Process-based analysis of electrosteric stabilization and surface site competition in copper butyl xanthate transport

The co-contamination of mining sites by heavy metals and xanthate processing reagents can lead to the formation of copper butyl xanthate (CuBX) complex colloids, posing potential risks for groundwater migration. This study investigates the process-level transport and retention mechanisms of CuBX in saturated porous media, specifically addressing the mechanistic distinction between suspension-mediated stabilization and collector-surface modification (site competition). Multi-spectroscopic characterization (XPS, TEM, and FTIR) suggested that CuBX particles possess a core-shell architecture, which dictates their colloidal stability. Under pristine, low-ionic-strength conditions, humic acid (HA) acted as an important regulator of CuBX transport, increasing the effluent mass recovery from 46.4% to 93.6%. Numerical simulations using a process-based two-site kinetic model based on BTC fitting indicated that HA facilitated transport by suppressing apparent particle retention, with decreases in both the attachment coefficients and the finite retention capacity. This HA-mediated stabilization was found to be sensitive to the electrolyte matrix, exhibiting a dynamic balance between electrosteric stabilization and valence-specific suppressive interactions. Divalent Ca²⁺ exerted a stronger inhibitory effect than monovalent Na⁺ at equivalent ionic strengths. Physicochemical measurements and xDLVO calculations supported cation bridging as an important mechanism for collapsing the protective HA coating, thereby eliminating the electrosteric barrier and enhancing particle deposition. Mechanistic decoupling experiments further demonstrated that suspension-mediated electrosteric stabilization exerts primary control over CuBX transport, whereas surface site competition contributes a secondary and saturable effect across diverse geochemical scenarios. These findings provide a process-based framework for refining site conceptual models and assessing the potential migration of complexed heavy metal contaminants in subsurface water resources.