METHOD FOR TRAPPING A CONTAMINATING SUBSTANCE IN A CONTAMINATED MATERIAL

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A method for treating a substantially solid material contaminated with metals, in particular a soil or a wastewater sludge, or demolition waste material polluted with heavy metals. The method provides a step of mixing the contaminated material with a prefixed amount of a solution of alkali metals or alkaline-earth metal polyphosphates, preferably sodium polyphosphates, followed, preferably after a given interval of time, by a step of mixing solid calcium hydroxide. A step may follow of pressing at a high pressure, said pressure higher than 700 Kilogram-force per square centimetre, preferably higher than 1000 Kilogram-force per square centimetre, most preferably higher than 2000 Kilogram-force per square centimetre, said pressure applied during at leas nine hours In such condition a material reduction size up to 1/20, preferably set between 1/4 to 1/6 is achieved. Due to such treatment, the contaminants are converted into substantially water insoluble compounds contaminants that are trapped within the compressed material, therefore their leaching and/or dissemination in the environment is prevented. The pressing can be carried out into moulds, to obtain such articles, as bricks, tiles or more simply filling materials to be used in asphalt or building industry. A method of treating a material that contains contaminants which does not react with polyphosphate, e.g. arsenic, is also provided comprising mixing the material with calcium hydroxide and pressing the mixture as above described.