Extracellular Polymeric Substances Facilitate the Adsorption and Migration of Cu²⁺ and Cd²⁺ in Saturated Porous Media

Heavy metal contamination in groundwater is a serious environmental problem. Many microorganisms that survive in subsurface porous media also produce extracellular polymeric substances (EPS), but little is known about the effect of these EPS on the fate and transport of heavy metals in aquifers. In this study, EPS extracted from soil with a steam method were used to study the adsorption behaviors of Cu²⁺ and Cd²⁺, employing quartz sand as a subsurface porous medium. The results showed that EPS had a good adsorption capacity for Cu²⁺ (13.5 mg/g) and Cd²⁺ (14.1 mg/g) that can be viewed using the Temkin and Freundlich models, respectively. At a pH value of 6.5 ± 0.1 and a temperature of 20 °C, EPS showed a greater affinity for Cu²⁺ than for Cd²⁺. The binding force between EPS and quartz sand was weak. The prior saturation of the sand media with EPS solution can significantly promote the migration of the Cu²⁺ and Cd²⁺ in sand columns by 8.8% and 32.1%, respectively. When treating both metals simultaneously, the migration of Cd²⁺ was found to be greater than that of Cu²⁺. This also demonstrated that EPS can promote the co-migration of Cu²⁺ and Cd²⁺ in saturated porous media.