Pertechnetate immobilization in aqueous media with hydrogen sulfide under anaerobic and aerobic environments

Summary

The basic chemistry for the immobilization of pertechnetate (TcO₄⁻) by hydrogen sulfide was investigated in aqueous solution under both aerobic and anaerobic environments. Pertechnetate immobilization was acid dependent, with accelerated rates and increased immobilization yields as the acid concentration increased. Oxygen had no effect under acidic conditions. Under anaerobic alkaline conditions, the pH, and therefore the speciation of sulfide, was the determining factor on the immobilization of pertechnetate. Only 53% of the TcO₄⁻ was immobilized at pH 8, while the yield increased to 83% at pH 9 as HS⁻ became the dominant sulfide species. The immobilization yield then decreased to 73% at pH 13. No reaction was observed between TcO₄⁻ and sulfide under aerobic alkaline conditions, indicating that oxygen suppressed this reaction.

Pertechnetate immobilization was found to be first order with respect to both sulfide and pertechnetate in acidic solutions, and in alkaline solution under anaerobic conditions. The results of stoichiometry studies and product analysis under alkaline anaerobic environments indicated that Tc₂S₇ was obtained at pH 9. EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) studies suggested that the samples obtained from acidic, aerobic solution and alkaline anaerobic solution were both Tc₂S₇. The stability of Tc₂S₇ is affected by O₂ with accelerated dissolution at high pH.