Analysis, occurrence and removal efficiencies of organophosphate flame retardants (OPFRs) in sludge undergoing anaerobic digestion followed by diverse thermal treatments

Organophosphate flame retardants (OPFRs) are a complex group of contaminants to deal with in sewage sludge, as cur-rently there is a lack of robust analytical methods to measure them and management strategies to remove them. To facil-itate quantifications of the occurrence of OPFRs in sludge and to establish their removal efficiencies (REs%) during thermal treatments, a simple, reliable, and rapid sample preparation methodology was developed for the determination of 21 OPFRs in diverse sludge, ash and biochar matrices. Matrix-solid phase dispersion (MSPD) tailored to ultra-performance liquid chromatography (UPLC) coupled to tandem mass spectrometry (MS/MS) was applied. Under optimal conditions, 0.5 g of freeze-dried sample were dispersed in 2 g of Bondesil C18, and 1.5 g of deactivated florisil were used as clean-up sorbent. The target analytes were extracted with 5 mL of acetone. The obtained extract was ready for analysis within 20 min without the need of any further treatment. The proposed methodology was assessed, providing absolute recoveries (Abs%) ranging from 50.4 to 112 % with good method repeatability (RSDs <17.9 %). Method limits of quan-tification ranged from 0.10 to 14.0 ng g-1 dry weight (d.w.). The optimized methodology was applied to raw-, digested-, combusted and pyrolyzed sludge samples collected from different waste treatment plants located in Norway, where 16 out of 21 OPFRs were detected in digested sludge samples up to 2186 ng g-1 (d.w.; sum concentration of OPFRs). Diverse thermal treatments of combustion and dry pyrolysis were assessed for the removal of OPFRs from sludge. Combustion at 300 degrees C reduced the concentrations of OPFRs by 98 % (in the ashes formed), whereas pyrolysis at temperatures >500 degrees C effectively removed the OPFRs in the produced biochar. Thermal treatments, in particularly dry pyrolysis, showed po-tential for achieving zero pollution management and recycling of OPFR contaminated sludge.