Co-doped N and Fe(III) improved amphoteric adsorption performance of raw chitosan.
Coexistent Cu(II) and Cr(VI) enhanced mutual adsorption capacities and speeds.
Common inorganic anions and heavy-metal cations promoted Cu(II) and Cr(VI) removal.
Cr(VI) induced H+ consumption and shielding effect to improve Cu(II) adsorption.
Cu(II) attracted Cr(VI) as cation-bridge and replaced H+ to enhance Cr(VI) redox.