Critical layer in liquid-solid system influencing the remediation of chromium using zeolite-supported sulfide nano zero-valent iron

•The form of chromium in critical layer affected by its initial concentrations.•The critical layer showed a significant influence in Cr(VI) capture.•The critical layer was composed of Cr(VI), O, H and a small amount of Fe(II).•Z/S-nZVI possessed high selectivity and stability. Sulfidated nano zero-valent iron particles were immobilized on ZSM-5 zeolite (Z/S-nZVI) and used for hexavalent chromium (Cr(VI)) remediation. The performance of Z/S-nZVI improved with the increase in Cr(VI) concentration (< 60 mg/L), while the performance significantly decreased for a Cr(VI) concentration of more than 60 mg/L. The adsorption behavior for Cr(VI) was different from that reported in previous studies. The improved performance can be tailored for increasing efficiency of nano zero-valent iron (nZVI) corrosion, while the degree of corrosion of nZVI was affected by the concentration of the pollutant as discussed by kinetics, X-ray diffraction (XRD) and X-ray photoelectron spectrometer (XPS) analyses. The experiments for the dissolution of ferrous ions and the dosage of adsorbent demonstrated that the critical layer in the liquid-solid system changed with the increase in the concentration of Cr(VI) (Cr(VI): Z/S-nZVI > 0.6). Moreover, the removal mechanisms of Cr(VI) were elucidated through XRD, transmission electron microscopy (TEM) and XPS techniques. This results demonstrate that the species of chromium in the critical layer changed from Cr(III) to Cr(VI) as the concentration of chromium increased from low to high. Furthermore, the critical layer was composed of Cr(VI), Fe(II), O and H elements. Additionally, the experiments of coexisting ions and aging time confirmed that Z/S-nZVI possessed high selectivity and stability to ensure efficiency and cost-effectiveness in practical applications. [Display omitted]