An ion exchange substance formed of a porous mineral support having a specific surface area greater than 10 sq.m/g and an organo silicon compound containing at least one amine group in an organic group attached to the silicon atom grafted onto the porous mineral support via the silicon atom of the organo silicon compound and an oxygen group on the support and in which the terminal radicals on the amino group are alkyl groups having from 1 to 3 carbon atoms. SPECIFICATION