Enhanced removal of Ni(II) by nanoscale zero valent iron supported on Na-saturated bentonite

Nanoscale zero valent iron (NZVI) can remove Ni(II) from wastewater through surface adsorption and then reduction into lower-toxic Ni⁰, but the reduction is often blocked by the iron oxide shell of NZVI. In this study, the negatively charged Na-saturated bentonite (Na-bent) with high adsorption capacity to Ni(II) was used to support NZVI for improving the removal and reduction of Ni(II), and the functions of Na-bent were investigated by X-ray photoelectron micro-spectroscopy (XPS), transmission electron microscope (TEM) and Fe(II) determination. The results showed that Na-bent as a carrier could enrich Ni(II) on the reaction surface, protect the surface of NZVI from oxidation, prevent the aggregation of NZVI particles, and decrease the iron oxides products on NZVI surface by pH buffering. Therefore, NZVI/Na-bent not only showed much higher removal efficiency of Ni(II) (98.5%) than the sum (48.8%) of those by bare NZVI removal (41.9%) and by Na-bent adsorption (6.9%), but also greatly enhanced the reduction efficiency of Ni(II) into Ni⁰ by facilitating the electron transfer from Fe⁰ core to the surface-adsorbed Ni(II). In general, the unique property of bentonites will provide effective solutions to support NZVI for enhancing the removal and transformation of various environmental contaminants.