Study on adsorption characteristics of dredged sediment-attapulgite vertical cutoff walls

Landfill leachate commonly contains mixed contaminants in which heavy metals coexist with organic acid, potentially altering the chemical attenuation and interfacial stability of vertical cutoff wall backfills. In this study, ZnCl₂ and butyric acid were selected as representative inorganic and organic solutes. A fluidity test determined an appropriate dredged sediment-attapulgite backfill proportion of attapulgite:water:dredged sediment = 31:36:33. Equilibrium batch adsorption tests and isotherm fitting were conducted for single and mixed solutes, and zeta potential was measured to probe interfacial electrical responses. Under single-solute conditions, Zn²⁺ showed capacity-limited retention consistent with the Langmuir model, with q_max = 23,617 mg/kg; Zn²⁺ removal exceeded 90% at ZnCl₂ ≤ 800 mg/L and remained above 80% at 1000 mg/L, whereas butyric acid removal stayed at 30%-40%. Under coexistence, interactions were strongly asymmetric: butyric acid substantially suppressed Zn²⁺ retention, while ZnCl₂ had a weaker effect on butyric acid removal. At 40,000 mg/L butyric acid, the apparent Zn²⁺ q_max decreased to 3132.59 mg/kg, corresponding to an effective capacity derating of 0.13 relative to the no-organic case, indicating that single-solute parameters can be nonconservative in environments enriched with organic acids. Zeta potential results further showed reduced magnitudes, especially with butyric acid, implying weakened electrostatic repulsion that may promote aggregation and microstructural rearrangement under seepage. These findings support compatibility screening and conservative parameter selection for dredged sediment-attapulgite cutoff wall backfills in leachate enriched with organic acids.