The influence of aluminum chloride on biosynthetic schwertmannite and Cu(ii)/Cr(vi) adsorption

Abstract

The iron oxyhydroxides schwertmannite and akaganèite are recognized as promising absorbents for heavy metal contaminated water treatment. In this study, aluminium chloride modified schwertmannite was biosynthesized using Acidithiobacillus ferrooxidans and characterized using SEM, FTIR, XRD, TGA, potentiometric titrations and XPS. The effect of pH, and the adsorption kinetics, isotherms and mechanism were systematically investigated. With increasing aluminium chloride, the adsorbent transformed from nanoscale villus covered spherical aggregates to rod like structures consisting of globes, and facilitated the decreasing crystallinity and akaganèite formation. The optimum pH for Cr(VI) adsorption was between 6.0 and 7.0, while Cu(II) adsorption was increased over the pH range of 4.0–8.0. The adsorption kinetics varied with pH and the adsorbents. Equilibrium was reached within 30 min for both metals, and the pseudo-second-order model fitted the adsorption process well. The maximum Cu(II) and Cr(VI) adsorption reached 23.95 and 57.60 mg g⁻¹ which occurred at an FeSO₄·7H₂O/AlCl₃ ratio of 15 : 5 and 15 : 10 respectively. As for the binary metal system, the maximum adsorption for Cu(II) and Cr(VI) was 14.10 and 28.89 mg g⁻¹, respectively, achieved with an FeSO₄·7H₂O/AlCl₃ ratio of 15 : 5. The modification enhanced the adsorption capacity effectively. Additionally, the adsorbent could be effectively regenerated through pH 2.0 water washing. FTIR, XPS and released proton correlation analysis revealed that –O–H, O–H–Cl and SO₄²⁻ were the key groups in adsorption. Heavy metals were adsorbed on schwertmannite through anion-exchange and surface complexation. Heavy metals can be efficiently removed by the modified biosynthetic schwertmannite.