Interactions of Actinide Analogues with Naturally Occurring Organic Matter
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INTERACTIONS OF ACTINIDE ANALOGUES WITH NATURALLYOCCURRING ORGANIC MATTER BARBARA E. STOUT AND CIELITO M. DERAMOS
University of Cincinnati, Department of Chemistry, Cincinnati, OH 45221-0172. ABSTRACT
We have studied the interaction of a low molecular weight alginic acid (ca. 7,000 daltons) with Ca2", Mg2 +,Eu3+and La3", using lanthanide luminescence spectroscopy (LLS) and "3C NMR. LLS results imply that nine-coordinate lanthanide ions lose six water molecules upon complexation with alginate, forming inner-sphere alginate complexes. In general, the "3C resonances in the NMR studies decreased in intensity as metal ion concentration increased, but the preference of a cation for repeating mannuronate and guluronate units (MM and GG blocks) varied greatly. The La3 +ion binds preferentially to the GG blocks, though some binding to the MM blocks is evident, while Eua+ binds equally well to GO and MM blocks. Ca2" shows a strong preference for the GG blocks only, while Mg2+shows no evidence of binding to alginate at all. These preferences may be explained in terms of cation size, charge and coordination number. The diequatorial linkage pattern of sequential mannuronate residues leads to a flat ribbon-like structure with shallow cavities for the metal ions while the diaxial linkages of the guluronates allow a deeper, more size-specific cavity. Calcium has the optimum ionic radius for the GG cavities compared to the larger Eu3+ and La3+ions. The smaller Mg2+ion does not bind at all over the concentration range studied. This could be due to its large charge density to coordination number ratio that would make dehydration of this ion and subsequent complexation by alginate thermodynamically unfavorable. INTRODUCTION
Because of the concern over the ultimate fate of the actinides in the environment, there is a need to develop methods to predict the transport properties of these elements in nature. Naturally-occurring organic matter (such as humic substances and polysaccharides) is an important constituent of soils and some ground waters and can be expected to play a significant role in the mobility of metal ions.' Carbohydrates are major constituents of humus, accounting for 5-25% of the organic matter found in soils.2 They occur predominantly in polymeric form and may be associated with humic and fulvic acids through ester linkages. Like humic acids, they can act as cation exchangers, a property generally attributed to the uronic acids present. Uronic acids are saccharide derivatives possessing carboxylate groups. Polyuronides are responsible for soil particle aggregation, a function related to structural features (length, ability to hydrogen bond through hydroxyl groups, etc.) that enable them to bridge the spaces between soil particles.2 Investigations of polyuronides can not only lay the foundations for studies of more complex organic matter like humic acid, but will yield valuable information on metal-polysaccharide interactions as well.
Mat. Res. Soc. Symp. Proc. Vol. 353 @1995 Materials Research Society
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Alginic
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