Biological treatment of toxic refinery spent sulfidic caustic at low dilution by sulfur-oxidizing fungi

One of the major concerns in petrochemical industries is eliminating hydrogen sulfide. Sodium hydroxide solutions are generally used for removing hydrogen sulfide and result in spent caustics which have a pH greater than 11 as well as high sulfide concentrations (2-3% w/v). In this study, SMHS-3 isolate was introduced with the ability to oxidize sulfur media containing sodium sulfide, thiosulfate and elemental sulfur. An analysis based on internal transcribed spacer (ITS) gene indicated that Sadat Mahale hot spring soil (SMHS-3) was closely related to Aspergillus genus. The oxidation results showed that Aspergillus sp. SMHS-3 has the highest sulfur oxidizing ability which produced sulfate in an average range of 0.85 g L-1 after 10 days. Response surface methodology (RSM) was applied to optimize the sulfur decomposition and resulted in a 1.2 fold increase in sulfur conversion rate. Aspergillus sp. SMHS-3 was used in a low-diluted spent caustic bio-treatment, resulting in a 16% reduction of COD, 10% reduction of TDS and 55% removal of active sulfite after 10 days of treatment. Results from this study suggests Aspergillus sp. SMHS-3 as a potential fungal strain for oxidizing sulfur compounds and a possible candidate for spent caustic biodesulfurization.