Binding of Pu(IV) to galacturonic acid and extracellular polymeric substances (EPS) fromShewanella putrefaciens, Clostridium sp. andPseudomonas fluorescens

Abstract

AbstractThe conditional stability constants for trace-level concentrations of Pu(IV) complexing with galacturonic acid and EPS, isolated from axenicClostridiumsp.,P. fluorescensandShewanella putrefaciensCN32 cultures, were determined at pH 4 and an ionic strength of 0.1 M NaCl using an ion-exchange technique. The analysis of ion-exchange data with Schubert´s technique indicates that the Pu binding by galacturonic acid and EPS fromClostridiumsp. andS. putrefacienscan be described based on the formation of 1:1 Pu(IV)-ligand complexes. However, the accurate description of Pu binding by EPS fromP. fluorescensrequires postulation of a mixture of 1:1/1:2 complexes between Pu(IV) and ligands under the experimental conditions studied.The results from the ion-exchange experiments were also modeled based on a non-electrostatic, discrete ligand approach in which bacterial EPS is conceptualized as being composed of a suite of monoprotic acids, HLi, of arbitrarily-assigned pKa(i) values (e.g., 4, 6 and 8). The examination of ion-exchange data in a chemical model suggested that only the pKa4 (L1) and 6 (L2) ligands are sufficient to accurately simulate the Pu(IV)/EPS binding, implying that carboxylic groups in EPS are the primary binding sites for complexing with Pu(IV) under the experimental conditions examined. The affinity of EPS for complexing Pu(IV) decreases in the order ofClostridiumsp.>S. putrefaciens>P. fluorescensalthough the concentrations of carboxylic groups in EPS decrease in the order ofP. fluorescens>S. putrefaciens>Clostridiumsp. This discrepancy may be due to differences in binding affinities between Na+ion in solution and EPS ligands. AtI=0.1 M, models demonstrated that the EPS fromP. fluorescensexhibits a much stronger affinity for the Na+ion compared to ligands from other EPS; therefore, the deprotonated carboxylic sites of EPS fromP. fluorescensare hypothesized to be mostly bound by Na+in solution.