Functionalized SiO₂ nanoparticle-mediated release and transport of Cd through soil porous media: implications for...

Silica-based nanomaterials have shown promise for remediating heavy metal-contaminated soils. However, their remediation stability may be compromised by natural and anthropogenic activities such as rainfall and irrigation. This work systematically investigates the leaching behavior of Cd from soils amended with unmodified SiO₂ and thiol-functionalized SiO₂ (SH-SiO₂) nanoparticles, along with nanoparticle-mediated Cd release and transport through soil porous media. The results demonstrated that the leaching concentrations of Cd²⁺ from the soil remediated by SH-SiO₂ were the least and followed the order of SH-SiO₂ < SiO₂ ≈ control (untreated Cd-contaminated soil) under simulated acid rain conditions (pH 5.6). This confirms that thiol-functionalized silica effectively immobilizes Cd due to its strong chelating ability. However, co-transport experiments revealed that both SiO₂ and SH-SiO₂ nanoparticles could act as carriers that enhanced Cd²⁺ transport, governed by their Cd-binding capacity and intrinsic mobility over a pH range of 5.0-8.0, with greater facilitation at higher pH levels. Notably, SH-SiO₂ exhibited a much stronger facilitation effect, attributable to its higher affinity for Cd²⁺, greater mobility in porous media, and resistance to desorption of SH-SiO₂-bound Cd²⁺. At a nanoparticle concentration of 50 mg/L, the maximum breakthrough of Cd²⁺ reached 10.1% for SiO₂ and 75.2% for SH-SiO₂. While SH-SiO₂ demonstrates excellent Cd immobilization capacity and acid-rain resistance, any disturbance that remobilizes these nanoparticles may facilitate the mobility of Cd²⁺, potentially expanding the scope of heavy metal contamination. Therefore, when applying remediation agents for soil rehabilitation, it is crucial to consider their long-term effectiveness and stability.