Polymetallic sulfide MSx (M = Ni, Fe, and Mo) microspheres for highly selective extraction of Ag+ over Cu2+ and reduction of Ag+ ions to Ag0 metals

Abstract

In this work, through a facile one-step hydrothermal reaction, MoO₄^2- intercalated NiFe-LDH is obtained, from which a series of multi-metal sulfides of MS_x-T (M = Ni, Fe, and Mo, and T refers to sulfuration temperature) are fabricated by sulfuration at different temperature. The NiFeMoS materials present super-fluffy structure with micron spheres assembled by ultrathin nanosheets, resulting in excellent adsorption performance toward silver ions. The optimized NiFeMoS-500 exhibits a maximum Ag(I) capture of 308 mg·g⁻¹ profiting from the hard and soft acid and base (HSAB) affinity and oxidation–reduction (REDOX) reaction. The 10 ppm of Ag⁺ can be reduced to 0.016 ppm (16 ppb) within 10 min, giving a ∼ 100 % removal. For trace Ag⁺ (∼1 ppm) with a high Cu:Ag ratio of 1000:1, a large separation factor SF_Ag/Cu (=K_d^Ag/K_d^Cu) of 1.80 × 10⁴ is achieved, and for a more dilute solution of Ag⁺ (∼0.5 ppm) with Cu:Ag ratio of 40:1, a much larger SF_Ag/Cu of 1.07 × 10⁵ is gained. The S^2- in MS_x as a soft Lewis base induces the capture of Ag⁺ as a soft Lewis acid, and the Mo⁴⁺ and S^2- of MoS₂ work as reducing agents to reduce Ag⁺ ions to Ag⁰ crystals with morphology of cypress leaves in nanoscale. The Mo⁴⁺ can be oxidized to MoO₄^2- which further binds with Ag⁺ forming Ag₂MoO₄, thus increasing the silver capture. This work provides inspiration to tailor effective adsorbents for trapping Ag⁺ from wastewater or extracting noble metals from silver-bearing copper ores leachates.