A new insight to explore toxic Cd(II) affecting denitrification: Reaction kinetic, electron behavior and microbial...

Denitrification process is a crucial step in nitrogen removal and is more vulnerable to external shocks due to the fact that anoxic process is always located before aerobic process in conventional sewage treatment. This study aims to elaborate the nitrogen conversion characteristics by investigating denitrification kinetics, electron behavior and microbial community under Cd(II) shock. Reaction kinetics showed that 10 mg/L of Cd(II) accelerated nitrate reduction rate by 52.29% but 80 mg/L of Cd(II) severely decelerated it by 95.41% with the accumulation of nitrite. High concentration of COD (C/N = 10.4) in the system caused by Cd(II) disrupting the integrity of cell membrane (lactate dehydrogenase increased by 328.7%) was proved to induce occurrence of Dissimilatory Nitrate Reduction to Ammonia (DNRA). The electron transport system activity (ETSA), electron consumption and electron distribution were combined to reveal the electron behavior regulated by Cd(II). The electron ratio of nitrate reductase to nitrite reductase increased from 1.48 (control) to 3.91 and 3.52 (40 and 80 mg/L of Cd(II)) indicated the electrons allocating tendency and further explained the nitrite accumulation. High concentration of Cd(II) also decreased ETSA by weakening the physiological activities of flavin adenine dinucleotide, flavin mononucleotide and cytochrome c or hindered the microbes to secrete these electron carriers. Furthermore, Cd(II) inhibited dominant bacteria genera containing napA gene (Azospirillum and Thauera) and nirS gene (unclassified_c_Betaproteobacteria). Enterobacteriaceae family was found to dominate the DNRA process.