  We consider the depletion of elements from the interstellar gas onto a
population of very small dust grains. Adopting a grain model in which of order
4% of the cosmic C abundance is in grains with radii <= 10 Angstroms, we find
that the rate of accretion onto these grains is adequately fast to account for
the observed large depletions of elements like Ti, without invoking
unreasonably high rates of mass transfer between interstellar phases or low
grain destruction rates. If these grains are composed of arene rings, then only
a limited number of metal atoms can be locked up in them. The depletion would
be quenched when this limit is reached, unless there is a mechanism for
transferring the metals to larger grains and refreshing the very small grain
population, for example by grain coagulation and shattering in the diffuse ISM.
If Fe depletes onto the very small grains, then for reasonable coagulation
rates there is at least one metal atom per five C atoms in the very small grain
population. Furthermore, approximately 60% of the cosmic Fe is associated with
the carbonaceous grain population. It is unclear whether this scenario is
compatible with observations. However, if there is another population of very
small grains, with a large capacity for holding Fe atoms, it might be the sink
for the most heavily depleted elements.
