Biodegradation kinetics of BTE-OX and MTBE by a diesel-grown biomass K. Acuna-Askar*, M.A. de la Torre-Torres*, M.J. Guerrero-Munoz*, M.T. Garza-Gonzalez**, B. Chavez-Gomez***, I.P. Rodriguez-Sanchez**** and H.A. Barrera-Saldana**** *Laboratorio de Biorremediacion Ambiental, Depto. de Microbiologia, Facultad de Medicina, UANL, Madero Pte. y Dr. Aguirre-Pequeno, 64460 Monterrey, N.L., Mexico (E-mail: kaskar@fm.uanl.mx; marco_delatorre81@yahoo.com.mx; jairgenoma@yahoo.com) **Facultad de Ciencias Quimicas, UANL, Pedro de Alba s/n, Cd. Universitaria, 66400 San Nicolas de los Garza, N.L., Mexico (E-mail: tgarza@fcq.uanl.mx) ***Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Delegacion Gustavo A. Madero, 07730 Mexico, D.F. (E-mail: bchavez@imp.mx) ****Unidad de Laboratorios de Ingenieria y Expresion Geneticas, Depto. de Bioquimica, Facultad de Medicina, UANL, Madero Pte. y Dr. Aguirre, 64460 Monterrey, N.L., Mexico (E-mail: irodriguez@fm.uanl.mx; hbarrera@fm.uanl.mx) Abstract The biodegradation kinetics of BTE-oX and MTBE, mixed all together in the presence of diesel-grown bioaugmented bacterial populations as high as 885 mg/L VSS, was evaluated. The effect of soil in aqueous samples and the effect of Tergitol NP-10 on substrate biodegradation rates were also evaluated. Biodegradation kinetics was evaluated for 54 h, every 6 h. All BTE-oX chemicals followed a first- order two-phase biodegradation kinetic model, whereas MTBE followed a zero-order removal kinetic model in all samples. BTE-oX removal rates were much higher than those of MTBE in all samples. The presence of soil in aqueous samples retarded BTE-oX and MTBE removal rates. The addition of Tergitol NP-10 to aqueous samples containing soil had a positive effect on substrate removal rate in all samples. Substrate percent removals ranged between 64.8 – 98.9% for benzene, toluene and ethylbenzene. O-xylene and MTBE percent removals ranged between 18.7 – 40.8% and 7.2 – 10.3%, respectively. Keywords BTEX; MTBE; Tergitol NP-10 Introduction Benzene, toluene, ethylbenzene and mixed xylenes (BTEX), along with methyl tertiary- butyl ether (MTBE), are among the unleaded gasoline compounds of major environmen- tal concern and are usually found in petroleum-contaminated sites. In the United States, BTEX are included in the National Primary Drinking Water Regulations (USEPA, 2001), and some states have set maximum soil contaminant concentrations for a number of hydrocarbons including BTEX (NCDENR, 2002). The maximum contaminant levels (MCLs) for BTEX in drinking water in Mexico are 0.01, 0.3, 0.7 and 0.5 mg/L, respect- ively (DOF, 2000). Hydrocarbon fractions of diesel, as well as BTEX-contaminated soil MCLs, have recently been established in Mexico environmental regulations. MCLs for BTEX-contaminated soil in Mexico are 6, 40, 10 and 40.0 mg/kg, for agricultural and residential settings, respectively, and 15, 100, 25 and 100 mg/kg for industrial use, respectively (DOF, 2005). Research has shown that among xylene isomers, o-xylene appears to be the most recalcitrant (Stewart and Kamarthi, 1997). Furthermore, the inability to degrade benzene has been correlated to the inability to degrade o-xylene (Gu ¨lensoy and Alvarez, 1999). In addition, chemical surfactants have been shown to increase substrate dissolution rate (Grimberg et al., 1996) and hydrocarbon uptake through emulsification (Van Hamme and Ward, 2001). Therefore, hydrocarbon polluted Water Science & Technology Vol 53 No 11 pp 197–204 Q IWA Publishing 2006 197 doi: 10.2166/wst.2006.353