Abstract
Surfactants are high-production-volume chemicals that are among the most abundant organic pollutants in municipal wastewater. In this study, sewage sludge samples of 36 Swiss wastewater treatment plants (WWTPs), serving 32% of the country's population, were analyzed for major surfactant classes by liquid chromatography mass spectrometry (LC-MS). The analyses required a variety of complementary approaches due to different analytical challenges, including matrix effects (which can affect adduct ion formation) and the lack of reference standards. The most abundant contaminants were linear alkylbenzene sulfonates (LAS; weighted mean [WM] concentration of 3700 μg g−1 dry weight), followed by secondary alkane sulfonates (SAS; 190 μg g−1). Alcohol polyethoxylates (AEO; 8.3 μg g−1), nonylphenol polyethoxylates (NPEO; 16 μg g−1), nonylphenol (NP; 3.1 μg g−1), nonylphenol ethoxy carboxylates (NPEC; 0.35 μg g−1) and tert-octylphenol (tert-OP, 1.8 μg g−1) were present at much lower concentrations. This concentration pattern agrees with the production volumes of the surfactants and their fates in WWTPs. Branched AEO homologues dominated over linear homologues, probably due to higher persistence. Sludge concentrations of LAS, SAS, and NP were positively correlated with the residence time in the anaerobic digester. Derivation of the per capita loads successfully revealed potential industrial/commercial emission sources. Comparison of recent versus historic data showed a decrease in NPEO and NP levels by one or two orders of magnitude since their ban in the 1980s. By contrast, LAS still exhibit similar concentrations compared to 30 years ago.
Highlights
Surface-active agents are amphiphilic organic compounds containing a hydrophilic head and a hydrophobic tail
Since the 1960s, Linear alkylbenzene sulfonates (LAS) have been used as substitutes for the branched alkylbenzene sulfonates that were responsible for foaming in effluents of wastewater treatment plants (WWTPs) and receiving natural waters (Sweeney and Anderson, 1989)
C16-LAS was used as the internal standards (IS) for LAS with C10–C13, C12-SAS for SAS with C14–C17, and C10EO8-Alcohol polyethoxylates (AEO) for AEO with C12–C18 and EO2–15
Summary
Surface-active agents (surfactants) are amphiphilic organic compounds containing a hydrophilic head and a hydrophobic tail. Surfactants are a diverse group with wide commercial and industrial applications as detergents (laundry detergents, cleaning agents, personal care products), emulsifiers, and wetting agents (Li et al, 2018b) They are high-production-volume chemicals (15 million tons per year), with anionic and non-ionic surfactants accounting for up to 90% of the annual production (Freeling et al, 2019; Lara-Martín et al, 2006). Linear alkylbenzene sulfonates (LAS) are a high-volume class of anionic surfactants with a yearly consumption of 0.43 million tons in Europe alone (HERA, 2013). They are the most consumed anionic surfactant class apart from soap (Berna et al, 2007).
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