Quaternary Fe-Ni-Co-S nanostructures: Unprecedented removal capacity of congo red and toxic metal ions

Abstract

A fresh adsorbent, quaternary Fe-Ni-Co-S nanostructures (FNCS) have been synthesized by a very simple precipitation method in the water at low temperatures. The newly synthesized nanostructure, its composition, and quaternary nanostructures were established by XRD, XPS, SEM, TEM, EDS, and BET analysis. The nanostructure was subsequently subjected to adsorption studies for organic dyes and metal ions. It was found unprecedently effective towards Congo red, Cr(VI) and As(V) ions with the maximum adsorption capacity of 4259 mg g−1, 1178 mg g−1, and 2771 mg g−1 respectively, from Zhu and Gu isotherm model. The adsorption mechanism was studied through FTIR, Zeta potential, and UV-Vis studies. A two-step adsorption mechanism was found to be involved in the adsorption of adsorbates. The first step is monolayer adsorption mainly through hydrogen bonding and metal coordination, and the second step is hydrophobic interaction (π-π interactions) between adsorbate molecules. The π-π interactions between adsorbate molecules cause the precipitation of adsorbates on the surface of the nanostructure, resulting in unprecedented adsorption of Congo red, Cr(VI), and As(V) ions. Furthermore, Cr(VI) removal mechanism was also investigated through XPS and EDS analysis, which elucidated the reduction of Cr(VI) to Cr(III) along with the precipitation of Cr(VI) on the surface of the nanostructure. The nanostructure was able to remove around 99% of the synthetic effluent at lower concentrations from synthetic wastewater. The thermodynamic study confirms that the adsorption is spontaneous, exothermic, and favored at low temperatures