Insights into the underlying effect of Fe vacancy defects on the adsorption affinity of goethite for arsenic immobilization.

Goethite is a commonly found iron (hydr)oxide in soils and sediments that has been proven to possess abundant defects in structures. However, the underlying impact of these defects in goethite on arsenic immobilization remains unclear. In this study, goethite samples with abundant, moderate, and sparse defects were synthesized to evaluate their arsenic adsorption capacities. The characteristics of the defects in goethite were investigated by extended X-ray absorption fine structure (EXAFS), high angle annular dark field-scanning transmission electron microscopy-energy dispersion spectrum (HAADF-STEM-EDS) mapping, vibrating-sample magnetometry (VSM), and electron spin resonance (ESR). The characterization analysis revealed that the defects in as-synthesized goethite primarily existed in the form of Fe vacancies. Batch experiments demonstrated that the adsorption capacities of defect-rich goethite for As(V) and As(III) removal were 10.2 and 22.1 times larger than those of defect-poor goethite, respectively. The origin of the impact of Fe defects on arsenic immobilization was theoretically elucidated using density functional theory (DFT) calculations. The enhanced adsorption of goethite was attributed to the improvement of the arsenic affinity due to the Fe vacancy defect, thus considerably promoting arsenic immobilization. The findings of this study provide important insight into the migration and fate of arsenic in naturally occurring iron (hydr)oxides.