Purification of high-arsenic groundwater by magnetic bimetallic MOFs coupled with PMS: Balance of catalysis and adsorption and promotion mechanism of PMS

Purification of high-arsenic (>10 µg/L) groundwater is of significance for drinking water safety all around the world. Considering the dominant presence of arsenite (As(III)) in groundwater and ineffective adsorption of As(III) by traditional adsorbents, the magnetic Zn/Co bimetallic MOFs (Zn/Co MOFs) which contain both catalytic and adsorption sites were prepared, and coupled with peroxymonosulfate (PMS) to realize the efficient simultaneous oxidation of As(III) and adsorption of evolved As(V). Considering the vital roles of oxidation and adsorption, the molar ratio of Zn and Co precursors was tuned ranging from 1: 1 to 9: 1 during Zn/Co MOFs synthesis to balance the catalysis and adsorption processes in the heterogeneous PMS system for As(III) immobilization. It was found that Zn/Co MOFs with Zn and Co molar ratio of 5: 1 possessing moderate catalytic performance and best adsorption capacity outperformed removal performance for As(III). The presence of PMS significantly promoted the elimination of both As(III) (the initial adsorption rate increased from 31.19 to 358.96 µg/g/min) and As(V) (the initial adsorption rate increased from 354.12 to 497.85 µg/g/min), which not only oxidized As(III) into As(V) by reactive species generated from PMS activation but also provided additional adsorption sites through enlarging pore size and changing chemical environment. Furthermore, Zn/Co MOFs coupled with the PMS system exhibited outstanding reusability and effectiveness for remediation of real high-arsenic groundwater.