Resonance-based Sensor for Detection of Nitrogen Oxide (NOₓ)-polluted Water in Industrial Effluents

Abstract

Water is a fundamental necessity for human existence. Unfortunately, water quality has been steadily deteriorating due to various pollutants, with industrial contamination being a major contributor. Industrial wastewater often contains hazardous substances among these pollutants, nitrogen-based compounds such as nitrates $\mathrm{NO}_{3}{ }^{-}$ and nitrites NO2 are of particular concern. In this study, we employ Ti3C2Tx-Fe3O4 to achieve this objective.Ti3C2Tx, a family of two-dimensional layered compounds, has displayed potential in gas sensing applications due to its unique attributes, including a high surface area, strong metallic conductivity, high hydrophilicity, good mechanical properties, and active surface chemistry. The selectivity of NO2 gas in Ti3C2TX can be further improved with Fe3O4 nanoparticles. Fe3O4 facilitates the electron transfer from NO2 to $\mathrm{NO}_{3}{ }^{-}$, while Ti3C2Tx offers ample surface area for this reaction to occur, along with providing additional electrons. The properties are studied as a function of variation of dielectric constant of Ti3C2Tx-Fe3O4 with gas interaction. Using our earlier studies, a well optimized complementary slit-ring resonator (CSRR) sensor, operating at 430MHz, was used as a unit cell sensor. Separately, Ti3C2Tx-Fe3O4 was exposed to NOx (ranging from 0 to 106 ppm), and the resulting NOx-purged Ti3C2Tx-Fe3O4 composite was later exposed to the CSRR device. As the purged gas concentration changed from 0 to 106 ppm, there was a systematic shift in frequency and power. The maximum power shift was ~ 18.4 dB. The frequency shift gave a sensitivity of 400 KHz / ppm with NOx-purged Ti3C2Tx-Fe3O4, which was two times higher than the reference Ti3C2Tx system. Thus, a two-stage apparatus for detection of hazardous industrial effluents is presented using a novel Ti3C2Tx-Fe3O4 composite system.