Construction of clay-mediated nZVI composites to alleviate the decline in Cr(vi) removal during co-transport with humic acid and phosphate

Abstract

The natural presence of phosphate and humic acid (HA) may impact the removal of Cr(VI) by green synthesized clay-supported nano-zero-valent iron (nZVI@GT) composites. This triggers the investigation of Cr speciation transformation by M-nZVI@GT and K-nZVI@GT, prepared with nZVI@GT loaded on montmorillonite and kaolinite clays. The Cr(VI) removal with the existence of phosphate and HA was investigated during cotransport in water-saturated sand columns. This study revealed that M-nZVI@GT exhibited a higher Cr(VI) removal capacity (82.08 mg g⁻¹) than K-nZVI@GT (61.74 mg g⁻¹). However, phosphate showed stronger competition with Cr(VI) for adsorption on the single-layer structured M-nZVI@GT, reducing its removal capacity to 61.98 mg g⁻¹ and inhibiting Fe⁰ core corrosion compared to the lamellar-structured K-nZVI@GT. Notably, phosphate enhanced the electrostatic repulsion between M-nZVI@GT and quartz sand, facilitating the mobility of M-nZVI@GT in the sand porous media (increasing from 41.03% to 52.20%). This enhancement was associated with a higher k_1d/k₁ value, which increased from 0.691 to 0.830. In particular, due to its steric hindrance of macromolecules, less HA entered the lamellar structure of K-nZVI@GT, lowering competitive adsorption with Cr(VI) for active sites. This research's findings indicate that K-nZVI@GT, as an environmentally friendly remediation material, can successfully alleviate the inhibitory effects of HA on Cr(VI) removal in contaminated soil. This study highlights the distinct structural construction of Fe-based nanomaterials via different clay meditations to improve the practical application of nanotechnology for soil in situ remediation.