We study by direct numerical simulation the role of spherical, solid, uniformly sized, moving particles immersed in a fluid in spreading a passive scalar with high Schmidt number dissolved in the fluid. The solid particles are one-way coupled to the fluid: they agitate the fluid but they do not feel the presence of the fluid (they move as a granular gas). The two independent variables in the simulations are the solids volume fraction that we vary in the range of 10-45%, and the granular Reynolds number, based on the particle diameter and the granular temperature, which has been varied from 2.8 to 280. It is shown that the scalar spreading rate strongly decreases with increasing solids volume fraction; the spreading appears to scale quite well with the mean free path of the particles. At increasing granular Reynolds number the fluid flow develops more small scale structures that enhance scalar spreading. We propose a relation for an effective diffusion coefficient that describes the scalar spreading in terms of the independent variables. (C) 2008 American Institute of Chemical Engineers.