a high proton competitive constant in solution was obtained, log 13=(-0.67+ 0.32)+3pH, but also a strong chemisorption, which at high concentrations ofhumic acid (above 0.05 g/I) indicates the formation of bridge humate complexes of plutonium on the humificated surface. Logarithms of heterogeneous interaction constants (~10, I/g) of the elements with surface humic acid are 4.47+0.23, 4.39+0.08, and 6.40+0.33 for Cs, Sr, and Eu(III), respectively, and the logarithm of the proton competitive constant (~24, I/g) for Pu(IV) -3.80+0.72. Distribution coefficients of humic acid and metal humates between 0.01 g HA/1 solution and montmorillonite were derived as log Ka(AH) =-1.04_+0.11, log Ka(EuA) = 1.56_+0.11 and log Ka(PUA ) = 2.25+0.04, while the values for Cs and Sr were obtained with very high uncertainty. Speciation of the elements on montmorillonite surface is illustrated as a function of equilibrium concentration of humic acid in solution and ofpH. Introduction In a previous paper, I the difference between distribution ratios of Cs and Sr ions on a free and humic acid coated ("blocked") alumosilicate surface was found to be less pronounced than expected, and no substantial complexation of the ions in solution was observed. Especially for cesium it was difficult to make the difference between the interaction with humificated surface and interactions with humic acid in solution, as reported for contaminated Chernobyl soil. 2 We decided to make a comparative study with polyvalent cations Eu 3+ and Pu 4+ with a regular humic acid (Aldrich) 3,4 and a well-defined montmorillonite (Jel~ov~ potok, Slovakia), 5-8 and to verify a model describing their behavior in humic acid - alumosilicate system with a minimal number of parameters. 9 Results of the complexation of strontium and europium in solutions of this humic acid may be also compared with those of several other reports. 1~ Unfortunately, quantitative data for plutonium(IV) are scarce and are mostly simulated by Th(IV) behavior. Generally, it is recognized that under deep underground conditions the reduction of PuO 2+ to Pu 4+ by humic acid would be very fast. 24,25 The same occurs in seawater, 26 presumably due to the presence of quinhydrone functional groups in the humic structure. 27 In the reducing conditions of a geological repository in clay 28 or in river estuary 29 plutonium is expected to be trivalent or tetravalent. A considerable effort, supported by international projects 3~ is oriented towards the assessment of plutonium properties at waste disposal sites. Published data quantify plutonium in various humates-containing environmental matrices: the humic and fulvic acids of soils, sands and clays. 31-36 It was found that plutonium accumulates in the clay-high molecular weight fractions of humic acids where it remains undissolved at their organic solvent extraction. 32 On the other hand, the transport with smaller organic fractions occurs in sandy aquifers. 34 Humic acid causes considerable increase of plutonium fluxes in clay layers 28 but not in cement pore water 33 and also enables passing of colloidal particles through quartz sand. 35 Humic substances dominate actinide (thorium, uranium, plutonium and americium) speciation at pH 4-7 and concentration of humic substances above 0.I mg/1 in absence of calcium and aluminum cations, and at 3-10mg/l in their presence. 37,38 Interaction constants for Pu(IV) with a sediment humic acid were obtained by the solvent extraction method of CHOPPIN and ALLARD 39 to be log 131=9.8 and log 132 = 16.0 for 1:1 and 1:2 stoichiometry. The constants for Pu(III) with a non- defined humic acid were found by MAHAJAN et al. 4~ to be log 131=2.8 and log 132=5.0. In a previous paper, the Sips isotherm for humic acid sorption, and two-site distribution model of ion sorption were found to suit well for description of mobility of cesium and strontium ions as a function of equilibrium humic acid concentration as the single variable at given pH. 1 Now, an extended 2-sites-2-species model of M z+ ion adsorption and chemisorption in presence of humic acid 9 was applied. * E-mail: macasek@fns.uniba.sk 0236-5 731/99/USD 17.00 9 1999 Akaddmiai Kiad6, Budapest All rights reserved Elsevier Science B. V., Amsterdam Akaddmiai Kiad6, Budapest