Mechanisms of arsenic oxidation and immobilization by structural and surface iron-modified nontronite

Abstract

Iron (Fe)-bearing clay minerals, which occur naturally, play a significant role as removal agents in the immobilization of redox-sensitive pollutants like arsenic (As) from natural soil environments. However, the effects of the structural position of Fe within the 2:1 clay mineral on the removal and transformation of contaminants are poorly understood. In this study, nontronite NAu-1 was transformed into redox-activated clay minerals by modifying the oxidation states of structural or surface Fe to investigate the mechanisms of As immobilization. The mineralogical, chemical, and spectroscopic properties of the redox-activated nontronite (RAN) were characterized, and the reaction mechanisms for the removal of As were investigated. X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy confirmed the Fe presence in di- or trioctahedral domains and Fe(II)/Fe(III) ratios. Following the reaction with aqueous As and various RANs, limitations in the As oxidation on structural Fe(II) and Fe(III) RANs were observed; however, improved As removal was observed in the presence of adsorbed Fe(II) on nontronite surface. Notably, the re-oxidized Fe(II)-reduced nontronite exhibited the highest As(III) oxidation capability, demonstrating the potential for As reactivity via redox-activated nontronite. In addition, density functional theory (DFT) calculations, through adsorption energy calculation and excess Bader charge analysis, demonstrated the positive effect of adsorbed Fe(II) on nontronite surface in facilitating As oxidation. This study proposes the development of eco-friendly adsorbents through redox activation of natural clays, providing fundamental insights into the mobility and fate of redox-sensitive elements, including metal(loid)s and radioactive elements