The invention relates to the use of a by-product from drinking water treatment, namely alum sludge, in a method and composition for the stabilisation of contaminants (e.g. arsenic and polycyclic aromatic hydrocarbon) in materials such as soils and other solid or semi-solid matrix types. The composition may also comprise a reactive carbon material such as activated carbon. Further, the composition may comprise a solidifier such as calcium silicate-based cement to enable the stabilisation and immobilisation of contaminants.
