Biology meets geology through chemistry n introductory chemistry and biology, students learn (tacitly) that the reconfiguration of electrons is why reactions occur and how life ekes out its existence. That isselectrons, given proper conditions, will associate among atomic nuclei of various types such that energy is minimized. Controlling when and how this occurs is the means to harnessing the energy that the system released upon electronic reconfiguration. Students begin to appreciate that “reaction”, “activity”, or “[concentration] gradient” refer to sweeping movement of electrons and exchanges of potentially useful energy. It is further learned that elemental metals tend to give up electrons (are oxidized) and elemental nonmetals tend to take on electrons (are reduced), with metalloids engaging in both. However, as chemistry is often practiced in solutions, students learn a single element can spawn numerous “species” that have differing tastes for electrons. The various interactions can form precipitating minerals such as metal oxides, sulfides, or halides, while electronically versatile elements like carbon can be rendered into gassy oxides or reduced organic matter. Thus oxidation-reduction, or redox, reactions underpin geochemistry and biochemistry alike.
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For decades, if not centuries, the prevailing wisdom was that rocks and slime, although often in spatio-temporal contact, conducted themselves with separate processes. This is despite Georg Pawer, aka Agricola, noting in his seminal De Re Metallica (1556) that a certain fungus is known to grow on soil overtop ore veins and thus direct a miner’s shovel. Yet through the 20th century, it became better understood that cells move about electrons to store and gain life-continuing energy. As mineral reactions too involve redox, it became well appreciated that “hardy” microbes can consume rocks for nutrients, sometimes under very “extreme” conditions. Thus to best understand the environment’s fundamental processes and thus to best avoid its fouling, Biogeochemical Redox Processes concern many researchers, and it comprises this Focus Issue. As established in the issue’s two review manuscripts (Environ. Sci. Technol. 2009, DOI 10.1021/es9026248 and Environ. Sci. Technol. 2009, DOI 10.1021/ es9018207), there are many redox couples that biology manipulates to its own ends. This can be beneficial, by sequestering toxic metals. Oppositely, redox can be highly deleterious by releasing metals from their crystal confines and further transforming them to a (more) harmful oxidation state. Yet studies such as those herein that advance the detectability and discernability of biogeochemical redox give clues as to how biology may be optimally exploited to control our anthropogenic messes. 10.1021/es9036176
2010 American Chemical Society
Published on Web 12/30/2009
As biology is relatively ubiquitous in the hydro- and lithosphere, the most ubiquitous elements that readily undergo redox transformations in these realms fill these pages. It is thus hardly ironic that the ferrous/ ferric (Fe[II]/Fe[III]) system and oxygen are commonly encountered. Many microbial species, when starved of oxygen (anoxic conditions) turn to metals to shuttle their electron transport chains along and store energy via organophosphates for later use in living processes such as reproduction or building protective coatings. Fe(II)/Fe(III) redox is addressed by Environ. Sci. Technol. 2009, DOI 10.1021/es9016848, Environ. Sci. Technol. 2009, DOI 10.1021/es901882a, Environ. Sci. Technol. 2009, DOI 10.1021/es9018699, Environ. Sci. Technol. 2009, DOI 10.1021/es901736t, and Environ. Sci. Technol. 2009, DOI 10.1021/es901669z, with Fe’s chemical sibling Mn concerning Environ. Sci. Technol. 2009, DOI 10.1021/es901577q and Environ. Sci. Technol. 2009, DOI 10.1021/es901572h. Microbes, alive and dead, slough organic matter (OM); it is therefore natural that humic acids, with all their zwitterionic states, get involved in this Fe redoxsa process considered by Environ. Sci. Technol. 2009, DOI 10.1021/es901585z. Two papers in particular focus on OM redox: Environ. Sci. Technol. 2009, DOI 10.1021/es901766r investigates the marine interplay of Fe, Cu, and spicy superoxide (O2-); Environ. Sci. Technol. 2009, DOI 10.1021/es902627p presents a novel method for analyzing the quixotic suites of OM redox processes with minimal system disruption to extract such measurement. That Fe is so mineralogically ubiquitous (e.g., hematite, magnetite, goethite, ferrihydrite, green rust, etc.), it is the foil to many microbes’ engaging in redox with other metal species. As speciation of metals can spell the difference between their being sequestered and un(bio)available to perniciously toxic, many environmental chemists and engineers investigate such redox couples with gusto. A most notorious case is how Fe influences the swing between relatively insoluble As(V) versus mobile and carcinogenic As(III). Manuscripts Environ. Sci. Technol. 2009, DOI 10.1021/es901274s, Environ. Sci. Technol. 2009, DOI 10.1021/es902100h, Environ. Sci. Technol. 2009, DOI 10.1021/es901472g, and Environ. Sci. Technol. 2009, DOI 10.1021/es902077q probe this balance and point to potential remediation strategies as suggested in Environ. Sci. Technol. 2009, DOI 10.1021/es900708s and Environ. Sci. Technol. 2009, DOI 10.1021/es901627e. Another notorious element is Cr, where its (III) form is the relatively insoluble one and Cr(VI) is nefarious. Herein Environ. Sci. Technol. 2009, DOI 10.1021/ es901759w studies the fundamentals of Cr(III) oxidation; Environ. Sci. Technol. 2009, DOI 10.1021/ January 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1
es9013882 suggests that anthropo-biogeochemical manipulation of S2- species could mitigate Cr(VI) contamination by indirectly promoting Cr(III) formationsthus gassing as opposed to dredging. While volatile species such as S2- can cause precipitation of Cr(III), creation of volatile organo-Se species can potentially liberate them from waste sites as described in Environ. Sci. Technol. 2009, DOI 10.1021/ es9006492. That manuscript is another of several papers again invoking Fe, this time to control radioactive wastes79Se in that case, a product of U fission. Thus it is hardly surprising to encounter Fe mentioned with U species themselves in Environ. Sci. Technol. 2009, DOI 10.1021/es9017464, Environ. Sci. Technol. 2009, DOI 10.1021/es902452u, or Environ. Sci. Technol. 2009, DOI 10.1021/es9014597. Like As, Cr, or Se, U has one oxidation states(IV)sthat sequesters, but U(VI) can complex up and move about. Cynically, one could say that investigating the biogeochemistry of a former U mine as described in Environ. Sci. Technol. 2009, DOI 10.1021/es902038e, where an Fe-rich creek has seemingly imbued greater mobility to Co, Ni, Zn, As, and U, suggests that humans should leave well enough alone. Yet using such
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aftermath investigations and drawing on similar lessons with Tc as a tracer species in Environ. Sci. Technol. 2009, DOI 10.1021/es9010866 or Environ. Sci. Technol. 2009, DOI 10.1021/es802885r, hindsight’s 20/ 20 vision could provide for more optimistic bioremediative foresight. Better yet, with an overall improvement in our understanding of biology’s interaction with geology, perhaps we can avoid the need for cleanup altogether. This Biogeochemical Redox Processes Focus Issue thus presents an opportunity to contemplate what has been done and what needs to be done. Looking forward, Environmental Science & Technology welcomes the continued submission of such manuscripts. Your furthering discoveries and applications will help to hasten the arrival of better remediation approaches, healthier living, and novel clean technologies for everyone’s betterment.
Darcy J. Gentleman, Managing Editor
