Chemical treatment of an anionic surfactant wastewater: Electrospray-MS studies of intermediates and effect on aerobic biodegradability

The effect of wet air oxidation on the aerobic biodegradability of a model wastewater containing 1000 mg L-1 of linear alkylbenzene sulfonate (LAS) ha s been investigated. Semibatch oxidation experiments were performed at a te mperature of 473 K, an oxygen partial pressure of 1.3 MPa and residence tim es varying from 40 to 390 min, while continuous oxidation experiments were performed at a residence time of 120 min. Oxygen uptake tests were performe d to assess the aerobic biodegradability of both the oxidised and the origi nal LAS solutions using cultures that had been adapted to both LAS and oxid ation intermediates. The concentration of total organic carbon, chemical ox ygen demand and active detergent were followed throughout the wet oxidation and biodegradation experiments, while the main intermediates formed during wet oxidation were identified by means of Electrospray-MS and high perform ance liquid chromatography. It was found that LAS could be easily oxidised at 473 K to yield a group of molecules with short alkyl chains which do not behave as active detergents. Sulfonated aromatics are produced as intermed iates which have had the alkyl chain shortened. The segments of alkyl chain s broken off the intermediate compounds appear primarily as short chain org anic acids. The original, unoxidised 1000 mg L-1 LAS solution was found to be readily biodegradable in the laboratory aerobic reactors operating at lo w organic loadings and substrate to microorganism concentration ratios. How ever, wet oxidation resulted in effluents that were less readily biodegrada ble than the original LAS with biodegradability decreasing with increasing degree of oxidation. These results suggest that, at the conditions under co nsideration, a combined chemical pre-oxidation and biological post-treatmen t process may be less effective in removing LAS than a single-stage biologi cal or chemical process. (C) 2001 Elsevier Science Ltd. All rights reserved .