Conventional　polyamide　nanofiltration　(NF)　membranes　(PA-M0)　exhibit　poor　As(III)　removal　efficiency　primarily　due　to　the　neutral　molecular　form　(H3AsO3)　of　As(III)　under　typical　conditions.　Although　alkaline　environments　can　convert　As(III)　into　an　anionic　species　to　enhance　separation,　most　NF　membranes　lack　chemical　stability　at　high　pH.　Herein,　we　develop　a　series　of　alkali-resistant　NF　membranes　(PA-MT,　T　=　1–24　h,　representing　the　reaction　time)　through　H₂SO₄/EtOH-induced　physical　swelling　and　chemical　reconstruction　of　PA-M0,　enabling　efficient　As(III)　removal　in　alkaline　media.　The　optimized　membrane,　PA-M6　h,　improves　As(III)　rejection　substantially　from　19.9　%　to　90.2　%　as　feed　pH　increases　from　7　to　10,　while　maintaining　a　high　water　flux　of　17.7　LMH/bar,　This　performance　considerably　surpasses　that　of　PA-M0　and　previously　reported　NF　systems　(typically　10　–　60　%　rejection).　Furthermore,　PA-M6　h　exhibits　exceptional　antifouling　properties,　sustaining　91.5　%　As(III)　rejection　and　a　stable　flux　of　13.7　LMH/bar　over　extended　cycles　in　the　presence　of　humic　acid　foulants　—　more　than　doubling　the　removal　efficiency　of　PA-M0　(5.9　LMH/bar).　This　study　advances　molecular-scale　interfacial　engineering　in　membrane　design,　promoting　the　development　of　environmentally　sustainable　separation　technologies　and　providing　a　scalable　solution　for　arsenic-safe　water　purification.　©　2025