Enhancing soil hydrophobicity through octadecylamine modification to prevent heavy metal pollution in red soil

Heavy-metal contamination from industrial activities and wastewater discharge poses a critical threat to soil environmental security, owing to the high mobility and toxicity of metal ions. Water infiltration is the principal pathway for vertical leaching and horizontal transport of heavy metals in soil; thus, establishing a hydrophobic barrier is regarded as an effective source-control strategy. In this study, octadecylamine (ODA), a long-chain organic amine, was employed to hydrophobically modify representative red soil. The water drop penetration test (WDPT) assessed the effects of compaction, particle-size distribution, temperature, and ODA dosage on hydrophobicity, while scanning electron microscopy (SEM) and X-ray diffraction (XRD) elucidated microscopic mechanisms. Inductively coupled plasma mass spectrometry (ICP–MS) quantified residual heavy metals. Results showed that ODA bonded with soil particles via amino groups and formed a stable hydrophobic layer through the C18 chain, rapidly elevating WDPT from slight to strong or severe levels and sustaining repellency after repeated wet–dry cycles. During modification, soil pH remained near neutral (6 – 7), favoring metal adsorption, whereas electrical conductivity fluctuated nonlinearly without impairing barrier performance. ICP–MS analysis revealed that 0.5% ODA achieved blocking efficiencies of 24.3%, 62.7%, and 87.5% for Cr, Cu, and Cd, respectively. Grass cultivation confirmed that ODA < 0.5% had minimal effects on plant growth, indicating ecological safety. In summary, ODA amendment enhanced soil hydrophobicity and mitigated heavy-metal migration, offering a promising, cost-efficient source-oriented pollution-control strategy. • ODA creates stable hydrophobic films, increasing WDPT to the strong or severe class. • Repellency remains effective after multiple wet–dry cycles. • ICP–MS shows that 0.5% ODA reduces Cr, Cu, and Cd leaching by 24.3%, 62.7%, and 87.5%, respectively. • Soil pH stays 6–7; EC varies but does not impair barrier. • ODA (≤0.5%) shows minimal effect on plant growth, ensuring ecological safety.