I don't think that the stuff on bike rims are mostly from the break pads... there isn't enough brake pad to supply all that dust. I think it is picked up (mostly in the rain or on wet roads) from dust and soot from cars. That stuff, comes from car brake pads, car tires, and possibly diesel particulates. So with out resorting to chromatography or taking out a mass spectrometer, I stand by the hypothesis that the soot that was filtered is probably much the same composition as the gunk on your rims.