What plants have all these qualities: large biomass, is a metal accumulator and grows fast?

Answer 1

Eichhornia or Water hyacinth( Family Pontaderiaceae.) is the answer for your question .

The seven species of water hyacinths comprise the genus Eichhornia of free-floating perennial aquatic plants native to tropical South America.

With broad, thick and glossy ovate leaves, water hyacinths may rise some 1 metre in height.

The leaves are 10-20 cm across, supported above the water surface by long, spongy and bulbous stalks.

The feathery, freely hanging roots are purplish black. An erect stalk supports a single spike of 8-15 conspicuously attractive flowers, mostly lavender to pinkish in colour with six petals.

When not in bloom, water hyacinth may be mistaken for frog's-bit (Limnobium spongia).
http://sfrc.ufl.edu/4h/frogsbit/frogcompare.jpg

One of the fastest growing plants known, water hyacinth reproduces primarily by way of runners or stolons, eventually forming daughter plants.

They may also reproduce via seeds.

The common water hyacinth (Eichhornia crassipes) is a vigorous grower known to double its population in two weeks.

http://en.wikipedia.org/wiki/Eichhornia.
http://www.museums.org.za/bio/images/enb5/enb05338.jpg
http://library.thinkquest.org/03oct/00946/pic_used/water_hyacinth_01.jpg

Answer 2

Are you referring to phytoremediation.
This is where they grow a certain plant in a polutted area. The roots of the plant absorb certain toxic metals and then they cut down the plant, burn it and then the metal is left in the ashes which they can dispose of or recycle the metal.
There are several plants that are used to do this and scientists are finding new ones.

these plants have a high resistance to toxic metals in the soil.

One plant that is used is a type of mustard,
http://en.wikipedia.org/wiki/Brassica_juncea
another is Lupinus albus. a member of the pea family.

Answer 3

A field full of dandelion.