A reagent-free on-site COD monitoring by a variable optical path UV-Vis spectrometer

Highlights

	● This spectrometer is characterized by variable optical path.
	● Versatility and detection range significantly broadened for diverse applications.
	● Enhanced sensitivity and accuracy for detecting subtle compositional changes.
	● Ideal for monitoring water samples with diverse turbidity and concentration levels.

Abstract

The variation in pollutant concentrations among different water bodies poses a significant challenge for environmental surveillance. Traditional UV-Vis spectrometers, with fixed optical paths, face limitations in accurately determining Chemical Oxygen Demand (COD) and other water quality parameters. High concentrations surpass the detection limit, while low concentrations yield weak response signals, thereby compromising measurement accuracy. This study tackles these challenges by enhancing a UV-Vis spectrometer with a variable optical path. By utilizing a right-angle reflector for reflection and a stepping motor for control, measurements are conducted within the wavelength range of 190–700 nm. The instrument incorporates a spectral fusion algorithm to optimize spectral measurements within its operational range. Furthermore, a Partial Least Squares (PLS) model has been established for COD inversion by using laboratory standard solutions and field samples. The spectrometer has been tested in the nearshore waters of Shenzhen Bay, China, validating its applicability and the model’s accuracy. The utilization of a variable optical path UV-Vis spectrometer facilitates the acquisition of precise monitoring data with wide measuring range, thereby enabling the prompt detection of anomalies and subsequent reduction in reaction time.

Graphical abstract

Keywords

Variable optical path / UV-Vis spectrometer / COD / Reagent-free

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CRediT Authorship Contribution Statement

Xinnan Qian: Formal analysis, Writing-original draft. Jianwu Sheng: Methodology, Writing-review and Funding acquisition. Qiyun Zhu: Methodology, Validation, Formal analysis, Data curation. Xiaohong Zhou: Supervision, Writing-review and editing. Wenjun Sun: Data collection, Funding acquisition. Xiangyun Xiong: Data collection, Funding acquisition. Siyu Zeng: Methodology, Supervision, Writing-review and editing, Funding acquisition. Miao He: Methodology, Supervision, Funding acquisition.

Conflict of Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was supported by the Bureau of Ecology and Environment of Shenzhen, Natural Science Foundation of Hunan Province (No. 2023JJ50346), and the Open Fund of National Engineering Research Center for Advanced Technology and Equipment of Water Environmental Pollution Monitoring (No. 2024KFJJ0106).

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