Mechanism of reduction and immobilization of Cr(VI) by application modified nano-zerovalent iron

Hexavalent chromium [Cr(VI)] is a highly toxic agent that seriously threatens the environment and human health. In-situ reduction and immobilization are potential strategies to treat Cr(VI)-contaminated soil. In this study, sodium alginate (SA) and carboxymethylcellulose (CMC) were used to modify nano-zero-valent iron (NZVI) to prepare SA-NZVI and CMC-NZVI to study its effect on the reduction efficiency of Cr(VI) in the soil. Scanning electron microscope (SEM) images found that compared with the agglomeration of NZVI and CMC-NZVI, SA-NZVI particles have a significant improvement, better dispersion, chain-like distribution, and a particle size of 50–100 nm. And the X-ray diffraction (XRD) pattern further showed that SA-NZVI has only Fe0 peaks and no Fe3O4 peaks, which further proved that it is pure nano-zero-valent iron. The batch tests showed that SA-NZVI could quickly and effectively reduce the Cr(VI) in the soil, and SA-NZVI could reduce the Cr(VI) by 96.68%. For Cr(VI)-contaminated soil, the Cr(VI) reduction rate of SA-NZVI was higher than NZVI and CMC-NZVI. After 28 d remediation, the fractions of Cr forms in soil, such as exchangeable (EX), carbonate-bound (CB), Fe-Mn oxides-bound (OX), and organic matter-bound (OM), were mainly transformed to residual fraction (RS) species because of the production of ferrochrome precipitates. X-ray photoelectron spectroscopy (XPS) showed that after SA-NZVI treatment, Cr(VI) was reduced to trivalent chromium [Cr(III)] and the formation of Cr(OH)3 precipitation. The research results provide a theoretical basis for applying SA-NZVI in Cr(VI) contaminated soil remediation.