Release of lead from Pb-clinoptilolite: managing the fate of an exhausted exchanger

The selectivity of clinoptilolite toward Pb2+ has stimulated many studies aimed to evaluate the metal uptake. Conversely, the management of a Pb-bearing clinoptilolite has not received the same attention, although it can release a harmful metal. This work aims to evaluate the possibility to control, through thermal treatments, the release of lead from a Pb-clinoptilolite, prepared to simulate the condition of highest dangerousness of an exhausted exchanger. A zeolite-rich rock from Sardinia (Italy) has been processed, obtaining a powder with almost 90 % (wt.) of clinoptilolite. This material has been initially Na- and then Pb-exchanged, reaching a Pb2+ content of 2.28 meq/g. The lead release has been tested before and after 2-h heating at eight different temperatures from 200 to 900 °C. The unheated material releases 64 % of the lead. Heating weakly affects lead release up to 400 °C (54 %), but higher temperatures determine an abrupt reduction from 44 % at 500 °C to 1 % at 700 °C, when the zeolite breakdown occurs. At 800 °C the nucleation of Pb-feldspar and silica polymorphs begins. Basically, the material heated at 900 °C does not release lead (0.03 %), because the metal is trapped in the lead feldspar, whose content attains 42 % (wt.). This solid-state transformation does not involve the emission of lead vapors, another significant environmental aspect.