BACKGROUNDFerric arsenate solids in the form of scorodite (FeAsO(4)2H(2)O) particles are suitable carriers for immobilization of arsenic-rich wastes. The stability of scorodite is, however, highly pH dependent (typically at 4 pH 7) and satisfactory only under oxic disposal conditions. In this work a new stabilization technology based on the concept of encapsulation with aluminum hydroxyl gels is investigated to enhance the stability of arsenical solids over a wider range of disposal conditions. RESULTSThe encapsulation system investigated involves blending and short-term ageing of synthetic scorodite particles with amorphous aluminum hydroxyl gels derived from partial hydrolysis of aluminum chloride or aluminum sulfate salts. Of the two gel types, the Al(SO4)(1.5)-derived gel proved to be the most effective, even at the very low Al(III)/As(V) molar ratio of 0.1, apparently due to in situ development of protective aluminum hydroxide matrix and not as a simple adsorption sink for soluble arsenate species. CONCLUSIONArsenic release from the scorodite-aluminum hydroxyl gel composites was found to be drastically reduced with respect to gel-free scorodite, making this system a very interesting candidate for further development as an effective hazardous material encapsulating material. (c) 2014 Society of Chemical Industry