Imaging VOC distribution and tracing emission sources in surface water by a mobile shipborne spray inlet proton transfer reaction mass spectrometry

Abstract

Volatile organic compound (VOC) pollution in surface water can affect the growth of microorganisms and even cause damage to human health. Quickly obtaining VOC distribution and tracing emission sources are of great significance to ensure the security of water quality. In this study, we developed a mobile shipborne spray inlet proton transfer reaction mass spectrometry (S-SI-PTR-MS) instrument for rapid and on-line detection of volatile organic compounds (VOCs) in surface water. It mainly consists of a shipborne platform, a spray inlet system, a proton transfer reaction mass spectrometer, and geographic information software (GIS). During the navigation, VOCs in water can be quickly extracted to gaseous state by the SI system and transferred into the proton transfer reaction mass spectrometry (PTR-MS) for detection. The GIS can receive VOC and longitude-latitude data at the same time. And then the identification and concentration information can be shown on an electronic map in real time. The limit of detection and response time were 1.32 μg/L∼9.43 μg/L and 56 s∼114 s, respectively. The spatial resolution is 0.14 m in a common navigation experiment (5 km/h, dwell time=0.1 s), allowing for high-resolution monitoring of VOCs in water. Finally, field experiments were performed on the lower Nanfeihe River and some surfaces of Chaohu Lake in China. During tracing pollution in Nanfeihe River, two VOC emission sources in the downstream were located, nearing a sewage outlet and a wharf, respectively. During imaging VOC distribution in the Chaohu Lake, we found the areas with high-concentration VOCs were mainly along the ship channel. The newly developed S-SI-PTR-MS can monitor water quality in surface water in real time and has the potential to assist environmental protection enforcement.

Introduction

Volatile organic compounds (VOCs) are one kind of the major pollutant in surface water. High concentrations of VOCs can deteriorate the water quality and cause fatal effects on aquatic plants, animals, and humans. Previous studies found eucalyptol and limonene can promote cyanobacteria to become dominant species and exert toxic effects on other algae, and even lead to water bloom (Zuo et al., 2018). Researchers also found benzene toluene ethylbenzene & xylene (BTEX) could damage multiple organs of fish (Folkerts et al., 2021). Moreover, VOCs can enter and accumulate in human body through drinking (Czub and McLachlan, 2004; Tao et al., 2018). And long-term intake of these VOCs will cause the risks of carcinogenic and teratogenic (Yang et al., 2019; Liu et al., 2020). The global freshwater resources are in short supply. And the VOC pollution will aggravate this phenomenon. Therefore, the VOC pollution in water has attracted more and more attentions from environmental protection departments and researchers.

Human activities are important sources of VOCs in water, such as agricultural production (Geng and Sharpley, 2019; Zhou et al., 2021), domestic waste (Hasan et al., 2019; Wang et al., 2019), and industrial production (Rajput et al., 2017; Li et al., 2022). In the agriculture production, chemical fertilizers and pesticides were heavily used, which has caused more and more pollution to surface water (Shen et al., 2012; Zou et al., 2020). On the other hand, a large amount of domestic wastewater, containing pesticides (Anju et al., 2010; Borsuah et al., 2020), detergents (Ibrahim, 2018; Song et al., 2021), and edible oils (Sharma et al., 2018; Mofokeng et al., 2020; Lu et al., 2021), was discharged in our daily life. And in some remote rural areas, the wastewater was directly discharged into rivers without any treatment (Mishell Donoso and Rios-Touma, 2020; Yi et al., 2021). Compared with the above agricultural and domestic production, VOCs emitted from industrial production have the characteristics of multiple types, high concentrations, and high toxicity (Rajput et al., 2017; Ahmad et al., 2019; Cheng et al., 2020; Rafiq et al., 2021). The emission sources are sometimes set at concealed places, such as underwater (Harari et al., 2019; Roveri et al., 2021), to dodge the supervision from the department of environment protection. And the discharge time is selected (Wu et al., 2018; Chen et al., 2020). For example, wastewater with low concentration VOCs is discharged in the daytime, while that with high concentration VOCs is discharged at night. As a result, the traditional detection instruments with point sampling method are hard to image the VOC distribution and trace the VOC emission sources in real time. Therefore, it is necessary to develop a mobile real-time and on-line method to detect and display the VOCs in surface water.

The common VOC extraction technologies in water include purge and trap (P&T), headspace solid phase microextraction (HS-SPME), membrane inlet (MI), equilibrator inlet (EI), and spray inlet (SI). P&T can extract VOCs and semi-volatile organic compounds with boiling point below 200 °C with fine reproducibility (Ueta et al., 2019, 2020). HS-SPME is a solvent-free sample pretreatment technology with an ability of enrichment (Maia et al., 2014; Abolghasemi and Piryaei, 2022). MI can extract VOCs according to different permeation rates (Wu et al., 2019; Kim et al., 2020). EI can achieve VOCs extraction through vapor-liquid equilibrium (Kameyama et al., 2009, 2010). However, these technologies also have some shortcomings. For example, P&T and HS-SPME are time consuming (Lo et al., 2021; Ma et al., 2023). MI has memory effect (Cheng et al., 2021), and The harsh field test conditions (acid-base exposure, temperature fluctuations, UV radiation, particle adhesion) are likely affect the service life of the membrane (Zhang et al., 2006, 2022; Rabuni et al., 2013; Kim et al., 2019). The extracting efficiency of EI technology is limited by the relatively small contact area between carrier gas bubbles and water. SI is a technology for rapid extraction of VOCs to gaseous state through pressurized atomization (Zou et al., 2016, 2017). The contact area between droplets and carrier gas was much bigger than that of EI. So, this technology has a shorter response time and is more suitable for rapid extraction of VOCs in water.

The common VOC detection technologies include gas chromatography (GC), gas chromatography mass spectrometry (GC-MS), and proton transfer reaction mass spectrometry (PTR-MS). GC has the advantages of high selectivity. It is often used in combination with hydrogen flame ionization detector (FID) (Djozan et al., 2020), electron capture detector (ECD) (Matin et al., 2020), flame photometric detector (FPD) (Marder et al., 2020), and photoionization detector (PID) (Frausto-Vicencio et al., 2021). GC-MS is the most common detection technology and has been regarded as a gold standard in laboratory. However, GC or GC/MS requires a long separation time and does not allow real-time and online detection of VOCs in water (Majchrzak et al., 2018). PTR-MS is a real-time, online, and sensitive VOC detection technology, which has been widely used in environmental monitoring, medical health, public safety, and food monitoring (Lindinger et al., 1998a, 1998b; Shen et al., 2009; Biasioli et al., 2011; Yuan et al., 2017; Sekimoto and Koss, 2021). So, it is suitable for detecting gaseous VOCs in real time.

In this study, we developed a mobile shipborne spray inlet proton transfer reaction mass spectrometry (S-SI-PTR-MS) instrument for imaging VOC distribution and tracing emission sources in surface water. The structure and composition of the instrument were introduced. To illustrate its performance, the shock resistance, limit of detection (LOD), response time, and repeatability were evaluated. To demonstrate its ability to image VOC distribution and trace emission sources, the S-SI-PTR-MS was applied for field experiments in Nanfeihe River and Chaohu Lake. The S-SI-PTR-MS instrument has the potential to assist environmental protection authorities in conducting water pollution investigations and protecting freshwater resources.