The Change of Coordination Environments Induced by Vacancy Defects in Hematite Leads to a Contrasting Difference between Cation Pb(Ii) and Oxyanion As(V) Immobilization

Abstract

Hematite is an iron oxide commonly found in terrestrial environments and plays an essential role in controlling the migration of heavy metal(loid)s in groundwater and sediment. Although defects are observed to extensively exist in naturally occurring and laboratory-synthesized hematite, their influences on the immobilization of heavy metal(loid)s remain poorly understood. In this study, hematite samples with tunable vacancy defect concentrations were synthesized to evaluate their adsorption capacities for the cation Pb(II) and oxyanion As(V). The defects in hematite were characterized using XRD, TEM-EDS mapping, position annihilation lifetime spectroscopy, and XAS. We found that Fe vacancies were the primary defect type in hematite structure. Batch experiments confirmed that Fe vacancies on hematite promoted As(V) adsorption, while they decreased Pb(II) adsorption. The reason for the opposite effects of Fe vacancies on Pb(II) and As(V) immobilization was investigated using DFT calculations and EXAFS analysis. The results revealed that Fe vacancies altered the As-Fe coordination from a monodentate to bidentate model and increased the bond length of Pb-Fe on the hematite surface, thereby leading to an increase in As(V) bonding strength, while decreasing Pb(II) adsorption affinity. Our study provides new insights into the migration and fate of toxic heavy metal(loid)s controlled by iron minerals.

Keywords: iron oxide, vacancy defect, arsenate, lead, mobility