At the macroscale, rocks and mineral don't appear to be very reactive - consider the millions of years it takes for mountains to react with water. At the nanoscale, however, many common minerals are able to undergo redox reactions - exchange one or more electrons - with other molecules in their environment, impacting soil and water, seawater as well as fresh. Among the most critical of these redox reactions is the formation or transformation of iron oxide and oxyhydroxide minerals by charge-transfer processes that cycle iron between its two common oxidation states iron(III) and iron(II).
"Because iron(II) is substantially more soluble than iron(III), reductive transformations of iron(III) oxide and oxyhydroxide minerals can dramatically affect the chemistry and mineralogy of soils and surface," Gilbert says. "In the case of iron(III) oxide, the reduction to iron(II) can cause mineral dissolution on a very fast timescale that changes the mineralogy and water flow pathways. There can also be a mobilization of iron into solution that can provide an important source of bioavailable iron for living organisms."
Gilbert also noted that many organic and inorganic environmental contaminants can exchange electrons with iron oxide phases. Whether it is iron(III) or iron(II) oxide is an important factor for degrading or sequestering a given contaminant. Furthermore, certain bacteria can transfer electrons to iron oxides as part of their metabolism, linking the iron redox reaction to the carbon cycle. The mechanisms that direct these critical biogeochemical outcomes have remained unclear because mineral redox reactions are complex and involve multiple steps that take place within a few billionths of a second. Until recently these reactions could not be observed, but things changed with the advent of synchrotron radiation facilities and ultrafast X-ra
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory