Elsevier

Talanta

Volume 221, 1 January 2021, 121450
Talanta

Wastewater monitoring by means of e-nose, VE-tongue, TD-GC-MS, and SPME-GC-MS

https://doi.org/10.1016/j.talanta.2020.121450Get rights and content

Highlights

  • An e-nose and a VE-tongue are used for the analysis of wastewater samples.

  • Using e-nose, headspace characterization is better than that of wastewater odors.

  • By exploiting chemometric methods, the wastewater samples are well discriminated.

  • TD-GC-MS and SPME-GC-MS identified the VOCs responsible for this discrimination.

  • E-nose and VE-tongue are effective for odor and taste wastewater analysis.

Abstract

The presence of wastewater and air pollution has become an important risk factor for citizens, not only in terms of problems related to health risks, but also because of its negative impact on the country's image. For this reason, malodorous emission monitoring and control techniques are in high demand in urban areas and industries. The aim of this work is first to build an electronic nose (e-nose) and a Voltammetric Electronic tongue (VE-tongue) in order to study their ability to discriminate between polluted and clean environmental samples. Secondly, Thermal Desorption-Gas Chromatography-Mass Spectrometry (TD-GC-MS), and Solid Phase Micro Extraction-Gas Chromatography–Mass Spectrometry (SPME-GC-MS) are utilized to explain this discrimination by identifying specific compounds from these samples. Indeed, the e-nose, consisted of metal oxide semiconductor gas sensors, is used for the assessment of the studied odorous air and headspace samples from water and wastewater sites. Moreover, the VE-tongue, based on metal electrodes, is utilized to determine the patterns of the sensor array responses, which serve as fingerprints profiles of the analyzed liquid samples. Chemometric tools, such as Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), and Support Vector Machines (SVMs) are operated for the processing of data from the e-nose and the VE-tongue. By using the both systems, the analyses of headspace and liquid samples from the seven sites allow better discrimination. To explain the cause of the obtained discrimination, TD-GC-MS and SPME-GC-MS analyses are well performed to identify compounds related sites. According to these outcomes, the proposed e-nose and VE-tongue are proved to be rapid and valuable tools for analysis of environmental polluted matrices.

Introduction

Over the past few decades, pollution has increasingly become a crucial factor affecting the quality of life and health status of urban populations. Indeed, very harmful pollutants from industries and households can be drained by wastewater, leading to a deterioration of water quality and causing serious damage to human health, aquatic ecosystems, animals, and the environment [[1], [2], [3], [4], [5], [6]]. Moreover, one of the irritating factors in wastewater is the presence of an unpleasant odor that comes from volatile compounds in wastewater passing into the air by evaporating and is transmitted by the wind and contaminates nearby areas [7]. That's why air and water pollution are reported to be the main cause of several diseases, such as cardiovascular dysfunction, inflammation, respiratory infections, and cancer leading to millions of deaths around the world every year [[8], [9], [10], [11], [12]].

The aim of wastewater analysis is to curb polluting elements. This will reduce or eliminate complaints from the population affected. In order to detect ultra-trace amounts of organic compounds in air and wastewater, different approaches, such as Gas Chromatography-Mass Spectrometry (GC-MS) [13], TD-GC-MS [14], SPME-GC-MS [15], Headspace Solid-Phase Micro-Extraction Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS) [16], Gas Chromatography-Flame Ionization Detector (GC-FID) [17], Gas Chromatography-Tandem Mass Spectrometry (GC-MS-MS) [18], and High Performance Liquid Chromatography-Fluorescence Detection (HPLC-FLD) [19] have been performed. These techniques provide very detailed information on the nature and concentration of the samples. However, they allow analysis of single compounds in odors. This is not enough sufficient to adequately characterize wastewater odors [20,21]. Indeed, they are relatively expensive, not transportable, often require samples pre-treatment, long measurement times, and qualified personnel [22].

Besides, for wastewater analysis, several devices have been devised including e-nose [23,24], e-tongue [[25], [26], [27]], e-nose correlated with olfactometry analysis [20,28], a portable and commercial e-nose (PEN3) in conjunction with GC-MS [29], voltammetric electronic tongue combined with HPLC as reference method [30]. Despite the advantages and potential of these instruments, complete wastewater analysis has not been fully explored because the reported studies only analyze odorous air or liquid wastewater samples individually. In the literature, it is difficult to find a study reporting the use of e-nose, VE-tongue, TD-GC-MS, SPME-GC-MS, and physic-chemical analysis for quasi-complete analysis of odorous air, headspace, and liquid samples from water and wastewater sites. To fill this gap, the devices mentioned above are used in this study.

For this purpose, Fez, an imperial and touristic city of Morocco, is selected for this investigation because it has a fast-growing urban workforce and is understudied for its outdoor air and water's quality. The city is crossed by two rivers “Oued Fez” and “Sebou”. The “Sebou” river (496 Km) flows to Fez and discharges into the atlantic sea (near to Kenitra city). It is widely used for different purposes like city water supply, industry, and agriculture [31]. The polluted “Oued Fez” river discharges into the “Sebou” river that makes it contaminated. They are affected by different types of untreated water discharges from industrial activities, tanneries, petrol stations, metallurgy, and pottery [32]. Wastewater pollution has a negative impact on the agricultural and economic area of Fez. It also has the same impact on the quality of water [33].

Due to the health problems caused by some Volatile Organic Compounds (VOCs) following volatilization processes in wastewater, e-nose and VE-tongue systems could be helpful because of their significant advantages for wastewater analysis. To easily interpret multidimensional data from the complex responses generated by the sensor arrays, pattern recognition methods have been used, including PCA, HCA, and SVMs.

The present research highlights the ability of an e-nose and VE-tongue technology in conjunction with pattern recognition methods to characterize and discriminate water and wastewater samples. In addition, analytical technics such as TD-GC-MS and SPME-GC-MS methods have been used to determine compounds of the water and wastewater samples.

The novelty is to carry out an almost complete analysis of different wastewaters by exploiting their odorous air, liquid, and their headspace samples using e-nose and VE-tongue systems. The obtained results are explained by TD-GC-MS, SPME-GC-MS, and physico-chemical analysis techniques. The both systems have the advantages of being easy to handle, fast, no qualified personnel required, and allow on-site measurements.

Thus, the carried tasks are summarized in three main points:

  • -

    A home-developed e-nose, in combination with chemometric tools, is used to analyze odorous air samples from water and wastewater sites. The obtained results are explained by TD-GC-MS technique, which allows the identification of compounds in these odorous air samples.

  • -

    The same e-nose is also utilized to analyze the liquid headspace samples from the seven sites. Compounds in these headspace samples are identified using the SPME-GC-MS technique, which helps explaining the VE-tongue results.

  • -

    Another multi-sensor system (VE-tongue) is operated for the analysis of the liquid samples. The results of this analysis are elucidated by physico-chemical analysis.

Section snippets

Methods of sampling

In general, every sample of unpleasant emissions from a pollutant source is affected by climatic conditions such as the collection period, temperature, humidity, rain, and wind. Some difficulties may arise in collecting and analyzing samples which can be represented in the results. Thus, in order to identify appropriate instruments, equipment and sampling methods, it is important to know the nature and characteristics of the emission source [34]. The purpose of sampling is to obtain

E-nose results

Fig. 1A and B shows the normalized conductance of the MQ-9 sensor exposed to odorous air samples collected above sites and the corresponding headspace of liquid samples, respectively. In both cases, the normalized conductance increases with exposure time. This can be explained by the presence of oxidizing chemicals on the sensing surface of n-type solid-state gas sensors. These responses of MQ-9 sensor reach a steady state after 120 s of sample exposure. In Fig. 1A, the smallest response

Conclusion

Wastewater has been shown to be the object of many human health problems and discomfort.

Detection systems that are simple to handle, on-site, affordable, and capable of analyzing both odorous air and liquid samples from wastewater are rare to find in the literature.

In the present study, the use of detection systems (e-nose and VE-tongue) fulfilling these criteria was highlighted. Their potential to distinguish between clean water and wastewater samples was the main focus of this study. They

Credit author statement

Mohamed Moufid: Investigation, Formal analysis, Methodology, Software, Experimentation, Writing. Nezha El Bari: Writing Reviewing, Funding acquisition, Carlos Tiebe, Michael Hofmann, Matthias Bartholmai: Writing Reviewing. Benachir Bouchikhi: Conceptualization, Supervision, Validation, Project administration, Ressources, Funding acquisition, Writing Reviewing.

Declaration of competing interest

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.

Acknowledgments

We would like to thank Moulay Ismaïl University of Meknes for financial support of the project “Research support”. This work is funded by the federal Ministry of Education and Research (Germany) and Ministry of Higher Education, Scientific Research and Executives training (Morocco) under the Framework program of Moroccan-German scientific research cooperation, project PMARS III N°2015–87. Additionally we have to thank Mr. Richter, Mrs. Brödner, Mr. Lindemann and Mr. Jörg of BAM for carrying out

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