Ageing
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Ageing is a concept where a chemical compound changes in the environment into a less reactive and often into a less soluble form. Important groups of chemicals where ageing is important, are e.g. combustion particles in air, or metal compounds in soil or sediments. In the former group, ageing usually happens in the order of hours, while in the latter group ageing may take place in from weeks to centuries. Metals typically transform into poorly soluble sulphide compounds. The concept of sulphide formation is called AVS/SEM.
Ageing is an important concept in impact assessment, because typically ageing reduces reactivity and toxicity of the compound.
Metal ageing
Metals age in a certain order: the first is silver, then ... Zinc is a critical metal because usually its concentrations are so high that it might consume the whole sulfide capacity.