Research Articles Biotransformation of Halogenated Benzenes Biotransformation of Halogenated in Anaerobic Sediments Benzenes Sridhar Susarla, Shigeki Masunaga, Yoshitaka Yonezawa National Institute for Resourcesand Environment, 16-30nogawa, Tsukuba, Ibaraki, 305 Japan Corresponding author: Dr. Sridhar Susarla Abstract The transformation kinetics of halogen substituted benzenes was examined in estuarine sediment. The sediment was sulfidogenic with sulfate concentration of 20 mmole/l. All compounds trans- formed without any lag period, with rate constants between0.0016 and 0.0342 day -l or half-lives of 20 and 433 days. For the com- pounds with different halogensubstituents on the aromatic ring, the transformation rate of the compound decreased in the order: I>Br>CI>E Key words: Benzenes, halogenated; halogenated benzenes; sediments, anaerobic; anaerobic sediments; bio- transformation, halogenated benzenes; organic contaminants, sinks 1 Introduction Sediments represent a major sink for organic contaminants (ADAMS et al., 1992). The pollutants in the sediment are subject to various physico-chemical and biological pro- cesses, i.e., adsorption on the mineral or organic fractions, biodegradation by the microbial community, or release by resuspension of the sediment. However, some of these processes remove the pollutants from the environment. Therefore, a major focus of current research is to improve the knowledge of the reaction mechanisms in the sediment. Recent research indicates that anaerobic processes remove halogens from haloaromatic compounds and produce de- halogenated compounds that are less toxic, less likely to bioaccumulate and more susceptible to further microbial attack (SIMS et al., 1990). Halogenated organic com- pounds, in particular, are more susceptible to reduction due to the presence of halogen substituents, which are highly electronegative. Thus, with increased halogenation, reduc- tion is more favourable than oxidation (VOGEL et al., 1987). Evidence for such a reaction include (i) the speci- ficity of the reduction reaction, (ii) the characteristic lag pe- riod, and (iii) the absence of activity in autoclaved sedi- ments (SIMS et al., 1990). In recent years, there have been a number of reports on the reductive dehalogenation of chlorobenzoates (SFFLITA et al., 1982; GENTHNER et al., 1989), chlorophenols (MADSEN and AAMAND, 1992; HENDRIKSENet al., 1992), chloroani- lines (STRuIJSand ROGERS, 1989; KUHN and SU~LITA,1989) and chlorobenzenes (BEURSKENS et al., 1994; RAMANANDet al., 1993), among others. Although a number of reports have been published on chlorobenzenes, to our knowledge, detailed investigations on the transformation of halo- genated benzenes (e.g., I, Br and F) have not been reported. The objective of this study was to determine the kinetic pa- rameters associated with the dehalogenation of several halogenated benzenes. The sediment used in this study was collected from the mouth of the Tsurumi river, which flows into Tokyo bay. A large number of factories discharge their treated waste- waters into the river. Contamination of the sediment with various industrial chemicals, such as toluene, xylene, ethyl- benzene, aniline, trichlorobenzene, hexachlorobenzene and decabromodiphenyl ether have been reported (Environ- ment Agency, 1993). 2 Experimental Section 2.1 Estuarine sediment The sediment used in the experiment was collected from the river bed in 1992 using an Ekman-Barge sediment sam- pier. The sediment was black in color indicating a sulfate reducing activity, however, macrobenthic organisms, such as small crabs, small fish and shellfish were present. The sediment was passed through a 2 mm sieve to remove de- bris. The concentrations of sodium chloride (NaC1), sulfate and nitrate in the sediment interstitial water were 1.7 %, 1.98 • 103 mg/1 (20.6 mmole/1) and below the detection limit, respectively. River water (24 ~ was also sampled just above the bot- tom of the sampling point and the NaCI, sulfate and nitrate concentrations were 1.5 %, 1.84 % • 103 mg/1 (19.2 mmole/1) and 4.0 rag/l, respectively. The sieved sediment sample was mixed with the river water to make a sediment slurry. The pH of the sediment slurry was 5.6. The total solid concentration and the ratio of ignition loss/total solid of the sediment slurry were 272 (_+ 2.8) g/kg (or 311 g/l) and 12.1 (• 0.033) %, respectively. These were an average of four replicate measurements. 2.2 Chemicals Halogenated benzenes were purchased from Tokyo Kasei Kogyo Co. Ltd. (TCI), Tokyo, Japan and were used with- out further purification. Stock solutions for all isomers at a ESPR - Environ. Sci. & Pollut. Res. 3 (2) 71-74 (1996) 9 ecomedpublishers, D-86899 Landsberg, Germany 71