Influence of oxidation reduction potential and pH on polysulfide concentrations and chain lengths in the biological desulfurization process under haloalkaline conditions

Biological desulfurization under haloalkaline conditions has been applied worldwide to remove hydrogen sulfide (H2S) from sour gas steams. The process relies on sulfide-oxidizing bacteria (SOB) to oxidize H2S to elemental sulfur (S8), which can then be recovered and reused. Recently, a dual-reactor biological desulfurization system was implemented where an anaerobic (sulfidic) bioreactor was incorporated as an addition to a micro-oxic bioreactor, allowing for higher S8 selectivity by limiting by-product formation. The highly sulfidic bioreactor environment enabled the SOB to remove (poly)sulfides (SX2−) in the absence of oxygen, with SX2- speculated as a main substrate in the removal pathway, thus making it vital to understand its role in the process. The SOB are influenced by the oxidation reduction potential (ORP) set-point of the micro-oxic bioreactor as it is used to control the product of oxidation (S8 vs. SO42−), while the uptake of SX2− by SOB has been qualitatively linked to pH. Therefore, to quantify these effects, this work determined the concentration and speciation of SX2- in the biological desulfurization process under various pH values and ORP set-points. The total SX2- concentrations in the sulfidic zone increased at elevated pH (8.9) compared to low pH (< 8.0), with on average 3.3 ± 1.0 mM-S more Sx2−. Chain lengths varied, with S72− only doubling in concentration while S52− increased 9 fold, which is in contrast with observations from abiotic systems. Changes to the ORP set-point of the micro-oxic reactor did not produce substantial changes in SX2- concentration in the sulfidic zone. This illustrates that the reduction degree of the SOB in the micro-oxic bioreactor does not enhance their ability to interact with Sx2− in the sulfidic bioreactor. This increased understanding of how both pH and ORP affect changes in SX2− concentration and chain length can lead to improved efficiency and design of the dual-reactor biological desulfurization process.