An anaerobic bioreactor allows the efficient degradation of HCH isomers in soil slurry J.C. Quintero a , M.T. Moreira b , J.M. Lema a , G. Feijoo a, * a Department of Chemical Engineering, Institute of Technology, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain b Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain Received 18 February 2005; received in revised form 23 August 2005; accepted 23 August 2005 Available online 8 November 2005 Abstract The insecticide gamma-hexachlorocyclohexane (c-HCH or lindane), which has been extensively used for agricultural and medical purposes, presents high persistence and toxicity to the environment and low solubility. This study intends to assess the efficiency of an anaerobic reactor to degrade HCH isomers contained in soil slurry cultures. This study was developed in two phases: experiments in flasks to optimize the process parameters, and assessment of the slurry process in the anaerobic slurry reactor operated for an approximate period of a year. The influence of different environmental conditions was evaluated: the HCH concentration (25–100 mg HCH kg À1 ), the type of substrate (volatile fatty acids or starch), the sludge concentration (2–8 g VSS l À1 ) and the replacement of spiked soil to simulate a fed-batch operation (10–50%). The best results were obtained when the reactor was operated with a sludge concentration of 8 g VSS l À1 , starch concentration of 2 g COD l À1 and soil replacements of 10–20%. Under these conditions, a- and c-HCH were completely degraded after 10 d while nearly 90% b- and d-HCH were removed only after 50 d. According to the obtained results related to the total degradation of the HCH isomers and the degradation rates, especially high for a- and c-HCH, the anaerobic slurry reactor appears to be a good alternative for the degradation of the HCH isomers present in polluted soil. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Hexachlorocyclohexane; HCH; Anaerobic degradation; Slurry soil remediation treatment 1. Introduction Lindane (c-HCH) is an organochloride insecticide that has been used worldwide for agriculture and public health (Breivik et al., 1999; Li, 1999). During its produc- tion, 85% of the product containing other isomers, mainly a-, b- and d-HCH, was dumped as waste, causing serious soil pollution (Braun et al., 1991). Although biologically mediated degradation of HCH isomers has been demonstrated under aerobic and anaerobic conditions (Deo et al., 1994; Singh et al., 2000), the high adsorption of the pollutant to the soil and its restricted availability for the biological action of endogenous or exogenous microorganisms limits the extent and rate of degradation (Bollag et al., 1992; Harms and Bosma, 1997). One possibility to favor the 0045-6535/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.08.043 * Corresponding author. Tel.: +34 981563100x16776; fax: +34 981547168. E-mail address: eqfeijoo@lugo.usc.es (G. Feijoo). Chemosphere 63 (2006) 1005–1013 www.elsevier.com/locate/chemosphere