Nano zero-valent iron loaded corn-straw biochar for efficient removal of hexavalent chromium: remediation performance and interfacial chemical behaviour

Abstract

To improve the poor stability of nano zero-valent iron (nZVI), corn-straw biochar (BC) was used as a support for the synthesis of composites of nZVI-biochar (nZVI/BC) in different mass ratios. After a thorough characterization, the obtained nZVI/BC composite was used to remove hexavalent chromium [Cr(vi)] in an aquatic system under varying conditions including composite amount, Cr(vi) concentration, and pH. The obtained results show that the treatment efficiency varied in the following order: nZVI-BC (1 : 3) > nZVI-BC (1 : 5) > nZVI alone > BC alone. This order indicates the higher efficiency of composite material and the positive effect of nZVI content in the composite. Similarly, the composite dosage and Cr(vi) concentration had significant effects on the removal performance and 2 g L^-1 and 6 g L^-1 were considered to be the optimum dose at a Cr(vi) concentration of 20 mg L^-1 and 100 mg L^-1, respectively. The removal efficiency was maximum (100%) at pH 2 whereas solution pH increased significantly after the reaction (from 2 to 4.13). The removal kinetics of Cr(vi) was described by a pseudo-second-order model which indicated that the removal process was mainly controlled by the rate of chemical adsorption. The thermodynamics was more in line with the Freundlich model which indicated that the removal was multi-molecular layer adsorption. TEM-EDS, XRD, and XPS were applied to characterize the crystal lattice and structural changes of the material to specify the interfacial chemical behaviour on the agent surface. These techniques demonstrate that the underlying mechanisms of Cr(vi) removal include adsorption, chemical reduction-oxidation reaction, and co-precipitation on the surface of the nZVI-BC composite. The results indicated that the corn-straw BC as a carrier material highly improved Cr(vi) removal performance of nZVI and offered better utilization of the corn straw.

Fig. 1. (a and b) TEM micrographs of biochar (BC); (c) TEM micrographs of nano zero-valent iron (nZVI); (d–f) TEM micrographs of nZVI–BC (W_Fe : W_BC = 1 : 3) materials; (g) HR-TEM images of nZVI–BC biochar; (h) size distribution curve of nZVI particles on BC surface.

Fig. 2. (a–d) EDS images of prepared biochar (BC); (e–i) EDS images of prepared nZVI–BC (W_Fe : W_BC = 1 : 3) materials.

Fig. 3. Comparison of removal capacity of different adsorbents addition: (a) net iron addition: 0.5 g L⁻¹; (b) net iron addition: 1.0 g L⁻¹; the Cr(vi) solution volume: 100 mL; initial pH value: 6.0 ± 0.2.

Fig. 4. The effect of nZVI–BC (W_Fe : W_BC = 1 : 3) dosage and Cr(vi) initial concentration on: (a) removal efficiency; (b) removal capacity (the Cr(vi) solution volume: 100 mL; initial pH value: 6.0 ± 0.2; (a–d) means the significance difference level).

Fig. 5. (a) Effect of pH on Cr(vi) remediation (Cr(vi) concentration: 100 mg L⁻¹; nZVI–BC (W_Fe : W_BC = 1 : 3) dosage: 2 g L⁻¹; Cr(vi) solution volume: 100 mL); (b) pH value change in remediation process (nZVI–BC (W_Fe : W_BC = 1 : 3) dosage: 2 g L⁻¹; Cr(vi) solution volume: 100 mL).

Fig. 6. Kinetics fitting results of Cr(vi) removal: (a and b) nZVI–BC (W_Fe : W_BC = 1 : 3) dosage: 2 g L⁻¹; (c and d) nZVI–BC (W_Fe : W_BC = 1 : 3) dosage: 6 g L⁻¹ (pH value: 6.0 ± 0.2) (solution volume: 100 mL; pH value: 6.0 ± 0.2).

Fig. 7. Isotherms fitting results of Cr(vi) removal: (a) nZVI–BC (W_Fe : W_BC = 1 : 3) dosage: 2 g L⁻¹; (b) nZVI–BC (W_Fe : W_BC = 1 : 3) dosage: 6 g L⁻¹ (solution volume: 100 mL; pH value: 6.0 ± 0.2).

Fig. 8. XRD scan of the material (a) BC; (b) nZVI–BC (W_Fe : W_BC = 1 : 3) composite; (c) nZVI–BC (W_Fe : W_BC = 1 : 3) composite after reaction with Cr(vi) at the concentration of 20 mg L⁻¹; (d) nZVI–BC (W_Fe : W_BC = 1 : 3) composite after reaction with Cr(vi) at the concentration of 100 mg L⁻¹ (dosage of nZVI–BC (W_Fe : W_BC = 1 : 3): 2 g L⁻¹; the Cr(vi) solution volume: 100 mL; initial pH value: 6.0 ± 0.2).

Fig. 9. XPS scan of nZVI–BC (W_Fe : W_BC = 1 : 3) composites before and after reaction with different concentration of hexavalent chromium: (a) C 1s scan; (b) full scan; (c) Fe 2p high spectral scan; (d) Cr 2p high spectral scan (dosage of nZVI–BC (W_Fe : W_BC = 1 : 3) 2 g L⁻¹; the Cr(vi) solution volume: 100 mL; initial pH value: 6.0 ± 0.2).

Fig. 10. Mechanism of Cr(vi) removal by nZVI–BC in aqueous medium.