Determination of stability constants of U(VI)-Fe(III)-citrate complexes.

Ion-exchange experiments were performed to evaluate the formation of the uranium-citrate and uranium-iron-citrate complexes over a wide concentration range; i.e., environmentally relevant concentrations (e.g., 10(-6) M in metal and ligand) and concentrations useful for spectroscopic investigations (e.g., 10(-4) M in metal and ligand). The stability of the well-known uranium-citrate complex was determined to validate the computational and experimental methods applied to the more complex system. Values of the conditional stability constants for these species were obtained using a chemical equilibrium model in FITEQL. At a pH of 4.0, the stability constant for uranium-citrate complex (log beta1,1) was determined to be 8.71+/-0.6 at I = 0. Analysis of the results of ion-exchange experiments for the U-Fe-citric acid system indicates the formation of the 1:1:1 and 1:1:2 ternary species with stability constants (log beta) of 17.10+/-0.41 and 20.47+/-0.31, respectively, at I= 0.

Protective effects of allozyme genotype during chemical exposure in the grass shrimp, Palaemonetes pugio.

The primary objective of this research was to determine if the genotype of the enzymes glucose phosphate isomerase (Gpi), phosphoglucomutase (Pgm), or mannose phosphate isomerase (Mpi) are factors affecting survival during acute endosulfan, fluoranthene, and chromium(VI) exposures. Palaemonetes pugio were exposed in the laboratory to 6.3 microg/L endosulfan, 100 mg/L chromium(VI), or 0.6 mg/L fluoranthene. Dead shrimp were removed at approximately 15-30 min intervals and the individual's genotypes for the Gpi, Mpi, and Pgm enzymes were determined. These data were used to establish whether allozyme genotype was related to tolerance as analyzed with non-parametric methods (i.e. development of survival curves using the Kaplan-Meier method of estimation) and parametric methods (i.e. proportional hazards models). Since the sex and/or size of the organism was expected to affect tolerance, these variables were accounted for in the proportional hazards models. Results indicated individuals that were heterozygous (MF) for the Gpi allozyme survived longer and had less overall mortality than the homozygous MM genotype when exposed to chromium(VI) and to fluoranthene. No allozyme genotypes were related to tolerance during the endosulfan exposure when sex was included as a covariate in the analysis. These results support the hypothesis that there is a genetic basis for tolerance in P. pugio during acute exposures to chromium(VI) and to fluoranthene. Although a relationship between Gpi genotype and contaminant tolerance has been identified in previous studies, no such relationship has been documented in P. pugio or with chromium(VI) as the contaminant.