Enhancement of bio-promoters on hexavalent chromium inhibited sulfur-driven denitrification: repairing damage, accelerating electron transfer, and reshaping microbial collaboration.

Proposing recovery strategies to recover heavy-metal-inhibited sulfur-driven denitrification, as well as disclosing recovery mechanisms, can provide technical support for the stable operation of bio-systems. This study proposed an effective bio-promoter (mediator-promoter composed of L-cysteine, biotin, cytokinin, and anthraquinone-2,6-disulfonate) to recover Cr(VI) inhibited sulfur-driven denitrification, which effectively reduced the recovery time of NO3--N reduction (18-21 cycles) and NO2--N reduction (27-42 cycles) compared with self-recovery. The mediator-promoter repaired microbial damage by promoting intracellular chromium efflux. Moreover, the mediator-promoter reduced the accumulated reactive oxygen species by stimulating the secretion of antioxidant enzymes, reaching equilibrium in the oxidative-antioxidant system. To improve electron transmission, the mediator-promoter restored S2O32- oxidation to provide adequate electron donors and increased electron transfer rate by increasing cytochrome c levels. Mediator-promoter boosted the abundance of Thiobacillus (sulfur-oxidizing bacterium) and Simplicispira (denitrifying bacterium), which were positively correlated, facilitating the rapid denitrification recovery and the long-term stable operation of recovered systems.

Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission.

Although the biotoxicity of heavy metals has been widely studied, there are few reports on the recovery strategy of the inhibited bio-system. This study proposed a combined promoter-I (Primary promoter: l-cysteine, biotin, and cytokinin + Electron-shuttle: PMo12) to recover the denitrification suppressed by Cr(VI). Compared with self-recovery, combined promoter-I shortened the recovery time of 28 cycles, and the recovered reactor possessed more stable long-term operation performance with >95 % nitrogen removal. The biomass increased by 7.07 mg VSS/(cm3 carrier) than self-recovery due to the promoted bacterial reproduction, thereby reducing the toxicity load of chromium per unit biomass. The combined promoter-I strengthened the toxicity remediation by promoting 92.84 % of the intracellular chromium release and rapidly activating anti-oxidative stress response. During toxicity remediation, ROS content quickly decreased, and the PN/PS value was 2.27 times that of self-recovery. PMo12 relieved Cr(VI) inhibition on NO3--N reduction by increasing NAR activity. The enhanced intracellular and intercellular electron transmission benefited from the stimulated NADH, FMN, and Cyt.c secretion by the primary promoter and the improved transmembrane electron transmission by Mo. PMo12 and the primary promoter synergized in regulating community structure and improving microbial richness. This study provided practical approaches for microbial toxicity remediation and maintaining high-efficiency denitrification.

Electrospinning micro-nanofibers immobilized aerobic denitrifying bacteria for efficient nitrogen removal in wastewater.

Electrospinning micro-nanofibers with exceptional physicochemical properties and biocompatibility are becoming popular in the medical field. These features indicate its potential application as microbial immobilized carriers in wastewater treatment. Here, aerobic denitrifying bacteria were immobilized on micro-nanofibers, which were prepared using different concentrations of polyacrylonitrile (PAN) solution (8%, 12% and 15%). The results of diameter distribution, specific surface area and average pore diameter indicated that 15% PAN micro-nanofibers with tighter surface structure were not suitable as microbial carriers. The bacterial load results showed that the cell density (OD600) and total protein of 12% PAN micro-nanofibers were 107.14% and 106.28% higher than those of 8% PAN micro-nanofibers. Subsequently, the 12% PAN micro-nanofibers were selected for aerobic denitrification under the different C/N ratios (1.5-10), and stable performance was obtained. Bacterial community analysis further manifested that the micro-nanofibers effectively immobilized bacteria and enriched bacterial structure under the high C/N ratios. Therefore, the feasibility of micro-nanofibers as microbial carriers was confirmed. This work was of great significance for promoting the application of electrospinning for microbial immobilization in wastewater treatment.