Abstract: Methods for waste treatment and compositions of stabilized waste elements are provided, typically for the disposal of arsenical wastes. Arsenic trioxide waste produced as a by-product of metal extraction is slurried in water for from 6 to 24 hours with finely divided quicklime to produce nonvolatile calcium arsenate (III). An excess of CaO stoichiometry with As is used to consume secondary waste species such as sulphate, silicate and iron oxides. The precipitated arsenate is oven dried at 120.degree. C. and then calcined in air between 15 minutes to several hours at 1000.degree. C. to 500.degree. C. respectively to oxidize arsenate (III) to arsenate (V) as Ca.sub.5 (AsO.sub.4).sub.3 (OH) (an apatite) as the major arsenic immobilization phase, with minor arsenic incorporated in Ca.sub.3 (AsO.sub.4).sub.2. Calcium sulphate anhydrate (sulphate immobilization) and calcium iron oxide (iron immobilization) are also produced.