SPECIATION-DEPENDENT TOXICITY OF NEPTUNIUM(V) TOWARD CHELATOBACTER-HEINTZII

This work investigates how chemical speciation controls the toxicity o f neptunium and the neptunium-NTA complex toward Chelatobacter heintzi i. We studied the effect of aquo and complexed/precipitated neptunium on the growth of C. heintzii in noncomplexing glucose and phosphate-bu ffered nitrilotriacetic acid (NTA) growth media. Equilibrium chemical speciation modeling and absorption spectroscopy were used to link nept unium speciation to biological growth inhibition. Our results show tha t metal toxicity of aquo NpO2+ significantly limits the growth of C. h eintzii at free metal ion concentrations greater than approximate to 1 0(-5) M. However, neptunium concentrations greater than or equal to 10 (-4) M do not cause measurable radiotoxicity effects in C. heintzii wh en present in the form of a neptunium-NTA complex or colloidal/precipi tated neptunium phosphate. The neptunium-NTA complex, which is stable under aerobic conditions, is destabilized by microbial degradation of NTA. When phosphate was present, degradation of NTA led to the precipi tation of a neptunium-phosphate phase.