Enhanced synergistic removal of Cr(VI) and Cd(II) with bi-functional biomass-based composites

Cr(VI) and Cd(II) are typical heavy metal ions and their co-removal is significant. Nitrogen-doped, bi-functional, and biomass-based composites (CP-BA) were successfully applied for efficient and synergistic removal of Cr(VI) and Cd(II). The mutual promotion ratios of adsorption capacity by Cr(VI) and Cd(II) were 161.32 % and 14.13 %, respectively. In addition, all coexisting inorganic anions could extremely promote the removal of Cd(II) with the best promotion ratio upon phosphate ions as high as 80.00 %. The following co-removal mechanisms were deeply revealed: (1) Cr(VI) combined with protonated imine groups could weaken the electrostatic repulsion between CP-BA and Cd(II) by electrostatic shielding, and further promoted the coordination of Cd(II) with hydroxyl, carboxyl and neutral imine groups. (2) Cd(II) which were coordinated to neutral imine groups could form cation bridges, and thus promoted the interaction with Cr(VI) because of the electrostatic attraction. Moreover, the removal capacity for Cr(VI) and Cd(II) did not display obvious reduction even after four cycles. Therefore, CP-BA showed a great potential in the co-removal of inorganic anion-cation complexes from wastewater.