  The deuterium enrichment in molecules in dark molecular cloud cores and
starforming regions is usually attributed to gas-phase chemistry. Here we
examine the effects of surface and mantle chemical reactions on the deuteration
of species. We use a simple kinetic chemistry model that includes gas, surface
and mantle pore phase reactions of deuterated species. The mantle is assumed to
be partially reactive due to pores with sufficient surface area for chemical
reactions, that are continuously transformed by cosmic-rays. Calculation
results show that surface reactions generally enhance the deuteration for at
least several molecules. However, once they are buried and become mantle
molecules, they lose their deuteration over a timescale of 10 million years due
to processes in the mantle. If deuterated species in young star-forming regions
come from grain mantles, a cautious conclusion is that the freeze-out of
molecules, perhaps, should not occur more than 10 Myr before the mantle
evaporates to the gas phase.
