Adsorption of Cr(VI) and Cu(II) by AlPO4 modified biosynthetic Schwertmannite

•Modifier affected the morphology, crystallinity, thermostability, acid–base property.•The maximum Cu(II)/Cr(VI) adsorption capacity reached 50.14 and 38.82mgg−1.•The modification enhanced the selective Cr(VI) adsorption in binary metals system.•Materials possess well reusability and adsorption performance in presence of anion.•Metals adsorbed on Schwertmannite through surface complexation and anion-exchange. Iron-based minerals play a pivotal role in controlling the mobility and bioavailability of heavy metals in natural environment. In this study, Schwertmannite modified with aluminum phosphate was biosynthesized by Acidithiobacillus ferrooxidans and characterized by SEM, FTIR, XRD, TGA, potentiometric titrations and XPS. The effect of pH, adsorption kinetic, isotherm and mechanism were investigated. The results showed that the biosynthesized materials transformed from villous spherical aggregates to smooth globule, and the crystallinity decreased with the aluminum phosphate content increasing. The optimum pH for Cr(VI) adsorption was at 5.0, while the adsorption capacity for Cu(II) increased over pH range 3.0–8.0. Adsorption kinetic varied with pH and adsorbents. The maximal Cu(II) and Cr(VI) adsorption capacity reached 50.14 and 38.82mgg−1, while the adsorption capacity ratio in binary metals system were 29.92/8.09, 29.06/7.34, 25.04/11.42 and 19.66/13.32mgg−1 with FeSO4·7H2O/AlPO4 ratio varied from 20/0.1 to 20/5g. The modification enhanced the Cr(VI) selective adsorption in binary metals system. Moreover, the materials could be effectively regenerated through pH 2.0 water washing. pH variation, FTIR and XPS analysis revealed that heavy metals adsorbed on Schwertmannite through surface complexation and anion-exchange. Heavy metals in solution can be selectively removed by the modified biosynthetic Schwertmannite.