Electrochemical nitrite sensing using mass transfer signal with a catalyst-free small-sized rotating disk electrode...

Electrochemical nitrite sensing (ENS) is a competitive method for online monitoring in the intelligent control of biological nitrogen removal process. However, its popularity is extremely low due to complex wastewater interference and low sensor durability. Here, we developed a novel ENS method that utilizes the mass transfer signal (MTS) of the nitrite oxidation reaction (NOR), making detection accuracy dependent solely on mass transfer process. These features enabled us to design a catalyst-free, small-sized glassy carbon rotating disk electrode for accurate MTS determination with exceptional durability. The linearity of MTS versus nitrite concentration surpasses that of conventional differential pulse voltammetry and amperometry. The method has a wide linear range of 100 μM-100 mM, a detection limit of 28 μM, and a high sensitivity of 1638 μA mM^-1 cm^-2. Importantly, solution pH and coexisting buffers show no significant effect on MTS determinations as long as pH does not exceed 10. Excellent immunity to interference from ionic strength, temperature, COD, inert salts, metal ions, dissolved oxygen, and hydrogen peroxide was observed. While reducing substances capable of oxidation reactions do cause interference, they are not common in environmental samples. Finally, a self-designed detection system requiring a sample volume of 4 mL was used for wastewater testing. The results demonstrate good capability for nitrite detection during practical wastewater treatment processes, although relative error increases with the complexity and content of organic pollutants in the wastewater. Overall, this ENS method holds great potential for achieving rapid, stable, and low-cost nitrite sensing in environmental applications.