Sunflowers removed more lead and copper from the soil than amaranth and Indian mustard in this Malaysian study. Sunflowers also removed significantly more lead (50-54%) and copper (34-34%) from contaminated industrial soils when nitrogen fertilizer was added to the soil.
Rahman, Motior M., Sofian M. Azirun, and Amru N. Boyce. (2013). Enhanced accumulation of copper and lead in amaranth (Amaranthus paniculatus), Indian mustard (Brassica Juncea) and sunflower (Helianthus annuus). PLOS ONE 8(5): e62941. doi:10.1371/journal.pone.0062941