Redox mediator improves sulfide-autotrophic denitrification under salinity stress via promoting electron transfer and...

Sulfur autotrophic denitrification (SADN) process is a promising nitrogen removal technology with significant low-carbon benefits. However, maintaining its efficient nitrogen removal in treating saline wastewater remains challenging due to the suppression of bioactivity by high salinity. Herein, the response of the SADN process in 2.0 % salinity conditions to four typical redox mediators (sodium anthraquinone-2-sulfonate, phosphomolybdic acid, sodium 1,2-naphthoquinone-4-sulfonate, and biochar) as well as the involved microbial mechanisms was investigated by electron transfer, functional enzymatic activity, and microbial community analyses. Results showed that the addition of redox mediators notably enhanced nitrate removal performance under the high salinity scenarios. Specifically, the nitrate removal efficiency increased from 17.6 % ± 2.2 %-61.4 % ± 2.4 % with the biochar amendment, representing a 240 %-276 % increase compared to the control without biochar. Following the addition of redox mediators, the electron transfer system activity of the SADN sludge increased 1.5 ± 0.3 times compared to that without redox mediators (i.e., from 0.04 ± 0.02 to 0.08-0.12 μg O₂/min mg protein). Subsequently, accelerated electrons were directed into denitrification enzymes, where the nitrate and nitrite reduction related functional genes (nar, nir) were upregulated. Furthermore, the addition of redox mediators stimulated the proliferation of the salt-tolerant bacterium Thiobacillus, which exhibited positive correlations with nitrogen-sulfur metabolism activities in the SADN sludge, supporting an efficient nitrate removal efficiency of 81.9 % ± 9.5 % during long-term operation. These results highlight the application potential of redox mediators for increasing SADN performance under high salinity conditions, and deepen our understanding of the underlying enhancement mechanisms.