The variation mechanisms from simultaneous nitrification and denitrification process to complete chromium reduction under Cr(VI) stress

Hexavalent chromium (Cr(VI)) disrupts both nitrification and denitrification processes while competing with nitrogen reduction. This study unraveled how Cr(VI) impacts nitrogen removal in simultaneous nitrification and denitrification (SND) systems by analyzing nitrogen removal efficiency, chromium distribution, electron transfer, enzyme activity, and microbial community dynamics. The results demonstrated that 0.5 mg/L Cr(VI) did not adversely affect the system's nitrogen removal performance. However, 5.0 mg/L and 20.0 mg/L Cr(VI) severely inhibited ammonia oxidation, reducing ammonia monooxygenase (AMO) activity by 14.01% and 16.71%, respectively, and lowering nitrogen removal efficiency to below 20.0%. These concentrations also resulted in complete chromium reduction.Exposure to 5.0 mg/L and 20.0 mg/L Cr(VI) enhanced chromium reductase (CHR) activity and electron transfer capacity within the system. Lactate dehydrogenase (LDH) levels increased by 2.33 and 1.33 times, respectively, indicating severe damage to the biological system. In response, cells secreted substantial amounts of catalase (CAT). Additionally, by the end of the 20.0 mg/L Cr(VI) shock, COD utilization was only 6.0 mg/L, with polysaccharides (PS) compensating for biological energy consumption. The system evolved two chromium-reducing and highly resistant genera, Diaphorobacter and Simplicispira. This study offers valuable insights into the complex biotransformation processes affected by heavy metal inhibition in SND systems.