The invention relates to a particular polymer which is derived from a copolymer polysaccharide formed by a main chain comprising similar or different anhydrohexose units and branches including at least one neutral or anionic anhydropentose and/or anhydrohexose unit. Moreover, the invention relates to the use of said polymer in the treatment of aqueous media, in particular the treatment of aqueous effluents containing heavy metals, the treatment of waste water, drinking water and for sludge conditioning. Said derivative polymer comprises one or more units bearing an oxime function at least at position C2.