article · Environmental Science & Technology
Partial denitrification stops the biological reduction of nitrate at nitrite, offering a promising method to supply nitrite to anammox bacteria for nitrogen removal in wastewater treatment. Maintaining a stable accumulation of nitrite is challenging because downstream enzymatic reduction often depletes it. A reliable strategy uses dimethyl sulfoxide, a biosafe aprotic solvent, to selectively inhibit the nitrite reductase enzyme without hindering nitrate reductase. Adding one percent dimethyl sulfoxide significantly suppresses nitrite reductase gene expression and enzyme activity, enabling stable partial denitrification. Increasing the concentration to three and a half percent reduces nitrite reductase activity to 0.95 milligrams of nitrite per minute. The solvent exhibits a stronger binding affinity for nitrite reductase than for nitrate reductase and competes directly with nitrite for the catalytic cavity of the enzyme. This selective inhibition provides a steady mechanism for partial denitrification.
Nitrogen removal from wastewater is vital for preventing environmental pollution and algal blooms. Pairing partial denitrification with anammox bacteria is an energy-efficient treatment route, but it requires a dependable nitrite supply. Demonstrating that a safe chemical additive can reliably halt the process at nitrite helps overcome a primary hurdle in managing microbial nitrogen removal systems.
This method could benefit municipal and industrial wastewater treatment plant operators seeking to implement partial denitrification coupled with anammox processes. By offering a chemical control mechanism to maintain nitrite accumulation, it addresses an operational stability issue. The research appears to be at an early laboratory stage, demonstrating biochemical feasibility and molecular binding mechanisms, with real-world application requiring further testing in continuous, large-scale bioreactors.
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The recently proposed partial denitrification (PD), terminating nitrate reduction to nitrite, has been regarded as a promising alternative to nitrite supplying for anammox bacteria. The most important aspect of the PD process for engineering application is the stable and continuous supply of nitrite. However, the activity of nitrate reductase is often higher than that of nitrite reductase (NIR), making it difficult to accumulate nitrite during the denitrification process. Herein, a strategy for achieving efficient and stable partial denitrification using the biosafe additive dimethyl sulfoxide (DMSO) was constructed, and the mechanism of DMSO inhibiting NIR was analyzed. DMSO addition reduced the expression of <i>NIR</i> gene, and 1% DMSO addition can significantly inhibit NIR enzyme activity to achieve a stable PD process. When the DMSO concentration increased to 3.5%, the NIR enzyme activity was almost inhibited with the enzyme activity of only 0.95 mg nitrite/min. However, the addition of DMSO has almost no inhibitory effect on the nitrate reductase (NAR) enzyme. The affinity constant of DMSO with the NAR enzyme is -2.4 kcal/mol, while the affinity constant of DMSO with the NIR enzyme is as high as -3.1 kcal/mol. DMSO shows a higher affinity for NIR. Moreover, DMSO and nitrite occupy the same catalytic cavity in the NIR enzyme, which is the fundamental reason why DMSO selectively inhibits the NIR enzyme. This study provides a new idea for realizing efficient and stable partial denitrification function.
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DOI: 10.1021/acs.est.4c08731
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