Reactive transport and retention of Cd, Pb, and Zn under coexisting multimetal-xanthate conditions in porous media

The geochemical fate of heavy metals in mining regions is significantly complicated by the coupled effects of residual flotation reagents and the co-existence of multiple metals. Combining column transport experiments and spectroscopic characterization, this study developed a Multisurface Speciation Model (MSM) to describe the reaction processes between cadmium (Cd), lead (Pb), and zinc (Zn) and key retention components including iron oxides, organic matter, clay minerals, and ethyl xanthate (EX), to quantitatively elucidate the impact of system complexity and EX-loaded concentration on their transport and retention behaviors in actual porous media. First, increasing the EX-loaded concentration from 0 to 1 mmol/L enhanced overall retention and inhibited the transport of all the three metals. Conversely, increasing system complexity from single-metal to multimetal systems diminished overall retention and enhanced transport. EX loading established EX as a dominant component for metal retention and concurrently suppressed the retention contribution of other soil components. However, the transition from single metal systems to ternary metal systems led to the intensified competition between metals and EX, thereby reducing the adsorption of EX onto the porous media and consequently weakening its overall capacity to retain the heavy metals, with the EX-retained concentrations of Cd, Pb, and Zn decreasing by 78.62 %, 52.44 %, and 80.46 %, respectively. The characterization results demonstrated that while the introduction of EX enhanced heavy metal retention through introducing new sulfur-containing functional groups, increasing the net negative surface charge, and promoting metal-immobilizing sulfidation reactions, the enhancement was significantly offset in multimetal systems by the intensified inter-metal competition and by a concurrent weakening of electrostatic attraction. This study successfully deconvolved the dual effects of EX loading and multimetal competition on the transport and retention of Cd, Pb, and Zn, offering a new perspective on the fate of heavy metals in xanthate-affected systems.