Diffusion-Based Continuous Real-Time Monitoring System for Total Volatile Organic Compounds
1. Introduction
Volatile organic compounds (VOCs) are hazardous and carcinogenic contaminants that readily vaporize into the atmosphere. They originate from diverse sources encompassing outdoor elements such as industrial processes and transportation as well as indoor contributors such as building materials and household items [1,2]. The origins of VOCs include inappropriate industrial waste management, biofuel production, combustion of fossil fuels, emissions from vehicles, and release from oil storage containers. A substantial contribution of VOC emission comes from the petroleum and chemical industries [3,4]. Notably, the leakage from underground storage tanks and inadequate emission management in the petrochemical plants can directly contaminate soil and groundwater, gradually spreading VOCs from the sources. Apparently, underground volatile compounds are likely to rise to the ground surface, eventually leading to vapor intrusion [5]. Since benzene, toluene, ethylbenzne, and xylene, collectively known as BETX, account for 80% of the total VOCs released by petrochemical plants [6], the on-site monitoring of BTEX migration to the ground level is crucial at the contaminated sites to ensure safe land use and proper site development.
VOCs are both air pollutants and contributors to the formation of other air pollutants. For example, VOCs act as precursors to the secondary formation of particulate matter [7]. In the troposphere, VOCs react with sunlight and nitrogen oxides, forming photochemical smog and contributing to an increase in the greenhouse effect [8]. Also, VOCs are introduced to the human body through various pathways such as ingestion, skin contact, and inhalation. This exposure occurs through transport mechanisms such as molecular diffusion from pollutants, precipitation penetration, and groundwater flow [9,10]. In particular, human health is vulnerable to highly volatile substances such as paints, ink, laundry detergents, etc. These substances are introduced to the body primarily through inhalation rather than ingestion or skin absorption [11,12]. Specifically, BTEX is known to pose a potential hazard to the human body, depending on the exposure level [13]. The International Agency for Research on Cancer classifies benzene as a human carcinogen (Group 1) and ethyl benzene as a substance that is likely to be carcinogenic to humans (Group 2B). While there is insufficient evidence supporting the carcinogenicity of toluene and xylene, they are lipophilic substances with high affinity for lipid-rich tissues such as brain cells. Therefore, long-term exposure can potentially affect the central nervous system [14,15].
VOCs are typically analyzed by gas chromatography (GC) with primary detectors such as flame ionization detector (FID) and photoionization detector (PID) for both quantitative and qualitative measurements [16,17,18]. The FID exhibits lower volatility, greater stability, and enhanced reliability [19,20]. In contrast, the PID is commonly employed as a cost-effective, portable sensor due to its ability to facilitate on-the-go analyses and prompt assessment. When examining low concentrations of VOCs, thermal desorption (TD) presents a feasible method to concentrate the gaseous VOC sample using a selective adsorbent for VOCs. This technique enables the reliable analysis of even minimal concentrations by enriching the samples at low temperatures, subsequently proceeding with thermal desorption and injection into the GC system [21,22]. Adsorbents made from porous polymer resin are employed for the preconcentration of VOCs in TD [23,24].
To assess BTEX in the contaminated site, it is crucial to develop instruments capable of directly measuring VOCs on-site to ensure a timely response. Though the portable GC apparatus demonstrates a potential to achieve detection limits as low as parts per billion, the utilization of carrier gas is an obstacle to application in the field [25,26]. Therefore, the on-site VOC measurement devices without carrier gas generally include PID sensors [27]. Though the PID sensor-equipped instruments are light and portable, an internal pump is required to introduce a sample with a flow rate of around 200~500 mL/min. Also, studies have explored sensors utilizing ion mobility spectrometry or infrared spectroscopy, demonstrating their superior capability for detection down to the pptv level, with a rapid response within 20 s [28,29]. However, all the existing real-time devices for monitoring VOCs are designed to facilitate the injection of gas into the analytical instrument. It is impractical to monitor precisely the concentration of VOCs within the soil stratum where the negative pressure buildup is undesirable [30,31,32]. Hence, it is necessary to design and operate measurement devices that overcome this limitation.
In this study, a total VOC (TVOC) monitoring system without a pump was developed employing a PID sensor. The reliability of the system was scrutinized across diverse conditions, drawing comparisons with the TD-GC system and a portable TVOC analyzer equipped with a pump. The TVOC monitoring system developed in this study was applied to measure TVOC migrated through a soil column to ensure a notable advantage by eliminating the need for GC or carrier gas, thus facilitating direct VOC measurements.
4. Conclusions
The TVOC monitoring system was developed by utilizing sample injection through diffusion, making it suitable for subsurface applications. The system aimed to incorporate the beneficial features of TD-GC and a portable VOC detector. The precision and accuracy of the TVOC monitoring system were in alignment with the TD-GC in established optimal experimental conditions. Even though the TVOC monitoring system exhibited a delayed response in comparison to the portable VOC detector, it demonstrated competitive responsiveness in cases characterized by a gradual and slow change in VOC concentration. Since the TVOC monitoring system is a diffusion-based gas monitoring system without altering the neighboring flows, it is another type of passive monitoring.
The TVOC monitoring system proposed in this study is expected to be a desirable device for monitoring VOC gas diffused from contaminated soil in the field. In particular, the TVOC monitoring system can find application in environmental and industrial policies aimed at regulating and controlling VOC emissions. Additionally, the data obtained by the TVOC monitoring system can support evidence-based decision-making in the development and implementation of policies related to VOC control and mitigation strategies. The sensor improvement needed to acquire high-precision measurement at a low concentration below 50 ppbv will allow further application of the TVOC monitoring system.
