Overlooked Pb–(Sn)–(Sb)–(As) oxide particles in incineration bottom ash: Particle-scale occurrence, alteration and potential relevance to antimony and arsenic mobility

Antimony (Sb) is a critical metal and an emerging hazardous pollutant, chemically akin to arsenic (As), and is widely used as antimonial lead in Pb–(Sn)–Sb–(As) alloys. A substantial fraction of the Sb fed to municipal solid waste (MSW) incinerators remains in incineration bottom ash (IBA), where Sb leaching, together with that of As, restricts IBA reuse. We document, at the particle scale, the occurrence of Pb–(Sn)–(Sb)–(As) oxide particles in the 0.25–2 mm fractions of IBA from several facilities in France. Seven particles were characterized in detail by optical microscopy, SEM–BSE–XEDS, PXRD, and S/TEM–XEDS with dual-range EELS. The particles span a continuum of apparent alteration states, from Pb–(Sn)–(Sb) metallic cores with incipient oxide crusts to hollow, fully oxidized, brittle polycrystalline shells with distinctive yellow to brown-reddish colours. EELS features are consistent with Sb(III) and higher Sb oxidation states and XEDS phase maps show that As is concentrated together with Sb in Ca-bearing Pb–Sb–As oxide domains of the alteration rims. The assemblages constitute an overlooked, dense host of Sb and As in IBA that complement the calcium-antimonate framework; Because these particles are much denser than the silicate-carbonate ash matrix, density-based separation of IBA fines is a natural candidate for their removal.