Geochemistry

Reflecting on Mica

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Science  03 Dec 2010:
Vol. 330, Issue 6009, pp. 1289-1291
DOI: 10.1126/science.330.6009.1289-d
CREDIT: GC MINERALS/ALAMY

Trace metals in soils and sediment predominantly exist as either tiny mineral grains or mobile aqueous ions in pore water. In both cases, adsorption and desorption reactions at the surface of mineral grains control their fate and transport through the subsurface. Because the size of ions and strength of the surrounding hydration shell are highly variable, predicting the way in which metals are distributed even at one specific mineral/water interface is challenging. To systematically address this question, Lee et al. measured—using an X-ray reflectivity technique sensitive enough to determine the thickness of a single layer of water—the manner in which divalent metal ions adsorbed on a hydrated muscovite surface. As expected, metal ions with weak hydration shells lost water and bound tightly to muscovite within just 2.5 Å; stronger hydration shells remained intact, pushing other metal ions further out (3.5 to 4.5 Å from the surface). The reflectivity profiles also revealed, surprisingly, an unexpected additional broad layer of metal ions 5 to 10 Å away from the mineral surface. Computational models of similar interfaces may need to be adjusted to account for this extended layer.

Langmuir 26, 16647 (2010).

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