Electroosmotic flow in microfluidic systems is limited to the low Reynolds number regime. As a result species mixing in electroosmotic flow systems is inherently diffusion dominated, requiring both a long mixing channel and retention time to attain a homogeneous solution. Recent studies have shown that the introduction of oppositely charged surface heterogeneities to microchannel walls can result in regions of localized flow circulation within the bulk flow. In this study we seek to investigate these circulation regions, through 3D finite-element based numerical simulations, and then use them as a method of enhancing species mixing in a T-shaped micromixer. While all cases of surface heterogeneity are shown to enhance mixing efficiency, greater improvement is found when both the size of the heterogeneous region and the degree of heterogeneity (i.e., the difference between the heterogeneous and homogeneous zeta-potentials) are increased. In some cases the presence of heterogeneous regions is shown to reduce the required mixing channel length by as much as 70%.