Sulfur oxidation kinetics of Acidithiobacillus caldus and its inhibition on exposure to thiocyanate present in cyanidation tailings wastewater.

The sulfur oxidation kinetics of an industrial strain of Acidithiobacillus caldus (At. caldus) cultured on elemental sulfur was explored in batch experiments in the absence and presence of thiocyanate (SCN-), a toxin inherent within cyanidation tailings wastewater. The Contois rate expression accurately described At. caldus sulfate generation (R2 > 0.93) and microbial growth (R2 > 0.87). For a culture maintained at 45 °C a maximum specific growth rate (µmax) of 0.105 h-1, sulfate yield from biomass (Ypx) of 4.8 × 10-9 mg SO42-.cell-1, and Contois affinity coefficient (Kx) of 1.56 × 10-8 mg S.cell-1 were established. The presence of SCN- (0 mg/L - 20 mg/L) in the bulk solution inhibited the microbial system competitively. Moreover, SCN- impeded microbial growth differentially; the rate expression was therefore partitioned with respect to SCN- concentration and inhibition constants (Ki) were determined for each region. Adaptation to discrete concentrations of SCN- (1 mg/L and 20 mg/L) improved SCN- tolerance in At. caldus; however, adapted strains exhibited reduced sulfur oxidation potential when cultured under thiocyanate-free conditions relative to the non-adapted control strain. To describe the adapted systems accurately, the Contois affinity coefficient (Kx) was revised to reflect the suspected metabolic decline. The derived Kx values increased in magnitude and affirmed an innate reduction in microbial substrate affinity or substrate adsorption capacity. Inclusion of these updated Kx constants within the rate equation suitably represented the experimental data for both adapted At. caldus strains with R2 > 0.94. Furthermore, the impact of adaptation on the inhibition kinetics and inhibition mechanism associated with SCN- exposure were reviewed. Thiocyanate inhibited sulfur oxidation non-competitively in the adapted strains, and the shift in inhibition mechanism may be attributed to the compromised metabolic state following adaptation.

Low-level thiocyanate concentrations impact on iron oxidation activity and growth of Leptospirillum ferriphilum through inhibition and adaptation.

Leptospirillum ferriphilum is the dominant iron-oxidising bacterium in traditional microbial communities utilised in bioprocesses for gold recovery from sulfidic minerals. Ferrous iron oxidation activity and growth of unadapted and thiocyanate-adapted L. ferriphilum HT was studied in batch culture across increasing thiocyanate (SCN-) concentrations in the range 0-2 mg/L to assess the feasibility of recycling remediated cyanidation wastewaters. Thiocyanate concentrations of 1 mg/L and 1.4 mg/L induced an inhibitory effect in the unadapted culture wherein ferrous iron oxidation rate and cell growth were compromised. A substantial lag in the onset of ferrous iron oxidation occurred at concentrations above 0.5 mg/L SCN-, with no oxidation activity above 1.75 mg/L SCN-. The adapted culture, however, was uninhibited across the SCN- concentration range investigated and demonstrated a higher specific ferrous iron oxidation rate owing to reduced growth. It is postulated that SCN- exposure in the absence of adaptation induces osmotic stress. Moreover, upregulation of genes associated with the synthesis of osmo-protectants may be responsible for the preservation of activity observed in the adapted culture. As L. ferriphilum is dominant within the biooxidation tank community, evidence of sustained iron oxidation activity at low-level SCN- concentrations affirms the potential of recycling bioremediated cyanidation wastewater.