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Vertical distribution profiles of linear alkylbenzene sulfonates and their long‐chain intermediate degradation products in coastal marine sediments

The variation with depth of the concentration of linear alkylbenzene sulfonates (LASs) and of the long‐chain sulfophenyl carboxylic acids (SPCs) resulting from LAS biodegradation was determined in coastal sediments. We analyzed samples of sediment cores taken from three locations in a littoral zone subjected to the discharge of untreated urban effluents in the Bay of Cádiz in the southwestern part of the Iberian Peninsula. The vertical profile of LAS concentrations showed a sharp reduction with depth, whereas the concentration of long‐chain SPCs (6–13 carbon atoms) was greatest at 10 to 14 cm depth. At this depth, the conditions in the interstitial water are strictly anoxic (Eh = −380 mV). The partition coefficients between the solid phase of the sediment versus the interstitial water are very different for LAS and for its degradation intermediates. For LAS, the organic carbon‐based partition coefficient values were between 2.4 × 103 and 6.6 × 1055 L/kg for the C10 and C13 homologues, respectively; these values are similar to those obtained from laboratory tests for the sorption of LAS onto marine sediments. For the long‐chain SPCs, the partition coefficients are several orders of magnitude less as a consequence of their lower hydrophobicity.