Microbial fuel cells (MFCs) are electrochemical devices focused on bioenergy generation and organic matter removal carried out by microorganisms under anoxic environments. In these types of systems, the anodic oxidation reaction is catalyzed by anaerobic microorganisms, while the cathodic reduction reaction can be carried out biotically or abiotically. Membranes as separators in MFCs are the primary requirements for optimal electrochemical and microbiological performance. MFC configuration and operation are similar to those of proton-exchange membrane fuel cells (PEMFCs)—both having at least one anode and one cathode split by a membrane or separator. The Nafion® 117 (NF-117) membrane, made from perfluorosulfonic acid, is a membrane used as a separator in PEMFCs. By analogy of the operation between electrochemical systems and MFCs, NF-117 membranes have been widely used as separators in MFCs. The main disadvantage of this type of membrane is its high cost; membranes in MFCs can represent up to 60% of the MFC’s total cost. This is one of the challenges in scaling up MFCs: finding alternative membranes or separators with low cost and good electrochemical characteristics. The aim of this work is to critically review state-of-the-art membranes and separators used in MFCs. The scope of this review includes: (i) membrane functions in MFCs, (ii) most-used membranes, (iii) membrane cost and efficiency, and (iv) membrane-less MFCs. Currently, there are at least 20 different membranes or separators proposed and evaluated for MFCs, from basic salt bridges to advanced synthetic polymer-based membranes, including ceramic and unconventional separator materials. Studies focusing on either low cost or the use of natural polymers for proton-exchange membranes (PEM) are still scarce. Alternatively, in some works, MFCs have been operated without membranes; however, significant decrements in Coulombic efficiency were found. As the type of membrane affects the performance and total cost of MFCs, it is recommended that research efforts are increased in order to develop new, more economic membranes that exhibit favorable properties and allow for satisfactory cell performance at the same time. The current state of the art of membranes for MFCs addressed in this review will undoubtedly serve as a key insight for future research related to this topic.

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The Sonobari Complex in northwestern Mexico preserves evidence of the consolidation of Pangaea and the Cordilleran orogenic cycle. Six Meso zoic magmatic pulses extending from the Early Triassic to the Paleocene are recognized in this complex. The volumetrically predominant rocks are calcalkaline metaluminous and peraluminous granitoids. Mafic rocks are mainly tholeiitic gabbros. All studied rocks show high concentrations of large ion lithophile elements (LILE) and negative Nb, Ta, and Ti anomalies. Initial Nd and Sr isotopic ratios in granitoids (εNd (i) = -5.7 to -0.5; 87 Sr/ 86 Sr (i) = 0.70630-0.71302) point to evolved continental sources, while radiogenic Pb isotopes ( 206 Pb/ 204 Pb (i) = 16.292-19.17; 207 Pb/ 204 Pb (i) = 15.503-15.666; 208 Pb/ 204 Pb (i) = 35.257-38.984) indicate a heterogeneous basement. Initial Nd and Sr isotope ratios in mafic rocks (εNd (i) = -1.9 to +5.0; 87 Sr/ 86 Sr (i) = 0.70384-0.70626) point to mantle sources with crustal assimilation, which is also supported by the radiogenic Pb values ( 206 Pb/ 204 Pb (i) = 18.412-19.081; 207 Pb/ 204 Pb (i) = 15.595-15.672; 208 Pb/ 204 Pb (i) = 38.147-38.988).Geochemical and isotopic signatures suggest that magmatic rocks in the complex orig inated from fractional crystallization with assimilation of a heterogeneous basement isotopically similar to the Grenville orogen of Mexico. Whole rock compositions are compatible with volcanicarc followed by backarc tectonic settings, where subduction and extensional processes occurred. Therefore, the main granitic pulses in the Sonobari Complex originated af ter the late Paleozoic GondwanaLaurentia collision, by subduction of oce anic plates or microplates along the western border of Pangea. Also, two extensionrelated magmatic pulses occurred after the Late Jurassic and in the Cenomanian, separated by a collisional orogenic event that is recorded by regional metamorphism. The continental arc setting of the Sonobari Complex differs from the oceanic arc context of the GuerreroAlisitos super terrane, indicating that there is no genetic relation between these blocks as previously proposed; rather, a relationship with the eastern Peninsular Ranges batholith is proposed.

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Los objetivos de este trabajo fueron: i) identificar y cuantificar metales pesados y metaloides potencialmente tóxicos presentes en los lodos residuales generados por la planta de tratamiento de agua residual “Taxco de Alarcón” en Guerrero, México; ii) determinar el potencial de geodisponibilidad de los metales pesados y metaloides potencialmente tóxicos mediante una extracción química secuencial, y, iii) proporcionar una metodología detallada para realizar una digestión total y extracción química secuencial empleando lodos residuales. La metodología principal consistió en realizar una digestión total y una extracción química secuencial empleando lodos residuales. El análisis de los metales pesados y metaloides potencialmente tóxicos se realizó mediante espectroscopía de emisión atómica por plasma acoplado inductivamente. A partir de la digestión total se logró identificar y cuantificar a Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, Sr, V y Zn. Siete metales pesados y metaloides potencialmente tóxicos analizados y considerados por la NOM-004-SEMARNAT-2002 están presentes en los lodos en concentraciones de 51, 50, 38, 752, 153, 190 y 3,134 mg/kg (base seca) para As, Cd, Cr, Cu, Pb, Ni y Zn, respectivamente. Sin embargo, únicamente la concentración de As, Cd, y Zn superan el límite máximo permisible establecido por la norma para poder clasificarlos como lodos de excelente calidad. Por otro lado, el análisis de las diferentes fracciones analizadas obtenidas mediante extracción química secuencial mostró que la mayor concentración de As, Cd y Zn se encuentra principalmente asociada a las fracciones de mayor geodisponibilidad (fracción intercambiable y asociada a carbonatos). Estas concentraciones fueron 34, 34 y 1,723 mg/kg de lodos (base seca) para As, Cd y Zn, respectivamente. Finalmente, en función de los siete metales pesados y un metaloide potencialmente tóxicos analizados y de su concentración total en los lodos residuales de la planta de tratamiento de agua residual de Taxco de Alarcón, los lodos se clasifican como de buena calidad. Esto significa que los lodos residuales tienen potencial para ser aprovechados en uso urbano sin contacto público directo durante su aplicación, en usos forestales, mejoramiento de suelos o uso agrícola en la región.