The stability of non-ionic surfactants and linear alkylbenzene sulfonates in a water matrix and on solid-phase extraction cartridges

Abstract

The stability of nonylphenol ethoxylates (NPEO), alcohol ethoxylates (AEO), coconut diethanol amides (CDEA) and linear alkylbenzene sulfonates (LAS) in a water matrix and preconcentrated on SPE cartridges was studied. A stability study was carried out in a water matrix (spiked ground water and real-world waste water) comparing different pretreatment procedures (addition of sulfuric acid to pH = 3, preservation with 1% and 3% of formaldehyde). When stored in a water matrix serious qualitative and quantitative changes occurred in waste water during the period of time studied (30 days). The losses of C₁₂–C₁₄ alcohol ethoxylates ranged from 72% to 88% when the sample was preserved with acid and from 17% to 86% when the sample was preserved with formaldehyde (3%). Simultaneously, an enrichment of the shorter alkyl chain homologues (C₇EO and C₁₀EO) was observed. The losses of NPEO were from 45% (sample preserved by acidification or by addition of 3% of formaldehyde) to 85% (sample preserved with 1% of formaldehyde). Additionally, an increase in concentration of polyethylene glycols (PEGs) and formation of different acidic forms, such as monocarboxylated (MCPEGs) and dicarboxylated polyethylene glycols (DCPEGs) were observed. The stability of surfactants preconcentrated on SPE cartridges was studied as a function of storage time and storage conditions (room temperature, 4 °C and –20 °C). The results indicate that disposable SPE cartridges can be recommended for the stabilization of non-ionic surfactants and LAS. Storage at –20 °C is feasible for long periods (up to ¶3 months for ground water and up to 2 months for waste water), while storage at 4 °C can be recommended for a maximum of 1 month. When cartridges were kept at –20 °C the losses of AEOs (n = 12, 13 and 14), preconcentrated from waste water, ranged from 17 to 29% (after 60 days) and other compounds suffered small losses (maximum of 14% for C₁₃LAS). At room temperature, after 7 days, the losses were less than 11%, indicating that shipping of samples by mail can be done without any special requirements.