Catálogo de publicaciones - revistas

<jats:p>Waste biomass is a cheap and relatively abundant source of electrons for microbes capable of producing electrical current outside the cell. Rapidly developing microbial electrochemical technologies, such as microbial fuel cells, are part of a diverse platform of future sustainable energy and chemical production technologies. We review the key advances that will enable the use of exoelectrogenic microorganisms to generate biofuels, hydrogen gas, methane, and other valuable inorganic and organic chemicals. Moreover, we examine the key challenges for implementing these systems and compare them to similar renewable energy technologies. Although commercial development is already underway in several different applications, ranging from wastewater treatment to industrial chemical production, further research is needed regarding efficiency, scalability, system lifetimes, and reliability.</jats:p>

<jats:p>Metals are infinitely recyclable in principle, but in practice, recycling is often inefficient or essentially nonexistent because of limits imposed by social behavior, product design, recycling technologies, and the thermodynamics of separation. We review these topics, distinguishing among common, specialty, and precious metals. The most beneficial actions that could improve recycling rates are increased collection rates of discarded products, improved design for recycling, and the enhanced deployment of modern recycling methodology. As a global society, we are currently far away from a closed-loop material system. Much improvement is possible, but limitations of many kinds—not all of them technological—will preclude complete closure of the materials cycle.</jats:p>

<jats:p>Most of the carbon-based compounds currently manufactured by the chemical industry are derived from petroleum. The rising cost and dwindling supply of oil have been focusing attention on possible routes to making chemicals, fuels, and solvents from biomass instead. In this context, many recent studies have assessed the relative merits of applying different dedicated crops to chemical production. Here, we highlight the opportunities for diverting existing residual biomass—the by-products of present agricultural and food-processing streams—to this end.</jats:p>

<jats:p>Polypropylene (PP) is a widely used plastic with consumer applications ranging from food packaging to automotive parts, including car battery casings. To differentiate it from other recyclable plastics, it is designated as #5. Here, the factors contributing to PP recycling rates are briefly reviewed. Considerations include collection and separation efficiency, processing chemistry, and market dynamics for the products derived from recyclates.</jats:p>

<jats:p>Mining and mineral-processing wastes are one of the world’s largest chronic waste concerns. Their reuse should be included in future sustainable development plans, but the potential impacts on a number of environmental processes are highly variable and must be thoroughly assessed. The chemical composition and geotechnical properties of the source rock determine which uses are most appropriate and whether reuse is economically feasible. If properly evaluated, mining waste can be reused to reextract minerals, provide additional fuel for power plants, supply construction materials, and repair surface and subsurface land structures altered by mining activities themselves.</jats:p>

<jats:title>Cycling Down</jats:title> <jats:p> The Mid-Pleistocene Transition, which lasted from approximately 1.25 million to 700 thousand years ago, was a period during which the dominant periodicity of Earth's climate cycles inexplicably changed from 41 thousand to 100 thousand years. This change is clearly apparent in the oxygen isotopic composition of many calcifying marine organisms, but changes in both ice volume and temperature affect the signal, and so exactly what the signal means has remained unclear. <jats:bold> Elderfield <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="704" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1221294">704</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6095" page="656" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1226335">Clark</jats:related-article> </jats:bold> ) separated these two effects by measuring both the oxygen isotopic makeup and the Mg/Ca (a proxy that reflects changes in temperature only) of certain benthic foraminifera. The findings reveal the contributions of ice volume and temperature to glacial cycles, suggest when and why the Mid-Pleistocene Climate Transition occurred, and clarify how carbon is lost from the ocean-atmosphere during deglaciations but also changes because of ocean circulation. </jats:p>

<jats:title>Lattice Trailing the Electrons</jats:title> <jats:p> Superconducting order recedes with decreased chemical doping in a typical iron-based superconductor family. Thus, the symmetry of the material is broken by almost simultaneous antiferromagnetic and structural phase transitions. However, some pnictides also exhibit an electronic nematic transition manifested by anisotropy of the electrical resistance. Because this anisotropy occurs at the same time as the structural transition, it is not clear whether it is a consequence of the broken crystal lattice symmetry or its cause. <jats:bold> Chu <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="710" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1221713">710</jats:related-article> ) performed constant strain experiments on the series Ba(Fe <jats:sub>1−x</jats:sub> Co <jats:sub>x</jats:sub> ) <jats:sub>2</jats:sub> As <jats:sub>2</jats:sub> , which can distinguish between the two scenarios, and confirm that the electrons drive the lattice transition. </jats:p>

<jats:title>Addressing a Burning Issue</jats:title> <jats:p> Complete combustion of methane is required in order to avoid the unproductive emission of this greenhouse gas into the atmosphere. Palladium catalysts can help to promote complete combustion, but high-temperature operating conditions also promote aggregation of catalyst particles (“sintering”) that lowers their surface area and overall activity. <jats:bold> Cargnello <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="713" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1222887">713</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6095" page="659" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1226310">Farrauto</jats:related-article> </jats:bold> ) report that cerium oxide–coated Pd catalyst particles could be fully dispersed on an alumina surface prepared with a hydrophobic coating. This treatment resisted Pd sintering up to temperatures of 800°C, and also enabled complete combustion of methane to occur at temperatures as low as 400°C. </jats:p>

<jats:title>UN Coordination</jats:title> <jats:p> Uranium is best known for its radioactivity. From the standpoint of lower-energy chemistry, uranium is also intriguing for its bonding motifs, which involve trinodal f orbitals. <jats:bold> King <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="717" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1223488">717</jats:related-article> , published online 28 June; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6095" page="652" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1225351"> <jats:bold>Sattelberger and Johnson</jats:bold> </jats:related-article> </jats:bold> ) synthesized and isolated a molecule bearing a uranium-nitrogen triple bond. Theoretical calculations allowed the mapping of the orbital interactions, distinguishing it from similar motifs in compounds of lighter metals. The preparation required use of a rigid, bulky ligand framework to keep the reactive uranium nitride group from binding to another molecule nearby, a pathway that has plagued prior attempts to prepare this class of compounds. </jats:p>

<jats:title>Constraining the Birthplace of Asteroids</jats:title> <jats:p> Many primitive meteorites originating from the asteroid belt once contained abundant water that is now stored as OH in hydrated minerals. <jats:bold> Alexander <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="721" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1223474">721</jats:related-article> , published online 12 July) estimated the hydrogen isotopic compositions in 86 samples of primitive meteorites that fell in Antarctica and compared the results to those of comets and Saturn's moon, Enceladus. Water in primitive meteorites was less deuteriumrich than that in comets and Enceladus, implying that, in contradiction to recent models of the dynamical evolution of the solar system, the parent bodies of primitive meteorites cannot have formed in the same region as comets. The results also suggest that comets were not the principal source of Earth's water. </jats:p>