Micropore sites in ferrihydrite are responsible for its higher affinity towards As(III) relative to As(V)

Poorly-crystalline ferrihydrite is ubiquitous in the environment and contributes to controlling the fate of arsenic in sediments and soils. Although there is evidence that ferrihydrite has a higher affinity towards As(III) relative to As(V), little is known about how and why As(III) is readily immobilized by ferrihydrite. In this study, ferrihydrite was employed to evaluate the As(III) and As(V) adsorption behavior. The properties of ferrihydrite such as morphology, pore size distribution, arsenic adsorption species, and adsorption energies of arsenic at different sites were carefully examined using TEM-EDS mapping, positron annihilation lifetime (PAL) spectroscopy, X-ray absorption spectroscopy (XAS), N2 adsorption isotherms, and theoretical calculations of density function theory (DFT). Batch adsorption experiments revealed that the maximum adsorption of As(III) (839.7 μmol g−1) on ferrihydrite, fitted by Langmuir model, was considerably larger than that of As(V) (372.4 μmol g−1). PAL characterization and pore size distribution analysis demonstrated that ferrihydrite had an abundance of vacancy cluster-like micropores, which consisted of 10–20 atom deficiencies (V10-20). The calculated hydrated ion size of As(III) species (4.22 Å) at pH 6.0 was smaller than the size of As(V) (5.90 Å). The higher As(III) adsorption to ferrihydrite was attributed to its surface adsorption sites as well as its abundant micropore adsorption sites, which are available only to As(III) due to their size matching well with the size of the ferrihydrite micropores, thus significantly contributing to greater As(III) immobilization.