The best catalysts for promoting char gasification are Group I metals, particularly lithium and potassium, although other metals are active to a lesser extent. The most prevalent metal naturally in biomass char is potassium, which is not only inherently active, but volatilises to become finely distributed throughout the char mass. The formation of an active carbon/potassium complex is frequently proposed. Calcium is the other most common active metal found in biomass, but is far less effective and less volatile. In a gasification system the metals remain as carbonate due to the action of carbon dioxide. The alkali metals can react with silica to form silicates, which prevents catalytic action. Transition metals can also participate in catalysis of gasification; iron accelerates gasification and nickel prevents carbon deposition, which helps in conditioning biomass-derived syngas. Volatile iron pentacarbonyl has been identified as a promoter of the char gasification step, with catalytic activity related to the finely dispersed low-valency metal atoms generated during the thermo-decomposition of biomass.

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Heavy metal pollution is a major environmental concern because of the toxicity to humans and plants. This toxicity is lethal even in trace quantities and metals have a great tendency to bioaccumulate. The efficiency of phosphate and apatites M 10 (PO 4 ) 6 (OH) 2 , M: Metal, in particular, in removing and immobilizing heavy metals from wastewater, groundwater (as permeable reactive barriers for in situ site remediation), fly ash, dredged sludges and contaminated soil has led to various studies to understand and explain the mechanisms involved. This paper will address the use of apatite as sorbent and stabilizing agents for the removal of heavy metals from various media. Efficient physico-chemical immobilization of heavy metals brings new perspectives for reuse of polluted waste water, soils and land treated by selected phosphates.