Effects of hydrogen-peroxide supply rate on schwertmannite microstructure and chromium(VI) adsorption performance

Schwertmannite has attracted increasing interest for its excellent sorption for pollutants such as arsenite [As(III)] and arsenate [As(V)]. Limited studies were conducted with hexavalent chromium [Cr(VI)], especially for schwertmannite synthesized through Fe²⁺ oxidation. The effect of the hydrogen-peroxide (H₂O₂) supply rate on the structural characteristics and Cr(VI) adsorption capacity of schwertmannite is unclear. The morphology, crystallinity, specific surface area (SSA), pore volume and Cr(VI) adsorption of schwertmannite through Fe²⁺ oxidation at different H₂O₂ supply rates were analyzed. A slow H₂O₂ supply could improve the schwertmannite SSA and pore volume. Schwertmannite changed from nanoparticle aggregates (169.31 m²/g SSA, 0.20 cm³/g pore volume) to ball-with-whisker-shaped particles (228.75 m²/g SSA, 0.30 cm³/g pore volume) with a lower H₂O₂ supply rate. The Cr(VI) adsorption capacity increased by 6.25%-11.70% at different given Cr(VI) concentrations. The maximum Cr(VI) adsorption capacity of schwertmannite was 1.89 mmol/g at pH 6.0. Cr(VI) adsorption onto schwertmannite may be attributed mainly to ion exchange with the structural SO₄^2-. More than 91% of the original Cr(VI) adsorption capacity was maintained after four recycles. This study provides novel insights into the effects of H₂O₂ supply rate on the microstructure of schwertmannite and its adsorption capacity for Cr(VI) in aqueous medium.