Characterisation of surface uptake and biosorption of cationic nuclear fission products by sulphate-reducing bacteria

Abstract

The treatment of radioactive fission products – ⁹⁰Sr²⁺, ⁶⁰Co²⁺ and ¹³⁷Cs⁺ – from simulated nuclear wastewater was evaluated using locally-isolated sulphate-reducing organisms. In this study, sulphate-reducing bacteria (SRB) were used as biosorbents for removal of the cationic fission products. The cultures achieved 90%, 100%, and 80% removal of Sr²⁺, Co²⁺ and Cs⁺, respectively, for a 75 mg×ℓ^-1 solution of each metal under a low ionic strength of 0.01 M. Increasing the ionic strength of the solution to 0.5 M resulted in a decreased metal uptake to 80%, 65% and 70% for Sr²⁺, Co²⁺ and Cs⁺, respectively. Approximately 68% of the adsorbed fraction on cell surfaces was exchangeable (i.e., was desorbed under acidic conditions). Using surface complexation models and equilibrium modelling analysis, reaction sites on the cell walls of the cultures were determined to belong to the –COOH and –H₂PO₄ groups (pK_a = 4–5 and 7–8, respectively). The distribution of the isoelectric equilibrium points for cell surfaces was consistent with the composition and characteristics of the identified microbial species in the culture which was dominated by the Gram(+ve) Bacilli – Lysinicibacillus boronitolerans AB199591 – and biofloc-forming Gram(–ve) SRBs, such as Desulfomonile tiedgei AF418162, Syntrophobacter wolinii X70905, and Desulforhopalus vacuolatus L42613. The high exchangeable fraction on the cells and the higher removal rates under lower ionic strength indicates that metal binding was non-electrostatic which was consistent with outer-sphere complexation behaviour.