The efficiency of grain growth in the diffuse interstellar medium
  Grain growth by accretion of gas-phase metals is a common assumption in
models of dust evolution, but in dense gas, where the timescale is short enough
for accretion to be effective, material is accreted in the form of ice mantles
rather than adding to the refractory grain mass. It has been suggested that
negatively-charged small grains in the diffuse interstellar medium (ISM) can
accrete efficiently due to the Coulomb attraction of positively-charged ions,
avoiding this issue. We show that this inevitably results in the growth of the
small-grain radii until they become positively charged, at which point further
growth is effectively halted. The resulting gas-phase depletions under diffuse
ISM conditions are significantly overestimated when a constant grain size
distribution is assumed. While observed depletions can be reproduced by
changing the initial size distribution or assuming highly efficient grain
shattering, both options result in unrealistic levels of far-ultraviolet
extinction. We suggest that the observed elemental depletions in the diffuse
ISM are better explained by higher initial depletions, combined with
inefficient dust destruction by supernovae at moderate ($n_{\rm H} \sim 30 {\rm
\, cm^{-3}}$) densities, rather than by higher accretion efficiences.
