Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 109, Issue 46, Pages 18689-18694Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1208184109
Keywords
aqueous geochemistry; kinetic isotope effect; ligand exchange
Categories
Funding
- Center for Isotope Geochemistry
- US Department of Energy Office of Basic Energy Sciences [DE-AC02-05CH11231]
- Center for Nano-scale Control of Geologic CO2, an Energy Frontier Research Center
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Molecular dynamics simulations show that the desolvation rates of isotopes of Li+, K+, Rb+, Ca2+, Sr2+, and Ba2+ may have a relatively strong dependence on the metal cation mass. This inference is based on the observation that the exchange rate constant, k(wex), for water molecules in the first hydration shell follows an inverse power-law mass dependence (k(wex) proportional to m(-gamma)), where the coefficient gamma is 0.05 +/- 0.01 on average for all cations studied. Simulated water-exchange rates increase with temperature and decrease with increasing isotopic mass for each element. The magnitude of the water-exchange rate is different for simulations run using different water models [i.e., extended simple point charge (SPC/E) vs. four-site transferrable intermolecular potential (TIP4P)]; however, the value of the mass exponent gamma is the same. Reaction rate theory calculations predict mass exponents consistent with those determined via molecular dynamics simulations. The simulation-derived mass dependences imply that solids precipitating from aqueous solution under kinetically controlled conditions should be enriched in the light isotopes of the metal cations relative to the solutions, consistent with measured isotopic signatures in natural materials and laboratory experiments. Desolvation effects are large enough that they may be a primary determinant of the observed isotopic fractionation during precipitation.
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