Mechanistic insight into the adsorption of mercury (II) on the surface of red mud supported nanoscale zero-valent iron composite.

Recently, nanoscale zero-valent iron (nZVI) particles have been efficiently used in the remediation of many heavy metals, yet potential agglomeration and loss of nZVI remain a critical area of research. In this study, we used red mud as a stable supporting medium to develop red mud modified nZVI to form (RM-nZVI) composite. We assessed its sorptive/reductive removal of mercury (Hg2+) from aqueous solutions. The RM-nZVI was synthesized through the reduction of ferric iron by sodium borohydride (NaBH 4) in the presence of red mud. Morphological characterization of RM-nZVI confirmed its diffusion state with lesser aggregation. The RM-nZVI has the BET surface area, pore diameter, and pore volume as 111.59 m2g−1, 3.82 nm, and 0.49 cm3g−1, respectively. Adsorption of mercury (Hg2+) by RM-nZVI exhibits pH-dependent behavior with increased removal of Hg2+ with the increase in pH up to 5, and the removal rate decreased gradually as the pH increased from 5 to 10. Extensive characterization of RM-nZVI corroborated the evidence that the removal of Hg2+ was initially by rapid physical adsorption, followed by a reduction of Hg2+ to Hg0. The adsorption data were best fitted with Langmuir isotherm with R2 (correlation coefficient) > 0.99 with high uptake capacity of 94.58 (mg g−1). The novel RM-nZVI composite with enhanced sorptive and reductive capacity is an ideal alternative for removing Hg2+ from contaminated water. [Display omitted] • RM-nZVI was more effective to remove Hg(II) ions from aqueous solution. • The removal capacity of Hg(II) by RM-nZVI was 94.58 mg g−1. • Hg2+ is completely reduced to Hg0 via a Hg2+-Fe0 replacement reaction. • Mechanisms of reduction of Hg2+ to Hg0 is explained by XRD, XPS and FTIR. [ABSTRACT FROM AUTHOR]