Adsorption and selective mechanism of Pb2+and Cd2+on the surface of calcined modified attapulgite

Abstract

The utilization of attapulgite (ATP for short) is widely employed in the remediation of heavy metal-contaminated wastewater and soil. However, its precise mechanism of action remains unclear. This study delves into the adsorption and selectivity mechanisms of Pb2+ and Cd2+ on calcined attapulgite surfaces. The investigation aims to elucidate the intricate processes by which these heavy metal ions interact with the modified attapulgite, shedding light on their respective affinities and preferences for binding sites. The modificated ATP (BS-ATP for short) was prepared through calcination at 450℃ to remove Pb2+ and Cd2+ from aqueous solutions. X-ray diffraction, thermogravimetric analysis, and infrared spectrum revealed that the crystal structure of BS-ATP remained fundamentally unaltered following the process of calcination. X-ray photoelectron spectroscopy confirmed the stable adsorption of both Pb2+ and Cd2+ by BS-ATP. The adsorption experiment revealed that BS-ATP exhibited a significantly greater equilibrium adsorption capacity for Pb2+ (84.64 mg/L) compared to Cd2+ (23.10 mg/L), indicating a pronounced selective adsorption preference for Pb2+. Density functional theory (DFT) calculations have revealed that the adsorption energy of BS-ATP for Pb2+ is lower (−111.2KJ/mol) than for Cd2+ (−35.2KJ/mol). Furthermore, a distinct bonding interaction has been observed between Pb and the hydroxyl oxygen in the BS-ATP surface, with the Pb-O bond length ranging from 2.52 Å to 2.92 Å. The combined experimental and theoretical findings show that Pb2+ has a higher affinity and greater stability in adsorbing onto BS-ATP surfaces compared to Cd2+. This research enhances our understanding of the molecular mechanisms behind the selective adsorption of Pb2+ by attapulgite.