Sodium polyacrylate-modified bentonite-fly ash composite for coal gangue backfill sites: A sustainable strategy for enhanced impermeability and immobilization of heavy metals.

The safe disposal of coal gangue, a massive solid waste in mining regions, is hindered by risks of heavy metal leaching and groundwater pollution. Conventional bentonite-fly ash (BF) liners face a fundamental trade-off between low hydraulic conductivity and adequate mechanical strength. To overcome this limitation, this study developed a novel, low-cost quaternary composite (BFS) by modifying BF with sodium polyacrylate (SP) and incorporating locally sourced silt (FS). The optimal mix ratio (Bent: FA: FS: SP = 65:35:37.5:4) was determined via response surface methodology to simultaneously meet strict engineering standards. The resulting BFS exhibited exceptional performance: an ultra-low hydraulic conductivity of 1.35 × 10-8 cm/s and a high unconfined compressive strength of 814.55 kPa. Furthermore, BFS demonstrated superior adsorption capacities for Pb2+ (98.77 %) and Cr6+ (88.92 %), significantly outperforming BF. Integrated characterization (XRD, FT-IR, SEM) and density functional theory (DFT) calculations revealed the underlying multi-scale mechanisms: (i) the FS/FA/Bent gradient skeleton ensures dense packing; (ii) pozzolanic reactions from FA generate cementitious C-S-H gel for chemical bonding; and (iii) the SP hydrogel network provides polymer bridging and dynamic pore sealing. This work provides a scientifically grounded, economically viable, and environmentally sustainable material solution for the dual challenges of seepage prevention and contaminant containment at coal gangue backfill sites, with significant implications for the global management of mining solid wastes.