|
ORNL geochemist Andrew Stack used metadynamics to examine complex reaction mechanisms during mineral growth and dissolution. Image: Oak Ridge National Laboratory
|
By using a novel
technique to better understand mineral growth and dissolution, researchers at Oak
Ridge National Laboratory (ORNL) are improving predictions of mineral reactions
and laying the groundwork for applications ranging from keeping oil pipes clear
to sequestering radium.
The mineral
barite was examined to understand mineral growth and dissolution generally, but
also because it is the dominant scale-forming mineral that precipitates in oil
pipelines and reservoirs in the North Sea. Oil
companies use a variety of compounds to inhibit scale formation, but a better
understanding of how barite grows could enable them to be designed more
efficiently.
Additionally,
barium can trap radium in its crystal structure, so it has the potential to contain
the radioactive material.
In a paper featured
in the Journal of the American Chemical
Society, the ORNL-led team studied barite growth and dissolution using
metadynamics, a critical technique that allows researchers to study much slower
reactions than what is normally possible.
"When a
mineral is growing or dissolving, you have a hard time sorting out which are
the important reactions and how they occur because there are many things that
could be happening on the surface," said Andrew Stack, ORNL geochemist and
lead author on the paper. "We can't determine which of many possible
reactions are controlling the rate of growth."
To overcome this
hurdle, ORNL Chemical Sciences Division's Stack started with molecular dynamics,
which can simulate energies and structures at the atomic level. To model a
mineral surface accurately, the researchers need to simulate thousands of
atoms. To directly measure a slow reaction with this many atoms during mineral
growth or dissolution might take years of supercomputer time. Metadynamics,
which builds on molecular dynamics, is a technique to "push"
reactions forward so researchers can observe them and measure how fast they are
proceeding in a relatively short amount of computer time.
With the help of
metadynamics, the team determined that there are multiple intermediate
reactions that take place when a barium ion attaches or detaches at the mineral
surface, which contradicts the previous assumption that attachment and
detachment occurred all in a single reaction.
"Without
metadynamics, we would never have been able to see these intermediates nor
determine which ones are limiting the overall reaction rate," Stack said.
To run computer
simulations of mineral growth, researchers used the Large-scale
Atomic/Molecular Massively Parallel Simulator, a molecular dynamics code
developed by Sandia National Laboratories.
SOURCE
