Detection of Polycyclic Aromatic Hydrocarbons and Emerging Contaminants in Environmental Wastewater

Wastewater includes liquids and waterborne solids from domestic, industrial or commercial uses, as well as other wastewater for man’s activities. The quality of the wastewater is very low, so the wastewater will be discharged into the sewage system. The waste from industrial wastewater comprises a lot of inorganic and organic pollutants, and the specific type of the pollutants depending on the industry. The organic part of wastewater contains some volatile organic compounds such as benzene, toluene and xylene, which are hardly soluble in water. organic solids such as fats and grease can also be present together with colloids and organics in real solution. Depending on the industry involved, it can contain sugars, starches, dyes, mercaptans, and the like.

There are several organic pollutants present in the storm water. These include herbicides, pesticides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), phenols, aliphatic and aromatic compounds from gasoline and oil, chlorinated ethenes from dry cleaners, and dry cleaners such as chlorinated ethylene, and many other contaminants that depend on the location and activities of the wastewater treatment plant.

Fig.1 Freshwater and wastewater cycle

Among these pollutants, polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds that present widely in the environment as the result of incomplete combustion of organic material. Trace amounts of PAHs have been found in industrial and domestic effluents. Their solubility in water is very low but can be enhanced by detergents and other organic solvents that may be present. Although PAHs are not very biodegradable, they tend to be taken out of solution by adsorption onto particulate matter. If present in raw waters, they are usually removed during coagulation, sedimentation and filtration. However, they can be re-introduced into the distribution system from mains that have been lined with coal tar pitch. In the UK, coal tar, which can contain up to 50% of PAHs, was used to produce iron water mains to prevent rusting until the 1970s. In some situations, this lining may eventually break down, releasing PAHs and particulates into the water. The concentration was found to have seasonal variations and the solubility was linked to the water temperature. There is also evidence that chlorine dioxide, when used as a disinfectant or used for taste and odour control, can result in elevated concentrations of PAHs at consumers’ taps.

Several PAHs are known to have a much higher carcinogenic concentration than those found in drinking water, and their main routes of exposure are food and cigarette smoke. Drinking water is normally monitored by five indicator parameters, namely benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, benzo(ghi)perylene and indeno(1,2,3-cd)pyrene, among which benzo(a)pyrene is considered to be the most harmful and has a separate standard.

 Fig.2 Different types of PAHs⁴

Both GC and LC methods are widely utilized in the determination of PAHs. GC with mass spectrometric detection offers the potential for high-resolution and selective separations of complex matrix samples. GC separations of PAHs are limited, however, by coelution of certain isomers with methylpolysiloxane stationary phases (e.g., chrysene and triphenylene [228 u]; benzo[b]fluoranthene, benzo[j]fluoranthene, and benzo[k]fluoranthene [252 u]; and dibenz[a,c]anthracene and dibenz[a,h]anthracene [278 u]). GC separations of isomers have been achieved by using liquid crystalline stationary phases and other phases in combination with optimized methods.

LC methods for the determination of PAHs in environmental samples are often based on fluorescence detection. Many, but not all, PAHs exhibit native fluorescent properties, and because fluorescent properties are relatively uncommon for other constituents in environmental samples, highly selective detection methods are possible. Selectivity is further enhanced through the use of time-programmed changes in the excitation and emission wavelengths, to maximize the detection sensitivity of specific analytes. Conditions can also be tuned to reduce the responses of potential interferences. The US Environmental Protection Agency (EPA) published an early application for the determination of PAHs by LC-fluorescence. EPA Method 610 is based on the use of a specific polymeric C18 column to resolve 16 high-priority pollutant PAHs in aqueous effluents, with subsequent fluorescence detection.

Major Analysis Trends of Emerging Contaminants

Fig.3 Emerging contaminants cycle¹

As a hot trend, the use of high-resolution mass spectrometry (HR-MS) and liquid chromatography (LC) for identifying unknown contaminants, especially environmental transformation products (TPs) and disinfection byproducts (DBPs), has grown exponentially. In these analyses, Orbitrap, time-of-flight (TOF), quadrupole (Q)-TOF, and sometimes Fourier transform (FT)-ion cyclotron resonance (ICR) mass spectrometers are used. Another growing trend is a combination of TOF-MS screening for large multianalyte analyses and subsequent target quantification. For example, a study performed an initial screening of 69 pharmaceuticals using ultraperformance liquid chromatography (UPLC)-TOF-MS and then performed a target quantification analysis for the analytes that were detected. This approach streamlines research processes and minimizes the use of analytical standards.

As a complementary analytical technique, the use of nuclear magnetic resonance (NMR) spectroscopy has also increased significantly to confirm tentative structures proposed by LC-HR-MS and LC-MS/MS. Since NMR lacks sensitivity compared to MS and it is difficult to analyze the mixtures, preparative LC coupled to fraction collection is often used to collect enough material to obtain an NMR spectrum. NMR techniques include traditional 1D techniques such as 1H NMR and 13C NMR, as well as 2D techniques such as nuclear Overhauser effect spectroscopy (NOESY) and correlation spectroscopy (COSY). Examples include the identification of TPs and DBPs from pharmaceuticals and the identification of new algal toxin cyanopeptides.

Another growing trend is the use of metabolomics tools. The METLIN database (a free online database of metabolite MS/MS data) and statistics-based profiling tools used in metabolomics are starting to be used more frequently for environmental research to assist with identification of TPs and to discriminate TPs from background signals. An example is the identification of TPs from the reaction of iopromide (a pharmaceutical medical imaging chemical) with UV-H2O2.

Novel analytical applications that may become trends in the future include the use of compound-specific isotope analysis (CSIA) and single crystal-X-ray analysis. For example, CSIA is used to identify the pharmaceutical diclofenac in environmental samples, based on carbon and nitrogen isotope measurements with gas chromatography (GC)-isotope ratio (IR)-MS, and it is also used to identify transformation pathways. Single-crystal X-ray analysis is used as an additional, more rigorous unknown chemical confirmation technique (along with GC/MS, LC-TOF-MS, and NMR) for structural confirmation of chlorine DBPs from the antidiabetic drug, metformin.

 Fig.4 Laboratory LC-TOF-MS analysis²

Finally, interesting new sensors are being developed. These include a new molecularly imprinted photoelectrochemical sensor for measuring PFOA under visible light irradiation, and a new nanochannel-based electrochemical sensor for real-time detection of the pharmaceutical ibuprofen in water.

In all, emerging contaminants are a growing concern for human health and the environment. The laboratory analyses is costly and there is currently no clear standard list of constituents as analytical methods are constantly developing. Due to their polarity, the majority of the aromatic compounds identified in environmental samples have been detected using liquid chromatography/mass spectrometry (LC/MS). This is the technique most commonly used for the identification and quantitation of aromatic compounds in water samples. Among the diverse LC/MS techniques commonly used for the routine monitoring and quantitation of aromatic compounds in water samples, the preferred one is tandem mass spectrometry (LC/MS-MS), which uses either collision cells or linear traps to obtain information on fragment ions.

References

• Richardson, S. D., & Chem, A. (2014) ‘Water analysis: emerging contaminants and current issues.’, Analytical Chemistry, 45(20): 2813.

• Trick, J. K., Stuart, M., & Reeder, S. (2008) ‘Chapter three – contaminated groundwater sampling and quality control of water analyses’, Environmental Geochemistry, 29-57.

• Schwarzenbach, R. P., Egli, T., Hofstetter, T. B., Gunten, U. V., & Wehrli, B. (2010) ‘Global water pollution and human health’, Social Science Electronic Publishing, 35(1).

• Hao, Y. L., Wang, Q., & Li, Q. Z. (2010) ‘Investigation on the pahs pollution in soil irrigating water in some area’, Modern Preventive Medicine.

• Rahman, M. F., Peldszus, S., & Anderson, W. B. (2014) ‘Behaviour and fate of perfluoroalkyl and polyfluoroalkyl substances (pfass) in drinking water treatment: a review.’ Water Research, 50(1): 318-340.

• Mitrano, D. M., Ranville, J. F., Bednar, A., Kazor, K., Hering, A. S., & Higgins, C. P. (2014) ‘Tracking dissolution of silver nanoparticles at environmentally relevant concentrations in laboratory, natural, and processed waters using single particle icp-ms (spicp-ms).’ Environmental Science Nano, 1(3): 248-259.

• Sánchez-Quiles, D., & Tovar-Sánchez, A. (2014) ‘Sunscreens as a source of hydrogen peroxide production in coastal waters.’ Environmental Science & Technology, 48(16): 9037-42.

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