ENVIRONMENTAL BIOTECHNOLOGY Kinetics during the redox biotransformation of pollutants mediated by immobilized and soluble humic acids Francisco J. Cervantes & Claudia M. Martínez & Jorge Gonzalez-Estrella & Arturo Márquez & Sonia Arriaga Received: 13 March 2012 / Accepted: 2 April 2012 / Published online: 9 May 2012 # Springer-Verlag 2012 Abstract The aim of this study was to elucidate the kinetic constraints during the redox biotransformation of the azo dye, Reactive Red 2 (RR2), and carbon tetrachloride (CT) mediat- ed by soluble humic acids (HA s ) and immobilized humic acids (HA i ), as well as by the quinoid model compounds, anthraquinone-2,6-disulfonate (AQDS) and 1,2-naphthoqui- none-4-sulfonate (NQS). The microbial reduction of both HA s and HA i by anaerobic granular sludge (AGS) was the rate- limiting step during decolorization of RR2 since the reduction of RR2 by reduced HA i proceeded at more than three orders of magnitute faster than the electron-transferring rate observed during the microbial reduction of HA i by AGS. Similarly, the reduction of RR2 by reduced AQDS proceeded 1.6- and 1.9- fold faster than the microbial reduction of AQDS by AGS when this redox mediator (RM) was supplied in soluble and immobilized form, respectively. In contrast, the reduction of NQS by AGS occurred 1.6- and 19.2-fold faster than the chemical reduction of RR2 by reduced NQS when this RM was supplied in soluble and immobilized form, respectively. The microbial reduction of HA s and HA i by a humus-reducing consortium proceeded 1,400- and 790-fold faster than the transfer of electrons from reduced HA s and HA i , respectively, to achieve the reductive dechlorination of CT to chloroform. Overall, the present study provides elucidation on the rate- limiting steps involved in the redox biotransformation of priority pollutants mediated by both HA s and HA i and offers technical suggestions to overcome the kinetic restrictions identified in the redox reactions evaluated. Keywords Humus . Immobilization . Recalcitrant pollutants . Redox mediator . Wastewater Introduction During the last two decades, evidence has been reported indicating that humus, the most abundant organic matter accumulating in terrestrial and aquatic environments, has active roles accelerating the redox biotransformation of azo dyes (Rau et al. 2002; Cervantes et al. 2011a; Liu et al. 2011), nitroaromatics (Bhushan et al. 2006; Kwon and Finneran 2006), polychlorinated compounds (Collins and Picardal 1999; Cervantes et al. 2004, 2011a; Alvarez et al. 2011), among other electron-accepting contaminants by serving as redox mediators (RMs) (Van der Zee and Cervantes 2009). The redox-mediating properties of hu- mus have mainly been attributed to quinone moieties (Scott et al. 1998), which are very abundant in humus; thus, quinoid model compounds have extensively been used as humus analogs in several studies (Van der Zee and Cervantes 2009). The information available in the literature provides a broad perspective of the different experimental conditions (pH, temperature, type and concentration of RM, type and concentration of electron donor, and redox potential), which affect the redox-mediating capacity of humus and quinones Electronic supplementary material The online version of this article (doi:10.1007/s00253-012-4081-5) contains supplementary material, which is available to authorized users. F. J. Cervantes : C. M. Martínez : J. Gonzalez-Estrella : A. Márquez : S. Arriaga División de Ciencias Ambientales, Instituto Potosino de Investigación Científica y Tecnológica (IPICyT), Camino a la Presa San José 2055, Col. Lomas 4ª Sección, San Luis Potosí 78216, Mexico F. J. Cervantes (*) Department of Biotechnology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway e-mail: francisco.cervantes@ntnu.no Appl Microbiol Biotechnol (2013) 97:2671–2679 DOI 10.1007/s00253-012-4081-5