Removal/adsorption mechanisms of Cr(VI) and natural organic matter by nanoscale zero-valent iron-loaded biochar in their coexisting system

Remediation of Cr(VI) pollution by nanoscale zero-valent iron-loaded biochar (nZVI-BC) is a hot research topic at present. However, the interaction of natural organic matter (NOM) and Cr(VI) on their adsorptions on nZVI-BC is still not well addressed. Here, a series of nZVI-BCs were prepared with different Fe:C ratios using reed straw biochar as the supporter. By screening, the optimal Fe:C ratio for nZVI-BC preparation was the 3:1, owing to its greatest Cr(VI) removal capacity (105 mg g−1). Then, the adsorption experiments under different addition conditions of Cr(VI) and NOM [using humic acid (HA) as the presentative] were conducted to systematically explore the interaction mechanism of NOM and Cr(VI) on their adsorptions. The obtained results demonstrated that the removal capacities of Cr(VI) or HA significantly decreased with rising HA or Cr(VI) concentration, respectively. Hence, a strong competitive adsorption between Cr(VI) and HA on the nZVI-BC was confirmed. The nZVI-BC samples, including the original one and those collected from different solution systems, were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscope (XPS) to further investigate interaction mechanisms of Cr(VI) and HA on their removal/adsorption. The enhanced electrostatic repulsion effect and FeOOH formation after adsorbing HA were suggested as the main mechanisms for the decreased Cr(VI) reduction and adsorption, and the formations of passivation layers and Fe(III)-Cr(III)(oxy)hydroxides on the surface of nZVI-BC were identified as the main mechanisms for the decreased adsorption of HA.