Determination of surfactants and some of their metabolites in untreated and anaerobically digested sewage sludge by subcritical water extraction followed by liquid chromatography-mass spectrometry

Enormous amounts of sewage sludge are worldwide generated and released into the environment. Analysis of the most common and/or toxic chemicals in sludge should be mandatory before deciding its destination. Surfactants and some of their breakdown products are invariably the most common organic contaminants in domestic sewage sludge. For determining these compounds, we have developed a method based on extraction with subcritical water followed by liquid chromatography-mass spectrometry. On extracting surfactants and their metabolites from 50 mg of sludge, the efficiency of the water extraction device was evaluated in terms of pH of the extractant, temperature, and time of the static extraction. The best extraction conditions were obtained by using carbonate buffer (pH 9.4) at 200degreesC as extractant, 10 min of static extraction at the pressure of 100 bar followed by 17 min of dynamic extraction. Analyte collection was performed by inserting a solid-phase extraction cartridge downstream the extraction cell. Compared to 16-h Soxhlet extraction with methanol, this procedure was remarkably more efficient in extracting anionic surfactants and acidic metabolites of nonylphenol ethoxylates (NPECs). A short survey was conducted to estimate concentration changes of target compounds after 14-d sludge anaerobic digestion. Results showed that 54-74% of both neutral and weakly acidic ethoxylate species were removed after residence of the sludge in the digester. On the contrary, little, if any, removal of anionic surfactants was observed after the digestion treatment. As expected, the level of nonylphenol increased under anaerobic conditions.