Determination of the Quaternary Ammonium Surfactant Ditallowdimethylammonium in Digested Sludges and Marine Sediments by Supercritical Fluid Extraction and Liquid Chromatography with Postcolumn Ion-Pair Formation

Abstract

To study the phasing-out of the quaternary ammonium surfactant ditallowdimethylammonium cation (DTDMAC), concentrations of the cation in anaerobically stabilized sewage sludges were determined before and after its replacement by better degradable compounds. DTDMAC was quantitatively extracted from digested sludges using 380 atm of supercritical CO₂ modified with 30% methanol at 100 °C. Determination of DTDMAC was performed by normal-phase HPLC with postcolumn ion-pair formation and extraction with no sample cleanup. Mean concentrations of DTDMAC in sludges from five different municipal sewage treatment plants in Switzerland decreased from 3.67 g/kg (in 1991) to 0.96, 0.47, and 0.21 g/kg of dry sludge in 1992, 1993, and 1994, respectively. The precision of the method in digested sludge for 0.1−6.0 g/kg of dry matter, as indicated by the relative standard deviation, was typically 7%. The influence of the sample matrix was studied by performing supercritical fluid extraction (SFE) in coastal marine sediments. While SFE and a conventional liquid extraction method gave equal DTDMAC concentrations in sludges, the extraction of marine sediment samples yielded 30−40% higher DTDMAC values for SFE compared to those obtained by liquid extraction. The 94% drop in DTDMAC concentrations in digested sludges is due to the replacement of this substance and is a clear result of the producers' voluntary ban on its use in Europe.