Using SPE and GC to Detect Harmful Isomers in Sewage

Water Treatment Plant at sunset | Image Credit: © tuastockphoto - stock.adobe.com

Scientists from the Guangdong University of Technology in Guangzhou, China recently developed a system for detecting nonylphenol (NP) isomers in sewage and wastewater sludge. Their findings were published in the Journal of Chromatography A (1).

Nonylphenols (NPs) are toxic xenobiotic compounds classified as endocrine disrupters capable of interfering with organisms’ hormonal systems (2). These isomers are regarded as important fine chemical intermediates, mainly used in the production of nonionic surfactants, such as nonylphenol ethoxylates (NPEs). NPs are also widely used as emulsifiers in pesticides, textiles, and more. During production and usage, NPs and NPEs can enter municipal wastewater treatment systems. In treatment processes, NPEs can degrade into NPs, which are more persistent and display stronger estrogenic activity. With wide distribution, high concentration levels, and persistent nature, the United Nations Environment Programme (UNEP) has designated NPs as priority persistent toxic substances for control.

Wastewater treatment plants are the main collection sites and potential pollution sources of NPs; however, current treatment technologies have limited capability for specific NP removal. As a result, residual NPs may be discharged into the environment through treated effluent, and sludge, a byproduct of wastewater treatment, can accumulate significant amounts of NPs. Left improperly managed, sludge poses a risk of secondary pollution to surface water, groundwater, and soil.

Many detection methods are used to combat NPs, with gas chromatography–mass spectrometry (GC–MS) widely used for NP analysis due to its high sensitivity and powerful separation capabilities. However, traditional GC–MS techniques often deal with matrix effects when analyzing complex samples, which can compromise the accuracy of both separation and quantification. Comprehensive two-dimensional GC (GC×GC) has been successfully applied to the separation of various NP isomers in industrial products and environmental waters. However, it has not been widely adopted in most laboratories due to high equipment costs.

For this study, the scientists aimed to create an effective and sensitive method for detecting NP isomers through combining solid-phase extraction (SPE) sample pretreatment with GC–MS and Deans Switch. Deans Switch is a flow path switching method that uses a central valve to divert specific fractions from the first chromatographic column to a second column with a different polarity for further separation. According to the scientists, the approach can significantly enhance isomer separation while effectively removing interferences, making it an effective means to improve detection sensitivity and specificity.

The technique was found to effectively transfer NPs from the system’s first gas chromatography column (DB-5MS) to the second column (CP-ChiraSil-DEXCB), achieving efficient separation of the 10 isomers. Sewage and sludge samples taken from wastewater treatment plants, were prepared with SPE and solid-liquid extraction. NP isomers in sewage samples were enriched with a phenol-specific SPE column, rinsed with 10 mL methanol-water (v/v 2:8), and eluted with 3 mL methanol and 5 mL dichloromethane. NP isomers in sludge samples underwent three ultrasonic extractions with 30 mL ethyl acetate-dichloromethane (v/v 1:1).

The results showed that the recovery rates for NP isomers ranged from 70.82–110.12% in sewage (LOQ: 0.09–0.31 μg/L; RSD: 1.82–10.12%) and from 72.82–114.12% in sludge (LOQ: 0.15–0.47 μg/kg; RSD: 0.23–11.28%), complying with the United States Environmental Protection Agency (U.S. EPA) standards (70–130%). When applied to real samples, the method detected all 10 NP isomers, with concentrations of 0.50–5.01 μg/L in sewage and 0.13–24.91 μg/g in sludge. With these findings, the scientists concluded that their method could both provide technical support for monitoring NPs pollution and offer scientific guidance for optimizing wastewater treatment processes and controlling pollution.

References

(1) Liu, L.; Zhang, J.; Zhao, Y.; Ning, X. Detection of Nonylphenol Isomers in Sewage and Sludge of Waste Water Treatment Plant by GC-FID-MS Combined with Deans Switch. J. Chromatogr. A 2025, 1744, 465717. DOI: 10.1016/j.chroma.2025.465717

(2) Soares, A.; Guieysse, B.; Jefferson, B.; Cartmell, E.; Lester, J. N. Nonylphenol in the Environment: A Critical Review on Occurrence, Fate, Toxicity and Treatment in Wastewaters. Environ. Int. 2008, 34 (7), 1033–1049. DOI: 10.1016/j.envint.2008.01.004