Reversible and irreversible retention of heavy metals in saturated porous media: association with kaolin

Contamination of heavy metals (HMs) has caused increasing concern due to their ecological toxicities and difficulties in degradation. The transport, retention, and release of HMs in porous media are highly related to their environmental fate and risk to groundwater. Column transport experiments and numerical simulations were conducted to investigate the retention and release behaviors of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ in the presence and absence of kaolin under varying ionic strengths and cation types. The interaction between HMs and soil colloids is critical to these processes, yet it remains poorly understood. In both single and multi-metal systems, the mobility of HMs ranked as Cd²⁺ > Zn²⁺ > Cu²⁺ > Pb²⁺, is influenced by their hydrolysis ability. Multi-metal systems showed higher mobility due to competition for retention sites, and Ca²⁺ enhanced transport more than Na⁺ due to greater affinity to the sand surface. Kaolin reduced HM transport by adsorption and led to irreversible retention. Cation exchange (Na⁺ replacing Ca²⁺) followed by reduced ionic strength promoted HM release due to the remobilization of kaolin associated with HMs. Uniform, nonmonotonic, and exponential retention profiles indicated variations in the spatial distribution of HMs. The Pb²⁺ and Cu²⁺ were more retained near the column inlet than Cd²⁺ and Zn²⁺, indicating limited mobility in the deep subsurface. Numerical simulations well described HM transport, considering the adsorption and desorption of HMs and the solid-water interface. These results enhance understanding of HM fate in terrestrial environments.