Assessment of trace organic chemicals in anaerobically digested sludge and their partitioning behaviour: Simultaneous Soxhlet chemical extraction and quantification via LC-MS/MS analysis

The increasing number of trace organic contaminants (TrOCs) detected in anaerobically digested sludge (ADS) is triggering increasing concern on its circular-economy reuse practices. A large scientific effort has been performed to define their concentration limits, partition behaviour, and innovative technologies for their removal, which require the definition of versatile and economically sustainable analytical methodologies. In this study, a Soxhlet extraction method coupled with LC-MS/MS analysis was developed to simultaneously determine 32 TrOCs in ADS, 11 of them being quantified in this matrix for the first time. The targeted TrOCs were selected based on the European Urban Wastewater Treatment Directive, and on their frequency of detection in municipal wastewater and/or sludge and chemical diversity. The use of methanol as solvent allowed good recovery efficiencies from ADS solid phase, with an extraction time of 3.5 h and without the need for subsequent clean-up procedures. The targeted LC-MS/MS method enabled high-sensitivity quantification of TrOCs in the liquid phase. At least 25 out of the 32 target compounds were detected in ADS samples from two wastewater treatment plants in Germany, providing their concentration data and highlighting the influence of TrOCs characteristics and sludge properties on contaminant partition coefficients (K_D). The experimental outcomes highlight the versatility of the Soxhlet method, which is effective in extracting compounds characterized by diverse properties and structures, and opens new perspectives for the analysis of various substrates. This could support the European Sewage Sludge Directive, expanding its application to soils and cultivated foods and offering insights into TrOCs transfer among different substrates and their influence when used as fertilizer, aiding in the efficient definition of risk assessment methodologies and regulatory concentration limits.