pH dependence of precipitation and adsorption of Pb(II) ions onto sepiolite

Quantifying the adsorption capacity of hydrolyzable heavy metals presents a significant challenge, as the formation of metal hydroxides at high pHs often obscures the true removal mechanism. Standard measurements based on solute depletion often fail to distinguish between surface complexation and bulk-phase separation, leading to inflated adsorption values. Addressing this ambiguity, the present study investigates the pH-dependent removal of Pb(II) ions by natural sepiolite, with a specific focus on isolating the “true adsorption” contribution from chemical precipitation. To achieve this, a comprehensive methodological approach combining electrokinetic measurements, ion-exchange monitoring, and thermodynamic solubility modeling was applied for an initial Pb(II) concentration of 2.5 × 10^–3 mol/L (approximately 518 mg/L). Kinetic analysis was consistent with a pseudo-second-order model (R² = 1), supporting—but not proving—the involvement of chemically controlled interactions and possible Pb²⁺/Mg²⁺ exchange. By superimposing theoretical speciation diagrams onto experimental removal curves, a system-specific, operational transition threshold was estimated at pH ≈ 5.32 for 2.5 × 10^–3 mol/L Pb(II) concentration. Below this point, removal is consistent with the adsorption of free Pb(II) ions; however, above pH 5.32, the onset of hydrolysis leads to the formation of species such as Pb(OH)⁺ and Pb(OH)_2(s), causing a sharp decline in true adsorption capacity despite high total removal rates. Consequently, model-assisted operational adsorption profiles were described by control-based subtraction of the precipitation component, suggesting that the affinity of sepiolite is strongly influenced by the speciation state of the metal ions.