Self-accelerating sulfur reduction via polysulfide to realize a high-rate sulfidogenic reactor for wastewater treatment

Sulfur reduction is a promising alternative to sulfate reduction as it can generate sulfide at a low cost for the precipitation of heavy metals or autotrophic denitrification in wastewater treatment. However, the extremely low water solubility of elemental sulfur limits its bioavailability and results in a low sulfur-reduction rate. Polysulfide, which is naturally generated through reactions between sulfur and sulfide, can enhance the bioavailability of sulfur and thus contribute to high-rate sulfur reduction. Based on this principle, a laboratory-scale sulfur-reducing bioreactor was designed in this study for wastewater treatment. After 164 days of operation, the sulfide production rate (SPR) in the bioreactor reached 126 mg S/L-h, which is significantly higher than those of other sulfate-reducing systems. Moreover, dissolved zero-valent sulfur (referred to as polysulfide) was detected in the sulfur-reducing reactor when the organics were completely depleted, indicating that polysulfide can form naturally and be readily reduced to sulfide in the bioreactor. We found that the produced sulfide promoted the formation of more polysulfide, which enabled a self-accelerating chain reaction of sulfur reduction via polysulfide. This stimulation effect was further validated by the 7-h batch tests. In the batch test without sulfide addition initially, a continuous increase in the hourly SPR was observed with increasing sulfide concentration. Furthermore, in the batch tests with the addition of 50-200 mg S/L sulfide at the beginning, the average SPR in the first 3 h increased with elevating initial sulfide concentration due to more polysulfide formation and reduction. However, high sulfide concentration (>250 mg S/L) hindered the continuous increase in SPR. Additionally, when polysulfide formation was prevented through the addition of Fe2+, the SPR dropped by 97.6% compared to that in the presence of polysulfide. This validates the key role of polysulfide in the high-rate sulfur reduction process. Overall, the findings suggest that high-rate sulfur reduction can be achieved for autotrophic denitrification or heavy-metal removal in wastewater treatment.