Nanoscale Fe(0)-zeolite composite derived from coal bottom ash for efficient treatment of Cr(VI)-contaminated groundwater: Unveiling the importance of locations for surface-bound Fe(II) and Fe(0) passivation products.

The synthesis of coal bottom ash-induced zeolite (Si-Al material) has been widely reported; however, the selective recovery of the three main elements, viz., Si, Al, and Fe, from coal bottom ash for the synthesis of reactive adsorbents has not yet been reported. In this study, we separated the magnetic and non-magnetic fractions of coal bottom ash to selectively recover Fe and Si-Al for synthesizing nanoscale zero-valent iron@zeolite (NZVI@ZBA) composites with uniform formation of Fe(0) nanoparticles on the ZBA surface. NZVI@ZBA exhibited a higher removal capacity for Cr(VI) (153.9 mg g) than bare NZVI (3.6 times), NZVI@AlO (4.1 times), and NZVI@SiO (3.5 times). The enhanced Cr(VI) removal was primarily attributed to well-distributed NZVI particles and the formation of surface-bound Fe(II) on the large surface area of ZBA. A significant portion of NZVI passivation products (i.e., CrFe(OH)) was formed on the ZBA surface rather than on the NZVI surface, whereas bare NZVI signified that the entire NZVI surface was covered by passivation products, blocking electron transfer from the core NZVI. Finally, an adverse effect of Ca²⁺ was observed during the groundwater test, because Ca occupied the adsorption sites of ZBA that were available for the released Fe²⁺ from NZVI. The novel findings in this study can provide insights into the complete recycling of coal bottom ash to produce value-added materials and highlight the importance of the formation location of surface-bound Fe(II) and Fe(0) passivation products when the NZVI@support composite is applied for the reductive removal of contaminants.