Synergistic role of pillared bentonite with single and binary Fe/Al-polyoxocations on Pb(II) adsorption recovery from hard water under competitive and non-competitive effects

Abstract

This study investigates the post-pillarization of bentonite clay using Fe/Al-polyoxocations—both singular and mixed—to enhance its surface and textural properties for improved adsorption-desorption recovery of Pb(II) ions from hard water. The adsorption-desorption capabilities of the pillared bentonites (referred to as Fe-, Al-, and Fe/Al-PILBCs) are investigated and optimized, taking into account the interactions between water components and operational conditions in both competitive and non-competitive scenarios. The incorporation of mixed Fe/Al pillars significantly increases the surface area, reactivity, and interlayer spacing of the clay, leading to improved pore diffusivity and enhanced interactions between Pb(II) and the Brönsted/Lewis acid sites in Fe/Al-PILBC. Consequently, the adsorption capability of Fe/Al-PILBC for Pb(II) ions is enhanced by 1.33 to 1.53 times compared to single-pillared clay adsorbents. Optimization using response surface methodology demonstrates that the interaction between solution pH and other operational factors is crucial for maximizing the Pb(II) adsorption capacity of Fe/Al-PILBC. This capacity can reach 67.85 mg/g at a pH of 4.5 after 10 min by adjusting the Pb(II) speciation and the surface charge density of Fe/Al-PILBC.Thermodynamic, kinetic, and isotherm studies indicate that the physisorption of Pb(II) onto Fe/Al-PILBC is facilitated by heat energy input (Ea = 21.22 kJ/mol and ΔH^ο = 20.32 kJ/mol), allowing for endothermic adsorption with sustained adsorption-desorption recovery of Pb over 10 cycles. In practical applications involving groundwater and petroleum wastewater, higher ionic strength enhances electrostatic interactions and ion exchange between Ca(II)/Mg(II) cations and the active sites of Fe/Al-PILBC, leading to changes in Pb(II) adsorption thermodynamics due to competitive effects at the solid/liquid interface. Nonetheless, the overall adsorption capacity of Fe/Al-PILBC for divalent metal cations increases by 2.9 times in hard water compared to single Pb adsorption in a non-competitive environment, underscoring the promising potential of Fe/Al-PILBCs for water-softening applications.