In　this　study,　we　demonstrated　the　effective　acquisition　of　magnetic　iron　oxide　(MIO)　for　As(V)　adsorption　by　high-temperature　pyrolysis　of　waste　iron　sludge　from　the　water　treatment　plant　under　a　confined　environment　without　adding　extra　chemical　reagents.　The　operating　temperature　and　time　in　the　pyrolysis　process　were　optimized　to　improve　the　yield　of　MIO　and　its　As(V)　adsorption　capacity.　MIO500–2(500　°C,　2　h)　had　both　relatively　high　yield　and　arsenic　adsorption　efficiency,　which　was　characterized　by　XRD　and　XPS　as　mainly　γ-Fe2O3　with　small　particle　size　(100–900　nm),　significant　mesopore　(12.43　nm),　high　specific　surface　area　(65.25　m2/g),　and　effective　saturation　magnetization　intensity　(14.45　emu/g).　The　maximum　adsorption　capacity　was　14.2　±　0.4　mg/g,　and　the　removal　rate　could　still　reach　about　80　%　after　five　times　of　adsorbent　regeneration.　Considering　this　facile　preparation　route　and　its　high　yield,　large-scale　production　of　MIO　from　waste　iron　sludge　is　feasible,　which　is　expected　to　provide　a　low-cost　and　efficient　adsorbent　for　the　treatment　of　arsenic-containing　water　in　less　economically　developed　areas.　©　2023