In-situ forming Sub-2 nm hydrous iron oxide particles in MOFs for deep-treatment and high anti-interference in arsenic removal

• In-situ forming sub-2 nm hydrous iron oxide particles in a MOF under room temperature. • A new composite rationally combining nanoparticles and MOFs. • A new method for MOF post-modification starting with the metal nodes. • Being able to deeply decrease As(V) concentration from 3900 μg L −1 to 5.9 μg L −1. An adsorbent with deep-treatment ability and high selectivity for arsenic removal is strongly in need in water treatment. A new composite structure combining ultrasmall (sub-2 nm) nanoparticles (NPs) with metal–organic frameworks (MOFs) has been prepared here. In this structure, ultrasmall (sub-2 nm) hydrous ferric oxide nanoparticles (HFeO) were in-situ generated inside a Fe-based MOF (MIL-100) through a room-temperature reduction process. The structure, denoted as HFeO@MIL-100, demonstrates extraordinary adsorption ability for As (V), with a high capacity and more than 20 times faster kinetics than that of MIL-100. More importantly, it can deeply treat As (V) in contaminated water from 3900 μgL −1 to 5 μg L −1 , directly meeting the WHO drinking water standard (<10 μg L −1 ). And due to the size-exclusion effect of uniform micropores originated from MOFs, the structure performs high anti-interference ability in As (V) adsorption, maintaining more than 97% capacity even within abundant competitive ions or species, such as humic acid, silicate and sulfate ions.