Due to experimental limitations, the solute distribution in polycrystalline materials is difficult to obtain directly, especially in the vicinity of grain boundaries. Using a newly developed computational method which mixes continuum diffusion equations and atomic scale jump rates, we study the interstitial diffusion in solids containing interfaces taking into account trapping kinetics. The model is applied to hydrogen diffusion in Ni in elementary configurations: fast intergranular diffusion with no segregation (in agreement with Fisher's model), slow intergranular diffusion with trapping, diffusion through a triple junction and solute redistribution due to stress gradients across the interface. It is shown that the classical diffusion modes can be captured and a new diffusion regime with the effect of grain boundary trapping is revealed.