Research review paper Impact and application of electron shuttles on the redox (bio)transformation of contaminants: A review Frank P. Van der Zee a , Francisco J. Cervantes b, ⁎ a IBB — Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal b División de Ciencias Ambientales, Instituto Potosino de Investigación Científica y Tecnológica (IPICYT), San Luis Potosí, SLP, Mexico abstract article info Article history: Received 10 October 2008 Received in revised form 6 January 2009 Accepted 9 January 2009 Available online 23 January 2009 Keywords: Degradation Electron shuttle Pollutants Remediation Redox mediator Wastewater treatment During the last two decades, extensive research has explored the catalytic effects of different organic molecules with redox mediating properties on the anaerobic (bio)transformation of a wide variety of organic and inorganic compounds. The accumulated evidence points at a major role of electron shuttles in the redox conversion of several distinct contaminants, both by chemical and biological mechanisms. Many microorganisms are capable of reducing redox mediators linked to the anaerobic oxidation of organic and inorganic substrates. Electron shuttles can also be chemically reduced by electron donors commonly found in anaerobic environments (e.g. sulfide and ferrous iron). Reduced electron shuttles can transfer electrons to several distinct electron-withdrawing compounds, such as azo dyes, polyhalogenated compounds, nitroaromatics and oxidized metalloids, among others. Moreover, reduced molecules with redox properties can support the microbial reduction of electron acceptors, such as nitrate, arsenate and perchlorate. The aim of this review paper is to summarize the results of reductive (bio)transformation processes catalyzed by electron shuttles and to indicate which aspects should be further investigated to enhance the applicability of redox mediators on the (bio)transformation of contaminants. © 2009 Elsevier Inc. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 2. Role of redox mediators on azo dye reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 2.2. Redox mediator properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 2.2.1. Oxidation–reduction potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 2.2.2. Ability to cross the cell membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 2.2.3. Solubility vs. non-solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 2.3. Mediator/dye-ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 2.4. Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 2.5. Electron donors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 2.6. Trophic groups and selective inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 3. Role of redox mediators on reductive dechlorinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 3.2. Main factors determining the input of RM in reductive dechlorinations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 3.2.1. pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 3.2.2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 3.2.3. Redox potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 3.2.4. Reactive species involved in reductive dechlorination reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 3.2.5. Variety of electron donors and natural sources of RM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 4. Role of redox mediators on the reduction of nitroaromatic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 4.2. Main factors determining the input of RM in the reduction of nitroaromatic pollutants. . . . . . . . . . . . . . . . . . . . . . . . 269 Biotechnology Advances 27 (2009) 256–277 ⁎ Corresponding author. División de Ciencias Ambientales, Instituto Potosino de Investigación Científica y Tecnológica (IPICyT), Camino a la Presa San José 2055, Col. Lomas 4 a . Sección, San Luis Potosí, SLP, 78216 Mexico. Tel.: +52 444 8342041; fax: +52 444 8342010. E-mail address: fjcervantes@ipicyt.edu.mx (F.J. Cervantes). 0734-9750/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2009.01.004 Contents lists available at ScienceDirect Biotechnology Advances journal homepage: www.elsevier.com/locate/biotechadv