Developing a Miniaturized Spectrophotometer Using 235 and 275 nm UVC-LEDs for Fast Detection of Nitrate in Natural Water and Wastewater Effluents
Yuze Han, Wen-Xiang Ji, Bicun Jiang, Yechao Tian, Shanqi Shen, Dong Zhou, Yan Li, Chendong Shuang, Wentao Li, Haiming Lu, Aimin Li
Abstract
In this study, miniaturized portable/online UVC-LED spectrophotometers have been developed for fast detection of nitrate nitrogen and dissolved organic matter, using 235 and 275 nm light-emitting diodes (LEDs) as the light sources and GaN-based photodiodes as UV detectors. The application of the wide-band gap GaN-based photodiodes eliminated the influences of parasitic visible emission of UVC-LEDs. Using Orange G as the test absorbing compound, the portable UVC-LED spectrophotometer showed high linear upper detection limits and a negligible stray light. The portable spectrophotometer showed good linearity for NO3––N concentration in the range of 0–15 mg/L (R2 > 0.99), while a loss of sensitivity was observed at higher NO3––N concentration. This nonlinearity was attributed to the intrinsic stray light arising from the incomplete overlap between the 235 nm LED emission spectra and the nitrate absorption spectra. With the empirical correction coefficient α being determined from natural water and municipal wastewater samples, the UVC-LED spectrophotometers have comparable accuracy to the standard spectrophotometric method. The results of size exclusion chromatography further elucidated that it was the fraction of humic substances that determined the correction coefficient α for the UV spectrophotometric detection of nitrate in natural water and municipal wastewater samples.