Chitosan composite alkali-activated geopolymer for solidification/stabilization of copper and chromium contaminated mudflat sediment

Mudflat sediments can pose dual challenges of engineering diseases and pollution risks due to unfavourable mechanical performances and potential heavy metal enrichment, impacting coastal engineering construction, ecological environments, and human health. Commonly used Portland cement has significant restrictions in ensuring mechanical stability and environmental sustainability during the remediation of heavy metal-contaminated mudflats. This study investigates a novel approach using chitosan-enhance alkali-activated geopolymer (CS-AGP), composed of slag, fly ash, and desulphurised gypsum, for solidifying/stabilizing highly toxic and concentrated Cu-/Cr(VI)-polluted sediments. The unconfined compressive strength, durability, and leaching toxicity of these sediments are assessed across varying binder incorporations, contamination concentrations, curing periods, and dry-wet cycles. The results demonstrate that the CS-AGP remarkedly increases both early and long-term strength as well as environmental stability of Cu-/Cr(VI)-polluted sediments, even after suffering serious dry-wet alternations and pollutant accumulation, far surpassing the USEPA strength criterion (0.35 MPa) for safe landfill and suiting for in-situ engineering applications. Moreover, the CS-AGP solidified/stabilized contaminated sediments exhibit excellent acid resistance and minimal environmental risk and leaching concentrations meet Cu ≤ 1.5 mg/L and Cr(VI) ≤ 0.1 mg/L, as these metals primarily redistribute to the residual fraction. Microstructure evolution reveals CS-AGP generates significant amounts of calcium silicate hydrate, calcium aluminium silicate hydrate, and ettringite to compact sediment skeleton structures, which is the improvement source of mechanical performance. Simultaneously, the comprehensive physical encapsulation, chemical bonding, and coordination effects promote the transformation of Cu/Cr(VI) into a low availability state. The study offers new insights for efficient remediation and safe development of coastal mudflats.