  In this paper we propose and validate a multiscale model for the description
of particle diffusion in presence of trapping boundaries. We start from a
drift-diffusion equation in which the drift term describes the effect of bubble
traps, and is modeled by a short range potential with an attractive term and a
repulsive core. The interaction of the particles attracted by the bubble
surface is simulated by the Lennard-Jones potential that simplifies the capture
due to the hydrophobic properties of the ions. In our model the effect of the
potential is replaced by a suitable boundary condition derived by mass
conservation and asymptotic analysis. The potential is assumed to have a range
of small size $\varepsilon$. An asymptotic expansion in the $\varepsilon$ is
considered, and the boundary conditions are obtained by retaining the lowest
order terms in the expansion.
  Another aspect we investigate is saturation effect coming from high
concentrations in the proximity of the bubble surface. The validity of the
model is carefully checked with several tests in 1D, 2D and different
geometries.
