Metal/Metalloid

The term 'metal' has been given to a number of elements that have a few characteristics in common for the pure elemental form:

1. Good electrical conductivity even at room temperature,
2. High thermal conductivity,
3. Bright metallic appearance.

In order to understand the uptake and bioaccumulation of metals by terrestrial plants, it is necessary to know the chemical properties and basic reaction types for different elements. Basically on the position of the element in the three major subdivisions (Streit and Stumm, 1993)

1. The 18 non-metals comprise the 6 noble gases as well as the following 12 elements: H, B, C, N, O, F, P, S, Cl, Br, I and At.
2. The 6 so-called semi-metals or metalloids are: Si, Ge, As, Se, Sb and Te. These elements either exhibit only a partial metallic character or they occur in a metallic as well as a non-metallic form. In the non-metallic forms they are used as semi conductors.
3. The remaining 88 elements (up to element 112; or 68 elements, if confined to the so-called natural elements up to uranium) are all metals.

Uptake of heavy metals does not simply follow physical rules but is regulated by the plant via physiological mechanisms. Higher plants are frequently used as accumulative bioindicators of heavy metals, Of the so-called lower plants, lichens, mosses, and to a lesser extent, fungi play an important role in accumulative bioindication, while algae, although present in terrestrial ecosystems, are not used for the bioindication of heavy metals which group and which species has to be chosen depends on the monitoring purposes and on the ecosystem under investigation.

Anthropogenic sources of specific heavy metals in the environment

Heavy metal	Sources	Reference
As	Pesticides and wood preservatives	Thangavel and Subbhuraam (2004)
Cd	Paints and pigments, plastic stabilizers, electroplating, incineration of cadmium-containing plastics, phosphate fertilizers	Salem et al., 2000 and Pulford and Watson, 2003
Cr	Tanneries, steel industries, fly ash	Khan et al. (2007)
Cu	Pesticides, fertilizers	Khan et al. (2007)
Hg	Release from Au–Ag mining and coal combustion, medical waste	Memon et al., 2001, Wuana and Okieimen, 2011 and Rodrigues et al., 2012
Ni	Industrial effluents, kitchen appliances, surgical instruments, steel alloys, automobile batteries	Tariq et al. (2006)
Pb	Aerial emission from combustion of leaded petrol, battery manufacture, herbicides and insecticides	Thangavel and Subbhuraam, 2004 and Wuana and Okieimen, 2011

Hyperaccumulator:-

Mine reclamation and biogeochemical prospecting depends upon right selection of plants species and sampling. The selection of heavy metal tolerant species is reliable tool to achieve success in phytoremediation. 163 plant taxa belonging to 45 families are found to be metal tolerant and are capable for growing on elevated concentrations of toxic metals. The use of metal tolerant species and their metal indicator and accumulation is a function of immense use for biogeochemical prospecting.

Some Hyperaccumulators