Zerovalent iron is the sure way to reduce Cr(VI). I've also read many papers and done some research with organic humic compounds, (quinones, etc.), which are regularly released by soil bacteria. However, this needs to be a completely anaerobic system. The presence of oxygen will return Cr(III) to Cr(VI). As for activated carbon, it works. I might attempt a zeolite, though. Zeolites have porosity similar to activated carbon, but they also have Cation exchange. Dr. Robert Bowman and St. Cloud Mining Co. have developed a iron impregnated modified zeolite for this very purpose, a reactive barrier.