Synergistic stabilization of heavy metals in MSWI fly ash using Ca-bearing chitosan and CFBC fly ash

This study investigates the rapid solidification/stabilization (S/S) of heavy metals (HM) in hazardous municipal solid waste incineration (MSWI) fly ash using circulating fluidized bed combustion (CFBC) fly ash and Ca-bearing chitosan (CBC) derived from sea shrimp shells. The aim is to develop an eco-friendly and efficient method for treating MSWI fly ash to meet landfill disposal criteria. MSWI fly ash, CFBC fly ash, and CBC were blended at a liquid-to-solid ratio of 2:10, pressed at 10 MPa, and cured for 24 h. Unconfined compressive strength (UCS) and toxicity characteristic leaching tests (TCLT) were used to assess mechanical performance and HM immobilization. CBC significantly enhanced UCS, reaching over 3 MPa and meeting landfill acceptance criteria within 24 h. TCLT results showed that CBC sharply reduced the leachability of Pb, Cr, Cd, Hg, As, Cu, Zn, Ni, Ba, and Se, with overall HM immobilization exceeding 90%. The synergistic mechanism involves the formation of calcium-silicate-hydrate (C-S-H) gels from CFBC fly ash hydration, which physically encapsulate HM, combined with chemical precipitation and chelation of metal ions by the amino and hydroxyl groups of CBC. Risk assessment code analysis classified treated fly ash as low to medium risk for most HM. The combined CFBC fly ash-CBC system provides an eco-friendly, rapid, and mechanically robust solution for HM-contaminated MSWI fly ash, fulfilling landfill disposal criteria while valorizing two waste streams.