Mitigating heavy metals leachability from CO2 carbonated coal-based solid waste backfill in high-salinity environments via biochar-clay-nanomaterial synergistic modification

Abstract

High-salinity mine water exerts an uncertain influence on the leaching behavior of heavy metals in CO₂-carbonated backfill material (CCB), posing potential environmental risks. To address this, this study develops a novel CCB using locally sourced solid waste and high-salinity mine water from a certain mining area, and proposes a control strategy involving rice husk biochar (RHB), natural clinoptilolite (NCP), sepiolite (SEP), and γ-phase nano-Al₂O₃ (NA) as a synergistic modifier. Through static and dynamic leaching tests, combined with ICP-MS, SEM, XRD, and FTIR analysis, this study investigates the leaching behavior of heavy metals before and after modification, while evaluating the mechanical properties and other parameters to assess the engineering applicability of the modified CCB. The results indicated that high-salinity environments (rich in SO₄²⁻ and Cl⁻) promoted the expansion and cracking of CCB, increasing the risk of heavy metals leaching. However, the introduction of the modifiers effectively suppressed the leaching of Cr, Ni, Mn, and other heavy metals through mechanisms such as electrostatic attraction, precipitation, ion exchange and complexation. Among the single-modifier systems, SEP demonstrated the most significant effect, reducing the average leaching of heavy metals by approximately 75.78%, though limitations remained for elements like Cr. The synergistic modification system ensures all detected heavy metals concentrations are below the Class IV groundwater quality standard (based on TCLP results), while enhancing porosity and adsorption capacity without sacrificing mechanical properties. The addition of NA enhanced mechanical strength but its effectiveness in controlling heavy metals leaching varied depending on the accompanying modifiers.