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| Detection of Dissolved CO2 in Water Based on Near-Infrared TDLAS and Degassed Membrane |
| WEI Zhi-han, ZHANG Jun-sen, GU Xuan, XU Pan, HE Bo-yang, YE Biao, WEI Xin-xin, MENG Xin, XUE Guo-gang*, WANG Jing-jing* |
School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
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Abstract Inland waters, as a key component of the carbon cycle, the monitoring of dissolved gases within them is of paramount importance for evaluating carbon sources/sinks and climate change. Existing in-situ dissolved gas detection technologies for water primarily focus on dissolved gas detection in seawater, characterized by low detection limits, high technical complexity, elevated costs, large instrument size, and limited applicability in inland waters. In light of the high solubility of CO2 in inland waters, the complex and variable environment, and the objective of cost reduction, this study constructed a measurement system for detecting the solubility of CO2 in water by employing low-cost domestic butterfly-shaped near-infrared (1 603 nm) lasers, compact optical multi-pass cells, and degassing membranes. This system comprises a CO2 gas detection system based on tunable diode laser absorption spectroscopy (TDLAS) and a water-gas separation device utilizing membrane degassing technology. The entire system can be completely integrated within two 15-inch instrument cases, achieving a high level of miniaturization. By measuring a set of CO2 standard gases of varying concentrations, the calculation formula and performance of the TDLAS gas detection system for CO2 gas phase concentration were determined. The lower limit of CO2 measurement by the system is 3.91×10-4 (integration time 5 s). An innovative and straightforward method for preparing standard aqueous solutions of CO2, based on a gas detection system and Teflon gas bags, was proposed. The relationship between the liquid-phase CO2 solubility and the gas-phase CO2 concentration after degassing was thoroughly explored. It was determined that, under constant degassing process and parameters, a linear relationship exists between the solubility of CO2 in water and the concentration of the evolved gas, and an inversion formula for the liquid-phase CO2 solubility was established through linear fitting. The noise situation and detection sensitivity of the system were deeply investigated using methods such as Allan variance. The system's sensitivity for detecting the solubility of CO2 in water reaches 1.15 μmol·L-1 (integration time: 100 s), with a detection lower limit of approximately 3.22 μmol·L-1. Finally, the solubility of CO2 in various water samples was measured. The results indicate that this system can effectively detect the solubility of CO2 in water samples such as tap water, pure water, river water, and fish tank water, and the measured values are largely consistent with the research and analysis results, demonstrating the outstanding performance and reliability of the device in practical application scenarios. This study offers an important technical reference for the development of small and low-cost in-situ dissolved gas detection equipment for water. It holds significant significance in water ecological environment monitoring and greenhouse gas management.
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Received: 2024-11-18
Accepted: 2025-04-23
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Corresponding Authors:
XUE Guo-gang, WANG Jing-jing
E-mail: jjwang@jsnu.edu.cn；ggxue@jsnu.edu.cn
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