Environ. Sci. Technol. 1999, 33, 4119-4125
Fate of Linear Alkyl Benzenesulfonates, Coproducts, and Their Metabolites in Sewage Treatment Plants and in Receiving River Waters A N T O N I O D I C O R C I A , * ,† LAURA CAPUANI,‡ FRANCESCA CASASSA,† ANTONIO MARCOMINI,§ AND ROBERTO SAMPERI† Dipartimento di Chimica, Universita` “La Sapienza”, Piazza Aldo Moro 5, 00185 Roma, Italy, Azienda Comunale Energia e Ambiente (ACEA), Via Eleniana 6, 00185, Roma, Italy, and Dipartimento di Scienze Ambientali, Universita` di Venezia, Calle Larga S. Marta, 30123 Venezia, Italy
While much attention has been paid to assess contamination levels of linear alkyl benzenesulfonate (LAS) surfactants in the environment, only a few papers have reported on the levels of breakdown products of LAS and coproducts. Major LAS coproducts are dialkyl tetralinsulfonates (DATS) and methyl-branched isomers of LAS (iso-LAS). A previous method (accompanying paper) based on solid-phase extraction (SPE) and liquid chromatography/mass spectrometry has been modified for monitoring the above analytes in aqueous samples of sewage treatment plants (STPs) and in river waters. Analytes were extracted from 25, 200, and 1000 mL of respectively raw sewage, treated sewage, and river water samples by a 0.5-g Carbograph 4 SPE cartridge. Recovery studies of some authentic short-chain LAS metabolites suggested that the SPE cartridge was able to quantitatively extract all the compounds of interest from the three types of aqueous matrixes. Under full-scan conditions, limits of detection of the method for metabolites of LAS and coproducts was set at 2 ng/L in river water. By this procedure, the concentrations in water of the analytes entering and leaving five activated sludge STPs were monthly monitored over several months. Field data indicated that about 50% of the metabolites of LAS coproducts were not mineralized by activated sludge treatment. More than 12% of them were dicarboxylated species. On the average, DATS metabolites amounted to more than 50% of the total analyte concentrations in STP effluents. A short survey of the fate of persistent metabolites of LAS coproducts discharged from a STP in river water gave us some evidence that isoLAS metabolites continued to degrade in the aquatic environment.
Introduction Surfactants constitute a major ingredient of detergent components. Usually, surfactants are disposed after use to * Corresponding author fax: +39-06-490631; e-mail: dicorcia@ axrma.uniroma1.it. † Universita ` “La Sapienza”. ‡ Azienda Comunale Energia e Ambiente (ACEA). § Universita ` di Venezia. 10.1021/es990596u CCC: $18.00 Published on Web 09/30/1999
1999 American Chemical Society
sewage treatment plants (STPs). Here, biodegradation processes and adsorption on sludge particles remove these chemicals from wastewaters to a greater or lesser extent, depending on the chemical structure of the surfactant molecule and on operating conditions of the STP. After treatment, residual surfactants, refractory coproducts, and biodegradation products dissolved in STPs effluents or adsorbed on sludges are discharged into the environment. By several transport mechanisms, these chemicals can enter the hydrogeological cycle. Assessment of the environmental contamination levels by surfactants and related compounds is also important to support a wide range of laboratory biodegradation tests and ecotoxicological studies. After soaps, linear alkyl benzenesulfonates (LAS) are the most widely used surfactants in domestic detergents. In 1995, the global production was ca. 2.8 × 106 ton (2). Commercial LAS mixtures usually contain about 15% of coproducts (3). The chemical nature of these impurities is reported in the companion paper (3). In that paper, the results of a laboratory aerobic biodegradation experiment of LAS and coproducts have substantially confirmed previous findings (4-6) indicating that, analogously to LAS, all of the homologues and isomers of DATS and iso-LAS undergo primary biodegradation. This process leads to formation of monocarboxylated (STACs and SPACs) and, to a lesser extent, dicarboxylated (STADCs and SPADCs) metabolites (see ref 3 for acronym explanation). But, differently from LAS intermediates (LASI), remarkable fractions of both DATS intermediates (DATSI) and iso-LAS intermediates (iso-LASI) were found resistant to ultimate biodegradation. Steric hindrances of the tworinged structure and methyl branching close to or attached on the benzylic carbon should be respectively responsible for the high resistance to catabolic attack of certain isomers of short-chain DATSI and iso-LASI. Because of the widespread usage of LAS-containing detergents, significant quantities of refractory organics can reach the environment. We estimated (3) that each year about 200 000 ton of these organics can reach the environment. Although the determination of the environmental levels of these metabolites is of relevant geochemical interest, only a few papers have reported on this matter. Field et al. (7, 8) surveyed extensively the fate and transport of LAS and DATS in a sewage-contaminated groundwater. These searchers identified a series of SPAC and STAC intermediates and estimated that concentrations of the latter species were remarkably larger than those of the former ones. On monitoring four U.S. activated sludge STPs for LAS, DATS, and related intermediates, Trehy et al. (9) found that STAC amounts released by the plants were 3.2 times larger than those of SPACs. Solid-phase extraction (SPE) with a graphitized carbon black (GCB) cartridge followed by ion-trap GC/ MS was used to determine concentration levels of LAS and SPACs in Taiwan STPs effluents and river waters (10). All the works cited above used gas chromatography (GC)/mass spectrometry (MS) instrumentation for analyzing analytes. Very selective detection by MS is of paramount importance for identifying and quantifying the great number of LASI and DATSI that can be present in complex aqueous environmental samples. A drawback of the GC-based procedures is that they require extensive sample manipulation to convert LASI and DATSI to species amenable to GC analysis. Some years ago, we developed a method involving SPE with a GCB cartridge, selective desorption of sulfonated species, and liquid chromatography (LC) with fluorescence detection for monitoring LAS and SPACs in STP aqueous samples (11). The weakness of this method was that VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Some Available Technical Characteristics of the Five Sewage Treatment Plants (STP) Considered in This Study STP
inhabitants
loading (m3/day)
HRTa (h)
effluent flow rate (m3/s)
inlet BOD (mg/L)
outlet BOD (mg/L)
inlet COD (mg/L)
outlet COD (mg/L)
Cobis Est Fregene Nord Ostia
40 000 800 000 120 000 800 000 350 000
10 000 265 000 42 000 354 000 112 000
12 14 12 14 14
0.12 2.9 0.45 4.1 1.3
270 140 210 120 220
