  The study of radiation emitted from black hole accretion discs represents a
crucial way to understand the main physical properties of these sources, and in
particular the black hole spin. Beside spectral analysis, polarimetry is
becoming more and more important, motivated by the development of new
techniques which will soon allow to perform measurements also in the X- and
{\gamma}-rays. Photons emitted from black hole accretion discs in the soft
state are indeed expected to be polarized, with an energy dependence which can
provide an estimate of the black hole spin. Calculations performed so far,
however, considered scattering as the only process to determine the
polarization state of the emitted radiation, implicitly assuming that the
temperatures involved are such that material in the disc is entirely ionized.
In this work we generalize the problem by calculating the ionization structure
of a surface layer of the disc with the public code CLOUDY , and then by
determining the polarization properties of the emerging radiation using the
Monte Carlo code STOKES . This allows us to account for absorption effects
alongside scattering ones. We show that including absorption can deeply modify
the polarization properties of the emerging radiation with respect to what is
obtained in the pure-scattering limit. As a general rule, we find that the
polarization degree is larger when absorption is more important, which occurs
e.g. for low accretion rates and/or spins when the ionization of the matter in
the innermost accretion disc regions is far from complete.
